HOW ARE MOUNTAINS FORMED?
By Igor REZANOV, Dr. Sc. (Geol. & Mineral.), S. I. Vavilov Institute of the History of Natural Science and Technology, Russian Academy of Sciences UNESCO has proclaimed 2002 a year of mountains. For centuries it has been an intriguing question to us: How did these giants come into being to form ridges thousands of kilometers long? We cannot tell for certain yet. Articles in this rubric reflect the opinion of the author. - Ed. Pages. 27 Back in the middle of the 19th century two Englishmen, George Airie (corresponding member of the St. Petersburg Academy of Sciences) and Joseph Pratt, proceeding from geodetic measurements in India, formulated the principle of isostasy (equal balance, equilibrium) whereby the earth's crust kind of "floats" on the heavier and denser mantle in compliance with Archimedes' principle. In the 20th century this idea was confirmed by seismic methods: the light earth crust was found to be thicker under high-mountain ridges. In the 1980s Russian geo-physicists built a unique seismic profile across the Pamir and the Hindu Kush Mountains. It showed that under the highland the earth's crust thickness reached 75 km, while it was only 40 km under the Hindustani lowland to the south. The earth's crust is also found to be thicker under the Caucasian Range, under the Alps and other high-mountain massifs. By contrast, the crust is much thinner under the negative relief features, that is seas and oceans. It may be just a mere 5 to 10 kilometers thick under deep-see troughs. Before elucidating the mountain-building mechanism, we should first find out what caused the earth's crust to become thicker under the mountain masses. We know it from geological history: high-mountain ridges often appear where there had been major down-warpings of the crust, a process accompanied by the accumulation of thick (10 - 15 km) strata of sedimental rock - what we call geosynclines. We should find out why these masses started rising in the Neogene- Quaternary pe ... Read more
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Sweden Online · 2047 days ago 0 595
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14.09.2018 (2047 days ago)
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