The Rock Glacier Inventory of the Italian Alps
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THE ROCK GLACIER INVENTORY OF THE ITALIAN ALPS M. Guglielmin1, C. Smiraglia2 1. Via Matteotti 22 20035 Lissone Italy e-mail: [email protected] 2. Earth Department, Milan University, Via Mangiagalli 34, Milan Italy e-mail: [email protected] Abstract An inventory of rock glaciers in the Italian Alps has recently been compiled. It contains data on 1594 rock glaciers. The rock glaciers were identified by aerial photo interpretation and some field surveys. Two hundred and ninety-seven (19%) of these landforms were considered active and 914 (59%) inactive. The total area occu- pied by these rock glaciers in the Italian Alps is about 220 km2 and the total volume of permafrost estimated on the basis of geophysical investigations carried out in the Upper Valtellina is at least 109 m3. The rock glaciers are concentrated mainly in the central part of the Alps with a maximum density of features of 1 per 9 km2 in the Atesine Alps. Rock glacier distribution is affected by lithology, glacial history and climatic conditions (especial- ly insolation and precipitation regime). Introduction rious scales (1:20,000 -1:56,000). To check the data obtained from this work, numerous site visits were This paper presents the results of an inventory of rock made. glaciers in the Italian Alps which represents one of the main objectives of the last ten years of research con- The rock glaciers were classified from the morphody- ducted by the Glaciological Section of the National namic point of view following Barsch (1988) as active or Group on Physical Geography and Geomorphology. inactive forms, with two additional categories of uncer- The recently published inventory (Guglielmin and tain activity and complex rock glaciers. For each rock Smiraglia, 1997) groups together data and analyses on glacier, relationships with glacial landforms or different about 1600 of these landforms. Some preliminary ice bodies located slightly above or adjacent to it were results have already been presented at previous recorded as well as its morphological setting (cirque, Permafrost Conferences (Carton et al., 1988; Belloni et slope etc.). al., 1993). Rock glaciers are widely distributed in high- altitude and high-latitude environments. Scientific The rock glaciers were then drawn on small-scale interest in these landforms pivots around a variety of (1:25,000-10,000) regional and national maps in order to elements: their geometry, the type of ice found internal- compute morphometric parameters such as: maximum ly, their genesis, movement and above all, their climatic length, width, minimum altitude of the front (MAF), and paleo-climatic significance. In addition to these ele- maximum altitude of the head of rock glacier (MARH) ments of interest, there are others of a more practical and aspect. The width and length were measured in the and applied nature, as well as those related to current normal direction and parallel to the direction of the climatic trend. In fact, further increases in temperature main flow of the rock glacier, respectively. Aspect is could heighten the melting of the ice inside rock glaci- divided into 16 classes from N to NNW. The lithology ers, causing debris flows that could reach catastrophic of the rock wall that feeds each rock glaciers was deter- proportions and create instability problems for build- mined from geological maps. ings and structures. Therefore, having an inventory available is essential - an inventory that is as complete Data treatments as possible and that also supplies information on the distribution and characteristic of these landforms. The average slope of the rock glacier surface was cal- Methods culated as the tangent of the ratio of the difference in height (head elevation minus the minimum altitude) to Rock glaciers were identified by interpretation of the maximum length of the rock glacier. colour and black and white aerial photographs at va- M. Guglielmin, C. Smiraglia 375 Figure 1. An example of an Inventory of Italian Alps Rock Glacier map. The area was calculated as the product of the maxi- The ratio of length to width that permits the separa- mum length multiplied by the width. This is, therefore, tion of tongue-shaped and lobate rock glaciers was also an over-estimation of area, because the real shape of calculated (Wahrhaftig and Cox , 1959). rock glaciers cannot be considered really rectangular. 