Glacial and periglacial processes

The analysis of multi-temporal DTMs provides detailed information on topographic changes in the Monte Rosa east face and, especially in combination with visual inspection of terrestrial photo- graphs (cf. Figure 6), enables conclusions to be drawn about processes affecting slope stability in such an area. The quantitative analysis of DTMs reveales complex patterns of terrain changes (cf. Figures 5 and 6). These diverse geomorphic patterns point to the simultaneous occurrence of dif- ferent processes in a high-mountain face. The following topographic changes could be distin- guished and quantitatively assessed:

- accumulation and ablation of ice over large areas of a glacier;

- rapid mass loss due to frequent small-scale or single large-scale rock and/or ice avalanche events;

- erosion of debris and/or glacier ice in the runout channel by rock and/or ice avalanche events; - accumulation of debris material from gravitational mass movement processes;

- rapid ice accumulation and build-up of steep glaciers in eroded areas after mass movement processes;

- accordion-like displacement pattern of steep glaciers with varying positions of ice fronts. The most striking observations were the exceptionally large loss of ice and bedrock. The DTM comparisons from 1988 to 2007 show how the glacierization of such a steep face, in contrast to valley glaciers, can change massively within a few years, remarkable in terms of both volume loss and gain. Rapid changes in surface geometry and the size of hanging glaciers are known to be comparably independent from climatic conditions, also for other high-mountain faces (Pralong and Funk, 2006; Post and Lachapelle, 1971). However, since about 1990 numerous large rock and ice avalanche events with a total volume of more than 20·106 m3 have occurred with no documented historical precedence of similar magnitude in the European Alps. We suspect that such develop- ments indicate the massive problems relating to permafrost, steep glaciation and rock-wall stability, which are expected to occur also in other steep high-mountain faces under similar conditions as those at the Monte Rosa east face.

The comparison of the slope failure zones (Fig. 5) with the modeled permafrost distribution (Figure 1) shows that all detachment zones are located in permafrost areas. The glacierization in these areas is assumed to consist of mainly of cold ice, however, hanging glaciers often have polythermal pat- terns with sections of much warmer or even temperate firn and ice (Alean, 1985; Haeberli, 2005). Such polythermal pattern may locally influence temperatures in the bedrock down to great depths. The small-scale rockfall events occurred in the area of the Imseng Channel, the Parete Innominata and elsewhere in the face can be related either to changes within the near-surface active layer of the permafrost or to enhanced frost weathering after the deglaciation. The large-volume rock ava- lanches such as 2007 event, however, cannot be only due to changes in the thermal field. For this rock avalanche, the striking and rapid topographic changes in the surrounding area with an enorm- ous mass loss of over 15 × 106 m3 ice and bedrock in this area may have had significant effects on the stability by influencing the topographic and geomechanical conditions of the remaining bedrock and thereby inducing changed stress fields in the bedrock. At the same time, the response of steep bedrock areas to glacier retreat is strongly conditioned by the geological setting, in particular by the geometrical and geotechnical characteristics of discontinuities, as in the 2007 case with an unfavor- able, surface-parallel setting of the rock discontinuities.

The DTM comparisons and additional imagery analyses have revealed that the sequence of the topographic changes and slope failures is strongly spatially correlated. The slope failures started in 1990 on a small part of the Parete Innominata with combined rock and ice avalanche activity, pro- ceeding with a chain reaction of mass wasting processes until the whole Parete Innominata and Imseng Channel was ice-free in 2001. Slope instabilities in the permafrost-affected bedrock can diminish the stability of hanging glaciers, and unloading effects due to glacier retreat may induce changed stress fields in the bedrock. Due to rock or ice avalanches, the terrain may become over- steepened and the internal stress field is changed. Subsequently, terrain adjustments implying mass movement activity occur to recover equilibrium. The analyses show a strong stability coupling between permafrost affected bedrock and adjacent hanging glaciers.

Looking at the glacierization, increased area-wide ice loss could be observed since 1956, probably to be explained by the trend towards increased air temperatures of about 0.5 to 1.5°C in this area. However, between 1988 and 2001, the lower part of the Monte Rosa glacier showed mass accumu- lation even though in other areas of the face, large ice loss occurred by ice avalanche events. Hence, simultaneous volume gain and volume loss can be observed in the glaciated areas at the

Fischer et al.: Monitoring topographic changes in high-mountain faces

same time. These observations suggest complex glaciation/deglaciation processes in such high- mountain faces.

Another noticeable phenomenon is the surprisingly fast accumulation of ice at distinct zones, where it was eroded by preceding mass movement events, e.g. in the Zapparoli Channel and the Imseng Channel. The accumulation rate is in these areas with up to 10 m per year higher than the assumed local precipitation rate. Strikingly, the areas with fast accumulation are located in flatter, depres- sion-like terrain. These high accumulation rates are most likely influenced by snow redistribution due to complex wind fields, and snow and ice avalanches from steeper parts. Considering the pho- tographs from 2003 and 2007 (Figure 6), it can be seen that the Marinelli Channel was totally ice- free in 2003 and shows again some ice cover in 2007. The ice and firn thickness in the Marinelli Channel is about 5 to 15 m. Such thin ice covers react extremely fast to temperature and precipita- tion changes, and can recover rapidly after having completely vanished.