Rock Avalanche Deposits Research Articles

Modeling the formation of toma hills based on fluid dynamics with a modified Voellmy rheology

Abstract. Toma hills are perhaps the most enigmatic morphological feature found in rock avalanche deposits. While it has been proposed that toma hills might emerge from the fluid-like behavior of rock avalanches, there still seems to be no consistent explanation for their occurrence. This paper presents numerical results based on a modified version of the Voellmy rheology, which was recently developed to explain the long runout of rock avalanches. In contrast to the widely used original version, the modified Voellmy rheology defines distinct regimes of Coulomb friction at low velocities and velocity-dependent friction at high velocities. When movement slows down, returning to Coulomb friction may cause a sudden increase in friction. Material accumulates in the region upstream of the point where this happens. In turn, high velocities may persist for some time in the downstream and lateral ranges, ultimately resulting in a thin deposit layer. In combination, both processes generate more or less isolated hills with shapes and sizes similar to those of toma hills found in real rock avalanche deposits. Thus, the modified Voellmy rheology suggests a simple mechanism for the formation of toma hills.

Characteristics of Rock Avalanche Deposit in Wangjiapo, Ludian Based on UAV Aerial Image Recognition

Rock avalanche disasters in alpine and gorge regions are frequent and large in scale and cause severe damage. The movement of a rock avalanche is complex and has not been fully studied. The deposits of a rock avalanche can provide valuable insights into its movement process, which is crucial in understanding the rock fragmentation mechanism and predicting disaster-affected areas. Taking the Wangjiapo rock avalanche in Yunnan Province of China as an example, the size, shape and distribution characteristics of the deposit were analyzed based on field surveys, unmanned aerial vehicle (UAV) photography and image recognition technology. Initially, 3062 deposited rock blocks were manually measured in the field. Subsequently, the Particles/Pores and Cracks Analysis System (PCAS) was employed to identify 11,357 rock blocks with an area greater than 0.1 m2 from UAV orthophotos. By comparing the characteristics of the rock blocks obtained through image recognition and manual measurement, the statistical analysis of UAV aerial imagery combined with PACS proved feasible in studying the Wangjiapo rock avalanche. The results showed that the rock block movement was accompanied by fragmentation and sorting processes; furthermore, the roundness increased with the migration distance. Small blocks were more prevalent at the foot of the slope, while irregularly shaped, large blocks dominated in source areas. The movement of huge blocks was characterized by significant potential energy-driven features and inertia advantages, allowing them to travel farther than smaller blocks, and they tended to be concentrated in the central area of the deposit. Additionally, affected by the cementation degree of breccia and the topography, the blocks in the eastern and western deposit areas exhibited different fragmentation and deposition characteristics.

Observations of avalanche–substrate interactions in the Iymek rock avalanche deposit: A possible causative mechanism

Ubiquitous and complex avalanche–substrate interactions during rock avalanche emplacement have attracted widespread attention in recent years and are regarded as vital processes influencing mobility and damage potential through rapid changes in avalanche mechanical properties. However, the fundamental interaction mechanisms of avalanche mass on substrate and the resultant effect on mobility have yet to be elucidated for natural events. To better understand the mechanics of avalanche–substrate interactions, we present a detailed study on the spectacular synsedimentary deformation structures at the bottom of the gigantic Iymek rock avalanche deposit, including undulose structures, flame structures, mixed textures, clastic dikes, and cracked gravels. The cracked gravels demonstrate that the overriding avalanche mass exited high-energy shearing on the avalanche–substrate interface, where the stress in substrate far exceeded the overburden from the avalanche mass during the avalanche–substrate interaction. Along with high-energy shearing, two different interaction modes with the change of substrate materials are identified, i.e., the Kelvin–Helmholtz instability (KHI) characterized by undulose-to-flame structure transitions, and the Raleigh–Taylor instability (RTI) characterized by the formation of clastic dikes. The KHI is interpreted as a result of the growth of shear instabilities induced by high-energy shearing along the avalanche–substrate interface. The RTI is associated with local liquefaction of the water-bearing sandy substrate and was mainly induced by the high-frequency ground vibrations generated by high-energy shearing. Therefore, we propose that the overriding avalanche mass exited high-energy shearing on the substrate during the avalanche–substrate interaction, which motivated two predominant physical processes of the KHI and RTI along the avalanche–substrate interface. The shear-induced KHI is a potential mechanism of erosion and entrainment in rock avalanches and is responsible for promoting the incorporation of substrate materials into moving avalanche mass. These results not only yield profound insights into the interaction behaviours between rock avalanches and substrates but also provide a fundamental geological prototype to motivate further modelling work to elucidate rock avalanche dynamics.

