The Dynamics of Active Rock Glaciers

Rock glaciers are debris-covered, slowly flowing mixtures of rock and ice common in many alpine and arctic regions; they are striking morphological expressions of permafrost creep and belong to the most spectacular and most widespread periglacial phenomenon on earth. Rock glaciers are important agents of geomorphic modification of the landscape, particularly of alpine landscapes. They are widespread in alpine regions, but are less well studied than their "true" ice-glacier counterparts. Despite more than 40 years of investigation, surprisingly little is understood about the origin and development of rock glaciers. Hypotheses about their genesis have been the subject of long debate and are highly controversial. One group of scientists states that, regardless of their similar appearance and distribution, rock glaciers are distinct from true glaciers and are strictly the result of periglacial processes. The other group, mainly scientists from North America, holds that the formation of rock glaciers involves a continuum of processes from glacial to periglacial . In 1959, Wahrhaftig and Cox in a classical paper on rock glaciers developed the idea that rock glaciers in the Alaska Range are composed of coarse rock debris connected by interstitial ice primarily derived from refrozen meltwater. Several researchers have extended this view, concluding that all rock glaciers are exclusively permafrost phenomena. During an AGU Chapman Conference in August 1996 an unanimous consensus was achieved that glacigenic rock glaciers ("ice-cored rock glaciers") do exist. However, as Martin and Whalley (1987) pointed out, confusion still exists in definitions, classifications, and hence also in the interpretation of the signification of rock glaciers. Studies of rock glaciers are also of great practical interest as they may create hazardous floods downstream. Furthermore, active rock glaciers are of great significance for construction and environmental problems, particularly on building foundations (e.g. cable-car stations, skilifts, roads, mountain huts, radar stations) on ice- bearing ground like active rock glaciers. Therefore it is necessary to develop models to predict the response of rock glaciers to short-term change, and to monitor changes in rock glaciers in important watersheds like the Alps for better water resource managment and hazard evolution planning. Nevertheless, in the past only three rock glaciers of the Austrian Alps have been studied in detail: Hochebenkar Rock Glacier near Obergurgl, Ölgruben Rock Glacier (Kaunertal), and recently Dösen Rock Glacier (Ankogelgruppe). In a pilot project funded by the Jubiläumsfonds der Oesterreichischen Nationalbank and Hohe Tauern National Park we started to study active rock glaciers in the western Stubai and Ötztal Alps and in the Schobergruppe, considering the following points: geology of the bedrock in the drainage area, morphology and composition of the rock glacier, thermodynamics of the rock glacier, particularly of the debris mantle, hydrology and flow velocity. For a better understanding of the dynamics of active rock glaciers and their response to climatic change (global warming), long-term measurements ("rock glacier monitoring") particularly focused on the thermodynamics and hydrology (discharge) are required. Therefore, these investigations are planned to be continued in the course of the applied project.

Investigations carried out in the course of this project provided important information on the evolution and dynamics of active rock glaciers, particularly concerning hydrology, flow velocities, internal structure and composition, and evolution. Discharge of active rock glaciers is mainly controlled by the local weather conditions, the thermal properties of the debris layer, and the physical mechanisms that control the flow of meltwater through the rock glacier. Discharge of active rock glaciers is characterized by strong seasonal and diurnal variations. Water derived from snowmelt and summer thunderstorms is quickly released causing floods. Fair weather periods with intense melting of snow and ice cause pronounced diurnal variations in discharge. Water temperature of active rock glacier springs is constantly below 1C during the whole melt season. At Reichenkar rock glacier flow rates on the lower part were very constant with highest velocities along the axis (2.3 - 3 m/year), and decreasing flow velocities towards both margins. Flow velocities on the lower part are very constant throughout the year with similar flow rates of 6 - 7 mm/day along the axis. At Ölgrube rock glacier highest flow velocities (up to 2 m/year) were measured near the front, velocities gradually decrease upward. Near the front highest velocities are recoded along the axis, decreasing towards both margins. For a period of 57 days in summer 2003 significantly higher daily mean flow rates were estimated compared to the daily mean flow rates for one year. This indicates significantly lower daily flow rates for the winter period. Flow velocities generally increased in 2003 and 2004, probably due to the unusually warm summers causing high amounts of meltwater. Refraction seismics, ground penetrating radar and gravimetry measurements showed differences between the three studied rock glaciers. All contain a frozen core, covered by an unfrozen debris layer. At Kaiserberg the frozen core is composed of a mixture of ice and debris. At Ölgrube the northern part of the composite rock glacier contains a core of massive ice, which represents the remains of a debris-covered glacier from the last high-stand around 1850. At Reichenkar ice exposures and geophysical data indicate that the rock glacier also derived from a debris-covered glacier. A small cirque glacier is still present and extends below the debris layer of the rock glacier. The ice core can be traced down to the front, where it disintegrates into several ice bodies.