The Mystery of The Indian Ocean’s ‘Gravity Hole’

A strange anomaly that covers 1.2 million square miles of the Indian Ocean has baffled scientists for centuries. Located 1100 miles from the Indian coast, this anomaly’s sea level lies around 100 meters below the global average. Called a gravity hole, not much was understood about its existence. However, in 2023 scientists from the Indian Institute of Science published a paper discussing the potential cause for this anomaly. 

What Is a Gravity Hole?

To better understand what a gravity hole is, we first need to learn about the force of gravity. This force attracts objects to the Earth’s center, however, it is not uniform across the entire globe. This force will vary slightly depending on what lies in the crust, core, and mantle below the surface of the Earth. The geoid is what we call the mean gravity surface of the Earth. It is an imaginary surface located at ocean surface level where gravity is all a uniform strength. [1] If this planet were a perfect sphere, then this imaginary surface would also be a perfect sphere. However, the interior of the Earth varies from one place to another, therefore so does the gravity’s strength. At the places where gravity’s pull is weaker, the geoid is lower. 

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The Indian Ocean Geoid Low

This specific gravity hole is known as the Indian Ocean Geoid Low. It was first discovered by a geophysicist from Holland called Felix Andries Vening Meinesz. The Indian Ocean Geoid Low was discovered during a gravity survey in 1948. It is the lowest point of the geoid, and up to now, its existence has never fully been understood. To look for potential answers, the scientists used computer models, creating a simulation of what the Earth was like 140 million years ago. This is because the oceans and continents were once situated in very different locations, as was the density structure of the Earth.

Computer Simulations of the Earth

The scientists then ran 19 various simulations from this point in the past up to the present day. In this manner, they recreated the shifting tectonic plates, in addition to the behavior of the molten rock inside the Earth’s mantle. In 6 different simulations, a geoid similar to the Indian Ocean Geoid Low formed. The common factor in these six simulations was the presence of magma plumes around the geoid low. 

The Magma Plumes

It’s these plumes of magma, along with the unique mantle structure of the location, that are believed to be the cause of the gravity hole. [2] The models were run with varying magma density parameters, and the low did not form in those models in which the plumes were not present. The plumes are believed to have originated millions of years ago when an ancient ocean disappeared as a result of India’s landmass drifting and colliding with Asia. As India drifted northward, the gap with Asia closed and the ocean disappeared. [3] As this oceanic plate sunk into the mantle, the plumes likely formed as a result, bringing material with a low density closer to the surface of the planet. 

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What Will Happen to the Gravity Hole?

According to the scientists’ calculations, this geoid low formed around 20 million years ago and it is difficult to know what will happen in the future. It could be around for many more years, it all depends on how these anomalies move around inside the Earth. If the plates moved in a certain way, the gravity hole could even disappear, but this would only be hundreds of millions of years from now. While there is still much to be learned, studies such as these encourage further investigation into the field. 

Comparison to Older Studies

Other scientists have welcomed this study as it vastly improves on older ones. Older studies only simulated the descent of cold material, whereas this study included the hot rising plumes as well. However, they have also pointed out a few potential flaws with the study. For example, it does not reproduce the forceful mantle plume that erupted around 65 million years ago. The eruptions of lava flow that stretched across half of the subcontinent have long been attributed to this particular mantle plume. However, this plume is nowhere to be seen in this new model simulation. 

The Bottom Line on Gravity Holes

Previously, many gravity hole studies focused on descending cold material in the mantle. However, this new research underscores the potential role of hot, rising magma plumes. This changing shift in perspective shows us that the Earth’s deeper layers are far more dynamic and complex than we once believed. That said, the study isn’t without its flaws and uncertainties. Nonetheless, it still opens up new avenues for exploring the Earth’s gravity anomalies. It also provides a stronger foundation for understanding how the planet’s interior has shaped its surface over millions of years. 

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