Earth's interior is divided into distinct layers, each with different compositions and physical properties. Understanding these layers is the foundation for making sense of plate tectonics, volcanism, earthquakes, and how the planet has evolved over billions of years.
Earth's Internal Structure and Composition
Layers of Earth's interior
Earth's interior is divided into four main layers, classified by both chemical composition and physical behavior.
Crust — the outermost and thinnest layer, ranging from about 5 to 70 km thick. There are two types:
- Oceanic crust is thinner (roughly 5–10 km) and denser, made of dark, iron-rich basalt.
- Continental crust is thicker (up to ~70 km under mountain ranges) and less dense, composed largely of granite-like rocks.
Mantle — sits beneath the crust and extends to a depth of about 2,900 km, making it by far the thickest layer. It's divided into the upper mantle and lower mantle and is primarily composed of peridotite, a silicate rock rich in iron and magnesium.
Outer core — extends from about 2,900 km to 5,100 km depth. It's composed mainly of liquid iron and nickel, with smaller amounts of lighter elements like sulfur and oxygen. The fact that this layer is liquid is what generates Earth's magnetic field.
Inner core — the very center of Earth, from about 5,100 km down to 6,371 km. It's also made of iron and nickel, but it's solid despite extreme temperatures because the pressure at that depth is so enormous that atoms are forced into a solid state.
Composition of Earth's layers
Each layer has a distinct chemical makeup and set of physical properties:
- Crust: Oceanic crust is mafic (rich in magnesium and iron), while continental crust is felsic (rich in silica and aluminum). The crust is relatively low density, brittle, and cool compared to everything beneath it.
- Mantle: Composed of peridotite, a dense ultramafic rock dominated by the minerals olivine and pyroxene. The mantle is hotter and denser than the crust. Heat from radioactive decay and leftover heat from Earth's formation keep it hot enough to flow slowly over long timescales, even though it's mostly solid.
- Outer core: Liquid iron and nickel with some lighter elements. Temperatures range from roughly 4,000 to 6,000 °C. Its fluid nature allows convection currents that produce Earth's magnetic field.
- Inner core: Solid iron and nickel. It has the highest density of any layer and reaches temperatures of 5,000 to 7,000 °C. It remains solid only because the immense pressure at Earth's center prevents the metal from melting.
Evidence for Earth's structure
No one has drilled anywhere close to the mantle, so how do we know what's down there? Three main lines of evidence:
Seismic waves are the most important tool. When an earthquake occurs, it sends two key types of waves through the planet:
- P-waves (primary waves) travel through solids, liquids, and gases.
- S-waves (secondary waves) travel only through solids.
By tracking how these waves speed up, slow down, bend, or disappear at different depths, scientists can map layer boundaries. The clearest example: S-waves vanish when they hit the outer core, which tells us that layer must be liquid.
Meteorites give us samples of the materials that originally formed Earth. Chondritic meteorites have a chemical composition similar to Earth's mantle, while iron meteorites closely match what we expect in Earth's core. Since Earth and these meteorites formed from the same cloud of material, the comparison is a strong clue.
Density and moment of inertia round out the picture. Earth's average density is about 5.51 g/cm³, but typical crustal rocks only have densities around 2.7–3.0 g/cm³. That gap means something much denser must exist deeper inside. Earth's moment of inertia (how its mass is distributed as it spins) confirms that dense material is concentrated at the center rather than spread evenly throughout.