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International audience; In the high mountain rock walls of the Mont Blanc massif, changes in the granite surface colour are related to its exposure age. The light grey colour of fresh rock surfaces turns orange when is long exposed to weathering. In order to study this colour/age relationship, reflectance spectroscopy was performed on 73 samples, and Terrestrial Cosmogenic Nuclide (TCN) dating was used to obtain their surface exposure age. The standard deviation of the reflectance values was calculated for each wavelength of the visible spectrum to study the behaviour of each spectral region. The aim was to find two colour regions that showed opposite behaviour, and once they are combined, they could provide a representative index of the rock colour changes that are linked to the degree of weathering.As an adaptation of the Green Red Vegetation Index used to measure colour changes in vegetation, the GReen-Infrared GRanite Index (GRIGRI), a normalized difference between the granite 770 nm and 530 nm reflectance values, was developed. The GRIGRI value of a weathered granite surface has a close relationship with its exposure age (R2 = 0.85). The reflectance spectra of seven samples for which TCN dating failed and two samples for which the TCN age was considered to be an outlier were used to calculate their GRIGRI value to assess the colour-based ages, that were plausible according to rock wall morphology and the TCN exposure ages of the surrounding surfaces. We propose a new method of surface dating for the rock walls of the Mont Blanc massif using reflectance spectroscopy.

Rockfalls and rock avalanches are a recurrent process in high mountain areas like the Mont Blanc massif. These processes are surveyed due to the hazard they present for infrastructure and alpinists. While rockfalls and rock avalanches have been documented for the last 150 years, we know very little about their frequency since the Last Glacial Maximum (LGM). In order to improve our understanding, it is imperative to date them on a longer timescale. A pilot campaign using Terrestrial Cosmogenic Nuclide (TCN) dating of five samples was carried out in 2006 at the Aiguille du Midi (3842 m a.s.l.). In 2011, a larger scale study (20 samples) was carried out in five other test sites in the Mont Blanc massif. This paper presents the exposure ages of the 2011 TCN study as well as the updated exposure ages of the 2006 study using newer TCN dating parameters. Most of these exposure ages lie within the Holocene but three ages are Pleistocene (59.87 ± 6.10 ka for the oldest). A comparison of these ages with air temperature and glacier cover proxies explored the possible relationship between the most active rockfall periods and the warmest periods of the Holocene: two clusters of exposure ages have been detected, corresponding to the Middle Holocene (8.2–4.2 ka) and the Roman Warm Period (c. 2 ka) climate periods. Some recent rockfalls have also been dated (