Scientists Find Ocean Floor Between Earth’s Mantle and Outer Core

  • Scientists believe they’ve discovered an ancient ocean floor comprising a new layer between Earth’s mantle and core.
  • Seen previously only in hints, a University of Alabama-led study shows the ancient ocean floor could cover the entirety of the core-mantle boundary.
  • Seismic waves produced by earthquakes help create an internal image of Earth.

If new data produced by studying earthquakes has it right, we’re going to need to add another layer to our understanding of Earth. This layer, which fits nicely between Earth’s mantle and outer core, is thought to be an ancient ocean floor that is now recreating our view of the core-mantle boundary.

In a study published in Science Advances—and led by a team from the University of Alabama, Arizona State University, and the University of Leeds—researchers traveled to Antarctica four times to collect three years’ worth of seismic data from 15 network stations buried in the continent. The data allowed them to map earthquake-induced seismic waves and create a new high-resolution image of Earth’s interior.

To mine for data, researchers literally went underground. The buried stations in Antarctica offered a first-ever a high-resolution look at the Earth’s mantle in the southern hemisphere. Using seismic waves created by earthquakes over a three-year span, researchers uncovered “unexpected energy” in the data.


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They found that geological forms—previously thought to happen in mere smatterings—were likely fully encompassing, offering up an entirely new layer of Earth. (In other new-layers-of-Earth news, we were recently told that Earth’s inner core actually has an even more innermost ball comprised mainly of iron.)

“Seismic investigations, such as ours, provide the highest resolution imaging of the interior structure of our planet, and we are finding that this structure is vastly more complicated than once thought,” Samantha Hansen, geological sciences professor at Alabama and the study’s lead author, says in a news release. “Our research provides important connections between shallow and deep Earth structure and the overall processes driving our planet.”

The images show that between the mantle and the outer core’s molten and metallic layer sits a “dense, yet think, sunk ocean floor.” This had been seen in previous studies, but was believed to be only an isolated occurrence. Known as an ultra-low velocity zone, something quite a bit denser than the surrounding mantle makeup, researchers say this ancient ocean floor covers the entire core-mantle boundary and likely slid deeper underground as Earth’s plates shifted.

The unique seismic data showed what they call a pencil-thin variable layer, which can range from three miles to more than 25 miles in thickness. “This suggests,” Edward Garnero of Arizona State University says the news release, “we are seeing mountains on the core, in some places up to five times taller than Mount Everest.”

Still, the ultra-low velocity zone is best described as an ancient ocean floor that sunk. “Oceanic material is carried into the interior of the planet where two tectonic plates meet and one dives beneath the other, known as subduction zones,” the researchers say. “Accumulations of subducted oceanic material collect along the core-mantle boundary and are pushed by the slowly flowing rock in the mantle over geologic time.”

Adding another layer of intrigue to the study, Earth’s molten layer’s position below this newfound ocean floor means that underground mountains may play a key role in determining the rate of heat escaping from the core.

Earth may not be growing, but its number of layers certainly is.

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Tim Newcomb is a journalist based in the Pacific Northwest. He covers stadiums, sneakers, gear, infrastructure, and more for a variety of publications, including Popular Mechanics. His favorite interviews have included sit-downs with Roger Federer in Switzerland, Kobe Bryant in Los Angeles, and Tinker Hatfield in Portland. 

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