Scientists solve mystery of Indian Ocean ‘gravity hole’

This image shows the gravity hole region illustrated with a dark blue color. — CNN/ESA/File

Recently, scientists discovered a “gravity hole” in the Indian Ocean, a place where Earth’s gravitational pull is weaker, its mass is lower than normal, and sea levels drop more than 100 meters.

Geologists have been baffled by the origin of the anomaly, which remained unknown, for a very long time until researchers from Bengaluru, India’s Indian Institute of Science, discovered what they believe to be a compelling explanation for its formation.

They postulate that the gravity hole was caused by plumes of magma emerging from the planet’s interior, similar to those that cause volcanoes to form.

Scientists used supercomputers to recreate the region’s formation up to 140 million years ago to come to this conclusion.

The results, described in a study recently published in the journal Geophysical Research Letterscentered on an ancient extinct ocean.

Humans are forced to believe that the Earth is a perfect sphere, which is not entirely true.

“Earth is basically a lumpy potato,” said study co-author Attreyee Ghosh, a geophysicist and associate professor at the Earth Science Center of the Indian Institute of Science. “So technically it’s not a sphere, but what we call an ellipsoid, because as the planet rotates, the middle part bulges outward.”

Our planet is not homogeneous in its density and properties, with some areas denser than others – this affects the Earth’s surface and its gravity, Ghosh added.

“If you pour water on the Earth’s surface, the level the water takes is called a geoid – and that’s controlled by these density differences in the material inside the planet, because they pull up to the surface very differently depending on how much mass there is underneath,” she said.

The Indian Ocean Geoid Depression, also known as the “gravity hole”, is the lowest point and largest gravitational anomaly in the ocean. It forms a circular depression off the southern tip of India and covers 1.2 million square miles. Although it was discovered in 1948, the anomaly remains a mystery.

“It’s by far the largest depression in the geoid, and it hasn’t been explained properly,” Ghosh said.

Ghosh and his colleagues used computer models to go back in time 140 million years to look at the big picture, geologically speaking, to uncover a potential solution.

“We have information and some confidence about what the Earth looked like at the time,” she said. “The continents and the oceans were in very different places, and the density structure was also very different.”

The team performed 19 simulations to simulate the movement of tectonic plates and the behavior of magma inside the Earth’s mantle. Six of the scenarios resulted in a geoid depression, similar to that of the Indian Ocean, CNN reported.

The “gravity hole” is thought to have formed as a result of the existence of magma plumes and mantle structures near the geoid depression. Different magma density parameters were used in the simulations, and lows did not emerge in those without plumes.

Tens of millions of years ago, as India’s landmass moved toward Asia and eventually collided with it, the ancient ocean disappeared, giving rise to the plumes themselves, according to Ghosh.

“India was in a very different place 140 million years ago, and there was an ocean between the Indian plate and Asia. India started moving north and as it did, the ocean disappeared and the gap with Asia closed,” she explained.

As the oceanic plate descended into the interior of the mantle, it could have stimulated the formation of the plumes, bringing low-density material closer to Earth’s surface.

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