Rock Temperature Measurements in Two Alpine Environments: Implications for Frost Shattering

Abstract

Temperature records are presented for average years in two vertical rockwalls situated in two extreme microenvironments of the alpine periglacial zone of the French Alps (Brianqonnais). Temperatures recorded at the southwest-facing site of the Crete de Vars (altitude 2450 m) show a variation pattern characterized by many superficial freeze-thaw cycles and uninterrupted frost at depth (48 cm) for 2 mo. Minimum annual temperatures are rarely lower than -8?C. At the north-facing site of the Roc Noir du Combeynot (altitude 3000 m) the record shows a different temperature pattern with few superficial freeze-thaw cycles and 7 mo of uninterrupted frost. Temperatures are often lower than -8 to -10IC. A comparison during a short period on a south-facing site of the Roc Noir document the influence of the orientation of the cliff. At both sites, the rock (limestone and granite) has a low porosity, 0.7 to 3.5 %,lo but is microand macrocracked with a fracture spacing from 10 to 25 cm. Winter and spring field observations suggest that rock breaking is essentially due to the dissociation of blocks along the macrofissures filled by ice. This process is more active at the north-facing site where the water supply is more constant and freezing longer and more intense.