Mantle Convection and Viscoelasticity

Abstract

The ongoing revolution in the Earth sciences, which began more than twenty years ago, was originally based upon the increasingly widespread acceptance of the idea that continental masses have moved horizontally with respect to one another throughout geological time. This hypothesis of continental drift, or at least the form of the hypothesis that came to be called seafloor spreading, now serves as the basic paradigm for the organization of most geological and geophysical research. At the center of this guiding principle is the recognition that the solid outer shell of the planet-its iron-magnesium silicate mantle, which occupies roughly half Earth's radial extent-must be able to deform as a viscous fluid when it is subjected to an applied shear stress over geological intervals of time. To the extent that this rheological ansatz is correct, it is clear that a thermally induced convective circulation must appear in the mantle in response to a radial temperature gradient that is sufficiently in excess of adiabatic. The observed spreading of the seafloor away from hot mid-oceanic ridges is presumably a surface manifestation of such deep­ seated mantle convection. Likewise, the deep ocean trenches are under­ stood to be regions where cold surface material returns to the mantle to complete the circulation. These and other aspects of the pattern of surface motions associated with mantIe convection have been described kinemati­ cally within the framework of a set of ideas that has come to be called plate tectonics. The development of this set of ideas has consummated the revolution at a descriptive level and has delivered as its main product a clear view of the velocity field of material at the Earth's surface at the present epoch of geological time. In so doing, it has also contributed in an