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

emplacement of the 18 may 1980 lateral blast deposit ene of mount st helens washington facies variations east northeast of mount st helens preserve a record of depositional processes in the 18 may 1980 lateral blast cloud this paper reports new field grain size and component data from the ene sector of the timber blowdown zone and presents a model for blast flow and sedimentation the first erupted ejecta was rich in juvenile components and extends to the distal blowdown limit the last erupted ejecta was rich in accidental lithics and reached no further than a few kilometres from the mountain due to waning discharge the blast cloud was a turbulent stratified flow which transported and deposited sediment in the manner of a high density turbidity current the possibility that the blast was emplaced as a giant shearing fluidised bed is not favoured by compositional zoning patterns depositional conditions were strongly influenced by the rate of suspended load fallout from the blast within about 8 km from vent rapid sedimentation caused deposition under moderate to high concentration conditions and formation of a basal hindered settling zone able to detach gravitationally and drain into local depressions the resulting proximal facies resembles a low aspect ratio ignimbrite fines depletion in the proximal facies is attributed to a combination of residual turbulence and rapid gas escape during particle settling and compaction through the hindered settling zone component data suggest that the blast head played no significant role in the generation of fines depletion in the blast deposit as suggested by previous workers with increasing distance from vent the rate of particle fallout declined and sedimentation took place under increasingly dilute and tractional conditions building up antidune like bedforms wavelengths of these bedforms range from 20 to 1 m and decrease away from vent there is a systematic relationship between antidune migration direction and depositional slope the transition from proximal ignimbrite like to distal surge like facies suggests a possible gradation in transport and deposition processes between conventional pyroclastic surges and high velocity pyroclastic flows

the architecture eruptive history and evolution of the table rock complex oregon from a surtseyan to an energetic maar eruption abstract the table rock complex trc pliocene pleistocene first documented and described by heiken heiken g h 1971 tuff rings examples from the fort rock christmas lake valley basin south central oregon j geophy res 76 5615 5626 is a large and well exposed mafic phreatomagmatic complex in the fort rock christmas lake valley basin south central oregon it spans an area of approximately 40 km2 and consists of a large tuff cone in the south trc1 and a large tuff ring in the northeast trc2 at least seven additional smaller explosion craters were formed along the flanks of the complex in the time between the two main eruptions the first period of activity trc1 initiated with a surtseyan style eruption through a 60 70 m deep lake the trc1 deposits are dominated by multiple 1 2 m thick fining upward sequences of massive to diffusely stratified lapilli tuff with intermittent zones of reverse grading followed by a finely laminated cap of fine grained sediment the massive deposits are interpreted as the result of eruption fed subaqueous turbidity current deposits whereas the finely laminated cap likely resulted from fallout of suspended fine grained material through a water column other common features are erosive channel scour and fill deposits massive tuff breccias and abundant soft sediment deformation due to rapid sediment loading subaerial trc1 deposits are exposed only proximal to the edifice and consist of cross stratified base surge deposits the eruption built a large tuff cone above the lake surface ending with an effusive stage which produced a lava lake in the crater 365 m above the lake floor a significant repose period occurred between the trc1 and trc2 eruptions evidenced by up to 50 cm of diatomitic lake sediments at the contact between the two tuff sequences the trc2 eruption was the last and most energetic in the complex general edifice morphology and a high percentage of accidental material suggest eruption through saturated trc1 deposits and or playa lake sediments trc2 deposits are dominated by three dimensional dune features with wavelengths 200 500 m perpendicular to the flow and 20 200 m parallel to the direction of flow depending on distance from source large u shaped channels 10 32 m deep run up features over obstacles tens of meters high and a large 13 m chute and pool feature are also identified the trc2 deposits are interpreted as the products of multiple erosive highly inflated pyroclastic surges resulting from collapse of an unusually high eruption column relative to previously documented mafic phreatomagmatic eruptions

erosional furrows formed during the lateral blast at mount st helens may 18 1980 nearly horizontal quasi periodic erosional features of 7 m average transverse wavelength and of order 100 m length occur in scattered locations from 3 5 to 9 km from the crater at mount st helens under deposits of the lateral blast of may 18 1980 we attribute the erosional features to scouring by longitudinal vortices resulting from flow instabilities induced by complex topography namely by streamline curvature in regions of reattachment downstream of sheltered regions and by the cross flow component of flow subparallel to ridge crests the diameter of the vortices and their transverse spacing inferred from the distance between furrows are taken to be of the order of the boundary layer thickness the inferred boundary layer thickness 14 m at 9 km from the source of the blast is consistent with the running length from the mountain to the furrow locations by using knowledge of ablation patterns on bodies and lofting of dust in high speed flow we are able to infer some features of the flow field within the blast within the furrows the erosion rate was of the order of 9 kg m 2 s 1 about 4 times greater than that expected from laboratory data obtained in flow free of longitudinal vortices the orientation of furrows induced by the cross flow instability can be used to measure the upwash angle and estimate the flow mach number at the central ridge of spirit lake the mach number is inferred to have been about 2 5 and the flow velocity approximately 235 m s the similarities and differences between the furrows reported here and channels observed at other volcanoes are discussed

