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The occurrence, mechanisms and hazards of large landslides along tablelands
Abstract
The largest terrestrial coalescent landslide areas of the Earth, spanning hundreds to thousands of square kilometres, occur along the fringes of relatively low-relief sedimentary and volcanic tablelands. However, difficulties in landslide recognition in these areas have led to underestimations of their frequency and likelihood. In this Review, we explore the global distribution, controls and dynamics of landslides occurring along tableland fringes. Landslide fringes are caused by the uninterrupted and extensive presence of weak sub-caprock lithologies below a more competent caprock. Topography, escarpment height and caprock thickness do not affect landslide size but can locally influence the type of displacement. Rotational landslides dominate most landslide fringes and will eventually lead to tableland consumption over million-year timescales. Some tableland rims can generate catastrophic long-runout rock avalanches or earthflows, which might in turn trigger tsunamis, river avulsion or outburst floods. Tablelands can also fail by slow (centimetre per year) landslide movements sufficient to cause damage to infrastructure. These hazards are increasing especially in high-latitude tablelands owing to cryosphere degradation, as observed in Western Greenland. A more detailed global inventory of landslide fringe activity is urgently needed to better quantify these potential hazards.
... Crosta et al., 2013;Delgado et al., 2022;Pánek et al., 2022). However, Pánek et al. (2024) pointed out that some of the largest coalescent landslide areas are not found in high mountains but in tableland landscapes such as the Colorado Plateau (Watkins & Rogers, 2022), West Greenland , extra-Andean Patagonia (Pánek et al., 2018(Pánek et al., , 2023Schönfeldt et al., 2020), Ustyurt Plateau in Kazakhstan (Aslan et al., 2021;Pánek et al., 2016), and in the central Sahara (Busche, 2001). However, surprisingly few detailed geomorphological maps of tablelands have been published to date (Ercolano et al., 2016;Migoń et al., 2020;Peulvast & Bétard, 2015) and only several geomorphological maps showing tablelands heavily affected by landslides (Devoto et al., 2012;Perego et al., 2011;Peulvast et al., 2011;Pike et al., 2024;Prampolini et al., 2017Prampolini et al., , 2018Zerboni et al., 2015). ...
... The majority of extra-Andean Patagonia is formed by tableland landscape (Somún Curá Massif, Northern Patagonian Plateau, Southern Patagonian Plateau) (Ramos, 1999a). Among tableland landscapes globally, the Patagonian tableland appears to be one of the most affected by landslides (Kilnar et al., 2024;Pánek et al., 2024;Schönfeldt et al., 2022), though geomorphological maps showing the distribution of landslides in context of other landforms in this area are lacking. In this map, we focus on the Sarmiento Basin, located in the central part of the Patagonian tableland. ...
... The Nuussuaq Basin in central West Greenland is a landslide hotspot (Dahl-Jensen et al. 2004;Svennevig 2019;Pánek et al. 2024). Two large historical landslides, specifically frozen debris avalanches, are known to have occurred near Paatuut (Fig. 1a). ...
On November 21, 2000 CE, the c. 48 × 106 m3 Paatuut landslide in West Greenland triggered a tsunami with a maximum runup height of c. 45 m. Although a field team examined the landslide in the immediate aftermath, prior events and processes, in addition to the cause of the landslide, were never studied. We combined field data, satellite images, and historical photos to bridge this knowledge gap. Our investigation unveiled that a hitherto unknown c. 55 × 106 m3 landslide occurred at the same slope in May or June of 1996. This landslide was a frozen debris avalanche, and we suggest a result of permafrost degradation since c. 1949. The subsequent 2000 landslide and tsunami removed and obscured the traces of the 1996 landslide. Interestingly, the 1996 landslide caused a tsunami with a runup height of only 15 m near the landslide impact area, one-third of the 2000 tsunami. We applied tsunami modelling and interpretation of morphological field evidence to explore why these volumetrically similar landslides on the same slope could produce markedly different tsunami runup heights. The deposit of the 1996 landslide on the coastal slope produced a large, unconsolidated, wet sediment volume that could be entrained in the 2000 landslide, and in addition, reducing the basal friction of this later event. Furthermore, differences in drop height and rheology between the two landslides may explain the different tsunamigenic potential. We see evidence of much older post-glacial landslide activity on the slope, constituting a static preconditioning factor for the landslides. The 1996 and 2000 landslides demonstrate the incomplete record of large landslides in the Arctic and the importance of considering the runout path, substrate, and entrainment in determining the tsunamigenic potential of landslides. Above all, they also demonstrate the sensitivity of these Arctic slopes to global warming and associated permafrost degradation.
... Although much work has been conducted in the fields of LS and landslide modelling, numerous challenges remain. First, the occurrence of landslides is influenced by various factors including geological landforms (e.g., soil type, riverine influences, and geological conditions) and hydrological conditions (e.g., fluvial influences and precipitation influences) [47,48]. The selection of these factors is crucial for LS. ...
Landslides are common natural disasters in mountainous regions, exerting considerable influence on socioeconomic development and city construction. Landslides occur and develop rapidly, often posing a significant threat to the safety of individuals and their property. Consequently, the mapping of areas susceptible to landslides and the simulation of the development of such events are crucial for the early warning and forecasting of regional landslide occurrences, as well as for the management of associated risks. In this study, a landslide susceptibility (LS) model was developed using an ensemble machine learning (ML) approach which integrates geological and geomorphological data, hydrological data, and remote sensing data. A total of nine factors (e.g., surface deformation rates (SDF), slope, and aspect) were used to assess the susceptibility of the study area to landslides and a grading of the LS in the study area was obtained. The proposed model demonstrates high accuracy and good applicability for LS. Additionally, a simulation of the landslide process and velocity was constructed based on the principles of landslide movement and the rule-based discrete grid model. Compared with actual unmanned aerial vehicle (UAV) imagery, this simulation model has a Sørensen coefficient (SC) of 0.878, a kappa coefficient of 0.891, and a total accuracy of 94.12%. The evaluation results indicate that the model aligns well with the spatial and temporal development characteristics of landslides, thereby providing a valuable reference basis for monitoring and early warning of landslide events.
The Vaigat strait (Sullorsuaq) in West Greenland is well known for its susceptibility to landslides and historical landslide-generated tsunamis. Recent mapping of the seabed in the Vaigat strait has revealed several prehistoric giga-scale (volumes of 109 m3) tsunamigenic landslides. However, the timing of these giga-scale tsunamis is largely unconstrained, but they are assumed to have occurred after the last deglaciation. Here, we report on lake sediment core records from four coastal lakes located between 19 and 91 m above sea level (a.s.l.) on the Saqqaq foreland at the eastern end of the Vaigat strait. We use a multiproxy approach including X-ray fluorescence (XRF) and magnetic susceptibility core scanning along with a screening for marine diatoms to identify at least two tsunami deposits in two of the four sediment cores. Radiocarbon dating of aquatic macrofossils and bulk samples suggest that the tsunami events occurred at ca. 7.6 and 7.3 ka cal BP. Using a previously published relative sea level curve from Vaskebugt, Arveprinsen Ejland (Alluttoq), located 40 km southeast of Saqqaq, we infer wave runup heights of ca. 41–66 and 45–70 m, respectively, for the two tsunami events. These runup heights from prehistoric tsunamis are 1 to 2 orders of magnitude higher than the historic landslide-tsunami runup heights at Saqqaq which only reached an elevation of ca. 3 m in November 2000. While we found deposits from two tsunami events in the lake sediments, landforms from at least nine giga-scale landslides are present on the seafloor of Vaigat. We infer that these deposits probably represent the two most recent tsunamis identified in the Vaigat strait and that the older tsunamis must have happened between the last deglaciation and the oldest sediment in the lakes, i.e., between ca. 10.0 and 8.5 ka cal BP.
Rock avalanche−triggered displacement waves (also termed tsunamis) have recently occurred in Greenland and Alaska, and they illustrate the presence of such hazards in polar regions. To improve understanding of the magnitude of this hazard for these areas, we investigated gigascale subaerial rock avalanches impacting a partially confined water body within the Vaigat strait (western Greenland). We present a new combined subaerial to subaqueous digital elevation model, alongside a new compilation of seismic data, which revealed nine deglacial to Holocene rock avalanche complexes that are between one and two orders of magnitude larger than nearby historical rock avalanches. The three largest complexes have deposit thicknesses up to 300 m, runout distances reaching 19 km, and best-estimate volumes from 1.7 to 8.4 km3. Based on the morphology and the volume−angle of reach relations, it is likely that each complex represents a single or few events, thus making them among the largest displacement wave−generating subaerial to submarine rock avalanches on Earth. We estimated displacement wave magnitude up to 280 m on the opposite shore. The ages of the deposits are poorly constrained but the main rock avalanche activity is referable to early Holocene times. With significant climatic changes predicted in the Arctic, we recommend that hazard assessments account for events not only from the historical record but also those from the recent geological past.
We report here a slow-moving landslide complex of the lateral spreading type revealed by Sentinel-1 interferometric time-series analysis. Located along the western coast of the Aral Sea, with a >150-km length and 3-km width, this is the world’s largest active landslide complex reported so far, with a constant velocity of up to 60 mm/yr. Systematic subsidence up to 5 mm/yr is also observed along narrow strips of terraces that appear to result from rotations of fault-bounded blocks. The landslide deformation velocity does not correlate with the annual variations of the water level in the southwestern lake of the Aral Sea during the observation period of 2014-2022, indicating a long-term forcing of this landslide that is rather interpreted to be caused by the long-term sea-level drop. The lateral spreading involves the competent limestone beds lying horizontally on plastic clay- and evaporite-rich layers. We propose a conceptual model for the kinematic of landslides that appear to be controlled by the attitude of bedding, lithological sequence, hydrogeology, and low angle faults.
