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CHAPTER VIII THE INFLUENCE OF ISOSTASY ON GEOLOGICAL THOUGHT HARRY FIELDING REID John.s Hopkins Universit?J The evidences for the truth of the doctrine of isost.asy have already bee- ~,;.ven.: The: are so strong, that we are -i'orced to accept then. 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) encl. extendi.n, down I'rom the earth's surface to a level surface some tens of' miles below sea-].evel, they will all contain the same amount of matters wherever they may be situated, under high mountains, under low plains ol unpiler the sea itself'. And as this equality is i'ound to exist to a -fair (leg-ree of' accuracy- otter large tracts of' the earth's surface, incleecl, wher- e-ver observations of' the ;nte:r~sity of gravity and of the deflection of the vertical leave been made, whether in regions of' ~eolo`~,ical activity, in regions which have lone, been ~,eolo`~,i.eallv stable, or at sea, we conclude that geological processes are unable 1:ermanentlv to alter this equality, ~ ., which is tallest :lSOSt:e been t~.~sl.'o~tecl by the rivets and deposited in the sea near the shore. Isostasy requires that an equivalent Class be abstracted from near:. the seasl.-~o-re awl be -fetus Dell to the area of erosion, aloud this call only be at ~ o~l.'lisl~etl by ~ sul~terra~ean flow. It does not mean there is a cl;~:ect u~rler~,rou~cl c Rent from the sea to the n~ount.ains but that there is a slow ~,ellel:al movement of' the -uncler~,-rouncl material which restores to a lame extel:~t the isostati.c equilibrium Which has been di.sturbecl by erosions ancl depos:itio~. 'l'l-~e I.~rotess is extren~el:T sloes. It tales about C,000 years to lower h\ e-~:osio~-~ the bash; o-.L' the Mississippi * This Bulletin Chap. VII. 116

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INFLUENCE OF ISOSTASY ON GEOLOGICAL THOUGHT 117 river one foot; and in the course of ages an equivalent amount of material must be returned by subterranean flow at about the same rate. Isostasy, therefore, teaches us that the rocl: at a small distance below the surface is sufficiently plastic to permit this flow under the long-continued action of' fox ces acting in the same direr lion. Some geologists dispute this characteristic o-T' the rock They claim that rock is too rigid and too strong to yield in this manner; they base their opinion principally on the experi- ments of Adams and Bancroft,~ '2 who showed that the rigidity of rocks increases greatly with the pressure. Je-~l'reys has pointed out 3 that we must distinguish between rigidity and strength. The former is the re- sistance to elastic deformation within the elastic limit, which disappears when the forces producing it are removed; the latter is the resistance to permanent deformation. With i.ncreasin~, temperatures the elastic limit gets smaller, as does also the strength, though the ri~,idity may be in- creased bv the pressure. But even at the earth's surface marble slabs (usecl for marking graves) about six feet lone, ar~cl an inch or so thick, supported at their ends, have bent down ~narl~eclly in the course of one or two hundred years under their own weight, though the marble is quite ri~,icl. This is nothing, but plastic flow under comparatively small fore es. Many petrographers claim that crystalline Nobles cannot flow plastically, but alter their shapes by recrystallization. Isostasy cannot determine what characteristic of rocl: permits flow; it only declares that flow does take plac e and that rocks are therefore prac t.ically plastic. Physicists would not deny that practically all solids change their shape slowly under l.on`, continued -forces. The rate. of' change in the rocks required by isostasy is small and the time provided is immensely long. 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. In 18~9 James I-Iall oracle a splendid contribution to our knowledge of the geologic history o-t' mountain ran~,es.4 Ile shoe that the sediments of' the Appalachian Mountains were laid down in a sinl~in~, syncline; their thinness was as much as 40,000 feet near the shore, and became pro~,ress:ively thinner to the west. Later they were :I'olded and faulted evidently. by horizontal i'orces; and he thought that a general continental elevation brought them to their present altitude.

