Toward Understanding the Conditions Required for Plate Tectonics to Occur on Earth-

Introduction. Previous studies of initiation of plate tectonics were concerned mostly with the present-day Earth where plate tectonics is actively occurring [1-3]. Understanding of how plate tectonics starts and can be sustained for a long time on an Earth-like planet is a different problem. Investigation of this problem began only recently [4-6]. This problem is also important for understanding evolution and tectonics of other terrestrial planets in the Solar System. These planets do not have plate tectonics at present but could have had plate tectonics in the past. Planets beyond the Solar System are of interest as well from the perspective of how life begins: the biosphere is strongly affected by plate tectonics and may not evolve to higher forms without plate tectonics. In particular, plate tectonics is known to play a key role in the long-term stability of planetary climate. These problems require some level of predictive power which in turn requires a better insight into the physical mechanisms of plate tectonics. The purpose of this study is to investigate initiation of plate tectonics on a most likely model of a terrestrial planet – with predominantly internal heat sources and in the strongly time-dependent regime of stagnant lid convection. The approach. The approach to understanding the conditions for initiation of subduction is as follows. First, the scaling laws for the stresses in the lithosphere are derived. Second, the critical yield stress for breaking the lithosphere is estimated based on the assumption that in the presence of yield stress, subduction starts when the effective viscosity contrast across the lithosphere drops below some critical value. This assumption is supported by previous studies and is also tested below for the particular problem considered here. Finally, the scaling laws for initiation of subduction are derived. Stresses in the lid. The stresses in the lithosphere increase gradually from the interior of the convective cell to-wards the surface of the convective layer and undergoes a large jump in a thin stress boundary layer near the surface (Fig. 1). This rather peculiar stress profile is characterized by three basic stress scales: (i) Relatively small stresses in the convective interiors. These are created by viscous shearing caused by sinking plumes: