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Instrumental monitoring of displacement rates of slow-moving rockslides is limited to an insufficient length of the pre-catastrophic failure deformation time. Prehistorical slip rates need to be considered to provide context for monitoring data in relation to the length of the failure process. Cosmogenic 10Be exposure ages (n = 10) on the backscarp of the Gamanjunni-3 rockslide (northern Norway) are between 5.3 ± 0.5 ka and 1.2 ± 0.1 ka. These ages were adjusted for prefailure nuclide production at depth based on the 10Be concentration in quartz from a stable horizontal bedrock surface above the rockslide. Displacement initiated between 6.6 and 4.3 ka with a relatively fast or instantaneous displacement rate that decelerated a few hundred years after initiation. Slide initiation coincided with the end of the Holocene thermal optimum in northern Norway. The age-height relationship on the backscarp yields an average paleo-slip rate of 28 (-5/+7) mm/yr. This is significantly slower than present-day rates of 54 mm/yr. However, during slide initiation, slip rates were faster than present-day rates. Considering its location within the modern permafrost zone and the observations from instrumental and Holocene displacement rates, we suggest that future global climate changes will influence the Gamanjunni-3 block slip rate. [ABSTRACT FROM AUTHOR]

Mudstones hosting Burgess Shale-type preservation of soft-bodied fossils are commonly held to be characterized by little to no bioturbation. This has been taken as evidence for bottom- water dysoxia or anoxia, along with anaerobic conditions in the sediment, which favored preservation of soft tissues by hindering decay. Although invisible on fresh and weathered surfaces, laminated claystone comprising the middle Cambrian (Drumian) Ravens Throat River Lagerstätte in the Rockslide Formation of the Mackenzie Mountains, northwestern Canada, is revealed by preparation of surfaces sawn parallel to bedding to exhibit extensive burrowing. Four types of burrows are distinguished: (1) rare large forms averaging 15 mm in diameter; (2) backfilled vertical burrows 3-6 mm wide; (3) oblique to horizontal burrows 2-4 mm wide and typically with meniscate backfilling; and (4) tiny, short, mostly vertical burrows 0.5-1 mm in diameter. The third group is the most common, locally completely bioturbating laminae and penetrating worm carcasses; it conforms to Planolites. A variety of ethologies is indicated, with the large type seemingly serving as a dwelling burrow and the smaller ones from deposit-feeding. Although dysoxic bottom conditions probably developed occasionally, the widespread burrowing argues for predominantly oxic conditions, and it indicates that restriction of bioturbation was probably not the most important factor leading to soft-tissue preservation. Bioturbation might be more common in other Cambrian Lagerstätten than is currently believed, and it is possible that low-oxygen conditions at the seafloor were not fundamentally necessary for Burgess Shale-type preservation. [ABSTRACT FROM AUTHOR]

Quaternary tectonic activity in the transition area between the Central and Patagonian Andes is closely associated with an anomalous cluster of rockslides: 19 rockslides with volumes up to 4 x 109 m³ developed in plateau basalts. We divided them into two groups: (A) rockslides related to neotectonic activity and (B) rockslides not related to neotectonic activity. Thirteen rockslides, with a total volume of ~10 km³, which lie on either folds or faults, have been displaced parallel to the structures and perpendicular to the valley axis, and they exhibit headscarps several kilometers away from the valley axis. Most of them are larger than 109 m³, and are generally of rock avalanche type with a high degree of crushing of rocks, although local relief in some cases does not exceed 200 m. Nine rockslides with a total volume of 8.9 km³ are related to folds, while four with a total volume of 1.3 km³ are related to faults. The six rockslides not related to neotectonic activity have a total volume of 0.25 km³ (of which the largest one accounts for 0.17 km³), and are rotational slides and block topples with a low degree of rock fragmentation, although local relief is up to 400 m. The ³He and 21Ne surface exposure ages for six of these slides, as well as relative age assessment based on stratigraphic relation with glacial deposits and the drainage development on the rockslide deposit, suggest that the rockslide ages spread rather randomly between pre-glacial and mid Holocene, discarding climatic conditions as a common triggering factor. The absence of structures that can represent ideal sliding planes shows that rock fracturing due to neotectonic activity is a major conditioning factor for failures and that the magnitude of landslides is strongly controlled by the type of deformation. [ABSTRACT FROM AUTHOR]

The collapse of Mount St. Helens (United States) on 18 May 1980 is one of the only incidents of its kind that was visually witnessed and instrumentally recorded. Previous analyses determined that the northern flank of the mountain failed, which resulted in a rockslide, and disintegrated during its mobilization to form a blocky facies and hummocky terrain in the downslope region. Gary Rosenquist's iconic photographs of the initial 18 May collapse were here analyzed by using a modern photogrammetric method to track portions of the moving flank across the photographs in the sequence. Thirty years after the 1980 Mount St. Helens rockslide, the digital image correlation technique enabled a precise investigation of the associated displacement vectors and strain. A listric basal detachment and several associated faults at the lateral edge of the rockslide were identifed. In addition, the heterogeneous movements and a number of shear zones were detected within the sliding block, which had previously been assumed to be cohesive. The results of my study demonstrate the value of using optical images for strain analyses of active volcanoes-even decades after recording. [ABSTRACT FROM AUTHOR]

