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  Contents   
    Page 
Table 18  1. Heat flow to the earth’s surface  269 
  2. Radioactive content and heat generation in rocks  270 
  3. Ultimate products of naturally occurring disintegration processes  271 
  4. Isotopic constitution of radioactive elements and their end products  272 
  5. Variations in the isotopic composition of common lead  273 
  6. Lead age ratios of selected radioactive minerals  274 
  7. Age results by lead isotope ratios  275 
  8. Helium age ratios of various magnetite deposits  276 
  Contents   
    Page 
Table 18  1. Heat flow to the earth’s surface  269 
  2. Radioactive content and heat generation in rocks  270 
  3. Ultimate products of naturally occurring disintegration processes  271 
  4. Isotopic constitution of radioactive elements and their end products  272 
  5. Variations in the isotopic composition of common lead  273 
  6. Lead age ratios of selected radioactive minerals  274 
  7. Age results by lead isotope ratios  275 
  8. Helium age ratios of various magnetite deposits  276 

THE EARTH’S HEAT

A. Loss of heat.—Only a limited amount of data is available on the heat flux transferred to the surface from within the earth. These observations are summarized in Table 18–1 and show a mean value of 1.3 ± 0.1 × 10−6 cal./cm.2/sec., when the necessary correction is made for the effect of the recent ice age. Considering the heat conductivity of the outer granitic layer as 0.006 cal./sec./cm.2/°C./cm., the corresponding surface geothermal gradient is about 2.0 × 10−4°C./cm., or 20°C./km.

This heat is partially transferred to the atmosphere by convection and radiation with subsequent reradiation by the atmosphere into space. The remainder of the heat is radiated directly through the blanketing atmosphere. The proportion of heat loss attributable to each of these processes is still a matter of speculation.

B. Sources of heat.—Aside from the residuum of heat associated with the earth at the time of its formation, the primary sources of heat in the earth are radioactivity, solar radiation, and exothermal chemical changes.

1. Radioactivity: Because . . .

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