essay about types of plate boundaries

The Types of Plate Tectonics Essay

Introduction, types of plate tectonics, works cited.

Plate tectonics refers to movements on Earth’s surface, that is, the lithosphere. This is a theory in science explaining such movements. The lithosphere is made up of large broken rock masses also referred as tectonic plates (Oreskes 424). These tectonic plates are suspended on molten layer of Earth’s crust that comes immediately below the lithosphere; this layer is called asthenosphere.

Given that the asthenosphere is molten, these plates move on it with ease. The movement occurs at boundaries namely; transform boundaries, divergent and convergent boundaries (Oreskes 16). These three different boundaries give rise to the different forms of plate tectonics known today.

According to United States Geological Survey (USGS), there are three different types of plate movements; that is, divergent, convergent, and lateral plate slipping resulting from the three different plate boundaries that exist. Divergent plate movements occur when two oceanic plate move away from each other to form new oceanic crust at a zone of divergence. The zone of divergence results as the Earth’s crust separates (Earth Science). The separation results from hot magma arising from the magma in the continental mantle. This magma has large pressure that causes the crust to crack and separate.

Convergent plate movements are the opposite of divergent and it occurs when two oceanic plates collide leading to loss of crust at a convergent point. Convergent movements involve collision between two plates and these two plates may be either continental or oceanic (USGS).

Convergent plate movements come after divergent plate movements because after the plates break up in the latter, they meet at another point and collide hence the subduction. On the other hand, lateral slipping occurs when two plates move in opposite direction slipping over each other at a transform boundary. The two plates eventually jerk apart due to pressure that mounts up in the mantle and this causes earthquakes (USGS).

The movement of these plates is facilitated by the fact that they float on the Earth’s molten magma on the region called asthenosphere, which lies, below lithosphere. As aforementioned, lithosphere is the outermost Earth’s crust that human beings can reach. Actually, lithosphere makes the tectonic plates (Rychert and Shearer 496). The molten magma heats up as the core of the Earth heats up which causes convectional currents within the molten magma. As the earth core cools, the molten magma cools and sinks and in the process, it pulls the plates attached to it hence the plate movement.

Earthquakes results from these plate tectonic movements along fault lines. Fault lines are cracks on lithosphere. As tectonic plates move, there is building up of pressure along the fault lines, and when this pressure exceeds the strength of lithosphere, earthquakes result to relieve the pressure mounting in the lithosphere. According to Rychert and Shearer, the lateral plate slipping form of movement is the one that causes many earthquakes around the world (498).

Plate tectonics describes the movement of fragments formed from broken lithosphere.

These fragments are suspended on the asthenosphere, which is molten hence offering good medium of movement. There are three different types of plate tectonics, that is, convergent, divergent, and lateral slipping. These movements cause earthquakes as the lithosphere releases mount up pressure in the Earth’s mantle. Earthquakes result mainly from lateral slipping moving and this occurs along fault lines, which are weak points on the lithosphere.

Earth Science. “Plate Tectonics.” Moorland School. N.d. Web.

Oreskes, Naomi. “Plate Tectonics: An Insider’s History of the Modern Theory of the Earth.” California: Westview Press, 2003.

Rychert, Catherine, & Shearer, Peter. “A Global View of the Lithosphere-Asthenosphere Boundary.” Science Journals. 324(5):5926. 2009.

United States Geological Survey. “ Understanding Plate Motions .” 1999. Web.

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Plate boundaries: divergent, convergent, and transform.

Movement in narrow zones along plate boundaries causes most earthquakes. Most seismic activity occurs at three types of plate boundaries—divergent, convergent, and transform.

As the plates move past each other, they sometimes get caught and pressure builds up. When the plates finally give and slip due to the increased pressure, energy is released as seismic waves, causing the ground to shake. This is an earthquake.

Some of the plates have ocean water above them. Other plates include continents, and some plates include both continents and ocean. The movements of the plates help shape the geological features of our planet. The three main types of plate movements include:

Divergent (Spreading) :This is where two plates move away from each other. Molten rock from the mantle erupts along the opening, forming new crust. The earthquakes that occur along these zones, called spreading centers, are relatively small. The Great Rift Valley in Africa, the Red Sea and the Gulf of Aden all formed as a result of divergent plate motion.

Convergent (Colliding) : This occurs when plates move towards each other and collide. When a continental plate meets an oceanic plate, the thinner, denser, and more flexible oceanic plate sinks beneath the thicker, more rigid continental plate. This is called subduction. Subduction causes deep ocean trenches to form, such as the one along the west coast of South America. The rocks pulled down under the continent begin to melt. Sometimes the molten rock rises to the surface, through the continent, forming a line of volcanoes. About 80% of earthquakes occur where plates are pushed together, called convergent boundaries.