migration direction of sand wave structures in pyroclastic surge deposits implications for depositional processes sand wave structures of both progressive downstream migrating and regressive upstream migrating types occur within the same pyroclastic surge deposit at both roccamonfina volcano southern italy and sugarloaf mountain arizona united states both wet and dry surge deposits are described that contain examples of regressive and progressive sandwave structures at sugarloaf mountain progressive and regressive sand waves occur within metres of each other within the same bedset indicating that penecontemporaneous deposition of the two different sand wave structures occurred in close proximity on the basis of this evidence it is suggested that the migration direction of sand waves in these pyroclastic surge deposits is controlled by the flow regime of the surge cloud rather than the temperature and or moisture content

origin and emplacement of a pyroclastic flow and surge unit at laacher see germany abstract within the pyroclastic ejecta sequence of laacher see volcano west germany are many repeated phreatomagmatic depositional cycles a complete cycle begins with a thin low aspect ratio fines depleted breccia layer followed by a sequence with several coarse to fine grained cross bedded units in turn overlain by a massive pyroclastic flow layer each sequence is interpreted in terms of water magma interaction and effects upon the eruption column the breccia represents the triggering eruption with a high eruption column where fines were winnowed out and fallback from high altitudes produced a low aspect ratio layer the cross bedded sequence containing more fine grained ash resulting from an increase in amount of water represents a fluctuating eruption column of sufficient height to produce large antidunes within 700 meters of the rim the massive pyroclastic flow layer which contains more fine grained ash than the underlying layers represents the culmination of the eruption where an increase in amount of water interacting with the magma resulted in a dense and very low eruption column producing a dense laminar moving pyroclastic flow

large eddy simulation of sediment transport currents over ripples a large eddy simulation has been performed to study the detailed sediment dynamics that occur in channel flow over ripples the code solves the navier stokes equations and an advection diffusion equation for the sediment and has been verified to produce quantitatively accurate hydrodynamic results for the flow domains used in this study the main features of interest in this flow are seen because of the high resolution of the grid especially near the ripple boundary this yields a detailed bottom shear stress distribution properly identified coherent structures and resolved sharp gradients in the sediment concentration globally the sediment is carried up into the flow from locations where the shear stress is high on the upslopes of ripple crests and is advected downstream by the current and upward by the vertical velocity component regions of positive vertical velocity are associated with gortler vortices which represent a significant sediment transport mechanism in this flow

depositional processes of the suwolbong tuff ring cheju island korea the suwolbong pyroclastic sequence in the western part of cheju island korea comprises partly preserved rim beds of a quaternary basaltic tuff ring whose vent lies about 1 km seaward of the present shoreline the sequence consists of breccia lapillistone lapilli tuff and tuff eighteen sedimentary facies are established and organized into six lateral facies sequences lfs and seven vertical facies sequences vfs the lfs 1 4 and 5 begin with massive lapilli tuff which transforms downcurrent into either planar bedded lfs 1 undulatory bedded lfs 4 or climbing dune bedded lfs 5 lapilli tuff units they are representative of relatively dry base surge whose particle concentration decreases downcurrent with a progressive increase in both tractional processes and sorting the lfs 2 begins with disorganized and massive lapilli tuff and transforms into crudely stratified units downcurrent it results from relatively wet base surge in which sorting is poor due to the cohesion of damp ash the lfs 3 comprises well sorted lapilli tuff and stratified tuff further downcurrent suggestive of deposition from combined fall and surge of relatively dry hydroclastic eruption all seven vertical facies sequences generally comprise two facies units of coarse grained fines depleted lapilli tuff and an overlying fine grained tuff these sequences are suggestive of deposition from base surge that consists of a turbulent head and a low concentration tail depositional processes in the suwolbong tuff ring were dominated by a relatively dry base surge the base surge comprises turbulent and high concentration suspension near the vent whose deposits are generally unstratified due to the lack of tractional transport as the base surge becomes diluted downcurrent through fallout of clasts and mixing of ambient air it develops large scale turbulent eddies and is segregated into coarse grained bedload and overlying fine grained suspension forming thinly stratified units further downcurrent the base surge may be either cooled and deflated or pushed up into the air depending on its temperature the suwolbong tuff ring comprises an overall wet to dry cycle with several dry to wet cycles in it suggestive of overall decrease in abundance of external water and fluctuation in the rate of magma rise

