For decades, a perplexing indentation in Earth's gravitational field, residing in the vast Indian Ocean, has baffled scientists. This region, known as the Indian Ocean Geoid Low, boasts a weaker gravitational pull than anywhere else on Earth, causing sea levels to dip by over 100 meters compared to the global average. The cause of this anomaly, nicknamed the "gravity hole," remained elusive – until recently.
Researchers from the Indian Institute of Science (IISc) believe they have finally unraveled the enigma. Their findings, published in Geophysical Research Letters, attribute the gravity hole's formation to a combination of ancient oceanic movements and subterranean plumes of hot, low-density magma.
Earth's uneven mass distribution results in variations in gravitational strength. Denser regions exert a stronger pull, while less dense areas have a weaker grip. The Indian Ocean Geoid Low exemplifies this phenomenon. Here, the culprit behind the reduced gravity is thought to be remnants of an ancient ocean floor, submerged during a colossal tectonic collision millions of years ago.
The IISc team theorizes that the sunken floor of the Tethys Ocean, which once separated India from Asia, plays a crucial role. As India drifted northward, around 50 million years ago, it eventually slammed into Asia, birthing the Himalayas in a titanic clash. This collision also resulted in the Tethys Ocean floor being subducted, or pushed beneath the colliding plates.
The researchers used computer simulations to recreate the past 140 million years of tectonic plate movements in the region. Their models indicated that the subduction of the Tethys Ocean floor triggered the rise of mantle plumes – scorching hot streams of molten rock originating from deep within the Earth's mantle.
These plumes, buoyed by their low density, are believed to have inflated the seafloor beneath the Indian Ocean, creating a bulge that pushed the overlying water column downwards. This subsidence, in turn, resulted in the observed decrease in gravitational pull and the characteristic dip in sea level.
The researchers estimate that the initial plume activity began roughly 20 million years ago and has intensified over time, gradually deepening the gravity hole. Their findings not only shed light on the origin of this geological oddity but also provide valuable insights into the complex interplay between surface and subsurface processes that sculpt Earth's features over vast stretches of time.
Further studies are needed to fully understand the long-term implications of the Indian Ocean Geoid Low. However, unraveling this mystery has deepened our understanding of Earth's dynamic interior and the forces that have shaped our planet over eons.