The absolute change in physical properties (e.g., temperature, density, and viscosity) from the mantle to the core is greater than that between solid rock and air. Hence, Earth’s core-mantle boundary (CMB) is host to various phenomena, including thin enigmatic regions with strongly reduced P- and S-wave velocities and increased density, dubbed ultralow velocity zones.
Ultralow velocity zones (ULVZs) are the most anomalous structures within the Earth’s interior. However, given the wide range of associated characteristics (thickness and composition) reported by previous studies, the origins of ULVZs have been debated for decades.
Using a recently developed seismic analysis approach, a new study by The University of Alabama found widespread, variable ULVZs along the core-mantle boundary (CMB) beneath a largely unsampled portion of the Southern Hemisphere.
Research headed by The University of Alabama found a layer between the core and the mantle that is probably a dense yet thin, submerged ocean floor using global-scale seismic imaging of the Earth’s interior.
The most recent research indicates that this ancient ocean floor layer, previously only observed in small patches, may have covered the core-mantle border. This ultra-low velocity zone, or ULVZ, is denser than the rest of the deep mantle and was subducted below long ago as the Earth’s plates changed, slowing seismic waves resonating below the surface.
Dr. Samantha Hansen, the George Lindahl III Endowed Professor in geological sciences at UA and lead author of the study, said, “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. Our research provides important connections between shallow and deep Earth structure and the overall processes driving our planet.”
For the first time, the team could probe a significant area of the southern hemisphere in high resolution using a thorough technique that looks at sound wave echoes from the core-mantle barrier. Hansen and the international team found unusual energy in the seismic data within seconds of the boundary-reflected wave.
Former oceanic seafloors that sank to the core-mantle boundary provide a good explanation for ULVZs. When two tectonic plates collide, and one subducts beneath the other, this process is known as subduction, and it transports oceanic material deep inside the earth. Throughout geologic time, the slowly moving rock in the mantle pushes the accumulations of subducted oceanic material along the border between the core and the mantle. The distribution and variability of such material can explain the variety of reported ULVZ features.
With heights ranging from less than 3 miles to more than 25 miles, the ULVZs can be compared to mountains along the core-mantle border.
Drs. Edward Garnero, Mingming Li, and Sang-Heon Shim from Arizona State University said, “Analyzing 1000’s seismic recordings from Antarctica, our high-definition imaging method found thin anomalous zones of material at the CMB everywhere we probed. The material’s thickness varies from a few kilometers to 10’s kilometers. This suggests we see mountains on the core, in some places up to 5 times taller than Mt. Everest.”
These underground “mountains” may play an important role in how heat escapes from the core, the portion of the planet that powers the magnetic field. Material from the ancient ocean floors can also become entrained in mantle plumes, or hot spots, that travel back to the surface through volcanic eruptions.