The traction force (ft) generated per unit square metre of the glide surface is given by

If we take the plate to extend 12,500 km in the direction of absolute motion, the total force f per metre width of the front of the plate will be

Over an area 50 km high by 1.0 m wide, this resistive force will be equivalent to an average resistive stress of

We have also assumed that the boundary and transform faults add a further 20 per cent of resistance to glide. Hence, the total resistive stress (ST) on the vertical plane within a plate 12,500 km from the spreading-ridge (which is 50 km high and 1 m wide) is 6.6*108 Pa. We note from Figure 3.11a that, at a distance of 12,500 km from the ridge, the average stress that can be attributed to the gravity-glide mechanism is 8.0 x10s Pa. Consequently, the nett horizontal stress (SN) that results from gravity-glide minus the total viscous restraint is given by

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