Another form of convergent boundary is a collision where two continental plates meet head-on. Since neither plate is stronger than the other, they crumple and are pushed up. This can lead to the formation of huge, high mountain ranges such as the Himalayas.

When two tectonic plates slide past each other, the place where they meet is a transform or lateral fault . The San Andreas Fault is one of the best examples of lateral plate motion.

This post is part of Exploring Earthquakes , a rich collection of resources co-presented by the California Academy of Sciences and KQED. This material is also available as a free iBooks textbook and iTunes U course .

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What are the different types of plate tectonic boundaries?

There are three kinds of plate tectonic boundaries: divergent, convergent, and transform plate boundaries..

This image shows the three main types of plate boundaries: divergent, convergent, and transform. Image courtesy of the U.S. Geological Survey. Download image (jpg, 76 KB) .

The Earth’s lithosphere, which includes the crust and upper mantle, is made up of a series of pieces, or tectonic plates, that move slowly over time.

A divergent boundary occurs when two tectonic plates move away from each other. Along these boundaries, earthquakes are common and magma (molten rock) rises from the Earth’s mantle to the surface, solidifying to create new oceanic crust. The Mid-Atlantic Ridge is an example of divergent plate boundaries.

When two plates come together, it is known as a convergent boundary . The impact of the colliding plates can cause the edges of one or both plates to buckle up into a mountain ranges or one of the plates may bend down into a deep seafloor trench. A chain of volcanoes often forms parallel to convergent plate boundaries and powerful earthquakes are common along these boundaries. The Pacific Ring of Fire is an example of a convergent plate boundary.

At convergent plate boundaries, oceanic crust is often forced down into the mantle where it begins to melt. Magma rises into and through the other plate, solidifying into granite, the rock that makes up the continents. Thus, at convergent boundaries, continental crust is created and oceanic crust is destroyed.

Two plates sliding past each other forms a transform plate boundary . One of the most famous transform plate boundaries occurs at the San Andreas fault zone, which extends underwater. Natural or human-made structures that cross a transform boundary are offset — split into pieces and carried in opposite directions. Rocks that line the boundary are pulverized as the plates grind along, creating a linear fault valley or undersea canyon. Earthquakes are common along these faults. In contrast to convergent and divergent boundaries, crust is cracked and broken at transform margins, but is not created or destroyed.

For More Information

What features form at plate tectonic boundaries?

Voyage to the Ridge 2022

The Northeast Caribbean – Plate Tectonics in Action

Plate Tectonics

The Earth's plates jostle about in fits and starts that are punctuated with earthquakes and volcanic eruptions.

There are a few handfuls of major plates and dozens of smaller, or minor, plates. Six of the majors are named for the continents embedded within them, such as the North American, African, and Antarctic plates. Though smaller in size, the minors are no less important when it comes to shaping the Earth. The tiny Juan de Fuca plate is largely responsible for the volcanoes that dot the Pacific Northwest of the United States.

The plates make up Earth's outer shell, called the lithosphere . (This includes the crust and uppermost part of the mantle.) Churning currents in the molten rocks below propel them along like a jumble of conveyor belts in disrepair. Most geologic activity stems from the interplay where the plates meet or divide.

The movement of the plates creates three types of tectonic boundaries: convergent, where plates move into one another; divergent, where plates move apart; and transform, where plates move sideways in relation to each other.

They move at a rate of one to two inches (three to five centimeters) per year.

Convergent Boundaries

Where plates serving landmasses collide, the crust crumples and buckles into mountain ranges . India and Asia crashed about 55 million years ago, slowly giving rise to the Himalaya , the highest mountain system on Earth. As the mash-up continues, the mountains get higher. Mount Everest, the highest point on Earth, may be a tiny bit taller tomorrow than it is today.

These convergent boundaries also occur where a plate of ocean dives, in a process called subduction, under a landmass. As the overlying plate lifts up, it also forms mountain ranges. In addition, the diving plate melts and is often spewed out in volcanic eruptions such as those that formed some of the mountains in the Andes of South America.

At ocean-ocean convergences, one plate usually dives beneath the other, forming deep trenches like the Mariana Trench in the North Pacific Ocean, the deepest point on Earth. These types of collisions can also lead to underwater volcanoes that eventually build up into island arcs like Japan.

