erosion by pyroclastic flows on lascar volcano chile pyroclastic flows generated in the 19 20 april 1993 eruption of lascar volcano chile produced spectacular erosion features scree and talus were stripped from the channels and steep slopes on the flanks of the volcano exposed bedrock and boulders suffered severe abrasion producing smoothed surfaces on coarse breccias and striations and percussion marks on bedrock and large boulders erosional furrows developed with wavelengths of 0 5 2 m and depths of 0 1 0 3 m furrows commonly nucleated downstream of large boulders or blocks which are striated on the upstream side and thereby produced crag and tail structures erosive features were produced where flows accelerated through topographic restrictions or where they moved over steep slopes the pyroclastic flows are inferred to have segregated during movement into lithic rich and pumice rich parts lithic rich deposits occur on slopes up to 14 whereas pumice rich deposits occur only on slopes less than 4 and mainly at the margins and distal parts of the 1993 fan the lithic rich deposits contain large up to 1 m lithic clasts eroded from the substrate and transported from the vent whereas pumice rich deposits contain only small typically 2 cm lithic clasts these observations suggest that lithic clasts segregated to the base of the flows and were responsible for much of the erosive phenomena the erosive features distribution of lithic clasts and deposit morphology indicate that the 1993 flows were highly concentrated avalanches dominated by particle interactions in some places the flows slid over the bedrock causing abrasion and long striations which imply that large blocks were locked in fixed positions for periods of about 1 s however shorter striae at different angles impact marks segregation of the deposits into pumice and lithic rich parts and mixing of bedrock derived lithic clasts throughout the deposits indicate that clasts often had some freedom of movement and that jostling of particles allowed internal mixing and density segregation to occur within the flows

numerical modeling of the emplacement of socompa rock avalanche chile 1 the 7 5 ka socompa sector collapse emplaced 25 km3 of fragmented rock as a thin but widespread 500 km2 avalanche deposit followed by late stage sliding of 11 km3 as toreva blocks most of the avalanche mass was emplaced dry although saturation of a basal shear layer cannot be excluded modeling was carried out using the depth averaged granular flow equations in order to provide information on the flow behavior of this well preserved long run out avalanche results were constrained using structures preserved on the surface of the deposit as well as by deposit outline and run up a proxy for velocity models assuming constant dynamic friction fail to produce realistic results because the low basal friction angles 1 to 3 5 necessary to generate observed run out permit neither adequate deposition on slopes nor preservation of significant morphology on the deposit surface a reasonable fit is obtained however if the avalanche is assumed simply to experience a constant retarding stress of 50 100 kpa during flow this permits long run out as well as deposition on slopes and preservation of realistic depositional morphology in particular the model explains a prominent topographic escarpment on the deposit surface as the frozen front of a huge wave of debris reflected off surrounding hills the result that socompa avalanche experienced a small approximately constant retarding stress during emplacement is consistent with a previously published analysis of avalanche data

the influence of slope angle ratio on the dynamics of granular flows insights from laboratory experiments laboratory experiments on granular flows using natural material were carried out in order to investigate the behaviour of granular flows passing over a break in slope sensors in the depositional area recorded the flow kinematics while video footage permitted reconstruction of the deposit formation which allowed investigation of the deposit shape as a function of the change in slope we defined the slope angle ratio as the proportion between slope angle in the depositional area and that of the channel when the granular flow encounters the break in slope part of the flow front forms a bouncing clast zone due to elastic impact with the expansion box floor during this process part of the kinetic energy of the dense granular flow is transferred to elutriating fine ash which subsequently forms turbulent ash cloud accompanying the granular flow until it comes to rest morphometric analysis of the deposits shows that they are all elliptical with an almost constant minor axis and a variable major axis the almost constant value of the minor axis relates to the spreading angle of flow at the end of the channel which resembles the basal friction angle of the material the variation of the major axis is interpreted to relate to the effect of competing inertial and frictional forces this effect also reflects the partitioning of centripetal and tangential velocities which changes as the flow passes over the break in slope after normalization morphometric data provided empirical relationships that highlight the dependence of runout from the product of slope angle ratio and the difference in height between granular material release and deposit the empirical relationships were tested against the runouts of hot avalanches formed during the 1944 ad eruption at vesuvius with differences among actual and calculated values are between 1 7 and 15 velocity measurements of laboratory granular flows record deceleration paths at different breaks in slope when normalized the velocity data show third order polynomial fit highlighting a complex behaviour involving interplay between inertial and frictional forces the theoretical velocity decays were tested against the data published for volcaniclastic debris flows of the 5 6 may 1998 event in the sarno area the comparison is very good for non channelized debris flows with significant differences between actual and calculated velocities for the channelized debris flows

pyroclastic density currents pdc of the 16 17 august 2006 eruptions of tungurahua volcano ecuador geophysical registry and characteristics abstract tungurahua located in the eastern cordillera of the ecuadorian andes is a 5023 m high active volcano notable for its extreme relief 3200 m steep sides and frequent eruptive cycles from 1999 until 2006 tungurahua experienced short periods of low to moderate strombolian activity characterized by fire fountaining explosions frequent ash falls and debris flows and no pdc events without warning tungurahua initiated pdc activity on 15 16 july 2006 which became more intense on the night of 16 17 august 2006 which is the focus of this study continuous monitoring of tungurahua has employed seismic both short period and broadband bb instruments so 2 gas emission cospec and doas and geodetic methods edm tilt meters and gps in addition to thermal imagery airborne and ground based acoustic flow monitors afm installed to monitor lahar activity were important for detecting pdc events acoustic signals were monitored at riobamba 40 km to the sw as well as by infrasound sensors at tungurahua s bb seismic stations based on geophysical parameters visual observations and pdc deposit characteristics four phases of distinct eruptive activity are recognized during the 16 17 august episode phase i 08h37 to 21h13 of 16 aug local time experienced low to moderate strombolian activity with occasional high energy impulsive bursts and small pdc phase ii 21h13 16 aug to 00h12 17 aug was characterized by a number of discrete events with high amplitude seismo acoustic signals followed by the generation of larger pdc that overran monitoring stations and had velocities of 30 33 m s after midnight phase iii 00h12 to 01h14 saw an intense period of unrelenting eruptive activity corresponding to the episode s greatest energy release it was characterized by subplinian activity accompanied by a series of high energy outbursts and constant low frequency jetting that together formed a continuous plume it was during this phase that the largest pdc were produced reaching the surrounding river valleys phase iv after 01h14 followed the cessation of the paroxysmal eruption but witnessed many granular pdc generated by degassed lava spill outs from the crater that developed lobe and channel morphology on the cone s lower flanks hours later a blocky lava flow issued from the crater during these episodes more than 30 pdc events were detected the majority being small flows that remained high on the cone the two largest pdc occurred after midnight probably generated by fountain collapse their descent down the cone s upper steep flanks 28 and 2 4 km in length favored air entrainment resulting in pdc with greater fluidity these flows had volumes of 9 to 17 10 6 m 3 and produced widespread but relatively thin 1 2 m thick normally graded deposits at their distal ends the character and evolution of the pdc activity apparently reflect decreasing volatile contents of the magma and a diminishing magma supply

the ad 1300 1700 eruptive periods at tungurahua volcano ecuador revealed by historical narratives stratigraphy and radiocarbon dating tungurahua is a frequently active and hazardous volcano of the ecuadorian andes that has experienced pyroclastic flow forming eruption in 1773 1886 1916 18 and 2006 08 earlier eruptions in late pre hispanic and early colonial times have remained poorly known and are debated in the literature to reconstruct the eruptive chronology in that time interval we examine relevant historical narratives recently found in sevilla spain and rome italy and we combine stratigraphic field constraints with 22 new radiocarbon age determinations results show that pyroclastic flow forming eruptions and tephra falls took place repeatedly since similar to 700 c 14 yr bp when the tungurahua region was already populated radiocarbon ages averaging around 625 yr bp reveal a period of notable eruptive activity in the 14th century late integration cultural period the associated andesitic eruptions produced ash and scoria falls of regional extent and left scoria flow deposits on the western flanks of the edifice the fact that tungurahua was known by the puruhas indians as a volcano at the time of the spanish conquest in 1533 perhaps refers to these eruptions a group of ages ranging from 380 to 270 yr bp is attributed to younger periods of activity that also predates the 1773 event and calibration results yield eruption dates from late 15th to late 17th centuries i e inca and early colonial periods the historical narratives mention an early colonial eruption between the spanish conquest and the end of the 16th century followed by a distinct eruptive period in the 1640s the descriptions are vague but point to destructive eruptions likely accompanied by pyroclastic flows the dated tephras consist of andesitic scoria flow deposits and the contemporaneous fallout layers occur to the west these findings reveal that the eruption recurrence rate at tungurahua is at least one pyroclastic flow forming event per century since the 13th century and the occurrence of such eruptions in 2006 08 is thus fully consistent with the rate inferred for the past seven centuries in addition historical chronicles suggest that a notch opened in the crater margin during the 1640 decade as has occurred repeatedly in the past millennium at tungurahua such small volume collapse events represent a previously unrecognized source of hazards which deserve special attention for risk assessment purposes in the context of the currently ongoing eruption c 2008 elsevier b v all rights reserved

numerical study on the entrainment of bed material into rapid landslides the entrainment of channel path material plays an important role during runout of many rapid landslides such a mechanism is able to change the mobility of a flow significantly through rapid chang

eruption chronologies plume heights and eruption styles at tungurahua volcano integrating remote sensing techniques and infrasound article i nfo satellite remote sensing allows volcanic ash plumes to be detected and tracked and routinely aids in hazard mitigation especially to the aviation community although satellite imagery provides valuable information about volcanic plume processes i e plume heights plume composition and plume transport it has its limitations in addition to the observational gaps produced by meteorological clouds as well as temporal and spatial resolution restrictions large volcanic clouds often obscure the vent during an eruption therefore some key information about eruption dynamics and processes are impossible to obtain from satellite images in this paper we investigate four eruptions at tungurahua volcano ecuador during the 2006 2008 time period we integrate satellite derived eruption chronologies plume heights and plume aspect ratios with infrasound data primarily acoustic power and hand held thermal imagery when available integrating these datasets allows us to identify accurate eruption onsets durations and cessations as well as define different types of eruption styles ranging from plinian to weak vulcanian transitions between the different eruption styles were also documented results show that there is a positive correlation between plume height and acoustic power we conclude that combining the two datasets allows better constraints on eruption processes and source parameters using these methods in real time monitoring will allow more accurate eruption monitoring and ash hazard mitigation

