decrypting silicic magma plug fragmentation at azufral crater lake northern andes insights from fine to extremely fine ash morpho chemistry azufral sw colombia is a dangerous silicic volcano hosting a crater lake which serves as an excellent example of an incipient plug disruption through phreatomagmatism we studied the youngest succession of dilute pyroclastic density currents pdcs onlapping the north eastern crater rim scanning electron microscopy coupled with energy dispersive x ray spectroscopy was used to carry out an automated single particle analysis of fine to extremely fine ash we were able to obtain fast and accurate chemical analysis and imaging of 15 098 particles within the 250 63 m and the 63 32 m size ranges the 2d form and roughness parameters were determined for 4895 juvenile glassy particles and validated by 3d micro x ray computer tomography there are two end members of high group 1 and low group 2 roughness juvenile glassy particles group 1 comprises high roughness glass particles with solidity values as low as 0 34 in 2d and 0 33 in 3d and convexity values as low as 0 33 in 2d and 0 26 in 3d group 2 comprises low roughness glass particles with 2d solidity values 0 79 and 3d solidity values typically 0 58 in this group 2d convexity values are 0 68 and 3d convexity values are 0 71 both end members are mostly discriminated by the 2d concavity index 0 14 to 0 77 in group 1 vs 0 05 0 35 in group 2 the remaining group 3 comprises particles of intermediate roughness values in this study we show how an incipient plug developed over a short repose time might be subjected to only a few cycles of vesicle nucleation collapse and densification retaining the characteristics of juvenile glass each glassy juvenile ash type defined by a particular morphology roughness and microtexture can be linked to a density stratified conduit model in azufral the capping and conduit lining dense regions and the permeable zones of the incipient plug likely cracked the newly formed cracks could allow hydraulic forcing caused by external water and induce phreatomagmatic interaction this interaction favoured the fine fragmentation of the plug while enhancing ongoing magmatic processes finally the variations of bulk componentry provided clues on dilute pyroclastic density current transport and physical fractionation processes by secondary fragmentation elutriation and interaction with the crater rim

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

impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 may 1980 mount st helens fallout deposit the 18 may 1980 eruption of mount st helens started with a lateral blast that fed a pyroclastic surge which then uplifted to form a co blast plume thirty minutes later plinian activity started at the vent and fluctuated in intensity for 9 h the resulting fallout deposit documented to 600 km from vent presents some striking features 1 displacement of the overall deposit to the north of the vent 2 a secondary thickness and mass maximum at 300 km from vent 3 a total grain size distribution dominated by fine ash 62 wt of the deposit 63 m and 4 individual grain size distributions that vary dramatically in the crosswind direction from strongly bimodal in the south to skewed unimodal in the north results from a new deconvolution of the individual grain size distributions show that they are a combination of a coarse subpopulation that decreases in size with distance from vent and a constant fine subpopulation with a mean of 15 m relative proportions of each subpopulation vary asymmetrically in the crosswind directions with the fine subpopulation preponderant toward the north and the coarse one dominating the south of the deposit both reach their absolute maxima in mass on the deposit axis componentry analyses of selected samples show that blast derived material is greatly enriched toward the north of the deposit these results indicate that the co blast plume dispersed fine grained material over great distances and dominated the fine subpopulation comparison with reanalysis data of atmospheric wind fields and satellite images of the spreading ash cloud suggests contrasting ash transport and depositional processes for the early co blast plume and the later vent derived plinian plumes the co blast plume is displaced to the north it had a high overshoot height and eastward dispersion via strong winds low in the stratosphere 10 15 km the plinian plumes were lower and dispersed most of the material to the southeast as the direction of high velocity winds shifted just before the late climactic plinian eruptive phase fine ash fine subpopulation was deposited continuously throughout the deposit with an increase of sedimentation rate 300 km from the vent where there is a secondary maximum in the deposit mass and thickness fine ash probably settled by a combination of enhanced sedimentation mechanisms including not only aggregation but also gravitational convective instabilities of cloud base hydrometeor formation and destruction and entrainment of small particles by larger ones finally we show that half of the deposit by mass in the medial area was deposited by the co blast plume and that a significant proportion of the mount st helens fallout deposit is nonjuvenile which has implications for the magmatic budget of this eruption

hydraulics of subaqueous ash flows as deduced from their deposits 2 water entrainment sedimentation and deposition with implications on pyroclastic density current deposit emplacement abstract turbidity currents are the most common flows of water and suspended and bed load sediment occurring in the relatively deep sea and lakes and act as large systems of sediment distribution due to the turbid nature of the flow they may impact subaqueous infrastructures as well as ecosystems during motion so quantifying the turbidity degree is important to predict the current impact a particular type of these currents is the so called volcaniclastic turbidity currents or subaqueous ash flows which are mostly composed of fine grained volcanic particles and are used here as synonymous sensu hydraulic with the meaning of secondary current in this paper a method to estimate the water entrainment column condition model as well as the sedimentation and deposition rates conveyer model in volcaniclastic turbidity currents is proposed by starting from the physical features of the deposits or inverse procedure some criteria of sediment mechanics are used to approximate the flow hydraulic parameters needed to quantify the water entrainment as well as the shear velocity in volcaniclastic turbidity currents the deposits used as case study are the impressive meters thick well sorted rhyolitic ash turbidites of late pliocene cropping out in southern italy particularly in the craco area matera the water entrainment coefficient of the currents is calculated in a range of particle concentration and slope angle whereas the slope angle giving the sedimentation rate is calculated in a range of flow shear velocity which in turns gives the deposition rate the results have a general validity for depositional turbidity currents laden with well sorted sediment and they show that the water entrainment is low for relatively dense slow moving subcritical flows but it increases as the richardson number decreases for relatively dilute fast supercritical flows moreover the sedimentation and deposition rates are high for relatively intense flows moving over gentle slopes but they decrease for relatively weak flows moving over steep slopes the deposit sedimentary structures thus infer sedimentation and particularly depositional processes finally the conveyer model is extended conceptually to pyroclastic density currents when approximating but not exclusively the column condition model in the flow above the flow boundary zone

sea surface temperature in the north tropical atlantic as a trigger for el nino southern oscillation events the el nino southern oscillation in the pacific ocean influences temperature in other tropical ocean basins reanalysis data and model simulations suggest that temperature anomalies in the north tropical atlantic may also influence the development of la nina events

the 15 september 1991 pyroclastic flows at unzen volcano japan a flow model for associated ash cloud surges large scale collapse of a dacite dome in the late afternoon of 15 september 1991 generated a series of pyroclastic flow events at unzen volcano pyroclastic flows with a volume of 1 106 m3 as dre descended the northeastern slope of the volcano changing their courses to the southeast due to topographic control after they exited a narrow gorge an ash cloud surge rushed straight ahead detaching the main body of the flow that turned and followed the topographic lows to the east the surge swept the kita kamikoba area which had been devastated by the previous pyroclastic flow events and transported a car as far as 120 m following detachment the surge lost its force after it moved several hundred meters but maintained a high temperature the deposits consist of a bottom layer of better sorted ash unit 1 a thick layer of block and ash unit 2 and a thin top layer of fall out ash unit 3 unit 2 overlies unit 1 with an erosional contact the upper part of unit 2 grades into better sorted ash at distal block and ash flow deposits the bottom part of unit 2 also consists of better sorted ash and the contact with the unit 1 deposits becomes ambiguous video footage of cascading pyroclastic flows during the 1991 1995 eruption traveling over surfaces without any topographic barriers revealed that lobes of ash cloud protruded intermittently from the moving head and sides and that these lobes surged ahead on the ground surface this fact together with the inspection by helicopter shortly after the events suggests that the protruded lobes consisted of better sorted ash and resulted in the deposits of unit 1 the highest ash cloud plume at the oshigadani valley exit and the thickest deposition of fall out ash over kita kamikoba and ohnokoba indicate that abundant ash was also produced when the flow passed through a narrow gorge in the model presented here the ash clouds from the pyroclastic flows were composed of a basal turbulent current of high concentration main body an overriding and intermediate fluidization zone and an overlying dilute cloud release of pressurized gas in lava block pores due to collisions among blocks and the resulting upward current caused a zone of fluidization just above the main body the mixture of gas and ash sorted in the fluidization zone moved ahead and to the side of the main body as a gravitational current where the ash was deposited as surge deposits the main body which had high internal friction and shear near its base then overran the surge deposits partially eroding them when the upward current of gas fluidization waned better sorted ash suspended in the fluidization zone was deposited on block and ash deposits in the distal places of block and ash deposits unit 2 probably was deposited in non turbulent fashion without any erosion of the underlying layer unit 1

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

experimental study of turbulence sedimentation and coignimbrite mass partitioning in dilute pyroclastic density currents abstract laboratory density currents comprising warm talc powder turbulently suspended in air simulate many aspects of dilute pyroclastic density currents pdcs and demonstrate links between bulk current behavior sedimentation and turbulent structures the densimetric and thermal richardson froude stokes and settling numbers match those of natural pdcs as does the ratio of thermal to kinetic energy density the experimental currents have lower bulk reynolds numbers than natural pdcs but the experiments are fully turbulent consequently the experiments are dynamically similar to the dilute portions of some natural currents in general currents traverse the floor of the experimental tank sedimenting particles and turbulently entraining heating and thermally expanding air until all particle sediments or the currents become buoyant and lift off to form coignimbrite plumes when plumes form currents often undergo local flow reversals current runout distance and liftoff position decrease with increasing densimetric richardson number and thermal energy density as those parameters increase total sedimentation decreases such that 50 of initial current mass commonly fractionates into the plumes in agreement with some observations of recent volcanic eruptions sedimentation profiles are best described by an entraining sedimentation model rather than the exponential fit resulting from non entraining box models time series analysis shows that sedimentation is not a constant rate process in the experiments but rather occurs as series of sedimentation erosion couplets that propagate across the tank floor tracking current motion and behavior during buoyant liftoff sedimentation beneath the rising plumes often becomes less organized auto correlation analysis of time series of particle concentration is used to characterize the turbulent structures of the currents and indicates that currents quickly partition into a slow moving upper portion and faster more concentrated lower portion air entrainment occurs within the upper region turbulent structures within the lower region track sedimentation erosion waves and indicate that eddies control deposition importantly both eddies and sedimentation waves track reversals in flow direction that occur following buoyant liftoff further these results suggest that individual laminations within pdc deposits may record passage of single eddies thus the duration of individual pdcs may be estimated as the product of the number of laminations and the current s turbulent timescale

particle velocity fields and depositional processes in laboratory ash flows with implications for the sedimentation of dense pyroclastic flows we conducted laboratory experiments on dam break flows of sub 250 m volcanic ash generated by the release of gas fluidized and variably non expanded to expanded up to 35 beds in order to gain insights into the internal kinematics of pyroclastic flows the flows were typically several cm thick and had frontal speeds of up to 2 m s 1 high speed videos taken through the transparent sidewall of the 3 m long channel were analyzed with a particle tracking algorithm providing a spatial and temporal description of transport and sedimentation the flows deposited progressively as they traveled down the flume being consumed by sedimentation until they ran out of volume deposition commenced 5 20 cm rearward of the flow front and for a given expansion proceeded at a rate independent of distance from the lock gate deposit aggradation velocities were equal to those inferred beneath quasi static bed collapse tests of the same ash at the same initial expansions implying that shear rates of up to 300 s 1 have no measurable effect on aggradation rate the initially non expanded and just fluidized flow deposited progressively at a rate indicative of an expansion of a few percent perhaps due to shear induced reynolds dilation during initial slumping the fronts of the flows slid across the flume floor on very thin basal shear layers but once deposition commenced a no slip condition was established at the depositional interface within the flows the trajectory of the constituent particles was linear and sub horizontal the velocities of the particles increased with height above the depositional interface reached a maximum then declined slightly towards the flow surface perhaps due to air drag at a given location the velocity profiles were translated upwards as the deposit aggraded the results show that even cm thin poorly expanded flows of ash deposit progressively as inferred for many pyroclastic flows the change from frontal slip to rearward no slip conditions at the bases of the laboratory flows are qualitatively consistent with some textural features of pyroclastic flow deposits

liftoff of the 18 may 1980 surge of mount st helens usa and the deposits left behind the distance that ground hugging pyroclastic density currents travel is limited partly by when they reverse buoyancy and liftoff into the atmosphere it is not clear however what deposits are left behind by lofting flows one current that was seen to liftoff was the surge erupted from mount st helens on the morning of 18 may 1980 before lofting it had leveled a large area of thick forest the blowdown zone the outer edge of the devastated area where trees were scorched but left standing the scorched zone is where the surge is thought to have lifted off deposits in the outer parts of the blowdown and in the scorched zone were examined at 32 sites the important finding is that the laterally moving surge traveled through the scorched zone and hence the change in tree damage does not mark the runout distance of the surge buoyancy reversal and liftoff are thus not preserved in the deposits where the surge lofted upwards we propose based on interpretation of eyewitness accounts and the impacts of the surge on trees and vehicles that the surge consisted of a faster dilute overcurrent and a slower undercurrent where most of the mass and heat was retained reasonable estimates for flow density and velocity show that dynamic pressure of the surge i e its ability to topple trees peaked near the base of the overcurrent we propose that where the overcurrent began to liftoff the height of peak dynamic pressure rose above the trees and stopped toppling them the slower undercurrent continued forward however scorching trees but lacked the dynamic pressure needed to topple them grain size variations argue that it slowed from 30 m s 1 when it entered the scorched zone to 3 m s 1 at the far end

