Abstract
Unsteady thermal convection within the upper mantle is studied in connection with the hypothesis that a trigger to generate tectogenesis is the undulation of isotherm at the ocean-continent boundary area. It is emphasized that there are two types of convection which are different each other in physical conditions. Calculations are performed until the time of 200 million years from the start of the convective movement for the cases with kinematic viscosity ν=5×1021 and 1022cm2/sec. The maximum tangential stress at the upper boundary of the convective layer reaches 3×107dyn/cm2. It is unlikely that either fissure or fault is produced in the crust by such a convection.
