The Earth, just like any other entity, has its share of oddities, and one such oddity is a “gravity hole” nestled in the Indian Ocean. This has been a long-standing conundrum since it was first identified in 1948 by Dutch geophysicist Felix Andries Vening Meinesz, who happened upon this anomaly during his gravity survey. Known as the Indian Ocean geoid low, it is a massive circular depression on the ocean floor, starting near the southern tip of India and sprawling over an area of 1.2 million square miles.
The intriguing aspect of this geoid low or gravity hole is that at its deepest point, where the sea level dips over 328 feet, Earth’s gravitational pull is weaker, and its mass is less than what is typically expected. However, after decades of befuddlement, a recent breakthrough in research seems to shed light on the existence of this geoid low.
A team of researchers at the Indian Institute of Science in Bengaluru took up the challenge and, with the help of advanced computer modeling, found that plumes of molten rock from deep within Earth might have played a significant role in forming this gravity hole. Their findings were published in the reputable journal Geophysical Research Letters in May.
The key to understanding this phenomenon lies in the fact that Earth, while being round, is not a perfect sphere. Attreyee Ghosh, a coauthor of the study and a geophysics professor at the Centre for Earth Sciences of the Indian Institute of Science, provides an apt analogy, describing Earth as a “lumpy potato,” an ellipsoid that bulges outward in the middle as it rotates. This unequal bulging results in different parts of our planet exerting varying gravitational pull due to the variable mass of Earth’s layers.
Backed by geological evidence of Earth’s past, Ghosh and her team conducted numerous simulations to explore possible ways in which tectonic plates and magma could have shaped the geoid low. They ran a total of 19 simulations tracing back to as far as 140 million years. In six of these simulations, they were successful in recreating the geoid low, and all six had a common feature: the fringing of the geoid low with hot, anomalous magma plumes.
These magma plumes, according to the team’s conclusion, along with the structure of the nearby mantle, are instrumental in forming the gravity hole. In simulations where the magma density was varied, the low never formed in the absence of magma. Further, the viscosity of the magma was also a determining factor in forming the low. Any alteration in the viscosity resulted in the failure to form the low. The team suggested that hot anomalies in both the upper mantle and deep below the Earth’s surface contributed to the formation. It is likely that these magma plumes originated from an ancient ocean that ceased to exist when present-day India collided with Asia tens of millions of years ago.