Plate tectonics (from the Late Latin tectonicus, from the Greek: 'pertaining to building') is a scientific theory that describes the large-scale motions of Earth's lithosphere. The theory builds on the concepts of continental drift, developed during the first decades of the 20th century. It was accepted by the geoscientific community after the concepts of seafloor spreading were developed in the late 1950s and early 1960s. The lithosphere is broken up into tectonic plates. On Earth, there are seven or eight major plates (depending on how they are defined) and many minor plates. Where plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The lateral relative movement of the plates typically varies from zero to 100 mm annually.
Contributions by Vsmith, SEWilco, and GeoGreg.