Rock avalanches in northeastern Baffin Island, Canada: understanding low occurrence amid high hazard potential

Rock avalanches in fjord environments can cause direct catastrophic damage and trigger secondary submarine landslides and tsunamis. These are well-documented in Greenland, Norway, and Alaska but have gone largely unreported in the extensive fjord terrain of the eastern Canadian Arctic. We provide the first inventory of rock avalanche deposits in northeastern Baffin Island—a region characterized by moderate to high seismic hazard, steep and high-walled fjords and glacial valleys, active deglaciation, and observed climate warming. Over a broad study area of ~60,000 km2, one sixth of the terrain had sufficient slope height and gradient to potentially generate rock avalanches. Within that hazard zone, we identified eight rock avalanche deposits at six locations. Only three rock avalanche deposits at two locations are dated, using aerial imagery (1958-present), to the last century while five deposits at four locations are inferred as syn- to post-glacial, likely occurring shortly after local debuttressing. These total numbers fall well below documented inventories from Greenland, Norway, and Alaska. We hypothesize that (1) continuous permafrost persists throughout this region and continues to act as a stabilizing factor and (2) rock mass quality is high in areas of most extreme relief contrast within the study region relative to analogous high-latitude fjord systems such as those in southwestern Greenland. We suggest that Baffin Island is currently in a period of quasi-stability that follows the intense instability during initial deglaciation, yet precedes the higher anticipated slope instability that may occur during permafrost degradation.

Distinctive shear zones demonstrate pervasive laminar cataclastic flow throughout the gigantic Iymek rock avalanche

Intensive fragmentation is a pervasive process during rock avalanche propagation, with a series of typical shearing characteristics being generated, indicating the occurrence of differential shear-induced comminution of clasts. However, much less is known about how shearing evolves within a rock avalanche over long runout. In the Iymek rock avalanche (IRA), pervasive shear zones characterized by multistoried, multiscale, and multistyle shear structures are well developed along the travel path of the IRA, providing an invaluable opportunity to decipher the evolution of shear-induced fragmentation occurred in rock avalanches. In these shear zones, a series of indicative structures, including preserved stratigraphic sequences, jigsaw structures, Riedel shear arrays, sigmoidal shear planes, asymmetrical folds, bookshelf and boudin-like structures, and anastomosing shear networks, are identified. These structures indicate that a laminar-like flow regime characterized by low disturbance but intensive shearing should dominate the emplacement of the IRA, contributing to the formation of these pervasive shear structures and material facies. As indicated by the progressive evolution of these pervasive shear structures, three facies are determined from the transition zone to the accumulation zone, i.e., low-fragmented facies, ductile-dominated facies and brittle-dominated facies. The low-fragmented facies is mainly distributed in the transition zone, similar to a semisolid phase rather than the solid phase of the bedrock in the source area. Conversely, the brittle-dominated facies features a high fragmentation degree and is predominant in the accumulation zone, which mainly consists of a semifluid to fluid phase. This indicates that the rock avalanche mass should evolve from a solid phase to a fully fluid phase during propagation, which involves clast fracturing and comminuting, frictional sliding and rotation. Therefore, we propose that the rock avalanche is a classical cataclastic flow driven by progressive and differential shearing comminution during its laminar-like propagation instead of a granular flow. These results provide a significant basis for interpreting similar structures in other rock avalanche deposits and yield new insights for understanding the emplacement mechanisms of rock avalanches.

