 Plate tectonics. Plate tectonics 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 sea floor spreading was validated in the late 1950s and early 1960s. The lithosphere, which is the rigid outer most 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. Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction, or one plate moving under another carries the lower one down into the mantle. The material lost is roughly balanced by the formation of new oceanic crust along divergent margins by sea floor spreading. In this way, the total surface of the lithosphere remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories, since disproven, proposed gradual shrinking contraction or gradual expansion of the globe. Tectonic plates are able to move because the Earth's lithosphere has greater mechanical strength than the underlying hastenosphere. Lateral density variations in the mantle result in convection, that is, the slow creeping motion of Earth's solid mantle. Plate movement is thought to be driven by a combination of the motion of the sea floor away from spreading bridges due to variations in topography. The ridge is a topographic high and density changes in the crust density increases as newly formed crust cools and moves away from the ridge. That subduction zones the relatively cold, dense crust is pulled or sinks down into the mantle over the downward convecting limp of a mantle cell. Another explanation lies in the different forces generated by tidal forces of the Sun and Moon. The relative importance of each of these factors and their relationship to each other is unclear and still the subject of much debate.