Strong dynamical effects arising from lateral viscosity variations are found near the heads of fast-upwelling plumes approaching the top boundary for unsteady 3-D compressible convection in a spherical geometry at an average Rayleigh number of O(106). The temperatures at the center of fast-upwelling plume are very hot due to the positive feedback interaction between viscous dissipation and temperature-dependence of the viscosity. Although the laterally averaged ratio of toroidal to poloidal energy varies only around 10%, the local toroidal velocities can reach about 40% of the poloidal velocities. The strongest toroidal motions always take place right above or close to the fast upwelling plume heads and behave as vortical flows with time. This finding may explain the origins of microplate rotation in regions with hotspot activity (e.g. Easter Island), which shows no direct relation to the transform-faulting system near the mid-ocean ridge.
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