At the Earth's surface this heat flow drives plate tectonics ( continental drift) ; large segments of the earth's crust (plates) separated along deep cracks called faults are forced into motion. When the plates collide, these powerful internal tectonic forces squeeze and fold solid rock, creating massive changes in Earth's crust (see Figure 1). Mountain uplift and associated volcanic activity where plates collide are only two aspects of the continual recycling and rebuilding of the crust