Researchers at the Indian Institute of Science (IISc) Bangalore have identified the cause of an over three million square kilometres-wide 'gravity hole' in the Indian Ocean.

Located just south of Sri Lanka, it is at this location the Earth's gravitational pull is weakest & sea level is over 100 metre lower than global average.

The researchers noted that in the absence of tides and currents on the oceans, all the water would settle onto a smoothly undulating shape called a geoid, rising wherever there is high gravity, and sinking where gravity is low.

The resulting unevenness in the ocean surface, known as “geoid anomalies”, are generated by uneven mass distribution within the deep Earth. One such anomaly is found just south of Sri Lanka, spanning a vast extent.

Known as the Indian Ocean Geoid Low (IOGL), it is dominated by a significant gravity low where the ocean surface plunges down to 106 metres.

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

In a study published in the journal of Geophysical Research Letters, the team from IISc, in collaboration with researchers from the GFZ German Research Centre for Geosciences, analysed the reasons behind the missing mass that is causing the geoid low.

Though several studies in the past have tried to answer this, most of them attributing it to a remnant of an earlier plate that dived into the Earth's mantle beneath another plate millions of years ago, there had been no convincing explanation of the source until now.

The new study employed numerical models of ‘mantle convection’ to explain the mass deficit. Mantle convection is a type of movement caused within the Earth's mantle or the middle layer, where hotter and lighter material rise to the top, and cooler and denser material sink due to gravity.

This convective movement within the mantle was driven by seismic tomography models that use seismic waves to obtain a 3-dimensional picture of the Earth's interior. The researchers found that 'low density anomalies' or the presence of lighter materials in the upper to mid mantle below the IOGL, were responsible for the gravity low in this region.

Mantle plumes or the rising up of abnormally hot rock within the Earth’s mantle can result in low density anomalies. However, no known mantle plume exists beneath the IOGL, ruling out this theory, the researchers said.

However, they found that there was hot material rising from the African large low-shear-velocity province (LLSVP) or the African superplume, in the neighbourhood of the IOGL, that was getting deflected eastward and terminating beneath the IOGL.

The deflection is possibly due to the fast motion of the Indian plate, the researchers said.

"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," Ghosh added.

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