measured shear rates in large dry and wet snow avalanches we present estimates of internal shear rates of real scale avalanches that are based on velocity measurements optical velocity sensors installed on the instrument pylon at the swiss vallee de la sionne test site are used to measure flow velocities at different flow heights of three large dry and wet snow avalanches possible sources of error in the correlation analysis of the time lagged reflectivity signals measured by optical sensors are identified for real size avalanches these include spurious velocities due to noise and elongated peaks an appropriate choice of the correlation length is essential for obtaining good velocity estimates placing restrictions on the maximum possible accelerations in the flow improves the analysis of the measured data coherent signals are found only in the dense flowing cores we observe the evolution of shear rates at different depths between the front and tail of the flowing avalanche at the front large shear rates are found throughout the depth at the tail plug flows overriding highly sheared layers near the bottom of the flow are observed the measured velocities change strongly with height above the ground and fluctuations around the measured mean velocity can be identified we find that the dense flows are laminar undergoing a transition from supercritical to subcritical flow behaviour from the head to the tail furthermore we provide real scale experimental evidence that the mean shear rate and the magnitude of velocity fluctuations increase with the mean discharge

slow drag in wet snow avalanche flow we report impact pressures exerted by three wet snow avalanches on a pylon equipped with piezoelectric load cells these pressures were considerably higher than those predicted by conventional avalanche engineering guidelines the time averaged pressure linearly increased with the immersion depth of the load cells and it was about eight times larger than the hydrostatic snow pressure at the same immersion depth the pressures were very similar for all three avalanches and no dependency between avalanche velocity and pressure was apparent the pressure time series were characterized by large fluctuations for all immersion depths and for all avalanches the standard deviations of the fluctuations were on average about 20 of the mean value we compare our observations with results of slow drag granular experiments where similar behavior has been explained by formation and destruction of chain structures due to jamming of granular material around the pylon and we propose the same mechanism as a possible microscale interpretation of our observations

the insular shelves of the faial pico ridge azores archipelago a morphological record of its evolution shelves surrounding reefless volcanic ocean islands are formed by surf erosion of their slopes during changing sea levels posterosional lava flows if abundant can cross the coastal cliffs and fill partially or completely the accommodation space left by erosion in this study multibeam bathymetry high resolution seismic reflection profiles and sediment samples are used to characterize the morphology of the insular shelves adjacent to pico island the data show offshore fresh lava flow morphologies as well as an irregular basement beneath shelf sedimentary bodies and reduced shelf width adjacent to older volcanic edifices in pico these observations suggest that these shelves have been significantly filled by volcanic progradation and can thus be classified as rejuvenated despite the general volcanic infilling of the shelves around pico most of their edges are below the depth of the last glacial maximum revealing that at least parts of the island have subsided after the shelves formed by surf erosion prograding lava deltas reached the shelf edge in some areas triggering small slope failures locally decreasing the shelf width and depth of their edges these areas can represent a significant risk for the local population hence their identification can be useful for hazard assessment and contribute to wiser land use planning shelf and subaerial geomorphology magnetic anomalies and crustal structure data of the two islands were also interpreted to reconstruct the long term combined onshore and offshore evolution of the faial pico ridge the subaerial emergence of this ridge is apparently older than previously thought i e before 850 ka

800 000 years of abrupt climate variability we constructed an 800 000 year synthetic record of greenland climate variability based on the thermal bipolar seesaw model our greenland analog reproduces much of the variability seen in the greenland ice cores over the past 100 000 years the synthetic record shows strong similarity with the absolutely dated speleothem record from china allowing us to place ice core records within an absolute timeframe for the past 400 000 years hence it provides both a stratigraphic reference and a conceptual basis for assessing the long term evolution of millennial scale variability and its potential role in climate change at longer time scales indeed we provide evidence for a ubiquitous association between bipolar seesaw oscillations and glacial terminations throughout the middle to late pleistocene