bypassing and diachronous deposition from density currents evidence from a giant regressive bed form in the poris ignimbrite tenerife canary islands exceptional longitudinal exposure through an ignimbrite sheet on the southern flanks of las canadas volcano tenerife canary islands reveals a giant regressive bed form 520 m long it comprises sourceward dipping diffuse lenses 50 m long of massive and diffuse bedded lapilli tuff its architectural relationships indicate that at any one moment in time deposition was restricted to an 50 m long longitudinal depositional zone bounded both upslope and downslope by extensive bypassing zones the regressive architecture shows that the zone of deposition gradually migrated sourceward as the current passed nonpersistent scours indicate that the current was locally and momentarily erosive although at many locations there is little evidence for substantial erosion or deposition and most of the pyroclastic load seems to have entirely bypassed subaerial slopes to enter the atlantic ocean 20 km from the source the ignimbrite is diachronous and any single vertical section through it represents only a short phase of the current9s protracted depositional history similarly protracted and complex flow histories elsewhere may be easily overlooked at typical field exposures of ignimbrites turbidites and lahar deposits leading to significantly underestimated flow durations and volumes

pyroclastic dune bedforms macroscale structures and lateral variations examples from the 2006 pyroclastic currents at tungurahua ecuador pyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions for much of their dynamics they behave as particulate density currents and share similarities with turbidity currents they occasionally deposit dune bedforms with peculiar lamination patterns from what is thought to represent the dilute low concentration and fluid turbulence supported end member of the pyroclastic currents here we present a high resolution dataset of sediment plates lacquer peels with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the august 2006 eruption of tungurahua ecuador most of the sedimentary features contain backset bedding and preferential stoss face deposition from the ripple scale few cm to the largest dune bedform scale several m length similar patterns of erosive based backset beds are evidenced recurrent trains of sub vertical truncations on the stoss side of structures reshape and steepen the bedforms in contrast sporadic coarse grained lenses and lensoidal layers flatten bedforms by filling troughs the coarsest clasts up to 10 cm least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend the stratal architecture exhibits strong lateral variations within tens of centimeters with very local truncations both in flow perpendicular and flow parallel direction we infer that the bedforms sedimentary patterns result from four formation mechanisms differential draping slope influenced saltation truncative bursts and granular based events whereas most of the literature makes a straightforward link between backset bedding and froude supercritical flows we reconsider this interpretation indeed features that would be diagnostic of subcritical dunes antidunes and chute and pools can be found on the same horizon and in a single bedform only laterally separated by short distances 10s of cm our data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as gortler vortices backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre existing morphology quantification of near bed flow velocities are made via comparison with wind tunnel experiments we estimate that shear velocities of ca 0 30 m s 1 equivalent to pure wind velocity of 6 to 8 m s 1 at 10 cm above the bed could emplace the constructive bedsets whereas the truncative phases would result from bursts with impacting wind velocities of at least 30 40 m s 1

interaction between particles transported by fallout and surge in the deposits of the agnano monte spina eruption campi flegrei southern italy abstract the agnano monte spina tephra amst is a complex sequence of beds generated by contrasting fragmentation and transportation dynamics the 4 1 ka eruption was accompanied by a volcano tectonic collapse part of which is the present agnano plain the pyroclastic sequence is subdivided into members and submembers each characterized by different lithological and sedimentological features plinian subplinian fallout deposits frequently alternate with base surge beds of phreatomagmatic origin analysis of lateral facies variations and vertical facies associations of correlated layers of submembers b2 d2 and e2 reveals that during some eruption phases the contrasting eruptive dynamics were almost contemporaneous base surge deposits of submember b2 formed during the declining stage of a plinian column they resulted from highly energetic and steady pyroclastic density currents that traveled long distances from the vent area and surmounted topographic obstacles such as the posillipo hill 150 m and the northern sector of the camaldoli hill 250 m submember d2 pyroclastic density currents formed when contemporaneous fallout from an eruptive column was present coarse particles from the column settled throughout the pyroclastic density current determining a significant increase of the solid load of the base surge the consequent increase of supply rate from the transportation to the depositional zone of the base surge led to the formation of unsteady flows that could not efficiently transport the solid load and did not have the ability to travel long distances base surge deposits of submember e2 were fed by a pulsating phreatomagmatic activity which was punctuated by a short lived fallout phase fallout material which was incorporated as an additional load to the base surges was partially transported by low energy steady pyroclastic density currents that traveled over the agnano plain but did not surmount either the camaldoli or the posillipo hills only the very fine material in continuous suspension in the upper no shearing part of the base surge cloud was dispersed higher in the atmosphere and quietly settled over a large area outside the caldera rim the phreatomagmatic origin of base surges which contrasts with the magmatic origin of fallout activity suggests that the pyroclastic density currents of the agnano monte spina eruption did not result from eruption column collapses they were most likely related to radially spreading clouds which were contemporaneous with fallout activity but issued from distinct zones in the vent area the turbulent nature and the high expansion of base surges made them capable under certain conditions of passing over high topographic obstacles with hazardous effects in distal areas

