Mount Everest is the highest mountain in the world – and it grows faster than most other Himalayan peaks. Geologists may now have discovered why this is so. Accordingly, it is not only the uplift caused by the collision of India with Eurasia that is to blame, but also a change in a river system at the foot of Mount Everest that dates back 89,000 years: the Arun River merged with the Kosi River and this increased the erosion caused by these rivers enormous. As a result, so much rock was removed around Mount Everest that the earth’s crust there rose isostatically. According to calculations, this process has raised Mount Everest and its surroundings by 15 to 50 meters since that time – enough to explain the difference in size from the rest of the Himalayan peaks, the team reports.
Mount Everest – called Chomolungma in Tibetan – is the highest mountain on our planet at 8,849 meters. Like the entire Himalayas, it owes its existence to the northward migration of the Indian plate, whose collision with Eurasia around 50 million years ago compressed and raised the entire area along the interface between the two plates. Over the course of millions of years, this tectonic uplift gave rise to the mountain giants of the Himalayas and adjacent mountain ranges. This tectonic uplift continues to this day and is stronger than the simultaneous erosion caused by wind, frost and precipitation that is gnawing away at the peaks. As a result, the peaks of the Himalayas are still growing, albeit only a few millimeters per year.
Unusual distance
But even among the eight-thousanders in the Himalayas, Mount Everest stands out: “The Chomolungma is around 250 meters higher than the other highest mountains in the Himalayas,” explain Xu Han from the Chinese University of Geosciences in Beijing and his colleagues. “This is surprising given the uniform tectonics, climatic conditions and erosion in the Himalayas.” Most of the other mountains in this range have an almost linear relationship, with a difference of around 120 meters between the peaks. In search of a reason for Mount Everest’s size advantage, Han and his team took another closer look at the geology at the foot of Mount Everest. A possible explanation for a regionally stronger uplift could be the isostatic springing back of the earth’s crust, as geologists explain. This process begins when the weight on the crust suddenly reduces. For example, the ground beneath Scandinavia is rising slightly today because the earth’s crust there is still reacting to the thaw of the mighty glaciers at the end of the last ice age.
Such a delayed, slow-motion springing back of the earth’s crust after a relief could – so Hand and his colleagues suspect – also have played a role in Mount Everest. Using a geophysical model and GPS data of the landscape and rivers around Mount Everest, they looked for possible traces of such relief. As it turned out, there was actually an event that could have caused isostatic uplift of the Everest region: around 89,000 years ago, there was a shift in the rivers at the foot of the mountain: the Arun and Kosi river systems merged and thereby There was increased flow and severe erosion at the foot of Everest. The raging Arun River carried away millions of tons of rock and sediment, carving the deep Arun Gorge underground. The loss of these rock masses caused isostatic uplift of the crust in the area directly east of Mount Everest and neighboring Makalu.
Additional uplift due to springing back of the earth’s crust
“Our research shows that the loss of the material through river erosion is helping to raise the mountain even further,” says co-author Adam Smith from University College London. The Lhotse and Makalu – the fourth and fifth highest mountains in the world – have also been raised slightly more over time than the remaining peaks of the Himalayas, which are only shaped by tectonics. “Our study has uncovered a previously unrecognized additional mechanism through which the crust in this area is rising,” report the researchers. As they determined, this effect caused an additional uplift of around two millimeters per year. Over the millennia, this added up to 15 to 50 meters that Mount Everest and its surroundings have grown more than the rest of the Himalayas.
“This isostatic uplift may explain part of the anomalous height difference of Chomolungma relative to K2 at 8,611 meters,” write Han and his colleagues. “This highlights the complex interrelationship between geological dynamics and the formation of topographic landmarks.”
Source: Xu Han (China University of Geosciences, Beijing) et al., Nature Geoscience, doi: 10.1038/s41561-024-01535-w