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Essay On The Volcano – 10 Lines, Short & Long Essay For Kids

Key Points To Remember When Writing An Essay On The Volcano For Lower Primary Classes

10 lines on the volcano for kids, a paragraph on the volcano for children, short essay on volcano in 200 words for kids, long essay on volcano for children, interesting facts about volcanoes for children, what will your child learn from this essay.

A volcano is a mountain formed through an opening on the Earth’s surface and pushes out lava and rock fragments through that. It is a conical mass that grows large and is found in different sizes. Volcanoes in Hawaiian islands are more than 4000 meters above sea level, and sometimes the total height of a volcano may exceed 9000 meters, depending on the region it is found. Here you will know and learn how to write an essay on a volcano for classes 1, 2 & 3 kids. We will cover writing tips for your essay on a volcano in English and some fun facts about volcanoes in general.

Volcanoes are formed as a result of natural phenomena on the Earth’s surface. There are several types of volcanoes, and each may emit multiple gases. Below are some key points to remember when writing an essay on a volcano:

• Start with an introduction about how volcanoes are formed. How they impact the Earth, what they produce, and things to watch out for.
• Discuss the different types of volcanoes and talk about the differences between them.
• Cover the consequences when volcanoes erupt and the extent of the damage on Earth.
• Write a conclusion paragraph for your essay and summarise it. 

When writing a few lines on a volcano, it’s crucial to state interesting facts that children will remember. Below are 10 lines on volcanoes for an essay for classes 1 & 2 kids.

• Some volcanoes erupt in explosions, and then some release magma quietly.
• Lava is hot and molten red in colour and cools down to become black in colour. 
• Hot gases trapped inside the Earth are released when a volcano erupts.
• A circle of volcanoes is referred to as the ‘Ring of Fire.’
• Volcano formations are known as seismic activities.
• Active volcanoes are spread all across the earth. 
• Volcanoes can remain inactive for thousands of years and suddenly erupt.
• Most volcanic eruptions occur underwater and result from plates diverging from the margins.
• Volcanic hazards happen in the form of ashes, lava flows, ballistics, etc.
• Volcanic regions have turned into tourist attractions such as the ones in Hawaii.

Volcanoes can be spotted at the meeting points of tectonic plates. Like this, there are tons of interesting facts your kids can learn about volcanoes. Here is a short paragraph on a volcano for children:

A volcano can be defined as an opening in a planet through which lava, gases, and molten rock come out. Earthquake activity around a volcano can give plenty of insight into when it will erupt. The liquid inside a volcano is called magma (lava), which can harden. The Roman word for the volcano is ‘vulcan,’ which means God of Fire. Earth is not the only planet in the solar system with volcanoes; there is one on Mars called the Olympus Mons. There are mainly three types of volcanoes: active, dormant, and extinct. Some eruptions are explosive, and some happen as slow-flowing lava.

Small changes occur in volcanoes, determining if the magma is rising or not flowing enough. One of the common ways to forecast eruptions is by analysing the summit and slopes of these formations. Below is a short essay for classes 1, 2, & 3:

As a student, I have always been curious about volcanoes, and I recently studied a lot about them. Do you know? Krakatoa is a volcano that made an enormous sound when it exploded. Maleo birds seek refuge in the soil found near volcanoes, and they also bury their eggs in these lands as it keeps the eggs warm. Lava salt is a popular condiment used for cooking and extracted from volcanic rocks. And it is famous for its health benefits and is considered superior to other forms of rock or sea salts. Changes in natural gas composition in volcanoes can predict how explosive an eruption can be. A volcano is labelled active if it constantly generates seismic activity and releases magma, and it is considered dormant if it has not exploded for a long time. Gas bubbles can form inside volcanoes and blow up to 1000 times their original size!

Volcanic eruptions can happen through small cracks on the Earth’s surface, fissures, and new landforms. Poisonous gases and debris get mixed with the lava released during these explosions. Here is a long essay for class 3 kids on volcanoes:

Lava can come in different forms, and this is what makes volcanoes unique. Volcanic eruptions can be dangerous and may lead to loss of life, damaging the environment. Lava ejected from a volcano can be fluid, viscous, and may take up different shapes. 

When pressure builds up below the Earth’s crust due to natural gases accumulating, that’s when a volcanic explosion happens. Lava and rocks are shot out from the surface to make room on the seafloor. Volcanic eruptions can lead to landslides, ash formations, and lava flows, called natural disasters. Active volcanoes frequently erupt, while the dormant ones are unpredictable. Thousands of years can pass until dormant volcanoes erupt, making their eruption unpredictable. Extinct volcanoes are those that have never erupted in history.

The Earth is not the only planet in the solar system with volcanoes. Many volcanoes exist on several other planets, such as Mars, Venus, etc. Venus is the one planet with the most volcanoes in our solar system. Extremely high temperatures and pressure cause rocks in the volcano to melt and become liquid. This is referred to as magma, and when magma reaches the Earth’s surface, it gets called lava. On Earth, seafloors and common mountains were born from volcanic eruptions in the past.

What Is A Volcano And How Is It Formed?

A volcano is an opening on the Earth’s crust from where molten lava, rocks, and natural gases come out. It is formed when tectonic plates shift or when the ocean plate sinks. Volcano shapes are formed when molten rock, ash, and lava are released from the Earth’s surface and solidify.

Types Of Volcanoes

Given below various types of volcanoes –

1. Shield Volcano

It has gentle sliding slopes and ejects basaltic lava. These are created by the low-viscosity lava eruption that can reach a great distance from a vent.

2. Composite Volcano (Strato)

A composite volcano can stand thousands of meters tall and feature mudflow and pyroclastic deposits.

3. Caldera Volcano

When a volcano explodes and collapses, a large depression is formed, which is called the Caldera.

4. Cinder Cone Volcano

It’s a steep conical hill formed from hardened lava, tephra, and ash deposits.

Causes Of Volcano Eruptions

Following are the most common causes of volcano eruptions:

1. Shifting Of Tectonic Plates

When tectonic plates slide below one another, water is trapped, and pressure builds up by squeezing the plates. This produces enough heat, and gases rise in the chambers, leading to an explosion from underwater to the surface.

2. Environmental Conditions

Sometimes drastic changes in natural environments can lead to volcanoes becoming active again.

3. Natural Phenomena

We all understand that the Earth’s mantle is very hot. So, the rock present in it melts due to high temperature. This thin lava travels to the crust as it can float easily. As the area’s density is compromised, the magma gets to the surface and explodes.

How Does Volcano Affect Human Life?

Active volcanoes threaten human life since they often erupt and affect the environment. It forces people to migrate far away as the amount of heat and poisonous gases it emits cannot be tolerated by humans.

Here are some interesting facts:

• The lava is extremely hot!
• The liquid inside a volcano is known as magma. The liquid outside is called it is lava.
• The largest volcano in the solar system is found on Mars.
• Mauna Loa in Hawaii is the largest volcano on Earth.
• Volcanoes are found where tectonic plates meet and move.

Your child will learn a lot about how Earth works and why volcanoes are classified as natural disasters, what are their types and how they are formed.

Now that you know enough about volcanoes, you can start writing the essay. For more information on volcanoes, be sure to read and explore more.

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Essays About Volcanoes: Top 5 Examples and 10 Prompts

Do you need to write essays about volcanoes but don’t know where to start? Check out our top essay examples and prompts to help you write a high-quality essay.

Considered the planet’s geologic architects, volcanoes are responsible for more than 80% of the Earth’s surface . The mountains, craters, and fertile soil from these eruptions give way to the very foundation of life itself, making it possible for humans to survive and thrive.  

Aside from the numerous ocean floor volcanoes, there are 161 active volcanoes in the US . However, these beautiful and unique landforms can instantly turn into a nightmare, like Mt. Tambora in Indonesia, which killed 92,000 people in 1815 .

Various writings are critical to understanding these openings in the Earth’s crust, especially for students studying volcanoes. It can be tricky to write this topic and will require a lot of research to ensure all the information gathered is accurate. 

To help you, read on to see our top essay examples and writing prompts to help you begin writing.

Top 5 Essay Examples

1. short essay on volcanoes by prasad nanda , 2. types of volcanoes by reena a , 3. shield volcano, one of the volcano types by anonymous on gradesfixer.com, 4. benefits and problems caused by volcanoes by anonymous on newyorkessays.com, 5. volcanoes paper by vanessa strickland, 1. volcanoes and their classifications, 2. a dormant volcano’s eruption, 3. volcanic eruptions in the movies, 4. the supervolcano: what is it, 5. the word’s ring of fire, 6. what is a lahar, 7. why does a volcano erupt, 8. my experience with volcanic eruptions, 9. effects of volcanic eruptions, 10. what to do during volcanic disasters.

“The name, “volcano” originates from the name Vulcan, a god of fire in Roman mythology.”

Nanda briefly defines volcanoes, stating they help release hot pressure that builds up deep within the planet. Then, he discusses each volcano classification, including lava and magma’s roles during a volcanic eruption. Besides interesting facts about volcanoes (like the Ojos del Salado as the world’s tallest volcano), Nanda talks about volcanic eruptions’ havoc. However, he also lays down their benefits, such as cooled magma turning to rich soil for crop cultivation.

“The size, style, and frequency of eruptions can differ greatly but all these elements are correlated to the shape of a volcano.”

In this essay, Reena identifies the three main types of volcanoes and compares them by shape, eruption style, and magma type and temperature. A shield volcano is a broad, flat domelike volcano with basaltic magma and gentle eruptions. The strato or composite volcano is the most violent because its explosive eruption results in a lava flow, pyroclastic flows, and lahar. Reena shares that a caldera volcano is rare and has sticky and cool lava, but it’s the most dangerous type. To make it easier for the readers to understand her essay, she adds figures describing the process of volcanic eruptions.

“All in all, shield volcanoes are the nicest of the three but don’t be fooled, it can still do damage.”

As the essay’s title suggests, the author focuses on the most prominent type of volcano with shallow slopes – the shield volcano. Countries like Iceland, New Zealand, and the US have this type of volcano, but it’s usually in the oceans, like the Mauna Loa in the Hawaiian Islands. Also, apart from its shape and magma type, a shield volcano has regular but calmer eruptions until water enters its vents.

“Volcanic eruptions bring both positive and negative impacts to man.”

The essay delves into the different conditions of volcanic eruptions, including their effects on a country and its people. Besides destroying crops, animals, and lives, they damage the economy and environment. However, these misfortunes also leave behind treasures, such as fertile soil from ash, minerals like copper, gold, and silver from magma, and clean and unlimited geothermal energy. After these incidents, a place’s historic eruptions also boost its tourism.

“Beautiful and powerful, awe-inspiring and deadly, they are spectacular reminders of the dynamic forces that shape our planet.”

Strickland’s essay centers on volcanic formations, types, and studies, specifically Krakatoa’s eruption in 1883. She explains that when two plates hit each other, the Earth melts rocks into magma and gases, forming a volcano. Strickland also mentions the pros and cons of living near a volcanic island. For example, even though a tsunami is possible, these islands are rich in marine life, giving fishermen a good living.

Are you looking for more topics like this? Check out our round-up of essay topics about nature .

10 Writing Prompts For Essays About Volcanoes

Do you need more inspiration for your essay? See our best essay prompts about volcanoes below:

Identify and discuss the three classifications of volcanoes according to how often they erupt: active, dormant or inactive, and extinct. Find the similarities and differences of each variety and give examples. At the end of your essay, tell your readers which volcano is the most dangerous and why.

Volcanoes that have not erupted for a very long time are considered inactive or dormant, but they can erupt anytime in the future. For this essay, look for an inactive volcano that suddenly woke up after years of sleeping. Then, find the cause of its sudden eruption and add the extent of its damage. To make your piece more interesting, include an interview with people living near dormant volcanoes and share their thoughts on the possibility of them exploding anytime.

Choose an on-screen depiction of how volcanoes work, like the documentary “ Krakatoa: Volcano of Destruction .” Next, briefly summarize the movie, then comment on how realistic the film’s effects, scenes, and dialogues are. Finally, conclude your essay by debating the characters’ decisions to save themselves.

The Volcanic Explosivity Index (VEI) criteria interpret danger based on intensity and magnitude. Explain how this scale recognizes a supervolcano. Talk about the world’s supervolcanoes, which are active, dormant, and extinct. Add the latest report on a supervolcano’s eruption and its destruction.

Identify the 15 countries in the Circum-Pacific belt and explore each territory’s risks to being a part of The Ring of Fire. Explain why it’s called The Ring of Fire and write its importance. You can also discuss the most dangerous volcano within the ring.

If talking about volcanoes as a whole seems too generic, focus on one aspect of it. Lahar is a mixture of water, pyroclastic materials, and rocky debris that rapidly flows down from the slopes of a volcano. First, briefly define a lahar in your essay and focus on how it forms. Then, consider its dangers to living things. You should also add lahar warning signs and the best way to escape it.

Use this prompt to learn and write the entire process of a volcanic eruption. Find out the equipment or operations professionals use to detect magma’s movement inside a volcano to signal that it’s about to blow up. Make your essay informative, and use data from reliable sources and documentaries to ensure you only present correct details.

If you don’t have any personal experience with volcanic eruptions, you can interview someone who does. To ensure you can collect all the critical points you need, create a questionnaire beforehand. Take care to ask about their feelings and thoughts on the situation.

Write about the common effects of volcanic eruptions at the beginning of your essay. Next, focus on discussing its psychological effects on the victims, such as those who have lost loved ones, livelihoods, and properties.

Help your readers prepare for disasters in an informative essay. List what should be done before, during, and after a volcanic eruption. Include relevant tips such as being observant to know where possible emergency shelters are. You can also add any assistance offered by the government to support the victims.Here’s a great tip: Proper grammar is critical for your essays. Grammarly is one of our top grammar checkers. Find out why in this  Grammarly review .

ENCYCLOPEDIC ENTRY

A volcano is an opening in a planet or moon’s crust through which molten rock and gases trapped under the surface erupt, often forming a hill or mountain.

Volcanic eruption

Volcanic eruptions can create colorful and dramatic displays, such as this eruption of this volcano in the Virunga Moutains of the Democratic Republic of the Congo.

Photograph by Chris Johns

A volcano is an opening in a planet or moon’s crust through which molten rock, hot gases, and other materials erupt . Volcanoes often form a hill or mountain as layers of rock and ash build up from repeated eruptions .

Volcanoes are classified as active, dormant, or extinct. Active volcanoes have a recent history of eruptions ; they are likely to erupt again. Dormant volcanoes have not erupted for a very long time but may erupt at a future time. Extinct volcanoes are not expected to erupt in the future.

Inside an active volcano is a chamber in which molten rock, called magma , collects. Pressure builds up inside the magma chamber, causing the magma to move through channels in the rock and escape onto the planet’s surface. Once it flows onto the surface the magma is known as lava .

Some volcanic eruptions are explosive, while others occur as a slow lava flow. Eruptions can occur through a main opening at the top of the volcano or through vents that form on the sides. The rate and intensity of eruptions, as well as the composition of the magma, determine the shape of the volcano.

Volcanoes are found on both land and the ocean floor. When volcanoes erupt on the ocean floor, they often create underwater mountains and mountain ranges as the released lava cools and hardens. Volcanoes on the ocean floor become islands when the mountains become so large they rise above the surface of the ocean.

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volcano , vent in the crust of Earth or another planet or satellite , from which issue eruptions of molten rock , hot rock fragments, and hot gases . A volcanic eruption is an awesome display of Earth’s power. Yet , while eruptions are spectacular to watch, they can cause disastrous loss of life and property, especially in densely populated regions of the world. Sometimes beginning with an accumulation of gas -rich magma (molten underground rock) in reservoirs near Earth’s surface, they can be preceded by emissions of steam and gas from small vents in the ground. Swarms of small earthquakes , which may be caused by a rising plug of dense, viscous magma oscillating against a sheath of more-permeable magma, may also signal volcanic eruptions, especially explosive ones. In some cases, magma rises in conduits to the surface as a thin and fluid lava , either flowing out continuously or shooting straight up in glowing fountains or curtains. In other cases, entrapped gases tear the magma into shreds and hurl viscous clots of lava into the air. In more violent eruptions, the magma conduit is cored out by an explosive blast, and solid fragments are ejected in a great cloud of ash-laden gas that rises tens of thousands of metres into the air. One feared phenomenon accompanying some explosive eruptions is the nuée ardente , or pyroclastic flow , a fluidized mixture of hot gas and incandescent particles that sweeps down a volcano’s flanks, incinerating everything in its path. Great destruction also can result when ash collects on a high snowfield or glacier , melting large quantities of ice into a flood that can rush down a volcano’s slopes as an unstoppable mudflow . ( See the table of the world’s major volcanoes by region.)

Strictly speaking, the term volcano means the vent from which magma and other substances erupt to the surface, but it can also refer to the landform created by the accumulation of solidified lava and volcanic debris near the vent. One can say, for example, that large lava flows erupt from Mauna Loa volcano in Hawaii , referring here to the vent; but one can also say that Mauna Loa is a gently sloping volcano of great size, the reference in this case being to the landform. Volcanic landforms have evolved over time as a result of repeated volcanic activity. Mauna Loa typifies a shield volcano, which is a huge, gently sloping landform built up of many eruptions of fluid lava. Mount Fuji in Japan is an entirely different formation. With its striking steep slopes built up of layers of ash and lava, Mount Fuji is a classic stratovolcano . Iceland provides fine examples of volcanic plateaus , while the seafloor around Iceland provides excellent examples of submarine volcanic structures.

Volcanoes figure prominently in the mythology of many peoples who have learned to live with eruptions, but science was late in recognizing the important role of volcanism in the evolution of Earth . As late as 1768, the first edition of the Encyclopædia Britannica gave voice to a common misconception by defining volcanoes as “burning mountains, which probably are made up of sulphur and some other matter proper to ferment with it, and take fire.” Today geologists agree that volcanism is a profound process resulting from the thermal evolution of planetary bodies. Heat does not easily escape from large bodies such as Earth by the processes of conduction or radiation . Instead, heat is transferred from Earth’s interior largely by convection —that is, the partial melting of Earth’s crust and mantle and the buoyant rise of magma to the surface. Volcanoes are the surface sign of this thermal process. Their roots reach deep inside Earth, and their fruits are hurled high into the atmosphere .

Volcanoes are closely associated with plate tectonic activity. Most volcanoes, such as those of Japan and Iceland, occur on the margins of the enormous solid rocky plates that make up Earth’s surface. Other volcanoes, such as those of the Hawaiian Islands, occur in the middle of a plate, providing important evidence as to the direction and rate of plate motion.

The study of volcanoes and their products is known as volcanology , but these phenomena are not the realm of any single scientific discipline . Rather, they are studied by many scientists from several specialties: geophysicists and geochemists , who probe the deep roots of volcanoes and monitor signs of future eruptions; geologists, who decipher prehistoric volcanic activity and infer the likely nature of future eruptions; biologists, who learn how plants and animals colonize recently erupted volcanic rocks; and meteorologists, who determine the effects of volcanic dust and gases on the atmosphere, weather , and climate .

Clearly the destructive potential of volcanoes is tremendous. But the risk to people living nearby can be reduced significantly by assessing volcanic hazards, monitoring volcanic activity and forecasting eruptions, and instituting procedures for evacuating populations. In addition, volcanism affects humankind in beneficial ways. Volcanism provides beautiful scenery, fertile soils , valuable mineral deposits, and geothermal energy . Over geologic time , volcanoes recycle Earth’s hydrosphere and atmosphere.

Volcanic eruptions

Volcanoes, explained

These fiery peaks have belched up molten rock, hot ash, and gas since Earth formed billions of years ago.

Volcanoes are Earth's geologic architects. They've created more than 80 percent of our planet's surface, laying the foundation that has allowed life to thrive. Their explosive force crafts mountains as well as craters. Lava rivers spread into bleak landscapes. But as time ticks by, the elements break down these volcanic rocks, liberating nutrients from their stony prisons and creating remarkably fertile soils that have allowed civilizations to flourish.

There are volcanoes on every continent, even Antarctica. Some 1,500 volcanoes are still considered potentially active around the world today; 161 of those—over 10 percent—sit within the boundaries of the United States .

