There’s A Giant Gravity Hole In The Indian Ocean, And We’ll Finally Know Why

The pull of gravity is uniform on Earth, but our planet is not a uniform sphere. It is covered in lumps and bumps, with geology of varying density pulling on nearby masses with subtly varying degrees of force in an undulating map known as a geoid.

In the depths of the Indian Ocean, that norther subsides to a very low level, leaving what is thought to be a massive gravity ‘hole’ about three million square kilometers in size where the seafloor sinks in a wide. depression.

One of the deepest gravitational anomalies on Earth, its presence has been mentioned for a while. Ship-based surveys and satellite measurements revealed long ago that sea level just off the tip of the Indian subcontinent has dropped due to the gravitational tug-of-war between the aptly named Indian Ocean geoid low and of the surrounding gravitational ‘highs’.

What caused this relative weakening is not yet clear. Now, two researchers from the Indian Institute of Science think they have a better idea of ​​the types of planetary phenomena that might be involved.

“All of this [past] The studies looked at the current anomaly and were not concerned with how this geoid low came into being,” geoscientists Debanjan Pal and Attreyee Ghosh explained in their published paper, describing 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 remains of an ancient ocean collapsed into a ‘slab graveyard’ under Africa about 30 million years ago. ago, which stirred up hot molten rock.

But their results, based on computer models, are unlikely to resolve a heated debate about the origins of the geoid low — at least until more data is collected.

In 2018, a shipload of scientists from India’s National Center for Polar and Ocean Research set out to deploy a string of seismometers along the seafloor of the deformation zone, to map the area.

Being so far offshore, little seismic data has been collected in the area before. Results from that 2018 survey pointed to the presence of hot plumes of molten rock rising from the bottom of the Indian Ocean and somehow contributing to its great churning.

But a longer view is needed to reconstruct the geoid low in its early stages. So Pal and Ghosh retraced the formation of the massive geoid by modeling how tectonic plates skimmed through Earth’s hot, viscous mantle over the past 140 million years.

Back then, the Indian tectonic plate was just beginning 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 motion and mantle movements, comparing the shape of the oceanic low to the models predicted with observations of the dent itself.

Models that reproduce the Indian Ocean geoid below in its current form have one thing in common: plumes of hot, low-density magma flowing below. These plumes, in addition to a unique mantle structure, are what create the geoid low; if they rise high, Pal and Ghosh predict.

“In other words, our results suggest that match the [shape and amplitude of the] observed low geoid, the plumes must be sufficiently buoyant to reach mid-mantle depths,” the pair wrote.

The first of these plumes appeared about 20 million years ago, south of the Indian Ocean geoid low, and about 10 million years after the old Tethys Sea sank into the lower mantle. As the plumes spread beneath the lithosphere and turned toward the Indian peninsula, the low intensified.

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Because their results are consistent with elements of Ghosh’s previous modeling work from 2017, the duo suggest that the telltale plumes were uplifted after the Tethys seafloor sank into the lower mantle, disrupting the famous ‘African blob’.

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

Such data may be revealed soon, and there is absolutely no rush – the geoid low is expected to continue for many more millions of years.

The study was published in Geophysical Research Letters.