356 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1964 



The pressure at the bottom of the mantle, 2,900 kilometers below 

 the surface, is about 1.37 million atmospheres, whereas at the center 

 of an entirely solid Earth it would be only just over 20 j)ercent greater. 

 Thus no more than a modest extrapolation of the law between pressure 

 and density is required. 



^K- D II ■ " ■! I ■■ n i < itl j l» W P ? i'>M f* " * *»* » ' »JM 



ti ^»wi i »j <ii j i; » |i f LM tji w a m f 



E 



c 



\n 



CO 

 LU 



a: 



Q. 



z 

 o 



U 



Z 



;/ 



T 



PRESSURE (10'^ dyne/cm.') 



Figure 2. — The incompressibility of each of the Earth's three main zones is a linear func- 

 tion of pressure. But the greater compressibility of the liquid core means that as radio- 

 active heating at great pressure causes more liquid to form, the Earth contracts. 

 Where the outer shell meets the solid mantle, the pressure is 0.141 x 10'^ dyne cm.~2; 

 at the boundary of the mantle and core, 1.36 x 10'^ dyne cm.~2; and at the Earth's 

 center, about 3.9 x 10'^ dyne cm."^; (10'^ dyne cm."^ is approximately 1 million 

 atmospheres). 