fabric analysis using the intercept method abstract the intercept method is a convenient yet powerful method for the numerical analysis of fabrics for geological applications it can be used when a set of objects e g pores grains or aggregates of a specific mineral lineaments etc with unique characteristics can be identified in an image the intercept method analyses boundaries of objects as a population of lines how intercepts are definined is therefore important in order to establish the significance of the intercept counts when the method is applied to digital images the subdivision of the information into square pixels must also be considered we apply a linear filter which minimizes the effects of grid anisotropy on the counting of intercepts analysis of the resulting rose of intercept counts by a fourier series considerably facilitates the interpretation of the data a rose of directions can readily be derived for the phase boundaries if a population of objects can be assumed to have started with an originally isotropic orientation and to have been deformed passively the fourier components of the rose of intercepts also give the sectional strain the fourier components also permit an objective test of the validity of describing a fabric as resulting from a homogeneous strain on an initially isotropic section two computer programs for the counting and analysis of intercepts on digital images are presented and applied in four examples which illustrate the usefulness of the intercept method for magmatic fabric measurement strain analysis and lineament analysis we also illustrate the importance of fourier series decomposition of rose of intercept counts for fabric interpretations intercept results are also shown to be compatible with those derived by other methods eigenvector surfor inertia tensor autocorrelation methods

simulation of block and ash flows and ash cloud surges of the 2010 eruption of merapi volcano with a two layer model a new depth averaged model has been developed for the simulation of both concentrated and dilute pyroclastic currents and their interactions the capability of the model to reproduce a real event is tested for the first time with two well studied eruptive phases of the 2010 eruption of merapi volcano indonesia we show that the model is able to reproduce quite accurately the dynamics of the currents and the characteristics of the deposits thickness extent volume and trajectory the model needs to be tested on other well studied eruptions and the equations could be refined but this new approach is a promising tool for the understanding of pyroclastic currents and for a better prediction of volcanic hazards

numerical modelling of entrainment deposition in rock and debris avalanches abstract rock and debris avalanches as well as debris flows are characterized by peculiar behaviour influenced by geometry of the initial failure surface topography involved materials involved and water content during their motion rock avalanches can entrain large volumes of sediments both in a dry and in a saturated state entrained dry material has generally the consequence to reduce the total landslide runout whereas entrainment of almost saturated material can have more complex consequences we use a finite element code to simulate the motion of a moving landslide mass on materials with different properties and along very sharp topographies erosion and deposition can be modelled as well as interaction with obstacles of different characteristics we present two examples of the modelling problems and capabilities we simulated the runout of a rock avalanche on a ground formed by alluvial material under dry and saturated conditions this material can be entrained and plowed by the impacting and running rock avalanche material sensitivity of the erosion and transport mechanisms has been analysed by performing different analyses the simulated case study is the 6 10 5 m 3 arvel rock avalanche 1922 switzerland for which a reliable set of pre and post failure observations is available final geometry mass redistribution velocities and runup are used to validate the model capabilities and study the effects of different material properties

tungurahua volcano ecuador structure eruptive history and hazards abstract tungurahua one of ecuador s most active volcanoes is made up of three volcanic edifices tungurahua i was a 14 km wide andesitic stratocone which experienced at least one sector collapse followed by the extrusion of a dacite lava series tungurahua ii mainly composed of acid andesite lava flows younger than 14 000 years bp was partly destroyed by the last collapse event 2955 90 years ago which left a large amphitheater and produced a 8 km3 debris deposit the avalanche collided with the high ridge immediately to the west of the cone and was diverted to the northwest and southwest for 15 km a large lahar formed during this event which was followed in turn by dacite extrusion southwestward the damming of the chambo valley by the avalanche deposit resulted in a 10 km long lake which was subsequently breached generating another catastrophic debris flow the eruptive activity of the present volcano tungurahua iii has rebuilt the cone to about 50 of its pre collapse size by the emission of 3 km3 of volcanic products two periods of construction are recognized in tungurahua s iii history from 2300 to 1400 years bp high rates of lava extrusion and pyroclastic flows occurred during this period the magma composition did not evolve significantly remaining essentially basic andesite during the last 1300 years eruptive episodes take place roughly once per century and generally begin with lapilli fall and pyroclastic flow activity of varied composition andesite dacite and end with more basic andesite lava flows or crater plugs this pattern is observed in the three historic eruptions of 1773 1886 and 1916 1918 given good age control and volumetric considerations tungurahua iii growth s rate is estimated at 1 5 106 m3 year over the last 2300 years although an infrequent event a sector collapse and associated lahars constitute a strong hazard of this volcano given the 3000 m relief and steep slopes of the present cone a future collapse even of small volume could cover an area similar to that affected by the 3000 year old avalanche the more frequent eruptive episodes of each century characterized by pyroclastic flows lavas lahars as well as tephra falls directly threaten 25 000 people and the agoyan hydroelectric dam located at the foot of the volcano

elementary theory of bed sediment entrainment by debris flows and avalanches 1 analyses of mass and momentum exchange between a debris flow or avalanche and an underlying sediment layer aid interpretations and predictions of bed sediment entrainment rates a preliminary analysis assesses the behavior of a coulomb slide block that entrains bed material as it descends a uniform slope the analysis demonstrates that the block s momentum can grow unstably even in the presence of limited entrainment efficiency a more detailed depth integrated continuum analysis of interacting deformable bodies identifies mechanical controls on entrainment efficiency and shows that entrainment rates satisfy a jump condition that involves shear traction and velocity discontinuities at the flow bed boundary explicit predictions of the entrainment rateeresult from making reasonable assumptions about flow velocity profiles and boundary shear tractions for coulomb friction tractions predicted entrainment rates are sensitive to pore fluid pressures that develop in bed sediment as it is overridden in the simplest scenario the bed sediment liquefies completely and the entrainment rate equation reduces toe 2 1gh1 cos 1 1 v 1 where is the slope angle 1 is the flow s coulomb friction coefficient h1 is its thickness 1 is its degree of liquefaction and v 1is its depth averaged velocity for values of 1ranging from 0 5 to 0 8 this equation predicts entrainment rates consistent with rates of 0 05 to 0 1 m s measured in large scale debris flow experiments in which wet sediment beds liquefied almost completely the propensity for bed liquefaction depends on several factors including sediment porosity permeability and thickness and rates of compression and shear deformation that occur when beds are overridden

incorporation and redistribution of locally derived lithic fragments within a pyroclastic flow the lower miocene peach springs tuff exposed in the newberry mountains california was deposited within a paleovalley trending s65 w exposures within the paleovalley contain lithic breccia intercalated with ash rich ignimbrite the clast assemblage of the lithic breccias matches the rock types of the paleovalley walls and therefore the clasts were not derived from a distant eruptive vent flow direction breccia bed thickness grain size data and sedimentary textures indicate that the lithic breccias were deposited from density currents within the pyroclastic flow that moved down tributaries and into the main paleovalley to be intermingled with the ash rich pyroclastic flow a model is proposed whereby a turbulent boundary layer at the base of the pyroclastic flow is induced by surface roughness of the substrate and incorporates loose material from the substrate to produce a high density ground layer that decouples from the lower density ash rich pyroclastic flow after decoupling has occurred the high density lithic rich ground layer moves independently from the ash rich pyroclastic flow the lithic breccia horizons have many characteristics of proximal lag breccias and caution must be used when inferring distance from vent in ancient ignimbrites based on the occurrence of coarse breccias

pyroclastic flow deposits of the 1991 eruption of volcan de colima mexico the april 16 1991 eruption of volcan de colima represents a classical example of partial dome collapse with the generation of progressively longer runout merapi type pyroclastic flows that traveled up to 4 km along the el cordoban gullies east central and west the flows filled the gullies with block and ash flow deposits up to 10 m thick of which after 7 years of erosion only remnants remained in the el cordoban west and east gullies the el cordoban central gully however provided a well preserved and incised longitudinal section of the 1991 deposits the deposits were emplaced as proximal and distal facies separated by a change in slope angle from 30 to 20 the proximal facies consists of massive clast supported flow units up to 1 m thick with andesite blocks locally supported by a matrix of coarse ash and devoid of segregation structures or grading the distal facies consists of a massive matrix supported deposit up to 8 m thick which contains dispersed andesite blocks in a fine ash matrix in the distal facies a train of blocks marks flow unit upper boundaries and although sorting is poor some grading is present thin finely stratified or dune bedded layers of fine ash material are locally present above or below units of both facies sedimentologic parameters show that the size or fraction of large pyroclasts larger than 1 decreases from proximal to distal facies as the percentage of matrix 0 to 4 increases especially immediately beyond the break in slope we propose that the propagation of the colima pyroclastic flows is critically dependent on local slope angle the presence of erodible slope debris and the decrease in grain size with distance from the vent the progressive fining is probably caused by some combination of erosion clast breakup and deposition of larger pyroclasts and is itself influenced by the slope angle in the proximal region the flows moved as granular avalanches in which interacting grains ground each other and erosion occurred to produce an overriding dilute ash cloud the maximum runout distance of the avalanches was controlled by the angle of repose of the material and the volume and grain size of source and eroded material because the slope angle is close to the repose angle for this debris granular avalanches were not able to propagate far beyond the change in slope if however an avalanche had enough mass in finer grain size fractions at least part of the flow continued beyond the break in slope and across the volcano apron propagating in a turbulent state and depositing surge layers or in an otherwise settling modified state and depositing block and ash flow layers

testing the suitability of frictional behaviour for pyroclastic flow simulation by comparison with a well constrained eruption at tungurahua volcano ecuador we use a well monitored eruption of tungurahua volcano to test the validity of the frictional behaviour also called mohr coulomb which is generally used in geophysical flow modelling we show that the frictional law is not appropriate for the simulation of pyroclastic flows at tungurahua with this law the longitudinal shape of the simulated flows is a thin wedge of material progressively passing over several hundreds of metres from an unrealistic thickness at the front 1 mm to some tens of centimetres simulated deposits form piles which accumulate at the foot of the volcano and are more similar to sand piles than natural pyroclastic deposits finally flows simulated with a frictional rheology are not channelised by the drainage system but affect all the flanks of the volcano in addition their velocity can exceed 150 m s 1 allowing pyroclastic flows to cross interfluves at bends in the valley affecting areas that would not have been affected in reality and leaving clear downstream areas that would be covered in reality instead a simple empirical law a constant retarding stress i e a yield strength involving only one free parameter appears to be much better adapted for modelling pyroclastic flows a similar conclusion was drawn for the socompa debris avalanche simulation kelfoun and druitt j geophys res 110 b12202 2005

