Theory of Plate Tectonics

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Printed: January 29, 2015

www.ck12.org Chapter 1. Theory of Plate Tectonics

CHAPTER 1 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.• List the three types of boundaries. Are they prone to earthquakes or volcanoes?• Describe how plate tectonics processes lead to changes in Earth’s surface features.

Vocabulary

• batholith• continental arc• continental rifting• convergent plate boundary• divergent plate boundary• epicenter• hotspot• intraplate activity• island arc• plate• plate boundary• plate tectonics• subduction• subduction zone• supercontinent cycle• transform fault• transform plate boundary

Introduction

When the concept of seafloor spreading came along, scientists recognized that it was the mechanism to explainhow continents could move around Earth’s surface. Like the scientists before us, we will now merge the ideas ofcontinental drift and seafloor spreading into the theory of plate tectonics.

Continental drift and the mechanism of seafloor spreading create plate tectonics: http://video.yahoo.com/watch/1595682/5390276 .

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Earth’s Tectonic Plates

Seafloor and continents move around on Earth’s surface, but what is actually moving? What portion of the Earthmakes up the “plates” in plate tectonics? This question was also answered because of technology developed duringwar times - in this case, the Cold War. The plates are made up of the lithosphere.

During the 1950s and early 1960s, scientists set up seismograph networks to see if enemy nations were testing atomicbombs. These seismographs also recorded all of the earthquakes around the planet. The seismic records could beused to locate an earthquake’s epicenter, the point on Earth’s surface directly above the place where the earthquakeoccurs.

Earthquake epicenters outline the plates. Mid-ocean ridges, trenches, and large faults mark the edges of the plates,and this is where earthquakes occur ( Figure 1.1).

FIGURE 1.1Earthquakes outline the plates.

The lithosphere is divided into a dozen major and several minor plates ( Figure 1.2). The plates’ edges can be drawnby connecting the dots that mark earthquakes’ epicenters. A single plate can be made of all oceanic lithosphere orall continental lithosphere, but nearly all plates are made of a combination of both.

Movement of the plates over Earth’s surface is termed plate tectonics. Plates move at a rate of a few centimeters ayear, about the same rate fingernails grow.

How Plates Move

If seafloor spreading drives the plates, what drives seafloor spreading? Picture two convection cells side-by-side inthe mantle, similar to the illustration in Figure 1.3.

1. Hot mantle from the two adjacent cells rises at the ridge axis, creating new ocean crust.2. The top limb of the convection cell moves horizontally away from the ridge crest, as does the new seafloor.3. The outer limbs of the convection cells plunge down into the deeper mantle, dragging oceanic crust as well.

This takes place at the deep sea trenches.4. The material sinks to the core and moves horizontally.5. The material heats up and reaches the zone where it rises again.

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FIGURE 1.2The lithospheric plates and their names.The arrows show whether the plates aremoving apart, moving together, or slidingpast each other.

FIGURE 1.3Mantle convection drives plate tectonics.Hot material rises at mid-ocean ridgesand sinks at deep sea trenches, whichkeeps the plates moving along the Earth’ssurface.

Mantle convection is shown in these animations:

• http://www.youtube.com/watch?v=p0dWF_3PYh4• http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_convection2.html

Plate Boundaries

Plate boundaries are the edges where two plates meet. Most geologic activities, including volcanoes, earthquakes,and mountain building, take place at plate boundaries. How can two plates move relative to each other?

• 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.

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The type of plate boundary and the type of crust found on each side of the boundary determines what sort of geologicactivity will be found there.

Divergent Plate Boundaries

Plates move apart at mid-ocean ridges where new seafloor forms. Between the two plates is a rift valley. Lava flowsat the surface cool rapidly to become basalt, but deeper in the crust, magma cools more slowly to form gabbro. Sothe entire ridge system is made up of igneous rock that is either extrusive or intrusive. Earthquakes are common atmid-ocean ridges since the movement of magma and oceanic crust results in crustal shaking. The vast majority ofmid-ocean ridges are located deep below the sea ( Figure 1.4).

FIGURE 1.4(a) Iceland is the one location where the ridge is located on land: the Mid-Atlantic Ridge separates the NorthAmerican and Eurasian plates; (b) The rift valley in the Mid-Atlantic Ridge on Iceland.

USGS animation of divergent plate boundary at mid-ocean ridge: http://earthquake.usgs.gov/learn/animations/animation.php?flash_title=Divergent+Boundary&flash_file=divergent&flash_width=500&flash_height=200 .

