May, 1922 dynamics of the lithosphere 197 



Support of load. — Physicists have pointed out that a Hquid 

 interior would not support continental masses rising 3,600 to 

 5,400'- meters above the ocean floor.' But probably this argu- 

 ment is of little value when isostatic measurements have shown 

 them to be of lower specific gravity. 



Relative density of solidified and molton rocks^^ is such that 

 a crust could not permanently form on a molten sphere in 

 which convection currents are possible, since solidified rocks 

 are denser than molten rocks. 



Behavior of rocks under pressure. — It is a well-known law 

 of physics that the melting point increases or rises with 

 pressure'^ Barus found the rise of the melting point of diabase 

 to increase directly as the pressure. This would give a melting 

 point of 76,000° C. at the center of the earth. Under such 

 pressures as exist at the center of the earth, many physicists 

 claim, it would be impossible to obtain molten rocks, because 

 such an adjustment would be necessary as gave the least space, 

 and it is certain that crystalline rocks occupy less space than 

 molten rocks do. An increment of temperature increase amount- 

 ing to about 1° C. for every 18 meters of descent has been 

 found. Assuming the radius to be 6,268 km., this would give 

 a temperature of 347,600° C. at the center, which is more than 

 four times the melting point of diabase under the pressures 

 assumed at the center. But it is unlikely that such a rate of 

 increase can be supposed. Deeper borings seem to indicate a 

 lessening rate, and a point may be attained, perhaps compar- 

 atively near the surface, where temperature ceases to rise. It 

 is not our problem to account for the heat. It may be due to 

 the mechanical work done, to radio-activity, to chemical action, 

 or compression, or all together. 



Another line of experiment giving some possible light on the 

 condition of the interior of the earth is furnished by those 

 investigations dealing with the crushing and flowage of rock. 

 Van Hise'^ states that more than 1,700 kilograms of pressure per 

 square centimeter are necessary to crush granite. 



Fisher^'' has estimated 3,000,000 atmospheres of pressure at 

 the earth's center. 



•^Chamberlin & Salisbury, Text II, 7. 



"Fisher: Physics of Earth's Crust. 



■■•Chamberlin and Salisbury: Textbook in Geology II, 7. 



'^Van Hise: Principles of Pre-Cambrian Geol., U. S. G. S., IGth Annual Report. 



"Fisher: Physics of Earth's Crust. 



