Scientists have discovered a new layer of partially molten rock beneath the Earth’s crust that could help settle a long-standing debate about how tectonic plates move. Scientists have previously identified patches of melt at a similar depth. But a new study led by the University of Texas at Austin has revealed for the first time the global extent of the layer and its part in plate tectonics. The findings were published in the journal Nature Geoscience.
The molten layer is roughly 100 miles below the surface and is part of the asthenosphere, which is located in the upper mantle beneath the Earth’s tectonic plates. Because it forms a relatively soft boundary that allows tectonic plates to migrate through the mantle, the asthenosphere is vital to plate tectonics.
However, the reasons why it is soft are not well understood. Scientists previously thought that molten rock might be a factor. However, this study shows that melt does not actually appear to significantly affect the flow of mantle rocks.
Junlin Hua, a postdoctoral fellow at UT’s Jackson School of Geosciences who led the research said “When we think about something melting, we intuitively think that the melt must play a big role in the viscosity of the material,”But we found that even when the melt fraction is quite high, its effect on mantle flow is small.”
According to research Hua began as a graduate student at Brown University, convection of heat and rock in the mantle is the predominant influence on plate motion. Although the Earth’s interior is largely solid, rocks can shift and flow like honey over long periods of time.
Showing that the melt layer has no effect on plate tectonics means one less tricky variable for computer models of the Earth, said co-author Thorsten Becker, a professor at the Jackson School.
“We can’t rule out that local melting doesn’t matter,” said Becker, who designs geodynamic models of the Earth at the University of Texas’ Jackson School’s Institute of Geophysics. “But I think it leads us to see these melting observations as an indicator of what’s happening on Earth, and not necessarily as an active contribution to anything.”
The idea to look 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 signs of partially molten rock beneath the crust, Hua compiled similar images from other seismic stations until he had a global map of the asthenosphere. What he and others thought was an anomaly was actually common around the world, showing up in seismic measurements wherever the asthenosphere was hottest.
Another surprise came when he compared his map of the melt to seismic measurements of tectonic movement and found no correlation, even though the molten layer covered nearly half the Earth.
“This work is important because understanding the properties of the asthenosphere and the origin of why it is weak is critical to understanding plate tectonics,” said co-author Karen Fischer, a seismologist and professor at Brown University who was Hua’s doctoral student when he began the research.
