The efficiency of plate tectonics and nonequilibrium dynamical evolution of planetary mantles
Consideration of the structure of dynamical equilibria in terrestrial planets using simplified descriptions of the relevant heat transport processes (rigid-lid convection, plate tectonics, and heat pipe volcanism) reveals that if the efficiency of plate tectonic heat transport decreases at higher mantle temperature, then it cannot govern quasi-equilibrium dynamical evolution, and the system is always evolving away from the plate tectonic regime. A planet on which plate tectonics is less efficient at higher temperature stays in heat pipe mode longer, spends less time undergoing plate tectonics, and has a low and ever-decreasing Urey number during this phase. These conclusions are based solely on the structure of the equilibria in a system with less efficient plate tectonics in the past and are independent of the mechanisms leading to this behavior. Commonly used quasi-equilibrium approaches to planetary thermal evolution are likely not valid for planets in which heat transport becomes less efficient at higher temperature.