Jul 8, 2023 - Science

A massive landslide may have "beheaded" a high Himalayan peak 800 years ago

Annapurna IV in the clouds, Pokhara, Nepal. Photo: Jon G. Fuller/VW Pics/Universal Images Group via Getty Images

A giant rockslide during medieval times may have taken hundreds of meters off a mountain peak high in the Himalayas, according to recent research.

The big picture: The finding provides new evidence about how the planet's highest peaks may evolve.

  • "On human timescales, mountain peaks seem eternal," researchers led by Jérôme Lavé at the Université de Lorraine in France write in the journal Nature.
  • "Yet, on geological timescales, mountain peaks are ephemeral: their shape and altitude are constantly evolving in response to the competition between tectonic uplift and erosion."
  • Glaciers can erode rock on the floors of mountain valleys, but higher in the peaks, glaciers may not form, and year-round cold temperatures limit the freezing and thawing of ice that can break rocks, leaving open questions about how the highest mountains evolve over time.

Details: Most of the valleys and basins of the Himalayas are filled with fertile soil but the Sabche Cirque basin in central Nepal is covered with rocks.

  • Researchers collected the samples from the remote location and found the deposits are breccia, or fragments of rock embedded in other rock. Features of the breccia indicated they were "generated by a single rockslide granular avalanche," they report.
  • Carbon dating of plant fossils in the rocks revealed they were all deposited at the same time, around 1200 CE.
  • The team of scientists estimated they came from 23 cubic kilometers of rock that slid from a mountain and buried the basin floor below one kilometer of rock.

What happened: The researchers think the landslide "beheaded" the summit of the nearby mountain Annapurna IV. The peak looks as though rock was sheared off but there is no evidence of erosion caused by a glacier or run-off.

  • The researchers estimate the mountain lost about 0.5 kilometers in height from the slide and was therefore once more than 8,000 meters high.
  • There are just 14 peaks of that size on Earth today — all of them in Asia's Himalayan and Karakoram mountain ranges.

The intrigue: The team suggests massive landslides like this one could shape other tall mountains.

  • Glaciers may not reach the highest peaks but they can still etch away at the lower slopes, making them steeper until the rock can collapse, possibly triggered by earthquakes.
  • The study "adds to the understanding that catastrophic processes can happen in this and similar regions," Louis Derry, a professor of earth and atmospheric sciences at Cornell University who wasn't involved in the study, writes in an email to Axios. "Fortunately such events would be rare on a human time scale."
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