Mount Everest is growing even taller, study finds

The world’s tallest mountain is getting taller.

Mount Everest, also known as Chomolungma, has grown about 15 to 50 meters (50 to 164 feet) higher over the past 89,000 years than expected, according to a modeling study released Monday. The culprit is a nearby river eroding and pushing down land, causing the ground under Mount Everest to rebound and lift.

“It’s a new additional component of uplift of Mount Everest,” said Matthew Fox, study co-author and geologist at University College London. He expects this spurt of Everest and its surrounding peaks to continue for millions of years.

He added “the biggest impact is probably on the climbers that have to climb another 20 meters or so to the top.” The additional height may also lead to the growth of more ice at the higher elevations.

Mount Everest, part of the Himalayan mountain range, towers along the Nepal-Tibet border at around 8,850 meters (29,000 feet) high. Not only is it the tallest worldwide, it leaves its surrounding peaks in the dust - rising around 250 meters above the next tallest mountain, the 8,611-meter (28,251-foot) K2 mountain.

But what could cause Everest’s anomalous height compared to its neighbors?

What is a river capture?

These extra meters on Mount Everest can be chalked up to a relatively rare “river capture event” from 89,000 years ago, according to the authors’ computer models. During such an event, one river changes it course, interacts with another and steals its water, Fox said.

In this case, the team said the Arun river network - about 75 kilometers east of Mount Everest - stole water from a river flowing north of Everest. Fox said the capture could have been initiated by a dramatic flood, which rerouted the water to a new drainage network. Today, the Arun River is a main tributary to the Kosi River to the south.

As more water then began flowing in the Arun River, erosion rates increased. Over millennia, the waterway carved a large gorge along its banks and washed away billions of tons of sediments and earth. That huge mass loss caused surrounding land to slowly uplift, a process known as isostatic rebound.

Mountains are a bit like icebergs because only a small portion is seen above surface. In their case, a big pile of thickened crust basically floats on top of mantle, said Fox. During this rebounding, the thickened crust pushes up to replace the material eroded away. He said it could take millions of years for the uplift to fully spread across the landscape.

How much is Everest growing?

The Himalayas formed around 50 million years ago from a collision by the Indian and Eurasian tectonic plates, which continue to slowly move today. The expected growth is around 1 millimeter per year, but GPS data show recent uplift is around 2 millimeters per year.

The rebound partly explains the difference, resulting at around a 15- to 50-meter increase of Everest since the capture event, according to the study. Other Himalayan peaks, such as Lhotse and Makalu, are also affected. In fact, the authors say Makalu is located closest to the Arun River and would experience even more uplift.

“It is well established that mountain peaks rise as rivers along intervening valleys erode downward and remove mass from the system,” said Paul Kapp, geoscience professor at the University of Arizona who was not involved in the research. The study is “novel and exciting” because it quantifies when, how, and to what extent changes in river drainages contributed to the accelerated uplift, he added.

River capture events are well-known but they are not very frequent, with only a couple of examples documented in history.

During the Arun River capture event, the area would have experienced a catastrophic landscape change, said geomorphologist Mikael Attal, who was also not involved in the research. High water levels would have caused landslides. Today, he said the deep gorge carved over past millennia is “very much impassable” while pushing up the largest mountain in the world.

At the same time, this uplift may have given Everest a slight advantage in a warming world. “It may have [lost] more ice if it had stayed at a lower elevation (warmer),” Attal, a professor at University of Edinburgh. wrote in an email.

Other scientists aren’t readily on board with the study’s findings.

Geologist Mike Searle, who was not involved in the research, said he was skeptical. For one, the modeling relies more on assumptions rather than observations, he said. Searle, who has conducted his own field research on rocks on Everest and Makalu, said dating river deposits or incision rates is very difficult.

He also said river capture happens all over active mountain ranges and is difficult to connect with uplift, especially when it happens tens of kilometers away.

“The main arguments are geographical,” Searle, a professor at Oxford, said in an email. “River incision, in my opinion, has little to do with mountain uplift.”

While Everest is about 250 meters above its neighbors, Attal said the river capture event would explain only a fraction of this anomaly. The main process causing uplift of rocks, influencing the height of Everest and Makalu, is tectonics: The thrusting of plates causes topography.

Fox said the team did not disentangle competing factors that could also influence Everest’s uplift, but the river capture does have an effect - that is special by itself.

“This is a completely new result: It is the first time such a mechanism is invoked to explain the growth of Mount Everest,” Attal said.