Raphael Pumpelly on secular rock disintegration |
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ContentsPumpelly's discussion of the role of secular disintegration in the origin of rock basins Pumpelly on the limited erosive power of continental glaciers Conclusion References & Bibliography Related Sites |
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About Raphael PumpellyRaphael Pumpelly (1837-1923), American geologist, explorer, and author, was born in Owego, NY. When he was seventeen, it was foretold that he would "make long journeys by sea and through dangerous lands, but that he would escape, unless taken unawares from behind by some one of his companions."In 1854 Pumpelly travelled to Europe with his mother, and he learned French, German, and Italian. In 1856 he attended the Royal Mining Academy at Freiburg, upon the advice of Emil Noeggerath, who encouraged his interest in geology. Pumpelly was employed as a mining engineer in Arizona in 1860. This was a perilous time. In 1861 he travelled to California, and was commissioned by the imperial Japanese government to make official surveys in Japan (1861-63) of Yezo (later Hokkaido), exploring for minerals. He surveyed the coal fields of China (1864) and made the first extensive survey (1865) of the Gobi. He journeyed across Siberia by sleigh. Later, he related these adventures in his book "Across America and Asia". Pumpelly found evidence that central Asia had once been occupied by vast inland seas or lakes, which had slowly diminished in size, leaving behind the Aral Sea and numerous small lakes. Their shorelines were still visible. In his account of his journey across the Gobi desert, he wrote [Ibid., p. 381]: At no very remote geological time this region was the bed of an extensive sea, which during one period of its existance formed part of an ocean, extending over much of Siberia, Tartary, western Asia, and eastern Europe, connecting the Polar sea with the waters of the Caspian and Mediterranean. The elevation of the plateau was, perhaps, the first step in the series of natural causes by which the area of this great body of water was gradually diminished till it was replaced by dry land.He initially interpreted the extensive mantle of loess in China as the deposit from a series of great lakes, which had disappeared. The thickness of the loess in China exceeds 500 feet in some locations. Pumpelly later accepted the theory proposed in 1877 by F. von Richthofen that the loess of China and Central Asia had been distributed by winds rather than water, but he noted: "The one weak point of Richthofen's theory is the evident inadequacy of the current disintegration as a source of material." [Pumpelly 1879 p. 135] Pumpelly was appointed professor of mining at Harvard (1866-75), but he was more interested in his researches on the iron and copper deposits in the region around Lake Superior, for which he used microscopes and thin sections for petrographic study. His major report was published in 1873. Pumpelly sensed the increasing importance of steel, and advised investors to search for iron rather than gold. Some who followed his advice made fortunes. He was state geologist of Missouri in 1871. In 1884 he was in charge of the New England section of the U.S. Geological Survey. Pumpelly was in charge of the mineral industries survey for the 10th U.S. census, and he served on the Northern Transcontinental Survey. He conducted (1903-4) explorations in Turkestan, seeking traces of past civilizations, financed by the Carnegie Institution. He wrote a number of reports on his findings. In 1905 he was elected president of the Geological Society of America. Rollo Walter Brown, in his biographical essay "Cosmic Prospector", p. 200, gives this description: He was tall of stature. Throughout his adult life he wore an incomparable beard that reached well down to his waist-a "golden" beard until it later became the gray beard of a patriarch. Though his face expressed a dignified kindness, his eyes seemed always to be looking far. He strode about with an easy self-forgetfulness, as though he were at home in the universe. No one could escape feeling that here was an extraordinary man. No one could escape feeling that this man lived some sweeping career of extraordinary quality.The mineral pumpellyite, a low temperature metamorphic mineral found in the iron and copper ore region around Lake Superior was named after him in 1925. Pumpelly's discussion of the role of secular disintegration in the origin of rock basinsPumpelly believed that the erosive power of ice sheets was not sufficient to explain the fiords and lake basins formed in rock; he proposed a mechanism of "secular disintegration" by which, over long ages, continental rocks underwent deep decay, forming loose, disintegrated material that was later easily eroded away by ice sheets, forming lake basins. The disintegrated material, he thought, was redeposited as drift by the ice sheets, or its fine fraction transported and deposited as loess by winds. He wrote [Pumpelly 1879 p. 139-141]:There are few problems in dynamical geology that have been considered more difficult to solve than the origin of rock-basins. Wherever my route between the great wall of China and the Siberian frontier lay through regions of crystalline rocks, I found that one of the characteristic features of the surface was the prevalence of basin-shaped depressions of all sizes hollowed out of the rock. In other words, if filled with water they would have been lakes without outlets and with unbroken sides of rock. In some exceptional instances it was clear that systems of intersecting dykes had been less acted upon by the basin-making process than the intervening rocks, and the basins were formed in these last. Pumpelly on the limited erosive power of continental glaciersThe following paragraph from the same paper [Pumpelly 1879 p. 141] shows Pumpelly considered glacial action ineffective to explain the extent of the distribution of drift material or its considerable thickness in many areas.In the warmly contested question as to the capacity of continental glaciers in excavating solid rock, the great extent of "till" and of the modified drift have been adduced as an evidence of this power. It seems to me rather that the abundance of the drift material may be equally well taken as in great part but not wholly a measure of the effect of long-continued and undisturbed disintegration and of the transporting power of ice, and that a period of glaciation may be, as Rutimeyer insists, a period of comparative rest as regards excavation of hard rock. ConclusionPumpelly's ideas about the evidence for a mechanism of in situ disintegration of rock, to considerable depths, and the subsequent erosion of unconsolidated material to excavate rock basins, to form lakes, were soundly based. Pumpelly also correctly understood the limited capacity of former ice sheets to break up the rocks beneath them. His theory did not go far enough, however; his thinking about the possible role of secular disintegration apparently was severely constrained by uniformitarianism, and the glacial interpretation of the drift championed by Louis Agassiz. But he can be credited with conceiving and proposing an embryonic form of the disintegration theory.The drift can be interpreted without invoking ice ages; a disintegration mechanism exists in the environment of rapid vertical movements of the crust, accompanied by unloading as overburden was removed. Erosion by currents generated as the flood waters flowed away from centers of uplift, removed the disintegration product, promoting disintegration by exposing fresh rock surfaces to decreased vertical stress. References & Bibliography
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