The theory of Plate Tectonics states that the lithosphere consists of several individual segments called plates. About twenty such plates have been identified. Of these, the largest is the Pacific plate while the Juan de Fuca plate, off the western coast of North America, is the smallest. Each plate moves as a distinct unit in relation to other plate and all major interaction between plates occur along plate boundaries. Most of the Earth,s seismic activity, vulcanism and mountain building occur along these dynamic margins. Depending on the type of movement, plate margins are of three types- Divergent, Convergent and Transform fault boundaries.