Friday, October 07, 2022

Grain size of rocks in Earth’s mantle affects tectonics

Grain size of rocks in Earth's mantle affects tectonics

Depending on how coarse-grained the rocks are in the upper mantle, they deform quite differently under stress. The picture shows two microscopic thin sections of mantle rocks. credit: Jonas Ruh / ETH Zurich

The planet is shaped by forces deep in its interior. These push the plates of the Earth’s crust against each other, forming mountains and volcanoes with colliding zones. But when reconstructing what is happening inside the Earth, we are limited to indirect observation; For example, by applying pressure to rocks from the Earth’s mantle or by analyzing seismic waves produced by earthquakes.

Yet all these observations provide only snapshots. If we want to understand the dynamics of what has happened over many millions of years, we need computer models that can simulate geological processes in fast motion. By feeding the above observational data and physical formulas into these models, researchers can show how Earth’s surface and interior change over time.

There is a flaw here, however: each model is based on simplification and thus prone to error. Factors that may not seem particularly important at first glance may also play an important role, as a new study published in nature geology Exhibited by the Structural Geology and Tectonics group in the ETH Department of Earth Sciences. With their new simulations, the researchers are able to show that previous models have not adequately considered a key factor, even though its potential effect is known: the grain size of mantle rocks. The latest simulations now show how big an effect grain size really is.

Displacement or diffusion?

Grain size is relevant because it affects how rocks are deformed in the upper mantle. If the grain size is in the range of a few millimeters, the minerals in the rocks are deformed mainly through the shifting of the crystal lattice of the minerals along the planes. This leads to what is called dislocation creep, which is thought to be the most important mechanism of rock deformation in Earth’s mantle.

On the other hand, if the grain size is small, another mechanism becomes more important: diffusion creep. The rocks are then deformed, not by dislocations in the crystal lattice of the minerals, but by individual atomic vacancies in the crystal lattice migrating through the crystal structure. Depending on which deformation mechanism predominates, the strength of the rocks changes accordingly.

Grain size of rocks in Earth's mantle affects tectonics

Simulations show how the grain size of mantle rocks evolves when continents break apart along a so-called rift zone. credit: Jonas Ruh / ETH Zurich

many unanswered questions

“Fine rocks form mainly in shear zones and are much weaker than deformable coarse-grained rocks,” explains Jonas Ruh, senior assistant in the group and lead author of the study. “But until now, we have not been able to realistically represent these differences in a dynamical model.” Some of the previous models only considered dislocation creep, which is an oversimplification. Other models use constant grain sizes for upper mantle rocks, which does not do this factor justice.

Ruh considered recent studies from other groups as well as laboratory experiments from his own research group for his new model. “In particular, we included a new growth model for the main mineral, olivine,” he explains. “And, based on the new research, we now also know that the mechanical energy in grain size reduction is significantly lower than previously thought.” If these new findings are taken into account, the processes in Earth’s mantle could be modeled more realistically.

the contradiction resolves itself

Ruh was able to show that the reduction in grain size, activation of diffusion creep, and consequent weakening of the upwelling significantly reduce the boundary forces required to initiate rafting, and facilitate continental break-up.

However, the new study was prompted by a different, seemingly paradoxical feature of plate tectonics: the topmost region of Earth’s mantle must be relatively solid, as this is the only explanation for why tectonic plates pushing beneath another plate are below. Why doesn’t the depth fall at a sharp angle?

But if this mantle field is as strong as the geometry of the plates going down requires, then the rocks in the topmost mantle must be brittle given the dominant stresses there. It follows that an earthquake would occur in this region of the Earth’s mantle which would cause a fit and release of stress initially. However, observations of such earthquakes are extremely rare to date.

The new model now resolves the paradox: “Fine, tensile shear regions relieve high stresses to the point where earthquakes can no longer occur,” explains Rooh. “At the same time, the uppermost part of Earth’s mantle remains strong enough to conform to the observed flexure geometry of the descending slab in the collision zones.”

Is Earth’s transition zone getting distorted like the upper mantle?

more information:
JB Ruh et al, Grain-size-evolution control on lithospheric dilution during continental transfer, nature geology (2022). DOI: 10.1038/s41561-022-00964-9

Citation: Grain Size of Rocks in Earth’s Mantle Affects Tectonics (2022, June 17) on June 17, 2022 retrieved from.

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