Unexpected molten rock layer discovered under Earth’s crust

The asthenosphere is a layer in the upper mantle of the Earth’s structure. It lies just beneath the crust and brittle outermost layer of the mantle, and is viscous rather than solid. The large plates of rock that form earth’s crust float on the asthenosphere, meaning that this layer plays a fundamental role in enabling the movements of these tectonic plates. The origin of the asthenosphere, and why it is soft enough to allow plate movement, are not understood.

Scientists previously thought that patches of molten rock they identified within the asthenosphere might be a factor. When Junlin Hua, at the time a graduate student at Brown University, was investigating seismic images of the mantle beneath Turkey, he became fascinated with the consequences of these molten rock patches under the mantle, and decided to investigate whether they contributed to the viscosity of the layer. He compiled similar images from other seismic stations around the world and produced a global map of the asthenosphere. 

What he and his colleagues discovered was a brand new viscous layer in the lower asthenosphere, lying around 100 miles below the Earth’s surface. This hidden layer, named PVG-150 (positive velocity gradient at 150 kilometers) was present in many locations. What Hua and others had taken to be anomalous patches was in fact commonplace around the world, appearing on seismic readings wherever the asthenosphere was hottest. 

The next surprise came when Hua, who is now at the Jackson School’s University of Texas Institute for Geophysics, and colleagues, compared this melt map with seismic measurements of tectonic movement. They found no correlation between these two variables, indicating that plate movement is not directly related to the areas where the molten layer of rock is present. These findings are published in the journal Nature Geoscience. 

“When we think about something melting, we intuitively think that the melt must play a big role in the material’s viscosity,” said Hua, who led the research. “But what we found is that even where the melt fraction is quite high, its effect on mantle flow is very minor.”

According to the researchers, it may not be the molten-ness of molten rock that is the key factor, but instead it may be the convection processes of heat and rock in the mantle that influence the shifting of the tectonic plates. Showing that the melt layer has no influence on plate tectonics means one less tricky variable for computer models of the Earth, said study co-author Thorsten Becker, a professor at the Jackson School.

“We can’t rule out that locally melt doesn’t matter,” said Becker, who designs geodynamic models of the Earth at the Jackson School’s University of Texas Institute for Geophysics. “But I think 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.”

While the details of the asthenosphere’s function may remain a mystery, this research will enable scientists to refine their models of plate tectonics in future, to reflect the latest findings. Knowing more about tectonic plates may help us to understand and predict earthquakes that result from plate movements, thereby reducing the destructive toll of these natural disasters. 

Study co-author Karen Fischer, a seismologist and professor at Brown University, said this work is important because understanding the properties of the asthenosphere and the origins of why it’s weak is fundamental to understanding plate tectonics

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By Alison Bosman, Earth.com Staff Writer

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