Divergent Boundaries

At divergent boundaries in the oceans, magma from deep in the Earth's mantle rises toward the surface and pushes apart two or more plates. Mountains and volcanoes rise along the seam. The process renews the ocean floor and widens the giant basins. A single mid-ocean ridge system connects the world's oceans, making the ridge the longest mountain range in the world.

On land, giant troughs such as the Great Rift Valley in Africa form where plates are tugged apart. If the plates there continue to diverge, millions of years from now eastern Africa will split from the continent to form a new landmass. A mid-ocean ridge would then mark the boundary between the plates.

Mountains and a rift can be seen along the San Andreas Fault.

Transform Boundaries

The San Andreas Fault in California is an example of a transform boundary, where two plates grind past each other along what are called strike-slip faults. These boundaries don't produce spectacular features like mountains or oceans, but the halting motion often triggers large earthquakes, such as the 1906 one that devastated San Francisco.

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• Types of Plate Boundaries

Introduction

The landscapes of our national parks, as well as geologic hazards such as earthquakes and volcanic eruptions, are due to the movement of the large plates of Earth’s outer shell. There are three types of tectonic plate boundaries:

• Plates rip apart at a divergent plate boundary , causing volcanic activity and shallow earthquakes;
• At a convergent plate boundary , one plate dives (“subducts”) beneath the other, resulting in a variety of earthquakes and a line of volcanoes on the overriding plate;
• Transform plate boundaries are where plates slide laterally past one another, producing shallow earthquakes but little or no volcanic activity.

Another large-scale feature is a hotspot , where a plate rides over a rising plume of hot mantle, creating a line of volcanoes on top of the plate. National Park Service lands contain not only active examples of all types of plate boundaries and hotspots, but also rock layers and landscapes that reveal plate-tectonic activity that occurred in the distant past.

Plate Boundaries and Hotspot Demonstration

Oreo® cookies are a fun way to demonstrate the three types of plate boundaries and a hotspot. (Modified from “Oregon's Island in the Sky: Geology Road Guide to Marys Peak, by Robert J. Lillie, Wells Creek Publishers, 75 pp., 2017, www.amazon.com/dp/1540611965).

Divergent Plate Boundary

Volcanic eruptions and shallow earthquakes are common where plates rip apart.

Convergent Plate Boundary

Where plates crash together, one dives (“subducts”) beneath the other, causing volcanoes (red triangles) to erupt on the overriding plate and earthquakes (black stars) at a variety of depths. The large white star represents the zone where plates lock together for centuries then suddenly let go, causing the largest earthquakes.

Transform Plate Boundary

Shallow earthquakes and little volcanism occur where one plate slides laterally past another.

In places like Hawaii and Yellowstone, a plate rides over a rising plume of hot mantle, causing earthquakes and a chain of volcanoes.

Photos and illustrations above modified from “Oregon's Island in the Sky: Geology Road Guide to Marys Peak, by Robert J. Lillie, Wells Creek Publishers, 75 pp., 2017, www.amazon.com/dp/1540611965.

How fast do the plates move?

Look at your fingernails and watch them grow. That will give you an idea of how fast the plates move relative to one another—about a fraction of an inch to a few inches per year! That doesn’t seem like much, but over time it adds up. For example, moving at about 2 inches (5 centimeters) per year, in our lifetime the Pacific Plate moves 10 to 15 feet (3 - 5 meters) past the North American Plate along the San Andreas Fault, a transform plate boundary in California. As Europe and Africa move away from North and South America at about 1½ inches (4 centimeters) per year, the Atlantic Ocean has opened to a width of 4,000 miles (6,000 kilometers) in the past 150 million years!

Geoscience Concepts—Plate Tectonics

Figures used, related links.

• NPS—Geoscience Concepts
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Site Index & Credits

Plate tectonics and our national parks—site index.

Plate Tectonics and Our National Parks

• Plate Tectonics—The Unifying Theory of Geology
• Inner Earth Model
• Evidence of Plate Motions
• Tectonic Settings of NPS Sites—Master List

Divergent Plate Boundaries

• Continental Rifts
• Passive Continental Margins

Convergent Plate Boundaries

• Subduction Zones
• Accreted Terranes
• Collisional Mountain Ranges

Transform Plate Boundaries

Oceanic Hotspots

Continental Hotspots

Plate Tectonics and Our National Parks (2020)

Text and Illustrations by Robert J. Lillie, Emeritus Professor of Geosciences, Oregon State University [ E-mail ]

Produced under a Cooperative Agreement for earth science education between the National Park Service's Geologic Resources Division and the American Geosciences Institute .