Effect of Rockfall Spatial Representation on the Accuracy and Reliability of Susceptibility Models (The Case of the Haouz Dorsale Calcaire, Morocco)

Rockfalls can cause loss of life and material damage. In Northern Morocco, rockfalls and rock avalanche-deposits are frequent, especially in the Dorsale Calcaire morpho-structural unit, which is mostly formed by Jurassic limestone and dolostone formations. In this study, we focus exclusively on its northern segment, conventionally known as “the Haouz subunit”. First, a rockfall inventory was conducted. Then, two datasets were prepared: one covering exclusively the source area and the other representing the entirety of the mass movements (source + propagation area). Two algorithms were then used to build rockfall susceptibility models (RSMs). The first one (Logistic Regression: LR) yielded the most unreliable results, where the RSM derived from the source area dataset significantly outperformed the one based on the entirety of the rockfall affected area, despite the lack of significant visual differences between both models. However, the RSMs produced using Artificial Neural Networks (ANNs) were more or less similar in terms of accuracy, despite the source area model being more conservative. This result is unexpected given the fact that previous studies proved the robustness of the LR algorithm and the sensitivity of ANN models. However, we believe that the non-linear correlation between the spatial distribution of the rockfall propagation area and that of the conditioning factors used to compute the models explains why modeling rockfalls in particular differs from other types of landslides.

Rock-avalanche deposit causes extreme high indoor radon concentrations in Kinsarvik, Ullensvang municipality, Western Norway

The radon problem in Kinsarvik, Ullensvang municipality in Western Norway, is among the most severe in the world. Annual average indoor radon concentrations as high as 56,000 Bq/m3 have been measured. The problem affects >100 houses, where average yearly indoor radon concentrations of 4340 Bq/m3 has been reported. Indoor radon concentrations vary during the year, which has been explained by thermally induced flows of radon-bearing air through a porous ice-margin deposit.Updated geological knowledge and new mapping methods (LIDAR data) have allowed for a re-examination of the problem. There is significant evidence that does not fit with the established geological understanding that classifies the Kinsarvik deposit to be an ice-marginal moraine. Instead, the landforms and internal structures of the deposit are characteristic of a rock-avalanche deposit. The LIDAR data suggests a rockslide deposit of approximately 50 million m3.Airborne and ground gamma-ray spectroscopy show elevated uranium content in the granitic bedrock north and east of Kinsarvik. These uranium-bearing rocks are also found in an open pit in the Kinsarvik deposit.Terrestrial cosmogenic nuclide dating of four surface boulders shows an average age of 10,900 ± 600 years. This suggests the rock avalanche fell immediately after the last deglaciation of Western Norway.Our results support that the radon problem in Kinsarvik is caused by uranium bearing bedrock and a porous rock-avalanche deposit that emanates radon, which in turn exhibits an alternating, temperature driven flow direction.

