Earth’s mantle is a solid layer that undergoes slow, continuous convective motion. But how do these rocks deform, thus making such motion possible? Given that the mineral olivine (the main constituent of the upper mantle) does not exhibit enough defects in their crystal lattice, what could…
Earth’s mantle is a solid layer that undergoes slow, continuous convective motion. But how do these rocks deform, thus making such motion possible? Given that the mineral olivine (the main constituent of the upper mantle) does not exhibit enough defects in their crystal lattice, what could be the cause?
The mineral olivine is a magnesium iron silicate with the formula (Mg+2, Fe+2)2SiO4. It is a common mineral in the Earth’s subsurface usually in the form of molten rock such as magma or lava.
The researchers suspected that the solution was to be found at the boundaries between the mineral grains that make up rocks. However, they lacked the conceptual tools needed to describe and model the role played by these boundaries in the plasticity of rocks.
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The research team led by Lille University of National Advanced Chemistry, Dept. of Materials and Transformations, France, has provided an unexpected answer to this question. It involves little known crystal defects known as ‘disclinations’.
These crystals are located at the boundaries between the mineral grains that make up rocks. Their findings have just been published in the scientific journal Nature. It goes well beyond the scope of the geosciences: they provide a new, extremely powerful tool for the study of the dynamics of solids and for the material sciences in general.
Earth continuously releases its heat via convective motion in Earth’s mantle, which underlies the crust. Understanding this convection is therefore fundamental to the study of plate tectonics. The mantle is made up of solid rocks. In order for convective motion to occur, it must be possible for the crystal lattice of these rocks to deform.
Until now, this was a paradox that science was unable to fully resolve. While defects in the crystal lattice (dislocations) provide a very good explanation of the plasticity of metals, they were not sufficient to explain the deformations undergone by certain mantle rocks.
New studies now show that the crystal lattice of the grain boundaries exhibits highly specific defects which had not been detected. The researchers succeeded in observing them for the first time in samples by using an electron microscope and specific image processing.
Flow in the mantle is thus no longer incompatible with its rigidity. This research goes beyond explaining the plasticity of rocks in Earth’s mantle: it is a major step forward in materials science. Consideration of disclinations should provide scientists with a new tool to explain many phenomena related to the mechanics of solids.
The scientists intend to continue their research into the structure of grain boundaries, not only in other minerals but also in other solids such as metals.