Last updated: February 11, 2020

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Plate tectonics is the grand, unifying theory of Earth sciences, combining the concepts of continental drift and sea-floor spreading into one holistic theory that explains many of the major structural features of the Earth's surface. It explains why the oceanic lithosphere is never older than about 180 Ma and why only the continents have preserved the Earth's geological record for the past 4000 Ma. It provides the framework to explain the distribution of earthquakes and volcanoes and a mechanism for the slow drift of the continents across the Earth's surface. The theory has now reached such a level of scientific acceptance that the movement of plates, both relative to one another and to the hot-spot reference frame, are being used to infer movement of the hot-spot reference frame with respect to the Earth's rotational axis.

Plate tectonics is an expression of the convective regime in the underlying mantle, but the link between individual convection cells and plate boundaries is not direct because plate boundaries are not fixed and, like the plates, move relative to one another. Plate movements are driven by gravity, largely by cold, dense lithospheric slabs pulling younger lithosphere towards a destructive boundary. A less-powerful driving force is generated by the potential energy of spreading centres, elevated some 2-3 km above the general level of the abyssal plains.

As ideas concerning plate tectonics have evolved since the 1970s, it has become apparent that while the theory can be applied rigorously to the oceans, the same cannot be said of the continents. Because of the strength and rigidity of oceanic plates, deformation is focused into narrow linear zones along plate margins. By contrast, when continental lithosphere approaches a plate boundary, deformation can extend hundreds of kilometres into the continental interior because continental plates are less strong. Such deformation gives rise to the major mountain belts of the Earth, as exemplified by the Alpine Himalayan Chain.

Plate Tectonics and the Ring of Fire

The Ring of Fire is a string of volcanoes and sites of seismic activity, or earthquakes, around the edges of the Pacific Ocean.

Earth Science, Geology, Geography, Physical Geography

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• National Geographic MapMaker: Plate Tectonics

The  Ring of Fire  is a string of  volcanoes and sites of  seismic  activity, or earthquakes , around the edges of the Pacific Ocean. Roughly 90 percent of all earthquakes occur along the Ring of Fire, and the ring is dotted with 75 percent of all  active volcanoes on Earth.

The Ring of Fire isn’t quite a circular ring. It is shaped more like a 40,000-kilometer (25,000-mile)  horseshoe . A string of 452 volcanoes stretches from the southern tip of South America, up along the  coast  of North America, across the  Bering Strait , down through Japan, and into New Zealand. Several active and  dormant volcanoes in Antarctica, however, “close” the ring.

Plate Boundaries

The Ring of Fire is the result of  plate tectonics .  Tectonic plates are huge slabs of Earth’s  crust , which fit together like pieces of a puzzle. The plates are not fixed but are constantly moving atop a layer of solid and  molten   rock  called the  mantle . Sometimes these plates  collide , move apart, or slide next to each other. Most tectonic activity in the Ring of Fire occurs in these  geologically active zones.

Convergent Boundaries

A  convergent plate boundary  is formed by tectonic plates crashing into each other. Convergent boundaries are often  subduction zones , where the heavier plate slips under the lighter plate, creating a deep  trench . This subduction changes the  dense  mantle material into  buoyant   magma , which rises through the crust to Earth’s surface. Over millions of years, the rising magma creates a series of active volcanoes known as a  volcanic arc .

If you were to drain the water out of the Pacific Ocean, you would see a series of deep ocean trenches that run  parallel  to  corresponding volcanic arcs along the Ring of Fire . These arcs create both  islands and  continental   mountain ranges .

The Aleutian Islands in the U.S. state of Alaska, for example, run parallel to the Aleutian Trench. Both geographic features continue to form as the Pacific Plate subducts beneath the North American Plate. The Aleutian Trench reaches a maximum depth of 7,679 meters (25,194 feet). The Aleutian Islands have 27 of the United States’ 65 historically active volcanoes.

The  Andes Mountains  of South America run parallel to the Peru-Chile Trench, created as the Nazca Plate subducts beneath the South American Plate. The Andes Mountains include the world’s highest active volcano, Nevados Ojos del Salado, which rises to 6,879 meters (over 22,500 feet) along the Chile-Argentina border. Many volcanoes in Antarctica are so geologically linked to the South American part of the Ring of Fire that some geologists refer to the  region  as the “Antarctandes.”

Divergent Boundaries

A  divergent boundary  is formed by tectonic plates pulling apart from each other. Divergent boundaries are the site of  seafloor spreading  and  rift valleys . Seafloor spreading is the process of magma welling up in the rift as the old crust pulls itself in opposite directions. Cold seawater cools the magma , creating new crust . The upward movement and eventual cooling of this magma has created high ridges on the ocean floor over millions of years.

