M A S T E R P A G E
The Mechanism of Plate Tectonics
Although the theory of plate tectonics is generally accepted, no one completely understands how the process
works. Wegener thought that centrifugal force, caused by the rotation of the Earth, and tidal forces caused
continental drift. Modern scientists have rejected this theory and replaced it with several others. Three
mechanisms called convection, ridge push, and slab pull may play a role.
Convection Currents
Convection currents are systems of heat exchange that operate in the mantle as it is heated by the core. The
mantle’s plastic-like material moves upward as it is heated, and sinks when it cools. You can see this kind of
movement if you boil water in a clear glass pot. Even though the heat on the stove is constant, the water on the
bottom of the pot is the hottest. As it heats, it becomes less dense and rises, while relatively cooler water from the
top of the pot takes its place on the bottom. This continuous exchange creates convection currents in the water.
According to this theory, convective movement acts as a drag on the underside of the plates. As mantle material
moves in large convection cells, the plates are rafted along the top of the cells, being pulled apart where the cell
rises and colliding where it sinks.
Ridge Push
The ridge-push mechanism derives from the bulging of the undersea ridge crest as the oceanic lithosphere below
it expands. The lithosphere expands because it is heated by mantle material pushing upward from below. As the
lithosphere is pushed up at the ridge, a large portion of a plate may come to rest on an inclined plane. Gravity will
cause this portion to slide down the inclined plane and push on the rest of the plate.
Slab Pull
Once this slab of oceanic lithosphere moves away from the ridge crest (and from the heat that is rising at the
ridge),
it will cool and contract, increasing in density. It will also thicken as underlying mantle material cools and
attaches itself to the bottom of the slab. Eventually, the slab of lithosphere becomes denser than the underlying
asthenosphere and sinks into the mantle. As it sinks, it pulls the rest of the plate along with it. At about 200 to 300
km, the difference in density between the descending slab and the mantle is at its greatest, so the slab pull force is
also at its greatest. Below this point, the mantle material becomes stronger and the resistance to sinking also
becomes stronger.
A Natural Cycle
One result of all this slow, steady motion is the gradual opening and closing of ocean basins. As tectonic
processes rearrange the surface of the planet, shifting lands and seas over millions of years, they also cause
earthquakes. Now, and for the foreseeable future, human beings have no way to affect these mighty tectonic
processes. By understanding them, however, we can learn to conduct our own lives in ways that minimize our risk
from any disturbances they may cause.
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