Rock falls in the Mont Blanc Massif in 2007 and 2008

Abstract

Due to a lack of systematic observations, the intensity and volume of rock falls and rock avalanches in high mountain areas
are still poorly known. Nevertheless, these phenomena could have burly consequences. To document present rock falls, a network
of observers (guides, mountaineers, and hut wardens) was initiated in the Mont Blanc Massif in 2005 and became fully operational
in 2007. This article presents data on the 66 rock falls (100m3 ≤ V ≤ 50,000m3) documented in 2007 (n = 41) and 2008 (n = 25). Most of the starting zones are located in warm permafrost areas, which are most sensitive to warming, and only four
rock falls are clearly out of permafrost area. Different elements support permafrost degradation as one of the main triggering
factors of present rock falls in high mountain areas.

KeywordsRock falls-Permafrost-High alpine environments-Mountains-Mont Blanc Massif

Full text

Landslides
DOI 10.1007/s10346-010-0206-z
Received: 26 June 2009
Accepted: 11 February 2010
© Springer-Verlag 2010
Ludovic Ravanel .Françoise Allignol .Philip Deline .Stephan Gruber .Mario Ravello
Rock falls in the Mont Blanc Massif in 2007 and 2008
Abstract Due to a lack of systematic observations, the intensity
and volume of rock falls and rock avalanches in high mountain
areas are still poorly known. Nevertheless, these phenomena could
have burly consequences. To document present rock falls, a
network of observers (guides, mountaineers, and hut wardens) was
initiated in the Mont Blanc Massif in 2005 and became fully
operational in 2007. This article presents data on the 66 rock falls
(100 m
3
≤V≤50,000 m
3
) documented in 2007 (n=41) and 2008 (n=
25). Most of the starting zones are located in warm permafrost
areas, which are most sensitive to warming, and only four rock falls
are clearly out of permafrost area. Different elements support
permafrost degradation as one of the main triggering factors of
present rock falls in high mountain areas.
Keywords Rock falls .Permafrost .High alpine
environments .Mountains .Mont Blanc Massif
Introduction
In recent years, large rock avalanches, such as the Kolka–
Karmadon event in the Caucasus in 2002 (Huggel et al. 2005),
have affected high mountain areas in the world. In the Alps, those
that have occurred on the Brenva glacier (Mont Blanc Massif in
1997; Deline 2001), the Punta Thurwieser (Ortles–Cevedale Massif
in 2004; Sosio et al. 2008), or the Drus (Mont Blanc Massif in 2005;
Ravanel and Deline 2008) are some of the most recent examples. A
high number of rock falls have affected alpine rock walls during the
hot summer 2003 that strongly increased the awareness of
mountaineers, mountain guides, hut keepers, and the general
public towards the connection between climate, permafrost, and
slope stability in the Alps. These phenomena can have strong
impacts on natural hazards, high mountain infrastructure stability,
and landscape evolution (cf. Haeberli et al. 1997; Davies et al. 2001;
Gruber and Haeberli 2007; Bommer et al. 2008).
The hypothesis of a relationship between these events and the
current global warming through a degradation of the rock wall
permafrost is supported by several evidences (Gruber and Haeberli
2007): (1) physical processes linking climate and collapses exist; (2)
many collapses originates from permafrost areas; (3) cracks filled
with ice are common in high mountain rock walls and, frequently,
ice is exposed in fresh detachment scars, or seeping water can be
observed, even in very dry conditions; (4) the intense rock fall
activity of the 2003 summer heat wave points to permafrost
degradation as the only plausible explanation (Gruber et al. 2004a);
and (5) permafrost degradation has been measured and is
consistent with atmospheric warming. The increase in mean
annual air temperature in the Alps during the twentieth century
exceeded 1.5°C above 2,500 m a.s.l., with an acceleration since the
early 1980s (Casty et al. 2005). Because the frequency and volume
of rock falls and rock avalanches remain poorly known, an
observation system was initiated in the French–Italian research
project PERMAdataROC (2006–2008; Deline et al. 2008a) and is
continued in the PermaNET project (Permafrost long-term
monitoring network;http://www.permanet-alpinespace.eu) since
2008. In particular, the identification and analysis of past and
present rock falls in high alpine rock walls and the establishment of
a corresponding database in support of further research are
pursued to provide a better scientific basis in the assessment of
climate change effects on rock wall stability.
