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Thousands of years ago, a river in the Himalayas ate a smaller river and gave an unexpected boost to the height of Everest, scientists have discovered.
Mount Everest, or Chomolungma (“Goddess Mother of the World” in the Tibetan language), is one of the highest mountains on Earth, reaching a height of 8,848.86 meters above sea level. Everest’s origin story began some 40 to 50 million years ago, when land masses on two plates of the Earth’s crust – the India Plate and the Eurasian Plate – collided in slow motion and crumpled the terrain, creating rocky peaks that became the Himalayas over millions of years. mountain range. Everest is the highest of those peaks at about 250 meters.
That age-old collision still lifts the Himalayas. However, recent GPS measurements showed that Everest was growing at about 2 millimeters per year, instead of the expected 1 millimeter per year; According to new research, this extra lift is the result of a more recent geological incident – an act of ‘piracy’.
About 89,000 years ago, the Kosi River in the Himalayas conquered part of a tributary: the Arun River. This process, known as river piracy, has set in motion a series of geological events that have reshaped the landscape, scientists reported Monday in the journal Nature Geoscience.
With a downstream flow enhanced by piracy, the Kosi system began to erode more rock from the valleys beneath Everest, the researchers wrote. As the rock mass crumbled, other parts of the Himalayas moved up to compensate for the loss. This balancing act, known as isostatic rebound, lifted Everest and two other nearby peaks – Lhotse and Makalu – increasing their height by at least 15 meters and perhaps as much as 50 meters, the study authors estimated. use of computer models.
“Our research shows how sudden changes in river systems can have far-reaching consequences for landscapes,” said co-author Jin-Gen Dai, professor of geology at the China University of Geosciences in Beijing. “The main cause of Everest’s height remains plate collision, but our discovery adds a new piece to this complex puzzle.”
Landscape limbo
That piece of the puzzle highlights a mechanism of mountain formation that has long been overlooked, Dai said in an email. As the river system eroded the bedrock, “the surrounding peaks actually rose due to the elastic recovery of the Earth’s crust,” he added.
“It’s as if the landscape is in limbo: lower in some places, higher in others.”
The link between river erosion and peak rise is well documented and has been studied in places such as the Alps, Antarctica and the Colorado Plateau, Dai said.
“Usually, rivers and mountains reach a kind of equilibrium, where erosion and uplift balance each other out,” Dai said. But if a river suddenly changes course, “it can shake things up dramatically. This sudden change can trigger rapid erosion, which in turn causes mountain uplift through isostatic recovery.”
The findings focus on two anomalies in the Himalayas: the unusual heights of Everest, Lhotse and Makalu compared to neighboring peaks, “and the unique path the Arun River takes from southern Tibet to the Kosi River in Nepal,” according to the report. Dr. Devon A. Orme. , an associate professor in the Department of Earth Sciences at Montana State University, who was not involved in the study.
“This paper convincingly highlights the interplay between surface and deeper tectonic processes in shaping Earth’s high topography,” Orme said in an email.
While some cases of river capture and landscape renewal began millions of years ago, others are happening today, she added.
Evidence of an ancient example still exists around the edges of the Himalayas, where deep gorges were long ago eroded by rivers. This caused two regions – Namche Barwa in the east and Nanga Parbat in the west – to rise about 0.2 to 0.4 inches (5 to 10 millimeters) per year for millions of years, according to Orme. And today, in the Amazon River basin, “ongoing river capture is documented” and believed to play a role in shaping the region’s steep topography.
While the new study’s computer models make a promising case for river piracy causing additional elevation changes in Everest, “future fieldwork within the drainage to test the timing of river capture will be crucial to testing the proposed ideas.” Orme said.
‘Flipping a Switch’
For the researchers, uncovering Everest’s growth spurt started with questions about Arun’s unusual course. Currently it flows east to west along the northern Himalayas, draining a large area north of Everest but then turning sharply south. During an expedition to the region, the scientists also found ancient lake sediments in the Arun River basin, indicating differences in water distribution millions of years ago.
“These features suggested that the upper and lower parts of the river were not always part of the same system,” Dai said. “This indicated an earlier river catch.”
A breakthrough came when lead study author Xu Han, a postdoctoral researcher at the School of Earth Sciences and Resources of China University of Geosciences, modeled landscape changes over time. Han’s simulations suggested that river capture would have dramatically increased water flow in the lower segments of the Kosi. In the models, the “supercharged” river cut deeper into the rocky landscape, and the subsequent rebound effect pushed Everest and nearby peaks higher.
“Everest and its neighbors, which were not directly eroded by the river, got a free ride up,” Dai said.
River capture, or piracy, can happen very quickly in geological terms, “like flipping a switch,” Dai added. The phenomenon can occur within a few years or decades. In 2017, another team of scientists reported a case of river piracy in Canada’s Yukon Territory; the formation of a gorge near the base of the Kaskawulsh Glacier had diverted meltwater that previously fed the Slims River into the Alsek River. When researchers visited the glacier earlier in 2013, the Slims River appeared untouched. Four years later it had all but disappeared.
Compared to river piracy, erosion and uplift disappear over a much longer period of time – and this is still happening with Everest, Lhotse and Makalu.
“Calculating the exact duration of this recovery is challenging,” Dai said. “There is still a lot of uncertainty in these calculations, especially about how long the isostatic recovery will last.”
However, growth is only part of Everest’s story. Even as the lingering effects of a tectonic collision and subsequent recovery continue to push Everest upward, extreme weather and glacier movements are causing the mountain to sink. For now, researchers expect Everest’s upward momentum to continue. But the mountain also stands tall figuratively: as a global icon and as a testament to the forces shaping our planet, Dai said.
“Understanding how it formed helps us understand the bigger picture of Earth’s dynamic evolution,” he added. “As we face a future of changing climates and changing weather patterns, understanding these processes could help us predict how our planet’s iconic landscapes might evolve in the future.”
Mindy Weisberger is a science writer and media producer whose work has appeared in the magazines LiveScience, Scientific American, and How It Works.
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