Mantle drag was the first driving mechanism to be proposed, and envisages plate motion in response to the viscous drag exerted on the base of the lithosphere by the lateral motion of the top of mantle convection cells in the asthenosphere (Fig. 12.9a). The convection cells would consequently rise beneath oceanic ridges and descend beneath trenches, being largely absent beneath continental regions. This mechanism predicts that the oceanic lithosphere would be in a state of tension at the ocean ridges and compression at the trenches.
Because oftheir relationship to accretive and destructive plate margins, the horizontal dimensions of the convection cells powering mantle drag would be expected to be about half the width of an ocean, that is, 3000 km. This great lateral extent implies that the cells should have a relatively simple form. It is conse quently difficult to explain how cells of simple geometry could drive plates with irregularly shaped margins, such as the Mid-Atlantic Ridge at equatorial latitudes where it is offset along a suite of transform faults. Also, the constant geometry of the convection cells cannot explain the relative movements between plate margins, such as is happening between the Mid-Atlantic and Carlsberg ridges. The large horizontal dimensions of the cells cannot account for the movements of small plates, such as the Caribbean and Philippine plates, which can hardly be powered by their own individual convective systems.
It would therefore seem that the classical mantle drag mechanism is not the main process causing the mobility of plates. It is possible, however, that our views on mantle drag are biased by the fact that the present continents are dispersed. Ziegler (1993) argues that mantle drag may have been a significant mechanism during supercontinent break-up and, indeed, Phanero-zoic plate motions appear to require this mechanism (Section 12.11).
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