Causes of Tectonic Plate Movement

  • 0:05 Background on Plate Movement
  • 2:23 Thermal Convection
  • 4:09 Ridge Push
  • 4:40 Slab Pull
  • 5:21 Trench Suction
  • 6:04 Lesson Summary
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Lesson Transcript
Instructor: Peter Jaeger

Pete currently teaches middle school Science, college level introductory Science, and has a master's degree in Environmental Education.

In this lesson, we explore the causes of plate movement, including thermal convection, ridge push and slab pull. Students will learn how these processes complement each other and form a theory for tectonic plate movement.

Background on Plate Movement

Do you ever wonder if the crazy thing someone just told you is true? For instance, you may have heard that person declare that at some point in the near future, California is going to break off and fall into the ocean. My older brother told me that the Golden State is hanging off the end of the continent and will fall into the Pacific Ocean during the next big earthquake. Do the tectonic plates, or giant pieces of the Earth's crust that fit together and move around on the Earth's surface, really move? If so, what causes them to move? The best stories are often based on fact, so it is helpful to know what is real versus what is not. Yes, tectonic plates move, and the mechanism that causes them to move is the subject of this lesson.

The part of my brother's story that is true is that the Western part of California, west of the San Andreas Fault, is moving in a different direction than the rest of the continent. The Western part of California is moving northwest at the rate of several centimeters per year. Looking to the future, California will indeed one day separate from the rest of North America and become an island. Where I think my brother got confused is the mechanism that will cause this to occur.

Tectonic plates move around and can cause earthquakes and volcanic eruptions.
Tectonic Plates Image

First of all, it is important to know that the Earth's crust is broken up into large pieces called tectonic plates. Remember, tectonic plates are giant pieces of the Earth's crust that fit together and move around on the Earth's surface. This movement can be observed and measured using GPS systems, and the edges of the plates can be detected, as their edges can be seen. We also know that these plates move around with respect to one another. Earthquakes and volcanoes are the results of such plate movement. One question geologists have been trying to answer is: what is causing the plates of the Earth to move? This can be a challenging question because we are not able to get into the Earth's interior to observe it. There are several mechanisms that scientists have developed based on the observations of the plates and a deeper understanding of the inner layers of the Earth. These mechanisms operate at different points in the Earth and very well could complement each other, each assisting in moving the plate in its own fashion.

Thermal Convection

Scientists believe that one of the primary forces behind plate movement is thermal convection. Thermal convection is when heat from the core of the Earth is transferred to the surface of the Earth by the mantle. The mantle is the thick, mostly solid layer of the Earth between the crust and the core. Thermal convection works a lot like a pot of boiling water, which can be seen in this animation.

In convection, heat from the stove warms up the water closest to the stove, causing the water to expand and rise. Cooler water near the surface of the pot sinks to take the place of the rising water. In doing so, a current of water is set up flowing toward the surface and back down again. Using this model, the stove is like the core and the water is the liquid mantle that rotates. The plates on the Earth's surface would be floating on top of the water. These currents push the plates along according to the direction of flow. Geologists think that this same phenomenon is what is happening inside the Earth. Liquid rock near the mantle is heated and rises toward the crust. The rock near the surface is cooler and sinks back down toward the core. This forms the same type of convection current that causes the plates to move. Scientists believe that this cycle of magma rising from the core to the crust and back again takes thousands of years to complete.

Convection currents in the liquid mantle cause the plates to move.
Thermal Convection Current

At the top of the mantle, the rock encounters the thin crust, and, as it pushes it aside, lava flows out from the mantle to form new oceanic crust. As this happens, the plates smash into each other, slide past each other or are pushed under another plate. This movement of the plate along with the upwelling of the mantle by the convection currents may also cause secondary actions that assist in plate movement.

Ridge Push

In ridge push, the mantle wells upward because of the convection and elevates the edges of spreading oceanic plates. Because these plates are higher at the spreading center, they are forced downhill due to gravity and eventually flatten out to the ocean floor. The gravity causes this movement down the ridge, and it gives the plate a push along as new crust forms behind the plate at the spreading center. This is like those coin bulldozers at the fair where one new coin may push forward and knock others out of the way!

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