Sudden large-volume detachments of low-angle mountain glaciers – 2 more frequent than thought ? 4

The detachment of large parts of low-angle mountain glaciers, resulting in massive ice-rock avalanches, have so far been 26 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 28 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 30 mountain glaciers at ten different sites in the Caucasus, the Pamirs, Tibet, Altai, Alaska’s St. Elias mountains, and the Southern Andes. Many of the detachments reached volumes in the order of 10–100 million m. The similarities and differences between 32 the presented cases suggest that glacier detachments often involve a coincidental combination of factors related to lowering of basal friction, high or increasing driving stresses, concentration of shear stress, or low resistance to exceed stability thresholds 34

period of mass-wasting activity, the glacier changed in unusual ways: it bulged and became heavily crevassed (Fig. 4), it developed a scarp at the location of the later detachment, and supraglacial ponds formed (Kotlyakov et al., 2004;Evans et al., 220 2009b;Kotlyakov et al., 2010b). Unusually high geothermal heat fluxes underneath the glacier (fumaroles and sulphur smell were reported in the glacier bed shortly after detachment), the impact energy of a large rock/ice fall, and successive loss of 222 shear stress due to excess water pressure have been proposed as possible factors and ultimate triggers of the 2002 detachment (Kotlyakov et al., 2004;Evans et al., 2009b;Kotlyakov et al., 2010b). Similarly, the additional loading of Kolka Glacier from 224 the rock and ice falls has more recently been proposed to have increased the basal shear stress until it exceeded a frictional threshold given by the glacier bed material, topography, and hydraulic conditions (Kääb et al., 2018). After the detachment, 226 lakes were visible on the Kolka Glacier bed, pointing to the involvement of large amounts of subglacial water in the detachment. The high mean avalanche velocities of 50-80 m/s (Huggel et al., 2005) suggest availability of large amounts of To roughly estimate the event volume we derive the ice thickness along the center flow line of the glacier based on an estimated 262 basal shear assuming a driving stress of 1.2 10 5 Pa as suggested for mountain glaciers, a slope of 16°, and a form factor of 0.8, which then results in a thickness of around 60 m (Cuffey and Patterson, 2010). Multiplying half of this depth (i.e. assuming a 264 triangular cross-section) with the detached area (ca. 200,000 m 2 ) gives a first volume estimate of roughly 6 10 6 m 3 . Whereas satellite images after detachment suggest that much of the glacier bed might actually have a triangular cross-section, it may 266 have been more shallow in the lowermost and uppermost parts. As an order of magnitude, we suggest a detachment volume of 5 10 6 m 3 and assign a conservative error of ±1 10 6 m 3 to this estimate.

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The cirque from which the 2017 event originated was also the source of other slope instabilities over recent years. Another (much smaller) ice-rock avalanche from a neighbouring glacier occurred between 15 and 24 July 2016 (dates from Planet This glacier also showed increased sliding speeds and crevassing around the later detachment area for at least 2-3 weeks before the failure. For the end of July 2019 we found surface speeds of roughly 2.5 m/day, a marked increase compared to roughly < surge-like advance. This condition is still visible in Landsat data 5-6 years later, though less certain due to the lower resolution of Landsat data (no other data is available to us between 2007 and 2015). Landsat data also suggest that the glacier experienced 288 a similar advance in the early 1990s. Under the limitation of the reduced spatial resolution of the Landsat data, however, we do not find signs of a large detachment event or large ice-rock avalanche. Nevertheless, we draw attention to a surprisingly 290 vegetation-free landform visible downstream of the 2019 detachment in pre-event imagery (Fig. 8). The lack of vegetation, the streamlined microtopography, and zones of rough and chaotic microtopography that resemble avalanche or debris flow Very-high resolution satellite images over the Peter the First Range suggest abundance of weak bedrock and fine sediments.

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All over the range, signs after large debris flows, rock avalanches or ice-rock avalanches are visible in high resolution satellite images (Maxar, CNES/Airbus, Planet; Leinss et al, 2020). The Pamirs in general are known to be geomorphologically very 300 active, with a number of associated hazards (Mergili et al., 2012;Gruber and Mergili, 2013;Strom and Abdrakhmatov, 2018) and a cluster of surge-type glaciers (Kotlyakov et al., 2008;Kotlyakov et al., 2010a;Gardelle et al., 2013;   Even if not documented in detail in an internationally accessible format so far, to our best knowledge, both the 2017 and 2019 detachments and their downstream effects were very likely noted by the local communities as the lowermost ice/rock deposits 310 stopped not far from settlements, agricultural fields, and pastures, and very-high resolution images (Maxar) show that flooding happened close to houses and two irrigation channel bridges were partially destroyed.        County, China) in the western Tibetan Plateau (termed Aru-1). The fragmented ice mass ran out 6 km beyond the glacier terminus, killing nine herders and hundreds of their animals, and reached the Aru Co lake (Tian et al., 2017;Kääb et al., 2018).

