Earth is layered like a peach: a thin skin, a thick middle and a dense pit. The layers formed when young Earth was hot enough for its materials to sort by density: iron sank and lighter rock floated up.
| Layer | Depth (km) | Made of | Solid or liquid | Density (g/cm³) |
|---|---|---|---|---|
| Crust | 0 to about 35 (5 to 70) | Light rock: granite in continents, basalt under oceans | Solid | 2.7 to 3.0 |
| Mantle | to about 2,900 | Hot rock rich in iron and magnesium | Solid (it creeps very slowly) | 3.3 to 5.6 |
| Outer core | 2,900 to 5,150 | Iron and nickel | Liquid | about 10 to 12 |
| Inner core | 5,150 to 6,371 (the center) | Iron and nickel | Solid | about 13 |
Key idea
Earth has four main layers: a thin rocky crust, a thick solid mantle, a liquid outer core and a solid inner core. Density increases toward the center.
Why it works
No one has drilled past the crust; the deepest hole is about 12 km. We know the layers from earthquake waves. Waves speed up, slow down and bend as they pass through different materials, and one kind of wave (S waves) cannot travel through liquid. S waves never cross the outer core, so it must be liquid. Earth's average density, about 5.5 g/cm³, is twice that of surface rock, so the inside must be much denser.
Worked example: Which layer?
A probe reads a depth of 4,000 km. Which layer is it in?
- The mantle ends at about 2,900 km.
- The outer core runs from 2,900 to 5,150 km, and 4,000 km is between them.
Answer: The outer core
Worked example: How thick?
Using the table, how thick is the outer core?
- It starts at 2,900 km and ends at 5,150 km.
- $5{,}150 - 2{,}900 = 2{,}250$ km.
Answer: 2,250 km
Common mistake
Picturing the mantle as a sea of liquid magma. The mantle is almost entirely solid rock; it is so hot that it can creep very slowly, a few centimeters a year, like very stiff putty. Only the outer core is liquid. Check: S waves pass through the mantle, which they could not do if it were liquid.