The Everest growth spurt

The highest mountain peak in the world grows taller every year.

Mount Everest – which goes by the name Chomolungma in Tibet and Sagarmāthā in Nepal – has an elevation of 8,849 metres above sea level. According to a new study (go.nature.com/3Y56KRp), Everest has grown by about 15-50 m due to a river phenomenon that happened thousands of years ago.

Published in Nature Geoscience, the study shows that a river ‘piracy’ event – in which a river robs another of its water as it changes course – is responsible for the elevation gain. "When this river sort of captured the water from the other river, there was an enormous amount of erosion and that erosion triggered isostatic uplift of Everest," explains Matthew Fox, study co-author and Associate Professor at University College London (UCL) in the U.K.

According to the research, the Arun River, a chief tributary of the Kosi River, captured another river 89,000 years ago, starting off landscape changes that led to the rise of Everest – and its neighbouring peaks – on top of the rise due to plate tectonics.

"We have a good idea what the uplift rates are in most mountain belts in the world now through measuring the elevation change of specific locations using GPS," says Fox. "And then over longer time periods, we can use a method called thermochronology, which allows us to measure the thermal histories of rocks as the rocks approach Earth’s surface – and that tells us how quickly rocks are coming to the surface." Scientists use these two tools to estimate the rate at which mountains are moving upwards.

According to long-term thermochronology data, Everest is rising by about a millimetre per year but its short-term uplift rate from GPS data is estimated to be 2 mm per year.

The uplift from river piracy is around 0.2-0.4 mm per year, "which is about 10-20% of the total amount of uplift that Everest experiences (it also is uplifted due to the collision of India and Eurasia)," says Adam Smith, co-author and PhD student at UCL, in an email.

The Arun River piracy event not only explains the faster uplift rate of Everest recorded by GPS in recent years but also why Everest is much higher than the other highest peaks in general. The Everest towers nearly 240 metres above the second-highest peak, K2. The next three highest peaks, however, don’t differ as much in elevation (less than 100 m).

Attreyee Ghosh, a geophysicist and Associate Professor at the Indian Institute of Science in Bengaluru, who was not involved in the work, finds the study convincing. "It is not very easy to tease out the exact factor that makes this mountain kind of stand out," says Ghosh.