At 8,849 meters high, Mount Everest (also called Chomolungma) is some 200 meters higher than other peaks in the Himalayas. This mountain range, created when the Indian tectonic plate smashed into Asia, is still very much active.
Recent GPS measurements suggest that the rock uplift rate of Chomolungma is higher than previously thought, averaging around 2 mm per year, compared to its long-term average of 1 mm per year. In a new study, researchers sought to understand whether this surprising increase in elevation is solely a product of tectonic uplift or if other mechanisms are at work. They found that one surprising process, called “river piracy” or “stream piracy” may have also contributed to this.
Shifting plates
Some 50 million years ago, the Indian Plate moved northward, towards the larger Eurasian Plate. They started colliding and because both are plates of continental crust, neither could easily subduct beneath the other. Instead, the collision caused the Earth’s crust to buckle and fold. The land was forced upwards between them to form the massive mountain range we see today. This process of continental collision continues to this day, driving the ongoing uplift of the Himalayas.
Essentially, the immense pressure and force generated by the collision causes the crust to thicken, resulting in the uplift of peaks like Mount Everest. Additionally, the ongoing movement of the Indian Plate into Eurasia continues to push the mountains higher each year.
Sometimes, this uplift can be balanced by erosion, resulting in a stable mountain height. But in the case of Everest, the net growth rate is at around 1 mm per year, or 1 meter every 1,000 years.
Jin-Gen Dai, from the China University of Geosciences, Beijing, China, and colleagues, wanted to investigate the effect of an unlikely culprit: rivers.
Rivers, pirates, and mountains
River drainage piracy refers to a geological process in which a river or stream captures the headwaters of another river, thus diverting the flow of water into a new drainage basin. This process can lead to increased rates of erosion in the newly formed river system, altering the landscape over time. For this study, the focus was on the Arun River, which is a major tributary of the Kosi River. Kosi flows through China, Nepal, and India.
varies from 8,481 m to 1,140 m over a distance of only 35 km. Image credits: Dai et al (2024).
The models indicate that around 89,000 years ago, the Arun River captured another river. This diversion of water caused a surge in erosion as the Arun River adjusted to its new course. Dai and colleagues suggest that while this erosion would have reduced the elevation along the river channel, the rapid removal of mass from the gorge would have prompted isostatic rebound, leading to surface uplift in the surrounding landscape, including Mount Everest.
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In a sense, the mountain behaved like a boat on water. As the Arun River cut into the landscape, it removed a significant volume of rock. This removal of mass is what caused the Earth’s crust to undergo isostatic rebound. Similar to how a boat rises when weight is removed, the crust under the Himalayas rises as the weight of the eroded material is lifted. This upward adjustment of the crust contributes to the surface uplift of the entire region, including Everest.
The researchers estimate that this isostatic response has added 15–50 meters to the height of Everest. As such, it accounts for some of the recent elevation gains not solely due to tectonic activity. In terms of rates, this process contributes an additional 0.16–0.53 mm per year to Everest’s uplift, which could represent 10–50% of the mountain’s total recent growth. This would essentially explain the surprising “growth spurt” exhibited by Everest.
This could be an underrated process
The findings of this study have important implications for our understanding of how Himalayan landscapes evolve over time. While tectonic processes remain the primary driver of mountain building in the region, the research highlights the role that fluvial erosion and drainage piracy can play in enhancing surface uplift.
This isn’t just about the Himalayas, either. Other mountain systems with similar geology could have undergone similar episodes in their history. This new work provides a new way to analyze the timing and dynamics of river capture events, which have likely occurred multiple times throughout the geological history of our planet.
The study has been published “Recent uplift of Chomolungma enhanced by river drainage piracy” has been published in Nature Geoscience. https://doi.org/10.1038/s41561-024-01535-w