Lava magma

[Tweed]

What process keeps lava and magma at their very high temperatures?

[wildwood]

Agitated molecules?

[dundurn]

Radioactive decay in the Earth's core and mantle. If the Earth had simply been a very hot body at the start of the Solar System it would have cooled down long ago, even with the input of energy from bombardment by meteors. It's the heat energy released by the decay of unstable isotopes that has provided the long-term heat in the Earth's interior.

[kwicky]

dundurn surely the mantle and skin of the Earth keeps the heat bottled up like an enormous thermos flask? Also I can think of recent underground fires that are impossible to distinquish.

[dundurn]

Thermos flasks work by using a vacuum, glass (a poor conductor of heat) and highly reflective surfaces to slow (but not stop) the exchange of heat between their contents and the outside world. The Earth's crust does do a similar but not as efficient job and certainly wouldn't have kept it as hot as it is for 4 billion years. There are other heat inputs to the system; Earth tides, contraction of the Earth under gravity, chemical changes within the regions below the crust and it's a very complex system. The point I'm making is that heat from radioactivity is a major continuing energy input to the system and the Earth would be a lot cooler now without it.
I'm not sure where the underground fires come in.

[kwicky]

Most of the underground fires are coal related but elsewhere may be just vegetation. They expect these to last for many centuries as there is no way of putting them out. As it combines to produce 3% of global emissions makes you wonder how the problem can ever be tackled.

http://www.buzzle.com/editorials/2-15-2003-355 76.asp

[Tweed]

Thanks a lot folks. Great answers.