Time dependence of mantle plumes: Some simple theory

Dynamical calculations indicate that plate tectonics are likely to have a major effect on the initiation of plumes from the basal boundary layer in the mantle and their subsequent evolution. Plume start because the boundary layer is hotter than the overlying mantle and hence unstable to perturbations. The growth rate of perturbations is too weakly dependent on wavelength to control the geographic distribution of plumes. Instead, the distribution of plumes is strongly influenced by the shape of large perturbations associated with slabs and surface plates. Plumes evolve because downward return flow is needed to replace the material tapped from the boundary layer. This flow downwarps the top of the boundary layer several hundred kilometers from the plume and thus restricts further flow of hot material toward an active plume. The geometry of this downwarp is strongly influenced by the initial relief of the top of the boundary layer and hence by plate processes. The actual history of plates and slabs is thus needed in numerical models for comparison with real data on plume fluxes.