a conceptual model for block and ash flow basal avalanche transport and deposition based on deposit architecture of 1998 and 1994 merapi flows the 1998 block and ash flow baf deposits on the western flanks of merapi were described both immediately following emplacement and after one and two wet seasons of erosion this afforded a unique combination of data on deposit morphology and internal structure from proximal to distal reaches the deposits of the basal avalanches from individual baf events were classified into three major and two minor facies in channelised reaches primarily resulting from effects of channel confinement and slope termination reaches were classified into three facies related to preevent topography the deposit facies architecture was interpreted to understand particle transport and deposition within the framework of granular flow systematics the transport model we propose for these basal avalanches involves unsteady flow conditions generated by pulses of collapse as the source and or by development of kinetic waves during the granular flow the coarsest clasts along with low density material migrated to the wave pulse fronts within these flows whereas in bulk flow grain dispersive pressures and kinetic sieving result in coarsest clasts being buoyed to the flow top with this tendency being greatest on higher slopes greater kinetic energy and with greater flow confinement maximising particle collisions deposition begins as the flow reaches near its full extent an initial pulse or flow wave freezes rapidly from its base upward and its rear forward as it looses kinetic energy and dispersive pressure such that an upper higher energy part of the flow is still able to move downstream over the frozen base later waves or pulses then accrete progressively horizontally upslope banking up onto or overtopping earlier deposited sediment hence the overall flow freezes from its front to its tail at termination reaches in unconfined areas the overflowing upper part of flow gradually deposits from its base upward resulting in a slightly tapering sheet formed deposit marked by a front made up of wood fragments and large boulders on sudden slope breaks in termination areas piling up of sediment occurs in flat reaches producing whaleback type forms as the flows rapidly lost competence one implication of this model is that each kinetic wave or flow pulse crest developed within the basal avalanche represents a portion of greater flow depth or stage height hence development of these crests control the ability of parts of the basal avalanche to escape from confined channels and generate unexpected hazards

computational dam break hydraulics over erodible sediment bed this paper presents one of the first dedicated studies on mobile bed hydraulics of dam break flow and the induced sediment transport and morphological evolution a theoretical model is built upon the conservative laws of shallow water hydrodynamics and a high resolution numerical solution of the hyperbolic system is achieved using the total variation diminishing version of the second order weighted average flux method in conjunction with the hllc approximate riemann solver and superbee limiter it is found that a heavily concentrated and eroding wavefront first develops and then depresses gradually as it propagates downstream in the early stage of the dam break a hydraulic jump is formed around the dam site due to rapid bed erosion which attenuates progressively as it propagates upstream and eventually disappears while the backward wave appears to migrate at the same speed as over a fixed bed the propagation of the forward wavefront shows a complex picture compared to its fixed bed counterpart as a result of the domination of rapid bed erosion initially the density difference between the wavefront and the downstream ambient water in the intermediate period and the pattern of the deformed bed profile in the long term it is also found that the free surface profiles and hydrographs are greatly modified by bed mobility which has considerable implications for flood prediction the computed wave structure in the intermediate period exhibits great resemblance to available experiments qualitatively and yet the existence of a shear wave is found in lieu of a secondary rarefaction postulated in an existing analysis finally the use of the complete rather than simplified conservation equations is shown to be essential for correct resolution of the wave and bed structures which suggests that previous models need reformulating

lidar derived morphology of the 1993 lascar pyroclastic flow deposits and implication for flow dynamics and rheology abstract pumice flows are potentially destructive volcanic events that derive from eruption column collapse and whose dynamics are poorly understood the challenges in studying these flows include the lack of constraints on the dynamics kinematics and initial conditions that control their emplacement we present a morphological study of the distal deposits lobes of the pumice flows resulting from the 1993 eruption at lascar chile the surface geometry of the lobes was measured in detail using a lidar device which allowed for detailed characterisation of their morphology consisting of central channel and lateral levees and terminal frontal snout in particular we find that the ratio of channel levee height as a function of the ratio of the distance between levees total width of the lobe has a characteristic curve for these pumice flow lobes our analysis of several of the lascar pumice lobe deposits south east sector identified several dimensionless groups of the available parameters which when compared against published results from both experimental and numerical investigations allowed us to constrain crucial kinematic and dynamic information on the terminal phase of the pumice flows notably we estimate the velocity of the terminal phase of pumice flows to be 5 10 m s froude numbers of 1 5 2 are comparable with values found for experimental granular flows height width aspect ratios for the levee channel section of the pumice lobes are similar to those for experimental flows although these same aspect ratios for the snout are much larger for the natural deposits than their small scale analogues finally we discuss the possible emplacement dynamics of the terminal lascar 1993 pumice flows a pseudo reynolds number based on the velocity estimation is found to be up to 100 times larger for the pumice flows than experimental scale flows this suggests that the flow retarding frictional forces for large scale flows are relatively unimportant compared to flows at smaller scales mechanical effects such as fluidisation mobilisation of material lying on the slopes over which they propagate and lubrication due to polydispersivity could provide an explanation for their ability to propagate on shallow slopes 6 11

dynamics of pyroclastic density currents conditions that promote substrate erosion and self channelization mount st helens washington usa abstract the may 18th 1980 eruption of mount st helens msh produced multiple pyroclastic density currents pdcs burying the area north of the volcano under 10s of meters of deposits detailed measurements of recently exposed strata from these pdcs provide substantial insight into the dynamics of concentrated currents including inferences on particle particle interactions current mobility due to sedimentation fluidization and internal pore pressure particle support mechanisms the influence of surface roughness and the conditions that promote substrate erosion and self channelization four primary flow units are identified along the extensive drainage system north of the volcano each flow unit has intricate vertical and lateral facies changes and complex cross cutting relationships away from source each flow unit is an accumulation from an unsteady but locally sustained pdc or an amalgamation of several pdc pulses the pdcs associated with units i and ii likely occurred during the pre climactic waxing phase of the eruption these currents flowed around and filled in the hummocky topography leaving the massive to diffusely stratified deposits of units i and ii the deposits of both units i and ii are generally more massive in low lying areas and more stratified in areas of high surface roughness suggesting that surface roughness enhanced basal shear stress within the flow boundary units iii and iv are associated with the climactic phase of the eruption which produced the most voluminous and wide spread pdcs both flow units are characteristically massive and enriched in vent derived lithic blocks these currents flowed over and around the debris avalanche deposits as evidenced by the erosion of blocks from the hummocks unit iii is massive poorly sorted and shows little to no evidence of elutriation or segregation of lithics and pumice suggesting a highly concentrated current where size density segregation was suppressed unit iv shows similar depositional features but typically has a basal lithic rich region is variably fines depleted and contains pumice lobes suggesting density segregation in a less concentrated current relative to unit iii deep erosive channels cut by the unit iii current and thick lithic levee deposits within unit iv occur in an area where debris avalanche relief is limited suggesting self channelization developed as a function of internal flow dynamics an increase in the proportion and size of lithic blocks is found 1 downstream of debris avalanche hummocks suggesting the pdcs were energetic enough to locally entrain accidental lithics from the hummocks and transport them tens of meters downstream and 2 within large channels cut by later pdcs into earlier pdc deposits suggesting self channelization of the flows increased the carrying capacity of the subsequent channelized currents finally the combination of thick massive deposits with a high percentage of fine ash within unit iii and in the medial distal depositional regions of units ii iv suggests that the pdcs developed and maintained a high internal pore pressure during transport and deposition the most important results include our ability to understand the role of internal pore pressure on current mobility the influence of self channelization on carrying capacity of the currents and the influence of surface roughness on substrate erosion these observations have critical consequences for understanding the flow dynamics and hazard potential of pdcs

dynamic pore pressure variations induce substrate erosion by pyroclastic flows field evidence shows that pyroclastic flows can entrain blocks from underlying substrates formed by earlier geological events yet counterintuitively they are less likely to erode unconsolidated layers of fine particles here we report laboratory experiments that reproduce these seemingly contradictory observations and also offer a means to infer pyroclastic flow velocity experiments demonstrate that the sliding head of a granular flow generates a dynamic upward pore pressure gradient at the flow substrate interface associated upward air flux is enough to fluidize a substrate of fines so that particles are not entrained individually but the substrate instead is subject to small shear instabilities in contrast coarse particles forming a non fluidized substrate are lifted at a critical upward force due to the pore pressure gradient according to their individual masses which provides a basis for a model to calculate the flow velocity application to the 18 may 1980 pyroclastic flow deposits at mount st helens washington state usa gives velocities of 9 13 m s 1 at 6 7 km from the vent on gentle slopes 4 6 in agreement with field observations at this volcano and at others

using tandem x to measure pyroclastic flow source location thickness and volume application to the 3rd june 2018 eruption of fuego volcano guatemala abstract the estimation of the volume of volcanic flows during an ongoing eruption is challenging but this information is crucial for improving risk assessment and for forecasting future events although previous studies have shown the ability of tandem x satellite data to derive the thickness and the volume of lava flow fields during effusive eruptions the method has not been explored yet for pyroclastic flows using bi static interferometry we produce tandem x dem on fuego volcano guatemala to measure the significant topographic changes caused by the 3rd june 2018 eruption which destroyed the town of san miguel los lotes we estimate the volume of the pyroclastic density currents pdcs to be 15 1 4 2 106 m3 the deposits are likely to be the source of lahars during future rainy seasons we identify the main channel of deposition positive elevation changes and the source region of pyroclastic material areas of significant substrate erosion and vegetation destruction negative elevation changes our results show that the june 3rd 2018 pyroclastic flow was predominantly composed of material which had gravitationally collapsed from a location close to the vent the eroded material increased the volume of the flow bulking and likely caused the run out distance of the 2018 pdc to be larger than previous eruptions 1999 2017 this study highlights the potential of remote sensing techniques for actively monitoring topography changes in inaccessible locations and to rapidly derive deposit volumes