This paper examines the distribution, stratigraphy and chronology of landslide deposits within the Gouffre Valley to provide a long-term context for the 1663 CE M 7.0-7.5 Charlevoix earthquake, Charlevoix-Kamouraska Seismic Zone (CKSZ), southern Quebec, Canada. Our mapping utilized a 1x1 m digital terrain model, and we interpreted deposit age and stratigraphy from 30 new AMS radiocarbon ages, supplemented by 51 regular radiocarbon ages and 23 dendrochronology ages compiled from previous studies. These data provide a robust chronological connection between the 1663 CE earthquake and large-scale landslide deposits that cover about 24 km2, extend about 20 km along the valley bottom, and originated from separate source areas. Eight, relatively small-scale (less than 0.48 km2), prehistoric landslide features are also present in the Gouffre Valley, including three dated landslides, aged between about 7950-7300, 6595-6087, and 1310-1200 cal BCE, a poorly-constrained deposit aged between 4782-4452 BCE and 1663 CE, and four landslides of unknown age. There is no evidence to support an earthquake trigger for any of these prehistoric landslides. Instead, the widespread deposits associated with the 1663 CE earthquake represent the only multi-event, landslide signature in the Gouffre Valley. This, in combination with a long gap between early postglacial and late Holocene mass movement activity in several nearby lakes, is consistent with the 1663 CE earthquake being the largest to occur in the CKSZ over the past about 10 ka.
Landslides are widespread occurrences that can become catastrophic when they occur near settlements and infrastructure. Detection, monitoring and prediction are fundamental to managing landslide risks and often rely on remote-sensing techniques (RSTs) that include the observation of Earth from space, laser scanning and ground-based interferometry. In this Technical Review, we describe the use of RSTs in landslide analysis and management. Satellite RSTs are used to detect and measure landslide displacement, providing a synoptic view over various spatiotemporal scales. Ground-based sensors (including ground-based interferometric radar, Doppler radar and lidar) monitor smaller areas, but combine accuracy, high acquisition frequency and configuration flexibility, and are therefore increasingly used in real-time monitoring and early warning of landslides. Each RST has advantages and limitations that depend on the application (detection, monitoring or prediction), the size of the area of concern, the type of landslide, deformation pattern and risks posed by landslide. The integration of various technologies is, therefore, often best. More effective landslide risk management requires greater leveraging of big data, more strategic use of monitoring resources and better communication with residents of landslide-prone areas. Sections
On December 15th 1952, at approximately 14:00 local time a mass of 5.9 × 10⁶ m³ of permafrozen talus deposits failed in a landslide close to the Niiortuut mountain on the south coast of the Nuussuaq peninsula, central West Greenland. Between 1.8 and 4.5 × 10⁶ m³ of the material entered the sea and generated a tsunami that propagated through the Vaigat strait (Sullorsuaq). Here we describe this catastrophic event for the first time by analysis of historical material supplemented by recent fieldwork and discuss the implications for the state of contemporary permafrozen slopes. The tsunami killed a fisherman working on the shore of southern Nuussuaq, 10 km south-east of the landslide. In the mining town of Qullissat, 30 km south of the landslide, it had a runup height of 2.2–2.7 m and caused minor material damage. Morphological evidence show that the basal surface of rupture was 80 m inside the permafrost cemented talus slope, whose degradation was a dynamic conditioning factor for the landslide. The 1952 Niiortuut landslide is the first historically recorded event of permafrost degradation induced landslide-tsunamis in the Arctic. We infer that the landslide and its cascading consequences occurred due to the early-twentieth century warming that started in the late 1910's in the Arctic. Warming is now increasingly affecting this region, as shown by an enhanced recent landslide activity.
Pleistocene periglacial activity in eastern Australia was widespread and has been predicted to have extended along much of the east coast. This paper describes block deposits in the New England Tablelands, Australia, as far north as 30°S. These deposits are characterized by openwork blocks on slopes below the angle of repose. The deposits are positioned where frost cracking was significant and range in area up to 8 ha. Surface exposure dating using the cosmogenic nuclide ³⁶ Cl from four block deposits indicate all sites were active late during the last glacial cycle, with a concentration of activity between 15–30 ka. Modern temperature measurements from block deposits highlight the importance of local topography for promoting freezing. Periglacial deposits are likely to have been more extensive than previously recognized at these northern limits, and mean annual temperature more than 8°C colder than today.
A large landslide (frozen debris avalanche) occurred at Assapaat on the south coast of the Nuussuaq Peninsula in Central West Greenland on June 13, 2021, at 04:04 local time. We present a compilation of available data from field observations, photos, remote sensing, and seismic monitoring to describe the event. Analysis of these data in combination with an analysis of pre- and post-failure digital elevation models results in the first description of this type of landslide. The frozen debris avalanche initiated as a 6.9 * 10 ⁶ m ³ failure of permafrozen talus slope and underlying colluvium and till at 600–880 m elevation. It entrained a large volume of permafrozen colluvium along its 2.4 km path in two subsequent entrainment phases accumulating a total volume between 18.3 * 10 ⁶ and 25.9 * 10 ⁶ m ³ . About 3.9 * 10 ⁶ m ³ is estimated to have entered the Vaigat strait; however, no tsunami was reported, or is evident in the field. This is probably because the second stage of entrainment along with a flattening of slope angle reduced the mobility of the frozen debris avalanche. We hypothesise that the initial talus slope failure is dynamically conditioned by warming of the ice matrix that binds the permafrozen talus slope. When the slope ice temperature rises to a critical level, its shear resistance is reduced, resulting in an unstable talus slope prone to failure. Likewise, we attribute the large-scale entrainment to increasing slope temperature and take the frozen debris avalanche as a strong sign that the permafrost in this region is increasingly at a critical state. Global warming is enhanced in the Arctic and frequent landslide events in the past decade in Western Greenland let us hypothesise that continued warming will lead to an increase in the frequency and magnitude of these types of landslides. Essential data for critical arctic slopes such as precipitation, snowmelt, and ground and surface temperature are still missing to further test this hypothesis. It is thus strongly required that research funds are made available to better predict the change of landslide threat in the Arctic.
Landslides in deglaciated and deglaciating mountains represent a major hazard, but their distribution at the spatial scale of entire mountain belts has rarely been studied. Traditional models of landslide distribution assume that landslides are concentrated in the steepest, wettest, and most tectonically active parts of the orogens, where glaciers reached their greatest thickness. However, based on mapping large landslides (> 0.9 km2) over an unprecedentedly large area of Southern Patagonia (~ 305,000 km2), we show that the distribution of landslides can have the opposite trend. We show that the largest landslides within the limits of the former Patagonian Ice Sheet (PIS) cluster along its eastern margins occupying lower, tectonically less active, and arid part of the Patagonian Andes. In contrast to the heavily glaciated, highest elevations of the mountain range, the peripheral regions have been glaciated only episodically, leaving a larger volume of unstable sedimentary and volcanic rocks that are subject to ongoing slope instability.
Submarine landslides pose a hazard to coastal communities as they can generate powerful tsunamis, and threaten critical offshore infrastructure such as seafloor cable networks that underpin global communications. Such events can be orders of magnitude larger than their onshore equivalents. Despite the hazard they pose, many aspects of submarine landslides remain poorly understood, such as why they fail on low angle (<2°), seemingly stable slopes. Many studies have proposed that failure on low slope angles, and the large areal extent of submarine landslides, may be controlled by the presence of laterally-extensive weak layers embedded within the slope stratigraphy, which precondition slopes to failure. Little remains known, however, about the characteristics and processes that control and form weak layers. We conducted a comprehensive review of published submarine landslide studies that examine failure planes and apparent weak layers associated with historical and ancient submarine landslides. Based on a new global landslide catalogue that comprises 64 case studies, this review aims to investigate the types of sediment that form weak layers and to understand the controls on their global variability. Existing classification schemes are based on mechanical process(es), and do not readily enable a diagnosis of weak layers from unfailed sediments. Here, a new and complementary classification of weak layers based on lithology is introduced. This classification enables weak layer recognition from sediment cores (including those sampling unfailed sediments), and allows us to attribute failure mechanisms to different environmental settings where distinct types of weak layers are more likely. The results show that failure planes usually form in the vicinity of an interface between distinct lithologies that together comprise a weak layer. The weak layers of 22 of our 64 case studies were related to characteristic sediment sequences within the slope stratigraphy, of which 19 were classified based on direct measurements from sediment cores and in-situ measurements: 16 weak layers were classified as siliciclastic, four as volcaniclastic, and two as fossiliferous sediment sequences. Only three submarine landsides were related to clay-dominated weak layers. In addition, failure along lithological contrasts was inferred for six case studies. Based on global depositional likely locations of these different types of weak layer can be inferred. These include oceanic gateways where long-term circulation can create distinct permeability interfaces within siliciclastic sequences, areas of high productivity where biogenic sediments may dominate, and regions that experience widespread ash fall from volcanic eruptions. We highlight that many submarine landslide studies have historically not collected sediment cores that characterise weak layers within intact sedimentary sequences and instead have focused on characterising the slope failure deposit. As weak layers can collapse or become heavily modified during failure, there is a widespread omission of key information required for geotechnical analysis to determine where and why certain slopes are predisposed to failure. We conclude by highlighting the need to combine detailed geotechnical measurements with sedimentological and geophysical analyses including grain-scaled observations (e.g. micro-Computed Tomography high-resolution 3D imagery), and emphasise the importance of a uniform workflow that will allow for a better comparison between individual studies.
