Physics of the Earth - II The Figure of the Earth Bulletin of the National Research Council (1931) / Chapter Skim
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Chapter VII. Isostasy
Chapter VII. Isostasy
Pages 103-115
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From page 103... ...
According, to the principle of isostasy, if' the earth's crust were cut into prisms of the same cross section by imaginary vertical planes the prisms would have the same Glass if the isostatic condition were perfect. This means that the pressure exerted by these prisms on the subcrustal material at the depth of' (40 miles below sea-level would be uniform throughout.
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From page 104... ...
This is corroborative evidence in favor of the existence of the isostatic condition of the earth's crust, a condition that calls for material in different places varying in density with the elevation of the surface above. The data secured from observations for the variation of latitude, earth and ocean tides, and the transmission of seismic waves indicate that the earth is either solid throughout with the rigidity of steel, or that it is solid to a distance approximately 2,000 miles below sea-level, with the solid portion having a rigidity greater than that of steel.
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From page 105... ...
Pratt had applied corrections to the astronomical latitudes in order to take into account the attraction of the nearby mountain masses. He realized that he had not advanced a wholly satisfactory hypothesis as to the geodetic and astronomical data of India for he closed his paper by saying: " The whole subject, however, deserves careful examination as no anomaly should, if' possible, remain unexplained in a work conducted with such care, labor and ability as the measurment of the Indian arc has exhibited." Airy in his paper re-l'erred to the one by Pratt and then proceeded to set forth his views as to how the anomalies, or unexplained differences between the latitudes bv astronomical observat~iolls and by triangulation, could best be interpreted.
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From page 106... ...
Airy makes it appear that every protuberance outside this thin crust must be accompanied by a protuberance inside, down into the fluid mass, would equally pros e that wherever there was a hollow, as in deep seas, in the outward surface, there must be one also in the inner surface of the crust corresponding to it; thus leading to a law of va.ryi:~g thickness which no process of cooling could have produced. However, he gave credit to Airy for first making the suggestion that there is a deficiency of material below elevated portions of the earth's surface, which also is the basis of the alternate hypothesis proposed by Pratt.
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From page 107... ...
This idea has now been abandoned, for the ana.l:-ses off- igneous rocks found on oceanic islands and on the continents i~.~clicate that the heavier elements are present in greater proportion in the rocks of the oceanic areas than in the continental areas. This leads to the definite conclusion that there is a fundamental difference in density due to the chemical composition of the rocks that causes a difference in the density of the suboceanic alla subcontinental crustal material.
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From page 108... ...
It was assumed that the isostatic condition of the earth's crust is perfect and confutations based on that hypothesis were made of the effect of land and water masses and of their compensat.ion on the astronomical data. Computations were made on the basis of a distribution of the isostatic compensation to various sleuths extending from the surface down to approximately 20V miles.
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From page 109... ...
The zones widen out as the distance increases from the station, since the attraction of the masses varies inversely as the square of the distance, and for any given mass its effect on the value of gravity decreases rapidly with the distance away. In making the isostatic reductions for the gravity stations it is not necessary to read the maps and charts all around the earth for each one of!
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From page 110... ...
After applying, the isostatic principle to deflections o-I' the vertical ancl values of' gravity, there will still be outstanding, differences called residuals or anomalies. The gravity anomaly is the difference between the observed value of gravity and the value computed according, to a definite method.
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From page 111... ...
lye know that there are variations i'rom each of:' these conditions at different gravity stations It is remarkable, however, that the isostatic anomalies without reboard to sign average less than 0.020 dyne for North America while for Europe and Asia, as far as is known, the average gravity anomaly is not greater than 0.020. On oceanic islands, however, the isostatic anomaly is considerably more than it is for continental areas.
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From page 112... ...
: and vertically, to the gravity station, and, conversely-, a large negative anomaly is due to material near the station that is lighter than Cornball It is rather significant that wherever any stations are t,roupecl ire ~ small area these are found to be many irregularities in the curves clra.wn to represent the gravity anomalies. If we could discover the exact distribution of densities from the earth's surface down to the depth of compensation, approximately 60 miles below sea-level, corrections could be applied to the computed value which would make it agree almost exactly with the observed value.
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From page 113... ...
Tbe equibbIium is bct~een the crust and the suhcrustu1 ~uteri~1 and Dot vitbin the crust itselL It is bused on the idea that the 1nuss of each unit section of the earth' Hurt exerts the same pressure OD the subcru~1 matehul. PLASTIC.- A term applied to m~teria1 ~bicb may brave rigidity under ~ su~ciently smog stress and ~y break if the stress is beyond ~ certain limit yet Blob may be distorted Abbott fracturing by ~ Cress within the rupture Wait acting for ~ su~cieDtly TODD time.
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From page 114... ...
On the computation of the effect of the attraction of mountain masses as disturbing the apparent astronomical latitude of stations in geodetic surveys.
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From page 115... ...
On the deflection of the plumb-line in India caused by the attraction of the Himalaya Mountains and of the elevated regions beyond; and its modification by the compensating, effect of a deficiency of matter below the mountain mass.
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