large scale experiments on the mechanics of pyroclastic flows design engineering and first results 1 a newly designed apparatus for experimental studies of pyroclastic flows consists of a cylindrical conduit that is filled with samples of natural volcanic products tephra blowing nozzles in the base plate of the conduit are connected to a volume of highly pressurized gas opening of fast solenoid valves results in impulse like coupling of the released gas to the sample the system was designed so that the range of mechanical energy transferred to the particle mass in the conduit reflects the mechanical energy observed and measured during fragmentation experiments with melts of similar composition depending on the specific mechanical energy sme of the system which results from pv m where p is gas overpressure i e pressure atmospheric v is gas volume and m is sample mass different behaviors are observed if sme 2 6 kj kg a dilute plume develops and particles are sedimented by fallout exclusively if sme 106 implying that flows are fully turbulent and that particle coupling to gas turbulence of natural pyroclastic flows is replicated by the experiments the measured shear current velocities are proportional to the impact mass flow rate i e the product of mixture density and impact velocity experimental data and grain size analysis of the produced particle deposits suggest that the scale of the experiment is large enough to reproduce the transport dynamics of natural pyroclastic flows

a method for the calculation of the impact parameters of dilute pyroclastic density currents based on deposit particle characteristics 1 a new method for the reconstruction of the physical characteristics of dilute pyroclastic density currents is introduced the velocity density and particle volumetric concentration profiles of the stratified current are calculated together with the profile of dynamic pressure which is a useful parameter for checking resistance of buildings the model links turbulent boundary layer shear flow theory with particle coupling to gas turbulence the working procedure starts with the recognition in the field of the fining upward sequence of layers formed during the time integrated depositional history of an individual current distinct processes of particle transportation and deposition are associated with the different particle modes composing the bedset the system of equations for the solution of the fluid dynamic parameters is implemented in two alternative ways the first one uses data of particles coming from both the basal coarse layer and the overlying laminated layer of the bedset the second uses features of two distinct components of the laminated layer a statistical test is performed for checking model results against experimental data of actual particles model calculations give the average solution as well as solutions corresponding to a range of 68 of probability around the average value the maximum solution can be considered as a safety value for impact parameters the methodology is applied to deposits of recent eruptions of vesuvius and campi flegrei in italy and results are discussed in terms of hazard and expected damage

the analysis of the influence of pumice shape on its terminal velocity 1 pumice particles represent the basic ingredient of many large explosive eruptions and form as a result of magma fragmentation inside the conduit at the onset of eruption fragmental pumices are expelled at high velocity from the crater by the overpressure of gas liberated on explosion the resulting multiphase flow is forced through atmosphere by a variety of transportation mechanisms and pumices eventually decouple from the gas flow settling down to form pyroclastic deposits here we propose new experimental data of terminal velocity together with a quantitative shape analysis of a wide range of pumice particles the resulting model allows predicting the terminal velocity of pumice by means of easily measured particle characteristics with an average error of 12 which compares favourably with previous models

volcaniclastic sediments process interaction and depositional setting of the mio pliocene miura group se japan abstract the mid miocene to early pliocene 14 3 ma miura group exposed on the miura and boso peninsulas around tokyo bay in southern honshu comprises at least a 2 km thick succession of volcaniclastic sediments these accumulated in a forearc basin or basins on the eastern side of a proto izu bonin arc prior to and during accretion of the miura block onto the honshu arc and its subsequent dextral transcurrent eduction the basin fill shows a complex admixture of facies that result from multiple processes which operated on a relatively shallow slope basin the main facies groups include dark coloured mostly coarse grained scoriaceous beds pale coloured muddy silty pumiceous bioturbated sediment yellowish and whitish coloured tuffaceous horizons and chaotic slump debris flow and injection units many of the facies and beds in the older misaki formation can be readily interpreted as the result of a direct pyroclastic fall through the air and water b downslope resedimentation via turbidity currents and related processes in some cases derived from subaqueous base surge pyroclastic flow or vertical density currents c hemipelagic settling commonly under the influence of thermohaline bottom currents in other cases composite beds are believed to result from the interaction of these processes in more complex events the younger hasse formation is dominated by more shallow water marine currents and storm events and in parts by fluvial or proximal fan delta processes the distribution of these different facies illustrates the evolution and shallowing of the miura boso forearc basins during accretion and emplacement