The Surprise Valley landslide complex is the name used here for a group of prominent river-damming landslides in Grand Canyon (Arizona, USA) that has shifted the path of the Colorado River several times in the past 2 m.y. We document a sequence of eight landslides. Three are Toreva-block landslides containing back-rotated but only mildly disrupted bedrock stratigraphy. The largest of these landslides, Surprise Valley landslide, is hypothesized to have dammed the Colorado River, cut off a meander loop through Surprise Valley, and rerouted the river 2.5 km south to near its present course at the Granite Narrows. Another bedrock landslide, Poncho’s runup, involved a mass detachment from the north side of the river that drove a kilometer-scale bedrock slab across the river and up the south canyon wall to a height of 823 m above the river. A lake behind this landslide is inferred from the presence of mainstem gravels atop the slide that represent the approximate spillway elevation. We postulate that this landslide lake facilitated the upriver 133 Mile slide detachment and Toreva block formation. The other five landslides are subsequent slides that consist of debris from the primary slides; these also partially blocked and diverted the Colorado River as well as the Deer Creek and Tapeats Creek tributaries into new bedrock gorges over the past 1 m.y.
The sequence of landslides is reconstructed from inset relationships revealed by geologic mapping and restored cross-sections. Relative ages are estimated by measuring landslide base height above the modern river level in locations where landslides filled paleochannels of the Colorado River and its tributaries. We calculate an average bedrock incision rate of 138 m/m.y. as determined by a 0.674 ± 0.022 Ma detrital sanidine maximum depositional age of the paleoriver channel fill of the Piano slide, which has its base 70 m above the river level and ~93 m above bedrock level beneath the modern river channel. This date is within error of, and significantly refines, the prior cosmogenic burial date of 0.88 ± 0.44 Ma on paleochannel cobbles. Assuming steady incision at 138 m/m.y., the age of Surprise Valley landslide is estimated to be ca. 2.1 Ma; Poncho’s runup is estimated to be ca. 610 ka; and diversion of Deer Creek to form modern Deer Creek Falls is estimated to be ca. 400 ka. The age of the most recent slide, Backeddy slide, is estimated to be ca. 170 ka based on its near-river-level position. Our proposed triggering mechanism for Surprise Valley landslides involves groundwater saturation of a failure plane in the weak Bright Angel Formation resulting from large volumes of Grand Canyon north-rim groundwater recharge prior to establishment of the modern Deer, Thunder, and Tapeats springs. Poncho’s and Piano landslides may have been triggered by shale saturation caused by 600–650 ka lava dams that formed 45 river miles (73 river km; river miles are measured along the Colorado River downstream from Lees Ferry, with 1 river mile = 1.62 river kms) downstream near Lava Falls. We cannot rule out effects from seismic triggering along the nearby Sinyala fault. Each of the inferred landslide dams was quickly overtopped (tens of years), filled with sediment (hundreds of years), and removed (thousands of years) by the Colorado River, as is also the potential fate of modern dams.
Sea-level rise of the Caspian Sea (CS) during the early Khvalynian (approximately 40-25 ka BP) generated hundreds of giant landslides along the sea's ancient coastlines in western Kazakhstan, which extended hundreds of kilometers. Although similar landslides have been observed along the present-day coastlines of the CS in the area of a prominent high escarpment, it remains unclear whether some of these ancient landslides are still active and whether the movement is slow or catastrophic, as previously suggested. The present study is the first to show evidence proving that the geomorphic responses to sea-level changes of the CS that were triggered in the Pleistocene are currently active. Using interferometric synthetic aperture radar (InSAR) data, we show that one of these giant landslides occurring along the western shore of the Kara-Bogaz-Gol (KBG) lagoon of the CS presents active transient motion, which makes it the world's largest active landslide reported thus far. Extending more than 25 km along the eastern coast of the inundated KBG depression in a N-S direction with maximum landward expansion of 5 km from the shoreline to the flat Ustyurt Plateau, this landslide conveys ~ 10 × 10 9 m 3 rocks toward the lagoon at a rate of ~ 2.5 cm/year. This event releases a nearly episodic aseismic moment of 6.0 × 10 10 Nm annually, which is equivalent to the response of an Mw 5.1 earthquake. We analyze the present-day evolution of this giant coastal landslide at high temporal and spatial resolutions using Sentinel-1 radar images acquired on descending and ascending modes every 12 days between 2014 and 2020. Modelling with elastic dislocations suggests that the KBG landslide was accommodated mostly by a shallow basal décollement with a nearly horizontal listric slip plane. Moreover, our analysis reveals week-long accelerating slip events at changing amplitudes that occur seasonally with slow, lateral spreading rather than sudden catastrophic motion. A strong correlation between the episodic slip events and seasonal water-level changes in the KBG lagoon suggests a causative mechanism for the transient accelerating slip events. Although water-level changes are widely acknowledged to trigger transient motion on a land mass, such movement, which is similar to a silent earthquake, has not been observed thus far at this mega scale; on an extremely low-angle detachment planes at < 5° with modulation by sea-level changes. This study suggests that present-day sea-level changes can reactivate giant landslides that originated 40-25 ka. Most giant terrestrial landslides > 10 8 m 3 usually occur in the steepest, deeply incised, and formerly glaciated landscapes of the world 1 , in addition to tectonically active mountain belts, flanks of volcanoes, and large escarpments 2. Nearly two-thirds of these kilometer-scale gigantic slope failures are triggered by catastrophic events such as ground shaking from strong earthquakes, volcanic eruptions, and heavy rainstorms 3. Recent observations have shown that some giant landslides, particularly those generated by Caspian Sea (CS) transgression in the late Pleistocene, can occur on a very gently inclined slip surface and in extremely low-relief landscapes far from active mountain belts 3-5. These landslides are believed to have been modulated by sea transgression/regression cycles 3. The geomorphic response to sea-level changes on sea coasts, particularly in the CS, remains enigmatic. It is widely accepted that sea transgression in the late Pleistocene during the Early Khvalynian (approximately 40-25 ka BP) inundated vast portions of the low-lying semi-desert of western Kazakhstan/eastern Turkmenistan to form highstands 3. Cliffs cut during the highstands generated the prominent escarpment that presently surrounds the Kara-Bogaz-Gol (KBG) lagoon of the CS, which intersects the area of a giant landslide. According OPEN
North African greening phases, during which large rivers ran through the Sahara Desert, occurred repeatedly during the Quaternary and are regarded as key periods for the development of past human populations. However, the timing and mechanisms responsible for the reactivation of the presently dormant fluvial systems remain highly uncertain. Here we present hydroclimate changes over the past 160,000 years, reconstructed from analyses of the provenance of terrestrial sediments in a marine sediment record from the Gulf of Sirte (offshore Libya). By combining high-resolution proxy data with transient Earth system model simulations, we are able to identify the various drivers that led to the observed shifts in hydroclimate and landscapes. We show that river runoff occurred during warm interglacial phases of Marine Isotope Stages 1 and 5 due to precession-forced enhancements in the summer and autumn rainfall over the entire watershed, which fed presently dry river systems and intermittent coastal streams. In contrast, shorter-lasting and less-intense humid events during glacial Marine Isotope Stages 3 and 4 were related to autumn and winter precipitation over the Libyan coastal regions driven by Mediterranean storms. Our results reveal large shifts in hydroclimate environments during the last glacial cycle, which probably exerted a strong evolutionary and structural control on past human populations, potentially pacing their dispersal across northern Africa.
The coastal Pacific Northwest USA hosts thousands of deep-seated landslides. Historic landslides have primarily been triggered by rainfall, but the region is also prone to large earthquakes on the 1100-km-long Cascadia Subduction Zone megathrust. Little is known about the number of landslides triggered by these earthquakes because the last magnitude 9 rupture occurred in 1700 CE. Here, we map 9938 deep-seated bedrock landslides in the Oregon Coast Range and use surface roughness dating to estimate that past earthquakes triggered fewer than half of the landslides in the past 1000 years. We find landslide frequency increases with mean annual precipitation but not with modeled peak ground acceleration or proximity to the megathrust. Our results agree with findings about other recent subduction zone earthquakes where relatively few deep-seated landslides were mapped and suggest that despite proximity to the megathrust, most deep-seated landslides in the Oregon Coast Range were triggered by rainfall.
Karst regions offer a variety of natural resources such as freshwater and biodiversity, and many cultural resources. The World Karst Aquifer Map (WOKAM) is the first detailed and complete global geodatabase concerning the distribution of karstifiable rocks (carbonates and evaporites) representing potential karst aquifers. This study presents a statistical evaluation of WOKAM, focusing entirely on karst in carbonate rocks and addressing four main aspects: (1) global occurrence and geographic distribution of karst; (2) karst in various topographic settings and coastal areas; (3) karst in different climatic zones; and (4) populations living on karst. According to the analysis, 15.2% of the global ice-free continental surface is characterized by the presence of karstifiable carbonate rock. The largest percentage is in Europe (21.8%); the largest absolute area occurs in Asia (8.35 million km2). Globally, 31.1% of all surface exposures of carbonate rocks occur in plains, 28.1% in hills and 40.8% in mountains, and 151,400 km or 15.7% of marine coastlines are characterized by carbonate rocks. About 34.2% of all carbonate rocks occur in arid climates, followed by 28.2% in cold and 15.9% in temperate climates, whereas only 13.1 and 8.6% occur in tropical and polar climates, respectively. Globally, 1.18 billion people (16.5% of the global population) live on karst. The highest absolute number occurs in Asia (661.7 million), whereas the highest percentages are in Europe (25.3%) and North America (23.5%). These results demonstrate the global importance of karst and serve as a basis for further research and international water management strategies.