assessing landslide movements in volcanic islands using near shore marine geophysical data south pico island azores marine geophysical data from around the submarine flanks of volcanic islands can potentially help to resolve whether large scale instability of an edifice has been geologically recently active we use geophysical data to investigate part of the coast of pico island of the azores where above sea level a major slump of topo volcano has been interpreted previously from arcuate escarpments and a rugged irregular topography seaward of them multibeam echo sounder data collected offshore of this feature show remarkably little evidence for slump fault movements in the island s submarine slope mid slope benches like those associated with the hilina slump of kilauea are absent the high resolution data extends onto the island s shelf allowing us to evaluate evidence for continuing activity there in particular as the shelf s rock platform will have been last modified by surf erosion during the postglacial period of sea level transgression it provides a reference surface of intermediate age 7 19 ka that can potentially reveal whether any movements occurred in pre historic times where the arcuate escarpments are continued offshore the modern seabed shows no bathymetric evidence for active faults where the shelf rock platform crops out in the multibeam data elsewhere mobile shelf sediments could be disguising evidence for active faulting so we examined boomer profiles able to image the rock platform beneath them the data reveal a platform that is steep 6 6 compared with the dips of platforms that we have studied previously around the coast of adjacent faial island and steeper than the platform outside the proposed slump this suggests that it was created by coastal erosion over a shorter period and hence is consistent with a younger age of the coastline as with the multibeam data where escarpments are continued offshore onto the shelf the rock surface imaged with these boomer data also shows no clear evidence of major slump related fault displacements this study therefore illustrates how high resolution boomer seismic and multibeam data could usefully contribute to hazard assessment of volcanic islands by helping to evaluate areas with no historical movements explanations to reconcile the onshore and offshore data here are also put forward

large scale catastrophic flank collapses in a steep volcanic ridge the pico faial ridge azores triple junction abstract large scale flank collapses are common in the geological evolution of volcanic ocean islands in the atlantic to date catastrophic lateral collapses in the azores islands have been difficult to identify leading to suggestions that a lack of events may relate to the relatively small size of the islands here we show evidence for two major collapses on the northern flank of pico island pico faial volcanic ridge central azores suggesting that this island had a collapse incidence similar to that of other atlantic volcanic islands the study is based on the analysis of 1 offshore and onshore high resolution digital elevation models 2 field data focused on the n flank and 3 new k ar ages on selected lava flow samples pico sub aerial northern flank is marked by two conspicuous arcuate shaped depressions concave towards the sea here interpreted as landslide scars a main debris field is observed offshore the largest depression this deposit has 20 km of maximum length covers ca 150 km2 is composed of meter to hectometer blocks and has an exposed volume here estimated between 4 and 10 km3 though the actual volume probably exceeds 10 km3 debris flow towards the ese was apparently determined by the slope of the narrow wnw ese s jorge channel young lava flows cascade over the interpreted scars thus concealing the older volcanic sequence s affected by the landslide s new k ar ages measured on these lava flows provide a minimum age of ca 70 ka for the large scale collapse s in pico s northern flank

gravity driven granular free surface flow around a circular cylinder snow avalanches and other hazardous geophysical granular flows such as debris flows lahars and pyroclastic flows often impact on obstacles as they flow down a slope generating rapid changes in the flow height and velocity in their vicinity it is important to understand how a granular material flows around such obstacles to improve the design of deflecting and catching dams and to correctly interpret field observations in this paper small scale experiments and numerical simulations are used to investigate the supercritical gravity driven free surface flow of a granular avalanche around a circular cylinder our experiments show that a very sharp bow shock wave and a stagnation point are generated in front of the cylinder the shock standoff distance is accurately reproduced by shock capturing numerical simulations and is approximately equal to the reciprocal of the froude number consistent with previous approximate results for shallow water flows as the grains move around the cylinder the flow expands and the pressure gradients rapidly accelerate the particles up to supercritical speeds again the internal pressure is not strong enough to immediately push the grains into the space behind the cylinder and instead a grain free region or granular vacuum forms on the lee side for moderate upstream froude numbers and slope inclinations the granular vacuum closes up rapidly to form a triangular region but on steeper slopes both experiments and numerical simulations show that the pinch off distance moves far downstream