Influence of intergranular friction weakening on rock avalanche dynamics

In this paper, we investigated the significance of intergranular friction weakening on the dynamics of the Jiweishan rock avalanche. First, high-velocity-friction tests simulating the shearing between the surfaces of adjacent grains were conducted to obtain a model of friction weakening of grains inside granular debris during transport. Then, the model was incorporated into an improved discrete element method that could consider intergranular friction weakening to simulate the dynamic course of the Jiweishan rock avalanche. Finally, we compared the results from the numerical models with and without granular friction weakening to investigate its influence on the dynamics of the rock avalanche. The results of the high-velocity-friction tests revealed that intergranular shearing could significantly reduce the friction of granular surfaces; this was determined by the overall results for normal stress, shear velocity and shear displacement. Our modeling results indicated that the rapid increase in normal stress and shear velocity were the main reasons for the rapid decrease in intergranular friction coefficients in the falling and collision stages. However, the friction coefficients further decreased due to increasing shear displacement between rock debris in the flowing stage. Furthermore, obvious intergranular friction coefficient distribution characteristics in rock avalanche deposits could be observed from our modeling; they tended to decrease with increased depth in the deposit and increasing distance from the source. With intergranular friction weakening, a faster velocity (∼10.0–15.5 m/s) and a longer transport time (∼12–44 s) of the rock debris were modeled due to the lower frictional energy consumption caused by intergranular friction weakening. Finally, the transport distance considering intergranular weakening was ∼ 395–711 m longer than that without weakening. Further analysis showed that intergranular friction weakening may be applied to explain the high mobility of rock avalanches. The results of our study may improve predictions of the speed and travel distance, and consequently the risk assessment, of widely distributed rock avalanches worldwide.

Vestige of a subaerial rock avalanche deposit in an upper Cretaceous synorogenic wedge-top succession (Gosau Group, Eastern Alps)

Modern subaerial catastrophic mass-wasting (rockslides, rock avalanches) are identified as being huge bulky masses with a surface littered with boulders to blocks. The identification of partly preserved deposits of ‘fossil’ subaerial catastrophic mass-wasting, in contrast, requires assessment of deposit fabrics. In the Alps, within the synorogenic Gosau Group (Turonian to Ypresian) that accumulated in an oblique-convergent setting, we had identified a ‘megabed’ (Wäschkogel megabed) up to 95 m thick that is interpreted as a vestige of a subaerial rock avalanche deposit. This megabed is an extremely poorly sorted, disordered, clast-supported deposit of sand- to boulder-sized dolostone fragments from an upper Triassic succession that comprises the local bedrock. In the megabed, numerous clasts display crackle, jigsaw and mosaic fabrics with fitted subclast boundaries. The matrix is a lithified gouge of fragmented dolomite crystals. In its upper part, the megabed displays very thick planar parallel stratification. Across strata, no change in sediment fabric is discerned.The distinctive features of the megabed, i.e., the sharp intercalation between fluvial deposits, large bed thickness with a uniform fabric, abundant clasts fractured in situ, and the gouge matrix all indicate that the megabed and lateral correlatives accumulated from a rock avalanche. The planar stratification in the upper part of the megabed is interpreted as a dynamic feature produced by internal simple shear during a terminal phase of rock avalanche propagation. Bedrock geology and the distribution of other Gosau Group deposits in the area suggest that the rock avalanche propagated along a SSW–NNE striking incised valley. This is the first identification of a subaerial rock avalanche deposit in the Gosau Group. Our case study may help to consolidate the awareness of deposits of fossil subaerial catastrophic mass-wasting.

Age and recurrence of coseismic rock avalanches in Sierra de la Sobia (Cantabrian Mountains, Spain)

Dating of rock avalanche clusters along faults may provide paleo-earthquake data for locations where direct fault-rupture evidence is rare. This study focuses on the rock avalanche cluster preserved in the western slope of Sierra de la Sobia (Cantabrian Mountains, Spain), where evidence of Quaternary faulting was previously reported at the Marabio Fault segment, a splay of the long-lived León Fault. In a previous study we interpret this cluster as seismically induced based on the kinematic analysis of local fault structures, the block size distribution analysis of several rock avalanche accumulation bodies, and the stability analysis of current slopes. In this new study we date these deposits to assess the timing of the recurrent events and evaluate a paleo earthquake record. We determine the ages of 20 boulders from the coarse carapace of two calcareous rock avalanche deposits using the 36Cl Cosmic Ray Exposure dating technique. Our results indicate that the youngest instability event occurred 8.5 ± 0.6 ka ago and was simultaneously recorded by the recent lobes of the two rock avalanches and could correspond to the latest severe earthquake striking this area. The analysis of boulder age clusters in the middle and distal lobes of both rock avalanches reflects up to four additional coseismic instability episodes between ∼22.0 and ∼10.9 ka, with a recurrence interval of ∼3.7 ka on average. Extending this chronological analysis to other rock avalanche clusters could improve our understanding on landscape evolution and its potential for developing regional paleo-earthquake catalogs for the Cantabrian Mountains and other similar contexts.

