164 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
But the geologic column fails to yield the one feature which might 
provide conclusive evidence on the earth’s origin. No rocks yet 
examined have the appearance of an original crust; they are all either 
old sediments or solidified magma which pushed up through sediments. 
TEMPERATURE AS A CLUE 
Trying another tack, we might expect that the earth’s thermal 
history could be traced back to determine its temperature at birth. 
In deep mines and wells the temperature increases 1° C. for each 125 
feet below the surface. Knowing how rocks conduct heat, we find 
that 10 million million calories of heat are flowing out from the earth’s 
interior each second. If the earth were solid granite, all 7,000 billion 
CRUST (30 miles thick) DENSITY 3 
720-mile layer DENSITY 4 
1050-mile layer DENSITY 5 to 6 
CENTRAL CORE DENSITY 10 to 13 
Figure 1.—Density stratification in the earth. Seismologists, using earthquake 
waves as a sounding device, have discovered these layers within the earth. 
They may indicate that the earth was once molten; or they may result from 
chemical compaction and plastic flow within the earth. 
billion tons of it, this escaping heat, would cool it about 1° C. in 3 
million years. From this measured rate of cooling is it possible to 
determine whether the earth was originally molten? 
One must be careful in such estimates; not all of this heat comes 
from cooling the earth, since the radioactive disintegration so useful 
in determining the age of rocks is also releasing energy. In fact if 
the measured radioactivity is constant with depth, the outer crust of 
the earth only 12 miles thick would provide all the 10 million million 
calories leaving the earth’s interior. If the radioactive material goes 
deeper than 12 miles, the earth must, willy-nilly, be heating up! 
So the heat now leaving the earth does not give a clue to its original 
temperature, although it does point to another approach. Since it is 
improbable that the earth is heating up rapidly, the radioactive 
material probably is not distributed uniformly throughout the earth 
but is concentrated in surface layers. 