Divergent plate boundary animation: http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/11/AOTM_09_01_Divergent_480.mov .

Can divergent plate boundaries occur within a continent? What is the result? In continental rifting ( Figure 1.5),magma rises beneath the continent, causing it to become thinner, break, and ultimately split apart. New ocean crusterupts in the void, creating an ocean between continents.

Convergent Plate Boundaries

When two plates converge, the result depends on the type of lithosphere the plates are made of. No matter what,smashing two enormous slabs of lithosphere together results in magma generation and earthquakes.

Ocean-continent: When oceanic crust converges with continental crust, the denser oceanic plate plunges beneaththe continental plate. This process, called subduction, occurs at the oceanic trenches ( Figure 1.6). The entire

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FIGURE 1.5The Arabian, Indian, and African plates are rifting apart, forming the GreatRift Valley in Africa. The Dead Sea fills the rift with seawater.

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 coastalvolcanic mountains are found in a line above the subducting plate ( Figure 1.7). The volcanoes are known as acontinental arc.

FIGURE 1.6Subduction of an oceanic plate beneatha continental plate causes earthquakesand forms a line of volcanoes known asa continental arc.

The movement of crust and magma causes earthquakes. A map of earthquake epicenters at subduction zones isfound here: http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_earthquakessubduction.html .

This animation shows the relationship between subduction of the lithosphere and creation of a volcanic arc: http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_subduction.html .

The volcanoes of northeastern California—Lassen Peak, Mount Shasta, and Medicine Lake volcano—along with therest of the Cascade Mountains of the Pacific Northwest are the result of subduction of the Juan de Fuca plate beneaththe North American plate ( Figure 1.8). The Juan de Fuca plate is created by seafloor spreading just offshore at theJuan de Fuca ridge.

If the magma at a continental arc is felsic, it may be too viscous (thick) to rise through the crust. The magma willcool slowly to form granite or granodiorite. These large bodies of intrusive igneous rocks are called batholiths,which may someday be uplifted to form a mountain range ( Figure 1.9).

Ocean-ocean: When two oceanic plates converge, the older, denser plate will subduct into the mantle. An oceantrench marks the location where the plate is pushed down into the mantle. The line of volcanoes that grows on theupper oceanic plate is an island arc. Do you think earthquakes are common in these regions ( Figure 1.10)?

An animation of an ocean continent plate boundary is seen here: http://www.iris.edu/hq/files/programs/education_

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FIGURE 1.7(a) At the trench lining the western mar-gin of South America, the Nazca plate issubducting beneath the South Americanplate, resulting in the Andes Mountains(brown and red uplands); (b) Conver-gence has pushed up limestone in theAndes Mountains where volcanoes arecommon.

FIGURE 1.8The Cascade Mountains of the PacificNorthwest are a continental arc.

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FIGURE 1.9The Sierra Nevada batholith cooled be-neath a volcanic arc roughly 200 millionyears ago. The rock is well exposedhere at Mount Whitney. Similar batholithsare likely forming beneath the Andes andCascades today.

FIGURE 1.10(a) Subduction of an ocean plate beneath an ocean plate results in a volcanic island arc, an ocean trench andmany earthquakes. (b) Japan is an arc-shaped island arc composed of volcanoes off the Asian mainland, asseen in this satellite image.

and_outreach/aotm/11/AOTM_09_01_Convergent_480.mov .

Continent-continent: Continental plates are too buoyant to subduct. What happens to continental material when itcollides? Since it has nowhere to go but up, this creates some of the world’s largest mountains ranges ( Figure1.11). Magma cannot penetrate this thick crust so there are no volcanoes, although the magma stays in the crust.Metamorphic rocks are common because of the stress the continental crust experiences. With enormous slabs ofcrust smashing together, continent-continent collisions bring on numerous and large earthquakes.

A short animation of the Indian Plate colliding with the Eurasian Plate: http://www.scotese.com/indianim.htm .

An animation of the Himalaya rising: http://www.youtube.com/watch?v=ep2_axAA9Mw .

The Appalachian Mountains are the remnants of a large mountain range that was created when North America

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FIGURE 1.11(a) In continent-continent convergence, the plates push upward to create a high mountain range. (b) The world’shighest mountains, the Himalayas, are the result of the collision of the Indian Plate with the Eurasian Plate, seenin this photo from the International Space Station.

rammed into Eurasia about 250 million years ago.

