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Asked by courtneywilliamson to Chris, Dave, David, Fiona, Jack on 19 Jun 2013.
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David Freeborn answered on 19 Jun 2013:
Hi courtneywilliamson,
Great question!
There are three main sources of heat in the earth’s core:
(1) left over heat from when the earth formed. The earth formed as a mass of molten rock, created from the remnants of stars and supernovae. The surface of the earth cooled more quickly than the inside, because it was closest to the cold of space. The inside still hasn’t fully cooled yet- 4.5 billion years later!
(2) Friction. Heavier materials in the core are always sinking to the centre due to gravity, and pushing lighter materials up. As they rub or flow past each other, this molten rock and metal created *a lot* of friction. That’s enough to help keep the earth warm.
(3) Radioactive elements: there are a few radioactive elements like Uranium deep inside the earth’s core. They give off heat when they decay.
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Jack Miller answered on 19 Jun 2013:
Great answer David, but don’t forget good, old-fashioned pressure! The pressure on rocks near the earth’s core is incredibly, incredibly high, and, as the rocks can’t really compress further, this raises their temperature.
This is best understood in the equation of state for an ideal gas — pV=nRT, which says pressure * volume = a constant * temperature. Increasing pressure and keeping volume the same will increase temperature.
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