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 explain how continents could move around Earth’s surface. Like the scientists before us, we will now merge the ideas of continental 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.

Earth’s Tectonic Plates

Seafloor and continents move around on Earth’s surface, but what is actually moving? What portion of the Earth makes up the “plates” in plate tectonics? This question was also answered because of technology developed during war 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 atomic bombs. These seismographs also recorded all of the earthquakes around the planet. The 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.

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 below).

The lithosphere is divided into a dozen major and several minor plates (Figure below). The plates’ edges can be drawn by connecting the dots that mark earthquakes’ epicenters. 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.

Movement of the plates over Earth’s surface is termed plate tectonics. Plates move at a rate of a few centimeters a year, 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 in the mantle, similar to the illustration in Figure below.

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.

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.

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.

Divergent Plate Boundaries

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

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 below), magma rises beneath the continent, causing it to become thinner, break, and ultimately split apart. New ocean crust erupts 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 beneath the continental plate. This process, called subduction, occurs at the oceanic trenches (Figure below). 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 coastal volcanic mountains are found in a line above the subducting plate (Figure below). The volcanoes are known as a continental arc.

The movement of crust and magma causes earthquakes. A map of earthquake epicenters at subduction zones is found 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 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 below). The Juan de Fuca plate is created by seafloor spreading just offshore at the Juan 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 will cool 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 below).

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

An animation of an ocean continent plate boundary is seen here: http://www.iris.edu/hq/files/programs/education_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 it collides? Since it has nowhere to go but up, this creates some of the world’s largest mountains ranges (Figure below). 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 of crust 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 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 opposite directions. Transform faults on continents bring massive earthquakes (Figure below).

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

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

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

Earth’s Changing Surface

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

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

1. What is the geologic origin of the Cascades Range? The Cascades are a chain of volcanoes in the Pacific Northwest. They are not labelled on the diagram but they lie between the Sierra Nevada and the Coastal Range.
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 evidence for his continental drift hypothesis. The Appalachian mountains formed at a convergent plate boundary as Pangaea came together (Figure below).

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

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

Intraplate Activity

A small amount of geologic activity, known as intraplate activity, does not take place at plate boundaries but within a plate instead. Mantle plumes are pipes of hot rock that rise through the mantle. The release of pressure causes melting near the surface to form a hotspot. Eruptions at the hotspot create a volcano. Hotspot volcanoes are found in a line (Figure below). Can you figure out why? Hint: The youngest volcano sits above the hotspot and volcanoes become 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 below).

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

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 by seafloor 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 below).

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 is this type of boundary found in California?
3. Next you find a chain of volcanoes along a coast on land, not too far inland from the ocean. The region experiences frequent large earthquakes. What type of plate boundary is this? What types of plates are involved? 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 fast do the plates move?
5. How does the theory of plate tectonics explain the locations of volcanoes, earthquakes, and mountain belts on Earth?
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 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 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?