Mystery of ‘gravity hole’ in the Indian Ocean finally solved after 75 years

A mysterious ‘gravity hole’ in the Indian Ocean is caused by plumes of magma beneath the crust (Image: Creative Commons / International Centre for Global Earth Models) A mysterious ‘gravity hole’ in the Indian Ocean where the Earth’s pull is significantly lower than elsewhere is caused by plumes of magma beneath the crust. This is the conclusion of a pair of researchers from India who simulated the evolution of plate tectonics in the region from 140 million years ago to the present day. Their models revealed that sinking slabs of crust from the ancient Tethys Sea disturbed a ‘superplume’ in the mantle and generated smaller plumes beneath the Indian Ocean. These smaller plumes — being made of lower-density material in comparison to the surrounding rock — reduce the strength of the Earth’s gravity in the area, creating the phenomenon known as a “gravity hole”. This region of low gravity is called the Indian Ocean Geoid Low, and it manifests as a large-scale depression in the ocean surface spanning millions of square miles and reaching some 348 feet deep at its lowest point. The geoid is the shape the oceans’ surface would take under the influence of Earth’s gravity alone (Image: Public Domain) A conceptual tool for geologists, the ‘geoid’ is the imaginary shape that the surface of the Earth’s oceans would take under the influence of Earth’s gravity, were other influences like the wind and tides not in play. This shape, incidentally, is that of a lumpy potato. The geoid also extended to cover the continental areas of Earth, by imaging narrow hypothetical canals that traverse the world’s landmasses. The geoid was first described by the German mathematician Carl Friedrich Gauss in 1828 — he described it as ‘the mathematical figure of the Earth’. While the geoid is smooth, its surface is highly irregular — a result of the Earth’s mass being unevenly distributed, leading to variations in the strength of gravity’s pull in different places. The Tethys Sea once seperated the ancient continents of Laurasia and Gondwana (Image: Creative Commons / Lennart Kudling) Where gravity is high locally, the geoid rises, and where it is low it falls — with a corresponding influence on the ocean’s surface. By far the most prominent of these ‘geoid anomalies’ is the Indian Ocean geoid low, which lies just south of Sri Lanka. Dr Attreyee Ghosh is a geophysicist at the Indian Institute of Science in Bangalore, and one of the authors of the new study, along with his colleague Debanjan Pal, a geodynamicist. Explaining the anomaly back in 2017, Dr Ghosh said: ‘The existence of the Indian Ocean geoid low is one of the most outstanding problems in earth sciences.’ Dr Ghosh continued: ‘It is the lowest geoid anomaly on Earth and so far no consensus existed regarding its source. ‘It is remarkable, as it means that there is some mass deficit in the deep mantle that’s causing the low.’ The new study appears to resolve the question of how a mantle plume could exist beneath the Indian Ocean in a place where no source of such was previously apparent. The full findings of the study were published in the journal Geophysical Research Letters .