Hot rockers

The Earth has been particularly active lately. It’s easy for us to forget, running around on the surface as we do, that there’s a lot more going on beneath our feet.

The rocks we’re familiar with are merely the crust on the surface, between 5 km (under the ocean) and 50 km thick. Beneath that is about 2,890 km of mantle, a 2,266 km  thick liquid outer core and a solid inner core, which has a radius of 1,216 km.

Cross section diagram of the Earth, showing the crust, mantle, outer core and inner core
Cross section diagram of the Earth, showing the crust, mantle, outer core and inner core (Image by NASA)

And of course, it’s really, really hot down there. Thanks to radioactive decay, warming from the sun and heat left over from the Earth’s formation 4.5 billion years ago (kept under great pressure), the inner core is believed to be as hot as 5505 °C. That’s about the same as the surface of the sun.

Such an enormous amount of heat has to be good for something, and it is: we can use it to generate geothermal energy.

Diagram showing the 3 different geothermal energy types, volcanic hydrothermal, hot sedimentary aquifer and enhanced geothermal, or hot rocks (click to embiggen)
Diagram showing the 3 different geothermal energy types, volcanic hydrothermal, hot sedimentary aquifer and enhanced geothermal, or hot rocks (Image from AGEA)

According to the Australian Geothermal Energy Association (AGEA), there are three different kinds of geothermal energy generation, all of which use hot water or steam to turn turbines and produce electricity:

  1. Volcanic systems use underground water heated by volcanic rock, in places like New Zealand’s North Island. Unfortunately we don’t have any suitable locations for this in Australia.
  2. Sedimentary geothermal, or hot sedimentary aquifers, uses naturally porous sandstone or limestone that contains water. In Australia, these conditions can be found in the Otway and Gippsland basins in Victoria and the Great Artesian Basin in Queensland and South Australia.
  3. Hot rocks, or enhanced geothermal, involves pumping water into deep, hot fractured granite. Suitable sites for this are also found in South Australia and Queensland.

Building geothermal power stations can be expensive, involving as they do really deep holes. And the fact that they’re often in remote areas means energy transmission becomes a factor too.

But in a time when averting climate change means finding alternative sources of energy, and options like nuclear power are looking less attractive, it’s comforting to know that Australia already has plenty of energy in the very rocks it’s made from.

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