Tectonic Plates May be Stretchy Instead of Rigid Deep Within Earth

A new study suggests that the common believe that the Earth's tectonic plates stay strong when they slide under another plate may not be universal. It could be that there's a stretchy material that causes them to flex when sliding.

Typically during subduction, plates slide down at a constant rate into the warmer, less-dense mantle at a fairly steep angle. However, in a process called flat-slab subduction, the lower plate moves almost horizontally underneath the upper plate.

In this latest study, the researchers studied the speed at which seismic waves travel in different direction through the same material, a phenomenon called seismic anisotropy. This revealed that the interior of the Nazca plate, which is the plate being subducted under the continental South American Plate, had been deformed during subduction.

"The process of consuming old seafloor at subduction zones, where great slabs of oceanic material are swallowed up, drives circulation in the Earth's interior and keeps the planet going strong," said Caroline Eakin, one of the researchers, in a news release. "One of the most crucial but least known aspects of this process is the strength and behavior of oceanic slabs once they sink below the Earth's surface. 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."

With their new observations, the researchers found that the slab's interior must have been stretched or deformed during subduction. This means that the slabs are weak enough to deform internally in the upper mantle over time.

The deformation associated with stretching of the slab as it depends to take on its flat-slab shape may have been enough to create a new alignment, which closely follows the contours of the slab bends.

The findings reveal a bit more about what's occurring beneath Earth's surface, which tells researchers a bit more about Earth itself.

The findings are published in the journal Nature Geoscience.

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