Scientists may finally have located the origins of the deep “gravity hole” in the Indian Ocean – a mysterious area where Earth’s gravitational pull is weaker than in other parts of our planet.
The Indian Ocean Geode Depression (IOGL) is a 1.2 million square mile (3 million square km) depression found 746 miles (1,200 km) southwest of India. Compared to its surroundings, the low’s gravity is so weak that a layer of its water has been ripped away—making the sea level over the crater 348 feet (106 meters) lower than the global average.
The dip is the result of our surprisingly fine-grained planet flattening at the poles, bulging at the equator and rippling lumps and bumps across its surface. But since its discovery in 1948, the origin of this abyss in the Indian Ocean has puzzled scientists.
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Now, a study is published May 5 in the journal Geophysical Research Letters indicates that IOGL was the result of low-density magma that was pushed into the Indian Ocean by sinking slabs of an ancient ocean.
“The origin of this geode dip has been obscure. Various theories have been put forward to explain this negative geode anomaly,” the researchers wrote in their study. However, “all of these studies looked at the present-day anomaly and were not concerned with how this low-lying geoid came to be.”
To search for a possible answer, the researchers used 19 computer models to simulate the movements of the mantle and tectonic plates in the region over the past 140 million years. They then compared the simulated lows that formed in each test to the real-world hollows.
The six models that best simulated the true depression share one common feature: plumes of hot, low-density magma have risen to replace higher-density material below the depression, reducing the area’s mass and weakening its gravity.
These plumes are outbursts of mantle rock that originated from an upheaval 600 miles (1,000 km) west under Africa. The bubble known as the “African Point” is a dense blob of crystallized material within Africa’s mantle the size of a continent and 100 times taller than Mount Everest.
But what could be propelling bits of this matter under the Indian Ocean? The final pieces of the tectonic puzzle are the “Tethyan plates,” or seafloor remnants of the ancient Tethys ocean, which existed between the supercontinents Laurasia and Gondwana more than 200 million years ago.
The researchers suggest that after the Indian plate separated from Gondwana to collide with the Eurasian plate, it passed over the Tethys plate, and pushed it under the Indian plate. As it was pushed into the mantle near modern-day East Africa, shattered pieces of the ancient Tethys Ocean slowly began to sink into the lower mantle. Eventually, about 20 million years ago, sinking Tethian plates displaced some of the trapped magma of the African massif to form the plumes.
“These plumes, together with the mantle structure in the vicinity of the low geode, are responsible for the formation of this negative geode anomaly,” the researchers wrote.
To confirm the researchers’ predictions, scientists will now need to detect the presence of the plumes using seismic data collected from around the geoid depression. Whether pillars are the real answer, or if deeper forces are at play remains to be seen.
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