Sudden large-volume detachments of low-angle mountain glaciers – more frequent than thought?
Authors: Kääb Andreas , Jacquemart Mylène , Gilbert Adrien , Leinss Silvan , Girod Luc , Huggel Christian , Falaschi Daniel , Ugalde Felipe , Petrakov Dmitry , Chernomorets Sergey , Dokukin Mikhail , Paul Frank , Gascoin Simon , Berthier Etienne , Kargel Jeffrey S. 
Published: April 2021

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.