characteristics of pyroclastic deposits at the margin of fond canonville martinique and implications for the transport of the 1902 nuees ardentes of mt pelee abstract the deposits which were studied at fond canonville represent a well defined record of the 1902 eruptions of the montagne pelee the sections are located at the margin of the valley where six eruption units with thicknesses ranging from 8 cm up to 2 m were identified overlying the pre 1902 paleosol the pyroclasts consist of lapilli coarse and fine ash composed of juvenile andesitic fragments of varying vesicularity dust free crystals dust free transparent vesicular glass and accretionary lapilli paleocurrent measurements on cross bedding support the stratigraphic evidence that the flows came down the valley of fond canonville rather than up as was previously proposed most eruption units are made of two or three beds always in sharp contact the basal bed relatively thin contains the coarsest grain size of the unit is massive or normally graded and fine depleted a thin division of parallel laminae may be present at the top the middle bed the thickest in all eruption units has a mean grain size which is intermediate between the lower and upper beds it may be simple or multiple normal grading is generally present on which may be superimposed in the upper portion of the bed 2 4 cm thick strata parallel to the upper bed surface or dipping from it at angles of less than 10 the upper bed the most regular in all units with thicknesses ranging from 2 to 8 cm is made of accretionary lapilli set in a matrix of fine ash a division of parallel and or oblique laminae is generally present at its top the characteristics of the deposits suggest that all three beds accumulated from a stratified turbulent flow normal grading of both coarsest and mean sizes sequences of internal structures including laminae evince that the flows were of relatively low concentration and that traction was an active process during the accumulation of the deposits for these structures to form the flows had to be turbulent for all three beds of the eruption units the thick strata superimposed on the graded middle bed are interpreted as having formed by the migration and aggradation of long wavelength antidunes the relationship between flow velocity and wavelength suggests that subaerial pyroclastic flows can only produce long wavelength antidunes with resulting internal stratification essentially parallel to bedding short wavelength antidunes in subaerial pyroclastic deposits require very high flow density at least close to the bed surface or low velocity which is unlikely considering the grain size present in the deposits

a facies interpretation of the eruption and emplacement mechanisms of the upper part of the neapolitan yellow tuff campi flegrei southern italy this study focuses on the upper part member b of the neapolitan yellow tuff nyt detailed measurements of stratigraphic sections within the unlithified pozzolana facies show that member b is composed of at least six distinct depositional units which each record a complex fluctuation between different styles of deposition from pyroclastic density flows six lithofacies have been identified 1 massive valleyponded facies the product of non turbulent flows 2 inverse graded facies formed by flows that were turbulent for the majority of transport but were deposited through a non tubulent basal regime 3 regressive sand wave facies the product of high concentration turbulent flows 4 stratified facies the product of deposition from turbulent low particle concentration flows 5 particle aggregate and 6 vesicular ash lithofacies both of which are considered to have formed by deposition from turbulent low concentration flows although the whole eruption may have been phreatomagmatic facies 1 4 are interpreted to be the product of dry eruptive activity whereas facies 5 and 6 are considered to be of wet phreatomagmatic eruptive phases small scale horizontal variations between facies include inverse graded lithofacies that pass laterally into regressive sand wave structures and stratified deposits this indicates rapid transition from non turbulent to turbulent deposition within the same flow thin vesicular ash and particle aggregate layers pass laterally into massive valley ponded vesicular lithofacies suggesting contemporaneous wet pyroclastic surges and cohesive mud flows three common vertical facies relations were recognised 1 massive valley ponded and inverse graded facies are overlain by stratified facies suggesting decreasing particle concentration with time during passage of a flow 2 repeated vertical gradation from massive up into stratified facies and back into massive beds is indicative of flow fluctuating between non turbulent and turbulent depositional conditions 3 vertical alternation between particle aggregates and vesicular facies is interpreted as the product of many flow pulses each of which involved deposition of a single particle aggregate and vesicular ash layer it is possible that the different facies record stages in a continuum of flow processes the deposits formed are dependent on the presence thickness and behaviour of a high concentration non turbulent boundary layer at the base of the flow the end members of this process are a flows that transported and deposited material from a non turbulent flow regime and b flows that transported and deposited material from a turbulent flow regime

