High School Earth Science/Theory of Plate Tectonics

Wegener's continental drift hypothesis had a great deal of evidence in its favor but it was largely abandoned because his theory on how the continents moved was disproved. In the meantime, scientists developed explanations to explain the locations of fossils on widely different continents (land bridges) and the similarity of rock sequences across oceans (geosynclines), which were becoming more and more cumbersome. When seafloor spreading came along, scientists recognized that the mechanism to explain drifting continents had been found. Like the scientists did before us, we are now ready to merge the ideas of continental drift and seafloor spreading into a new all-encompassing idea: the theory of plate tectonics.

Lesson Objectives

Describe what a plate is and how scientists can recognize its edges.
Explain how mantle convection moves lithospheric plates.
Describe the three types of plate boundaries and whether they are prone to earthquakes and volcanoes.
Describe how plate tectonics processes lead to changes in Earth's surface features.

Earth's Tectonic Plates

Now you know that seafloor and continents move around on Earth's surface. But what is it that is actually moving? In other words, what is the "plate" in plate tectonics? This question was also answered due to war, in this case the Cold War.

Although seismographs had been around for decades, during the 1950s and especially in the early 1960s, scientists set up seismograph networks to see if enemy nations were testing atomic bombs. Seismographs record seismic waves. Modern seismographs are sensitive enough to detect nuclear explosions.

While watching for enemy atom bomb tests, the seismographs were also recording all of the earthquakes that were taking place around the planet. These seismic records could be used to locate an earthquake's epicenter, the point on Earth's surface directly above the place where the earthquake occurs. Earthquakes are associated with large cracks in the ground, known as faults. Rocks on opposite sides of a fault move in opposite directions.

Earthquakes are not spread evenly around the planet, but are found mostly in certain regions. In the oceans, earthquakes are found along mid-ocean ridges and in and around deep sea trenches. Earthquakes are extremely common all around the Pacific Ocean basin and often occur near volcanoes. The intensity of earthquakes and volcanic eruptions around the Pacific led scientists to name this region the Pacific Ring of Fire (Figure 6.12). Earthquakes are also common in the world's highest mountains, the Himalaya Mountains of Asia, and across the Mediterranean region.

Scientists noticed that the earthquake epicenters were located along the mid-ocean ridges, trenches and large faults that mark the edges of large slabs of Earth's lithosphere (Figure 6.13). They named these large slabs of lithosphere plates. The movements of the plates were then termed plate tectonics. A single plate can be made of all oceanic lithosphere or all continental lithosphere, but nearly all plates are made of a combination of both.

The lithosphere is divided into a dozen major and several minor plates. The plates' edges can be drawn by connecting the dots that are earthquake epicenters. Scientists have named each of the plates and have determined the direction that each is moving (Figure 6.14). Plates move around the Earth's surface at a rate of a few centimeters a year, about the same rate fingernails grow.

How Plates Move

We know that seafloor spreading moves the lithospheric plates around on Earth's surface but what drives seafloor spreading? The answer is in lesson one of this chapter: mantle convection. At this point it would help to think of a convection cell as a rectangle or oval. Each side of the rectangle is a limb of the cell. The convection cell is located in the mantle. The base is deep in the mantle and the top is near the crust. There is a limb of mantle material moving on one side of the rectangle, one limb moving horizontally across the top of the rectangle, one limb moving downward on the other side of the rectangle, and the final limb moving horizontally to where the material begins to move upward again.

Now picture two convection cells side-by-side in the mantle. The rising limbs of material from the two adjacent cells reach the base of the crust at the mid-ocean ridge. Some of the hot magma crystallizes and creates new ocean crust. This seafloor moves off the axis of the mid-ocean ridge in both directions when still newer seafloor erupts. The oceanic plate moves outward due to the eruption of new oceanic crust at the mid-ocean ridge.

Beneath the moving crust is the laterally moving top limb of the mantle convection cells. Each convection cell is moving seafloor away from the ridge in opposite directions. This horizontal mantle flow moves with the crust across the ocean basin and away from the ridge. As the material moves horizontally, the seafloor thickens and both the new crust and the mantle beneath it cool. Where the limbs of the convection cells plunge down into the deeper mantle, oceanic crust is dragged into the mantle as well. This takes place at the deep sea trenches. As the crust dives into the mantle its weight drags along the rest of the plate and pulls it downward. The last limbs of the convection cells flow along the core. The material is heated and so is ready to rise again when it reaches the rising limb of the convection cell. As you can see, each convection cell is found beneath a different lithospheric plate and is responsible for the movement of that plate.