Slope instability phenomena such as mudslides represent a major geohazard in Brazil, which has caused devastation in many states and affected the lives of people, particularly in self-built settlements on steep slopes. This paper presents and discusses slope stability issues encountered in Caxias do Sul in the State of Rio Grande do Sul, which exemplifies the existing situation of landslide risk assessment in southern Brazil. Local geology and ground conditions of the area in relation to slope instability were reviewed and gaps in information required for mitigating risk were identified, such as inadequate geotechnical information and lack of full inspection and continuous monitoring of active landslides. Although risk assessment has been developed for the city and regarded as a fundamental management tool in the mitigation of landslide hazards, the study showed that the risk assessment works are outdated and not effectively considered for the development of the city. With significant unplanned urban expansion (where houses have been self-built on very steep terrains without geotechnical assessment of the ground and slope conditions), new geohazard mapping is essentially required. Several key recommendations were provided for mitigating the destructive effect of landslides and improving their management in mountainous urban settings.
The landslide of 17 June 2017 in Karrat Fjord, central West
Greenland, highlighted the need for a better understanding
of landslides and landslide-generated tsunamis in Greenland
and motivated a landslide screening project in 2018, led by
the Geological Survey of Denmark and Greenland (GEUS;
see also Svennevig et al. this volume). A central part of this
project was to conduct a preliminary mapping of Quaternary
and historical landslides in Greenland – the first effort of its
kind. The main objective was to establish a landslide inventory
database that can be used to identify areas prone to landslides
and serve as a tool for gaining a better understanding
of where, when and why catastrophic landslides take place in
Greenland.
This paper describes the workflow used to produce the
preliminary landslide inventory of Greenland and discusses
some of the initial results. To date (June 2019), I have
mapped 564 landslides with the vast majority situated in
the Nuussuaq Basin between Sigguup Nunaa (Svartenhuk
Halvø), and Qeqertarsuaq (Disko) in West Greenland (Fig.
1). The inventory mapping is mainly based on observations
and analyses of remotely sensed imagery and pre-existing
geological maps. The mapping coverage was not systematic
for all of Greenland, but focused on postglacial, potentially
tsunamigenic landslides in inhabited coastal regions, i.e. on
relatively large landslides on coastal slopes, mainly in West
Greenland and small areas of East Greenland. However,
smaller and inland landslides were included when they were
encountered. Similarly, the less inhabited parts of Greenland
were provisionally screened, but call for more thorough, systematic
mapping in the future.
Rock avalanches are high velocity flows of fragmented rock that can dramatically alter landscapes, and impact people and infrastructure far from their source. These catastrophic events have been studied for over a century, however, a consensus regarding the mechanism(s)that govern their motion has yet to emerge. This work details the results of the back-analysis of 45 rock avalanche case histories using a semi-empirical runout model that simulates motion over 3D terrain. These simulations account for topographic effects and bulk basal shear resistance variation during motion. For a subset of the cases, we find that a volume-dependent mechanism appears to influence basal resistance in the source zone. However, once the material has vacated the source zone, our results suggest that the character of the path material strongly controls mobility, and appears to dominate over other, potentially volume-dependent, mechanisms that may also be at work. The results presented in this paper support the longstanding hypothesis that the interaction of flowing debris with the underlying substrate is an important consideration in rock avalanche runout prediction, and that this mechanism warrants further research.
In active mountain belts with steep terrain, bedrock landsliding is a major erosional agent. In the Himalayas, landsliding is driven by annual hydro-meteorological forcing due to the summer monsoon and by rarer, exceptional events, such as earthquakes. Independent methods yield erosion rate estimates that appear to increase with sampling time, suggesting that rare, high-magnitude erosion events dominate the erosional budget. Nevertheless, until now, neither the contribution of monsoon and earthquakes to landslide erosion nor the proportion of erosion due to rare, giant landslides have been quantified in the Himalayas. We address these challenges by combining and analysing earthquake- and monsoon-induced landslide inventories across different timescales. With time series of 5 m satellite images over four main valleys in central Nepal, we comprehensively mapped landslides caused by the monsoon from 2010 to 2018. We found no clear correlation between monsoon properties and landsliding and a similar mean landsliding rate for all valleys, except in 2015, where the valleys affected by the earthquake featured ∼5–8 times more landsliding than the pre-earthquake mean rate. The long-term size–frequency distribution of monsoon-induced landsliding (MIL) was derived from these inventories and from an inventory of landslides larger than ∼0.1 km2 that occurred between 1972 and 2014. Using a published landslide inventory for the Gorkha 2015 earthquake, we derive the size–frequency distribution for earthquake-induced landsliding (EQIL). These two distributions are dominated by infrequent, large and giant landslides but under-predict an estimated Holocene frequency of giant landslides (> 1 km3) which we derived from a literature compilation. This discrepancy can be resolved when modelling the effect of a full distribution of earthquakes of variable magnitude and when considering that a shallower earthquake may cause larger landslides. In this case, EQIL and MIL contribute about equally to a total long-term erosion of ∼2±0.75 mm yr-1 in agreement with most thermo-chronological data. Independently of the specific total and relative erosion rates, the heavy-tailed size–frequency distribution from MIL and EQIL and the very large maximal landslide size in the Himalayas indicate that mean landslide erosion rates increase with sampling time, as has been observed for independent erosion estimates. Further, we find that the sampling timescale required to adequately capture the frequency of the largest landslides, which is necessary for deriving long-term mean erosion rates, is often much longer than the averaging time of cosmogenic 10Be methods. This observation presents a strong caveat when interpreting spatial or temporal variability in erosion rates from this method. Thus, in areas where a very large, rare landslide contributes heavily to long-term erosion (as the Himalayas), we recommend 10Be sample in catchments with source areas > 10 000 km2 to reduce the method mean bias to below ∼20 % of the long-term erosion.
Landslide is one of the devastating natural phenomenon that threatens human life and property. Every year a number of persons lost their lives due to the landslides. Therefore, a better understanding and characterization of landslide is very essential for adopting mitigation strategies to contain the adversities of this natural hazard. Information on landslides from different climatic setup are very essential for better understanding of the influence of weathering, rainfall, or topography on landslide generation. Weathering is one of the important causative factor for landslide generation in the moderate topography or inactive mountainous terrain. The Western Ghats including the Deccan Traps, an inactive mountain range, receives torrential rainfall. Intense rainfall in these areas enhances the weathering processes and fabricates thick soil covers. Mahabaleshwar area, Maharashtra was chosen as a case study, where high elevated part is covered by lateritic layer and each lava flow unit is separated by a thin weathered bed of red bole. The area experiences series of landslides during the summer monsoon months. Mainly two types of landslides have been identified in the area confined with the red bole bed and powdery lateritic soil. The first type of landslides occur at higher elevations (≥1200m) where horizontal beds of permeable laterites underlined by impermeable thick basalt beds. The rain water infiltrates down and spread laterally within the permeable lateritic beds. It finally spouts at lower plateau elevations and triggers mainly debris flows. The other category of landslides occurs where the weathered red bole bed separates two successive lava flows. The percolating water from the secondary porosities (joints and inter connected vugs) comes out from the contact zones of basalt and red bole bed in the form of seepages. It erodes the red bole bed and as a result the overlying masses hang and consequently lead to rock fall. The Chemical Index of Alteration (CIA) of the representative samples from landslide locations indicates significant weathering. The CIA values for the fine lateritic soil are up to 98% whereas for the red bole bed it varies from 77 to 85%. This suggests a high chemical weathering and higher erodibility. The association of active landslide locations with the red bole bed and fine lateritic soil suggests a close relation between weathering and landslide occurrences in the area.
The Pontocaspian (Black Sea - Caspian Sea) region has a very dynamic history of basin development and biotic evolution. The region is the remnant of a once vast Paratethys Sea. It contains some of the best Eurasian geological records of tectonic, climatic and paleoenvironmental change. The Pliocene-Quaternary co-evolution of the Black Sea-Caspian Sea is dominated by major changes in water (lake and sea) levels resulting in a pulsating system of connected and isolated basins. Understanding the history of the region, including the drivers of lake level and faunal evolution, is hampered by indistinct stratigraphic nomenclature and contradicting time constraints for regional sedimentary successions. In this paper we review and update the late Pliocene to Quaternary stratigraphic framework of the Pontocaspian domain, focusing on the Black Sea Basin, Caspian Basin, Marmara Sea and the terrestrial environments surrounding these large, mostly endorheic lake-sea systems.