Transform Plate Boundaries

Transform plate boundaries are seen as transform faults, where two plates move past each other in oppositedirections. Transform faults on continents bring massive earthquakes ( Figure 1.12).

FIGURE 1.12At the San Andreas Fault in California, the Pacific Plate is sliding northeastrelative to the North American plate, which is moving southwest. At thenorthern end of the picture, the transform boundary turns into a subductionzone.

California is very geologically active. What are the three major plate boundaries in or near California ( Figure 1.13)?

1. A transform plate boundary between the Pacific and North American plates creates the San Andreas Fault, theworld’s most notorious transform fault.

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2. Just offshore, a divergent plate boundary, Juan de Fuca ridge, creates the Juan de Fuca plate.3. A convergent plate boundary between the Juan de Fuca oceanic plate and the North American continental

plate creates the Cascades volcanoes.

FIGURE 1.13This map shows the three major plateboundaries in or near California.

A brief review of the three types of plate boundaries and the structures that are found there is the subject of thiswordless video (3b): http://www.youtube.com/watch?v=ifke1GsjNN0 (4:50).

MEDIAClick image to the left or use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/8524

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Earth’s Changing Surface

Geologists know that Wegener was right because the movements of continents explain so much about the geologywe see. Most of the geologic activity that we see on the planet today is because of the interactions of the movingplates.

In the map of North America ( Figure 1.14), where are the mountain ranges located? Using what you have learnedabout plate tectonics, try to answer the following questions:

FIGURE 1.14Mountain ranges of North America.

1. What is the geologic origin of the Cascades Range? The Cascades are a chain of volcanoes in the PacificNorthwest. They are not labelled on the diagram but they lie between the Sierra Nevada and the CoastalRange.

2. What is the geologic origin of the Sierra Nevada? (Hint: These mountains are made of granitic intrusions.)3. What is the geologic origin of the Appalachian Mountains along the Eastern US?

Remember that Wegener used the similarity of the mountains on the west and east sides of the Atlantic as evidencefor his continental drift hypothesis. The Appalachian mountains formed at a convergent plate boundary as Pangaeacame together ( Figure 1.15).

Before Pangaea came together, the continents were separated by an ocean where the Atlantic is now. The proto-Atlantic ocean shrank as the Pacific ocean grew. Currently, the Pacific is shrinking as the Atlantic is growing. This

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FIGURE 1.15About 200 million years ago, the Ap-palachian Mountains of eastern NorthAmerica were probably once as high asthe Himalaya, but they have been weath-ered and eroded significantly since thebreakup of Pangaea.

supercontinent cycle is responsible for most of the geologic features that we see and many more that are long gone( Figure 1.16).

FIGURE 1.16Scientists think that the creation and breakup of a supercontinent takes place about every 500 million years. Thesupercontinent before Pangaea was Rodinia. A new continent will form as the Pacific ocean disappears.

This animation shows the movement of continents over the past 600 million years beginning with the breakup ofRodinia: http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_plate_reconstruction_blakey.html .

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Intraplate Activity

A small amount of geologic activity, known as intraplate activity, does not take place at plate boundaries but withina plate instead. Mantle plumes are pipes of hot rock that rise through the mantle. The release of pressure causesmelting near the surface to form a hotspot. Eruptions at the hotspot create a volcano. Hotspot volcanoes are foundin a line ( Figure 1.17). Can you figure out why? Hint: The youngest volcano sits above the hotspot and volcanoesbecome older with distance from the hotspot.

An animation of the creation of a hotspot chain is seen here: http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_hawaii.html .

Geologists use some hotspot chains to tell the direction and the speed a plate is moving ( Figure 1.18).

Hotspot magmas rarely penetrate through thick continental crust. One exception is the Yellowstone hotspot ( Figure1.19).

Plate Tectonics Theory

Plate tectonics is the unifying theory of geology. Plate tectonics theory explains why:

• Earth’s geography has changed through time and continues to change today.• some places are prone to earthquakes while others are not.• certain regions may have deadly, mild, or no volcanic eruptions.• mountain ranges are located where they are.

Plate tectonic motions affect Earth’s rock cycle, climate, and the evolution of life.

Lesson Summary

• Plates of lithosphere move because of convection currents in the mantle. One type of motion is produced byseafloor spreading.

