The
researchers discovered that ‘low-density anomalies’—the presence of
lighter materials in the upper to mid-mantle beneath the IOGL—caused the
gravity low in this region.
(CREDIT: International Centre for Global Earth Models / Wikimedia, CC BY 4.0)
(CREDIT: International Centre for Global Earth Models / Wikimedia, CC BY 4.0)
Beneath the vast expanse of the Indian Ocean, a strange phenomenon has puzzled scientists for decades—a massive gravitational anomaly where the ocean surface dips an astonishing 106 meters lower than surrounding areas.
Known as the Indian Ocean Geoid Low (IOGL), this region experiences weaker gravitational forces than anywhere else on Earth, and until now, its origins remained a mystery.
New research, published in Geophysical Research Letters, suggests that the anomaly may be linked to mantle convection processes dating back over 140 million years.
This groundbreaking study, which used advanced computer simulations, provides the clearest explanation yet for this gravitational oddity—and in doing so, challenges our understanding of how Earth’s deep interior evolves over time.
This shows the area chosen for calculating the regional geoid correlation. (b) and (c) Temperature profiles taken along AA’ and BB’ for Case 1. (d) and (e) 3D views of temperature profiles in and around the IOGL for Case 1.
(CREDIT: Geophysical research Letters)
The Earth Is a “Lumpy Potato”: Why Gravity Isn’t the Same Everywhere
Despite appearing as a smooth blue sphere from space, Earth is anything but uniform.
Its shape is constantly influenced by gravitational variations, which result from uneven mass distribution beneath the surface.
These distortions, known as geoid anomalies, create areas where gravity is either stronger or weaker, causing sea levels to rise or fall in response.
“The existence of the Indian Ocean geoid low is one of the most outstanding problems in Earth Sciences,” says Prof.
Attreyee Ghosh, an Assistant Professor at the Centre for Earth Sciences, Indian Institute of Science, Bangalore.
“It is the lowest geoid/gravity anomaly on Earth, and so far, no consensus existed regarding its source.”
Unlike other geoid lows, which are often linked to well-understood tectonic and mantle processes, the IOGL’s origins have remained elusive.
Some scientists previously speculated that it was caused by a subducted tectonic plate sinking into the mantle, but no existing theory fully explained the scale of the anomaly—until now.
In their study, researchers used geodynamic models to reconstruct the region’s geological past, running simulations that tracked mantle activity over the last 140 million years.
Their findings revealed a surprising culprit: hot, low-density material rising from deep within the mantle, likely stemming from the African superplume, which extends eastward and terminates beneath the IOGL
Despite appearing as a smooth blue sphere from space, Earth is anything but uniform.
Its shape is constantly influenced by gravitational variations, which result from uneven mass distribution beneath the surface.
These distortions, known as geoid anomalies, create areas where gravity is either stronger or weaker, causing sea levels to rise or fall in response.
“The existence of the Indian Ocean geoid low is one of the most outstanding problems in Earth Sciences,” says Prof.
Attreyee Ghosh, an Assistant Professor at the Centre for Earth Sciences, Indian Institute of Science, Bangalore.
“It is the lowest geoid/gravity anomaly on Earth, and so far, no consensus existed regarding its source.”
Unlike other geoid lows, which are often linked to well-understood tectonic and mantle processes, the IOGL’s origins have remained elusive.
Some scientists previously speculated that it was caused by a subducted tectonic plate sinking into the mantle, but no existing theory fully explained the scale of the anomaly—until now.
In their study, researchers used geodynamic models to reconstruct the region’s geological past, running simulations that tracked mantle activity over the last 140 million years.
Their findings revealed a surprising culprit: hot, low-density material rising from deep within the mantle, likely stemming from the African superplume, which extends eastward and terminates beneath the IOGL
Contour plot of the predicted Indian Ocean geoid low (IOGL) superimposed over predicted temperature anomalies in Case 1. (c) Contour plot of the observed IOGL superimposed over temperature anomalies in S40RTS. Both are plotted at 200 km depth. The contour intervals are 10 m.
(CREDIT: Geophysical research Letters)
To uncover the origins of the Indian Ocean Geoid Low, researchers turned to seismic tomography data and high-powered computer models.
These tools allowed them to peer deep into the Earth’s interior and trace the movement of materials beneath the ocean floor.
They discovered that the gravity low is linked to anomalous mantle activity—specifically, the presence of lighter, hotter material stretching from a depth of 300 km to nearly 900 km beneath the Indian Ocean.
“A geoid low or a negative geoid anomaly would be caused by a mass deficit within the deep mantle,” explains Prof.
Ghosh.
“Our study explains this low with hotter, lighter material stretching from a depth of 300 km up to ~900 km in the northern Indian Ocean, most likely stemming from the African superplume.”
This mantle activity is not a random occurrence.
The study suggests that the IOGL’s origins may be linked to the disappearance of an ancient ocean as India’s landmass drifted northward over the last tens of millions of years.
A map showing how water elevation and distribution would change due to gravity if the effects of tides and currents were removed.
(Image credit: European Space Agency)
The Lost Ocean That Created a Gravity Hole
The Indian tectonic plate wasn’t always connected to Asia.
140 million years ago, a vast ocean separated the two landmasses.
Over time, India drifted north, closing the gap and causing the oceanic plate beneath it to sink into the mantle.
This subduction event likely triggered mantle plumes, bringing low-density material closer to the Earth’s surface—ultimately leading to the formation of the IOGL.
“The Earth is basically a lumpy potato,” said Ghosh.
“Technically, it’s not a sphere, but what we call an ellipsoid, because as the planet rotates, the middle part bulges outward.” This irregular shape, combined with mantle convection processes, helps explain why gravity behaves differently across different regions of the planet.
To test this theory, the researchers ran 19 different simulations, replicating tectonic shifts and mantle dynamics over the last 140 million years.
In six of these models, a geoid low similar to the one in the Indian Ocean appeared—with the key factor being the presence of mantle plumes around the anomaly.
The Indian tectonic plate wasn’t always connected to Asia.
140 million years ago, a vast ocean separated the two landmasses.
Over time, India drifted north, closing the gap and causing the oceanic plate beneath it to sink into the mantle.
This subduction event likely triggered mantle plumes, bringing low-density material closer to the Earth’s surface—ultimately leading to the formation of the IOGL.
“The Earth is basically a lumpy potato,” said Ghosh.
“Technically, it’s not a sphere, but what we call an ellipsoid, because as the planet rotates, the middle part bulges outward.” This irregular shape, combined with mantle convection processes, helps explain why gravity behaves differently across different regions of the planet.
To test this theory, the researchers ran 19 different simulations, replicating tectonic shifts and mantle dynamics over the last 140 million years.
In six of these models, a geoid low similar to the one in the Indian Ocean appeared—with the key factor being the presence of mantle plumes around the anomaly.
Links :
- TheBrighterside : Mysterious ‘Gravity Hole’ discovered at the center of the Indian Ocean
- LiveScience: Indian Ocean gravity hole: The dent in Earth's gravitational field created by the death of an ancient ocean
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