The new Layer found hidden under the earth’s crust could shed light on tectonic plate movement, according to the researchers.

A long-running controversy over how tectonic plates move may finally be resolved thanks to the discovery by scientists of a new layer of partially molten rock under the Earth’s crust.

Patches of melt have previously been found by researchers at a comparable depth. However, a recent research headed by The University of Texas at Austin showed the layer’s global size and role in plate tectonics for the first time.

The findings were published in the journal Nature Geoscience today.

The asthenosphere, which lies in the upper mantle under the Earth’s tectonic plates, contains the molten layer, which is roughly 100 miles from the surface. As a relatively supple border that permits tectonic plates to migrate through the mantle, the asthenosphere is crucial for plate tectonics.

However, the causes behind its softness remain unknown. Previously, scientists hypothesized that molten rocks may play a role. But as this research demonstrates, melt really doesn’t seem to have a significant impact on the movement of mantle rocks.

“When we think about something melting, we intuitively think that the melt must play a big role in the material’s viscosity,” comments lead author Junlin Hua. 

However, what they “found is that even where the melt fraction is quite high, its effect on mantle flow is very minor.”

The study, which Hua started as a doctoral student at Brown University, claims that the primary factor affecting the motion of the plates is the convection of heat and rock in the mantle. Although the Earth’s core is mostly solid, rocks may move and flow like honey over extended periods of time.

Coauthor Thorsten Becker, a professor at the Jackson School, stated that the absence of an impact from the melt layer on plate tectonics simplifies computer models of the Earth by reducing the number of complex variables.

Becker said they “can’t rule out that locally melt doesn’t matter.”

But he thinks “it drives us to see these observations of melt as a marker of what’s going on in the Earth, and not necessarily an active contribution to anything.”

Hua got the idea to look for a new layer in the Earth’s interior when he was doing his doctoral research and looking at seismic images of the mantle under Turkey.

Intrigued by indications of partially molten rock under the crust, Hua gathered comparable photos from other seismic sites to create a worldwide map of the asthenosphere. What he and others thought was a strange occurrence actually happened all over the world and showed up in seismic readings wherever the asthenosphere was the hottest.

The next thing that surprised him was that there was no link between his melt map and seismic measurements of tectonic movement, even though the molten layer covered almost half of the Earth.

“This work is important,” remarks coauthor Karen Fischer, “because understanding the properties of the asthenosphere and the origins of why it’s weak is fundamental to understanding plate tectonics.”

Source: 10.1038/s41561-022-01116-9

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