The active rockglacier at Hohe Gaisl - implications on genesis

Active rockglaciers are less common in the mountain ranges composed of carbonatic rocks, such as the Northern Cretaceous Alps ort he Dolomites, even if here more than lithology probably the minor mean elevation plays a role.
Although few active rockglaciers are present in the Dolomites, they have never been studied in detail.
One studied rockglacier is located in the “Gletscherkar (glacier cirque)” on the north-eastern side of the Hohe Gaisl (3146m). The rockglacier lies in a deeply incised cirque, surrounded by steep walls composed of upper Triassic dolomite and limestone.

Fig.1. Air photos and digital terrain modell of the Hohe Gaisl mountain group with two active rockglaciers in the north-eastern cirques (Autonomous Province of Bozen/Bolzano - South Tyrol)

Fig.2. View to west on the Hohe Gaisl mountain group with two active rockglaciers in the north-eastern exposed, deeply incised cirques. The visited rockglacier can be found in the right cirque (north).

Debris of the rockglacier is mainly derived from a prominent, NW-SE-trending fault, along which the bedrock is intensively deformed. The rockglacier is 850m long, 300-550m wide and covers an area of 0,3 square kilometres. The rockglacier extends from an altitude of 2340m at the front to about 2500m. The eastern lobe shows well developed surface topography of transverse ridges and furrows. The surface is coarse grained and varies from place to place, manly constituted of poorly sorted gravel and sand, huge boulders are missing, great blocks exceeding 1m are rare.

Fig.3. View to west on front of the "Gletscherkar" rockglacier.

In the upper part massive ice is exposed during the summer months at several places below a less than 1m thick debris layer. The ice is coarse-grained, banded, and contains thin, fine-grained debris layers parallel to the banding. Rarely larger clasts occur within the ice.
During the melting season small thermokarst lakes may be developed on the upper part of the rockglacier.

Fig.4a. Ice-exposure on the rockglacier (15.09.2007).
Fig.4b. Fine-grained debris layers parallel to the banding in the ice (15.09.2007).

Georadar measurements provided information on the internal structure and thickness of the rockglacier. The data indicate that the rockglacier has a total thickness of approximately 25m. In the lower and middle part the debris layer is 3-5m thick. Below the debris layer numerous, well developed reflectors are visible indicating the presence of shear planes in the frozen body of the rockglacier, which according to ice exposure in the upper part is composed of coarse (glacier) ice with numerous thin debris layers parallel to the banding. A thin sediment layer (?lodgement till) may be present at the base of the rockglacier.

Internal structure and ice exposure clearly indicate that the rockglacier in the Gletscherkar developed from a debris-covered cirque glacier. It is suggested that the glacier has developed from a small cirque glacier during retreat trough inefficiency of sediment transfer from the glacier ice to the meltwater. The presence of a cirque glacier at Gletscherkar is documented in the older literature and on older maps, for example on a topographic map published in 1902 (FREYTAG 1902).

References:

KRAINER, K. & LANG, K. (2007): Active rock glaciers at Hohe Gaisl (eastern Dolomites). Geo.Alp 4, 127-131
LANG, K. (2006): Geologie des Hohe Gaisl Massivs (Pragser- und Ampezzaner Dolomiten) unter besonderer Berücksichtigung der aktiven Blockgletscher. Unveröff. Diplomarbeit, Institut für Geologie und Paläontologie Leopold-Franzens-Universität Innsbruck, 170S.
SHRODER, J.F.; BISHOP, M.P.; COPLAND, L. & SLOAN, V.F. (2000): Debris-covered glaciers
and rock glaciers in the Nanga Parbat Himalya, Pakistan. Geografiska Annaler 82(A):
17 - 31