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Geophysicists at ETH Zurich are employing models of the lower mantle to locate regions where seismic waves exhibit behavior contrary to what was previously anticipated. This suggests the presence of rock formations that are cooler or possess a different composition compared to the surrounding material. This discovery challenges our prevailing conception of the Earth’s tectonic plates and presents the researchers with an intriguing enigma.
No one can peer inside the Earth. Nor can anyone drill sufficiently deep to extract rock specimens from the mantle, the layer between the Earth’s core and the outermost rigid envelope, the lithosphere, or assess temperature and pressure at that depth. Consequently, geophysicists rely on indirect techniques to uncover what exists deep beneath the Earth’s surface.
For instance, they utilize seismograms, or records of earthquakes, to ascertain the velocity at which seismic waves travel. They then employ this data to infer the internal architecture of the Earth. This process closely resembles how medical professionals utilize ultrasound to visualize organs, muscles, or veins within the body without performing open surgery.
This is how it operates: when seismic activity occurs, waves propagate outward from the epicenter in all directions. As they traverse the Earth, they are refracted, diffracted, or reflected. The rate at which the waves travel depends on the type of wave and the density and elasticity of the materials they traverse. Seismographic stations capture these various waves, allowing geophysicists to deduce insights regarding the Earth’s structure and composition and to investigate the processes occurring within.
By analyzing seismic data, Earth scientists have located the position of submerged tectonic plates throughout the Earth’s mantle. They have consistently found them where anticipated: in regions identified as subduction zones, where two plates converge, and one descends beneath the other into the Earth’s interior. This has facilitated researchers in examining the tectonic plate cycle, i.e., the formation and destruction of plates on the Earth’s surface through geological history.
Plate remnants in unexpected locales
However, a group of geophysicists from ETH Zurich and the California Institute of Technology has made an astonishing discovery: employing a new high-resolution model, they have identified additional areas in the Earth’s interior resembling remains of submerged plates.
Yet, these zones are not situated where they were prognosticated; rather, they lie underneath expansive oceans or within continental interiors—far from tectonic plate boundaries. There is also an absence of geological proof of historical subduction in those regions. This research was recently published in the journal Scientific Reports.
The innovative aspect of their modeling technique is that the ETH researchers are engaging not just a singular type of seismic wave for analyzing the Earth’s internal structure; they are utilizing all varieties. Experts refer to this methodology as full-waveform inversion. This makes the model computationally demanding, which is why the researchers enlisted the Piz Daint supercomputer at the CSCS in Lugano. Is there an undiscovered world beneath the Pacific Ocean?
“Evidently, such areas in the Earth’s mantle are far more widespread than previously believed,” asserts Thomas Schouten, lead author and doctoral candidate at the Geological Institute of ETH Zurich.
One newly identified area is situated beneath the western Pacific. Nevertheless, based on existing theories and knowledge regarding plate tectonics, there should be no remnants of subducted plates present since it’s improbable that subduction zones were in proximity during recent geological times.
The researchers remain uncertain regarding the material involved and its implications for the dynamics within the Earth. “That’s our conundrum. Using the advanced high-resolution model, we observe such anomalies across the Earth’s mantle. However, we are clueless about their exact nature or the material producing the patterns we have detected.”
It’s akin to a physician who has been assessing blood circulation with ultrasound for years and identifies arteries precisely where expected, notes ETH professor Andreas Fichtner. “Then, with a new, superior examination tool, he suddenly observes an artery in the buttock that shouldn’t be there at all. That’s precisely how we react to the new discoveries,” elaborates the wave physicist, who developed the model within his team and wrote the corresponding code.
Gaining deeper insights from waves
Currently, the investigators can merely hypothesize. “We suspect that the anomalies within the lower mantle stem from multiple sources,” states Schouten. He surmises it is plausible that they consist of more than just the cold plate substance that has descended over the past 200 million years, as was earlier believed.
“It may either be ancient, silica-laden material that has existed since the formation of the mantle approximately 4 billion years ago, enduring the convective movements within the mantle, or regions where iron-rich rocks have formed as a result of these mantle movements over billions of years,” he explains.
For the doctoral candidate, this signifies that further investigation with advanced models is crucial to uncover additional details about Earth’s interior.
“The waves we utilize for the model primarily reflect one characteristic, namely the velocity at which they traverse through the Earth’s interior,” remarks the Earth scientist. However, this fails to capture the intricacy of the Earth’s composition.
“We need to compute the various material parameters that could account for the observed velocities of the different wave types. Essentially, we must delve deeper into the material characteristics that influence wave speed,” adds Schouten.
Additional information:
Thomas L. A. Schouten et al, Full-waveform inversion reveals diverse origins of lower mantle positive wave speed anomalies, Scientific Reports (2024). DOI: 10.1038/s41598-024-77399-2
Reference:
Submerged realms beneath the Pacific? Detailed models unveil the mechanisms of Earth’s mantle (2025, January 7)
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