flow of variably fluidized granular masses across three dimensional terrain 1 coulomb mixture theory rock avalanches debris flows and related phenomena consist of grain fluid mixtures that move across three dimensional terrain in all these phenomena the same basic forces govern motion but differing mixture compositions initial conditions and boundary conditions yield varied dynamics and deposits to predict motion of diverse grain fluid masses from initiation to deposition we develop a depth averaged three dimensional mathematical model that accounts explicitly for solid and fluid phase forces and interactions model input consists of initial conditions path topography basal and internal friction angles of solid grains viscosity of pore fluid mixture density and a mixture diffusivity that controls pore pressure dissipation because these properties are constrained by independent measurements the model requires little or no calibration and yields readily testable predictions in the limit of vanishing coulomb friction due to persistent high fluid pressure the model equations describe motion of viscous floods and in the limit of vanishing fluid stress they describe one phase granular avalanches analysis of intermediate phenomena such as debris flows and pyroclastic flows requires use of the full mixture equations which can simulate interaction of high friction surge fronts with more fluid debris that follows special numerical methods described in the companion paper are necessary to solve the full equations but exact analytical solutions of simplified equations provide critical insight an analytical solution for translational motion of a coulomb mixture accelerating from rest and descending a uniform slope demonstrates that steady flow can occur only asymptotically a solution for the asymptotic limit of steady flow in a rectangular channel explains why shear may be concentrated in narrow marginal bands that border a plug of translating debris solutions for static equilibrium of source areas describe conditions of incipient slope instability and other static solutions show that nonuniform distributions of pore fluid pressure produce bluntly tapered vertical profiles at the margins of deposits simplified equations and solutions may apply in additional situations identified by a scaling analysis assessment of dimensionless scaling parameters also reveals that miniature laboratory experiments poorly simulate the dynamics of full scale flows in which fluid effects are significant therefore large geophysical flows can exhibit dynamics not evident at laboratory scales

the 1984 nuee ardente deposits of merapi volcano central java indonesia stratigraphy textural characteristics and transport mechanisms for many centuries merapi volcano has generated hot avalanches of blocks lapilli and ashes derived from the destruction of partially solidified viscous lava domes merapi type nuees ardentes on 15 june 1984 at least four nuees ardentes came down the southwest slope of the merapi the first and the last being responsible for more than 99 of the deposits which are now exposed the first nuee ardente a merapi type nuee ardente was produced by the destruction of the dome travelled 7 km from the crater leaving a measured deposit 2 7 m thick 4 km from the crater near its upper depositional limit regularly increasing to a maximum measured thickness of 12 m at the front of the deposit the lower contact is sharp non erosive with pines still rooted in the underlying paleosol the deposit consists of 50 ash 33 lapilli and 17 blocks with two subpopulations one rosin and one normal and is finespoor with less than 4 of fine ash d finer than 4 the deposit displays reverse population grading of both vesiculated and massive clasts and of the maximum grain size the maximum size significantly increases regularly down current over most of the exposed length of unit 1 and bed thickness increases for the entire length of the deposit the deposit of the second nuee ardente is only 6 21 cm thick and of very limited lateral extent it is a normally graded coarse to fine ash with a finespoor base the third unit consists of fines poor normally graded coarse ash exposed in low amplitude 20 40 cm 12 m wavelength dunes the deposit of the fourth nuee ardente rests in sharp erosive contact on the underlying unit increasing in thickness down flow it consists of transitional coarse and fine grained strata 6 130 c cm thick dipping 5 10 down flow the deposit made up of two subpopulations one rosin and one normal is normally graded over the entire bed but coarsegrained strata are reversely graded the relative content of vesiculated clasts increases up bed in both strata types from 12 at the base to 40 at the top the characteristics of unit 1 suggest that it accumulated from a concentrated suspension of cohesionless solids exhibiting non newtonian behavior where dispersive pressure played an important role in the suspension of the clasts units 2 and 3 were probably deposited from dilute turbulent suspensions whereas the upper unit 4 is a classic example of deposition from a high density turbulent suspension leading to the formation of multiple traction carpets driven by the overlying lower density surge the horizontal distance travelled by a hot rock avalanche may be influenced by its transport mechanism debris flows are mobile on very low slopes as low as 1 whereas grain flows even density modified grain flows require relatively high slopes more than 6 at merapi to maintain their mobility if the present merapi dome were to collapse and produce a debris flow its present volume coupled with the minimal 1 5 km vertical drop could travel a distance ranging between 15 and 30 km however if transport were by grain flow mechanisms the mass could come to rest as it reaches a 5 10 slope

dynamic modelling of entrainment in rapid landslides entrainment of path material is an important feature of many rapid landslides the associated increases in volume and changes in flow character can significantly influence mobility a simple material entrainment algorithm based on the assumption of natural exponential growth with displacement has been incorporated into a new computer model designed to simulate rapid landslide motion across 3d terrain the user controls the growth rate and can also implement a change in rheology at the onset of entrainment a hypothetical example is used to demonstrate the influence of the mass and momentum transfer assumptions a back analysis of the 1999 nomash river landslide is included to show that the simple model is capable of simulating the bulk characteristics of a complex event involving substantial entrainment key words landslides debris flows rock avalanches entrainment erosion dynamic modelling

erosion and mobility in granular collapse over sloping beds 1 we describe laboratory experiments of granular material flowing over an inclined plane covered by an erodible bed designed to mimic erosion processes of natural flows travelling over deposits built up by earlier events two controlling parameters are the inclination of the plane and the thickness of the erodible layer we show that erosion processes can increase the flow mobility i e runout over slopes with inclination close to the repose angle of the grains r by up to 40 even for very thin erodible beds erosion efficiency is shown to strongly depend on the slope of the topography entrainment begins to affect the flow at inclination angles exceeding a critical angle c r 2 runout distance increases almost linearly as a function of the thickness of the erodible bed suggesting that erosion is mainly supply dependent two regimes are observed during granular collapse a first spreading phase with high velocity followed by a slow thin flow provided either the slope or the thickness of the erodible bed is high enough surprisingly erosion affects the flow mostly during the deceleration phase and the slow regime the avalanche excavates the erodible layer immediately at the flow front waves are observed behind the front that help to remove grains from the erodible bed steep frontal surges are seen at high inclination angles over both rigid or erodible bed finally simple scaling laws are proposed making it possible to obtain a first estimate of the deposit and emplacement time of a granular collapse over a rigid or erodible inclined bed

daily monitoring of ecuadorian volcanic degassing from space we present daily measurements of sulfur dioxide so2 emissions from active volcanoes in ecuador and southern colombia between september 2004 and september 2006 derived from the ozone monitoring instrument omi on nasa s eos aura satellite omi is an ultraviolet visible spectrometer with an unprecedented combination of spatial and spectral resolution and global coverage that permits daily measurements of passive volcanic degassing from space we use non interactive processing methods to automatically extract daily so2 burdens and information on so2 sources from the omi datastream maps of monthly average so2 vertical columns retrieved by omi over ecuador and s colombia are also used to illustrate variations in regional so2 loading and to pinpoint sources the dense concentration of active volcanoes in ecuador provides a stringent test of omi s ability to distinguish so2 from multiple emitting sources our analysis reveals that tungurahua reventador and galeras were responsible for the bulk of the so2 emissions in the region in the timeframe of our study with no significant so2 discharge detected from sangay at galeras and reventador we conclude that omi can detect variations in so2 release related to cycles of conduit sealing and degassing which are a critical factor in hazard assessment the omi so2 data for reventador are the most extensive sequence of degassing measurements available for this remote volcano which dominated regional so2 production in june august 2005 at tungurahua the omi measurements span the waning stage of one eruptive cycle and the beginning of another and we observe increasing so2 burdens in the months prior to explosive eruptions of the volcano in july and august 2006 cumulative so2 loadings measured by omi yield a total of 1 16 tg so2 emitted by volcanoes on mainland ecuador s colombia between september 2004 and september 2006 as much as 95 of this so2 may originate from non eruptive degassing approximate apportionment of the total so2 loading indicates that 40 originated from tungurahua with 30 supplied by both reventador and galeras these measurements of volcanic so2 degassing in ecuador confirm omi s potential as an effective economical and risk free tool for daily monitoring of so2 emissions from hazardous volcanoes

3 d finite element modelling of deformation and stress associated with faulting effect of inhomogeneous crustal structures summary modelling of deformation and stress caused by earthquake faulting or fault motion based on a homogeneous elastic half space model does not include crustal rigidity layering and lateral variations geological data and seismic surveys indicate that the crust is elastically inhomogeneous and that rigidity layering and heterogeneities are likely to affect the magnitude and pattern of deformation and stress we use the finite element method fem to investigate deformation and stress generated by strike slip tensile and thrust faulting considering rigidity layering in the crust for a fault with a dimension of 25 10 km and a slip value of 0 5 m cutting through the top 10 km of the crust the deformation and stress change associated with the three modes of faulting are calculated for an island arc crustal structure represented by three models of rigidity layering the results indicate i the maximum effect in horizontal displacements due to crustal layering for the investigated models is 19 7 mm and 28 8 mm for strike slip and tensile faulting respectively for thrust faulting the effect is up to 58 3 mm this remarkable effect is due to the offset of layers by thrust faulting which dynamically changes the crustal structure both horizontally and vertically ii the maximum effect in vertical deformation due to crustal rigidity layering for strike slip and tensile faulting is 5 mm in contrast the effect for thrusting is 30 3 mm iii the effect in the shear stress change at a depth of 7 5 km for the different styles of faulting ranges from 0 02 mpa to 0 10 mpa an additional modelling analysis of subduction thrusting at the japan trench with a detailed structural model based on seismic surveys further confirms the significant distortion caused by a homogeneous model that ignores crustal structural information overall the effect of crustal rigidity layering is at least 20 per cent for surface deformation and 15 per cent for stress changes at depth in light of the present accuracy of global positioning system gps observations 2 mm and the estimate of stress change required for earthquake hazard analysis ca 0 01 mpa the results from the numerical experiments indicate that crust inhomogeneity rigidity layering and lateral variations should be included in any quantitative models of faulting

