23 4.5 Mechanisms for Plate Motion

Mantle convection is often said to be critical to plate tectonics. While this is certainly true, there is still debate about the actual forces that make the plates move. One side of the argument holds that the plates are only moved by the traction caused by mantle convection, and that friction between the asthenosphere and lithosphere pulls the lithosphere along as the mantle convects. The other side holds that traction plays only a minor role and that ridge-push and slab-pull are more important (Figure 4.37).

Ridge-push refers to gravity causing lithosphere to slide downhill away from the elevated mid-ocean ridges. Slab-pull refers to the weight of subducting slabs dragging the rest of the plate down into the mantle.

Kearey and Vine (1996) have listed some compelling arguments in favour of the ridge-push/slab-pull model:

• Plates that are attached to subducting slabs (e.g., Pacific, Australian, and Nazca Plates) move the fastest, and plates that are not (e.g., North American, South American, Eurasian, and African Plates) move significantly slower.

• In order for the traction model to apply, the mantle would have to be moving about five times faster than the plates are moving  because the coupling between the partially liquid asthenosphere and the plates is not strong. Such high rates of convection are not supported by geophysical models.

• Although large plates have the potential for much higher convection traction, plate velocity is not related to plate area.

References

Kearey, P., & Vine, F. (1996). Global Tectonics (2nd E.). Oxford: Blackwell Science Ltd.