Geologists discover mysterious subduction zone beneath Pacific, reshaping understanding of Earth’s interior

University of Maryland scientists uncovered evidence of an ancient seafloor that sank deep into Earth during the age of dinosaurs, challenging existing theories about Earth’s interior structure. Located in the East Pacific Rise (a tectonic plate boundary on the floor of the southeastern Pacific Ocean), this previously unstudied patch of seafloor sheds new light on the inner workings of our planet and how its surface has changed over millions of years. The team’s findings were published in the journal Science Advances on September 27, 2024.

Led by geology postdoctoral researcher Jingchuan Wang, the team used innovative seismic imaging techniques to peer deep into Earth’s mantle, the layer between our planet’s crust and core. They found an unusually thick area in the mantle transition zone, a region located between about 410 and 660 kilometers below the Earth’s surface. The zone separates the upper and lower mantles, expanding or contracting based on temperature. The team believes that the newly discovered seafloor may also explain the anomalous structure of the Pacific Large Low Shear Velocity Province (LLSVP)—a massive region in Earth’s lower mantle—as the LLSVP appears to be split by the slab.

“This thickened area is like a fossilized fingerprint of an ancient piece of seafloor that subducted into the Earth approximately 250 million years ago,” Wang said. “It’s giving us a glimpse into Earth’s past that we’ve never had before.”

Subduction occurs when one tectonic plate slides beneath another, recycling surface material back into Earth’s mantle. The process often leaves visible evidence of movement, including volcanoes, earthquakes and deep marine trenches. While geologists typically study subduction by examining rock samples and sediments found on Earth’s surface, Wang worked with Geology Professor Vedran Lekic and Associate Professor Nicholas Schmerr to use seismic waves to probe through the ocean floor. By examining how seismic waves traveled through different layers of Earth, the scientists were able to create detailed mappings of the structures hiding deep within the mantle.

“You can think of seismic imaging as something similar to a CT scan. It’s basically allowed us to have a cross-sectional view of our planet’s insides,” Wang said. “Usually, oceanic slabs of material are consumed by the Earth completely, leaving no discernible traces on the surface. But seeing the ancient subduction slab through this perspective gave us new insights into the relationship between very deep Earth structures and surface geology, which were not obvious before.”

What the team found surprised them—material was moving through Earth’s interior much more slowly than previously thought. Wang believes that the unusual thickness of the area the team discovered suggests the presence of colder material in this part of the mantle transition zone, hinting that some oceanic slabs get stuck halfway down as they sink through the mantle.

“We found that in this region, the material was sinking at about half the speed we expected, which suggests that the mantle transition zone can act like a barrier and slow down the movement of material through the Earth,” Wang explained. “Our discovery opens up new questions about how the deep Earth influences what we see on the surface across vast distances and timescales.”

Looking ahead, the team plans to extend their research into other areas of the Pacific Ocean and beyond. Wang hopes to create a more comprehensive map of ancient subduction and upwelling (the geological process that occurs when subducted material heats up and rises to the surface again) zones, as well as their effects on both deep and surface Earth structures. With the seismic data acquired from this research, Wang and other scientists are improving their models of how tectonic plates have moved throughout Earth’s history.

“This is just the beginning,” Wang said. “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.”

The paper, “Mesozoic intraoceanic subduction shaped the lower mantle beneath the East Pacific Rise,” was published in Science Advances on September 27, 2024.