The force of gravity is not constant throughout the Earth. Especially in the Indian Ocean
The Earth is neither a perfect sphere nor is it composed homogeneously of the same materials. This means that the force that gravity exerts on us (or any other object that is on its surface) can vary from one point in geography to another. That’s the theory, because the practice is a little more complex.
A new explanation. Although in school we learned that the force of Earth’s gravity causes an acceleration of 9.81 meters per square second on the Earth’s surface, this measurement is not homogeneous. Thanks to a study conducted by researchers at the Indian Institute of Science, we have a better idea of what modulates these changes.
Probing the Indian Ocean. The work focuses on the so-called Geoid Minimum of the Indian Ocean (Indian Ocean geoid low, or IOGL), a region in the Indian Ocean where this force exerted by gravitational interaction is particularly weak. The team of researchers investigated the reasons and formulated a hypothesis about the reason for this gravitational anomaly, one of the most pronounced on our planet.
The Tethys Ocean. The key would be in tectonic plate movements that occurred over the last 30 million years. More specifically, in the movement that would have caused the Earth’s crust on which the now-disappeared Tethys Ocean was located to end up buried under what is now the African continent.
Large plumes of molten rock. According to the hypothesis, this gravitational “gap” would have occurred after the Indian subcontinent broke away from the ancient continent of Gondwana and began its journey along the Tethys Sea towards what is now the Eurasian continent. During this journey the plate that served as Tethys’ seabed would have descended towards the Earth’s mantle.
India’s transit north would have created the current Indian Ocean, but it would also have allowed the appearance of magma plumes, a relatively low-density rock that would have risen to the upper layers of the Earth thanks to this low density. According to the models, these plumes would have begun to appear 20 million years ago, and over time they proliferated and made the gravitational minimum increasingly intense.
Surveys and simulations. The study, published in the journal Geophysical Research Letters, is based on different simulations carried out by the team of researchers. They build on similar work published in 2017 and complemented by data collected in ocean surveys carried out in 2018. The expedition placed a series of seismographs across hundreds of kilometers of seabed in the Indian Ocean.
From these data, those responsible for this latest study simulated the sub-tectonic movements of the last 140 million years.
A hypothesis to be tested. The models created by the team have served to offer a plausible explanation, but verifying that the magma plumes are located where the models predict will be a very complicated task. The interior of our planet is one of those things that is so close and so difficult to observe. Any effort to verify with our own eyes what lies beneath our feet has important limitations. No matter how deep we dig.
That is why seismographic analysis is our best tool, but collecting data in this way is also an arduous task that requires seismic and tectonic movements. It is likely that surveys in the area will offer us new data that will allow us to test the new hypothesis.
Image | Pal and Gosh, 2023 / NASA
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*An earlier version of this article was published in July 2023
