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Mount Everest is many things. It’s called Chomolungma in Tibetan, and Sagarmatha in Nepali. It’s an iconic part of Earth’s topography, a potentially lethal climbing challenge and a geologic marvel.
It’s also staggeringly tall — and, with a peak 29,032 feet above sea level, it easily achieves the status of the world’s tallest mountain by about 800 feet over other Himalayan peaks like Kangchenjunga and Lhotse. But what gave Everest its height advantage and contributes to making it grow taller by millimeters every year?
Research published on Monday in the journal Nature Geoscience points to a surprising cause: a river, stolen long ago by another marauding waterway.
Rivers are agents of erosion, but their consumption of land can have unexpected effects. About 89,000 years ago, a powerful river annexed another one nearby. Combined, the two grew more erosive. This led to the washing away of far more of the Himalayan landscape, and a huge weight was shorn from the crust, the layer of Earth we live on.
This lightweight crust, the study says, was able to float more easily atop the underlying mantle layer. Ultimately, this has added anywhere from 50 to 165 feet to the height of Everest, serving as a reminder that nothing, not even a colossal pyramid of rock, is immutable.
“Although mountains may appear to stand still from the perspective of a human lifetime, they are in fact constantly in motion,” said Jin-Gen Dai, a geoscientist at the China University of Geosciences in Beijing who is an author of the study.
Although this explanation can account only for part of Everest’s additional height, scientists see it as a step toward accounting for how the mountain grew to what it now is. “It’s a really neat story, and one that makes a lot of sense,” said Tom Gernon, a geoscientist at the University of Southampton in England who was not involved with the new work.
Everest began to form around 45 million years ago, when the tectonic plate India rests on crashed into the Eurasian plate, and then started descending below it. The crust buckled and crumpled on a gargantuan scale, creating the Himalayas.
Earth’s crust may seem rigid, but it’s not. When something massive, like an ice sheet or a mountain range, weighs down the crust, it flexes downward. But the mantle below is buoyant, nudging the sagging crust upward. It’s as if the Himalayas “are standing on a bouncy castle,” said Adam Smith, a geoscientist at University College London in England who is an author of the new study.
When the crust neither rises nor sinks, it’s referred to as being in isostatic equilibrium. And Everest should be in that state. Material is always being scraped off the Indian plate and added to the mountain range as the plate plunges downward. Rock is also being removed from the mountain’s exposed surfaces by precipitation and the grinding movement of glaciers.
These forces should balance each other, and Everest should neither shrink nor grow. But Everest’s superlative height suggests that something had been putting its thumb on the tectonic scales. And there is nothing better at creating that imbalance than a rock-eating river.
In Everest’s shadow lies the Arun River. And, just as Everest outgrew the other Himalayan peaks, the Arun distinguishes itself from neighboring waterways. The Arun takes a suspiciously circuitous route: It runs along the northern Himalayas and then turns abruptly and cuts through a ridge near Everest. That suggests the upper and lower segments of the river were not always unified, and that something significant forced them to become one.
Reproducing the region’s myriad rivers in computer simulations, the researchers identified that big moment. Some 89,000 years ago, a river network, the Kosi, cut backward into the Himalayas, eventually reaching the Arun, plundering its water and merging with it.
The Himalayas were no match for this rock-eroding liquid behemoth. The gossamer crust was pushed skyward by the buoyant mantle sea, various peaks were uplifted and Everest gained up to 165 extra feet in the blink of a geologic eye.
The tallest mountain on Earth is still growing, by about the width of a strand of spaghetti each year. A combination of factors, including this crustal rebound, explains these gains.
But its growth spurt probably won’t continue forever. The balance could tip the other way, shaving some of Everest’s great height. The Himalayas will rise and fall, as all mountain ranges do.
The rocky skin of a dynamic planet is never stationary. “On long time scales,” Mr. Smith said, “it’s almost as if the Earth is breathing.”
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