But each volcano is different. Some burst to life in explosive eruptions, like the 1991 eruption of Mount Pinatubo , and others burp rivers of lava in what's known as an effusive eruption, like the 2018 activity of Hawaii's Kilauea volcano. These differences are all thanks to the chemistry driving the molten activity. Effusive eruptions are more common when the magma is less viscous, or runny, which allows gas to escape and the magma to flow down the volcano's slopes. Explosive eruptions, however, happen when viscous molten rock traps the gasses, building pressure until it violently breaks free.

How do volcanoes form?

The majority of volcanoes in the world form along the boundaries of Earth's tectonic plates—massive expanses of our planet's lithosphere that continually shift, bumping into one another. When tectonic plates collide, one often plunges deep below the other in what's known as a subduction zone .

As the descending landmass sinks deep into the Earth, temperatures and pressures climb, releasing water from the rocks. The water slightly reduces the melting point of the overlying rock, forming magma that can work its way to the surface—the spark of life to reawaken a slumbering volcano.

Not all volcanoes are related to subduction, however. Another way volcanoes can form is what's known as hotspot volcanism. In this situation, a zone of magmatic activity —or a hotspot—in the middle of a tectonic plate can push up through the crust to form a volcano. Although the hotspot itself is thought to be largely stationary, the tectonic plates continue their slow march, building a line of volcanoes or islands on the surface. This mechanism is thought to be behind the Hawaii volcanic chain .

Where are all these volcanoes?

Some 75 percent of the world's active volcanoes are positioned around the ring of fire , a 25,000-mile long, horseshoe-shaped zone that stretches from the southern tip of South America across the West Coast of North America, through the Bering Sea to Japan, and on to New Zealand.

This region is where the edges of the Pacific and Nazca plates butt up against an array of other tectonic plates. Importantly, however, the volcanoes of the ring aren't geologically connected . In other words, a volcanic eruption in Indonesia is not related to one in Alaska, and it could not stir the infamous Yellowstone supervolcano .

What are some of the dangers from a volcano?

Volcanic eruptions pose many dangers aside from lava flows. It's important to heed local authorities' advice during active eruptions and evacuate regions when necessary.

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One particular danger is pyroclastic flows, avalanches of hot rocks, ash, and toxic gas that race down slopes at speeds as high as 450 miles an hour . Such an event was responsible for wiping out the people of Pompeii and Herculaneum after Mount Vesuvius erupted in A.D. 79 .

Similarly, volcanic mudflows called lahars can be very destructive. These fast-flowing waves of mud and debris can race down a volcano's flanks, burying entire towns.

Ash is another volcanic danger. Unlike the soft, fluffy bits of charred wood left after a campfire, volcanic ash is made of sharp fragments of rocks and volcanic glass each less than two millimeters across. The ash forms as the gasses within rising magma expand, shattering the cooling rocks as they burst from the volcano's mouth. It's not only dangerous to inhale , it's heavy and builds up quickly. Volcanic ash can collapse weak structures, cause power outages, and is a challenge to shovel away post-eruption.

Can we predict volcanic eruptions?

Volcanoes give some warning of pending eruption, making it vital for scientists to closely monitor any volcanoes near large population centers. Warning signs include small earthquakes, swelling or bulging of the volcano's sides, and increased emission of gasses from its vents. None of those signs necessarily mean an eruption is imminent, but they can help scientists evaluate the state of the volcano when magma is building.

However, it's impossible to say exactly when, or even if, any given volcano will erupt. Volcanoes don't run on a timetable like a train. This means it's impossible for one to be “overdue” for eruption —no matter what news headlines say.

What is the largest eruption in history?

The deadliest eruption in recorded history was the 1815 explosion of Mount Tabora in Indonesia. The blast was one of the most powerful ever documented and created a caldera —essentially a crater—4 miles across and more than 3,600 feet deep. A superheated plume of hot ash and gas shot 28 miles into the sky, producing numerous pyroclastic flows when it collapsed.

The eruption and its immediate dangers killed around 10,000 people. But that wasn't its only impact. The volcanic ash and gas injected into the atmosphere obscured the sun and increased the reflectivity of Earth, cooling its surface and causing what's known as the year without a summer. Starvation and disease during this time killed some 82,000 more people, and the gloomy conditions are often credited as the inspiration for gothic horror tales, such as Mary Shelley's Frankenstein .

Although there have been several big eruptions in recorded history, volcanic eruptions today are no more frequent than there were a decade or even a century ago. At least a dozen volcanoes erupt on any given day. As monitoring capacity for—and interest in—volcanic eruptions increases, coverage of the activity more frequently appears in the news and on social media. As Erik Klemetti, associate professor of geosciences at Denison University, writes in The Washington Post : “The world is not more volcanically active, we’re just more volcanically aware.”

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Home — Essay Samples — Science — Geology — Different Types of Volcanoes

Different Types of Volcanoes

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Published: Mar 20, 2024

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Shield volcanoes, stratovolcanoes, cinder cones, submarine volcanoes.

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Volcanoes and Their Characteristics Essay

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Volcanoes always presented a broad area for researches in terms of their close relationship between their forms, structures, the styles of their eruption, and the mineral composition of their magma and lava. In the following paper, different types of volcanic mountains will be examined and compared in order to make conclusions concerning the relationship between the volcanoes’ structure and their features. Generally, after evaluating data, it appears that depending on the process of volcano formation it may be related to one of the four existing volcano types which are known for their different characteristics and “conduct” while eruptions.

First of all, speaking about different types of volcanoes’ structure, it should be mentioned that depending on their geographic location and, thus, appurtenance to a certain lithosphere platform with varied features, volcanoes demonstrate different ways of eruption ranging from the so-called “quiet” ones to the very disastrous and dangerous (Furniss 56). In general, volcanoes are subdivided into the following categories: shield volcanoes, composite volcanoes, lava domes or plug domes, and, finally, cinder cones which can be distinguished by their sizes, forms, and functioning. All of these volcano types have their peculiarities in structure and way of eruption.

In addition, the very notion of “volcano” is to be explained. A volcano is a geological landform that is made up of two parts – the upper one called a cone, and the lower one called fissure where volcanic material is accumulated (Hess 391). According to Furniss (57),

The vast majority of volcanoes are found along the boundaries of tectonic plates. Convergent boundaries, where the plates are crashing into each other, host around 90 percent of what we generally think of as volcanoes. Here, as one plate is pushed below another–a process is known as subduction–it melts, the resultant magma rising and causing volcanic eruptions. Volcanoes also form away from plate boundaries, above so-called volcanic hotspots.

Volcanoes are known for their disastrous nature which many times led to serious tragedies for humanity. During volcanoes’ eruptions, numerous developing processes in the crust of the earth are taking place. According to Furniss (58),

Several factors are used to assign eruptions a score, including the volume of erupted material, the height of the eruption column, and the duration. The index ranges between zero and eight, with each increase in score representing a ten-fold increase in the various factors. The highest score on the list, VEI8, is reserved for eruptions that emit more than 1,000 cubic kilometers of material.

Below, all the four types of volcanoes will be addressed along with their features and popularities.

Discussing shield volcanoes, it should be said that they are formed as a result of a huge amount of free lava spilling from a vent and coming up abundantly and widely; gradually congealing lava forms a low and wide mountain of dome shape which is called a shield volcano. These types of volcanoes can be very high. Among them are popular volcanoes in Hawaii.

With regards to composite volcanoes, it should be stated that they are formed as a result of the eruption of both lava and tephra which occurs from a central vent. At the end of the erupting process lava and tephra form a cone in the shape of a tower which is called a composite volcano. These volcanoes tend to develop into mountains of beautiful and symmetrical forms. Among the most famous volcanoes of this type are Mount Fuji in Japan and Mount Rainer in Washington (Hegner 88).

Addressing plug domes, it should be stated that they are formed as a result of congealing of very viscous lava (for example, rhyolite one) which is too thick to flow at a remote distance. As a result of this process, the mountain grows from below and from within. This mountain is called a plug dome. The volcanoes of this type are rather young and can be found in numerous parts of the earth including Mono Lake in California.

And finally, speaking about cinder cones, it should be stated that they are formed as a result of tephra’s building up. A cone-shaped mountain that is the result of this process is called a cinder cone volcano. These volcanic mountains are the smallest ones of all types and are generally less in their size than 500 meters. An example of such a volcanic mountain is SP Mountain situated in Arizona, and belonging to the Colorado Plato.

Concluding on all the information related above, it should be stated that there exists a close connection between the structure of any particular volcano and the style of its functioning and its magma and lava nature. Evaluating a row of facts about the functioning of different types of volcanoes, it appears that depending on the process of volcano formation it may be related to one of the four existing volcano types which are known for their different characteristics and “conduct”. In general, volcanoes are subdivided into the following categories: shield volcanoes, composite volcanoes, lava domes or plug domes, and, finally, cinder cones.

Furniss, Charlie. “Volcanoes: They’re the Most Powerful Expressions of Nature’s Might, Responsible for Mass Extinctions, Global Climate Change, and the Demise of Entire Civilisations, but How Much Do We Really Know About Volcanoes? and Just How Close Are We to the Holy Grail of Accurately Predicting When They’re Going to Explode?.” Geographical Mar. 2007: 52+. Questia . Web.

Hegner, E., et al. “Testing Tectonic Models with Geochemical Provenance Parameters in Greywacke.” Journal of the Geological Society (2005): 87+. Questia . Web.

Hess, Darrel. Physical Geography Laboratory Manual (10th ed.), The United States: Prentice-Hall, 2010. Print.

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Essay on Volcano

Students are often asked to write an essay on Volcano in their schools and colleges. And if you’re also looking for the same, we have created 100-word, 250-word, and 500-word essays on the topic.

Let’s take a look…

100 Words Essay on Volcano

What is a volcano.

A volcano is a crack in the Earth’s surface. Through this crack, melted rock, ash, and gases can escape from deep inside the Earth. Think of it like a soda bottle. If you shake it and then open the top, everything rushes out. That’s similar to what happens during a volcanic eruption.

Types of Volcanoes

There are mainly three types: shield, cone, and composite. Shield volcanoes are broad and flat. Cone volcanoes are steep and pointy. Composite volcanoes are tall and can be very explosive. Each type acts differently when it erupts.

Why Do Volcanoes Erupt?

Deep inside the Earth, it’s so hot that rocks melt into liquid called magma. When magma is lighter than the rock around it, it moves up. If it reaches the Earth’s surface, it erupts. This can happen because of the Earth’s plates moving and creating pressure.

Living with Volcanoes

People live near volcanoes for the fertile soil, which is good for farming. But, living close to a volcano can be dangerous. Scientists help by monitoring volcanoes to predict eruptions and keep people safe.

250 Words Essay on Volcano

A volcano is a crack in the Earth’s surface where molten rock, ash, and gases from deep inside the Earth come out. Think of it like a soda bottle that’s been shaken up. When you open the cap, everything rushes out because of the pressure. In the same way, when a volcano erupts, it releases pressure from beneath the Earth’s crust.

There are different kinds of volcanoes, mainly based on their shape and how often they erupt. Some are called shield volcanoes because they’re broad and low, like a warrior’s shield. Others are called stratovolcanoes, which are tall and steep. They usually have more explosive eruptions. Then there are cinder cone volcanoes, which are smaller and made of bits of rock and ash.

Volcanoes erupt because of the movement of tectonic plates, which are big pieces of the Earth’s surface. When these plates move, they can cause magma from deep inside the Earth to push its way up to the surface. This magma then becomes lava when it comes out of the volcano.

The Impact of Volcanoes

Volcanoes can be dangerous, destroying homes and forests with their lava flows and ash. But they also create new land and bring important nutrients to the soil, which can help plants grow. Plus, the gases they release into the atmosphere can affect the Earth’s climate.

Understanding volcanoes helps us prepare for their eruptions and appreciate the powerful forces that shape our planet.

500 Words Essay on Volcano

Volcanoes: nature’s fiery breath.

Volcanoes are fascinating natural wonders that capture our imaginations. These colossal mountains showcase the immense power of nature, capable of awe-inspiring eruptions and destruction. Let’s explore the world of volcanoes and delve into some of their most intriguing aspects.

A Peek Inside a Volcano

Imagine a giant underground chamber filled with molten rock, known as magma. This magma is incredibly hot, and it’s constantly pushing against the Earth’s crust. When the pressure becomes too intense, it finds a way to escape, and that’s when a volcano erupts.

Types of Volcanic Eruptions

There are various types of volcanic eruptions, each with its own characteristics. Some eruptions are explosive, sending ash and lava soaring high into the air. Others are more gentle, with lava flowing slowly out of the volcano. Some eruptions produce glowing clouds of ash, called pyroclastic flows, which can race down the volcano’s slopes at high speeds.

Volcanic Hazards

While volcanoes can be a sight to behold, they also pose potential hazards. Lava flows can destroy entire villages and forests, and ash clouds can disrupt air travel. Volcanic eruptions can also trigger earthquakes, landslides, and tsunamis.

Predicting Volcanic Eruptions

Volcanoes and the environment.

Volcanic eruptions can have both positive and negative impacts on the environment. On the one hand, they can release harmful gases and ash into the atmosphere, which can affect air quality and climate. On the other hand, volcanic eruptions can create new landforms, provide fertile soil for agriculture, and support unique ecosystems.

Conclusion: The Majestic Force of Nature

Volcanoes are a powerful reminder of the Earth’s dynamic nature. They can be both destructive and awe-inspiring, showcasing the incredible forces that shape our planet. By studying volcanoes, we can better understand the Earth’s processes and prepare for potential hazards, while still appreciating their majestic beauty.

Apart from these, you can look at all the essays by clicking here .

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Essay on volcanoes | geology.

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After reading this article you will learn about:- 1. Introduction to Volcanoes 2. Volcano Formation 3. Volcanic Landforms 4. Major Gases Emitted by Volcanoes 5. Lightning and Whirlwinds 6. Features Produced by the Escape of Gases from Volcanic Lavas 7. Volcanic Products 8. Source of the Explosive Energy 9. Classification of Pyroclastics 10. Lahars-Mudflows on Active and Inactive Cones and Other Details.

Essay Contents:

• Essay on the Volcanoes and Atmospheric Pollution

Essay # 1. Introduction to Volcanoes :

A volcano is a cone shaped hill or mountain which is built-up around an opening in the earth’s surface through which hot gases, rock fragments and lavas are ejected.

Due to the accumulation of the solid fragments around the conduit a conical mass is built which increases in size to become a large volcanic mountain. The conical mass so built-up is called a volcano. However the term volcano is taken to include not only the central vent in the earth but also the mountain or hill built around it.

Volcanoes are in varying sizes, varying from small conical hills to loftiest mountains on the earth’s surface. The volcanoes of the Hawaiian Islands are nearly 4300 metres above sea level since they are built over the floor of the Pacific ocean which at the site is 4300 to 5500 metres deep, the total height of the volcano may be about 9000 m or more.

The very high peaks in the Andes, in the Cascade Range of the Western United States, Mt. Baker, Mt. Adams, Mt. Hood etc. are all volcanoes which have now become extinct. Over 8000 independent eruptions have been identified from earth’s volcanoes. There are many inaccessible regions and ocean floors where volcanoes have occurred undocumented or unnoticed.

The eruption of a volcano is generally preceded by earthquakes and by loud rumblings like thunder which may continue on a very high scale during the eruption. The loud rumblings are due to explosive movement of gases and molten rock which are held under very high pressure. Before eruption of a volcano fissures are likely to be opened, nearby lakes likely to be drained and hot springs may appear at places.

The eruptive activity of volcanoes is mostly named after the well-known volcanoes, which are known for particular type of behaviour, like Strambolian, Vulcanian, Vesuvian, Hawaiian types of eruption. Volcanoes may erupt in one distinct way or may erupt in many ways, but, the reality is, these eruptions provide a magical view inside the earth’s molten interior.

The nature of a volcanic eruption is determined largely by the type of materials ejected from the vent of the volcano. Volcanic eruptions may be effusive (fluid lavas) or dangerous and explosive with blasts of rock, gas, ash and other pyroclasts.

Some volcanoes erupt for just a few minutes while some volcanoes spew their products for a decade or more. Between these two main types viz. effusive and explosive eruptions, there are many subdivisions like, eruption of gases mixed with gritty pulverised rock forming tall dark ash clouds seen for many kilometres, flank fissure eruptions with lava oozing from long horizontal cracks on the side of a volcano.

There is also the ground hugging lethally hot avalanches of volcanic debris called pyroclastic flows. When magma rises, it may encounter groundwater causing enormous phreatic, i.e., steam eruptions. Eruptions may also release suffocating gases into the atmosphere. Eruptions may produce tsunamis and floods and may trigger earthquakes. They may unleash ravaging rockslides and mudflows.

Volcanoes which have had no eruptions during historic times, but may still show fairly fresh signs of activity and have been active in geologically recent times are said to be dormant. There are also volcanoes which were formerly active but are of declining activity a few of which may be emitting only steam and other gases.

Geysers are hot springs from which water is expelled vigorously at intervals and are characteristics of regions of declining volcanic activity. Geysers are situated in Iceland, the Yellowstone park in USA and in New Zealand.

In contrast to the explosive type of volcanoes, there exist eruptions of great lava flows quietly pouring out of fissures developed on the earth’s surface. These eruptions are not accompanied by explosive outbursts. These are fissure eruptions.

Ex: Deccan Trap formations in India. The lavas in these cases are mostly readily mobile and flow over low slopes. The individual flows are seldom over a few meters in thickness; the average thickness may be less than 15 meters. If the fissure eruptions have taken place in valleys however, the thickness may be much greater.

A noteworthy type of volcano is part of the world encircling mid-ocean ridge (MOR) visible in Iceland. The MOR is really a single, extremely long, active, linear volcano, connecting all spreading plate boundaries through all oceans. Along its length small, separate volcanoes occur. The MOR exudes low-silica, highly fluid basalt producing the entire ocean floor and constituting the largest single structure on the face of the earth.

Essay # 2. Location of Volcanoes:

Volcanoes are widely distributed over the earth, but they are more abundant in certain belts. One such belt encircles the Pacific ocean and includes many of the islands in it. Other volcanic areas are the island of West Indies, those of the West coast of Africa, the Mediterranean region and Iceland.

Most volcanoes occur around or near the margins of the continents and so these areas re regarded as weak zones of the earth’s crust where lavas can readily work their way upward. There are over 400 active volcanoes and many more inactive ones. Numerous submarine volcanoes also exist.

Since it is not possible to examine the magma reservoir which fees a volcano our information must be obtained by studying the material ejected by the volcano. This material consists of three kinds of products, viz. liquid lava, fragmented pyroclasts and gases. There may exist a special problem in studying the gases, both in collecting them under hazardous conditions or impossible conditions.

It may also be difficult to ascertain that the gases collected are true volcanic gases and are not contaminated with atmospheric gases. Investigation of the composition of extruded rock leads to a general, although not very detailed, correlation between composition and intensity of volcanic eruption.

In general, the quite eruptions are characteristic of those volcanoes which emit basic or basaltic lavas, whereas the violent eruptions are characteristic of volcanoes emitting more silicic rocks.

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Essay # 3 . formation of volcanoes :.

The term volcano is used to mean both the opening in the earth’s crust, i.e. the vent through which the eruption of magma occurs as well as the hill built- up by the erupted material. Volcanoes occur where the cracks in the earth’s crust lead to the magma chamber.

The liquid magma which is lighter than the surrounding rocks is under high pressure is pushed up towards the surface through these cracks. In this process the gases dissolved in the magma which expand are released providing an upward push to the magma.

As the magma gets closer to the surface, due to the reducing confining pressure to overcome, the magma and the gases flow faster. The magma, depending on its viscosity may quietly pour to the surface in the form of a flood of molten rock or it may explosively spurt out the molten rock to considerable heights as showers on the surrounding region with solid rock fragments and globs of molten rock. The liquid magma discharged to the surface is called lava.

Essay # 4 . Volcanic Landforms :

Many surface features of volcanic origin are created. These features range from towering peaks and huge lava sheets to small and low craters. The features created by a volcano vary depending on the type of eruption, the material erupted and the effects of erosion.

Four types of volcanic landforms are formed:

i. Ash and Cinder Cones or Explosion Cones:

These appear where explosive eruptions take place. When very hot solid fragments from a central crater (or a subsidiary crater) are ejected. A concave cone of height not exceeding 300 m is formed.

ii. Lava Cones:

These are formed from slowly upwelling lava.

