Frequency of rockfalls in the permafrost-affected rock walls of the Mont Blanc massif in relation with post-glacial warm periods
Résumé
The characterization of the evolution of rockfall (> 100 m3) in high mountain areas is a prerequisite to any risk management. This study aimed to collect and analyse data on past rockfall in the Mont Blanc massif in order to investigate the possible correlation between periods of warming and rockfall and, beyond, between permafrost degradation and rockfall. This required to compile exhaustive inventories and to compare them with climatic data. We first documented the occurrence of the 58 rockfalls at the Drus and the Aiguilles de Chamonix, documented by photo-comparison since the end of the Little Ice Age (LIA) and direct observation in the recent period. We then crossed these rockfall data with available climate data. In the West face of the Drus, air temperature seems to control the triggering of the 12 documented rockfalls, as suggests by their concomitance with the hottest periods. The main one (2005, 265 000 m3) seems to have been promoted by the combination of heat and heavy rainfalls, resulting in high fluid pressure in the rock fractures. Surface runoff has been observed in the scar of this event and massive ice was also observed after the rockfall of October 2011. The role of climate is also demonstrated by the analysis of the 46 rockfalls documented on the North side of the Aiguilles de Chamonix after the LIA, with a very strong correlation between these and the hottest periods: 70 % of the rockfalls occurred during the past two decades, characterized by the increasing global warming. Heat waves are particularly prone to rockfall: the maximum rockfall frequency occurred during the 2003 Summer heat wave. To investigate beyond the last century and a half, we carried out surface exposure dating on 20 samples of granite from 6 rockwalls of the massif by the determination of 10Be-concentrations in their surface. The surface ages found vary from less than a few hundred years to tens of thousands years. These dates are generally corresponding to warm periods.