evaluation of geophysical mass flow models using the 2006 block and ash flows of merapi volcano java indonesia towards a short term hazard assessment tool abstract the dynamics and depositional processes associated with block and ash flows bafs are most commonly inferred to be a function of granular or inertial grain flow similar to debris flows and cold rock avalanches existing geophysical mass flow models are either based on frictional mohr coulomb behavior the titan2d model developed at the state university of new york at buffalo usa or another rheological law i e a constant retarding stress eventually adding some viscous and turbulent components the volcflow model developed at the laboratoire magmas et volcans clermont ferrand france the 2006 bafs of merapi present a rare opportunity to test these two well established models against a well constrained field example integration of high resolution field based data into numerical simulations allows the validity of these models to be tested and rapid quantification of best fit input parameters we first show that with the incorporation of spatially varying bed friction angles titan2d is capable of reproducing the paths runout distances areas covered and deposited volumes of the 2006 merapi flows over highly complex topography however some discrepancies with field data are noted and the velocity and travel time of the flows do not match entirely using a single free parameter a constant retarding stress simulations obtained with the volcflow model also reproduce the morphology and distribution of the natural deposits as well as the time of emplacement and velocities of the flows the results suggest that the performance of these models in simulating actual events is critically dependent on 1 the calibration of the model by using extensive field based data such as deposit distribution and processes of flow generation transport and deposition 2 the incorporation of a suitable numerical topographic dataset i e high resolution digital elevation model and 3 the choice of input parameters such as location and volume of the initial pile of material and source characteristics single or multiple dome collapse dome collapse duration and total volume of collapsed material sensitivity analyses and inundation maps based on the probability of impact were used to produce a suite of potentially inundated areas from future gravitational dome collapse events affecting the gendol valley and adjacent areas on the southern flank of the volcano our results provide the basis for defining hazard zonations of key areas at risk from bafs which will be generated during future comparable eruptions at merapi

emplacement of pyroclastic flows during the 1998 1999 eruption of volcan de colima mexico after three years of quiescence volcan de colima reawakened with increasing seismic and rock fall activity that reached its peak on november 20 1998 when a new lava dome forced its way to the volcano s summit the new lava rapidly reached the s sw edge of the summit area beginning the generation of merapi type pyroclastic flows that traveled down la lumbre and the el cordoban western and eastern ravines reaching distances of 3 4 5 and 3 km respectively on december 1 1998 the lava flow split into three fronts that in early 1999 had reached 2 8 3 1 and 2 5 km in length advancing down the el cordoban ravines the lava flow fronts disaggregated into blocks forming pyroclastic flows one of the best examples occurred on december 10 1998 as the lava flow ceased moving in early 1999 activity became more explosive strong blasts were recorded on february 10 may 10 and july 17 1999 the last event developed a 10 km high eruptive column from which a pyroclastic flow developed from the base traveling 3 3 km sw from the summit into the san antonio montegrande ravines regardless of the mechanism of pyroclastic flow generation each flow immediately segregated into a basal avalanche that moved as a granular flow and an upper ash cloud in which particles were sustained in turbulent suspension when the basal avalanche lost velocity and eventually stopped the upper ash cloud continued to move independently as a dilute pyroclastic flow that produced a massive pyroclastic flow deposit and an upper dune bedded surge deposit the dilute pyroclastic flow scorched and toppled maguey plants and trees and sandblasted vegetation in the direction of the flow at the end of the dilute pyroclastic flow path the suspended particles lifted off in a cloud from which a terminal ash fall was deposited the basal avalanche emplaced block and ash flow deposits up to 8 m thick that filled the main ravines and consisted of several flow units each flow unit was massive monolithologic matrix supported and had a clast supported steep front ca 1 5 to 2 m thick composed of boulders up to 1 7 m in diameter the juvenile lithic clasts had an average density of 1800 kg m3 the dilute pyroclastic flow emplaced overbank deposits found on valley margins or beyond the tip of block and ash flow deposits they consist from bottom to top of a massive medium to coarse sand size flow layer 2 4 cm thick a dune bedded surge layer 2 10 cm thick and a massive silt size layer 0 5 cm thick the total estimated volume of the pyroclastic flow deposits produced during the 1998 1999 eruption is 24 105 m3

positive feedback and momentum growth during debris flow entrainment of wet bed sediment debris flows typically occur when intense rainfall or snowmelt triggers landslides or extensive erosion on steep debris mantled slopes the flows can then grow dramatically in size and speed as they entrain material from their beds and banks but the mechanism of this growth is unclear indeed momentum conservation implies that entrainment of static material should retard the motion of the flows if friction remains unchanged here we use data from large scale experiments to assess the entrainment of bed material by debris flows we find that entrainment is accompanied by increased flow momentum and speed only if large positive pore pressures develop in wet bed sediments as the sediments are overridden by debris flows the increased pore pressure facilitates progressive scour of the bed reduces basal friction and instigates positive feedback that causes flow speed mass and momentum to increase if dryer bed sediment is entrained however the feedback becomes negative and flow momentum declines we infer that analogous feedbacks could operate in other types of gravity driven mass flow that interact with erodible beds the mechanisms by which debris flows acquire mass and momentum as they entrain material are unclear large scale experiments suggest that the pore pressure of wet bed sediment increases as the flow moves over the bed leading to reduced friction and progressive scouring of the base

a debris avalanche at suphan stratovolcano turkey and implications for hazard evaluation the quaternary suphan debris avalanche deposit is located in eastern anatolia turkey the avalanche formed by the sector collapse of a major stratovolcano towards the north possibly during a single catastrophic event the deposit has an estimated volume of 4 km3 and ran out over 25 km to cover an area of approximately 200 km2 products of the collapse are overlain by younger eruptive units from the suphan volcano we have tested the numerical code volcflow to first reproduce the emplacement of the quaternary suphan debris avalanche and then to develop a hazard assessment for potential future sector collapses and subsequent emplacement of debris avalanches and associated tsunami the numerical model captures the main features of the propagation process including travel distance lateral spread and run up the best fit obtained for the existing flow has a constant retarding stress of 50 kpa and a collapse scar volume of 4 km3 analysis of potential future collapse scenarios reveals that northern sector debris avalanches up to 6 km3 could affect several towns in the case of a sector collapse towards the south a tsunami will reach the city of van and several of the biggest towns on the southern shoreline of lake van cities most affected by the larger amplitude waves would be van edremit gevas tatvan and to a lesser extent ercis with wave amplitudes first waves after the onset of the collapse between 8 and 10 m

riemann wave description of erosional dam break flows this work examines the sudden erosional flow initiated by the release of a dam break wave over a loose sediment bed extended shallow water equations are formulated to describe the development of the surge accounting for bed material inertia a transport layer of finite thickness is introduced and a sharp interface view of the morphodynamic boundary is adopted approximations are sought for an intermediate range of wave evolution in which equilibration of the sediment load can be assumed instantaneous but momentum loss due to bed friction has not yet been felt the resulting homogeneous hyperbolic equations are mathematically tractable using the riemann techniques of gas dynamics dam break initial conditions give rise to self similar flow profiles the wave structure features piecewise constant states two smoothly varied simple waves and a special type of shock an erosional bore forming at the forefront of the wave profiles are constructed through a semi analytical procedure yielding a geomorphic generalization of the stoker solution for dam break waves over rigid bed for most flow properties the predictions of the theoretical treatment compare favourably with experimental tests visualized using particle imaging techniques

sedimentology and geomorphology of the deposits from the august 2006 pyroclastic density currents at tungurahua volcano ecuador the deposits of the pyroclastic density currents from the august 2006 eruption of tungurahua show three facies associations depending on the topographic setting the massive proximal cross stratified and distal cross stratified facies 1 the massive facies is confined to valleys on the slopes of the volcano it contains clasts of 1 m diameter to fine ash material is massive and interpreted as deposited from dense pyroclastic flows its surface can exhibit lobes and levees covered with disk shaped and vesicular large clasts these fragile large clasts must have rafted at the surface of the flows all along the path in order to be preserved and thus imply a sharp density boundary near the surface of these flows 2 the proximal cross stratified facies is exposed on valley overbanks on the upper part of the volcano and contains both massive coarse grained layers and cross stratified ash and lapilli bedsets it is interpreted as deposited from a dense pyroclastic flows that overflowed the gentle ridges of valleys of the upper part of the volcano and b dilute pyroclastic density currents created from the dense flows by the entrainment of air on the steep upper flanks 3 the distal cross stratified facies outcrops as spatially limited isolated and wedge shaped bodies of cross stratified ash deposits located downstream of cliffs on valleys overbanks it contains numerous aggrading dune bedforms whose crest orientations reveal parental flow directions a downstream decrease in the size of the dune bedforms together with a downstream fining trend in the grain size distribution are observed on a 100 m scale this facies is interpreted to have been deposited from dilute pyroclastic density currents with basal tractional boundary layers we suggest that the parental flows were produced from the dense flows by entrainment of air at cliffs and that these diluted currents might rapidly deposit through pneumatic jumps three modes are present in the grain size distribution of all samples independently of the facies which further supports the interpretation that all three facies derive from the same initial flows this study emphasizes the influence of topography on small volume pyroclastic density currents and the importance of flow transformation and flow stripping processes

