Mount Everest continues to rise as river merger boosts its height by up to 50 metres (164 feet). Photo: ReutersMount Everest is Earth’s tallest mountain – towering 8.85km above sea level – and it is still growing.
Along with the rest of the Himalayas, it is continuing an inexorable uplift that dates back to their birth roughly 50 million years ago when the Indian subcontinent collided with Eurasia. Everest is growing more than expected from this alone. Scientists now think they know the reason why, and it has to do with the monumental merger of two nearby river systems.
Everest has gained roughly 15 to 50 metres (40 to 164 feet) in height due to this change in the regional river system. The Kosi river merged with the Arun river approximately 89,000 years ago as per researchers. That translates to an uplift rate of roughly 0.2 to 0.5mm annually.
The geological process at work, they said, is called isostatic rebound. It involves the rise of land masses on Earth’s crust when the weight of the surface diminishes (see graphic). The crust, Earth’s outermost layer, essentially floats atop a mantle layer made of hot, semi-liquid rock.
In this case, the merger of the rivers is more like a hostile takeover. The Kosi subjugated the Arun as the rivers changed course over time. This caused accelerated erosion that has carried off huge amounts of rock and soil, reducing the weight of the region near Everest.
“Isostatic rebound can be likened to a floating object adjusting its position when weight is removed,” said geoscientist Jingen Dai of China University of Geosciences in Beijing.
He is one of the leaders of the study published last month in the journal Nature Geoscience.
“When a heavy load, such as ice or eroded rock, is removed from the Earth’s crust, the land beneath slowly rises in response, much like a boat rising in water when cargo is unloaded,” Dai added.
The main gorge of the merged river system is located approximately 45km east of Everest.
The researchers used numerical models to simulate the evolution of the river system. They estimated that isostatic rebound accounts for about 10 per cent of Everest’s annual uplift rate.
The study’s co-author Adam Smith is a University College London doctoral student in Earth sciences. He said GPS measurements reveal the continued rising of the Himalayas. This uplift outpaces the continued surface erosion caused by factors such as wind, rain and river flow. As this erosion continues, Everest’s uplift rate from isostatic rebound may increase, Smith said.
“This research underscores our planet’s dynamic nature. Even a seemingly immutable feature like Mount Everest is subject to ongoing geological processes, reminding us that Earth is constantly changing, often in ways imperceptible in our daily lives,” Dai said.
Everest is situated on the border between Nepal and the Tibet autonomous region of China. It was named for George Everest, a 19th century British surveyor in India.
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Is Everest the only mountain that is “growing”, and are there other places in the world that are undergoing isostatic rebound?
Other nearby mountains, including Lhotse, the world’s fourth highest, and Makalu, the fifth highest, are also growing for the same reason.
Lhotse is growing at a rate similar to Everest. Meanwhile, Makalu, which is situated closer to the Arun, is growing at a faster rate.
In addition to the Himalayas, Scandinavia is another region where the land is still rising in response to the melting of thick ice sheets that covered the region during the last Ice Age. On average, it is increasing by 9mm yearly, and the process is ongoing today.
The rising land is causing problems along the coastlines of countries such as Sweden because the ports are getting shallower, making it harder for ships to navigate the area.