Mantle ‘Blobs’ Analyzed in New Research

Two Large Low-Shear-Velocity Provinces, or “blobs”, in Earth’s mantle turn out to have different compositions and possible different evolutions.

IMAGE: A 3D view of the blob in Earth’s mantle beneath Africa, shown by the red-yellow-orange colors. The cyan color represents the core-mantle boundary, blue signifies the surface, and the transparent gray signifies continents. CREDIT: Mingming Li/ASU

So Earth is like an onion. Thank you, Shrek. It has many layers, starting with the continental crust, where all our land hangs out. Under that is the mantle, which is thick and viscous and sticky and warm, and the convection currents in the mantle make our plates move (more on that later). Beneath the mantle is a fluid outer core and a solid inner core, and those drive Earth’s magnetic field, without which we probably wouldn’t exist. Yay for magnetic fields!

In a new paper published in Nature Geoscience, scientists researched a couple of interesting features in our mantle called blobs. Well, formally, they are known as Large Low-Shear-Velocity Provinces or LLSVPs but “blob” is easier to say. And it’s rather descriptive since you can think of it like the blobs in a lava lamp. Remember lava lamps? No? Okay, we’re getting old around here.

Anyway, there are two major blobs in our mantle — one under Africa and one under the Pacific Ocean. These things are huge. They’re the size of an entire continent and over 100 times as tall as Mt. Everest. And they’re, well, blobby. Also, they’re not well understood, so these scientists did a seismic study of the two blobs to get a better handle on their size and shape and possibly how they affect the surrounding mantle.

Surprisingly, the African blob is 1,000 kilometers higher than the Pacific Ocean blob because it’s less dense, and that could further mean that these two blobs may have different compositions and different evolutions. Lead author Qian Yuan explains: Our calculations found that the initial volume of the blobs does not affect their height. The height of the blobs is mostly controlled by how dense they are and the viscosity of the surrounding mantle.

And co-author Mingming Li notes: The Africa LLVP may have been rising in recent geological time. This may explain the elevating surface topography and intense volcanism in eastern Africa.

This work is potentially the starting point for really understanding how our mantle processes came to be and how they operate as they do on a global scale, affecting and influencing plate tectonics.

More Information

ASU press release

Instability of the African large low-shear-wave-velocity province due to its low intrinsic density,” Qian Yuan and Mingming Li, 2022 March 10, Nature Geoscience

This story was written for the Daily Space podcast/YouTube series. Want more news from myself, Dr. Pamela Gay, and Erik Madaus? Check out DailySpace.org.

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Beth Johnson

Beth Johnson

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Planetary scientist, podcast host. Communication specialist for SETI Institute and Planetary Science Institute. Buy me a coffee: https://ko-fi.com/planetarypan