I started researching this question so that I could present the answer in the simple format that I try to use for the school blog. But then I got drawn into the whole world of continental drift, tectonic plates, earthquakes, volcanoes and lots more. It is fascinating and I will do my best to pass some of it on over the next few months. My source is mostly a US Government publication “This Dynamic Planet”.

But let’s talk about the first question first.

Under the oceans, the thickness is fairly consistently just 5 km, which is remarkably thin. On land, however, it averages about 30km but can be as much as 100km.

Considering that the distance from the surface to the centre of the earth is about 6 400 km, a crust of even 100km is strikingly thin. This diagram is to scale. The thin blue outer ring is the crust. Below that is the magma (orange in the image), the partially melted rock that forms the lava flows when a volcano erupts.

What prompted me to ask the question in the first place was that, because the temperature below the crust is sufficiently high to melt rock, this heat must be an inexhaustible source of clean energy. It turns out that this geothermal energy is already being utilised in many places and often from only a few metres below ground.

Hot springs are one example of the use of geothermal energy, but would you believe that in some places this energy produces steam which is used directly to power electricity generating turbines. In fact, one fifth of California’s renewable energy is produced in this way.

Practically all of Iceland’s electricity is produced from geothermal energy.

Where steam is not produced naturally, water is pumped into the hot area and turns into steam to drive the turbines. The diagram is also compliments of the US government.

So why is this not being used as the solution to our power problems?

In some places (Iceland and California being two) it is easier to tap into the energy source than in others. However, wherever you are, you’ve only got to drill deep enough to find it. That suggests that cost is the determining factor. Having said that, once you’ve drilled, the heat is for free. You’d still need power stations, but would no longer need to burn coal and pollute the atmosphere.