Plate tectonics is a scientific theory which describes the large scale motions of Earth's lithosphere. The theory builds on the older concepts of continental drift. The lithosphere (the shell of a rocky planet) is broken up into what are called tectonic plates. In the case of the Earth, there are currently seven or eight major and many minor plates. The lithospheric plates ride on the asthenosphere (the upper mantle just below the lithosphere). These plates move in relation to one another at one of three types of plate boundaries: convergent, or collisional boundaries; divergent boundaries, also called spreading centers; and conservative transform boundaries. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The movement of the plates varies, from 0 to 100 mm annually.