How to explain the two great contradictions discovered within the earth?

La composition du manteau inférieur n'est pas homogène. © TuMeggy, Adobe Stock

At our feet are game processes essential to the dynamics of the earth. However, this dynamic is closely linked to the texture and composition of the coat, which is far from identical, this new study shows.

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[EN VIDÉO] Seismology, the stethoscope that hears the heart of the earth
In the preface to this video, Jean-Paul Montagner, a researcher at the IPGP (Institut de physique du Globe de Paris), explains that “the earth is not opaque but exposed to seismic waves”. Thanks to that, we can immerse ourselves in the bowels of the earth, observe its structure and understand the origin of earthquakes.

What is the structure Code Deep? If this sounds like a story, then this question is important for understanding the whole dynamics of the Earth. Convection This is the gown Plate tectonics engine. Located at a depth of more than 2,800 km, this zone also has an impact on the volcano we see on the surface. Dynamo Terrain and The magnetic field of our planet.

Two big speed contrasts in the deep coat

The Earth quakeThanks Study of seismic waves It allows us to cross the globe and capture these inner zones more and more accurately Earth. In particular, it made it possible to highlight two large structures at the base of the lower mantle. These massive structures, one of which is located under Africa and the other under the Pacific Ocean, are similar to the lowlands. Speed From Seismic waves Cutting (LLSVP) Large low-cut-speed province) These two structures are thus identified Contradictions Compared to other low coats, its speed is considerably higher due to its enormity Pressure Prevail at this depth. This velocity variation indicates that the two LLSVPs have large differences with the other mantle. However, we do not know what these differences are: composition, density, ViscosityTemperature … The difficulty for scientists in understanding the origin and nature of these two major anomalies comes mainly because the properties of the entire undercoat are still very poorly controlled.

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Why such strong asymmetry?

However, researchers are interested Morphology These two contradictions attempt to understand their dynamics and their evolution over time. The anomaly under Africa is particularly imposing: it rises 1,300 to 1,800 km above the core-mantle limit, while the anomaly under the Pacific reaches only 500 to 800 km. So the African anomaly is twice as large as the Pacific anomaly. What is this asymmetry related to? However, the two structures are surprisingly opposed, one on each side of the world.

Some studies suggest the spread of these abnormalities Is controlled by tectonic processes occurring at the surface, And especially by sub-regions. In fact, The Layers Penetrating deep into the mantle (plowing plates) will disturb the heating system at the interface with the center and lead to cutting LLSVPs.

The anomaly under Africa is less dense than under the Pacific

In a new study, two researchers from the University of Arizona (USA) conducted Numerical simulations To study the evolution of these paradoxes according to different parameters. Results, published Natural Earth Science, Show that the height of the contrasts is mainly controlled by the viscosity of the surrounding coat and the density of the material creating the irregularity. Their hypothesis is that the African anomaly has a different composition and a lower density than the anomaly, especially under the Pacific, thus explaining the difference in height between the two structures.

This distinction is supported by a recent study of composition Basalts Of Hot spots Located directly above the two LLSVPs, and it shows that lava flows from the sub-African mantle were concentrated in continental (not very dense) material previously subjected during the life cycle. Super Continent Pangea. That Pollution No continent was found in the volcano above the Pacific anomaly. This distinction suggests that the two contradictions evolved differently over time, and that the exchanges with the surrounding mantle may have been different, combining elements of a continent that came to depth through African disorder and oppression.

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The vertical extension of the African anomaly brings it “relatively” closer to the surface, which explains the existence of super volcanoes on the African plate for hundreds of millions of years.

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