The absolute change in physical properties (eg, temperature, density, and viscosity) from the mantle to the core is greater than that between solid rock and air. Thus, the Earth’s central-mantle boundary (CMB) is host to various phenomena, including thin enigmatic regions of strongly reduced P- and S-wave velocities and increased density, known as ultralow velocity zones.
Ultralow velocity zones (ULVZs) are the most unusual structures in Earth’s interior. However, given the wide range of relevant properties (thickness and composition) reported by previous studies, the origin of ULVZs has been debated for decades.
Using a recently developed seismic analysis approach, a new study from the University of Alabama found widespread, variable ULVZs at the core-mantle boundary (CMB) beneath a largely unsampled region of the Southern Hemisphere.
Research led by the University of Alabama used global-scale seismic imaging of the Earth’s interior to identify a layer between the core and the crust of a thick and thin, submerged ocean floor.
Recent research indicates that this ancient ocean floor layer, previously seen only in small patches, may comprise the core-mantle boundary. This ultra-low-velocity zone, or ULVZ, is denser than the rest of the deep mantle and was long suppressed below as Earth’s plates shifted, reducing the speed at which seismic waves reverberated below the surface.
Dr. Samantha Hansen, George Lindahl III Assistant Professor of Geological Sciences at the UA and lead author of the study, said, “Seismic studies like ours provide extremely high-resolution imaging of our planet’s interior structure, and we’re finding that this system is much more complex than once thought. Our research provides important connections between the shallow and deep Earth structure and the overall processes that drive our planet.”
For the first time, the team was able to study a significant area of the Southern Hemisphere at high resolution using a holistic technique that looks at sound wave echoes from the core-mantle barrier. Hansen and the international team detected unusual energy in the seismic data within seconds of the reflected wave at the boundary.
Ex-oceanic platforms submerged at the core-mantle boundary provide a good explanation for ULVZs. When two tectonic plates collide and slide beneath each other, the process is called subduction, and it transports oceanic material deep inside the Earth. Throughout geologic time, slowly moving rock in the mantle pushes an accumulation of oceanic material across the boundary between the core and the mantle. The distribution and variability of such material may explain the various reported ULVZ features.
Ranging in height from less than 3 miles to over 25 miles in height, ULVZs can be compared to mountains at the core-mantle boundary.
Dr. Edward Carnero, Mingming Li and Sang-Heon Shim of Arizona State University said: “Analyzing over 1000 seismic records from Antarctica, our high-definition imaging method detected thin anomalous regions in the CMB everywhere we examined. The thickness of the material varies from a few kilometers to 10 kilometers. This suggests that in some places we see mountains 5 times higher than Mount Everest.
These underground “mountains” may play an important role in how heat escapes from the planet’s core, driving the magnetic field. Material from ancient ocean floors can travel back to the surface through volcanic eruptions and accumulate in mantle plumes or hot spots.
- Samantha E. Hansen, Edward J. Carnero et al. Globally distributed entrained materials at the Earth’s core-mantle boundary: Implications for ultralow velocity zones. Scientific advances. DOI: 10.1126/sciadv.add4838