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

eruption and deposition of the fisher tuff alaska evidence for the evolution of pyroclastic flows abstract recognition that the fisher tuff unimak island alaska was deposited on the leeside of an 500 700 m high mountain range tugamak range more than 10 km away from its source played a major role in defining pyroclastic flows as momentum driven currents we reexamined the fisher tuff to evaluate whether deposition from expanded turbulent clouds can better explain its depositional features we studied the tuff at 89 sites and sieved bulk samples from 27 of those sites we find that the tuff consists of a complex sequence of deposits that record the evolution of the eruption from a buoyant plume 22 km that deposited 0 2 km3 of dacite magma as a pyroclastic fall layer to erupting 10 100 km3 of andesitic magma as scoria rich pyroclastic falls and flows that were mainly deposited to the north and northwest of the caldera including those in valleys within the tugamak range the distribution of the flow deposits and their welding internal stratification and the occurrence of lithic breccia all su

addressing complexity in laboratory experiments the scaling of dilute multiphase flows in magmatic systems the kinematic and dynamic scaling of dilute multiphase mixtures in magmatic systems is the only guarantee for the geological verisimilitude of laboratory experiments we present scaling relations that can provide a more complete framework to scale dilute magmatic systems because they explicitly take into account the complexity caused by the feedback between particles crystal bubble or pyroclast and the continuous phase liquid or gas we consider three canonical igneous systems magma chambers volcanic plumes and pyroclastic surges and we provide estimates of the proposed scaling relations for published experiments on those systems dilute magmatic mixtures can display a range of distinct dynamical regimes that we characterize with a combination of average eulerian properties and instantaneous lagrangian variables the eulerian properties of the mixtures yield the reynolds number re which indicates the level of unsteadiness in the continuous phase the lagrangian acceleration of particles is a function of the viscous drag and gravity forces and from these two forces are derived the stokes number st and the stability number rt two dimensionless numbers that describe the dynamic behavior of the particles within the mixture the compilation of 17 experimental studies relevant for pyroclastic surges and volcanic plumes indicates that there is a need for experiments above the mixing transition ren10 4 and for scaling st and rt among the particle dynamic regimes present in surges and plumes some deserve special attention such as the role of mesoscale structures on transport and sedimentary processes or the consequences of the transition to turbulence on particle gathering and dispersal the compilation of seven experimental studies relevant to magma bodies indicates that in the laminar regime crystals mostly follow the motion of the melt and thus the physical state of the system can be approximated as single phase in the transition to turbulence magmas can feature spatially heterogeneous distributions of laminar regions and important velocity gradients this heterogeneity has a strong potential for crystals sorting in conclusion the re st rt framework demonstrates

ignimbrite depositional facies the anatomy of a pyroclastic flow a model is presented for the depositional regimes in a pyroclastic flow to explain the origin of facies in the taupo ignimbrite and to compare these with other published examples in this model a pyroclastic flow consists of a head a body and a tail the head which is where fluidization caused by air ingestion occurs generates layer 1 deposits on lithological grounds these are divided into layers 1 p which is fines depleted and rich in pumice and 1 h which is fines depleted and rich in lithics and crystals layer 1 p represents material thrown forwards from the flow head and is termed the jetted deposits layer 1 h represents material sedimented from within the flow head and is termed the ground layer the body represents the bulk of the flow and the tail is its trailing part which is slowed by ground friction these parts generate the layer 2 deposits which include the valley pond ignimbrite and its associated ignimbrite veneer deposit localized depositional modes within the body and tail generate distinctive coarse pumice concentration zones and pumiceous lee side lenses behind obstacles at its outer limits all of the flow on interfluves is affected by air ingestion fluidization producing the distant facies which combines features of both layer 1 and 2 deposits one important conclusion of this work is that the thicknesses and compositions of the various facies are not related in any simple way to the thickness and composition of the parent flow

the effects of topography on sedimentation from particle laden turbulent density currents abstract we present results from a series of laboratory experiments that illustrate the influence of changes in channel topography depth or width on the sedimentation patterns produced by steady particle laden currents on a planar surface in a channel with constant width such currents created deposits that thinned exponentially with distance as long as a current was not blocked to produce a bore topographic features consisting of constrictions ridges and sudden openings caused no significant deviation from exponential deposit thinning or discontinuity in thickness even when there was a transition in flow regime caused by the topographic feature changes in the channel width did affect the distance over which the deposit thinned with the deposits associated with wider parts of the channel thinning more rapidly with distance in contrast if the topographic change was large enough to partially reflect the flow producing an upstream propagating bore then the deposit did not thin exponentially the results are consistent with a model in which the current is assumed to be turbulent and well mixed in this case sedimentation occurs at a rate proportional to the channel width the settling speed of the particles and their concentration within the current and inversely proportional to the current discharge such a model predicts that as a flow passes through a topographic control that does not produce a bore the deposit continues to thin exponentially even if the flow undergoes a transition from the subcritical to the supercritical regime the results suggest that in natural systems changes in current speed and flow regime do not in themselves produce changes in deposit thickness or gradient in thickness unless the flow is partially blocked and a fraction of the flow is reflected upstream

dynamics of co ignimbrite plumes generated from pyroclastic flows of mount st helens 7 august 1980 four co ignimbrite plumes were generated along the flow path of the pyroclastic flow of 7 august 1980 at mount st helens three of the plumes were generated in discrete pulses which can be linked to changes in slope along the channel one plume was generated at the mouth of the channel where the flow decelerated markedly as it moved onto the lower slopes of the pumice plain plume generation here may be triggered by enhanced mixing due to a hydraulic jump associated with an abrupt slope change measurements of plume ascent velocity and width show that the co ignimbrite plumes increased in velocity with height the plumes have initial velocities of 1 2 m s two of the plumes reached a velocity maximum 4 6 and 8 8 m s respectively at heights of 270 and 315 m above the flow and thereafter decelerated the other plumes reached velocities of 6 2 and 13 m s the four plumes become systematically less energetic downstream as measured by their ascent rates which can be interpreted as a consequence of decreasing interaction of the pyroclastic flow front with the atmosphere theoretical models of both co ignimbrite plumes and discrete co ignimbrite clouds assume that there is no initial momentum and both are able to predict the observed acceleration stage the rising plumes mix with and heat air and sediment out particles causing their buoyancy to increase theoretical models agree well with observations and suggest that the initial motion of the ascending material is best described as a discrete thermal cloud which expands as it entrains air whereas the subsequent motion of the head may become influenced by material supplied from the following plume the models agree well with observations for an initial temperature of the ash and air mixture in the range of 500 600 k which is in turn consistent with the measured initial ash temperature of around 920 k ash masses of 3 4 105 to 1 8 106 kg are estimated

transport and deposition processes of the hydrothermal blast of the 6 august 2012 te maari eruption mt tongariro the 2012 eruption of tongariro volcano new zealand produced highly mobile low temperature blast derived pyroclastic density currents after partial collapse of the western flank of the upper te maari crater despite a low volume 340 000 m3 the flows traveled up to 2 5 km from source covering a total area of 6 1 km2 along one of the blast axes freshly exposed proximal to distal sedimentary structures and grain size data suggest emplacement of the fining upward tripartite depositional sequence massive stratified and laminated under a dilute and strongly longitudinally zoned turbulent density current while the zoning formed in the deposit in the first 1500 m of runout the current progressively waned to the extent where it transported a nearly homogenous grain size mixture at the liftoff position our data indicate that after the passage of an erosive flow front massive unit a was deposited under a rapid suspension sedimentation regime unit b was deposited under a traction dominated regime generated by a subsequent portion of the flow moving at lower velocities and with lower sediment transport capacity than the portion depositing unit a the final and slowest flow zone deposited the finest particles under weakly tractive conditions transport and emplacement dynamics inferred in this study show strong similarities between hydrothermal explosions magmatic blasts and high energy dilute pdcs the common occurrence of hydrothermal fields on volcanic flanks points to this hazard being an under appreciated one at stratovolcanoes worldwide

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

enhanced relationship between the tropical atlantic sst and the summertime western north pacific subtropical high after the early 1980s the western north pacific subtropical high wnpsh in boreal summer shows a remarkable enhancement after the early 1980s whereas the sea surface temperature sst in the north indian ocean nio and the equatorial eastern pacific had been noted to have remarkable local or remote effects on enhancing the wnpsh the influence of the atlantic sst so far is hardly explored this article reports a new finding enhanced relationship between the tropical atlantic ta sst and the wnpsh after the early 1980s regression study suggests that the warm ta sst produced a zonally overturning circulation anomaly with descending over the equatorial central pacific and ascending over the tropical atlantic eastern pacific the anomalous descending over the equatorial central pacific likely induced low level anticyclonic anomaly to the west and therefore enhanced the wnpsh one implication of this new finding is for predictability the well known spring predictability barrier i e the influence of el nino southern oscillation enso falls dramatically during boreal spring does not apply to the ta sst wnpsh relationship the ta sst shows consistently high correlation starting from boreal spring when the enso influence continues declining the ta sst extends the predictability of the wnpsh in boreal summer approximately one season earlier to boreal spring

experimental volcanic ash aggregation internal structuring of accretionary lapilli and the role of liquid bonding explosive volcanic eruptions can release vast quantities of pyroclastic material into earth s atmosphere including volcanic ash particles with diameters less than two millimeters ash particles can cluster together to form aggregates in some cases reaching up to several centimeters in size aggregation alters ash transport and settling behavior compared to un aggregated particles influencing ash distribution and deposit stratigraphy accretionary lapilli the most commonly preserved type of aggregates within the geologic record can exhibit complex internal stratigraphy the processes involved in the formation and preservation of these aggregates remain poorly constrained quantitatively in this study we simulate the variable gas particle flow conditions which may be encountered within eruption plumes and pyroclastic density currents via laboratory experiments using the procell lab system of glatt ingenieurtechnik gmbh in this apparatus solid particles are set into motion in a fluidized bed over a range of well controlled boundary conditions particle concentration air flow rate gas temperature humidity liquid composition experiments were conducted with soda lime glass beads and natural volcanic ash particles under a range of experimental conditions both glass beads and volcanic ash exhibited the capacity for aggregation but stable aggregates could only be produced when materials were coated with high but volcanically relevant concentrations of nacl the growth and structure of aggregates was dependent on the initial granulometry while the rate of aggregate formation increased exponentially with increasing relative humidity 12 45 rh before overwetting promoted mud droplet formation notably by use of a broad granulometry we generated spherical internally structured aggregates similar to some accretionary pellets found in volcanic deposits adaptation of a powder technology model offers an explanation for the origin of natural accretionary pellets suggesting them to be the result of a particular granulometry and fast acting selective aggregation processes for such aggregates to survive deposition and be preserved in the deposits of eruption plumes and pyroclastic density currents likely requires a significant pre existing salt load on ash surfaces and rapid aggregate drying prior to deposition or interaction with a more energetic environment our results carry clear benefits for future efforts to parameterize models of ash transport and deposition in the field

