Tectonic plates are great slabs of rock which form the Earth's crust. Both the continents and the oceans rest on tectonic plates, which float on the superheated, plastic mantle below. Over millions of years, these tectonic plates float around, driven by convection currents in the mantle, gathering into supercontinents and scattering again. This is called the theory of plate tectonics. Tectonic plates move about as fast as your fingernails grow — 5 to 10 centimeters per year.

The Earth has 14 major tectonic plates, and 38 minor plates, for 52 plates total. The major plates are the African Plate, Antarctic Plate, Arabian Plate, Australian Plate, Caribbean Plate, Cocos Plate, Eurasian Plate, Indian Plate, Juan de Fuca Plate, Nazca Plate, North American Plate, Pacific Plate, Philippine Plate, Scotia Plate, South American Plate. The largest are the Antarctic Plate, the Eurasian Plate, and the North American Plate. A continent may include several different plates: for instance, India and Arabia are both separate from the Eurasian Plate. The Himayala Mountains are a result of the Indian Plate pushing north into the Eurasian Plate.

Minor plates break off major plates during rifting events. Some minor plates subduct under other plates as they move. This causes them to be pushed down into the mantle and melt. The Farallon Plate to the west of San Francisco is almost completely subducted beneath the North American Plate, for example. Sometimes a part of a plate will end up on top of another plate, in which case it will be called an accreted terrane. Parts of North America and Europe are terranes.

It is thought that the boundaries of tectonic plates are partially determined by upwelling events called mantle plumes. These split apart the crust and cause it to crack, creating rifts, like the Red Sea, which separates Africa from Arabia. A typical upwelling event creates three rifts at roughly 120° from each other. This is called a triple junction. Usually, two of these rifts will continue to spread while one fails. A failed rift on land is called a rift valley.