In the depths of our planet, where the earth’s core meets the mantle, there are two gigantic regions that have been disconcerating the geologists for 50 years. Two giant rock masses other than the rest, discovered in the last decade of the 70s and … that are sunk thousands of km deep under our feet: one, just below Africa, Spain and part of the Atlantic; The other under the Pacific Ocean.
Both rock masses, the size of continents, with up to 900 km high and several thousand km wide, are known as LLSVPS, Large Low-Shear-Velocity Province (large provinces of low cutting speed), and are made of much more dense and hard materials than those around them. That is, its origin cannot be the same as that of the rest of the mantle. Its size also means that both are able to generate their own disturbances in the magnetosphere, including the great phenomenon that the magnetic field of the Earth weakens and that is known as “anomaly of the South Atlantic.” Together, the two structures cover more than a quarter of the earth’s core, which means that it is something too important for the planet to not understand it.
Fragments of another planet?
Until now, several hypotheses have been formulated about the nature of the LLSVPS. One of the most common is that they are formed by ancient oceancing cortex, pushed to the depths of the mantle in the subduction areas of the tectonic plates, where some slide under others. According to that idea, that material of the cortex was stirred through the mantle for millions of years and ended up accumulating to form the LLSVPS.
In March 2021, however, a team of researchers led by the geologist Qian Yuan, then from Arizona State University, surprised the world with a “crazy but possible” idea: those two mysterious fragments could, in fact, the remains of Theia, the object of the size of Mars that crashed into the earth 4.5 billion years ago and that gave rise to the moon. In other words, in the very heart of the earth there would be two huge fragments of another world, of an already missing extraterrestrial planet.
Two years later, in 2023, another team of researchers, including Qian Yuan himself, obtained new data that seemed to confirm that intriguing possibility. The investigation then deserved the cover of ‘Nature’.
They are not the same
One way or another, scientists had always assumed that the two LLSVPS are similar to each other both in their nature and in their chemical composition and age, because the seismic waves with which geologists study them travel through them in a similar way. But a new work led by researchers from Cardiff, Oxford, Bristol and Michigan universities has just revealed something totally unexpected and puts everything ‘legs up’. According to the study, in effect, these two mysterious underground regions have different stories: they were not formed at the same time and their chemical compositions are different. The findings, which question all the hypotheses formulated so far, have just been published in ‘Scientific Reports’.
According to the new studies, the African LLSVP is considerably older and is ‘better mixed’ than that of the Pacific, whose composition is constantly renewed with fresh materials of the oceanic crust for 300 million years. This is because, on the surface, this LLSVP is surrounded and ‘fed’ by a large circle of subduction zones, known as the ‘Fire Ring’ of the Pacific. On the contrary, the African LLSVP does not receive new material at the same speed, so it has mixed better with the surrounding mantle, reducing its density.
“As numerical simulations are not perfect,” explains James Panton, lead author of the study – we have executed multiple models for a whole range of parameters. And every time, we find that the LLSVP of the Pacific is enriched with subduced oceanic crust, which implies that the recent history of subduction of the Earth is promoting these differences ».
Differences that until now had been overlooked due to the fact that both LLSVPS have the same temperature (which is the dominant factor to explain how quickly travel seismic waves through a material), which explains why they seem seismically so similar. The finding highlights the importance of combining different scientific disciplines to closely examine the internal functioning of our planet.
«The fact that these two LLSVPS differ in their composition, but not in their temperature – says Paula Koelemeijer, co -author of the study – is key to history and explains why they seem to be the same seismically. It is fascinating to be able to see the links between the movements of the plates on the surface of the earth and the structures more than 3,000 km deep ».
The findings, of course, question the previous theory that LLSVPS are a legacy of the titanic collision that formed the moon.
Imbalances in the magnetic field
The high temperatures of the LLSVPS, and their location, in the deep mantle on each side of the planet, imply that both affect the way the heat is extracted from the nucleus of the earth. And that has consequences in the convection of the liquid external nucleus, a process that drives the earth’s magnetic field, the natural shield that protects the surface of the harmful cosmic rays and maintains the qualities that allow the existence of life on our planet.
But if African and Pacific LLSVPS are different, it means that heat can no longer be extracted symmetrically, which could lead to ‘anomalies’, such as that of the South Atlantic, or even the general instability of the magnetic field. That is why it is important to understand the structure of these huge rock masses and the way they influence the heat extraction of the nucleus.
From now on, therefore, scientists must take into account this asymmetry in the density of the mantle in their deep earth models. And calculate with greater precision the effects that it could have on the defensive shield of the planet. Something that represents new challenges for future observations.
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