The Onset of Plate tectonics in a Compositionally Stratified mantle

We investigate the geodynamical evolution of the Earth after the crystallization of the magma ocean. This situation is resembled by a scenario, displaying a compositionally stratified mantle being heated from below and within, while also cooled from above. The evolution of the system is dependent on initial conditions and further strongly on the assumed rheology. We have employed numerical models in 2D and 3d cartesian , as well as in spherical geometry to explore possible evolutionary paths of the Early Earth. A common feature of all models featuring a strongly temperature dependent viscosity is the self-organized generation of separately convecting layers. Such layers reduce the heat flow from the core and also delay instabilities from the top layer. Thus this type of model favors a late onset of l arge scale plate tectonics, possibly as late as 2Gyears after formation. A typical feature of this scenario is the intermittent breakdown of layers in the mantle leading to strong pulses of the heat flow. A different picture emerges for a more pressure dominated rheology. Even a slightly stable compositional stratification is sufficient to suppress global convection. Instead the surface is destabilized and small scale plate tectonics sets in immediately. We further investigated the effect of fractional crystallization and a resulting unstable compositional gradient. In all models we observed an Rayleigh-Taylor instability and subseuqtly a long term evolution being very similar to the scenario with the initially stable gradient. According to these results both, a fractional or homogeneous crystallization would lead to a similar long term evolution of the mantle.