Using global-scale seismic imaging of the Earth’s interior, researchers led by the University of Alabama have discovered a layer between the core and the mantle, possibly a dense but thin submerged ocean floor, according to results published today in Science Advances.
Recent data, previously observed only in isolated areas, suggest that this layer of the ancient ocean floor may span the core-mantle boundary. This ultra-low-velocity region, or ULVZ, plunged underground long ago when Earth’s plates moved, denser than the rest of the deep mantle, slowing seismic waves reflected below the surface.
George Lindahl III Professor Dr. “Seismic studies like ours provide the highest resolution images of our planet’s interior, and we find that this structure is much more complex than previously thought,” said Samantha Hansen. He is from the Department of Geological Sciences in Ukraine and is the lead author of the study. “Our research establishes important links between Earth’s fine and deep structure and the overall processes that drive our planet.”
These thin signals were used to map the layer of variable material in the study region to a thin pencil that is tens of kilometers across compared to the thickness of Earth’s dominant layers. Features of the anomalous coating at the core-mantle interface include a strong reduction in wave velocity, giving rise to the name ultra-low velocity region.
The ULVZ can be well explained by an ancient ocean floor that submerged into the core-mantle interface. Oceanic material is transported to the interior of the planet, known as subduction zones, where two tectonic plates collide and sink one under the other. Submerged oceanic material accumulation collects along the core-mantle boundary and is pushed upward by slowly flowing rock in the mantle over geologic time. The distribution and variability of such materials explains the range of ULVZ properties observed.
The ULVZ can be thought of as mountains that range from less than 3 miles to 25 miles in height along the core-mantle boundary.
“After analyzing 1,000 seismic records from Antarctica, our high-definition imaging revealed subtle regions of anomalous material everywhere we explored in the CMB,” said Garnero. “The thickness of the material ranges from a few kilometers to 10 kilometers. This suggests that in the core we see mountains in some places that are 5 times higher than Mount Everest.”
These subterranean “mountains” may play an important role in how heat escapes from the core, the part of the planet that powers the magnetic field. Material from the bottom of the ancient ocean can also be dragged by mantle plumes or hotspots brought back to the surface by volcanic eruptions.
Hansen and co-authors of the paper, Dr. Edward Garnero, Minmin Lee and Sang-Heung Shim of Arizona State University and Dr. Sebastian Rost. About 2,000 miles below the surface, Earth’s rocky mantle meets the molten metallic outer core. The changes in physical properties along this boundary are greater than the changes between the solid rock at the surface and the air above it.
Understanding the composition of the core-mantle boundary on a large scale is difficult, but a seismic network deployed by Hansen, his students and others during four trips to Antarctica has collected data over three years. Similar to a medical body scan, the network’s 15 stations buried in Antarctica used seismic waves produced by earthquakes around the world to create images below the Earth.
Within the scope of the project, a large part of the southern hemisphere could be studied in high resolution for the first time using a detailed method of examining the echoes of sound waves from the core-mantle boundary. Hansen and an international team discovered an unexpected energy in the seismic data coming within seconds of the reflected wave at the boundary.
Source: Port Altele
