Earth is unique among the known planets: it has an abundance of water. Other worlds — including a few moons — have atmospheres, ice, and even oceans, but only Earth has the right combination to sustain life.
Earth's oceans cover about 70 percent of the planet's surface with an average depth of 2.5 miles (4 kilometers). Fresh water exists in liquid form in lakes and rivers and as water vapor in the atmosphere, which causes much of Earth's weather.
Earth has multiple layers. The ocean basins and the continents compose the crust, the outermost layer. Earth's crust is between three and 46 miles (five and 75 km) deep. The thickest parts are under the continents and the thinnest parts are under the oceans.
Earth's crust is made up of several elements: oxygen, 47 percent; silicon, 27 percent; aluminum, 8 percent; iron, 5 percent; calcium, 4 percent; magnesium, potassium and sodium, 2 percent.
The crust is divided into huge plates that float on the mantle, the next layer. The plates are constantly in motion; they move at about the same rate as fingernails grow. Earthquakes occur when these plates grind against each other. Mountains form when the plates collide and deep trenches form when one plate slides under another plate. Plate tectonics is the theory explaining the motion of these plates.
The mantle under the crust is about 1,800 miles deep (2,890 km). It is composed mostly of silicate rocks rich in magnesium and iron. Intense heat causes the rocks to rise. They then cool and sink back down to the core. This convection — like a lava lamp — is believed to be what causes the tectonic plates to move. When the mantle pushes through the crust, volcanoes erupt.
At the center of the Earth is the core, which has two parts. The solid, inner core of iron has a radius of about 760 miles (about 1,220 km). It is surrounded by a liquid, outer core composed of a nickel-iron alloy. It is about 1,355 miles (2,180 km) thick. The inner core spins at a different speed than the rest of the planet. This is thought to cause Earth's magnetic field. When charged particles from the solar wind collide with air molecules above Earth's magnetic poles, it causes the air molecules to glow, causing the auroras — the northern and southern lights.
Editor's note: This article was updated on September 29, 2015, to correct the percentages of elements in Earth's crust.