SOCIAL SCIENCES • GEOGRAPHY

Basic Structure of the Earth

Earth may look like a solid ball from space, but its inside is made of several layers. The three broad parts are the crust, mantle and core; the core itself is divided into the outer core and inner core.

We cannot travel to the centre of Earth. Scientists learn about its interior by studying earthquake waves, rocks, gravity and Earth's magnetic field. Let us explore these layers with pictures and diagrams.

Earth: Our Home Planet

Satellite-based Blue Marble views of Earth showing continents, oceans and clouds
Earth seen from space. This NASA Blue Marble montage helps us observe the surface, oceans and clouds; it does not show the planet's internal layers. Image courtesy of NASA.

Earth is almost spherical, although it is slightly flattened at the poles and bulges near the equator. Its average radius is about 6,371 kilometres. The layers inside Earth differ in thickness, composition, temperature and physical state.

Diagram: The Four Main Internal Layers

This cutaway diagram shows the order of the layers from the surface towards the centre. The thicknesses and sizes are simplified so that each layer can be seen clearly; the drawing is not to scale.

Labelled cross-section of Earth's internal structure A cutaway Earth showing a thin crust, thick orange mantle, liquid outer core and solid inner core, with labels and approximate depth ranges. CrustThin, solid outer skin MantleHot, mostly solid rock Outer coreLiquid iron-rich metal Inner coreSolid iron-rich metal Earth's centre is about6,371 km below the surface. Simplified cutaway • not to scale
Cross-section of Earth's main internal layers. The crust is extremely thin compared with the mantle and core.

Remember: The crust is the outermost layer, but it is not the same as the whole solid surface system. The crust together with the rigid uppermost mantle forms the lithosphere, which is broken into tectonic plates.

1. Crust — Earth's Thin Outer Layer

Depth: surface to roughly 5–70 km

The crust is the outermost solid layer. We live on it, and it includes the continents and the ocean floors. Compared with the size of Earth, the crust is very thin—rather like the skin of an apple.

  • Continental crust: generally thicker, often about 30–70 km, and mainly made of lighter rocks rich in silica and aluminium.
  • Oceanic crust: generally thinner, often about 5–10 km, and mainly made of denser basaltic rocks.
  • The crust and the rigid uppermost mantle together make up the lithosphere.

2. Mantle — The Thickest Layer

Extends to about 2,900 km below the surface

The mantle lies below the crust and is Earth's thickest layer. It is made mostly of hot silicate rock rich in magnesium and iron. Most of the mantle is solid, but over long periods it can slowly deform and flow.

  • Heat inside Earth helps drive slow movement in the mantle.
  • Movement in the mantle contributes to the motion of tectonic plates over geological time.
  • Some mantle rock melts to form magma, which may rise and erupt as lava at the surface.

The mantle is not a giant ocean of liquid rock. It is mostly solid rock that can move very slowly because of the high temperature and pressure.

3. Outer Core — A Layer of Liquid Metal

About 2,900–5,150 km below the surface

The outer core is made mainly of liquid iron and nickel, along with smaller amounts of other elements. The metal is liquid because the temperature is high enough for it to melt at the pressure found there.

  • Flowing liquid metal in the outer core helps generate Earth's magnetic field.
  • The outer core surrounds the solid inner core.
  • Scientists infer its liquid state partly because certain earthquake waves do not travel through it.

4. Inner Core — The Solid Centre

About 5,150–6,371 km below the surface

The inner core is Earth's central region. It is made mainly of iron and nickel and is solid, even though it is extremely hot. The enormous pressure at Earth's centre prevents it from melting in the same way as the outer core.

  • Its radius is about 1,220 km.
  • It is the deepest layer of Earth.
  • Its temperature is estimated to be similar to the temperature at the Sun's surface, but the pressure is enormously greater.

Compare the Four Layers

LayerPositionMain materialPhysical state
CrustOutermostSolid rocksSolid
MantleBelow the crustHot silicate rocksMostly solid; slowly deformable
Outer coreBelow the mantleIron-rich metalLiquid
Inner coreCentre of EarthIron-rich metalSolid

Diagram: How Deep Are the Layers?

Depths are measured from Earth's surface. The boundary values are approximate and can vary slightly between scientific models.

Earth layer depth guide A vertical depth guide showing the crust near the surface, mantle down to about 2,900 kilometres, outer core down to about 5,150 kilometres, and inner core down to Earth's centre at about 6,371 kilometres. Depth below Earth's surface The bands are simplified for learning. CrustA very thin outer shell (about 5–70 km) MantleTo about 2,900 km Outer coreAbout 2,900–5,150 km Inner coreAbout 5,150–6,371 km SurfaceEarth's centre Increasing depth
The mantle occupies most of the distance between the crust and the core. The crust is too thin to appear to scale in a simple diagram.

How Do Scientists Know What Lies Inside Earth?

  • Earthquake waves: their speed and paths change as they pass through different materials. S-waves cannot travel through the liquid outer core.
  • Volcanic rocks: some rocks brought to the surface give clues about materials from deeper inside Earth.
  • Gravity and density: measurements show that Earth's inside is much denser than its surface rocks.
  • Magnetic field: the field provides evidence of moving electrically conducting material in the outer core.

Why Is Earth's Internal Structure Important?

  • Movement of tectonic plates helps explain earthquakes, mountain building and the formation of ocean basins.
  • Heat from Earth's interior powers many geological processes, including volcanism.
  • The liquid outer core generates the magnetic field that helps shield Earth from charged particles from space.
  • Knowledge of Earth's layers helps scientists interpret earthquake records and understand the planet's history.

Quick Facts to Remember

  • Earth has four commonly described internal layers: crust, mantle, outer core and inner core.
  • The mantle is the thickest layer.
  • The outer core is liquid; the inner core is solid.
  • The crust is thinner beneath oceans than beneath most continents.
  • Earth's average radius is approximately 6,371 km.

Memory tip: From the surface to the centre, remember C–M–O–I: Crust → Mantle → Outer core → Inner core.

Frequently Asked Questions

Which is the thinnest layer of Earth?

The crust is the thinnest of the four main internal layers.

Which layer is the thickest?

The mantle is the thickest layer, extending to about 2,900 km below the surface.

Why is the inner core solid but the outer core liquid?

The inner core is under much greater pressure, which keeps its iron-rich material solid despite its very high temperature. The outer core is liquid under its different pressure and temperature conditions.

How do we know the outer core is liquid?

One important clue comes from earthquake waves: S-waves do not pass through the outer core, which indicates that it is liquid.

Is the mantle completely liquid?

No. The mantle is mostly solid rock. Over very long periods, it can deform and flow slowly.

Test Your Understanding

  1. Name the four main internal layers of Earth in order from the surface to the centre.
  2. Which layer is made mostly of liquid iron-rich metal?
  3. Why is the mantle described as mostly solid rather than liquid?
  4. State one way scientists study Earth's interior.
  5. Explain one reason Earth's internal structure matters to life on the surface.