This article presents an inventory and first synopsis of rock falls
having a volume >100 m
3
in the Mont Blanc Massif during 2007
and 2008. Due to its high elevation—highest peaks exceed 4,000 m
a.s.l.—and strong precipitations, there are about 100 glaciers in the
massif and permafrost is generally present in steep bedrock above
2,800–3,000 m a.s.l. The risk that results from the combination of
steep topography, dense infrastructure below or within rock walls,
and large tourist fluxes adds direct practical relevance to those
investigations.
Study area
The Mont Blanc Massif (Figs. 1and 2), oriented SW–NE, has an
area of approximately 350 km
2
and its highest point is at
4,810 m a.s.l. Bordered by the deep valley of Chamonix in the
NW, the Val Veny in the E, and the Val Ferret in the SE, it is
characterized by an extraordinary combination of peaks and
ridges, with glaciers covering about 40% of its surface. Many of
its granitic, fractured faces and summits stand well above
3,000 m a.s.l.: the drainage divide between Rhône and Pô
basins forms a 35-km-long crest line which is continuously
above 3,300 m and locally exceeds 4,000 m a.s.l.
The Mont Blanc is mainly a granitic batholith (Fig. 1) formed
during the Hercynian orogeny by granite intrusion in the
gneissic basement (micaschists and gneiss). The Mont Blanc
summit is on the contact of these two units. The granite changes
from an intrusive position in gneiss in the SW to a tectonic
contact in the NE. Tilted towards the NW, the massif is cut in
panels by large subvertical Variscan, recurrent faults (north–
south), and alpine faults (N40–N60°E) with mylonitized zones
(shear zones). The Mont Blanc granite has a very coarse-grained
texture, with facies varying from microgranite to porphyroidic
granite. Multiple tectonic phases have broken up the rock with
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multiple direction planes that may overlap. Finally, the
combination of past and present glaciations, steep and fractured
rock walls, and strong relative relief results in high-magnitude
morphodynamics.
Method
First, data were collected in 2005 through observations made by a
small number of Italian and French mountain guides. Since 2007,
the observer network is operational with about 30 French and
Italian guides and additionally several hut keepers and rescue
teams. The Swiss and SW sides of the massif are not surveyed. In
addition, educational posters in huts and a website (http://edytem.
univ-savoie.fr/eboulements) invite mountaineers to send their own
observations. A form is filled for each observed rock fall or its
deposit, with the characteristics of the event: date, location,
weather and snow conditions, and volume. For each year, data on
identified events have been verified and completed on the field by
the beginning of autumn by one of the first authors to ensure a
good homogeneity of the recorded data. Furthermore, for 2007, the
number of rock falls that formed supraglacial deposits has also
been checked using aerial photographs at 1:20,000, dated
September 16, 2007. For the 2 years, this checking phase has not
revealed rock falls that were not reported by the observer network,
even in less frequented areas of the massif.
For each event, scar elevation, slope angle, and aspect of the
affected slopes are calculated using GIS ArcGIS 9.2 and a 50-m
digital elevation model (DEM; Fig. 3)—enhanced to 10 m for
affected areas—for the French side of the Mont Blanc Massif and a
10-m DEM for the Italian side—no DEM at a higher resolution is
available. If aspects values and slope geometry before failures have
been as far as possible checked and corrected based on maps and
orthophotos where necessary, slope angle values have to be taken
with caution because of the small scale resolution. Deposits have
been mapped on the field or from aerial photographs for 2007,
even for the smallest rock falls which usually produce deposits of
several hundreds of square meters. Their areas have been
computed with the polygon tool of the Bayo-IGN PhotoExplorer
software. The collapsed volumes and the maximum scar depths
have been computed from the dimensions of the scars, surveyed on
the field with a Laser Technology TruPulse 200 laser rangefinder or,
when impossible, from altitudes reported on scar photographs.
