The Indian Ocean features a massive gravity anomaly that has puzzled scientists for years. Now, a new study proposes that the ancient remains of another ocean that sank deep into the Earth itself gave rise to the Indian Ocean geoid low (IOGL), as the anomaly is called.
The IOGL is a region covering nearly two million square miles south of the Indian peninsula where the ocean surface plunges over 300 feet. The anomaly was first discovered in 1948, Scientific American noted in its coverage of the new study. But while scientists agree it must be due to gravity and the physical properties of Earth itself—which in reality looks more like a lumpy ball of dough than a smooth sphere—the exact mechanisms of the IOGL’s formation have long eluded a definitive explanation.
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A recent study published in Geophysical Research Letters and authored by researchers Depanjan Pal and Attreyee Ghosh from the Indian Institute of Science used mantle convection models covering the Mesozoic era, which ended 60 million years ago, to the present in order to narrow down the solution. The result was that the IOGL is mainly caused by a geological formation known as a large low-shear-velocity province (LSVP) under Africa, also called a “blob.” According to the study, this region was “perturbed” by sinking slabs of the sea floor belonging to the ancient Tethys Ocean.
The Tethys Ocean does not exist anymore, but in prehistoric times it was situated between the ancient landmasses of Gondwana and Laurasia. When these continents broke up and shifted, Tethys disappeared, and the modern Indian and Atlantic oceans were formed. The slabs that made up the ancient Tethyian sea floor sank into the Earth’s mantle, producing plumes that reach the upper mantle.
“These plumes, along with the mantle structure in the vicinity of the geoid low, are responsible for the formation of this negative geoid anomaly,” the authors wrote in the study.
So, the IOGL is a product of a planet in the midst of seismic change. Such change never stopped, however. “When the temperature anomalies causing this low geoid shift out of the present-day location,” Pal told Scientific American, “the geoid low will start to dissipate.”
One day, even this massive gravitational anomaly will disappear just like the ancient Tethys Ocean that gave rise to it.
