| 09 Dec 2025
How ice apron loss and permafrost degradation promote the Platteikogel rock slide: A thermo-mechanical reconstruction
Abstract. The Alpine cryosphere changes at unprecedented speed, affecting the thermal, hydrological, and mechanical state and behaviour of rock slopes. While numerous studies investigated singular drivers for progressive rock slope failures, the knowledge of hydro-thermo-mechanically coupled processes remains scarce. In this manuscript, we investigate the 2024 permafrost rock slide at Platteikogel with a volume of 50,000 m³ (3,395 m a.s.l., above Vernagtferner, Austria). We aim to assess how observed ice apron loss and related permafrost warming promote the release mechanism. We reconstructed multidecadal thermal evolution accounting for the thermal impact of ice apron loss. Based on field observations, we derived a conceptual model on how ice apron loss potentially affects rock slope destabilization. Integrating the outcome of the preceding steps, we performed a mechanical stability analysis assuming that the rock slide failed along ice-filled discontinuities. The mechanical model indicates that the rock slide can not be solely explained by a warming-driven decrease in shear strength of ice-filled discontinuities, suggesting that other failure processes superimpose or even dominate. The implemented system feedback related to ice apron loss suggests that hydrostatic pressure buildup due to water infiltration and rockfall-induced unloading thereby promoted the Platteikogel rock slide release. In summary, we demonstrate that ice apron loss not only leads to increased rockfall activity but also accelerates progressive rock slope failure, promoting the release of rock slides. In upcoming decades, ice aprons on steep rock slopes above 3000 m in the European Alps are expected to experience drastic area loss, exposing potential source zones for future rock slides.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Earth Surface Dynamics.
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This manuscript is a back analysis of the Plattikogel rock slope failure, a mountain that is characterized by permafrost and a retreating ice apron. The manuscript is based on a detailed description of the event including the lowering of the surrounding glaciers and decay of ice aprons. It includes a rock fall analysis in the years prior to the event. The model includes both a thermal model using the model CryoGrid and a rock mechanical model using the model UDEC. Assumptions in the model are based on field observations and are sound. The manuscript is well written and figures in general are very good and appropriate. I recommend publication after minor revisions:
The event is called a rock slide in the title and abstract and indeed is the stability strongly controlled by the stability of the toe following the UDEC model. However, structural geology is complex. This allows taht the rock slope failure might have occurred in small subsequent events (also suggested in Figure A5). The event was not observed by eyewitnesses during the event nor days later. It was not recorded in any other records (It is indicated that nearby seismic stations were checked and had no apparent signal). Rock slope failures that form rock avalanches (suggested in the manuscript on lines 54, 61 and table title 1 have sudden strong energy releases, that can be detected in seismic data. (e.g. Karasözen, E., & West, M. E. (2024). Toward the Rapid Seismic Assessment of Landslides in Coastal Alaska. The Seismic Record, 4(1), 43-51. doi:10.1785/0320230044.). This event is with 50.000 m3 extremely small for a rock avalanche. See the used terminology in this paper is by: Hungr, O., Leroueil, S., & Picarelli, L. (2014). The Varnes classification of landslide types, an update. Landslides, 11(2), 167-194.) which suggests a volume of about 1.000.000 m3 for rock avalanches. Hermanns et al., 2022 (Hermanns, R. L., Penna, I. M., Oppikofer, T., Noël, F., & Velardi, G. (2022). 5.06 - Rock Avalanche. In J. F. Shroder (Ed.), Treatise on Geomorphology (Second Edition) (pp. 85-105). Oxford: Academic Press. doi:https://doi.org/10.1016/B978-0-12-818234-5.00183-8.) suggest that rock avalanche can be of smaller size than earlier suggested but 50.000 m3 is at the absolute end of rock avalanches ever recorded. This opens the question if the material indeed failed in a single short (seconds) event but may be in several failures over hours or even days and thus would more classify as a rock collapse (following the extended definition of Hermanns et al. 2022 after Hungr et al., 2014). The H/L ratio and Fahrböschung as given in table 1 indeed suggest a rock avalanche but this high mobility might just be related to the steepness of the terrain so that the failed material tumbled further downslope and never moved as a fluidized stream of rock material as in a rock avalanche. Anyhow the authors should be more carful and accept different failure scenarios than a single event failure. Thus, I would also recommend neither using rock slide nor rock avalanche as specific terms but the broader term “rock slope failure”.
I actually still have some problems putting the event in front of an inner eye. This is likely due to the lack of a detailed map. I would call Figure 1A unreadable as the only map and would strongly recommend a detailed map that also allows to assess the steepness of the terrain. Similar to Figure 1 are also Figure 2, 5, 6b, 11, A3, A5 below a size that is acceptable to read.
Beside these comments, I have only a few other minor observations:
Line 66 and 67: why is schistose and muscovite written in capital letters?
Line 68: numbering of supplementary material different to numbering of supplementary material in line 101.
Line 70: I actually see gullies on the image with two orientations.
Line 86,87: I cannot see this on the figures.
Figure 2: Include orientation of figure.
Line 102: I think the profile line should be included in the main body of the paper as many figures reference to that.
Line 213: This was nicely documented by: Kuhn, D., Hermanns, R. L., Fuchs, M., Schüßler, N., Torizin, J., Aga, J., et al. (2025). Warming-induced destabilization of polar coastal rock cliffs and the role of thermokarst: A case study of Forkastningsfjellet on Svalbard. Science of The Total Environment, 968, 178807.
Figure 5: This figure strongly suggests that several subsequent failures are feasible.
Line 399, 400: This concept was discussed much earlier using different terminology: static boundary conditions (conditions that do not change in time), dynamic boundary conditions (conditions that change in time), triggering factors. (Hermanns, R., Niedermann, S., Villanueva Garcia, A., & Schellenberger, A. (2006). Rock avalanching in the NW Argentine Andes as a result of complex interactions of lithologic, structural and topographic boundary conditions, climate change and active tectonics. Landslides from massive rock slope failure, NATO Science Series IV, 49, 539-569.). However, ice aprons and permafrost were not included in the list by Hermanns et al., 2006.
Discussion: I think a short paragraph could be included discussing if the event was indeed a one event failure in seconds or if it could have occurred in more length in time or in several events. Does the geomechanically model suggest something?
I was happy reviewing this manuscript as it changed my view looking on such high alpine landscapes. Very nice contribution to science.