In a “gravity hole” discovered in 1948, the sea level drops by more than 328 feet (100 meters). According to a new study, experts believe the hole in the Indian Ocean was formed by jets of lava from deep within the earth.
The Indian Ocean has a “gravity hole” – a location where the Earth’s gravitational attraction is weaker, its mass is lower than normal, and sea level drops by more than 328 feet (100 meters).
This anomaly has long perplexed geologists, but now researchers from the Indian Institute of Science in Bengaluru, India, believe they have discovered a plausible explanation for its formation: plumes of magma coming from deep within the planet, similar to those that lead to the formation of volcanoes.
To get at this theory, the scientists used supercomputers to simulate how the area could have evolved 140 million years ago. The findings, which were recently published in the journal Geophysical Research Letters,
A disappearing ocean
Humans are used to thinking about Earth as a perfect sphere, but that’s far from the truth.
“The Earth is basically a lumpy potato,” said study coauthor Attreyee Ghosh, a geophysicist and associate professor at the Centre for Earth Sciences 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 its properties, with some areas being more dense than others — that affects Earth’s surface and its gravity, Ghosh added. “If you pour water on the surface of the Earth, the level that the water takes is called a geoid — and that is controlled by these density differences in the material inside the planet, because they attract the surface in very different ways depending on how much mass there is underneath,” she said.
The “gravity hole” in the Indian Ocean — officially called the Indian Ocean geoid low — is the lowest point in that geoid and its biggest gravitational anomaly, forming a circular depression that starts just off India’s southern tip and covers about 1.2 million square miles (3 million square kilometers). The anomaly was discovered by Dutch geophysicist Felix Andries Vening Meinesz in 1948, during a gravity survey from a ship, and has remained a mystery.
It is by far the biggest low in the geoid, and it hasn’t been explained properly,” Ghosh said.
To find a potential answer, Ghosh and her colleagues used computer models to set the clock back 140 million years in order to see the big picture, geologically. “We have some information and some confidence about what the Earth looked like back then,” she said. “The continents and the oceans were in very different places, and the density structure was also very different.”
From that starting point, the team ran 19 simulations up to the present day, recreating the shifting of tectonic plates and the behavior of magma, or molten rock, inside the mantle — the thick layer of Earth’s interior that lies between the core and the crust. In six of the scenarios, a geoid low similar to the one in the Indian Ocean formed.
The distinguishing factor in all six of these models was the presence of plumes of magma around the geoid low, which along with mantle structure in the vicinity are believed to be responsible for the formation of the “gravity hole,” Ghosh explained. The simulations were run with different parameters of density for the magma, and in those in which the plumes were not present, the low did not form.
The plumes themselves originated from the disappearance of an ancient ocean as India’s landmass drifted and eventually collided with Asia tens of millions of years ago, Ghosh said.
“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 went down inside the mantle, it could have spurred the formation of the plumes, bringing low-density material closer to Earth’s surface.