The two colossal massive structures that border the outer core of the Earth give rise to the South Atlantic Anomaly and the intense African volcanism, but to the surprise of geologists, their composition and structure are different.
The interior of the Earth is structured in layers, with a thin outer crust, followed by a thick, viscous mantle, a fluid outer core, and a solid inner core.
Within the lower part of the mantle, which surrounds the outer core, are two massive blob-like structures located on opposite sides of the planet: one is under the African continent, the other under the Pacific Ocean.
The patches, known as Large Low Shear Rate Provinces (LLSVPs), are the size of a continent and 100 times higher than Mount Everest. They are characterized by slow shear wave velocities and were discovered by seismic tomography of deep within the Earth.
Currently there are many doubts about the nature of these structures. The main hypothesis for the LLSVP indicates that they have been produced by the accumulation of subducted oceanic plates.
A second proposed origin for LLSVPs is that their formation is related to the Big Impact hypothesis, which states that the Moon formed after the early Earth collided with a planet-sized body called Theia.
This hypothesis suggests that the LLSVPs are fragments of Theia’s mantle that sank to the Earth’s core-mantle boundary after the Big Impact.
new tracks
New research, carried out at the University of Arizona, does not clarify the origin of these spots, but it has discovered new clues about their nature.
To carry out their research, Qian Yuan and Mingming Li of the University’s School of Earth and Space Exploration designed and ran hundreds of mantle convection model simulations.
Through these simulations, they were able to determine the maximum heights that the spots reach, and how their volume and density, as well as the surrounding viscosity in the mantle, might control their height. The results have been published in Nature Geoscience.
The first thing they discovered was that the slick under the African continent is about 1,000 kilometers higher than the slick under the Pacific Ocean.
different compositions
According to Yuan and Li, the possible explanation for the large height difference is that the slick below the African continent is less dense (and therefore less stable) than the one below the Pacific Ocean, indicating that the two may have a different composition and evolution.
Africa’s LLVP would have been increasing in height in recent geologic time, the researchers believe, which may explain the elevated surface topography and intense volcanism in East Africa.
The unstable nature of the slick beneath the African continent would be related to changes in topography, gravity, surface volcanism and plate movement in that African region, the researchers believe.
Another characteristic of these rocky provinces is that, being so massive, they generate disturbances such as the well-known South Atlantic Anomaly, which causes a significant reduction in the strength of the Earth’s magnetic field.
These findings about these massive structures may fundamentally change the way scientists think about deep mantle processes and how they can affect Earth’s surface, the researchers say.
And the origin?
Regarding the origin of these large provinces, the new research does not bring anything new, although in a study published last year, Qian Yuan and his team suggested that they could be the remains of Theia.
Given that both the terrestrial spots and the Great Impact that gave rise to the Moon are approximately the same age, everything indicates that these underground spots may indeed be the remains of Theia.
The Great Impact is the most accepted scientific theory to explain the formation of the Moon: it originated as a result of a collision between the primitive Earth or proto-Earth and a protoplanet the size of Mars, called Theia.
When Theia grew large enough to escape from its initial orbit, it entered a chaotic dynamic and the impact with the young Earth became inevitable, because both bodies occupied the same orbit. It has been calculated that the Great Impact occurred around 4.533 million years ago.
The enigmatic spots could therefore be the remains of Theia, although the fact that they have marked differences in their composition and evolution casts doubt on whether they may have the same origin and nature. The authors acknowledge in their article that it is not yet possible to fully understand the origin and dynamics of LLSVP.
Reference
Instability of the African large low-shear-wave-velocity province due to its low intrinsic density. Qian Yuan, Mingming Li. Nature Geoscience (2022). DOI:https://doi.org/10.1038/s41561-022-00908-3