Expert: Ed - 7/6/2007

Question
as far as i am aware there is no continuing source of combustion or nuclear process feeding the molten core of the earth.

it is still hot as a result of it's state following the earth's birth. yes?

now i know the core is well insulated from the surface, but still, after FIVE BILLION YEARS, why hast it cooled off completely and totally solidified.
that's an awfully long time, there must be some significant radiation and conduction of the heat, that has been continuing for these eons.
i don't know the math, but this is HIGHLY counter-intuitive

thank you

Answer
Feiv,

This is more of a geology question than an astronomy one.  But, here is what I can find.

Look at this article:
http://en.wikipedia.org/wiki/Geothermal_(geology)

Scroll down to the section about "Sources," about halfway down the page.  Some of the heat does come from the origin of the Earth, billions of years ago.  However, apparently, lots of the heat at the center of the Earth comes from radioactive decay of heavy elements that have dropped down to the core.

This actually does make sense to me.  Radioactive elements are usually very massive, so it makes sense that during the origin of Earth, most of them dropped all the way to the core.  And they woud still be radiating heat because that's what radioactive elements do!  Maybe in a few billion more years, they will have decayed enough that they no longer give off heat, but until then the Eart's core will continue to be very hot.

Click on the "references" at the bottom of the article for more information.  Also, try googling various relevant keywords.

I wish I could be more helpful, but I'm not a geologist.  I recommend asking one of these people:
http://www.allexperts.com/browse.cgi?catLvl=3&catID=1359

Good luck,
--Ed