Pulling and sucking together to move the plates

The various plates of the earth move at quite different velocities.  Some are speeding along at over 10 cm/y, while others are taking it easy, at less than 2 cm/y.  The fastest plates are those that are primarily made up of oceanic crust and are being actively subducted, such as the Pacific, Indo-Australian, Nazco and Cocos Plates - while the slowest are those that have significant areas of continental crust and are not being subducted, such as the North and South American and the African and Eurasian Plates.

Several models have been proposed to explain what makes the plates move, including:

1)  that plates are driven by convective forces within the mantle,

2)   that plates pushed away from the oceanic ridges (which are significantly elevated regions of the ocean floor), or

3)   that plates are pulled along by gravitational forces operating on their subducting oceanic slabs.

Geologists from Michigan State University have used a mathematical model in an effort to understand why the different plates move at different velocities (Conrad and Lithgow-Bertelloni, 2002).  The model is based on two assumptions: firstly that the gravitational pull on an oceanic crustal slab that is descending into the upper part of the mantle (the upper 600 km) will pull the plate towards the subduction zone (slab-pull), and secondly that convection in the mantle that is induced by the descending slab will also pull the descending plate, and other plates in the region, towards a subduction zone (slab-suction).  The model assumes that the ridge-push effect does not make a significant contribution to plate motion, and that any inherent convection systems in the mantle do not contribute significantly to plate motion.

When the model is run with only the slab-suction process operating the plates tend to move too slowly, and non-subducting plates move almost as fast as subducting plates.  In effect, the induced mantle flow (ie. the red arrows in the diagram above)  exerts traction equally on subducting and non-subducting plates.

When the model is run with only the slab-pull process operating the subducting plates move at approximately the right velocity towards the subduction zones, but the non-subducting plates move in the wrong directions – away from the subduction zones, rather than towards them.

When the model is run with both slab-pull and slab-suction processes operating, the plates move at roughly the correct velocities and in the general directions that they are observed to move.

Conrad and Lithgow-Bertelloni conclude that the motions of the plates can be explained by the slab-pull and the slab-suction forces operating together – each contributing approximately half of the driving force.

Reference

Conrad C P and Lithgow-Bertelloni C, How mantle slabs drive plate tectonics, Science, Vol. 298, p. 207-209. (October 2002)

(This paper is available as a pdf file: http://www.mala.bc.ca/~earles/slab-ps.pdf)

Steven Earle, 2002. Return to Earth Science News