According to the latest study, led by The University of Texas at Austin, scientists have uncovered a hidden layer of partially molten rock beneath the Earth’s surface that may solve long-standing questions surrounding the movements of tectonic plates.
The molten layer is roughly 100 miles below the surface and is part of the asthenosphere, located in the upper mantle beneath the Earth’s tectonic plates. Because it forms a relatively soft border that allows tectonic plates to migrate through the mantle, the asthenosphere is vital for plate tectonics.
However, the reasons behind its softness remain unknown. Molten rocks were once assumed to be a factor by scientists. However, this study found that melt does not appear to significantly impact 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 Junlin Hua, a postdoctoral fellow at UT’s Jackson School of Geosciences who led the research.
“But what we found is that even where the melt fraction is quite high, its effect on mantle flow is very minor.”
According to the research, which Hua began as a doctoral student at Brown University, the primary influence on plate motion is the convection of heat and rock in the mantle. Although the Earth’s interior is primarily solid, rocks can shift and flow like honey over extended periods of time.
According to coauthor and Jackson School professor Thorsten Becker, the discovery that the melt layer does not affect plate tectonics is one less complicated variable for computer models of the Earth.
“We can’t rule out that locally melt doesn’t matter,” said Becker, who designs geodynamic models of the Earth at the Jackson School’s University of Texas Institute for Geophysics. “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 crust, asthenosphere, and upper mantle are depicted in this figure. The partial melt layer is depicted as a speckled red layer beneath the crust. Subduction is indicated by a tectonic slab running down into the interior.
During his doctoral studies, Hua had the idea to look for a new layer in the Earth’s interior while investigating seismic images of the mantle beneath Turkey.
Intriguing hints of partially molten rock beneath the crust piqued Hua’s interest, and he collated comparable photos from additional seismic sites until he had a global map of the asthenosphere. What he and others thought was an abnormality was normal worldwide, showing up on seismic readings wherever the asthenosphere was heated.
The second surprise occurred when he compared his melt map to seismic readings of tectonic movement and discovered no correlation, even though the molten layer covered about half of 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 coauthor Karen Fischer, a seismologist and professor at Brown University who was Hua’s Ph.D. advisor when he began the research.
The research was published in the journal Nature Geoscience.