Plate Tectonics is a principle of Geology. Basically, the Earth's crust is cracked into pieces called plates. (Plates are actually part of the Lithosphere, which are the plates and a small portion of the upper mantle just below it.) These plates move around, but very slowly. The dissipation coming from heat in the mantle is what drives these plates. They can also be between 8 to 50 kilometers thick, depending on being in the ocean or the crust.