Beneath the vast expanse of the Pacific Ocean, a mysterious phenomenon has captivated geophysicists, challenging their understanding of our planet's inner workings. A new high-resolution seismic model has revealed something unexpected, leaving scientists with a perplexing enigma.
The Pacific's Hidden Enigma
Deep beneath the Pacific, far from the reach of drilling, scientists have stumbled upon a peculiar discovery. A structure, unlike anything expected, has been found in the lower mantle, defying the textbook image of our planet. This finding has sparked curiosity and raised questions among researchers.
The seismic model unveiled giant zones of unusually fast-moving rock in the lower mantle, resembling the remnants of ancient tectonic plates. However, these formations exist under open ocean and continental interiors, where subduction zones, typically associated with such structures, are not known to occur.
Unraveling the Mystery with Earthquakes
So, how do geophysicists 'see' what lies so deep without physically going there? The answer lies in earthquakes. Each powerful tremor generates waves that ripple through the planet, and seismographs record their arrival and behavior, akin to a global medical scan. Traditionally, most global mantle images were created by measuring the travel times of specific waves, primarily the direct P and S phases.
While this approach produced the first 3D maps of the mantle, it had its limitations. It favored regions with frequent earthquakes and dense seismic networks, leaving large areas under old ocean plates or quiet continents less defined. The new study, however, takes a different approach, utilizing 'full waveform inversion'.
Full Waveform Inversion: A New Perspective
The team at ETH Zurich and their collaborators employed a method that analyzes complete earthquake seismograms, incorporating reflected and refracted waves that were previously overlooked. This technique enhances sensitivity throughout the mantle, even in areas lacking nearby quakes or instruments. However, running such a model requires powerful computing, and the researchers relied on the Piz Daint supercomputer in Switzerland to process vast amounts of data.
A Forest of Hidden Anomalies
The REVEAL model, a high-resolution global model, revealed a mantle far more heterogeneous than previous maps suggested. In the mid and lower mantle, the team discovered numerous large regions with faster-than-average seismic wave movement, not only beneath familiar subduction zones but also under the Pacific, Atlantic, and Indian oceans, as well as stable continental interiors.
Interpreting the Anomalies
Historically, geophysicists have interpreted such anomalies as 'slabs,' the cold remnants of old ocean plates that have sunk into the mantle at subduction zones. However, the new model challenges this simple picture. When compared with detailed plate boundary reconstructions, the fast anomalies do not align with expected subduction zones, suggesting that many 'blobs' may not be what they seem.
The Puzzle Unveiled
Thomas Schouten, the lead author, bluntly states that the team can now see anomalies 'everywhere in the Earth's mantle,' yet they still 'do not know exactly what they are or what material is creating the patterns.' The study proposes diverse origins for these structures, including ancient plate fragments, silica-rich mantle material, and zones where iron-rich rocks have pooled over billions of years.
Implications for Our Understanding of the Planet
This discovery has significant implications for our understanding of the planet's inner workings. The same mantle circulation that shapes these hidden structures drives plate motions, influences volcanic activity, and impacts long-term sea levels and carbon cycling. If many fast anomalies are not simple cold slabs, scientists will need to reevaluate their climate models and estimates of past atmospheric carbon dioxide levels.
The Next Steps
For Schouten and his colleagues, the next step is clear: they aim to go beyond mapping wave speeds and delve into the material properties that could produce them, from mineral mix to temperature to grain size. As our 'ultrasound' of Earth improves, the familiar picture of a tidy, layered planet is giving way to a more intricate and surprising reality. The hidden structures beneath the Pacific serve as a reminder that even on our well-studied home world, there are still deep mysteries waiting to be unraveled.
