A mysterious “gravity hole” in the Indian Ocean has puzzled geologists for years, as it represents a spot where Earth’s gravitational pull is weaker than usual, leading to a sea level dip of over 328 feet (100 meters). Now, researchers from the Indian Institute of Science in Bengaluru, India, claim to have uncovered a plausible explanation for its formation.
Using supercomputers, the team simulated the possible origins of this anomaly, dating back as far as 140 million years. Their findings, published in the journal Geophysical Research Letters, center around an ancient ocean that no longer exists.
While people often think of Earth as a perfect sphere, it is far from it. According to Attreyee Ghosh, a geophysicist and associate professor at the Centre for Earth Sciences of the Indian Institute of Science, Earth is more like an “ellipsoid” due to its uneven density. This non-uniformity in density affects Earth’s surface and gravity, resulting in what is called a “geoid” — the water level on the planet’s surface.
The “gravity hole” in the Indian Ocean, known officially as the Indian Ocean geoid low, is the lowest point in this geoid and the largest gravitational anomaly of its kind. It was first discovered in 1948 by Dutch geophysicist Felix Andries Vening Meinesz during a gravity survey from a ship.
To shed light on its origin, Ghosh and her team used computer models to go back 140 million years and analyze various scenarios, taking into account shifting tectonic plates and the behavior of magma in the mantle — the layer between Earth’s core and crust. In six out of 19 simulations, they found a geoid low similar to the one observed in the Indian Ocean.
The key factor in these simulations was the presence of magma plumes around the geoid low, along with the mantle structure nearby. These plumes, arising from the disappearance of an ancient ocean when India’s landmass shifted and collided with Asia millions of years ago, seem to be responsible for the formation of the “gravity hole.”
Ghosh clarified that India was once located differently, with an ocean separating the Indian plate from Asia. As India moved northward, the ocean vanished, and the gap between India and Asia closed. The oceanic plate’s descent into the mantle may have triggered the formation of magma plumes, bringing low-density material closer to the Earth’s surface.
According to the team’s calculations, the geoid low formed around 20 million years ago. Whether it will disappear or shift away depends on the movements of these mass anomalies in the Earth over time. It could persist for an extended period or vanish in hundreds of millions of years.
The research has been deemed interesting and encourages further investigations by experts like Professor Huw Davies from Cardiff University and Professor Alessandro Forte from the University of Florida. However, Forte pointed out some flaws in the study, including the failure to reproduce a powerful mantle dynamic plume that erupted 65 million years ago and mismatches between the computer simulation and the actual geoid. Despite these challenges, Ghosh and her team remain confident in their overall explanation for the “gravity hole” in the Indian Ocean.