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As consequence of ongoing climate change, permafrost degradation is thought to be increasingly affecting slope stability in periglacial environments. This is of growing concern in Iceland, where in the last decade, permafrost degradation has been identified among the triggering factors of landslides. The role of ground ice in conditioning the morphology and dynamics of landslides involving loose deposits is poorly understood. We show the geomorphological impact of the Móafellshyrna and Árnesfjall landslides that recently occurred in ice-cemented talus deposits in northern Iceland. Using field and aerial remote-sensing measurements of the morphological and morphometric characteristics of the landslides, we assess the influence of thawing ground ice on their propagation style and dynamics. The two mass movements are complex and are similar to rock- and debris-ice avalanches, changing trajectory and exhibiting evidence of transitioning their style of motion from a dry granular mass to a debris flow-like movement via multiple pulses. We infer that the thawing of ground ice together with the entrainment of saturated material provided the extra fluid causing this change in dynamics. The hazardous consequences of permafrost degradation will increasingly affect mountain regions in the future, and ground-ice thaw in steep terrain is a particularly hazardous phenomenon, as it may induce unexpected long-runout failures and can cause slope instability to continue even after the landslide event. Our study expands our knowledge of how landslides develop in unstable ice-cemented deposits and will aid assessment and mitigation of the hazard that they pose in Iceland and other mountainous periglacial areas. [ABSTRACT FROM AUTHOR]

Molards have been defined in the past as conical mounds of debris that can form part of a landslide's deposits. We present the first conclusive evidence that molards in permafrost terrains are cones of loose debris that result from thawing of frozen blocks of ice-rich sediments mobilised by a landslide, and hence propose a rigorous definition of this landform in permafrost environments. We show that molards can be used as an indicator of permafrost degradation, and that their morphometry and spatial distribution give valuable insights into landslide dynamics in permafrost environments. We demonstrate that molards are readily recognisable not only in the field, but also in remote sensing data; surveys of historic aerial imagery allow the recognition of relict molards, which can be used as an indicator of current and past permafrost conditions. The triggering of landslides as a result of permafrost degradation will arguably occur more often as global atmospheric temperatures increase, so molards should be added to our armoury for tracking climate change, as well as helping us to understand landslide-related hazards. Finally, we have also identified candidate molards on Mars, so molards can inform about landscape evolution on Earth and other planetary bodies. • Molards in permafrost terrains form from degradation of blocks of ice-rich deposits into mounds of debris. • Molards in periglacial environments can be an indicator of permafrost degradation. • Morphometry and distribution of molards reveal landslide dynamics. • Molards can be readily recognised in the field and from remote sensing. • Molards can reveal the influence of ice thaw on landscapes on Earth and other planets. [ABSTRACT FROM AUTHOR]

Permafrost degradation is one of the main controlling factors of slope instabilities in glacial and periglacial environments (e.g., Gruber and Haeberli, 2007). In permafrost terrains, ground ice can occur in pores, cavities, voids in soils or rocks, and its thaw can cause slope failures. A plethora of studies exists on the destabilisation of bedrock slopes due to permafrost degradation (e.g., Harris et al., 2001; Magnin et al., 2015). However, the role of thawing ground ice in conditioning and controlling the dynamics of rapid mass movements involving loose deposits is not well constrained, and has been rarely explored through geomorphometric analysis.In this research, we investigate two landslides induced by ground-ice thaw in Iceland, whose source materials comprised ice-cemented talus deposits. We apply quantitative terrain analysis using high-resolution DEMs to describe and quantify the morphometric characteristics of these landslides. Our morphometric approach allows us to show that different dynamic processes were involved during both failures due to the presence of ground ice. This caused the movement to evolve during the failure event, changing the mobility and trajectories of the landslides. Improving our knowledge on this type of landslides through morphometric analysis is important, as it can aid in assessing their hazard and in predicting similar rapid mass movements in comparable settings.References: Gruber, S. and Haeberli, W., 2007. Permafrost in steep bedrock slopes and its temperature‐related destabilization following climate change. Journal of Geophysical Research: Earth Surface, 112(F2).Harris, C., Davies, M.C. and Etzelmüller, B., 2001. The assessment of potential geotechnical hazards associated with mountain permafrost in a warming global climate. Permafrost and Periglacial Processes, 12(1), 145–156.Magnin, F., Deline, P., Ravanel, L., Noetzli, J. and Pogliotti, P., 2015. Thermal characteristics of permafrost in the steep alpine rock walls of the Aiguille du Midi (Mont Blanc Massif, 3842 m asl). The Cryosphere, 9(1), 109–121. [ABSTRACT FROM AUTHOR]