Geologists track the mysterious origin of giant 'gravity hole' in the Indian Ocean

Of course, this isn't an actual "hole" as one might assume after hearing the term.

Mrigakshi Dixit
Created: Jul 04, 2023
SCIENCE


A massive, mysterious "gravity hole" deep beneath the Indian Ocean has perplexed scientists for decades.

Of course, this isn't an actual "hole" as one might assume after hearing the term.

Instead, it is a huge anomaly in the Earth's crust with significantly lower gravity than the average.

This depression is scientifically known as the Indian Ocean geoid low (IOGL), and it covers over two million square miles and sits around 600 miles beneath the Earth's crust.

It was found in 1948 by Dutch geophysicist Felix Andries Vening Meinesz during a ship-based gravity study.

However, what causes this "gravity hole" is still being studied.

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And researchers from the Indian Institute of Science in Bangalore have illustrated one probable explanation for this mysterious gravity anomaly.

Tracing the origin of the hole

It is not the first time geologists have researched the IOGL. Multiple previous studies concentrated on its current status without looking into its origins. And this new study focuses on the latter to establish how ancient influences shaped IOGL's current condition.

The researchers investigated the gravity dip using multiple computer models that were fed information on how the region altered over the last 140 million years due to the movement of Earth's tectonic plates. They also employed several factors to simulate molten material convection within the mantle

The findings indicated that IOGL exists due to a different mantle structure combined with another mantle anomaly, the African blob or large low-shear velocity province (LLSVP).

They discovered that certain tectonic plate portions had plunged into the mantle beneath Africa, triggering plumes under the Indian Ocean.

“What we’re seeing is that hot, low-density material coming from this LLSVP underneath Africa is sitting underneath the Indian Ocean and creating this geoid low,” Attreyee Ghosh, one of the authors of this study, told Scientific American.
This geoid started to form 20 million years ago

The team believes the geoid gained its current form some 20 million years ago and will likely continue as long as mantle material flows.

Geologists believe that this mantle structure beneath the Indian Ocean was probably produced by the leftovers of a long-lost Tethys Ocean's seabed. About 200 million years ago, this ancient ocean divided two supercontinents, Laurasia and Gondwana.

Africa and India were formerly part of Gondwana, but around 120 million years ago, what is now India drifted north into the Tethys Ocean, establishing the Indian Ocean behind it.

The authors highlight that there could also be other underlying facts for the IOGL's existence. Nevertheless, the "Gravity Hole" serves as a reminder that the Earth still harbors many mysteries that must be studied and understood.

The results have been published in Geophysical Research Letters.

Study abstract:

The origin of the Earth's lowest geoid, the Indian Ocean geoid low (IOGL), has been controversial. The geoid predicted from present-day tomography models has shown that mid to upper mantle hot anomalies are integral in generating the IOGL. Here we assimilate plate reconstruction in global mantle convection models starting from 140 Ma and show that sinking Tethyan slabs perturbed the African Large Low Shear Velocity province and generated plumes beneath the Indian Ocean, which led to the formation of this negative geoid anomaly. We also show that this low can be reproduced by surrounding mantle density anomalies, without having them present directly beneath the geoid low. We tune the density and viscosity of thermochemical piles at core-mantle boundary, Clapeyron slope and density jump at 660 km discontinuity, and the strength of slabs, to control the rise of plumes, which in turn determine the shape and amplitude of the geoid low.