In geology, subduction is when a tectonic plate slides underneath another plate. The collective belief is that these tectonic plates stay rigid when they go under. A new study from the University of Southampton in the UK found this common belief does not apply to all instances of subduction.

During subduction, the plates slide underneath at a steep slope. During a flat-slab process, one plate slides underneath the other horizontally, making the two overlapping slides parallel. The world's biggest flat-slab can be found in Peru. Here the mainland South American Plate overlaps with the oceanic Nazca Plate.

Researchers found that the flat-slab is weaker than what was previously thought and could collapse easily. They discovered that the Nazca plate was distorted when the subduction took place.

"Our findings provide some of the first direct evidence that subducted slabs are not only weaker and softer than conventionally envisioned," said Caroline Eakin from the University of Southampton. Eakin is an Ocean and Earth Science research fellow who led the study. She added that the findings can give the scientific community a look inside the slabs and analyze the slabs' behavior as they sink deeper into the Earth.

Seismic anisotropy is a phenomenon wherein seismic waves move towards various directions through the same material. Looking at the speed in which the waves travel enabled the researchers to study the condition of the Nazca plate. From 2010 to 2013 and in coordination with the Peru Lithosphere and Slab Experiment (PULSE), the team measured the seismic waves in 15 local Peruvian stations and seven more stations positioned on other continents.

Olivine is a green, rock-forming mineral that readjusts with the plate's growth direction during subduction. The researchers discovered that the primary olivine structure in the Nazca plate had been altered and is now facing a different direction. Eakin described that the slab's interior might have become elongated when the subduction took place, suggesting that the weakened slab can deform internally and manifest in the upper mantle over time.

Lara Wagner, the principal investigator for PULSE, added that the findings provide information about the plates' ultimate fate and how the planet's scorching interior modifies the materials within the slabs.

The findings were published in the Nature Geoscience journal on Nov. 23.