A close-up of mylonite made up of a green form of muscovite mica. Scientists think mylonite plays a key role in initiating the tectonic movement of plates. Credit: Ian Geoffrey Stimpson, CC BY-NC-SA 2.0

The idea of plate tectonics—that Earth’s plates smash into each other to form mountains, slide underneath one another to form ocean trenches, and pull apart to form new oceans and continents—is well known. The underlying mechanism driving these processes, which scientists think may be vital to the evolution of life, remains unclear. No one knows for sure how plate tectonics even evolved.

At a global scale, individual plates can be easy to see—their borders are defined by where earthquakes occur. Global perspectives have also allowed scientists to precisely map the movement of plates over millennia by tracking magnetic signatures on the bottoms of the oceans. In addition, vast networks of GPS receivers can also track minute movements of plates today.

To fully understand plate tectonics, he said, “We need to zoom in from the global scale into the microscale.”

However, investigating the factors that first triggered plate tectonics requires a different perspective, said David Bercovici, a geophysicist from Yale University. He explained this perspective in a session at the 2015 annual conference of the American Association for the Advancement of Science in San Jose, Calif.

To fully understand plate tectonics, he said, “We need to zoom in from the global scale into the microscale.”

It All Comes Down to Grain Size

At many plate boundaries, scientists find a metamorphic rock made of deformed, very fine grained minerals called mylonite. The origin of mylonite is still unknown, Bercovici said.

The grains within mylonite are much smaller than the rocks in the plates around them, which makes mylonite relatively weak. Because of this relative weakness, mylonite “seems to support or permit very, very rapid, focused deformation,” said Bercovici.

Bercovici and colleagues suggest that the small-grained mylonite fuels a feedback mechanism that creates the weak spots on Earth that we know as active plate boundaries.

“As you deform [mylonite], somehow the grains in rocks become so small that it softens the rock up, and softened rock supports rapid deformation to allow you to have these plate boundaries,” Bercovici said.

Small-grained mylonite fuels a feedback mechanism that creates the weak spots on Earth that we know as active plate boundaries.

Scientists remain in the dark about exactly how mylonite forms at a granular level. However, decades of collaborative research gave Bercovici’s team an idea.

In most rocks, minerals grow grain by grain, gobbling up the grains next to them—much like how the bubbles in foam get bigger by “eating” neighboring bubbles. When a growing mineral grain swells up against a different type of mineral, its growth is blocked by the boundary between the two minerals in a process called “pinning.” This process forces the grains into smaller and smaller sizes by further damaging the grain-to-grain interface.

As the grains get smaller and smaller, the resulting mylonite gets weaker and weaker.

“By damaging the [grain] interface, we can drive the grains to smaller sizes and therefore get the self-softening feedback mechanism,” Bercovici said.

Origins of Plate Boundaries

The last piece of the puzzle required a peek back in time—via exhaustive research on samples of 4.4-billion-year-old zircon. This zircon likely formed when granites crystallized from magma heated by the hot fluids that sweat off subduction zones.

The age of the zircon falls as much as 1 billion years before scientists think plate tectonics became a global phenomenon. This presents a puzzle—how did a mineral likely formed by subduction processes crystallize before subduction became mainstream? Bercovici speculates that primitive subduction zones might have formed on Earth’s surface early in its history, when cool, heavy mantle rock near the surface began to drip down deeper into the mantle, pulling overlying crust down with it.

Bercovici applied his theoretical model of mylonite formation to simulations that mimicked the formation of subduction zones. He found that where primitive subduction formed, a mylonitic-type weak zone formed.

“When the drip-like subduction ceased and started again elsewhere, it left behind a weak zone that would persist without healing for many millions of years,” Bercovici said.

More weak zones accumulated, which would eventually connect to make a continuous plate boundary, complete with a spreading center and strike-slip faults. Bercovici compared these weak zones to scars on the Earth’s surface that never healed. “Once I’ve built up enough of these scars, it persists long enough to give me a full-blown plate,” Bercovici said.

Bercovici also applied his calculations to models of Venus. However, he found that the feedback mechanism could not exist on Venus because, thanks to high surface temperatures, mineral grains would be able to grow fast enough to “heal” the deformed areas of lithosphere. On Earth, the relatively low temperature slows down this healing process.

A Large-Scale Mystery

Although the research may be the first to show how initial damage in surface plates can propagate through tectonic cycles, it needs to be tested using more realistic rock mechanics, said Jun Korenaga, professor of geophysics at Yale University.

In addition, he noted that the comparison to Venus is moot because the planet’s soaring temperatures are probably due to its lack of plate tectonics.

“In the hypothetical early Venus, the surface temperature could be as low as the current Earth,” said Korenaga, which would mean its surface would not have been able to heal from weak-zone fractures.

Although Bercovici’s work gives clues about how plates on Earth started to move, it does not solve the plate tectonic mystery completely. For example, what happens when two continents collide? In the future, Bercovici hopes to include the effects of continent-to-continent interaction in his models of mylonite-induced tectonics.

—JoAnna Wendel, Staff Writer

Citation: Wendel, J. (2015), Tiny mineral grains could drive plate tectonics, Eos, 96, doi:10.1029/ 2015EO024967. Published on 24 February 2015.

Text © 2015. The authors. CC BY-NC 3.0
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