Floppy Tectonic Plates Are Under the Earth
Maybe tectonic plates are not staying firm as they slip under another plate in the process called subduction. Or at least, not all of the time, says a study published recently in the journal Nature Geoscience by researchers at the U.K.'s University of Southampton.
Usually, plates during subduction slide with a constant rate into the less-dense mantle, using a relatively steep angle. But during flat-slab subduction, the lower plate sort of inserts itself horizontally below the upper plate, according to a release.
In the study, researchers learned about the Earth's largest case of flat slab, which is beneath Peru. There, the Nazca Plate from the ocean is subducted under the South American Plate, which is continental. The latter might be fairly weak and it changes shape easily, said the release.
The team learned about this deformation during subduction by looking at the speed for seismic waves traveling in different directions but passing through the same material. This is called seismic anisotropy, according to a release.
"Our findings provide some of the first direct evidence that subducted slabs are not only weaker and softer than conventionally envisioned, but also that we can peer inside the slab and directly witness their behavior as they sink," Dr. Caroline Eakin at the University of Southampton, said in the release.
At 15 local (Peruvian) seismic stations, from 2010 to 2013, the team measured seismic waves. They also conducted measurements at seven other stations on different continents. In part, they were looking at the structure of the mineral olivine in the area. Olivine realigns with the plate-growth direction when oceanic plates meet with ridges in the mid-ocean and move away from them. Then the olivine is made a solid part of the oceanic plate as it moves along. The seismic waves as a result move at differing speeds in different directions. In the study, the olivine structure inside the slab had changed to a different alignment facing in a different direction than before.
"Imaging Earth's plates once they have sunk back into the Earth is very difficult," said Lara Wagner from the Carnegie Institution for Science and with the study, in the release. "It's very exciting to see results that tell us more about their ultimate fate, and how the materials within them are slowly reworked by the planet's hot interior. The original fabric in these plates stays stable for so long at the Earth's surface, that it is eye opening to see how dramatically and quickly that can change," Lara added in the release.
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