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The ice debris covered 8-9 km 2 and a volume of the detached glacier part of 68 10 6 m 3 was calculated. On 21 September 2016, a second glacier (Aru-2) detached just a few km south of Aru-1 ( Fig. 9). Similar to the July event, the glacier ice fragmented  (Gilbert et al., 2018). It showed that the two glaciers were close to their steady state geometry with no/little sliding until 2010. Thereafter, decreasing friction under the whole detachment area of Aru-1 and in more localized zones of Aru-2 356 started to trigger the surge-like mass transfer. Modelling the glaciers' thermal regimes revealed that the frictional changes likely occurred in temperate areas of the two glaciers and that stress concentration occurred on the cold-ice margins (Gilbert 358 et al., 2018). The surge-like changes of basal friction under the Aru glaciers were thus likely not associated with a change of the glaciers' thermal regimes but rather with a change in friction due to increasing water pressure in the already temperate    (Wenying, 1983). The first ice-rock avalanche happened between 26 January and 3 February 2004, 384 and the involved volume was estimated to be 20-25 10 6 m 3 , perhaps up to 36 10 6 m 3 according to a local information signboard (Paul, 2019). Three years later, between 23 September and 2 November 2007, a second detachment followed a surge-   The event seems to have had a severe impact on Sedongpu Glacier. We generated two elevation models from 13 Nov 2015 428 Spot6 and 30 Dec 2018 Pleiades tri-stereo data that produced robust results despite the extreme topographic conditions. Differencing the two DEMs indicates that the October 2017 rock avalanche removed around 17 and 33 10 6 m 3 of material 430 from two close-by but separated areas, respectively (Fig. 11c). If both failures happened as part of the same event, the total volume of 50 10 6 m 3 makes this one of the larger rock avalanches detected in recent decades. Based on visual inspection of 432 satellite data, we consider it very likely that the avalanche also involved small glaciers from the west wall of Gyala Peri and incorporated ice from the surface of the glaciers lower down, as it ran over them. The Chinese seismic database registered two     Jacquemart et al. (2020) found that the water availability during the exceptionally warm summer of 2013 was primarily melt driven and up to 4.8 standard deviations (σ) above the long term mean . No detachments were detected in 2014, 544 when water availability was below average (-0.5 σ). Water availability was again higher in 2015 (+ 1 σ), when the second detachment occurred.

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The volume of the deposit was estimated at 8.1 10 6 m 3 (Marangunic, 1980). Five mountaineers witnessed the event and noted https://doi.org/10.5194/tc-2020-243 Preprint. Discussion started: 22 October 2020 c Author(s) 2020. CC BY 4.0 License. several supraglacial ponds and 2-3 cm of wet snow on the surface of the glacier (Marangunic, 1980). These observations 568 suggest that the triggering mechanism of the glacier detachment likely involved an extreme reduction of the basal drag due to high water saturation of the glacier bed. Aparejo glacier appears to sit on a glacier bed composed primarily of weak subglacial 570 till , and the slope on the lower two thirds of the glacier averages 7°. Snowmelt infiltration and warm precipitation due to a sudden increase of the zero-degree isotherm elevation could have provided the main source of infiltrated water, leading to 572 enhanced water pressure at the glacier bed. During a field inspection on 12 March 1980, Marangunic (1980) found that the nearby debris-covered glacier to the east, glacier no 51 according to the Chilean glacier inventory at the time (Fig. 14), also 574 showed significant signs of surge-like instability, such as a heavily crevassed front and patches of freshly exposed ice along its entire length. The prominent terminal moraine of this glacier may have contained its detachment, though.

