Driving Mechanism Of Plate Tectonics

The energy available to drive plate motions is the heat generated in the core and mantle that is brought to the surface by convection in the mantle. It now remains to consider the manner in which this thermal energy is employed in driving the lithospheric plates. The proposal by Morgan (1971, 1972b) that plates are driven by the horizontal flow of material brought to the base of the lithosphere by hotspots was discounted initially (Chapple & Tullis, 1977), as the lateral flow would probably be equal in all horizontal directions and thus would not apply a directional force to the plates. Two models have been proposed. The classical, or mantle drag, model considers that the upper, cool, boundary layer of the convecting system is represented by the upper part of the asthenosphere, and that plates are driven by the viscous drag of the asthenosphere on their bases. By

Convection The Mantle Black And White
Figure 12.9 Two concepts of plate driving mechanism: (a) cellular convection, with the cells exerting a mantle drag on the lithosphere; (b) Orowan-Elsasser-type convection, with plates driven by edge forces (redrawn from Bott, 1982, by permission of Edward Arnold (Publishers) Ltd).

contrast, the edge-force model recognizes the lithosphere itself as the upper, cool boundary layer of the convection cells and proposes that the plates are driven by forces applied to their margins. The two models thus differ in the importance placed on the various forces acting on plates (Section 12.6) described by Forsyth & Uyeda (1975).

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