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

enhanced influences of tropical atlantic sst on wnp nio atmosphere ocean coupling since the early 1980s abstractthis paper reports a new finding and related mechanism the forcing effect of the tropical atlantic ta sea surface temperature sst on the atmosphere ocean coupling in the western north pacific wnp and northern indian ocean nio since the early 1980s the ta sst has increased and notably exhibited an enhanced interannual statistical relationship with the wnp subtropical high and nio sst in boreal summer empirical diagnostics reveal the following spatial pattern linking the ta sst and the atmosphere ocean coupling in the pacific and indian ocean 1 a cyclonic anticyclonic circulation pair straddling the equator over the eastern pacific 2 an anticyclonic cyclonic circulation pair straddling the equator in the wnp and indian ocean 3 overturning circulation with ascending descending and descending ascending anomalies over the ta and tropical western pacific respectively and 4 positive negative sst anomaly in the ta and nio the characteristics of this pattern are consistent

effects of tropical north atlantic sst on tropical cyclone genesis in the western north pacific the tropical cyclone genesis number tcgn in july october jaso over the western north pacific wnp exhibits a robust interannual variation it shows a longitudinally tri pole pattern with a high in the eastern wnp and south china sea scs and a low in the western wnp which explain 42 2 and 23 4 of total tcgn variance in the eastern wnp and scs respectively the high low high pattern is similar to that derived from a tc genesis potential index gpi to understand the cause of the longitudinal distribution of the dominant interannual mode we examine the contributions of environmental parameters associated with gpi it is found that relative humidity and relative vorticity are important factors responsible for tc variability in the scs while vertical shear and relative vorticity are crucial in determining tc activity in eastern wnp a simultaneous correlation analysis shows that the wnp tcgn in jaso is significantly negatively correlated with a correlation coefficient of 0 5 with sea surface temperature anomalies ssta in the tropical north atlantic tna the longitudinal distribution of tc genesis frequency regressed onto tna ssta resembles that regressed upon the wnp tcgn series the spatial patterns of regressed environmental variables onto the ssta over the tna also resemble those onto tcgn in the wnp that is an increase of relative humidity in the scs and a weakening of vertical shear in the eastern wnp are all associated with cold ssta in the tna further analyses show that the cold ssta in the tna induce a negative heating in situ in response to this negative heating a low upper level anomalous aniti cyclonic cyclonic flows appear over the subtropical north atlantic and eastern north pacific and to east of the cold ssta anomalous low level westerlies appear in the tropical indian ocean given pronounced mean westerlies in northern indian ocean in boreal summer the anomalous westerly flows increase local surface wind speed and surface evaporation and cool the sst in situ cold ssta in northern indian ocean further suppress local convection inducing anomalous westerlies to its east leading to enhanced cyclonic vorticity and low surface pressure over the wnp monsoon trough region idealized numerical experiments further confirm this indian ocean relaying effect through which cold ssta in the tropical atlantic exert a remote impact to circulation in the wnp

the role of tropical atlantic sst anomalies in modulating western north pacific tropical cyclone genesis the connection between north tropical atlantic nta sea surface temperature sst anomalies and tropical cyclone tc genesis over the western north pacific wnp and associated physical mechanisms are investigated in this study we demonstrate a remarkable negative correlation of wnp tc genesis frequency with the preceding boreal spring nta sst anomalies our analysis suggests that major factors for tc genesis including distributions of large scale vorticity and midtropospheric humidity are rendered unfavorable by remote teleconnections while barotropic energy conversion from the large scale flow is suppressed as shown in recent studies the remote teleconnection from the atlantic is sustained and enhanced throughout the typhoon season through local air sea interactions these results suggest that boreal spring nta sst anomaly could be a new predictor for the seasonal wnp tc activity

variations in column height and magma discharge during the may 18 1980 eruption of mount st helens abstract peak eruption column heights for the b1 b2 b3 and b4 units of the may 18 1980 fall deposit from mount st helens have been determined from pumice and lithic clast sizes and models of tephra dispersal column heights determined from the fall deposit agree well with those determined by radar measurements b1 and b2 units were derived from plinian activity between 0900 and about 1215 hrs b3 was formed by fallout of tephra from plumes that rose off pyroclastic flows from about 1215 to 1630 hrs a brief return to plinian activity between 1630 and 1715 hrs was marked by a maximum in column height 19 km during deposition of b4 variations in magma discharge during the eruption have been reconstructed from modelling of column height during plinian discharge and mass balance calculations based on the volume of pyroclastic flows and coignimbrite ash peak magma discharge occurred during the period 1215 1630 hrs when pyroclastic flows were generated by collapse of low fountains through the crater breach pyroclastic flow deposits and the widely dispersed co ignimbrite ash account for 77 of the total erupted mass with only 23 derived from plinian discharge a shift in eruptive style at noon on may 18 may have been associated with increase in magma discharge and the eruption of silicic andesite mingled with the dominant mafic dacite increasing abundance of the silicic andesite during the period of highest magma discharge is consistent with the draw up and tapping of deeper levels in the magma reservoir as predicted by theoretical models of magma withdrawal return to plinian activity late in the afternoon when magma discharge decreased is consistent with theoretical predictions of eruption column behavior the dominant generation of pyroclastic flows during the may 18 eruption can be attributed to the low bulk volatile content of the magma and the increasing magma discharge that resulted in the transition from a stable convective eruption column to a collapsing one

initiation of dilute and concentrated pyroclastic currents from collapsing mixtures and origin of their proximal deposits numerical solution of the time dependent conservation equations for mass momentum specific internal energy and granular temperature in flows involving gas particle mixtures is used to explore the initiation of pyroclastic currents from collapsing mixtures such as during fountaining eruptions one objective is to determine when a depth averaged granular flow model or a box model for dilute currents is most applicable for hazards modeling of pyroclastic currents produced by column collapse a second objective is to gain insight into the formation of proximal breccia facies of ignimbrites collapsing gas particle mixtures impacting a flat surface are modeled with mixtures of coarse particles that are poorly coupled with the gas phase and well coupled fine particles resulting lateral flows are sensitive to the impact speed overall particle concentration and proportions of fine and coarse particles for total particle concentrations of around 1 vol an impacting mixture consisting of at least 50 coarse particles relative to fines will tend to form a concentrated lateral underflow which can be approximated by a depth averaged granular flow model for hazard assessment purposes starting from the impact zone low total particle concentrations e g total concentrations of 0 1 vol tend to produce dilute lateral flows that could be simplified to box model approaches for dilute pyroclastic currents larger total particle concentrations in impacting mixtures 10 vol produce granular underflows if they have any coarse particles but these can be complicated by mach number effects for intermediate concentrations of the impacting mixture a rough threshold for development of a concentrated underflow versus a dilute only current is based upon the flux per unit area of coarse particles to the impact and their stokes numbers in general some knowledge of the eruption column fountain conditions is required in order to make an informed decision as to which hazards modeling approach is most applicable for a given scenario modeling indicates that proximal breccias are related to influxes of coarse wall rock material into an eruptive mixture which increase both the total particle concentration and the proportion of coarse dense clasts in the mixture that subsequently collapses and impacts the ground the breccias record concentration of dominantly coarse clasts immediately upon impact and formation of concentrated flows that propagate laterally while expelled fines and gas flow rapidly overhead as dilute currents lateral and vertical heterogeneity in proximal deposits likely record rapid time and space variations in avalanches of material into eruptive vents and the occurrence of breccia hummocks might record the temporary positions of impact zones

impact of the equatorial atlantic on the el nino southern oscillation observations indicate that the atlantic zonal mode influences el nino southern oscillation enso in the pacific as already suggested in previous studies here we demonstrate for the first time using partial coupled experiments that the atlantic zonal mode indeed influences enso the partial coupling experiments are performed by forcing the coupled general circulation model echam5 mpi om with observed sea surface temperature sst in the tropical atlantic but with full air sea coupling allowed in the pacific and indian ocean the ensemble mean of a five member simulation reproduces the observational results well analysis of observations reanalysis and coupled model simulations all indicate the following mechanism sst anomalies associated with the atlantic zonal mode affect the walker circulation driving westward wind anomalies over the equatorial pacific during boreal summer the wind stress anomalies increase the east west thermocline slope and enhance the sst gradient across the pacific the bjerknes positive feedback acts to amplify these anomalies favouring the development of a la nina like anomalies the same mechanisms act for the cold phase of atlantic zonal mode but with opposite sign in contrast to previous studies the model shows that the influence on enso exists before 1970 furthermore no significant influence of the tropical atlantic on the indian monsoon precipitation is found in observation or model

indian ocean capacitor effect on indo western pacific climate during the summer following el nino significant climate anomalies persist through the summer june august after el nino dissipates in spring over the equatorial pacific they include the tropical indian ocean tio sea surface temperature sst warming increased tropical tropospheric temperature an anomalous anticyclone over the subtropical northwest pacific and increased mei yu baiu rainfall over east asia the cause of these lingering el nino effects during summer is investigated using observations and an atmospheric general circulation model gcm the results herein indicate that the tio warming acts like a capacitor anchoring atmospheric anomalies over the indo western pacific oceans it causes tropospheric temperature to increase by a moist adiabatic adjustment in deep convection emanating a baroclinic kelvin wave into the pacific in the northwest pacific this equatorial kelvin wave induces northeasterly surface wind anomalies and the resultant divergence in the subtropics triggers suppressed convection and the anomalous anticyclone the gcm results support this kelvin wave induced ekman divergence mechanism in response to a prescribed sst increase over the tio the model simulates the kelvin wave with low pressure on the equator as well as suppressed convection and the anomalous anticyclone over the subtropical northwest pacific an additional experiment further indicates that the north indian ocean warming is most important for the kelvin wave and northwest pacific anticyclone a result corroborated by observations these results have important implications for the predictability of indo western pacific summer climate the spatial distribution and magnitude of the tio warming rather than simply whether there is an el nino in the preceding winter affect summer climate anomalies over the indo western pacific and east asia

a box model for non entraining suspension driven gravity surges on horizontal surfaces the propagation of and the deposition from a turbulent gravity current generated by the release of a finite volume of a dense particle suspension is described by a box model the approximate model consists of a set of simple equations a predetermined depth dependent leading boundary condition and one experimentally determined parameter describing the trailing boundary condition it yields predictions that agree well with existing laboratory observations and more complex theoretical models of non eroding non entraining suspension driven flows on horizontal surfaces the essential features of gravity surge behaviour have been observed and are captured accurately by the box model these include the increased rate of downstream loss of flow momentum with increased particle setting velocity the existence of maxima in the thickness of proximal deposits and the downstream thinning of distal deposits our approximation for the final run out distance xr of a surge in deep water is given by xr3 g oq3o w2s 1 5 where g o is the initial reduced gravity of the surge qo the initial two dimensional volume and ws the average settling velocity of the particles in the suspension a characteristic thickness of the resulting deposit is given by oqo xr where oo is the initial volumetric fraction of sediment suspended in the surge our analysis provides additional insight into other features of gravity surge dynamics and deposits including the potential for the thickening of currents with time the maintenance of inertial conditions and the potential for strong feedback in the sorting of particle sizes in the downstream direction at travel distances approaching xr box model approximations for the evolution of gravity surges thus provide a useful starting point for analyses of some naturally occurring turbidity surges and their deposits