We present quantitative analyses of recent large rock falls in Yosemite Valley, California, using integrated high-resolution imaging techniques. Rock falls commonly occur from the glacially sculpted granitic walls of Yosemite Valley, modifying this iconic landscape but also posing signifi cant potential hazards and risks. Two large rock falls occurred from the cliff beneath Glacier Point in eastern Yosemite Valley on 7 and 8 October 2008, causing minor injuries and damaging structures in a developed area. We used a combination of gigapixel photography, airborne laser scanning (ALS) data, and ground-based terrestrial laser scanning (TLS) data to characterize the rock-fall detachment surface and adjacent cliff area, quantify the rock-fall volume, evaluate the geologic structure that contributed to failure, and assess the likely failure mode. We merged the ALS and TLS data to resolve the complex, vertical to overhanging topography of the Glacier Point area in three dimensions, and integrated these data with gigapixel photographs to fully image the cliff face in high resolution. Three-dimensional analysis of repeat TLS data reveals that the cumulative failure consisted of a near-planar rock slab with a maximum length of 69.0 m, a mean thickness of 2.1 m, a detachment surface area of 2750 m2, and a volume of 5663 ± 36 m3. Failure occurred along a surfaceparallel , vertically oriented sheeting joint in a clear example of granitic exfoliation. Stress concentration at crack tips likely propagated fractures through the partially attached slab, leading to failure. Our results demonstrate the utility of high-resolution imaging techniques for quantifying far-range (>1 km) rock falls occurring from the largely inaccessible, vertical rock faces of Yosemite Valley, and for providing highly accurate and precise data needed for rock-fall hazard assessment. [ABSTRACT FROM AUTHOR]

We present a structural analysis of subaerial natural and analogue rockslide avalanches. Such deposits often have well- developed faults, folds, and hummocks. These structures can be used to determine the kinematics and dynamics of emplacement. Large-scale terrestrial rockslide avalanches show large runout distances compared to their fall height. Most attempts to explain this phenomenon invoke fluidizing mechanisms or lubricating agents to reduce forces opposed to momentum, especially at the base. However, the properties and mechanics of low friction are still poorly understood. Any model for motion and emplacement must integrate geometric, morphologic, and structural features, all crucial in constraining kinematics and dynamics. Here we fi rst examine the morphological and structural features displayed by 13 natural rockslide avalanche deposits; we then use simple and well-constrained analogue models involving the slide of stratified granular material down smooth, curved ramps. These differ from previous analogue models in that we concentrate on observing the structures produced by brittle deformation and use a low-friction sliding surface. Models show that variations in the sliding surface curvature, lateral profile, roughness, and modifications in material cohesion can successfully reproduce the majority of rockslide-avalanche deposit features. After discussing the geometrical and dynamic similarity between experiments and natural examples, we propose a model for structure formation and a fourfold classification based on model and natural deposit morphology and dynamics: hummocky, nonhummocky, dominantly extensional, and dominantly compressional rockslide avalanches. The models require a brittle core and surface that spreads and contracts by adjustment on large numbers of faults that bottom into a low-friction décollement layer. Spreading is accommodated by normal and strike-slip faults, while on deceleration, thrust faulting generates thickening. To be realistic, any physical predictive model must take into account these fundamental kinematic and structural aspects. [ABSTRACT FROM AUTHOR]

This study examines the geomorphic effects of large (>106 m³) rock-slope failures on long profiles of rivers in the Swiss Alps and the New Zealand Southern Alps. Regression of channel slope versus drainage basin area objectively highlights knickpoints separating incised from aggraded reaches that often correspond to locations of large rock-slope failures. For a fixed concavity index, the highest values of the steepness index and erosion index along a given profile spatially coincide with breach channels cut into formerly river-damming rockslide debris as old as 10 k.y. Assuming that the knickpoints do not predate slope failure, data show that high profile steepness and inferred specific stream power are not always the cause, but often a result, of large river-blocking rock-slope failure in mountain basins. Omission of rockslide data from slope-area plots lowers the steepness index and increases the concavity index on average, yet only in few cases more than one standard deviation. [ABSTRACT FROM AUTHOR]

Many rock avalanches entrain and liquefy saturated soil from their paths. Evidence for this includes mud displaced from the margins of rock avalanche deposits, substrate material smeared along the base of deposits, extrusion of liquefied soil upward through the deposits, and increases of volume. A hypothesis first suggested in 1881 and since reinforced by several authors suggests that entrainment of substrate material increases mobility. Although the process has been discussed in the literature for more than 100 years, few detailed and quantitative descriptions exist. The main purpose of this paper is to describe two recent cases from British Columbia, Canada, where rockslides entrained substrate on a very large scale, influencing the character of the events. Estimated volume balance curves, based on detailed field mapping, are provided for both cases. Dynamic analyses are carried out using a numerical model and using the same set of rheological parameters. The mechanism of material entrainment and displacement is discussed. The data suggest that rapid rock failures entraining very large quantities of saturated substrate material represent a special type of landslide, transitional between rock avalanche and debris avalanche. Many rock avalanches can thus be seen as end members of a continuum of phenomena involving rock failure followed by interaction with saturated substrate. [ABSTRACT FROM AUTHOR]

Describes the effects of an unusual rock fall that occurred at Happy Isles in Yosemite National Park, California. Description of the event; Comparison with other rock falls; Importance of considering geological hazards such as airblasts in the development of mountainous areas.