Paraglacial Rock Slope Failures Research Articles

Schmidt-hammer exposure-age dating of seven paraglacial rock-slope failures in the Bolungarvík-Suðureyri area (Westfjords, Iceland)

Many rock-slope failures (RSF) impact Iceland's Westfjords morphology. Five RSF areas, totalling seven RSFs, were studied in the western fjords around Bolungarvík-Suðureyri: Óshólar, Minni-Hlið, Meira-Hraun, the Vatnadalur valley and part of the Sunddalur valley.The aim of this article is to provide chronological data of the RSFs and to subsequently obtain a better chronosequence of post-glacial events, none of which have yet been dated in the Westfjords. More particularly, we discuss a paraglacial hypothesis of their occurrence.For this purpose, the Schmidt-hammer exposure-age dating (SHD) method is used on the RSF deposits, and the rebound values obtained from the impacted blocks are converted to age estimates using radiocarbon ages obtained from the different sites in order to calculate a SHD age calibration. Then, we establish a chronology of the setting up of the events. In this way, seven RSFs have been dated and different chronological generations of deposit emplacement have been exhibited for some of them. Thus, various periods of instability affected mountain slopes. The Óshólar RSF is thought to have been deposited firstly at around 12,500 cal BP, the Minni-Hlið RSF at ∼9800 cal BP, the Meira-Hraun RSF at ∼8700 cal BP, the Vatnadalur RSFs at ∼9000 cal BP, ∼ 6900 cal BP, and ∼ 4300 cal BP, and the Sunddalur RSF at ∼9600 cal BP. These RSFs therefore correspond to so-called paraglacial events, which took place at the beginning of deglaciation of the area during the first half of the Holocene.

Rock slope failure in the Southern Carpathians (Romania): Range-wide inventory and links with long-term mountain landscape evolution

Rock slope failure (RSF) is now widely recognised as a significant contributor to the denudation of mountain ranges. Its spatial incidence can consequently be a valuable indicator of prevailing process domains, tectonic regimes and climatic variations, whether active or past. Here we provide the first systematic inventory of RSF in the Southern Carpathians, a mountain range marginally glaciated during the Pleistocene, cut by valleys up to 1.3 km deep, and displaying low-gradient summit erosion surfaces that suggest late Neogene uplift and incision of an extensive plateau. We discuss the spatial distribution of RSF in terms of process interactions and landscape evolution scenarios. Widespread metamorphic bedrock outcrops are confirmed as highly susceptible to RSF, particularly to large-scale slope deformation, but RSF also occurs on other lithologies as smaller rockslides and avalanches. Substantial spatial densities of RSF cluster around the residual, plateau-like range-top surface rims. We ascribe this to the ‘raised plateau’ features of the mountain range, where its fat, flat massifs engender greater slope stresses than would thin, serrated ones where arêtes and pyramidal peaks are gravitationally more stable. A large minority (42 %) of RSFs are spatially associated with glacial cirques and trough flanks. While these nominally qualify as paraglacial RSFs, glacial erosion may not be the main driver because the majority of RSFs occur along ridgetops and upper hillslopes in previously unglaciated fluvial contexts. Many of them thus qualify as parafluvial RSFs – i.e., occurring within a fluvial catchment but not directly generated by stream undercutting. We introduce the concept of fluvial–parafluvial and periglacial–parafluvial cycles as complementary to the recognised glacial–paraglacial cycle. All three process regimes may have prevailed concurrently or in succession, emphasising that cirque glaciation was probably not a principal agent in shrinking range-top erosion surfaces during the Quaternary. We point to post-orogenic (Neogene) tectonic uplift as the overarching driver of RSF in the Southern Carpathians, with pervasive fluvial incision keeping slopes close to failure thresholds. The role of RSF (extant and in earlier cycles) as a primary agent in perimeter scarp retreat and massif dissection here offers a ‘Transylvanian model’ of global interest whether as a comparator or a contrast for other ranges.

