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The Earth is made up of different layers.

1. The inner core is in the centre and is the hottest part of the Earth. It is solid and made up of iron and nickel with temperatures of up to 5,500°C.
2. The outer core is the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel.
3. The mantle is the widest section of the Earth. It has a thickness of approximately 2,900 kilometres. The upper part of the mantle is called the asthenosphere. It is made up of semi-molten rock called magma.
4. The crust is the surface of the Earth. It is a rock layer forming the upper part of the lithosphere. The lithosphere is split into tectonic plates.

The theory of plate tectonics

The crust is the outer layer of the Earth. It is the solid rock layer upon which we live. It is either continental or oceanic. Continental crust is typically 30-50 kilometres thick, whilst oceanic crust is only five to ten kilometres thick. Oceanic crust is denser, can be subducted and is constantly being destroyed and replaced at plate boundaries. Continental crust is older, lighter and cannot be destroyed.

Why do plates move?

One explanation for plate movements is slab pull. Plates are extremely heavy so gravity acts upon them, pulling them apart.

Alternatively, as shown in the diagram, convection currents under the Earth’s crust transfer heat, which rises through the surface and cools back down in a circular motion.

The convection currents move the plates. Where convection currents diverge near the Earth's crust, plates move apart.

Where convection currents converge, plates move towards each other, plates converge and the plates move together, also known as ridge push. The movement of the plates, and the activity inside the Earth, is described as the theory of plate tectonics.