The inner core is made up of solid rock at very high temperature (about 4,300°C) and pressure. It has a high density (about 15 g cm−1). The temperature is maintained by the insulating effects of the outer layers of the Earth, and is due to radioactive decay of naturally occurring potassium, thorium, and uranium giving out large amounts of energy as they do so. The diameter of the inner core is about 2,530 km/1,570 mi.
The outer core is very dense liquid rock (about 11 g cm−1). It is where the Earth's magnetic field arises and has a diameter of about 6,930 km/4,300 mi.
The mantle makes up the rest of the Earth (apart from the thin crust). The mantle is a layer of thick dense rock. Within the mantle is molten rock called magma – it is this which erupts to the surface in volcanoes.
The crust is the very thin layer of solid rock upon which we live. It has a maximum thickness of 50 km/31 mi. Where the crust contains continents, it is found to be rich in silicon and aluminium, and where it is oceanic crust it is found to be rich in silicon and magnesium. Oceanic crust is more dense than continental crust. The crust is divided into a number of plates which move about on the surface of the mantle – this is known as plate tectonics.
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