A long-running controversy over how tectonic plates move may finally be
resolved thanks to the discovery by scientists of a new layer of partially
molten rock beneath the Earth's crust.
Patches of melt had previously been found by researchers at a comparable
depth. But a recent research headed by The University of Texas at Austin
made the layer's worldwide breadth and role in plate tectonics public for
the first time.
The study was released in the journal Nature Geoscience today, February 6,
2023.
The asthenosphere, which lies in the upper mantle under the Earth's
tectonic plates, contains the molten layer, which is roughly 100 miles from
the surface. As a relatively supple border that permits tectonic plates to
migrate through the mantle, the asthenosphere is crucial for plate
tectonics.
However, the causes of its softness are not fully known. Previously,
scientists hypothesized that molten rocks may play a role. But as this study
demonstrates, melt really doesn't seem to have a significant impact on the
movement of mantle rocks.
The study's principal investigator, Junlin Hua, a postdoctoral researcher
at the University of Texas at Austin's Jackson School of Geosciences, said:
"When we think about anything melting, we instinctively imagine that the
melt must have a large influence in the material's viscosity. However, we
discovered that even in cases when the melt percentage is extremely large,
the impact on mantle flow is minimal.
The study, which Hua started as a doctoral student at Brown University,
claims that the primary factor affecting the motion of the plates is the
convection of heat and rock in the mantle. Despite the fact that the
interior of the Earth is essentially solid, over a very long time, rocks may
move and flow like honey.
According to cofounder and professor at the Jackson School Thorsten Becker,
demonstrating that the melt layer has no impact on plate tectonics removes
one challenging variable from Earth computer models.
According to Becker, who creates geodynamic models of the Earth at the
Jackson School's University of Texas Institute for Geophysics, "We can't
exclude out that locally melt doesn't matter." But I believe it forces us to
see these melt data as a marker of what's happening in the Earth rather than
as an active cause of anything.
During his dissertation studies, Hua looked at seismic photographs of the
mantle under Turkey and had the notion to hunt for a new layer in the
Earth's interior.
Hua gathered comparable photos from additional seismic sites after becoming
intrigued by indications of partially molten rock beneath the crust.
Eventually, he produced a worldwide map of the asthenosphere. In reality,
what he and others had thought to be an oddity was widespread, showing up on
seismic data wherever the asthenosphere was warmest.
The second surprise occurred when, despite the molten layer covering over
half of the Earth, he compared his melt map with seismic readings of
tectonic activity and discovered no association.
Hua's Ph.D. adviser at Brown University, Karen Fischer, a seismologist, and
a coauthor of the study said, "This work is essential because knowing the
features of the asthenosphere and the origins of why it is weak is key to
understanding plate tectonics."
