Climbing Mount Everest has always been a feat, but it seems the task might be getting harder: researchers say Everest is having something of a growth spurt.
The Himalayas formed about 50m years ago, when the Indian subcontinent smashed into the Eurasian tectonic plate – although recent research has suggested the edges of these plates were already very high before the collision.
With the process still going on, the mountain range continues to be pushed upwards, though landslides and other events mean rock is also being lost.
But now experts say Everest – which currently stands at 8,849m (29,032ft) – has been experiencing an additional boost to its height as a result of erosion by its neighbouring rivers.
The team say the process has resulted in Everest rising an extra 15 to 50 metres over the last 89,000 years, with the uplift continuing today.
“Our study demonstrates that even the world’s highest peak is subject to ongoing geological processes that can measurably affect its height over relatively short geological timescales,” said Prof Jingen Dai, co-author of the study from China University of Geosciences in Beijing.
Dai noted Everest is something of an anomaly, with its peak about 250 metres higher than the Himalayas’ other tallest mountains. In addition, data has suggested a discrepancy between Everest’s long-term and short-term rates of uplift.
“This raised the question of whether there was an underlying mechanism making Everest’s anomalous elevation even higher,” said Dai.
Writing in the journal Nature Geoscience, Dai and colleagues report how they created computer models to explore the evolution of river networks in the Himalayas.
Their results suggest that about 89,000 years ago the upper reach of the Arun River that lies to the north of Everest – and which would have flowed eastward on the Tibetan plateau – merged with its lower reach, as a result of the latter eroding northward. The upshot was that the entire length of the Arun River became part of the Kosi River system.
The team suggest the rerouting arising from this “river capture” resulted in an increase in river erosion near Everest, and the formation of the Arun River gorge.
“At that time, there would be an enormous amount of additional water flowing through the Arun River, and this would have been able to transport more sediment and erode more bedrock, and cut down into the valley bottom,” said Dr Matthew Fox, co-author of the research, from University College London.
The researchers say the reduction in weight on the Earth’s crust as this material was removed has led to an uplift of the surrounding land – a process known as isostatic rebound.
The team estimates the process is propelling Everest upwards by about 0.16mm to 0.53mm a year, with its neighbouring peaks Lhotse and Makalu, the world’s fourth and fifth highest peaks respectively, experiencing a similar uplift.
“This effect will not continue indefinitely,” said Dai. “The process will continue until the river system reaches a new equilibrium state.”
Prof Mikaël Attal of the University of Edinburgh, who was not involved in the work, said while river capture was a well-known phenomenon, it was relatively rare.
“What is unique in this study is the demonstration that erosion resulting from river capture can lead to such a dramatic response of the Earth’s surface, with an area the size of Greater London going up a few tens of metres in tens of thousands of years, which is fast,” he said.
However, Attal notes this rebound only explains a fraction of the unusual height of the highest peaks of the Himalayas. Indeed, Fox noted other mechanisms such as tectonic stresses associated with earthquake cycles, and loss of mountain glaciers, could also cause uplift.
Dr Elizabeth Dingle of Durham University said the study’s findings could be important beyond Everest.
“There are other river captures known to have occurred in the Himalaya,” she said, “So it would be interesting to know whether similar effects are preserved elsewhere, or in other tectonically active mountain ranges more broadly.”