In active mountain belts with steep terrain bedrock landsliding is a major erosional agent. In the Himalayas, landsliding is driven by annual hydro-meteorological forcing due to the summer monsoon and by rarer, exceptional events, such as earthquakes. Independent methods yield erosion rate estimates that appear to increase with sampling time, suggesting that rare, high magnitude erosion events dominate the erosional budget. Nevertheless, until now, neither the contribution of monsoon and earthquakes to landslide erosion, nor the proportion of erosion due to rare, giant landslides have been quantified in the Himalayas. We address these challenges by combining and analyzing earthquake and monsoon induced landslide inventories across different timescales. With time-series of 5m satellite images over four main valleys in Central Nepal, we comprehensively mapped landslides caused by the monsoon from 2010 to 2018. We found no clear correlation between monsoon properties and landsliding, and a similar mean landsliding rate for all valleys, except in 2015, where the valleys affected by the earthquake featured ~5–8 times more landsliding than the pre-earthquake mean rate. The long-term size-frequency distribution of monsoon induced landslides (MIL) was derived from these inventories and from an inventory of landslides larger than ~0.1km² that occurred between 1972 and 2014. Using a published landslide inventory for the Gorkha 2015 earthquake, we derive the size-frequency distribution for earthquake-induced landslides (EQIL). These two distributions are dominated by infrequent, large and giant landslides, but underpredict an estimated Holocene frequency of giant landslides (>1km³) which we derived from a literature compilation. This discrepancy can be resolved when modelling the effect of a full distribution of earthquake of variable magnitude and considering that shallower earthquake may cause larger landslides. In this case, EQIL and MIL contribute about equally to a total long-term erosion of ~2±0.75mm.yr−1 in agreement with most thermochronological data. Independently of the specific total and relative erosion rates, the heavy-tailed size-frequency distribution from MIL and EQIL and the very large maximal landslide size in the Himalayas indicate that mean landslide erosion rates increase with sampling time, as has been observed for independent erosion estimates. Further, we find that the sampling time scale required for adequately capturing the frequency of the largest landslides, which is necessary for deriving long-term mean erosion rates, is often much longer than the averaging time of cosmogenic ¹⁰Be methods. This observation presents a strong caveat when interpreting spatial or temporal variability of erosion rates from this method.
Landslides have been observed in different terrestrial environments and also on planets, satellites, and asteroids. Long runout landslides are strongly dependent on the initial mass position, material and slope path properties, topographic relief, and presence of volatiles. Therefore, landslides represent a means for the description of rock properties and environment of deposition prevailing at the time of occurrence, and may assist understanding the geological and climatological history of the planetary surfaces. Concerning Mars, previous studies have concentrated on Valles Marineris, where among the largest and longest landslides have been observed. Using different imagery, we present and analyse an original database of 3,118 Martian landslides of deposit area greater than 0.1 km2 throughout the planet between 60°n and 60°S, resulting in a dataset far richer than previously done. After a distinction is made between different typologies of landslides, their position and the statistical distribution of their geometrical properties are examined. Large landslides cluster along the Noctis Labyrinthus – Valles Marineris – Margaritifer Terra system. Rock avalanches within craters are widespread, but no significant large landslides have been found at latitudes higher than 40°S and 46°N. The magnitude-frequency distribution follows a power-law with scaling exponent ranging between 1.02 and 1.57, for the entire dataset, and varies according to the geomorphological settings, the landslide typology, and mobility. A volume-area power law relationship (exponent: 1.12-1.24) is proposed, based on the reconstruction of 222 landslide geometries, and compared to those for similar terrestrial landslides (1.39). Similarities with respect to terrestrial landslide, distribution with respect to impact craters and impact energy, and cryosphere extent are also discussed.
The fjords of eastern Iceland display many landslide landforms. These phenomena, visible by remote observation of satellite images and Digital Elevation Models (DEM), were inventoried then measured and analysed. A total of 290 landslides were recorded in a spreadsheet database and in a Geographic Information System (GIS). For each landslide, location, morphometry (length, width, surface area, thickness, estimated volume, etc.) data, as well as potential control variables, particularly geological (lithology, dip) or explanatory (orientation, age of deglaciation of the slope affected by the landslide) were recorded. These variables and their distribution were studied by spatial and statistical analysis. This study highlights a higher density of landslides in the northern part of the study area, which could be explained by past ice sheet current directions, which would have reinforced the pressure exerted on the slopes leading to the postglacial decompression phenomenon. The over-representation of west- and south-facing landslides leads to the hypothesis of climatic control: the sunnier slopes underwent more rapid deglaciation, which favoured the instability of the slopes. The data collected also suggest that the lithology (Tertiary basalts) is a control variable and the period during which the landslide area is deglaciated an explanatory variable for landslide initiation. The landslides observed are thus part of a paraglacial dynamic of slope instability after their deglaciation.
One of the largest concentrations of giant landslides (!10 8 m 3) in Patagonia is in the eastern part of Lago Cochrane/Pueyrred on (LP) valley in Argentina. In addition to minor earthflows and rock slides, this landslide cluster is dominated by rock and debris avalanches that affect the northern slope of Meseta Belgrano, the largest of which have volumes >1 km 3 and a runout of >10 km. To determine the chronology of these large landslides and their relationship to the geological setting and the glacial history related to the Last Glacial Maximum (LGM)~20e18 ka ago, we combined geomorphological mapping with absolute dating (luminescence and radiocarbon dating) and numerical modelling of slope stability. Dating and cross-cutting relationships with glaciolacustrine deposits suggest that some of the largest rock avalanches collapsed directly into a glacial lake between~17 and~12 ka, soon after deglaciation, but some were pre-glacial and landslide activity continued until today, posing a potential hazard to the area. In agreement with these data, numerical modelling suggests that slope stability was only marginally affected by ice retreat and glacial lake drainage, and landslides were most likely favoured by relatively low rock strength, related glacially-conditioned topography, and, possibly, seismic activity. A newly identified active fault at the base of the Meseta Belgrano, whose activity was likely enhanced by post-glacial rebound, was probably the key factor that concentrated postglacial rock avalanches into the LP valley. We conclude that exceptionally large (km-scale) landslides can occur on slopes made of relatively weak rocks in a glacially-conditioned topographic setting even without a strong direct triggering effect of deglaciation, while fatigue due to long-term seismicity may promote collapse.
The main conceptual and terminological issues related to lateral spreading are presented and accompanied by a brief outline of the state-of-the-art on the topic. Then the geomorphic features related to the two main types of spreading (rock spreading and soil spreading) are illustrated, with reference to the geological conditions in which they take place, as well as to their causes and evolution. The role of monitoring and modelling is then considered as useful tools to better understand the mechanical behavior of unstable slopes affected by such processes. Finally, some considerations on the hazard and planning implications are provided.
Coarse clastic sedimentary successions cover approximately-one fourth of the continental surface and give rise to distinctive landforms at a variety of scales. Rock-mass strength differences between members of layered successions account for the presence of escarpments, typically capped by thick sandstone or conglomerate beds, usually also with mid-slope cliffs and benches reflecting variable resistance of individual members of the succession. Depending on the dip of strata, two main types of regional landscapes are plateaus and plains, or homoclinal ridges (cuestas), but higher degree of deformation may occur in the vicinity of major faults, resulting in hogback morphology. Medium-scale landforms include residual hills of various types such as mesas and buttes, rock cities and assemblages of ruiniform relief, whereas canyons and slots are common valley forms. Escarpment retreat is usually considered as an overarching concept in geomorphology of layered successions, but it does not seem to be a universal pathway of landscape evolution and even retreat itself may occur in different ways. We propose that juxtaposition of strong and weak rocks in the vertical succession is the viable unifying theme, as it has profound geomorphological implications, influencing processes and patterns of evolution at a variety of spatial scales. However, depending on lithological characteristics, mechanical and hydrogeological properties, dominant processes may vary, explaining considerable landform diversity within tablelands, even though at the grand scale stepped topography becomes a repetitive theme. Synthetic graphical presentation of morphogenetic systems on coarse clastic successions is also presented.
Hundreds of basaltic plateau margins east of the Patagonian Cordillera are undermined by numerous giant slope failures. However, the overall extent of this widespread type of plateau collapse remains unknown and incompletely captured in local maps. To detect giant slope failures consistently throughout the region, we train two convolutional neural networks (CNNs), AlexNet and U-Net, with Sentinel-2 optical data and TanDEM-X topographic data on elevation, surface roughness, and curvature. We validated the performance of these CNNs with independent testing data and found that AlexNet performed better when learned on topographic data, and UNet when learned on optical data. AlexNet predicts a total landslide area of 12,000 km² in a study area of 450,000 km², and thus one of Earth's largest clusters of giant landslides. These are mostly lateral spreads and rotational failures in effusive rocks, particularly eroding the margins of basaltic plateaus; some giant landslides occurred along shores of former glacial lakes, but are least prevalent in Quaternary sedimentary rocks. Given the roughly comparable topographic, climatic, and seismic conditions in our study area, we infer that basalts topping weak sedimentary rocks may have elevated potential for large-scale slope failure. Judging from the many newly detected and previously unknown landslides, we conclude that CNNs can be a valuable tool to detect large-scale slope instability at the regional scale. However, visual inspection is still necessary to validate results and correctly outline individual landslide source and deposit areas.
The Vermilion and Echo Cliffs form a nearly continuous escarpment more than 160 km long within the Colorado Plateau physiographic province of North America. The cliffs overlie the Marble Platform in northern Arizona and are located along the Colorado River, just upstream of the Grand Canyon. Large rotational block landslides mantle the erosional escarpment along most of its extent. Although these landslides have been noted for over 100 years, their likely origin has never been explained. Landslide failure surfaces appear to be influenced by the Petrified Forest Member of the Triassic Chinle Formation, a shale layer containing smectite clay weathered from volcanic ash. Although landslides are common along the majority of escarpments comprising the Colorado Plateau where the Petrified Forest Member and other shales outcrop, most appear to have been inactive since the early Holocene. Multiple generations of landslides and remnants of previous slides exist up to 3 km from the present cliff face. Multiple working hypotheses explaining these landslides are explored in this article, including past landslides and/or lava dams along the Colorado River within the Grand Canyon, periods of wetter climate with higher groundwater levels, and earthquakes related to nearby faulting and volcanism. Various sliding modes along these cliffs are described along with potential triggering mechanisms. Back-analysis of these landslides has been conducted using mechanical properties of the formations involved as well as varying groundwater levels. Calculated factors of safety for existing slides under present conditions are greater than unity, consistent with their apparent stability.
