The Discovery Of A Layer Of Molten Rock Hidden Beneath The Earth's Tectonic Plates
New research has found that, some 150 kilometers (93 miles) below Earth's surface, there is an unknown layer of molten rock, which could help scientists learn more about the movements of Earth's tectonic plates.
Discovering this viscous layer will help researchers better understand how tectonic plates "float" above this mantle layer, study lead author Gonlin Hua, a postdoctoral researcher in geosciences at the University of Texas at Austin, told Live Science.
The molten rock lies in the asthenosphere (asthenosphere, or flow zone), the uppermost layer of the mantle between about 80 km (50 miles) and 200 km (124 miles) below Earth's surface.
The only way to probe this layer of the mantle is with seismic waves. Scientists can draw on wave data at seismic stations around the world and look for subtle changes in waveforms that indicate the types of material the waves are traveling through.
Previously, scientists learned from these types of studies that some parts of the asthenosphere were hotter than others, Hua said, and patchy melting zones were discovered. But little was known about how deep and widespread the melt was.
To find out, Hua and his colleagues collected data from thousands of seismic waves detected at 716 stations around the world. They found that rather than retaining small areas of melt, the asthenosphere appears to contain a partially thawed layer that extends around the globe, under at least 44 percent of the planet.
The researchers found that this area is widely distributed around the world and could be much larger, because they were unable to investigate under the ocean, which is likely to cover a layer of melt that occupies a much larger area than the continents.
Oddly enough, this melted layer does not seem to affect the motions of tectonic plates. The scientists found that regions of melting did not affect the viscosity of the mantle, or the tendency to flow.
"These molten rocks plus hard rocks are not as easy to deform as compared to hard rocks alone," Hua said. So, contrary to expectations, this magma, although present, will not affect how easily tectonic plates can move over the asthenosphere.”
This is useful information, the study authors say, for building computer models of how the plates move.
"We cannot rule out that local melting is not significant," Thorsten Becker, a geophysicist at the University of Austin and one of the study's authors, said in a statement. But I think it prompts us to see these melt observations as a sign of what's going on in the ground, not necessarily an active contributor to anything."
Hua added that there is still more work to be done to map this melted mantle layer.
He continued: “In this study, we mainly use seismic instruments on the continents, and although we also used some instruments from ocean islands, there are definitely some degrees of data gap in the ocean. Hence, a follow-up study will use other types of data or seismic tools found on the ocean floor to fill in this gap.”
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