Physics of the Earth - II The Figure of the Earth Bulletin of the National Research Council (1931) / Chapter Skim
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Chapter VIII. The Influence of Isostasy on Geological Thought
Chapter VIII. The Influence of Isostasy on Geological Thought
Pages 116-122
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From page 116... ...
We shall see, in this chapter, how ;sostasv influences our ideas of the processes oft' dynamical geology, for it is oilily to this branch of geological science that isostasy has any relation; it has no relation to historical ~,eolo~,:; nor to areal geology, except in so far as faults due to isostatic readjust.ment snap alter the areal distribution of rocks. NVe mav define isostasv as follows: Given a number of' vertical columns ,J eJ of the same cross-section (not less than about ~,000 square miles)
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From page 117... ...
It is the element o:t' time that makes it so difficult to prove by experiment that rocks yield under long continued forces. Geodetic observations in mountainous regions show that there is no rlel'ect of matter there, due to erosion, and similar observations in regions of heavy clepositior.~, such as the basin of' the Ganges river, the NIississ:ippi e~hav~nent and delta, show no excess of' matter; so that, in spite of all objections, the return und.er~rouncl flow of' roil: seems pretty cleDnitel: p r oved Another important geolo¢,;ca.l problem is the cause of the elevation of mountain ranges.
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From page 118... ...
He did not suppose the Hags to be liquid but sufficiently plastic to lead to practically hydrostatic equilibrium. This has been called the " loots of Mountains " hypothesis, because the lighter strata near the surface is supposed to project downwards into the denser magnet below.
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From page 119... ...
liVhe~ we apply these facts to Pratt's ideas of isostasy, we find no discrepancies; and we have the 'following picture of:' the course of events leaclint, to a folded mountain range: After the accumulation o:t' sediments to a considerable thickness, forces compress and i'olcl the strata. This necessarily increases the amount of' matter ill the compressed region and would naturally cause some elevation of the surl'ace.
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From page 120... ...
This r squires, in ~ mountainous ~:e¢,io~, an excess of mass equal in volume to that of the visible mountains; awl isostasy will have none of it. It may lie discarded without further coIlsicleration, -l'or mountains are in isostatic equilibrium and it is impossible to believe that a region in which tens of thousands o-L' feet of sediments have beets accumulated and in which still more ~at.erial has been c oncentrated by horizontal c oppression, should then be so unclerwe:i,hte~l that material, equal in volume and greater in mass than the later visible mountain range, should be neeessa.r: to restore it to isostatic equilibrium.
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From page 121... ...
The elevated alla tilt.e`~l leaches in North America and iI1 Norway find their simplest explanation ilk the restoration oiL' isostatic equilibrium after the melting, of' the great Pleistocene ice-cap. This explanation naturally calls I'or all earlier depression under the weight of the ice.
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From page 122... ...
strong, influence on geological thought. It doffs not explain the origin of all the vertical and horizontal forces that have caused the breast movements in the crust of the earth; it does not tell us where or when these movements will occur; but it puts definite limits to our speculations and requires us to reject any hypothesis which calls for the concentration or the abstraction of large amounts of material from any region.
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