The newly-detected molten rock layer is located about 150 km (93 miles) from the surface of our planet and is part of the asthenosphere, which sits under tectonic plates in the upper mantle.
Hua et al. discovered a previously unknown layer of partly molten rock in a key region just below the tectonic plates. Image credit: Leonello Calvetti / Dreamstime.
The asthenosphere is the denser, weaker layer beneath the lithospheric mantle. It lies between about 100 and 700 km (62-435 miles) beneath Earth’s surface.
It is important for plate tectonics because it forms a relatively soft boundary that lets tectonic plates move through the mantle. The reasons why it is soft, however, are not well understood.
Geoscientists previously thought that molten rocks might be a factor. But the new study shows that melt, in fact, does not appear to notably influence the flow 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,” said Dr. Junlin Hua, a postdoctoral researcher at the University of Texas at Austin.
“But what we found is that even where the melt fraction is quite high, its effect on mantle flow is very minor.”
The convection of heat and rock in the mantle are the prevailing influence on the motion of the plates.
Although the Earth’s interior is largely solid, over long periods of time, rocks can shift and flow like honey.
“Showing that the melt layer has no influence on plate tectonics means one less tricky variable for computer models of the Earth,” said University of Texas at Austin’s Professor Thorsten Becker.
“We can’t rule out that locally melt doesn’t matter. 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.”
The idea to look for a new layer in Earth’s interior came to Dr. Hua while studying seismic images of the mantle beneath Turkey.
Intrigued by signs of partly molten rock under the crust, he compiled similar images from other seismic stations until he had a global map of the asthenosphere.
What he and others had taken to be an anomaly was in fact commonplace around the world, appearing on seismic readings wherever the asthenosphere was hottest.
The next surprise came when Dr. Hua compared his melt map with seismic measurements of tectonic movement and found no correlation, despite the molten layer encompassing almost half the Earth.
“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,” said Brown University’s Professor Karen Fischer.
The results appear this week in the journal Nature Geoscience.
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J. Hua et al. Asthenospheric low-velocity zone consistent with globally prevalent partial melting. Nat. Geosci, published online February 6, 2023; doi: 10.1038/s41561-022-01116-9
