a layer of low hardness, strength, and viscosity in the earth’s upper mantle. It is identical to Gutenberg’s layer. It is found at depths of about 100 km under the continents and about 50 km under the ocean floor; the lower boundary occurs at 250 to 350 km. The layer may not be continuous. Seismic studies have shown that the rate of propagation of transverse and possibly longitudinal seismic waves within the asthenosphere is somewhat slower than in the covering and underlying layers of the upper mantle. The viscosity of the substance of the asthenosphere is 1019 to 1023 poises, whereas below and above the boundaries of the asthenosphere it is at least 1023 poises. It is conjectured that there is a slow overflow of masses in a horizontal direction within the asthenosphere because of the low yield point. The overflow is caused by uneven stress from the earth’s crust.
The presence of the asthenosphere is explained by the high geothermal gradient, the high temperature of the substance of the asthenosphere (close to the melting point), and processes of relaxation. Volcanic materials usually originate in the asthenosphere, and the subcrustal masses that accomany the main tectonic processes are displaced. The term “asthenosphere” was introduced in 1914 by the American geologist J. Barrell.
V. A. MAGNITSKII