The mysterious gravity ‘hole’ in the Indian Ocean has puzzled scientists for years

Unlike “black holes” that attract and swallow the surrounding cosmic matter, a “gravitational hole” pushes the material around due to the lack of gravitational force. When it forms in an ocean, a gravity well pulls water away and creates pockets of air where water should have been, lowering sea levels. Take the example of the world’s largest and deepest gravity hole discovered in the Indian Ocean. Causing sea levels to drop 348 feet (106 meters), the hole baffled geologists for decades until 2023, when several researchers offered a possible explanation for it in a study published in the journal Geophysical Research Letters. The mystery lies in an ancient sunken Indian sea, CNN reported.

Called the “Indian Ocean Geoid Low (IOGL),” this gravity hole is located about 1,200 kilometers (750 miles) southwest of Kanyakumari, the southernmost tip of the Indian subcontinent. It is a circular depression of 1.2 million square kilometers (3 million square miles) that hides in the ocean waters. Compared to its surroundings, gravity is weaker in this area. “It is by far the lowest level of the geoid and it has not been properly explained,” said study co-author Attreyee Ghosh, a geophysicist and associate professor at the Indian Institute of Science’s Center for Earth Sciences.

Originally, a Dutch geophysicist, Felix Andries Vening Meinesz, discovered the hole, in 1948, during a gravity survey from a ship. Felix had invented a device called the “Golden Calf” to measure gravity in the seas, according to Big Think. Since then, researchers have been trying to explain the existence of this oceanic abyss. “The origin of this low geoid has been enigmatic. Various theories have been put forward to explain this negative geoid anomaly,” the researchers wrote in the study. In 2023, some researchers from the Indian Institute of Science, Bengaluru, announced that they had found an explanation. Their hypothesis is that the hole formed as a result of an ancient ocean that no longer exists. They believe that plumes of magma rising from deep within the planet are responsible for the existence of this gravity hole.

To lay the groundwork for understanding, Ghosh explained that the secret lies in the Earth’s geometry. Contrary to what most people believe, the Earth is not a perfect sphere. “The Earth is basically a bumpy potato,” she said, “So technically it’s not a sphere, but what we call an ellipsoid, because as the planet rotates, the middle part bulges outward.”

In addition, the Earth is not even uniform in its density and properties. Some areas are denser than others, which affects the surface of the Earth and its gravity, she explained, adding, “If you pour water on the surface of the Earth, the level the water takes is called the geoid – and it’s controlled by these differences in density in the material inside the planet because they pull on the surface in very different ways depending on the mass of the mass underneath.” Live Science described this geometric anomaly behind the geoid trough as: “The low is a consequence of our strangely squidgy planet, which flattens at the poles, swells at the equator, and undulates between bumps and bumps across its surface.”

Ghosh, with her fellow researchers, pushed the entire story back to 140 million years ago. She said at the time, “the continents and oceans were in very different places, and the density structure was also very different.” Starting from that time scale, the team built 19 simulation models up to the present day, recreating the tectonic history and magma behavior within the mantle. In six of the models, a geoid low similar to that in the Indian Ocean was formed.

In each of these six models, they observed the presence of magma plumes around the lower geoid, which they believed were responsible for the formation of the “gravity hole,” Ghosh said. They further suggested that these plumes were formed when an ancient Indian Ocean disappeared millions of years ago.

“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,” she explained. According to the team, when the oceanic plate squeezed within the mantle, it could have spurred the formation of plumes, bringing low-density material closer to Earth’s surface, reducing the region’s mass and weakening gravity. Over 100 million years ago, the Indian plate broke away from the Gondwana supercontinent and collided with the Eurasian plate. This collision eventually ended up forming the Himalayas, but before that happened, the Indian plate passed over the Tethys plate, pushing it under the Indian plate.

It was thrown into the mantle which is today’s location near East Africa. Eventually, about 20 million years ago, the subducting Tethyan plates moved the trapped magma of the African plume, leading to the formation of the plumes. “These plumes, along with the mantle structure in the vicinity of the low geoid, are responsible for the formation of this negative geoid anomaly,” the researchers wrote in the study.

Discussing whether the gravity hole will stay or shift or disappear, Ghosh told CNN, “It all depends on how these massive anomalies move around on Earth. It could go on for a very long time.” long. But it may also be that plate motions will act in such a way as to obliterate it, several hundred million years into the future.”