Formation of steep‐sided topography from compositionally distinct dense material at the base of the mantle

In this study we use 3‐D, two‐layer, thermochemical convection experiments to explore the development of a thin, compositionally distinct, dense layer at the base of the mantle. We investigate the conditions under which steep‐sided, flat‐topped topography will form on an initially ubiquitous layer as a result of the convecting mantle. Previous authors found that discrete, steep‐sided piles of material form when compressibility, temperature‐dependent viscosity, or a large viscosity increase in the lower layer is assumed. We observe, however, that this topographic style develops in calculations of incompressible convection with layers of constant viscosity. Experiments involving simple convection planforms are used to systematically investigate the factors which encourage or inhibit this topographic style. The shape of the topography that develops on the density interface can be linked to the convection coupling style between the two layers: viscous coupling leads to discrete, steep‐sided piles and thermal coupling to a ubiquitous layer with cusp‐like peaks. The dependence of the topographic style on the Rayleigh number, density ratio, and volume ratio can largely be explained by considering their effect on the convection coupling style. Consequently, we find that it is crucial for the lower layer to be thin compared with the overlying convection wavelength and have a density contrast of the order 1% in order for discrete, steep‐sided piles to develop.