We have investigated the effects of mantle phase‐transitions on the Earth's thermal evolution by numerical modelling. Our results show that the tendency of layering grows with increasing vigor of convection. A self‐consistent model which couples the convective dynamics of the mantle to the secular cooling of the core is constructed. For a temperature‐dependent effective viscosity, we found that the thermal history of the Earth is characterized by two timescales, one is associated with layered convection in the early period, which is followed by a transition period with dramatic overturns. As the Rayleigh number decreases sufficiently low, whole mantle convection prevails and a faster cooling timescale ensues.

The effects of multiple phase‐transitions on the dynamics of the Venusian mantle have been investigated with a two‐dimensional finite‐element method. A depth‐dependent thermal expansivity has been employed in a purely basal heating configuration with an aspect‐ratio of four. The addition of the olivine → spinel transition promotes layered convection more so than models with a single spinel → perovskite phase‐change. The shifts in the phase‐transition depths and the presence of a conductive lid increase the tendency of the Venusian mantle circulation to flow through the transition zone. The potentially lower Rayleigh number in Venus from lack of volatiles will also enhance whole mantle convection. The style of convection in Venus may have changed from layered convection in the past to whole‐mantle convection today. This transition might have been responsible for the major resurfacing event on Venus.

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Numerical models of mantle convection using the Boussinesq, extended Bousinessq and anelastic‐liquid approximation are compared. For steady state solutions there is good quantitative agreement between the results if they are scaled in a proper way. Time‐dependent extended Boussinesq and anelastic‐liquid flows show only qualitative agreement, the main difference being a distortion of timescale. Compressibility induces an asymmetry in the structure of upper and lower boundary layers that cannot be observed in Boussinesq fluids.