by Anatoly NIKISHIN, Dr. Sc. (Geol. & Mineral.), Head of the Chair of Regional Geology and History of the Earth of the Geological Department of Moscow State University
Today we know reasonably well what the geological structure of our planet Earth is like. Owing to progress in Soviet (Russian) and American space programs, we have at our disposal detailed information on the terrestrial planets. Proceeding from it, we may try to understand how the Earth was formed, and look into some of the stages of its history.
First, let us recall what is known for certain about the structure of our planet. The hard Earth with a radius of 6,378 km consists of three chemical mantles, namely, a mostly iron core in the center (depth, 2,890-6,378 km), a silicate mantle formed of ultrabasic* magmatic rocks (35-2,890 km) and a crust formed of silicates with a composition ranging from basic to acid (0-35 km). On the surface, the lithosphere, there is a stone "mantle" of 50-300 km thick, hard contrary to the underlying asthenos-phere and, therefore, able to move in the horizontal. Besides, the bottom of this drifting "iceberg" passes approximately in the isotherm of 1,250°, i.e. deeper its heated substance may be in for convection.
The earth crust under the oceans and continents (together with shelves) is not uniform. In oceans its thickness is around 6 km, it has a basalt composition and is formed in the axial parts of mid-oceanic ridges, in areas of lithosphere plate shifting as the occurring cracks are filled with mantle melts. The continental crust has a typical thickness of 25-60 km and an andesitic** composition by and large. Besides, its upper part is acid (granitoid) and the lower part, basic (basalt). Its substance is formed by mantle components with the partial melting of basalts.
Nowadays the majority of specialists agree that the lithosphere plate tectonics* is active on the Earth. The foundations of this theory were formed by many geologists a ... Читать далее