Indian Ocean Geoid Low (IOGL)

Prelims & Mains

Indian Ocean Geoid Low (IOGL)

To study the nature and origin of the geoidal low in the Indian Ocean centered around south of Sri Lanka.

Indian Ocean Geoid Low (IOGL)  found in the Indian ocean south of Srilanka. It is the lowest geoid/gravity 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.”

Indian Ocean Geoid Low (IOGL) – Background

Many of us might have seen those beautiful pictures of our round, blue planet taken from space, but did you know that our planet actually looks like a bumpy potato? It has its own share of deformations, non-uniform gravity because of the unequal distribution of mass and occasionally, mountains and valleys created by the movements of tectonic plates.

Considering that around three-fourths of our planet’s surface is made up of oceans, these deformities affect the shape of the oceans too. If we removed the tides and currents from the oceans on the planet, they would settle onto a smoothly undulating shape called a geoid, rising wherever there is high gravity, and sinking where gravity is low, creating what are known as “geoid anomalies.” These highs and lows are generated by uneven mass distribution within the deep Earth.

One such point of low gravity is found just south of the Indian peninsula, called the Indian Ocean Geoid Low (IOGL). The geoid low spans a vast extent south of the Indian subcontinent, and is dominated by a significant low of minus 106 metres, or roughly 348 feet, south of Sri Lanka.

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The existence of the Indian Ocean geoid low is one of the most outstanding problems in Earth Sciences

It is the lowest geoid/gravity 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.”

A low gravitational potential would mean that the ocean surface itself would go down.

So, for a 100 meter (328 feet) geoid low the  surface would dip down by 100 meters at that region.

 Indian Ocean Geoid Low (IOGL)  –  Reasons

The reasons behind the mass deficit that is causing the geoid low.

The remnant of an earlier plate that dived into the Earth’s mantle beneath another plate millions of years ago. However, there has been no convincing explanation of the source—until now.

The study showed that lighter material (low density anomalies) in the upper to mid mantle below the IOGL seem to be responsible for the existence of the gravity low in this region. But what causes these low density anomalies?

Mantle plumes, upwellings of abnormally hot rock within the Earth’s mantle, are generally thought to be the reason behind such anomalies. But in this case, no known mantle plume exists in this part of the world.

The researchers found that there was hot material arising from the African large low-shear-velocity province (LLSVP) or the African superplume, that was getting deflected eastward and terminating at the base of the low density region beneath the Indian Ocean low. The deflection is possibly due to the fast motion of the Indian plate.

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The researcher explains this low with hotter, lighter material stretching from a depth of 300 km, or 186 miles, up to ~900 km, or 559 miles, in the northern Indian Ocean, most likely stemming from the African superplume.”

This study is definitely a breakthrough in convincingly explaining the occurrence of the Indian Ocean Geoid Low.

Justification: Geoid is an equipotential surface of the Earth’s gravity fields that best fits the global mean sea level in a least squares sense. Recent gravity models and satellite based observations show that geoid rises and falls over spheroid as much as-100 m to +100 m.

The undulations in geoid are caused due to subsurface density heterogeneities and long wavelength geoid anomalies are often interpreted as present-day mantle density heterogeneities. These have a direct bearing on the physical and chemical properties of deep mantle and processes that are responsible for phenomena such as mantle convection, plate tectonics etc. Thus elucidating large wavelength Geoid anomalies is of significance in global geodynamical studies.

Despite the significance of the gravity low in the context of global geodynamics, no systematic study of this anomaly has been undertaken. The available seismic stations in the vicinity are few and far between and there are no ocean bottom observatories (OBOs) in the area. Ray paths are scanty, particularly over central low.

Raja Raja Cholan
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