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Indian Ocean Geoid Low (IOGL), Large Low Shear Velocity Province (LLSVP)

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Indian Ocean Geoid Low (IOGL)

  • Context (TG | WION): Earth’s gravitational pull is weakest at the Indian Ocean Geoid Low (IOGL).
  • Indian Ocean Geoid Low (IOGL) is a pronounced dip in the geoid because less mass under that spot makes the gravitational pull weakest.
  • IOGL covers more than 3 million km2 and is about 1,200 km southwest of the southern tip of India.

Gravity (or Gravitational Pull)

  • Gravity is a fundamental force of nature between any objects with mass.
  • The force of gravity between two objects depends on their masses and distance. This relationship is known as Newton’s law of universal gravitation.

Earth’s Gravity

  • The strength of Earth’s gravity is approximately 9.8 m/s² near the surface.
  • The force of gravity is not constant across the Earth’s surface due to variations/irregularities in mass distribution.


  • The model of the Earth that represents the Earth’s gravitational field by acknowledging the variations in gravity across the Earth is called the geoid.


  • Earth’s ellipsoid or spheroid represents the shape that the Earth acquired due to its rotation. It is flatter than a true sphere around the North and South Poles and bulges near the equator.

Indian Ocean Geoid Low (IOGL)

  • As a result of the low gravitational pull at IOGL, coupled with the higher gravitational pull from the surrounding areas, the sea level over the IOGL is 106 m lower than the global average.

Why IOGL Exists?

  • The existence of IOGL is attributed to the African blob, which is a large low-shear velocity province (LLSVP) underneath the African continent.
  • The hot, low-density material coming from the African LLSVP is sitting underneath the Indian Ocean and creating Indian Ocean Geoid Low (IOGL).
  • The African blob is formed by Tethyan slabs. These slabs are ancient remnants of the Tethys Ocean’s seafloor, located between the supercontinents of Laurasia and Gondwana more than 200 mya.
  • Both Africa and India were part of Gondwana, but India slowly moved north into the Tethys Ocean, creating the Indian Ocean behind it about 120 million years ago.

Large Low Shear Velocity Province (LLSVP)

  • A Large Low Shear Velocity Province (LLSVP) or blob is a region within the Earth’s mantle where seismic waves travel slower than in the surrounding mantle.
  • This happens because LLSVPs are relatively denser and cooler than the surrounding mantle.
  • There are two hypotheses regarding the existence of blobs:
    1. First is that LLSVPs consist of accumulations of crust that have subducted from the Earth’s surface to deep inside the mantle by tectonic plate convergence.
    2. Second, LLSVPs are the remnants of an ocean of magma that has cooled and crystallised, leaving denser areas than the rest of the mantle.
  • Effects of LLSVP’s instability on Earth’s surface: LLSVPs may be a source of hot plumes of mantle material that rise upward. These plumes might cause supervolcanic eruptions, tectonic upheaval, and continental breakup.
  • There are two LLSVPs discovered so far: the African blob and the Pacific blob.

    Indian Ocean Geoid Low (IOGL) - African blob, Pacific blob - Large Low Shear Velocity Province (LLSVP)

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