Pieces of crust and brittle uppermost mantle, perhaps 100 kilometers thick and hundreds or thousands of kilometers wide, that cover the Earth's surface. The plates move very slowly over, or possibly with, a viscous layer in the mantle at rates of a few centimeters per year. (See Figure 8. )
The surface of the Earth is broken into large plates that move very gradually over time. Where the edges of two plates come together, there is often intense and sometimes dangerous geological activity, involving earthquakes and volcanoes. The massive pressure that results from plate movements can build mountain ranges, which continue to push upwards for many years, as plates buckle against each other.