Convection with decaying heat sources: constant viscosity

A series of two-dimensional numerical convection models with time-dependent radioactive heating rates are studied in order to investigate the role of convective adjustment in the thermal evolution of the mantle. For uniform viscosity models it is found that two time-scales are important. Some disturbances act on an overturn time-scale, which is on the order of 100 Myr for the upper mantle. This is typical of processes affecting only the thermal boundary layers. Convective adjustment to decaying heat sources, which is a process involving changes in the temperature of the layer as a whole, acts on a time-scale that is long compared to an overturn time (∼1 × 109 yr for the upper mantle). This results in large time lags between surface heat loss and instantaneous heat production.