Why do tectonic plates move?

Tectonic plates are large, relatively rigid segments of the Earth's lithosphere (the hard outer shell of the Earth which contains the crust, continents and outer mantle). The lithosphere is about 35 km (20 miles) thick. It sits on top of the relatively softer asthenosphere and lower mantle. Convection currents within this softer rock are responsible for the movement of the overlying tectonic plates, which move at about the rate your fingernails grow: several centimeters (about one inch) per year.