Formation and growth of deep mantle plumes

Abstract

Summary. If whole mantle convection occurs in the Earth’s mantle, then the core-mantle boundary constitutes the lower boundary layer for mantle convection. This boundary layer appears to be unstable on a small scale, and thus may be a source of plumes of hot matter which penetrate the mantle and occasionally even the lithosphere (producing hot spots). A finite-amplitude numerical code is used to study the formation of such plumes and their growth through the mantle. The plumes are restricted to being two-dimensional sheets rather than cylinders. The initial conditions consist of a steadily convecting mantle, and plumes are produced by introducing a perturbation in the form of either a pulse or a steady stream of heat into the bottom of the mantle. Two main results are obtained: (1) A critical perturbation size has been found for a mantle with a Rayleigh number of lo’. Small perturbations produce plumes which fail to penetrate the mantle, and instead are swept up by the pre-existing convective pattern, while large perturbations succeed in penetrating the mantle and reaching the lithosphere. The critical perturbation size is shown empirically to be proportional to the effective bouyancy and to a factor related to the shape of the perturbation. A perturbed region 150km wide and 60 km deep should produce a successful plume when the temperature perturbation is 200K or more. (2) Deep mantle plumes appear to require on the order of 50-100Myr to penetrate the mantle; episodic plumes on shorter time-scales appear unlikely. A similar time is required for plumes forming in an initially static, uniform temperature mantle.