The plates of the Earth’s crust push against each other, causing mountains and volcanoes to form along the collision zones. But when modeling what exactly is happening inside the Earth, we are limited to indirect observation; for example, by performing pressure experiments on rocks from the Earth’s mantle or by analyzing seismic waves triggered by earthquakes.
However, if we want to understand the dynamics of what has happened over several million years, we need computer models that can simulate geological processes in fast motion.
With their simulation, the researchers were able to demonstrate that one crucial factor has not been adequately considered in previous models: the grain size of mantle rocks.
Grain size is important because it affects how the rocks deform in the upper mantle. If the grain size is a few millimeters, the minerals in the rocks deform mainly through the shifting of the minerals crystal lattice.
However, if the grain size is smaller, diffusion creep occurs. The rocks deform not by dislocations in the crystal lattice of the minerals, but by individual atomic vacancies in the crystal lattice. Depending on the deformation mechanism, the strength of the rocks changes accordingly.
The uppermost region of the Earth’s mantle must be relatively solid. This is the only explanation for why tectonic plates that are pushed under another plate do not plunge into the depths at a steeper angle.
The new model shows: “The fine-grained, ductile shear zones relieve the high stresses to the point where earthquakes can no longer occur,” study author Jonas Ruh explains.
https://phys.org/news/2022-06-grain-size-earth-mantle-affects.html