Lateral spreading is a complex geomorphological process occurring through the interplay of different factors. Due to their low rates of displacement, lateral spreads in rock are much less investigated than other landslide types even though sometimes they can evolve into faster and more hazardous movements such as topples. The lack of long-term monitoring data means that the deformation mechanisms of these landslides remain uncertain. Along the northwestern coast of the island of Malta (central Mediterranean Sea), the presence of a thick layer of clay underlying a brittle cap rock made of limestone has led to extensive rock spreading and associated block sliding. Two sites affected by such processes were monitored by GNSS (Global Navigation Satellite Systems) from 2005 to 2019. A network consisting of 17 benchmarks were surveyed twice per year, providing a 14-year displacement history. Coupling this exceptionally long monitoring dataset with Limit Equilibrium and Finite Difference slope stability modelling, the failure mechanisms of the landslides have been investigated to identify predisposing and driving instability factors. This research provides new knowledge on the kinematic behavior of extremely slow landslides and insights into landslide hazard assessments in areas extensively affected by lateral (rock) spreading.
A new database of volcanic debris avalanche deposits (VDADs) from 594 volcanoes in 52 countries has been compiled based on published inventories and our own unpublished data. This work presents an overview of the distribution of VDADs around the world, their sizes, recurrence intervals, and a quantitative characterization of their controlling parameters. Around 50% of the catalogued deposits are located in Japan, the Americas and Russia. Multiple lateral collapses are common at a single volcanic edifice, with a dozen or more events at some volcanoes. The deposits of volcanic debris avalanches are similar to those of non-volcanic rock avalanches (RAs) in terms of shape, deposit morphology and texture. A comparative analysis of the deposit parameters volume, area, drop height and runout length, however, shows that their main difference lies in the maximum volume RAs can typically reach. This is mainly due to differences in source scar geometry. At the same drop height, VDAs can produce larger volumes because their scars are deeper-seated, more bowl-shaped, and extend along longer slopes than is common for rock slopes. Although the new database is still incomplete, this compilation shows great potential for future analyses and emphasizes the importance of strengthening such inventories with more well-studied cases for sound scientific studies and hazard assessments.
Giant catastrophic landslides (>10 8 m 3) dot the formerly glaciated mountain forelands of the eastern Patagonian Andes. From geomorphic mapping, sedimentological logs and radiocarbon dating, we infer the emplacement kinematics and approximate timing of giant landslides in moraines and other glacial deposits in the Lago Pueyrred on valley (LPV), Argentina. For the first time, we report in detail examples of giant low-gradient landslides with hummocky lobes derived from unconsolidated glacial deposits and weak bedrock. We find that at least 4.5 km 3 of debris and weak bedrock were mobilized by slope failures in an area of~500 km 2 since the Last Glacial Maximum (LGM;~25e18 ka). Nearly 90% of this landslide volume originated along the shores of, or as subaqueous failures in, a postglacial moraine-dammed meltwater lake. The larger landslides (>1 km 3) detached from moraines fringing the lake, whereas other landslides displaced glacial and lake deposits either on the paleolake bed, or in a river gorge that was cut upon drainage of the glacial lake. Sequences of till, glaciofluvial and glaciolacustrine deposits overlying weak Early Miocene marlstone are mostly conducive to major landslides in the LPV. Cross-cutting relationships indicate that largest landslides in the area originated during rapid glacial lake-level drops. Distribution and internal structure of hummocks within landslide lobes suggest that these landslides were emplaced as catastrophic debris avalanches. It suggests that giant catastrophic landslides in the glacier forelands of Patagonia can result from layered weak rocks, changes in meltwater-lake levels, and possibly as a consequence of strong earthquakes linked to rapid post-glacial rebound following the retreat of the Patagonian Ice Sheet (PIS).
Many of the volcanic plateau margins of the eastern, formerly glaciated, foreland of the Patagonian Andes are undermined by giant landslides (≥10⁸ m³). One cluster of such landslides extends along the margin of the Meseta del Lago Buenos Aires (MLBA) plateau that is formed mainly by Neogene-Quaternary basalts. The dry climate is at odds with numerous >2-km long earthflows nested within older and larger compound landslides. We present a hydrological analysis, a detailed geomorphic map, interpretations of exposed landslide interiors, and radiocarbon dating of the El Mirador landslide, which is one of the largest and morphologically most representative landslide.
We find that the presence of lakes on top of the plateau, representing low infiltration rates, correlates negatively with the abundance of earthflows on landslide debris along the plateau margins. Field outcrops show that the pattern of landslides and earthflows is likely controlled by groundwater seepage at the contact between the basalts and underlying soft Miocene molasse. Numerous peat bogs store water and sediment and are more abundant in earthflow-affected areas than in their contributing catchment areas.
Radiocarbon dates indicate that these earthflows displaced metre-thick layers of peat in the late Holocene (<2.5 ka). We conclude that earthflows of the MLBA plateau might be promising proxies of past hydroclimatic conditions in the Patagonian foreland, if strong earthquakes or gradual crustal stress changes due to glacioisostatic rebound can be ruled out.
The Paleocene volcanic rocks in the Nuussuaq Basin on Disko and Nuussuaq comprise the Vaigat Formation (c. 62–61 Ma) and the Maligât Formation (c. 60 Ma). The Vaigat Formation in this area is 0–1600 m thick and is dominated by olivine-rich picrites. The formation was deposited during three volcanic episodes and is divided into 10 formally defined members and about 20 informal units. The first episode gave rise to the Anaanaa Member. The second episode gave rise to the Naujánguit Member, which is intercalated with the minor, crustally contaminated Nuusap Qaqqarsua, Nuuk Killeq, Asuk, Tunoqqu and Kûgánguaq members and the uncontaminated Qordlortorssuaq Member. The third episode gave rise to the Ordlingassoq Member and the minor alkaline Manîtdlat Member. Contemporaneous sediments deposited during the first two episodes are the marine Eqalulik Formation, and during the third episode the nonmarine Atanikerluk Formation. During the second episode, the polarity of the geomagnetic field changed from normal (Chron C27n) via a transition zone to reversed (C26r). The deposits of the first volcanic episode are situated on western Nuussuaq. During the second and third episodes, the volcanism gradually spread eastwards and southwards so that the Vaigat Formation now forms a domed structure, thickest in the north, thinning out on northern Disko and reaching eastwards to the high gneiss country on central Nuussuaq. The earliest eruptions took place on the sea floor and quickly built up a subaerial lava plateau. All three episodes gave rise to complicated facies changes between subaqueous and subaerial eruption products caused by the eastmoving volcanism, subsidence, volcanic aggradation and blockage of the sea connection against the elevated eastern gneiss country. Eruption sites are widespread for all three volcanic episodes. Within certain time periods, a number of contemporaneous high-level magma reservoirs developed within sediments of the Nuussuaq Group, and the crustally contaminated members formed in these reservoirs by reaction between Mg-rich magmas and sediments. The uncontaminated rocks in the Vaigat Formation are picrites with 12–31 wt% MgO and subordinate basalts with 7–12 wt% MgO. The crustally contaminated rocks range from silicic picrites with 12–16 wt% MgO (Nuusap Qaqqarsua Member) to native-iron-bearing magnesian andesites with 6–10 wt% MgO and up to 62 wt% SiO2 (Asuk Member). The Asuk Member includes unique, strongly reduced rock types with native iron, graphite and sulfide. The contaminated units have individually distinct compositions, indicating individually different contamination events. The alkaline Manîtdlat Member contains an enriched lithospheric component. Present-day seeps of migrated oil are widespread in the oldest part of the volcanic succession on western Nuussuaq. Some of the contaminated magmas in the Asuk and Kûgánguaq members have fractionated sulfides with Cu and Ni and have been explored for nickel and platinum-group elements.
