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Question:
I can compress dirt and it becomes warm but begins cooling instantly. After 4.5 billion years, wouldn't all of that heat be dispersed? How hot would the earth have had to have been 4.5 billion years ago to still be this hot at the center or is there some continually "fueling" process that has also defied the second law of thermodynamics?



Replies:
The earth is continuously cooling still, even as we write. Sometimes it's hard to grasp just how massive the earth is, and there are other things that keep it from cooling off very fast.

One, it's got an atmosphere, which captures some of the radiative heat loss from the earth and keeps it near, and thus the heat loss is slowed. Two, it's got the sun, which is continuously radiating energy to the earth and its atmosphere.

Don
Donald Yee Ph.D.


Updated - September 2009
The core, mantle and crust of Earth contain radioactive elements. These are primarily uranium-238, uranium-235, thorium-232, and potassium-40, with half-lives of roughly 4.47 billion years, 704 million years, 14.1 billion years, and 1.28 billion years, respectively. Aside from the daughter atoms and radiation, thermal energy is released. This thermal energy not only changes the cool-down time of Earth (it extends it) but is a major source of energy for plate tectonics.



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