Scientists have been intrigued by the presence of a ‘Gravity Hole’ in the Indian Ocean for years now. The hole will not drain the ocean but it will have an impact on the gravitation. Located just south of Sri Lanka, it is here that the Earth’s gravitational pull is at its weakest. As a result, the sea level is 100 metres lower than the global average.

In the recent study published in Geophysical Research Letters, the Indian Ocean geoid low (IOGL) is the lowest geoid anomaly on Earth. It is a circular geoid low situated just south of the Indian peninsula. An equipotential surface coinciding with the mean sea level, the geoid is controlled by internal mass distribution within the mantle and their related surface and core-mantle boundary (CMB) topography, and as such, the IOGL is a consequence of, mass deficit inside the Earth's mantle beneath the Indian Ocean.

The gravity hole, called the Indian Ocean geoid low (IOGL) and was discovered in 1948 during a survey by Dutch geophysicist Felix Andries Vening Meinesz. 

The origin of the deepest geoid on Earth, the Indian Ocean geoid low (IOGL), is debated. Several competing hypotheses exist, amongst which, a recent study employing tomography models suggested that hot anomalies at mid to upper mantle depths are crucial in generating this elusive feature.

What is a gravity hole?

A pronounced dip in the geoid under the Indian Ocean—called the Indian Ocean geoid low (IOGL)—is the planet’s most prominent gravitational anomaly. It covers more than three million square kilometers and is centered about 1,200 km southwest of the southern tip of India. (Its enormity, as well as the fact that the ocean looks relatively flat at any given point, means the dip isn’t visible at the surface.) As a result of the low pull of gravity there, combined with the higher gravitational pull from the surrounding areas, the sea level of the Indian Ocean over the hole is a whopping 106 meters lower than the global average, says the new study’s senior author Attreyee Ghosh, a geophysicist at the Indian Institute of Science (IISc) in Bangalore.

The results, published in Geophysical Research Letters, indicate the IOGL is present because of a distinctive mantle structure.

How was the gravity hole studied?

For their research, the scientists used 19 mantle convection computer models from the Mesozoic era to the present day to understand the connection between the incidents in the two periods. Mantle convection refers to the movement caused within the Earth’s mantle, or the middle layer, where hotter and lighter material rises to the top, and cooler and denser material sinks due to gravity.

Through these models, which replicate tectonic plate movements from the last 140 million years, researchers found that “low density anomaliesâ€_x009d_, or the existence of lighter elements in the upper to mid-mantle under the IOGL, were the cause of low gravity in this area. 

Here, researchers found hot material originating from the African superplume — a large section of the Earth’s mantle that carries heat from near the core up to the crust — was being deflected eastwards, possibly due to the Indian plate’s rapid motion. This material then ended under the IOGL, according to researchers. 

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Through these models, which replicate tectonic plate movements from the last 140 million years, researchers found that “low density anomaliesâ€_x009d_, or the existence of lighter elements in the upper to mid-mantle under the IOGL, were the cause of low gravity in this area. 

The new study has opened doors to a whole new world, where scientists are learning new things about the planet.