Paraglacial rock-slope deformations: sudden or delayed response? Insights from an integrated numerical modelling approach

Glacial and paraglacial processes have a major influence on rock slope stability in alpine environments. Slope deglaciation causes debuttressing, stress and hydro-mechanical perturbations that promote progressive slope failure and the development of slow rock slope deformation possibly evolving until catastrophic failure. Paraglacial rock slope failures can develop soon after or thousands of years after deglaciation, and can creep slowly accelerating until catastrophic failure or nucleate sudden rockslides. The roles of topography, rock properties and deglaciation processes in promoting the different styles of paraglacial rock slope failure are still elusive. Nevertheless, their comprehensive understanding is crucial to manage future geohazards in modern paraglacial settings affected by ongoing climate change. We simulate the different modes and timing of paraglacial slope failures in an integrated numerical modelling approach that couples realistic deglaciation histories derived by modelling of ice dynamics to 2D time-dependent simulations of progressive failure processes. We performed a parametric study to assess the effects of initial ice thickness, deglaciation rate, rock-slope strength and valley shape on the mechanisms and timing of slope response to deglaciation. Our results allow constraining the range of conditions in which rapid failures or delayed slow deformations occur, which we compare to natural Alpine case studies. The melting of thicker glaciers is linked to shallower rockslides daylighting at higher elevation, with a shorter response time. More pronounced glacial morphologies influences slope lifecycle and favour the development of shallower, suspended rockslides. Weaker slopes and faster deglaciations produce to faster slope responses. In a risk-reduction perspective, we expect rockslide differentiation in valleys showing a strong glacial imprint, buried below thick ice sheets during glaciation.

Glacial conditioning and paraglacial sediment reworking in Glen Croe (the Rest and be Thankful), western Scotland

Glen Croe, located near the western edge of the Loch Lomond and Trossachs National Park, is a well-known landslide hazard site in Scotland. Debris flows have repeatedly closed the A83 Rest and be Thankful road that passes through the valley, and considerable investment has been directed towards hazard risk reduction. However, little research has focused on the former glaciation and paraglacial response that have played an important role in governing current landscape processes at the site. This paper addresses the knowledge gap by investigating the glacial processes that have shaped and conditioned Glen Croe, and pathways that have characterised subsequent paraglacial sediment transfer. The large-scale valley form results from watershed breaching and interaction between glacier erosion and paraglacial rock slope failures. The distribution of thick glacigenic sediment is conditioned by deposition at former lateral ice margins, which was influenced by topography. Sediment reworking has resulted in accumulation of debris cones and alluvial fans in the upper part of the catchment, and growth of a delta at the outlet. Spatial connectivity mapping supports an interpretation whereby upper Glen Croe is poorly connected to the valley outlet, influencing sediment storage. In contrast, slopes in the lower part of Glen Croe are well connected. Sediment distribution in Glen Croe fits within the context of glaciated valley and paraglacial landsystems, allowing an understanding of sources and transport pathways. In upland infrastructure corridors this type of information is potentially helpful for understanding how landscapes might be affected by renewed sediment reworking under altered threshold conditions.

A paraglacial rock-slope failure origin for cirques: a case study from Northern Iceland

The contribution of paraglacial rock-slope failure (RSF) to the excavation of cirques in glacial landscapes is explored in the Tindastóll Mountain, Skagi peninsula, northern Iceland. We analyze 8 cirques that developed during the Quaternary and 13 paraglacial RSF cavities that developed during the Holocene in this paleo-plateau. From a reconstruction of the pre-Quaternary surface of the plateau, we calculate the excavated volumes of the cirques and RSF cavities and quantify the contribution of Holocene RSF to cirque growth. By extrapolating this contribution to the whole Quaternary, we find that paraglacial RSF is a first-order contributor to the development of cirques. This contribution is twofold: first, paraglacial RSF creates cavities along valley slopes, in which cirque glaciers can develop during subsequent glaciations; second, paraglacial RSF along pre-existing cirque walls promotes cirque widening and deepening. Our results also reveal that Quaternary glacial/paraglacial erosion rates range from 0.02 to 0.17 mm yr‑¹ in the studied cirques. We infer that glaciers are (i) efficient preparatory factors for the destabilization of slopes by paraglacial RSF, (ii) efficient conveyors to evacuate deposits produced by paraglacial RSF in cirques, but (iii) not necessarily predominant agents in the excavation of cirque bedrocks.

