Figure 3.6 Slab-pull mechanism. F=velocity of movement of oceanic lithosphere, d=3.3 g cm-3 and 3.1 g cm-3 for density of oceanic lithosphere and asthenosphere respectively, r=the retardation force on the upper and lower surface of the down-going slab and F=the tensile force generated by the difference in density of the oceanic lithosphere and the asthenosphere.

resistance at the upper and lower boundaries of the slab, or to the resistance to penetration by the front end of the slab. The values of these resistive forces are not readily determined, for they are based on a range of boundary conditions which are not easily demonstrated or quantified.

We accept the 'judgement of Solomon' approach taken by Turcotte and Schubert who suggest that, 'Trench (slab) pull must be mostly off-set by large resistive forces encountered by the descending lithosphere as it penetrates the mantle. The nett force at the trench is probably comparable with that of the ridge-push'. One could infer from this statement that the forces of trench-pull and 'ridge-push' would give rise to a differential stress of not more than 108 Pa (1.0kb). However, we have indicated that the 'ridge-push' mechanism may have little effect upon the elastic layer of the oceanic lithosphere. Hence, at the trench, we are theoretically left with a small 'slab-pull'.

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