The Fall of the Drus

The Drus before the 1977 rock fall

From Ludovic Ravanel, a young geomorphologist who has studied in depth the evolution of the Drus over the years, we learn that from 1860, date of the first valid photo of the Drus until the beginning of the XXth century nothing much was observed. From 1905 untill 2011, 11 rock falls occured. The first one - 20000 m3 - in 1950 modified the West face of the Drus over a 100 m between the heights of 3220 m and 3330 m.

The second significant rock fall - 27000 m3 - occured in September 1977.
Two alpinists were bivouaking on the face. They were be taken off by helicopter.

This photo is taken before this rock fall.

To visualise the volume represented note that 1000 m3 of rock fall is equivalent to at least a block of 20 m2.

The Drus is of course not the only mountain to suffer. In 1997 2 millions m3 disappeared from the Brenva on the Italian side of Mont Blanc. However, it is the best observed and best researched of all so greatly helping the understanding of the causes. Pillars and crests are the most exposed parts of the mountains.

The Drus after the 2005 huge rock fall

In 2003, the summer was so warm, that rock avalanches occured everywhere. The danger was such that even the classic route of the Mont Blanc by the Dome du Gouter had to be banned. A rock fall of 6500 m3 occured on The Drus but went unnoticed as the mountain range was deserted due to the very high rock fall threats. On the summit of the Matterhorn, 1000 m3 of rock fall down requiring the rescue of 90 alpinists by helicopter.

Two years later, on the 29th and 30th of June 2005 a serie of huge rock falls distroyed completely the Bonatti pillar - 90000 m3 in all - Between June and October of 2005 in all 265000 m3 of rocks fell down. The damage covered 700 m in height and 100 m wide and two more followed in September.

The Drus at the end of 2011

During 6 years, nothing much happened. Then, on the 10th and 11th of September 43000 m3 of rock fell down. Ice appeared from the crack: the thermafrost!

This photo was taken by myself on the 11/09/2011 at 13h26 from Les Mottets facing the West face of the Drus, after having climbed the 'Caline' route. The cloud of dust must have remained for at least a good hour and totally obscured the bottom of the face. There were two rock avalanches, the second some 10 minutes after the first. The previous one on the 10th was not as big: only 13000 m3!

Evolution until 2005

Successive Rock falls on the Drus until 2005 (Ludovic Ravanel et Philippe Deline)

Conclusion by the specialist Ludovic Ravanel:

From 1855 until 1950, the situation stayed stable. Then in the second half of the XXth century the rock falls were increasing. During the two decades of 1990 and 2000, the warmest, "it exploded in frequency and volume".

"The falls occur during the warmest periods or at the end of them".
"The permafrost is the triggering factor".
"The global warming +2% in Chamonix since 1936 will increase the phenomenom".
"Most probably rock falls high in our mountains will occur more frequently and be bigger according to the increase of temperatures, even during the colder seasons'.

Details of the 1997 rock fall

Fig. 6 – Upper half of the rockfall scar of September 1997 (ph. J.-C. Marmier, 1998).

White arrows: fractures plans N 30°E – 75°N; black arrows: fractures plans N 120°E – 80° NW; dotted line: scar of the 1992 rock fall.

Links

The superb study done by Ludovic Ravanel and Philip Deline

The 1905 Rock fall

Left: ca1900 (Jullien brothers) from la Pierre à Bayer, 2090 m a.s.l, view angle, N115°; right: 1908 (unknown author) from le Montenvers, 1913 m a.s.l, view angle, N100°; centre: the 1905 rockfall scar, seen during the 1905-1906 winter (unknown author); dotted line: scar of a rockfall that occurred before the end of the LIA.

This Rock fall was due to an earthquake on the 13th August 1905. It destroyed the top of the Argentière church, strength estimated at 5-6 and VII intensity, and as well the top of the Pic sans Nom reduced by 9 m (15 m in 1860). No more rock fall was observed then until the mid 1930s. A photo taken by frendo in 1940 shows a small clear mark, typical of a small rock fall estaimated at around 5500 m3.

The 2005 rock fall in detail

Characteristics of the last rock avalanches on the Brenva Glacier (Mont Blanc massif).
A: scars (thick lines) and tracks (thin lines) of the rock avalanches of the 19 November 1920 (white lines) and 18 January 1997 (black lines); B: 1997 scar on the Eperon de la Brenva (G. Mortara, February 1997); C: 25-m-high ridges of the 1997 mixed deposit in the distal area (E. Dal Molin, 18 January 1997).
A huge rock fall estimated at 2 millions m3 on the Brenva spur triggered a snow avalanche and land slide on the 18th January 1997 killing 2 skiers and damaging a hotel.

The Brenva huge 2 millions m3 rock fall

Characteristics of the last rock avalanches on the Brenva Glacier (Mont Blanc massif).
A: scars (thick lines) and tracks (thin lines) of the rock avalanches of the 19 November 1920 (white lines) and 18 January 1997 (black lines);

B: 1997 scar on the Eperon de la Brenva (G. Mortara, February 1997); C: 25-m-high ridges of the 1997 mixed deposit in the distal area (E. Dal Molin, 18 January 1997).

Images

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