the dynamics and thermodynamics of volcanic clouds theory and observations from the april 15 and april 21 1990 eruptions of redoubt volcano alaska on april 15 1990 and april 21 1990 two relatively small explosive eruptions occurred at redoubt volcano alaska lasting about 4 and 8 minutes respectively on both occasions the erupted material travelled as a pyroclastic flow down an ice canyon on the north flank of the volcano using slow scan television recordings of the eruption and the seismic record in both the near and far field we deduce that after a few minutes the upper parts of these pyroclastic flows became buoyant and a large and hot ash cloud ascended off the flow these thermals rose to a height of about 12 km at which point they began to spread laterally as umbrella clouds using a simple thermodynamic model we estimate that the clouds had a temperature in the approximate range 600 700 k as they rose buoyantly from the flow after entraining and heating air and melting and vaporizing ice we also estimate that in each eruption approximately 109 kg of fine ash was injected into the atmosphere during the april 15 eruption a slow scan television camera recorded the ascent of the cloud these observations are described analysed and compared with the predictions of a new model for the dynamics governing the ascent of such clouds in accord with the observations our model predicts that the cloud initially ascended rather sluggishly since it is only just buoyant on rising from the pyroclastic flow as it ascends it entrains and heats up more air and hence generates more buoyancy therefore it accelerates upwards only much higher in the cloud does the velocity decrease again as the thermal energy of the cloud becomes exhausted the model also predicts that the height of rise of such coignimbrite thermals is a function of the initial mass and temperature of the cloud but is almost independent of the initial velocity during the april 21 eruption a sequence of photographs recorded the lateral spreading of the umbrella cloud during an interval of about 10 minutes after the eruption these photographs are analysed and successfully compared with a simple model for the spreading of the umbrella cloud as a gravity current in a stratified environment using an avhrr satellite image of the air fall deposit from the april 15 eruption combined with a simple model we suggest that the primary mechanism of ash dispersal was transport by the ambient wind

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

in situ production of ash in pyroclastic flows 1 abrasion and comminution of pumice clasts during the propagation of pyroclastic flows have long been recognized as a potential source for the enhanced production of volcanic ash however their relative importance has eluded quantification the amount of ash produced in situ can potentially affect runout distance deposit sorting the volume of ash introduced in the upper atmosphere and internal pore pressure we conduct a series of laboratory experiments on the collisional and frictional production of ash that may occur during different regimes of pyroclastic flow transport ash produced in these experiments is predominately 10 100 microns in size and has similar morphology to tephra fall ash from plinian events we find that collisional ash production rates are proportional to the square of impact velocity frictional ash production rates are a linear function of the velocity of the basal particle enriched bed load region of these flows using these laboratory experiments we develop a subgrid model for ash production that can be included in analytical and multiphase numerical procedures to estimate the total volume of ash produced during transport we find that for most flow conditions 10 20 of the initial clasts comminute into ash with the percentage increasing as a function of initial flow energy most of the ash is produced in the high energy regions near the flow inlet although flow acceleration on steep slopes can produce ash far from the vent on level terrain collisionally and frictionally produced ash generates gravity currents that detach from the main flow and can more than double the effective runout distance of these flows ash produced at the frictional base of the flow and in the collisional upper regions of the flow can be redistributed through the entirety of the flow although frictionally produced ash accumulates preferentially near its source in the bed load flows that descend steep slopes produce the majority of their ash in the collisonally dominant flow head and flow snouts likely develop subangular to rounded pumice during this process

the dual beam mini doas technique measurements of volcanic gas emission plume height and plume speed with a single instrument the largest error in determining volcanic gas fluxes using ground based optical remote sensing instruments is typically the determination of the plume speed and in the case of fixed scanning instruments also the plume height we here present a newly developed technique capable of measuring plume height plume speed and gas flux using one single instrument by simultaneously collecting scattered sunlight in two directions the angle between the two measurement directions is fixed removing the need for time consuming in field calibrations the plume height and gas flux is measured by traversing the plume and the plume speed is measured by performing a stationary measurement underneath the plume the instrument was tested in a field campaign in may 2005 at mt etna italy where the measured results are compared to wind fields derived from a meso scale meteorological model mm5 the test and comparison show that the instrument is functioning and capable of estimating wind speed at the plume height

distal ash hurricane pyroclastic density current deposits from a ca 2000 yr b p plinian style eruption of mount pelee martinique distribution grain size characteristics and implications for future hazard plinian style activity has not occurred on any volcano in the lesser antilles since european settlement however pumice rich deposits from this eruptive style are found throughout the lesser antilles and such eruptions were witnessed by pre columbian native american populations one of these eruptive events from mount pelee on martinique occurring between 1800 and 2200 yr b p produced pumice and ash flow deposits that followed the main drainage channels on the volcano ash hurricane deposits that both mantle the volcano s flanks slopes up to 25 and extended at least 20 km from the crater we examined the latter in the context of modern developments in understanding transport and depositional mechanics of pyroclastic density currents based on grain size analysis they are subdivided into proximal and distal deposits the distal deposits become progressively more depleted in fines and enriched in crystals with distance from the crater we conclude these are deposits of dilute or inertia dominated pyroclastic overcurrents which decoupled from their underflows as they surmounted the topographic barrier formed by the older pitons du carbet volcanic center a combination of topography and ingestion of air and water vapor from tropical vegetation perhaps caused the considerable expansion and liftoff and resulting extreme crystal enrichment their areal distribution suggests much of the northern half of martinique must have been severely affected and the zone of devastation extended at least to the northern outskirts of the capital fort de france although their final runout distance may have extended significantly further south major coignimbrite plumes dispersing ash and aerosols over thousands of kilometers would today pose hazards to aviation

a fast calibrated model for pyroclastic density currents kinematics and hazard abstract multiphase flow models represent valuable tools for the study of the complex non equilibrium dynamics of pyroclastic density currents particle sedimentation flow stratification and rheological changes depending on the flow regime interaction with topographic obstacles turbulent air entrainment buoyancy reversal and other complex features of pyroclastic currents can be simulated in two and three dimensions by exploiting efficient numerical solvers and the improved computational capability of modern supercomputers however numerical simulations of polydisperse gas particle mixtures are quite computationally expensive so that their use in hazard assessment studies where there is the need of evaluating the probability of hazardous actions over hundreds of possible scenarios is still challenging to this aim a simplified integral box model can be used under the appropriate hypotheses to describe the kinematics of pyroclastic density currents over a flat topography their scaling properties and their depositional features in this work multiphase flow simulations are used to evaluate integral model approximations to calibrate its free parameters and to assess the influence of the input data on the results two dimensional numerical simulations describe the generation and decoupling of a dense basal layer formed by progressive particle sedimentation from the dilute transport system in the boussinesq regime i e for solid mass fractions below about 0 1 the current froude number i e the ratio between the current inertia and buoyancy does not strongly depend on initial conditions and it is consistent to that measured in laboratory experiments i e between 1 05 and 1 2 for higher density ratios solid mass fraction in the range 0 1 0 9 but still in a relatively dilute regime particle volume fraction lower than 0 01 numerical simulations demonstrate that the box model is still applicable but the froude number depends on the reduced gravity when the box model is opportunely calibrated with the numerical simulation results the prediction of the flow runout is fairly accurate and the model predicts a rapid non linear decay of the flow kinetic energy or dynamic pressure with the distance from the source the capability of pdc to overcome topographic obstacles can thus be analysed in the framework of the energy conoid approach in which the predicted kinetic energy of the flow front is compared with the potential energy jump associated with the elevated topography to derive a condition for blocking model results show that although preferable to the energy cone the energy conoid approach still has some serious limitations mostly associated with the behaviour of the flow head implications of these outcomes are discussed in the context of probabilistic hazard assessment studies in which a calibrated box model can be used as a fast pyroclastic density current emulator for monte carlo simulations

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

transport and deposition in the may 18 1980 mount st helens blast flow a new statistical sampling strategy for pyroclastic deposits has yielded data that reveal the depositional characteristics of the may 18 1980 blast flow of mount st helens samples were collected from each layer along a profile parallel to flow lines as interpreted from tree blow down directions the decrease in the mass of sediment of different grain sizes with distance supports a turbulent gravitational mechanism of suspended sedimentation and transport within a dilute blast cloud having a density only slightly greater than that of the atmosphere traditional sampling strategies would have been unable to yield data suitable to test this settling mechanism hypothesis

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

remote forcing of the northern tropical atlantic sst anomalies on the western north pacific anomalous anticyclone this study uses both observations and numerical modeling experiments to investigate the lead lag relationship and the associated physical mechanism of the western north pacific anomalous anticyclone wnpac with sea surface temperature sst anomalies over the northern tropical atlantic nta the results show that the wnpac from late spring to the middle of autumn has a significant in phase relationship with the nta sst anomalies up to two seasons ahead this relationship reaches a peak when the nta sst leads by approximately 1 2 months and is nearly independent of the el nino southern oscillation variability diagnosis based on observations and numerical experiments using the community atmospheric model version 5 3 reveals that the nta warming favors intensified local convection activity during the spring autumn seasons causing enhanced low level convergence and upper level divergence i e ascending motion over the nta and opposite flow anomalies over the central tropical pacific the enhanced subsidence over the central tropical pacific in turn triggers an anomalous low level anticyclone over the western north pacific moreover the intensity of the anomalous local convection activity that is associated with the nta sst is closely related to the seasonal migration of the atlantic intertropical convergence zone itcz as the atlantic itcz migrates northward the nta sst induced local convection activity extends northward from a narrow band near the equator during early spring to nearly the entire nta region during the middle of summer leading to the strongest remote effect of the nta sst anomalies on the wnpac during late summer and early autumn

effects of topography on pyroclastic density current runout and formation of coignimbrites laboratory experiments of dilute mixtures of warm talc powder in air simulate dilute pyroclastic density currents pdcs and show the effects of topography on current runout buoyancy reversal and liftoff into buoyant plumes the densimetric and thermal richardson froude stokes and settling numbers for our experiments match those of natural pdcs the laboratory currents are fully turbulent although the experiments have lower reynolds numbers than pdcs in sum our experiments are dynamically similar to natural currents comparisons of currents traversing flat topography or encountering barriers show that runout distance is not significantly reduced for currents that traverse barriers with height

a new age within mis 7 for the homo neanderthalensis of saccopastore in the glacio eustatically forced sedimentary successions of the aniene river valley rome abstract field observations as well as borehole sedimentological and geochronologic data allow us to reconstruct the geologic setting of the aniene river valley in northern rome framing it within the recently recognized picture of temporally constrained glacio eustatically forced aggradational successions of this region the sedimentary successions cropping out in this area include those described in the literature of the early 20th century in saccopastore where two skulls of homo neanderthalensis were recovered based on the geometry elevation and sedimentologic features of the investigated sedimentary deposits the stratigraphic record of saccopastore is correlated with the aggradational succession deposited in response to sea level rise during glacial termination iii at the onset of mis 7 i e 250 ka corresponding to the local vitinia formation as opposed to previous correlation with the mis 5 interglacial and a locally defined tyrrhenian stage 130 ka this previous attribution was based on the interpretation of the sedimentary succession of saccopastore occurring between 15 and 21 m a s l as a fluvial terrace formed around 130 ka during the riss wurm interglacial ca 6 m above the present day alluvial plain of the aniene river in contrast to this interpretation a 40ar 39ar age of 129 2 ka determined for this study on a pyroclastic flow deposit intercalated in a fluvial lacustrine sequence forming a terrace 37 m a s l near the coast of rome constrains the aggradational succession in this area to mis 5 precluding the occurrence of an equivalent fluvial terrace at lower elevation in the inland sector of saccopastore we therefore interpret the stratigraphic record of saccopastore as the basal portion of the aggradational succession deposited in response to sea level rise during mis 7 whose equivalent fluvial terrace occurs around 55 m a s l in this region we also review the published paleontological and paleoethnological records recovered in saccopastore and demonstrate their compatibility with the faunal assemblages and lithic industries occurring in the sedimentary deposits of the vitinia formation while we show the lack of any unequivocal late pleistocene mis 5 affinity we therefore propose that the chronostratigraphic position of the saccopastore deposits containing the two skulls should be around 250 000 years as opposed to a previously preferred age of 130 000 years the revised age makes these skulls the oldest italian occurrences of h neanderthalensis and provides evidence for a substantially coeval appearance and evolutionary path with respect to central northern europe

