An ancient act of 鈥渞iver piracy鈥� may be partly responsible for Mount Everest鈥檚 soaring heights.
At 8849鈥塵etres, Mount Everest 鈥� also called Chomolungma or Sagarm膩th膩 鈥� is the world鈥檚 highest peak. Most of this stature is due to the tectonic uplift that formed the rest of the Himalayas. But Everest is an outlier, jutting above neighbouring giants by around 250 metres.
at University College London and his colleagues considered how the distinctive pattern of rivers flowing around the mountain may have contributed to this height anomaly by eroding away the rock that lies close to the flanks of Mount Everest.
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鈥淭he thick rocks that make up the Himalayas are floating on the underlying mantle,鈥� says Fox. When rivers cut through the landscape, eroding rock and forming a deep gorge, the mantle responds by rebounding upwards. This means that 鈥� paradoxically 鈥� the erosion process that removes rock actually causes the land surface to rise.
Fox and his colleagues focused on the mighty Arun river, which flows north from Everest, before circling back to the south and passing through the narrow but deep Arun gorge. A long-standing debate is whether the river already took this circuitous route before the rise of the Himalayas, or whether it changed course after the mountains rose. One possibility is that the river initially drained a relatively small area 鈥� but that as the mountains formed there was a 鈥渄rainage piracy鈥� event that saw it capture the waters draining from a much larger area.
The researchers modelled different scenarios of how such a capture event may have played out. If it occurred, it would have led to a dramatic increase in the volume of water flowing down the Arun river. This, in turn, would have given the river a greater capacity to erode through the landscape and create the Arun gorge 鈥� which would have affected patterns of elevation in the region and added to Mount Everest鈥檚 height.
The scenario that gave the closest match to today鈥檚 pattern of river elevation involved a capture event around 89,000 years ago. At that point 鈥渢he Arun river continued to erode backwards and stole the upstream drainage area鈥�, says Fox. A final nudge may have come from extreme precipitation or a landslide that pushed the river to change course.
The researchers estimated how much extra rock would have been eroded by the Arun river after such an event. They say it would correspond to a rebound effect that adds 0.16 to 0.53鈥塵illimetres per year to Everest鈥檚 elevation, equivalent to a 15 to 50 metre rise since the capture.
The principle that river erosion can raise elevation is well known, says at the University of Oxford. However, he is sceptical that these estimates based on modelling tell the full story. 鈥淭he question here is whether the river is causing the uplift or not. To me, it鈥檚 not. It鈥檚 the tectonics,鈥� he says.
at the University of Southern California points out that tectonic activity may have triggered the drainage piracy event itself, making it the ultimate source of Everest鈥檚 majesty.
Nature Geoscience