Plate Boundaries

Back at the planet's surface, the edges where two plates meet are known as plate boundaries. Most geologic activity, including volcanoes, earthquakes, and mountain building, takes place at plate boundaries where two enormous pieces of solid lithosphere interact.

Think about two cars moving around a parking lot. In what three ways can those cars move relative to each other? They can move away from each other, they can move toward each other, or they can slide past each other. These three types of relative motion also define the three types of plate boundaries:

Divergent plate boundaries: the two plates move away from each other.
Convergent plate boundaries: the two plates move towards each other.
Transform plate boundaries: the two plates slip past each other.

What happens at plate boundaries depends on which direction the two plates are moving relative to each other. It also depends on whether the lithosphere on the two sides of the plate boundary is oceanic crust, continental crust, or one piece of each type. The type of plate boundary and the type of crust found on each side of the boundary determines what sort of geologic activity will be found there: earthquakes, volcanoes, or mountain building.

Divergent Plate Boundaries

Plates move apart, or diverge, at mid-ocean ridges where seafloor spreading forms new oceanic lithosphere. At these mid-ocean ridges, lava rises, erupts, and cools. Magma cools more slowly beneath the lava mostly forming the igneous intrusive rock gabbro. The entire ridge system, then, is igneous. Earthquakes are also common at mid-ocean ridges since the movement of magma and oceanic crust result in crustal shaking. Although the vast majority of mid-ocean ridges are located deep below the sea, we can see where the Mid-Atlantic Ridge surfaces at the volcanic island of Iceland (Figure 6.15).

**Figure 6.16**: The Arabian, Indian, and African plates are rifting apart, forming the Great Rift Valley in Africa. The Red Sea fills the rift with seawater.

Although it is uncommon, a divergent plate boundary can also occur within a continent. This is called continental rifting (Figure 6.16). Magma rises beneath the continent, causing it to thin, break, and ultimately split up. As the continental crust breaks apart, oceanic crust erupts in the void. This is how the Atlantic Ocean formed when Pangaea broke up. The East African Rift is currently splitting eastern Africa away from the African continent.

Convergent Plate Boundaries

What happens when two plates converge depends on the types of crust that are colliding. Convergence can take place between two slabs of continental lithosphere, two slabs of oceanic lithosphere, or between one continental and one oceanic slab. Most often, when two plates collide, one or both are destroyed.

When oceanic crust converges with continental crust, the denser oceanic plate plunges beneath the continental plate. This process occurs at the oceanic trenches and is called subduction (Figure 6.17). The entire region is known as a subduction zone. Subduction zones have a lot of intense earthquakes and volcanic eruptions. The subducting plate causes melting in the mantle. The magma rises and erupts, creating volcanoes. These volcanoes are found in a line above the subducting plate. The volcanoes are known as a continental arc. The movement of crust and magma causes earthquakes. The Andes Mountains, which line the western edge of South America, are a continental arc. The volcanoes are the result of the Nazca plate subducting beneath the South American plate (Figure 6.18).

The volcanoes of northeastern California—Lassen Peak, Mount Shasta, and Medicine Lake volcano—along with the rest of the Cascade Mountains of the Pacific Northwest, are the result of subduction of the Juan de Fuca plate beneath the North American plate (Figure 6.19). Mount St. Helens, which erupted explosively on May 18, 1980, is the most famous and currently the most active of the Cascades volcanoes.

Sometimes the magma does not rise all the way through the continental crust beneath a volcanic arc. This usually happens if the magma is rich in silica. These viscous magmas form large areas of intrusive igneous rock, called batholiths, which may someday be uplifted to form a mountain range. The Sierra Nevada batholith cooled beneath a volcanic arc roughly 200 million years ago (Figure 6.20). Similar batholiths are likely forming beneath the Andes and Cascades today.

**Figure 6.21**: A convergent plate boundary subduction zone between two plates of oceanic lithosphere. Melting of the subducting plate causes volcanic activity and earthquakes.

When two oceanic plates converge, the older, denser plate will sink beneath the other plate and plunge into the mantle. As the plate is pushed deeper into the mantle, it melts, which forms magma. As the magma rises it forms volcanoes in a line known as an island arc, which is a line of volcanic islands (Figure 6.21).

The Japanese, Indonesian, and Philippine islands are examples of island arc volcanoes. The volcanic islands are set off from the mainland in an arc shape as seen in this satellite image of Japan (Figure 6.22).

**Figure 6.23**: When two plates of continental crust collide, the material pushes upward forming a high mountain range. The remnants of subducted oceanic crust remain beneath the continental convergence zone.

