The Earth seems vast and solid. But inside, it is mostly molten or semi-molten, and is always on the move. The whole Earth is some 7,930 miles (12,800 km) in diameter. But the hard outer Layer, the crust, is only about 15-22 miles (25-35 km) under the major Land masses or continents, and 3-6 miles (5-10 km), beneath the oceans. Below this is the thickest layer, the mantle, which is 1,800 miles (2,900 km) deep. Within the mantle is the Earth's two-part core. The outer core, 1,360 miles (2,200 km) thick, is composed of almost liquid iron-rich rocks. The solid inner core, 1,550 miles (2,500 km) across, is also mainly iron and nickel. If you could travel down a drill hole into the Earth, the temperature would soon be unbearable even in the crust. At the core, it is nearly 8,000°F (5,000°C).
The bulk of the Earth's volume is the mantle. This is fairly firm in its upper region, but becomes semi-melted or plastic deeper down. The mantle is made of rocky minerals, rich in silicon, magnesium and iron. The fairly rigid upper 35-56 miles (60-90 km) of mantle, plus the crust above, forms the lithosphere. This is cool and strong and is divided into huge lithospheric plates. In the crust, temperatures rise by about 4-6°F (2-3°C) for every 300 feet (90 m) of depth, but this rate soon becomes less. (World of Science)