The Paleocene volcanic rocks in the Nuussuaq Basin on Disko and Nuussuaq comprise the picritic Vaigat Formation (c. 62–61 Ma) and the overlying basaltic Maligât Formation (c. 60 Ma). The Maligât Formation is up to 2000 m thick on western Disko where the top of the formation is least eroded. The formation is divided into four members, the Rinks Dal, Nordfjord, Niaqussat and Sapernuvik members, which are formally defined here. On central and eastern Disko and Nuussuaq the Maligât Formation lavas are interbedded with fluvial and lacustrine sandstones and mudstones of the Atanikerluk Formation.The Rinks Dal Member is the lowest member and originally constituted around 61% by volume of the formation. It is divided into 12 informal units based on chemically recognisable oscillations in the fractionation state of the basalts. The oldest units are present on central and south Disko close to the Disko Gneiss Ridge. The younger lavas spread farther to the east, north and west, filled the Assoq Lake basin east of the ridge and gradually onlapped the shield of the earlier Vaigat Formation that rose to the north. Only the lavas of the upper Rinks Dal Member reached far into Nuussuaq. The lavas are generally not crustally contaminated and comprise evolved basalts with 4.4–9.2 wt% MgO and a few picrites. The most evolved basalts with 3.2–4.8 wt% TiO2 occur in the middle part of the member where they form the Akuarut unit. The Nordfjord Member originally constituted around 6% by volume of the formation. It is not subdivided because the lithological variability is local. The member is widespread but has its depocentre on north-western Disko where thicknesses reach 350 m and eruption sites, intermediate lavas and acid tuffs are present. Over most of the area the member consists of just a few lava flows with combined thicknesses of 30–100 m. The member has a very diverse lithology with rock types ranging from silicic basalt with 5.3–10.0 wt% MgO through magnesian basaltic andesite and andesite with 2.4–10.6 wt% MgO to dacite with 1.2–2.2 wt% MgO. Rhyolite with 0.2–1.2 wt% MgO and up to 77 wt% SiO2 occur in tuffs and conglomerate clasts. All rocks are crustally contaminated and some are native-iron-bearing. The Niaqussat Member originally constituted around 33% by volume of the formation. It is subdivided into three informal units. The member is widespread, but much of it has been removed by erosion. Lithologies in the lower unit range from silicic picrite with up to 15 wt% MgO to basalt with 6–12 wt% MgO and a few basaltic andesite flows. The middle and upper parts of the Niaqussat Member comprise more evolved basalts with respectively 6.1–7.2 wt% MgO and 4.9–6.4 wt% MgO. All rocks are crustally contaminated and a few lava flows are native-iron-bearing. The Sapernuvik Member comprises three uncontaminated basalt flows with 7.5–10.7 wt% MgO. It is only preserved in a small area on western Disko. Dyke systems with up to 80 km long dykes and subvolcanic intrusions associated with the Nordfjord and Niaqussat members occur on western and north-eastern Disko. The rocks are crustally contaminated and range from silicic basalt with 4–13 wt% MgO to magnesian andesite with 3–10 wt% MgO. They commonly form composite intrusions, some of which contain accumulations of native iron and sulfides. The contaminants are carbon- and sulfur-bearing sediments of the Nuussuaq Group. Major contamination mechanisms were mixing with partial melts from the sediment sidewall and xenoliths and selective exchange of some elements, including carbon and sulfur, between magma and sediment. Degrees of contamination vary from 2−5% in the basalts to 10−50% in the more silicic rocks. No rocks more evolved than basalt were produced by ordinary fractional crystallisation.
Past climates and environments experienced by the Saharo-Arabian desert belt are of prime importance for palaeoclimatic and palaeoanthropological research. On orbital timescales transformations of the desert into a grassland landscape in response to higher precipitation provided “windows of opportunity” for hominin dispersal from Africa into Eurasia. On long timescales, palaeoenvironmental reconstructions for the region are predominantly derived from marine sediments and available terrestrial records from the Arabian Peninsula are limited to 450 ka before present (BP). Here, we present a new stalagmite-based palaeoclimate record from Mukalla Cave in Yemen which extends back to ∼1.1 million years BP or Marine Isotope Stage (MIS) 31, as determined by Uranium-lead dating. Stalagmite Y99 grew only during peak interglacial periods and warm substages back to ∼1.1 Ma. Stalagmite calcite oxygen isotope (δ¹⁸O) values show that every past interglacial humid period was wetter than the Holocene, a period in which large lakes formed in the now arid areas of southern Arabia. Carbon isotope (δ¹³C) values indicate habitable grassland environments developed during these pluvial periods. A total of 21 pluvial periods with precipitation of more than 300 mm yr⁻¹ occurred since ∼1.1 Ma and thus numerous opportunities for hominin dispersals occurred throughout the Pleistocene. New determinations of hydrogen (δDFI) and oxygen (δ¹⁸OFI) isotopes in stalagmite fluid inclusion water demonstrates that enhanced precipitation in Southern Arabia was brought by the African and Indian Summer Monsoons. When combined with sub-annual calcite analysis of δ¹⁸O and δ¹³C, these data reveal a distinct wet (summer) and dry (winter) seasonality.
The first comprehensive global database of giant landslides on volcanic islands is outlined in this report. This database comprises a total of one hundred and eighty-two entries: the Atlantic Ocean hosts seventy-five giant landslides; the Pacific Ocean hosts sixty-seven giant landslides; and the Indian Ocean hosts forty giant landslides. To determine the spatial characteristics of each giant landslide, it has been necessary to georeference published maps using ArcGIS software coupled with global DTMs. Using the georeferenced outputs, it has been possible to measure the basic morphometric characteristics of each landslide such as its length, width, perimeter, area, and fall height. Landslide volumes have been calculated with a higher degree of certainty in thirty-five cases and with a lower degree of certainty in sixty-three cases while complete outlines of the landslide area have been defined in ninety-six cases. On the basis of these data, it has been possible to interrogate relationships between potentially significant variables. The age distribution of giant landslides on volcanic islands demonstrates that more than half of the records in the database occurred during the last 0.5 Ma. This global database of giant landslides on volcanic islands is hosted on the website of the Institute of Rock Structure & Mechanics: https://www.irsm.cas.cz/ext/giantlandslides. From there, the records can be downloaded as a spreadsheet or as a kml file for interrogation in a number of geospatial software programs including ArcGIS and Google Earth. This work is part of the activities of the International Consortium on landslides, namely, its International Programme on Landslides (Project No. 212).
Moderate-relief landscapes, such as the Czech Flysch Outer Western Carpathians (COWC), in comparison with alpine regions are rarely subject to extensive landslide inventory mapping. An understanding of landslides in such a landscape is needed, because densely populated hilly landscapes in temperate zones are usually of major socio-economic importance. In this study, we performed the first LiDAR-based landslide mapping for the entire COWC area (~7539 km²), one of the most landslide-prone regions in Europe. By calculating various landscape and landslide metrics, we infer the distribution, frequency-area relationships, kinematics and controls of mass movements with special attention on large landslides (≥0.1 km²) and deep-seated gravitational slope deformations (DSGSDs). We mapped a total of 13,611 landslides, of which 1357 failures are large landslides and DSGSDs. Whereas the lower and more subdued areas in the southern part of COWC are hotspots in terms of the total number of landslides, the higher and more topographically pronounced areas in the northeast are affected predominantly by large landslides and DSGSDs. However, landslides with ≥0.1 km² are widespread throughout the whole territory of COWC. A discrepancy also exists in the spatial distribution of different types of landslides. Rock slides and DSGSDs are dominant in the north-east, while flow-type landslides are dominant in the southern lower topographic relief with claystone-dominated flysch. We conclude that 1) distinct geological units (nappes) produce landslide populations with different frequency-area distributions; 2) stratigraphic composition alongside the tectonic style of flysch formations control the type of landslides; 3) DSGSDs affect mainly slopes formed by rigid rocks sitting atop soft formations; and 4) geological conditions, rather than topography, control distribution of large landslides and DSGSDs in COWC.
Although the general concept that escarpments in layered rocks retreat through time was offered more than 150 years ago, recognizing the exact mechanisms, patterns and rates of retreat remains a challenge. In this paper, we provide the state-of-the-art of the theme of escarpment retreat, reviewing both the classic contributions, often forgotten but important case studies, and the latest research. In separate parts of the paper we discuss: (1) terminology applied to the geomorphology of escarpments; (2) the main assumptions and pathways of long-term evolution of escarpments, including conceptual models presented so far; (3) processes involved in cliff retreat, mainly weathering, mass movements and erosion; (4) the role of spring erosion and fluvial processes, leading to fluvial dissection and development of canyons present in many tablelands; (5) the possible role of climate change in escarpment evolution; (6) rates of escarpment retreat. In conclusions, we summarize the major advances in our understanding of escarpment evolution of the last two-three decades such as the recognition of the key role of emerging groundwater in non-uniform escarpment retreat, of an important role of silica dissolution and underground removal of dissolved solids, increasing appreciation of rock properties in governing the patterns of escarpment retreat, the proposal that cliffs do not necessarily retreat via catastrophic rock slope failures but may disintegrate in situ, and the first attempts to date escarpment retreat using cosmogenic isotopes. Finally, the remaining gaps and possible research priorities for the future are identified.
Rock slope failures are a potential source of danger in polar regions. A causal connection between slope failures and climate-related glacial and deglacial processes has been inferred for the growing number of documented events. In this context, we investigated a large-scale rotational rock slide affecting the coastal ridge of Spitsbergen's Forkastningsfjellet. Based on a detailed structural description, we discuss the kinematics, timing and potential drivers of rock slide activity and present a preliminary landslide hazard assessment. The Forkastningsfjellet rock slide has a footprint of at least 2.03 km². A minimum rock mass volume of 0.10 km³ was displaced either catastrophically or over a longer time period. Initial movement in the hanging wall of a NW-dipping listric sliding surface led to the fragmentation of the sliding mass into separated tilt blocks that created the present-day, stair-stepped morphology. The main rock slide release was probably related to the deglaciation of Isfjorden and the resulting instability of the weakened rock mass along the oversteepened slopes during Allerød times (~13,900-12,700 BP). Mass wasting and sea-cliff erosion, mainly controlled by the inherent discontinuities of the fractured and tilted rock masses, currently take place along the steep slopes of the coastal tilt blocks. A preliminary hazard analysis suggests a medium to high hazard for a reactivation of the slide or individual blocks, but uncertainty margins for this classification are large due to a lack of data. Poor control of total displacement data in particular contributes to the uncertainty. A high-acceleration reactivation of a large compartment of the slide (e.g., on the order of 10 million m 3) could cause a displacement wave several metres high in Longyearbyen. These results indicate a need for further multidisciplinary investigations to better understand the extent and nature of the rock slide and parameters such as displacement velocities to support a more reliable hazard and risk assessment for the Longyearbyen region.
