Thousands of years ago in the Himalayas, a river ate a smaller river and gave an unexpected boost to Everest's height, scientists have discovered.
Mount Everest, or Chomolungma ("Goddess Mother of the World" in the Tibetan language), is one of Earth's tallest mountains, standing 29,031.69 feet (8,848.86 metres) above sea level.
Everest's origin story began about 40 million to 50 million years ago, when landmasses on two slabs of Earth's crust — the India Plate and the Eurasian Plate — collided in slow motion and crumpled the terrain, raising rocky peaks that over millions of years became the Himalayan mountain range. Everest is the highest of those peaks by about 820 feet (250 metres).
That ancient collision is still lifting the Himalayas. However, recent GPS measurements showed that Everest was growing at a rate of about 0.08 inches (2 millimetres) per year, rather than the expected 0.04 inches (1 millimetre) per year; according to new research, this extra lift results from a more recent geological incident — an act of "piracy."
Around 89,000 years ago, the Kosi River in the Himalayas captured part of a tributary: the Arun River. This process, known as river piracy, set in motion a chain of geological events that reshaped the landscape, scientists reported Monday in the journal Nature Geoscience.
With a downstream flow strengthened by piracy, the Kosi system began eroding more rock from the valleys below Everest, the researchers wrote. As rocky mass crumbled away, other parts of the Himalayas shifted upward to compensate for the loss. This balancing act, known as isostatic rebound, lifted Everest and two other nearby peaks — Lhotse and Makalu — boosting their height by at least 49 feet (15 metres) and perhaps by as much as 164 feet (50 metres), the study authors estimated using computer models.
"Our study shows how sudden changes in river systems can have far-reaching effects on landscapes," said coauthor Jin-Gen Dai, a professor of geology at China University of Geosciences in Beijing. "The main driver of Everest's height remains the plate collision, but our discovery adds a new piece to this complex puzzle."
That puzzle piece highlights a mechanism of mountain formation that has long been overlooked, Dai said in an email. As the river system eroded rock, "the surrounding peaks were actually rising due to the elastic rebound of the Earth's crust," he added.
"It's like the landscape was doing the limbo — lower in some places, higher in others."
The connection between river erosion and peak uplift 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 when a river suddenly changes course, "it can shake things up dramatically. This sudden change can kick-start rapid erosion, which in turn triggers mountain uplift through isostatic rebound."
The findings address two anomalies in the Himalayas: the unusual heights of Everest, Lhotse and Makalu compared with neighbouring peaks, "and the unique path the Arun River takes from southern Tibet towards the Kosi River in Nepal," said Dr. Devon A. Orme, an associate professor in the department of Earth sciences at Montana State University, who was not involved in the research.
"This paper convincingly highlights the interplay of surface and deeper tectonic processes in shaping high topography on Earth," Orme said in an email.
While some instances of river capture and landscape remodeling began millions of years ago, others are happening today, she added.
Evidence of one ancient example still exists around the edges of the Himalayas, where long-ago river capture eroded deep gorges. 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 millimetres) per year, over millions of years, according to Orme. And today, in the Amazon drainage basin, "ongoing river capture is documented" and is thought to play a part in shaping the region's steep topography.
While the new study's computer models build a promising argument for river piracy causing extra elevation in Everest, "future boots-on-the-ground fieldwork within the drainage to test the timing of the river capture will be crucial for testing the ideas proposed," Orme said.
For the researchers, uncovering Everest's growth spurt began with questions about the unusual course of the Arun. It currently flows from east to west along the northern Himalayas, draining a large area to the north of Everest, but then turns sharply to the south. In an expedition to the region, the scientists also found ancient lake sediments in the Arun River Basin, hinting at differences in water distribution millions of years ago.
"These features suggested that the upper and lower sections of the river may not have always been part of the same system," Dai said. "This hinted at a past river capture event."
A breakthrough came when lead study author Xu Han, a postdoctoral researcher in the School of Earth Sciences and Resources at China University of Geosciences, modelled landscape changes over time. Han's simulations suggested that river capture would have dramatically increased the flow of water in the Kosi's lower segments. In the models, the "supercharged" river carved deeper into the rocky landscape, and the subsequent rebound effect pushed Everest and nearby peaks higher.
"Everest and its neighbours, which weren't directly eroded by the river, got a free ride upwards," Dai said.
River capture, or piracy, can be very rapid in geological terms, "like flipping a switch," Dai added. The phenomenon can happen in just 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 canyon near the foot of Kaskawulsh Glacier had rerouted meltwater that previously fed the Slims River, diverting it into the Alsek River. When the researchers previously visited the glacier in 2013, the Slims River appeared unaffected. Four years later, it had all but vanished.
Compared with river piracy, erosion and uplift unspool over a much longer time span — and are still happening with Everest, Lhotse and Makalu.
"Calculating the exact duration of this rebound is challenging," Dai said. "There's still a lot of uncertainty in these calculations, especially regarding how long the isostatic rebound will continue."
However, growth is just one part of Everest's story. Even as the lingering effects of tectonic collision and the later rebound continue to push Everest upward, extreme weather and glacier movement are wearing the mountain down. For now, the researchers expect that Everest's upward momentum will continue. But the mountain stands tall metaphorically, too — as a global icon and as a testament to the forces that shape our planet, Dai said.
"Understanding how it formed helps us grasp the bigger picture of Earth's dynamic evolution," he added. "As we face a future with changing climates and shifting weather patterns, understanding these processes could help us predict how our planet's iconic landscapes might evolve in the future."
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