The  East Pacific Rise  is a site of major seafloor spreading in the Ring of Fire. The East Pacific Rise is located on the divergent boundary of the Pacific Plate and the Cocos Plate (west of Central America), the Nazca Plate (west of South America), and the Antarctic Plate. In addition to volcanic activity, the rise also has a number of hydrothermal vents.

Transform Boundaries

A  transform boundary  is formed as tectonic plates slide  horizontally past each other. Parts of these plates get stuck at the places where they touch.  Stress  builds in those areas as the rest of the plates continue to move. This stress causes the rock to break or slip, suddenly  lurching the plates forward and causing earthquakes . These areas of breakage or slippage are called  faults . The majority of Earth’s faults can be found along transform boundaries in the Ring of Fire .

The San Andreas Fault, stretching along the central west coast of North America, is one of the most active faults on the Ring of Fire. It lies on the transform boundary between the North American Plate, which is moving south, and the Pacific Plate, which is moving north. Measuring about 1,287 kilometers (800 miles) long and 16 kilometers (10 miles) deep, the fault cuts through the western part of the U.S. state of California. Movement along the fault caused the 1906 San Francisco earthquake, which destroyed nearly 500 city blocks. The earthquake and accompanying fires killed roughly 3,000 people and left half of the city’s  residents homeless.

The Ring of Fire is also home to hot spots, areas deep within Earth’s mantle from which heat rises. This heat  facilitates the melting of rock in the  brittle , upper portion of the mantle. The melted rock, known as magma, often pushes through cracks in the crust to form volcanoes.

Hot spots are not generally associated with the interaction or movement of Earth’s tectonic plates . For this reason, many  geologists do not consider hot spot volcanoes part of the Ring of Fire .

Mount Erebus, the most southern active volcano on Earth, sits over the eruptive zone of the Erebus hot spot in Antarctica. This  glacier -covered volcano has a  lava lake  at its  summit and has been consistently  erupting since it was first discovered in 1841.

Active Volcanoes in the Ring of Fire

Most of the active volcanoes on the Ring of Fire are found on its western edge, from the Kamchatka Peninsula in Russia, through the islands of Japan and Southeast Asia, to New Zealand.

Mount Ruapehu in New Zealand is one of the more active volcanoes in the Ring of Fire , with yearly minor eruptions , and major  eruptions occurring about every 50 years. It stands 2,797 meters (9,177 feet) high. Mount Ruapehu is part of the Taupo Volcanic Arc , where the dense Pacific Plate is subducting beneath the Australian Plate.

Krakatau, perhaps better known as  Krakatoa , is an island volcano in Indonesia. Krakatoa erupts less often than Mount Ruapehu, but much more  spectacularly . Beneath Krakatoa , the denser Australian Plate is being subducted beneath the Eurasian Plate. An  infamous   eruption in 1883 destroyed the entire island , sending  volcanic gas ,  volcanic ash , and rocks as high as 80 kilometers (50 miles) in the air. A new island volcano , Anak Krakatau , has been forming with minor eruptions ever since.

Mount Fuji, Japan’s tallest and most famous mountain, is an active volcano in the Ring of Fire . Mount Fuji last erupted in 1707, but recent earthquake activity in eastern Japan may have put the volcano in a “critical state.” Mount Fuji sits at a “ triple junction ,” where three tectonic plates (the Amur Plate, Okhotsk Plate, and Philippine Plate) interact.

The Ring of Fire’s eastern half also has a number of active volcanic areas, including the Aleutian Islands, the Cascade Mountains in the western U.S., the Trans-Mexican Volcanic Belt, and the Andes Mountains.

Mount St. Helens, in the U.S. state of Washington, is an active volcano in the Cascade Mountains. Below Mount St. Helens, the Juan de Fuca plate is being subducted beneath the North American Plate. Mount St. Helens lies on a particularly weak section of crust, which makes it more  prone  to eruptions. Its historic 1980 eruption lasted nine hours and covered nearby areas in tons of volcanic ash.

Popocatépetl is one of the most dangerous volcanoes in the Ring of Fire. The mountain is one of Mexico’s most active volcanoes, with 15 recorded eruptions since 1519. The volcano lies on the Trans-Mexican Volcanic Belt, which is the result of the small Cocos Plate subducting beneath the North American Plate. Located close to the  urban areas of Mexico City and Puebla, Popocatépetl poses a risk to the more than 20 million people that live close enough to be  threatened by a  destructive  eruption.

