Rock Glacier Flow

Rock glacier flow–law exponent determined from geomorphological observations: A case study from the Murtèl rock glacier (Engadin)

Student : Dominik Amschwand

Supervisor : Dr. Marcel Frehner, Dr. Isabelle Gärtner-Roer

Active rock glaciers are the visible expression of the steady-state creep of ice-supersaturated mountain permafrost. Recently observed signs of permafrost degradation due to global warming raise the question how rock glaciers respond to these changing environmental conditions. Numerical modelling of their dynamics has proven to be a successful approach, but often simplifying assumptions about the rheology are made. The aim of this thesis is to constrain the viscous flow-law that governs the creep of the Murtèl rock glacier, an exceptionally well studied rock glacier in the Upper Engadin Valley. In a purely descriptive approach, it is attempted to back-fit the stress exponent n in Glen’s flow law to deformation data. The deformation of a borehole and the curved superficial transverse ridges are combined to provide a pseudo-3D picture. The stress exponent n of the non-linear viscous flow law describing the entire deforming layer of the rock glacier lies between 4.8 and 6.5, deviating substantially from the stress exponent for pure ice. If the basal shear zone is excluded, a better fit is achieved with n between 0.8 and 1.6, in agreement with the value of 1.3 that the ridges suggest. The findings imply that the deforming upper part of the Murtèl rock glacier should preferentially be considered as a two-layer system with a basal shear zone and an upper layer. The latter can conveniently be approximated by a linear Newtonian rheology. Since shear horizons are a common feature of rock glaciers regardless of their internal structure, these implications are likely to apply generally.