The Flims rock avalanche: structure and consequences

The Flims rock avalanche has a gliding surface that cuts down section in a limestone sequence and does not follow a weak horizon. The gliding surface is parallel to bedding and/or to the penetrative Alpine foliation in the limestone that is characterized by a shape-preferred orientation of calcite grains. Predisposition was governed by structural weaknesses in form of sub-vertical fault zones within solid limestone. Faults controlled the orientation of lateral scarps of the rock avalanche. The main body of the rock avalanche behaved as semi-coherent mass, which preserved the original overall structure. The internal deformation occurred by dynamic fragmentation, which was distributed rather heterogeneously. Fragmentation starts by the formation of veins consisting of comminuted limestone. In a later stage crosscutting veins coalesce to form a texture with entirely comminuted limestone (“rock flour”) containing angular shattered fragments of limestone of variable sizes. The involvment of the post-glacial water-saturated substrate contributed to the long-runout of the Flims rock avalanche. The substrate was plowed bulldozer-wise at the front of the rock avalanche and escaped upward through the moving rock avalanche as clastic dikes and blow-out pipes. Bulldozing raised the valley floor in front of the rock avalanche by more than 100 m and, on the margins, entrained pieces of the valley flank and the neighboring, older Tamins rock avalanche deposit. The liquefied substrate breached the Tamins rock avalanche at Reichenau and entrained fragments thereof down the valley. The fragments now form the famous tumas of Domat/Ems, Felsberg and Chur. Lake Ilanz, dammed the by Flims rock avalanche, experienced a first major outburst shortly after its formation.

Geology, geomorphology and geochronology of the coseismic? Emad Deh rock avalanche associated with a growing anticline and a rising salt diapir, Zagros Mountains, Iran

This work documents for the first time the prehistoric, but morphologically pristine, Emad Deh rock avalanche of the Zagros Mountains. The ca. 420 Mm3 rock avalanche was initiated as a rockslide on a dip slope affecting a weak unit of marls with interbedded limestones overlain by a thick and competent limestone formation. The slope is located on the northern limb of the growing Gavbast Anticline and at the edge of the inflating Gavbast Dome, related to a buried salt diapir of Hormuz salt. The unconfined rock avalanche deposits, covering 32 km2, were accumulated on coalescing alluvial fans and a floodplain environment. The depositional lobe shows sectors with distinctive morphological features attributable to the variable flow-depositional behavior of the debris stream, controlled by the nature of the substrate: (1) proximal continuous breccia flanked by levees; (2) intermediate depression; and (3) flat-topped polygonal hills and conical hills. A morphometric and spatial distribution analysis has been performed with the 550 conical hills (hummocks) mapped in the distal sector. The rock avalanche, with 914 m of maximal height drop (H) and 9280 m of runout (L), displays an extraordinarily high mobility (H/L index of 0.09), that can be attributed to the combined effect of dynamic rock fragmentation and basal lubrication by the soft substrate in the floodplain. Relief rejuvenation and slope over-steepening related to the growth of the anticline and the inflation of the Gavbast Dome are considered the main long-term preparatory factors that shifted the slope to a state of marginal stability. The slope failure was likely triggered by a large M ≥ 6.5–7 earthquake at ca. 5.4 ka, as indicated by OSL ages obtained from folded alluvium situated just beneath the rock avalanche deposit. The source of the earthquake was probably a blind reverse fault situated beneath the asymmetric Gavbast Anticline, as support the structural and geomorphic features of the fold. The identification and dating of large coseismic landslides in the Zagros Mountains, where large earthquakes are rarely accompanied by primary surface ruptures, could help to improve both landslide and seismic hazard assessments.