Thus, the maximum depths are sometimes unknown, often given a
Fig. 1 Geological map of the Mont
Blanc area (after Leloup et al. 2005 and
Rolland et al. 2003, modified). 1
Quaternary, 2Dauphinois and Helvetic
Mesozoic sediments, 3Triassic, 4
carboniferous, 5Mont Blanc granite, 6
Variscan metamorphic rocks (gneiss), 7
undifferentiated granites, 8Penninic
klippe, 9Mont Blanc shear zone
(gneiss), 10 Versoyen + Valais, 11
internal zones, 12 mapped shear zone
network, 13 thrust, 14 late reverse fault
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minima, and the uncertainty on volumes may reach 25%. The
possible presence of permafrost is estimated from an approximate
model of the mean annual ground surface temperatures (MAGST)
of the massif, carried out using an energy balance model (TEBAL;
see Gruber et al. 2004b). MAGST values are not presented because
rock wall surface temperatures are modeled for the period 1982–
2002 based on meteorological data from Corvatsch and Jungfrau-
joch stations (Switzerland) and not from the Aiguille du Midi
station (where air temperature is the only data measured, since
only February 2007); only a qualitative index of the probability of
Fig. 3 A part of the 50-m DEM used
for the study (enhanced to 10 m in the
areas affected by rock falls). Circles rock
falls in 2007 (red) and 2008 (yellow)
Fig. 2 Rock falls occurred in the Mont
Blanc Massif in 2007 (red) and 2008
(yellow). Dashed red line surveyed zone;
1Droite, 2Tour des Grandes Jorasses, 3
Dent de Jethoula, 4Tré-la-Tête
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Table 1 Characteristics of the 45 rock falls of 2007 in the Mont Blanc Massif
Site Date Location (extended Lambert II
coordinate system)
Elevation of the scar
centre (m a.s.l.)
Slope
(°)
Rock type Aspect
(°)
Deposit
area (m
2
)
Collapsed
volume (m
3
)
Max. scar
depth (m)
Permafrost occurrence Ice seen in
the scar
Granite Gneiss Unlikely Possible Likely
Dent de Jethoula R 01/08 X0958.096 Y2103.945 2,810 65 × 180 >60,000 15,000 >10 × No
Tour des Grandes
Jorasses
R 30/09 X0960.968 Y2106.695 3,830 70 × 160 20,000 10,000 >4 × Yes
Droites F 06/09 X0960.895 Y2114.435 3,360 69 × 30 20,000 7,000 >6 × Yes
Aiguille Passon F 31/08 X0960.105 Y2119.294 3,060 43 × 15 4,500 5,000 2 to 3 × ?
Aiguilles Marbrées F 12/09 X0957.370 Y2104.984 3,430 58 × 240 10,000 4,000 <2 × ?
Lex Blanche R 01/08 X0947.223 Y2097.516 3,500 53 × 90 700 3,500 ∼6 × Yes?
Rognon inf. du Plan F 17/01 X0954.830 Y2108.538 3,310 77 × 220 450 3,000 ∼7×?
Brêche des Périades F 12/09 X0958.758 Y2107.888 3,400 48 × 310 2,500 2,500 3 to 4 × ?
Aiguilles Marbrées R 20/09 X0957.323 Y2105.093 3,430 62 × 290 ? 2,500 ∼4 × Yes
Pointes des Hiron-
delles
F 04/08 X0961.940 Y2107.913 3,410 57 × 150 20,000 2,000 ? × ?
Aiguilles du Tacul F 12/09 X0958.677 Y2108.761 3,280 43 × 180 6,000 2,000 <2 × ?
Arête des Grands
Mulets
R 07/07 X0951.168 Y2105.665 3,270 60 × 65 3,000 1,500 ? × ?
