20 The Evolution of the Universe 



others that its temperature changed Httle with time, and still others 

 that it began as a cool object and has since become warmer. Some 

 believe it was molten; others believe it was solid. Depending upon 

 their various premises, some believe it is shrinking, others that it is 

 expanding, and still others that the earth has reached an equilibrium 

 in its size and that its heat production and heat loss are in equi- 

 librium. The origin of the three divisions of the earth, core, mantle, 

 and crust, is also a major problem and has been discussed by Urey 

 (1952), Wilson (1959), and Kennedy (1959) to name only a few. 

 Some including Urey ( 1952 ) believed the earth was originally 

 more homogeneous and that the three divisions of the earth are 

 the result of density segregation of minerals as the earth cooled. 



Crustal Changes 



Perhaps one of the most significant realizations in the study of 

 earth history has been the discovery that continental areas are far 

 from being as stable as we commonly envisage. Indeed, in the 

 geologic past, almost the only land areas persistent at any one place 

 have been the continental shield areas, such as those found in 

 eastern Canada, Brazil, Australia, and Siberia. Mountain ranges and 

 huge troughs in the surface, called geosynclines, commonly formed 

 at the edges of the continental shield areas. Shallow but extensive 

 seas commonly flooded the interior of the shield areas. The end of 

 the Cenozoic era in which we are living is perhaps exceptional 

 because it is a time of widespread glaciation and because it has an 

 unusual amount of emergent land. The continual changes of surface 

 features and redistribution of land and sea areas on the continental 

 blocks show that the crust is by no means a stable structure. Earth- 

 quakes, fault movement, and volcanic activity are but a few 

 phenomena which give us day-by-day evidence of the dynamics of 

 crustal instability. 



The many diametrically opposed theories attempting to explain 

 this instability fall into two major schools of thought. According 

 to one school, the continents are essentially fixed in their relative 

 positions and the crustal instability is the result of compressional 

 forces in the earth's crust or mantle. According to the other school, 

 the continents are not stationary but are "floating" blocks which 

 respond to long-term stress and are free to "drift" over the sima. 

 Various hypotheses modify these premises. According to Gamow's 

 views (1948), the continental shields may have become fixed in 

 their present position at a time before the possibility of the existence 