These are of two types:

(a) Steep Sided Volcanoes:

These are formed from sticky acid lava which gets hardened quickly. The highly viscous lava which is squeezed out makes spines like tower.

(b) Shield Volcanoes:

These show gently sloping dome features. These are formed from runny lava which flows long distances, before getting hardened.

iii. Composite Cones or Strato-Volcanoes or Strato Cones:

These volcanoes have concave cone shaped sides of alternating ash and lava layers. These are common in most very high volcanoes. In some cases solid lava may plug the main pipe to the crater. Then pent up gases may blast the top off.

When the magma chamber empties, the summit of the volcano collapses. As a consequence, the feature produced is a vast shallow cavity called a Caldera. Strato volcanoes are the accumulated products of many volcanoes. Chemically most of these products are andesite. Some are dacite and a few are basalt and rhyolite. Due to this chemical mix and characteristic interlayering of lava flows, this volcano is called strato volcano.

iv. Shield Volcanoes:

When a volcano vent produces many successive basaltic lava flows stacked one on top of another in eruptive order, the resulting landform is called a shield volcano. A cinder cone and its associated lava flow can be thought of as the initial building blocks of a shield volcano.

A cinder cone is monogenetic because it forms from a single short-lived eruption (of a few years to a decade or two in duration). In contrast, a shield volcano that is an accumulation of the products of many eruptions over a period of say thousands to hundreds of thousands of years is polygenic.

On land these volcanoes have low angle cones. When they form under water they start with a steeper shape because the lava freezes much faster and does not travel far. The shape fattens to the shield form as the cone builds above the sea level.

v. Plateau Basalts or Lava Plains:

These form the bulk of many volcanic fields. These are features which occur where successive flows of basic lava leaks through fissures, over land surface and then cools and hardens forming a blanket-like feature.

The surface appearance of a flow provides information on the composition and temperature of the magma before it solidified. Very hot low viscosity basalt flows far and fast and produces smooth ropy surfaces. Cooler and less-fluid basalt flows form irregular, jagged surfaces littered with blocks.

The lava flows have blanketed to about 2000 m thickness covering 6,50,000 sq.km. in the Indian Deccan Plateau. Such lava flows have also created the U.S. Columbia River Plateau, the Abyssinian Plateau, the Panama Plateau of South America and the Antrim Plateau of Northern Ireland.

Magmas like dacite and rhyolite that have high silica contents are cooler and more viscous than basalt and hence they do not flow far resulting in the features, lobes, pancakes and domes. Domes often plug up the vent from which they issued, sometimes creating catastrophic explosions and may create a crater.

Eroded volcanoes have their importance. They give us a glimpse of the interior plumbing along which the magma rose to the surface. At the end of an eruption, magma solidifies in the conduits along which it had been rising. The rock so formed is more resistant than the shattered rock forming the walls and hence these lava filled conduits are often left behind when the rest of the volcano has been eroded away.

The filling of the central vertical vent is somewhat circular in section and forms a spire called a neck. The filling of cracks along which lava rose forms nearly vertical tabular bodies called dikes. Sometimes magma works its way along cracks that are nearly horizontal, often along bedding planes of sedimentary rocks. This results in the formation of table-like bodies called sills.

Essay # 5 . Major Gases Emitted by Volcanoes :

Volcanic gases present within the magma are released as they reach the earth’s surface, escaping at the major volcanic opening or from fissures and vents along the side of the volcano. The most prevalent gases emitted are steam, carbon dioxide and hydrogen sulphide. Carbon dioxide is an invisible, odourless poisonous gas. The table below shows the gases emitted from volcanoes.

Essay # 6 . Lightning and Whirlwinds :

Lightning flashes accompany most volcanic eruptions, especially those involving dust. The cause of this lightning is believed to be either contact of sea water with magma or generation of static electricity by friction between colliding particles carried in the erupting gases. Lightning is characteristic of vulcanian eruptions and is common during glowing avalanches.

Whirlwinds are seen during many volcanic eruptions. They are seen above hot lavas. Sometimes they form inverted cones extending a little below the eruption cloud. Energy for the whirlwinds might be from the hot gases and lava, high velocity gas jets in the eruption, heat released into the atmosphere during falls of hot tephra or where lava flows into the sea creating steam.

Essay # 7 . Features Produced by the Escape of Gases from Volcanic Lavas :

The gases of volcanic lavas produce several interesting features while they escape. They expand in the lava of the flow and thus cause the formation of Scoriaceous and Pumiceous rocks. By their explosion, they blow the hardened lava above them in the conduit, into bits and thus produce pyroclastic material.

They form clouds above volcanoes, the rain from which assists in the production of mud flows. When the volcano becomes inactive, they escape aiding in the formation of jumaroles, geysers and hot springs. Scoriaceous rocks are extremely porous. They are formed by the expansion of the steam and other gases beneath the hardened crust of a lava. The final escape of the gases from the hardening lava leaves large rounded holes in the rock.

Pumice is a rock also formed by the expansion and escape of gases. In pumice, many of the holes are in the form of long, minute, closed tubes which make the rock so light that it will float on water.

These tubes are formed by the expansive force of large amounts of gases in an extremely viscous lava that cools very rapidly, forming a glassy rock. Pumice is the rock that is usually formed from the lava ejected from explosive volcanoes. It can be blown to kilometres by explosions.

Essay # 8 . Volcanic Products :

Volcanoes give out products in all the states of matter – gases, liquids and solids.

Steam, hydrogen, sulphur and carbon dioxide are discharged as gases by a volcano. The steam let out by a volcano condenses in the air forming clouds which shed heavy rains. Various gases interact and intensify the heat of the erupting lavas. Explosive eruptions cause burning clouds of gas with scraps of glowing lava called nuees ardentes.

The main volcanic product is liquid lava. Sticky acid lava on cooling, solidifies and hardens before flowing long distances. Such lava can also block a vent resulting in pressure build-up which was relieved by an explosion. Basic fluid lava of lesser viscosity flows to great distances before hardening.

Some lava forms are produced by varying conditions as follows. Clinkery block shaped features are produced when gas spurted from sluggish molten rock capped by cooling crust. These are called Aa.

Pahoehoe is a feature which has a wrinkled skin appearance caused by molten lava flowing below it.

Pillow lava is a feature resembling pillows. This feature piles up when fast cooling lava erupts under water.

Products in explosive outbursts are called Pyroclasts. These consist of either fresh material or ejected scraps of old hard lava and other rock. Volcanic bombs include pancake-flat scoria shaped on impacting the ground and spindle bombs which are twisted at ends as they whizzle through the air. Acid lava full of gas formed cavities produces a light volcanic rock.

Pumice which is so light it can float on water. The product Ignimbrite shows welded glassy fragments. Lapilli are hurled out cinder fragments. Vast clouds of dust or very tiny lava particles are called volcanic ash. Volcanic ash mixed with heavy rain creates mudflows.

Sometimes mudflows can bury large areas of land. Powerful explosions can smoother land for many kilometres around with ash and can hurl huge amount of dust into the higher atmosphere. Violent explosions destroy farms and towns, but volcanic ash provides rich soil for crops.

i. Hot springs:

The underground hot rocks heat the spring waters creating hot springs. The hot springs shed minerals dissolved in them resulting in crusts of calcium carbonate and quartz (geyserite).

ii. Smoker:

This is a submarine hot spring at an oceanic spreading ridge. This submarine spring emits sulphides and builds smoky clouds.

iii. Geyser:

Periodically steam and hot water are forced up from a vent by super-heated water in pipe like passage deep down. Famous geysers are present in Iceland and Yellowstone National Park.

iv. Mud volcano:

This is a low mud cone deposited by mud-rich water gushing out of a vent.

v. Solfatara:

This is a volcanic vent which emits steam and sulphurous gas.

vi. Fumarole:

This is a vent which emits steam jets as at Mt. Etna, Sicily and Valley of Ten Thousand smokes in Alaska.

vii. Mofette:

This is a small vent which emits gases including carbon dioxide. These occur in France, Italy and Java.

Various terms used while describing volcanic features are given below:

i. Magma Chamber:

Magma is created below the surface of the earth (at depth of about 60 km) and is held in the magma chamber until sufficient pressure is built-up to push the magma towards the surface.

This is a pipe like passage through which the magma is pushed up from the magma chamber.

This is the outlet end of the pipe. Magma exits out of the vent. If a vent erupts only gases, it is called fumarole.

iv. Crater:

Generally the vent opens out to a depression called crater at the top of the volcano. This is caused due to the collapse of the surface materials.

v. Caldera:

This is a very big crater formed when the top of an entire volcanic hill collapses inward.

When the erupted materials cover the vent, a volcanic dome is created covering the vent. Later as the pressure of gas and magma rises, another eruption occurs shattering the dome.

A mountain-like structure created over thousands of years as the volcanic lava, ash, rock fragments are poured out onto the surface. This feature is called volcanic cone.

viii. Pyroclastic Flow :

A pyroclastic flow (also known as nuee ardentes (French word) is a ground hugging, turbulent avalanche of hot ash. pumice, rock fragments, crystals, glass shards and volcanic gas. These flows can rush down the steep slopes of a volcano at 80 to 160 km/li, burning everything in their path.

Temperatures of these flows can reach over 500°C. A deposit of this mixture is also often referred to as pyroclastic flow. An even more energetic and dilute mixture of searing volcanic gases and rock-fragments is called a pyroclastic surge which can easily ride up and over ridges.

ix. Seamounts :

A spectacular underwater volcanic feature is a huge localized volcano called a seamount. These isolated underwater volcanic mountains rise from 900 m to 3000 m above the ocean floor, but typically are not high enough to poke above the water surface.

Seamounts are present in all the oceans of the world, with the Pacific ocean having the highest concentration. More than 2000 seamounts have been identified in this ocean. The Gulf of Alaska also has many seamounts. The Axial Seamount is an active volcano off the north coast of Oregon (currently rises about 1400 m above the ocean floor, but its peak is still about 1200 m below the water surface.

Essay # 9 . Source of the Explosive Energy :

The energy for the explosive violence comes from the expansion of the volatile constituents present in the magma, the gas content of which determines the degree of commination of the materials and the explosive violence of the eruption.

This energy is expanded in two ways, firstly in the expulsion of the materials into the atmosphere and secondly, due to expansion within the magma leading to the development of vesicles. The most important gas is steam, which may form between 60 to 90 per cent of the total gas content in a lava. Carbon dioxide, nitrogen and sulphur dioxide occur commonly and hydrogen, carbon monoxide, sulphur and chlorine are also present.

Essay # 10 . Classification of Pyroclastics :

Pyroclastics refer to fragmental material erupted by a volcano. The larger fragments consisting of pieces of crystal layers beneath the volcano or of older lavas broken from the walls of the conduit or from the surface of the crater are called blocks.

Volcanic bombs are masses of new lava blown from the crater and solidified during flight, becoming round or spindle shaped as they are hurled through the air. They may range in size from small pellets up to huge masses weighing many kilonewtons.

Sometimes they are still plastic when they strike the surface and are flattened or distorted as they roll down the side of the cone. Another type called bread crust bomb resembles a loaf of bread with large gaping cracks in the crust.

This cracking of the crust results from the continued expansion of the internal gases. Many fragments of lava and scoria solidified in flight drop back into the crater and are intermixed with the fluid lava and are again erupted.

In contrast to bombs, smaller broken fragments are lapilli (from Italian meaning, little stones) about the size of walnuts; then in decreasing size, cinders, ash and dust. The cinders and ash are pulverized lava, broken up by the force of rapidly expanding gases in them or by the grinding together of the fragments in the crater, as they are repeatedly blown out and dropped back into the crater after each explosion.

Pumice is a type of pyroclastic produced by acidic lavas if the gas content is so great as to cause the magma to froth as it rises in the chimney of the volcano. When the expansion occurs the rock from the froth is expelled as pumice. Pumice is of size ranging from the size of a marble to 30 cm or more in diameter. Pumice will float in water due to many air spaces formed by the expanding gases.

Lava fountains in which steam jets blow the lava into the air produce a material known as Pele’s hair which is identical with rock wool which is manufactured by blowing a jet of steam into a stream of molten rock (Rock wool is used for many types of insulation).

Coarse angular fragments become cemented to form a rock called volcanic breccia. The finer material like cinders and ash forms thick deposits which get consolidated through the percolation of ground water and is called tuff. Tuff is a building stone used in the volcanic regions. It is soft and easily quarried and can be shaped and has enough strength to be set into walls with mortar.

i. Agglomerate:

The debris in and around the vent contains the largest ejected masses of lava bombs which are embedded in dust and ash. A deposit of this kind is known as agglomerate. The layers of ash and dust which are formed for some distance around the volcano and which builds its cone, become hardened into rocks which are called tuffs.

Ash includes all materials with size less than 4 mm. It is pulverized lava, in which the fragments are often sharply angular and formed of volcanic glass; these angular and often curved fragments are called shards.

Since the gas content of ash on expulsion is high it has considerable mobility on reaching the surface; it is also hot and plastic, the result of these conditions being that the fragments often become welded together. The finest of ash is so light that wind can transport it for great distances.

The table below sets out a general classification of pyroclastic rocks based on the particle size of the fragments forming the rocks.

Essay # 14. Quiet and Violent Volcanoes :

Volcanoes are in various sizes and shapes and their behaviour ranges from a quiet state to violently destructive state. Such diversity in the activity of volcanoes is related to the chemistry of the erupting magma. Chemical compositions that produce thin, easy flowing magmas are associated with non-violent eruptions, whereas compositions that produce thick, sluggish (highly viscous) magmas are associated with explosive eruptions.

Magma is a silicate liquid (with rare exception). The most abundant chemical building block is a pyramid-shaped assembly of the element silicon (Si) surrounded by four oxygen (O) atoms. Other elements that are common rock-forming elements including aluminium, iron, magnesium, potassium and calcium occupy the spaces between and around the silicate building blocks.

The overall mixture forms the hot and sticky stuff called magma. Magma takes on a spectrum of chemical compositions, with different relative amounts of these chemical compositions. Magma which forms in the mantle carries the name basalt. Though fairly uniform in composition, even this mother magma is somewhat variable.

Much of this variation in composition depends upon what happens to the basalt magma once it leaves the mantle and begins its upward journey through the crust. For instance, rocks encountered in the crust sometimes are partly melted by and dissolved into the rising mantle-derived magma, and sometimes crystals grow and then separate from magma as it begins to cool.

Both of these processes actually result in an increased silicon content for the modified daughter magma, and thus the natural tendency is for basaltic magma to change to become a more silicic composition. The extent to which such change advances depends on the duration of the magma journey and the chemical character of the crust traversed.

Magma composition is classified and named mainly on the basis of the amount of silicon in the form of silica or silicon dioxide SiO 2 .

The chart in Fig. 15.3 summarizes the names of the common magmas and their associated ranges in silica. A very important property of magma that determines the eruption style and the eventual shape of the volcano it builds, is its resistance to flow, namely its viscosity.

Magma viscosity increases as its silica content increases. Eruptions of highly viscous magmas are violent. The highly viscous rhyolite magma piles up its ticky masses right over its eruptive vent to farm tall steep sided volcanoes.

On the contrary the basaltic magma flows great distances from its eruptive vent to from low, broad volcanic features. Magma in the intermediate viscosity spectrum say the andesite magma tends to form volcanoes of profile shapes between these two extremes.

An additional important ingredient of magma is water. Magmas also contain carbon dioxide and various sulphur-containing gases in solution. These substances are considered volatile since they tend to occur as gases at temperatures and pressures at the surface of the earth.

As basaltic magma changes composition toward rhyolite the volatiles become concentrated in the silica-rich magma. Presence of these volatiles (mainly water) in high concentration produces highly explosive volcanoes. It should be noted that these volatiles are held in magma by confining pressure. Within the earth, the confining pressure is provided by the load of the overlying rocks.

As the magma rises from the mantle to depths about 1.5 km or somewhat less, the rock load is reduced to that extent that the volatiles (mainly water) start to boil. Bubbles rising through highly viscous rhyolitic magma have such difficulty to escape their way, that many carry blobs of magma and fine bits of rock with them and they finally break free and jet violently upward resulting in a violent buoyant eruption column that can rise to kilometres above the earth.

The fine volcanic debris in such a powerful eruption gets dispersed within the upper atmosphere, hide the sunlight affecting the weather. The greater the original gas concentration in a magma and the greater the volume rate of magma leaving the vent, the taller is the eruption column produced.

The gases escaping from magma during eruption mix with the atmosphere and become part of the air humans, animals and plants breath and assimilate. However as magma cools and solidifies to rock during eruption, some of the gas remains trapped in bubbles creating vesicles. Generally all volcanic rocks contain some gas bubbles. A variety of vesicular rhyolite is pumice. Pumice is vesicular to such an extent, it floats in water.

Essay # 15. Classification of Volcanic Activity:

A classification of volcanic activity based on the type of product is shown in Fig. 15.4. The basic subdivision is based on the proportions of the gas, liquid and solid components, which can be represented on a triangular diagram. The four basic triangles represent the domain of four basic kinds of volcanic activity.

Essay # 16. Cone Topped and Flat Topped Volcanoes:

Generally rhyolite volcanoes are flat-topped because rhyolite magma which is extremely viscous, oozes out of the ground, piles up around the vent and then oozes away a bit to form a pancake shape. In contrast basalt volcanoes generally feed lava flows that flow far from the vent, building a cone.

Basaltic tephra (large particles of different size) is a spongy-looking black, rough material of pebble or cobble. Commercially this tephra is known as cinder and is used for gardening and rail-road beds. In some situations basaltic volcanoes develop flat top profile.

Flat topped volcanoes of basalt can form when there is an eruption under a glacier. Instead of getting ejected as tephra to form a cone, it forms a cauldron of lava surrounded by ice and water and eventually solidifying. When the ice melts, a steep-sided, table-shaped mountain known as a tuya remains. Volcanoes of this type are common in Iceland and British Columbia, where volcanoes have repeatedly erupted under glaciers.

Surprisingly, the Pacific ocean is a home to many flat-topped undersea basaltic mountains. These are called seamounts. How these seamounts were formed was a mystery for a long time. Surveying and dredging operations revealed that most seamounts were formerly conical volcanoes projecting above the water.

Geologists found that the conical volcanoes got lowered due to subsidence and the tops of the volcanoes came near the sea water level and the powerful waves mowed them flat. Continued subsidence caused them to drop below the water surface.

Essay # 17. Types of Volcanoes :

There are many types of volcanoes depending on the composition of magma especially on the relative proportion of water and silica contents. If the magma contains little of either of these, it is more liquid and it flows freely forming a shallow rounded hill.

Large water content with little silica permits the vapour to rapidly rise through the molten rock, throwing fountains of fire high into the air. More silica and less water in the magma make the magma more viscous. Such magma flows slowly and builds-up a high dome.

High content of both water and silica create another condition. In such a case the dense silica prevents the water from vaporizing until it is close to the surface and results in a highly explosive way. Such an eruption is called a Vulcan eruption.

Other types of eruption are named after people or regions associated with them. Vesuvian eruption named after Vesuvius is a highly explosive type occurring after a long period of dormancy. This type ejects a huge column of ash and rock to great heights upto 50 km.

A peleean eruption named after the eruption of Mt. Pelee in Martin que in 1902 is a highly violent eruption ejecting a hot cloud of ash mixed with considerable quantity of gas which flows down the sides of the volcano like a liquid. The cloud is termed nuee ardente meaning glowing cloud. Pyroclastic or ash flow refers to a flow of ash, solid rock pieces and gas. Hawaiian eruptions eject fire fountains.

Essay # 18. Violence of Volcanic Eruptions :

Volcanic activity may be classified by its violence, which in turn is generally related to rock type, the course of eruptive activity and the resulting landforms. We may in general distinguish between lava eruptions associated with basic and intermediate magmas and pumice eruptions associated with acid magmas.

The percentage of the fragmentary material in the total volcanic material produced can be used as a measure of explosiveness and if calculated for a volcanic region can be adopted as an Explosion Index (E), useful for comparing one volcanic region with others. Explosion Index for selected volcanic regions by Rittmann (1962) are shown in the table below.