Aiguille du Peigne F 30/08 X0953.964 Y2109.958 2,860 59 × 35 20,000 1,500 ∼4×No
Petites Aiguilles
Rouges du Dolent
F 06/09 X0964.161 Y2115.395 3,450 64 × 350 2,000 1,500 ? × ?
Arête des Grands
Mulets
F 25/05 X0951.168 Y2105.665 3,270 60 × 65 2,500 1,200 ? × ?
Aiguille du Tacul F 12/09 X0958.375 Y2108.914 3,010 57 × 325 4,000 1,200 2 to 3 × ?
Rognon des Grands
Charmoz
F 30/08 X0954.716 Y2110.956 2,700 63 × 340 5,000 1,000 2 to 3 × No
Eperon de la Brenva F 21/09 X0953.133 Y2102.975 3,560 43 × 150 9,000 1,000 ? × Yes
Aiguille du Midi F 21/07 X0953.855 Y2108.800 2,940 50 × 320 4,000 900 ∼5 × Yes
Aiguille des Pélerins F 30/08 X0954.225 Y2109.855 2,960 65 × 10 17,000 800 <2 × ?
Aiguille à Bochard F 07/09 X0957.738 Y2115.763 3,010 60 × 350 2,000 800 <2 × No
Arête des Grands
Mulets
F 22/04 X0951.168 Y2105.665 3,270 60 × 65 2,000 700 ? × ?
Rognon du Dolent F 06/09 X0963.620 Y2115.548 3,280 46 × 315 5,000 600 ? × ?
La Noire F 12/09 X0957.048 Y2106.918 3,200 43 × 180 2,500 600 <2 × ?
Arête Freshfield R 19/06 X0955.083 Y2103.898 3,610 56 × 70 3,000 448 4 × Yes
Arête Freshfield F 28/08 X0955.083 Y2103.898 3,610 56 × 70 1,000 Yes
Col sup. de la Noire F 12/09 X0957.752 Y2106.352 3,470 54 × 260 600 200 <2 × ?
Arête inf. des Cosmi-
ques
R 16/07 X0952.990 Y2107.169 3,600 39 × 140 100 180 2 to 3 × Yes
Dent du Géant F 29/06 X0958.094 Y2105.878 3,650 55 × 260 500 150 <2 × Yes?
Pointe Isabelle F 29/06 X0962.473 Y2112.373 3,270 57 × 315 800 150 <2 × ?
Pointes des Hiron-
delles
F 04/08 X0962.033 Y2108.013 3,250 58 × 40 800 150 ? × ?
La Noire F 12/09 X0957.256 Y2106.822 3,340 52 × 210 700 150 2 to 3 × ?
Aiguille du Midi F 14/07 X0953.560 Y2108.765 2,780 53 × 340 8,000 >100 <2 × ?
Grands Charmoz R 17/07 X0955.655 Y2110.868 3,060 54 × 100 ? >100 ? × ?
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existence of permafrost for each sector is proposed. Permafrost is
considered unlikely, possible, and likely when MAGST is >1°C,
between 1°C and −1°C, and <−1°C, respectively.
Results: 45 observed rock falls in 2007, 21 in 2008
The database compiles the characteristics of the 45 and 21 rock falls
observed in 2007 and 2008, respectively, in the Mont Blanc Massif
(Tables 1and 2; Fig. 2).
Most of these events took place between July 15 and September
15. Few were later (up to September 30 for the Tour des Grandes
Jorasses event in 2007 on the Italian side of the massif) or earlier
(especially the first two rock falls of the Arête des Grands Mulets, in
April and May 2007)—sometimes much earlier (collapse of the
Rognon Inférieur du Plan, in January 2007).
Fifty-five rock fall events (83%) occurred above 3,000 m a.s.l.
while only 48% of the studied rock walls (elevation >2,000 a.s.l.,
slope angle >37°, not covered by glaciers) are located above this
altitude. Thirty-two rock fall events (48%) occurred between 3,200
and 3,500 m a.s.l. (19% of the study area), none above 3,900 m (4%
of the study area). The highest scar is the one on Tour des Grandes
Jorasses (3,830 m a.s.l.; Fig. 4), on the Italian side.
