Earth differs from other planets in that its crust is made up of slabs of thin rock called plates. Driven by currents in the upper mantle, the plates, which number more than a dozen, carry the continents on their backs like rafts.
Meteorologist Alfred Wegener argued for continental drift (as he called it) in the early 1900s. However, the idea was not generally accepted until the 1960s, when geophysicists found that new oceanic crust was constantly being formed in mid-ocean ridges on the seabed. The theory of how new crust moves away from the mid-ocean ridges and disappears at the edges of continents is called plate tectonics, and it has revolutionized geology.
As the edges of crustal plates collide, they rift, overlap, and fold, recycling their rocks and remaking the surface of the planet. The Himalaya Mountains, for example, mark where the Indian plate is being forced under the edge of the Asian plate. The Rift Valley of East Africa, on the other hand, marks where a continent is rifting apart, to eventually form a new ocean.