2.6 Mechanisms for Plate Motion

It has often been repeated in this text and elsewhere that convection of the mantle is critical to plate tectonics. While this is almost certainly so, other forces likely play a significant role. One side of the argument holds that the plates are only moved by the traction caused by mantle convection. The other side holds that traction plays only a minor role and that two other forces, ridge-push and slab-pull, are more important (Figure 2.35). Some argue that the real answer lies somewhere in between.

Kearey and Vine (1996)(Kearey and Vine, 1996, Global Tectonics (2ed), Blackwell Science Ltd., Oxford) have listed some compelling arguments in favour of the ridge-push/slab-pull model as follows: (a) 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; (b) 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), and such high rates of convection are not supported by geophysical models; and (c) although large plates have potential for much higher convection traction, plate velocity is not related to plate area.

In the ridge-push/slab-pull model, which is the one that has been adopted by most geologists working on plate-tectonic problems, the lithosphere is the upper surface of the convection cells, as is illustrated in Figure 2.36.

Although ridge-push/slab-pull is the widely favoured mechanism for plate motion, it’s important not to underestimate the role of mantle convection. There would be no ridges to push from without convection because upward convection brings hot, buoyant rock to the surface. Furthermore, many plates, including our own North American Plate, move along nicely—albeit slowly—without any slab-pull happening.

Image Descriptions

Figure 2.35 image description: In this model, there are three forces working to move the plates. Ridge-push forces cause two plates to pull apart on the surface. Slab-pull forces pull the plates down. This movement out and down is also encouraged by convection traction or clockwise and counterclockwise currents present beneath the plates. [Return to Figure 2.35]

Media Attributions

• Figures 2.35, 2.36: © Steven Earle. CC BY.