Plate tectonics

Plate tectonics is the theory that Earth's outer shell, the lithosphere, is broken into rigid plates that move slowly over the softer mantle below. It grew from Alfred Wegener's idea of continental drift, proposed in 1912 but rejected for lack of a mechanism, and from the discovery of seafloor spreading, advanced by Harry Hess in the early 1960s. Magnetic stripes on the ocean floor and the study of earthquakes strongly supported the idea, and in 1965 the Canadian geophysicist J. Tuzo Wilson described transform faults and named the plates. The theory explains where earthquakes and volcanoes occur, how mountain ranges and ocean trenches form, and why continents have drifted over time. Plates move at a few centimetres a year, about as fast as fingernails grow.

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