Whole mantle convection and the thermal evolution of the earth

Abstract

Three recent results in the fields of geophysics and geochemistry have combined to rejuvenate the study of the Earth's thermal history. The first, and perhaps most important, of these is the new understanding of the planetary accretion process which has been realized as a consequence of work by V.S. Safronov and others. This work shows that the terrestrial planets accrete on a time scale of ∼10 8 y by the accumulation of planetesimal sized objects and that they probably form sufficiently hot to induce ‘immediate’ differentiation of iron from silicates. The second important recent contribution has been from geochemistry and concerns the net complement of radioactivity of the Earth as a whole. Although the net complement of radioactive elements has usually been inferred on the basis of the assumption of equilibrium with the surface heat flow, a priori analysis based upon elemental condensation temperatures in the primitive solar nebula yields a model in which the net radioactivity is at least a factor of two less than would be required to explain the mean surface heat flow. There are good reasons to believe that this model is valid for the Earth. The third result has been the successful development of a technique for incorporating the influence of convective heat transfer in a thermal evolution model which does not require explicit solution of the dynamical equations. This paper begins with an analysis of the mantle convective circulation which is required to understand plate tectonics and continental drift, an analysis which presents the arguments for and against the crucial notion that the main circulation has a depth scale equal to the mantle thickness. Using the idea of ‘parameterized’ convection, the radial heat transfer by the mantle convective circulation is included in a thermal history model of the Earth. This model turns out to contain within it an interesting explanation of the fact that no crustal rocks with ages in excess of 3.8 Ga have ever been found, an observation that has heretofore proven enigmatic.