susceptibility of mid ocean ridge volcanic islands and seamounts to large scale landsliding 1 with a view to assessing the incidence of large scale landsliding a morphologic database was created for volcanic islands and seamounts on young oceanic lithosphere the database included 44 mid ocean ridge seamounts jasper seamount and the islands of ascension bouvet guadalupe and several of the galapagos and azores islands supplemented with published reports from a further five volcanic edifices the data reveal that major landslides are common on edifices taller than 2500 m but are rare in shorter edifices implying a threshold of instability at around 2500 m a number of causes of this threshold are discussed for example many structures taller than 2500 m are or were originally volcanic islands and therefore their flanks probably include extensive weak hyaloclastite built up from lava sea interactions around coasts compaction of hyaloclastites in larger edifices lead to regions of low permeability which may help to explain the more deeply seated slope failures it is intriguing that the threshold also coincides with the edifice height at which volcanic ridges become observable a stage at which dike intrusion is probably common because slope oversteepening or excess pore pressure associated with dike intrusions have been proposed elsewhere as landslide triggers the current indications of landsliding reveal no observable relation to rainfall as landslides occur equally in wet and dry climates and no relation to tectonic setting as there are relatively few major landslides in the seismically active azores group

geodar data and the flow regimes of snow avalanches geodar a custom radar system images avalanches over the entire slope with high spatial and temporal resolution at the experimental test site vallee de la sionne in switzerland between winter seasons 2009 10 and 2014 15 data has been acquired from 77 avalanches these data sets describe a wide variety of avalanches which we classify in terms of seven flow regimes and combinations thereof these flow regimes expand on previous classifications with four identifiable dense flow regimes where interaction between granules and with the flow bed dominates dynamics and two different dilute flow regimes where interaction between snow particles and the air becomes dominant there is a further regime identified where snow balls simply roll down the mountain a cold dense regime and a warm shear regime behave like non cohesive granular flows with velocity shear throughout the flow a sliding slab regime and a warm plug regime occur when cohesion dominates and causes the flow units to act as solid like objects sliding on a thin shear zone an intermittent regime connects the cold dense regime with the suspension regime and is characterised by highly fluctuating density and surging activity geodar enables localization of these flow regimes and transitions between them in time and space we discuss flow regime transitions in terms of snow properties topography speed and size of the avalanches this paper also serves as a reference for the data set which is made publicly available and should prove to be an invaluable resource for the development of physically based avalanche models

reply to comment by marques et al on the insular shelves of the faial pico ridge azores archipelago a morphological record of its evolution

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

large scale active slump of the southeastern flank of pico island azores we report evidence for ongoing lateral slump of part of the southeastern flank of the pico volcanic ridge in the azores data from a high resolution digital elevation model field work gps and radar interferometry show that 1 the slumping sector is several cubic kilometers in size 2 the structure involves several curved scars with normal fault kinematics 3 the central part is undergoing little horizontal displacement toward the southeast 1 6 1 3 mm yr but significant downward movement 5 12 mm yr and 4 the outer part of the southeastern flank of pico is subsiding faster than the inner parts this likely reflects recent individualization of a steep seaward dipping fault in the moving mass the slump shares similarities with active slumps recognized elsewhere although the studied area may represent only the proximal part of a much larger complex potentially affecting the deep submarine base of the island displacement of the subaerial part of the southeastern flank of pico seems to be accommodated by the movement and rotation of large blocks along listric normal faults

