There’s a massive “hole” in the Indian Ocean, researchers say—but it’s not the kind that could drain away all that water. Instead it’s a term geologists use to describe a spot where Earth’s gravity is lower than average. And a new study may have finally revealed its origins: it appears to be caused by plumes of molten rock rising from deep beneath Africa at the edges of the sinking remnants of an ancient ocean bed. In an ideal universe, Earth would be a perfect sphere, and its gravity would be exactly the same at every point on its surface. But in reality, Earth is flatter than a true sphere around both the North and South Poles, and it bulges out near the equator. Additionally, different regions exert a different gravitational pull depending on the mass of Earth’s crust, mantle and core beneath them. Local gravity measurements taken by ground-based sensors and satellites can be combined to show what the ocean’s surface would look like from those varying gravitational tugs alone, stripping out other influences such as winds and tides. This produces an exaggerated visualization of our planet’s gravitational high and low spots called the global geoid. A pronounced dip in the geoid under the Indian Ocean—called the Indian Ocean geoid low (IOGL)—is the planet’s most prominent gravitational anomaly. It covers more than three million square kilometers and is centered about 1,200 km southwest of the southern tip of India. (Its enormity, as well as the fact that the ocean looks relatively flat at any given point, means the dip isn’t visible at the surface.) As a result of the low pull of gravity there, combined with the higher gravitational pull from the surrounding areas, the sea level of the Indian Ocean over the hole is a whopping 106 meters lower than the global average, says the new study’s senior author Attreyee Ghosh, a geophysicist at the Indian Institute of Science (IISc) in Bangalore. According to the study’s lead author Debanjan Pal, an IISc doctoral student, the IOGL was discovered in 1948 during a ship-based gravity survey by Dutch geophysicist Felix Andries Vening Meinesz. It has since been confirmed by other shipboard expeditions and by measurements from satellites. But scientists didn’t know why it was there. To answer that question, Pal and Ghosh compared more than a dozen computer models of how the region formed over the past 140 million years as Earth’s tectonic plates have shifted around. Each model used different variables for the convection of molten material within the mantle. The results, published in Geophysical Research Letters, indicate the IOGL is present because of a distinctive mantle structure, combined with an adjacent disturbance under Africa called a large low shear velocity province (LLSVP) that is more commonly known as the “African blob.” “What we’re seeing is that hot, low-density material coming from this LLSVP underneath Africa is sitting underneath the Indian Ocean and creating this geoid low,” Ghosh says.
