Ancient Seafloor Discovered Beneath Pacific Ocean Rewrites Earth’s Tectonic History and Offers Clues About Planetary Evolution

Geologists have uncovered evidence of a long-lost seafloor buried deep beneath the Pacific Ocean, offering crucial insights into Earth's geological history.

This remarkable discovery, led by researchers from the University of Maryland, challenges long-standing theories about the behavior of the planet's deep interior, specifically within the mantle transition zone. The findings suggest that remnants of an ancient tectonic plate, which sank over 100 million years ago, are influencing the mantle’s structure and dynamics today, offering a new lens through which to study the geological forces that shaped our planet.

Uncovering the Buried Seafloor: A Seismic Breakthrough

Using cutting-edge seismic imaging technology, a team led by Jingchuan Wang, a postdoctoral researcher in geology at the University of Maryland, has mapped out a mysterious portion of Earth's mantle transition zone, located approximately 410 to 660 kilometers beneath the ocean floor. This section of the mantle, spanning a vast area east of the East Pacific Rise, was found to be unusually thick and cold. The researchers believe this anomaly represents the remains of an ancient oceanic plate that subducted into the Earth’s interior during the Mesozoic era, between 250 and 120 million years ago.

Wang and his colleagues used a seismic technique known as SS precursor analysis, which involves examining the way seismic waves bounce off boundaries within Earth's deep layers before reaching the surface. Through this method, they were able to detect what Wang described as “a fossilized fingerprint of an ancient piece of seafloor that subducted into the Earth approximately 250 million years ago.” The slab, preserved in the mantle transition zone, has remained trapped for over 100 million years, providing researchers with a unique glimpse into Earth’s distant past.

The Impact on Mantle Dynamics and Plate Tectonics

One of the most intriguing aspects of the discovery is the effect the ancient sunken plate has on the Large Low Shear Velocity Province (LLSVP), a massive region of Earth's lower mantle characterized by slower-than-average seismic waves. The LLSVP, which lies beneath the Pacific Ocean, has long puzzled scientists due to its unusual structure. The team’s findings reveal that the ancient seafloor may have split the LLSVP, acting like a wedge as it descended into the mantle.

This new information not only helps explain the curious structure of the Pacific LLSVP, but also provides a deeper understanding of how mantle convection—the slow, churning movement of Earth’s interior—affects the planet’s surface over millions of years. According to Wang, “Our discovery opens up new questions about how the deep Earth influences what we see on the surface across vast distances and timescales.” The research suggests that the mantle transition zone, which separates Earth’s upper and lower mantles, acts as a barrier that can slow down the sinking of subducted plates, a finding that challenges previous models of how material moves through the planet.

The Phoenix Plate: A Relic from The Age of Dinosaurs

The researchers propose that the ancient subducted slab may belong to the Phoenix Plate, a tectonic plate that once dominated a large portion of the Pacific Ocean before it was consumed by intraoceanic subduction. This process, which occurs when one oceanic plate is forced beneath another, resulted in the plate sinking deep into Earth’s mantle. As it descended, the plate carried cooler material from the ocean floor into the hot mantle, leaving behind a cold thermal signature that is still detectable today.

This subduction event, which occurred during the age of dinosaurs, may have shaped many of the features of Earth’s mantle that scientists are only now beginning to understand. “We found that in this region, the material was sinking at about half the speed we expected,” Wang explained, “which suggests that the mantle transition zone can act like a barrier and slow down the movement of material through the Earth.” This unexpected finding indicates that some oceanic slabs may become "stuck" in the mantle transition zone for extended periods, rather than descending directly into the lower mantle.

A New Understanding of Earth’s Geological Past

The discovery of this ancient seafloor has significant implications for how scientists understand Earth's geological processes, particularly those related to subduction and mantle dynamics. Typically, subduction zones are associated with surface-level phenomena like volcanic eruptions and earthquakes, but Wang’s research shows that ancient subducted plates can remain preserved deep within Earth’s interior, influencing mantle structures for hundreds of millions of years. This new information could lead to revisions in models of plate tectonics and provide a better understanding of how Earth's surface has evolved over geological timescales.

The study, published in Science Advances on September 27, 2024, marks the beginning of a new era in the study of Earth’s deep interior. The researchers plan to extend their seismic imaging work to other parts of the Pacific and beyond, with the hope of discovering additional ancient subducted structures. “This is just the beginning,” Wang noted. “We believe that there are many more ancient structures waiting to be discovered in Earth’s deep interior. Each one has the potential to reveal many new insights about our planet’s complex past—and even lead to a better understanding of other planets beyond ours.”

Wang’s work not only opens up new avenues for studying Earth’s deep mantle but also has the potential to offer clues about the geological processes of other planets. By understanding how tectonic plates have moved and interacted over Earth's history, scientists may be able to apply these models to the study of Mars, Venus, and other rocky planets in our solar system. The insights gained from this research could help explain the geological evolution of planets that lack plate tectonics, offering a broader perspective on planetary formation and behavior.