A vast area of the Indian Ocean is 106 meters below the global mean sea level due to the large gradient in Earth’s gravity. Scientists now think they know why. – writes Scientific American.
There is a massive hole in the Indian Ocean – but not a hole that drains water, but a place where Earth’s gravity is below average. A new study may have deciphered its origin: it appears to be caused by clouds of molten rock rising from the depths of Africa at the edges of the sunken remains of the ancient ocean floor.
In a perfect universe, the Earth would be a perfect sphere and gravity would be exactly the same at every point on its surface. But in reality, the Earth is flat at the corners and bulging around the equator. In addition, different regions exert different gravitational effects depending on the mass of the Earth’s crust, underlying rock, and underlying core.
They combined local gravity measurements from ground-based sensors with measurements from satellites to show what the ocean surface looked like with only gravity changing, removing other influences such as wind and tides. This created the global Earthshape, an exaggerated perception of our planet’s strong and weak gravitational positions. One of the most famous of these forms is the so-called “Potsdam Gravity Potato” (hence its name because it resembles a tuber and was developed at the Potsdam Research Institute).
The depression is called the Indian Ocean Geode Low (IOGL) – the most prominent gravitational anomaly on the planet. More than 3 million km2 It extends and is concentrated about 1,200 km southwest of the tip of southern India. All points in the ocean seem to be relatively flat, which means that the depression is not visible on the surface.
Atrey Ghosh, a geophysicist at the Indian Institute of Science in Bangalore and one of the study’s authors, says,
Because of the weaker gravity there and the stronger gravity of the surrounding areas, sea level in the Indian Ocean over the crater is 106 meters lower than the global average.
The depression was discovered in 1948 by a Dutch geophysicist, Felix-Andries Venning Mainz, during a shipboard gravity survey, says Dipanjan Pal, one of the study’s authors, also from the Indian Institute of Science. It has since been confirmed by other ship missions and satellite measurements. But scientists did not know the reason for the existence of this hole.
Pal and Josh compared more than a dozen computer models of how the region formed over the past 140 million years as Earth’s tectonic plates shifted around it. Each model used different convection variables for intra-mantle melting.
The results, published in the journal Geophysical Research Letters, show that the geoid depression is caused by a combination of the characteristic mantle structure and the African Spot.
African spot is a neighboring disorder in Africa called large low shear velocity province (LLSVP). What they’re seeing is hot, low-density material coming from the LLSVP under Africa, under the Indian Ocean, Ghosh says, creating this geode basin.
The African Spot, which is mainly caused by the geoid drop, is probably caused by the Tethys plates deep in the mantle. Geologists believe these plates are the ancient remnants of the Tethys ocean floor, located between the supercontinents Laurasia and Gondwana, more than 200 million years ago. Africa and India were once part of Gondwana, but what is now India moved north into the Tethys Ocean, creating the Indian Ocean behind it, about 120 million years ago. Ball says that molten clouds form when collapsed and fused rock plates of the ancient Tethys ocean collapse into the mantle and reach the core-mantle boundary. They also found that in addition to magma clouds, surrounding mantle structures also play a role in creating the depression.
Ball says the depressions in the geode may have taken their current shape 20 million years ago, when clouds began to disperse in the upper mantle. And it will likely take a long time as mantle material continues to flow along the nebula from the African Spot – potentially millions of years. But as soon as these flows stop, so does the depression. When temperature anomalies move this geoid basin from its current location, the geode basin will begin to dissipate.
(Source: Scientific American: https://www.scientificamerican.com/)
