Discriminating viscous creep features (rock glaciers) in mountain permafrost from debris-covered glaciers – a commented test at the Gruben and Yerba Loca sites, Swiss Alps and Chilean Andes

Abstract. Viscous flow features in perennially frozen talus/debris called rock glaciers are being systematically inventoried as part of global climate-related monitoring of mountain permafrost. In order to avoid duplication and confusion, guidelines were developed by the International Permafrost Association for discriminating between the permafrost-related landform “rock glacier” and the glacier-related landform “debris-covered glacier”. In two regions covered by detailed field measurements, the corresponding data- and physics-based concepts are tested and shown to be adequate. Key physical aspects, which cause the striking morphological and dynamic difference between the two phenomena/landforms concern: • tight mechanical coupling of the surface material to the frozen rock-ice mixture in the case of rock glaciers as contrasting with essential non-coupling of debris to glaciers they cover; • talus-type advancing fronts of rock glaciers exposing fresh debris material from inside the moving frozen bodies as opposed to massive surface ice exposed by advancing fronts of debris-covered glaciers; and • increasing creep rates and continued advance of rock glaciers as convex landforms with structured surfaces versus predominant slowing down and disintegration of debris-covered glaciers as concave landforms with primarily chaotic surface structure. Where debris-covered surface ice is, or has recently been, in contact with thermally-controlled subsurface ice in permafrost, complex conditions and interactions can develop morphologies beyond simple “either-or”-type landform classification. In such cases, remains of buried surface ice mostly tend to be smaller than the lower size limit of “glaciers” as applied in glacier inventories, and to be far thinner than the permafrost in which they are embedded.