conditions for formation of massive turbiditic sandstones by primary depositional processes it can be shown from bedform stability diagrams that each turbidity current regardless of whether its density is low or high or whether its history is simple e g surging flow or complex e g sustained flow should pass through a final waning phase in which flow conditions are prone to bedform development yet many thick deep marine sandstones of supposed turbidity current origin are totally devoid of tractional structures such as plane parallel lamination and ripple cross lamination tdure a mathematical model for the calculation of duration of sediment deposition from high density turbidity currents hdtcs is used to show that the absence of bouma tb and tc divisions cannot be explained by rapid temporal flow deceleration only the model calculations imply that discernible tractional structures are formed even at residence times within the upper stage plane bed and ripple stability regimes of several seconds and several tens of seconds respectively therefore other explanations have to be sought for the absence of lamination in turbiditic massive sandstones these include incompatible grain size post depositional removal or obstruction of lamination by bioturbation and liquefaction and erosion of laminated divisions followed by rapid burial

the interaction of ash flows with ridges using both laboratory experiments and theoretical models we examine the different flow regimes that may develop when an ash flow encounters a ridge for very small ridges all the flow may pass over the ridge for intermediate size ridges the flow may be partially blocked with a fraction of the flow reflected upstream as a travelling bore in this case the remainder of the flow which does pass over the ridge is hydraulically controlled at the ridge crest finally if the ridge is sufficiently high then the flow will be totally blocked new laboratory experiments show that the sedimentation patterns associated with these flow regimes may be very different most importantly flows that involve partial blocking and the formation of upstream propagating bores display enhanced sedimentation upstream of the ridge analogous to valley ponded and caldera fill deposits in contrast under some circumstances if the flow is able to scale a ridge the deposit may be relatively unaffected by the presence of the ridge the minimum ridge height that leads to total blocking of the flow increases with mass eruption rate and has a complex variation with distance from the source in a one dimensional channel the minimum ridge height that causes blocking increases with distance downstream this is because the flow becomes less dense through sedimentation of particles and entrainment of air and so requires less energy to scale a ridge of a particular height in axisymmetric flow the minimum ridge height initially decreases with distance downstream as the flow spreads radially but subsequently increases as the flow becomes less dense through sedimentation and entrainment a new quantitative model of dilute ash flows propagating over ridges indicates that flows with mass fluxes in excess of 108 109 kg s can partially scale barriers as high as 1000 m at distances of tens of kilometres from the source whereas smaller flows are likely to be totally blocked by such an obstacle our results shed new insight on the possible long range transport mechanism of several large flows including the ata fisher and aniakchak pyroclastic flows

an experimental investigation of density stratified inertial gravity currents turbidity currents and pyroclastic density currents may originate as stratified flows or develop stratification during propagation analogue density stratified laboratory currents are described using layers of salt solutions with different concentrations and depths to create the initial vertical stratification the evolving structure of the flow depends on the distribution of the driving buoyancy between the layers b proportional to the layer volumes and densities and their density ratio q when the lower layer contains more salt than the upper layer and so has a greater proportion of the driving buoyancy b 0 ae5 this layer travels to the nose of the current by mixing into the back of the head along the body wake density interface to produce a homogeneous flow q fi 1 or overtaking leading to streamwise stratification q fi 0 timescales describing the mixing between the layers and the streamwise separation of the layers are used to understand these flow behaviours and are in accordance with the experimental observations distance time measurements of the flow front show that strongly stratified flows initially travel faster than weakly stratified flows but during their later stages they travel more slowly in natural flows that are stratified in concentration and grain size internal features such as stepwise grading gradual upward fining and reverse grading could be produced depending on b and q stratification may also be expected to affect interactions with topography and overall fan architecture

