14 years of LiDAR monitoring and insights into ice of rockwall permafrost: the east face of the Tour Ronde (3792 m, Mont Blanc massif)

The increasing rockfall frequency in high mountain rockwalls is generally associated with global warming via the permafrost warming but long series of high resolution data on rockfall are still necessary to better appreciate the evolution of their frequencies and volumes, and to better understand their triggering factors.

Here we present an inventory of rockfalls surveyed by terrestrial laser scanning (LiDAR) since 2005 in the east face of the Tour Ronde (3792 m a.s.l.) in the Géant glacial basin (Mont Blanc massif).

Between 2005 and 2018, the rockwall was scanned 12 times, giving 11 comparisons of 3D models [1]. These highlighted a very intense morphodynamics with 91 destabilizations with volumes between 1 and 15,578 m³ for a total volume of 31,610 m³ (mean erosion rate: 29,8 mm.yr^-1). In the first year of measurement, the Bernezat spur was affected by a collapse of more than 700 m³ [2]. Then, it was affected by rockfalls not exceeding a few tens of m³. On the other hand, in the rest of the face, there is a very strong increase in rockfall activity, especially during the hot summer 2015 at the end of which (August 27) the most voluminous collapse of the whole period occurred.

The modelled surface temperature distribution at the scale of the Mont Blanc massif [3] attests to the presence of permafrost throughout the rockslope, confirmed by temperature measurements carried out at 3, 30 and 55 cm deep in the rock at the base of the Bernezat spur between October 2006 and May 2009. In addition, the main collapses left massive ice, at the level of their scar, more or less mixed with rock debris. These different elements, associated with the fact that collapses occur essentially during and following the highest summer heat, point to the role of degradation of permafrost [4]. A collapse on December 4, 2018 at the level of the small spur located at the foot of the Bernezat and whose volume is estimated at 7000 m³ reinforces this hypothesis since the detachment surface was covered - except for its margins - by massive ice. This has been sampled and its dating will perhaps confirm the age of the ice present in the cracks of the permafrost-affected rockwalls of the Mont Blanc massif. In 2017, a collapse of 44,000 m³ in the north face of the Aiguille du Midi (3842 m a.s.l.) had exposed 4060 calBP ice. In the Tour Ronde case, ice/snow cover changes and glacial debutressing could also partly explain the rockfall activity.

 

 

[1] Ravanel L. et al. (2010). Revue Française de Photogrammétrie et de Télédétection, 192 : 58-65.

[2] Rabatel A. et al. (2008). Geophysical Research Letters, 35: L10502.

[3] Magnin F. et al. (2015). Geomorphologie, 21: 145-162.

[4] Ravanel L. et al. (2017). Science of the Total Environment, 609: 132-143.