Newhall and Self (1982) proposed a Volcanic Explosivity Index (VEI) which helps to summarize many aspects of eruption and is shown in the table below.

Essay # 19. Famous Volcanoes around the World :

Many volcanoes are present around the world. Some of the largest and well known volcanoes are listed in the table below.

Essay # 20. Volcanic Hazards :

Volcanic eruptions have caused destruction to life and property. In most cases volcanic hazards cannot be controlled, but their impacts can be mitigated by effective prediction methods.

Flows of lava, pyroclastic activity, emissions of gas and volcanic seismicity are major hazards. These are accompanied with movement of magma and eruptive products of the volcano. There are also other secondary effects of the eruptions which may have long term effects.

In most cases volcanoes let out lava which causes property damage rather than injuries or deaths. For instance, in Hawaii lava flows erupted from Kilauea for over a decade and as a consequence, homes, roads, forests, cars and other vehicles were buried in lavas and in some cases were burned by the resulting fires but no lives were lost. Sometimes it has become possible to control or divert the lava flow by constructing retaining walls or by some provision to chill the front of the lava flow with water.

Lava flows move slowly. But the pyroclastic flows move rapidly and these with lateral blasts may kill lives before they can run away. In 1902, on the island of Martinique the most destructive pyroclastic flow of the century occurred resulting in very large number of deaths.

A glowing avalanche rushed out of the flanks of Mount Pelee, running at a speed of over 160 km/h and killed about 29000 people. In A.D. 79 a large number of people of Pompeii and Herculaneum were buried under the hot pyroclastic material erupted by Mount Vesuvius.

The poisonous gas killed many of the victims and their bodies got later buried by pyroclastic material. In 1986, the eruption of the volcano at Lake Nyos, Cameroon killed over 1700 people and over 3000 cattle.

When magma moves towards the surface of the earth rocks may get fractured and this may result in swarms of earthquakes. The turbulent bubbling and boiling of magma below the earth can produce high frequency seismicity called volcanic tremor.

There are also secondary and tertiary hazards connected with volcanic eruptions. A powerful eruption in a coastal setting can cause a displacement of the seafloor leading to a tsunami. Hazardous effects are caused by pyroclastic material after a volcanic eruption has ceased.

Either melt water from snow or rain at the summit of the volcano can mix with the volcanic ash and start a deadly mud flow (called as lahar). Sometimes a volcanic debris avalanche in which various materials like pyroclastic matter, mud, shattered trees etc. is set out causing damage.

Volcanic eruptions produce other effects too. They can permanently change a landscape. They can block river channels causing flooding and diversion of water flow. Mountain terrains can be severely changed.

Volcanic eruptions can change the chemistry of the atmosphere. The effects of eruption on the atmosphere are precipitation of salty toxic or acidic matter. Spectacular sun set, extended period of darkness and stratospheric ozone depletion are all other effects of eruptions. Blockage of solar radiation by fine pyroclastic material can cause global cooling.

Apart from the above negative effects of volcanisms there are a few positive effects too. Periodic volcanic eruptions replenish the mineral contents of soils making it fertile. Geothermal energy is provided by volcanism. Volcanism is also linked with some type of mineral deposits. Magnificent scenery is provided by some volcanoes.

The study of volcanoes has great scientific as well as social interest. Widespread tephra layers inter-bedded with natural and artificial deposits have been used for deciphering and dating glacial and volcanic sequences, geomorphic features and archeological sites.

For example, ash from Mt. St. Helens Volcano in Washington travelled at least 900 km into Alberta. North American Indians fashioned tools and weapons out of volcanic glass, the origin of which is used to trace migratory and trading routes.

Volcanoes are windows through which the scientists look into the interiors of the earth. From volcanoes we learn the composition of the earth at great depths below the surface. We learn about the history of shifting layers of the earth’s crust. We learn about the processes which transform molten material into solid rock.

From the geological historical view point, volcanic activity was crucial in providing to the earth a unique habitat for life. The degassing of molten materials provided water for the oceans and gases for the atmosphere – indeed, the very ingredients for life and its sustenance.

Essay # 21. Volcanoes and Atmospheric Pollution :

During eruptions volcanoes inject solid particles and gases into the atmosphere. Particles may remain in the atmosphere for months to years and rain back on to the earth. Volcanoes also release chlorine and carbon dioxide.

The main products injected into the atmosphere from volcanic eruptions however are volcanic ash particles and small drops of sulphuric acid in the form of a fine spray known as aerosol. Most chlorine released from volcanoes is in the form of hydrochloric acid which is washed out in the troposphere. Volcanoes also emit carbon dioxide.

During the times of giant volcanic eruptions in the past the amount of carbon dioxide released may have been enough to affect the climate. In general global temperatures are cooler for a year or two after a major eruption.

A large magnitude pyroclastic eruption such as a caldera-forming event can be expected to eject huge volumes of fine ash high into the atmosphere where it may remain for several years, carried around the globe by strong air currents in the upper atmosphere.

The presence of this ash will increase the opacity of the atmosphere, that is, it will reduce the amount of sunlight reaching the earth’s surface. Accordingly, the earth’s surface and climate will become cooler. Various other atmospheric effects may be observed. Particularly noticeable is an increase in the intensity of sunsets.

i. Global Warming :

Besides blocking the rays of the sun, the vast clouds of dust and ash that result from a volcanic eruption can also trap ultraviolet radiation within the atmosphere causing global warming.

Volcanic eruptions usually include emissions of gases such as carbon dioxide which can further enhance this warming. Even if it lasted only for a relatively short time, a sudden increase in temperature could in turn have contributed to extinctions by creating an environment unsuitable for many animals.

ii. Geothermal Energy :

Geothermal energy is the heat energy trapped below the surface of the earth. In all volcanic regions, even thousands of years after activity has ceased the magma continues to cool at a slow rate. The temperature increases with depth below the surface of the earth. The average temperature gradient in the outer crust is about 0.56° C per 30 m of depth.

There are regions however, where the temperature gradient may be as much as 100 times the normal. This high heat flow is often sufficient to affect shallow strata containing water. When the water is so heated such surface manifestations like hot springs, fumaroles, geysers and related phenomena often occur.

It may be noted that over 10 per cent of the earth’s surface manifests very high heat flow and the hot springs and related features which are present in such areas have been used throughout the ages, for bathing, laundry and cooking.

In some places elaborate health spas and recreation areas have been developed around the hot-spring areas. The cooling of magma, even though it is relatively close to the surface is such a slow process that probably in terms of human history, it may be considered to supply a source of heat indefinitely.

Temperatures in the earth rise with increasing depth at about 0.56°C per 30 m depth. Thus if a well is drilled at a place where the average surface temperature is say 15.6°C a temperature of 100°C would be expected at about 4500 m depth. Many wells are drilled in excess of 6000 m and temperatures far above the boiling point of water are encountered.

Thermal energy is stored both in the solid rocks and in water and steam filling the pore spaces and fractures. The water and steam serve to transmit the heat from the rocks to a well and then to the surface.

In a geothermal system water also serves as the medium by which heat is transmitted from a deep igneous source to a geothermal reservoir at a depth shallow enough to be tapped by drilling. Geothermal reservoirs are located in the upward flowing part of a water – convective system. Rainwater percolates underground and reaches a depth where it is heated as it comes into contact with the hot rocks.

On getting heated, the water expands and moves upward in a convective system. If this upward movement is unrestricted the water will be dissipated at the surface as hot springs; but if such upward movement is prevented, trapped by an impervious layer the geothermal energy accumulates, and becomes a geothermal reservoir.

Until recently it was believed that the water in a geothermal system was derived mainly from water given off by the cooling of magma below the surface. Later studies have revealed that most of the water is from surface precipitation, with not more than 5 per cent from the cooling magma.

Production of electric power is the most important application of geothermal energy. A geothermal plant can provide a cheap and reliable supply of electrical energy. Geothermal power is nearly pollution free and there is little resource depletion.

Geothermal power is a significant source of electricity in New Zealand and has been furnishing electricity to parts of Italy. Geothermal installations at the Geysers in northern California have a capacity of 550 megawatts, enough to supply the power needs of the city of San Francisco.

Geothermal energy is versatile. It is being used for domestic heating in Italy, New Zealand and Iceland. Over 70 per cent of Iceland’s population live in houses heated by geothermal energy. Geothermal energy is being used for forced raising of vegetables and flowers in green houses in Iceland where the climate is too harsh to support normal growth. It is used for animal husbandry in Hungary and feeding in Iceland.

Geothermal energy can be used for simple heating processes, drying or distillation in every conceivable fashion, refrigeration, tempering in various mining and metal handling operations, sugar processing, production of boric acid, recovery of salts from seawater, pulp and paper production and wood processing.

Geothermal desalinization of sea water holds promise for abundant supply of fresh water. In some areas it is a real alternative to fossil fuels and hydroelectricity and in future may help meet the crisis of our insatiable appetite for energy.

iii. Phenomena Associated with Volcanism :

In some regions of current or past volcanic activity some phenomena related to volcanism are found. Fumaroles, hot springs and geysers are the widely known belonging to this group. During the process of consolidation of molten magma either at the surface or at some depths beneath the surface gaseous emanations may be given off.

These gas vents constitute the fumaroles. The Valley of Ten Thousand Smokes in Alaska is a well-known fumarole and is maintained as a national monument. This group of fumaroles was formed by the eruption of Mount Katmai in 1912. This valley of area of about 130 square kilometres contains thousands of vents discharging steam and gases.

These gases are of varied temperatures and the temperatures vary from that of ordinary steam to superheated steam coming out as dry gas. Many of the gases escaping from the vents may be poisonous, such as hydrogen sulphide and carbon monoxide which are suffocating and may settle at low places in the topography. For example, the fumaroles at the Poison Valley, Java discharge deadly poisonous gases.

Solfataras are fumaroles emitting sulphur gases. At some places, the hydrogen sulphide gases undergo oxidation on exposure to air to form sulphur. The sulphur accumulates in large amount so that the rocks close to the solfataras may contain commercial quantities of sulphur.

Hot springs are also phenomena associated with volcanic activity. Waters from the surface which penetrate into the ground can get heated either by contact with the rocks which are still hot or by gaseous emanations from the volcanic rocks. The water so heated may re-emerge at the surface giving rise to hot springs. In some situations the hot springs may be intermittently eruptive. Such intermittently hot springs are called geysers.

Related Articles:

• Lava: Types and Eruptions | Volcanoes
• Submarine and Sub Glacial Eruptions | Volcanoes

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Volcanoes: compilation of essays on volcanoes | disasters | geology.

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Essay on Volcanoes

Essay Contents:

• Essay on the Effects of Volcanic Activity

Essay # 1. Meaning of Volcanoes:

Sometimes the molten rock, ash, steam and other gases find their way to the surface of the earth through some vents or openings. These ejected materials accumulate around the vent and give rise to a volcanic cone or a hill. The conical hill along with the vent is known as volcano.

The mouth of a volcano is funnel shaped and hollow. This funnel-shaped hole is called the crater and the opening through which molten rock materials come out from inside is called the pipe or vent.

Fuji Yama in Japan, Visuvius in Italy, Cotopaxi in Equador and Barren Island are good examples of volcanic mountains.

The ejection of materials through the crater is called volcanic eruption. Volcanoes are generally classified into three types. These are active, dormant and extinct.

The active volcanoes generally erupt fairly frequently and are of two types, namely, permanent and intermittent:

(a) Permanent volcanoes:

The volcanoes that eject molten materials continuously are called permanent volcanoes.

(b) Intermittent volcanoes:

These are those volcanoes in which an eruption occurs at the end of a certain period of time. Stromboli of Italy and Etna of Sicily are the examples of active volcanoes.

A volcano which erupted sometime in the past, and is now passive, but may erupt again, is called a dormant volcano. Fuji Yama of Japan and Vesuvius of Italy are the dormant volcanoes.

Extinct volcanoes are those whose activity has not been known to us during the historical time. Kilimanjaro of Africa and Chimborazo of Equador are the examples of extinct volcanoes.

Sometimes it is difficult to say whether a volcano is extinct or dormant. For example, the Vesuvius, the Krakatoa, Mt. Unzen of Japan and Mt. Pintubo of Philippines once thought to be extinct became active recently.

Essay # 2. Formation of Volcanoes :

The actual causes of the formation of volcanoes are not yet definitely known.

The probable causes of their origin are given below:

(a) Difference of Pressure on the Surface:

Pressure below the earth’s surface increases with depth. The interior of the earth is under high pressure. It is at the same time very hot. Because of high pressure and high temperature the materials of the earth’s interior are in the viscous state. Owing to the release of pressure from the top portion for some reason, the viscous materials get expanded. There is then a natural tendency for molten materials to make an upward drive through the fissures or weak zones of the earth. By this way a volcano may be formed.

(b) Chemical Reaction:

There are some radioactive minerals in the interior of the earth. They emit heat continuously and as a result of this, the other materials get expanded and create enormous pressure. Then the underlying expands and molten materials may come out through some fissures forming volcanoes.

(c) Percolation of W ater to Earth’s Interior:

Besides those two causes, a volcano may be caused when water percolating through the surface meets the molten materials. In this case water turns into steam of great volume. Steam, lava, and other materials may then come out to the surface forming a volcano.

Essay # 3. Arisen of Volcanoes:

It is a well-known fact that in all the northern and central part of Europe at the present time there are no active volcanoes. But, the geologist professes to be perfectly certain that there were, during many of the past geological periods, many volcanoes in our island. Before we can understand how he has come to this conclusion we must, according to Hutton’s method, find out how any known volcano has arisen and what traces it has left after it has become extinct.

The nearest European volcano to our islands is Vesuvius, and it is fortunate that we know a great deal about Vesuvius during the last two thousand years. The Neapolitan volcanoes extend from Vesuvius on the east to the Islands of Procida and Ischia on the west.

Before the Christian era, as far back as any tradition is known, there seems to have been no activity at Vesuvius. But a Greek colony, which settled on Ischia about 380 B.C., had to abandon the island owing to an eruption, and there were traditions of volcanic eruptions driving earlier colonists away as well.

Since the date above mentioned there has been complete cessation of vulcanicity in that island except at one time, when there was an emission of lava. On the mainland to the west of Naples there was Lake Avernus, occupying the inside of an ancient crater, where probably poisonous gases were emitted, if what Lucretius tells us is true, that birds could not fly over it without being stifled.

Thus we see that just before and during the beginning of the Christian epoch there was some evidence that the volcanic forces, which had in far-off ages built up high mountains near the Bay of Naples, were not quite extinct.

Vesuvius, which we now recognise as a modern cone within an ancient one, the remains of the latter being called Monte Somma, was, up to the first century, a single regular truncated cone. Plutarch writes of its old crater having steep cliffs and being covered with wild vines. The flanks of the mountain were of a rich soil and were covered with fertile fields stretching down towards the busy towns of Pompeii and Herculaneum.

An especially violent shock on the 5th of February A.D. 63 gave warning of a recrudescence of vulcanicity. Pompeii was badly damaged, many buildings being thrown down. But the inhabitants were rich and at once set about restoring what was damaged. After sixteen years most of the houses were in good repair, and at least two damaged temples had been rebuilt. In August A.D. 79 the great catastrophe occurred, and when the Forum was dug out a few years ago, it was seen that the west side of the colonnade, which had been almost completed when the earthquake of A.D. 63 threw it down, had been only partly rebuilt, and the area was strewn with blocks which were being made ready for the rebuilding by the stone masons.

On August 24 the elder Pliny, who was commander of the Roman fleet, went with his ships to rescue the people at the foot of Vesuvius from danger. Arriving too late he went to Stabiae where, on the next morning, he was suffo­cated by fumes..

His nephew, the younger Pliny, tells the story of what he saw in a letter to Tacitus. It is easy to follow the course of events. A column of vapour ascended from Vesuvius and then spread out on all sides. This caused intense dark­ness, which was lit up at times by flashes of lightning.

Volcanic ashes, as the fragments blown out from a volcano are called, fell on ships many miles away, earthquake shocks were frequent, and the sea went back, leaving marine animals high and dry.

Pompeii and Herculaneum:

But the ashes did not only fall on ships and the earthquakes were only, as it were, premonitory warnings of what was to follow. For at last a series of gigantic explosions blew one side of Monte Somma into the air and the enormous mass of material on falling buried up the prosperous cities of Pompeii and Herculaneum.

On the west side the descending ashes mingled with torrents of rain and so gave rise to a huge stream of mud, which poured down upon Herculaneum and sealed it up, on the south side, driven by the wind, the ashes spread out like a cloud and covered Pompeii. From this cloud there descended first broken fragments of pumice about the size of a walnut, which rained down to a depth of about 9 feet, then came fine ash wet with rain and settled for a depth of some 6 or 7 feet over the doomed town.

When the sun set the storm of ashes had ceased, but by that time Herculaneum had vanished and only the roofs of the houses of Pompeii projected above the surface. As far as we can tell, no lava flowed from the mountain during this period. But after this year other periods of activity occurred, and in 1036 we learn that a lava stream did make its way down the mountain-side.

Other times of activity were in the years 1049, 1138, 1306, 1500, and 1631, though the activity in 1500 was very slight. During the long pause before 1631 there occurred in the Phlegraean Fields to the west an event of some importance. In 1538 a new mountain was suddenly formed; it was named Monte Nuovo, and good accounts of its formation have come down to us.

Monte Nuovo:

After several shocks during September 28, we are told that very early on the 29th flames of fire were seen, and shortly after the earth burst open and ashes and pumice were blown out. Later on the inhabitants of the neighbouring town fled in terror to Naples, and it is stated that the sea had retired and left multitudes of fish to die on the shore.

The eruption continued for two days and nights, and on the third day, signs of activity having ceased, several people went up the new hill which was over 400 feet high and 8000 feet round its base. When they looked down into the crater they saw a boiling cauldron. Some days afterwards fresh explosions took place, but with decreasing activity.

No lava was extruded during the eruption, and the whole mountain was built up of large and small fragments which were blown up into the air by explosions and fell close to the orifice. No further volcanic action has occurred here since the time of the formation of the hill and it is now completely covered with grass.

For nearly a century after the birth of Monte Nuovo Vesuvius continued tranquil, and there was no violent eruption for nearly five hundred years. At the bottom of its crater cattle grazed, and its sides were covered with bushes. But in 1631 these grassy flats and woods were blown into the air and seven streams of lava poured down from the mountain and overwhelmed several villages. From the seventeenth century to the present day there has been a constant series of eruptions and no great convulsion.

In fact, the behaviour of the district is very much like that of a safety valve; when there is an opening the forces below cause more or less quiet extrusion of molten rock, but if the opening gets plugged up then the activity ceases, it may be for many years, but there comes a time when the imprisoned forces get sufficiently great to blow away the overlying tract which had sealed up any opening that had before existed.

Probably the most violent volcanic eruption of recent times occurred in the Islands of Krakatoa in August 1883. The Royal Society appointed a committee to investigate the eruption, and the report of this committee is a very remarkable production and stands for all time as an example of what such a report should be.

Krakatoa is an island lying between Java and Sumatra. Although prior to the catastrophe this island had attracted but little attention, it lies in the heart of the great focus of volcanic activity in the world at the present time. Java itself contains forty-nine great volcanic mountains, and from it a chain of volcanoes is continued to the east and to the west.

Krakatoa did not show the regular conical shape of a volcano, but was only a part of a huge crater of which other smaller islands close by were also fragments.

The volcano of which these islands are the remnant must have been formerly one of gigantic dimensions, probably some twenty-five miles in circumference at the sea-level and over 10,000 feet in height. It was built up almost entirely of lava, and rested on beds of post-Tertiary age.

At some unknown date the central mass of the volcano was blown away, and only irregular portions of its crater- ring were left. Then occurred quiet eruptions at the bottom of the crater, causing the formation of small cones which filled up the older crater in parts, and raised these above sea-level. At one edge of the old crater eruptions built up the cone of Krakatoa from lava extrusions and outbursts of ashes.

In historic times the lateral cone of Krakatoa and the other land which was part of the original volcano were seen to be covered with luxuriant vegetation, and the products of its forests attracted the inhabitants of neighbouring islands.

