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Scientists Find Reason for Giant ‘Hole’ in Indian Ocean

Researchers unravel the origin of the hole in Indian Ocean forming a IOGL.
Researchers unravel the origin of the hole in Indian Ocean forming a IOGL. Credit: Richard Mortel / Flickr / CC BY 2.0

Researchers have recently identified a significant anomaly in the Indian Ocean, and while it might be referred to as a “hole,” it’s not the kind that could suck up all the water. Instead, geologists use this term to describe an area where the Earth’s gravity is lower than average.

A new study suggests that its origin can be traced back to molten rock rising from deep beneath Africa, specifically at the edges of a sunken ancient ocean bed.

Measurement of gravitational forces

Earth would have a perfectly spherical shape in an ideal world, and its gravity would be uniform across its surface. However, in reality, our planet is slightly flattened near the North and South Poles and bulges out near the equator.

Moreover, different regions exhibit varying gravitational forces based on the mass of Earth’s crust, mantle, and core beneath them.

Scientists utilize measurements of local gravity captured by sensors on the ground and satellites. By combining these measurements, they can visualize how the ocean’s surface would appear solely due to gravitational forces without the influence of other factors like winds and tides.

This visualization showcases the highs and lows of Earth’s gravitational field on a global scale and is known as the global geoid. One well-known model of the global geoid is called the “Potsdam gravity potato,” named after its resemblance to the tuber and the German research institute where it was developed.

Indian Ocean geoid low

The Indian Ocean geoid low (IOGL) is a significant gravitational irregularity, standing out as the most prominent on our planet. This anomaly extends over an impressive area of more than three million square kilometers, with its center located approximately 1,200 kilometers southwest of the southern tip of India.

However, the dip is not visible to the naked eye due to its vastness and the fact that the ocean’s surface appears relatively flat from any given point.

According to a recent study led by geophysicist Attreyee Ghosh from the Indian Institute of Science (IISc) in Bangalore, the Indian Ocean geoid low is characterized by a remarkable reduction in gravitational force.

This, in combination with the higher gravitational pull from the surrounding regions, causes the sea level over the anomaly to be an astonishing 106 meters lower than the global average.

Discovery and identification of IOGL

Back in 1948, a remarkable discovery was made during a gravity survey conducted by Dutch geophysicist Felix Andries Vening Meinesz on a ship. This led to the identification of the Indian Ocean geoid low (IOGL).

Since then, subsequent shipboard expeditions and satellite measurements have confirmed its existence. However, the cause behind this anomaly remained a mystery for scientists.

To shed light on this enigma, Debanjan Pal, a doctoral student at the Indian Institute of Science (IISc) and lead author Attreyee Ghosh, undertook a thorough investigation.

They analyzed over a dozen computer models that simulated the geological changes occurring in the region over the past 140 million years, encompassing the shifting of Earth’s tectonic plates. These models incorporated various factors related to the movement of molten material within the Earth’s mantle.

Discovery of the African blob

The findings of their study, published in Geophysical Research Letters, revealed that the IOGL is attributed to a distinctive structure in the mantle, combined with a neighboring disturbance beneath Africa known as a large low shear velocity province (LLSVP), colloquially referred to as the “African blob.”

Ghosh explains that the presence of this geoid low can be attributed to hot, low-density material originating from the LLSVP beneath Africa, which accumulates beneath the Indian Ocean.

Pal further elaborates on the origins of the African blob, which primarily contributes to the IOGL. Geologists believe it is formed by ancient remnants of seafloor known as “Tethyan slabs” that reside deep within the Earth’s mantle.

These slabs are remnants of the Tethys Ocean, which existed over 200 million years ago between the supercontinents of Laurasia and Gondwana. While both Africa and India were part of Gondwana, India gradually moved northward into the Tethys Ocean, forming the Indian Ocean behind it around 120 million years ago.

The plumes of molten rock are generated when these subducted slabs from the old Tethys Ocean sink into the mantle and reach the boundary between the Earth’s core and mantle. The surrounding mantle structures also contribute to creating this gravitational low, alongside these plumes.

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