by Lev KRASNY, Corresponding Member, USSR Academy of Sciences, department head at the USSR Geological Research Institute, Academician Mikhail SADOVSKY, Honorary Director of O. Schmidt Earth Physics Institute, USSR Academy of Sciences The earth's crust was once thought to be firm and unmoving. It is not for nothing that the Latin continentis means "stable, steady, firm". In the 20th century geology, an essentially descriptive science, has accumulated an enormous body of facts. New methods of study (e.g., geophysical) have emerged. Employing seismic and blast waves, and also thermal, electric, magnetic and gravitational fields, geologists have learned to see deep into the Earth's interior. The 1960s of this century saw a geologic revolution. Scientists found that the crust of continents and of the ocean floor differs both in thickness and in composition. This gave rise to the theory of plate tectonics of lithospheric plate movement. The lithosphere was assumed to consist of six relatively rigid plates bounded by seismic belts. Most of the plates occupy entire continents and adjacent areas of the ocean floor as far as the mid-oceanic ridges where the lithosphere spreads causing rifts to open and new oceanic crust to form. Thus identified were the North-and South- American plates with adjacent areas of the Atlantic, the African plate with portions of the Indian and Atlantic oceans, and other plates. Advocates of this theory considered it to be almost infallible and were convinced that it described the Earth's evolution in its totality and accounted for all its fundamental laws. According to the authors of this article, however, certain patterns of occurrence of mineral resources can be explained more convincingly from the standpoint of the geoblock conception. That continental and oceanic crustal structures are different cannot be denied. Indeed, the continental crust is normally thick (up to 70 km) and is mainly composed of vast fields and chains of granitoids, w ... Read more

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Точикистон Онлайн
Душанбе, Tajikistan
15.09.2018 (1648 days ago)
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