Earth

Earth Third major planet from the Sun, and the largest of the four inner, or terrestrial planets. Water covers c.70% of the Earth's surface. This fact, and the Earth's average surface temperature of 13°C (55°F), makes it suitable for life. Continental land masses make up the other 30%. Earth has one natural satellite, the Moon. Like all the terrestrial planets, there is a dense core, rich in iron and nickel, surrounded by a mantle of silicate rocks. The thin, outermost layer of lighter rock is the crust, which can vary in depth from between 50km (30mi) – the thickest continental crust – to 5km (3mi) – the thinnest oceanic crust. The boundary between the crust and the mantle is called the Moho (Mohorovičić) discontinuity. The solid, inner core rotates at a different rate from the molten outer layers, and this, together with currents in the outer core, gives rise to the Earth's magnetic field. The crust and the uppermost mantle together form the lithosphere, which consists of tightly fitting slabs called plates. The plates, which float on a semi-molten layer of mantle called the asthenosphere, move with respect to one another in interactions called plate tectonics. See also atmosphere