Scientists have discovered a new layer of partially molten rock beneath Earth’s crust that may help settle a long-running debate about how tectonic plates move.
The researchers had previously located patches of melt at a similar depth. But a new study led by the University of Texas at Austin reveals for the first time the global extent of the plate and its role in plate tectonics.
The research was published on February 6, 2023 in the journal Natural Earth Sciences.
The magma lies about 100 miles from the surface and is part of the asthenosphere, which lies beneath Earth’s tectonic plates in the upper mantle. The asthenosphere is important to plate tectonics because it forms a relatively smooth boundary that allows tectonic plates to move through the mantle.
However, the reasons why it is soft are not well understood. Scientists previously thought that molten rock might be a factor. But this study shows that melting does not, in fact, appear to appreciably affect mantle rock flow.
“When we think of something melting, we intuitively think that melting must play a big role in the viscosity of the material,” said Junlin Hua, a postdoctoral fellow at the University of Texas Jackson School of Geosciences who led the research. “But what we found is that even when the fraction of melt is very high, its effect on mantle flow is very small.”
According to the research, which Hua began as a graduate student at Brown University, convection of heat and rocks in the mantle is the dominant influence on plate motion. Although Earth’s interior is pretty much solid, over long periods of time, rocks can move and gush like honey.
Showing that the melting layer has no effect on plate tectonics means a less challenging variable for computer models on Earth, said co-author Thorsten Becker, a professor at the Jackson School.
“We can’t rule out that local melting is not significant,” said Baker, who models Earth geodynamics at the University of Texas Institute for Geophysics at the Jackson School. “But I think it prompts us to see these melt observations as a sign of what’s going on in the ground, not necessarily an active contributor to anything.”
The idea to search for a new layer in the Earth’s interior came to Hua while studying seismic images of the mantle beneath Turkey during his doctoral research.
Intrigued by the signs of partially molten rock beneath the crust, Hua collected similar images from other seismic stations until he had a global map of the asthenosphere. What he and others considered an anomaly was actually common around the world, showing up in seismic readings wherever the asthenosphere was hotter.
The next surprise came when he compared the melt map with seismic measurements of tectonic motion and found no correlation, even though the molten layer includes nearly half of the Earth.
“This work is important because understanding the properties of the asthenosphere and the origins of why it is weak is central to understanding plate tectonics,” said co-author Karen Fisher, a seismologist and professor at Brown University who earned her PhD at Hua. advisor when the search began.
The research was funded by the US National Science Foundation. Collaborating institutions included the UT Oden Institute of Engineering and Computational Science and Cornell University.