Abstract
Abstract. The detachment of large parts of low-angle mountain glaciers resulting in massive ice–rock avalanches have so far been believed to be a unique type of event, made known to the global scientific community first for the 2002 Kolka Glacier detachment, Caucasus Mountains, and then for the 2016 collapses of two glaciers in the Aru range, Tibet. Since 2016, several so-far unrecognized low-angle glacier detachments have been recognized and described, and new ones have occurred. In the current contribution, we compile, compare, and discuss 20 actual or suspected large-volume detachments of low-angle mountain glaciers at 10 different sites in the Caucasus, the Pamirs, 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 cases indicate that glacier detachments often involve a coincidental combination of factors related to the lowering of basal friction, high or increasing driving stresses, concentration of shear stress, or low resistance to exceed stability thresholds. Particularly soft glacier beds seem to be a common condition among the observed events as they offer smooth contact areas between the glacier and the underlying substrate and are prone to till-strength weakening and eventually basal failure under high pore-water pressure. Partially or fully thawed glacier bed conditions and the presence of 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 of thick and thus large-volume glaciers while also being steep enough to allow critical driving stresses to build up. We construct a simple slab model to estimate ranges of glacier slope and width above which a glacier may be able to detach when extensively losing basal resistance. From this model we estimate that all the detachments described in this study occurred due to a basal shear stress reduction of more than 50 %. Most of the ice–rock avalanches resulting from the detachments in this study have a particularly low angle of reach, down to around 5∘, likely due to their high ice content and connected liquefaction potential, the availability of soft basal slurries, and large amounts of basal water, as well as the smooth topographic setting typical for glacial valleys. Low-angle glacier detachments combine elements and likely also physical processes of glacier surges and ice break-offs from steep glaciers. The surge-like temporal evolution ahead of several detachments and their geographic proximity to other surge-type glaciers indicate the glacier detachments investigated can be interpreted as endmembers of the continuum of surge-like glacier instabilities. Though rare, glacier detachments appear to be more frequent than commonly thought and disclose, despite local differences in conditions and precursory evolutions, the fundamental and critical potential of low-angle soft glacier beds to fail catastrophically.
Highlights
After the detachment of Kolka Glacier in the Russian Caucasus 14 years ago, the 17 July and 21 September 2016 detachments of two neighbouring glaciers in Tibet’s Aru range directed attention to a new type of glacier instability that had been rarely observed and little described before (Tian et al, 2017; Gilbert et al, 2018; Kääb et al, 2018)
Neglecting practical obstacles, events that repeat after a few years – Amney Machen, Flat Creek (?) – could be a suitable place to learn more about the governing physical processes by setting up detailed field investigations and monitoring facilities. In this contribution we described around 20 ice–rock avalanche events that we characterize as sudden largevolume detachments of low-angle glaciers
We described one new event in the same size-class as the 2002 Kolka and the 2016 Aru glacier detachments (Sedongpu 2018) and as a side-result quantified one of the larger high-mountain rock avalanches of recent decades (Gyala Peri 2017)
Summary
After the detachment of Kolka Glacier in the Russian Caucasus 14 years ago, the 17 July and 21 September 2016 detachments of two neighbouring glaciers in Tibet’s Aru range directed attention to a new type of glacier instability that had been rarely observed and little described before (Tian et al, 2017; Gilbert et al, 2018; Kääb et al, 2018). The Aru twin glacier detachments – which released 68 and 83 × 106 m3 of glacier ice without known conditions of high geothermal flux – have recently raised the questions of whether and where such events may have happened before or need to be expected in the future, what conditions allow low-angle glaciers to detach catastrophically from their beds, and what this means for mountain hazard management. The urgency of these questions is highlighted by the fact that several detachments similar to the Aru events, though smaller, have been detected subsequently (Falaschi et al, 2019; Paul, 2019; Jacquemart et al, 2020). The lowering of basal glacier friction that is associated with surging involves abnormally high water pressure, changes in the thermal regime, and/or responses of subglacial till to increasing shear stress and water input (Clarke et al, 1984; Kamb, 1987; Truffer et al, 2000; Fowler et al, 2001; Murray et al, 2003; Frappe and Clarke, 2007; Sevestre et al, 2015; Benn et al, 2019)
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