Indian Ocean Geoid Low (IOGL)

The Indian Ocean Geoid Low (IOGL) is a remarkable and enigmatic feature of Earth’s gravity field: a pronounced dip or “dimple” in the geoid surface located south of the Indian subcontinent. It is considered the largest and deepest negative geoid anomaly on Earth.
The geoid is the equipotential surface of Earth’s gravity field that best approximates mean sea level (in the absence of tides, currents, atmospheric pressure effects, etc.). Variations in the geoid reflect irregularities in Earth’s internal mass distribution.

Location, Size, and Magnitude

• The IOGL is centred to the southwest of Sri Lanka and Kanyakumari (southern India), and east of the Horn of Africa.
• It spans an area of about 3 million km² (≈ 1.2 million square miles), roughly comparable to the land area of India.
• The anomaly is such that the geoid is depressed by up to 106 metres (≈ 348 feet) relative to the reference ellipsoid (i.e. the geoid is that much “lower” there than average)
• Because sea surface (ignoring dynamic effects) conforms to the geoid, this dip corresponds to a “sea-level” depression of about 106 m in the absence of tides, currents, etc.

Thus, the IOGL is a large and deep “gravity hole”—not a physical sink in the ocean floor, but a region where Earth’s mass distribution causes a weaker gravitational potential.

Historical Discovery and Context

• The geoid low was first detected in 1948 by Dutch geophysicist Felix Andries Vening Meinesz, through shipborne gravimetric measurements.
• For many decades, its origin remained mysterious.
• In recent years, improved satellite gravity models, seismic tomography, and geodynamic simulations have advanced hypotheses explaining the IOGL’s formation.

Understanding the IOGL is important because it offers insight into Earth’s internal structure and mantle dynamics over large scales.

Causes and Hypotheses

Because the geoid anomaly is fundamentally linked to variations in mass density beneath Earth’s surface, the explanation for the IOGL centers on “mass deficits” (regions of lower density) in Earth’s mantle, or compensating dynamic effects.

Deficit of Mass (Low-density Zones)

• One hypothesis is that hot, low-density material is present beneath the region, producing a weaker gravitational pull above.
• In particular, models by Ghosh, Pal, and collaborators suggest that mantle plumes or upwellings of lighter (less dense) mantle material, deflected from deeper structures, contribute to the geoid low.
• Their 2023 study used 19 numerical geodynamic simulations over ~140 million years, concluding that many of the plausible matches include low-density anomalies stemming from the African Large Low Shear Velocity Province (LLSVP) or “African superplume,” which are deflected eastward beneath the Indian Ocean region.
• According to that model, these anomalies extend over depths from about 300 km to 900 km beneath the surface.
• This explanation links the IOGL to large-scale mantle flow and the dynamics of deep structure in Africa and below.

Alternative or Complementary Mechanisms

• Some older theories invoked remnants of subducted slabs from the Tethys Ocean (or other ancient oceanic crust) that sank into the mantle beneath this region, leaving behind mass deficiencies.
• There is also interplay between dynamic topography (uplift or subsidence caused by mantle flows) and density anomalies, which can partly mask or enhance the geoid signal.
• Some critics argue that the current models are not fully conclusive, and that uncertainties remain in mantle viscosity, exact densities, and the detailed flow patterns.

Overall, the prevailing view is that a combination of deep mantle anomalies and dynamic flow, rather than crustal effects, is responsible for the IOGL.

Geological Significance & Implications

• The IOGL is an extreme case of a long-wavelength geoid anomaly, and hence serves as a natural laboratory for studying mantle convection, density structure, and large-scale geodynamics.
• Its magnitude (≈106 m depression) represents one of the largest deviations of the geoid from the reference ellipsoid around the Earth.
• It suggests that the Earth’s mantle is not homogeneous, but hosts large-scale heterogeneities that influence surface gravity.
• Because the geoid is used in precise geodesy, satellite altimetry, sea-level studies, and the reference frame for height measurements, the IOGL is also relevant in calibrating and understanding those datasets.
• In plate tectonics and Earth history, the IOGL connects to the breakup of Gondwana, the closure of Tethys, drift of India, and mantle plume evolution.

Challenges, Criticism & Open Questions

• The parameter space in mantle modelling (viscosity, temperature, density, flow patterns) is vast, so even good matches do not constitute proof.
• Some scientists point out that the proposed models may not fully explain analogous geoid features elsewhere (e.g., in the Pacific or under Africa).
• Direct observational evidence (e.g. seismic signatures of plumes) under the IOGL region is still limited.
• The temporal evolution: when exactly did the anomaly form, how stable is it, and how will it evolve in the future remain topics of research. The new models suggest formation began ~20 million years ago.

Thus, while the 2023 simulations are promising, the IOGL remains a subject of active geophysical investigation.