2023-07-09 17:25:40
In the Indian Ocean lies a gravity anomaly that scientists have long sought to explain. A new study suggests that it is linked to the disturbance of the super plume present under Africa.
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Except for a few die-hard “flat-earth” people who limit themselves to denying one of the most basic scientific evidences, the Earth is round. Aristotle demonstrated this more than 2,300 years ago. No more going back. However, the shape of the Earth is still the subject of scientific discussion. Because there are nuances. Indeed, the Earth is not a perfect sphere. It is slightly flattened at the pole, due to its rotation.
Distribution of masses, geoid and shape of the Earth
But the complexity goes further if we consider the gravity field. If we walk on the so-called reference ellipsoid, which is used to model the shape of the Earth, we realize that the gravitational field varies slightly. The reason: the uneven distribution of the masses that make up the Earth. Thus, scientists have defined another (theoretical) surface which accounts for these variations in the gravity field and the distribution of masses: this is the geoid. The geoid thus represents what is called an equipotential of gravity, that is to say that at any point of this theoretical surface, the value of g is constant and the direction of the gravity field is perpendicular to it. In a way, the shape of the geoid is therefore the closest to the shape of the Earth. By zooming out and looking with an extreme degree of precision, the shape of the Earth would thus rather resemble a “potatoid”, with bumps and hollows which testify to the unequal distribution of the masses in depth. It should however be kept in mind that the variations of the geoid do not exceed +/- 100 meters. The approximation of the ellipsoid is therefore more than satisfactory!
A “trough” in the Indian Ocean
But back to our potato. Because there is indeed an area that has intrigued scientists for a long time. Indeed, one of the most striking anomalies in the shape of the geoid is that located in the Indian Ocean. This is a negative anomaly, which forms a “trough” in the geoid. It would therefore be associated with a mass defect somewhere below the earth’s surface. But what is it exactly?
A rise of hot material following a disturbance at the level of the African super-plume
A new study, published in the journal Geophysical Research Letters, proposes a new hypothesis. To understand, you have to dive more than 1,000 kilometers under the earth’s crust, in the heart of the mantle. This is where the culprit is believed to be: an ancient piece of crust from the ancient Tethys Ocean that plunged beneath Africa at a subduction zone 30 million years ago. This piece of crust, cold and dense, would have disturbed the mantle super-plume (LLSVP for Large Low Shear Velocity province) that roots at the core-mantle interface beneath the African continent. This disturbance would have caused the rise of hot material within several mantle plumes under the Indian Ocean. However, who says large mass of hot and not very dense material, says negative gravity anomaly! These results come from numerical modeling having taken into account the geodynamic and tectonic evolution over the past 140 million years.
A hypothesis that does not yet have consensus within the scientific community, however, with some researchers highlighting the fact that there is no clear seismological evidence of these mantle plumes under the Indian Ocean.
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