Major tectonic phenomena and the hypothesis of convection currents in the earth

Abstract

The cooling of the earth in its relation to the formation of convection currents is a subject which has long occupied the attention of geologists and geophysicists. Jeffreys, Holmes, Schwinner, Pekeris and Griggs have all studied the subject, which is one of the major problems of the history of the earth. It is clear that such currents, if they were formed, must have subjected the earth's crust to drag-forces which may hold the explanation of tectonic phenomena. Certain other phenomena may likewise thus be accounted for. The hypothesis of convection currents, besides assuming that the earth is indeed cooling, is based on the supposition that below the rigid crust there exists a plastic and homogeneous layer sufficiently thick to allow of the formation of such currents. There can be no doubt that at a depth of a few tens of kilometres below the surface, the material has a plastic character, since otherwise it would be impossible to explain isostasy in areas where erosion has removed great surface masses or sedimentation has added them; for the readjustment of the isostatic balance after these changes of load we must assume mass movements at depths less than 50 kilometres, and this assumption requires the presence of currents flowing under the effect of relatively small pressure differences. As we shall presently argue, the lower limit of the resulting shear-stress cannot be much more than 10 kg/cm ² . The question is, however, whether the layer below the rigid crust is homogeneous to a sufficient depth. The