island edifice failures and associated tsunami hazards volcanic ocean islands are prone to structural failure of the edifice that result in landslides that can generate destructive tsunamis these island landslides range enormously in size varying from small rock falls to giant sector failures involving tens of cubic kilometers of debris a survey of literature has allowed us to identify twenty three processes that contribute to edifice collapse these have been divided into endogenetic and exogenetic sources of edifice failure endogenetic sources of instability and failure include unstable foundations volcanic intrusions thermal alteration edifice pore pressures unbuttressed structures and buried faults exogenetic sources of instability and failure include collapse of subaerial or submarine deposits endo upwelling karst megaporosity fractures oversteepening overloading sea level change marine erosion weathering including hurricanes glacial response volcanic activity regional uplift or subsidence tectonic seismicity and anthropogenic agents while the endogenetic sources dominate during periods of active volcanism and cone building the exogenetic sources may cause failure at any time tsunamis both small and large are associated with these edifice failures

catastrophic flank collapses and slumping in pico island during the last 130 kyr pico faial ridge azores triple junction abstract the pico island constitutes the easternmost sub aerial domain of a steep wnw ese volcanic ridge which has developed within the nubia eurasia diffuse plate boundary azores triple junction the island comprises three volcanic systems from older to younger the topo volcano the fissural system and the pico stratovolcano from a high resolution digital elevation model 10 m and new bathymetric stratigraphic structural and high precision k ar data we reconstruct the main successive stages of growth and partial destruction of the island over the last 200 kyr we especially concentrate on the central sector of the island which has recorded gradual movements through slumping and catastrophic flank collapses since ca 130 kyr the remmants of the topo volcano are partly exposed on pico s se flank and are here dated between 186 5 and 115 4 ka topo was significantly destroyed by n and s directed large scale flank collapses between ca 125 and 70 ka on pico s n flank collapse seems to have removed all the unstable material but in the s the collapse structure is composite including a major flank collapse and a remnant slump complex that is still active a first episode of deformation occurred between ca 125 and 115 ka along the master fault of the slump between ca 115 and 69 ka most of the unstable material was removed by a major flank collapse leaving behind a still considerable volume of unstable material that comprises the active slump this first collapse was catastrophic and generated a large debris deposit recognized on the high resolution bathymetry with a minimum run out of ca 17 km the scar was partially filled by volcanic products erupted from volcanic cones developed within the slump depression and possibly also from the early wnw ese fissural system subsequent deformation in the slump area affected in part the filling units leading to the individualization of secondary curved faults younger volcanic products have gradually masked the mass wasting scars unlike the well known hilina slump hawaii pico s slump evolution might be controlled by an active regional tectonics

large scale active slump of the southeastern flank of pico island azores comment we were interested to read the paper by hildenbrand et al 2012 1 outlining geodetic and geologic evidence for active movement of a giant slump in pico island azores such movement is potentially a concern to local communities so the issues arising from the paper need addressing we wish to

palaeoclimate climate in phase abrupt climate changes during the last glacial period affected most of the northern hemisphere greenland ice core data suggest that for the penultimate abrupt warming event climate change was nearly synchronous at high and low latitudes

comment on construction and destruction of a volcanic island developed inside an oceanic rift graciosa island terceira rift azores by sibrant et al 2014 and proposal of a new model for graciosa s geological evolution abstract volcanoes rising above sea level within extensional oceanic plate boundaries provide accessible locations with which to study the effects of plate tectonic and volcanic processes of such areas however relying solely on subaerial observations can lead to biased interpretations reconciling the information provided by multibeam echo sounders on the submarine parts of volcanic islands with geology and geomorphology observable above sea level can potentially provide more robust interpretations in this comment of the study of sibrant et al 2014 which is based almost solely on subaerial observations we show how the published multibeam sonar data around graciosa reveals that their proposed successive phases of destruction of the volcanic edifices composing the island by massive landslides is incompatible with the high resolution bathymetry the data reveal no large scale debris avalanche deposits or characteristic flank collapse scars where sibrant et al 2014 propose these landslides to have occurred instead the data show volcanic constructional areas some of which have simply been eroded by wave abrasion the interpretation of collapse structures appears to have originated partly from a misreading of the volcano stratigraphy and tectonic structures overall wave erosion coupled with subaerial erosion and tectonic activity can more easily explain the onshore observations of sibrant et al 2014 providing a less catastrophic explanation for the evolution of graciosa island

