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Resisting Forces

Slab Resistance

This force acts as a plate collides with another plate boundary. It directly resists all the driving forces associated with plate tectonics. This force occurs only at subduction zones (ie ocean-continent and ocean-ocean boundaries). The subducting plate exerts a force directly onto the over-riding more bouyant plate which acts to oppose the forces driving the subduction.

Collisional Resistance

This force directly opposes the slab pull force. It occurs as the heavy basaltic plate is pulled down into the mantle. The collisional force occurs because the mantle, although less dense than the subducting plate,  still resists subduction to some extent due to friction. This force is most prevalent at young subduction zones where the angle of dip of the subducting slab is not well developed and still quite shallow thus increasing friction (Wilson, 1993).

This force is illustrated under the description of slab pull.

Transform Fault Resistance

One of the most prominent features of mid-oceanic ridges is ridge-offset.  This means that along the ridge, the actual spreading centre may be broken up, sometimes by hundreds of kilometres. This is shown in the diagram below.

Fig.3.4. Diagram illustrating ridge off-set and transform faults
Between two off-set spreading centres, the plates move past one another forming transform faults. Since the direction of movement is opposite, drage forces are establishd between these sections of the plate. This drag force resists plate movement away from the spreading centre.

Drag Force

The drag force operates on almost all parts of a moving lithospheric plate. This force was initially considered to be the main reason why Wegner's theory of continental drift was disgarded, the forces required to force a continent around the globe was simply too large. As seen, this is not true considering the large shear zone created by the aesthenosphere that allows lithospheric plates to slide around the Earth. However, the basal drag force still acts to resist plate motion at the interface between the lithosphere and upper mantle.

Fig.3.5. Diagram illustrating Basal Drag and Collisional Resistance
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