experimental study of dense pyroclastic density currents using sustained gas fluidized granular flows we present the results of laboratory experiments on the behaviour of sustained dense granular flows in a horizontal flume in which high gas pore pressure was maintained throughout the flow duration by continuous injection of gas through the flume base the flows were fed by a sustained 0 5 30 s supply of fine 75 15 m particles from a hopper the falling particles impacted an impingement surface at concentrations of 3 to 45 where they densified rapidly to generate horizontally moving dense granular flows when the gas supplied through the flume base was below the minimum fluidization velocity of the particles i e aerated flow conditions three flow phases were identified i an initial dilute spray of particles travelling at 1 2 m s 1 followed by ii a dense granular flow travelling at 0 5 1 m s 1 then by iii sustained aggradation of the deposit by a prolonged succession of thin flow pulses the maximum runout of the phase 2 flow was linearly dependent on the initial mass flux and the frontal velocity had a square root dependence on mass flux the frontal propagation speed during phase 3 had a linear relationship with mass flux the total mass of particles released had no significant control on either flow velocity or runout in any of the phases high frequency flow unsteadiness during phase 3 generated deposit architectures with progradational and retrogradational packages and multiple internal erosive contacts when the gas supplied through the flume base was equal to the minimum fluidization velocity of the particles i e fluidized flow conditions the flows remained within phase 2 for their entire runout no deposit formed and the particles ran off the end of the flume sustained granular flows differ significantly from instantaneous flows generated by lock exchange mechanisms in that the sustained flows generate by prolonged progressive aggradation deposits that are much thicker than the flowing layer of particles at any given moment the experiments offer a first attempt to investigate the physics of the sustained pyroclastic flows that generate thick voluminous ignimbrites

deposits and physical properties of pyroclastic density currents during complex subplinian eruptions the ad 472 pollena eruption of somma vesuvius italy small scale pyroclastic density currents pdcs associated with the ad 472 pollena eruption of somma vesuvius italy were generated by both magmatic and phreatomagmatic explosive fragmentation the resulting deposits were emplaced under flow boundary conditions dominated by varying combinations of grain interaction fluid escape and traction processes stratigraphic and lithofacies analysis of these pdcs offers a new perspective on the en masse versus progressive aggradation debate for pdc deposition in particular the analyses indicate that pdcs were density stratified with a basal underflow dominated by grain interactions the underflows comprised trains of self organized granular pulses of variable thickness and magnitude depending on the overall particle concentration and fluid turbulence a change in gradient between the upper and lower slopes of the volcano promoted deposition and the different pulses aggraded sequentially stepwise in this model each pulse stops en masse and the whole deposit aggrades progressively particle concentration density mean velocity and flow height were assessed for the studied pdcs using differaent methods for massive and stratified deposits the calculated mobility of the flows was 0 2 to 0 3 in the expected range for small scale pdcs

comments on turbulent boundary layer shear flows as an approximation of base surges at campi flegrei southern italy by dellino et al 2004 dellino et al 2004 use some principles of sediment mechanics to estimate the flow behaviour of base surge deposits at astroni and agnano monte spina in southern italy to do so they assume quasisteady incompressible shear flow for the basal part of the surges hypothesizing that at the transition between a basal inversely graded bed and an overlying finely laminated bed the shields criterion and suspension criterion are simultaneously valid they combine the two criteria to get

an ignimbrite veneer deposit the trail marker of a pyroclastic flow abstract the term ignimbrite veneer deposit ivd is proposed for a new kind of pyroclastic deposit which is found associated with and passes laterally into taupo ignimbrite of valley pond type in new zealand it forms a thin layer mantling the landscape over 15 000 km 2 and is regarded as the deposit from the trailing tail of a pyroclastic flow where a relaxation of shear stress favoured the deposition of the basal part of the flow the ivd differs little in grain size from the associated ignimbrite but it shows a crude internal stratification attributed to the deposition of a succession of layers one after the passage of each pulse of the pyroclastic flow it locally contains laterally discontinuous lenses of coarse pumice lee side lenses on the far vent side of topographic obstacles in nearvent exposures the taupo ivd shows lensoid and cross stratified bed forms even where it stands on a planar surface attributed to deposition from a flow travelling at an exceedingly high velocity an ivd can be distinguished from a poorly sorted pyroclastic fall deposit because the beds in it show more rapid lateral variations in thickness it may show a low angle cross stratification and it contains carbonised wood from trees not in the position of growth from the deposit of a wet base surge because it lacks vesicles and strong antidune like structures and contains carbonised vegetation and from a hot and dry pyroclastic surge deposit because it possesses a high content of pumice and fines the significance of an ivd is that it records the passage of a pyroclastic flow where the flow itself has moved farther on

