The theory that the surface of Earth is made of a number of plates, which have moved throughout geological time to create the present-day positions of the continents. Plate tectonics explains the location of mountain building, as well as earthquakes and volcanoes. The rigid plates consist of continental and oceanic crust together with the upper mantle, which "float" on the semi-molten layer of the mantle beneath them, and move relative to each other across the planet. Six major plates (Eurasian, American, African, Pacific, Indian, and Antarctic) are recognized, together with a number of smaller ones. The plate margins coincide with zones of seismic and volcanic activity.