The lithosphere (from the Greek for "rocky" sphere) is the solid outermost shell of the Earth. It includes the crust and the uppermost solid layer of the mantle. Below the lithosphere lies the asthenosphere, the semi-plastic or partially molten upper part of the mantle. The lithosphere essentially "floats" on the asthenosphere. The lithosphere has broken up into what are called tectonic plates. The Earth's ten plates move in relation to one another at one of three types of plate boundaries: convergent (moving together), divergent (moving apart) and transform (moving laterally). This movement of lithospheric plates is described as plate tectonics. Divergent plate boundaries can cause regional extension and basins and oceans to form, as is currently the case for Atlantic Ocean and the Red Sea. Convergent plates cause regional compression and mountains and volcanoes to form as is currently the case in the Pacific Northwest and the Japan. The collision of continents on convergent plate boundaries can create mountains, such as the Himalayas of Nepal. Transform plate boundaries, like the San Andreas Fault in California, cause earthquakes as the two plates grind against each other along the fault.