Mount Everest (Chomolungma) may thank a river curving through the Himalayan massif for a portion of its towering height. A long-ago change of course may have led to increased erosion that allowed Earth’s crust to stretch just a tiny bit higher in the region, lofting Everest’s great bulk skyward. That’s according to new simulations published in Nature Geoscience.
The work “highlights that these landscapes are quite dynamic. And that river networks can change,” said Matthew Fox, a coauthor of the research and a geologist at University College London. “The results of that change can be significant.”
The process has added at most 50 meters (165 feet) to Everest’s 8,848-meter (29,032-foot) height. But nevertheless, it reveals the complexity of the forces that together have created the world’s tallest mountain, which include tectonic collisions, scouring winds, and, evidently, rivers.
“I kind of chuckled,” said Richard Lease after reading the study. “You have the world’s highest mountain, as if it needed any extra help.” Lease is a research geologist with the U.S. Geological Survey who wasn’t involved with the research.
From the River to the Mountain
The study authors didn’t set out to investigate Everest’s height. They were initially interested in the Arun River, which begins north in Tibet and circles east of Everest through a deep gorge to join the Kosi River to the south. That defies convention—most rivers flow away from mountains, not through them.
“For it to cut through the Himalayas where the Himalayas are the highest is unexpected,” Fox said.
The reasons for this have been debated: Some say the river’s course predates the Himalayas and simply etched its way deep into their bedrock as they rose, whereas others suggest it’s a more recent phenomenon.
Also odd is the river’s elevation profile (which you can imagine as its course as seen from the side). Most rivers start out steep and gradually flatten out downstream, looking something like a bowl cut in half. But the Arun has a steep drop at one point.
“The river there really sticks out like a sore thumb,” Lease said. “It has this pronounced convexity suggesting it’s responding to recent geologic change.”
For a better look, Fox and his coauthors used information about the river’s drainage area and steepness to simulate how it has been eroding the ground beneath it. By tuning various parameters of the model, such as how quickly water can break down the riverbed, how much sediment the river holds, and how long erosion has been going on, they tested a theory they had that the river’s path suddenly changed at some point in the recent past.
The Arun River may have once been two separate rivers, Fox said. As what is now the lower part of the river eroded its headwaters, it eventually broke through to the channel of the current upper portion. This is an event known as river capture or, more colorfully, river piracy, and it can change the course of a river entirely as it’s diverted into a new channel.
The simulation that best fit the real Arun River suggested that capture happened on the Arun River around 90,000 years ago.
“All of the precipitation that would have fallen across this part of Tibet now flowed through and down the lower reaches of the Arun,” Fox said. The extra water led to more erosion, carrying soil and rock away from that part of the Himalayas. With that burden gone, Earth’s crust began to rise, a process known as rebound.
Growing Up
Most of Everest’s height comes from the northward movement of the Indian continental plate, which is folding Earth’s crust upward and creating the Himalayas, which are still rising about 1 millimeter (0.04 inch) per year. Everest and some of the nearby peaks, however, seem to be growing at a slightly faster rate—2 millimeters (0.8 inch) per year. That accelerated rate may, in fact, be a growth spurt caused as Earth’s crust responds to the erosive force of the Arun River.
“There’s a dynamic interplay between those processes that build mountains up and those that tear them down,” Lease said. “Because it’s the Himalayas, everything is magnified in terms of effects.”
Depending on the strength of Earth’s crust beneath the Himalayas, which affects how strongly it rebounds, the process could have added anywhere between 15 and 50 meters (50 and 165 feet) to Everest and other peaks near it, the researchers estimated. Uplift is likely still ongoing, meaning the world’s tallest mountain still has room to grow.
Similar processes could be happening in other mountain ranges, such as the Andes, or elsewhere in the Himalayas, Lease said. The new research provides one of the most detailed looks yet at how rivers and mountains interact with Earth’s crust and could help inform further studies of the process across the globe.
—Nathaniel Scharping (@nathanielscharp), Science Writer