The Indian Ocean Geoid Low (IOGL) is a perplexing phenomenon that has puzzled scientists for decades. This “gravity hole,” as it’s often called, is a region in the Indian Ocean where the Earth’s gravitational pull is significantly weaker than average.
This anomaly causes a depression in sea level of up to 106 meters (348 feet) lower than the global mean. Spanning 1.2 million square miles southwest of India, the IOGL is the most prominent gravitational anomaly on the planet.
Discovery and Characteristics
The IOGL was first discovered in 1948 by Dutch geophysicist Felix Andries Vening Meinesz during a ship-based gravity survey. Since then, it has been confirmed by numerous expeditions and satellite measurements. The anomaly is centered southwest of Sri Lanka and Kanyakumari, the southernmost tip of India, and east of the Horn of Africa.
Possible Causes
The exact cause of the IOGL remained a mystery for a long time. However, a recent study published in Geophysical Research Letters in May 2023 has shed light on its origins. Using supercomputers to simulate Earth’s geological processes over the past 140 million years, researchers have linked the formation of the IOGL to the demise of an ancient ocean called Tethys.
The Tethys Ocean once separated the supercontinents of Laurasia and Gondwana. As Gondwana fragmented about 180 million years ago, portions of the Tethys crust were pushed (or subducted) beneath the Eurasian plate. Over millions of years, these crustal fragments sank into the Earth’s mantle. Around 20 million years ago, they reached the lower mantle, displacing high-density material from the “African blob,” a large, crystallized magma formation beneath Africa.
This displacement triggered the rise of low-density magma plumes, reducing the overall mass in the region and weakening its gravitational pull. These mantle plumes, similar to those that create volcanoes, are believed to be the primary cause of the IOGL. The lower density of these plumes results in a weaker gravitational pull in the region.
Competing Theories and Future of the IOGL
While the magma plume theory is currently the most credible explanation, some scientists have raised concerns about the modeling approach used in the study. Alessandro Forte, a geology professor at the University of Florida, points out potential issues with the team’s simulations and the differences between the simulated and actual geoids.
The future of the IOGL remains uncertain. It is believed to have taken its present shape around 20 million years ago when the plumes began to spread within the upper mantle. The anomaly’s persistence depends on the continued flow of mantle material from the African blob. If these flows cease, the IOGL will likely dissipate.
Broader Implications
The study of the IOGL has implications beyond our planet. Similar investigations on Mars have revealed subsurface anomalies, suggesting that such gravitational peculiarities may be more common in planetary bodies than previously thought. By understanding the dynamics of Earth’s interior, we can gain insights into the processes that shape other planets as well.
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