How does plate tectonics work?

Well, surely we ought to know, 52 years after W. Jason Morgan proposed that the Earth’s surface consists of 12 rigid plates that move relative to each other. But that is not completely true, although most of its mechanisms expressed by external and internal Earth processes are known in great detail. It is still a ‘chicken and egg’ issue: do convective motions in the mantle drive the superficial plates around by dragging at the base of the lithosphere or is it the subduction of plates and slab-pull force that result in overturn of the mantle? Nicolas Coltice of the University of Paris and colleagues from those of Grenoble, Rome and Texas consider that posing plate tectonics in such a manner is an abstraction; rather like the plot for a novel that is yet to be written (Coltice, N. et al. 2019. What drives tectonic plates? Science Advances, v. 5, online eaax4295; DOI: 10.1126/sciadv.aax4295). Instead, all the solid Earth’s vagaries and motions have to be considered as an indivisible whole rather than the traditional piecemeal approach of focussing on the forces that act on the interfaces between plates.

Their approach is to model a combination of mechanisms throughout the Earth as a single, evolving three-dimensional system without the constraint of perfectly rigid plates, which of course they are not. The physical parameters boil down to those involved in relative buoyancy, viscosity, and gradients of temperature, pressure and gravitational potential energy within a spherical planet. Designing the algorithms and running the model on a supercomputer took 9 months to reconstruct the evolution of the planet over 1.5 billion years.

Still from a movie of simulated breakup of a supercontinent, in bland blue-grey, showing what happens at the surface (left) and, at the same time, in the mantle (right): note the influence of rising plumes (credit: Nicolas Coltice)

The result is a remarkable series of unfolding scenarios. In them, 2/3 of the planet’s surface moves faster than does the underlying mantle, suggesting that the surface is dragging the interior. For the remainder, mantle motions exceed those of the surface. Continents are dragged by the underlying mantle to aggregate in supercontinents, which in turn are torn apart by the sinking of cold oceanic slabs. The model takes on a highly visual form, showing in 3-D, for instance: ocean closure and supercontinent assembly; and example of continental breakup; how subduction is initiated.

It will be fascinating to see the reaction of the authors’ peers to their venture, and the extent to which the technicalities of the paper are translated into a form that is suitable for teaching. My suspicion is that most Earth scientists will be happy to stay with the old conceptions until the latter is achieved, and laptops are able to run the model(!)

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