Mantle Convection: Rayleigh-Bénard Plus Plumes and Plates? Or Pekeris with Intrinsic Smaller Scales?

There are three distinct mechanisms which might give rise to convection in the Earth's mantle: Rayleigh-Bénard takes place if the radial superadiabatic gradient exceeds a critical value, for a fluid heated from below. Convection may also be driven by internal heat sources or secular cooling. Convection grows from unstable perturbations to a static equilibrium. No equilibrium is possible if horizontal temperature gradients are present; convection will start if there is any horizontal thermal gradient no matter how small. There is no critical Rayleigh number. This type of convection was first discussed by Pekeris (1935) in the context of an irregular distribution of continental crust on the surface.