Context: Recently, a team of Indian scientists has identified what caused this giant ‘gravity hole’ covering more than three million square kilometers in the Indian Ocean.

• Our planet Earth looks like a uniform blue sphere, but it is actually like a potato with its share of deformations.

• This uneven surface results from non-uniform gravity because of the unequal distribution of matter within the planet’s interior.

• In addition, the movement of tectonic plates occasionally creates mountains and valleys on the surface of the Earth, adding to the deformations.

• Since the oceans cover 71% of the planet’s surface, these deformities affect the shape of the oceans too.

• If there were no tides and currents in the oceans, all the water in the oceans would settle onto a smoothly undulating shape called a geoid, rising wherever there is high gravity, and sinking where gravity is low.

What is the Indian Ocean Geoid Low (IOGL)?

• There is an enigmatic “gravity hole” in the Indian Ocean.

• It is not the kind of “hole” that would allow the entire ocean to be drained. However, there is a massive anomaly in the Earth’s crust that has a much weaker gravitational attraction than usual.

• It is located about 600 miles below the surface of the Earth.

• This “gravity hole” is actually the Indian Ocean Geoid Low (IOGL), which spans an area of more than two million square miles.

Type of structure: Humongous structures

• Shape: The gravity hole is not actually a perfect sphere, as Earth's gravitational field is not uniform due to variations in density and mass distribution.

What forms it?

• The distribution of mass within Earth is not homogeneous, with denser regions in some areas and less dense regions in others.

• These variations in density and mass create differences in gravitational attraction, leading to the formation of the geoid.

• The irregularity in the gravitational field causes anomalies, resulting in differences in the strength of gravity at different locations on the Earth's surface, which influences the Earth's shape.

• The shape is also affected by the planet's rotation, gravitational forces, and internal structure.

How are they formed (Key-findings)?

• The scientists looked inside Earth's surface, nearly 1,000 kilometers beneath the crust where once an ancient ocean plunged nearly and stirred up hot molten rock, nearly 30 million years ago.

• The team then looked at how the tectonic plates moved along each other in the past 140 million years when the Indian plate had just begun separating from the larger Gondwanaland.

• Every time for the gravity hole anomaly to form plumes of hot, low-density magma was required, and the first such plume appeared nearly 20 million years ago, and as the plumes intensified so did the gravity hole.

• Low density anomalies: 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: Plumes are integral in generating the IOGL. Mantle plumes or the rising up of abnormally hot rock within the Earth’s mantle can result in low density anomalies.

Cause of Indian Ocean Geoid Low

• In the recent study, the researchers employ 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 rises 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.

• Low-density anomalies or the presence of lighter materials in the upper to mid mantle below the Indian Ocean Geoid Low, were responsible for the gravity low in this region.

• Mantle plumes or the rising of abnormally hot rock within the Earth’s mantle can result in low-density anomalies.

• However, no known mantle plume exists beneath the Indian Ocean Geoid Low, ruling out this theory.

• However, they found that there was hot material rising from the African large low-shear-velocity province (LLSVP), or the African superplume, in the neighborhood 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, argues the researchers.

Gravity anomalies

• The gravity anomaly on the Earth’s surface is the difference between the observed value of gravity and the value predicted by a theoretical model.

Gravity anomalies reveal rock density variations

• Short-wavelength (< 250 km) gravity anomalies are usually correlated with crustal structures.

• Long-wavelength (< 1000 km) gravity anomalies are correlated with variations in mantle densities.

The largest positive anomalies over the continents were associated with the Andes, the East European Platform, the Alpine–Mediterranean fold belt, and the central southeastern part of North America.

• The largest negative anomalies, indicating a thin lithosphere, are associated with vast Cenozoic regions of plume–lithosphere interaction: the East African Rift and the Basin and Range Province of western North America.