C. Grigné

Mantle convection in the Earth and Mars are dominated by large‐scale flow, and this can be due at least in part to the effect of thermally insulating continent‐like heterogeneities. Indeed, the presence of a finitely conducting lid on top of a convective isoviscous fluid at infinite Prandtl number induces the formation of a zone of hot upwelling centered beneath the lid and a large horizontal cellular circulation on each side of the lid. In a previous paper, this lateral cellular circulation was limited in size by the dimensions of the model. We now use very large aspect ratio models in order for this circulation to be unhampered. The large‐scale circulation consists of the constructive superposition of hot and cold plumes attracted to and expelled from, respectively, the conductive lid. The width of the cellular circulation increases with the Rayleigh number of the fluid, and we propose a scaling law to explain this expansion. Scaling laws for the velocity of the plumes and heat transfer by this large‐scale circulation are also developed. Although the model used is much simpler than actual planetary mantles, the driving mechanism for the large‐scale circulation must operate in a similar fashion. Important implications in terms of heat transfer can therefore be predicted.

A scaling law for the heat flux out of a convective fluid covered totally or partially by a finitely conducting lid is proposed. This scaling is constructed in order to quantify the heat transfer out of the Earth's mantle, taking into account the effect of the dichotomy between oceans and continents, which imposes heterogeneous thermal boundary conditions at the surface of the mantle. The effect of these heterogeneous boundary conditions is studied here using simple two‐dimensional models, with the mantle represented by an isoviscous fluid heated from below and continents represented by nondeformable lids of finite thermal conductivity set above the surface of the model. We use free‐slip boundary conditions under the oceanic and continental zones in order to study in an isolated way the possible thermal effect of continents, independently of all mechanical effect. A systematic study of the heat transfer as a function of the Rayleigh number of the fluid, of the width of the lid, and of its thermal properties is carried out. We show that estimates of continental lithosphere thickness imply a strong insulating effect from continents on mantle heat loss, at least locally. The heat flux below continents was low in the past and of the order of the present one if the continental thickness has remained broadly constant over the Earth's history.