The first eruption recorded as having taken place at Krakatoa is in 1680, though tradition points to former ones having occurred while man lived in the neighbourhood. The eruption was of a moderate type and the island soon recovered its former state, and the vegetation which had been destroyed once more spread and for two hundred years the volcanic forces lay dormant.

In 1880 earthquakes were frequent, and in May 1883 the inhabitants of the town of Batavia, 100 miles away from Krakatoa, heard booming sounds, and for many hours their doors and windows were rattled. On the next day ashes fell on Java and Sumatra at places opposite to Krakatoa, and later a column of steam from Krakatoa showed the position of the disturbance.

The height of the cloud of dust above the island was estimated to be seven miles, and falls of dust were noticed three hundred miles away. This eruption soon decreased in intensity.

An excursion party from Batavia landed on the island and found some depth of fine ash scattered over it, and many” trees had been stripped of their leaves and branches by the fall of this deposit.

On August 11 another visit to Krakatoa showed that the forests had been completely destroyed, and that the fine ash covered the surface of the ground near the shore to a depth of 20 inches. Some fourteen foci of eruption were seen, from which steam and dust were being sent out.

This recrudescence of activity finally culminated on August 26 and 27 in a series of tremendous explosions. During these days three European ships were actually in the Straits in which Krakatoa lies, and as they escaped destruction those on them have been able to give a record of their observations.

The course of the events which happened seem to have been as follows- During the night of the 26th white-hot fragments of lava were blown out and rolled down the mountain-sides, smaller cinders were blown out to sea, and finer ashes, remaining in the air, caused intense darkness in the early morning.

From sunset till midnight there was a continuous roar which moderated in the early morning. Each explosive outburst of steam blew off the crust which formed on the lava and left the white-hot molten material to throw its light on to the overhanging pall of ash, and so lit up the scene for miles around.

But now the somewhat peculiar situation of the volcano made itself felt. It lies close to the sea-level, and so the removal of so much material by the constant explosions allowed the waters of the sea to make their way into the heated mass of lava. This must have chilled and frozen much of the molten rock.

Some flashing into steam of the water no doubt occurred, and this gave rise to waves in the sea which travelled to the shores of Java and Sumatra, and were noted on the evening of the 26th and through the night.

The general result of this action of the sea-water was the closing of the safety-valve while the forces below it were still active, and finally there resulted four tremendous explosions between 5.30 a.m. and 10.52 a.m. on the morning of the 27th.

What happened in this space of time to the island itself was that the whole of the northern and lower portion of the island was blown into the air, while a large part of the cone which existed there also vanished. Where the island had risen to heights of between 300 and 400 feet was now in some places more than 1000 feet below sea-level.

These terrible explosions caused huge waves to travel away from the centre of destruction and, rushing up on to the coasts of Java and Sumatra, stranded all vessels near the shore, devastated towns and villages, destroyed two lighthouses, and caused the deaths of over 36,000 people.

But not only were sea-waves produced, but there were also air-waves. Some were so rapid that they produced sounds even at a place 3080 miles away, others were larger and broke in windows and cracked walls a hundred miles away at Batavia, where also lamps were thrown down, gas- jets extinguished, and a gasometer leaped out of its well.

Other air-waves were traced in their course by barometers at various places scattered widely over the surface of the world, and so we know that they travelled several times round the globe.

At Greenwich the depressions of the barometer due to these air-waves were recorded on every day between noon on August 27 to early in the morning of September 1. During the whole of August 27 eruptive action seems to have continued. Three vessels remained beating about in the Straits near the island, and their crews were busy for hours shovelling off their decks the volcanic dust which descended.

The finer particles of dust, however, floated still farther away. Between 7 and 10 in the morning the sky began to darken at Batavia, soon after lamps had to be lit, and a descent of fine dust began about 10.30 and lasted for nearly an hour, when complete darkness came on, the heavy dust- rain continuing till 1, and a less heavy precipitation till 3 P.M.

Complete darkness was experienced still farther to the east at a place one hundred and fifty miles from the volcano.

What happened during the days succeeding August 27 is doubtful; there may have been several small eruptions, but we know that on October 10 there was an explosion of some magnitude, and a large sea-wave. The commotion in the sea due to the cataclysmic convulsion of August 27 travelled far and wide.

Its terrible effects on the neighbour­ing islands of Java and Sumatra have already been noticed, and, of course, as it passed away from the centre of dis­turbance it gradually lost its intensity, still, tide-gauges and eye-observations tell us that to the west it was noticed up both the coasts of India, along the south of Arabia, round the south coast of Africa, to the east coast of Central America and Terra del Fuego, while it also spread to the north and reached the coast of France and was recorded at Devonport and Portland.

To the east it spread to Australia and New Zealand, to Alaska and San Francisco.

In Australia sudden rises of 5 and 6 feet were recorded on the west coast; on the shores of Java the rise was one of about 50 feet, while in the English Channel it was only barely noticeable. But if the effect on the sea near England was so slight there was another effect so pronounced that it must have forced itself on the attention of almost everyone in our islands.

The coarser particles which were shot into the air fell close to the island, finer ones rained down, as we have noted, a hundred miles away in Java, but still finer ones, shot up into the air to enormous heights by the terrific explosions, were carried in the upper regions of the atmosphere all round the globe. One effect of these fine particles floating in the air was to give most extraordinary sunset effects on clear evenings.

The writer very vividly remembers some of them he saw in Kent. The brilliant, almost lurid glows in the western sky lasted long after the usual sunset effects would have passed away, and were far brighter than the normal sunset colours.

These splendid sunset effects were noted in Australia, in India, and throughout Europe and America.

In Surrey particularly fine sunset colourings were noted on September 8, and on several occasions between that date and early November. On November 9 the effect was most wonderful and magnificent. Afterglows were recorded throughout November, December, and January, but during February and March the coloration decreased, and after March no peculiar illumination was observed.

Essay # 4. General Course of an Volcanic Eruption:

The general course of a volcanic eruption becomes plain when we consider such cases as those of Vesuvius, Monte Nuovo, and Krakatoa.

A district may have been quiescent for centuries, but then from some cause the earth’s internal heat produces stresses at some point in the crust below that district and the crust gives. Such a crack even though it be exceedingly small in amount causes a tremor, and things on the surface are shaken. It is rare that any actual Crack appears on the actual surface of the ground, but this does occur some­times.

The molten rock which is pressed up in some cases finds its way along these cracks and may eventually solidify in them without ever reaching the surface; these rock masses are spoken of as Intrusions. At other times water makes its way down to the heated area and, flashing into steam, produces an explosion.

This, if powerful enough, may blow off a huge amount of the superincumbent crust and cause vast damage, but if an opening has once been drilled the explosions are less violent and merely cause the ejection of blocks of stone or masses of molten rock. These may either fall back into the crater or be scattered over the mountain-sides round it.

Such mountains as Monte Nuovo and many of the so-called cinder cones of Central France have been formed in this way and show no other signs of volcanic activity. But in other cases molten rock wells up the central crater and pours over or breaches its cone and flows down its sides. In this way we get lava streams, and these, owing to subsequent periods of explosive violence, may get covered by volcanic ashes.

But the story of a volcano is not quite so simple as this, for as the volcano rises higher the pressure needed to force the column of molten rock up to near its summit must grow greater, and so it often happens that the internal forces find it easier to produce an opening on the sides of the mountain and so get formed those lateral cones, which in some cases are of such frequent occurrence. These may be merely cinder cones or they may pour out molten rock.

The wind which prevails during an eruption sends the ashes this way or that, and so the deposit of volcanic detritus round a volcano is not so continuous as an aqueous sediment. Lavas and ashes may be mixed up, and the lava stream may end off with great abruptness both at its lower end and along its sides. Moreover, when the whole mass has become firm, subsequent shocks may crack it, and into those cracks molten rock may be squeezed, giving rise to what are called dykes.

In general, therefore, beds which owe their formation to igneous activity do not show anything like the same regularity that is seen in the case of aqueous sediments. Moreover, as a rule, volcanic detritus encloses no life, for though the living things, such as the inhabitants of Pompeii, may get killed on the surface of the earth when it has ashes poured down on to it, yet the mass of the ash above contains no life of the period. It may, however, contain blocks of rocks which enclose fossils of their own age, and so be useful in giving something of a clue to the age of the ash.

Fossiliferous Ash Beds:

But there is another possibility; some ash may fall into the sea, and then as it settles it will entomb some of the life of the period, and successive erup­tions may build up a great thickness of rock in which after­wards fossils in plenty may be found. Such a deposit will often be in part like a terrestrial ash bed, and in part like a marine sediment, and these volcanic deposits are the most hopeful ones in which to search for fossils.

When denudation gets to work on a volcano it is obvious that loose ash beds will be scoured away with rapidity, and so when the denuding agents get either to the solid plug of frozen lava in the vent, or to streams of lava down the sides, or to the dyke intrusions, these hard rocks will be left as prominences above the softer and more easily eroded beds.

The discussion of vulcanicity is, however, incomplete unless one considers the origin of the molten rock; here we cannot appeal to active or modern volcanoes, as in their cases we can only see surface phenomena. But, after ages of denudation, ancient volcanoes have been dissected, and their roots, as it were, disclosed.

We then find that there was a large mass of molten rock below them which no doubt was compressed at different times and squeezed upwards. These huge masses of liquid material ultimately froze, and so have arisen the enormous masses of granite so well known in parts of our islands. Such a rock is spoken of as plutonic to distinguish it from a molten rock which solidified on the earth’s surface and is called volcanic.

If one asks what caused the pressure one must remember that our earth has a hot interior, as proved by every coal-mine and deep boring, and a cooled crust, and that as the earth slowly gives out its heat it shrinks, and the crust settles on to the shrinking nucleus. This settling down must give rise to enormous stresses, and it is no wonder that cracks appear and molten material is squeezed up them.

Fissure Eruptions:

But though the Vesuvian type of eruption is the one nearest to our doors and the one most commonly found at present on the earth, if we go to Iceland we shall find another and a very different type of eruption allied to what is called the Fissure type.

In Iceland, though there are volcanoes of the Vesuvian type, the most common eruptions take place from a chain of volcanoes arranged along a large fissure. Small ash cones may be built up at points on the fissure and the position of actual eruption moves on so that the rings of the cones intersect, but at times flows of lava pour out from the fissure without any cones at all, flood over the surrounding country, fill up the valleys, and instead of an undulating plain produce a more or less level lava desert. Such was the course of events in 1783 when one of the greatest extrusions of lava in modern times occurred in Iceland.

When such an outpouring takes place, the rivers which afterwards arise cut their way into the lavas and gradually remove them from the older rocks which they overlie or rest against. Ultimately the lava plain may become a plateau scored by streams that wind over it.

This type of volcanic action differs from the Vesuvian type in that it possesses few, if any, ash beds, and in its occurring with great regularity over a vast area.

The fissure type of rock being pre-eminently a lava there seems little hope of finding fossil evidence of its age. But when we remember that extrusion is not continuous and that rivers make their way over the plateau and form gravels, and that these gravels may be covered by a sub­sequent flow, we see that it is possible for fossil evidence to be found even amongst the lavas of the fissure type. Where the centre of volcanic activity lay beneath the sea the beds due to the eruption will get covered by marine deposits and so evidence will accumulate to give the age of the eruption, if in after times the sea-floor is raised above sea-level.

Essay # 5. Volcanic Deposits in the British Islands:

Now when we examine the rocks of our islands we find repeated evidence that in various periods there existed centres of vulcanicity in many places. The Pre-Cambrian rocks of Charnwood in Leicestershire, of Shropshire, of the Malverns and else­where contain undoubted beds of volcanic ash, while when we come to the succeeding Primary rocks we find abundant proof that volcanic action took place during their formation.

North Wales is one of the districts which must again and again have included volcanoes throughout Cambrian and Ordovician times, while on the other side of. St. George’s Channel Ordovician lavas and ashes are found to the north of Dublin and in the Waterford area. Nearer the centre of Ireland such rocks are seen close to Kildare, and in the west, in Galway and Mayo, extensive coarse and fine ash deposits and lavas have been proved to be of Ordovician age.

In South Wales Ordovician lavas and ashes are seen forming Cader Idris, and they occur in Shropshire, in the Lake District, and in Ayrshire. The Ordovician period, in fact, was one, as far as our islands are concerned, of almost universal volcanic activity. It will be noticed that much of our mountainous country owes its ruggedness and its height to the Ordovician volcanic rocks, which by their hardness have been able to withstand denuding forces.

After this widespread outburst of volcanic forces came the Silurian period, and for long it was thought that there was then a complete cessation of the ejectment of ashes and the outflow of lavas. Comparatively recently, however, it has been shown that Silurian lavas and ashes are to be seen in Kerry in Ireland, to the north of Bristol, and in the Mendips, and in South-west Wales. But elsewhere, as far as we know, throughout our islands deposition of marine sediment went on quietly throughout Silurian times.

After Silurian times there came a period of uplift from Central England northwards, and the formation of a series of large sea lochs in which the Old Red Sandstone was laid down.

The tract of land stretching north from the Cheviots contained many active volcanoes. In South Devon also there was much lava poured out and ash accumulated near what must have been the northern margin of a great ocean stretching southwards.

In South Scotland many of the plugs of igneous rocks which filled up old vents in Old Red Sandstone times have proved much harder to wear down than the surrounding ash cones, and so have remained forming hills standing up somewhat abruptly amongst softer strata. This volcanic tract of Argyllshire is continued to the west in Ulster.

To the south of this line volcanoes of Old Red Sandstone age occurred in the Pentland Hills, where numerous “necks ” of this Age are found. These form the western end of a long line of active volcanoes which stretches westwards to the Ayrshire coast.

The upper Old Red Sandstone beds but rarely enclose evidence of igneous activity, but in Carboniferous times once more the subterranean forces burst into vigorous action.

Carboniferous volcanoes were most abundant in Scotland, and they persisted from the lowest beds to the basement beds of the coal-measures. They were distributed over the central valley from the south of Kintyre to the Firth of Forth.

In England Carboniferous volcanic rocks are seen in Derbyshire, in the Mendips, and near Weston-super-Mare. In the Isle of Man there are the relics of a group of Carboni­ferous vents, but in Ireland the only evidence of activity during this period is in King’s County and near Limerick.

In Scotland the predominant type of outflow of lava is of the plateau type, but the cinder-cone type of volcano is seen abundantly in the Firth of Forth region and forms the regular type seen elsewhere in the British Isles.

The plateau type of eruption built up enormous sheets of igneous rock which now form the prominent escarp­ments of many hills in South Scotland, and the vents up which the molten rock rose were plugged up by the frozen lava, and now remain as numerous prominent hills rising abruptly above the softer sandstone around them. Instances of these hills are North Berwick Law and the Bass Rock.

Eventually, but before the close of the Carboniferous period, the plateau extrusions were submerged and buried under the Carboniferous limestone series, and then a new type of eruption began, the cinder-cone type. The relics of these cones are abundant in the Carboniferous beds of Scotland.

After a very considerable time earth movements ridged up the coal-measures and formed a series of inland seas, and in the Permian period we find feeble and short-lived volcanoes in Ayrshire, in East Fife, and in South Devon. In Ayrshire there are several volcanic necks, which descend vertically through the surrounding rocks and form vertical columns of volcanic material. This material is usually a coarse ash, but in some cases molten rock has risen up the vent.

In East Fife, along the shore near St. Andrews and Elie, there are abundant small necks piercing the Carboniferous strata. That they are older than the beds they pierce is proved by the blocks which fill them being of the same mineral character as the Carboniferous beds, while they also contain the same fossils.

At Largo a coarse volcanic ash lies unconformably on the ridges of the newest Carboniferous beds, and though no Permian sedimentary beds are known in this district, these volcanic beds, being post- Carboniferous, are considered to be of Permian age.

In East Fife some sixty necks can be seen, and the larger ones form conspicuous hills such as Largo Law.

In Devonshire lavas and volcanic ashes are found near Exeter, but, as in Ayrshire, there is no thick accumula­tion of the ashes. In Devon, however, the volcanic activity was far feebler than in Scotland.

The long story of Igneous activity in our islands now comes to an end for an enormous length of time. It is to be noticed that a region of vulcanicity frequently continues to be one throughout the Primary period. Thus in South Scotland we have volcanoes in Cambrian and Ordovician times, then came a rest during Silurian times, which was followed by a renewed outburst on a gigantic scale in the Old Red Sandstone and Carboniferous periods; this activity then became feebler, and though we do find evidences of volcanic action there in Permian times, the volcanic forces were evidently dying out.

After this for untold ages throughout the whole of the Secondary period came silence. No explosions rent the ground, no faulting caused earth­quakes, there was no ejectment of lava, the Jurassic period passed and was succeeded by the Cretaceous, and bed after bed was laid down over the greater part of our island. Time after time parts of the land were submerged or were upraised, but of all the enormous thickness of rock which accumulated not one inch is of volcanic origin.

Then we enter Tertiary times, and then once more the subterranean forces got to work and produced, effects on a grander scale than had ever before been witnessed in our island region.

The Tertiary Fissure-Eruptions:

The type of eruption which was characteristic of this Tertiary period of activity was the fissure type; very occasionally there was a small amount of crater building, as is proved by the occurrence of ashes, but the mass of igneous rock extruded was through almost innumerable fissures.

The tract of ground covered by these lavas was immense, and though the outpouring took place in Tertiary times the effect of marine and surface denudation has been so pronounced that the vast original plateau is now represented by mere scattered fragments above sea-level. The chief places where it is to be seen are in Antrim, in Staffa and Iona, in Mull and in Skye, but the plateau extended north­wards to the Faroe Islands, and the Iceland volcanoes of to-day are the sole remaining places where the eruptive forces still effect the extrusion of molten rock as they did  in days gone by.

The lava is of the type called a basalt, an almost black rock and one which frequently gives rise to a columnar structure, as is well seen in Staffa. This basalt rests on chalk in the Antrim district, but in Skye is found on much older rocks.

This basalt, which builds up huge thicknesses of rock in this north-western area, must not be imagined as coming out in one continuous flow. There were pauses between extrusions, and during those pauses rivers flowed over the plateaus, cut channels in them and produced beds formed of rolled blocks of the lava and of other rocks brought down from the hills; then more molten rock flowed out and buried these channels and their beds of conglomerate or gravel.

Thus we find in amongst the lavas water-formed beds, and some of them in Antrim and in the islands off the coast of Scotland have yielded plant remains, leaves and wood and a portion of a fresh-water fish.

After the building up of vast thicknesses of rock by repeated outwellings of lava there came a later period when igneous rock of a somewhat similar nature was intruded in sheets amongst the lavas. This rock withstands the action of the weather better than the softer basalts, and so as the basalts are now as a rule more or less horizontal, the two sets of rocks produce a very definite type of scenery.

The hills which have been carved out of them are flat-topped, and the sides are ringed with a series of horizontal scarps with almost vertical edges, these being formed by the hard intrusions. As the basalts on weathering form fertile ground, the slopes below the dark cliffs of intrusive rock are commonly covered by rich green grass.

Such scenery is well seen along the west coast of Skye and any one sailing up the west coast of Scotland will again and again see instances of it.

A consideration of the height to which the basalt plateau now rises enables one to see what an enormous amount of denudation has gone on since the Tertiary time, during which the lavas were poured out.

Originally the plateau extended from the island of Mull to the mainland to the north-east, where the promontories of Morven and Ardnamurchan are now seen. But between Mull and the mainland the Sound of Mull now exists twenty miles long and two miles broad. From the deepest part of the Sound to the top of the plateau in Mull is nearly 4000 feet, and the huge mass of basalt which used to occupy and cover the present Sound has been completely washed away.

Elsewhere the same story is told, and we must imagine that in Tertiary times a huge plateau of Basalt stretched from Antrim up between the present west coast of Scotland and the Hebrides, and beyond to the Faroe Islands.

From this general consideration of the history of British vulcanicity throughout past geological ages certain general facts become obvious.

The regions of vulcanicity in our islands all lie to the west of a line from the most south-easterly point of Scotland to Exeter. In this western region volcanic activity has persisted from the very earliest times of which we have any knowledge down to Tertiary times. Not only is this general persistence to be noted, but also the fact that particular portions of the volcanic region have been the sites of recrudescence of action again and again.

In the south-west of England we find volcanoes in Silurian, in Old Red Sandstone, in Carboniferous, and in Permian times.

