Did Light Crust Hamper Plate Motion?

DAVIES (1992) suggested that plate tectonics was not operative for the early Earth, because hotter mantle produced thick, buoyant oceanic crust, which resisted subduction. We re-examine this possibility using a simple 2-D model of convection heated from within, based on the boundary layer theory. We found that a range of mantle temperatures existed in which the plate velocity decreases with increasing mantle temperature owing to the formation of thick buoyant crust, as suggested by DAVIES (1992). However, when the mantle temperature increases further still, the plate velocity again increases with increasing mantle temperature, in spite of the thick crust. This is because hot mantle produces higher density crust (such as komatiitic crust). Thus, the plate velocity and heat flux have minimum values at a certain mantle temperature, but plate motion does not stop. Hence, our result shows that the existence of an early hot mantle and the formation of thick crust does not necessarily prevent subduction. We also found that the heat flux is insensitive to changes in the mantle temperature, while it is around the minimum value.