Study of Earth’s Mantle Reveals Structures of Unexpected Height and Density

Researchers have discovered two huge structures within our planet that are roughly opposite to one another. These structures, referred by experts as “blobs,” have unexpected heights and densities.

The surface of our planet can be described as being layered like an onion; planet Earth has a thin outer crust, a thick mantle, a liquid outer core, and a solid inner core.

Within the mantle, there are two huge structures that are roughly opposite to one another.

Large Low-Shear-Velocity Provinces (LLSVPs), as the blobs–structures–are officially called, are each the size of a continent and about one hundred times as tall as Mount Everest. One lies beneath the African continent, and the other is believed to be located beneath the Pacific Ocean.

Scientists are able to determine the shape and structure of these two blobs by measuring seismic waves. Although the blobs appear odd, we don’t know what they are made of or why they exist.

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Geodynamic modeling and analysis of published seismic studies by Arizona State University scientists Qian Yuan and Mingming Li have shed light on these two blobs.

Their research enabled them to determine the maximum height of the blobs and how the density, volume, and surrounding viscosity of the mantle might contribute to controlling their height. Their study was recently published in Nature Geoscience.

Seismologists determined that the blob under the African continent is about six times larger than the blob beneath the Pacific Ocean after analyzing seismic data. There is probably less density (and therefore less stability) in the blob under the African continent than in the blob under the Pacific Ocean, which explains the vast height difference between the two.

For their research, Yuan and Li simulated hundreds of mantle convection models.

Researchers thoroughly investigated the differences in the height of the blobs caused by the volume of the blobs. In addition, they also investigated the contrast in density and viscosity between the blobs and their surroundings.

Accordingly, the blob under the African continent has to have a lower density than the one under the Pacific Ocean. This indicates that they may have different compositions and evolutionary histories.

Lead author Yuan explained that volume does not affect the height of blobs. “The height of the blobs is mostly controlled by how dense they are and the viscosity of the surrounding mantle.”

“The Africa LLVP may have been rising in recent geological time,” co-author Li added. “This may explain the elevated surface topography and intense volcanism in eastern Africa.”

Researchers may have to rethink their understanding of deep mantle processes and how they influence the Earth’s surface in light of these results. For example, topography, gravity, surface volcanism, and plate motion may all play a role in maintaining the instability of the blob under the African continent.

“Our combination of the analysis of seismic results and the geodynamic modeling provides new insights into the nature of the Earth’s largest structures in the deep interior and their interaction with the surrounding mantle,” Yuan said. “This work has far-reaching implications for scientists trying to understand the present-day status and the evolution of the deep mantle structure, and the nature of mantle convection.”