Timing of paraglacial rock‐slope failures and denudation signatures in the Cantabrian Mountains (North Iberian Peninsula)

AbstractGlacial erosion of hillslopes and stress changes induced by the transition from glacial to nonglacial conditions exert a strong influence on slope instability and are considered among the scope of paraglacial geomorphology. Failure mechanisms and coupling between paraglacial rock‐slope failures (RSFs) and fluvial erosion are difficult to define. Here we show a preliminary spatio‐temporal framework of paraglacial RSFs in a small catchment of the central Cantabrian Mountains, the San Isidro valley, with a dense concentration of RSFs. Preliminary radiocarbon dates obtained from two floodplain sequences deposited upstream from RSFs indicate that their sedimentation started as consequence of valley impoundment by RSFs after glacier retreat (after approximately 16.1 ka), consistent with the deglaciation pattern of nearby valleys. RSFs continued during the Holocene. Glacier erosion, debuttressing, and stress‐release conditions played an important role in slope destabilization as preparatory factors in all cases and probably triggered the oldest events. However, the long prefailure endurance (approximately 12 ka) between RSFs points to other factors such as rainfall and fluvial down‐cutting of hillslopes as triggers for Holocene events. Postglacial fluvial incision rates of 2.2–2.5 mm a−1 were estimated along gullies carved into bedrock areas nonaffected by RSFs. These values are one order of magnitude higher than previous rates based on other geomorphological proxies (~ 0.2 mm a−1), suggesting accelerated fluvial incision following the last deglaciation. Local RSFs contributed to increase in fluvial incision rates by a factor of three. This study provides a quantitative perspective of postglacial land degradation relevant for understanding postorogenic landscape evolution.

Is gravitational spreading a precursor for the Stífluhólar landslide (Skagafjörður, Northern Iceland)?

In Iceland, the Late Weichselian deglaciation led to paraglacial topographic readjustments in the form of extensive deep-seated gravitational slope deformation (DSGSD) and large rockslope failures (landslides and rock-avalanches). Here we provide age estimates for the Sti uholar landslide (Trollaskagi Peninsula, Northern Central Iceland), which indicate that this occurred between the end of the Late Weichselian deglaciation (15-13 ka cal. yr. BP) and 8200 ± 1400 cal. yr. BP. Our surface observations also suggest that the Stifluholar landslide was probably initiated by propagation and connection of DSGSD failure planes. This observation suggests that the DSGSD can act as a precursor for landsliding in paraglacial settings. However, further investigations are still required to fully assess the role of DSGSD as a landslide precursor, both at the local and at a regional scale. Comparisons with other examples of paraglacial rock slope failures in Northern Iceland and in the world suggest that DSGSD is not the main or suficient precondition for large landslides. The landslide of Stifluholar occurred due to some combination of favorable preconditioning, predisposing and triggering controls. Predisposing controls include: the favourable dip of the bedding planes in the basaltic bedrock and the presence of intercalated poorly cohesive layers of palagonite between the lava layers, probably acting as basal sliding planes for the landslide.

Large‐scale rock slope failures in the eastern pyrenees: identifying a sparse but significant population in paraglacial and parafluvial contexts

This first overview of large‐scale rock slope failure (RSF) in the yrenees addresses the eastern third of the range. Around 30 principal RSFs greater than 0.25 km2 and 20 lesser or uncertain cases have been identified from remote imagery and groundtruthing. Compared with other European mountain ranges, RSF incidence is relatively sparse, displays no obvious regional trend or spatial clustering, and occurs across diverse landscape types, if mainly on metamorphic rocks. A transition is observed from paraglacial RSFs in formerly glaciated valleys to what are here termed ‘parafluvial’ RSFs, within wholly or mainly fluvial valleys but where slope failure is not directly provoked by or linked to river erosion. RSFs are particularly found in three topographic settings: at cirque and trough‐head thresholds (transition zones of elevated instability between cirque and main glaciated trough walls); near the upper or outer periphery of the ice field, where glacial adaptation of fluvial valleys is incomplete; and in fluvial valleys beyond glacial limits where incision is locally intense. RSF is absent from the range divide, from within cirques, and from most main valleys. In the montane areas, RSF is strongly associated with vestiges of preglacial summit surfaces, confirming that plateau ridges are less stable than sharpened crests and horns. RSF is contributing significantly to the progressive destruction of this paleic relief. The overall sparsity of RSF indicates insufficient rock mass stresses, including rebound after concentrated bedrock erosion. This may reflect a relatively weak imprint of glacial erosion, including breaching, in a context of relatively low mean rates of neotectonic uplift, possibly signalling overall that eastern yrenees landscapes are close to dynamic equilibrium.

