Mumbai: The Indian subcontinent formed when the Indian plate broke off from Antarctica about 132 million years ago, and collided into the Eurasian landmass 55 million years ago. It is thought that the process involved two separation events as the plate was just breaking off from Antarctica.
The first event caused the Indian subcontinent to break away from Antarctica 132 million years ago, while the second event — estimated to have occurred 120 million years ago — saw the separation of two structures, known as the Elan Bank and the Southern Kerguelen Plateau (SKP), which are currently embedded in Antarctica.
A study conducted by researchers from the Alfred Wegener Institute (AWI) in Germany rejects this theory and proposes that the Indian plate had only one break-up event, that of the subcontinent breaking away from the Antarctic plate. It states that the structures embedded in Antarctica didn’t break away from the Indian plate, but were instead created by volcanism.
The AWI research proposes a simpler northward drift of Greater India — a landmass comprising Sri Lanka, with Madagascar and Seychelles on the western side — where there was only one separation event.
“The drift of India is much more simpler than people thought,” says AWI geoscientist Wilfried Jokat, who conceptualised this research and is the first author of the scientific publication, said in an interview to ThePrint.
The study was published in Nature Scientific Reports last month.
If it gains traction, the new theory would mean reimagining the land movement that led to the separation of the Indian plate from Antarctica, and its flow to its current location on the Eurasian plate.
However, in a slowly developing battle-of-facts, this new finding is already being challenged by the advocates of the two-phase separation theory, which was first proposed in 2007 by a team of researchers from Norway, Australia, Japan and Russia, and currently holds general currency among researchers.
The Indian subcontinent is believed to have been a part of an ancient supercontinent called Gondwana, which also comprised modern-day Antarctica, Africa, South America, Australia, Arabia and Madagascar, and took shape 600 million years ago. The first breakup of this supercontinent is estimated to have begun 180 million years ago.
Around 132 Ma (mega annum, million years) ago, when Greater India broke away from Antarctica and Australia, there was a small lake between them.
As it started to drift northwards, this lake began to expand to form the Indian Ocean. At the same time, under the ocean, a large volcanic feature called the Kerguelen Plateau, which still exists, formed, very close to Antarctica.
An underwater plateau is a flat elevated stretch of crust that is higher than the surrounding land.
“It was a big party down there,” says Tabea Altenbernd, co-author of the study. “There was a lot of action going on during that time down there, not a good place to be in at that point.”
About 130 million years ago, when the Greater India block was still closer to Antarctica than Eurasia, Sri Lanka started to separate from India while the Indian plate was still supporting it.
According to the two-part separation theory, about 120 million years ago, a bit of land and some continental fragments broke off from the Indian plate. These came to be known as the Elan Bank and the Southern Kerguelen Plateau (SKP), which lie embedded on the ocean floor of the Antarctic plate.
The new theory, however, claims the Elan Bank and SKP did not separate from the Indian plate at all, but instead were formed by volcanism after the plate separated.
Original theory methodology
To determine how the present-day landmasses came to be, geologists drill, scan and date rocks and seafloor beds.
The advocates of the two-part separation theory came up with their conclusion after drilling in the Kerguelen plateau and the Elan Bank. They found pieces of continental rocks, which are generally less dense and much older than oceanic rocks, surrounding and underneath the newer oceanic rocks on the seafloor.
This, researchers say, indicated that they came from a continent rather than formed right there on the seafloor.
To ascertain that Greater India was once attached to the Kerguelean plateau, Indian earth scientists also assessed the Rajmahal hills in Jharkhand, which was the site of long-duration, slow basaltic volcanism from the volcanic region called the Rajmahal traps.
A previous study, conducted in 2002, proposed that Rajmahal hills and the Southern Kerguelen Plateau formed at the same time.
The scientists then compared this with the volcanic rock of the Kerguelen plateau.
They found the rocks from these regions to be of the same age and type.
“These continental pieces found through drilling on the Kerguelen plateau and the Elan Bank were attached to the Indian mainland once upon a time,” says K.S. Krishna of the Centre for Earth, Ocean and Atmospheric Sciences, University of Hyderabad. “The Kerguelen plateau was attached to the Rajmahal-Sylhet Line near the Shillong Plateau.”
Krishna is one of the researchers involved in the 2016 study that established the connections between the Greater India split from Antarctica, and is one of the proponents of the two-part separation theory.
New study’s claim
In the new study, Jokat and his collaborators reviewed the database that advocates the two-part separation theory.
At the time that the two-part separation theory was derived, the AWI researchers say, key interpretations were based on widely spaced and unevenly distributed data points in the ocean floor.
In their study, the AWI researchers tested the two-part separation model by conducting magnetic and seismic surveys through helicopters and ships, in an effort to make estimates about the age of the rock and thickness of the crust more accurate.
Magnetic anomalies are magnetic variations within rocks that help in understanding the formation of the seafloor. Seismic data helps to know the composition of the rocks.
Using magnetometers and seismometers, the researchers made observations in areas south of Sri Lanka and in the Enderby Basin, the structure that was created at the point where the Indian plate broke off from Antarctica.
The researchers found that the Indian block started to separate in the eastern part of Antarctica at 132 Ma and drifted northwards more or less continuously. The southern tip of the Indian plate, Sri Lanka, was detached from Antarctica at some 112 Ma, much later than the two-phase model predicts.
The study also suggests this later separation is the only break-off that happened. It claims the Elan Bank and SKP didn’t break off from the drifting Indian plate, but were formed by volcanism from a large area of hot material in the Earth’s mantle.
Magma from this volcanism also migrated under the floating Indian block to create eruptions of the Rajmahal hills, the researchers say.
Questions from advocates of 2-part separation theory
Krishna isn’t convinced by the results of this study.
“This seismic experiment is very good, but it cannot provide the age of the oceanic rocks,” he told ThePrint. “It can only provide the nature of the rocks.”
The age of oceanic rocks can be determined by either drilling in the crust, collecting fragments and dating them in a laboratory, or by magnetic surveys.
But, according to Krishna, magnetic surveys cannot be entirely relied upon, as this study has done.
This is because, to date oceanic rocks, they shouldn’t be disturbed by newer geological processes that deposit magma on top of them, burying the original rock underneath.
“Once secondary geological processes take place and initial signatures are disturbed, it becomes difficult to correctly identify the original age of the rocks,” said Krishna. “The researchers have collected the data but they haven’t kept subsequent geological processes in mind while interpreting the age of the rocks.”
Since the researchers didn’t drill, Krishna said, this study wrongly proposes the formation of the Elan Bank and SKP by newer ocean tectonics and not by break-away from Greater India, made up of more ancient continental rock. Thus, the authors of the two-part separation theory allege the study wrongly questions their conclusion.
Krishna and his colleagues have written to Nature Scientific Reports with a rebuttal to the study. They are yet to hear back from the journal, said Krishna, adding that he has asked for the researchers to incorporate the drilling data and come up with more convincing observations. If the AWI researchers were to do this, Krishna and team think, their results would align with the two-part separation of Greater India.
Vrushal Pendharkar is a Mumbai-based freelance journalist covering the natural world
(Edited by Sunanda Ranjan)