MANTLE CONVECTION

A wide range of geophysical and geochemical observations pertaining to convection in the earth's mantle and the dynamics of the tectonic plates is discussed. It is inferred that the dominant mode of mantle convection is a plate-scale flow and that the plates are an integral part of this flow. Upwelling buoyant plumes, that cause volcanic hotspots, are inferred to comprise a secondary model of convection arising from a relatively weak thermal boundary layer at the base, of the mantle. The balance of a large range of evidence weighs against the transition zone being a barrier to flow, the strongest evidence coming from seafloor topography and the gravity field. We infer a significant viscosity increase, by perhaps two to three orders of magnitude, through the depth of the mantle, with a large part of this increase occurring through the transition zone. This viscosity increase can account for the low velocities of hotspots relative to plates and the apparent lag between surface plate configuration and deep mantle structure, as probed by seismology and the gravity field. The viscosity increase also enhances the survival of chemical heterogeneities in the mantle and produces an increase in heterogeneity and mean residence time with depth. There may also be inefficient gravitational settling of old subducted oceanic crust near the bottom of the mantle, which might account for seismological complications in the D" layer at the bottom of the mantle and for trace elements exceeding bulk-earth concentrations in hotspot sources. With ridges sampling the top of the mantle and plumes sampling the bottom, these features offer explanations for the main geochemical characteristics of, and differences between, mid-ocean ridge basalts and oceanic island- (hotspot) basalts.