What controls how the Earth’s tectonic plates are fragmented?

If you look at the shapes of the continents you’ll see that, similarly to puzzle pieces, they seem to fit. In the early 20th century, the geologist Alfred Wegener also realised that and he was convinced that this feature couldn’t simply be a coincidence. Then he couldn’t quite explain this property but later, in the 1950s and 1960s, a theory came along to explain it: it was the theory of tectonic plates.

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Fig1. The outermost shell of our planet is broken in smaller plates known as tectonic plates. (Mallard et al., 2016)

According to the theory of tectonic plates the lithosphere, which is the rigid outermost shell of our planet (regions called crust and upper mantle), is broken. Each piece is called a tectonic plate. There are two major groups of tectonic plates: a group of seven large plates of similar area covering up to 94% of the planet and a group of smaller plates. These plates can move relative to one another and this motion can explain many geological phenomena such as: earthquakes, volcanic activity, mountain-building, and oceanic trench formation. The relative movement of the plates typically ranges from zero to 100 mm annually.

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Fig2. There are two groups of plates (small and large) and there was a debate as to how they were formed. New model can self-consistently reproduce the frequency distribution of plate size. The figure shows the cumulative plate count versus the logarithm of plate size for earth and for the model (for four different values of stress between the plates). (Mallard et al., 2016)

The mechanism that let to the existence of two different groups of tectonic plates (the large and the small plates) is a matter of scientific debate. Some scientists believe that it reflected the existence of two different evolutionary laws. The large tectonic plates would be associated with mantle flow and the small group with lithosphere dynamics. Others believe that plate layout was produced by processes in the surface of the planet. Since most of the work done in this field involves only statistical tools, resolving this controversy has proven challenging. Though statistical tools can be quite powerful they do not provide an understanding of the underlying forces and physical principles behind the organization of the plate system. Now, a new work from scientists led by Claire Mallard from the University of Lyon (France) can settle this controversy.

The scientists used a 3D spherical model to describe the dynamics of the convection within the Earth’s mantle, that is, the movement of fluids in this layer. The model self-consistently produce the plate size-frequency distribution observed for Earth. It allows the scientists to evaluate what sets of parameters can lead to the structure of tectonic plates that is found today in our planet.

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Fig 3. The new mathematical model takes into account physical properties of the earth’s mantle and presents an explanation as to how the two types of plates came to be. (Mallard et al., 2016)

The results show that the plate pattern on Earth is produced by a dynamic feedback between mantle convection and the strength of the lithosphere. The larger plates are associated with convection whereas a mechanism called subduction plays a major role in the fragmentation into smaller plates. Subduction is a geological process that takes place at some types of boundaries between tectonic plates. In convergent boundaries, where tectonic plates are moving towards each other, one of the plates can move under the other and is forced down into the mantle. Regions where this process occurs are known as subduction zones.

Besides solving an old debate, the model can open the way to new research approaches. It shows that convection simulations with plate-like behaviour can be used learn more about the dynamical connection between global tectonics and mantle.

The full paper can be found in the edition of June (2016) of Nature magazine.

By Kellen Manoela Siqueira.

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