The hidden explanation behind the plasticity of earth’s mantle

The movement of continents is reflected in the crystal structure of rocks

A few million years ago, the earth didn't look like it does now. The continents have been, and still are, constantly in motion, drifting on the solid outer mantle of the planet. However, until now, there were no distinctive changes in the structure of the rocks of that layer reflective of those movements. Researchers have used a poorly understood deformation at the interface between mineral grains to explain these tectonic movements. The results will not only help understand plate tectonics, but also provide a powerful tool to study solid dynamics and material science in general.

A few million years ago, the earth didn't look like it does now. The continents have been, and still are, constantly in motion, drifting on the solid outer mantle of the planet. However, until now, there were no distinctive changes in the structure of the rocks of that layer reflective of those movements. Researchers have used a poorly understood deformation at the interface between mineral grains to explain these tectonic movements. The results will not only help understand plate tectonics, but also provide a powerful tool to study solid dynamics and material science in general.


© S. Demouchy, Montpellier

The key to understanding plate tectonics was hidden at the interface between the mineral grains of the rocks in the earth’s mantle. Until recently, researchers lacked the conceptual tools to visualize and model their role in plasticity. However, researchers in Lille and Montpellier, France, have managed to identify changes in the crystal structure, called disclinations, which hadn't been taken into account before.

Continents on earth are constantly drifting on the outer mantle of the planet. Understanding how they move will help better pinpoint the underlying mechanisms of plate tectonics. The outer mantle, although consisting entirely of solid rocks, moves over the long period of geological time. For this to be possible, the crystal structure of the rocks needs to be malleable and yielding. However, defects in crystal position, called dislocations, which explain the plasticity of metals, don't reflect the movements of the continental crust. This paradox had persisted until now.

Looking at olivine, a rock that constitutes 60% of the composition of the mantle, the researchers observed rotational crystal defects, called disclinations, corresponding to the movements of the tectonic plates. These allow the grains of olivine to move when mechanical pressure is applied, inducing a deformation in any direction. The researchers then went further, developing a mathematical model of these alterations that can finally explain the plasticity of the rocks.

This research not only helps explain the distortions of rocks in the mantle involved in plate tectonics, but is also a huge step forward for material science. The study should lead to new tools for solid mechanics and new ideas on how to explore the evolution of rocks and other solids, such as metals.