geothermal energy-Box 1

Published by

Australian Academy
of Science

Rocking on with hot rocks geothermal energy

Box 1 | Layers of the Earth

The Earth is made up of three main layers: the relatively thin crust, the mantle and the core. Immediately beneath our feet is the crust, which ranges in depth from five to 70 kilometres (it is thinnest below the oceans). Below the crust the mantle is 2,900 kilometres deep and makes up 84 per cent of the Earth by volume. Below the mantle is the outer core, then the inner core at Earth’s centre.

These layers are distinguished by their composition, density and temperature; both density and temperature increase towards the centre of the Earth. The inner core, which is slightly smaller in diameter than the moon, is thought to consist mostly of a nickel-iron alloy. Surprisingly even though its temperature is around 5,000°C, it is solid due to the intense pressure at these depths below Earth’s surface. The outer core, although cooler than the inner core, is liquid because it is under less pressure. The deepest part of the mantle reaches temperatures close to 4,000°C but it is less than 1,000°C at its outer edge.
The movement of heat from the ultra-hot core, through the mantle to the crust and then into space is an essential part of the creation and maintenance of many geothermal energy resources. When heated sufficiently, rock melts to form magma; because magma is less dense than the surrounding rock it rises towards the surface. In some places, such as along the boundaries of tectonic plates, magma appears at the surface in the form of lava during a volcanic eruption. In other places, magma is unable to reach the surface and remains in the crust heating the rocks around it. Most of the geothermal energy below Australian soils is generated by radioactive decay, but heat coming from the centre of the Earth also contributes to our geothermal resources.

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