sedimentation by river induced turbidity currents field measurements and interpretation turbidity currents initiated from spring runoffs of an influent river were observed in the upper region of a reservoir in hokkaido japan by measuring water temperature velocity and suspended sediment concentration their profiles offer some physical parameters for the sedimentary conditions assuming the turbidity currents to be quasi uniform the bottom sediment deposited by the turbidity currents was then collected by a portable core sampler the bottom sediment consists of more than 90 silt and clay and thus offers a hydraulically smooth bed for shear flow a plane bed as a bed configuration was formed on the reservoir bed probably because of the low shear velocity and small grain size of sediment using a graphic method with log normal probability paper the bottom sediment is divided into several overlapping log normal subpopulations grain size analysis indicates that the bottom sediment may be regarded as cohesionless criteria for complete deposition of transported grains can then be incorporated into the extended shields diagram giving the minimum shear stress to erode bottom sediment applying the new diagram to the grain size distribution of the bottom sediment it is suggested that each of the log normal subpopulations was deposited in each of four different modes of deposition i e traction saltation or intermittent suspension suspension and suspension under equilibrium the last mode may be observed under a sedimentary condition where upward flux of suspended sediment by eddy diffusion is almost equal to its depositional flux due to gravity the mean and critical grain sizes for bottom sediment and each of the corresponding subpopulations decrease consistently with an increase of fd2 log10re fd is the densimetric froude number and re is the flow reynolds number correlates inversely with shear velocity which bears a linear relationship to mean velocity these results lead to the conclusion that relatively fine suspended sediment is deposited as a result of decreasing bottom friction with a relative decrease of turbulent energy

quantifying velocity and turbulence structure in depositing sustained turbidity currents across breaks in slope two dimensional experiments investigating sediment transport and turbulence structure in sustained turbidity currents that cross breaks in slope are presented as analogue illustrations for natural flows the results suggest that in natural flows turbulence generation at slope breaks may account for increased sand transport into basins and that the formation of a hydraulic jump may not be necessary to explain features such as the occurrence of submarine plunge pools and the deposition of coarser grained beds in the bottomsets of gilbert type fan deltas experimental flows were generated on 0 3 6 and 9 slopes of equal length which terminated abruptly on a horizontal bed two component velocities were measured on the slope at the slope break and downstream of the slope break flows were depositional and non uniform visibly slowing and thickening with distance downstream one dimensional continuous wavelet transforms of velocity time series were used to produce time period variance maps peaks in variance were tested against a background red noise spectrum at the 95 level a significant period banding occurs in the cross wavelet transform at the slope break attributed to increased formation of coherent flow structures kelvin helmholtz billows variance becomes distributed at progressively longer periods and the shape of the bed normal velocity spectral energy distribution changes with distance downstream this is attributed to a shift towards larger turbulent structures caused by wake stretching mean velocity reynolds shear stress and turbulent kinetic energy profiles illustrate the mean distribution of turbulence through the currents a turbulent kinetic energy transfer balance shows that flow non uniformity arises through the transfer of mean streamwise slowing to mean bed normal motion through the action of reynolds normal stresses net turbulence production through the action of normal stresses is achieved on steeper slopes as turbulence dissipation due to mean bed normal motion is limited at the slope break an imbalance between the production and dissipation of turbulence occurs because of the contrasting nature of the wall and free shear boundaries at the bottom and top of the flows respectively a rapid reduction in mean streamwise velocity predominately affects the base of the flows and steeper proximal slope flows have to slow more at the break in slope the increased turbulent kinetic energy limited bed normal motion and strong mixing imposed by steep proximal slopes means rapid slowing enhances turbulence production at the break in slope by focusing energy into coherent flow structures at a characteristic period thus mean streamwise slowing is transferred into turbulence production at the slope break that causes increased transport of sediment and a decrease in deposit mass downstream of the slope break the internal effects of flow non uniformity therefore can be separated from the external influence of the slope break

