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
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
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.
Click here to return to the Environmental and Earth Science Archives
Update: June 2012