Scientists find why there is giant ‘hole' in Indian Ocean, where Earth’s gravity is at weakest

The Earth’s gravitational pull is weakest somewhere around the middle of the Indian Ocean. And that’s because there is a giant “hole” on that spot.

The hole being referred to here is not a typical hole where water will drain. Instead, the geologists say that there is less mass under that spot, making the gravitational pull weaker than any other place on the planet.

Scientists have for years tried to figure out the origins of such geological phenomena until now.

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Two leading geologists—Debanjan Pal and Attreyee Ghosh—from the Centre for Earth Sciences at the Indian Institute of Science in Bengaluru city of India's southern state Karnataka, have put forward a compelling explanation.

They argue that it has origins to an ancient ocean bed that sank due to plate tectonic movements.

Origins in ancient ocean

By reconstructing the last 140 million years of tectonic plate movements and underlying mantle that accompanied the rearrangement of Earth’s crust, they say that it may have been caused by plumes of molten rock rising from deep beneath Africa at the edges of the sinking remnants of an ancient ocean bed.

Earth is not a perfect sphere. Had it been thecase, then gravity would have been the same at every point on its surface.

But in reality, the blue planet is flatter than a true sphere around the North and South Poles, and it bulges out near the equator.

Furthermore, different regions exert different gravitational pull depending on the mass of Earth’s crust, mantle and core beneath them.

In the Indian Ocean, there is a pronounced dip in the geoid, which is called the Indian Ocean geoid low (IOGL). It is the planet’s most prominent gravitational anomaly.

It covers more than three million square kilometres and is centered about 1,200 km southwest of the southern tip of India.

Given the fact that the ocean looks relatively flat at any given point, it means that the dip isn’t visible at the surface. As a result of the low gravitational pull there, coupled with the higher gravitational pull from the surrounding areas, the sea level of the Indian Ocean over the hole is a whopping 106 metres lower than the global average, says the new study.

Indian Ocean geoid low

According to the study’s lead author Debanjan Pal, the IOGL was discovered in 1948 during a ship-based gravity survey by Dutch geophysicist Felix Andries Vening Meinesz.

Since then, numerous ship expeditions were undertaken and satellites used to measure the hole, but scientists could find why it was there until now.

They argue that it is present because of a distinctive mantle structure, combined with an adjacent disturbance under Africa called a large low shear velocity province (LLSVP) that is more commonly known as the “African blob.”

“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,” Ghosh says, according to Scientific American magazine.

Pal says that the African blob, which largely causes the IOGL, is probably formed by “Tethyan slabs” deep in the mantle.

Geologists think these slabs are ancient remnants of seafloor from theTethys Ocean, which was located between thesupercontinentsof Laurasia and Gondwana more than 200 million years ago.

Both Africa and India were part of Gondwana, but what is now India slowly moved north into the Tethys Ocean, creating the Indian Ocean behind it about 120 million years ago.

“Plumes [of molten rock] arise when subducted slabs belonging to the old Tethys Ocean sink inside the mantle and reach the core-mantle boundary,” he said.

“We also show that the surrounding mantle structures play a role in giving rise to this low, in addition to these plumes.”

(With inputs from agencies)

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