• Plate boundaries can be located by outlining earthquake epicenters.• Plates interact at three types of plate boundaries: divergent, convergent and transform.• Most of the Earth’s geologic activity takes place at plate boundaries.• At a divergent boundary, volcanic activity produces a mid ocean ridge and small earthquakes.• At a convergent boundary with at least one oceanic plate, an ocean trench, a chain of volcanoes develops and

many earthquakes occur.• At a convergent boundary where both plates are continental, mountain ranges grow and earthquakes are

common.• At a transform boundary, there is a transform fault and massive earthquakes occur but there are no volcanoes.• Processes acting over long periods of time create Earth’s geographic features.

Review Questions

Use this diagram to review this chapter ( Figure 1.20).

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FIGURE 1.17The Hawaiian Islands are a beautiful example of a hotspot chain. Kilauea volcano lies above the Hawaiian hotspot.Mauna Loa volcano is older than Kilauea and is still erupting, but at a lower rate. The islands get progressivelyolder to the northwest because they are further from the hotspot. Loihi, the youngest volcano, is still below thesea surface.

1. What are the three types of plate boundaries and what type of geologic activity is found at each?2. As a geologist, you come across a landscape with a massive fault zone that produces a lot of large earthquakes

but has no volcanoes. What type of plate boundary is this? What are the movements of plates there? Where isthis type of boundary found in California?

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FIGURE 1.18The Hawaiian chain continues into theEmperor Seamounts. The bend in thechain was caused by a change in the di-rection of the Pacific plate 43 million yearsago. Using the age and distance of thebend, geologists can figure out the speedof the Pacific plate over the hotspot.

FIGURE 1.19Volcanic activity above the Yellowstonehotspot on the North American Plate canbe traced from 15 million years ago to itspresent location.

3. Next you find a chain of volcanoes along a coast on land, not too far inland from the ocean. The regionexperiences frequent large earthquakes. What type of plate boundary is this? What types of plates areinvolved? Where is this type of boundary found in California?

4. What is the driving force behind the movement of lithospheric plates on the Earth’s surface? About how fastdo the plates move?

5. How does the theory of plate tectonics explain the locations of volcanoes, earthquakes, and mountain belts onEarth?

6. What causes earthquakes and at what types of plate boundaries are earthquakes common? Explain.7. Thinking about the different types of plate boundaries, where do mountain ranges that do not include volcanoes

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FIGURE 1.20Plate boundaries

occur and why?8. Why are there no volcanoes along transform plate boundaries? At continent-continent convergent plate

boundaries?

Points to Consider

• On the map in Figure 1.2, 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, wheresubduction 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 howcontinents 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 toavoid volcanic eruptions and earthquakes?

References

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Domain3. CK-12 Foundation. CK-12 Foundation . CC BY-NC 3.04. (a) Courtesy of US Geological Survey; (b) Adam W.. (a) http://pubs.usgs.gov/gip/dynamic/understanding.h

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9. User:Geographer/Wikipedia. http://commons.wikimedia.org/wiki/File:Mount_Whitney_2003-03-25.jpg . CCBY 1.0

10. (a) Courtesy of US Geological Survey; (b) Courtesy of Jeff Schmaltz, MODIS Rapid Response Team, NASA/GSFC.(a) http://commons.wikimedia.org/wiki/File:Oceanic-oceanic_convergence_Fig21oceanocean.gif; (b) http://commons.wikimedia.org/wiki/File:Satellite_image_of_Japan_in_May_2003.jpg . Public Domain

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13. Courtesy of US Geological Survey, modified by CK-12 Foundation. http://pubs.usgs.gov/gip/dynamic/understanding.html . Public Domain

14. Image copyright VanHart, 2014; modified by CK-12 Foundation. http://www.shutterstock.com . Used underlicense from Shutterstock.com

15. Courtesy of National Park Service. http://commons.wikimedia.org/wiki/File:Appalachians_NC_BLRI9242.jpg . Public Domain

16. Jodi So. CK-12 Foundation . CC BY-NC 3.017. Courtesy of Joel E. Robinson, Will R. Stettner, US Geological Survey. http://commons.wikimedia.org/wiki/Fi

le:Hawaii_hotspot_poster.jpg . Public Domain18. Courtesy of National Oceanic and Atmospheric Administration. http://commons.wikimedia.org/wiki/File:P

acific_elevation.jpg . Public Domain19. User:Metrodyne/Wikipedia. http://commons.wikimedia.org/wiki/File:HotspotsSRP.jpg . Public Domain20. Courtesy of José F. Vigil/U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research

Laboratory. The main types of plate boundaries . Public Domain

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