Scientists detect molten rock layer hidden under Earth's tectonic plates


Date:
February 6, 2023
Source:
University of Texas at Austin
Summary:
Scientists have discovered a new layer of partly molten rock under
the Earth's crust that might help settle a long-standing debate
about how tectonic plates move. The molten layer is located about
100 miles from the surface and is part of the asthenosphere, which
is important for plate tectonics because it forms a relatively soft
boundary that lets tectonic plates move through the mantle. The
researchers found, however that the melt does not appear to
notably influence the flow of mantle rocks. Instead, they say,
the discovery confirms that the convection of heat and rock in
the mantle are the prevailing influence on the motion of the plates.


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FULL STORY ========================================================================== Scientists have discovered a new layer of partly molten rock under the
Earth's crust that might help settle a long-standing debate about how
tectonic plates move.


========================================================================== Researchers had previously identified patches of melt at a similar
depth. But a new study led by The University of Texas at Austin revealed
for the first time the layer's global extent and its part in plate
tectonics.

The research was published Feb. 6, 2023, in the journal Nature Geoscience.

The molten layer is located about 100 miles from the surface and is part
of the asthenosphere, which sits under the Earth's tectonic plates in
the upper mantle. The asthenosphere is important for plate tectonics
because it forms a relatively soft boundary that lets tectonic plates
move through the mantle.

The reasons why it is soft, however, are not well understood. Scientists previously thought that molten rocks might be a factor. But this study
shows that melt, in fact, does not appear to notably influence 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 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 the melt layer has no influence on plate tectonics means
one less tricky variable for computer models of the Earth, said coauthor Thorsten Becker, a professor at the Jackson School.

"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 idea
to look for a new layer in Earth's interior came to Hua while studying
seismic images of the mantle beneath Turkey during his doctoral research.

Intrigued by signs of partly molten rock under the crust, Hua compiled
similar images from other seismic stations until he had a global map of
the asthenosphere. What he and others had taken to be an anomaly was in
fact commonplace around the world, appearing on seismic readings wherever
the asthenosphere was hottest.

The next surprise came when he compared his melt map with seismic
measurements of tectonic movement and found no correlation, despite the
molten layer encompassing almost half 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 funded by the U.S. National Science
Foundation. Collaborating institutions included the UT Oden Institute
for Computational Engineering and Sciences and Cornell University.

* RELATED_TOPICS
o Earth_&_Climate
# Geology # Earthquakes # Natural_Disasters #
Earth_Science # Tsunamis # Environmental_Issues #
Atmosphere # Ozone_Holes
* RELATED_TERMS
o Crust_(geology) o Mantle_plume o Mid-ocean_ridge o Volcano
o Earth o Lithosphere o Basalt_rock o Earthquake

========================================================================== Story Source: Materials provided by University_of_Texas_at_Austin. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Junlin Hua, Karen M. Fischer, Thorsten W. Becker, Esteban Gazel,
Greg
Hirth. Asthenospheric low-velocity zone consistent with globally
prevalent partial melting. Nature Geoscience, 2023; DOI:
10.1038/s41561- 022-01116-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/02/230206130628.htm

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