late pliocene volcaniclastic products from southern apennines distal witness of early explosive volcanism in the central tyrrhenian sea two volcaniclastic successions intercalated in pliocene basinal clays from the southern apennines have been analysed to determine their provenance and their relationship with the geodynamic evolution of the western mediterranean the studied deposits are exclusively made up of ashy pyroclasts dominated by fresh acidic to intermediate glass mostly in the form of shards pumice fragments and groundmass fragments with vitrophyric texture crystals include pl opx cpx hbl and rare bt sedimentological features suggest that the volcanic material accumulated near the basin margin by primary fallout processes and was later remobilized by density currents 40 ar 39 ar geochronology allowed dating of one succession at 2 24 0 06 ma corresponding to the late pliocene composition of the volcaniclastic material is typical of a transitional high k calc alkaline series the age and chemical composition constrain the provenance of the volcaniclastic rocks from the southern tyrrhenian domain here volcanic centres were active during pliocene time approximately at the northern end of a volcanic arc formed before the opening of the southernmost part of the sea this paper shows that a detailed study of volcaniclastic products from the southern apennines and calabria can be very useful in collecting new pieces of information on the eruption history of the southern tyrrhenian domain since they record additional data not available from the study of exposed volcanic edifices

turbidity current sediment waves on the submarine slopes of the western canary islands two sediment wave fields have been identified on the flanks of the western canary islands of la palma and el hierro using a high quality 2 d and 3 d dataset that includes geosea and tobi imagery 3 5 khz profiles and short sediment cores the la palma sediment wave field covers some 20 000 km2 of the continental slope and rise and consists of sediment waves with wave heights of up to 70 m and wavelengths of up to 2 4 km the wave crestlines have a complex morphology with common bifurcation and a clear sinuosity waves have migrated upslope through time cores recovered from the wave field contain volcaniclastic turbidites interbedded with pelagic hemipelagic layers the wave field is interpreted as having formed beneath unconfined turbidity currents a simple previously published two layer model is applied to the waves revealing that they formed beneath turbidity currents flowing at 10 100 cm s 1 with a flow thickness of 60 400 m and a sediment concentration of 26 427 mg l the el julan sediment wave field lies within a turbidity current channel on the southwest flank of el hierro the sediment waves display wave heights of about 6 m and wavelengths of up to 1 2 km the waves are migrating upslope and migration is most rapid in the centre of the channel where the flow velocity is highest this wave field has been formed by channelised turbidity currents originating on the flanks of el hierro

laboratory experiments and numerical simulation of sediment wave formation by turbidity currents abstract this paper presents the results of laboratory experiments and numerical simulations carried out in order to understand the mechanism of sediment wave formation by turbidity currents experimental turbidity currents were generated in a 10 m long laboratory flume so as to investigate the topographic effects of slope and ridges on turbidite deposition the results indicate that preferential deposition occurs on the upstream side of the ridges this preferential deposition is considered to result in possible upstream migration of the topography a common feature of deep sea sediment waves provided that such deposition were repeated the preferential deposition occurred under a subcritical turbidity current implying that antidune flow conditions 0 844