In the south of Scotland we find plenty of evidence of vulcanicity all through the Ordovician period, in the Old Red Sandstone, in Carboniferous, and in Permian times, while there are also plenty of dykes there originally full of molten rock which rose up the cracks in the Tertiary period.

But not only are certain well-marked areas again and again the scenes of volcanic action, others are quite as con­spicuous for the absence of that action.

The Central Highlands of Scotland, though they abut on to the Old Red Sandstone, Carboniferous, and Permian volcanic areas to the south, themselves contain no traces of vents. The southern uplands, almost surrounded by volcanic rocks of various ages, are themselves perfectly free of any opening through which either lavas or ashes were sent out.

There seem to be, therefore, certain regions which are regions of weakness through which the volcanic forces were able to make openings to the surface, and these regions remained weak even though at one time or another those forces failed to continue ejecting matter through them.

Another point to be noticed is that we have no evidence of any slackening of the forces which produce volcanic action, and a further point is that the types of vulcanicity now known as existing at the present day have been seen again and again during the history of the world.

The volcanoes such as Vesuvius can be matched in the Primary period, during which, for instance, material in our English Lake District was accumulated to a depth of some 8000 feet.

The small ash-cones of the Neapolitan area are strikingly alike to the Tertiary cones of the Puy de Dôme district in France, and to the small cones of the Carboniferous period as seen in Scotland.

Lastly, the modern eruptions of Iceland resemble the Tertiary fissure-eruptions of North-east Ireland and West Scotland, and the Carboniferous fissure-eruptions of South Scotland.

Although the Tertiary outburst proves that as a whole the volcanic forces had by that time suffered no diminution, yet when one traces the history of volcanic action it is seen to have waxed and waned.

The enormous outpouring of lavas and ejection of ashes in Ordovician times was followed by an almost complete cessation of volcanic action in the following Silurian period. The succeeding Old Red Sandstone period gives evidence of a vulcanicity somewhat less pronounced than that of Ordo­vician times, and is succeeded by smaller outpourings in the Carboniferous period, while the following Permian vol­canoes are comparatively few and unimportant.

Then for the whole of the Secondary period there was a complete absence of any vulcanicity at all. Then in the older times of the Tertiary age came a period of great violence, which died down, and from that time to this the sound of a volcanic outburst has been unheard in our islands. But the history of our islands shows that from such a long spell of quiescence we cannot draw the con­clusion that those sounds will never be heard again.

The time may come when Scotland, or Wales, or the Midland district of England will be deluged with lava, or their lands desolated by downfalls of volcanic ash, when Glasgow or Birmingham may repeat the story of Pompeii, and future ages, by excavation at those places, may learn of the civilisa­tion of the twentieth century.

Essay # 6. Distribution of Volcanoes :

Almost all the active volcanoes are in the young folded mountain region, and the fault zones of Africa. There are more than 1000 volcanoes in the world and out of these about 500 are active or dormant.

These are distributed in three belts:

(b) Mid-World Mountain Belt:

This belt is extended from the Mediterranean Alps to the Himalayan region. Visuvius, Stromboli, Barren Island, Krakatoa, etc., are some of the best known volcanoes of this belt. There are 83 active volcanoes in this belt.

(c) African Rift Valley Belt:

This belt goes from Palestine in the north to Malagasy Island, through the East African region. The Kilimanjaro in Tanzania is a well-known dormant volcano of this belt.

Over and above these, some scattered volcanoes occur in some islands of the Pacific, Atlantic and Indian Oceans.

Essay # 7. Effects of Volcanic Activity :

Like earthquake, volcanic activity also changes the features of the surface of the earth. These changes are both destructive and constructive.

Great calamities take place due to volcanic activity. Often volcanic eruptions bury many beautiful towns. For example, Harculium and Pompii, the two beautiful Italian towns were completely buried by the erupted materials of Visuvius. Volcanic eruptions may be violent if it originates under sea. It causes strong waves which are highly destructive to life and settlements of coastal regions.

On the other hand, volcanoes may have some good effects. It sometimes creates extensive basaltic plateaus. Generally, the volcanic region is rich in minerals and the volcanic soils are fertile.

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• Earth Science

A volcano is a landform (usually a mountain) where molten rock erupts through the surface of the planet. There are a huge number of active volcanoes present worldwide. In this article, we will learn about the definition, formation and types of volcanoes.

What Are Volcanoes?

A volcano is a landform, a mountain, where molten rocks erupt through the surface of the planet. The volcano mountain opens downwards to a pool of molten rocks underneath the surface of the earth.

Pressure builds up in the earth’s crust and this is the reason why eruptions occur. Gases and igneous rocks shoot up and splash over or fill the air with lava fragments. The volcano eruption can cause hot ash, lateral blasts and lava flow, mudslides, and more.

Formation of Volcanoes

A volcano mountain is formed by the surface eruption of magma from within the earth’s upper mantle. The magma that erupts to the surface and forms a lava flow that deposits ash. As the volcano continues to erupt, a new layer of lava is added to the surface, accumulating to form a mountain.

Different Stages of Volcanoes

They tend to be conical although there are a variety of forms, depending upon:

• The nature of the material erupted
• The type of eruption
• The amount of change since the eruption

Volcanoes are categorised into three main categories:

• Active Volcanoes: A volcano will be classified as an active volcano if at the present time it is expected to erupt or is erupting already.
• Dormant Volcanoes: The classification of volcanoes which is called dormant would be a volcano that is not erupting or predicted to erupt in the near future.
• Extinct Volcanoes: An extinct volcano is a volcano that no one expects will ever have another eruption.

Reason Behind the Eruption of Volcanoes

The volcano eruption begins with the formation of magma in the lower section of the earth’s crust. The earth’s crust is made up of massive slabs called plates, which fit together like a jigsaw puzzle. The friction during the movement of plates causes earthquakes and volcanic eruptions.

With pressure, it travels upwards with tremendous force hitting solid rocks and other materials and creates a new passage to the earth’s surface. Once the magma reaches the air it is called lava.

Types of Volcanoes

These are grouped into four types:

• Cinder cones
• Composite volcanoes
• Shield volcanoes
• Lava volcanoes

Cinder Cones: These are the simplest type of volcano. They occur when particles and blobs of lava are ejected from a volcanic vent. The lava is blown violently into the air, and the pieces rain down around the vent. Over time, this builds up a circular or oval-shaped cone, with a bowl-shaped crater at the top. Cinder cone volcanoes rarely grow larger than about 1,000 feet above their surroundings.

Composite Volcanoes: Composite volcanoes are some of the Earth’s grandest mountains, and they are also called stratovolcanoes. They are typically symmetrical cones of large dimension built of alternating layers of lava flows, steep-sided, volcanic ash, blocks, bombs, and cinders and may rise as much as 8,000 feet above their bases.

Shield Volcanoes: A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They have very gentle slopes and are developed horizontally. Shield volcanoes are built by effusive eruptions, which flow out in all directions. They almost never have violent eruptions, with basic lava simply flowing out.

Lava Domes: Lava domes are the fourth type of volcano that we are going to discuss. Unlike composite and shield volcanoes, lava domes are of tiny stature. They are formed when the lava is too viscous to flow to a great distance. As the lava dome slowly grows, the outer surface cools and hardens as the lava continues to pile within. Eventually, the internal pressure can shatter the outer surface, causing loose fragments to spill down its sides. Generally, such lava domes are found on the flanks of larger composite volcanoes.

The video about the eruption of volcanoes

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Your burning questions about volcanoes, answered

ASU experts explain these molten mysteries

Volcano! That little word brings so much to our minds — streams of lava and clouds of ash, rumbling mountains, the might of a planet’s fiery underbelly, and our own nervous anticipation, curiosity and fear.

In fact, if it seems like more and more people have volcanoes on the brain, there’s a good reason.

It’s not necessarily that the number of volcanic eruptions is increasing, though media coverage of dangerous eruptions, such as the one in Indonesia on Aug. 10 or other recent ones in New Zealand and the Philippines, may make it appear that way. Scientists can’t say without more data from Earth’s history.

What is certain is that humans (and our stuff) take up more space on the planet than ever before, putting more people in the paths of volcanoes.

“The impact of volcanic eruptions is increasing,” volcanologist Amanda Clarke said. “As the global population grows, more people are being affected by eruptions, so we care about them more.”

Despite their growing effect on our lives, volcanoes seem to retain their air of mystery, leaving many of us with questions. Where do they come from? What causes eruptions? How do scientists predict them?

Clarke and fellow volcanologist Christy Till — both faculty in the Arizona State University  School of Earth and Space Exploration  — answer these questions and more to help us understand how to safely live in the shadows of these mighty forces of nature.

Click to view larger image. Illustration by Shireen Dooling

How does a volcano form?

There are two sides to the making of a volcano: what happens below ground and what happens above.

Events below ground have to do with plate tectonics. This is the theory that the Earth’s crust — the outer shell on which we live — is broken up into plates that move around on top of Earth’s mantle like ice cubes in a glass of water. Scientists see it as the force behind earthquakes, mountains, continent migration and volcano formation.

“Scientists for a long time have scratched their heads trying to figure out why these volcanoes occur where they do.”  — Christy Till

There are three basic types of tectonic environments where volcanoes grow.

The first is a convergent plate boundary, where two plates crash and an oceanic plate slips underneath another plate, bringing water and carbon dioxide into the mantle. This triggers a magma-melting process and creates more explosive volcanoes. This process created the Ring of Fire, an arch of volcanoes that wraps around the Pacific Ocean.

The second, a divergent plate boundary, occurs when a gap opens up between two plates. The gap is filled in by the mantle underneath, causing magma to melt. These volcanoes are common on the ocean floor and erupt continuously as the plates keep going their separate ways.

Volcanoes that form in the middle of a plate are called hot spot volcanoes.

“Scientists for a long time have scratched their heads trying to figure out why these volcanoes occur where they do,” Till said. “Our best guess is that there’s magma or mantle rising up underneath, and for some reason, it’s just hotter than in other places, so we get a volcano.”

Above ground, the part of the volcano we can see is formed by eruptions.

For example, Mount St. Helens, a composite volcano in Washington, grew over time as layers of debris from a mix of effusive eruptions (think gooey lava) and explosive eruptions (think pumice stone and ash) built on top of each other.

Sunset Crater, a cinder cone volcano in Arizona, ejected glowing fountains of lava and ash when it erupted, which then fell around the crater to create its steep slopes.

And Kilauea, a shield volcano in Hawaii, formed its wide but shallow slopes as its lava spread out in all directions and built up in layers over time.

However, the type of eruption, and therefore volcano, circles back to another underground element.

“The composition of the magma, and the process deep in the earth that forms it, controls the eruption style to a large extent,” Till said.

What is magma?

Magma is the molten material that sits under or inside the Earth’s crust. (Lava is magma that has reached the surface through a volcano.) Till’s lab, the  Experimental Petrology and Igneous processes Center , looks at how magma forms on Earth and on other planets, as well as the underground processes that lead up to an eruption.

One of the surprises that researchers have learned in the last 10 years, she says, is that the magma below a volcano is not the cauldron of bubbling, liquid goo we might imagine.

“In fact, what’s below a volcano is more like a slushie. In a slushie, you have mostly ice crystals and some liquid, and at first, it’s hard to suck it through a straw because it’s mostly ice. You have to wait until it melts a little to get it through a straw.”

Magma, too, is composed of crystals (the geological kind) with just a little bit of liquid. Something must happen to the magma underground to warm it up, making it liquid enough to erupt. To study those processes, Till gathers samples of those crystals, which she likens to “little black boxes,” from volcanic deposits on the surface and examines them with microscopes.

“These crystals have little zones in them, much like tree rings. They can tell us about the temperature, pressure and composition of the magma chamber, and also how long before an eruption these specific events happened,” she said.

Video by ASU Research

What happens during a volcanic eruption?

First, a fresher, hotter, more liquid magma rises from deeper in the Earth’s mantle and warms the slushie magma in the volcano’s chamber. One way for it to arrive there is via an earthquake, which might push up fresh magma or open new pathways for it to travel upward. However, not every earthquake can warm a magma chamber and cause an eruption, Till notes.

“There’s also a possibility that the seismic waves passing through the crust can kind of jiggle a magma body and cause it to fizz. Just like with a soda, those bubbles can generate overpressure and buoyancy, driving an eruption,” Clarke said.

As the new and old magmas mix, the crystal mush heats up and comes to the surface. It could be an effusive eruption of syrupy, flowing lava, or it could be an explosive eruption of ash, cinders and hunks of molten rock known as lava bombs. The amount of gas in the body of magma determines how violent the eruption is.

For those that are more explosive, the volcano could generate an ash cloud that travels great distances, which could have indirect effects like roof damage, bad air quality or crop devastation. It could also unleash the significantly more destructive pyroclastic flow, which is a searing wave of dense ash and gases that rushes along the ground, killing and burning everything in its path.

“The plume is the big footprint, but only indirectly dangerous,” Clarke said. “The pyroclastic flows are the smaller footprint, but much more dangerous.”

If the volcano is near a body of water, there is another opportunity for additional destruction — pyroclastic flows entering the sea can cause tsunamis.

How do scientists predict eruptions?

“The bread and butter of prediction is seismic data,” Clarke said. Volcanologists take seismic stations, which measure vibrations in the earth, and distribute them all around a volcano to get the best read on what’s happening underneath.

Another important tool is the tiltmeter, which, as its name suggests, measures any miniscule changes in the level of the earth. Typically, before a volcano erupts, the ground around it inflates slightly, which scientists call deformation.

Observatories typically also monitor gas emissions, such as sulfur dioxide and carbon dioxide, which may indicate changes happening deeper in the volcano.

“If you want to know what a volcano is capable of doing in the future, the first thing you have to do is look at what it did in the past.”  — Amanda Clarke

And finally, cameras — both standard and thermal — help volcanologists keep an eye on activity. Clarke explains that thermal cameras are especially helpful for tall volcanoes whose tops may often be obscured by clouds.

“Using these kinds of data together, you can even predict how much magma there is, and at what depth,” Clarke said.

Having an idea of what a particular volcano can do once it’s ready to erupt is also a critical piece of prediction that allows volcanologists to make safety recommendations.

“If you want to know what a volcano is capable of doing in the future, the first thing you have to do is look at what it did in the past,” Clarke said.

Researchers do this by collecting ash deposits from a wide area and dating them. This gives them an idea of how large a volcano’s eruptions were and how frequently they occurred. However, the method has its limitations. Hardened magma is much harder to date than ash, and supervolcanoes have eruptions so large that the ash travels thousands of miles, making it difficult to determine their true size.

There’s also the trouble of inconsistent eruptions. Volcanoes tend to fluctuate in the size of their eruptions; a big one may be followed by several smaller ones before another large one happens. That’s why it’s crucial, Clarke said, to look over long timespans for an accurate picture of a volcano’s history.

How far in advance scientists can predict an eruption depends on a host of factors, one of which is whether the eruption is large or small. Large eruptions are farther apart, so they might have longer warning times — from weeks away to even decades — while the magma slowly heats up after the last eruption. Small eruptions are closer together, so their warning times are shorter — months to hours. However, an abundance of data means that those predictions are typically more precise than for large eruptions.

Click to view larger image. Illustration by Shireen Dooling  

How can you stay safe in an area with volcanic activity?

Clarke has seen too many volcanic eruptions to count, but she says that her time on the island of Montserrat while getting her PhD was when she learned how to be safe around them.

“I think some people take a bit of a macho attitude about trying to get close to volcanoes,” she said.

Proper precautions, she argues, help people stay alive.

“The main thing is to understand what the local observatories and scientists are doing. They collect data. They know what’s going on,” she said.

Till has not experienced a volcanic eruption and, despite an academic interest in seeing one, is largely happy to keep it that way.

“I’ve been to volcanoes that could erupt at any time, but I was fortunate enough not to be there when they were erupting,” she said. Like Clarke, by checking in with observatories, she’s managed to keep herself safe in dangerous environments.

In the U.S., you can find the latest reports on activity at the  U.S. Geological Survey website . Abroad, other nations may have an equivalent database online, or you can visit the Smithsonian’s  Global Volcanism Program website , which gathers data from around the world.

These resources can help you find out what the alert level is in the area (and what colored or numbered alert system locals use), and whether there has been any activity recently. Clarke said it’s not a good idea to assume that other groups are communicating with the local observatory and recommends always checking for yourself.

“If you get a permit from the forest service to hike to a crater, that doesn’t mean it’s safe. That doesn’t mean they’ve checked the data.”

What do classifications like active, dormant and extinct mean?

Not much, it turns out.

Clarke explains that people used to classify a volcano as “active” if it had erupted in historic time. The problem with this is that historic time varies from culture to culture, because it refers to the time when written records became available. Volcanoes in Italy have extensive documentation going back thousands of years, but volcanoes in the U.S. don’t have as deep of a written history.

“Having had a historic eruption is a meaningless classification, because there’s no number that goes along with that,” Clarke said.

A dormant volcano is one that is active but not currently erupting, while an extinct volcano has not erupted in historic time and is unlikely to erupt in the future.

A handier — and globally applicable — way to determine if a volcano is active is whether it has erupted during the Holocene, our present epoch which began over 11,000 years ago. However, this marker ultimately has its own flaws. A volcano can have an incredibly long lifespan, sometimes lasting millions of years. Silence in recent millennia doesn’t mean its erupting days are over.

“Whether it erupted in the Holocene is meaningless when it comes to someplace like Yellowstone or the Valles Caldera, whose timescales are way longer than we even have the capacity to document,” Clarke said.

Can a volcanic eruption be stopped?

Ideas for stopping eruptions range from venting gases to relieve volcanic pressure to plugging the top like a cork in a bottle. However, these concepts remain untested, and most volcanologists don’t take such efforts seriously.

What has found some success, though, is using barriers to redirect lava and pyroclastic flows away from towns and important structures. Clarke gives the example of Heimaey, a harbor town in Iceland that experienced a nearby eruption in 1973. The resulting lava flow threatened to close off the bay that was their main economic resource.

“As it started to enter the bay, they got out all the water hoses they had and sprayed it, and it solidified there. They used the lava itself as a barrier,” Clarke said.

Do volcanoes affect the climate?

Volcanic eruptions have both positive and negative effects on the climate. For example, their plumes carry gases like sulfur dioxide, which reach above the clouds into the stratosphere. There, the gas forms into droplets of sulfuric acid.

“The sulfur compounds can be circulated around the globe, and they can filter out the sun’s light and heat to cool global temperatures,” Clarke said.

Researchers speculate that such an event — an 1815 eruption of Mount Tambora in Indonesia — was behind the 1816 “year without a summer” that caused low temperatures and heavy rains in Europe and North America, leading to food shortages.

Whether an eruption can have a worldwide effect may depend on the size and composition of the ash cloud, as well as the volcano’s position on Earth. The cooling effect is always temporary. The longest documented cooling period lasted about three years, though Clarke believes that super eruptions in Earth’s history may have had longer temperature effects.

If you’re thinking that this sounds like a good way to combat today’s warming temperatures, you’re not alone. Some scientists are beginning to research the possibilities of solar engineering — a strategy inspired by volcanoes that would use planes to spray sulfur dioxide into the stratosphere.

Another climate effect of volcanoes is that their ash makes super fertile soil, creating lush environments in the areas surrounding them. The plants and trees that grow in this rich soil capture and store carbon dioxide from the atmosphere.

“What’s in fertilizer? Phosphorus, nitrogen and potassium. Those are abundant in volcanic products,” Clarke said. “Basically, they act as a fertilizer just like you might buy at Agway or ACE Hardware.”

Nutrients from falling ash easily leach into the soil, she adds, making it an excellent delivery system as well.

What are volcanoes like on other planets?

Planets, and moons as well, can have volcanoes very different from those on Earth. Jupiter’s moon Io has more volcanic activity than any other object in our solar system; its lava fountains can be many miles high. And the dwarf planet Ceres has ice volcanoes, or cryovolcanoes. They erupt water instead of magma, which freezes on its surface.

“The compositions of planets are different, so the kinds of magma they have are different, which then gives them unique eruptive behavior,” Till said.

Her lab works to understand the magma of other celestial bodies by creating it in a special device called a piston cylinder, which simulates conditions on the interior of a planet.

