The rocky history of a missing 26,000-foot Himalayan peak

Date:
Thu, 06 Jul 2023 16:30:00 +0000

Description:
The base camp at Annapurna in Nepal, one of the tallest mountains on Earth. Depositphotos A massive mountain summit crumbled around 1190 CE, leaving evidence in the plains below. The post The rocky history of a missing 26,000-foot Himalayan peak appeared first on Popular Science .

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The base camp at Annapurna in Nepal, one of the tallest mountains on Earth. Depositphotos

Earth is home to 14 eight-thousanders, summits that top off at more than
8,000 meters, or 26,247 feet, above sea level. All of these grand mountains tower over the Himalayas , the highest place in the world.

But our planet is dynamiccould there have been additional peaks like these, since lost? We wanted to know whether, 830 years ago, the Earth and the Himalayas had one more, says Jrme Lav , a geomorphologist at the National Centre for Scientific Research (CNRS) and the University of Lorraine in France.

The answer, according to Lav and his colleagues, appears to be yes. In a new paper, published in the journal Nature on July 6, theyve found evidence of an ancient landslide that reshaped South Asias geographyand linked that to the collapse of a peak that would have once been one of the tallest mountains on Earth.

Lav says his team first spotted the fingerprints of this medieval landslide not in the Himalayas, but far to the south, near the India-Nepal border, in the flat plains around the Narayani River.

To look for missing mountains, these plains are prime land for geomorphologistsscientists who study the evolution of the land under our feet (or, in this case, the land towering well above everyone but the hardiest mountaineers). Rivers like the Narayani carry sediments downslope, and those sediments can reveal much about the mountains where they originated.

For instance, Lav and colleagues found medieval sediments with a carbonate content five times higher than average. This mineral fingerprint indicated that something had disrupted the Narayanis flow. A giant landslide occurringseemed to me the most obvious avenue to explore, Lav says.

[Related: How to start mountain biking this summer ]

They began plying uphill to find out more. The Narayani flows through the
city of Pokhara, nestled in a valley less than 3,000 feet above sea level.
But this is one of the steepest landscapes on Earth: looming over Pokhara is the Annapurna massif, a section of the Himalayas. (The massifs crown jewel is its tallest peak: also named Annapurna, a proud member of the eight-thousand club.)

By studying images of the Annapurna massif, the team found geographic signs
of an old landslide. In one subsection of the massif, called the Sabche cirque, they spotted strange features like pillars and pinnacles, markers of erosion.

The authors needed more samples. Collecting fragments from the plains is one thing. It was another to gather wood and rock from the Sabche cirquethey ventured up into the massif by helicopter. From these parts, they began to build the hazy image of a mountain that existed, long ago, until one catastrophic day around 1190 CE.

They really managed to capture this eventboth at the source as well as at the far sink of these sediments, says Wolfgang Schwanghart , a geomorphologist at the University of Potsdam in Germany, who was not an author of the paper.

This is what Lav and colleagues think happened: There once rose a second eight-thousander from the Annapurna massif. Then, it collapsed. The resulting rockslide thoroughly eroded the Himalayan landscape and poured sediment into the valley that now contains Pokhara, from where waters carried it
downstream. This event played a major role in eroding the rock, reshaping the massif closer to what we see today.

The paper suggests that large, dramatic landslides may be a significant
driver of erosion at high altitudes like this. This is a mechanism that still needs to be further investigated, but this hypothesis may open new insights, says Odin Marc , a geomorphologist at CNRS who was also not involved in the research.

What caused the mountain to collapse isnt clear. A warming medieval climate might have melted mountaintop permafrost that otherwise strengthens the peak. Schwanghart, who has also studied the regions geology, believes the answer
may be earthquakes. He says the chronology indicates that three earthquakes struck Nepal around the time that Lav and colleagues suggested the mountain collapsed, and one of them may have caused the mountain to topple in the
first place.

[Related: There might be underground mountains near Earths core ]

Whatever happened, the new report reinforces the fact that mountains are constantly changing environments. We might see summits as eternal fixtures on the landscape, but if anything, they are the complete opposite.

After all, Himalayan landslides arent consigned to the past. In 2021, an avalanche and rockslide careened down a mountainside in Uttarakhand, India, around 300 miles northwest of Annapurna. The disaster burst a dam , and the resulting flood left some 200 people dead or missing.

If such a rockslide were to happen to Pokhara today, the results could be devastating. Pokhara is Nepals second-largest city (after the capital Kathmandu) and home to more than half a million people. Moreover, globally, evidence is mounting that a warming climate exacerbates the risk of mountain landslides . Just last month, the Alpine summit of Fluchthorn, nestled on the Swiss-Austrian border, abruptly collapsed in an event that scientists
ascribed to thawing permafrost.

Mountain collapses like these may be more common than we realize. In Alaska, you would find similar eventsbut often they go unnoticed, because there is no one around, says Schwanghart.

The post The rocky history of a missing 26,000-foot Himalayan peak appeared first on Popular Science . Articles may contain affiliate links which enable us to share in the revenue of any purchases made.



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Link to news story:
https://www.popsci.com/science/himalaya-mountain-landslide/


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