University of Maryland scientists have found remnants of an ancient seafloor buried deep within Earth’s mantle, offering a new glimpse into the planet’s interior and how it has evolved over millions of years.

It’s an important discovery that sheds light on a tectonic boundary that subducted beneath the Pacific Ocean during the age of dinosaurs — approximately 250 million years ago. And it could redefine our understanding of Earth’s geological history.

A Fossilized Record in Earth’s Mantle

Geologists, led by postdoctoral researcher Jingchuan Wang, used seismic imaging techniques to probe deep into the mantle, the layer between Earth’s crust and core. It’s almost like a CT scan of the Earth that shows a cross-section view of our planet’s interior. In the process, they discovered a thickened section of the mantle transition zone, located about 410 to 660 kilometers below the Earth’s surface. This zone separates the upper and lower mantles and can expand or contract based on temperature.

Wang describes this discovery as a “fossilized fingerprint” of an ancient seafloor that subducted during the Triassic period. Subduction is a process where one tectonic plate slides beneath another, which typically consumes slabs of oceanic crust entirely. Yet, in this case, the ancient slab remains trapped deep within the mantle, providing an unprecedented glimpse into the Earth’s geological past.

A CT Scan of the Earth

To make this discovery, Wang’s team used seismic waves — vibrations caused by earthquakes — to create cross-sectional maps of the Earth’s interior. By tracking how these waves traveled through different layers (each layer has a different signal), they detected the subducted slab in the mantle beneath the East Pacific Rise, a tectonic boundary on the floor of the southeastern Pacific Ocean.

This seismic “scan” revealed not only the ancient subduction slab but also the surprising behavior of the material in this region. It was sinking at about half the speed researchers had expected.

The team’s discovery suggests that the mantle transition zone can act as a barrier, slowing the movement of subducted material through the Earth. Wang believes that the unusually thick area they identified contains colder material, which may explain why some oceanic slabs may become stuck in the transition zone rather than sinking further into the lower mantle.

“Our discovery opens up new questions about how the deep Earth influences what we see on the surface across vast distances and timescales,” Wang noted.

New Questions About Deep Earth Structures

This discovery offers new clues about the behavior of the Pacific Large Low Shear Velocity Province (LLSVP). It’s a giant, continent-sized zone, sitting about 2,900 kilometers beneath the surface, where seismic waves travel at a much slower rate than in the surrounding mantle. But what makes this region truly puzzling is how little we understand about it, despite its profound role in shaping our planet’s behavior. The newly found slab may provide a key to understanding this complex structure.

Geodynamic simulations also suggest that ancient slabs like the one found under the East Pacific Rise may interact with deep mantle structures called super plumes, rising columns of hot rock that feed volcanic hotspots like the one under Hawaii. Another plume is believed to be fueling the volcanic activity beneath the Easter Islands.

Looking ahead, Wang and his team plan to expand their research, mapping other ancient subduction zones across the Pacific Ocean and beyond. “We believe there are many more ancient structures waiting to be discovered in Earth’s deep interior,” Wang said. “Each one has the potential to reveal new insights about our planet’s complex past — and even help us understand other planets beyond our own.”

A New Chapter in Earth’s History

This discovery rewrites part of Earth’s story, revealing a hidden chapter in the planet’s evolution. As scientists continue to probe deeper into the Earth’s mantle, these ancient slabs could unlock long-lost secrets about how the Earth formed and how its tectonic plates have moved over millions of years.

The remnants of this ancient seafloor might be the first of many yet-to-be-discovered slabs lurking deep within our planet, waiting to tell their stories. As such, this research marks a significant step toward uncovering these mysteries, bridging the gap between deep-Earth structures and the planet’s surface geology.

In Wang’s words, “This is just the beginning.”

The findings appeared in the journal Science Advances.