origin and stratigraphy of phreatomagmatic deposits at the pleistocene sinker butte volcano western snake river plain idaho abstract sinker butte is the erosional remnant of a very large basaltic tuff cone of middle pleistocene age located at the southern edge of the western snake river plain phreatomagmatic tephras are exposed in complete sections up to 100 m thick in the walls of the snake river canyon creating an unusual opportunity to study the deposits produced by this volcano through its entire sequence of explosive eruptions the main objectives of the study were to determine the overall evolution of the sinker butte volcano while focusing particularly on the tephras produced by its phreatomagmatic eruptions toward this end twenty three detailed stratigraphic sections ranging from 20 to 100 m thick were examined and measured in canyon walls exposing tephras deposited around 180 of the circumference of the volcano three main rock units are recognized in canyon walls at sinker butte a lower sequence composed of numerous thin basaltic lava flows an intermediate sequence of phreatomagmatic tephras and a capping sequence of welded basaltic spatter and more lava flows we subdivide the phreatomagmatic deposits into two main parts a series of reworked mostly subaqueously deposited tephras and a more voluminous sequence of overlying subaerial surge and fall deposits most of the reworked deposits are gray in color and exhibit features such as channel scour and fill planar stratification high and low angle cross stratification trough cross stratification and bouma turbidite sequences consistent with their being deposited in shallow standing water or in braided streams the overlying subaerial deposits are commonly brown or orange in color due to palagonitization they display a wide variety of bedding types and sedimentary structures consistent with deposition by base surges wet to dry pyroclastic fall events and water saturated debris flows proximal sections through the subaerial tephras exhibit large regressive cross strata planar bedding and bomb sags suggesting deposition by wet base surges and tephra fallout medial and distal deposits consist of a thick sequence of well bedded tephras however the cross stratified base surge deposits are thinner and interbedded within the fallout deposits the average wavelength and amplitude of the cross strata continue to decrease with distance from the vent these bedded surge and fall deposits grade upward into dominantly fall deposits containing 75 95 juvenile vesiculated clasts and localized layers of welded spatter indicating a greatly reduced water melt ratio overlying these dryer deposits are massive tuff breccias that were probably deposited as water saturated debris flows lahars the first appearance of rounded river gravels in these massive tuff breccias indicates downward coring of the diatreme and entrainment of country rock from lower in the stratigraphic section the wetter nature of these deposits suggests a renewed source of external water the massive deposits grade upward into wet fallout tephras and the phreatomagmatic sequence ends with a dry scoria fall deposit overlain by welded spatter and lava flows field observations and two new 40 ar 39 ar incremental heating dates suggest the succession of lavas and tephra deposits exposed in this part of the snake river canyon may all have been erupted from a closely related complex of vents at sinker butte we propose that initial eruptions of lava flows built a small shield edifice that dammed or disrupted the flow of the ancestral snake river the shift from effusive to explosive eruptions occurred when the surface water or rising ground water gained access to the vent as the river cut a new channel around the lava dam water levels dropped and the volcano returned to an effusive style of eruption

saltation threshold for pyroclasts at various bedslopes wind tunnel measurements abstract pyroclastic density currents represent one of the most destructive hazards associated with explosive volcanism this destructive nature does not only urge the need for but also prevents the obtainment of in situ measurements of their physical characteristics the resulting deposits offer however evidence of the physics of their sedimentation phase deposits of dilute pyroclastic density currents frequently exhibit repeated cycles of deposition and erosion yielding insights into the turbulent shearing along the ground the utilization of such field observations can be greatly enhanced by the calibration of physical properties of such flows under well constrained laboratory conditions here wind tunnel measurements were performed using pyroclastic particles the saltation threshold and surface roughness length were calculated for wind above a pyroclastic bed the results serve as an aid in linking field observations to quantitative values of turbulent shear at the base of a flow scoria and pumice particles were investigated as a function of grain size 1 fractions between 0 125 and 4 mm as well as the influence of bedslope 20 to 25 in 10 steps the results point to the dominant control of density grain size and contrary to previous assumptions differ moderately from results obtained for round beads properly utilized the dataset enables the establishment of a link between the grain size of natural deposits and the shearing extant during their emplacement depending on the type of sedimentary structure observed in the field the saltation threshold can be used as a minimal or a maximal shearing limit during emplacement of dilute pyroclastic density current deposits stoss aggrading laminations likely involve the saltation threshold as an upper limit whereas for truncation events it must have been overcome the effect of particle concentration within the flow a critical parameter for pyroclastic density currents and the extent of validity of the data are discussed

the leaking bucket of a maldives atoll implications for the understanding of carbonate platform drowning abstract seismic and multibeam data as well as sediment samples were acquired in the south male atoll in the maldives archipelago in 2011 to unravel the stratigraphy and facies of the lagoonal deposits multichannel seismic lines show that the sedimentary succession locally reaches a maximum thickness of 15 20 m above an unconformity interpreted as the emersion surface which developed during the last glacial sea level lowstand such depocenters are located in current protected areas flanking the reef rim of the atoll or in infillings of karst dolinas much of the 50 m deep sea floor in the lagoon interior is current swept and has no or very minor sediment cover erosive current moats line drowned patch reefs whereas other areas are characterized by nondeposition karst sink holes blue holes and karst valleys occur throughout the lagoon from its rim to its center lagoonal sediments are mostly carbonate rubble and coarse grained carbonate sands with frequent large benthic foraminifers halimeda flakes red algal nodules mollusks bioclasts and intraclasts some of them glauconitic as well as very minor ooids finer grained deposits locally are deposited in current protected areas behind elongated faros i e small atolls which are part of the rim of south male atoll the south male atoll is a current flushed atoll where water and sediment export with the open sea is facilitated by the multiple passes dissecting the atoll rim with an elevated reef rim and tower like reefs in the atoll interior it is an example of a leaky bucket atoll which shares characteristics of incipiently drowned carbonate banks or drowning sequences as known from the geological record

