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University of Utah researchers say they have discovered a previously unknown layer of earth that is 900 miles deep and stiffer than believed.
Findings published today in Nature Geoscience also suggest Earth is hotter than previously thought. The new study could explain some deep earthquakes and account for differences in volcanic magma from Iceland to Hawaii.
Until now, scientists had believed that the stiffness of the Earth's layers varied more slightly in the planet's interior.
"Essentially, we have discovered a new layer in the Earth," researcher Lowell Miyagi said in a prepared statement. "This layer isn't defined by the minerals present, but by the strength of these minerals."
Earth's thin crust about 4 miles deep under oceans and 50 miles under continents covers a "mantle" layer that is 1,800 miles deep to the iron core of the planet. Tectonic plates about 60 to 90 miles thick are clustered in the crust and upper mantle.
Miyagi and German physicist Hauke Marquardt of the University of Bayreuth worked together to identify a "superviscous" or very stiff layer in the lower mantle.
The researchers simulated pressures in Earth's lower mantle by pressing ferropericlase, a mineral found in the lower mantle, between diamond anvils. They found the mineral gets stronger at pressures equivalent to those at about 410 miles deep – the upper-lower mantle boundary – and intensifies threefold by the time it reaches the pressure level of a 930-mile depth. At that point, ferropericlase becomes 300 times stiffer than at the first boundary.
"The result was exciting," Miyagi said.
Previous seismic images show that many slabs, which pull plates along as they sink into Earth's interior, stall at around 930 miles, some under Indonesia and South America's Pacific coast a phenomenon thought to cause earthquakes and volcanic eruptions.
The research team believes the study explains why magma from Hawaii and other island volcanoes is different. It's from a deeper, older, less well-mixed layer of Earth.
The findings imply that Earth's interior is hotter than believed, Miyagi said in a press release. If rock cannot mix as easily, heat cannot escape as easily.
The study was paid for by the National Science Foundation and the German Science Foundation.