Notifications

Log in to see your notifications.

Internal Structure of the Earth

12 min read

Layers of the Earth

The Earth is made of layers, like an egg. Our knowledge comes from seismic waves from earthquakes, deep mines and boreholes, and volcanic material.

The layers of the EarthA cross-section of the Earth. A thin solid crust lies on top, then a thick mantle of hot rock that flows very slowly, then a liquid outer core of iron and nickel, and a solid inner core at the centre which is very hot but kept solid by pressure.crust (5–70 km)mantle (2 900 km thick)outer core: liquid (2 200 km)inner core: solid (radius 1 200 km)
LayerThicknessMaterialState
Crust5–70 kmGranite (continents), basalt (ocean floors)Solid
MantleAbout 2 900 kmDense rock rich in iron and magnesiumMostly solid; the upper part (asthenosphere) is soft and flows slowly
Outer coreAbout 2 200 kmMolten iron and nickelLiquid
Inner coreAbout 1 250 km radiusIron and nickelSolid, because of high pressure

Temperature and pressure increase with depth. The temperature at the centre is about 5 000 °C. The Mohorovičić discontinuity (the Moho) separates the crust from the mantle, and the Gutenberg discontinuity separates the mantle from the core.

Seismic waves

An earthquake produces P waves (primary, compression waves, which can travel through solids and liquids) and S waves (secondary, shear waves, which cannot travel through liquids). Because S waves do not pass through the outer core, scientists know that it is liquid. P waves travel faster (about 8 km/s) than S waves (about 4.5 km/s), so the P waves arrive first. The time gap shows how far away the earthquake was.

Earthquake wavesAn earthquake starts at the focus underground. The point on the surface directly above it is the epicentre. P waves are longitudinal and fast and travel through solids and liquids. S waves are transverse, slower and cannot pass through liquids. Surface waves travel along the ground and do the most damage.focusepicentreP waveS wavesurface waves
d = distance to the earthquake, Δt = time between the arrival of the P and S waves, v_P = 8 km/s, v_S = 4.5 km/s.
The longer the gap between the P and S waves, the farther the earthquake.
Try itLocate an earthquake

Choose the time gap between the arrival of the P and S waves at a seismograph.

Why it matters

The movement of the plates of the crust over the mantle causes earthquakes, volcanoes and mountain building. Heat from the Earth’s interior is used as geothermal energy, and valuable minerals come from the crust.

Check yourself Which layer is liquid?
Check yourself Which wave arrives first?
Check yourself The crust under the oceans is mostly

Key terms

Crust
The thin outer layer of the Earth.
Mantle
The thick middle layer.
Core
The central part, mostly iron and nickel.
Seismic waves
Waves of energy from an earthquake.

Summary

  • The Earth has a crust, mantle, outer core and inner core.
  • Temperature and pressure increase with depth.
  • S waves do not pass through the liquid outer core.
  • The gap between the P and S waves shows how far away an earthquake is.
Earth Systems