Shear rate effect on the residual strength of saturated clayey and granular soils under low- to high-rate continuous shearing

The residual strength of sliding basal soils plays a pivotal role not only in the movement of bedding landslides controlled by clay-rich interlayers but also in remobilized granular deposits. Many studies have focused on better understanding the residual shear behaviors of clayey and granular soils under relatively low shear rates. However, the residual strength varies with various clay fractions for clayey soils and fine-size fractions for granular soils, as well as shear rates (especially in fast ranges) need further examined. In the present research, results of a low- to high-rate continuous ring shear test on a clayey soil and three granular soils at the shear rates ranging from 0.5 to 50.0 mm/s were reported. The clayey soil was remolded from the shear zone soil of a bedding landslide controlled by a clay-rich interlayer with main minerals of illite-smectite, and the granular soils included two silica sands and one mixture of silica sand with fine granite residual soil particles. The results show that saturated clayey soil has an obvious positive shear rate effect on its residual strength after a certain shear rate; the mechanism is the transformation of shear mode, i.e., from sliding shearing to turbulent shearing with the increasing of shear rate. The high clay fraction may promote this transformation of shear mode. In addition, the critical shear rate of the clayey soil that has a positive shear rate dependency of the residual strength (i.e., rate-strengthening) is related to the clay fraction, i.e., a smaller critical shear rate appears in the clayey soil with higher clay fraction. The residual strength of saturated granular soils with different fine-size fractions do not show any shear rate-dependent, while the fine-size fraction affects the residual strength both in drained and undrained conditions; that is, the higher the fine-size fraction is, the lower the residual strength is. Moreover, the neutral shear rate effect of granular soils in this study may be related to the low normal stress and low shear rate range. The authors believe that, results such as those obtained in this work, may be useful for developing a more suitable constitutive model to better understand the postfailure motion (or reactivation kinematics) of bedding landslides controlled by clay-rich interlayers, as well as rock avalanche deposits remobilized by heavy rainfall.

The Namaras rock avalanche: Evidence of mid-to-late Holocene paraglacial activity in the Central Taurus Mountains, SW Turkey

The Namaras rock avalanche (NRA) deposit originated from the northern flank of the glaciokarstic Geyikdağ Mountain in the Central Taurus Range, SW Turkey. The deposit has an area of ~0.430 km2 and an estimated average thickness of 10 m, corresponding to 4.3 million m3 volume. The fan-shaped deposit area consists of house-sized Jurassic-Cretaceous neritic limestone boulders that overlie lateral and hummocky moraines down into the valley. We used geomorphological mapping and 36Cl surface exposure dating to obtain six boulder ages which unravelled the NRA age, the number of events and their geometric features. Our results indicate that the NRA consists of two successive mid-to-late Holocene events; the first high magnitude main event with a weighted average age of 4.59 ± 0.25 ka followed by a second low magnitude event with a weighted average age of 3.77 ± 0.20 ka. 36Cl exposure dating of the lateral moraine covered by the rock avalanche deposit yielded approximately an age of ~12.30 ± 1.20 ka. The older event with a maximum runout distance of 1550 m and 600 m elevation loss yielded in a travel angle of 21°. Similarly, the younger event with 1720 m maximum runout length and 640 m vertical elevation loss resulted in a travel angle of 20°. The bedded limestone with cross-joints in the glacial cirque preconditioned the NRA slope failure. The significant lag time between deglaciation and the rock avalanche indicates that the glacial erosion and debuttressing acted as preparatory factors. Based on the low seismic activity in the Central Taurus Range and the synchronicity of the rock avalanche with mid-to-late Holocene climatic transitions, we propose that climatic factors may have triggered the NRA failure. The age of the older main event coincides with the warm drought period of the mid-Holocene associated with torrential rainfalls in the Taurus Mountains, while the age of the younger event correlates with the long wet period in the late Holocene. Thus, the main NRA event may have been triggered by the influence of warm temperatures and intense rainfall, while the second event may have been triggered by prolonged period of high precipitation. The two events of the NRA are coeval with the enhanced rock slope failures dating back to 5–3 ka in the Alps.