Lateglacial rock slope failures in north‐west Ireland: age, causes and implications

ABSTRACTNine postglacial quartzite rock slope failures (RSFs) in north‐west Ireland were dated using cosmogenic 10Be. Weighted mean RSF ages range from 17.7 ± 0.9 to 12.5 ± 0.7 ka or 16.6 ± 0.7 to 11.7 ± 0.5 ka, depending on assumed 10Be production rate. All dated RSFs occurred within ∼5000 years following ice‐sheet deglaciation at ∼17.4ka (∼16.3 ka) and all but two occurred within 2000 years after deglaciation. The timing of RSFs rules out glacial ‘debuttressing’, permafrost degradation and enhanced deglacial cleft‐water pressures as triggers of failure in most cases. We infer that paraglacial stress release and associated fracture propagation were critical in reducing rock masses to critical stability, although earthquakes caused by Lateglacial glacio‐isostatic rebound and/or release of stored tectonic stresses may have triggered failure in some or all cases. In conjunction with data from related studies, our results imply that most undated RSFs outside the limit of Younger Dryas glaciation in the British Isles are of Lateglacial age, and that numerous Lateglacial RSFs occurred inside these limits, with subsequent removal of debris by glaciers. They support the view that paraglacial RSF activity in tectonically stable intraplate terrains was concentrated within a few millennia following deglaciation.

Timing and periodicity of paraglacial rock-slope failures in the Scottish Highlands

Many formerly-glaciated mountains exhibit a high density of postglacial rock-slope failures (RSFs) in the form of rockslides, rock avalanches or deep-seated gravitational slope deformations. Such RSFs are often termed ‘paraglacial’ as they reflect pre-conditioning by glaciation and deglaciation. The temporal pattern of paraglacial RSFs is unknown. We employ 47 cosmogenic isotope exposure ages obtained for 17 catastrophic RSFs in the Scottish Highlands to test models of changes in RSF frequency since deglaciation. Our results show that RSF activity spans almost the entire postglacial period from ~17ka until ~1.5ka, and that the periodicity of sampled postglacial RSFs is statistically indistinguishable from a pattern of uniform periodicity (~1ka−1). Our data do not conform to proposed models of declining RSF frequency with time elapsed since deglaciation, but suggest that the temporal distribution of postglacial RSFs is best described by a combination of rapid response (enhanced RSF frequency during or immediately after deglaciation) followed by approximately constant periodicity. Our findings are consistent with near-surface fracturing of rock when glacially-induced confining stresses are removed, causing some failures during or immediately after deglaciation and reducing other slopes to critical conditional stability so that failure is triggered by progressive rock-mass weakening or by transient triggering mechanisms. Neither permafrost degradation (thaw of ice in joints) nor seismic activity offers a general explanation for triggering Scottish RSFs, but could be contributory in some cases. The quasi-constant periodicity inferred for Holocene RSFs implies that the probability of RSFs in this seismically-quiescent intraplate area has not changed in the past ~10ka, and that further (though infrequent) catastrophic RSFs are likely to occur in the future.

