Recent Brenva rock avalanches (Vallev of Aosta): new chapter in an old story?

Abstract

Two rock avalanches have affected the Brenva Glacier (Mont Blanc Massif, Italy) in the 20th Century, in 1920 and 1997. The low-friction substrate offered by the glacier caused excessive run-out distances, and both avalanches reached the valley floor. In both cases, vertical and horizontal travel distances exceeding respectively 2 and 5 km were attained. The 1920 deposit on the glacier caused an advance until 1940, the only one of this kind in the Alps. These rock avalanches, each triggered by 2-3 x 106 m3 rockfall, occurred during opposite phases of the glacial cycle (advance in 1920, retreat in 1997), but a large part of the mobilized rock and ice mass was spread on the distal side of the right-lateral moraine in both events. Historical and mainly geomorphological evidence suggests that several rock avalanches have occurred during the Holocene. The most recent, except those of this century, might have taken place in the 14th Century. These rockfalls might be in relation with the modification of rockwall permafrost due to global warming (+0.50°C during the 20th Century, +1.25°C in the Alps), although the understanding of this possible link remains insufficient.