23 - Thermal history

Summary

Preamble

A thermal history of the mantle can be calculated almost independently of the core. The logic for this is that core heat is carried up through the mantle by narrow, buoyant plumes that have only a weak interaction with the plate tectonic convection process that cools the mantle. The converse is not true. The core is cooled by loss of heat into a thermal boundary layer at the base of the mantle and so depends on the temperature difference between the core and the deep mantle, 100 to 200 km above the boundary, as well as on mantle rheology. The boundary layer must have developed, that is, the mantle must have cooled substantially, before significant core cooling could occur.

Mantle rheology also controls the cooling of the mantle itself, but it is a mutual control. Tozer (1972) drew attention to the fact that the strong dependence of viscosity on temperature (Eq. (10.27)) has a stabilizing effect on both. If the mantle were to become too cool and viscous to convect at the ‘normal’ rate, convection would slow until radioactive heating caught up. But this does not mean that the heat loss is in equilibrium with the heat source, because the source is not constant. With diminishing radiogenic heat, convection slows down and this means that the mantle is cooling, as is most convincingly demonstrated by a consideration of the heat balance equation (Eq. (23.14)).