Found an explanation for the “gravitational hole” discovered in the Indian Ocean

This anomaly has puzzled geologists for a long time, but now researchers at the Indian Institute of Science in Bangalore have found what they believe is a plausible explanation for its formation: magma ejections emanating from the planet’s depths are very similar to those that lead to the formation of volcanoes.

To come up with this hypothesis, the team used supercomputers to model how the area might have formed from 140 million years ago, according to CNN. The findings, detailed in a study published recently in the journal Geophysical Research Letters, center around an ancient ocean that no longer exists.

People used to think of the Earth as a perfect sphere, but that’s far from the truth, CNN continues.

“Earth is basically a lumpy potato,” says study co-author geophysicist and associate professor at the Center for Geosciences at the Indian Institute of Science, Attreyi Ghose. “So, technically, it’s not a sphere, but what we call an ellipsoid, because as the planet rotates, the middle part sticks out.”

Our planet is heterogeneous in density and properties, with some areas denser than others, which affects the Earth’s surface and gravity, Ghose added. “If you pour water on the surface of the Earth, the level that the water occupies is called the geoid – and it is controlled by these differences in the density of material inside the planet, because they attract the surface in completely different ways depending on how much mass is under it,” she said.

The “gravity hole” in the Indian Ocean, officially called the Indian Ocean geoid low, is the lowest point of this geoid and its largest gravity anomaly, forming a circular trough that begins just beyond the southern tip of India and covers an area of ​​about 3 million square kilometers. The anomaly was discovered by the Dutch geophysicist Felix Andris Wenning Meinesch in 1948 during a gravity survey from a ship and has remained a mystery to this day.

To find a potential answer, Ghose and her colleagues used computer models to “set the clock” back 140 million years to see the big picture from a geological perspective. “We have some information and some confidence in what the Earth looked like then,” she said. “The continents and oceans were in completely different places, and the density structure was also very different.”

From that starting point, the team has run 19 simulations up until today, recreating the shifting of tectonic plates and the behavior of magma, or molten rock, inside the mantle, the thick layer of the Earth’s interior that lies between the Earth’s core and crust. In six scenarios, a geoid basin similar to that found in the Indian Ocean was formed.

A distinguishing factor in all six of these models was the presence of magma plumes around the bottom of the geoid, which, along with mantle structure nearby, are thought to be responsible for the formation of the “gravity hole,” Ghosh explained. Modeling was carried out with various magma density parameters, and in cases where plumes were absent, a low level did not form.

According to the scientist, the plumes themselves arose as a result of the disappearance of the ancient ocean, when the landmass of India drifted and eventually collided with Asia tens of millions of years ago.

“India was in a completely different place 140 million years ago, and there was an ocean between the Indian Plate and Asia. India began to move north, and at that moment the ocean disappeared, and the gap with Asia narrowed, ”explains the scientist. As the oceanic plate sank into the interior of the mantle, this could have triggered the formation of plumes, bringing low-density material closer to the Earth’s surface.

According to the team’s calculations, the geoid basin formed about 20 million years ago. It’s hard to say if it will ever disappear or move aside.

“It all depends on how these massive anomalies move around on Earth,” notes Ghose. “Perhaps it will last for a very long time. But it could also happen that the movement of the plates will lead to its disappearance – in a few hundred million years in the future.

Hugh Davies, a professor in the School of Earth and Environmental Sciences at Cardiff University in the UK, said the study was “certainly interesting and presents interesting hypotheses that should stimulate further work on this topic.”

Dr. Alessandro Forte, professor of geology at the University of Florida at Gainesville, said there is good reason to run computer simulations to determine the origin of the Indian Ocean geoid floor, and that this study is an improvement over previous ones. Past studies have only modeled the sinking of cold material through the mantle, and have not also included hot rising mantle plumes.

However, Forte said he found a couple of flaws in the conduct of the study, CNN emphasizes.

“The biggest problem with the modeling strategy adopted by the authors is that it does not fully reproduce the powerful mantle dynamic plume that erupted 65 million years ago under the present-day location of Réunion Island,” he said. “The eruption of lava flows that covered half of the Indian subcontinent at this time, forming the famous Deccan Traps, one of the largest volcanic formations on Earth, has long been attributed to a powerful mantle plume that is completely absent from model simulations.”

Another problem, Dr. Forte added, is the difference between the geoid, or surface shape predicted by computer simulations, and the real one: “These differences are particularly noticeable in the Pacific, Africa and Eurasia. The authors mention that there is a moderate correlation, around 80%, between predicted and observed geoids, but they do not provide a better estimate of how well they match numerically (in the study). This discrepancy suggests that there may be some flaws in computer simulations.”

For her part, Ghose noted that not all possible factors can be taken into account in the simulation. “We don’t know with absolute certainty what the Earth looked like in the past. The further you go into the past, the less confidence in the models. We cannot take into account every possible scenario, and we also have to accept the fact that there may be some discrepancies in how the plates have moved over time, she said. “But we believe that the general reason for such a low level is quite clear.”