A large number of rock falls (23%) detached on slopes with an
angle in the range 53–57° (17% of the study area), but this value is
probably underestimated, given the low resolution of the DEM.
Rock falls particularly affect west and north faces, which do not
correspond to the main class of the rock walls distribution in terms
of aspect (SW).
The most important rock fall was at Tré-la-Tête (50,000 m
3
,
aspect 80°; Deline et al. 2008b; Fig. 5) in September 2008, on the
Italian side. The second largest rock fall occurred at the Dent de
Jethoula in August 2007 (15,000 m
3
, aspect 180°), also on the Italian
side, at one of the lowest elevations (2,810 m a.s.l.). The two other
main events affected the Tour des Grandes Jorasses (10,000 m
3
,
aspect 160°, Italian side; Fig. 4) and the Droites (7,000 m
3
, aspect
30°, French side), in 2007. Their scars are both located at high
elevation (3,830 and 3,620 m a.s.l., respectively).
Discussion
Rock falls, as most of the instabilities in rock slopes, are usually
related to existing fractures (mesoscale and fine-scale fracturation
is poorly studied in the Mont Blanc Massif), along which a rock
mass is destabilized by a triggering factor. The permafrost
degradation could be an important one. Only four events (6%)
are clearly out of the permafrost area. The 41 events (61%) that
occurred where permafrost presence is likely could be related to
permafrost degradation (active layer formation, active layer
thickening, or warming at depth). Historical studies that are
currently developed (e.g., Ravanel and Deline 2008) support this.
They point out a clear evolution with a strong correlation between
rock fall occurrences and the warmest periods over the last
150 years (see also Evans and Gardner 1989). It is to note that the
years 2007 and 2008 have, respectively, the seventh and eighth
highest mean annual temperatures in Chamonix for a century
(MétéoFrance data) and probably for at least 500 years (see Casty
et al. 2005). About 90% of the events took place during summer,
i.e., the hottest period of the year (Fig. 6). Massive ice has besides
been observed in about 12 scars (Fig. 4). This observation largely
corroborates the ice-filled fractures thawing. Bonding of ice-filled
fractures and its reduction or loss during degradation can be
related to a combination of ice/rock interlocking and ice/rock
Site Date Location (extended Lambert II
coordinate system)
Elevation of the scar
centre (m a.s.l.)
Slope
(°)
Rock type Aspect
(°)
Deposit
area (m
2
)
Collapsed
volume (m
3
)
Max. scar
depth (m)
Permafrost occurrence Ice seen in
the scar
Granite Gneiss Unlikely Possible Likely
Aiguille du Midi F 12/09 X0953.383 Y2108.495 3,020 48 × 330 ? >100 ? × ?
Aiguille de Talèfre F 12/09 X0961.795 Y2110.515 3,430 48 × 235 8,000 >100 <2 × ?
Aiguille de Thoules F 12/09 X0955.985 Y2104.245 3,310 38 × 120 3,500 >100 <2 × ?
Courtes F 12/09 X0961.545 Y2113.318 3,320 44 × 220 ? >100 <2 × ?
Aiguille Pierre Joseph F 29/06 X0960.579 Y2110.935 3,060 43 × 325 800 100 <2 × ?
Aiguille de Blaitière F 16/07 X0954.483 Y2110.565 2,870 69 × 290 700 100 4 × No
Aiguille des Pélerins F 21/07 X0954.218 Y2109.700 3,250 38 × 310 ? 100 3 to 4 × ?
Arête inf. des Cosmi-
ques
R 29/07 X0952.972 Y2107.156 3,580 48 × 140 300 100 < 2 × ?
Aiguille du Tacul R 24/08 X0958.264 Y2108.845 2,880 67 × 280 500 100 ? × ?
Rognon du Dolent F 06/09 X0963.558 Y2115.531 3,200 64 × 275 400 100 <2 × ?
Pointe de Pré Bar F 06/09 X0964.498 Y2113.460 3,230 59 × 335 300 100 <2 × ?
