Getty ImagesA new study suggests that Mount Everest is 15-50m taller than it otherwise is because rivers are eroding the rock and soil at its base, helping to push it upward.
Loss of land area in the Arun River basin, 75 kilometers (47 miles) away, is causing the world’s highest peak to rise by 2 millimeters a year, researchers at University College London (UCL) said.
“It’s a bit like throwing a bunch of cargo off a ship,” study co-author Adam Smith told the BBC. “The ship becomes lighter, so it floats a little higher. Likewise, when the crust becomes lighter… it can float higher. Higher.
The pressure from the collision of the Indian and Eurasian plates 40-50 million years ago formed the Himalayas, and plate tectonics remains the main reason for their continued rise.
But the UCL research team said the Arun river network was a factor in causing the mountains to rise.
As the Arun River flows through the Himalayas, it strips material (in this case, the river bed) from the earth’s crust. This reduces the forces on the mantle (the next layer beneath the Earth’s crust), causing the thinned crust to bend and float upward.
This is an effect called isostatic rebound. The study, published in Nature Geoscience, adds that this upward push is causing Everest and other nearby peaks to move upward, including Lhotse and Makalu, the fourth and fifth tallest mountains in the world.
Dr Matthew Fox, co-author of the study, told the BBC: “Everest and its neighboring peaks are growing because isostatic rebound is lifting them faster than erosion is wearing them down.”
“We can see them growing about two millimeters per year using GPS instruments, and now we have a better understanding of what’s driving it.”
Some geologists not involved in the study say the theory is sound, but that there are still many uncertainties in the study.
Getty ImagesMount Everest is located on the border between China and Nepal, with its northern part on the Chinese side. The Arun River flows from Tibet into Nepal, then joins two other rivers to become the Kosi River, and then enters northern India to join the Ganges.
This is a very mud-producing river because the mountains it flows through are steep and its strength allows it to wash away so much rock and soil along the way.
But researchers at University College London say it most likely gained its real power when it “occupied” another river or body of water in Tibet 89,000 years ago, a recent event on geological timescales.
Dr. Han Xu of China University of Geosciences is the lead author of the study, which he conducted during an academic visit to University College London.
“The changes in Everest’s height really highlight the dynamic nature of the Earth’s surface,” he said.
“The interaction between the erosion of the Arun River and the upward pressure of the Earth’s mantle gave Everest the push it needed to push it higher than it originally was.”
Research from University College London suggests that the Arun River likely gained the ability to cut large amounts of rock and other materials after taking over another river or water system in Tibet.
Professor Hugh Sinclair from the University of Edinburgh’s School of Earth Sciences, who was not involved in the study, said the basic process identified by the UCL team made perfect sense.
But, he adds, there is considerable uncertainty about the exact amount and time scale of river incision (or how rivers cut down into their beds and deepen their channels) and the consequent surface uplift of surrounding mountain peaks.
“First, predicting river incision in such a large watershed due to drainage capture (where one river captures another river or lake) is challenging,” he said.
The authors acknowledged this uncertainty in their study.
Secondly, Professor Sinclair said, the distance at which mountains rise from points of intense local erosion is extremely difficult to predict.
“However, even taking these reservations into account, the possibility that some of Everest’s special elevations are related to rivers represents exciting insights.”
