Ancient seabed discovered between Earth’s core and mantle

Seismic waves from earthquakes in the southern hemisphere are recorded by sensors in Antarctica. Photo credits: Edward Garnero and Mingming Li, Arizona State University

Through global-scale seismic imaging of Earth’s interior, a new study revealed a layer between the core and mantle that is likely a dense but thin submerged seafloor.

The latest data suggest that this layer of ancient seafloor, previously seen only in isolated areas, may cover the core-mantle boundary (CMB).

This ultra-low-velocity zone (ULVZ), submerged underground long ago as the tectonic plates shifted, is denser than the rest of Earth’s deep mantle and slows down seismic waves echoing beneath the surface.

“Seismic surveys like ours provide the highest-resolution image of our planet’s internal structure, and we find that this structure is far more complicated than previously thought,” said geologist Samantha Hansen, lead author of the study.

“Our research provides important connections between the shallow and deep Earth structure and the overall processes that drive our planet.”

The study was published in scientific advances.

These subtle signals were used to map a variable layer of material across the study area, pencil-thin compared to the thickness of Earth’s dominant strata, measuring tens of kilometers.

Characteristics of the anomalous CMB coating include a sharp reduction in wave velocity, leading to the designation “Ultra Low Velocity Zone”.

ULVZs can be explained by former oceanic seafloors that have subsided to the CMB. Oceanic material is transported into the planet’s interior, where two tectonic plates meet and one subducts beneath the other, called subduction zones.

Aggregates of subducted oceanic material accumulate along the CMB and are pushed down by the slowly flowing rocks in the mantle over geologic time. The distribution and variability of this material explains the range of observed ULVZ properties.

The ULVZs can be thought of as mountains along the CMB, ranging in elevation from less than about 3 miles to more than 25 miles.

“In analyzing thousands of seismic records from Antarctica, our high-resolution imaging method found thin anomalous zones of material on the CMB wherever we examined it,” said Edward Garnero, co-author of the study.

“The thickness of the material varies from a few kilometers to tens of kilometers. This suggests that at the core we are seeing mountains that are up to five times taller than Mount Everest in some places.”

These underground “mountains” may play an important role in how heat escapes from the core, the part of the planet that drives the magnetic field.

Material from the old seafloor can also be entrained in mantle clouds or hot spots brought back to the surface by volcanic eruptions.

“This study is unique because it maps widespread, variable ULVZs along the core-mantle boundary under a largely unsampled portion of the Southern Hemisphere,” said Michael Jackson of NSF’s Office of Polar Programs.

“The research also underscores the importance of NSF’s investment in high-quality seismic observations in Antarctica for elucidating global Earth structures.”

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