Temporal variations in the convective style of planetary mantles

Investigations of mantle convection with temperature- and strain-rate-dependent viscosity have shown the existence of fundamentally different convective styles: By varying e.g. the Rayleigh number, the viscosity contrast or the strain-rate dependency of viscosity, the planform of convection in the asymptotic stationary state changes from the so-called stagnant lid regime to an episodic behaviour and further to a state characterised by a permanently mobilised surface. Our studies suggest that this transition may not only be induced by a change of parameters but also occurs temporally for fixed parameters. We have in fact observed convective systems in the stagnant lid regime that show isolated events of surface mobilisation occurring out of a thermally equilibrated state. We use a 3D numerical mantle convection model to investigate mantle convection and surface dynamics as a coupled fluid dynamical system. Our studies focus on the existence of a transitional regime in which temporal variations between the stagnant lid and the episodic regime occur. We were able to deduce a mobilisation criterion that describes the stability of the stagnant surface, thus allowing for a quantitative analysis of the transition to a (temporarily) mobilised surface. This criterion is also suitable to predict the occurrence of surface mobilisation events.