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The Indian Ocean has a location known as a “gravity hole,” where the Earth’s gravitational attraction is weaker than usual, its mass is lower than usual, and the sea level drops by more than 328 feet (100 meters). Geologists have long been perplexed by this anomaly, but now scientists at the Indian Institute of Science in Bengaluru, India, have discovered what they think is a plausible explanation for its formation: magma plumes that originate from deep within the planet and are similar to those that cause volcanoes to form. The scientists used supercomputers to recreate the formation of the area going back as far as 140 million years in order to arrive at this theory. The results, which were presented in a paper that was just published in the journal Geophysical Research Letters, are centered on an extinct ancient ocean.

An ocean that is vanishing

Although that isn’t true, humans are accustomed to thinking of Earth as a perfect spherical. According to coauthor of the study Attreyee Ghosh, an associate professor at the Indian Institute of Science’s Centre for Earth Sciences and geophysicist, “the Earth is basically a lumpy potato.” The planet’s center bulges outward as it rotates, making it an ellipsoid rather than a sphere in theory.

According to Ghosh, the density and characteristics of our planet are not uniform, with certain regions having higher densities than others. This has an impact on the gravity and surface area of Earth. Water on the Earth’s surface takes on a level known as a geoid, which is determined by density variations in the planet’s interior material. Depending on the amount of mass under the surface, these density differences attract the surface in very different ways, the speaker explained. The largest gravitational anomaly and lowest point of the Indian Ocean geoid, known as the “gravity hole” or Indian Ocean geoid low, forms a circular depression spanning approximately 1.2 million square miles (3 million square kilometers) and begins just off the southern tip of India. The anomaly has remained a mystery since it was discovered in 1948 by Dutch geophysicist Felix Andries Vening Meinesz while conducting a gravity survey from a ship.

“It hasn’t been explained properly, and it is by far the biggest low in the geoid,” Ghosh stated.

Ghosh and her colleagues utilized computer models to understand the larger picture, geologically speaking, by going back 140 million years in search of a possible solution. “We possess a certain amount of knowledge and assurance regarding the appearance of the Earth during that era,” she stated. “The density structure was very different, and the continents and oceans were in very different places.”Beginning at that period and continuing through the present, the researchers conducted 19 simulations to replicate the movement of tectonic plates and the actions of molten rock, or magma, within the mantle, the thick layer of the Earth’s interior that sits between the core and the crust. A geoid low resembling the one in the Indian Ocean developed in six of the scenarios. The presence of magma plumes surrounding the geoid low, together with nearby mantle structure, is thought to be responsible for the formation of the “gravity hole,” which set these six models apart, according to Ghosh. Different magma density assumptions were used in the calculations, and the low did not occur in the cases where plumes were absent.Tens of millions of years ago, as India’s landmass moved and finally collided with Asia, the ancient ocean vanished, giving rise to the plumes that we see today, according to Ghosh. An ocean separated Asia and the Indian plate 140 million years ago, and India was in a totally different location. The ocean vanished and the distance between India and Asia shrank as it began to move north, the woman added. Low-density material may have formed in the plumes as a result of the oceanic plate descending deeper into the mantle, putting it closer to the surface of the Earth.

What lies next for the geoid low

The team calculated that the formation of the geoid low occurred approximately 20 million years ago. It’s difficult to predict if it will ever go away or change. According to Ghosh, “that all depends on how these mass anomalies in the Earth move around.” It is possible that it endures for an extended period of time. However, it’s also possible that the movements of the plates will work in a way that causes it to vanish a few hundreds of millions of years down the road.

Scientist Huw Davies of Cardiff University in the United Kingdom’s School of Earth and Environmental Sciences remarked that the study is “definitely interesting, and describes interesting hypotheses, which should encourage further work on this topic.” Davies was not part of the research team. Professor of geology at the University of Florida in Gainesville, Dr. Alessandro Forte, who was not involved in the study, thinks this study is an improvement over previous ones and that there is good reason to use computer simulations to figure out the origin of the Indian Ocean geoid low. Previous studies did not account for hot mantle plumes ascending to the surface; instead, they simply modeled the descent of cold material across the mantle.

But Forte said he discovered a few shortcomings in the way the study was carried out.

“The primary issue with the modeling approach utilized by the writers is its total inability to replicate the potent mantle dynamic plume that erupted 65 million years ago beneath the current location of Réunion Island,” the author stated. “A strong mantle plume that is totally absent from the model simulation has long been attributed to the eruption of lava flows that covered half of the Indian subcontinent at this time, producing the celebrated Deccan Traps, one of the largest volcanic features on Earth.” The disparity between the geoid, or surface shape, anticipated by the computer simulation and the real one, according to Forte, is another problem. “These differences are especially noticeable in the Pacific Ocean, Africa, and Eurasia,” he said. Although the predicted and observed geoids show a moderate correlation (of about 80%), the authors do not give a more accurate estimate of how well they match numerically (in the study). This discrepancy raises the possibility that the computer simulation has some flaws.

According to Ghosh, the simulations are unable to take into consideration every potential component.

This is due to the fact that our knowledge of the Earth’s historical appearance is incomplete. There is less faith in the models the further back in time you go. We have to acknowledge that there might be some variations in the way the plates shifted over time and that we are unable to account for every situation that could occur,” the speaker stated. “However, we think the main cause of this low is fairly obvious.”

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