296 



Status of the Theory of Isostasy. 



is balanced by a deficiency of mass below sea level. The 

 deficiency of mass is known as the isostatic compensation. 

 For the oceanic segments the isostatic compensation is 

 an added mass. For convenience the sea level is used 

 as the datum surface. Any other datum between the 

 mountain summits and ocean depths could be used, since 

 it is the difference of mass between A and B which is 

 the quantity involved and this is independent of the 

 datum. For convenience in computation, assumptions 

 are introduced in regard to change of density at the 

 datum surface which may require minor corrections ac- 

 cording to the level of the surface. 



Fig. 1. 



Lihhosphere 



Dept-h of 



CDE 



iLocal 

 compensation 



i i i 



compensation 



Asl-henosphere 



Fig. 1. Diagram to illustrate the principles of isostasy. In nature there 

 are of course no sharp boundaries such as are drawn to make clear a dia- 

 gram; the proportions also are different. 



At a certain depth, known as the depth of compensa- 

 tion, the difference in density above will, for perfect 

 isostasy, have completely neutralized the difference in 

 surface elevations, and two sections A and B of figure 1, 

 if of equal area, will contain the same mass. 



The depth of compensation will depend upon the ver- 

 tical distribution of compensation. If the depth of com- 

 pensation is assumed equal to the radius of the earth, 

 6,378 kilometers, and the mean difference in elevation 

 of A and B is 5 kilometers, the densities of A and B 

 differ in the ratio of 5 to 6,378, or less than one part 

 in a thousand. The body of the earth would not in that 

 case be relieved of the loads due to the surface relief, 

 for isostasy does not involve the absence of distortional 

 stress in the shell above the depth of compensation. 

 Compensation extending to the center of the earth is, 

 therefore, equivalent to no isostasy. 



