Sudden Large-Volume Detachments of Low-Angle Mountain Glaciers – More Frequent Than Thought

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 unknown glacier detachments have been discovered and described, and new ones occurred. Here, we compile, compare and discuss 19 actual or possible large-volume detachments of low-angle mountain glaciers at 9 different sites in the Caucasus, the Pamirs, Tibet, Alaska’s St. Elias mountains, and the Southern Andes, many of them reaching volumes in the order of 10–100 million m³. Commonalities and differences between the cases investigated suggest that a set of different conditions drives a transient combination of factors related to low basal friction, low resistance and high driving stress to exceed stability thresholds. Particularly soft bedrocks below the detached glaciers seem to be a common condition among the observed events, as they are prone to till-strength weakening and eventually basal failure under high pore-water pressure. The lowest surface slope found for a detached glacier part was around 8–9°. All ice-rock avalanches resulting from detachments in this study have a particularly low friction angle, down to around 0.1, likely due to their high ice content and connected liquefaction potential, the ready availability of soft basal slurries and large amounts of basal water, and 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 detachments or their geographic proximity to other surge-type glaciers suggests the glacier detachments investigated in this study can be viewed as endmembers of the continuum of surge-like glacier instabilities. Though rare, glacier detachments appear more frequent than previously thought and disclose, despite local differences in conditions and pre-failure evolutions, the fundamental and critical potential of low-angle soft glacier beds to fail catastrophically.