The Mount Everestalso called Chomolungma in Tibetan e Sagarmāthā in Nepali, the highest peak in the world with its 8848.86 m in the Himalayas, it is rising by 2 mm per year. The discovery comes from a study by the Chinese University of Geosciences and University College London published on Natureaccording to which Everest has grown between 15 and 20 m in the last 89,000 years. It is a result that may seem counterintuitivebecause the erosion of atmospheric agents should instead gradually lower the peak. Well, paradoxically it would be precisely erosion that caused the rise of Mount Everest: in particular, according to the study, theerosive action of a river which flows near the Himalayan massif, between Nepal and Tibet. River erosion, which increased dramatically 89,000 years ago due to a process called “river capture”, made the portion of the earth’s crust of which Everest is part lighter, which thus rose. The process helped make Everest the highest peak in the world.
What is the river capture phenomenon underlying the erosion of the Himalayas
The Mount Everest reaches 8849 m in altitude and is higher than the surrounding peaks (the summit of K2 is approximately 250 m lower). The researchers, via GPS measurementsthey calculated the lifting rate, equal to 2 mm per year. Through mathematical models they also managed to simulate the phenomenon responsible for the continuous uplift of the area starting from 89,000 years ago. At that time the Arun Riverwhich flowed at the foot of the Himalayas, thanks to its enormous erosive capacity it had managed to dig a gorge through the mountains, until it reached another watercourse: the Kosi. In this way the Arun and the Kosi (perhaps also with the contribution of river floods) ended up joining, giving rise to a large watercourse which today flows north of the Himalayas towards the east and then deviates sharply towards the south, cutting through the mountains. The phenomenon is called river capture and causes the “captured” river to undergo a diversion, called catching elbow. However, the models did not allow us to calculate the duration of the capture process.
Why Everest is rising: the isostatic adjustment
River trapping caused the river’s flow to increase dramatically, as did its own erosive capacity. The watercourse has removed the base of the surrounding hills enormous quantities of sedimenttransporting them downstream. In this way the earth’s crust “lightened” by rising very slowly. In particular, for the last 89,000 years Everest would have risen by an estimated amount between 15 and 50 m. The phenomenon can be defined with the term “isostatic adjustment”, which describes the vertical movement that the crustal blocks perform to reach a condition of equilibrium above the underlying mantle. If, as in this case, the weight of a crustal block decreases, the weight increases isostatic thrustand the block rises. The isostatic thrust follows the Archimedes’ principleaccording to which “every body immersed in a fluid receives a push from the bottom upwards equal to the weight of the displaced fluid”: in this case the fluid is the mantle which behaves in a viscous way by deforming, on which it floats and in in which the crust is partially immersed.
A similar phenomenon is that of post-glacial rebound corresponding to the Scandinavian Peninsula: here during the last glaciation the weight of the ice caused the crust to sink into the mantle, but when these began to melt due to the rise in temperatures the crust lightened and lifted.
As for Everest, the co-author Jin Gen Daiprofessor of geology at the China University of Geosciences in Beijing points out:
Our study shows how sudden changes in river systems can have far-reaching effects on landscapes. The main factor determining Everest’s height remains plate collisions, but our discovery adds a new piece to this complex puzzle.
