Indian scientists have uncovered the possible cause behind the Indian Ocean Geoid Low (IOGL), the lowest geoid anomaly on Earth or “Gravity Hole,” where Earth’s gravitational pull is weaker, its mass is lower than normal, and the sea level dips by over 106 meters. The anomaly was first discovered by Dutch geophysicist Felix Andries Vening Meinesz in 1948, during a gravity survey from a ship, and it has remained a mystery since then.
When seawater settles into a shape known as a geoid — a wavy, gravity-defined surface giving rise to some areas where gravity is stronger, and dip where it is weaker. The Indian Ocean’s “gravity hole” is the lowest point and the biggest gravitational anomaly, forming a circular depression off India’s southern tip, covering about 1.2 million square miles.
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“India was in a very different place 140 million years ago, and there was an ocean between the Indian plate and Asia. India started moving north and as it did, the ocean disappeared, and the gap with Asia closed,” says lead author Attreyee Ghosh from the Indian Institute of Science (IISc), Bangalore.
India’s rapid northward drift forced massive slabs of oceanic crust into the lower mantle beneath Southeast Asia, says the study. The consequent disturbance affected the African Large Low Shear Velocity Province (LLSVP), a zone of hot, slow-moving rock, leading to the rise of hot material beneath the Indian Ocean, which shaped the geoid low, she explains.
“Our study explains this low with hotter, lighter material stretching from a depth of 300 km up to ~900 km in the northern Indian Ocean, most likely stemming from the African superplume,” the authors of the study note.
Using CitcomS, a powerful computer model, they recreated the movement of tectonic plates over millions of years, incorporating real-world data like reconstructed seafloor ages and past plate motions.
After running 19 different simulations, adjusting factors like temperature changes, viscosity or resistance to flow, and the chemical composition of deep mantle layers, seven models successfully recreated the IOGL as observed today.
The study revealed that the IOGL took over 100 million years to develop. Initially, sinking slabs shaped the region, but around 20 million years ago, the pooling of hot material beneath the lithosphere intensified the geoid low.
Using time-dependent simulations for the first time, the study confirms that both sinking cold slabs and rising hot plumes are necessary for the geoid low’s formation. Without the plumes, the anomaly would be broader and less distinct, they point out.
Professor Alessandro Forte at the University of Florida, not involved in the study, supports using computer simulations to explain the Indian Ocean geoid low but sees two flaws in the approach. He points out that the model fails to show a powerful mantle plume that erupted 65 million years ago, forming Réunion Island and the Deccan Traps, one of Earth’s largest volcanic features.
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Forte also notes that the predicted geoid shape differs from reality, especially in the Pacific, Africa, and Eurasia. While the study claims an 80% match, he believes it lacks precise numerical comparisons.
However, the study is significant in providing a credible explanation for one of Earth’s most intriguing gravitational anomalies.