Cooling Ring The Pacific Plate, which drives much of the tectonic activity in the Ring of Fire, is cooling off. Scientists have discovered that the youngest parts of the Pacific Plate (about two million years old) are cooling off and contracting at a faster rate than older parts of the plate (about 100 million years old). The younger parts of the plate are found in its northern and western parts—the most active parts of the Ring of Fire.

Jolting Japan The island nation of Japan lies along the western edge of the Ring of Fire, and is one of the most tectonically active places on Earth. As much as 10 percent of the world’s volcanic activity takes place in Japan.

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Home — Essay Samples — Science — Plate Tectonics — The Theory of Plate Tectonics and the Three Types of Plate Boundaries

The Theory of Plate Tectonics and The Three Types of Plate Boundaries

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Published: Nov 6, 2018

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Plate Tectonics

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11.2: Earthquakes and Plate Tectonics

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• Page ID 7838

• Steven Earle
• Vancover Island University via BCCampus

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The distribution of earthquakes across the globe is shown in Figure \(\PageIndex{1}\). It is relatively easy to see the relationships between earthquakes and the plate boundaries. Along divergent boundaries like the mid-Atlantic ridge and the East Pacific Rise, earthquakes are common, but restricted to a narrow zone close to the ridge, and consistently at less than a 30 kilometre depth. Shallow earthquakes are also common along transform faults, such as the San Andreas Fault. Along subduction zones, as we saw in Chapter 10, earthquakes are very abundant, and they are increasingly deep on the landward side of the subduction zone.

Earthquakes are also relatively common at a few intraplate locations. Some are related to the buildup of stress due to continental rifting or the transfer of stress from other regions, and some are not well understood. Examples of intraplate earthquake regions include the Great Rift Valley area of Africa, the Tibet region of China, and the Lake Baikal area of Russia.

Earthquakes at Divergent and Transform Boundaries

Figure \(\PageIndex{2}\) provides a closer look at magnitude (M) 4 and larger earthquakes in an area of divergent boundaries in the mid-Atlantic region near the equator. Here, as we saw in Chapter 10, the segments of the mid-Atlantic ridge are offset by some long transform faults. Most of the earthquakes are located along the transform faults, rather than along the spreading segments, although there are clusters of earthquakes at some of the ridge-transform boundaries. Some earthquakes do occur on spreading ridges, but they tend to be small and infrequent because of the relatively high rock temperatures in the areas where spreading is taking place.

Earthquakes at Convergent Boundaries

The distribution and depths of earthquakes in the Caribbean and Central America area are shown in Figure \(\PageIndex{3}\). In this region, the Cocos Plate is subducting beneath the North America and Caribbean Plates (ocean-continent convergence), and the South and North America Plates are subducting beneath the Caribbean Plate (ocean-ocean convergence). In both cases, the earthquakes get deeper with distance from the trench. In Figure \(\PageIndex{3}\), the South America Plate is shown as being subducted beneath the Caribbean Plate in the area north of Colombia, but since there is almost no earthquake activity along this zone, it is questionable whether subduction is actually taking place.

There are also various divergent and transform boundaries in the area shown in Figure \(\PageIndex{3}\), and as we’ve seen in the mid-Atlantic area, most of these earthquakes occur along the transform faults.

The distribution of earthquakes with depth in the Kuril Islands of Russia in the northwest Pacific is shown in Figure \(\PageIndex{4}\). This is an ocean-ocean convergent boundary. The small red and yellow dots show background seismicity over a number of years, while the larger white dots are individual shocks associated with a M6.9 earthquake in April 2009. The relatively large earthquake took place on the upper part of the plate boundary between 60 kilometers and 140 kilometers inland from the trench. As we saw for the Cascadia subduction zone, this is where large subduction earthquakes are expected to occur.

In fact, all of the very large earthquakes — M9 or higher — take place at subduction boundaries because there is the potential for a greater width of rupture zone on a gently dipping boundary than on a steep transform boundary. The largest earthquakes on transform boundaries are in the order of M8.

The background seismicity at this convergent boundary, and on other similar ones, is predominantly near the upper side of the subducting plate. The frequency of earthquakes is greatest near the surface and especially around the area where large subduction quakes happen, but it extends to at least a 400 kilometre depth. There is also significant seismic activity in the overriding North America Plate, again most commonly near the region of large quakes, but also extending for a few hundred kilometers away from the plate boundary.

The distribution of earthquakes in the area of the India-Eurasia plate boundary is shown in Figure \(\PageIndex{5}\). This is a continent-continent convergent boundary, and it is generally assumed that although the India Plate continues to move north toward the Asia Plate, there is no actual subduction taking place. There are transform faults on either side of the India Plate in this area.