Identification of a Quaternary rock avalanche deposit (Central Apennines, Italy): Significance for recognition of fossil catastrophic mass-wasting.

Whereas deposits of extremely‐rapid, ‘catastrophic’ mass wastings >105 m3 in volume (for example, the Marocche di Dro rock avalanche in the Southern Alps and the Flims rockslide in the Western Alps) are easily recognized by their sheer mass and blocky surface, the identification of fossil catastrophic mass wastings partly removed by erosion must be based on deposit characteristics. Herein, a ‘fossil’ (pre‐last glacial) rock avalanche, previously interpreted as either a till or debris flow, is described. The deposit, informally called ‘Rubble Breccia’, is located in the intramontane Campo Imperatore halfgraben that is bounded by a master fault with up to ca 900 m topographic throw. Based on documentation from field to thin section, and by comparative analysis with post‐glacial rock avalanches, tills and debris flows, the Rubble Breccia is reinterpreted as a rock avalanche. The Rubble Breccia consists of an extremely‐poorly sorted, disordered mixture of angular clasts from sand to block size. Many clasts show fitted subclast boundaries in crackle, jigsaw and mosaic fabrics, as diagnostic of catastrophic mass wasting deposits. Intercalated layers of angular to well‐rounded clasts of coarse sand to fine pebble size, and deformed into open to recumbent folds, may represent shear belts folded during terminal avalanche propagation. The clast spectrum of the Rubble Breccia – mainly shallow‐water bioclastic limestones, Saccocoma wackestones and other deep‐water limestones and dolostones – is derived from the front range along the northern margin of the basin. Calcite cement found within the Rubble Breccia was dated with the U/Th disequilibrium method to 124.25 ± 2.76 ka bp, providing an ante‐quam age constraint to the rock avalanche event. Because catastrophic mass wasting is a common erosional process, fossil deposits thereof should be more widespread than have been identified to date, although this may be a consequence of misidentification. The criteria outlined here provide a template to identify fossil catastrophic mass wasting deposits of any age.

On the genesis of a massive Holocene rock avalanche deposit in the Yeso River catchment, Andes Cordillera of central Chile

The Andes Cordillera of central Chile is one of the most active mountain ranges on the Earth, where tectonics and volcanism strongly imprint its Quaternary landscape evolution, and superficial processes, such as catastrophic mass movements, modulate it. This article addresses the genesis and flow mechanisms of a massive rock avalanche deposit in the Yeso River catchment within the Maipo River Andean watershed in front of Santiago, through geomorphological and geological analyses. The deposit has the characteristics of a huge rock avalanche (>108 m3) with the Mount Marmolejo, an ancient volcano reaching up to 6100 m a.s.l., as a source area, from which horizontal and vertical travel distances of 20.5 km and 3475 m, respectively, can be estimated until the maximum extent of the deposit recognisable in the field. Our results suggest that the flow that generated this deposit responded more fluidly than what is hypothesised for a rock avalanche. We propose that the avalanche incorporated part of the glaciers of the Mount Marmolejo that collapsed during this event leading to an avalanche flow. This occurred after 2447 ± 95 cal. years BP according to radiocarbon results from peat buried by the avalanche deposit. It may have been triggered by shallow seismicity nucleated along active faults present in the study area, even though the rock avalanche may have also been favoured by hydrothermal alteration and tectonised rocks, as well as by steep morphologies caused by glacial erosion affecting the Mount Marmolejo, in the context of a warming regional climate during the late Holocene.

