Geologists Discover a Hidden Subduction Zone Beneath the Pacific Ocean, Transforming Our Knowledge of Earth’s Interior.

Scientists from the University of Maryland have found indications of an ancient seafloor that sank deep into the Earth during the era of dinosaurs. This discovery challenges our current understanding of the Earth’s interior. Situated at the East Pacific Rise, a boundary of tectonic plates on the ocean floor in the southeastern Pacific, this previously unexplored area of seafloor provides fresh insights into the dynamics of our planet and the changes its surface has undergone over millions of years. The results of this study were published in the journal Science Advances on September 27, 2024.

Under the leadership of Jingchuan Wang, a geology postdoctoral researcher, the team utilized advanced seismic imaging methods to investigate the Earth’s mantle, which lies between the crust and the core. They detected an unexpectedly thick region within the mantle transition zone, located roughly 410 to 660 kilometers beneath the Earth’s surface. This zone separates the upper and lower mantles and can either expand or contract due to temperature variations. The researchers believe that this newly identified seafloor might also account for the unusual structure of the Pacific Large Low Shear Velocity Province (LLSVP) — an extensive region in Earth’s lower mantle — as it seems to be divided by the subducted slab.

“This thickened area resembles a fossilized fingerprint of an ancient seafloor that sank into the Earth around 250 million years ago,” Wang stated. “It is providing insights into Earth’s history that we’ve never encountered before.”

Subduction happens when one tectonic plate descends beneath another, sending surface materials back into the mantle. This process often produces tangible signs such as volcanoes, earthquakes, and deep ocean trenches. While geologists typically study subduction by analyzing rock samples and sediments found at the surface, Wang, along with Geology Professor Vedran Lekic and Associate Professor Nicholas Schmerr, opted to utilize seismic waves to investigate the ocean floor. By studying the way seismic waves moved through various layers of the Earth, the scientists were able to create detailed maps of the structures hidden deep within the mantle.

“Think of seismic imaging as akin to a CT scan. It essentially gives us a cross-sectional view of the inner workings of our planet,” Wang explained. “Typically, oceanic slabs are completely consumed by the Earth, leaving no visible signs on the surface. However, observing the ancient subduction slab through this method revealed new connections between deep Earth structures and surface geology that had not been previously apparent.”

The findings were unexpected — the movement of materials within Earth’s interior was found to be significantly slower than earlier projections. Wang theorizes that the unusual thickness identified by the team indicates colder materials in this segment of the mantle transition zone, suggesting that certain oceanic slabs may become trapped midway as they descend through the mantle.

“We discovered that materials in this area were sinking at approximately half the anticipated speed, implying that the mantle transition zone may function as a barrier, impeding the flow of material through the Earth,” Wang clarified. “Our findings raise new questions about how deep Earth processes impact what we observe on the surface over extensive distances and across long timescales.”

Looking forward, the team aims to expand their investigations into additional regions of the Pacific Ocean and beyond. Wang aspires to compile a more comprehensive map of ancient subduction and upwelling zones, as well as their implications for both deep and surface Earth structures. Utilizing the seismic data obtained from this research, Wang and fellow scientists are refining their models regarding the movements of tectonic plates throughout Earth’s history.

“This is merely the beginning,” Wang remarked. “We are confident that there are many more ancient structures waiting to be uncovered in the depths of the Earth. Each one holds the potential to offer new insights into our planet’s intricate past — and could even enhance our understanding of other planets beyond our own.”