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

3d particle size distributions from 2d observations stereology for natural applications abstract we have developed a general formulation for stereological analysis of particle distributions which is applicable to any particle size or size distribution not limited to log normal unimodal etc we have applied numerical techniques to define intersection probability distributions for any shapes previously only known for spheres and quantified the errors involved in using spherical coefficients for various non spherical particles this stereological technique is based on knowing the probability distribution of random cross sections through various particles so that small circle cross sections can be subtracted from an observed population to provide the 3d size distribution of particles the results indicate that the most important parameter controlling calculated size distribution is particle aspect ratio for a distribution of particles with a specific aspect ratio or range of aspect ratios variations of particle form spherical vs cubic rectangular vs elliptical do not alter the results so the technique can be applied to a range of particle shapes applications can be made in a petrology volcanology and other fields only a few of which can also be treated using expensive x ray techniques

explosive activity and generation mechanisms of pyroclastic flows at arenal volcano costa rica between 1987 and 2001 explosive activity at arenal and associated tephra fall that has occurred over the 14 year period from 1987 2001 is described explosions have been notably variable in both frequency and size a marked decrease in both frequency and quantity of tephra fallout occurred in early 1998 until the end of 2001 grainsize distributions of cumulative tephra samples collected once a month are typically bimodal aggregation causing premature fallout of fine ash and possibly fallout from ash plumes produced by pyroclastic flows are considered responsible for the bimodality of fallout scanning electron microscopy of the glass component of tephra from single explosions show predominantly blocky and blocky fluidal clast types interpreted as being the product of vulcanian type explosions fragmentation of a mainly rigid degassed magma body and a minor molten component is inferred for these explosions pyroclastic flows were produced either associated with the larger explosions by a mechanism of column collapse 1987 1990 or unrelated to explosions by partial collapse of the crater wall 1993 1998 2000 2001 pyroclastic flow activity has migrated from west to north during the period reported pyroclastic flow deposits are variable in the quantity of juvenile material and any associated surge component large juvenile blocks were partially molten on emplacement and many have a typical cauliform texture blocks with both juvenile and lithic textures indicate that at the summit magma was in intimate contact with the pre existing edifice rather than as a simple open crater or lava pool crater wall collapse may have been promoted by the reduction in explosive activity which has increased the lava accumulation at the summit and in turn increased instability of the summit region thus although explosive activity has waned if the lava output is maintained the hazard of pyroclastic flows is likely to continue

degassing patterns of tungurahua volcano ecuador during the 1999 2006 eruptive period inferred from remote spectroscopic measurements of so2 emissions this paper presents the results of 7 years aug 1999 oct 2006 of so2 gas measurements during the ongoing eruption of tungurahua volcano ecuador from 2004 onwards the operation of scanning spectrometers has furnished high temporal resolution measurements of so2 flux enabling this dataset to be correlated with other datasets including seismicity the emission rate of so2 during this period ranges from less than 100 to 35000 tonnes day t d with a mean daily emission rate of 1458 t d and a standard deviation of 2026 t d average daily emissions during inferred explosive phases are about 1 75 times greater than during passive degassing intervals the total amount of sulfur emitted since 1999 is estimated as at least 1 91 mt mostly injected into the troposphere and carried westwards from the volcano our observations suggest that the rate of passive degassing at tungurahua requires so2 exsolution of an andesitic magma volume that is two orders of magnitude larger than expected for the amount of erupted magma two possible and not mutually exclusive mechanisms are considered here to explain this excess degassing gas flow through a permeable stagnant magma filled conduit and gas escape from convective magma overturning in the conduit we have found that real time gas monitoring contributes significantly to better eruption forecasting at tungurahua because it has provided improved understanding of underlying physical mechanisms of magma ascent and eruption

experimental evidence links volcanic particle characteristics to pyroclastic flow hazard abstract pyroclastic flows represent the most hazardous events of explosive volcanism one striking example being the famous historical eruption of vesuvius that destroyed pompeii ad 79 much of our knowledge of the mechanics of pyroclastic flows comes from theoretical models and numerical simulations valuable data are also stored in the geological record of past eruptions including the particles contained in pyroclastic deposits but the deposit characteristics are rarely used for quantifying the destructive potential of pyroclastic flows by means of experiments we validate a model that is based on data from pyroclastic deposits the model allows the reconstruction of the current s fluid dynamic behaviour model results are consistent with measured values of dynamic pressure in the experiments and allow the quantification of the damage potential of pyroclastic flows

the 17 july 1999 block and ash flow baf at colima volcano new insights on volcanic granular flows from textural analysis abstract on july 17 1999 a strong explosion occurred at colima volcano mexico that produced a 10 km high eruptive column the partial column collapse originated a block and ash flow baf that flowed to the south along the san antonio and montegrande ravines travelling 3 3 km from the volcano summit the flow filled the ravines with a volume estimated at 7 9 105 m3 the erosion of these deposits occurred between 1999 and 2002 time of sampling providing excellent longitudinal outcrops that allowed their detailed textural study the study was carried out by means of quantitative textural analysis 1 rosiwal intersections for carrying out vertical granulometric profiles 2 total grain size analysis from 11 to 9 and 3 fourier and fractal analysis of the particle morphology grain size and morphometric parameters obtained with these methods were used to identify vertical and longitudinal variation patterns in the baf deposit the grain size variations allowed to infer the main particle segregation mechanisms that acted during transport and deposition of the studied bafs the two methods used for studying the particle shape morphologies yielded results with different accuracy and reliability in particular fractal analyses have been found to be the most effective in describing the particle support mechanisms that acted during transport and deposition of the studied bafs the results highlight the importance of the information obtained by means of these techniques and provide new insights in transportation and deposition mechanisms of bafs

insights into the october november 2010 gunung merapi eruption central java indonesia from the stratigraphy volume and characteristics of its pyroclastic deposits abstract the 2010 eruption of merapi was the second most deadly in the historic record of this volcano claiming over 380 lives by relating the observations of this eruption with detailed examination of deposit distribution stratigraphy and sedimentology a reconstruction of the properties of the pyroclastic density currents pdcs is presented including the valley controlled block and ash flows bafs and widespread energetic pyroclastic surges the distribution volume and mobility characteristics of all types of pdc during the eruption sequence show evidence for levels of intensity unseen since the large scale 1872 and 1930 eruption phases especially during the climactic events of october 26 and november 5 many tephra falls interbedded with pdc units show that most dome collapse events occurred along with and between explosive vulcanian eruptions the 2010 eruption produced very long runout bafs reaching 16 1 km in the kali gendol on november 5 this runout could be explained by its large volume 20 million m 3 around 10 times that of previous merapi bafs during the last 130 years major avulsion of these dense bafs to form overbank deposits became more common through the eruptive sequence as the valley was progressively filled with successive pdc deposits spreading avulsed bafs were a particular hazard downstream of 10 km where the landscape is less dissected less clear however is why pyroclastic surges extended up to 10 km from the vent on november 5 and 6 4 km on october 26 these expanded much farther from baf margins 2 km than ever seen before at merapi in one location they were decoupled from valley centered bafs with high momentum traveling initially laterally across steep valley systems before draining downslope at this site on the western side of the upper gendol at around 3 km from source surge decoupling was apparently exacerbated by upstream collision and deflection of high flux hot and gas rich bafs against the cliffs of gunung kendil the 1 4 km long cliff face was impacted directly for the first time in 2010 events and may have been responsible for the formation of larger than normal turbulent ash rich clouds above bafs these results imply that future eruption events under the present summit and upper flow path configuration are also highly likely to generate wide dispersal pyroclastic surges and extreme hazard especially now that dense forest has been destroyed on the upper southern slopes of the volcano

parallel adaptive numerical simulation of dry avalanches over natural terrain high fidelity computational simulation can be an invaluable tool in planning strategies for hazard risk mitigation the accuracy and reliability of the predictions are crucial elements of these tools being successful we present here a new simulation tool for dry granular avalanches using several new techniques to create a highly accurate tool with a combination of good models and numerical methodologies highlights of our methodology are the use of a depth averaged model of the conservation laws and an adaptive grid godunov solver to solve the resulting equations the software is designed to run on distributed memory supercomputers and make use of digital elevation data dynamically i e refine the grid and input data to finer resolutions to better capture flow features as the flow evolves our simulations are validated using quantitative and qualitative comparisons to tabletop experiments and data from field observations our software is freely available and can be used on a range of hardware from inexpensive desktop computers to the largest supercomputers

sediment entrainment by debris flows in situ measurements from the headwaters of a steep catchment 1 debris flows can dramatically increase their volume and hence their destructive potential by entraining sediment yet quantitative constraints on rates and mechanics of sediment entrainment by debris flows are limited using an in situ sensor network in the headwaters of a natural catchment we measured flow and bed properties during six erosive debris flow events despite similar flow properties and thicknesses of bed sediment entrained across all events time averaged entrainment rates were significantly faster for bed sediment that was saturated prior to flow arrival compared with rates for sediment that was dry bed sediment was entrained from the sediment surface downward in a progressive fashion and occurred during passage of dense granular fronts as well as water rich inter surge flow en massefailure of bed sediment along the sediment bedrock interface was never observed large magnitude high frequency fluctuations in total normal basal stress were dissipated within the upper 5 cm of bed sediment within this near surface layer concomitant fluctuations in coulomb frictional resistance are expected irrespective of the influence of pore fluid pressure or fluctuations in shear stress if the near surface sediment was wet as it was overridden by a flow additional large magnitude high frequency pore pressure fluctuations were measured in the near surface bed sediment these pore pressure fluctuations propagated to depth at subsonic rates and in a diffusive manner the depth to which large excess pore pressures propagated was typically less than 10 cm but scaled as d fi 0 5 in which d is the hydraulic diffusivity and fiis the frequency of a particular pore pressure fluctuation shallow penetration depths of granular normal stress fluctuations and excess pore pressures demonstrate that only near surface bed sediment experiences the full dynamic range of effective stress fluctuations and as a result can be more easily entrained than deeper sediment these data provide robust tests for mechanical models of entrainment and demonstrate that a debris flow over wet bed sediment will be larger than the same flow over dry bed sediment

suitability of simple rheological laws for the numerical simulation of dense pyroclastic flows and long runout volcanic avalanches 1 the rheology of volcanic rock avalanches and dense pyroclastic flows is complex and it is difficult at present to constrain the physics of their processes the problem lies in defining the most suitable parameters for simulating the behavior of these natural flows existing models are often based on the coulomb rheology sometimes with a velocity dependent stress e g voellmy but other laws have also been used here i explore the characteristics of flows and their deposits obtained on simplified topographies by varying source conditions and rheology the coulomb rheology irrespective of whether there is a velocity dependent stress forms cone shaped deposits that do not resemble those of natural long runout events a purely viscous or a purely turbulent flow can achieve realistic velocities and thicknesses but cannot form a deposit on slopes the plastic rheology with e g bingham or without a velocity dependent stress is more suitable for the simulation of dense pyroclastic flows and long runout volcanic avalanches with this rheology numerical flows form by pulses which are often observed during natural flow emplacement the flows exhibit realistic velocities and deposits of realistic thicknesses the plastic rheology is also able to generate the frontal lobes and lateral levees which are commonly observed in the field with the plastic rheology levee formation occurs at the flow front due to a divergence of the driving stresses at the edges once formed the levees then channel the remaining flow mass the results should help future modelers of volcanic flows with their choice of which mechanical law corresponds best to the event they are studying

