Evolution of an isoviscous, thermal/chemical boundary layer convection model with Ra=10⁵, H=0, ICD=0.02, and Ra_c/Ra=2 (this nominal value assumes reference crustal and mantle densities of 2800 and 3300 kg m^{− 3}, a non‐adiabatic temperature drop of 2500 K and a thermal expansion coefficient of 3 × 10^{− 5} K^{− 1}). (a) Isotherms over composition shade plots for the full model domain (grey shading indicates crust, and isotherms span the hot two‐thirds of the thermal field). (b) Isotherms over composition shade plots for the upper 0.2 of the model domain (isotherms span the cold half of the thermal field). (c) Non‐dimensional surface heat‐flux profiles (the ‘final’ profile is from the last time frame in (a); the ‘initial’ profile is just after model start time).