Mount Everest is tall. In other news, the sky is blue.

But Everest (also called Chomolungma and Sagarmāthā) is taller than it logically should be – towering 238 metres above the world’s next highest peak, K2, and more than 250 metres higher than any of its counterparts in the relatively uniform Himalaya range.

Plus, it’s growing at about 2mm a year, faster than the expected rate for the range.

A team of Chinese and UK scientists have now suggested why this is the case.

The researchers think the culprit is a nearby river which “captured” another river 89,000 years ago, causing erosion that made Everest more buoyant.

They’ve published their findings in Nature Geoscience.

Recent uplift of Mount Everest may have been assisted by the landscape response to a river capture event, which occurred about 89,000 years ago, according to a @NatureGeosci paper. https://t.co/3HMHrD8hlk pic.twitter.com/l1yZBqNl8L

— Nature Portfolio (@NaturePortfolio) September 30, 2024

The Himalayan peaks get their extraordinary height from the collision of the Indian and Eurasian tectonic plates, causing the Earth’s crust to thicken and the mountain range to push upwards.

“An interesting river system exists in the Everest region,” says co-author Dr Jin-Gen Dai, from China University of Geosciences.

“The upstream Arun river flows east at high altitude with a flat valley. It then abruptly turns south as the Kosi river, dropping in elevation and becoming steeper. This unique topography, indicative of an unsteady state, likely relates to Everest’s extreme height.”

The team used numerical modelling to see how the river changed over time.

They found that, about 89,000 years ago, the Arun river “captured” another nearby river.

This event, referred to as “river piracy”, happens when a river diverts its course and takes up the discharge of another river or stream.

“Our research shows that as the nearby river system cuts deeper, the loss of material is causing the mountain to spring further upwards,” says co-author Adam Smith, a PhD student at University College London, UK.

The team estimates that the river piracy has made Everest between 15 metres and 50 metres higher than it would otherwise be.

It’s also made neighbouring peaks, Lhotse and Makalu, unusually tall. These are the fourth and fifth highest mountains in the world, respectively.

“Mount Everest and its neighbouring peaks are growing because the isostatic rebound is raising them up faster than erosion is wearing them down,” says co-author Dr Matthew Fox, also from University College London.

“The changing height of Mount Everest really highlights the dynamic nature of the Earth’s surface,” says lead author Dr Xu Han, from China University of Geosciences.

“The interaction between the erosion of the Arun river and the upward pressure of the Earth’s mantle gives Mount Everest a boost, pushing it up higher than it would otherwise be.”

This article first appeared in Cosmos Magazine. See the original here