According to a study published in Scientific Reports, anomalies deep within the Earth's mantle appear to be remnants of submerged tectonic plates—but located far from where such plates are typically found.
Using high-resolution models and the Piz Daint supercomputer of the Swiss National Supercomputing Centre (CSCS) in Lugano, the researchers detected these anomalies beneath large oceans and within the interiors of continents, far from current plate boundaries. "With the new high-resolution model, we can see this type of anomaly everywhere in the Earth's mantle, but we don't know exactly what they are or what material creates the patterns we've discovered," explained Professor Andreas Fichtner from ETH Zurich, author of the study, according to El Comercio.
Traditionally, submerged plates have been discovered where two tectonic plates meet and one subducts beneath the other into the Earth's interior. However, the newly found anomalies defy these expectations. According to current theories about plate tectonics, there should not be subducted plate material in these areas, as it is impossible that there have been subduction zones nearby in recent geological history.
The research team suggests that these anomalies could be formed by primordial materials from the mantle or accumulations of iron-rich or silica-rich rock, raising questions about Earth's internal dynamics. "We believe that the anomalies in the lower mantle have a variety of origins," stated Thomas Schouten, the first author and PhD student at the Geological Institute of ETH Zurich, as reported by Phys.org.
To uncover these hidden structures, the scientists employed full waveform inversion, a method that uses information from all frequencies of seismic waves—including P, S, and surface waves. This approach improves the accuracy of the models compared to methods that only analyze specific frequencies. However, full waveform inversion is computationally intensive, necessitating the use of the Piz Daint supercomputer, one of the most powerful in Europe.
"The waves we use for the model essentially only represent one property, namely the speed at which they travel through the Earth's interior," explained Schouten. "We have to calculate the different material parameters that could generate the observed speeds of the different types of waves. Basically, we have to delve into the material properties behind the wave speed."
One of the newly discovered anomalies is located beneath the western Pacific Ocean. The presence of such anomalies in unexpected locations has led scientists to reconsider current models of mantle dynamics. The remnants of submerged plates could have implications for our understanding of how materials mix and recycle within the mantle, potentially requiring revisions to previous geological models that assume rapid mixing and recycling.
"It's like if a doctor has been examining blood circulation with ultrasounds for decades and finds arteries exactly where he expects them. If you give him a new and better exploration instrument, suddenly he sees an artery in the buttock that shouldn't be there. That's exactly how we feel with the new findings," Fichtner explained, as per Exclsior.
Despite the discovery, the researchers acknowledge that they cannot yet say conclusively what exactly is hidden within these anomalies. They can only study the zones indirectly via the speed of seismic waves. "However, this does not do justice to the complex interior of the Earth," Schouten added, as reported by El Comercio.
The discovery raises fundamental questions about subduction processes and the geological evolution of the Earth. It could suggest older tectonic events or unknown mechanisms that have yet to be uncovered. "Apparently, such zones in the Earth's mantle are much more widespread than previously thought," said Schouten, according to Phys.org.
Geophysicists use seismograms, or earthquake recordings, to determine the speed at which seismic waves propagate. They use this information to calculate the internal structure of the Earth. Computational models like the one used in this study can process data in a way that identifies anomalies on scales that were previously impossible to detect.
This article was written in collaboration with generative AI company Alchemiq