early summer southern china rainfall variability and its oceanic drivers rainfall in southern china reaches its annual peak in early summer may june with strong interannual variability using a combination of observational analysis and numerical modeling the present study investigates the leading modes of this variability and its dynamic drivers a zonal dipole pattern termed the southern china dipole scd is found to be the dominant feature in early summer during 1979 2014 and is closely related to a low level anomalous anticyclone over the philippine sea psac and a eurasian wave train pattern over the mid high latitudes linear regressions based on observations and numerical experiments using the cam5 model suggest that the associated atmospheric circulation anomalies in early summer are linked to decaying el nino southern oscillation like sea surface temperature sst anomalies in the tropical pacific basin scale sst anomalies in the tropical indian ocean and meridional tripole like sst anomalies in the north atlantic in the previous winter to early summer the tropical pacific and indian ocean sst anomalies primarily exert an impact on the scd through changing the polarity of the psac while the north atlantic tripole like sst anomalies mainly exert a downstream impact on the scd by inducing a eurasian wave train pattern the north atlantic tripole like sst anomalies also make a relatively weak contribution to the variations of the psac and scd through a subtropical teleconnection modeling results indicate that the three basin combined forcing has a greater impact on the scd and associated circulation anomalies than the individual influence from any single oceanic basin

lithic breccias in intermediate volume phonolitic ignimbrites tenerife canary islands constraints on pyroclastic flow depositional processes abstract lithic occurrences within quaternary phonolitic ignimbrites on tenerife canary islands fall into three broad groupings 1 massive ignimbrite with large floating lithics 2 basal lithic rich bedforms and 3 matrix to clast supported lithic breccias within ignimbrite massive ignimbrite divisions commonly contain large outsized either solitary or scattered lithics which are mostly monomictic and derived locally from substrate erosion by pyroclastic density currents low and high aspect ratio lithic rich bedforms occur at the base of several ignimbrites most bedforms are asymmetric with steeper lee sides and they are buried by stratified and or massive ignimbrite internally most bedforms are massive some however exhibit an upstream inclined stratification indicating upcurrent migration of the bedform which is interpreted to record antidune like flow regimes locally derived basalt clasts are important constituents of some of these bedforms which have also been sourced through substrate erosion matrix to clast supported pebble to boulder sized lithic breccias occur at various heights within massive ignimbrite lithic breccias at or near the top of most ignimbrites contain vent derived lithics and record a late flux of lithic material from the vent during a sustained pyroclastic flow event extensive boulder sized breccias at the top of two ignimbrites are interpreted to record the onset of caldera collapse late in the pyroclastic flow forming eruptions some breccias contain boulders that are oriented with long axes transverse to flow indicating late stage tractional transport thin bedded matrix supported lithic breccias or lithic trains and stratified ignimbrite are commonly developed adjacent to or immediately above topographic highs these facies laterally merge into massive ignimbrite infilling topographic lows massive ignimbrite is interpreted to be deposited from a high particle concentration hindered settling zone inverse grading and long axis parallel to bedding imbrication of lithics within the thin bedded breccia facies are interpreted to record deposition under modified grain flow conditions a variety of depositional processes are thus recorded and the relationship between lithic breccias ignimbrite facies and topography indicates that there were spatial variations in depositional mechanisms at any one instant the vertical sequence of bedform stratified ignimbrite massive ignimbrite characterises the lower part of several ignimbrites and is interpreted to record increasing rates of deposition from a sustained current multilayered lithic breccias at various heights within massive ignimbrite are a form of clast compositional zonation and reflect temporal changes in supply flow competence and capacity

entrance of hot pyroclastic flows into the sea experimental observations the entrance of hot pyroclastic flows into water has been observed in a series of experiments in which shooting granular flows of hot ignimbrite ash 403 c of bulk density near that of water run down a smooth chute and enter a water filled tank at an angle of 26 flows of relatively cool ash 250 c mixing across the water surface over some distance from shore generates steam explosions forming fountains of wet and dry ash and convectively rising fine ash plumes steam explosions increase in strength and lateral extent towards higher ash temperatures and mass fluxes the explosions generate water waves that remain driven by massive fountain fallout across some distance from shore the ash fountains feed pyroclastic surges which advance down tank over water at high speeds underwater plumes of sediment falling from the ash fountains and surges drop coarse load onto the floor whereas the finer load forms a turbidity current high temperature and poor size sorting of pyroclastic flows are key parameters that determine the processes of interaction with water and the associated hazards poor sorting allows for the formation of ash cloud surges moving over water even from cool flows high temperatures cause littoral explosions and facilitate extensive mass transport over water all flows generate tsunami waves by different mechanisms

dune bedforms produced by dilute pyroclastic density currents from the august 2006 eruption of tungurahua volcano ecuador a series of pyroclastic density currents were generated at tungurahua volcano ecuador during a period of heightened activity in august 2006 dense pyroclastic flows were confined to valleys of the drainage network while dilute pyroclastic density currents overflowed on interfluves where they deposited isolated bodies comprising dune bedforms of cross stratified ash exposed on the surface here the description measurement and classification of more than 300 dune bedforms are presented four types of dune bedforms are identified with respect to their shape internal structure and geometry length width thickness stoss and lee face angles and stoss face length 1 elongate dune bedforms have smooth shapes and are longer in the flow direction than wide or thick internal stratification consists of stoss constructional thick lensoidal layers of massive and coarse grained material alternating with bedsets of fine laminae that deposit continuously on both stoss and lee sides forming aggrading structures with upstream migration of the crests 2 transverse dune bedforms show linear crests perpendicular to the flow direction with equivalent lengths and widths internally these bedforms exhibit finely stratified bedsets of aggrading ash laminae with upstream crest migration steep truncations of the bedsets are visible on the stoss side only 3 lunate dune bedforms display a barchanoidal shape and have stratification patterns similar to those of the transverse ones finally 4 two dimensional dune bedforms are much wider than long exhibit linear crests and are organized into trains elongate dune bedforms are found exclusively in proximal deposition zones transverse lunate and two dimensional dune bedforms are found in distal ash bodies the type of dune bedform developed varies spatially within an ash body transverse dune bedforms occurring primarily at the onset of deposition zones transitioning to lunate dune bedforms in intermediate zones and two dimensional dune bedforms exclusively on the lateral and distal edges of the deposits the latter are also found where flows moved upslope elongate dune bedforms were deposited from flows with both granular based and tractional flow boundaries that possessed high capacity and competence they may have formed in a subcritical context by the blocking of material on the stoss side we do not interpret them as antidune or chute and pool structures the dimensions and cross stratification patterns of transverse dune bedforms are interpreted as resulting from low competence currents with a significant deposition rate but we rule out their interpretation as antidunes a similar conclusion holds for lunate dune bedforms whose curved shape results from a sedimentation rate dependent on the thickness of the bedform finally two dimensional dune bedforms were formed where lateral transport exceeds longitudinal transport i e in areas where currents were able to spread laterally in low velocity zones we suggest that the aggrading ash bedsets with upstream crest migration were formed under subcritical flow conditions where the tractional bedload transport was less important than the simultaneous fallout from suspension this produced differential draping with no further reworking we propose the name regressive climbing dunes for structures produced by this process a rapid decrease in current velocity possibly triggered by hydraulic jumps affecting the entire parent flows is inferred to explain their deposition this process can in principle hold for any kind of particulate density current

investigation of variable aeration of monodisperse mixtures implications for pyroclastic density currents the high mobility of dense pyroclastic density currents pdcs is commonly attributed to high gas pore pressures however the influence of spatial and temporal variations in pore pressure within pdcs has yet to be investigated theory suggests that variability in the fluidisation and aeration of a current will have a significant control on pdc flow and deposition in this study the effect of spatially heterogeneous gas pore pressures in experimental pdcs was investigated sustained unsteady granular currents were released into a flume channel where the injection of gas through the channel base was controlled to create spatial variations in aeration maximum current front velocity results from high degrees of aeration proximal to the source rather than lower sustained aeration along the whole flume channel however moderate aeration i e 0 5 minimum static fluidisation velocity umf st sustained throughout the propagation length of a current results in greater runout distances than currents which are closer to fluidisation i e 0 9 umf st near to source then de aerating distally additionally although all aerated currents are sensitive to channel base slope angle the runout distance of those currents where aeration is sustained throughout their lengths increases by up to 54 with an increase of slope from 2 to 4 deposit morphologies a primarily controlled by the spatial differences in aeration where there is a large decrease in aeration the current forms a thick depositional wedge sustained gas aerated granular currents are observed to be spontaneously unsteady with internal sediment waves travelling at different velocities

processes in the pacific la nina onset triggered by the atlantic nino previous observational and model studies have shown that a warm cold event in the equatorial atlantic during the boreal summer are related to the development of a pacific la nina el nino event that is fully developed in the following winter although the connection takes place via atmospheric bridge the processes at work have not been clarified for such a remote and lagged relationship the present paper uses a partially coupled atmosphere ocean model to infer a mechanism by which a pacific el nino event can be developed in this way enhanced equatorial convection in the equatorial atlantic during a warm event results in enhanced subsidence and surface wind divergence over the equatorial pacific around the dateline this wind anomaly contributes to pile up water in the western equatorial pacific triggering a perturbation in the depth of the oceanic thermocline which propagates eastward as an equatorial kelvin wave from autumn to winter the thermocline shallowing as the wave propagates allows for cooling of the oceanic mixed layer through anomalous temperature advection by anomalous zonal currents and by mean vertical entrainment velocity zonal advective and thermocline feedbacks reinforce the surface winds anomalies over the central eastern equatorial pacific setting up the conditions for the development of a cold event in this ocean the sequence during an atlantic cold event is similar with the appropriate change in signs these findings are relevant to enso predictability at seasonal timescales

boiling over dense pyroclastic density currents during the formation of the 100 km 3 huichapan ignimbrite in central mexico stratigraphic and lithofacies analysis abstract a lithofacies analysis of the huichapan ignimbrite has been undertaken to evaluate its depositional history from large pyroclastic density currents the huichapan ignimbrite is a massive ignimbrite sheet with a maximum runout of at least 55 km and thickness variations between 6 and 80 m the lower portion of the huichapan ignimbrite consists of a large plateau 100 km3 69 km3 as dense rock equivalent dre of massive ignimbrites with welding variations from densely welded to partly welded devitrification and high temperature vapor phase alteration the lower part grades laterally to moderately welded and non devitrified ignimbrites these variations are interpreted as the sedimentation of density stratified pyroclastic density currents erupted as boiling over pulses from the huichapan donguinyo caldera complex at a continuous rate supporting deposition by quasi steady progressive aggradation of sustained and hot currents to the north of the caldera the lower portion of the ignimbrite consists of a small plateau

structures and grain size distribution in surge deposits as a tool for modelling the dynamics of dilute pyroclastic density currents at la fossa di vulcano aeolian islands italy abstract turbulent dilute pyroclastic density currents surges represent the most common and hazardous type of eruptive event at la fossa di vulcano during the volcanic history these currents have frequently passed over the topographic barrier of la fossa caldera maintaining a high velocity we interpret lateral facies variation in terms of fluid dynamic behaviour by analyses of structures thickness and grain size of correlated surge layers from the palizzi succession using these data we validate a sedimentation model of traction fed by a low concentration turbulent and stratified current by the use of a sedimentological model we have then reconstructed the fluid dynamic parameters of the current and calculated the maximum horizontal distance that solid particles travelled before deposition results have been checked against actual deposit features consistency of data indicates that the method used in this study even if it is approximate and needs further refinement by specific experimental approaches and numerical simulations gives a reasonable indication of surge characters such as velocity and density which are relevant for hazard calculation and risk assessment

