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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.

An illustrative diagram showing the ancient subducted 'slab' the team resolved at present day. It has a direct impact on the large-scale lowermost mantle structures known as 'superplumes'. Credit: Jingchuan Wang, University of Maryland.

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