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Theoretically, mountains that grow upward due to the bending of the earth’s crust following the collision of tectonic plates, can rise to an unlimited height. However, this does not happen in practice. A series of geological processes, including the effect of erosion caused by natural glaciers, the effect of rain and continuous cracking due to freezing and melting of water, reduces their height. In a paper recently published in the journal Nature, geologist Jerome Lave of the University of Lorraine describes another intriguing mechanism.
According to the magazine Economist, Dr. Lave has collected evidence that around 1190, a large landslide probably reduced the height of Annapurna 4 by 500 meters. Annapurna 4 is a mountain in the Himalayan mountain range, which today is about 7500 meters high.
If Lave’s argument is correct, this would be one of the largest landslides ever recorded. The fallen mountain peak could have displaced 27 cubic kilometers of rock. The energy released from the collapse of the rocks was almost six times the energy of the tsar’s bomb; The largest atomic bomb ever detonated.
“I can’t imagine what that would have sounded like,” said Ann Rowan, a geologist at the University of Bergen who was not involved in Dr. Lave’s study.
Dr. Laveh suspected this when he was doing fieldwork in the Ganga plains of Nepal in 2012. He noticed that the ground under his feet had an unusual composition. In the 50-meter core that was taken out of the rock, the concentration of limestone was about 10%, but in a four-meter piece, its concentration increased to nearly 50%, which was very high and completely unnatural. This indicated that the stones in question had made their way from the Annapurna massif and from hundreds of kilometers away to the Ganga plain, and indicated a large landslide in the recent past (from a geological point of view).
After reviewing satellite images of the rock mass and using a helicopter to survey the site, Dr. Lave observed a large debris field that appeared to be from the same event. By visiting this place again the following year, he became the second known geologist to do such a thing and collect samples.
Examining the surrounding cliffs for signs of rockfalls, Laveau noticed that Annapurna was consistent in appearance with this event. Dr. Lave sent samples of rocks, cores, and other materials collected from the rock field to his colleagues to determine their age. If their age was almost the same, it would indicate that they are related to the same event.
His colleagues dated the samples by measuring the abundance of chlorine 36 (one of the radioactive isotopes that accumulates in surface rocks and decays after they are buried) and carbon 14 (another isotope that accumulates in living organisms and decays after their death). They determined the end of the 12th century and a few decades apart, which was within the accuracy range of dating techniques
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