Means 3,253 55 40 5 202 ∼7,000 >1,600 ∼3.3
Total (45) >260,000 >73,000 3 12 30 >10
Uncertainty on volumes can reach 25%
Rthe date of the rock fall, Fthe date of the first observation of the deposit
Table 1 Continued
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Table 2 Characteristics of the 21 rock falls of 2008 in the Mont Blanc Massif
Site Date Location (extended Lambert II
coordinate system)
Elevation of the scar
centre (m a.s.l.)
Slope
(°)
Rock type Aspect
(°)
Deposit
area (m
2
)
Collapsed
volume (m
3
)
Max. scar
depth (m)
Permafrost occurrence Ice seen in
the scar
Granite Gneiss Unlikely Possible Likely
Epaule orientale de
Tré-la-Tête
R 10/09 X0948.895 Y2097.758 3,400 66 × 80 150,000 50,000 >20 × ?
Aiguille de Thoules R 09/07 X0956.068 Y2104.465 3,410 49 × 162 10,000 5,000 ∼6×?
Arête des Grands
Montets
F 07/08 X0958.879 Y2115.033 3,600 40 × 290 ? 3,500 8 × Yes
Aiguille du Midi F 29/06 X0952.905 Y2108.520 2,920 56 × 315 7,000 3,000 ? × ?
Périades F 26/06 X0958.397 Y2106.941 3,270 52 × 299 4,000 1,500 5 × ?
Aiguille du Char-
donnet
F 03/08 X0960.505 Y2118.177 3,060 53 × 287 3,000 1,500 3 × ?
Aiguille à Bochard F 23/06 X0957.710 Y2115.740 3,070 60 × 358 4,000 1,200 5 × ?
Aiguille du Passon F 26/06 X0960.295 Y2119.220 3,110 52 × 344 2,500 1,200 2 × ?
Aiguille du Tacul F 26/06 X0958.440 Y2109.044 3,090 41 × 276 8,000 1,000 > 2 × ?
Aiguille du Tacul F 14/08 X0958.750 Y2109.273 3,000 58 × 339 ? 900? ∼5 × Yes
Grands Charmoz R 23/08 X0954.720 Y2110.950 2,720 69 × 339 2,000 500 2 × ?
Aiguilles Marbrées F 14/08 X0957.385 Y2104.983 3,480 56 × 244 1,000 400 4 × ?
Tour Ronde R 20/08 X0955.093 Y2104.250 3,450 43 × 48 1,200 400 ? × ?
Droites R 24/06 X0960.015 Y2113.873 3,640 49 × 211 ? >300 <2 × ?
Droites F 23/07 X0960.885 Y2114.395 3,520 44 × 61 ? 300? ? × ?
Aiguille de Blaitière F 26/06 X0954.493 Y2110.518 2,880 72 × 174 ? 250 <1 × ?
Aiguilles d’Entrèves F 29/06 X0955.938 Y2103.822 3,250 55 × 59 800 250 2 × ?
Aiguille du Midi R 05/08 X0952.403 Y2107.696 3,000 65 × 295 ? >200 ? × ?
Aiguille du Goûter F 12/06 X0949.154 Y2105.317 3,380 40 × 8 300 200 3 × ?
Mont Dolent F 18/08 X0965.270 Y2113.390 3,530 55 × 85 ? >100 ? × ?
Pointe Adolphe
Rey
R 05/08 X0954.460 Y2105.146 3,420 49 × 22 ? >100 ? × ?
