If you want to know why Mount Everest is so tall, imagine squeezing and bending a tube of toothpaste. The same process has been happening on a grand scale ever since India smashed into the rest of Asia – an unexpectedly prolonged collision that threw up the Himalayas and then caused them to grow ever taller. Today these mountains host most of the world's 100 highest peaks, including Everest.
Earth's surface is divided into several slow-moving plates, and often one gets pushed under its neighbour in a process called subduction. But although thin ocean floor is easily pushed down, the thicker rocks of a continent can get stuck, blocking subduction. So if plate movements lead to one continent crashing into another, then instead of one being pushed deep into the Earth, its rocks can be pushed up into mountain ranges.
Continental collisions are messy, with the ground twisting and undulating over geological time. Because the rocks deform in a complex way, nobody had been able to figure out what kind of terrain will result and whether there is a common underlying process.
That has left geophysicists puzzling over the formation of many of the features we see on Earth's surface. In particular, it is unclear how India has continued to slide into Eurasia for the past 20 million years, creating the Himalayas, instead of grinding to a halt. "The obvious big grand challenge problem of tectonics has always been the Himalayas, and how India hits Eurasia," says Louis Moresi of the University of Melbourne, Australia.
Very slow smash
Now Moresi and colleagues have built a computer model that explains what happens when continents collide. It shows that when one continent bears thick or buoyant crust that blocks subduction, the other continent gets squeezed like a tube of toothpaste and folds around the blockage, creating a complex array of geophysical features (see video).
They tested the model against what happened in Australia hundreds of millions of years ago when a small continent crashed into its eastern coast and was absorbed, throwing up mountain ranges. The model explained several mysterious features of the landscape. For instance, it accurately reproduced oroclines – curves in mountain ranges – suggesting they are a result of the squeezing and folding process.
Then the team turned to the Himalayas. Their model suggests that as India shoves into Eurasia, China and South-East Asia initially resist being pushed underneath, and then get pushed aside instead. That unclogs the subduction zone and allows India to keep pushing into Eurasia, raising up Mount Everest and its towering siblings.
"Without this process, India almost certainly would have stopped moving northwards," says David Foster of the University of Florida in Gainesville, who was not part of the team. "If it did stop 20 million years ago, then the Himalayas would have looked more like the Alps. They would have stopped growing and they would have started receding."
"India is like a bulldozer," says Bill Collins of the Australian National University in Canberra, who was also not involved in the work. "I can tell you this is a seriously good breakthrough."
Other geologists had suggested that continents could be pushed aside like this, says Foster, but until now it was mostly speculation.
Journal reference: Nature, DOI: 10.1038/nature13033
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