The Pangal landslide complex, Cachapoal basin, central Chile (34°S): An example of a multi-temporal slope instability cluster in the Andes

A landslide cluster located in the Pangal river valley, Cachapoal river basin, in the Andes of central Chile is investigated. Deposits of five rock avalanche deposits of volumes varying from 1.5 to 150 million cubic metres are distinguished by detailed geomorphological mapping and dating. The landslides were originated in volcanic rocks affected by hydrothermal alteration. The largest three avalanche deposits, formerly considered as a single deposit, were dated using Terrestrial Cosmogenic Nuclide methods. The results show three distinct ages from Late Pleistocene to Holocene, confirming the hypothesis of separate events. The presence of clay minerals detected by X-ray diffraction in matrix samples confirm the presence of hydrothermal alteration in the landslide source areas detected by satellite image multispectral analyses, suggesting an important control of such alteration in the landslide generation. The present river morphology and outcrops of lacustrine deposits and outburst terraces evidence that the largest rock avalanches, with deposit thicknesses of up to about 100 m, have dammed the valley to be later eroded by the Pangal river. The repetition of large volume landslides in the valley and their potential of river damming should be considered as a hazard that may endanger critical infrastructure and human settlements downstream. This case study is an example of potential hazards of large slope collapses and related river damming in the Chilean Andes that should be addressed in more depth along with further investigations on the landslide controlling factors and failure mechanisms in the region.

Particle Project Correction for Rock Avalanche: Particle Crushing in Aerzhai Rock Avalanche, China

Abstract Strong fluidized rock avalanches with various particle sizes are one of the typical disasters in the mountainous area of southwestern China. Statistical analysis on rock avalanche particles can help understand the traveling behavior, distance, and deposition of rock avalanche, which are useful for potential threat range prediction and disaster prevention. Rock avalanche deposit has more than one oblique slopes different in attitude. Image distortion will happen when particles on the slopes are projected to one plane. Area projection conversion coefficient Ki is applied in this paper to correct the distortion and improve the precision of the geometric dimension parameters of the rock avalanche particle. A rock avalanche (Aerzhai rock avalanche) with a volume of about 10×104 m3 took place in Longxi Township, Wenchuan County, China, on April 8, 2018. Being corrected by the coefficient Ki, the mean area and the standard deviation of the particles within different zones have been obtained, and we analyzed their changes in vertical and transverse directions. Here, we show that the mean particle area decreases by about 61.1% from the source to deposition region and area standard deviation decreases by about 37.4%. The deposition region shows a higher proportion of small crashed particles than the source region. The proportion of small particles gets larger with the longitudinal motion distance and lateral distance. For big particles’ significant potential energy-driving ability and inertia advantages, the longitudinal motion routes can be maintained, and inertia decreases as the particles get smaller, while small particles offset from the center area laterally. These statistical characters are easy to be ignored if projection correction is not performed.

The intrinsic mobility of very dense grain flows

High mobility of natural disasters with severe consequences is often reported in large landslides and in earthquake fault ruptures. The mechanism of high mobility keeps to be mysterious to date. In this study, we used high-speed rotary shear experiments under normal stress 1 MPa and with a rotary speed of 2 m/s on grains of four different minerals to show that the tested dry dense grain flows with crushable grains are highly mobile intrinsically. Granular flow quickly weakens to a very low viscosity (about 500 Pa.s), which is independent of grain composition. We find that when flow begins, grains are crushed to a special fractal structure in which most larger grain is surrounded by much smaller grains. This special structure provides a favourable condition for generating and propagating acoustic energy from the abrasion indicated by the scratches on the grains. The scratching generates very high frequency elastic energy (vibration) in the larger grains, which profoundly weakens all grain contacts so that the grain mass flows like a giant flood. Chatter-marked scratches made by smaller grains scouring across boulders in rock-avalanche deposits suggest that the same processes occur in nature. This finding is important for the explanation of the behaviour of dense grain flows at large strains and high strain rates.

Rock-Avalanche Deposit Causes Extreme High Indoor Radon Concentrations in Kinsarvik, Ullensvang Municipality, Western Norway

Rock-Avalanche Deposit Causes Extreme High Indoor Radon Concentrations in Kinsarvik, Ullensvang Municipality, Western Norway