The region of the Earth with the lowest relative gravity is located just south of India in the Indian Ocean. This gravity “hole” may be the result of low-density plumes of magma disturbed by the sinking slabs of a former tectonic plate.
Neither Earth nor its gravitational field are a perfect sphere. Because gravity is proportional to mass, the shape of the Earth’s gravitational field depends on the distribution of mass within it. One way of representing this field is to imagine the planet’s surface covered by a calm ocean; the variation in the Earth’s gravitational field would create bulges and depressions across this idealized ocean, corresponding to areas of more and less mass. The resulting shape, which is known as a geoid, is irregular in appearance.
The lowest point on Earth’s geoid is a circular depression located in the Indian Ocean that falls 105 metres below average sea level. Geophysicists have long debated the cause of this dip.
Geoid lows in other parts of the Earth are due to deformations at the core-mantle boundary and structures within the mantle. But these factors are not enough to account for the shape or amplitude of the extreme low in the Indian Ocean.
Debanjan Pal at the Indian Institute of Science led a study simulating 19 different scenarios for the movement of tectonic plates and changes in the mantle over the last 140 million years. In each simulation, they varied parameters that affect the formation of magma plumes in the mantle beneath the Indian Ocean. Then they compared the shape of the geoid resulting from different simulations with Eath’s actual geoid as observed by satellites.
In six of the simulations, the amplitude and shape of the Indian Ocean geoid low closely matched the observed low. In each of these simulations, but not the others, the low was surrounded by plumes of hot, low density magma.
Pal suggests that as the Indian plate broke off from the supercontinent of Gondwana to collide with the Eurasian plate, the Tethys plate that formed an ocean between them was subducted into the mantle. Over the course of tens of millions of years, slabs of the Tethys plate sank into the lower mantle where they “churned” up a region of hot magma beneath east Africa, generating plumes.