Dislocation creep theory is cited to postulate an unstable tetrahedral convection pattern in the earth's mantle. This pattern is then assumed to cooperate with other convection mechanisms to account for the gross features of plate tectonics. Instability of the pattern arises from the combination of high Rayleigh numbers and high Prandtl numbers, which causes an ideal tetrahedral convection pattern to degenerate into four largely randomly wandering upwelling plumes which act as nodes connecting narrow, weaving, ascending curtains. These curtains enclose broad regions of downwelling, and define the convection cell boundaries. Convection flow may also be discontinuous. Hence the four convection cells are continually changing in size, shape, and position with respect to each other, and this may be the dynamic controlling sea-floor spreading and continental drift. Convection so visualized has considerable inherent versatility, and the model is fitted to one reconstruction of Phanerozoic crustal events to demonstrate its utility. Polar ice sheets may be a natural laboratory where this and other models of mantle convection can be tested. © 1973.
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