Fast deformation of perennially frozen debris in a warm rock glacier in the Swiss Alps:: An effect of liquid water

Surface movement, internal deformation, and temperature were monitored over 5 years on Buz North rock glacier, a small rock glacier located at the lower limit of the permafrost belt in the Swiss Alps. The permafrost in the rock glacier mainly consists of pebbles and cobbles filled with interstitial ice. Two inclinometers installed at 4 and 5 m depths showed fast deformation with large seasonal and interannual variations, while the permafrost temperatures remained almost at the melting point. The movement of the inclinometers coincided with changes in the surface velocities. The deformation rapidly accelerated during snowmelt periods, whereas it gradually decelerated below a dry snow cover in winter. The frozen debris was more deformable than typical glacier ice at the melting point. These phenomena suggest that the frozen debris is permeable to snowmelt water. The fast deformation should result from significant annual relocation of debris particles, which probably creates a network of air voids in the frozen debris that eventually allows water infiltration. The meltwater infiltration accelerates the deformation by reducing effective stress, resulting in the reduced strength of the frozen debris. The refreezing of the pore water, which depends on the cooling intensity in winter, decelerates the deformation. The combination of these processes controls the temporal variations in the deformation.