Beneath the Earth, ancient ocean floor likely surrounds the core

Adam Jones

Through global-scale seismic imaging of Earth’s interior, research led by The University of Alabama team revealed a layer between the core and the mantle that is likely a dense, yet thin, sunk ocean floor, according to results published in Science Advances.

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

Roughly 2,000 miles below the surface, Earth’s rocky mantle meets the molten, metallic outer core. The changes in physical properties across this boundary are greater than those between the solid rock on 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, her students and others during four trips to Antarctica collected data for three years.

The project was able to probe in high-resolution a large portion of the southern hemisphere for the first time using a detailed method that examines sound wave echoes from the core-mantle boundary. Hansen and the international team identified unexpected energy in the seismic data that arrives within several seconds of the boundary-reflected wave.

These subtle signals were used to map a variable layer of material across the study region that is pencil thin, measuring in the tens of kilometers, compared to the thickness of the Earth’s dominant layers. The properties of the anomalous core-mantle boundary coating include strong wave speed reductions, leading to the name of ultra-low velocity zone.

ULVZs can be well explained by former oceanic seafloors that sunk to the core-mantle boundary. Oceanic material is carried into the interior of the planet where two tectonic plates meet and one dives beneath the other, known as subduction zones.

The ULVZs can be thought of as mountains along the core-mantle boundary, with heights ranging from less than about 3 miles to more than 25 miles.

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.

Article: Hansen, Garnero, Li, Shim and Rost Globally distributed subducted materials along the Earth’s core-mantle boundary: Implications for ultralow velocity zones