event stratigraphy and emplacement mechanisms of the last major caldera eruption on sete cidades volcano sao miguel azores the 16 ka santa barbara formation sete cidades is an active yet dormant central volcano located in the westernmost part of sao miguel island azores archipelago characterized by a 5 km wide multi stage caldera formed in the last 36 ka in this work we present new stratigraphic grain size petrographic and geochemical data of the 16 ka santa barbara formation related to the last stage of caldera formation we define the lithostratigraphy of this pyroclastic sequence into three members based on stratigraphic position lithofacies and distribution of the deposits the preserved deposits outcrop in a narrow area over 5 km between the caldera rim and the coastal cliffs and include ash beds with dispersed lapilli lower member coarse grained pumice and lithic rich lapilli tuffs middle member and pumice fall deposits upper member the minimum bulk volume preserved on land is about 0 03 km3 however the majority of the pyroclastic products were deposited in the ocean the presence of magmatic mafic enclaves 48 wt sio2 within the juvenile pumice clasts 63 wt sio2 is testimony to the injection of trachybasaltic magma into the trachytic magma reservoir shortly prior to eruption the mingling of these two magmas acted as the eruption trigger the eruption history can be divided into three main phases each characterized by different eruption styles and deposit emplacement mechanisms accompanied by enlargement of the caldera the eruption started with a sequence of short lived explosions generating unstable columns that produced ash fallout and generated dilute pyroclastic density currents mostly deposited in the sw sector of the volcano the climax of the eruption was marked by successive low fountaining events that generated high particle concentration pyroclastic density currents emplaced on the sw and ne flanks the final phase of the eruption was characterized by establishment of a sub plinian eruption column that produced pumice fall deposits on the ne flank of the volcano our study allows delineation of eruptive scenarios for future major explosive eruptions at sete cidades a volcano which is located on the most populated island of the azores sao miguel island and can provide a framework for the assessment of volcanic hazards on oceanic islands elsewhere

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

mis 9 to mis 5 terraces along the tyrrhenian sea coast of latium central italy assessing interplay between sea level oscillations and tectonic activity abstract we present a review of the geomorphology of the tyrrhenian sea coast of central italy integrated by a novel structural geomorphological study coupled with statistical analysis of topographic culminations and comparison with a digital elevation model aimed at reconstructing a suite of paleo surfaces corresponding to remnant portions of marine terraces we performed geochronological sedimentological micromorphological and mineralogical investigations on the deposits forming the different paleo surfaces between civitavecchia and anzio towns in order to provide chronostratigraphical paleogeographical and paleoenvironmental constraints using the newly achieved dataset we correlate these paleo surfaces with the coastal terraces formed during past sea level highstands as recognized by previous studies and we refine their correlation with the marine isotope stage mis timescale in particular we have extended our geomorphological analyses landward in the area between the tiber river mouth and anzio in order to include the oldest paleo surface developed above the deposits of the last large explosive eruption at 365 4 ka in the colli albani volcanic district results of this study allow us to recognize a set of higher paleo surfaces at elevation ranging 108 thought 71 m a s l which we interpret as one tectonically displaced widespread coastal terrace originated during the mis 9 1 highstand we correlate the previously identified paleo surfaces of 66 62 m a s l and 56 52 m a s l with the equivalent coastal terraces developed during the sea level highstands of sub stages 7 5 and 7 3 7 1 moreover based on data from literature on relative elevation of maximum sea level during the highstands of mis 11 through mis 5 1 we assess the regional uplift and the concurrent tectonic displacements that have occurred since 900 ka in this area

mass budget partitioning during explosive eruptions insights from the 2006 paroxysm of tungurahua volcano ecuador how and how much the mass of juvenile magma is split between vent derived tephra pdc deposits and lavas i e mass partition is related to eruption dynamics and style estimating such mass partitioning budgets may reveal important for hazard evaluation purposes we calculated the volume of each product emplaced during the august 2006 paroxysmal eruption of tungurahua volcano ecuador and converted it into masses using high resolution grainsize componentry and density data this dataset is one of the first complete descriptions of mass partitioning associated with a vei 3 andesitic event the scoria fall deposit near vent agglutinate and lava flow include 28 16 and 12 wt of the erupted juvenile mass respectively much 44 wt of the juvenile material fed pyroclastic density currents i e dense flows dilute surges and co pdc plumes highlighting that tephra fall deposits do not depict adequately the size and fragmentation processes of moderate pdc forming event the main parameters controlling the mass partitioning are the type of magmatic fragmentation conditions of magma ascent and crater area topography comparisons of our dataset with other pdc forming eruptions of different style and magma composition suggest that moderate andesitic eruptions are more prone to produce pdcs in proportions than any other eruption type this finding may be explained by the relatively low magmatic fragmentation efficiency of moderate andesitic eruptions these mass partitioning data reveal important trends that may be critical for hazard assessment notably at frequently active andesitic edifices this article is protected by copyright all rights reserved

pacific east asian teleconnection how does enso affect east asian climate observational evidence is presented to show a teleconnection between the central pacific and east asia during the extreme phases of enso cycles this pacific east asian teleconnection is confined to the lower troposphere the key system that bridges the warm cold events in the eastern pacific and the weak strong east asian winter monsoons is an anomalous lower tropospheric anticyclone cyclone located in the western north pacific the western north pacific wind anomalies develop rapidly in late fall of the year when a strong warm or cold event matures the anomalies persist until the following spring or early summer causing anomalously wet dry conditions along the east asian polar front stretching from southern china northeastward to the east of japan kuroshio extension using atmospheric general circulation and intermediate models the authors show that the anomalous philippine sea anticyclone results from a rossby wave response to suppressed convective heating which is induced by both the in situ ocean surface cooling and the subsidence forced remotely by the central pacific warming the development of the anticyclone is nearly concurrent with the enhancement of the local sea surface cooling both the anticyclone and the cooling region propagate slowly eastward the development and persistence of the teleconnection is primarily attributed to a positive thermodynamic feedback between the anticyclone and the sea surface cooling in the presence of mean northeasterly trades the rapid establishment of the philippine sea wind and sst anomalies implies the occurrence of extratropical tropical interactions through cold surge induced exchanges of surface buoyancy flux the central pacific warming plays an essential role in the development of the western pacific cooling and the wind anomalies by setting up a favorable environment for the anticyclone sst interaction and midlatitude tropical interaction in the western north pacific

complex proximal deposition during the plinian eruptions of 1912 at novarupta alaska proximal 3 km deposits from episodes ii and iii of the 60 h long novarupta 1912 eruption exhibit a very complex stratigraphy the result of at least four transport regimes and diverse depositional mechanisms they contrast with the relatively simple stratigraphy and inferred emplacement mechanisms for the previously documented better known medial distal fall deposits and the valley of ten thousand smokes ignimbrite the proximal products include alternations and mixtures of both locally and regionally dispersed fall ejecta and numerous thin complex deposits of pyroclastic density currents pdcs with no regional analogs the locally dispersed component of the fall deposits forms sector confined wedges of material whose thicknesses halve radially from and concentrically about the vent over distances of 100 300 m cf several kilometers for the medial distal fall deposits this locally dispersed fall material and many of the associated pdc deposits is rich in andesitic and banded pumices and richer in shallow derived wall rock lithics in comparison with the coeval medial fall units of almost entirely dacitic composition there are no marked contrasts in grain size in the near vent deposits however between locally and widely dispersed beds and all samples of the proximal fall deposits plot as a simple continuation of grain size trends for medial distal samples associated pdc deposits form a spectrum of facies from fines poor avalanched beds through thin bedded landscape mantling beds to channelized lobes of pumice block rich ignimbrite the origins of the novarupta near vent deposits are considered within a spectrum of four transport regimes 1 sustained buoyant plume 2 fountaining with co current flow 3 fountaining with counter current flow and 4 direct lateral ejection the novarupta deposits suggest a model where buoyant stable regime 1 plumes characterized most of episodes ii and iii but were accompanied by transient and variable partitioning of clasts into the other three regimes only one short period of vent blockage and cessation of the plinian plume occurred separating episodes ii and iii which was followed by a single pdc interpreted as an overpressured blast involving direct lateral ejection in contrast regimes 2 and 3 were reflected by spasmodic sedimentation from the margins of the jet and perhaps lower plume which were being strongly affected by short lived instabilities these instabilities in turn are inferred to be associated with heterogeneities in the mixture of gas and pyroclasts emerging from the vent of the parameters that control explosive eruptive behavior only such sudden and asymmetrical changes in the particle concentration could operate on time scales sufficiently short to explain the rapid changes in the proximal 1912 products

decadal shift in el nino influences on indo western pacific and east asian climate in the 1970s abstract el nino s influence on the subtropical northwest nw pacific climate increased after the climate regime shift of the 1970s this is manifested in well organized atmospheric anomalies of suppressed convection and a surface anticyclone during the summer june august of the el nino decay year jja 1 a season when equatorial pacific sea surface temperature sst anomalies have dissipated in situ observations and ocean atmospheric reanalyses are used to investigate mechanisms for the interdecadal change during jja 1 the influence of the el nino southern oscillation enso on the nw pacific is indirect being mediated by sst conditions over the tropical indian ocean tio the results here show that interdecadal change in this influence is due to changes in the tio response to enso during the postregime shift epoch the el nino teleconnection excites downwelling rossby waves in the south tio by anticyclonic wind curls these rossby waves propagate slowly westward causing persistent sst warmi

nature and velocity of pyroclastic density currents inferred from models of entrainment of substrate lithic clasts abstract deposits of pyroclastic density currents pdcs often contain accidental lithic clasts of typical size of 0 1 1 m captured from an underlying substrate by the parent flows at distances up to several tens of kilometers from the eruptive vent in order to gain insights into the nature of pdcs this study investigates the conditions required for entrainment of particles from a granular substrate by a gas particle density current with special emphasis to ignimbrite forming currents whose dynamics are controversial the two types of physics of emplacement of pdcs proposed in literature are considered the first model deals with a hydraulically rough dilute turbulent pdc of bulk density of 1 10 kg m 3 and considers that entrainment through both traction and saltation is controlled by a shield criterion at high 10 4 particle reynolds number the second model considers entrainment by a pdc consisting of a dense basal flow of bulk density of the order of 10 3 kg m 3 and with high interstitial gas pore pressure this model involves uplift of substrate particles caused by an upward pressure gradient at the flow substrate interface and then transport and deposition on the aggrading basal deposit of the flow as demonstrated by recent laboratory experiments results show that a dilute pdc can entrain blocks of maximum size of 10 15 cm for a block density of 2000 3000 kg m 3 if maximum current velocities up to 100 m s are taken into account this in turn suggests that larger heavier blocks found in deposits were captured by pdcs if these had a dense basal flow the dense flow model predicts that pdcs have the potential to entrain metric blocks whose maximum size up to 2 5 m decreases with decreasing atmospheric pressure i e increasing altitude application of the model considering published data on the characteristics of accidental blocks in several well studied ignimbrites indicates that the velocity of the parent dense pdcs was up to 25 30 m s

el nino modoki and its possible teleconnection 1 using observed data sets mainly for the period 1979 2005 we find that anomalous warming events different from conventional el nino events occur in the central equatorial pacific this unique warming in the central equatorial pacific associated with a horseshoe pattern is flanked by a colder sea surface temperature anomaly ssta on both sides along the equator empirical orthogonal function eof analysis of monthly tropical pacific ssta shows that these events are represented by the second mode that explains 12 of the variance since a majority of such events are not part of el nino evolution the phenomenon is named as el nino modoki pseudo el nino modoki is a classical japanese word which means a similar but different thing the el nino modoki involves ocean atmosphere coupled processes which include a unique tripolar sea level pressure pattern during the evolution analogous to the southern oscillation in the case of el nino hence the total entity is named as el nino southern oscillation enso modoki the enso modoki events significantly influence the temperature and precipitation over many parts of the globe depending on the season the impacts over regions such as the far east including japan new zealand western coast of united states etc are opposite to those of the conventional enso the difference maps between the two periods of 1979 2004 and 1958 1978 for various oceanic atmospheric variables suggest that the recent weakening of equatorial easterlies related to weakened zonal sea surface temperature gradient led to more flattening of the thermocline this appears to be a cause of more frequent and persistent occurrence of the enso modoki event during recent decades