376 The 7th International Permafrost Conference The density of rock glaciers was calculated for the areas above an altitude of 1000 m a s.l. for each of the nine traditional geographical sectors of the Italian Alps: Maritime, Cottian, Graian, Pennine, Lepontine, Rhaetian, Atesine, Dolomite and Carnic Alps. Statistical analyses discussed below refer to rock gla- ciers for which all the data were available. In addition, cluster analysis using the CANOCO pro- gram (Ter Braak, 1987) was performed to elucidate the complex, multiple interactions that constrain rock glaci- er occurrence. The main characteristics of the rock glaciers in the italian alps: results and discussion A total of 1594 rock glaciers were identified in the Italian Alps. The mean density of these landforms is equal to 6 per 100 km2 but ranges between 2 per 100 km2 in Dolomite Alps to 1 per 9 km2 in Atesine Alps. The rock glaciers are concentrated in areas, generally where peaks are less than 3500 m a.s.l. An example of the Italian rock glacier inventory maps is shown in Figure 1. Almost 60% of rock glaciers were classified as inac- tive, whereas only 19% are active (Figure 2a). As con- cerns the morphological locations, the greatest number of rock glaciers are located in cirques and on slopes but this varies considerably among the different Alpine sec- tors (Figure 2b). Rock glaciers are located mainly on slopes in the Graian, Pennine and Lepontine Alps, whereas the cirque locations are more important in the other sectors. Table 1. Geographical distribution of rock glacier in the sectors of Italian Alps 1) Number of rock glaciers per sector ; 2) MAF of active rock glaciers; 3) MAF of inactive rock glaciers; 4) total surface area (ha); 5) estimated permafrost volume (*10,000 m3); 6) density of rock glacier expressed as area (km2) for 1 rock glacier. Figure 2. Main characteristics of Italian rock glaciers: (a) classification based on degree of morphodynamic activity; (b) morphological location; (c) relationships with glacial landforms and ice bodies; (d) relationships with lithology. M. Guglielmin, C. Smiraglia 377 Figure 3. Results of Principal Component Analysis (PCA) of the main parameters of active and inactive rock glaciers. Figure 2c shows the topographic relationships Table 1 contains information on altitude, that is, the observed between rock glaciers and different types of means for the minimum altitudes reached by the fronts ice bodies and morainic forms. Only 27% of rock glaci- of active and inactive rock glaciers in the various sec- ers are clearly close to these features, and within this tors of the Italian Alps. The mean minimum altitude for group, semi-permanent snow banks and moraines are active rock glacier fronts proved to be 2564 m, with a the most important. maximum for the Graian Alps and a minimum for the Lepontine Alps. Over 80% of the rock glaciers have sources in meta- morphic rocks (Figure 2d). This high percentage is due The total surface covered by rock glaciers extends to the rock types that are characteristic of the Italian over 22000 ha of the Italian Alps; almost a half of this is Alps, as metamorphic rocks clearly prevail. However, concentrated in the Rhaetian Alps. as previously demonstrated for several sectors of the Italian Alps (Guglielmin, 1997), by comparing surface The volume of the permafrost existing in the active areas of the same dimension, the density of rock gla- rock glaciers in the Italian Alps is estimated to be ciers is at least doubled for metamorphic rocks com- 109 m3 assuming a mean thickness of 12 m (Guglielmin, pared to carbonate rocks. 1997). More than two-thirds of the rocks glaciers yielded a Figure 3 shows the plots of axes (X1 and X2) obtained length: width ratio greater than 1 and can be defined as by principal component analysis (PCA) using a data tongue-shaped. This ratio was almost equal to 1 only in matrix of MAF, MARH, aspect, slope, length and width the Pennine Alps and thus lobate rock glaciers prevail of active and inactive rock glaciers, respectively. The there. slope and aspect lie near the origin of the principal As concerns rock glacier morphometry, one may note component axes and are less discriminant factors than that over two-thirds of the rock glaciers have lengths MAF, MARH and length or width. Above all it should ranging between 100 and 600 m, with a mean of 448 m. be noted that MARH-MAF and slope-aspect are A substantial 85% of these landforms have widths ran- inversely related. ging between 50 and 500 m, with a mean of 281 m. The prevailing slope is between 10 and 35¡ with a mean The prevalent aspect proved to be northern (for 21% slightly exceeding 22¡. of the rock glaciers), followed by a NW aspect (14%). If 378 The 7th International Permafrost Conference Figure 4. Relationships between aspect and MAF and MARH. Note the different trends for active and inactive landforms. only the active rock glaciers are taken into considera- dation. The less pronounced increase in altitude resul- tion, the percentage of rock glaciers facing the northern ting for inactive rock glaciers that face south, suggests sectors (NNW to NNE) increases, reaching 34% (of that the climatic conditions were once governed by an which 26% facing northward). atmospheric circulation that was quite different from todayÕs. Additional aspect-related information can be found in Figure 4, which shows the distribution of MARH and The pattern of distribution of MAF and MARH with MAF with respect to the orientation of both active and respect to orientation of the rock glaciers in cirques and inactive rock glaciers.