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

how coriolis forces can limit the spatial extent of sediment deposition of a large scale turbidity current abstract the rotation rate of the earth controls the deposition scale of turbidity currents when their flow transit time is comparable to one day using laboratory experiments it is shown that the maximum length scale l of deposition is set by the scale where the rossby number of the current is equal to one the dimensionless rossby number is defined as ro u fl where u is a depth averaged velocity of the turbidity current and f is the coriolis parameter in these new laboratory experiments the coriolis parameter is varied by changing the platform rotation rate and there is good agreement between experimental observations and the prediction that the extent of the deposition scales as l 4 1 4 g v f 1 4 where g is the reduced gravity and v the initial volume of the sediment laden material this scaling implies that the depositional patterns will vary with latitude and that at higher latitudes one would expect smaller thicker deposits using this scaling implies that the 200 400 km spatial extent of the turbidite arising during the 1929 grand banks earthquake had ro 1 and so coriolis forces controlled the length scale of the turbidite deposition

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

stratified flow in pyroclastic surges stratified flow theory is applied to pyroclatic surges in an effort to gain insight into transport dynamics during explosive eruptions particle transport is assumed to be by turbulent suspension and calculations contained herein show that this is likely for many cases including the 18 may 1980 blast at mount st helens the discussion centers on the rouse number pn which represents a ratio of particle settling velocity to scale of turbulence the brunt vaisala frequency n which is the maximum possible frequency of internal waves the froude number fr representing the ratio of inertial forces to gravitational forces and the richardson number ri a ratio of buoyant restoring forces to turbulent mixing forces the velocity or flow power dependence of bed form wavelength in surge deposits is related to a velocity dependence of wavelength of internal waves in the turbulent surge this produces a decrease in dune wavelength with increasing distance from vent migration direction of bed forms is related tofr as it is defined for a continuously stratified flow proximal to distal facies variations in surge deposits reflect increasingpn andri as the flows move away from their sources this produces the progression from sandwave to massive to planar facies with increasing distance from vent where the long axis of topography is at low angles to the flow direction massive facies in topographic lows may from concurrently with sandwave facies on highs due to the higher particle concentration in the lows where long axis of topography is at high angles to flow direction denser lower parts of the surge may be dammed or blocked blocked material tends to form massive flows that may move down slope independent of the overriding surge a model incorporating turbulent transport stratified flow and time evolution of pyroclastic surges is proposed for deposits which have been attributed to both pyroclastic flow and pyroclastic surge transport by various workers during the initial high energy waxing phase of the eruptive event pn is sufficiently low that only coarse but poorly sorted material is deposited to form relatively coarse bottom layers as the event wanes remaining finer material is deposited through a thin bed load to produce overlying bedded and cross bedded veneer deposits throughout most of the event blocking occurs to produce relatively thick and massive deposits in valley bottoms

reconciling pyroclastic flow and surge the multiphase physics of pyroclastic density currents abstract two end member types of pyroclastic density current are commonly recognized pyroclastic surges are dilute currents in which particles are carried in turbulent suspension and pyroclastic flows are highly concentrated flows we provide scaling relations that unify these end members and derive a segregation mechanism into basal concentrated flow and overriding dilute cloud based on the stokes number s t the stability factor t and the dense dilute condition d d we recognize five types of particle behaviors within a fluid eddy as a function of s t and t 1 particles sediment from the eddy 2 particles are preferentially settled out during the downward motion of the eddy but can be carried during its upward motion 3 particles concentrate on the periphery of the eddy 4 particles settling can be delayed or fast tracked as a function of the eddy spatial distribution and 5 particles remain homogeneously distributed within the eddy we extend these concepts to a fully turbulent flow by using a prototype of kinetic energy distribution within a full eddy spectrum and demonstrate that the presence of different particle sizes leads to the density stratification of the current this stratification may favor particle interactions in the basal part of the flow and d d determines whether the flow is dense or dilute using only intrinsic characteristics of the current our model explains the discontinuous features between pyroclastic flows and surges while conserving the concept of a continuous spectrum of density currents

