Tiny, mineral grains could explain a fundamental tectonic force

The theory may provide an origin story for subduction.

Subduction happens when one tectonic plate slides under another plate and then sinks into the Earth’s mantle. Its role in geological and measures is huge: It is the main engine for tectonic motion. It builds mountains, triggers quakes, structures volcanoes, and drives the geologic carbon cycle.

Yet, scientists have been uncertain about what initiates subduction.

Another examination by Yale University recommends that small mineral grains — squeezed and mixed over millions of years- set in motion the chain of events that plunge massive tectonic plates deep into the Earth’s interior.

David Bercovici, Yale’s Frederick William Beinecke Professor and chair of Earth and Planetary Sciences, said, “Why Earth even has subduction, unlike other terrestrial planets as far as we know, is a mystery. Mantle rock near the surface that has cooled for hundreds of millions of years has two competing effects.”

“While it’s gotten colder and heavier and wants to sink, it’s also gotten stiffer and doesn’t want to sink. The stiffening effect should win out, as it does on most planets, but on Earth, for some reason, it doesn’t.”

A conceptual sketch of the ocean basin
A conceptual sketch of the ocean basin setting for the new model. Inset images from a computer model show mineral fraction, grain size, and weakness. (Credit: Elvira Mulyukova and David Bercovici)

The model suggests that subduction may initiate at the margins between Earth’s seafloor and continents.

The model shows that tectonic stresses in an oceanic plate cause its mineral grains to mix, become damaged, and eventually shrink. Over approximately 100 million years, this process weakens the oceanic plate and makes it susceptible to vertical shear and bending, associated with the start of subduction.

Elvira Mulyukova, a research scientist at Yale, said, “The real bottleneck for tectonic plate activity on a terrestrial planet is how fast its massive, rocky layers can deform. The rocks can deform only as fast as their tiny mineral grains allow. Our model explains how these changes in mineral grains can dramatically weaken the rock and make subduction possible on a planet like Earth.”

The study appears in the Proceedings of the National Academy of Sciences.

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