The Evolution of the Universe 19 



mantle which shears under concentrated, short-term stress but 

 which flows plastically under diffuse, long-term stress. The crust, 

 8 to 40 km thick, is the outermost part of the earth. It flows plas- 

 tically only under extremely long-term stress. Each of these parts 

 of the earth's interior is separated by more or less abrupt discon- 

 tinuities which may be related to their density and physical prop- 

 erties. 



The crust is extremely complex, and although we live on its 

 surface and know much more about it than about the mantle and 

 the core, we have only begun to unravel its structure. The crust 

 extends under the oceans and across the continents. The oceanic 

 areas of the crust lie beneath the great ocean basins and are 8 to 10 

 km thick. The rocks which compose this part of the crust are 

 believed to be Fe-Mg-rich silicates, such as rock basalt, having a 

 specific gravity of 2.9 to 3.1. They are commonly termed sima. 

 These rocks are usually covered by only a few hundred feet of 

 sediments consisting in part of fine clay particles washed in from 

 the continents and in part of atmospheric dust and organic shells 

 which form vast areas of "oozes." 



The continental areas of the crust are believed to be strikingly 

 different in most respects from those of the ocean areas, although 

 locally there may be a gradation between the oceanic and con- 

 tinental types of crust. The upper portions of the continents are 

 believed to be composed primarily of aluminum silicate rocks, 

 such as granite, having a specific gravity of about 2.7. They are 

 commonly termed sial. These low density rocks are about 30 km 

 thick and rest on sima. The mountains of the world commonly have 

 greatly thickened "roots" of sial and sima which extend into the 

 mantle to much greater depths than is normal for continental 

 areas, in much the same manner as icebergs extend farther beneath 

 the water surface as they grow higher. Thus the thickness of the 

 crust beneath the Himalaya Mountains, India, is nearly twice that 

 of the crust beneath the Great Plains area of the United States. 



Early History of the Earth 



Age determinations from radioactive elements indicate the crust 

 of the earth is at least 4 billion years old. The earth itself probably 

 was formed about 4.5 or 5 billion years ago (J. T. Wilson, 1959; 

 Hales, 1960). Hypotheses as to the process of its formation are 

 about as numerous as the number of students of the field. Some 

 believe that the earth started as a hot object and cooled with time. 



