What Is Plate Tectonics?

Earth lithosphere — the rigid outer layer comprising the crust and upper mantle — is broken into about 15 major tectonic plates that float on the semi-fluid asthenosphere below. Convection currents in the mantle, along with gravitational pull at subduction zones, drive these plates at speeds of 1 to 15 cm per year. Where plates diverge, new crust forms (mid-ocean ridges); where they converge, one may slide beneath the other (subduction), creating deep trenches and volcanic arcs; and where they slide past each other, transform faults like the San Andreas produce earthquakes.