<P> Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός "pertaining to building") is a scientific theory describing the large - scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, since tectonic processes began on Earth between 3 and 3.5 billion years ago . The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century . The geoscientific community accepted plate - tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s . </P> <P> The lithosphere, which is the rigid outermost shell of a planet (the crust and upper mantle), is broken into tectonic plates . The Earth's lithosphere is composed of seven or eight major plates (depending on how they are defined) and many minor plates . Where the 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 (or faults). The relative movement of the plates typically ranges from zero to 100 mm annually . </P>

The part of the earth on which the tectonic plates are able to move is the