The Problem of the Mantle-Crust Mix: Lateral Inhomogeneity in the Uppermost Part of the Earth's Mantle

Abstract

Publisher Summary The material with compressional wave velocities of 7.4 to 7.7 km/sec, existing beneath many active tectonic belts, is postulated to be a mixture of mantle- and crustal-type rocks. The belts with such velocities include mid-ocean ridges, island arcs, and rift valleys. The uplift and lateral extension, as well as the high heat flow and volcanism generally characteristic of the active belts, indicate expansion and an upwelling of convection currents in the mantle, thus giving lateral inhomogeneities in the uppermost mantle. The mantle-crust mix is postulated to comprise a mixture of eclogite and basalt in the phase-transformation zone, which may be of considerable thickness. This chapter discusses some of the evidence to date for the possible lateral inhomogeneity of the Earth's uppermost mantle and gives possible explanations of this inhomogeneity. The topic is speculative and much more data is needed before firm conclusions can be reached. It is hoped, however, that this chapter may stimulate more thought and discussion—and, above all, the gathering of more experimental data —on the subject.