42 EVOLUTION OF THE EARTH 



Rise of basic magmas from the as theno sphere. The series 

 of radioactive elements slowly break down into elements of 

 lower atomic weight and give off in the process enormous quan- 

 tities of energy. Uranium, in degenerating through radium to 

 the stable element lead, develops more than a million times 

 the heat given by the combustion of an equal weight of coal, 

 but the disintegration of the element and the liberation of its 

 heat are so slow that the whole duration of geologic time has 

 not sufficed to eliminate uranium from the crust of the earth. 

 Therefore it has acted as a permanent generator of heat in 

 the rocks which contain it. 



Uranium and thorium, the parents of the radioactive series, 

 are widely though sparsely diffused through the lithosphere. 

 It has been calculated that, if they extend in their surface 

 amount to a depth of 40 miles, they must supply heat to the 

 surface as fast as it is lost by radiation into space. The earth 

 therefore appears not to be growing colder, though losing heat. 

 The small content of radioactive elements in the basaltic 

 shell below the granitic crust of the continents would then 

 supply that slow increment of heat which is necessary to 

 generate new molten rocks. The granitic shell above, though 

 somewhat richer in radioactive elements, is sufficiently near 

 the surface to lose its excess heat by conduction. The excess 

 heat generated in the asthenosphere is, on the contrary, so 

 deeply buried that it cannot escape in that manner but must 

 slowly transform some of the solid rock into liquid form. 

 Reservoirs gather, until their mass, combined with their de- 

 creased density in the fluid form, enables them to work their 

 way through the crust above and demonstrate their existence 

 in igneous activity at the surface of the earth. The magma 

 which thus comes from the greatest depth and in greatest 

 volume would, because of the initial density stratification, 

 produce a notable increase in the density of the outer crust. 



