Plate tectonics: a phase a planet goes through?

Stagnant lid convection is generally identified as an end-member state of planetary evolution, applicable to old, cold worlds with thick lithosphere and sluggishly convecting interiors which are unable to generate sufficient stress to break the thick immobile lid. However, recent results have shown stagnant lid regimes can also occur forvery hot planets; here high internal temperatures results in low mantle viscosities, and the stress coupling between the vigorously convecting mantle and plates breaks down, leading to stagnation of a thick lithosphere. The implication of these two stagnant end-members is that plate tectonic may be an impermanent tectonic regime in planetary evolution, between two stagnant lid extremes. To test this hypothesis we ran a suite of convection simulations over a range of basal temperatures and internal heating rates, and show that for high ratios of internal to basal heating, applicable to the Hadean, stagnant lid convection is the stable tectonic regime. We also ran a number of evolutionary models, where the basal temperatures and internal heat production decay through time. While sensitive to the starting state, the simulations show an evolution from stagnant lid convection, to an episodic subduction regime, through to continuous plate tectonics, and finally to a cold stagnant lid mode; implying plate tectonics is a transient phase in planetary evolution.