facies variation of eruption units produced by the passage of single pyroclastic surge currents hopi buttes volcanic field usa abstract base surges are a significant volcanic hazard associated with phreatomagmatism but their transport and depositional dynamics are incompletely understood stratigraphic analysis and lateral tracing of surge eruption units produced by the passage of single base surges from haskie tuff ring hopi buttes volcanic field usa extend to 1 km from the vent and show similar downcurrent changes in lithofacies within 200 m of the vent eruption units are primarily disorganized lapilli tuff that is rich in juvenile pyroclasts at distances 200 m from the vent eruption units transform into a couplet of juvenile rich lapilli that grades upward into bedform bearing ash tuff at distances 900 m upper and lower beds of the eruption unit couplet transform into planar stratified beds at distances 1100 m couplets composing different eruption units are almost entirely composed of planar beds and amalgamate to form almost identical planar bedded ash tuff the collapse of dense eruption columns following explosive magma wet sediment interaction generated turbulent surge currents as the surges traveled away from the haskie vent they expanded and partitioned into a basal juvenile rich traction carpet and overlying accidental rich ash cloud surge near the limit of their runout the surges cooled and lost their pyroclastic load leaving planar beds of accidental rich ash the deposition of multiple surge eruption units with similar lateral and vertical facies transitions and a paucity of interbedded fallout suggests an eruption characterized by recurring phreatomagmatic explosions of similar size and the generation of surges with similar flow characteristics

a fluid dynamic model of volcaniclastic turbidity currents based on the similarity with the lower part of dilute pyroclastic density currents evaluation of the ash dispersal from ash turbidites subaqueous sediment density currents have variable flow structures depending on particle size and volumetric concentration they form both in ocean basins and in volcanic districts where subaqueous density currents are generated by collapse of slope and primary pyroclastic material entering the sea respectively a particular type of such flows is represented by volcaniclastic turbidity currents which form by the subaqueous remobilization of primary pyroclastic deposits in this work a fluid dynamic model of volcaniclastic turbidity currents is presented and applied to the currents that generated the craco southern italy volcaniclastic deposits the deposits consist of thick laminated to massive beds which formed from the remobilization of thin fine ash pyroclastic fall deposits in an area far away from the original volcanic source our model is based on the sedimentological features of the deposits and particle physical characteristics it allows the calculation of velocity thickness and reynolds number of the currents furthermore a relationship between the current thickness laminae thickness and paleo slope angle is obtained which allows to evaluate the thickness of the primary pyroclastic fallout deposits our model can be useful for obtaining an estimation of the primary distal ash dispersal by the characteristics of the associated subaqueous volcaniclastic deposits

constraining the efficiency of turbidity current generation from submarine debris flows and slides using laboratory experiments abstract results from a small set of laboratory experiments are presented here that help further constrain the processes governing the production of turbidity currents from impulsive failures of continental shelf and slope deposits three mechanisms by which sediment can be transferred from a parent debris flow to a less dense turbidity current were observed and quantified these mechanisms are grain by grain erosion of sediment from the leading edge of the parent flow detachment of thin layers of shearing material from the head of the parent flow and turbulent mixing at the head of the parent flow which transfer process dominates an experimental run depends on whether the large dynamic stresses focused on the head of the debris flow are sufficient to overcome a effective yield strength for the parent sediment water mixture and on whether the dynamic stresses are sufficient to induce the turbulent flow of the parent mixture analysis of data from marr et al geol soc am bull 113 2001 1377 and mohrig et al geol soc am bull 110 1998 387 support the use of a shear strength to dynamic stress ratio in constraining necessary critical values for occurrence of the different production mechanisms direct sampling of turbidity currents using racks of vertically stacked siphons was used to measure both the quantity of sediment eroded from the heads of non mixing parent flows and the distribution of particle sizes transported by the developing turbidity currents acoustic backscatter imaging was used to better resolve the internal boundary separating any turbulent mixing zone near the front of a flow from unmodified parent material

