The interiors of rocky planets and moons tend to be pretty hot compared with their surfaces. This heat, which can be caused by a number of sources — such as tidal stretching and compression, the initial accretion of the planet, and the radioactive decay of heavy elements — can drive large convection currents of rocky materials inside these bodies, similar to the circular motion of water boiling in a pot.
Earth's solid surface, or lithosphere, extends more than a hundred miles into the planet. It's broken into large chunks, and the convection currents of molten rock below exert pressure on these chunks, causing them to push up against, slide by and separate from one another. This process, called plate tectonics, is responsible for earthquakes, volcanoes, ocean ridges and vast mountain ranges on Earth's surface.
