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Scientists have identified an old chunk of the Pacific Ocean – the ancient remains of its ancient seabed – stretching for hundreds of miles below China, as it is pulled down into the mantle transition zone earthly.
This rocky slab that once lined the bottom of the Pacific is a relic of the ocean lithosphere, the outermost layer of the Earth’s surface, made up of the crust and the solid outer parts of the upper mantle.
However, the lithosphere is not always meant to take advantage of the views at the top. The upper surface layer is made up of several fragmented tectonic plates, which move and move slowly on the surface, sometimes colliding with each other.
During these collisions, a geological process called subduction can occur, where one plate is forced under the other in subduction zones, and ends up being driven ever deeper into the planet.
In a new study, Chinese and American scientists have now witnessed this epic phenomenon unfolding at greater depths than ever before.
Prior to that, scientists had recorded subduction slabs probing the boundaries at depths of about 200 kilometers (about 125 miles).
Now, thanks to a giant network of more than 300 seismic stations spread across northeast China, researchers were able to see the event at a much lower point, imagining parts of the tectonic plate that once stood under the Pacific Ocean being pushed into the middle of the mantle. level transition zone, at depths between 410 and 660 kilometers (254 and 410 miles) below the Earth’s surface.
To interpret the sinking slab, the team identified two seismic velocity discontinuities, deep underground regions where seismic waves encounter anomalies. In this case, two anomalies were encountered, which, according to the team, involve both the upper and lower sides of the dip plate.
“Based on detailed seismological analyzes, the upper discontinuity was interpreted as the Moho discontinuity of the subductured slab,” explains geophysicist Qi-Fu Chen of the Chinese Academy of Sciences.
“The smaller discontinuity is probably caused by the partial melting of the sub-slab asthenosphere under water conditions in the sea-side portion of the slab.”
While the plate’s subduction can be seen in progress under the China, the subduction zone itself lies far to the east, with the slab tilted at a relatively shallow angle of 25 degrees downward.
“Japan is located roughly where the Pacific Plate reaches a depth of about 100 kilometers,” says seismologist Fenglin Niu of Rice University.
Thanks to the new imagery, scientists have a better idea of what happens to a subducted slab when it reaches that part of the transition zone, including how much it deforms and how much water it loses from its oceanic crust.
“Many studies suggest that the slab deforms a lot in the mantle transition zone, that it becomes soft, so it deforms easily,” Niu explains.
“We are still debating whether this water is fully released at this depth. There is growing evidence that some of the water stays inside the plate and goes much, much deeper.
The results are reported in Geoscience of nature.
This article was originally published by Science Alert. Read the original article here.
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