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The Earth is not a perfect sphere and, as such, the gravitational potential on the surface can change depending on where it is and the density of the material beneath your feet.

A map referred as a 'geoid' has long found that there's a massive piece of mass – more than 100 metres in depth – missing from under the Indian Ocean.

Scientists have been trying to work out why this is the case, and earlier theories argued that cold, dense oceanic plates sank into the Earth’s mantle in the past, causing the depression.

One of the authors of the study, Professor Attreyee Ghosh, from the Indian Institute of Science, said in a statement:

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. It is remarkable as it means that there is some mass deficit in the deep mantle that's causing the low.

A low gravitational potential would mean that the ocean surface itself would go down. So, for a 100 meter (328 feet) geoid low the ocean surface would dip down by 100 meters at that region.

However a study, which is published in the Geophysical Research Letters, has a new theory arguing that the depression may be the result of hot material rising from the deep mantle beneath Africa.

Our study finds that the source of this low stems from a low-density anomaly stretching from a depth of 300km down to 900km in the northern Indian Ocean region.

This density anomaly potentially originates from hot buoyant material rising from deep mantle beneath Africa and movement towards the northeast, facilitated by the movement of the Indian plate in the same direction.

H/T iflscience

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