When an unstoppable force like the Indian subcontinent crashes into an immovable object like the Eurasian plate, the consequences include the tallest mountains in the world and a cadence of earthquakes like the magnitude 7.8 one that struck Nepal last month and a major aftershock in the same region last week.
Many of the geological questions about the collision remain unanswered. How did the Indian subcontinent get so quickly to where it is today? How big was India originally? Even the simplest of questions — when did India meet Eurasia, the tectonic plate that Europe and Asia sit on? — is up for debate, with researchers offering answers that differ by some 30 million years.
“It’s going to be hard to convince anyone,” said Oliver E. Jagoutz, a geologist at M.I.T. and part of a team that outlined its ideas about the collision in the journal Nature Geoscience.
Another mystery is why India is still moving at a quick pace — one and a half to two inches a year — driving the devastating earthquakes. “That is one of the biggest problems that we have in plate tectonics,” said Douwe J.J. van Hinsbergen, a professor of earth sciences at Utrecht University in the Netherlands. “It may not seem much, but it’s the rate at which your fingernails grow.”
The geologists are like accident investigators trying to decipher what happened from the wreckage, pondering how rocks from the ocean floor ended up high in the Himalayas. Much of the evidence, namely the chunk of India that is now jammed under Tibet and the Himalayas, is out of reach.
Still, until about a decade ago, scientists thought they had the story essentially figured out.
Throughout Earth’s four-and-a-half-billion-year history, the hulking chunks of land have alternated between periods of combining into supercontinents like Pangea 300 million years ago and periods like today when they are moving apart.
At the height of the age of dinosaurs, Pangea had broken into two giant continents, Laurasia and Gondwana, and India was a piece of Gondwana, in the Southern Hemisphere, attached to Antarctica and nestled between Africa and Australia.More than 100 million years ago, India broke away and accelerated northward. According to the widely accepted picture, that runaway continental fragment collided with Eurasia 50 million to 55 million years ago in one of the few places today where a piece of continent runs into a continent rather than an ocean plate.But not all of the pieces of the conventional wisdom fit together.
New analyses of the magnetism preserved in rocks suggested that the southern edge of Eurasia was farther north than some had thought, raising the question of whether India was close enough to make contact with Asia by then.
“In the old days, we thought that when India collided with southern Asia, southern Tibet was 2,000 kilometers south of where it is now,” said Peter H. Molnar, a professor of geological sciences at the University of Colorado. “And that all made sense. If you collide 40 to 50 million years ago, and southern Asia was down there, you have one collision, bang, you’re done.”
The revised magnetic measurements suggest that southern Tibet was 1,000 kilometers south at most. “Suddenly you have this huge space,” Dr. Molnar said. “Where was India 50 million years ago? Well, India was way south of where southern Asia was. So you have a choice: You have to make northern India be enormous or you have to put an ocean in there.”
Others pointed out that the most obvious effects of a continental collision, like the raising of the Tibetan plateau, did not start until later, perhaps 10 million years later. The rocks in the western Himalaya region also point to two collisions, not one. “The jury was in,” said Simon L Klemperer, a geophysicist at Stanford. “The jury is out again.”
Another key question: How much of India has disappeared? If the collision occurred more than 50 million years ago, almost 2,000 miles of India must have been pushed under Asia. A piece of continent this large would appear to be too big to fit the available space in ancient Gondwana. And yet scientists agree that something collided 50 million to 55 million years ago, clearly seen in an uplifted seafloor of this age in the northern Himalaya region.
“That is why people said there is a collision between two continents,” Dr. van Hinsbergen said. “Which is true. That is the best conclusion you can draw. The question is not so much whether there is a continent colliding 50 million years ago. The question is whether that is India.”
In 2007, Jonathan Aitchison, now a professor of geosciences at the University of Queensland in Australia, put together an alternative timeline. The collision that people thought was India meeting Asia was actually India running into an arc of islands south of Asia, and India then pushed these islands like a snowplow into Asia 20 million years later. “We believe the evidence shows India bumped into other things before Asia,” Dr. Aitchison said. His hypothesis was widely dismissed. Dr. van Hinsbergen came up with a different idea. He proposed that 70 million to 120 million years ago, India split in two as it was moving north. The first piece reached Asia 50 million to 55 million years ago, as dictated by the conventional picture, but the main part lagged behind, not colliding until 20 million to 25 million years ago.
That would make India small enough to fit into Gondwana. Tectonic plates are known to break apart — India was once connected to Madagascar, before splitting away in its northward sprint — but other scientists are skeptical, citing the lack of any geological evidence of an ocean crust separating two pieces of India.
Dr van Hinsbergen replies that missing ocean crust is no more unlikely than missing continent. “Whatever was there disappeared without a trace,” he said. “Then the question is, what was it?”
Dr van Hinsbergen does not find the island arc hypothesis compelling, and Dr Aitchison is equally unconvinced by the suggestion of India breaking in two before colliding with Asia.
Dr. Jagoutz and Leigh H. Royden, a professor of geology and geophysics at MIT. looking at rocks in the western Himalaya, have come to a conclusion similar to Dr. Aitchison’s — that India ran into an island arc before it hit Asia — put the second collision about five million years earlier.
In the Nature Geoscience paper, Dr Royden and Dr. Jagoutz show that the island arc could explain the swiftness of India’s travels. About 80 million years ago, India was barreling north at half a foot a year, a pace it maintained for 30 million years.
Generally, the motion of continents is driven by subduction zones — where one tectonic plate passes beneath another and then descends into the Earth’s mantle, pulling everything behind it.
Geologists knew there was one subduction zone where the Indian plate dived under Asia. With an island arc between Asia and India, there would have been two subduction zones pulling on India, which could explain the high velocity of India.
But none of this explains why India is still so moving so fast. Whatever was north of India has long since disappeared into the mantle, and continental crust does not provide the same downward pull.”It’s yet to be resolved what keeps India moving to the north,” Dr. Royden said. “At the moment, it’s still a work in progress.”
Geologists will be hard pressed to deduce what happened to India from the seismological data. Dr. Klemperer says he hopes that the mix of helium isotopes in geothermal springs will help tell whether the mantle below the Himalayan crust is part of the Indian plate or Asian plate. “I agree the jury should still be out,” Dr. Klemperer said. “If I want to hold onto my view, then I need to eventually be able to demolish the arguments.”