Beneath the vast expanse of the Pacific Ocean lies a mysterious phenomenon that challenges our understanding of the Earth's inner workings. A recent study, utilizing advanced seismic modeling, has revealed the presence of something unexpected, leaving geophysicists perplexed.
Deep beneath the ocean floor, far from the reach of drilling, scientists have discovered peculiar zones of exceptionally fast-moving rock within the Earth's lower mantle. These regions bear resemblance to the remnants of ancient tectonic plates, yet they exist under open ocean and continental areas where no known subduction zones exist. This intriguing finding has sparked curiosity and raised numerous questions.
One of these enigmatic structures lies beneath the western Pacific, at a depth of approximately 900 to 1,200 kilometers. Interestingly, this region has no geological evidence of plate subduction in the past 200 million years. How can we 'see' something so deep without physically exploring it? Geophysicists employ earthquakes as a powerful tool. Each major earthquake generates waves that travel through the Earth, and seismographs record their arrival and behavior, akin to a global medical scan.
For decades, most mantle imaging relied on measuring the travel times of specific waves, primarily the direct P and S phases. While this approach yielded the first 3D maps of the mantle, it had limitations. It favored regions with frequent earthquakes and dense seismic networks, such as those surrounding the Pacific Rim. Consequently, large areas beneath ancient ocean plates or quiet continents remained less defined.
The new study introduces a novel approach called 'full waveform inversion.' Instead of selecting specific wave arrivals, researchers analyze complete earthquake seismograms. This method incorporates reflected and refracted waves that were previously overlooked, enhancing sensitivity across the entire mantle, even in areas lacking nearby earthquakes or instruments.
Running such a complex model requires substantial computational power. The researchers utilized the Piz Daint supercomputer in Switzerland to process data from numerous earthquakes and construct a high-resolution global model named REVEAL. This model reveals a mantle far more heterogeneous than previously imagined.
REVEAL's findings indicate numerous large regions within the mid and lower mantle where seismic waves travel faster than average. These fast-wave zones are not limited to subduction zones but also exist beneath the Pacific, Atlantic, and Indian oceans, as well as stable continental interiors.
Historically, geophysicists interpreted these anomalies as 'slabs,' which are the cold remnants of old ocean plates that have sunk into the mantle at subduction zones and continued to descend vertically over millions of years. However, the new model challenges this neat explanation.
When comparing fast anomalies with detailed plate boundary reconstructions, the study reveals that only about 60-70% of reconstructed subduction zones align with positive wave speed regions in the lower mantle. This correlation diminishes when accounting for sampling biases. Consequently, many 'blobs' do not correspond to where subducted plates are expected.
Thomas Schouten, the lead author, succinctly summarizes the puzzle. The new model reveals anomalies throughout the Earth's mantle but fails to pinpoint their exact nature or the material responsible for these patterns. The study suggests diverse origins for these structures.
Some may be fragments of old plates, while others could be ancient, silica-rich mantle material that has survived the slow convection currents over billions of years. Additionally, some anomalies might represent zones where iron-rich rocks have gradually accumulated as mantle currents redistributed materials.
Previous geochemical and geodynamic research hinted at the Earth's mantle being a 'marble cake' of mixed rock types rather than a perfectly uniform layer. The new seismic images support this view, demonstrating how compositional differences can mimic or obscure temperature variations in wave speed signals.
The implications of these findings extend far beyond the depths of the Earth. The same mantle circulation that shapes these hidden structures also drives plate motions, influences volcanic activity, and plays a role in long-term sea levels and the slow cycling of carbon between the interior and the atmosphere.
Climate reconstructions have utilized deep mantle slab images, but if many fast anomalies are not simple cold slabs, scientists must carefully address this uncertainty. The Earth's interior engine remains active, but we are now uncovering its more complex mechanisms.
Looking ahead, Schouten and his colleagues aim to delve deeper into the material properties that could produce these wave patterns, considering factors such as mineral composition, temperature, and grain size. As our 'ultrasound' of the Earth becomes more sophisticated, our understanding of its intricate layers continues to evolve, revealing surprises even on a planet we thought we knew well.
