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The hidden world beneath our feet -Se

Seismic waves from earthquakes in the Southern Hemisphere sample ULVZ structures along Earth’s CMB and are recorded by sensors in Antarctica. Credit: Edward Garnero and Mingming Li, Arizona State University

Researchers at the University of Alabama have discovered a thick layer of the ancient sea floor, or ultra-low velocity zone (ULVZ), between Earth’s core and mantle using seismic imaging. These subterranean “mountains” may play a key role in escaping heat from the core and the planet’s magnetic field.

Through global-scale seismic imaging of Earth’s interior, research led by The University of Alabama has revealed a layer between the core and mantle that is likely a dense, yet thin, submerged sea floor, according to findings published April 5 in the journal Science advances.

Previously seen only in isolated patches, recent data suggest that this layer of ancient seafloor may cover the core-mantle boundary. As the Earth’s plates migrated long ago underground, this ultra-low velocity zone, or ULVZ, is denser than the rest of the deep mantle, slowing seismic waves below the surface.

“Seismic probes, like ours, provide the highest resolution imaging of our planet’s internal structure, and we can see that this structure is much more complex than once thought,” said Dr. Samantha Hansen, George Lindahl III Endowed Professor. Geological Sciences and lead author of the study at UA. “Our research provides important links between the structure of the shallow and deep Earth and the overall processes that drive our planet.”

Along with Hansen, the paper’s co-authors include Drs. Edward Garnero, Mingming Lee, and Sang-Hyun Shim from Arizona State University and Dr. from the University of Leeds in the UK. Sebastian Rost.

About 2,000 miles below the surface, Earth’s rocky mantle meets the molten, metallic outer core. Changes in physical properties across this boundary are greater than those between the solid rock at the surface and the air above it.

Seismic Equipment Antarctic Station

Researchers lowered seismic equipment to one of the Antarctic stations in 2012 Credit: Lindsey Kenyon

Understanding the structure of the core-mantle boundary on a large scale is difficult, but a seismic network deployed by Hansen, his students and others collected data for three years during four trips to Antarctica. Like a medical scan of the body, the network’s 15 stations buried in Antarctica use seismic waves generated by earthquakes around the world to create an image of the Earth below.

The project was able to probe a large part of the Southern Hemisphere in high-resolution for the first time using a detailed method that examines sound wave echoes from the core-mantle boundary. Hansen and the international team detected unexpected energy in seismic data that comes within seconds of boundary-reflected waves.

These subtle signals were used to map a variable layer of material across the study area that was measured in tens of kilometers compared to the thickness of pencil thin, Earth-dominant layers. Characteristics of the unusual core-mantle boundary layer include strong wave speed reduction, which is named the ultra-low velocity region.

ULVZs can be well explained by former oceanic seafloors that sank at the core-mantle boundary. Oceanic material is carried into the interior of the planet where two tectonic plates meet and dive beneath the other, known as subduction zones. Accumulations of subducted oceanic material accumulate along the core-mantle boundary and are slowly pushed into the mantle over geologic time by drifting rocks. The distribution and variability of such components explains the range of observed ULVZ properties.

ULVZs can be thought of as mountains along the core-mantle boundary, ranging in height from less than 3 miles to more than 25 miles.

“By analyzing thousands of seismic recordings from Antarctica, our high-definition imaging method found thin anomalous regions of material in the CMB everywhere we looked,” Garnero said. “The thickness of the material varies from a few kilometers to 10 kilometers. This indicates that we are looking at mountains in the core, in some places up to 5 times higher than Mount Everest.”

These subterranean “mountains,” part of the planet that powers the magnetic field, may play an important role in how heat escapes from the core. Material from the ancient ocean floor can also enter mantle plumes or hot spots brought back to the surface by volcanic eruptions.

Reference: “Globally Distributed Subducted Materials Along Earth’s Core-Mantle Boundary: Implications for Very Low Velocity Zones” Samantha E. Hansen, Edward J. By Garnero, Mingming Lee, Sang-Heon Shim, and Sebastian Rost, 5 April 2023. Science advances.
DOI: 10.1126/sciadv.add4838

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