The entire northern India and southern Asia region is very seismically active. Earthquakes are common in northern India, Nepal, Bhutan, Bangladesh and adjacent parts of China, and throughout Pakistan and Afghanistan. Many of the earthquakes are related to the transform faults on either side of the India Plate, and most of the others are related to the significant tectonic squeezing caused by the continued convergence of the India and Asia Plates. That squeezing has caused the Asia Plate to be thrust over top of the India Plate, building the Himalayas and the Tibet Plateau to enormous heights. Most of the earthquakes of Figure \(\PageIndex{5}\) are related to the thrust faults shown in Figure \(\PageIndex{6}\) (and to hundreds of other similar ones that cannot be shown at this scale). The southernmost thrust fault in Figure \(\PageIndex{6}\) is equivalent to the Main Boundary Fault in Figure \(\PageIndex{5}\).

There is a very significant concentration of both shallow and deep (greater than 70 kilometers) earthquakes in the northwestern part of Figure \(\PageIndex{5}\). This is northern Afghanistan, and at depths of more than 70 kilometers, many of these earthquakes are within the mantle as opposed to the crust. It is interpreted that these deep earthquakes are caused by northwestward subduction of part of the India Plate beneath the Asia Plate in this area.

This map shows the incidence and magnitude of earthquakes in British Columbia over a one-month period in March and April 2015.

• What is the likely origin of the earthquakes between the Juan de Fuca (JDF) and Explorer Plates?
• The string of small earthquakes adjacent to Haida Gwaii (H.G.) coincides closely with the rupture surface of the 2012 M7.8 earthquake in that area. How might these earthquakes be related to that one?
• Most of the earthquakes around Vancouver Island (V.I.) are relatively shallow. What is their likely origin?
• Some of the earthquakes in B.C. are interpreted as being caused by natural gas extraction (including fracking). Which of the earthquakes here could fall into this category?

See Appendix 3 for Exercise 11.1 answers .

Image descriptions

Figure \(\PageIndex{7}\) image description: The incidence and magnitude of earthquakes in British Columbia over a one-month period in March and April 2015: There were a few dozen smaller earthquakes spread out around Vancouver Island and the sunshine coast with a magnitude of 2. Farther west along the Explorer Plate, which is between the North American plate, the Juan de Fuca Plate, and the Pacific Plate, there were quite a few earthquakes with a magnitude of 3 and at least one earthquake with a magnitude of 4. Between the North American Plate and the Pacific Plate off the south-west coast of Haida Gwaii, there was a large cluster of earthquakes with magnitudes of 2. Along the Alaskan panhandle, there was a collection of 2- and 3-magnitude earthquakes. In addition, there were two 3-magnitude earthquakes west of Fort St. John in northern British Columbia and one or two 2-magnitude earthquakes. In total, this map shows one hundred and forty-nine earthquakes with a magnitude less than 2, ninety-seven earthquakes with a magnitude of 2, thirty-nine earthquakes with a magnitude of 3, and two earthquakes with a magnitude of 4 [Return to Figure \(\PageIndex{7}\)]

Media Attributions

• Figure \(\PageIndex{1}\): Global Earthquakes © Dale Sawyer, Rice University. Used with permission.
• Figure \(\PageIndex{2}\): Earthquakes Around the Mid-Atlantic Ridge © Steven Earle after Dale Sawyer, Rice University.
• Figure \(\PageIndex{3}\): Earthquakes Around the Central-American Region © Steven Earle after Dale Sawyer, Rice University.
• Figure \(\PageIndex{4}\): Earthquakes Around the Kuril Islands. © Steven Earle after Gavin Hayes, from data from the USGS [PDF] .
• Figure \(\PageIndex{5}\): Earthquakes Around the India Plate © Steven Earle after Dale Sawyer, Rice University.
• Figure \(\PageIndex{6}\): India-Asia Convergent Boundary © Steven Earle based on D. Vouichard, from a United Nations University document .

H1 Sample Answer | Plate Tectonics - 2018

Keywords in the question are very useful to highlight because they help shape your answer. The keywords in this question are;

• destructive boundaries.

In your answer, you will need to demonstrate that you understand what a destructive boundary is. You should start by defining destructive boundaries. As there are three types, it would be a good idea to divide your answer into three sections accordingly; this will help you keep to the point.

For each type of destructive boundary, describe its characteristics or main features and define the boundary. Give a step-by-step account of what happens at the boundary, say why it is happening and explain the processes involved at each step.

