Beneath the Indian Ocean lies a gravitational enigma — a vast depression where the ocean surface dips 106 meters lower than surrounding areas. Known as the Indian Ocean Geoid Low (IOGL), this anomaly has baffled scientists for decades. Now, a study published in Geophysical Research Letters offers the clearest explanation yet, linking the phenomenon to deep-seated mantle processes dating back 140 million years.
Using advanced computer simulations, researchers have reconstructed the geological past of the region, uncovering a surprising culprit: a mass of hot, low-density material rising from the deep mantle. Their findings suggest that this anomaly is linked to the African superplume, a massive upwelling of mantle material that extends beneath the Indian Ocean.
“The existence of the Indian Ocean geoid low is one of the most outstanding problems in Earth Sciences,” says Professor 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.”
Unlike other geoid lows, typically linked to subducted tectonic plates, the IOGL’s origins have remained elusive. Some earlier theories speculated that sinking plates could be responsible, but none fully explained the scale of the anomaly.
To resolve this mystery, researchers turned to seismic tomography and geodynamic models, tracing mantle movements over 140 million years. Their analysis revealed a striking correlation between the IOGL and an ancient tectonic event: the slow disappearance of an ocean as the Indian subcontinent drifted northward.
“The Earth is basically a lumpy potato,” Ghosh explains. “Technically, it’s not a sphere, but what we call an ellipsoid, because as the planet rotates, the middle part bulges outward.” This irregular shape, combined with mantle convection, helps explain why gravity behaves differently across various regions.
As India’s landmass moved, subducted oceanic plates sank into the mantle, triggering mantle plumes that pushed lighter material upward — altering gravity in the process. This process likely led to the formation of the IOGL around 20 million years ago.
To test their theory, the team ran 19 different simulations, recreating plate shifts and mantle dynamics over millions of years. In six of these models, a geoid low resembling the one in the Indian Ocean emerged with the presence of mantle plumes playing a key role.
Not all experts are convinced. Dr. Alessandro Forte, a geologist at the University of Florida, told Daily Galaxy that the study may overlook the role of a massive mantle plume responsible for the volcanic eruptions of Réunion Island and the Deccan Traps. Moreover, while the models show an 80% correlation between predicted and observed geoids, discrepancies persist in other regions like the Pacific, Africa, and Eurasia.
For now, the IOGL remains a window into Earth’s shifting interior — one that may hold more secrets about the forces shaping our planet.