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118 FIGURE OF THE EARTII The folding was greatest where the sediments severe thickest, passing off into gentle folds towards the west, and disappearing at some distance. He thought that the sinking was due to the weight of the sediments; isostasy supports this idea to some extent, for the extra weight must have driven out an equal amount of material from below. But on account of the smaller density of the sediments the volume of the material driven out must have been smaller than that of the sediments and the syncline would soon have been filled up and the deposits carried further to the west. There must have been some additional cause for the sinking of the syncline. We are warned not to ascribe all earth movements to isostas y. If the sinking of the syncline were clue to the added weights of the sediments, tending to drive out material below and reestablishing isostat:ic equilibrium, the later elevation of the range could not be due to the same cause. There are notary deeps in the ocean, such as the Virgin Island Deep, the Tonga Deep, the Tuscarora Deep, aloud others, which have sum; without any material deposit of sediments; and mountain ranges gener- ally have certainly not been elevated because the region they occupied was insufficiently loaded. . NVe must recognize that there is some other cause of great earth movements. ~ mountain range is not a permanent feature of the earth's surface. TVe know that the regions where mountain ranges now exist were once below sea-level. We must account not only for the wait in which mountains ranges maintain their altitudes hut also for the way in which they were originally elevated. Airy 5 was the first to reali.~.e that the anomalies in the deflection o:t the vertical i. India indicated a delici.eney of mass under the Himalayas; and he proposed the hypothesis that mountains float in. the magma as logs float in water. He did not suppose the Hags to be liquid but sufficiently plastic to lead to practically hydrostatic equilib- rium. This has been called the " loots of Mountains " hypothesis, be- cause the lighter strata near the surface is supposed to project down- wards into the denser magnet below. Pratt ~ ~ accepted Airy's idea of the deficiency of matter under mountain ranges, but rejected the idea of roots; he proposed his own hypothesis, based on a line of reasoning which would not be accepted at the present time, that the deficiency under mountains and exc ess under of cans is due to a general variation in the density down to a more or less de-finite depth. He thought the eleva- tion of mountains was due to an expansion of the underlying material which would not disturb the isost.a.tic equilibrium. Later studies have done much to remedy the clelects in Pratt's reasoning. * And other papers.

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INFLUENCE OF ISOSTASY ON GEOLOGICAL THOUGHT 119 What has isostasy to say to these two hypotheses? Evidently neither one contravenes isostasy. We must go back to the geological history of mountain ranges to decide between them. Button ~ (see p. 201) mentions that the Sierra Nevada.s were folded in the Mesozoic and elevated in the Cenozoic and it may be added that in the interval the region was reduced to a peneplain and lay, therefore, at a small elevation above sea- level. This peneplain now forms the western slope of the mountain and has been only slightly warped. Many other ranges have a similar history; the Rockies, the Appalachians, the Andes, the Lesser Himalayas, were all peneplained after the folding and were later elevated without material warping of the peneplain. The Catskills have a base of intensely folded Silurian strata, on which lie uncomfortably only slightly disturbed marine Devonian. Therefore, after the folding the region was depressed for a long time below sea-level and was later elevated without folding. Nearly the same history applies to the Blue Mountains of southeastern Australia. Nearly horizontal carboniferous and other deposits, some of them marines rest uncomfortably on strongly folded Devonian. Some geologists thinly that the great nappes of the Alps were produced at a low level and subse- quently elevated. Criteria such as the foregoing, and others based on i'aultin`,, show without question that all the mountain ranges, sufficiently well letdown to be used as tests, have been elevated long after the folding; and, therefore, that their present elevation is clue to vertical forces and Cot to horizontal compression.* 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 eleva- tion of the surl'ace. On account of the disturbance of' the isost.atic equilib :~i-u~:n, forces are T~:ou,ht into play which cause the region to silly and. drive out matter from below. NYhen the equilibrium is reestablished the region would be slightly higher than before, on account of the accumu- lation of the lighter surface rocl:, which lowers the average density of the mass. How ,rea.t the actual elevation, due to compression, may be, clepends on the amount of compression and on the relation between the rates of compression and of' readjustment of equilibrium. We do not l~now this relation, and, in the absence of observations of deflection and. gravity during the time of' compression, we can gain an idea of it only through geological observations which dist.ing~ish between folding and * For further details see literature reference 8.

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120 FIGURE OF -THE EARTH uplift. Finally, expansiol1 of the underlying, mass raises the region into a true mountain range. The cloctrine of' isostasy does not deny the possibility that elevation by expansion, and :t'oldin~, by compression could occur at the same time; but geological observations show that, in at least a. very large number of' cases, the elevation has taken place a long time after the folding,. According, to Airy's idea of roots, the surface strata were crushed and folded by horizontal ]:'orees, and the weight o:t' the ,~,rea.t excess of material thus concentrated caused them to sink into the underlying, magma until equilibrium was established. On account of small difference in density between the sur-i'ace rock and the magma, the roots would be many times as extensive as the visible mountains. There is no other way to imagine the production of' the roots. Mountains, from this point of' view, are quite analogous to the pressure ridges formed in the floating, ice of the Arctic regions; the mountains are wholly due to compression and are elevated only at times of compression. But the geological history of mountain ranges, outlined above, is in direct contradiction to this idea. This is a case where geology comes to the help of geodesists and helps them to decide between two hypotheses where `~eocletic observations alone . ~ . . . are 1naeclslve.r There is one other hypothesis to account for the elevation of mountains; namely, that they are ralser1 by material forced in under them. 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. Mountains rise slowly but some of them have risen too rapidly to allow the rock to readjust itself' to the strain bar plastic yielcling; and great -fractures are produced, developing steep boundary :E'aults (with a.ttendi~-~g earthquakes) which increase in throw as the mountains rise. The Sierra NevaLlas, the \Vasatch, the Alps, the Himalayas, are conspicuous examples. As a mountain range rises under the action of' vertical forces, erosion carries oft' material i'rom its surface and isostas~r requires that an equal mass be i'orce(1 in below, arid this adds to the upward movement. A:t't.er the original elevating fort es have Thrown weal: or have ceased to act, the * See literature r eference 9.

