There's an enormous gravity hole in the middle of the Indian Ocean. Scientists say they've finally worked out what's causing it.
ESA
- The Indian Ocean has a gravity hole where sea levels are about 300 feet lower than nearby areas.
- Local gravity is slightly lower there, which has puzzled scientists for decades.
- The gravity hole may have been caused by an ancient ocean bed that sank millions of years ago.
There's a huge gravity hole in the middle of the Indian Ocean that has stumped scientists for decades.
The million-square-mile anomaly isn't a physical hole but an area of the ocean where the Earth's gravity is lower than average.
Scientists studying the "hole" have long thought that something underneath was causing the strange effect.
But a new study suggests researchers should have been looking around — not under — the gravity hole to solve the mystery of how it formed.
The study says plumes of molten rock rising from the remnants of an ancient ocean bed could be to blame.
A million-square-mile gravity hole is lowering the oceans
ESA – GOCE High Level Processing Facility
Gravity varies very slightly over the surface of the globe.
Most of these variations can be easily explained. But scientists have struggled to explain the gravity hole in the Indian Ocean, known as the Indian Ocean geoid low.
The difference in gravity is not huge. You certainly wouldn't be able to notice it if you were standing right in the middle of the anomaly, Bernhart Steinberger, a geodynamics researcher at the GFZ German Research Centre of Geosciences, told Insider.
But it is significant enough that ocean levels in the 1.1-million-square-mile patch are about 300 feet lower than in the surrounding oceans.
"I think what people generally assume is that there must be something low density underneath that's causing that," Steinberger said.
"But in that paper, they have actually a different theory," he said.
To understand the anomaly, scientists had to look around the hole
NASA/Goddard Space Flight Center Scientific Visualization Studio
To understand what was causing the hole, the geophysicist Attreyee Ghosh and the doctoral student Debanjan Pal at the Indian Institute of Science in Bangalore asked a computer to simulate what may have happened.
They plotted 19 scenarios of how tectonic plates may have moved around the hole in the past 140 million years.
Their study, published in Geophysical Research Letters in May, found that only a few scenarios could account for the gravity hole, and in none of these models was the low gravity caused by what was directly underneath it.
Instead, they found the hole was likely shaped by plumes of low-density magma.
"It's something you could have thought of before, you just wouldn't think of it because you tend to think there must be something underneath," Steinberger, who was not involved in the study, said.
"You have like a negative cutout," he said.
It goes back 120 million years ago
The most likely explanation for the gravity hole goes back to the separation of Gondwana, the supercontinent at the origin of Africa, Australia, and India, about 120 million years ago, according to the study.
As India separated from Africa and smashed into the European plate, the ocean that used to be there, called Tethys, was split apart and squished between the continental plates.
Some tiny parts of the plate are still present in the Mediterranean, but most of that plate is still slowly melting back into the deep Earth's interior around Eastern Africa.
This pulls the surface down, causing a large area of lower gravity, said Steinberger.
As the dense mantle melts away, it creates plumes of low-density magma. These and other surrounding masses like the Tibetan plateau create a relative gravity high, amplifying the effect, Steinberger said.
Himangshu Paul, a scientist at the National Geophysical Research Institute in India, told NewScientist that future ocean surveys would have to confirm whether these plumes exist in real life, not just on computers.
Correction — July 7, 2023: A previous version of this story erroneously stated that continents are denser, hence they exert more gravity locally. This has been removed from the copy. A quote explaining how the Tethys slab pulls the ocean floor down and therefore contributes to amplifying the area of low gravity was also added to provide the full context of the scientific findings. An earlier version of this story misidentified the space agency that operates the GOCE satellite. It is the European Space Agency, not NASA.