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 and 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.