Scientists have discovered a new layer of partially melted rock under the crust. This could help end the longstanding debate about how tectonic plates move.

Researchers had previously identified melt spots at similar depths. However, new research led by the University of Texas at Austin reveals for the first time the global extent of this layer and its role in plate tectonics.

Located approximately 100 miles (160 kilometers) from the surface, the molten layer is part of the asthenosphere beneath the Earth’s tectonic plates in the Earth’s upper mantle. The asthenosphere is important for plate tectonics because it forms a relatively soft boundary that allows tectonic plates to move through the mantle. 

However, the reason for the softness is not well understood. Scientists previously thought lava could be the cause. However, this study shows that melting does not actually appear to have a significant effect on mantle flow.

“When we think of something as being molten, our intuition is that the melt must have played a large role in the viscosity of the material,” said the UT’s who led the study. said Junlin Hua, a postdoctoral fellow at the Jackson Graduate School of Geosciences. “However, we found that even at very high melt fractions, the effect on sheath flow was very small.” 

According to the research, which Hua began as a graduate student at Brown University, 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 melted layers do not affect plate tectonics means fewer variables are tricky for computer models of the Earth, says co-author Torsten Becker, a professor at the Jackson School.

“We cannot rule out that local melting does not play a role,” said Becker, who designs geodynamic models of the Earth at the University of Texas Jackson School Geophysical Laboratory. I think we’ll come to think of it as an indication of what’s going on on Earth, and not necessarily as an active contributor to anything.”

The idea to search for new layers in the Earth’s interior came to Hua during his doctoral studies, studying seismic images of the Earth’s mantle beneath Turkey. 

Intrigued by signs of partially molten rock beneath the crust, Hua stitched together similar images from other seismic stations to create a world map of the asthenosphere. What he and others thought was anomalous was actually common around the world, showing up in seismic measurements where the asthenosphere was at its hottest.

The next surprise came when Melt compared his map with seismic measurements of tectonic motion. No correlation was found, even though the melt layer covered almost half of the Earth.

“This study is important because understanding the properties of the asthenosphere and what causes its weakness is fundamental to understanding plate-hi tectonics,” said a consultant when he began his research.