a two layer depth averaged model for both the dilute and the concentrated parts of pyroclastic currents pyroclastic currents are very destructive and their complex behavior makes the related hazards difficult to predict a new numerical model has been developed to simulate the emplacement of both the concentrated and the dilute parts of pyroclastic currents using two coupled depth averaged approaches interaction laws allow the concentrated current pyroclastic flow to generate a dilute current pyroclastic surge and inversely the dilute current to form a concentrated current or a deposit the density of the concentrated current is assumed to be constant during emplacement whereas the density of the dilute current changes depending on the particle supply from the concentrated current and the mass lost through sedimentation the model is explored theoretically using simplified geometries as a proxies for natural source conditions and topographies it reproduces the relationships observed in the field between the surge genesis and the topography the increase in surge production in constricted valleys the decoupling between the concentrated and the dilute currents and the formation of surge derived concentrated flows the strong non linear link between the surge genesis and the velocity of the concentrated flow beneath it could explain the sudden occurrence of powerful and destructive surges and the difficulty of predicting this occurrence a companion paper compares the results of the model with the field data for the eruption of merapi in 2010 kelfoun et al submitted and demonstrates that the approach is able to reproduce the natural emplacement of the concentrated and the dilute pyroclastic currents studied with good accuracy

erosion transport and segregation of pumice and lithic clasts in pyroclastic flows inferred from ignimbrite at lascar volcano chile abstract investigations have been made on the distribution of pumice and lithic clasts in the lithic rich soncor ignimbrite 26 5 ka and the 1993 pumice flow deposits of lascar volcano chile the soncor ignimbrite shows three main lithofacies which grade into one another coarse lithic breccias range from matrix poor stratified varieties irregular shaped sheets and elongate hummocks in proximal environments to breccia lenses with pumiceous ignimbrite matrix massive lithic rich facies comprise the bulk of the ignimbrite pumice rich facies are bimodal with abundant large pumice clasts often with reverse grading rare lithic clasts and occur distally and on high ground adjacent to deep proximal valleys in the 1993 pyroclastic flow deposits lithic rich facies are deposited on slopes up to 14 whereas pumice rich facies are deposited only on slopes

the porosity of pyroclasts as an indicator of volcanic explosivity abstract the porosity of pyroclasts is a valuable indicator of the character of a volcanic eruption here we analyse the porosity distribution of deposits of 17 different volcanic eruptions covering a broad range of inferred magma properties and observed eruption styles histograms of porosity versus pyroclast frequency show distinctive features in terms of the mean variance modality and skewness we present characteristic porosity distributions for five different eruption style categories dome forming explosive basaltic cryptodome forming subplinian plinian ultraplinian and phreatomagmatic we find that the mean porosity values of the deposits correlate positively with the volcanic explosivity index vei of newhall and self 1982 regardless of the degree of complexity of the individual distributions we then propose causes of the deviations from a general trend in terms of magma viscosity degassing efficiency and cap rock confinement finally a generalised correlation between clast porosity and explosivity of an eruption is developed that should help together with conventional field techniques facilitate the interpretation of pyroclastic sequences even in cases where no direct observation of the eruption exists

textural studies of vesicles in volcanic rocks an integrated methodology abstract vesicles in volcanic rocks are frozen records of degassing processes in magmas for this reason their sizes spatial arrangements numbers and shapes can be linked to physical processes that drive magma ascent and eruption although numerous techniques have been derived to describe vesicle textures there is no standard approach for collecting analyzing and interpreting vesicular samples here we describe a methodology for techniques that encompass the entire data acquisition process from sample collection to quantitative analysis of vesicle size and number carefully chosen samples from the lower mean and higher density vesicularity endmembers are characterized using image nesting strategies we show that the texture of even microvesicular samples can be fully described using less than 20 images acquired at several magnifications to cover efficiently the range of existing vesicle sizes a new program foams was designed to perform the quantification stage from vesicle measurement to distribution plots altogether this approach allows substantial reduction of image acquisition and processing time while preserving enough user control to ensure the validity of obtained results we present three cameo investigations on basaltic lava flows scoria deposits and pumice layers to show that this methodology can be used to quantify a wide range of vesicle textures which preserve information on a wide range of eruptive conditions

field experiments and numerical modeling of mass entrainment in snow avalanches 1 snow entrainment alters the speed and hence the run out distance of avalanches yet little is known about this significant process we studied entrainment in snow avalanches using observations from 1 the swiss vallee de la sionne test site 2 the italian pizzac site 3 catastrophic avalanches that occurred during the winter 1998 1999 in switzerland and 4 a medium sized spontaneous avalanche that occurred in 2000 in davos switzerland we determined mass and energy balances for 18 avalanche events on average the mass increased by a factor of 4 the primary mode of entrainment appeared to be frontal ploughing although entrainment behind the avalanche front was also observed step entrainment where a snow cover layer fractures and is entirely consumed by the avalanche also occurred basal erosion was negligible mass availability and snow cover structure were the limiting factors governing entrainment other factors such as track topography and avalanche dimension played a secondary role using the experimental results we introduced an entrainment model into a saint venant type flow model where the internal shear deformation of the avalanche is governed by a bagnold law and the shear stress at the basal layer is treated as a voellmy fluid the model with entrainment not only improves the prediction of the velocities and flow heights in comparison to measurements but also reproduces the variations in run out distances which characterize avalanches with similar terminal velocities but different masses

infrasound from tungurahua volcano 2006 2008 strombolian to plinian eruptive activity abstract strombolian to plinian activity from tungurahua volcano ecuador has been recorded by the autonomous infrasound arrays of the acoustic surveillance for hazardous eruptions ashe project since early 2006 our studies suggest that acoustic energy release during large eruptions does appear to broadly scale with eruption intensity this manuscript provides a detailed chronology and characterization of tungurahua s eruptive activity between 2006 and 2008 and demonstrates the ability to constrain source parameters of significant eruptions such as onset duration and escalation at regional distances by combining infrasound and remote sensing techniques the ashe system in ecuador automatically detected over 20 000 volcanic explosions at an array 37 km from tungurahua and was successful at notifying the onset escalation and cessation of a hazardous february 2008 eruption with a latency of 5 min elevated infrasonic energy from sustained and intense tungurahua eruptions correlates well with ash column heights and their lateral extent during the study period the spectra of these sustained explosive eruptions appear to be recurrent readily recognizable and indicative of volcanic jetting and significant atmospheric ash injection the paroxysmal plinian phase of the august 2006 eruption produced an ash cloud that extended well into the stratosphere 24 km coinciding with a shift of the dominant jetting frequency from 0 25 hz to below 0 1 hz and radiation of over 5 107 w of acoustic power transient explosions were often marked by minor or no ash release and are presumed to be more gas rich a change in the acoustic spectrum of volcanic jetting was also detected in the transition from a sustained to collapsed eruption column at the end of the july 14 2006 eruption the jetting spectrum at tungurahua during a period of sustained pyroclastic density current production changes from a typical double peaked to a single peaked spectrum suggesting remote acoustic monitoring can help ascertain the stability and dynamics of an eruptive column

pore fluid pressure and internal kinematics of gravitational laboratory air particle flows insights into the emplacement dynamics of pyroclastic flows 1 the emplacement dynamics of pyroclastic flows were investigated through noninvasive measurements of the pore fluid pressure in laboratory air particle flows generated from the release of fluidized and nonfluidized granular columns analyses of high speed videos allowed for correlation of the pressure signal with the flow structure the flows consisted of a sliding head that caused underpressure relative to the ambient followed by a body that generated overpressure and at the base of which a deposit aggraded for initially fluidized flows overpressure in the body derived from advection of the pore pressure generated in the initial column and decreased by diffusion during propagation relatively slow diffusion caused the pore pressure in the thinner flow to be larger than lithostatic at early stages furthermore partial auto fluidization revealed in initially nonfluidized flows also occurred and contributed to maintain high pore pressure whereas dilation or contraction of the air particle mixture with associated drag and or pore volume variation transiently led the pressure to decrease or increase respectively the combination of all these processes resulted in long lived high pore fluid pressure in the body of the flows during most of their emplacement in the case of the initially fluidized and slightly expanded 3 4 flows at least 70 100 of the weight of the particles was supported by pore pressure which is consistent with their inertial fluid like behavior dense pyroclastic flows on subhorizontal slopes are expected to propagate as inertial fluidized gas particle mixtures consisting of a sliding head possibly entraining basement derived clasts and of a gradually depositing body

combining historical and 14 c data to assess pyroclastic density current hazards in banos city near tungurahua volcano ecuador pyroclastic density currents pdcs from tungurahua volcano ecuador recurrently rush down the northern vazcun valley and threaten the small tourist city of banos located on its mouth estimating pdc hazards i e pdc return rate and extent is difficult in banos and vazcun because data from the literature are rare and debated while geological exposures are few and vegetated to improve our awareness of pdc hazards in the banos area we examined historical documents conducted new geological research in the vazcun valley and obtained additional 14c data we highlight complex radiocarbon results that may lead to difficulties when interpreting the pdc chronology and offer recommendations to improve sampling strategies for 14c age determinations of pdc deposits overall our results show that the 1640 1773 and 1886 pdc forming eruptions are recorded in the valley while products of the 1918 and 2006 events were likely removed through considering recorded unrecorded bias in the reconstruction of past pdc emplacement in vazcun we calculate a minimal average return rate of 18 pdc forming events for pre columbian and historical eruptions since the devastating 3 ka bp eruption which translates into a maximum average return rate of 150 200 years in the valley however the archives suggest that most pdcs did not affected the now urbanized banos area e g 1918 2006 and we estimate that the city is impacted on average every 350 500 years a duration that is likely perceived as fairly long for people living in the area the banos case study thus raises questions on how to communicate on uncertain threats and impacts and how to improve alert messages in a town where economic incomes are essentially based on tourist activities to better address these issues we plan in future works to focus on cognitive perception of risk and volcanic hazard judgment by populations of banos and nearby rural communities