The timing of paraglacial rock-slope failures: testing the exhaustion model using 10Be and 36Cl exposure dating

The timing of paraglacial rock-slope failures: testing the exhaustion model using 10Be and 36Cl exposure dating

Paraglacial rock slope failure as an agent of glacial trough widening

Abstract Rock slope failure (RSF) generates the largest single erosional events in the glacial–paraglacial land system, leaving numerous obvious cavities and less obviously weakened valley walls. Its contribution to trough widening in a mountain range has not previously been systematically quantified. Map-based measures of RSF ‘depth of bite’ are applied to five sample areas in the Scottish Highlands, and a comparator area in north Norway, all in metasediments structurally conducive to mass deformation and block sliding. Problems in applying map-based measures include bedrock cavities remaining partially occupied by failed debris or subsequent infill, and multiple planes of reference. The most practical measure is of maximum recess depth on any single contour ( D MAX ). This is a standardizable single-point indicator of visible impact, not a measure of actual cavity depth, nor an average applying to the whole RSF. In four of the five areas, average D MAX is consistent at 40–45 m. RSF breadth averages 270–600 m over the five areas. RSF affects 9% and 14% of total valley wall length in the two densest RSF areas, rising to 47% and 52% on two specific valley sides. The depth:breadth ratio in areas dominated by slope deformation can be twice that in areas of translational sliding. An evolutionary model of glacial–paraglacial cycling proposes a ‘zone of paraglacial relaxation’ in which RSF is intense in early cycles as fluvial profiles adjust to ice discharge, diminishing with maturity as trough walls become stress-hardened, and reviving in response to neotectonic and glaciological perturbations, notably ice piracy via transfluent breaching. However, a major unknown is the efficacy of glacial exploitation of RSFs: if it takes several cycles to evacuate debris and pare back cavity angles, cumulative RSF impact is lessened. Glacial–paraglacial cycling is a classic positive feedback loop, promoting valley widening beyond the parabolic norm. Preferential exploitation of structure by RSF promotes asymmetrical trough profiles. RSF acts both as a scarp retreat process, and as a slope reduction counterpoint to glacial slope steepening. In landscape evolution, it is a powerful agent in destruction of paleic relief, notably around watersheds that are undergoing breaching by transfluent ice, where trough development and widening is still vigorous.

Exposure dating and reinterpretation of coarse debris accumulations (‘rock glaciers’) in the Cairngorm Mountains, Scotland

AbstractRelict rock glaciers have considerable potential for contributing to palaeoclimatic reconstruction, but this potential is often undermined by lack of dating control and problems of interpretation. Here we reinvestigate and date four proposed ‘rock glaciers’ in the Cairngorm Mountains and show that the morphology of only one of these appears consistent with that of a true rock glacier produced by creep of underlying ice or ice‐rich sediment. All four features comprise rockslide or rock avalanche runout debris, and the possibility that all four represent unmodified runout accumulations cannot be discounted. Surface exposure dating of the four debris accumulations using cosmogenic 10Be produced uncertainty‐weighted mean ages of 15.4 ± 0.8 ka, 16.2 ± 1.0 ka, 12.1 ± 0.6 ka and 12.7 ± 0.8 ka. All four ages imply emplacement under cold stadial conditions, two prior to the Windermere Interstade of ca. 14.5–12.9 cal. ka BP and two during the Loch Lomond Stade of ca. 12.9–11.5 cal. ka BP. The above ages indicate that paraglacial rock‐slope failure on granite rockwalls occurred within a few millennia after deglaciation. The mean exposure ages obtained for runout debris at two sites – Strath Nethy (16.2 ± 1.0 ka) and Lairig Ghru (15.4 ± 0.8 ka) – are consistent with basal radiocarbon ages from Loch Etteridge, 22 km to the southwest (mean = 15.6 ± 0.3 cal. ka BP) and imply widespread deglaciation of the Cairngorms and adjacent valleys before 15 ka and possibly 16 ka. Copyright © 2008 John Wiley & Sons, Ltd.

Large rock slope failures in the Highlands of Scotland: Characterisation, causes and spatial distribution

