The nature of the Mohorovicic discontinuity

Abstract

New evidence for the composition of the silicate mantle of the primary meteorite body suggests that a considerable portion of the Earth's upper mantle may well be composed of silicate material of basaltic composition. Consequently the Mohorovicic discontinuity may not separate basaltic crustal material from ultrabasic upper‐mantle material but rather mark a zone in which basaltic material is transforming into a high‐pressure modification possibly represented by the rock‐type eclogite. Estimates of the temperatures and pressures at the average levels of the Mohorovicic discontinuity under oceans and continents give two points on the ‘equilibrium line’ for the transformation basalt → eclogite which is roughly given by the relationship p = 21.80 — 488 (p in bars, 9 in °C). The geophysical consequences of this hypothesis are far reaching. It removes difficulties arising from the observed equivalence of oceanic and continental heat flows, provides satisfactory ‘parent magma’ from which the granitic and basaltic zones in the crust have differentiated, and suggests a mechanism for elevating or depressing regions of the Earth's surface simply by increasing or decreasing the temperature at the level of the Mohorovicic discontinuity beneath that region.