synthesizing large scale pyroclastic flows experimental design scaling and first results from pele pyroclastic flow eruption large scale experiment pele is a large scale facility for experimental studies of pyroclastic density currents pdcs it is used to generate high energy currents involving 500 6500 m3 natural volcanic material and air that achieve velocities of 7 30 m s 1 flow thicknesses of 2 4 5 m and runouts of 35 m the experimental pdcs are synthesized by a controlled eruption column collapse of ash lapilli suspensions onto an instrumented channel the first set of experiments are documented here and used to elucidate the main flow regimes that influence pdc dynamic structure four phases are identified 1 mixture acceleration during eruption column collapse 2 column slope impact 3 pdc generation and 4 ash cloud diffusion the currents produced are fully turbulent flows and scale well to natural pdcs including small to large scales of turbulent transport pele is capable of generating short pulsed and sustained currents over periods of several tens of seconds and dilute surge like pdcs through to highly concentrated pyroclastic flow like currents the surge like variants develop a basal 0 05 m thick regime of saltating rolling particles and shifting sand waves capped by a 2 5 4 5 m thick turbulent suspension that grades upward to lower particle concentrations resulting deposits include stratified dunes wavy and planar laminated beds and thin ash cloud fall layers concentrated currents segregate into a dense basal underflow of 0 6 m thickness that remains aerated this is capped by an upper ash cloud surge 1 5 3 m thick with 100 to 10 4 vol particles their deposits include stratified massive normally and reversely graded beds lobate fronts and laterally extensive veneer facies beyond channel margins

recurrence of volcanic activity along the roman comagmatic province tyrrhenian margin of italy and its tectonic significance 1 the roman comagmatic province rcp developed along the nw se striking extensional tyrrhenian margin of italy since middle pleistocene and includes ultrapotassic small eruptive centers and large volcanic districts by means of a review and statistical analysis of a large data set of radiometric ages we demonstrate that in the rcp major eruptive cycles occurred at different volcanic districts almost simultaneously and with an almost constant recurrence time our analysis started with the alban hills volcanic district where a detailed 40ar 39ar chronology of the eruptive cycles has been recently achieved and then extended to include a database of 264 radiometric ages of the rcp volcanics published since 1980 statistical treatment of the data set shows a marked non poissonian behavior of volcanic activity through the whole magmatic province with climax of each major eruptive cycle occurring simultaneously at the different districts approximately every 48 kyr the concomitant eruption of many volcanic districts spreading over a 250 km long chain implies a control mechanism acting at a regional scale after discussing the possible relationship between tectonics sea level changes and volcanic activity we hypothesize that constant rates of slab retreat during middle pleistocene could have been a key factor controlling the rate of magma upwelling and the major cycles of recharge of the magma chambers as a working hypothesis we propose that the duration of the cycles may be related to 1 progressive weakening of the crust due to magma underplating or 2 cyclic injection of convective plumes into the mantle wedge

numerical analysis of the effect of topography on deposition from dilute pyroclastic density currents abstract pyroclastic density currents pdcs vary between two end members concentrated and dilute when a pdc interacts with an uneven topography the flow field variables velocity pressure bulk density particle concentration may drastically change near the flow substrate boundary these changes may significantly affect the sediment flux and the resulting deposits can record the effects in their facies architecture here we show by means of numerical simulations how a dilute pyroclastic density current interacts with four different types of simple topography namely flat one hill one valley and two hills our numerical scheme treats the very fine particles as being in full thermo mechanical equilibrium with the volcanic gas i e a dusty gas a dusty gas air mixture is defined as a mixture of dusty gas and atmospheric air the trajectories of the coarser particles or discrete phase three grain size classes of 1 mm 5 mm and 10 mm and density of 1500 kg m 3 are tracked as lagrangian particles that interact with the dusty gas air mixture through two way momentum and energy coupling numerical results are used to analyze the local effects of topography on the deposition of the lagrangian particles by monitoring with time and space the local changes at the boundary between the current and the substrate the results show that the sediment flux in the flow boundary zone increases near the stoss sides of hills and in the valleys relative to the flat reference case whereas it decreases along the lee flanks and on top of the hills we use the sediment flux in the flow boundary zone and the grain size distribution of the lagrangian particles as proxies of the deposit features and by these parameters we qualitatively compare simulations with deposits of known eruptions

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

tropical sst forcing on the anomalous wnp subtropical high during july august 2010 and the record high sst in the tropical atlantic in summer of 2010 the western north pacific subtropical high wnpsh was extremely strong and exhibited unusual westward extension which resulted in record breaking warmth in japan and considerably below normal and westward shifted tropical cyclone activity in the western north pacific wnp although a moderate la nina occurred the sea surface temperature sst in the northern indian ocean nio and tropical atlantic ta was considerably high in this study we argued that the la nina cold sst alone was not sufficient to maintain the strong wnpsh of 2010 and that the unusually warm ssts in the nio and ta markedly contributed to the enhancement and westward shift of the wnpsh in the boreal summer of that year we focused on the effects of sea surface temperature anomalies in the tropical atlantic ta sstas which have been seldom explored and are poorly understood compared with the effects of sstas in the tropical pacific and nio the warm ta sst forced a westward extending overturning circulation with a sinking branch over the central pacific ocean which produced a remote response similar to the la nina condition and enhanced the wnpsh the warm ta sst also induced the cyclonic anomaly in the tropical eastern north pacific a distinct phenomenon not observed in a canonical la nina event furthermore we demonstrated that the anomalous near surface circulation associated with the negative north atlantic oscillation might play a more dominant role than that of the 2009 el nino in inducing the record high sst in the ta in 2010

history of glacial terminations from the tiber river rome insights into glacial forcing mechanisms 1 we document the aggradational history of the tiber river delta through the last 17 000 years by means of 17 new 14c ages from peat or wood collected from the delta sediment an abrupt change in sediment clast size grading from gravel to clay occurred between 13 63 0 20 and 12 80 0 15 ka indicating that it was synchronous with the last glacial termination with no appreciable phase lag knowing this phase relationship enables us to reduce the magnitudes of age uncertainties for aggradational sections corresponding to glacial terminations ix through iii which we had dated previously by 40ar 39ar methods glacial terminations viii vi and iv precede beyond 95 confidence the ages predicted by northern hemisphere summer insolation maxima additionally we find that each of these seven glacial terminations follows particularly mild insolation minima which we suggest may be regarded as the preconditioning factor to trigger a glacial termination

recognizing unsteadiness in the transport systems of dilute pyroclastic density currents laboratory density currents generated with unsteady source conditions provide insight into the distances and timescales over which unsteadiness at the vent should persist in natural dilute pyroclastic density currents pdcs the laboratory experiments comprise heated 20 m talc particles turbulently suspended in air and introduced as density currents into an 8 5 6 2 6 m air filled chamber the densimetric and thermal richardson froude stokes and settling numbers are all similar to those of natural dilute pdcs although the experiments reynolds numbers are several orders of magnitude less than natural pdcs the experiments are fully turbulent and are thus dynamically similar to some dilute natural pdcs unsteadiness in the experiments is generated by adding two pauses of duration t to the eruption e g a 100 s experiment comprising three 33 s pulses separated by two pauses of t 10 s propagation distance of the leading head is proportional to the square root of time positions of the trailing pulses have similar time dependence but they generally travel slightly faster than and thus catch and merge with the leading current trailing pulses are more easily distinguished from the body of the preceding current when t is large analysis of turbulent structures through space and time shows that unsteadiness at the eruption source can be distinguished from turbulent fluctuations in the currents when t is greater than the integral turbulent timescale of the current body body a statistical measure of the characteristic timescale of unsteadiness within a turbulent flow the distances over which unsteadiness in the transport system persist scale with t body and the ratio of the thermal richardson numbers of the trailing and leading pulses as t body increases from 2 to 4 trailing pulses remain distinct over an increasing fraction of the lead runout distance when subsequent pulses have higher rit than the leading current they coalesce with the leading pulse at more proximal distances but when rit is much lower they remain distinct over distances similar to that of the leading head even when t is similar to body for currents that have depositional timescales comparable to or shorter than body deposits should be expected to preserve a record of unsteadiness over the distances that unsteady pulses persist

internal flow variations and diachronous sedimentation within extensive sustained density stratified pyroclastic density currents flowing down gentle slopes as revealed by the internal architectures of ignimbrites on tenerife during a protracted explosive eruption at least four laterally extensive and sustained pyroclastic density currents radiated across the flanks of las canadas volcano tenerife each pyroclastic current developed marked local and regional spatial variations in response to the incised gently concave substrate topography the locations of these variations shifted in space as rapid sedimentation from the current progressively buried and modified the topography this complex shifting response of the density currents to minor topographic variations has been reconstructed in high resolution over a wide area 500 km2 using the internal architecture of cryptic time surfaces entrachrons marked by compositional zoning in the deposit including variations in clast types valley side field relations reveal that the currents were density stratified yet at a single instant in time the lower levels of each current comprised a granular fluid at some locations but were fully dilute and turbulent at others moreover the locations of these variations shifted geographically as the topography changed during the eruption the variations within the current are recorded by numerous superbly exposed gradational transitions from various stratified to massive lithofacies both laterally and in the downcurrent direction individual currents were regionally widespread and travelled 15 km but deposited only in longitudinally restricted localised zones that spanned several small valleys and interfluves the currents bypassed slopes up and downcurrent of the restricted depositional zones without depositing the locations of deposition then gradually shifted with time such that the extensive deposit sheet was gradually assembled beneath the sustained current in a diachronous fashion onlap relationships of internal entrachrons reveal that the base of the ignimbrite sheet and even the bases of individual flow units are markedly diachronous deposition of a flow unit commenced and ceased at different times in different places this study suggests that in hazard assessments a models of density currents that incorporate only pre existing topography e g from dems may give misleading results in the case of sustained currents because sedimentation from these significantly modifies the topography during emplacement altering flow paths b frequencies and scales of previous pyroclastic currents determined from pyroclastic successions are likely to be significantly under estimated because currents commonly bypass without leaving a deposit record and c even where preservation appears to be complete an ignimbrite at a single exposure commonly will not record the current s entire flow history at that site

transport and deposition of pyroclastic the complex eruption sequence from the 1000 a d caldera forming eruption of volcan ceboruco known as the jala pumice offers an exceptional opportunity to examine how pyroclastic material is transported and deposited from pyroclastic density currents over variable topography three main pyroclastic surge deposits sl s2 and s3 and two pyroclastic flow deposits marquesado and north flank pfds were emplaced during this eruption pyroclastic surge deposits are massive planar or cross bedded poor to well sorted and display fluctuations in thickness median diameter sorting and lithology as a function of distance topography and flow dynamics marquesado pyroclastic flow deposits reveal lateral variations from massive poorly sorted deposits located within 5 km of ceboruco to planar bedded moderately well sorted deposits located 15 km away over the nearly horizontal topography to the south of ceboruco north flank pyroclastic flow deposits also reveal lateral variations from massive poorly sorted deposits located within 4 km of ceboruco to planar bedded moderately well sorted deposits located 8 km away atop an escarpment that steeply rises 230 m from the northern valley floor field observations granulometric analyses component analyses and crystal sedimentation calculations along flow parallel sampling transects all suggest that both surges and flows were density stratified currents where deposition occurred from a basal region of higher particle concentration that was supplied from an overlying dilute layer that transports particles in suspension this supports the idea of a transition between flow and surge end members with variations in particle concentration topography greatly affects the transport and depositional capacity of the pyroclastic density currents as a result of blocking either by topographic obstacles or by abrupt breaks at the base of volcano slopes whereas the origin of jala pumice surge deposits phreatomagmatic versus magmatic appears to have little impact on their flow dynamics