“In the same way that you’d mix flour and sugar and eggs to make a cake, we mix silica and magnesium and iron and other elements in the proportion we want to study. Then we put them in our equivalent of an oven to make magma at high pressures and temperatures,” Till said. “When we do this, we can discover how magmas on other planets are different.”

Her team has begun work on a new project that will study the types of magma that may exist on planets outside our solar system, known as exoplanets. Knowing more about their magma will give researchers glimpses into those planets’ volcanic behavior.

“Over 4,000 exoplanets have been confirmed in the last five years or so, and we’re just starting to investigate them,” Till said. “It’s an exciting time.”

Top photo from Shutterstock.

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Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing (2017)

Chapter: 1 introduction, 1 introduction.

Volcanoes are a key part of the Earth system. Most of Earth’s atmosphere, water, and crust were delivered by volcanoes, and volcanoes continue to recycle earth materials. Volcanic eruptions are common. More than a dozen are usually erupting at any time somewhere on Earth, and close to 100 erupt in any year ( Loughlin et al., 2015 ).

Volcano landforms and eruptive behavior are diverse, reflecting the large number and complexity of interacting processes that govern the generation, storage, ascent, and eruption of magmas. Eruptions are influenced by the tectonic setting, the properties of Earth’s crust, and the history of the volcano. Yet, despite the great variability in the ways volcanoes erupt, eruptions are all governed by a common set of physical and chemical processes. Understanding how volcanoes form, how they erupt, and their consequences requires an understanding of the processes that cause rocks to melt and change composition, how magma is stored in the crust and then rises to the surface, and the interaction of magma with its surroundings. Our understanding of how volcanoes work and their consequences is also shared with the millions of people who visit U.S. volcano national parks each year.

Volcanoes have enormous destructive power. Eruptions can change weather patterns, disrupt climate, and cause widespread human suffering and, in the past, mass extinctions. Globally, volcanic eruptions caused about 80,000 deaths during the 20th century ( Sigurdsson et al., 2015 ). Even modest eruptions, such as the 2010 Eyjafjallajökull eruption in Iceland, have multibillion-dollar global impacts through disruption of air traffic. The 2014 steam explosion at Mount Ontake, Japan, killed 57 people without any magma reaching the surface. Many volcanoes in the United States have the potential for much larger eruptions, such as the 1912 eruption of Katmai, Alaska, the largest volcanic eruption of the 20th century ( Hildreth and Fierstein, 2012 ). The 2008 eruption of the unmonitored Kasatochi volcano, Alaska, distributed volcanic gases over most of the continental United States within a week ( Figure 1.1 ).

Finally, volcanoes are important economically. Volcanic heat provides low-carbon geothermal energy. U.S. generation of geothermal energy accounts for nearly one-quarter of the global capacity ( Bertani, 2015 ). In addition, volcanoes act as magmatic and hydrothermal distilleries that create ore deposits, including gold and copper ores.

Moderate to large volcanic eruptions are infrequent yet high-consequence events. The impact of the largest possible eruption, similar to the super-eruptions at Yellowstone, Wyoming; Long Valley, California; or Valles Caldera, New Mexico, would exceed that of any other terrestrial natural event. Volcanoes pose the greatest natural hazard over time scales of several decades and longer, and at longer time scales they have the potential for global catastrophe ( Figure 1.2 ). While

the continental United States has not suffered a fatal eruption since 1980 at Mount St. Helens, the threat has only increased as more people move into volcanic areas.

Volcanic eruptions evolve over very different temporal and spatial scales than most other natural hazards ( Figure 1.3 ). In particular, many eruptions are preceded by signs of unrest that can serve as warnings, and an eruption itself often persists for an extended period of time. For example, the eruption of Kilauea Volcano in Hawaii has continued since 1983. We also know the locations of many volcanoes and, hence, where most eruptions will occur. For these reasons, the impacts of at least some types of volcanic eruptions should be easier to mitigate than other natural hazards.

Anticipating the largest volcanic eruptions is possible. Magma must rise to Earth’s surface and this movement is usually accompanied by precursors—changes in seismic, deformation, and geochemical signals that can be recorded by ground-based and space-borne instruments. However, depending on the monitoring infrastructure, precursors may present themselves over time scales that range from a few hours (e.g., 2002 Reventador, Ecuador, and 2015 Calbuco, Chile) to decades before eruption (e.g., 1994 Rabaul, Papua New Guinea). Moreover, not all signals of volcanic unrest are immediate precursors to surface eruptions (e.g., currently Long Valley, California, and Campi Flegrei, Italy).

Probabilistic forecasts account for this uncertainty using all potential eruption scenarios and all relevant data. An important consideration is that the historical record is short and biased. The instrumented record is even shorter and, for most volcanoes, spans only the last few decades—a miniscule fraction of their lifetime. Knowledge can be extended qualitatively using field studies of volcanic deposits, historical accounts, and proxy data, such as ice and marine sediment cores and speleothem (cave) records. Yet, these too are biased because they commonly do not record small to moderate eruptions.

Understanding volcanic eruptions requires contributions from a wide range of disciplines and approaches. Geologic studies play a critical role in reconstructing the past eruption history of volcanoes,

especially of the largest events, and in regions with no historical or directly observed eruptions. Geochemical and geophysical techniques are used to study volcano processes at scales ranging from crystals to plumes of volcanic ash. Models reveal essential processes that control volcanic eruptions, and guide data collection. Monitoring provides a wealth of information about the life cycle of volcanoes and vital clues about what kind of eruption is likely and when it may occur.

1.1 OVERVIEW OF THIS REPORT

At the request of managers at the National Aeronautics and Space Administration (NASA), the National Science Foundation, and the U.S. Geological Survey (USGS), the National Academies of Sciences, Engineering, and Medicine established a committee to undertake the following tasks:

• Summarize current understanding of how magma is stored, ascends, and erupts.
• Discuss new disciplinary and interdisciplinary research on volcanic processes and precursors that could lead to forecasts of the type, size, and timing of volcanic eruptions.
• Describe new observations or instrument deployment strategies that could improve quantification of volcanic eruption processes and precursors.
• Identify priority research and observations needed to improve understanding of volcanic eruptions and to inform monitoring and early warning efforts.

The roles of the three agencies in advancing volcano science are summarized in Box 1.1 .

The committee held four meetings, including an international workshop, to gather information, deliberate, and prepare its report. The report is not intended to be a comprehensive review, but rather to provide a broad overview of the topics listed above. Chapter 2 addresses the opportunities for better understanding the storage, ascent, and eruption of magmas. Chapter 3 summarizes the challenges and prospects for forecasting eruptions and their consequences. Chapter 4 highlights repercussions of volcanic eruptions on a host of other Earth systems. Although not explicitly called out in the four tasks, the interactions between volcanoes and other Earth systems affect the consequences of eruptions, and offer opportunities to improve forecasting and obtain new insights into volcanic processes. Chapter 5 summarizes opportunities to strengthen

research in volcano science. Chapter 6 provides overarching conclusions. Supporting material appears in appendixes, including a list of volcano databases (see Appendix A ), a list of workshop participants (see Appendix B ), biographical sketches of the committee members (see Appendix C ), and a list of acronyms and abbreviations (see Appendix D ).

Background information on these topics is summarized in the rest of this chapter.

1.2 VOLCANOES IN THE UNITED STATES

The USGS has identified 169 potentially active volcanoes in the United States and its territories (e.g., Marianas), 55 of which pose a high threat or very high threat ( Ewert et al., 2005 ). Of the total, 84 are monitored by at least one seismometer, and only 3 have gas sensors (as of November 2016). 1 Volcanoes are found in the Cascade mountains, Aleutian arc, Hawaii, and the western interior of the continental United States ( Figure 1.4 ). The geographical extent and eruption hazards of these volcanoes are summarized below.

The Cascade volcanoes extend from Lassen Peak in northern California to Mount Meager in British Columbia. The historical record contains only small- to moderate-sized eruptions, but the geologic record reveals much larger eruptions ( Carey et al., 1995 ; Hildreth, 2007 ). Activity tends to be sporadic ( Figure 1.5 ). For example, nine Cascade eruptions occurred in the 1850s, but none occurred between 1915 and 1980, when Mount St. Helens erupted. Consequently, forecasting eruptions in the Cascades is subject to considerable uncertainty. Over the coming decades, there may be multiple eruptions from several volcanoes or no eruptions at all.

The Aleutian arc extends 2,500 km across the North Pacific and comprises more than 130 active and potentially active volcanoes. Although remote, these volcanoes pose a high risk to overflying aircraft that carry more than 30,000 passengers a day, and are monitored by a combination of ground- and space-based sensors. One or two small to moderate explosive eruptions occur in the Aleutians every year, and very large eruptions occur less frequently. For example, the world’s largest eruption of the 20th century occurred approximately 300 miles from Anchorage, in 1912.

In Hawaii, Kilauea has been erupting largely effusively since 1983, but the location and nature of eruptions can vary dramatically, presenting challenges for disaster preparation. The population at risk from large-volume, rapidly moving lava flows on the flanks of the Mauna Loa volcano has grown tremendously in the past few decades ( Dietterich and Cashman, 2014 ), and few island residents are prepared for the even larger magnitude explosive eruptions that are documented in the last 500 years ( Swanson et al., 2014 ).

All western states have potentially active volcanoes, from New Mexico, where lava flows have reached within a few kilometers of the Texas and Oklahoma borders ( Fitton et al., 1991 ), to Montana, which borders the Yellowstone caldera ( Christiansen, 1984 ). These volcanoes range from immense calderas that formed from super-eruptions ( Mastin et al., 2014 ) to small-volume basaltic volcanic fields that erupt lava flows and tephra for a few months to a few decades. Some of these eruptions are monogenic (erupt just once) and pose a special challenge for forecasting. Rates of activity in these distributed volcanic fields are low, with many eruptions during the past few thousand years (e.g., Dunbar, 1999 ; Fenton, 2012 ; Laughlin et al., 1994 ), but none during the past hundred years.

1.3 THE STRUCTURE OF A VOLCANO

Volcanoes often form prominent landforms, with imposing peaks that tower above the surrounding landscape, large depressions (calderas), or volcanic fields with numerous dispersed cinder cones, shield volcanoes, domes, and lava flows. These various landforms reflect the plate tectonic setting, the ways in which those volcanoes erupt, and the number of eruptions. Volcanic landforms change continuously through the interplay between constructive processes such as eruption and intrusion, and modification by tectonics, climate, and erosion. The stratigraphic and structural architecture of volcanoes yields critical information on eruption history and processes that operate within the volcano.

Beneath the volcano lies a magmatic system that in most cases extends through the crust, except during eruption. Depending on the setting, magmas may rise

___________________

1 Personal communication from Charles Mandeville, Program Coordinator, Volcano Hazards Program, U.S. Geological Survey, on November 26, 2016.

directly from the mantle or be staged in one or more storage regions within the crust before erupting. The uppermost part (within 2–3 km of Earth’s surface) often hosts an active hydrothermal system where meteoric groundwater mingles with magmatic volatiles and is heated by deeper magma. Identifying the extent and vigor of hydrothermal activity is important for three reasons: (1) much of the unrest at volcanoes occurs in hydrothermal systems, and understanding the interaction of hydrothermal and magmatic systems is important for forecasting; (2) pressure buildup can cause sudden and potentially deadly phreatic explosions from the hydrothermal system itself (such as on Ontake, Japan, in 2014), which, in turn, can influence the deeper magmatic system; and (3) hydrothermal systems are energy resources and create ore deposits.

Below the hydrothermal system lies a magma reservoir where magma accumulates and evolves prior to eruption. Although traditionally modeled as a fluid-filled cavity, there is growing evidence that magma reservoirs may comprise an interconnected complex of vertical and/or horizontal magma-filled cracks, or a partially molten mush zone, or interleaved lenses of magma and solid material ( Cashman and Giordano, 2014 ). In arc volcanoes, magma chambers are typically located 3–6 km below the surface. The magma chamber is usually connected to the surface via a fluid-filled conduit only during eruptions. In some settings, magma may ascend directly from the mantle without being stored in the crust.

In the broadest sense, long-lived magma reservoirs comprise both eruptible magma (often assumed to contain less than about 50 percent crystals) and an accumulation of crystals that grow along the margins or settle to the bottom of the magma chamber. Physical segregation of dense crystals and metals can cause the floor of the magma chamber to sag, a process balanced by upward migration of more buoyant melt. A long-lived magma chamber can thus become increasingly stratified in composition and density.

The deepest structure beneath volcanoes is less well constrained. Swarms of low-frequency earthquakes at mid- to lower-crustal depths (10–40 km) beneath volcanoes suggest that fluid is periodically transferred into the base of the crust ( Power et al., 2004 ). Tomographic studies reveal that active volcanic systems have deep crustal roots that contain, on average, a small fraction of melt, typically less than 10 percent. The spatial distribution of that melt fraction, particularly how much is concentrated in lenses or in larger magma bodies, is unknown. Erupted samples preserve petrologic and geochemical evidence of deep crystallization, which requires some degree of melt accumulation. Seismic imaging and sparse outcrops suggest that the proportion of unerupted solidified magma relative to the surrounding country rock increases with depth and that the deep roots of volcanoes are much more extensive than their surface expression.

1.4 MONITORING VOLCANOES

Volcano monitoring is critical for hazard forecasts, eruption forecasts, and risk mitigation. However, many volcanoes are not monitored at all, and others are monitored using only a few types of instruments. Some parameters, such as the mass, extent, and trajectory of a volcanic ash cloud, are more effectively measured by satellites. Other parameters, notably low-magnitude earthquakes and volcanic gas emissions that may signal an impending eruption, require ground-based monitoring on or close to the volcanic edifice. This section summarizes existing and emerging technologies for monitoring volcanoes from the ground and from space.

Monitoring Volcanoes on or Near the Ground

Ground-based monitoring provides data on the location and movement of magma. To adequately capture what is happening inside a volcano, it is necessary to obtain a long-term and continuous record, with periods spanning both volcanic quiescence and periods of unrest. High-frequency data sampling and efficient near-real-time relay of information are important, especially when processes within the volcano–magmatic–hydrothermal system are changing rapidly. Many ground-based field campaigns are time intensive and can be hazardous when volcanoes are active. In these situations, telemetry systems permit the safe and continuous collection of data, although the conditions can be harsh and the lifetime of instruments can be limited in these conditions.

Ground-based volcano monitoring falls into four broad categories: seismic, deformation, gas, and thermal monitoring ( Table 1.1 ). Seismic monitoring tools,

TABLE 1.1 Ground-Based Instrumentation for Monitoring Volcanoes

including seismometers and infrasound sensors, are used to detect vibrations caused by breakage of rock and movement of fluids and to assess the evolution of eruptive activity. Ambient seismic noise monitoring can image subsurface reservoirs and document changes in wave speed that may reflect stress. changes. Deformation monitoring tools, including tiltmeters, borehole strainmeters, the Global Navigation Satellite System (GNSS, which includes the Global Positioning System [GPS]), lidar, radar, and gravimeters, are used to detect the motion of magma and other fluids in the subsurface. Some of these tools, such as GNSS and lidar, are also used to detect erupted products, including ash clouds, pyroclastic density currents, and volcanic bombs. Gas monitoring tools, including a range of sensors ( Table 1.1 ), and direct sampling of gases and fluids are used to detect magma intrusions and changes in magma–hydrothermal interactions. Thermal monitoring tools, such as infrared cameras, are used to detect dome growth and lava breakouts. Continuous video or photographic observations are also commonly used and, despite their simplicity, most directly document volcanic activity. Less commonly used monitoring technologies, such as self-potential, electromagnetic techniques, and lightning detection are used to constrain fluid movement and to detect

ash clouds. In addition, unmanned aerial vehicles (e.g., aircraft and drones) are increasingly being used to collect data. Rapid sample collection and analysis is also becoming more common as a monitoring tool at volcano observatories. A schematic of ground-based monitoring techniques is shown in Figure 1.6 .

Monitoring Volcanoes from Space

Satellite-borne sensors and instruments provide synoptic observations during volcanic eruptions when collecting data from the ground is too hazardous or where volcanoes are too remote for regular observation. Repeat-pass data collected over years or decades provide a powerful means for detecting surface changes on active volcanoes. Improvements in instrument sensitivity, data availability, and the computational capacity required to process large volumes of data have led to a dramatic increase in “satellite volcano science.”

Although no satellite-borne sensor currently in orbit has been specifically designed for volcano monitoring, a number of sensors measure volcano-relevant

TABLE 1.2 Satellite-Borne Sensor Suite for Volcano Monitoring

NOTE: AIRS, Atmospheric Infrared Sounder; ALOS, Advanced Land Observing Satellite; ASTER, Advanced Spaceborne Thermal Emission and Reflection Radiometer; AVHRR, Advanced Very High Resolution Radiometer; CALIPSO, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation; COSMO-SkyMed, Constellation of Small Satellites for Mediterranean Basin Observation; GOES, Geostationary Operational Environmental Satellite; IASI, Infrared Atmospheric Sounding Interferometer; MISR, Multi-angle Imaging SpectroRadiometer; MLS, Microwave Limb Sounder; MODIS, Moderate Resolution Imaging Spectroradiometer; OMI, Ozone Monitoring Instrument; OMPS, Ozone Mapping and Profiler Suite; SAGE, Stratospheric Aerosol and Gas Experiment.

parameters, including heat flux, gas and ash emissions, and deformation ( Table 1.2 ). Thermal infrared data are used to detect eruption onset and cessation, calculate lava effusion rates, map lava flows, and estimate ash column heights during explosive eruptions. In some cases, satellites may capture thermal precursors to eruptions, although low-temperature phenomena are challenging to detect. Both high-temporal/low-spatial-resolution (geostationary orbit) and high-spatial/low-temporal-resolution (polar orbit) thermal infrared observations are needed for global volcano monitoring.

Satellite-borne sensors are particularly effective for observing the emission and dispersion of volcanic gas and ash plumes in the atmosphere. Although several volcanic gas species can be detected from space (including SO 2 , BrO, OClO, H 2 S, HCl, and CO; Carn et al., 2016 ), SO 2 is the most readily measured, and it is also responsible for much of the impact of eruptions on climate. Satellite measurements of SO 2 are valuable for detecting eruptions, estimating global volcanic fluxes and recycling of other volatile species, and tracking volcanic clouds that may be hazardous to aviation in near real time. Volcanic ash cloud altitude is most accurately determined by spaceborne lidar, although spatial coverage is limited. Techniques for measuring volcanic CO 2 from space are under development and could lead to earlier detection of preeruptive volcanic degassing.

Interferometric synthetic aperture radar (InSAR) enables global-scale background monitoring of volcano deformation ( Figure 1.7 ). InSAR provides much higher spatial resolution than GPS, but lower accuracy and temporal resolution. However, orbit repeat times will diminish as more InSAR missions are launched, such as the European Space Agency’s recently deployed Sentinel-1 satellite and the NASA–Indian Space Research Organisation synthetic aperture radar mission planned for launch in 2020.

1.5 ERUPTION BEHAVIOR

Eruptions range from violently explosive to gently effusive, from short lived (hours to days) to persistent over decades or centuries, from sustained to intermittent, and from steady to unsteady ( Siebert et al., 2015 ). Eruptions may initiate from processes within the magmatic system ( Section 1.3 ) or be triggered by processes and properties external to the volcano, such as precipitation, landslides, and earthquakes. The eruption behavior of a volcano may change over time. No classification scheme captures this full diversity of behaviors (see Bonadonna et al., 2016 ), but some common schemes to describe the style, magnitude, and intensity of eruptions are summarized below.

Eruption Magnitude and Intensity

The size of eruptions is usually described in terms of total erupted mass (or volume), often referred to as magnitude, and mass eruption rate, often referred to as intensity. Pyle (2015) quantified magnitude and eruption intensity as follows:

magnitude = log 10 (mass, in kg) – 7, and

intensity = log 10 (mass eruption rate, in kg/s) + 3.

The Volcano Explosivity Index (VEI) introduced by Newhall and Self (1982) assigns eruptions to a VEI class based primarily on measures of either magnitude (erupted mass or volume) or intensity (mass eruption rate and/or eruption plume height), with more weight given to magnitude. The VEI classes are summarized in Figure 1.8 . The VEI classification is still in use, despite its many limitations, such as its reliance on only a few types of measurements and its poor fit for small to moderate eruptions (see Bonadonna et al., 2016 ).

