What Are These Enormous Blobs 1,800 Miles Beneath Our Feet?

Meet the strange structures at Earth’s core-mantle boundary.

Scientists discovered strange structures at the boundary between Earth’s mantle and molten iron outer core.

The structures, called ultra-low velocity zones, lie about 1,800 miles beneath our feet.

Scientists are able to develop a rough picture of Earth’s interior thanks to seismic waves. 

Roughly 1,800 miles below Earth’s surface, at the boundary between our planet’s mantle and molten iron outer core, lurk strange, giant structures. So what are they?

Doyeon Kim, a post-doctoral student at the University of Maryland, and his colleagues used an algorithm to analyze over 7,000 seismic readingsā€”specifically shear waves generated during an earthquakeā€”recorded between 1990 and 2018. All of the earthquakes, which had a magnitude of 6.5 or greater and were more than 125 miles or deeper below Earth’s surface, occurred in the Pacific Ocean Basin. Kim and his colleagues published the results of their study last week in the journal Science.

The analysis revealed a massive, previously undiscovered structure beneath the Marquesas Islands in the Pacific, as well as a newly discovered structure below Hawaii. The structure perched below the Marquesas measures about 620 miles in diameter and is 15 miles thick. 

Researchers are still puzzling over what makes up these structures, dubbed ultra-low velocity zones because seismic waves pass through them more slowly. They seem to have a different composition and density from the molten iron-nickel outer core and the slushy rock that makes up the mantle. 

These aren’t the first such structures that scientists have discovered. Since the 1970s, geologists have used seismic tomography to piece together what Earth’s interior looks like. When an earthquake erupts, the seismic waves it generates travel through Earth’s interior. Some, like S-waves, can’t pass through certain layers beneath the surface, such as Earth’s liquid outer core. This helps paint a picture of what’s happening deep below Earth’s surface.

The more information we have about this peculiar place, the better chance we’ll have at understanding how our dynamic planet formed. 


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