When two continental plates collide, they are too thick to subduct. Just like if you put your hands on two sides of a sheet of paper and bring your hands together, the material has nowhere to go but up (Figure 6.23)! Some of the world's largest mountains ranges are created at continent-continent convergent plate boundaries. In these locations, the crust is too thick for magma to penetrate so there are no volcanoes, but there may be magma. Metamorphic rocks are common due to the stress the continental crust experiences. As you might think, with enormous slabs of crust smashing together, continent-continent collisions bring on numerous earthquakes.

The world's highest mountains, the Himalayas, are being created by a collision between the Indian and Eurasian plates (Figure 6.24). The Appalachian Mountains are the remnants of a large mountain range that was created when North America rammed into Eurasia about 250 million years ago.

**Figure 6.24**: The Himalaya Mountains are the result of the collision of the Indian Plate with the Eurasian Plate, seen in this photo from the International Space Station. The high peak in the center is world's tallest mountain, Mount Everest (8,848 meters; 29,035 feet).

Transform Plate Boundaries

**Figure 6.25**: At the San Andreas Fault in California, the Pacific Plate is sliding northeast relative to the North American plate, which is moving southwest. At the northern end of the picture, the transform boundary turns into a subduction zone.

Transform plate boundaries are seen as transform faults. At these earthquake faults, two plates move past each other in opposite directions. Where transform faults bisect continents, there are massive earthquakes. The world's most notorious transform fault is the 1,300 kilometer (800 mile) long San Andreas Fault in California (Figure 6.25). This is where the Pacific and North American plates grind past each other, sometimes with disastrous consequences.

California is very geologically active. A transform plate boundary creates the San Andreas Fault. A convergent plate boundary between an oceanic plate and a continental plate creates the Cascades volcanoes. Just offshore, the Juan de Fuca ridge is subducting beneath the North American plate at a divergent plate boundary.

Earth's Changing Surface

Geologists now know that Wegener was right when he said that the continents had once been joined into the supercontinent Pangaea and are now moving apart. Most of the geologic activity that we see on the planet today is due to the interactions of the moving plates. Where plates come apart at a divergent boundary, there is volcanic activity and small earthquakes. If the plates meet at a convergent boundary, and at least one is oceanic, there is a chain of volcanoes and many earthquakes. If both plates at a convergent boundary are continental, mountain ranges grow. If the plates meet at a transform boundary, there is a transform fault. These faults do not have volcanic activity but they have massive earthquakes.

If you look at a map showing the locations of volcanoes and earthquakes in North America, you will see that the plate boundaries are now along the western edge. This geologically active area makes up part of the Pacific Ring of Fire. California, with its volcanoes and earthquakes, is an important part of this region. The eastern edge of North America is currently mostly quiet, although mountain ranges line the area. If there is no plate boundary there today, where did those mountains come from?

Remember that Wegener used the similarity of the mountains in eastern North America, on the west side of the Atlantic, and the mountains in Great Britain, on the eastern side of the Atlantic, as evidence for his continental drift hypothesis. These mountains were formed at a convergent plate boundary as the continents that made up Pangaea came together. So about 200 million years ago these mountains were similar to the Himalaya today (Figure 6.26)!

Before the continents collided they were separated by an ocean, just as the continents rimming the Pacific are now. That ocean crust had to subduct beneath the continents just as the oceanic crust around the Pacific is being subducted today. Subduction along the eastern margin of North America produced continental arc volcanoes. Ancient lava from those volcanoes can be found in the region.

Currently, Earth's most geologically active area is around the Pacific. The Pacific is shrinking at the same time the Atlantic is growing. But hundreds of millions of years ago, that was reversed: the Atlantic was shrinking as the Pacific was growing. What we’ve just identified is a cycle, known as the supercontinent cycle, which is responsible for most of the geologic features that we see and many more that are long gone. Scientists think that the creation and breakup of a supercontinent takes place about every 500 million years.

Intraplate Activity

While it is true that most geological activity takes place along plate boundaries, some is found away from the edges of plates. This is known as intraplate activity. The most common intraplate volcanoes are above hotspots that lie beneath oceanic plates. Hotspot volcanoes arise because plumes of hot material that come from deep in the mantle rise through the overlying mantle and crust. When the magma reaches the plate above, it erupts, forming a volcano. Since the hotspot is stable, when the oceanic plate moves over it, and it erupts again, another volcano is created in line with the first. With time, there is a line of volcanoes; the youngest is directly above the hot spot and the oldest is furthest away. Recent research suggests that hotspots are not as stable as scientists once thought, but some larger ones still appear to be.

The Hawaiian Islands are a beautiful example of a chain of hotspot volcanoes. Kilauea volcano on the south side of the Big Island of Hawaii lies above the Hawaiian hot spot. The Big Island is on the southeastern end of the Hawaiian chain. Mauna Loa volcano, to the northwest, is older than Kilauea and is still erupting, but at a lower rate. Hawaii is the youngest island in the chain. As you follow the chain to the west, the islands get progressively older because they are further from the hotspot (Figure 6.27).

