Puzzled over the large geoid anomalies in the Indian Ocean just south of Sri Lanka, where a deep-seated earth structure exists, scientists from the National Centre for Polar and Ocean Research (NCPOR) have started to uncover the possible reason behind this enigma.
Using a series of ocean bottom seismometers, researchers are taking a peek into the earth’s deep mantle layer, which is around 700km below our planet’s crust, to find out what lies beneath.
Known as the Indian Ocean Geoid Low or simply IOGL, this is a depression where there is a significant gravity drop, causing the ocean surface to plunge by 106 metres. Scientists say that this is the largest deficit in the earth’s shape, but have never been able to conclusively figure out why. But now, a team of researchers from NCPOR may be close to unravelling this mystery.
The geoid is the shape of an imaginary global ocean dictated by gravity in the absence of tides and currents. The video below shows a Geoid Animation/Map of Earth’s Gravity from ESA. Red are positive anomalies and blue are negative anomalies:
For the first time, scientists have conducted an experiment in IOGL region using 34 battery-operated ocean bottom seismometers. For over 28 months, scientists observed the minutest seismic activities on the oceanic sea floor continuously, that allows analysis of movement of mass underneath the tectonic plates and even cyclonic data.
“We have wonderful data recorded; many earthquakes, noise of the underwater animals, cyclone movements,” said scientist in NCPOR’s geo-scientific studies department, Dhananjai Pandey. “This information provides information on what is happening almost 1000km deep. There appears to be a huge depression in the mantle transition zones, which is because of the high temperature anomaly. Our hypothesis is that this is what is causing the geoid depression.”
Several suggestions have been made to explain the IOGL depression in recent years.
As shown in the video below, there are a lot of gravitational anomalies on Earth:
Extensive seismic data collected by NCPOR scientists
Initially, researchers felt that it is because of a depression in the earth’s core-mantle boundary, while others scientists suggested that it is because of the density variation in the earth’s mantle.
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While the anomaly in the Indian Ocean Region has been known for close to half a century, scientists have not been able to study the intriguing phenomenon due to logistical shortcomings. The ministry of earth science (MoES) finally took it upon itself to exploring the origin of the IOGL as part of an ambitious deep ocean mission programme to create an image of the earth’s insides. A team led by Pandey has been carrying out long-term geophysical observations to find an answer to why the earth’s mass in this region is so deficient.
NCPOR scientists collected extensive seismic data, which also shows an extremely deformed oceanic crust. Scientists were also able to directly record multiple earthquakes which were occurring in the middle of the tectonic plates. The initial findings also found a huge depression around 410km to 660km deep in the earth, which is spread across an area around 800km wide in the Indian Ocean.
“This significant observation potentially implies a very hot mantle material beneath the IOGL region, which in turn could be responsible for creating an extreme geoid deficit,” said Pandey. [Tectonophysics, TOI]