Smaller VEI events are relatively common, whereas larger VEI events are exponentially less frequent ( Siebert et al., 2015 ). For example, on average about three VEI 3 eruptions occur each year, whereas there is a 5 percent chance of a VEI 5 eruption and a 0.2 percent chance of a VEI 7 (e.g., Crater Lake, Oregon) event in any year.

Eruption Style

The style of an eruption encompasses factors such as eruption duration and steadiness, magnitude, gas flux, fountain or column height, and involvement of magma and/or external source of water (phreatic and phreatomagmatic eruptions). Eruptions are first divided into effusive (lava producing) and explosive (pyroclast producing) styles, although individual eruptions can be simultaneously effusive and weakly explosive, and can pass rapidly and repeatedly between eruption styles. Explosive eruptions are further subdivided into styles that are sustained on time scales of hours to days and styles that are short lived ( Table 1.3 ).

Classification of eruption style is often qualitative and based on historical accounts of characteristic eruptions from type-volcanoes. However, many type-volcanoes exhibit a range of eruption styles over time (e.g., progressing between Strombolian, Vulcanian, and Plinian behavior; see Fee et al., 2010 ), which has given rise to terms such as subplinian or violent Strombolian.

1.6 ERUPTION HAZARDS

Eruption hazards are diverse ( Figure 1.9 ) and may extend more than thousands of kilometers from an active volcano. From the perspective of risk and impact, it is useful to distinguish between near-source and distal hazards. Near-source hazards are far more unpredictable than distal hazards.

Near-source hazards include those that are airborne, such as tephra fallout, volcanic gases, and volcanic projectiles, and those that are transported laterally on or near the ground surface, such as pyroclastic density currents, lava flows, and lahars. Pyroclastic density currents are hot volcanic flows containing mixtures of gas and micron- to meter-sized volcanic particles. They can travel at velocities exceeding 100 km per hour. The heat combined with the high density of material within these flows obliterates objects in their path, making them the most destructive of volcanic hazards. Lava flows also destroy everything in their path, but usually move slowly enough to allow people to get out of the way. Lahars are mixtures of volcanic debris, sediment, and water that can travel many tens of kilometers along valleys and river channels. They may be triggered during an eruption by interaction between volcanic prod-

TABLE 1.3 Characteristics of Different Eruption Styles

ucts and snow, ice, rain, or groundwater. Lahars can be more devastating than the eruption itself. Ballistic blocks are large projectiles that typically fall within 1–5 km from vents.

The largest eruptions create distal hazards. Explosive eruptions produce plumes that are capable of dispersing ash hundreds to thousands of kilometers from the volcano. The thickness of ash deposited depends on the intensity and duration of the eruption and the wind direction. Airborne ash and ash fall are the most severe distal hazards and are likely to affect many more people than near-source hazards. They cause respiratory problems and roof collapse, and also affect transport networks and infrastructure needed to support emergency response. Volcanic ash is a serious risk to air traffic. Several jets fully loaded with passengers have temporarily lost power on all engines after encountering dilute ash clouds (e.g., Guffanti et al., 2010 ). Large lava flows, such as the 1783 Laki eruption in Iceland, emit volcanic gases that create respiratory problems and acidic rain more than 1,000 km from the eruption. Observed impacts of basaltic eruptions in Hawaii and Iceland include regional volcanic haze (“vog”) and acid rain that affect both agriculture and human health (e.g., Thordarson and Self, 2003 ) and fluorine can contaminate grazing land and water supplies (e.g., Cronin et al., 2003 ). Diffuse degassing of CO 2 can lead to deadly concentrations with fatal consequences such as occurred at Mammoth Lakes, California, or cause lakes to erupt, leading to massive CO 2 releases that suffocate people (e.g., Lake Nyos, Cameroon).

Secondary hazards can be more devastating than the initial eruption. Examples include lahars initiated by storms, earthquakes, landslides, and tsunamis from eruptions or flank collapse; volcanic ash remobilized by wind to affect human health and aviation for extended periods of time; and flooding because rain can no longer infiltrate the ground.

1.7 MODELING VOLCANIC ERUPTIONS

Volcanic processes are governed by the laws of mass, momentum, and energy conservation. It is possible to develop models for magmatic and volcanic phenomena based on these laws, given sufficient information on mechanical and thermodynamic properties of the different components and how they interact with each other. Models are being developed for all processes in volcanic systems, including melt transport in the mantle, the evolution of magma bodies within the crust, the ascent of magmas to the surface, and the fate of magma that erupts effusively or explosively.

A central challenge for developing models is that volcanic eruptions are complex multiphase and multicomponent systems that involve interacting processes over a wide range of length and time scales. For example, during storage and ascent, the composition, temperature, and physical properties of magma and host rocks evolve. Bubbles and crystals nucleate and grow in this magma and, in turn, greatly influence the properties of the magmas and lavas. In explosive eruptions, magma fragmentation creates a hot mixture of gas and particles with a wide range of sizes and densities. Magma also interacts with its surroundings: the deformable rocks that surround the magma chamber and conduit, the potentially volatile groundwater and surface water, a changing landscape over which pyroclastic density currents and lava flows travel, and the atmosphere through which eruption columns rise.

Models for volcanic phenomena that involve a small number of processes and that are relatively amenable to direct observation, such as volcanic plumes, are relatively straightforward to develop and test. In contrast, phenomena that occur underground are more difficult to model because there are more interacting processes. In those cases, direct validation is much more challenging and in many cases impossible. Forecasting ash dispersal using plume models is more straightforward and testable than forecasting the onset, duration, and style of eruption using models that seek to explain geophysical and geochemical precursors. In all cases, however, the use of even imperfect models helps improve the understanding of volcanic systems.

Modeling approaches can be divided into three categories:

• Reduced models make simplifying assumptions about dynamics, heat transfer, and geometry to develop first-order explanations for key properties and processes, such as the velocity of lava flows and pyroclastic density currents, the height of eruption columns, the magma chamber size and depth, the dispersal of tephra, and the ascent of magma in conduits. Well-calibrated or tested reduced models offer a straightforward ap-

proach for combining observations and models in real time in an operational setting (e.g., ash dispersal forecasting for aviation safety). Models may not need to be complex if they capture the most important processes, although simplifications require testing against more comprehensive models and observations.

• Multiphase and multiphysics models improve scientific understanding of complex processes by invoking fewer assumptions and idealizations than reduced models ( Figure 1.10 ), but at the expense of increased complexity and computational demands. They also require additional components, such as a model for how magma in magma chambers and conduits deforms when stressed; a model for turbulence in pyroclastic density currents and plumes; terms that describe the thermal and mechanical exchange among gases, crystals, and particles; and a description of ash aggregation in eruption columns. A central challenge for multiphysics models is integrating small-scale processes with large-scale dynamics. Many of the models used in volcano science build on understanding developed in other science and engineering fields and for other ap-

plications. Multiphysics and multiscale models benefit from rapidly expanding computational capabilities.

• Laboratory experiments simulate processes for which the geometry and physical and thermal processes and properties can be scaled ( Mader et al., 2004 ). Such experiments provide insights on fundamental processes, such as crystal dynamics in flowing magmas, entrainment in eruption columns, propagation of dikes, and sedimentation from pyroclastic density currents ( Figure 1.11 ). Experiments have also been used successfully to develop the subsystem models used in numerical simulations, and to validate computer simulations for known inputs and properties.

The great diversity of existing models reflects to a large extent the many interacting processes that operate in volcanic eruptions and the corresponding simplifying assumptions currently required to construct such models. The challenge in developing models is often highlighted in discrepancies between models and observations of natural systems. Nevertheless, eruption models reveal essential processes governing volcanic eruptions, and they provide a basis for interpreting measurements from prehistoric and active eruptions and for closing observational gaps. Mathematical models offer a guide for what observations will be most useful. They may also be used to make quantitative and testable predictions, supporting forecasting and hazard assessment.

Volcanic eruptions are common, with more than 50 volcanic eruptions in the United States alone in the past 31 years. These eruptions can have devastating economic and social consequences, even at great distances from the volcano. Fortunately many eruptions are preceded by unrest that can be detected using ground, airborne, and spaceborne instruments. Data from these instruments, combined with basic understanding of how volcanoes work, form the basis for forecasting eruptions—where, when, how big, how long, and the consequences.

Accurate forecasts of the likelihood and magnitude of an eruption in a specified timeframe are rooted in a scientific understanding of the processes that govern the storage, ascent, and eruption of magma. Yet our understanding of volcanic systems is incomplete and biased by the limited number of volcanoes and eruption styles observed with advanced instrumentation. Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing identifies key science questions, research and observation priorities, and approaches for building a volcano science community capable of tackling them. This report presents goals for making major advances in volcano science.

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100 Words Essay On Volcanic Eruptions In English

Wikipedia defines the term as follows: “A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.” A volcanic eruption is when lava and gas are released from a volcano—sometimes explosively. 

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Ever since we were small kids, we always were fascinated about what was in the sky. Ever thought about what is going on beneath the land on which we stand? Let us dive down inside the Earth’s surface and familiarize ourselves with a phenomenon known as the Volcano. You may have seen Volcanic eruptions in movies. Let us learn volcanoes in detail.

Introduction to Volcanoes

Volcanoes are openings on the Earth’s crust through which molten metal comes out. Volcanoes form on the boundaries of the tectonic plates that continuously move inside the Earth. There are 17 such tectonic plates inside the Earth’s Crust. The Earth is divided into several parts such as the Crust, Upper Mantle, Mantle, Outer Core, Inner Core. These tectonic plates exist in the Mantle region inside the Earth. Earthquakes also occur because of the movement of such tectonic plates. Places on the Earth that are on the edges of these tectonic plates are affected the most as these regions are subjected to more amount of underground activity. According to geographical studies, volcanoes generally exist in places where the tectonic plates are either converging or diverging. Volcanic eruption results in polluting the environment as it includes harmful gases and other hot toxic elements including ash.

Formation of Volcanoes and Volcanic Eruptions

Volcanoes are scattered across the planet. Almost about 1500 volcanoes are there on the Earth’s crust and several others are there on the ocean bed. All volcanoes emit gases no matter where they are present. The journey of these emissions begins deep down near the core of the Earth. The core is as hot as the surface of the Sun. Above the core is the mantle layer which is fully present of rocks. The Earth’s core is of very high temperature, because of which the heat converts some portion of the rocky mantle into Magma which has a lower density than the surrounding rocky environment and as a result, it travels upwards and burst out through the openings on the earth’s crust which we know as a volcanic eruption. Once this magma reaches Earth’s surface it is known as lava and has a temperature of near about.

Classification of Volcanoes

There are various kinds of volcanoes primarily classified on the shape and size basis. They are-

• Strato Volcanoes: These appear as tall steep mountains over the earth’s crust.
• Shielded Volcanoes: These appear as dome-shaped over the ground.
• Calderas: These types of volcanoes have large depressions and look like plateaus. A continuous amount of smoke keeps coming out from these types of volcanoes.
• Ocean- ridges: These are underwater chains of volcanic mountains that exist deep inside the ocean bodies.

Again based on eruptions volcanoes are classified as active or inactive type volcanoes. The volcanoes which erupt on regular basis are termed as active volcanoes and the ones which erupted after a long time (maybe during historical times) are referred to as inactive or dormant type volcanoes.

Ring of Fire

The Ring of Fire is nothing but the path that traces the boundaries of several tectonic plates around the Pacific Ocean which consist of about the world’s 75% of the volcanoes. The Ring of fire covers up regions from Eastern Asia to both North and South America. It is a long chain of about 25000 miles and touches some parts of New Zealand as well.

Famous Volcanic Eruptions

Since the inception of Earth, there has been a lot of volcanic eruptions which were also catastrophic for mankind. Some of them are:

• Mount Vesuvius in 79 A.D in Italy.
• Mount Ruiz in 1985 in Columbia.
• The Mount Tambora in 1815 in Indonesia.
• Mount Pelee in 1902 in the Caribbean Islands.
• Mount Krakatoa in 1883 in Indonesia.

In America, there is the famous Yellowstone National Park which consists of the supervolcano. Scientists think that if this supervolcano erupts then it will destroy almost half of the planet. Also, this supervolcano is said to have erupted about 7 lakh years ago.

Effects of Volcanic Eruption on the Environment

Volcanic eruptions releases tons of harmful and toxic elements into the atmosphere. These harmful gases include oxides of carbon and sulfur which have a deadly impact on the ozone layer. Nitrogen dioxide formed also affects the environment. Again, gas emissions as a result of the volcanic eruptions lead to the creation of acid rain as well. Also, aerosols are injected into the Earth’s Atmosphere. When high-temperature ash is thrown out into the environment then sometimes it happens that those particles hit the aeroplanes flying in the sky.

Again when pyroclastic materials mix up with the water in the nearby lakes and rivers, it affects the marine life as well as pollutes the water bodies. Earthquakes are also a result of a high VEI type volcanic eruption. These types of earthquakes are volcano-tectonic earthquakes. These types of earthquakes can have a reading of as high as 7 on the Richter scale. Tsunamis are also a consequence of a devastating volcanic eruption.

Volcanic Eruptions In Solar System

As Earth is a planet and volcanic eruptions have been taking place since its inception, so it is quite obvious that the other planets also experience such a phenomenon. Mars is said to have several dormant type volcanoes. Some moons of Jupiter are said to have an active volcanic system. The planet Venus has a Basalt surface which has been generated because of several volcanoes erupting together. In a nutshell, we can say that a celestial body whose core is hot enough to generate molten metal will experience volcanic eruptions.

FAQs about Volcanoes

Q.1. What is VEI?

Answer: VEI is the volcanic explosivity index. This property depicts various factors of a volcanic eruption such as the volume of lava, gas, and other emissions from the volcano. The VEI scale begins from 0. The most destructive volcanic eruption has a rating of 8 on the VEI scale. Mount Tambora in Indonesia erupted in 1815 with a VEI scale of 7. Also, this was the most destructive Strato volcanic eruption ever witnessed by mankind. This leads to earthquakes and tsunamis as well. This volcanic eruption immediately killed around 10,000 people and later around with various diseases coming into the picture took away more than 1,00,000 lives.

Q.2. What are tectonic plates?

Answer: The tectonic plates are large pieces on the Earth’s crust and upper mantle region which move and collide continuously with each other. The formation of tectonic plates is considered to begin about 3.2 billion years ago. Because of the movement of these tectonic plates, mountains and plateaus, volcanoes form.

Q.3.  What is a pyroclastic flow?

Answer: When a volcano erupts, lots of toxic elements(gases) and molten metal having very high temperature coming out of the volcano opening and flows down through the outer volcanic surface. This can burn down anything in its path. This phenomenon is the pyroclastic flow. The speed of such flow is as high as 725 kilometres per hour.

Q.4. What are aerosols?

Answer: Aerosols are the suspension of solid particles or liquid droplets in another gaseous medium. The volcanic eruptions include aerosols, i.e. solid particles mix with harmful gases and eject out.

Q.5. What is Basalt?

Answer: Basalt is an igneous rock that forms by the cooling of the lava rich in elements such as iron and magnesium.

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Short Essay on Volcanoes

A volcano is an opening, in the planet’s surface which allows hot, molten rock, ash and gases to escape from below the surface.

The name, “volcano” originates from the name Vulcan, a god of fire in Roman mythology. Volcanoes are like giant safety valves that release the pressure that builds up inside the Earth. The Hawaii islands were formed by 5 volcanoes.

Classified by the extent of their activity volcanoes are of four types. An ‘active’ volcano is one that erupts regularly. There are about 500 known active volcanoes on Earth, not counting those that lie beneath the sea.

A ‘dormant’ volcano is one that has not erupted for many years, although there is still some activity deep inside it. An ‘extinct’ volcano is one which has ceased to be active.

A volcanic eruption occurs when hot rocks and lava burst from a volcano; and geysers and springs are actually just volcanoes that throw boiling water high in the air. They are caused by volcanic heat warming trapped ground water.

Image Source: 745515a37222097b0902-74ef300a2b2b2d9e236c9459912aaf20.ssl.cf2.rackcdn.com

The liquid rocks inside a volcano are called magma and when it flows out it is called as lava. Fresh lava has temperatures from 700 degrees C to 1200’C and glows red-hot to white hot as it flows. The most dangerous volcanic eruption recorded is the eruption of Mount St. Helens in Washington.

The tallest volcano in the world is the Ojos del Salado, a volcano in Chile. The world’s largest volcano is the Muano Loa in Hawaii.

Volcanoes are generally concentrated on the edge of continents, along the island chain, or beneath the sea forming long mountain ranges. A major part of the world’s active volcanoes above sea level encircle the Pacific Ocean forming the “Ring of Fire.”

Volcanoes can have serious affects on the lands and people around them when they erupt. The destruction they leave in their wake accounts for the total annihilation of the surrounding landscape. Around 2, 00,000 people have lost their lives to volcanic eruptions in the past five hundred years.

Buildings are destroyed, people are rendered homeless, people are killed, plant and animal life are both destroyed and the poisonous gases that emanate from the volcanoes can cause death and diseases like pneumonia in the people who survive it.

However not everything associated with the volcanoes is negative. The crust of the earth exists due to the large volumes of magma that did not erupt but instead cooled below the surface. It results in rich soil which is good for cultivation.

The volcanic ash that blows out of the volcano increases soil fertility by adding nutrients to the soil. Ground water heated by magma can be tapped for geothermal energy. Most of the metallic minerals like copper, gold, silver, lead and zinc are mined from the magmas found deep within the roots of extinct volcanoes.

With the increasing studies done by scientists on volcanoes it is becoming possible to gauge the activity level of a volcano. With this information although it might not be possible to prevent the erupting of a volcano at least the massive destruction of lives can be avoided by getting people evacuated in time.

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One of Russia's most active volcanoes erupts after a powerful earthquake off the east coast of the country's Kamchatka Peninsula

Topic: Disasters, Accidents and Emergency Incidents

A volcano has erupted in Russia's far east after a magnitude-7.0 earthquake struck early on Sunday, local time. 

Plumes of of ash erupted 5 kilometres into the sky over the far eastern Kamchatka Peninsula and briefly triggered a "code red" warning for aircraft.

What's next?

The Institute of Volcanology and Seismology said a potential second quake could come "within 24 hours" with a magnitude approaching 9.0.

One of Russia's most active volcanoes has erupted, spewing plumes of ash 5 kilometres into the sky over the far eastern Kamchatka Peninsula and briefly triggering a "code red" warning for aircraft.

The Shiveluch volcano began sputtering shortly after a powerful magnitude-7.0 earthquake struck off Kamchatka's east coast early on Sunday, local time, according to volcanologists from the Russian Academy of Sciences.

They warned that another, even more potent earthquake may be on the way.

The academy's Institute of Volcanology and Seismology released a video showing the ash cloud over Shiveluch. It stretched over 490km east and south-east of the volcano.

The eruption of the Shiveluch volcano is seen in Kamchatka Peninsul. ( Institute of Volcanology and Seismology of the Far Eastern Branch of the Russian Academy of Sciences )

The Ebeko volcano located on the Kuril Islands also spewed ash 2.5km high, the institute said.

It did not explicitly say whether the earthquake touched off the eruptions.

A "code red" ash cloud warning briefly put all aircraft in the area on alert, the Kamchatka Volcanic Eruption Response Team reported.

A separate report on Sunday carried by the official Tass news agency said that no commercial flights had been disrupted and there was no damage to aviation infrastructure.

The tremors in the area may be a prelude to an even stronger earthquake in south-eastern Kamchatka, Russian scientists warned.

The Institute of Volcanology said a potential second quake could come "within 24 hours" with a magnitude approaching 9.0.

There were no immediate reports of injuries from Sunday's earthquake, which struck at a depth of 6km under the sea bed with the epicentre 108km south-east of the nearest city, according to Russian emergency officials.

Russian news outlets cited residents of Petropavlovsk-Kamchatsky, a port city of more than 181,000 people that sits across a bay from an important Russian submarine base, reporting some of the strongest shaking "in a long time".

On November 4, 1952, a magnitude-9.0 quake in Kamchatka caused damage but no reported deaths despite setting off 9.1m waves in Hawaii.