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Ancient Ocean Floor Found at Core-Mantle Boundary

This image depicts an underground scan where seismic waves generated by earthquakes in the southern hemisphere are used to sample the ULVZ structure at the Earth's core-mantle boundary. The recorded seismic waves are captured by sensors located in Antarctica. Credit: Drs. Edward Garnero and Mingming Li at Arizona State University.

Scientists have made an incredible discovery at the core-mantle boundary: evidence of ancient ocean floor. This finding has significant implications for our understanding of Earth's geology and history.

A Thin, Dense Layer Discovered

Researchers from The University of Alabama have uncovered a dense, thin layer of what is likely an ancient ocean floor between the Earth’s core and mantle. The findings, published in Science Advances, were achieved through global-scale seismic imaging.

Ancient Ocean Floor at Core-Mantle Boundary

Previously seen only in isolated patches, the latest data indicates that this layer of ancient ocean floor may cover the core-mantle boundary. Subducted underground long ago as Earth’s plates shifted, this ultra-low velocity zone (ULVZ) is denser than the rest of the deep mantle, slowing seismic waves beneath the surface.

Seismic Imaging Reveals Complex Interior Structure

Dr. Samantha Hansen, the George Lindahl III Endowed Professor in geological sciences at UA, explained that seismic investigations provide the highest resolution imaging of Earth’s interior structure. The research establishes important connections between shallow and deep Earth structure and the processes driving the planet.

An International Effort

Along with Dr. Hansen, co-authors on the paper include researchers from Arizona State University and the University of Leeds in the United Kingdom. The team collected data for three years, using 15 stations in the Antarctic network to create an image of the Earth below.

Mapping a Variable Layer of Material

The project was able to examine a large portion of the southern hemisphere for the first time in high resolution. The researchers identified unexpected energy in the seismic data, mapping a variable layer of material that is pencil-thin compared to the thickness of Earth’s dominant layers. This layer is referred to as an ultra-low velocity zone (ULVZ) due to its strong wave speed reductions.

Ancient Ocean Floors and ULVZs

ULVZs can be explained by the sinking of former oceanic seafloors to the core-mantle boundary. Oceanic material is carried into Earth’s interior at subduction zones, where tectonic plates meet and one dives beneath the other. Over time, the slow movement of mantle rock pushes subducted oceanic material to gather along the core-mantle boundary.

“Mountains” at the Core-Mantle Boundary

ULVZs, akin to subterranean mountains on the core-mantle boundary, vary in height from less than 3 miles to over 25 miles. These “mountains” play a crucial part in regulating the planet’s magnetic field by facilitating heat dissipation from the core. Moreover, ancient ocean floor material gets swept into mantle plumes, which through volcanic eruptions, make their way back to the surface.

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