Scientists think they've finally found the cause of one of the weirdest anomalies on the planet.
We might be able to blame a ghost ocean for one of the wildest gravitational anomalies on Earth .
The existence of the deepest gravitational dip on our planet, known as the Indian Ocean geoid low (IOGL), has long puzzled scientists. For context, the “geiod” is a model that shows what the surface of the Earth would look like if the only influences were gravity and rotation—no land, no wind, nothing else that could disturb the surface. If the Earth were a uniform sphere, that geoid would be even, smooth, and largely uninteresting.
But the Earth isn’t a uniform sphere, and the geoid shows all the ways in which it can vary gravitationally. Areas of lower-than-average mass density, and therefore lower gravity, show up as dents in the geoid, whereas areas of higher-than-average mass density and higher gravity show up as peaks. These peaks and dents, known collectively as gravitational anomalies, indicate regions of particular strength and weakness in Earth’s gravitational field.
In a new study published in Geophysical Research Letters , a team of researchers ran dozens of computer simulations on the origin of the Indian Ocean geoid low, which spans 8 million square miles and is located south of Sri Lanka. Researchers now believe that this “elusive feature” is, in part, the result of some undersea movements that happened pre-Indian Ocean.
“Assimilating plate motion in global mantle convection models from the Mesozoic till the present day,” the authors write , “we attempt to trace the formation of this geoid low. We show that flow induced by downwelling Tethys slabs perturbs the African Large Low Shear Velocity province and gives rise to plumes that reach the upper mantle.”
Attreyee Ghosh, an assistant professor at the Indian Institute of Science and one of the authors on the study, says in a news release that, while there have been previous studies focused on this anomaly, most attributed it to a remnant of an earlier plate that dived beneath another plate and into Earth’s mantle beneath millions of years ago, none of them have provided what she would deem a sufficient explanation for the anomaly.
Ghosh and her team were not satisfied with that lack of explanation, and attempted to with their study to fill in the blanks. Over the course of their research, the scientists discovered that the gravity low could be attributed to the presence of lighter materials in the upper- to mid-mantel below the IOGL. This can be attributed to mantle plumes—caused by the rising of abnormally hot rock—and the team discovered there was hot material coming from the nearby African large low-shear-velocity providence, known as the African superplume.
“A geoid low or negative geoid anomaly would be caused by a mass deficit within the deep mantle,” Ghosh says . “Our study explains this low with hotter, lighter material stretching from a depth of 300 kilometers up to around 900 kilometers in the northern Indian Ocean, most likely stemming from the African superplume.”
Also known as the African blob, this geological feature that helped form the IOGL likely came from oceanic slabs from the Tethys Ocean, a “ghost ocean” that was possibly in place before India shifted and helped form the Indian Ocean.
Though the researchers say that the lower mantle slabs from the Tethys Ocean likely contributes less to the formation of the IOGL than the plumes , the team also call them “necessary” for the generation of this geoid low. So, apparently, combining plumes with a unique region of mantle structure makes the perfect recipe for the formation of a negative geoid anomaly.
Tim Newcomb is a journalist based in the Pacific Northwest. He covers stadiums, sneakers, gear, infrastructure, and more for a variety of publications, including Popular Mechanics. His favorite interviews have included sit-downs with Roger Federer in Switzerland, Kobe Bryant in Los Angeles, and Tinker Hatfield in Portland.
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