There’s a giant gravity hole in the Indian Ocean, and maybe we’ll finally know why

The pull of gravity is a constant on Earth, but our planet is not a uniform sphere. It’s covered in lumps and bumps, with geology of varying density that tugs at nearby landmasses with slightly different degrees of strength in an undulating map known as a geoid.

Deep in the Indian Ocean, that pull weakens to an extreme minimum, leaving what is thought to be a massive three million square kilometer gravity “hole” where the seafloor sinks into a vast depression.

One of the deepest gravitational anomalies on Earth, its presence has been alluded to for a while. Naval surveys and satellite measurements revealed long ago that sea levels just off the tip of the Indian subcontinent subsided due to the gravitational tug-of-war between the aptly named Indian Ocean geoid low and the surrounding gravitational “highs”.

What caused this relative weakening has never been clear. Now two researchers at the Indian Institute of Science think they have a better idea of ​​the kinds of planetary phenomena that might be involved.

“All of these [past] studies looked at today’s anomaly and didn’t worry about how this low-lying geoid came to be,” explain geoscientists Debanjan Pal and Attreyee Ghosh in their published paper, which describes their new working hypothesis.

They think the answer lies more than 1,000 kilometers (621 miles) beneath the Earth’s crust, where the cold, dense remnants of an ancient ocean plunged into a “graveyard of slabs” under Africa some 30 million years ago. kicking up hot molten rock.

But their findings, based on computer models, are unlikely to settle a heated debate about the origins of the low-lying geoid until more data is collected.

In 2018, a shipload of scientists from the Indian National Center for Polar and Oceanic Research decided to deploy a series of seismometers along the seabed of the deformation zone, to map the area.

Being so far from the coast, little seismic data had previously been collected in the area. Findings from that 2018 survey indicated the presence of hot plumes of molten rock rising under the Indian Ocean and contributing in some way to its large dent.

Map showing the gravitational depression in blue under the Indian Ocean and the location of the seismometers deployed on the seabed.
The gravitational “hole” in the Indian Ocean and the position of the seismometers (black triangles) deployed on the seabed. (Ningthoujam, Negi and Pandey/EOS, 2019)

But a longer view was needed to reconstruct the low-lying geoid in its early stages. Then Pal and Ghosh traced the formation of the massive geoid by modeling how tectonic plates grazed Earth’s hot, sticky mantle over the past 140 million years.

Back then, the Indian tectonic plate was just starting to break away from the supercontinent, Gondwana, to begin its northward march. As the Indian plate advanced, the seabed of an ancient ocean called the Tethys Sea sank into the Earth’s mantle, and the Indian Ocean opened up behind it.

Pal and Ghosh ran simulations using more than a dozen computer models of plate movement and mantle movements, comparing the shape of the ocean floor those models predicted with observations of the dent itself.

Models of the Indian Ocean low-geoid in its present form all had one thing in common: plumes of hot, low-density magma rising beneath the low. These plumes, plus a distinctive mantle structure, are what created the low-lying geoid; if they climb high enough, Pal and Ghosh speculate.

“In short, our results suggest that to match the [shape and amplitude of the] geoid observed low, the plumes must be buoyant enough to reach the depths of the mid-mantle,” the couple write.

The first of these plumes appeared about 20 million years ago, south of the lower geoid of the Indian Ocean, and about 10 million years after the old Tethys Sea sank into the lower mantle. As the plumes widened under the lithosphere and slowly drifted towards the Indian peninsula, the low intensity intensified.

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Given that their findings are consistent with elements of Ghosh’s previous modeling work from 2017, the duo suggest that the telltale plumes were kicked up after the Tethys seabed sank into the lower mantle, disturbing the famous “African blob.”

However, some researchers not involved in the work are not convinced, saying New scientist there is still no clear seismographic evidence that the simulated plumes are actually present under the Indian Ocean.

Such data may soon come to light, and there’s no hurry, in fact the low-lying geoid is expected to persist for many more millions of years to come.

The study was published in Geophysics research letters.

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