Scientists have identified a previously unknown layer of partially molten rock hidden below the Earth’s crust — one that will help settle a long-standing puzzle about how tectonic plates move. The molten layer, researchers led from the University of Texas at Austin explained, sits at a depth of around 100 miles beneath the surface and is an element of the so-called asthenosphere, the mechanically weak and deformable a part of the upper mantle. The asthenosphere is very important for continental drift, because it forms a comparatively soft boundary that permits the bottom of tectonic plates to maneuver through the mantle.
The explanation why the asthenosphere is soft, nevertheless, isn’t well-understood.
Previous studies had identified patches of melt at 100 miles deep — but the brand new study is the primary to disclose that these belong to a layer that’s found around many of the globe.
In accordance with Dr Hua, the concept to search for a latest layer inside the Earth got here when he was studying images of the mantle beneath Turkey — formed by the passage of seismic waves — during his doctoral studies.
Intrigued by signs of partially molten rock under the crust in these images, he began to compile similar data from world wide, until he had put together essentially a worldwide map of the asthenosphere.
This “map” revealed that such molten rock was commonplace, encompassing almost half of the Earth and appearing on the seismic readings wherever the asthenosphere was hottest — and never, as had been previously assumed, only a series of localised anomalies.
It had previously been suggested the presence of molten rock within the asthenosphere might account for its “softness”.
Nonetheless, the researchers found, the presence of the melt layer doesn’t actually appear to have a major influence on the flow of mantle rocks.
Specifically, when Dr Hua compared his map of melt within the asthenosphere with seismic measurements of tectonic movement, he found no correlation.
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Paper writer and seismologist Dr Junlin Hua said: “After we take into consideration something melting, we intuitively think that the melt must play an enormous role in the fabric’s viscosity.
“But what we found is that even where the melt fraction is kind of high, its effect on mantle flow may be very minor.”
As an alternative, Dr Hua and colleagues found, the principal influence on the motion of Earth’s tectonic plates appears as a substitute to be the convention of warmth and rock within the mantle. While the Earth’s mantle is essentially solid, on long timescales it could actually flow like treacle.
Demonstrating that the newly-identified melt layer has no influence on plate tectonics means one less thing to fret about when creating models of the Earth, the team said.
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Paper co-author and geodynamicist Professor Thorsten Becker commented: “We will’t rule out that, locally, melt doesn’t matter.
“But I believe it drives us to see these observations of melt as a marker of what’s occurring within the Earth, and never necessarily an energetic contribution to anything.”
Seismologist and fellow co-author Professor Karen Fischer of Brown University concluded: “This work is very important because understanding the properties of the asthenosphere and the origins of why it’s weak is key to understanding plate tectonics.”
The total findings of the study were published within the journal Nature Geoscience.
