Structure and plate motion

A layered planet

Earth has three main layers: the crust, mantle and core. These names distinguish materials with different compositions. The crust contains relatively light rocks, the mantle contains silicate rocks rich in iron and magnesium, and the core is dense metal, mainly iron with nickel.

A layer's physical state describes whether its material is solid or liquid. Its density describes how much mass is packed into a particular volume. Temperature, pressure and density generally increase towards Earth's centre, but increasing temperature does not mean every deeper layer is liquid: pressure also affects whether material can melt.

From the crust to the core

The crust is thin compared with the rest of Earth. Oceanic crust is typically about 5–10 km thick, whereas continental crust is much thicker, approximately 25–100 km. A thicker layer is not necessarily denser: continental crust is thicker but less dense than oceanic crust.

The mantle extends about 2,900 km below the crust and makes up most of Earth's volume. Although mantle rock is very hot, most of it remains solid. Over very long timescales, some of this solid rock can deform and flow rather than behaving like a rigid block.

The core has two parts with different physical states. The outer core is liquid, while the inner core is solid. The inner core is not cooler: the immense pressure at Earth's centre keeps it solid despite its very high temperature.

The lithosphere and asthenosphere

To understand plate movement, it helps to group Earth's outer layers by how they behave, rather than only by their composition.

The lithosphere is the rigid outer shell. It includes both the crust and the rigid uppermost mantle. It is broken into tectonic plates, so a plate is not simply a piece of crust.

Beneath it lies the asthenosphere, a hotter, softer region of the upper mantle. Its rock is mostly solid, with some partial melting, but can deform and flow very slowly. This allows the rigid lithospheric plates above it to move. Deeper mantle rock remains solid under greater pressure.

Reading an Earth cross-section

A cross-section shows an imaginary cut through Earth. Start at the outside and trace the layers towards the centre: crust → mantle → outer core → inner core. The thin outer rim represents the crust; the broad surrounding layer is the mantle; the core occupies the central region.

Then examine the enlarged outer-layer view. The lithosphere crosses the boundary between crust and mantle because it includes both. The asthenosphere sits beneath the lithosphere and is part of the mantle, not a separate layer between the mantle and core.

Use labels and a key to identify composition or physical state rather than assuming that a particular colour means liquid. Check the scale too: diagrams often exaggerate the crust's thickness so it can be seen. In the explanatory diagram here, the outer-layer detail is enlarged and the convection arrows show a process, not exact paths inside Earth.

Internal heat drives convection

Earth's interior contains a large store of heat. Radioactive decay releases energy as unstable elements break down, helping maintain this internal heat. Heat from the core is transferred into the mantle above it.

Where mantle material becomes hotter, it expands and becomes less dense than surrounding cooler material. It rises slowly. Near the cooler outer part of Earth, it loses heat, becomes denser and sinks. Continued heating and cooling can therefore produce a circulating movement called a convection current.

The important distinction is that material rises and sinks, carrying heat with it. Heat itself does not become dense and sink. Although convection is familiar in heated water, mantle convection is much slower and mainly involves deforming solid rock rather than a freely flowing liquid.

Connecting convection to plate movement

Convection provides a key connection between Earth's internal heat and movement at its surface. Slow movement of mantle material beneath the lithosphere can exert forces on the overlying plates and help them move. The softer asthenosphere allows movement beneath the rigid plates.

The causal chain is therefore: internal heat → density differences → mantle convection → forces acting on tectonic plates. This explains why the temperature and physical properties of Earth's layers matter for plate tectonics.

Convection is not the only contribution to plate movement. For example, a dense oceanic slab sinking into the mantle can pull the rest of its plate behind it, a process called slab pull. Nevertheless, internal heat and mantle movement provide the foundation for understanding the moving plate system.