Paleo-temperature data indicates that the Earth’s mantle did not cool at a constant rate over geologic time. The data are consistent with slow cooling from 3.8 to ~2.5 billion years ago with a transition to more rapid cooling extending to the present. This has been suggested to indicate a change in global tectonics from a single plate to a plate tectonic mode. However, a tectonic change may not be necessary. Multi-stage cooling can result from deep water cycling coupled to thermal mantle convection. Melting and volcanism removes water from the mantle (degassing). Dehydration tends to stiffens the mantle, which slows convective vigor and plate velocities causing mantle heating. An increase in temperature tends to lower mantle viscosity which acts to increase plate velocities provided that mantle viscosity offers resistance to plate motion. If these two tendencies are in balance, then mantle cooling can be weak. If the balance is broken, by a switch to net mantle rehydration, then the mantle can cool more rapidly. We use coupled water cycling and mantle convection models to test the viability of this hypothesis. Within model and data uncertainty, the hypothesis that deep water cycling can lead to a multi-stage Earth cooling is consistent with present day and paleo data constraints on mantle cooling. It is also consistent with constraints that indicate a change from net mantle dehydration to rehydration over the Earth’s geologic evolution. Probability distributions, for successful models, indicate that plate and plate margin strength play a relatively minor role for resisting plate motions relative to the resistance from interior mantle viscosity.

Mantle convection is driven by the transport of heat from a planetary interior. This heat may come from the internal energy of the mantle or may come from the core beneath and in general there will be contributions from both. Past investigations of mixed-mode heating have revealed unusual behavior that confounds our intuition based on boundary layer theory applied to end-member cases. In particular, increased internal heating can cause a decrease in convective velocity despite an increase in surface heat flow. We investigate this behavior using numerical experiments and develop a scaling for velocity in the mixed-heating case. We identify a planform transition that impacts both heat flux and convective velocities. More significantly, we demonstrate that increased internal heating leads not only to a decrease in internal velocities but also a decrease in the velocity of the upper thermal boundary layer (a model analog of the Earth’s lithosphere). This behavior is connected to boundary layer interactions and is independent of any partic- ular rheological assumptions. In simulations with a temperature-dependent viscosity and a finite yield stress, increased internal heating does not cause an absolute decrease in surface velocity but does cause a decrease in sur- face velocity relative to the purely bottom or internally heated cases as well as a transition to rigid-lid behavior at high heating rates. The differences between a mixed system and end-member cases have implications for under- standing the connection between plate tectonics and mantle convection and for planetary thermal history modeling.