More
Where Is Earth’s Radioactivity? Three types of measurements each show that Earth’s radioactivity is concentrated in the relatively thin continental (granite) crust. In 1906, some scientists recognized that just the heat from the radioactivity in the granite crust should explain all the heat now coming out of the Earth. If radioactivity were occurring below the crust, even more heat should be exiting. Because it is not, radioactivity should be concentrated in the top “few tens of kilometers” of the Earth—and have begun recently.
The distribution of radioactive material with depth is unknown, but amounts of the order of those observed at the surface must be confined to a relatively thin layer below the Earth’s surface of the order of a few tens of kilometers in thickness, otherwise more heat would be generated than can be accounted for by the observed loss from the surface.
45
Later, holes drilled into the ocean floor showed slightly more heat coming up through the ocean floors than through the continents. But basaltic rocks under the ocean floor contain little radioactivity.
46 Apparently, radioactive decay is not the primary source of Earth’s geothermal heat.
A second type of measurement occurred in Germany’s Deep Drilling Program. The concentration of radioactivity measured down Germany’s deepest hole (5.7 miles) would account for all the heat flowing out at the Earth’s surface if that concentration continued down to a depth of only 18.8 miles and if the crust were 4-billion years old.
47
However, the rate at which temperatures increased with depth was so great that if the trend continued, the rock at the top of the mantle would be partially melted. Seismic studies have shown that this is not the case.
48 Therefore, temperatures do not continue increasing down to the mantle, so the source of the heating is concentrated in the Earth’s crust.
A third measurement technique, used in regions of the United States and Australia, shows a strange, but well-verified, correlation: the amount of heat flowing out of the Earth at specific locations correlates with the radioactivity in surface rocks at those locations. Wherever radioactivity is high, the heat flow will usually be high; wherever radioactivity is low, the heat flow will usually be low. However, the radioactivity at those hotter locations is far too small to account for that heat.
49 What does this correlation mean?
First, consider what it does not necessarily mean. When two sets of measurements correlate (or correspond), people often mistakenly conclude that one of the things measured (such as radioactivity in surface rocks at one location) caused the other thing being measured (surface heat flow at that location). Even experienced researchers sometimes make this mistake. Students of statistics are repeatedly warned of this common mistake in logic, and hundreds of humorous
50 and tragic examples are given; nevertheless, the problem abounds in all research fields.
This correlation could be explained if most of the heat flowing up through the Earth’s surface was generated, not by the radioactivity itself, but by the events that produced that radioactivity. If more heat is coming out of the ground at one place, then more radioactivity was also produced there. Therefore, radioactivity in surface rocks would correlate with surface heat flow.