On the Upper Mantle

Abstract

Publisher Summary The term “mantle” signifies the region of the earth that extends from the Mohorovicic discontinuity (lower boundary of the crustal layers) down to the central core. The “upper mantle” delimits the outer part of the earth's mantle and can be regarded as a spheroidal shell lying from the Mohorovicic discontinuity to a depth of nearly 950–1,000 km. However, this is only a rough estimate without any pretensions of accuracy and serves to distinguish the outer part of the earth, nearly 1,000 km thick. It is obvious, therefore, that the crustal layers are included in a wide program of research, which aims at clearing up the basic problems dealing with the physicochemical compositions of the upper mantle. Only a good knowledge of crustal features will facilitate research on the region immediately below. Among the fundamental problems that need a more complete solution, apart from the earth's crust, should include those relating to the constitution of the asthenosphere as a channel guide of seismic energy and the so-called “20” discontinuity.” As related problems, this chapter mentions: Origin of deep earthquakes, correlations between middle-depth earthquakes and active volcanism, “mixed” regions, possibility of independent oscillations of the upper mantle, convection currents, continental drift, possible expansion of the earth, secular and non-secular phenomena, periodical and non-periodical variations of the apparent vertical, magnetic and gravitational anomalies, heat flux, etc. Research on the upper mantle will have made substantial progress when its chemical composition as well as its behavior with regard to elasticity, viscosity, plasticity, and internal friction is known. Some attempts in this direction have already been made. A short report of these is given here to indicate which course should be followed after having clarified the knowledge of physical features.