Mysterious ‘Gravity Hole’ discovered at the center of the Indian Ocean

The ground beneath your feet may feel solid, but Earth is constantly shifting. While we have mapped the surface in detail, its deep interior remains a mystery. Even with modern technology, no probe has ever reached beyond the thin crust, which is only about 35 kilometers deep. To study the Earth's core and mantle, scientists must rely on indirect methods.

From space, Earth appears as a smooth blue sphere, but its shape is far from perfect. Beneath the surface, unevenly distributed mass creates gravitational variations, distorting its form. The movement of tectonic plates further reshapes the planet, building mountains, carving valleys, and adding to its irregularity.

These distortions extend to the oceans, which cover 71% of the surface. Without tides or currents, seawater would settle into a shape known as a geoid—a wavy, gravity-defined surface. Some areas rise where gravity is stronger, while others dip where it is weaker. These variations, called "geoid anomalies," reveal how mass is distributed deep within the Earth.

One of the most striking geoid anomalies lies south of Sri Lanka. Called the Indian Ocean Geoid Low (IOGL), it features an enormous dip in the ocean’s surface, dropping 106 meters lower than surrounding areas. This "gravity hole" has baffled scientists for decades.

“The existence of the Indian Ocean geoid low is one of the most outstanding problems in Earth Sciences,” says Prof. Attreyee Ghosh, an Assistant Professor at the Centre for Earth Sciences, Indian Institute of Science, Bangalore. “It is the lowest geoid/gravity anomaly on Earth and so far no consensus existed regarding its source.”

To solve this puzzle, Prof. Ghosh and her team collaborated with researchers from the GFZ German Research Centre for Geosciences. Their findings, published in Geophysical Research Letters, suggest a long-missing explanation for this strange gravitational dip.

Earlier theories pointed to a sunken fragment of an ancient tectonic plate that plunged into the mantle millions of years ago. However, none of these ideas fully accounted for the extent of the anomaly. Until now, the true cause remained unknown.

Using advanced numerical models, the researchers traced the anomaly to mantle convection—the slow, churning movement of material inside the Earth. In this process, hot, lighter material rises while cooler, denser material sinks, creating a flow driven by gravity. Their models, informed by seismic tomography data, provided the first convincing explanation of the missing mass.

The researchers discovered that 'low-density anomalies'—the presence of lighter materials in the upper to mid-mantle beneath the IOGL—caused the gravity low in this region. Mantle plumes, or rising abnormally hot rock, can result in low-density anomalies.

However, no known mantle plume exists beneath the IOGL. Instead, they found hot material rising from the African large low-shear-velocity province (LLSVP) or the African superplume, near the IOGL, which gets deflected eastward and terminates beneath the IOGL. The deflection is possibly due to the fast motion of the Indian plate.

The researchers used supercomputers to simulate how the area could have formed, going as far back as 140 million years. "The Earth is basically a lumpy potato," said Ghosh. “Technically, it’s not a sphere, but what we call an ellipsoid, because as the planet rotates, the middle part bulges outward.”

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

From that starting point, the team ran 19 simulations up to the present day, recreating the shifting of tectonic plates and the behavior of magma inside the mantle. In six scenarios, a geoid low similar to the one in the Indian Ocean formed. The distinguishing factor in all six models was the presence of plumes of magma around the geoid low, believed to be responsible for the formation of the "gravity hole."

The plumes themselves originated from the disappearance of an ancient ocean as India's landmass drifted and eventually collided with Asia tens of 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," Ghosh explained.

As the oceanic plate went down inside the mantle, it could have spurred the formation of the plumes, bringing low-density material closer to Earth's surface.

“A geoid low or a negative geoid anomaly would be caused by a mass deficit within the deep mantle. 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,” says Prof. Ghosh.

The geoid low formed around 20 million years ago, according to the team's calculation. It’s hard to say whether it will ever disappear or shift away. "That all depends on how these mass anomalies in the Earth move around," Ghosh said. "It could be that it persists for a very long time. But it could also be that the plate movements will act in such a way to make it disappear — a few hundreds of millions of years in the future."

Dr. Alessandro Forte, a professor of geology at the University of Florida, believes there is good reason to carry out computer simulations to determine the origin of the Indian Ocean geoid low. However, Forte identified a couple of flaws in the study’s execution.

“The most outstanding problem with the modeling strategy adopted by the authors is that it completely fails to 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 — producing the celebrated Deccan Traps, one of the largest volcanic features on Earth — have long been attributed to a powerful mantle plume that is completely absent from the model simulation.”

Another issue, Forte added, is the difference between the geoid, or surface shape, predicted by the computer simulation and the actual one: “These differences are especially noticeable in the Pacific Ocean, Africa, and Eurasia. The authors mention that there is a moderate correlation, around 80%, between the predicted and observed geoids, but they don’t provide a more precise measure of how well they match numerically (in the study). This mismatch suggests that there may be some deficiencies in the computer simulation.”

Ghosh acknowledged that not every possible factor can be accounted for in the simulations. "That’s because we do not know with absolute precision what the Earth looked like in the past. The farther back in time you go, the less confidence there is in the models. We cannot take into account each and every possible scenario, and we also have to accept the fact that there may be some discrepancies on how the plates moved over time,” she said. “But we believe the overall reason for this low is quite clear.”

This study is a significant step in understanding the IOGL, providing a credible explanation for one of Earth's most intriguing gravitational anomalies. Future research will continue to refine these models and explore the complex dynamics of Earth's interior.

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