Science Questions

How does the Earth's core stay hot?

Mon, 4th Jul 2016

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Jan F asked:

I believe the Earth has a solid core made of Nickel and Iron at a temperature of several thousand DegC (correct me if I'm wrong). Why does this core keep its temperature? Why did the core not cool down over the last 3-4 billion years or so?


Keep up the good work, love the show!


Kat Arney put this to Professor Marian Holness, geologist from the University of earthCambridge...

Kat - So Marian, you are our Earth expert. What's going on here? Why does the core keep its temperature or does it?

Marian - It doesnít. it is actually cooling and it has been, all the way through Earth history. Itís been doing it very slowly because itís in the centre of the Earth and all that heat has got to get out through thousands of kilometres of solid rock. What's interesting is that because itís cooling down, it started out all liquid. So a great big globe of liquid iron and nickel as Jan says. As it cooled down, it started to solidify. So, in the centre of the Earth now, we have an inner core which is about a thousand kilometres in diameter which is solid and then the outer core is liquid. Itís the movement of this outer core in fact that makes the Earthís magnetic field. But what we can do is use the fact that itís solidifying to work out how hot it is because what we can do is we know how big the core is because we can listen to earthquakes and find out where all these different divisions of the Earth are. The division between the solid and the liquid core is about 5,000 kilometres below the surface of the Earth. So we can work out how much pressure there is there and then we do some experiments and say, ďWhen does iron solidify at these sorts of pressures?Ē And the answer is, 6,000 degrees Celsius.

Chris - Andrew.

Andrew - I was just wondering, does radioactive decay in the Earth contribute to that heating? Does that put some heat back in if you like as well?

Marian - Yes, it does. The Earth started out hot because of all the potential energy that was released when you made the planet in the first place.

Kat - That's everything just like slamming together.

Marian - Thatís everything slamming together, yes, under gravity. But itís actually cooling much slower than you'd expect if it just had that original heat because as Andrew says, weíre generating heat radiogenically by the decay of isotopes like uranium, thorium, lead, potassium, some early isotopes, and tungsten isotopes that are now completely dead, so there's nothing left of them. But in the core, you donít really have that. You donít really have radiogenic heat generation in the core. Itís all in the outer mantle, the rocky bit.

Kat - Absolutely fascinating! Thank you.



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Heat released by the decay of radioactive elements like Uranium keep it toasty. RD, Mon, 15th Nov 2010

Potassium is much more abundant than either uranium and thorium- the decay of radioactive isotopes of Potassium likely contributes much of the radioactive heat in the core and mantle. Bass, Mon, 15th Nov 2010

Also rock is a very poor conductor of heat, so convenction and advection of heat is extraordinarily slow. frethack, Mon, 15th Nov 2010

just thought of two other possible sources of heat input to Earth...

Body tides adding heat via deformation.

The interaction of the magnetic fields of Earth and Sun as they rotate/orbit, (cf magnetic brake) RD, Mon, 15th Nov 2010

Physicist calculate that the core of the earth is 500į C cooler than it was 2.5 billion years ago. JimBob, Sun, 10th Apr 2011

Your all forgetting one thing, apparently 86% of us think that there is this red dude with a trident and horns who lives down there and supposedly he is going to keep it eternally hot, no one told him about the entropic nature of the universe!

Sorry I apologise for my completely non scientific response to your valid question but I am just sick and tired of Intelligent Design.  Aaron_Thomas, Mon, 11th Apr 2011

Aye! The world is full of whacko's!

The core is slowly cooling down but don't worry - it will still stay hot enough to see us out. Farcanal, Tue, 19th Apr 2011

Don't you mean "see us oot"? Geezer, Tue, 19th Apr 2011

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