Our planet has a huge “gravitational hole”. And we are one step closer to understanding the reason.

The force of gravity is not constant throughout the Earth. Especially in the Indian Ocean

The earth is not a perfect sphere and is not made of the same materials. This means that the force that gravity exerts on us (or any other object on its surface) can vary from one point in geography to another. This is theory because practice is a little more complicated.

New explanation. Although we learned in school that the Earth’s gravitational force causes an acceleration of 9.81 meters per second squared on the Earth’s surface, this measurement is not uniform. Thanks to a study conducted by researchers at the Indian Institute of Science, we have a better understanding of what modulates these changes.

Indian Ocean exploration. The work focuses on the so-called Indian Ocean geoid minimum (Indian Ocean geoid low, or IOGL), a region in the Indian Ocean where this gravitational force is particularly weak. A team of researchers investigated the reasons and formulated a hypothesis about the cause of this gravitational anomaly, one of the most pronounced on our planet.

Ocean Tethys. The clue may lie in plate tectonic movements that have occurred over the past 30 million years. More precisely, in a movement that would lead to the fact that the earth’s crust, on which the now disappeared Tethys Ocean was located, was buried under what is now the African continent.

Large plumes of molten rock. According to the hypothesis, this gravitational “rip” would have occurred after the Indian subcontinent broke away from the ancient continent of Gondwanaland and began its journey along the Tethys Sea to what is now the Eurasian continent. During this journey, the plate that served as the seabed of Tethys would have descended towards the Earth’s mantle.

India’s northward transit would have created the present Indian Ocean, but it would also have allowed for plumes of magma, relatively low-density rock that, thanks to that low density, would have risen to the upper layers of the Earth. According to models, these plumes began to appear 20 million years ago, and over time they multiplied and made the gravitational minimum more and more intense.

Research and modeling. Research published in the journal Letters on Geophysical Research, is based on various simulations conducted by a team of researchers. They build on similar work published in 2017 and expanded with data collected from ocean surveys conducted in 2018. The expedition placed a series of seismographs on hundreds of kilometers of the Indian Ocean seafloor.

Based on these data, those responsible for this latest study modeled subtectonic movements over the past 140 million years.

A hypothesis to be tested. The team’s models provided a plausible explanation, but verifying that the magma plumes are located where the models predict will be very challenging. The interior of our planet is one of those places that is so close and so difficult to observe. Any attempt to see with our own eyes what lies beneath our feet has important limitations. No matter how deep we dig.

This is why seismographic analysis is our best tool, but collecting data in this way is also a difficult task, requiring seismic and tectonic movements. It is likely that research in this area will provide us with new data that will allow us to test a new hypothesis.

Image | Pal and Ghosh, 2023 / NASA

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*A previous version of this article was published July 2023.