Means 3,248 54 18 3 205 ∼15,000 >3,200 ∼4,6
Total (21) >194,000 >67,000 1 9 11 2
Uncertainty on volumes can reach 25%
Rthe date of the rock fall, Fthe date of the first observation of the deposit
Recent Landslides
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adhesion (Gruber and Haeberli 2007). Moreover, many events
have originated from ridges and spurs, possibly due to more
rapid thaw in such geometries (Noetzli et al. 2007). Two of the
three main events, the Tour des Grandes Jorasses and the Tré-la-
Tête events, occurred in September, i.e., when the active layer
(i.e., the top layer of the permafrost that thaws during the
summer) is almost the deepest (see Gruber et al. 2004a). The
parameter “permafrost”could also explain the development of
collapses in cold and deemed stable north faces. The average
altitude of scars on north-facing slopes is indeed well smaller
(3,090 m a.s.l.) than the one of the west-facing (3,270 m a.s.l.) and
especially the ones of the east-facing (3,390 m a.s.l.) and south-
facing slopes (3,370 m a.s.l.). This asymmetry is consistent with
the temperature distribution at and below the surface of steep
rock walls (see Noetzli et al. 2007). However, there is no clear
trend regarding the orientation of the rock walls affected by the
most important rock falls: among the six events with a volume
≥5,000 m
3
, three have affected south faces, one a west face, one a
NE face, and one a north face. Several years of observations are
probably necessary to establish a relationship between aspect and
volume of the scars.
Mean annual air temperature of 2007 and 2008 at the Aiguille
du Midi are quite the same (−7.5°C and −7.8°C, respectively) as the
summer temperatures (Fig. 6) and cannot explain the significant
difference in number of events between 2007 and 2008. Only the
April mean temperature was really higher in 2007 (−5.8°C) than in
2008 (−12.2°C). So, the thawing period should have begun earlier in
2007 than in 2008. Concerning precipitations, summer 2007 has
been largely wetter than summer 2008 (Fig. 6). With higher air
temperatures, percolating water in fractures could have more
degraded permafrost by advection of heat, in complement of
slower heat conduction from the surface (see Gruber and Haeberli
2007). This may explain, at least in part, the difference in number
of events (45 rock falls in 2007, 21 in 2008).
Fig. 4 The September 2007 rock fall at
the Tour des Grandes Jorasses, seen
from the bottom of the rock wall, and
its scar. Black arrow shows massive ice
still present in the scar 2 weeks after
the rock fall. The seeping water in the
lower part of the scar corresponds to
just melt ice
Landslides

Fig. 5 The rock fall of Tré-la-Tête
occurred in September 2008. Left
September 2005, right October 2008
(ph. M. Tamponi)
Fig. 6 a 2007 and 2008 monthly
means of the daily temperature at the
Aiguille du Midi (3,842 m a.s.l.); b2007
and 2008 monthly precipitation
amounts in Chamonix (1,042 m a.s.l.).
Data from MétéoFrance
Recent Landslides
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Concluding remarks and prospects
Developed since 2005, a network of rock fall observers in the Mont
Blanc Massif surveys for the first time and as exhaustively as
possible the rock instability in high alpine steep rock walls. In 2007
and 2008, 66 events were observed and documented. Most of the
starting zones are located in warm permafrost areas (0 to −5°C; see
Noetzli et al. 2003), which is most sensitive to warming. For several
rock falls, massive ice has been observed in the detachment zone;
this supports the relevance of the thaw of the ice, which fills
fractures in high alpine rock walls (“ice-cemented”).
Permafrost conditions seem today more and more important
because warming is thought to be a mechanism through which
climate controls rock wall stability and, consequently, natural
hazard in mountain areas. Thus, to study the role of permafrost
degradation in rock fall triggering, subsurface rock temperature
has to be modeled for each rock fall scar. A standard statistical
analysis of the distribution of rock walls, according to elevation
and aspect, is in progress. It is complemented by historical
research to characterize the recent evolution of the frequency and
volume of rock falls, which is essential to argue that global
warming is affecting rock fall triggering through permafrost
degradation.
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L. Ravanel ()).F. Allignol .P. Deline
Laboratoire EDYTEM, Université de Savoie, CNRS,
73376 Le Bourget-du-Lac, France
e-mail: Ludovic.Ravanel@univ-savoie.fr
S. Gruber
Glaciology, Geomorphodynamics and Geochronology, University of Zurich,
Winterthurerstrasse 190,
CH-8057 Zurich, Switzerland
M. Ravello
11015 La Salle, AO, Italy
Landslides