insights into the formation and dynamics of coignimbrite plumes from one dimensional models coignimbrite plumes provide a common and effective mechanism by which large volumes of fine grained ash are injected into the atmosphere nevertheless controls on formation of these plumes as a function of eruptive conditions are still poorly constrained herein two 1 d axysymmetric steady state models were coupled the first describing the parent pyroclastic density current and the second describing plume rise global sensitivity analysis is applied to investigate controls on coignimbrite plume formation and describe coignimbrite source and the maximum plume height attained for a range of initial mass flow rates between 108 and 1010 kg s modeled liftoff distance the distance at which neutral buoyancy is attained assuming radial supercritical flow is controlled by the initial flow radius gas mass fraction flow thickness and temperature the predicted decrease in median grain size between flow initiation and plume liftoff is negligible calculated initial plume vertical velocities assuming uniform liftoff velocity over the pyroclastic density current invasion area are much greater several tens of m s than those previously used in modeling coignimbrite plumes 1 m s such velocities are inconsistent with the fine grain size of particles lofted into coignimbrite plumes highlighting an unavailability of large clasts possibly due to particle segregation within the flow prior to plume formation source radius and initial vertical velocity have the largest effect on maximum plume height closely followed by initial temperature modeled plume heights are between 25 and 47 km comparable with plinian eruption columns highlighting the potential of such events for distributing fine grained ash over significant areas

mobility of a large volume pyroclastic flow emplacement of the campanian ignimbrite italy abstract the trachytic campanian ignimbrite originally exposed over a 30 000 km 2 area around naples italy is the product of a highly energetic gas rich eruption the deposit lies in valleys and isolated watersheds and in its medial and distal extent to the south north and east of naples the campanian pyroclastic current encountered mountains exceeding 1000 meters anisotropy of magnetic susceptibility ams measurements indicate that the campanian pyroclastic current the transport system traveled radially outward from the phlegrean fields area but the ignimbrite forming flow the deposition system that developed from the base of the transport system moved downslope from mountainsides to valleys including slopes facing the eruption source and flowed down drainage systems from intermontane basins the campanian pyroclastic current flowed 35 km over the water of the bay of naples to deposit 43 m of ignimbrite on the south shore and also overtopped a 685 1000 m high ridge of the sorrento peninsula to deposit more on the other side the distribution of the ignimbrite and the measured flow directions suggest that the campanian pyroclastic current moved across the landscape as an expanded and therefore turbulent decompressing flow rather than as a high density nonturbulent sheet like current moving over mountains by momentum acquired by eruption column collapse strong expansion is corroborated by shard morphology that indicates derivation from highly inflated pumice and suggests vesicles must have been at least 80 by volume of the original magma

the contribution of experimental volcanology to the study of the physics of eruptive processes and related scaling issues a review abstract the experimental approach has become a major tool increasingly used by volcanologists in recent decades to investigate the physics of eruptive processes in complement to field and theoretical works researchers have developed various methodologies to study volcanic phenomena at reduced length scale the works involve natural or analogue materials and their types range from first order tests to identify fundamental processes and make qualitative comparison with field observations to more sophisticated experiments in which precise data obtained in a controlled environment can be used to validate outputs of theoretical models scaling is a central issue to ensure dynamic similarity between the small scale experiments and the large scale volcanic phenomena when natural conditions cannot be simulated due to inherent length scale difference and or technical limitations in this respect dimensionless numbers are used to map physical regimes and to define scaling laws which allow experimental results to be extrapolated to natural scale this review presents the variety of experimental studies conducted to investigate subterraneous and aerial volcanic phenomena involving in particular fluid particle mixtures we focus on the major scaling issues and the physical regimes investigated and we also highlight in a historical perspective some of the major advances achieved through experimental studies finally we conclude on some perspectives for future works

tropical atlantic sea surface temperature variability and its relation to el nino southern oscillation past analyses of tropical atlantic sea surface temperature variability have suggested a dipole behavior between the northern and southern tropics across the intertropical convergence zone itcz by analyzing an improved 43 year 1950 1992 record of sst smith et al 1996 and other data derived from the comprehensive ocean atmosphere data set coads it is shown that the regions north and south of the itcz are statistically independent of each other at the seasonal to interannual timescales dominating the data confirming the conclusions of houghton and tourre 1992 some dipole behavior does develop weakly during the boreal spring season when there is a tendency for sst anomaly west of angola to be opposite of that in the tropical north atlantic it is further shown that tropical atlantic sst variability is corre lated with pacific e1 nifio southern oscillation enso variability in several regions the ma jor region affected is the north atlantic area of ne trades west of 40ow along 10on 20on and extending into the caribbean there about 50 80 of the anomalous sst variability is associ ated with the pacific enso with atlantic warmings occurring 4 5 months after the mature phases of pacific warm events an analysis of local surface flux fields derived from coads data shows that the enso related atlantic warmings occur as a result of reductions in the surface ne trade wind speeds which in turn reduce latent and sensible heat losses over the region in ques tion as well as cooling due to entrainment this enso connection is best developed during the boreal spring following the most frequent season of maximum enso anomalies in the pacific a region of secondary covariability with enso occurs along the northern edge of the mean itcz position and appears to be associated with northward migrations of the itcz when the north at lantic warmings occur although easterly winds are intensified in the western equatorial atlantic in response to pacific warm events they do not produce strong local changes in sst contrary to expectations from studies based on equatorial dynamics these teleconnected wind anomalies do not give rise to significant correlations of sst in the gulf of guinea with the pacific enso as the teleconnection sequence matures strong se trades at low southern latitudes follow the de velopment of the north atlantic sst anomaly and precede by several months the appearance of weak negative sst anomalies off angola and stronger positive anomalies extending eastward from southern brazil along 15o 30os

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

hazard map of el chichon volcano chiapas mexico constraints posed by eruptive history and computer simulations abstract el chichon volcano consists of a 2 km wide somma crater compound cone 0 2 ma old with peripheral domes with a central crater reactivated several times during the holocene the most recent eruption at el chichon occurred from march 28 to april 4 1982 resulting in the worst volcanic disaster during historical times in mexico killing more than 2000 people and destroying nine towns and small communities the volcanic hazard map of el chichon is based on detailed field work that documented twelve eruptions during the last 8000 years and computer simulations to validate the results computer simulations were first performed over pre 1982 topography mimicking the extent of the actual deposits produced and afterwards run over post 1982 topography these eruptions have produced pyroclastic fall surge flow and lahar deposits pyroclastic flows have different volumes and heim coefficients varying from 0 2 pumice flows to 0 15 block and ash flows and 0 10 ash flows simulations using flow3d and titan2d indicate that pumice flows and block and ash flows can fill the moat area and follow main ravines up to distances of ca 3 km from the crater with no effect on populations around the volcano on the other hand more mobile ash flows related to column collapse events can reach up to 4 km from the vent but will always follow the same paths and still not affect surrounding populations the energy cone model was used to simulate the outflow of pyroclastic surges based on the 1982 event h l 0 1 and 0 2 and shows that surges may reach some towns around the volcano explosive activity also produced plinian columns higher than 27 km that were dispersed towards the ne by prevailing winds future fallout will seriously disturb the lives of more than 70 000 inhabitants of cities and towns located in a radius of 35 km from the volcano lahar events in the past originated from unconsolidated pyroclastic deposits with volumes varying from 1 to 4 10 6 m 3 for single pulses to a large one from dam breakout with a volume of 40 10 6 m 3 we used the laharz code to model future events with volumes of 1 2 and 3 10 6 m 3 that will reach as far as 10 14 and 16 km from the crater posing some danger to the communities of nicapa volcan and ostuacan even some time after the climax of the eruption

interannual relationships between the tropical sea surface temperature and summertime subtropical anticyclone over the western north pacific 1 the interannual variability of the western north pacific subtropical high wnpsh in boreal summer is investigated with the use of the ncep ncar reanalysis data the most significant change of the 500 hpa geopotential height field appears at the western edge of the wnpsh with dominant 2 3 year and 3 5 year power spectrum peaks the 2 3 year oscillation of the wnpsh and associated circulation and sea surface temperature sst patterns possess a coherent eastward propagating feature with a warm sst anomaly ssta and anomalous ascending motion migrating from the tropical indian ocean in the preceding autumn to the maritime continent in the concurrent summer of a strong wnpsh a strong wnpsh is characterized by anomalous anticyclonic circulation and maximum subsidence in the western north pacific wnp the anomalous wnpsh circulation has an equivalent barotropic vertical structure and resides in the sinking branch of the local hadley circulation triggered by enhanced convection over the maritime continent a heat budget analysis reveals that the wnpsh is maintained by radiative cooling the 3 5 year oscillation of the wnpsh exhibits a quasi stationary feature with a warm ssta anomalous ascending motion located in the equatorial central eastern pacific and indian ocean and a cold ssta anomalous descending motion located in the western pacific the anomaly pattern persists from the preceding winter to the concurrent summer of a high wnpsh the greatest descent is located to the southeast of the anomalous anticyclone center where a baroclinic vertical structure is identified the zonal phase difference and the baroclinic vertical structure suggest that the anomalous anticyclone on this timescale is a rossby wave response to a negative latent heating associated with the persistent local cold ssta echam4 model experiments further confirm that the 2 3 year mode is driven by the ssta forcing over the maritime continent while the 3 5 year mode is driven by the local ssta in the wnp

the rate of sedimentation from turbulent suspension an experimental model with application to pyroclastic density currents and discussion on the grain size dependence of flow runout

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

the run out distance of large scale pyroclastic density currents a two layer depth averaged model abstract we have developed a new two layer pyroclastic density current pdc model that considers the effects of the entrainment and thermal expansion of ambient air in the upper dilute layer in order to investigate the dominant factors controlling the run out distance of large scale pdcs numerical simulations show a dilute current spreading radially from the collapsing eruption column edge and forming a thin dense basal current through particle settling i e a two layer pdc forms forward propagation of the two layer pdc stops owing to lift off of the dilute current and or deposition of the dense current and eventually the two layer pdc converges to a steady state our parametric study classifies the flow patterns of steady state two layer pdcs into one of two flow regimes according to the relative magnitude of the run out distances of the dilute and dense currents this relative magnitude critically depends on the ratio of the deposition speed at the base of the dense current d to the speed of particles settling from the dilute current to the dense current ws the run out distance of the whole two layer pdc is determined by that of the dilute current when d ws is large 4 10 3 otherwise when d ws is small 4 10 3 the run out distance of the dense basal current exceeds that of the dilute current the former increases as d ws decreases the run out distance of the dilute current is too short to explain some of the long run out distances of pdcs observed in the field e g the 1991 pinatubo and 2014 kelud eruptions it is suggested that the dense current traveling beyond the lift off point of the parent dilute current plays a significant role in the emplacement of pdcs with such long run out distances

impact of the indian ocean sst basin mode on the asian summer monsoon 1 following an el nino event a basin wide warming takes place over the tropical indian ocean peaks in late boreal winter and early spring and persists through boreal summer our observational analysis suggests that this indian ocean warming induces robust climatic anomalies in the summer indo west pacific region prolonging the el nino s influence after tropical east pacific sea surface temperature has returned to normal in response to the indian ocean warming precipitation increases over most of the basin forcing a matsuno gill pattern in the upper troposphere with a strengthened south asian high near the ground the southwest monsoon intensifies over the arabian sea and weakens over the south china and philippine seas an anomalous anticyclonic circulation forms over the subtropical northwest pacific collocated with negative precipitation anomalies all these anomaly patterns are reproduced in a coupled model simulation initialized with a warming in the tropical indian ocean mixed layer indicating that the indian ocean warming is not just a passive response to el nino but important for summer climate variability in the indo west pacific region the implications for seasonal prediction are discussed