Active Rock Glaciers at Croda Rossa/Hohe Gaisl, Eastern Dolomites (Alto Adige/South Tyrol, Northern Italy)

Abstract

Two active rock glaciers occur in the eastern and northeastern cirques Cadin del Ghiacciaio and Cadin di Croda Rossa at Croda Rossa in the Eastern Dolomites, South Tyrol (northern Italy). Both rock glaciers display a tongue-shaped morphology with typical surface morphology of transverse ridges and furrows. The rock glaciers are composed of limestone and dolomite debris derived from Upper Triassic and Lower Jurassic carbonate rocks. Compared to rock glaciers of regions with metamorphic bedrock the debris of both rock glaciers is finer grained and the surface morphology is less well developed. Due to the karstified bedrock beneath both rock glaciers almost all meltwater is released along karst cavities and there is almost no surface discharge. The thermal regime within the debris layer is strongly influenced by the local weather conditions. Ground temperatures are significantly lower than on permafrost-free ground outside the rock glaciers. Annual flow velocities are low compared to other rock glaciers, ranging mostly between 5 and 20 cm. Internal structures (shear planes) interpreted by georadar data, flow velocities and particularly ice exposures at the upper part of Cadin del Ghiacciaio rock glacier clearly indicate that this rock glacier developed from a debris-covered cirque glacier and is under permafrost conditions still today. We suggest that Cadin del Ghiacciaio rock glacier has developed from a small avalanche-fed cirque glacier during retreat through inefficiency of sediment transfer from the glacier ice to the meltwater. Cadin di Croda Rossa rock glacier lacks ice exposures and shows different internal structures indicating that this is probably an ice-cemented rock glacier.