The asthenosphere plays a fundamental role in current plate tectonics, as its low viscosity determines how convection in the mantle below is reflected on the Earth’s surface above. The origin of the asthenosphere, including the role of partial melting in reducing viscosity and facilitating deformation, remains unclear.

In a new study, scientists led by the University of Texas at Austin analyzed receiver function data from globally distributed seismic stations to image the lower reaches of the low seismic velocity asthenospheric zone. They discovered a new layer of partially molten rock beneath the Earth’s crust that could help resolve a long-standing debate about how tectonic plates move.

This newly discovered molten layer was located about 100 miles from the surface and is part of the asthenosphere. However, the causes of the softness are not fully known. Previously, scientists hypothesized that molten rock might play a role. But as this study shows, melting doesn’t appear to significantly affect the flow of mantle rocks.

A diagram of the asthenosphere, supporting plate tectonics, where researchers at the UT Austin Jackson School of Geosciences say they have detected a global layer of partial melting (shown in speckled red). Credit: Junlin Hua, UT Jackson School of Geosciences

Junlin Hua, a postdoctoral fellow at UT’s Jackson School of Geosciences who led the research, said: “When we think of something that melts, we intuitively think that the melt must play a big part in the viscosity of the material. But we found that even if the melt fraction is quite high, the effect on the mantle current is small.”

According to the research, the convection of heat and rock in the mantle is the predominant influence on the movement of the plates. While the Earth’s interior is mostly solid, rocks can shift over long periods of time and flow like honey.

Co-author Thorsten Becker, a professor at the Jackson School, said: “Showing that the melt layer does not affect plate tectonics means a less tricky variable for computer models of the Earth.”

“We cannot exclude that local smelting does not matter. But I think it drives us to see these melting observations as a marker of what’s happening on Earth, and not necessarily as an active contributor to anything.”

The idea to search for a new layer in the Earth’s interior came during his PhD research while studying seismic images of the mantle under Turkey.

Scientists collected similar photos from other seismic sites after becoming intrigued by evidence of partially molten rock beneath the crust. Finally, they had a global map of the asthenosphere. In reality, what they thought was an aberration was widespread, appearing on seismic data where the asthenosphere was hottest.

The second surprise came when, despite the molten layer covering more than half of the Earth, they compared the melt map to seismic measurements of tectonic activity and found no correlation.

Co-author Karen Fischer, a seismologist and professor at Brown University who received Hua’s Ph.D. adviser when he began the investigation said: “This work is important because understanding the properties of the asthenosphere and the origins of why it is weak is fundamental to understanding plate tectonics.”

Magazine reference:

  1. Junlin Hua, Low Velocity Asthenospheric Zone Consistent With Globally Occurring Partial Melt, Nature Geoscience (2023). DOI: 10.1038/s41561-022-01116-9