Re: power of natural radioactivity in the earth

[jrcamero@facstaff.wisc.edu (John Cameron)]

I submitted the following questions to "Ask a Geologist" :  What is the
total radiogenic heat flow from the earth's surface and references to the
total amount?  What is the effect of radiogenic heat on the formation and
evolution of life on earth?
        I received the following reponse which might be of interest to some
members of radsafe.Note that the "nuclear power" of the earth is roughly
equal to that of 2 700 nuclear power plants of one gigawatt each!  The
address for ask a geologist is: ask-a-geologist@octopus.wr.usgs.gov
         Apparently various retired geologists answer the questions. 

>     The accepted value for heat loss from the earth is revised from 
>time to time.  The latest, and perhaps the best, estimation of total heat 
>flow from the earth is 44.2 x 10-12 (read as 10 to the 12th power) watts 
>(Pollach and others, 1993).  This figure is for total heat loss, not just 
>radiogenic heat.  According to Holmes (1965), only about 80% of the 
>earth's heat flow is produced by radioactive decay in areas of granitic 
>continental crust.  In areas of basaltic oceanic crust where radioelement 
>content is much lower the percentage is much lower.  Holmes (1965) 
>calculated the total heat loss from the earth to be 64 x 10-12 calories 
>per second (268 x 10-12 watts).  Also according to Holmes (1965), 94 x 10-
>12 watts comes from the land alone.  The 2700 billion (0.27 x 10-12) watts 
>reported by Skinner and Porter (1989) is about 100 times less than that 
>estimated by Pollach and others, 1993).
>
>Regarding the importance of radiogenic heat or radioactivity to the 
>formation and evolution of life on earth, several possibly important 
>factors come to mind.
>
>     1.  There was about twice as much radiogenic heat or radioactivity 
>produced early in the earth's history than now.  Radioactive elements have 
>been decaying throughout the earth's history and the quantities are less 
>now.  For instance U-238 has a half-life (4.5 billion years) that is 
>approximately equal to the age of the earth, and therefore only about half 
>as much remains.  Ambient radioactivity was also higher early in the 
>earth's history at the time when life probably first formed. 
>     2.  Radioelements are more concentrated in the sialic crust of the 
>earth than in the mantle.  Holmes (1965) suggests that there is very little 
>radiogenic heat transferred from the mantle to the crust.
>     3.  More radiogenic heat is produced from the granitic continental 
>crust, which contains higher concentrations of the radioactive elements and 
>is thicker than the basaltic oceanic crust.
>     4.  The effect of heat flow on temperatures at the earth's surface is 
>insignificant because mosth of the heat there is form solar radiation.
>     5.  The earth' radiogenic heat is important to life on earth more 
>because of its  influence on convection currents in the asthenosphere, in 
>turn influencing plate movement and the location of volcanic belts than 
>because of its effect on surface temperatures. 
>     6.  Some scientists believe that natural nuclear reactors such as the 
>one at Oklo in Gabon, Africa (Draganic and others, 1990) were much more 
>common during early earth history.  These reactors may have raised 
>radioactive levels at many places on the earth.  During early earth history 
>fissionable U-235 made up about 25% of the uranium.  Today U-235  makes up 
>only about 3/4 of one percent of uranium due to its shorter half-lifel (0.7 
>billion years)
>
>                               References
>
>Draganic, I.G., Draganic, Z.D., and Adloff, J., 1990, Radiation and 
>     Radioactivity: CRC Press Inc., Boca Raton, Florida, 347 pages.
>
>Holmes, Arthur, 1965, Principles of Physical Geology: Ronald Press, New 
>     York, 1288 p.
>
>Pollach, H.N., Hurter, J.J., and Johnson, J.R., 1993, Heat flow from the 
>     earth's interior: analysis of the global data set: Reviews of 
>     Geophysics, American Geophysical Union, v. 31, n.3, p.267-280.
>
>Skinner, B.J. and Porter, S.C., 1989, The Dynamic Earth: John Wiley and 
>     Sons, New York, 541 p.
>
John R. Cameron 2678 SW 14th Drive, Gainesville, FL 32608-2050 
phone: 352/371-9865; fax 352/371-9866  e-mail: jrcamero@facstaff.wisc.edu