Somewhere in the Indian Ocean, between south of Sri Lanka and the Maldives, can be found a region with the lowest gravity on Earth. This area of low gravity known as the Indian Ocean geoid low (IOGL) is approximately 3 million square kilometres (1.2 million miles) long and scientists believe that it was formed around 20 million years ago. Now researchers think they may have found the reason why this region of Earth exhibits such low gravity, which is so weak that sea level over the “gravity hole” is 348 feet or 106 meters lower than the global average.
This phenomenon was discovered in 1948 by Dutch geophysicist Felix Andries Vening Meinesz, as he sailed around the planet to survey its gravity. And since then, scientists have been flummoxed as to why it exists.
The reason for this anomaly is the shape of our planet, which is more like a potato than round. It is flat at the poles and bulges out at the equator. Plus, it has various other lumpy bits across its surface as can be seen in the image below.
“The Earth is basically a lumpy potato,” study co-author Attreyee Ghosh, a geophysicist and associate professor at the Indian Institute of Science, told CNN. “So technically it’s not a sphere, but what we call an ellipsoid, because as the planet rotates the middle part bulges outward.”
Now a new study published in the journal Geophysical Research Letters says that the IOGL was caused by low-density magma that was pushed into the Indian Ocean by the sinking pieces of an ancient ocean known as the Tethys Sea.
“The origin of this geoid low has been enigmatic. Different theories were put forward to explain this negative geoid anomaly. All these studies looked at the present-day anomaly and were not concerned with how this geoid low came into existence”, write the the researchers in the study.
To answer this question, they used 19 computer models to simulate the motions of the mantel and the tectonic plates in the particular region, over a span of 140 million years and then compared the gravity lows derived from the simulated models to the one actually present today.
In 6 of the best simulations, they observed low-density magma plumes rising up and displacing the high-density material below the IOGL. This displacement resulted in reducing the region’s mass and lowering its gravity.
We know that the Indian plate of our planet’s crust broke off from Gondawana and started to collide with the Eurasian plate starting 50 million years ago. According to the scientists, after the plate broke off, it passed over another plate known as the Tethys plate, pushing it under (known as subduction) the Indian plate into the mantle near modern-day East Africa. As this happened, the pieces of the ancient Tethys Ocean started to sink into the lower mantle and around 20 million years ago the sinking Tethys plates displaced the African blob’s (a bubble of dense crystalized material inside Africa’s mantle) magma to form the plumes.
“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 researchers wrote.
So basically, an ancient extinct sea caused the “gravity hole” at the bottom of the Indian Ocean. This certainly needs more research to confirm the predictions, such as finding the existence of the plumes using earthquake date from the IOGL area. But still what a cool thing to have happened on our planet.
Read the full study here.