Large rockfall on a small glacier - Case study of a periglacial landform formation in the Horlachtal, Stubai Alps, Austria

Deglaciation in high mountain areas signifies the transition from glacial to periglacial conditioned landscapes. Due to the reduced melt rate of debris coved glacier ice, these parts of the glacier might persist long after the surrounding glacier has melted, forming periglacial landforms in the post-glacial landscape. Therefore, in this case study, we examine the geomorphological development of a recent 19267 m³ ± 204 m³ rockfall from the glacier headwall on the small, low elevated Zwieselbachferner in the Horlachtal, Stubai Alps, Austria. The multi-epochal analysis is based on different remote sensing data (photogrammetrically and airborne laserscanning derived digital elevation models, orthophotos and satellite data) and covers the period from the occurrence of the initial rockfall in 2003/2004 until 2022. Results show that the headwall in this area is still very active, supplying 13 further rockfalls of varying magnitude to the debris covered glacier part during the study period. The debris cover created by rockfall, estimated to be several meters to a few decimeters thick, causes the surface elevation change of the glacier to decrease by a factor of 5 to 6 compared to the surrounding glacier. This results in the formation of a steep front and flanks, which become progressively covered and thus isolated by debris redistribution. In contrast to the surrounding glacier, whose thickness and length has strongly decreased during the study period, the mean ice thickness of the debris-covered area only decreases from 23.5 m to 21.8 m between 2006 and 2022. The extrapolation of ice thickness development shows that this part of the glacier will remain as a debris covered, ice-cored landform after the complete melting of the surrounding glacier. As glaciers melt rapidly, ELA rises and glacier headwalls become more unstable due to glacier melt and permafrost warming, we expect this process to occur more frequently in the future and in some cases to shape the appearance of formerly glaciated landscapes.