Layers: Crust, Mantle and Core

If you could cut the Earth in half, you would see layers, like the layers of an onion. From the outside in, they are the crust, the mantle, the outer core and the inner core. The deeper you go, the hotter and denser the rock becomes.

1. Crust

• The thin, solid outer layer we live on. Compared with the whole Earth, it is as thin as the skin of an apple.
• Oceanic crust (under the oceans): about 5–10 km thick, made mostly of dark, heavy rock called basalt.
• Continental crust (under the land): usually 30–50 km thick, and up to about 70 km under high mountains like the Himalaya and Karakoram. Made mostly of lighter rock such as granite.
• The crust is mostly oxygen (about 46%) and silicon (about 28%), followed by aluminium, iron, calcium, sodium, potassium and magnesium.

2. Mantle

• The thickest layer, reaching about 2,900 km down. It makes up about 84% of the Earth's volume.
• It is solid rock, but so hot that it can flow very slowly, a few centimetres a year, like very thick toffee.
• These slow movements (convection currents) move the plates of the crust and cause earthquakes and mountains.

3. Outer core

• From about 2,900 km to 5,150 km deep.
• Liquid iron and nickel, at about 4,000–5,000 °C.
• The moving liquid iron produces the Earth's magnetic field.

4. Inner core

• From about 5,150 km to the centre at 6,371 km.
• Solid iron and nickel, at about 5,200 °C, roughly as hot as the surface of the Sun.
• It stays solid, even though it is hotter than the outer core, because the pressure is enormous.

How do we know? Earthquake waves

No one has drilled through even the crust; the deepest hole is about 12 km. Scientists learn about the inside of the Earth from earthquake (seismic) waves:

• P waves (primary) are faster and can travel through solids and liquids.
• S waves (secondary) are slower and can travel only through solids.
• Seismometers more than about 104° away from an earthquake receive no S waves. So something liquid must block them: the liquid outer core.
• Between about 104° and 140°, no direct P waves arrive either. Waves travel faster deeper down, so their paths curve; where they enter the slower liquid outer core they bend sharply, leaving a gap at the surface. This is the shadow zone.