A comprehensive database of larger Rock Slope Failures (RSFs) has been compiled and characterised for a whole mountain range, possibly for the first time apart from Iceland. Most RSFs in the Scottish Highlands occurred soon after deglaciation. A threshold size of 0.25 km 2 focuses on RSFs which contribute significantly to gross erosion in the glacial–paraglacial cycle, and on the spatial concentration of this poorly recognised process. Database verification indicates that at least 90% of the total extant population in the Highlands is covered. Standard mass movement typologies are broadly applicable, but require some refinement to characterise RSFs in a passive-margin glaciated mountain range comprised mainly of ancient metasediments. Most larger RSFs here are complex and difficult to categorise. A continuum from ‘cataclasmic’ failures (including rock avalanches) through arrested translational slides to almost in situ slope deformations is identified. The prevailing modes of large RSF are arrested (short-travel) translational sliding and gravitational sagging. Cataclasmic failure is rare, and sub-cataclasmic outcomes are more evident. A significant class of deformations exhibits compressional rather than extensional features, and may be responding to localised glacio-isostatic rebound stresses. The large RSF population is widely but very unevenly distributed across the main mountain areas: 65% are in seven main clusters, with the rest non-randomly scattered. Previous glaciological, lithological, and seismotectonic explanations are insufficient to explain this spatial pattern, and overlook the basic engineering geology prerequisites for failure. Massifs of similar relief and lithology can display intense or sparse RSF incidence. Geomorphological appraisal suggests an association with recently active glacial breaches of main and secondary watersheds, with RSF being scarce or absent in valleys inherited from Tertiary uplift, and in mountain groups away from the main watersheds. Valleys which adapted early to ice discharge are seen as ‘stress-hardened’, with their slopes unlikely to reach critical thresholds for failure after subsequent reoccupations by ice. Concentrated erosion in the last glaciations is most likely to be found close to where breaches are actively forming or enlarging. Such erosion in the slope foot and valley floor may have daylighted failure planes or generated rebound stresses augmenting those from other endemic sources sufficiently to provoke widespread paraglacial RSF. Slope stress models for deglaciated valleys and comparative studies in other glaciated mountain ranges are required to test this concept, and may assist in geohazard assessments.

Paraglacial rock-slope failures in Wasdale, western Lake District, England: morphology, styles and significance

Five sites of rock-slope failure are described from glaciated hill slopes in Wasdale, western Lake District. There are some distinct morphological variations and clear differences in failure styles among the sites but none of them appears to have been modified by glacier ice. It is inferred that they post-date the Last Glacial Maximum ( c. 21±3 cal. ka BP) and, in one case, possibly the Loch Lomond Stade glaciation (12.9–11.5 cal. ka BP). More precise dates for the failures are unavailable and they might be of different ages. The failures are regarded as paraglacial (glacially-conditioned), that is, resulting from readjustments to rock-slope stress fields following loss of ice. It is possible that stress release and seismic activity combined to trigger failure. Three of the failures have scarp-edge locations and these are probably residual components of greater failures of unknown size and style that provided substantial amounts of coarse rock debris to the Wast Water screes and also contributed to the widening of Wasdale. Based on these sites, a preliminary conceptual model of talus slope development is proposed. The morphology and aspect of the other two sites suggest they could become sites of cirque initiation following removal of their failed materials. All five failures have led to reductions in the mass of their respective mountain blocks, suggesting that paraglacial rock-slope failures may have played a greater role than previously appreciated in the evolution of the Lake District landscape.

Rock slope failure and landscape evolution in the caledonian mountains, as exemplified in the abisko area, northern sweden

Paraglacial rock slope failure (RSF) is here studied as a locally major contributor to mountain landscape evolution in the Caledonian ranges. Dense RSF clusters exist in Scotland and Norway, but overall RSF distribution in Scandinavia is poorly known. In the Abisko area, air photo scrutiny confirms the reported incidence of sparse but significant RSF. In the Kärkevagge complex, the Rissa RSF is one of the largest in northern Europe, with a scar volume of 42 Mm3. The well–known Giant Boulder Deposit (GBD) is a rock avalanche emanating from the Rissa RSF scar, the interpretation of wholesale valley wall retreat at deglaciation being discounted. In the adjacent valley of Vassivagge, a major RSF on Vuoitasrita has a similar area and morpholocation, but lacks a GBD. It has consumed 5–10% of the relict preglacial mountain surface. Both RSFs are near incipient watershed breaches in valleys which may have undergone vigorous enlargement during the last stadial. Glaciation history may explain spatial incidence as well as neotectonic and other triggers. The localised geomorphic impact of RSF in the Abisko mountains is high by comparison with contemporary slope processes. The cumulative impact of paraglacial RSF over the Quaternary may have been considerable, and RSF may be an indicator of concentrated late–stage glacial erosion.