Initiation of subduction by small-scale convection

The possibility of initiation of subduction by sublithospheric small-scale convection is investigated with the help of Fowler's theory, numerical constraints on the lithospheric stresses generated by convection, and direct numerical simulations of subduction. The derived scaling laws show that the critical yield stress for initiation of subduction by small-scale convection is of the order of 30 MPa. This value is comparable with the critical yield stress required for initiation of subduction by plate forces and with the estimates of stresses in the subduction zone. This implies that small-scale convection can play an important role in the initiation of subduction on the present-day Earth. This also implies that small-scale convection can initiate subduction in the absence of already occurring plate tectonics and thus can be a trigger mechanism for plate tectonics on the early Earth and other terrestrial planets. The critical yield stress for initiation of subduction by either mechanism is at least one order of magnitude smaller than the estimates of the lithospheric strength suggested by laboratory experiments on rock deformation. Such a low lithospheric strength is most likely caused by a relatively high abundance of water in the interior of the Earth. Since the critical yield stress depends only weakly on planetary parameters, water seems to be the key condition required for plate tectonics to occur on a terrestrial planet.