**Figure 6.27**: This view of the Hawaiian islands shows that the youngest islands are in the southeast and the oldest in the northwest. Kilauea volcano, which makes up the southeastern side of the Big Island of Hawaiian, is located above the Hawaiian hotspot.

The chain continues into the Emperor Seamounts, which are so old they no longer reach above sea level. The oldest of the Emperor seamounts is about to subduct into the Aleutian trench off of Alaska; no one knows how many older volcanoes have already subducted. It's obvious from looking at the Emperor seamounts that the Pacific plate took a large turn. Radiometric dating has shown that turn to have taken place about 43 million years ago (Figure 6.28). The Hawaii hotspot may also have been moving southward during this time. Still, geologists can use some hotspot chains to tell not only the direction but the speed a plate is moving.

Hot spots are also found under the continental crust, although it is more difficult for the magma to make it through the thick crust and there are few eruptions. One exception is Yellowstone, which creates the activity at the Yellowstone hotspot. In the past, the hotspot produced enormous volcanic eruptions, but now its activity is best seen in the region's famous geysers.

Lesson Summary

Driven by mantle convection, the plates of lithosphere move around Earth’s surface. New oceanic crust forms at the ridge and pushes the older seafloor away from the ridge horizontally.
Plates interact at three different types of plate boundaries, divergent, convergent and transform fault boundaries, which are where most of the Earth’s geologic activity takes place.
These processes acting over long periods of time are responsible for the geographic features we see.

Review Questions

What are the three types of plate boundaries? For each type, what sort of geologic activity do you find?
As a working geologist, you come across a landscape with a massive fault zone that produces lots of large earthquakes, but has no volcanoes. What type of plate boundary have you come across? What are the movements of plates relative to each other at this type of boundary? Where would you find a plate boundary of this type in California?
You continue on your geologic tour to a location where there is a chain of volcanoes on land, but not too far inland from the edge of the continent. The region experiences frequent large earthquakes. What type of plate boundary have you come across? What types of plates are involved? Where would you find a plate boundary of this type in California?
What is the driving force behind the movement of lithospheric plates on the Earth's surface? About how fast do the plates move?
How does the theory of plate tectonics explain the locations of volcanoes, earthquakes and mountain belts on Earth?
Thinking about the different types of plate boundaries, explain why continental crust is much thicker than oceanic crust.
Why are there few (if any) volcanoes along transform plate boundaries?

Vocabulary

batholith: An enormous body of granitic rock that is formed from a large number of plutons.
continental arc: A line of volcanoes sitting on a continental plate and aligned above a subducting oceanic plate near a deep sea trench.
continental rifting: A divergent plate boundary that forms in the middle of a continent.
convergent plate boundary: A location where two lithospheric plates come together.
divergent plate boundary: A location where two lithospheric plates spread apart.
epicenter: The point on the Earth's surface directly above an earthquake's focus, which is the place where the ground breaks.
fault: A fracture along which there has been movement of rock on one or both sides.
intraplate activity: Geologic activity such as volcanic eruptions and earthquakes that takes place away from plate boundaries.
island arc: A line of volcanoes sitting on an oceanic plate above a subducting oceanic plate near a deep sea trench.
plate: A slab of the earth's lithosphere that can move around on the planet's surface.
plate boundary: A location where two plates come together.
plate tectonics: The theory that the Earth's surface is divided into lithospheric plates that move on the planet's surface. The driving force behind plate tectonics is mantle convection.
pluton: A relatively small body of igneous intrusive rock.
subduction: The sinking of one lithospheric plate beneath another.
subduction zone: The area where two lithospheric plates come together and one sinks beneath the other.
supercontinent cycle: The cycle in which the continents join into one supercontinent on one side of the planet and then break apart.
transform fault: An earthquake fault where relative motion is sliding past.
transform plate boundary: The type of plate boundary where two plates slide past one another.

Points to Consider

On the map in Figure 6.14 above, the arrows show the directions that the plates are going. The Atlantic has a mid-ocean ridge, where seafloor spreading is taking place. The Pacific ocean has many deep sea trenches, where subduction is taking place. What is the future of the Atlantic plate? What is the future of the Pacific plate?
Using your hands and words, explain to someone how plate tectonics works. Be sure you describe how continents drift and how seafloor spreading provides a mechanism for continental movement.
Now that you know about plate tectonics, where do you think would be a safe place to live if you wanted to avoid volcanic eruptions and earthquakes?

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