Antarctica sits above the strongest negative gravity anomaly on Earth, a region where the planet’s gravitational pull dips so low that the sea surface sags by more than 100 meters compared to the global average. Scientists have long known about this so-called gravity hole beneath the Indian Ocean, but explaining how it formed and why it has intensified over geological time remained an open question. A series of mantle convection studies now offers a clear mechanism. Ancient ocean floor that sank into Earth’s interior hundreds of millions of years ago set off a chain of deep-mantle disruptions that persist to this day.
What the Gravity Hole Actually Is
Gravity is not uniform across Earth’s surface. Variations in the density and composition of rock deep below the crust create zones where gravitational acceleration is slightly stronger or weaker than the planetary average. The Indian Ocean Geoid Low, or IOGL, is the most extreme example of a weak zone. It represents Earth’s largest negative geoid anomaly, meaning the hypothetical ocean surface in that region sits dramatically lower than it does elsewhere. An institutional overview from the Indian Institute of Science reports that the geoid depression exceeds 100 meters, a figure large enough to be clearly visible in satellite-derived gravity maps and to stand out among global anomalies.
The anomaly stretches beneath the Indian Ocean south of the Indian subcontinent and extends toward Antarctica, where it coincides with what researchers describe as Earth’s strongest gravity hole. A detailed University of Florida analysis notes that gravity varies naturally across the planet, but the IOGL stands out for both its magnitude and its link to deep-seated mantle processes. The same research group emphasizes that this anomaly is not a mere surface quirk of seafloor topography. Instead, it reflects a long-lived deficit in mass within the mantle column beneath the Indian Ocean, which subtly reshapes the surrounding geoid.
Ancient Ocean Slabs and Deep-Mantle Plumes
The leading explanation centers on the Tethys Ocean, a vast body of water that once separated the ancient supercontinents of Laurasia and Gondwana. As tectonic plates shifted, the Tethyan ocean floor was driven downward into the mantle through subduction zones that ringed the closing ocean basin. A study in Geophysical Research Letters used global mantle convection models that assimilate plate reconstructions from roughly 140 million years ago to the present to track this process. The authors concluded that sinking Tethyan slabs disturbed a massive structure near the base of the mantle known as the African Large Low Shear Velocity Province, or African LLSVP, generating buoyant plumes of hot, less dense material beneath the Indian Ocean and lowering the geoid there.
Because these plumes are hotter and lighter than the surrounding mantle, they reduce the average density in the affected region, leading to a deficit of mass that manifests as a negative gravity anomaly. The IISc summary indicates that the gravity hole began to form about 20 million years ago, when subducted Tethyan material would have reached depths sufficient to interact with the LLSVP boundary. A separate University of Florida report underscores that this gravity hole has intensified rather than faded with time, consistent with a scenario in which subducted slabs continue to perturb the deep mantle and feed rising plumes that sustain and deepen the anomaly.
Earlier Work on Upper-Mantle Heterogeneity
The 2023 formation study did not emerge in a vacuum. Earlier work had already shown that structures in the upper mantle contribute significantly to the IOGL signal. A paper by Ghosh, Thyagarajulu, and Steinberger, published in 2017, demonstrated that upper-mantle heterogeneity plays an important role in shaping the geoid low. By systematically varying density distributions within the top few hundred kilometers of the mantle, the authors found that shallow anomalies could account for a substantial fraction of the observed geoid depression, challenging models that focused exclusively on deep-mantle features such as LLSVPs.
This recognition that both shallow and deep structures matter shifted the scientific conversation. Rather than treating the IOGL as a purely deep-rooted phenomenon, researchers began to explore how interactions between subducted slabs, upper-mantle flow, and deep thermal anomalies might combine to produce the observed gravity field. The 2017 study effectively framed the IOGL as a multi-layered problem, where density variations from the base of the lithosphere down to the core-mantle boundary contribute to the final geoid pattern. That perspective laid the groundwork for more sophisticated, time-dependent simulations that could track how heterogeneity evolves over tens of millions of years.
Refining Mantle Models With the Geoid
Building on that foundation, Pal and Ghosh extended the convection modeling framework in a later study that incorporated both seismic and gravity constraints. Their work, published in the Geophysical Journal International, used the geoid itself as an additional benchmark when comparing modeled present-day mantle structure to seismic tomography images. In practice, this meant adjusting mantle density and viscosity distributions until the simulations reproduced not only the broad pattern of seismic velocity anomalies but also the amplitude and shape of the IOGL as inferred from satellite data.
By treating the gravity anomaly as both an outcome and a calibration target, the researchers were able to narrow the range of plausible mantle structures that could have produced the modern IOGL. Their results support a scenario in which subducted Tethyan slabs sink into the lower mantle, accumulate near the African LLSVP, and trigger thermochemical plumes that rise beneath the Indian Ocean. At the same time, variations in composition and temperature within the upper mantle amplify the geoid signal. This integrated approach strengthens the case that the IOGL is the product of a long, complex history of plate motions, slab subduction, and deep-mantle reorganization rather than a single isolated event.
How Satellites Mapped the Anomaly
None of these modeling advances would have been possible without precise measurements of Earth’s gravity field from orbit. The European Space Agency’s Gravity field and steady-state Ocean Circulation Explorer, or GOCE, was specifically designed to map subtle variations in Earth’s gravity by measuring gravity gradients along its orbit. Over its mission lifetime, GOCE produced high-resolution geoid and gravity anomaly grids that clearly delineate features such as the IOGL, providing the empirical backbone for studies that link surface gravity to deep-mantle structure. These satellite-derived products allowed geophysicists to quantify the depth and lateral extent of the Indian Ocean gravity hole with much greater confidence than was possible using terrestrial measurements alone.
Complementary observations from satellite laser ranging have further refined global gravity field models. Missions tracked by NASA’s International Laser Ranging Service, including the long-running LAGEOS series described in ILRS documentation on orbital tracking, provide extremely accurate measurements of satellite positions by timing laser pulses bounced off retroreflectors. Tiny perturbations in these orbits reveal how mass is distributed within Earth, offering an independent check on gravity models derived from missions like GOCE. Together, gradient measurements and laser ranging data underpin the geoid reconstructions that mantle convection studies rely on to test hypotheses about the origin and evolution of the Indian Ocean Geoid Low.
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*This article was researched with the help of AI, with human editors creating the final content.
