Enormous boulders—10 meters or more in diameter—litter many river channels in the Himalayas. Scientists have now age dated several of these behemoths and estimated the flow velocities necessary to heave them.
The boulders were likely set in motion thousands of years ago by the powerful forces of glacial lake outburst floods, the researchers suggested. These findings shed light on how infrequent events can shape landscapes.
A Rocky Mystery
In 2016, Marius Huber, a geoscientist at the University of Lorraine in Nancy, France, and his colleagues traveled to Nepal to solve a rocky mystery: the origin of the house-sized boulders often found in or near Himalayan river channels. “No one really knows where they’re coming from,” said Huber.
Boulders of that size can have a significant impact on the local hydrology, said Mike Turzewski, a geomorphologist at Pacific Lutheran University in Tacoma, Wash., not involved in the research. “They can completely change the direction of the channel.”
The boulders stick out like sore thumbs not only because of their sizes but also because of their compositions—their lithologies tend to differ from those of their immediate surroundings. That’s a telltale sign that they’ve been transported at some point in the past, the researchers concluded. But the Trishuli and Sunkoshi river channels—where the scientists did their fieldwork—are at too low an elevation to have been glaciated in the past, meaning that the rocks couldn’t have hitched a ride with a glacier.
Measuring “Sunburn”
Huber and his collaborators focused on 16 boulders ranging in diameter from about 5 to 30 meters. They clambered to the top of each rock to collect samples for cosmic ray exposure dating to estimate how long ago the rocks had settled into their current positions. The technique hinges on measuring minute changes in rock chemistry, which arise from energetic protons—emitted by distant supernova explosions—slamming into the boulders over time. “Cosmogenic radiation alters the surface of the rock over time,” said Huber. “It’s like a sunburn.”
The researchers found that the boulders’ ages ranged from fewer than 500 years to up to about 13,000 years. However, more than half of the rocks had ages that clustered around 5,000 years. That pattern was striking, said Huber, and informative. It meant that the boulders probably weren’t being heaved by earthquakes. Repeated temblors over time—a region’s earthquakes tend to strike every few hundred or thousand years—would have yielded a larger spread in ages rather than a single cluster, the researchers concluded.
Lots of Water
Huber and his team next used three different metrics to estimate the water flow velocities necessary to move the boulders. They found velocities in the range of roughly 4–17 meters per second, which, when translated into peak discharge values for either the Trishuli or Sunkoshi river channel, yielded values ranging from about 1,300 to 300,000 cubic meters per second. (For comparison, the Mississippi River discharges roughly 16,000 cubic meters of water per second into the Gulf of Mexico.)
That’s higher than the levels associated with even monsoonal flooding, the researchers calculated. “Our discharges are considerably bigger than most monsoonal discharges,” said Huber. “You need lots of water.”
A plausible culprit, Huber and his colleagues suggested, is a glacial lake outburst flood. These events, which occur worldwide, involve the sudden drainage of a glacier-fed lake. (Many such lakes are bounded by fragile glacial moraine, which is apt to give way.) Glacial lake outburst floods have frequently struck in the Himalayas; one roared down the Sunkoshi River valley in 2016.
An uptick in glacial lake outburst flood activity roughly 5,000 years ago makes sense, Huber and his team proposed. Climate proxies such as ice and sediment cores record drier-than-normal conditions around that time, and glaciers tend to shrink when there’s less precipitation, said Huber. Because receding glaciers form moraines, setting up the conditions for glacial lake outburst floods, this time period was essentially primed for heaving around big boulders, the researchers concluded. These results were published in September in Earth Surface Dynamics.
It’s worth returning to the Himalayas to measure more boulders in the future, said Huber. “We’ll get a better picture of what’s going on.”
—Katherine Kornei (@KatherineKornei), Science Writer