Indian Ocean Geoid Low

The Indian Ocean Geoid Low (IOGL), is Earth’s most prominent gravitational anomaly. Recently, a team of Indian scientists has identified what caused this giant ‘gravity hole’ covering more than three million square kilometers in the Indian Ocean. Read here to learn more about it.

The massive “gravity hole” in the Indian Ocean, which spans more than three million square kilometers, has been explained by a team of Indian scientists from IISc, Bengaluru.

The region where the Earth’s gravitational pull is weakest is just south of Sri Lanka. As a result, the sea level is 100 meters lower than the average for the world.

While we know quite a few things about the Earth’s surface or the crust, its interior is still a mystery. We have launched spacecraft to explore the outer reaches of the galaxy, but we haven’t yet designed probes that go up to even the base of the crust, a mere 35 km deep.

Indian Ocean Geoid Low

Our planet Earth looks like a uniform blue sphere, but it is actually like a potato with its share of deformations.

• This uneven surface results from non-uniform gravity because of the unequal distribution of matter within the planet’s interior.
• In addition, the movement of tectonic plates occasionally creates mountains and valleys on the surface of the Earth, adding to the deformations.
• Since the oceans cover 71% of the planet’s surface, these deformities affect the shape of the oceans too.

If there were no tides and currents in the oceans, all the water in the oceans would settle onto a smoothly undulating shape called a geoid, rising wherever there is high gravity, and sinking where gravity is low.

The resulting unevenness in the ocean surface, known as “geoid anomalies”, is generated by uneven mass distribution within the deep Earth.

One such anomaly is found just south of Sri Lanka, spanning a vast extent. Known as the Indian Ocean Geoid Low (IOGL), it is dominated by a significant gravity low where the ocean surface plunges to 106 meters.

Though several studies in the past have tried to answer this, most of them attributing it to a remnant of an earlier plate that dived into the Earth’s mantle beneath another plate millions of years ago, there had been no convincing explanation of the source until now.

Cause of Indian Ocean Geoid Low

In the recent study, the researchers employ numerical models of ‘mantle convection’ to explain the mass deficit.

• Mantle convection is a type of movement caused within the Earth’s mantle or the middle layer, where hotter and lighter material rises to the top and cooler and denser material sink due to gravity.
• This convective movement within the mantle was driven by seismic tomography models that use seismic waves to obtain a 3-dimensional picture of the Earth’s interior.

Low-density anomalies or the presence of lighter materials in the upper to mid mantle below the Indian Ocean Geoid Low, were responsible for the gravity low in this region.

• Mantle plumes or the rising of abnormally hot rock within the Earth’s mantle can result in low-density anomalies.
• However, no known mantle plume exists beneath the Indian Ocean Geoid Low, ruling out this theory.
• However, they found that there was hot material rising from the African large low-shear-velocity province (LLSVP), or the African superplume, in the neighborhood of the IOGL, that was getting deflected eastward and terminating beneath the IOGL.
• The deflection is possibly due to the fast motion of the Indian plate, argues the researchers.

Gravity anomalies

The gravity anomaly on the Earth’s surface is the difference between the observed value of gravity and the value predicted by a theoretical model.

Gravity anomalies reveal rock density variations-

• Short-wavelength (< 250 km) gravity anomalies are usually correlated with crustal structures.
• Long-wavelength (< 1000 km) gravity anomalies are correlated with variations in mantle densities.

The largest positive anomalies over the continents were associated with the Andes, the East European Platform, the Alpine–Mediterranean fold belt, and the central southeastern part of North America.

The largest negative anomalies, indicating a thin lithosphere, are associated with vast Cenozoic regions of plume–lithosphere interaction: the East African Rift and the Basin and Range Province of western North America.

Way forward

The study into the Indian Ocean Geoid low can pave to further understanding of the origins of Earth.

• The ancient Tethys Ocean began to close as the Indian continent separated from the African, Antarctic, and Australian continents during the Late Cretaceous to early Paleocene, leading to the opening of the Indian Ocean.
• Researchers believe that during this period, slabs of oceanic origin were subducted into a slab graveyard in the lower mantle which possibly contributed to the IOGL.

Such uncertainties regarding the formation of the earth, as we see it today, can be answered through IOGL studies.

The data obtained can also understand science beneath the unexplored parts of oceanic plates but will also help to quantify deep-ocean wave dynamics.

-Article written by Swathi Satish