ANALYZING 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 perio d 1970 to 1995 results in a unique time series documenting the three-d imensional surface kinematics of creeping mountain permafrost. In plac es, the area under study is affected by historical fluctuations of the polythermal Gruben glacier. Changes in elevation and surface velociti es were measured over five consecutive five-year periods using an adva nced photogrammetric monoplotting technique of multitemporal stereo mo dels. The measurements are based on a regular grid with a mesh width o f 25 metres and have an accuracy of a few centimetres per year. Althou gh surface lifting occurred in places and within individual time inter vals, surface subsidence predominated at an average rate of a few cent imetre 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 gl acier which still contains some dead glacier ice in permafrost. Fluctu ations in horizontal surface velocities seem to correlate with tempora l changes in surface elevation. Analysing flow along principal traject ories and interpreting the advance rate of the front leads to an age e stimate of the rock glacier of a few millennia. Dynamic effects of thr ee-dimensional straining within the creeping permafrost as computed fr om the measured surface velocity field are estimated to potentially co ntribute to surface heave or subsidence in the same order of magnitude as the observed vertical changes. Temporal variations of surface alti tudes at Gruben rock glacier show distinct similarities with mass bala nce 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 pre dominant influence on permafrost aggradation/degradation as well as on glacier mass balance in mountain areas. (C) 1997 John Wiley & Sons, L td.