Everest grows a few millimeters more every year. It shouldn’t be surprising, the entire Himalayan mountain range is doing it. Dinosaurs were still alive when the collision of the Indian and Eurasian plates began to lift it. But the highest mountain in the world is between 238 meters (compared to K2) and 822 meters (compared to Shisha Pangma) higher than the rest of the eight thousand. Now, Chinese and British geologists believe they have found the reason for this geological anomaly. As detailed in the scientific journal Nature Geosciencethe Arun River, which surrounds it, is eroding its base, lightening its mass and raising it higher than the rest of the neighboring peaks.
“The Arun River helps lift Mount Everest through a process called isostatic rebound,” says geologist at China University of Geosciences and first author of this research, Jingen Dai. If you consider the Earth’s crust as a flexible surface that floats on denser material below, the mantle, “when the river erodes and carries enormous amounts of rock from the valleys, it is like taking weight away from this floating surface; Just as a ship rises higher in the water when its load is removed, the land responds by slowly rising upward,” explains Dai. This rebound effect had been observed in areas of the planet covered by large masses of ice, such as Greenland, which are thawing rapidly. In a process called isostasy, the geosphere tends to gravitational balance. There the causal agent would be the loss of water that goes to the sea, on Everest, this role would be played by the erosive force of the river. “In the case of the Arun, as it carves deeper valleys, the surrounding land, including Mount Everest, rises in response. This elevation is a way for the Earth to maintain balance, compensating for the mass eliminated by river erosion,” concludes the Chinese geologist.
For millennia and millennia, Everest rose alongside the rest of the mountain range. Then, the Arun ran along the northern slope, along the Chinese and Tibetan sides, and from east to west. But about 69,000 years ago (a short period in geological terms), the river changed its course to the south, becoming trapped by the river network of another larger one, the Kosi. The researcher at the Geological and Mining Institute of Spain (IGME), Luis Carcavilla, says that, “the river made a 90-degree turn, turning north-south, excavating one of the deepest gorges in the world.” In this process, called fluvial incision, and given its elevation and trajectory, the Arun causes great erosion at the base. “What they come to say [los autores de esta nueva investigación] “It’s that that canyon so wedged and that anomalous route that goes west to east and suddenly north to south could have something to do with the elevation of Everest,” he indicates. According to calculations by Chinese geologists, who insist that the main factor continues to be tectonic, the river would be responsible for up to a third of the height differential with its neighboring mountains.
The geologist from the Pablo de Olavide University, Francisco Moral, who has not participated in this research, points out what could have happened: “A small river, north-south, when it captures another river, west to east, enormously increases the size of the basin, of its flow. If before we had a river that, let’s say, had a catchment area of 10 km², when the capture occurs it can increase to 80 km². We are multiplying the slope count by eight. From that moment on, the flow increases and the erosive capacity increases.” According to the study’s calculations, the Arun has fallen about 700 meters downwards in this time. “This erosion in the river bed and in the slope eliminates enormous amounts of geological material. As a consequence, we have less weight in that area of the cortex and around that throat an isostatic lifting occurs due to the loss of that mass,” explains Moral.
The conclusion, supported by various models and the geological record, would be confirmed by the fact that the other two mountains closest to Everest, Lhotse and Makalu, are also rising at a faster rate. The isostatic rebound of the world’s fourth and fifth highest peaks would have increased their height by an amount similar to that of Everest. In fact, the second, located closer to the Arun River, would be experiencing a slightly higher rate of uplift.
Carcavilla, from IGME, highlights that in the last two decades, “external geological processes, such as rain, glaciers… that influence or can trigger certain internal processes” have been investigated. Some of them are better studied, such as isostatic rebound due to snowmelt. “But fluvial incision or glacial erosion is more recent,” he says. The geologist remembers that these processes and their impact are very difficult to quantify. But also that it is relevant to try: “In fact, there are studies that maintain that the area of the Himalayas that rises the highest is the one where the monsoon is most intense, because it produces more erosion and that produces more rebound.” And the fate of a third of the world’s population largely depends on the monsoon.
How long will Everest grow? Neither the authors of the study nor the geologists consulted know this. There are many factors involved. Some are recounted by Chinese researcher, Dai: “The ongoing tectonic collision, which remains the main driver of the uplift; isostatic rebound caused by river erosion; and glacial erosion at high altitudes, which gradually wears away the summit. The interaction of these processes (tectonic uplift, isostatic rebound and erosion) will determine its final height.” But his colleague Carcavilla remembers that Everest is already at the limit. In the past, there could only have been mountains this high once in Earth’s 4.5 billion history. “The lithosphere has a limit to how much it can hold before it collapses,” he recalls.
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