Abstract
The theory of isostasy supposes that, in regions which have not been disturbed recently, each vertical column of the earth's crust with a certain minimum radius and extending to a depth of about 100 km has approximately the same mass. To find the deviation from this approximation in a given region, the density must be assumed as a function of depth. Such assumptions used at present for calculations are discussed critically. The resulting errors are greater than it is normally beleaved; errors in the calculated isostatic gravity anomalies exceeding ten milligals must be expected in certain regions. Systematic errors result from the usual assumption in routine calculations that the mean density in the earth's crustal layers under the bottom of the Pacific and in the continental areas is the same, and that in both the difference between the density of the layers above about 30 km and the layers below this depth is 0.6. The processes producing and maintaining isostatic equilibrium are discussed.
Highlights
The theory of isostasy supposes that, in regions which have not been disturbed recently, each vertical column of the earth's crust with a certain minimum radius and extending to a depth of about IOO km has approximately the same mass
\ is an ap roximation, the question cannot be raised w ether the theory of isostasy is true or false, but how good the approximation is in a given region
The first is, to use observations of t e density and distribution of rocks in the earth's crust and of gravity at the earth's surface and to calculate the residuals against an assumed equilibrium condition; the second is to consider the processes involved in establishing isostasy
Summary
’ i assumed for the crustal ,a ers is too small, considering the fact that va ues assumed for the gravity calculations have to represent the density of the uppermost surface layers, and of the deeper more basic and heavier layers down to the material in which the upper layers are “floating”. Their approximate thickness is given in the last column of Table 11. With velocity of longitudinal waves of s1/2-61/4 kni/sec
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