The edges of the Ethiopian Plateau are affected by frequent landslides predisposed by pronounced seasonality in precipitation, thick weathering mantle of volcanic material and rough relief. We analyzed the three‐dimensional dynamics of three large landslides in Dessie using digital elevation models and ortho‐images derived from a time series of aerial photographs reaching back to 1936. Furthermore, we utilized repeated photography based on terrestrial photographs from the 1930s and 1940s to analyze landscape changes. It was revealed that the large sliding zone (25.4 ha) in lacustrine sediments at Kerra locality existed approximately in the present extent already before 1936. The volume of depleted material of the 1986‐1994 rock slide at Doro Mezleya locality was assessed as 1.82 106 m3 with mean vertical thickness of 48 m. Additionally, we described the nowadays inactive Hot Spring landslide (12.9 ha). We documented a large scale reforestation of the area carried out as a remediation measure and rapid changes in land cover and settlement structure. We conclude that two out of three studied large landslides existed before 1936 and thus are not induced by the increased human pressure on the landscape of the last decades. Additionally, we provide an overview about collections of historical aerial photographs of Ethiopia and we discuss their potential and drawbacks for mass wasting studies.
Oversteepened valley walls in western Norway have high recurrences of Holocene rock-slope failure activity causing significant risk to communities and infrastructure. Deposits from six to nine catastrophic rock-slope failure (CRSF) events are preserved at the base of the Mannen rock-slope instability in the Romsdal Valley, western Norway. The timing of these CRSF events was determined by terrestrial cosmogenic nuclide dating and relative chronology due to mapping Quaternary deposits. The stratigraphical chronology indicates that three of the CRSF events occurred between 12 and 10 ka, during regional deglaciation. Congruent with previous investigations, these events are attributed to the debuttressing effect experienced by steep slopes following deglaciation, during a period of paraglacial relaxation. The remaining three to six CRSF events cluster at 4.9 ± 0.6 ka (based on 10 cosmogenic ¹⁰Be samples from boulders). CRSF events during this later period are ascribed to climatic changes at the end of the Holocene thermal optimum, including increased precipitation rates, high air temperatures and the associated degradation of permafrost in rock-slope faces. Geomorphological mapping and sedimentological analyses further permit the contextualisation of these deposits within the overall sequence of post-glacial fjord-valley infilling. In the light of contemporary climate change, the relationship between CRSF frequency, precipitation, air temperature and permafrost degradation may be of interest to others working or operating in comparable settings.
Rotational–translational landslides are common in Neogene basins throughout the world and are of high risk to the public. To understand the mechanism of rotational–translational failure, the spatial distribution and deformation of rotational–translational landslides in the northeast of the Tibetan Plateau are investigated in this study. The spatial distribution of these landslides is dependent on the regional tectonics and geomorphology, crustal stresses, and lithological properties. The rotational–translational landslides are concentrated in the Neogene mudstone basins, and the intensities of these landslides are observed to gradually decrease from the hinterland of the plateau to the marginal basins. The bedding-parallel shear zones within the location of the rotational–translational landslides are present in the overconsolidated Neogene mudstones with high clay content. Nearly horizontal tectonic stresses and erosion cause the formation of horizontal shear stresses in the sliding masses. In this stress environment, materials with low internal friction angle (<10°) are observed to develop in the shear zones. A weak layer with high clay content and low calcium content are observed in all the bedding-parallel shear zones of rotational-translational landslides. Further, illite–montmorillonite and illite are the main clay minerals of all the shear zones with no montmorillonite. Horizontal shearing is further accelerated by increasing the pore-water pressure and creep.
Quaternary glaciations have repeatedly shaped large tracts of the Andean foreland. Its spectacular large glacial lakes, staircases of moraine ridges, and extensive outwash plains have inspired generations of scientists to reconstruct the processes, magnitude, and timing of ice build-up and decay at the mountain front. Surprisingly few of these studies noticed many dozens of giant (≥10⁸ m³) mass-wasting deposits in the foreland. We report some of the world's largest terrestrial landslides in the eastern piedmont of the Patagonian Ice Sheet (PIS) along the traces of the former Lago Buenos Aires and Lago Puyerredón glacier lobes and lakes. More than 283 large rotational slides and lateral spreads followed by debris slides, earthflows, rotational and translational rockslides, complex slides and few large rock avalanches detached some 164 ± 56 km³ of material from the slopes of volcanic mesetas, lake-bounding moraines, and river-gorge walls. Many of these landslide deposits intersect with well-dated moraine ridges or former glacial-lake shorelines, and offer opportunities for relative dating of slope failure. We estimate that >60% of the landslide volume (∼96 km³) detached after the Last Glacial Maximum (LGM). Giant slope failures cross-cutting shorelines of a large Late Glacial to Early Holocene lake (“glacial lake PIS”) likely occurred during successive lake-level drop between ∼11.5 and 8 ka, and some of them are the largest hitherto documented landslides in moraines. We conclude that 1) large portions of terminal moraines can fail catastrophically several thousand years after emplacement; 2) slopes formed by weak bedrock or unconsolidated glacial deposits bordering glacial lakes can release extremely large landslides; and 3) landslides still occur in the piedmont, particularly along postglacial gorges cut in response to falling lake levels.
Taking into consideration the different situations observed in the field, this chapter proposes a model of evolution of the Patagonian volcanic landscape developed from the outcrop of basaltic flows. The different geomorphological processes that act upon the evolution of these landscapes are exposed, particularly fluvial erosion and mass movement processes, and the factors that contribute to the modification or interruption of the evolutionary sequence proposed. The term “landscape of lobes and hummocks” is proposed for the final evolutionary stage of these landscapes. The rate of relative elevation of the basaltic mesetas is also estimated.
This chapter presents the problems discussed in this book and the methodology used for their study, based upon a wide use of remote sensing techniques. The spatial relationships established between the two major and typical elements of the Patagonian landscapes, the basaltic plateaus and the wet meadows, known in this region as “escoriales” and “mallines”, respectively, are analyzed. The characteristics of both landscape components are described and a classification system is proposed, based upon a six-digit system which synthetizes the geological, geomorphological, and hydrological characteristics of each “escorial”. The full structure of the book, organized in 10 chapters, is herein presented.
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https://drive.google.com/file/d/1JgUHmM0fX98QSWfw6MzlbpvdbJ4oGJmf/view?usp=share_link _____________________________________________________________________________________Lake Baikal is Earth's deepest lake and an iconic site of scientific study. This vast basin holds sedimentary archives of environmental change dating back to the Miocene and its array of palaeoshorelines and surrounding relief record the past ~ 1-3 Ma of lake-levels and outflows. Here we present an extensive review of previous work alongside a new set of observations concerning the Quaternary development of Lake Baikal, with special focus on lake-level fluctuations and the formation and evolution of the lake's three known outlets. The sequence of shoreline terraces indicates that lake-levels were both higher and lower in the past. Lake Baikal stood ~ 200 m higher during the Last Interglacial, i.e. Marine Isotope Stage (MIS) 5e and dropped to 40 m below (present-day) during the Last Glacial Maximum (MIS 2). The relative lake-level variations reflect climate factors and gradual or sudden (coseismic) tectonic impacts on the elevation of the lake's outlet thresholds. Three successive outlets are known: i) the palaeo-Goloustnaya-Manzurka, associated with the Manzurka Alluvium; ii) the palaeo-Irkut, and iii) the currently-active Angara River outlet. We propose that the Manzurka Alluvium is the product of catastrophic events in Lake Baikal. The sudden (possibly coseismic) collapse of the ~ 15 x 3 km Goloustnaya fault-block into Lake Baikal triggered a mega-tsunami that thrust overwash deposits across neighbouring drainage divides above Lake Baikal and the valleys of the Goloustnaya-Manzurka River system. The age of the Manzurka Alluvium remains poorly constrained, but the mega-tsunami is potentially traceable to an unconformity in drill-core sediments at ~ 0.8-1.0 Ma, although older (late Pliocene) and younger (~125 ka) ages have also been proposed. The Irkut outlet existed between MIS 6 and MIS 5 when lake-level was ~ 200 m higher than present (~ 640-650 masl) and a large bay extended into the Tunka rift at Baikal's south-west tip. Lake Baikal retreated from the Tunka rift when lake-level fell by up to 100 m in early MIS 5e. We propose that the lake-level fall is connected to a partial collapse of Primorsky Ridge at Listvenichny Bay, which caused Baikal to overspill into the Angara River thereby forming a new outlet. The release of a > 4000 km3 megaflood down the Angara River valley caused large-scale modification and reworking of valley-fills (MIS 5e). At the end of MIS 2, further collapse of Primorsky Ridge lowered the outlet threshold an additional 50-60 m and prompted a second megaflood down the Angara River valley, which left a widespread unconformity where horizontal-bedded sands (dated at ~ 11.8-13.4 ka) overlie cryoturbated deposits of the earlier megaflood. The central role of catastrophic processes at Lake Baikal suggests that, rather than being rare events, coseismic landsliding and mega-tsunami may be more frequent than hitherto recognised.
Data on large rockslides presented in a book by Olafur Jónsson (1976) are analyzed in terms of their magnitude and frequency in the Postglacial (10 000 a BP). Size has been examined by division into four groups, the modal size is from 1 to 10 × 106 m³. Age of rockslide events was assessed by relative methods by O. Johnsson and is thought to be internally consistent. Five age categories have been used and the modal group (III ≈ 3–5000 a BP) has 34 % of rockslides. The period 3–7000 a BP contains 60% of all events; less than 5% have occurred in the last 1000 years. Age-size distributions suggest that the smallest slides are also the youngest; the largest events are the oldest. Problems associated with interpretation of the data are discussed in terms of a probability model but it is concluded that only a very simple interpretation is possible by its use. Climatic controls are not considered to be of prime importance in determining either size or age of slides. The majority of slides are found in lavas (Quaternary or Tertiary) and it is concluded that cleft water pressure (in some cases as a result of waning ice sheets) is the prime cause of slope failure.