Scientists at the Indian Institute of Science (IISc) Bangalore have uncovered the origin of a massive "gravity hole" spanning over three million square kilometers in the Indian Ocean. Situated south of Sri Lanka, this region experiences the weakest gravitational pull on Earth, with sea levels more than 100 meters lower than the global average.

The researchers observed that in the absence of oceanic tides and currents, water would settle into a smooth undulating shape known as a geoid, rising where gravity is stronger and sinking where gravity is weaker. These variations in the ocean surface, referred to as "geoid anomalies," are caused by uneven distribution of mass within the Earth. One such anomaly, known as the Indian Ocean Geoid Low (IOGL), features a significant gravity low where the ocean surface plunges to 106 meters. Attreyee Ghosh, an Assistant Professor at the Indian Institute of Science, Bangalore, described the IOGL as the lowest geoid/gravity anomaly on Earth and a long-standing challenge in the field of Earth Sciences.

In a study published in the journal Geophysical Research Letters, the team from IISc collaborated with researchers from the GFZ German Research Centre for Geosciences to investigate the source of the missing mass causing the geoid low. Previous studies had proposed various explanations, including remnants of a past tectonic plate that had descended into the Earth's mantle below another plate millions of years ago, but none provided a convincing answer until now. The new study employed numerical models of "mantle convection" to elucidate the mass deficit. Mantle convection refers to the movement occurring within the Earth's middle layer, where hotter and lighter material rises while cooler and denser material sinks due to gravity.

The researchers utilized seismic tomography models, which employ seismic waves to create a 3D depiction of the Earth's interior, to drive their convective mantle models. They discovered that the presence of "low density anomalies" or lighter materials in the upper to mid mantle beneath the IOGL accounted for the gravity low in this area. Although mantle plumes, which involve the ascent of abnormally hot rock within the Earth's mantle, can lead to low density anomalies, the researchers ruled out this possibility since no known mantle plume exists beneath the IOGL.However, they did find that hot material from the African large low-shear-velocity province (LLSVP) or the African superplume was rising near the IOGL and being deflected eastward before terminating beneath it.

The researchers attributed this deflection to the rapid motion of the Indian plate. Ghosh further explained that a geoid low or negative geoid anomaly results from a mass deficit within the deep mantle. Their study provides an explanation for this low, attributing it to hotter, lighter material stretching from depths of 300 km to 900 km in the northern Indian Ocean, likely originating from the African superplume.