https://www.economist.com/science-and-technology/2023/07/06/a-gigantic-landslide-shows-the-limit-to-how-high-mountains-can-grow

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[                    ]
Science & technology | Hitting peak peak

A gigantic landslide shows the limit to how high mountains can grow

Enough rock fell off a Himalayan peak to bury Paris to the height of
the Eiffel Tower

[20230708_STP001]Not what it used to be image: Alamy
Jul 6th 2023
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In geology, unlike business, nothing is too big to fail. Mountains
offer the most spectacular example. Pushed up by the crumpling of
Earth's crust following the collision of tectonic plates, they could
in theory keep rising almost indefinitely. In practice, they do not.
A suite of geological processes--including the grinding of glaciers,
the gentle impact of rain, and forcible cracking by freezing and
thawing of water--erode them down to size.

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In a paper published in Nature Jerome Lave, a geologist at the
University of Lorraine, describes another, much more spectacular
mechanism. Dr Lave has collected evidence suggesting that, in around
1190, an enormous landslide slashed perhaps 500 metres from the
height of Annapurna IV, a mountain in the Himalayas that stands about
7,500 metres high today. If he is right, it would be one of the
biggest landslips ever recorded. The falling mountain top would have
displaced up to 27 cubic kilometres of rock--roughly enough to bury
the entirety of Manhattan to about the height of the Empire State
Building.

As the rubble crashed down, the energy released would have been
equivalent to around six times that of the Tsar Bomba, the biggest
nuclear weapon ever detonated. "I don't think I could imagine what it
would sound like," says Ann Rowan, a geologist at the University of
Bergen who was not involved in Dr Lave's work.

Dr Lave's suspicions were aroused while doing fieldwork in the Ganga
plain in Nepal in 2012. He noticed that the ground beneath his feet
had an unusual composition. A 50-metre core drilled out of the rock
showed an average concentration of limestone of around 10%. But for
one 4-metre stretch the concentration rose to nearly 50%, "which is
enormous, and completely abnormal", he says.

This suggested that the rocks in question had made their way to the
Ganga plain from the Annapurna massif, hundreds of kilometres away.
That, in turn, hinted at a massive landslide in the (geologically)
recent past. After examining satellite images of the massif, and
taking a helicopter ride to have a look for himself, Dr Lave spotted
a large rubble field which looked like it could have been caused by
the same event. So he visited the site the following year, becoming
only the second geologist known to have done so, and took some
samples. Examining the surrounding cliffs for signs of a collapse, he
noticed that a peak known as Annapurna IV offered a relatively
smooth, steep face which seemed to fit.

Back home, he sent samples from the rubble field, the rock core and
others from the path the landslide might have taken for dating.
Should their ages roughly correspond, that would suggest they were
linked to the same event. By measuring the abundance of chlorine-36
(a radioactive isotope which accumulates in surface rocks and decays
once they are buried), and carbon-14 (another which accumulates in
living matter and decays after death), his colleagues dated the
samples to the late 12th century, and to within a couple of decades
of each other. That is within the accuracy limits of the dating
techniques themselves.

Besides shedding light on a previously unknown cataclysm, Dr Lave's
work could plug a gap in the dominant explanation for why mountains
stop growing, which is known as the "glacial buzzsaw" hypothesis.
Under this model, it is glaciers, which are extremely effective at
carving scoops out of mountains, that are mostly responsible for
curbing their growth.

The problem with that theory, says Dr Rowan, is that there are some
peaks that manage to escape the erosive effect of glaciers, and then
grow so steeply that glaciers can no longer stick to their sides.
"The question is," she asks, "what stops these mountains getting
bigger?"

Landslides could well be one answer. While the exact trigger for the
Annapurna landslide is unknown, Dr Lave's idea is that, with nothing
to shave rock off their tips, very high mountains simply keep growing
until their weight is too much for their lower slopes--which do still
experience erosion--to support.

Working out exactly how and when the tipping point is reached will
require examining other such rockslides. Unfortunately, due to the
actions of both glaciers and swollen rivers during the monsoon
season, the rubble from the Annapurna landslip is vanishing fast. Dr
Lave reckons that only about 10% of the dislodged material now
remains in place. Older rockslides, assuming there were any, may
already be impossible to reconstruct. #

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This article appeared in the Science & technology section of the
print edition under the headline "When mountains reach peak peak"

Science & technology July 8th 2023

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    grow
  * A Belgian company wants to create woolly-mammoth burgers

The future of war: A special report

From the July 8th 2023 edition

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