The interaction of plume heads with compositional discontinuities in the Earth's mantle

The effects of compositional discontinuities of density and viscosity in the Earth's mantle on the ascent of mantle plume heads is studied using a boundary integral numerical technique. Three specific problems are considered: 1) a plume head rising away from a deformable interface, 2) a plume head passing through an interface, and 3) a plume head approaching the surface of the Earth. For the case of a plume attached to a free-surface, the calculated time-dependent plume shapes are compared with experimental results. Two principal modes of plume head deformation are observed: plume head elongation or the formation of a cavity inside the plume head. If the plume head moves from a high-to low-viscosity region, the plume head becomes significantly elongated and, for the viscosity contrasts thought to exist in the Earth, could extend from the 670km discontinuity to the surface. As the plume head approaches the surface and spreads, the dynamic topography becomes plateau-shaped. The largest stresses are produced in the early stages of plume spreading when the plume head is still nearly spherical, and the surface expression of these stresses is likely to be dominated by radial extension. -from Authors