turbulent boundary layer shear flows as an approximation of base surges at campi flegrei southern italy abstract small volume base surge deposits of astroni and agnano monte spina eruptions at campi flegrei consist of sets of beds formed by the passage of multiple pyroclastic density currents each set is made by a fining upward sequence consisting from base to top of 1 an inversely graded layer made of bombs and lapilli 2 a finely laminated layer with wavy bedforms made of coarse ash and fine lapilli 3 a structureless fine ash bed lack of discontinuities at layer contacts allows us to hypothesize that each set of beds represents the passage of one pyroclastic density current the sequence formed as a function of the decreasing particle transportation capability of the base surge during the waxing sustained waning phases of flowage we make the simplifying assumptions that the flow on passing from the deposition of the inversely graded layer to the deposition of the finely laminated layer had low particle concentration was quasi steady and incompressible by such assumptions some of the equations that regulate sediment mechanics are applicable to the base surges of campi flegrei to reconstruct flow density and shear velocity in between the deposition of the two layers structural and textural features of deposits stress the importance of shear and turbulence during transportation and suggest that the turbulent boundary layer shear flow is a good approximation to such pyroclastic density currents we used the coles formula to reconstruct velocity as a function of flow height during the formation of the finely laminated layer of astroni and agnano monte spina base surges the methodology we used allows the reconstruction of flow parameters just at a particular point in time and space of base surge evolution and does not provide the complete picture of the flow history of pyroclastic density currents nevertheless flow velocity and density data as obtained with the proposed approach point to dynamic pressure values that are compatible with those reported for similar currents on other volcanoes we conclude therefore that the results obtained give approximate but helpful information on the nature of such hazardous events at campi flegrei

late ordovician climbing dune cross stratification a signature of outburst floods in proglacial outwash environments climbing dune scale cross statification is described from late ordovician paraglacial successions of the murzuq basin sw libya this depositional facies is comprised of medium grained to coarse grained sandstones that typically involve 0 3 to 1 m high 3 to 5 m in wavelength asymmetrical laminations most often stoss depositional structures have been generated with preservation of the topographies of formative bedforms climbing dune cross stratification related to the migration of lower flow regime dune trains is thus identified related architecture and facies sequences are described from two case studies i erosion based sandstone sheets and ii a deeply incised channel the former characterized the distal outwash plain and the fluvial subaqueous transition of related deltaic wedges while the latter formed in an ice proximal segment of the outwash plain in erosion based sand sheets climbing dune cross stratification results from unconfined mouth bar deposition related to expanding sediment laden flows entering a water body within incised channels climbing dune cross stratification formed over eddy related side bars reflecting deposition under recirculating flow conditions generated at channel bends associated facies sequences record glacier outburst floods that occurred during early stages of deglaciation and were temporally and spatially linked with subglacial drainage events involving tunnel valleys the primary control on the formation of climbing dune cross stratification is a combination between high magnitude flows and sediment supply limitations which lead to the generation of sediment charged stream flows characterized by a significant relatively coarse grained sand sized suspension load concentration with a virtual absence of very coarse to gravelly bedload the high rate of coarse grained sand fallout in sediment laden flows following flow expansion throughout mouth bars or in eddy related side bars resulted in high rates of transfer of sands from suspension to the bed net deposition on bedform stoss sides and generation of widespread climbing dune cross stratification the later structure has no equivalent in the glacial record either in the ancient or in the quaternary literature but analogues are recognized in some flood dominated depositional systems of foreland basins

an unusually energetic basaltic phreatomagmatic eruption using deposit characteristics to constrain dilute pyroclastic density current dynamics abstract multiple highly erosive base surges of the table rock complex tuff ring trc2 oregon produced dune bedded deposits with crest to crest bedform wavelengths up to 200 m which are amongst the largest ever recognized in the deposits of pyroclastic density currents here we use bedform wavelength surmounted obstacles and a large chute and pool feature to estimate near source velocities 118 233 m s 1 lower bound velocities at radial distances of 1 6 2 and 4 7 km from source 34 29 and 20 m s 1 respectively and corresponding column collapse heights up to 2 8 km this paper represents one of the few studies that attempt to quantify flow characteristics such as emplacement velocities at different distances from source eruption column collapse height and eruptive energy based on deposit characteristics