flow and deposition of pyroclastic granular flows a type example from the 1975 ngauruhoe eruption new zealand small volume pyroclastic density currents pdcs are generated frequently during explosive eruptions with little warning assessing their hazard requires a physical understanding of their transport and sedimentation processes which is best achieved by the testing of experimental and numerical models of geophysical mass flows against natural flows and or deposits to this end we report on one of the most detailed sedimentological studies ever carried out on a series of pristine small volume pdc deposits from the 1975 eruption of ngauruhoe volcano whose emplacement were also witnessed during eruption using high resolution gps surveys a series of lateral excavations across the deposits and bulk sedimentological analysis we constrained the geomorphology internal structure and texture of the deposits with respect to laterally varying modes of deposition deposition from these pdcs began only on slopes at or around the material s angle of repose c 30 in unconfined settings the granular pdcs are interpreted to have been quasi steady forming sheets and lobes around the angle of repose where flows were confined sheet like proximal facies made up around 10 of the deposit volume at the angle of repose but 90 of the material was deposited from apparently unsteady inertial granular pdcs as a distal levee and channel facies on slopes well below the repose angle hence confined pdcs were able to travel up to 50 farther than unconfined flows in the distal facies the deposit width is inversely correlated to the local slope and the height of the levees above the deposit centreline is positively correlated with slope internally the deposits comprise three parts a coarse grained fines free sole layer that laterally connects to levees zone i an ashy matrix supported central body zone ii and an overlying coarse plaster of clasts zone ii trends in grain size data suggests these zones derive from a continuous un mixing of coarse particles from the initial bulk material by granular segregation that preferentially drives large particles to the upper free surface of the flow where they are concentrated at the front of flow before being deposited and overrun by comparison to analogue experiments we suggest a model of flow and deposition where the temporally and spatially varying mode of deposition is determined by the flow velocity the local slope the vertical velocity gradient the velocity gradient at the free surface and the vertical deposition rate using this model estimated vertical deposition rates of c 5 cm s 1 from the ngauruhoe pdcs agree with those determined in laboratory experiments on inertial granular flows

predicting the block and ash flow inundation areas at volcan de colima colima mexico based on the present day february 2010 status abstract maps of areas potentially affected by block and ash flows and associated ash clouds are here presented for the volcan de colima titan2d 2 0 1 code has been used to simulate block and ash flows using as an input volume that of the actual summit dome assessed at 2 10 6 m 3 while the energy cone model has been used to delimit the possible inundated area from associated ash clouds both merapi and soufriere type block and ash flows were generated using different basal friction angles and maintaining fixed the volume and the internal friction angle the setting of the input parameters takes into account some flow characteristics such as the stepwise aggradation of different pulses that piled up to form the total thickness of the block and ash flow deposits the outputs of the computational routines are reported as two maps describing the total thickness of the final deposits they predict that thick deposits will engulf the ravines descending from the main cone to the west south and southeast with expected maximum runouts between 4 5 and 7 km the associated ash clouds have slightly longer runouts and the model predicts they will inundate some higher grounds that are not affected by the concentrate underflows the presented maps represent useful tools for managing the current block and ash flow hazard at volcan de colima

influence of slope angle on pore pressure generation and kinematics of pyroclastic flows insights from laboratory experiments the influence of slope angle on pore pressure generation and kinematics of fines rich pyroclastic flows was investigated through laboratory experiments granular flows were generated by the release of a column of fine glass beads d 0 08 mm in an inclined channel 0 30 the granular column could be fluidized while the channel base was either smooth or made rough by glued beads of 3 mm diameter pore pressure measurements reveal that the degree of autofluidization caused by air escaping from the substrate interstices into which flow particles settled was high at all slope angles flow runout increase due to autofluidization however was reduced at slope angle higher than 12 because of the occurrence of a strong deceleration phase that limited the flow duration this is probably caused by the combination of flow head thinning at increased slope angle and settling of particles into the substrate interstices until the flow ran out of mass analysis of high speed videos suggests that ingestion of ambient air at the flow front did not occur even on steep slopes of 30 experiments at inclinations close to 25 or slightly higher 30 than the repose angle of the granular material 28 5 revealed the formation of a thin basal deposit that was then eroded as the flow thickness and velocity gradually decreased our study suggests that air escape from substrate interstices in nature can be a significant external cause of pore pressure generation that favors low energy dissipation and long runout distances of pyroclastic flows on moderate topographies

friction marks on blocks from pyroclastic flows at the soufriere hills volcano montserrat implications for flow mechanisms friction marks on surfaces of blocks 1 to 10 m in pyroclastic flow deposits generated by lava dome collapse in the 1995 1999 eruption of the soufriere hills volcano montserrat consist of a thin 5 to 10 mm layer of cataclasite with a polished slickensided surface in some examples thin surface layers and veins of pseudotachylyte are found the friction marks occur in several orientations on all block surfaces they are typically 0 5 to 10 cm wide and up to 1 m long the marks are similar to tectonic structures on faults they indicate that the blocks were tumbling and sliding within the flows large transient stresses result when the mass of a large sliding block is applied to a small contact area on a neighboring block or the ground compositional data indicate that the pseudotachylytes involved melting of the flow matrix these observations support models of pyroclastic flows as rapid granular avalanches

evaluation of the impact of the 2010 pyroclastic density currents at merapi volcano from high resolution satellite imagery field investigations and numerical simulations the 2010 pyroclastic density currents pdc at merapi have presented a rare opportunity to collect a uniquely detailed dataset of the source extent lateral variations and impact of various pdc deposits on a densely populated area using traditional volcanological field based methods and a multi temporal dataset of high resolution satellite imagery a total of 23 pdc events have been recognized including 5 main channeled flows 15 overbank flows derived from overspill and re channelization of the main pdcs into adjacent tributaries and two main surge events the 2010 pdc deposits covered an area of 22 3 km2 unequally distributed between valley filling 6 9 overbank 22 4 and surge and associated fallout deposits 71 7 their total estimated non dre volume is 36 3 106 m3 with 50 2 of this volume accounting for valley filling deposits 39 3 for overbank deposits and 10 5 for surge and associated fallout deposits more than 70 of the total volume was deposited during the third eruptive phase 4 5 november and only 16 6 11 5 and 0 9 during the first 26 29 october second 30 october 3 november and fourth phase 6 23 november respectively the internal architecture and lithofacies variations of the 2010 pdc deposits were investigated using data collected from 30 stratigraphic sections measured after one rainy season of erosion the results show that complex local scale variations in flow dynamics and deposit architectures are apparent and that the major factors controlling the propagation of the main flows and their potential hazards for overbanking were driven by 1 the rapid emplacement of several voluminous pdcs associated with the steady infilling of the receiving landscape after the two first phases of the eruption 2 longitudinal changes in channel capacity following increased sinuosity in the valley and decreased containment space and 3 the effects of varying source mechanisms gravitational dome collapse vertical or lateral dome explosions and column collapse and source materials involved during individual pdc forming events integration of these data into numerical simulations of the channeled and overbank pdcs of the third eruptive phase 4 5 november using two well established geophysical mass flow models titan2d and volcflow allow us to evaluate the ability of these models to reproduce the main features of the natural deposits and some of the flow overbanking processes observed in the field using such a multi technique approach the dataset obtained in this study not only characterizes the pdcs and related hazards at merapi but will allow comparisons with similar events at other volcanoes around the globe

deposit architecture and dynamics of the 2006 block and ash flows of merapi volcano java indonesia the internal architecture of the 2006 block and ash flow deposits of merapi volcano java indonesia was investigated using data collected from 27 stratigraphic sections measured immediately after flow emplacement and after one and two rainy seasons of erosion identification of different depositional units and their longitudinal and lateral facies variations provide detailed information about i the distribution volumes and sedimentological characteristics of the different units ii flow types and mobility as inferred from associated deposits and iii changes in the dynamics of the different flows and their material during emplacement two main types of block and ash flows short runout to medium runout block and ash flows and long runout block and ash flows are defined based on flow generation mechanism flow volume travel distance deposit morphology distribution lithology and grain size distribution conceptual models for the transport and depositional mechanisms of these two types of block and ash flows are presented variations in the runout distances observed for short runout to medium runout block and ash flows are linked directly to different initial flow volumes degree of fragmentation and material properties of the moving mass during transport with the largest and finer grained flows having the greatest mobility deposition occurs only over a narrow range of basal inclinations close to the angle of repose for pyroclastic material indicating that such flows behave in a similar way to granular free surface flows on unconfined planes the flow mechanisms of long runout block and ash flows at merapi are interpreted to be similar in many respects to unsteady cohesionless grain flows with an inertial flow regime where collisional forces largely overcome frictional forces flow unsteadiness causes the main body to be segmented into different pulses that run closer to each other as the flow moves downslope deposition occurs stepwise with rapid aggradation of stacked sub units from different parts of the major flow pulses in such a model the arrival of each flow pulse front at selected sites in the main river valley controls the generation and development of highly mobile unconfined pyroclastic flows outside valley regions and their associated overbank deposits

pyroclastic flow erosion and bulking processes comparing field based vs modeling results at tungurahua volcano ecuador pyroclastic density currents pdcs are high temperature and high velocity mixtures that threaten populations in the vicinity of many active volcanoes deciphering the cause of their remarkable mobility is essential for volcanic hazard analysis but remains difficult because of the complex processes occurring within the flows here we investigate the effect of bulking on dense pdc mobility by means of a double approach first we estimate the amount of material incorporated into scoria flows emplaced during the august 2006 eruption of tungurahua volcano ecuador for this we carry out a detailed analysis of 3d corrected digital images of well exposed scoria flow deposits componentry analysis indicates that pdc bulking occurs principally on the steep 25 upper slope of the volcano and the deposits typically comprise 40 50 wt of non juvenile i e accessory and accidental material secondly we develop a simple stress related grain by grain equation of erosion combined with two simple depth averaged geophysical mass flow models that compare the bulking mechanism to a non fluidized and a fluidized flow two behaviors based on coulomb and plastic rheologies are used to reproduce on a first order basis the 2006 tungurahua pdcs cross check comparisons between these modeled cases and the erosion pattern inferred from field based data allow us to evaluate the accuracy of our modeling assumptions regardless of the rheological regime the pdc induced erosion pattern of the 2006 tungurahua eruption can only be reproduced by fluctuations of the flow s basal shear stress during emplacement such variations are controlled by flow thinning thickening processes notably through the formation of a thick non erosive flow body that pushes a thin frictional erosive front during pdc emplacement the input volume of juvenile material as well as the thickness of the erodible layer available prior to the eruption are additional key parameters our work highlights complexities in pdc erosion and bulking processes that deserve further study in terms of hazard assessment our findings reveal that incorporation and bulking translate into increased flow mobility i e the augmented flow mass enhances both flow velocity and runout distance up to 20 these outcomes should be considered closely for hazard analysis at many other andesitic volcanoes worldwide where similar pdc events are common