Support what you are saying with relevant evidence and examples. A diagram would be a great way of supporting your explanation.

Student answer

The earth's crust is broken into plates that move around the mantle. The edges of the plates are called boundaries; they mark the point where plates meet. At destructive plate boundaries, two plates collide and land is destroyed. 2 SRPs Different plate boundaries exist as plate boundaries move by convection currents.

This sentence is a vague statement that would require further discussion and explanation in order to qualify for an SRP.

This student mentioned convection currents which are essential to the movement of the plates which is very relevant to this question. This means that you need to make a big deal about them and you should explain what they are and how they cause plates to collide at destructive boundaries as this will get you marks.

At continental-continental destructive plate boundaries, 2 SRPs - type of destructive boundary fold mountains are formed, e.g. the Himalayas, 2 SRPs - location example formed where the Indian and Eurasian plates 2 SRPs - location example slowly collided over millions of years. 2 SRPs Continental crust is 40-60km thick, made of sial-rich rock, and is older than oceanic crust. The Eurasian plate crumpled and rose to form the Tibetan Plateau 2 SRPs - location example and the Himalayas. When 2 continental plates collide, neither sinks; they squeeze together and are compressed, then buckle upwards and the pressure creates shallow earthquakes. For example, in Pakistan in 2005, a shallow earthquake occurred and 89,000 people died. 2 SRPs Fold mountains are usually made from sedimentary rock as they form underwater. Marine fossils are found in the Himalayas because the rock was underwater before it was uplifted by plate movement. 2 SRPs

I always recommend including a diagram and leaving out some points. Diagrams are handy because they require less writing. Less writing means less time spent which you could use elsewhere to develop points further or add surplus points.

Oceanic-continental destructive boundaries   2 SRPs - type of destructive boundary

Oceanic plates are heavier than continental plates so when they collide, the heavier oceanic plate subducts under the lighter continental plate. 2 SRPs As it moves downwards it melts, turns to magma and moves up through the continental crust, causing an explosive volcano to form, e.g. Mount Saint Helens, USA. 2 SRPs Deep earthquakes are also common at these boundaries because the plate movement is not smooth and they can sometimes get stuck. When they suddenly start moving again the pressure is released as an earthquake. 2 SRPs A trench is created at the junction of two colliding plates. This marks the subduction zone, e.g. Peru-Chile Trench. 2 SRPs The Pacific Ring of Fire is an area that contains 452 active volcanoes at the subduction zone at the edge of the Pacific Plate. 75% of the world’s active volcanoes are here, e.g. Mount Pinatubo and Mount Fuji, Japan. 2 SRPs At the point of collision, the continental plate buckles upwards and fold mountains are formed, e.g. the Andes formed due to the collision between the Nazca plate and the South American plate. 2 SRPs 

At a boundary where two oceanic plates collide, the heaviest one subducts beneath the lighter one and a deep ocean trench forms, like the Mariana Trench in the Pacific ocean. This trench marks where the Pacific plate subducts under the Philippine plate. SRPs - surplus Strong earthquakes can occur at trenches to release pressure, e.g. Japan 2011. They lead to Tsunamis as they happen under the sea. SRPs - surplus The subducted plate sinks and melts, causing raising plumes of magma to break through the crust, creating active volcanoes under the sea. SRPs - surplus The material from the volcanoes builds upon the sea floor until it is visible on the surface to form an island arc, e.g. the Philippine Islands. SRPs - surplus An island arc is a curved line of volcanic islands marking a subduction zone; they are parallel to the ocean trench. SRPs - surplus 

While the answer is well laid out, consider using bullet points/smaller paragraphs containing 2-3 pieces of information to break apart the long paragraphs in each section. This will make it easier for you to see at a glance how many SRPs you have included.

In this answer, there is surplus information. This is good because it acts as a safety net. It is clever to include some extra SRPs just in case the marking scheme changed or one of your other points wasn’t valid.

Just remember to watch your timing to make sure you haven't gone over the 12-15 minute timing recommendation for a 30-mark question.

Location examples = 3 x 2 marks = 6 marks (maximum of 3 examples were allowed by marking scheme @ 2 marks each) Type of destructive boundary identified = 2 x 2 marks = 4 marks (maximum of 2 allowed by marking scheme @ 2 marks each) SRPs = 10 x 2 marks = 20 marks (surplus SRPs were given in this answer) 

Total = 30/30 = 100% = H1

Useful links

SRP stands for 'Significant Relevant Point'. Find out more about SRPs

These essays were reviewed by an experienced teacher of Geography who has corrected the Leaving cert Geography exams