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. The detachment of large parts of low-angle mountain glaciers resulting inmassive ice–rock avalanches have so far been believed to be a unique typeof event, made known to the global scientific community first for the 2002Kolka Glacier detachment, Caucasus Mountains, and then for the 2016collapses of two glaciers in the Aru range, Tibet. Since 2016, severalso-far unrecognized low-angle glacier detachments have been recognized anddescribed, and new ones have occurred. In the current contribution, wecompile, compare, and discuss 20 actual or suspected large-volume detachmentsof low-angle mountain glaciers at 10 different sites in the Caucasus, thePamirs, Tibet, Altai, the North American Cordillera, and the Southern Andes.Many of the detachments reached volumes in the order of 10–100 million m3. The similarities and differences between the presented casesindicate that glacier detachments often involve a coincidental combinationof factors related to the lowering of basal friction, high or increasing drivingstresses, concentration of shear stress, or low resistance to exceedstability thresholds. Particularly soft glacier beds seem to be a commoncondition among the observed events as they offer smooth contact areasbetween the glacier and the underlying substrate and are prone totill-strength weakening and eventually basal failure under high pore-waterpressure. Partially or fully thawed glacier bed conditions and the presenceof liquid water could thus play an important role in the detachments.Surface slopes of the detached glaciers range between around 10∘and 20∘. This may be low enough to enable the development ofthick and thus large-volume glaciers while also being steep enough to allowcritical driving stresses to build up. We construct a simple slab model toestimate ranges of glacier slope and width above which a glacier may be ableto detach when extensively losing basal resistance. From this model weestimate that all the detachments described in this study occurred due to abasal shear stress reduction of more than 50 %. Most of the ice–rockavalanches resulting from the detachments in this study have a particularlylow angle of reach, down to around 5∘, likely due to their highice content and connected liquefaction potential, the availability of softbasal slurries, and large amounts of basal water, as well as the smoothtopographic setting typical for glacial valleys. Low-angle glacierdetachments combine elements and likely also physical processes of glaciersurges and ice break-offs from steep glaciers. The surge-like temporalevolution ahead of several detachments and their geographic proximity toother surge-type glaciers indicate the glacier detachments investigated canbe interpreted as endmembers of the continuum of surge-like glacierinstabilities. Though rare, glacier detachments appear to be more frequentthan commonly thought and disclose, despite local differences in conditionsand precursory evolutions, the fundamental and critical potential oflow-angle soft glacier beds to fail catastrophically.

Sudden large-volume detachments of low-angle mountain glaciers – more frequent than thought?