Edgeforce mechanism

In this mechanism the oceanic lithosphere represents the top of the convection system, and the plates move in response to forces applied to their edges (Fig. 12.9b). The mechanism was first proposed by Orowan (1965) and Elsasser (1969, 1971) and is sometimes referred to as Orowan-Elsasser-type convection (Davies & Richards, 1992).

Only a small percentage of the energy supplied from the mantle is available to drive the plates, but this fraction is adequate to power the present plate motions (Bott, 1982). The energy is utilized by the lithosphere to drive the plates in several ways. The ridge-push force (Section 12.6) originates from the uplift of the ridge crest caused by the anomalously hot asthenosphere beneath it. This provides a lateral push to the rear of accreting oceanic lithosphere. The slab-pull force (Section 12.6) arises from the negative buoyancy of the downgoing slab at trenches, and is assisted by phase changes to denser forms that affect minerals in the slab at increased pressure. The slab-pull force is potentially some four times larger than the ridge-push force, although in practice much of this force is probably utilized in overcoming slab resistance (Chapple & Tullis, 1977). The trench suction force (Section 12.6) originates from the geometry of the downgoing slab and also provides a significant driving force.

The edge-force mechanism can account for many phenomena more satisfactorily than the mantle drag mechanism, in particular:

1 It is more acceptable thermodynamically and is much more effective in transporting heat from the mantle.

2 It is consistent with the observed pattern of intraplate stress. As discussed in Section 12.7.1, the mantle drag mechanism implies tension at ocean ridges and compression at trenches. The edge-force mechanism would give rise to the opposite stress configuration, and this is in accord with the stress regime indicated by focal mechanism solutions of intraplate earthquakes.

3 It is reconcilable with the present plate motions, in particular with the observations of Forsyth

(a) plate velocity is independent of plate area (Fig. 12.10a). If mantle drag were operative it would be expected that the greatest velocities would be experienced by plates with the greatest area over which the mantle drag would act;

(b) plates attached to downgoing slabs move more rapidly than other plates (Fig.

Figure 12.10 Correlations of plate parameters with plate velocity: (a) plate area; (b) plate circumference connected to downgoing slab (open bar, total length; filled bar, effective length); (c) continental area of plate (redrawn from Forsyth & Uyeda, 1975, with permission from Blackwell Publishing).

Figure 12.10 Correlations of plate parameters with plate velocity: (a) plate area; (b) plate circumference connected to downgoing slab (open bar, total length; filled bar, effective length); (c) continental area of plate (redrawn from Forsyth & Uyeda, 1975, with permission from Blackwell Publishing).

12.10b). This is in accord with the slab-pull force being greater than other forces affecting the plates; (c) plates with a large area of continental crust move more slowly (Fig. 12.10c). This implies that mantle drag inhibits the motion of such plates rather than driving them.

The mechanism also provides a reasonable explanation of the motions of small plates.

Consequently, the edge-force mechanism of plate movement appears to be much more successful in explaining all observed phenomena, and has been adopted by most workers, certainly for present-day plate motions.

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How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

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