The Earth’s crust and lithosphere are broken into over a dozen thin, rigid shells, or plates, that move around the planet over the plastic aesthenosphere in the upper mantle. The interaction between these plates is called tectonics, from the Greek tekon for “builder”; plate tectonics describes the deformation of the Earth’s surface as these plates collide, pass by, go over, or go under each other. In other words, plate tectonics describes how these plates move, but not why.

Overall, plate tectonics combines Wegener’s theory of continental displacement (or drift) and Hess’s discovery of seafloor spreading (see below). The theory has truly revolutionized the study of the Earth’s crust and deep interior. It allows scientists to study and understand the formation of such features as mountains, volcanoes, ocean basins, mid-ocean ridges, and deep-sea trenches, and to understand earthquakes and volcano formation. It also gives clues as to how the continents and oceans looked in the geologic past, and even how the climate and life forms evolved.