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Our transcription: Most geologists are in general agreement regarding plate tectonic theory. However, there is considerable debate about the forces which drive plate movement, and a great deal of current research focuses on finding a mechanism. This is a particularly challenging problem because the mechanism operates deep within the Earth's interior, which we cannot see or sample directly. The Earth's crust and uppermost mantle act together as a rigid unit. This layer which comprises the plates is called the "lithosphere," and it's about a hundred kilometers thick. These lithospheric plates float, crowded together like lily pads in a pond, in a deeper layer of the mantle called the "asthenosphere". Unlike the lithosphere, the asthenosphere is soft and partially molten, so the relatively cool rigid lithospheric plates can move through the asthenosphere if enough force is applied. Several different theories have been advanced to try to explain the driving force behind this process. All agree, however, that the mechanism for plate movement is somehow related to the unequal distribution of heat within the mantle. When mantle rocks are heated unevenly from below, most geologists believe that they can circulate in a cyclic fashion called "convection." Currently, some form of convection in the mantle is the most widely accepted mechanism for plate movement.
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