primary or secondary distal volcaniclastic turbidites how to make the distinction an example from the miocene of new zealand mahia peninsula north island abstract miocene marine volcaniclastic deposits occur in mahia peninsula north island new zealand and were sedimented in a forearc setting related to the hikurangi trench subduction system these deposits are interbedded with hemipelagic marls and correspond to simple or amalgamated centimetric to metric thick turbiditic sequences volcaniclastic material is mainly composed of vitric particles with crystals quartz plagioclase with minor biotite amphibole pyroxene and oxides which are well represented in the coarse grained fraction the glass shards are mainly rhyolitic in composition three types of volcaniclastic turbidites were distinguished with geochemical data because distinction is impossible on sedimentary characteristics 1 primary monomagmatic turbidites contain both magmatic bubble wall pumice and shards and phreatomagmatic blocky shards with few vesicles and hydroclastically fragmented pyroclasts vitric particles the chemical compositions of the vitric particles and the crystals are very homogeneous suggesting a cogenetic origin these turbidites directly result from unique eruptive events and are probably related to the entrance of hot subaerial pyroclastic flows into the sea which also led to their transformation into subaqueous gravity flows 2 secondary monomagmatic turbidites never contain phreatomagmatic pyroclasts and the glass compositions display a trend from andesites to rhyolites there is a strong linear correlation in the compositions that suggest that the glass particles are derived from the same magma crystals also show a compositional homogeneity these deposits reflect the succession of several eruptions related to a unique magmatic system and result of the reworking of volcaniclastic material after relatively short storage on the shelf 3 secondary multimagmatic turbidites do not display compositional homogeneity of their vitric and mineral components this implies that the volcaniclastic material has been stored during a relative long period on the shelf before remobilization and that this sedimentation records the volcanic activity of multiple magmatic sources consequently it has been possible to distinguish primary volcaniclastic turbidites that are directly related to the volcanic activity from secondary turbidites that result from reworking of previously deposited material on the shelf monomagmatic turbidites can be used as stratigraphic and magmatic markers whereas multimagmatic cannot multimagmatic turbidites however record the evolution of the volcanic arc during longer periods this leads to the conclusion that the mahia peninsula volcaniclastic turbidites distally record the evolution of the source volcanic arc

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

anatomy of a submarine pyroclastic flow and associated turbidity current july 2003 dome collapse soufriere hills volcano montserrat west indies the 12 to 13 july 2003 andesite lava dome collapse at the soufriere hills volcano montserrat provides the first opportunity to document comprehensively both the sub aerial and submarine sequence of events for an eruption numerous pyroclastic flows entered the ocean during the collapse depositing approximately 90 of the total material into the submarine environment during peak collapse conditions as the main flow penetrated the air ocean interface phreatic explosions were observed and a surge cloud decoupled from the main flow body to travel 2 to 3 km over the ocean surface before settling the bulk of the flow was submerged and rapidly mixed with sea water forming a water saturated mass flow efficient sorting and physical differentiation occurred within the flow before initial deposition at 500 m water depth the coarsest components 60 of the total volume were deposited proximally from a dense granular flow while the finer components 40 were efficiently elutriated into the overlying part of the flow which evolved into a far reaching turbidity current

climatic and morphologic relationships of rivers implications of sea level fluctuations on river loads the characteristics of 279 rivers that discharge into the world oceans are analyzed in terms of their basin hydrology river discharge morphometry basin slope and area river length extension of the continental shelf seaward of the river mouth and climate precipitation statistically reliable relationships are found between discharge and basin area and between sediment load and a combined function of basin area and slope these functions are used to demonstrate how river hydrologic features would be strongly influenced by sea level fluctuations particularly under the influence of continental shelf emergence a fall in sea level toward a glacioeustatic lowstand would induce the merging of rivers on the subaerial continental shelf thereby allowing giant rivers to form for example rivers of western europe would reorganize themselves into two or three very large rivers sediment concentration carried by these mega rivers would decrease and thus the number of hyperpycnal plumes generated at river

