Reply To: Animated nuclear reaction.

[jamesr]

I wasn’t sure about the 1/10 figure either so I did a bit of googling.

The book:
The solid earth: an introduction to global geophysics By C. M. R. Fowler gives detailed analysis of the figures for the radioactive heating and gives a bottom line figure of 2.1*10^13W = 21TW for the radioactive heat of the crust and mantle (not including the core). NB it wasn’t thought the concentration of Potassium-40 was very high in the core itself. However recent studies have found potassium does actually dissolve in iron when at very high pressures. So if there is a higher concentration of potassium-40 in the core then there will be some more heat generated there. (source)

If the 3.75TW figure for tidal heating is correct, and using the 21TW figure from above, this would make the ratio of tidal/radioactive 3.75/21 = 1/5.6. So the 1/10 figure is correct to 1sf.

The total rate of heat loss currently by the earth is given as 4.2-4.4*10^13W. ie. we are loosing twice as much heat as is is now being made – hence the earth is cooling. In the past the radioactive heating component would have been much larger, exponenitally so, given the nature of radioactivity. Whereas the tidal heating would have been roughly the same. Hence over the age of the earth the proportion of the stored heat due to radioactivity will be much higher than the current heat generation ratio.