What can we learn from ice collected in high mountain rockfall scars? Three examples from the Mont Blanc massif (France)

International audienceThe study of the ice (cryostructures, cryostratigraphy, types and age) that partly fills the fractures in rockwall permafrost is necessary to better understand the increasing frequency/volume of rockfalls in the high alpine rockwalls. Typically inaccessible, this ice is visible only when a rockfall occurs, exposing previously hidden ice. Over the past two years, the two largest rockfalls of the decade in the Mont Blanc massif and a smaller one allowed to start the documentation of the rockwall permafrost ice. This poster aims to present the three cases. Up to now, dating has been carried out for only one of the three cases. How to date permafrost ice? Dating ice cores from alpine glaciers has shown the wide time span covered by these, from a few decades to several thousand years. Several dating methods are available to quantify the age. Counting the annual layers, which proved to be effective, is widely used in the study of mountain glaciers but cannot be used for the internal ice of the permafrost since it does not present these annual layers. Structure of the ice in the scar of the Trident of Tacul rockfall scar shows nevertheless a series of layers. We will therefore use radionuclide dating to obtain the absolute age of the ice. So far, only one age is available for interstitial ice from permafrost-affected rockwall: the ice sampled from a felt block that is part of the deposit of the 150'000 m 3 rockfall that occurred in Feb. 2014 at Piz Kesch (Swiss Alps