transport and deposition of a pyroclastic surge across an area of high relief the 18 may 1980 eruption of mount st helens washington the 08 32 pdt 18 may 1980 mount st helens eruption began as an explosion blast that produced a laterally flowing pyro clastic current blast surge it moved outward in a 180 arc at supersonic speeds and then slowed to subsonic speeds supersonic flow severely eroded the ground and produced a ground layer formerly layer a0 above the ground layer lies the deposit derived from the blast surge the blast surge deposits formed in three continuous and concurrent stages the first stage was movement of the transport system that is the blast surge carrying fragments most of the distance from the vent the next two stages involved the depositional system this system developed from rapidly accumulating sediment by gravity segregation at the base of the blast surge two stages of sediment gravity flows with independent flow regimes formed sequentially in rapid succession from the moving blast surge each moved downhill due to gravity independently of the generating blast surge that was driven by explosive expansion units repeated in some sections are deposits derived from divergent and crossing surge lobes caused by irregular topography locally ponded massive deposits in valleys and depressions formed from gas rich pyroclastic flows and originated in two ways 1 topographic blocking by steep volcano facing slopes blocking includes diversion or damming of the lower elevation high density part of the blast surge that could not surmount high ridges 2 also occurring to form valley ponds was drainage of just deposited or almost deposited material from steep slopes 30 35 moving downhill in all slope directions into separate valleys or depressions distances traveled by the blast surge and rates at which coarsest fragments decrease laterally are related to the topographic grain surge runout was farther where flow directions paralleled topographic trends and least where trends were at right angles to the flow obstacle interference depositional patterns thinning thickening rapid fine to coarse facies changes caused a spread in median diameter values that are as great in local areas near the volcano as they are over the width of the devastated region

upper mantle discontinuity structure beneath east anatolian plateau turkey from receiver functions abstract imaging of seismic velocity discontinuities with receiver functions suggests a complex mantle structure beneath the east anatolian plateau by stacking all the receiver functions as a function of ray parameter we examined the moveout characteristics of the data set and tested different velocity models to fit the arrival times of converted phases from the moho and the lithosphere asthenosphere boundary and their multiples the best fitting model shows anomalously thin crust 40 km and lithosphere 65 km and low uppermost mantle velocities causing systematic delays on later discontinuity arrivals these findings are consistent with the high plateau supported by partially molten and buoyant asthenosphere the upper mantle discontinuities are all visible in different frequencies 0 15 and 0 4 hz especially the 410 km discontinuity does not show frequency dependence in amplitude and suggests a sharpness of 10 km or less spatial variations of the 410 and 660 km discontinuities reveal anticorrelated topography and distinct zones of diminished amplitude associated with a detached slab and delaminated fragments of lithospheric mantle in the south the slab becomes deeper and horizontally deflected towards the east suggesting westward migration of slab detachment and resistance to slab penetration at the 660 km discontinuity in the north the mantle transition zone is characterized by two spatially limited lithospheric fragments with complex geometry at the center of the study area the transition zone is thin 230 km suggesting excess temperature of 120 c this warm mantle anomaly most likely is due to hot material trapped within the upper mantle between slab in the south and delaminated fragments in the north

base surge deposits eruption history and depositional processes of a wet phreatomagmatic volcano in central anatolia cora maar abstract cora maar is a quaternary volcano located to the 20 km northwest of mount erciyes the largest of the 19 polygenetic volcanic complexes of the cappadocian volcanic province in central anatolia cora maar is a typical example of a maar diatreme volcano with a nearly circular crater with a mean diameter of c 1 2 km and a well bedded base surge dominated maar rim tephra sequence up to 40 m in thickness having a diameter depth ratio d d of 12 cora is a relatively mature maar compared to recent maar craters in the world cora crater is excavated within the andesitic lava flows of quaternary age the tephra sequence is not indurated and consists of juvenile clasts up to 70 cm non juvenile clasts up to 130 cm accretionary lapilli up to 1 2 cm in diameter and ash to lapilli sized tephra base surge layers display well developed antidune structures indicating the direction of the transport both progressive and regressive dune structures are present within the tephra sequence wavelength values increase with increasing wave height and with large wavelength and height values cora tephra display similarities to taal and laacher see base surge deposits impact sags and small channel structures are also common lateral and vertical facies changes are observed for the dune bedded and planar bedsets according to granulometric analyses cora maar tephra samples display a bimodal distribution with a wide range of md values characteristic for the surge deposits very poorly sorted bimodal ash deposits generally vary from coarse tail to fine tail grading depending on the grain size distribution while very poorly sorted lapilli and block rich deposits display a positive skewness due to fine tail grading