New hypotheses for two giant anomalies detected inside Earth

Well below our feet, fundamental processes for the dynamics of the world are at play. However, this dynamic is closely linked to the nature and composition of the mantle, which is far from homogeneous, as this new study shows.

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[EN VIDÉO] seismology, stethoscope listening to the heart of the earth
“The Earth is opaque but transparent to seismic waves,” explains Jean-Paul Montagner, a researcher at the IPGP (Institut de physique du Globe de Paris) in the preface to this video. Thanks to him, we dive into the bowels of the Earth to observe its structure and understand the origin of earthquakes.

what is the structure coat deep? Although it may seem anecdotal, this question still covers all the dynamics of the Earth and especially convection the mantle plate tectonics engine. Located at a depth of more than 2,800 kilometers, this region is also volcanism that we observe on the surface, but also closely related to the responsible process dynamo terrestrial and the magnetic field of our planet.

Two major velocity anomalies in the deep mantle

Thanks to seismology study of seismic waves It makes it possible to image these interior parts of the world with increasing precision, traversing the globe. soil. In particular, it made it possible to highlight two large structures at the base of the lower mantle. These gigantic structures, one of which is located under Africa and the other under the Pacific Ocean, correspond to the lowlands. speed shear seismic waves (for LLSVP Large Low Shear Speed ​​Zone). These two structures can thus be defined anomalies compared to the rest of the lower mantle, whose velocity is significantly higher due to its enormous To make pressure at this depth. This speed variation indicates that the two LLSVPs have major differences with the rest of the mantle. However, we do not know exactly what these differences consist of: difference in composition, density, viscositytemperature… The difficulty scientists have in understanding the origin and nature of these two main anomalies is mainly due to the fact that the properties of the entire lower mantle are still very weakly constrained.

Why such a strong asymmetry?

However, researchers were interested in the issue. morphology Trying to understand the dynamics and evolution of these two anomalies over time. The anomaly found below Africa is particularly imposing: The core rises 1300 to 1800 kilometers above the mantle boundary, while the anomaly below the Pacific reaches an altitude of only 500 to 800 kilometers. The African anomaly is therefore about twice the Pacific anomaly. What is this asymmetry about? However, the two structures are surprisingly opposite, with one on each side of the world.

Some studies have shown that the distribution of these abnormalities controlled by tectonic processes occurring on the surfaceand especially by subduction zones. Really, slabs Penetrating deep into the mantle (subduction plates) can disrupt the thermal structure at the interface with the core and lead to shearing of LLSVPs.

The anomaly below Africa will be less intense than below the Pacific.

In a new study, two researchers from the University of Arizona (USA) numerical simulations To study the evolution of these anomalies according to different parameters. results, published Nature Geology, indicating that the height of the anomalies is mainly controlled by the viscosity of the surrounding mantle and the density of the material constituting the anomaly. Their hypothesis is that the African anomaly would have a different composition and specifically a lower density than the sub-Pacific anomaly, thus explaining the difference in height between the two structures.

This difference is supported by a recent study on its composition. basalts hotspots located just above the two LLSVPs, and lavas It is enriched with materials of continental origin (low density) from the lower African mantle that previously plowed through its life cycle. supercontinent Pangea. he is infection The continent was not found in lava above the Pacific anomaly. This difference suggests that the two anomalies evolved differently over time and exchanges with the surrounding mantle differ, suggesting that the African anomaly integrated a continental component that probably reached depth through subductions.

The vertical extension of the African anomaly brings it “relatively” to the surface, which may explain the existence of the African anomaly. supervolcanoes on the African plate for several hundred million years.

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