Everest Is Still Growing — and the Reason Isn't What You'd Think

Everest is not finished growing. Every year, the world's tallest mountain gains a little more height — and until recently, nobody could fully explain why. A 2024 study in Nature Geoscience found that a big part of the answer has nothing to do with the collision of the Indian and Eurasian plates that built the Himalaya in the first place. It's a river, 75 kilometres away, that has spent the last 89,000 years quietly eating away at a gorge — and lifting Everest as it goes.

The process is called river piracy: roughly 89,000 years ago, the Arun river was captured by the Kosi river system, merging into a single, much more powerful drainage network. That combined river has spent the millennia since carving a deep gorge through the rock roughly 75 kilometres from Everest. As millions of tonnes of rock and sediment have been washed away, the crust beneath the region has lost weight — and, like a cushion released from pressure, sprung back upward. Geologists call this isostatic rebound. The research team, led by Dr Xu Han at the China University of Geosciences with contributions from Adam Smith and Dr Matthew Fox at UCL, modelled the effect and found it could account for Everest's extra 15 to 50 metres — and for an ongoing uplift of about 2mm a year, over and above whatever the Indian and Eurasian plates are doing underneath.

Not just Everest

Lhotse, the world's fourth-highest peak, and Makalu, the fifth-highest, sit in the same stretch of the Himalaya and are being pushed upward by the same rebound. Makalu, positioned closest to the Arun's new course, appears to be rising slightly faster than its two neighbours. All three peaks, in other words, owe part of their extreme height not only to the ancient plate collision that raised the Himalaya, but to a river that changed course tens of thousands of years after the mountains already existed.

It's a strange thought for anyone who has stood at Everest Base Camp: the mountain overhead is not a static fact, fixed the moment the plates first collided, but something still quietly rising underfoot — by roughly the thickness of a coin, every single year. Nepal and China's most recent joint survey, using GPS receivers carried to the summit, put Everest's official height at 8,848.86 metres in 2020 — a figure that will already be very slightly out of date by the time you read this. For how the first-ever calculation of that height was made, a full century before GPS existed, see our piece on the mathematician who computed Everest's height from more than 150 kilometres away.

Source: UCL News, reporting on research published in Nature Geoscience.

Cover photo: Prabin Sunar via Pexels (Pexels License).