Why Everest, the world's tallest mountain, is growing even taller

Mount Everest is astoundingly tall at 29,032 feet above sea level, besting its Himalayan neighbors by hundreds of feet.

But the world’s tallest peak is still growing, scientists say, thanks in part to the merger of two nearby river systems tens of thousands of years ago, NBC News reports.

Everest has gained between about 50 and 160 feet as a result of that merger, researchers revealed in a study published Monday in the journal Nature Geoscience.

“Even a feature as seemingly permanent as Mount Everest is subject to ongoing changes driven by various geological forces,” Dai Jingen, a geoscientist at China University of Geosciences in Beijing and a co-author of the study, told NBC News in an email Tuesday.

The mountain peak has been steadily gaining height along with the rest of the Himalayas since its birth about 45 million years ago from the collision of the tectonic plates beneath the Indian subcontinent and Eurasia. 

But that didn’t fully explain how much Everest was growing. Researchers now say that when the Kosi river took over the Arun river about 89,000 years ago, it gave rise to a combined river of such immense power that it eroded large quantities of rock and soil from the Himalayan base nearly 50 miles away from Everest.

In a geological process known as isostatic rebound, the erosion reduced the weight of the region, allowing the rise of land masses on the Earth’s crust, its outermost layer that floats atop a mantle layer made of hot, semi-liquid rock.

“Essentially, as the river carved away more rocks, the Earth’s crust rebounded, rising like a boat when weight is removed,” said Dai, adding that even though the river itself didn’t directly make Everest taller, the erosion and crust movement it caused contributed to the mountain’s elevation.

Scientists estimate the rebound is causing Everest to grow 0.16 to 0.53 millimeters a year, accounting for as much as half of its annual uplift rate. They said Everest had gained as much as 2 millimeters annually in recent years.

Researchers also say this could explain why Everest is unusually tall, reaching nearly 800 feet higher than its neighbors.

Dai said the idea that river capture and erosion-related isostatic rebound play a role in Everest’s elevation adds a “surprising” dimension to the study of mountain formation, which is traditionally explained by tectonic activity.

“While not entirely revolutionary, these findings are certainly surprising,” Dai said, adding that they could lead to a re-examination of current models of Himalayan formation and evolution.

“It also emphasizes the importance of viewing the Earth as an interconnected system, where changes in one area can have surprising and significant impacts elsewhere,” he said.

This story first appeared on NBCNews.com. More from NBC News:

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