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.
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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!
Slab Pull
Another mechanism that forms as a result of plate movement is slab pull. As the plate is pushed along, it may run into another plate. Oceanic crust is easily forced under another plate and back into the mantle. These converging boundaries can be identified by deep ocean trenches that mark the location where one plate is sliding under another one. As it slides under the other plate and is forced back into the mantle, gravity again works to pull it along, giving the plate another force to keep it moving along. These two mechanisms are different. In slab pull, gravity is pulling on the front part of the plate; in ridge push, it is forcing movement from the back end.
Trench Suction
A diagram of the trench suction process
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A third force that forms as a result of plate movement is known as trench suction. As a plate gets pushed back into the mantle (as we saw in slab pull), it is forced down at an angle. Not only does gravity pull it down in the slab pull model, but another force can act on it as well. Underneath and behind the plate, a small convection current caused by the diving crust can form and help pull the plate into the Earth. It is very similar to the force that keeps a raft trapped in suction under a waterfall. This force aids in pulling the plate back to the Earth's interior to re-melt. This also differs from ridge push and slab pull because it is not the result of gravitational pull.
Lesson Summary
So, to recap, there are several mechanisms that scientists use to help explain the movement of the giant pieces of the Earth's crust (called tectonic plates). The force that causes most of the plate movement is thermal convection, where heat from the Earth's interior causes currents of hot rising magma and cooler sinking magma to flow, moving the plates of the crust along with them.
Additional mechanisms that may aid in plates moving involve ridge push, slab pull and trench suction. In ridge push and slab pull, gravity is acting on the plate to cause the movement. Areas where the crust is pushed upward by magma welling up are also pulled downward by gravity, forcing edges of plates back into the Earth's interior. These processes pull the whole plate along. In trench suction, a small convection current is formed by the re-melting plate as it pushes back into the interior, and the current creates a force that pulls the plate downward.
Learning Outcome
After completing this lesson, you should be able to describe thermal convection, ridge push, slab pull and trench suction.
