Analysing the creep of mountain permafrost using high precision aerial photogrammetry: 25 years of monitoring Gruben rock glacier, Swiss Alps

Aerophotogrammetrical monitoring of Gruben rock glacier over the period 1970 to 1995 results in a unique time series documenting the three‐dimensional surface kinematics of creeping mountain permafrost. In places, the area under study is affected by historical fluctuations of the polythermal Gruben glacier. Changes in elevation and surface velocities were measured over five consecutive five‐year periods using an advanced photogrammetric monoplotting technique of multitemporal stereo models. The measurements are based on a regular grid with a mesh width of 25 metres and have an accuracy of a few centimetres per year. Although surface lifting occurred in places and within individual time intervals, surface subsidence predominated at an average rate of a few centimetres per year in the ‘periglacial’ part of the rock glacier and of a few decimetres per year in the ‘glacier‐affected’ part of the rock glacier which still contains some dead glacier ice in permafrost. Fluctuations in horizontal surface velocities seem to correlate with temporal changes in surface elevation. Analysing flow along principal trajectories and interpreting the advance rate of the front leads to an age estimate of the rock glacier of a few millennia. Dynamic effects of three‐dimensional straining within the creeping permafrost as computed from the measured surface velocity field are estimated to potentially contribute to surface heave or subsidence in the same order of magnitude as the observed vertical changes. Temporal variations of surface altitudes at Gruben rock glacier show distinct similarities with mass balance and surface altitude variations determined on nearby glaciers but at a greatly reduced amplitude. This similarity may indicate that the same climatic forcing (summer temperatures?) could possibly have a predominant influence on permafrost aggradation/degradation as well as on glacier mass balance in mountain areas