CHAPTER IX 

 THERMAL METHODS 



The generation and dissipation of heat governs the life history of the 

 milhons of stars distributed throughout celestial space. After the initial 

 period of condensation of the nebula which formed our earth, when intense 

 heat was generated, a period was ultimately reached when the internal 

 energy of the earth was radiated more rapidly than it was generated. From 

 this period onward the radioactive matter in the earth became an increas- 

 ingly important source of heat. With any reasonable assumption regarding 

 the distribution of radioactive material with depth, it can be shown that 

 % to % of the present total surface flow of heat is supplied by radioactivity. 

 During the estimated 3,000 million years' life of our earth there has been 

 a decrease in radioactivity heat generation of about 50 per cent. The internal 

 heating of the earth has therefore reached a fairly stable phase and will 

 continue to decline at a slower rate, i.e. exponentially with time. 



The present temperature of the earth is governed by many f actors. f 

 From a geophysical viewpoint however, we are concerned chiefly with only 

 one factor; i.e., the dissipation of heat coming from the interior of the 

 earth. The heat from the interior must flow through the outer crust, and in 

 so doing, it affects the temperature of the upper surface layers of the earth. 

 This temperature will not be uniform but will vary with the thermal con- 

 ductivities of the various materials in the surface and near-surface of the 

 crust, and with their geometric distribution. From studies of these tem- 

 perature variations, predictions often may be made of the nature and sub- 

 surface distribution of these materials. 



Temperature measurements of the earth's outer crust can be used to 

 furnish fundamental information relating to the origin and the history of 

 the areas under observation. In addition, temperature measurements in 

 favorable cases will yield valuable information regarding the zonal distribu- 

 tion of ores ; the configuration of intrusive bodies ; the contacts between 

 sedimentary and igneous rocks, or between (different) sedimentary rocks; 

 the location, hade, and throw of faults ; the ground water distribution 

 and local regional subsurface flow. The practical application of geothermal 

 measurements to the solution of various problems of economic geology is 

 usually one of two types : (a) near-surface temperature measurements, to 

 study the lateral variations of temperature; (b) subsurface measurements 



t Wm. D. Urry, "Significance of radioactivity in geophysics; thermal history of the earth," 

 Trans. Amer. Geophysical Union, Vol. 30, No. 2, April, 1949. 



B. Gutenberg, "Physics of the Earth," McGraw-Hill, 1939. 