granulometric investigations of snow avalanches avalanche deposits consist of rounded granules composed of aggregates of snow and ice particles the size of the granules is related to vertical shear gradients within the flow studying the granule size distribution may be useful in understanding the flow and stopping of avalanches we applied a sediment size sampling method to measure snow granule size distributions at different depositional environments on two dry and two wet avalanche deposits at three field sites the granule size distributions are approximately log normal similar to many natural sediment deposits the median granule size in the wet and dry avalanches varies between 65 and 162 mm wet avalanches tend to produce more large granules than dry avalanches indicating both smaller flow velocities and near surface shear gradients granule size is similar in frontal lobes and levee deposits suggesting that levee formation occurs independently of the size segregation at the avalanche front

in situ tensile tests of snow pack layers during the winter of 1987 88 an average of seven tensile tests was made for each of 66 snow layers in the rocky mountains of western canada the precision of the mean strength for seven tests expressed in terms of the coefficient of variation was 15 with 90 confidence snow with a faceted micro structure was approximately half as strong as partly settled or rounded snow of the same density notch sensitivity in the strength data and critical strains of 1 or less indicate that the test fractures were essentially brittle

prodigious submarine landslides on the hawaiian ridge the extensive area covered by major submarine mass wasting deposits on or near the hawaiian ridge has been delimited by systematic mapping of the hawaiian exclusive economic zone using the side looking sonar system gloria these surveys show that slumps and debris avalanche deposits are exposed over about 100 000 km2 of the ridge and adjacent seafloor from kauai to hawaii covering an area more than 5 times the land area of the islands some of the individual debris avalanches are more than 200 km long and about 5000 km3 in volume ranking them among the largest on earth the slope failures that produce these deposits begin early in the history of individual volcanoes when they are small submarine seamounts culminate near the end of subaerial shield building and apparently continue long after dormancy consequently landslide debris is an important element in the internal structure of the volcanoes the dynamic behavior of the volcanoes can be modulated by slope failure and the structural features of the landslides are related to elements of the volcanoes including rift zones and fault systems the landslides are of two general types slumps and debris avalanches the slumps are slow moving wide up to 110 km and thick about 10 km with transverse blocky ridges and steep toes the debris avalanches are fast moving long up to 230 km compared to width and thinner 0 05 2 km they commonly have a well defined amphitheater at their head and hummocky terrain in the lower part oceanic disturbance caused by rapid emplacement of debris avalanches may have produced high level wave deposits such as the 365 m elevation hulopoe gravel on lanai that are found on several islands most present day submarine canyons were originally carved subaerially in the upper parts of debris avalanches subaerial canyon cutting was apparently promoted by the recently steepened and stripped slopes of the landslide amphitheaters

millennial scale variability during the last glacial the ice core record the oxygen isotope records from greenland ice cores show a very strong reproducible pattern of alternation between warm greenland interstadials gi and cold greenland stadials gs at millennial scale during the last glacial period here we summarise what is known about this variability from ice core records the typical cycle has a sawtooth pattern with a very rapid warming event occurring in a few decades a slow cooling trend and then a final fast cooling 25 such events have been numbered in the last glacial the recent gicc05 age scale provides the best available age scale that can be directly applied to this stratigraphy and we summarise the timing of the warming events and the length and strength of each event the greenland stratigraphy can be transferred to other records if we make assumptions about the contemporaneous nature of rapid events in different archives other parameters such as the snow accumulation rate and the concentration of terrestrial dust and sea salt recorded in the greenland cores also show a strong contrasting pattern between gi and gs methane concentrations are generally high during gi and lower during gs with the increase from gs to gi occurring within a century antarctic ice cores show a different pattern each gi has an antarctic counterpart but antarctica appears to warm while greenland is in a gs and cool during gi these changes are consistent with a mechanism involving ocean heat transport but the rapid nature of warmings poses a challenge for modellers while the rapid methane changes pose questions about the pattern of land biosphere emissions during the glacial that are also relevant for understanding glacial interglacial methane variability

