Beneath the vast expanse of the Indian Ocean lies a strange and puzzling phenomenon—a massive gravitational anomaly that has stumped scientists for decades. This anomaly, known as the Indian Ocean Geoid Low (IOGL), is a region where the ocean surface dips an astonishing 106 meters lower than surrounding areas, creating weaker gravitational forces than anywhere else on Earth. The cause of this strange phenomenon remained a mystery—until now. A groundbreaking study, published in Geophysical Research Letters, has provided the clearest explanation yet.
What is the Indian Ocean Geoid Low?
The Indian Ocean Geoid Low (IOGL) is a significant geophysical anomaly that has intrigued researchers and geophysicists for many years. Unlike other regions of the Earth, which are generally subject to gravitational forces shaped by tectonic movements and variations in the planet’s mass, the IOGL is characterized by its lower-than-expected gravitational pull. This has resulted in a region where sea levels are unusually low, defying the expected behavior of oceanic and continental systems.
According to Prof. Attreyee Ghosh, an assistant professor at the Centre for Earth Sciences, Indian Institute of Science in Bangalore, the IOGL represents one of the most puzzling problems in Earth sciences: “It is the lowest geoid/gravity anomaly on Earth, and so far, no consensus existed regarding its source.”
The mystery of the IOGL has been the subject of various hypotheses over the years. Some researchers speculated that a subducted tectonic plate might be sinking into the mantle below, while others pointed to the complex dynamics of the mantle itself. But no single explanation has fully accounted for the massive scale of the anomaly—until now.
The Origins of the Anomaly
The study has shed light on the IOGL’s origins. Using advanced computer simulations and geodynamic models, researchers have traced the gravitational anomaly back over 140 million years to the tectonic and mantle processes that have shaped the region over geological time.
The study reveals that the anomaly is likely linked to mantle convection processes, which are responsible for the movement of material from the Earth’s deep interior toward its surface. Researchers have identified the likely culprit as hot, low-density material rising from deep within the mantle, possibly linked to the African superplume—a large upwelling of molten rock that stretches from the Earth’s core to near the surface.
To uncover these findings, scientists used seismic tomography data and high-powered computer models to peer deep beneath the Earth’s surface. Their findings suggest that the gravity anomaly is caused by a mass deficit in the Earth’s mantle, specifically in the upper to mid-mantle beneath the IOGL, where lighter, hotter material has accumulated. This material extends from a depth of 300 km to nearly 900 km below the ocean floor.
A Hidden Force Beneath the Ocean
The origins of this gravity anomaly seem to be tied to a deep geological history that stretches back over tens of millions of years. The Indian tectonic plate, which today forms much of the Indian subcontinent, was once part of a much larger supercontinent. Over the last 140 million years, as the Indian plate began drifting northward, it closed a gap with the Asian plate, leading to the disappearance of a vast ocean between the two landmasses.
As the oceanic plate beneath the Indian plate was subducted into the Earth’s mantle, mantle plumes were triggered, bringing low-density material closer to the surface and causing the formation of the gravity anomaly we see today. These mantle plumes are likely responsible for the Indian Ocean Geoid Low, as they bring lighter, hotter material to the upper portions of the mantle, which in turn influences the gravitational pull of the region.
Prof. Ghosh explains, “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.”
The Fate of the Gravity Hole
A lingering question surrounding the Indian Ocean Geoid Low is whether this anomaly will persist over time or eventually disappear. The research team suggests that the anomaly likely formed around 20 million years ago, which is relatively recent in geological terms. However, whether it will continue to exist in the future remains uncertain.
The fate of the IOGL depends on how plate tectonics and mantle dynamics evolve. As Prof. Ghosh notes, “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.”
While this study provides an important insight into the formation of the Indian Ocean Geoid Low, not all scientists are convinced that the research fully explains the phenomenon. Dr. Alessandro Forte, a geologist at the University of Florida, questions the model’s ability to account for the massive mantle plume responsible for the volcanic eruptions of Réunion Island, which created the Deccan Traps, one of the largest volcanic formations on Earth. Additionally, there are some discrepancies in the model’s predictions, particularly in regions such as the Pacific Ocean, Africa, and Eurasia.
