Evolution of the mantle temperature (a) and viscosity (b) for a planet with Earth-like composition (Mg/Si = 1.2, 8 wt% FeO) in the PT- and SL-cases as a function of pressure (GPa), ranging from the surface (0 GPa) to the core–mantle boundary (138 GPa, 2891 km). Profiles are shown for the initial condition (t = 0 Gyr), current age of the Earth (t = 4.55 Gyr), and for the estimated lifetime of the Sun (t = 10.0 Gyr). Independently estimated profiles for the present-day Earth are shown as black lines (Stacey & Davis 2008).