Understanding the Mysterious “Gravity Hole” in the Indian Ocean

An enigmatic phenomenon known as the “gravity hole” exists in the depths of the Indian Ocean, where the Earth’s gravitational pull is weaker, and the sea level dips significantly by over 328 feet (100 meters). Geologists have long been puzzled by this unusual occurrence, but now, researchers from the Indian Institute of Science in Bengaluru, India, claim to have discovered a credible explanation for its formation. This captivating theory revolves around plumes of magma originating deep within the Earth, similar to those responsible for the creation of volcanoes.

Unraveling the Geologic Enigma

To arrive at this groundbreaking hypothesis, the team utilized supercomputers to simulate the possible mechanisms behind the formation of this gravity hole, reaching as far back as 140 million years in time. Their findings, recently published in the journal Geophysical Research Letters, focus on an ancient ocean that no longer exists.

Earth’s Unusual Shape and Density

While many envision the Earth as a perfect sphere, it is far from being so. In reality, our planet is more akin to a lumpy potato, described by study coauthor Attreyee Ghosh, a geophysicist and associate professor at the Centre for Earth Sciences of the Indian Institute of Science. This irregular shape, termed an ellipsoid, occurs due to bulging at the middle as the Earth rotates.

Moreover, Earth’s density and properties vary throughout its structure, leading to variations in its gravitational pull and surface features. The geoid, the level at which water settles on the planet’s surface, is influenced by these density differences in the Earth’s interior.

The Indian Ocean Geoid Low

The “gravity hole,” officially designated as the Indian Ocean geoid low, encompasses a circular depression spanning approximately 1.2 million square miles (3 million square kilometers) just off the southern tip of India. It was first discovered by Dutch geophysicist Felix Andries Vening Meinesz during a gravity survey from a ship in 1948, and ever since, it has remained a perplexing mystery in the scientific community.

Unveiling the Formation Process

To unearth a potential answer, Ghosh and her colleagues conducted computer models to rewind the clock 140 million years and gain insight into the geological landscape of that era. “We have some information and some confidence about what the Earth looked like back then,” she explained. During this period, continents and oceans occupied vastly different positions, and the density structure of the Earth differed significantly.

Based on this initial setup, the team ran 19 simulations, progressing to the present day, in an attempt to recreate the movements of tectonic plates and the behavior of magma within the Earth’s mantle. Out of these simulations, six displayed a geoid low akin to the one observed in the Indian Ocean.

The Role of Magma Plumes

In all six models, the defining factor was the presence of plumes of magma surrounding the geoid low. The interaction between these magma plumes and the mantle structure in the vicinity appears to be responsible for the formation of the gravity hole, as Ghosh explained. The simulations also explored different parameters of magma density, revealing that when the plumes were absent, the geoid low did not form.

The origins of these plumes trace back to the disappearance of an ancient ocean that once separated India from Asia. As India gradually moved northward and collided with Asia over millions of years, the intervening oceanic plate was driven down into the mantle. This action likely prompted the formation of the plumes, bringing low-density material closer to the Earth’s surface.

The Fate of the Geoid Low

According to the team’s calculations, the geoid low emerged approximately 20 million years ago. Whether it will endure or undergo shifts in the future remains uncertain and dependent on the dynamic movement of mass anomalies within the Earth. It could persist for eons or eventually vanish over hundreds of millions of years.

While the research has garnered interest from fellow experts, some have pointed out potential flaws in the study’s execution. Despite these challenges, Ghosh emphasized that certain factors remain beyond the precision of the simulations, especially when exploring the Earth’s distant past. Nevertheless, the team believes that the overall reasoning behind the gravity hole in the Indian Ocean is now better understood, shedding light on this captivating geologic enigma.