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INFLUENCE OF ISOSTASY ON GEOLOGICAL THOUGHT 121 mountain continues to rise as a result of' erosion at the surface and supply down below, and the throw on the boundary fault would probably be increased, even though the movement might not be fast enough to produce faults in sound rolls In the earlier stages of' erosion the material carried off comes almost entire!\ furor the valleys carvecl out by the streams, and the interfile sections, as po;~-~te~l out by :Xansen,3 will continue to rise until these sections are reduced to ricl~,es and peaks. The mean height of' the mass, however, steadily grows less, for the material supplied below, being denser than the material eroded from the surf'a.ce, has a smaller volume than the latter. And so, in time, unless new forces come into alar. the region is degraded to a peneplain. Isostasy rejects the idea, , ,, cow (A ''' ~ . '' ~ . ~ ~ ~ ~ , ~ 1 ~ 1 1 ~ ~ A ~+ ~ 1 ~ ~ A ~ ; ~ ~ held by many ~eolo~,~sts, tnat a penep~a~r~ ca~ orgy u~ ueve~up region where all movement has ceased for a long time. This process has in~port.ant geological applications; for accordin`, to it a very much larger quantity of material could be eroded from a region than might exist there, above sea-level, at and given time. The immense quantity ol' sediments that were deposited in the Appalachian trough and for some hundreds of miles to the westward, during Paleozoic times, calve :I'rom a land area to the east of the present Appalachian mountains. If this area, called Appalachia, remained perfectly still, it must have been very extensive to supply the required amount of sediments; but if ~ according, to the doctrine of' isostasyr, it was steadily rising, under erosion its area Blah have been very n~uch less. Dutton's studies among, the Leigh plateaus of the Nicest led him to conclude that eight or ten miles o:t' strata had been erodec;l from the surface, though the elevation there is still about a mile. Ile did not believe that the region had ever had a height of' nine to eleven miles but thought that it continued to rise as the strata were swept awe:: and that the rise was a consequence of the unloacling. It was these observations, a~.~parentlv, that turned his atten- tion to the doctrine of' isostas\r, the name ;tsell' Rein,= due to him. 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. From geological observations Nansen ~ has concluded that Norway has nearly reached the elevation it :l~acl before the ice invasion, and that the isosta.tic adjustment " has practically been completed in less tl-~an 18,000 years." This is the only quantitative est.in~ate we have, and it is shorter than isostasists would hale expected. But it nicest lie re~ne~nbered that geodetic measures show that the Hi~nala~ra Mountains remain in good * Loc. cit., p. 81.

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122 FI G1 RE OF THE EAT? TlI isostatic adjustment in spite of the enormous erosion.; which shows that the lag in the readjustment is not 'greats We realize now that the doctrine of' isostas,v has a. 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. It is strikingly analogous to the doctrine of the conservation of energy in physical phenomena. R:EFERENCES 1. Adams, Frank D., and Bancroft, T. Austin. On the amount of internal fric- tion developed in rock during deformation and on the relative plasticity of different kinds of rocks. J. Geol. 25: 592-637 (1917). 2. Adams, Frank D. An experimental contribution to the question of the depth of the zone of flow in the earth's crust. J. Geol. 20: 97-118 (1912). 3. Jeffreys, Harold. The earth: Its origin, history, and physical constitution. 2d ed. London, Cambridge Univ. Press, 1929. (Chap. NI, Theory of isostasy) . 4. Hall, James. Paleontology. Vol 3. Albany, 1859. (Natural history of New York, part 6), p. 50-85. 5. Airy, George B. On the computation of the eJect of the attraction of moun- tain masses' as disturbing the apparent astronomical latitude of sta- tions in geodetic surveys. Roy. Soc., Phil. Trans. 145: 101-104 (1855); Roy. Astron. Soc., Mo. Notices 16: 42-43 (1855-56). 6. Pratt, J. H. On the effect of local attraction upon the plumb line on the Eng- lish arc of the meridian between Dunnose and Burleigh Moor; and a method of computing its amount. Roy. Soc., Phil. Trans. 146: 31-52 (1856) . 7. Dutton, C. E. On some of the greater problems of physical geology. Phil. Soc. Wash., Bull. 11: 51-64 (1889); J. Wash. Acad. Sci. vol 15, no. 15: 359-369 (1925); Bull. Geod., no. 9 (1926); this Bulletin, Chap. NIII. 8. Reid, Harry F. Isostasy and earth movements. Geol. Soc. Am., Bull. 33: 317- 326 (1922). 9. Bowie, Wm. Notes on the Airy or " Roots of Mountains " theory. Science, n. s. 63: 371-374 (1926) . 10. Nansen, F. The earth's crust, its surface forms and isostatic adjustment. Avhand. utgitt TV Det Norske Videnskaps-Akad i Oslo. Mat. Naturv. Klasse '1927, no. 12, p. 91-102.