velocity profile inversion in dense avalanche flow we present a velocity time series of a dry mixed real scale avalanche that was recorded at different heights on a mast at the swiss vall ee de la sionne avalanche test site the dataset is of extraordinary quality in terms of completeness and spatio temporal resolution and allows the evolution of distinct flow phases of the avalanche to be traced in particular we found velocity profile inversions at the transition from the avalanche front to the avalanche body we provide a tentative empirical description of the profile inversions and briefly discuss our observations in the light of theoretical considerations

the morphology of the submarine flanks of volcanic ocean islands a comparative study of the canary and hawaiian hotspot islands the submarine flanks of volcanic islands are shaped by volcanic constructional processes landslides erosion sediment deposition and tectonic movements we use a newly acquired multibeam sonar dataset from the westerly canary islands el hierro la palma and tenerife to develop a comparison with the hawaiian islands which suggests differences in the processes constructing and modifying their flanks landslides affect the flanks of both island groups debris avalanches fast moving shallow landslides have left smooth chutes and blocky deposits in both cases but blocks within some hawaiian avalanche deposits are markedly larger we attribute the larger block sizes in the hawaiian islands to the fact that their avalanches were relatively unconfined whereas many canary and hawaiian avalanches with small block sizes appear to have been constrained down narrow chutes forcing interactions between blocks within the flows and encouraging disintegration furthermore the hawaiian avalanches with the largest blocks initiated near sea level whereas many of the canary avalanches initiated above sea level so hydraulic resistance of water entering cracks may be an additional factor in resisting block disintegration during flow slow moving deep seated slumps or volcanic spreading have produced submarine benches and tabular escarpments due to thrust faulting adjacent to several hawaiian rift zones but are not well developed in the canaries although volcanic morphology is partly obscured by sedimentation in the canaries we are able to interpret lava terraces around the deep flanks of el hierro which are similar to those found in the hawaiian islands however cones rather than terraces are the most common volcanic forms in the canary islands implying that flank eruptions have involved magma with significant volatile contents assuming that volatile contents dictate whether cones or terraces are formed these differences may ultimately originate from the different building rates of the two island groups for example the lack of evidence for high level magma chambers in the canaries associated with their lower outputs implies that there is less possibility for degassing of magma below the summit before lateral intrusion down rift zones hence cones rather than lava terraces are more commonly observed the apparent lack of slumping or volcano spreading could also reflect a lack of driving pressure from extensive high level magma chambers in the canaries

impact pressures and flow regimes in dense snow avalanches observed at the vallee de la sionne test site 1 a fundamental problem in avalanche engineering is to determine the impact pressures exerted on structures this task is complicated because snow avalanches flow in a variety of regimes primarily depending on snow temperature and moisture content in this paper we address this problem by analyzing measured impact pressures flow velocities and flow depths of five vallee de la sionne avalanches the measurements are made on a 20 m high tubular pylon instrumented with high frequency pressure transducers and optoelectronic velocity sensors in the observed avalanches we find both subcritical and supercritical flow regimes typical froude numbers were smaller than 6 the subcritical regime fr 1 is characterized by a flow plug riding above a highly sheared basal layer the measured pressures are large and velocity independent in contradiction to calculation procedures pressure fluctuations increase with flow depth indicating a kinematic stick slip phenomena which is largest at the basal layer supercritical flow regimes 1 fr 6 are characterized by a sheared flow all over the avalanche depth in this regime the impact pressure is velocity dependent we derive relationships governing impact pressure as a function of the froude number and therefore flow regime encompassing all the observed avalanches