Key properties of the Earth’s mantle through time. (a) The mantle and core gradually cool over time. (b) As the core and mantle cool, the heat flow across the upper mantle and the CMB and the heat flow due to radiogenics in the core and mantle decrease. (c) Both the upper- and lower-mantle boundaries grow deeper as the core–mantle system cools and the mantle solidifies downward. (d) The lower-mantle TBL is initially less viscous than the upper-mantle TBL. As water subducts into the cooling mantle, both the upper- and lower-mantle viscosities gradually increase. (e) As the interior cools, the upper-mantle melt fraction rapidly decreases. (f) Similarly, the melt flux from the mantle also decreases as the mantle cools. On all plots where applicable, dots represent measurements of PIE (4.5 Gyr) properties with uncertainties.