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

odp leg 107 in the tyrrhenian sea insights into passive margin and back arc basin evolution leg 107 of the ocean drilling program drilled a west northwest east southeast transect of seven sites across the tyrrhenian sea the youngest of the sub basins of the mediterranean sea sites 654 653 652 and 656 document the rifting and subsidence of the sardinia passive continental margin on the upper margin site 654 we cored a classic transgressive sequence subaerial conglomerates overlain by oyster bearing sands overlain by marine marl comparison between the recovered lithologies and seismic reflection profiles suggests that the synrift sediments on the upper margin are tortonian late miocene to messinian latest miocene in age whereas synrift sediments on the lower margin are messinian to pliocene in age during the messinian desiccation of the mediterranean sites 654 and 653 now on the upper sardinian margin apparently occupied a basinal setting where they received nannoplankton bearing clays interbedded with laminated gypsum sites 656 and 652 now on the lower sardinia margin were apparently higher standing during the desiccation event their messinian facies are subaerial and lacustrine respectively we infer from these lines of evidence that tilting and subsidence occurred more than a million years earlier on the upper margin than on the lower margin such diachroneity can be interpreted in terms of migration of the zone of maximum extension above a rolling back subduction zone or in terms of extension of continental crust by shear along a deep detachment fault sites 655 651 and 650 were drilled into two small basalt floored basins of the central and eastern tyrrhenian emplacement of basaltic crust in the central tyrrhenian vavilov basin apparently began more than a million years before emplacement of basaltic crust in the eastern tyrrhenian marsili basin this observation is compatible with previous suggestions that the tyrrhenian has grown southeastward in response to rollback of the down going slab that currently dips northwestward under the toe of italy at the easternmost site high vesicularity of the basalt and benthic foram assemblages in the oldest sediments imply that the basalt erupted in water shallower than 2 500 m it has apparently subsequently subsided to its present depth of 4 100 m below sea level nearly three times as fast as normal subsidence of crust formed at a mid ocean ridge

depositional processes in large scale debris flow experiments this study examines the depositional process and characteristics of deposits of large scale experimental debris flows to 15 m3 composed of mixtures of gravel to 32 mm sand and mud the experiments were performed using a 95 m long 2 m wide debris flow flume that slopes 31 following release experimental debris flows invariably developed numerous shallow 10 cm deep surges sediment transported by surges accumulated abruptly on a 3 runout slope at the mouth of the flume deposits developed in a complex manner through a combination of shoving forward and shouldering aside previously deposited debris and through progressive vertical accretion progressive accretion by the experimental flows is contrary to commonly assumed en masse sedimentation by debris flows despite progressive sediment emplacement deposits were composed of unstratified accumulations of generally unsorted debris hence massively textured poorly sorted debris flow deposits are not emplaced uniquely en masse the depositional

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

emplacement of the taupo ignimbrite by a dilute turbulent flow an ignimbrite is a pumice rich deposit that records the passage of a ground hugging ash flow a pyroclastic flow generated by the collapse of a volcanic eruption column1 geologists study such deposits to reconstruct the parent eruptions and to predict the consequences of future eruptions the 1 800 yr old taupo ignimbrite of new zealand has been interpreted to represent en masse emplacement by an avalanche like flow with a volumetric solids concentration in excess of 30 ref 2 the evidence for this is equivocal however and here we propose the alternative view that the deposit was emplaced by a relatively dilute and turbulent density current3 we present an isothermal hydraulic model the results of which taken together with existing observations suggest that the total flow rate of solids and gas was about 40km3s 1 for around 15 minutes this intense flux resulted in a flow which had a near vent solids concentration of 0 3 by volume was about 1 km thick and travelled outward from the vent with a typical speed of 200 ms 1 in view of the good agreement between predicted and observed radial trends in the taupo deposit we suggest that the origin of other ignimbrites with similar characteristics should be reconsidered