- Scientists are gaining a better understanding of two immense blob-shaped structures in Earth’s mantle.
- Located low in the mantle layer, the blobs influence the motions of the mantle and crust, and also fuel the making of mountains and volcanoes. One of the blobs is drifting up towards the crust.
- Researchers are investigating the blobs’ physical characteristics. They don’t yet know why—or when—such large structures formed.
Two massive blob-like structures deep in Earth’s mantle are revealing a bigger picture about geological phenomena like volcanoes and plate tectonics. One blob lies deep beneath the African continent, and the other is on the opposite side of the planet, beneath the Pacific Ocean.
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Planetary scientists at Arizona State University (ASU) think that these structures—enmeshed about 400 to 1,600 miles below Earth’s crust—can influence both ongoing changes in the mantle and shifts in the deepest, core layers of the planet. They published their findings last week in the peer-reviewed journal Nature Geoscience.
What Are These Blobs?
The blobs are “seismic anomalies” because they are still not understood well, the researchers say in their paper. “We can’t get samples, we can’t send people or robots into the deep mantle, because the pressure and temperature are so high,” Qian Yuan, a geodynamicist at ASU’s School of Earth and Space Exploration, who was involved in the new work, tells Popular Mechanics.
Yuan and fellow geodynamicist Mingming Li don’t know why these particular blobs—especially the African anomaly—are so large. But it could have to do with the processes that created them.
The blobs are more properly called “Low-Shear-Velocity Provinces” (LLSVPs). They’re thought to be made of extremely hot “thermochemical piles” of iron-rich material that accumulated during eons of oceanic crust subduction, the process of dense oceanic sections of Earth’s crust sinking beneath less dense sections of continental crust. LLSVPs can stick around for up to three billion years, and are known to influence volcanic and other thermal activity at the crust.
How the Scientists Investigated Blob Behavior
The mantle is 1,802 miles deep. Movement within it contributes to earthquakes, volcanos, and seafloor spreading. Yuan’s team ran simulations of mantle activity and analyzed 20 years of previously published seismic tomography models, which create three-dimensional images of the action beneath Earth’s crust. They noticed trends in the data that indicated the borders of the blobs. According to their research, the blob under the African continent is about 621 miles higher than the blob under the Pacific Ocean, and it’s slowly rising.
The researchers were puzzled by the much higher position of the African blob compared to the deep-seated Pacific blob. “We wanted to know, what’s the reason for that, because they are similar,” Yuan says. At about 990 to 1,100 miles high, the African blob is also larger than its counterpart below the Pacific, which is between 430 and 500 miles high.
Yuan and Li’s mantle-convection simulations helped them figure out what contributed to that difference. They altered four parameters (blob volume, density, and viscosity, plus the viscosity of the surrounding mantle) and observed the blobs’ behaviors under the new conditions. Of the four possible factors, the answer lay in a combination of blob density and the viscosity of the mantle surrounding it. Higher viscosity means it is thicker and doesn’t flow as easily.
The African blob is less dense than the Pacific one, the scientists believe; the lower density could explain why it’s rising. It may be less stable and have a different evolutionary history as well. The viscosity of the surrounding mantle is also a major factor in the blob’s higher position in the mantle. “It can influence the mantle flow, so the flow can have a large viscous force and drag this blob higher,” Yuan explains.
The African Blob is Linked to Volcanic Action
This larger blob’s position is probably linked to high volcanic activity in parts of Africa. Earth’s mantle is not static. It moves along the convection currents that transfer heat from the extremely hot interior toward the crust.
The blobs hold themselves separate from the surrounding mantle because they are especially hot spots. It’s important to know where these spots lie, because they can lead to a phenomenon called “mantle plumes,” localized columns of hot magma that ascend toward Earth’s surface.
“Many plumes rising from a hot place to the surface can cause a lot of super volcanoes, like in Hawaii and also Iceland,” Yuan says. Past research has uncovered a link between locations of LLSVPs and eruptions. “Scientists have actually found that in the past 250 million years, 80 to 90 percent of large eruptions are located around the edges of the blobs,” he says. So when you project the edges of these two massive blobs upward to the Earth’s surface, Yuan explains, you will find that most volcano eruptions are like a map to the blobs’ boundaries.
The seismic history of parts of Africa appears to connect the active East African Continental Rift zone—with its high plateaus and volcanic action—to the blob far beneath the continent, Yuan says. It’s possible the blob itself is to blame for the upheaval, or there could be something else about the mantle flow causing these upheavals in the crust. Scientists still can’t say for sure.
Seismic action such as earthquakes probably can’t be linked directly to deep-seated LLSVPs like the ones in this study, leading earthquake researcher Chris Goldfinger tells Popular Mechanics. “Earthquakes are a shallow brittle phenomenon, so something like these anomalies so deep in the mantle might reflect something of the driving forces, or perhaps be related to past subduction, but I’d have to guess no relation to modern seismicity,” he says in an email.
Many Questions About Mantle Anomalies Remain
As for why the blobs seem so iron-rich, there could be a few different explanations, Yuan says. The subducted oceanic crust tends to be iron-rich. The other possibility involves a magma ocean in the early days of Earth, when the fledgling planet was even hotter. “At the core-mantle boundary, there was a lot of magma surrounding the core. When that magma ocean slowly crystallized, it could have produced some iron-rich materials,” Yuan says.
Many questions remain after the original discovery of these blobs 30 years ago, such as why they are different shapes, and how exactly they affect mantle behavior. Ultimately, researchers want to know why they formed in the first place.
Could the African blob ever reach the surface? It’s only moving at a rate of one to two centimeters per year, so it may take 100 million years to get there.
Before joining Popular Mechanics, Manasee Wagh worked as a newspaper reporter, a science journalist, a tech writer, and a computer engineer. She’s always looking for ways to combine the three greatest joys in her life: science, travel, and food.
