The outermost solid layer of the earth is the 50- to 100- kilometer-thick lithosphere, which is broken up into a series of rigid plates. The lithosphere is underlain by a plastic layer of the mantle, the asthenosphere, over which the plates can move. This plate motion gives rise to earthquakes and volcanic activity at the plate boundaries. At seafloor spreading ridges, which are divergent plate boundaries, new sea floor is created from magma rising from the asthenosphere. The sea floor moves in conveyor belt fashion, ultimately to be destroyed in subduction zones, a type of convergent plate boundary, where the sea floor is carried down into the asthenosphere and eventually remelted. Magma rises up through the overriding plate to form volcanoes above the subduction zone. Where continents ride the leading edges of converging plates, continent-continent collision may build high mountain ranges.

Evidence for seafloor spreading includes the age distribution of seafloor rocks, and the magnetic stripes on the ocean floor. Continental drift can be demonstrated by such means as polar-wander curves, fossil distribution among different continents, and evidence of ancient climates revealed in the rock record, which can show changes in a continent's latitude. Past supercontinents can be reconstructed by fitting together modern continental margins, matching similar geologic features from continent to continent, and correlating polar-wander curves from different landmasses.

Present rates of plate movement average a few centimeters a year. One possible driving force for plate tectonics is slow convection in the asthenosphere; another is gravity pulling cold, dense lithosphere down into the asthenosphere, dragging the asthenosphere along. Plate-tectonic processes appear to have been more or less active for much of the earth's history. They play an integral part in the rock cycle—building continents to weather into sediments, carrying rock and sediment into the warm mantle to be melted into magma that rises to create new igneous rock and metamorphoses the lithosphere through which it rises, subjecting rocks to the stress of collision—assisting in the making of new rocks from old.