- Scientists are gaining a better understanding of two huge teardrop-shaped structures in the Earth coat.
- Located low in the mantle layer, blobs influence mantle and crustal movements, and also fuel the formation of mountains and volcanoes. One of the drops drifts towards the crust.
- Researchers study the blobs’ physical characteristics. They don’t yet know why – or when – such large structures formed.
Two massive blob-shaped structures deep in the Earth’s mantle reveal a larger picture of geologic phenomena like volcanoes and tectonic plates. One drop is deep under the African continent and the other is on the other side of the planet, under the Pacific Ocean.
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Planetary scientists at Arizona State University (ASU) believe these structures – tangled about 400 to 1,600 miles below the Earth’s crust – may influence both ongoing changes in the mantle and changes in the innermost layers. depths of the planet. They published their findings last week in the peer-reviewed journal nature geoscience.
What are these blobs?
The drops are “seismic anomalies” because they are still not well understood, 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 the temperature are so high,” Qian Yuangeodynamics engineer at ASU School of Earth and Space Explorationwho was involved in the new work, tells Popular mechanics.
Yuan and his fellow geodynamicist Mingming Li I don’t know why these particular spots, especially the African anomaly, are so large. But he could have to do with the processes that created them.
The blobs are more correctly called “low shear rate provinces” (LLSVP). They are thought to be made up of extremely hot “thermochemical stacks” of iron-rich materials that have accumulated over eons of oceanic crustal subduction, the process of dense oceanic sections of the earth’s crust sinking beneath less dense sections dense continental crust. LLSVPs can stick around up to three billion years old, and are known to influence volcanic activity and other thermal activities in the crust.
How scientists studied blob behavior
The mantle is 1,802 miles deep. Movement within it contributes to earthquakes, volcanoes, and seabed spreading. Yuan’s team performed simulations of mantle activity and analyzed 20 years of previously published seismic tomography models, which create three-dimensional images of the action beneath the Earth’s crust. They noticed patterns in the data that indicated the borders of the blobs. According to their research, the drop under the African continent is about 621 miles higher than the drop under the Pacific Ocean, and it is slowly rising.
The researchers were intrigued by the much higher position of the African spot compared to the deep Pacific spot. “We wanted to know what the reason was because they are similar,” says Yuan. At around 990 to 1,100 miles, the African drop is also larger than its sub-Pacific counterpart, which is between 430 and 500 miles.
Yuan and Li’s mantle convection simulations helped them understand what contributed to this difference. They modified four parameters (volume, density and viscosity of the drops, as well as the viscosity of the surrounding mantle) and observed the behavior of the drops under the new conditions. Of the four possible factors, the answer lies in a combination of the density of the spots and the viscosity of the surrounding mantle. Higher viscosity means it’s thicker and doesn’t flow as easily.
The African patch is less dense than the Pacific patch, scientists believe; the lower density could explain why it increases. It may be less stable and also have a different evolutionary history. The viscosity of the surrounding mantle is also a major factor in the drop’s higher position in the mantle. “It can influence the flow of the mantle, so the flow can have a large viscous force and drive that drop higher,” Yuan says.
The African blob is linked to volcanic action
The position of this larger blob is likely related to strong volcanic activity in parts of Africa. The Earth’s mantle is not static. It moves along convection currents that transfer heat from the extremely hot interior to the crust.
The blobs stand apart from the surrounding mantle because they are particularly hot spots. It’s important to know where these points are because they can lead to a phenomenon called “mantle plumes,” localized columns of hot magma rising toward the Earth’s surface.
“Lots of plumes rising from a hot place to the surface can cause lots of super volcanoes, like in Hawaii and also in Iceland,” Yuan says. Previous research has found a link between LLSVP locations and flares. “Scientists have actually found that over the past 250 million years, 80-90% of large eruptions are on the edges of drops,” he says. So when you project the edges of these two massive blobs up onto the Earth’s surface, Yuan says, you’ll find that most volcanic eruptions are like a map of the blobs’ boundaries.
The seismic history of parts of Africa appears to link East Africa’s active continental rift zone – with its high plateaus and volcanic action – to the drop far below the continent, Yuan says. It’s possible that the drop itself is to blame for the upheaval, or there could be something else in the mantle flow that’s causing these upheavals in the crust. Scientists still can’t say for sure.
Seismic action such as earthquakes probably cannot be directly linked to deep LLSVPs like those in this study, according to a leading earthquake researcher chris goldfinger recount Popular mechanics. “Earthquakes are a shallow and fragile phenomenon, so something like these anomalies so deep in the mantle could reflect something of the driving forces, or maybe be related to past subduction, but I would have to guess it has nothing to do with modern seismicity,” he said. in an email.
Many questions remain about mantle anomalies
As for why the drops seem so iron-rich, there could be several different explanations, Yuan says. Subducted oceanic crust tends to be iron-rich. The other possibility involves an ocean of magma in Earth’s early days, when the fledgling planet was even hotter. “At the core-mantle boundary, there was a lot of magma surrounding the core. When this ocean of magma slowly crystallized, it could have produced iron-rich materials,” says Yuan.
Many questions remain after the original discovery of these drops 30 years ago, such as why they have different shapes and how exactly they affect the behavior of the mantle. Ultimately, researchers want to know why they formed in the first place.
Could the African spot ever reach the surface? It only moves at a rate of one to two centimeters per year, so it may take 100 million years to get there.
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