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The Aparejo glacier is situated in a region of complex geology with a number of weak rock formations, including sandstones and fine-grained conglomerate in the immediate vicinity of the glacier. (Ugalde, 2016). Ugalde (2016) sampled the grain size 578 distribution of the remains of the lower ice-rock avalanche deposits and did not find them to contain more fines that a typical moraine, but notes that spatial variability was high and that the 35 years since the detachment mav have depleted the deposits 580 of fine particles. In the former avalanche path, modern satellite images show streamlined striations similar to those reported from several other detachments in this contribution. Remarkably, similar striations are also visible in Hycon airphotos from 582 1956. Interestingly, the geomorphology of the deposit area is similar between the 1956 airphotos and the post 1980-event highresolution satellite images. One possible interpretation of this is that large mass flows had already originated from the Aparejo 584 cirque at earlier times. A detailed field investigation would be required to determine whether the striations consist of glacial flutes formed under a previous glacier extent or stem from a catastrophic detachment.

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In 2015, the glacier had around 15% of its pre-detachment volume, covering much of the original area, and with a surface slope of around 20° (Ugalde, 2016). The current glacier terminus lies at around 3400 m a.s.l., slightly below the lower regional   The role of basal water pressure in the detachments is difficult to examine in detail, but most detachments could have involved severe reduction in friction due to high basal water pressure (likely at least for Kolka 2002, Aru, Flat Creek, Sedongpu, 738 Aparejo). Ways to rapidly increase basal water pressure include: an increase in water input (e.g., large high-altitude rain events (Kääb et al., 2018) or increased surface snow/ice melt) into a subglacial drainage system not capable of adjusting fast enough; 740 inefficiencies or blockages of this drainage system; or increased permeability of the glacier through enhanced crevassing (Dunse et al., 2015). Sudden weakening of the strength of subglacial till under high pore water pressure and over large parts 742 of the glacier bed was shown to be a key process leading to the Aru detachments (Gilbert et al., 2018). Ongoing surge-like activity may enhance sensitivity to water input (Flowers et al., 2016).

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In Figure 18 we attempt to summarize the main drivers of the glacier detachments described here. We define a detachment's disposition as the sum of long-term factors that might promote glacier detachments and refer to triggers to describe short-term 746 factors that might suddenly tip the scale toward a catastrophic failure. Fundamentally, it seems that different combinations of dispositions and triggers are able to produce instability. Aside from the commonalities mentioned above, the observed and planning. The particularly low friction coefficients (H/L) involved in the detachments enable them to travel over low slopes (where other types of ice-rock avalanches would stall) and to cover large distances. The detachment events seem very 802 rare, but their large volumes, fast evolution, and the exceptionally long reaches and high speeds hold the potential for severe impacts even far away from the source. Our compilation of all (so far) known cases shows that low-angle glacier detachments 804 might have, though rare, more frequently occurred than thought. The differences between the events suggest that there is no straightforward way to predict where they might occur, but the following list of the most common conditions might support a 806 more systematic assessment.  high driving stresses, the resulting concentration of high shear stresses, and the lack of sufficient resistance develop fastly, or are combined fastly, preventing the glacier to adjust to changing forces in a steady way. Several of the factors 838 potentially involved in the detachment are subsequently also able to strongly reduce basal friction of the resulting icerock avalanche and lead thus to particularly low angles of reach. Boxes in the figure indicate main physical conditions, 840 grey italic text indicates different actual processes that can fulfil these conditions, sorted from long-term (left) to shortterm (right) variability.

Conclusions
In this contribution we describe around a dozen ice-rock avalanche events that we characterize as sudden large-volume 844 detachments of low-angle glaciers. Overall, these events seem to be more frequent than previously thought. The detached volumes ranged from a few up to more than 100 10 6 m 3 . We described one new event in the same size-class as the 2002 Kolka 846 https://doi.org/10.5194/tc-2020-243 Preprint. Discussion started: 22 October 2020 c Author(s) 2020. CC BY 4.0 License.
• Repeated events or geomorphological imprints of potential earlier collapses or other violent ice-rock mass flows can 878 be further investigated, but events can also happen without historical precedence through shifts in the array of failure conditions.

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• Several of the glaciers investigated here showed abnormal crevassing and enhanced precursory surface speeds in the days to weeks before detachment.

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• The surface slopes found in this study for the detached glaciers ranged between roughly 10° and 20°.
Due to the large amounts of snow and ice involved in glacier detachments, the high chance of lubrication and of liquefaction 884 of glacier ice and subglacial sediments, and smooth geometries of glacial valleys, avalanche friction is typically greatly reduced. This results in the particularly high mobility of the ice-rock avalanches resulting from low-angle glacier detachments 886 and can lead to substantial damage far from the source. Between the large runout distances and the varying factors that can impact a glacier's detachment probability, high-mountain hazard management will, after the first general assessment provided 888 in this study, benefit from more detailed investigations of glacier detachments, the conditions that lead to them and the mechanics that drive them.