the mechanism of surtseyan volcanism during eruption the vent of a surtseyan volcano is occupied by a highly mobile slurry of tephra hyaloclastite and water as magma ascends rapidly through the slurry mixing occurs due to velocity shear acceleration of the fluid fluid contacts and mass incorporation consequent expansion of the admixed water causes the jetting and continuous up rush activity that characterizes surtseyan volcanism there is no evidence of the widely accepted fuel coolant interaction mechanism

occurrence stratigraphy and geochemistry of late quaternary tephra layers in the aegean sea and the marmara sea abstract several discrete tephra layers occur in 2 5 10 3 m long cores from the aegean sea whereas a single tephra is identified in 3 5 9 7 m long cores from the marmara sea major and trace element data on these tephras delineate eight distinct groups the geochemical data and multivariate statistics allow the correlation of the tephra layers with six previously described marine tephras from the eastern mediterranean sea potential terrestrial sources are suggested for the others the chronostratigraphy of the tephra layers was established using several radiocarbon dates and the oxygen isotope stratigraphy from two long piston cores four widespread tephra layers occur within the upper quaternary sediments of the aegean sea the two youngest widespread tephra layers were derived from the santorini volcano of south central aegean sea z2 the minoan eruption ca 3 6 ka and y2 the cape riva eruption ca 21 7 ka the y2 tephra is also identified in three piston cores from the marmara sea the two older widespread tephra layers originated from the eruptions which emplaced the campanian ignimbrite of the italian volcanic provinces i e the y5 tephra ca 39 3 ka and from the nisyros volcano of the southeastern aegean sea the consistent occurrence of the nisyros tephra below the y5 tephra confirms similar findings in a lake core on the island of lesbos linear extrapolations using the oxygen isotope stratigraphy suggest an age of 42 44 ka for the nisyros eruption three additional tephra layers with a limited regional distribution are identified in the aegean sea cores two of these tephras originated from the yali volcano of the southeastern aegean sea and from the aeolian islands i e x1 tephra a series of six mugearite tephra beds in the south skiros basin is interpreted as a set of redeposited turbidites

towards the reconstruction of the shallow plumbing system of the barombi mbo maar cameroon implications for diatreme growth processes of a polygenetic maar volcano abstract understanding the mechanisms involved in the formation of maars and their diatreme growth processes has been a subject of contention while there is no direct evidence of the presence of diatremes beneath most of the young maars their existence is inferred based on the amount and type of country rocks excavated at different depths and deposited as pyroclastic ejecta around their craters properly tracing fragmented country rocks in ejecta to interpret their depths of origin and thus the depths of phreatomagmatic explosions require good and detailed information on the substrate geology that is generally lacking at many maars as an alternative this paper explores the role of juvenile components in deposits of a maar for understanding the cratering and growth of diatremes during maar forming eruptions based on field investigations pyroclast distribution componentry and grain morphology examinations this study reports on the eruptive mechanisms that led to the formation of the barombi mbo maar bmm a polygenetic maar volcano in cameroon the bmm consists of three diatremes that formed during distinct eruptive events and coalesced to produce an amalgamated maar diatreme two end member types of eruption styles from the dry magmatic to the wet phreatomagmatic explosions governed the formation of the maar in total a minimum of 0 0658 km 3 of magma dense rock equivalent corrected was ejected based on calculation by applying interpolation techniques on digital elevation models obtained from srtm30m data corrected by rock textural data collected from the field the distribution of juvenile clasts throughout the stratigraphic sequence suggests a complex subsurface eruptive process which originated probably within the uppermost part of the diatreme from the distribution and morphology of juvenile clasts in the deposits it is inferred that cratering and country rock excavation during the growth of each of the small diatremes developed mainly from shallow level explosions sometimes with lateral and vertical variations in the position of the explosion loci a prospective juvenile based conceptual model is proposed for the formation of the bmm the model suggests that during maar forming eruptions explosions taking place at a deeper position might entrain extensive amount of lithics from the mostly lithic dominated upper crater infill to deposit juvenile poor 90 vol

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

drivers of explosivity and elevated hazard in basaltic fissure eruptions the 1913 eruption of ambrym volcano vanuatu sw pacific abstract fissure eruptions along linear structures can extend for several tens of kilometres with distinct separate manifestations of volcanism along their length they typically involve low viscosity mafic magmas forming long lava flows and cinder cones eruptions in 1894 and 1913 on ambrym volcano vanuatu showed how these mildly explosive eruptions can rapidly transform into violent explosive events producing significant hazard and widespread volcanic ash clouds during the 1913 episode a fissure began in the central caldera and basaltic magmas broke out in a series of locations down the island s western flank in all sites over 100 m in elevation fissure outbreaks produced vigorous lava fountains and highly fluid lava flows that travelled rapidly to the shoreline when the outbreaks propagated along the island s axis into coastal plain areas a climactic series of explosive eruptions occurred producing a 1 2 km long by 600 m wide maar and tephra ring a further small tuff ring was formed later creating a temporary island 400 m offshore the onshore tephra ring destroyed a hospital and associated buildings its last evacuating occupants were close witnesses to the eruption processes deposits exposed in the lower portion of the tephra ring show that this part of the eruption began with a mild phreatomagmatic explosive eruption from a narrow vent followed by a magmatic scoria producing phase subsequently a complex sequence of highly explosive phreatomagmatic eruptions occurred producing pyroclastic surges along with repeated distinctive breccia horizons rich in coral and lava country rock these features tally with eye witness accounts to indicate that the main eruption phase was produced by a periodically shifting locus of phreatomagmatic fragmentation and eruption along a single e w fissure the glassy and vesicle poor pyroclasts produced during this eruption phase were dominantly fragmented in a brittle manner by magma water interaction low volatile content of the magma upon fragmentation is confirmed by ftir analysis showing

sedimentological processes in a scarp controlled rocky shoreline to upper continental slope environment as revealed by unusual sedimentary features in the neogene coquimbo formation north central chile abstract exceptionally good outcrops of miocene to pliocene deposits in the vicinity of submarine paleozoic basement scarps at carrizalillo north of la serena reveal a wealth of sedimentary features not commonly observed the most proximal facies consist of rock fall and coarse grained debris flow deposits directly abutting the basement wall from which they originated angular basement clasts are mixed with well rounded cobbles which probably formed as a basal gravel on a wave cut platform at the beginning of marine flooding subsequently accumulated at the scarp edge and were incorporated into the debris when the latter collapsed the poor sorting inverse grading and protruding cobbles and boulders are classical debris flow features with good clast imbrication indicating a laminar shearing action a medial facies is represented by secondary channels running parallel to the major scarp about 1 km downslope of the first locality in the largest channel megaflutes at the base indicate the passage of highly turbulent nondepositing flows eroding the soft silty substrate in the deepest central part of the channel a pebbly coquina shows horizontal and trough cross stratification with most of the bivalves oriented convex side up meter scale rip up clasts of the underlying siltstone are also present indicating turbulent flow with a density sufficiently high to retard settling the coquina is interpreted as a detachment deposit resulting from a hydroplaning debris flow along the central part of the channel where the velocity and rate of pore pressure decay were highest this deposit is overlain by fining upward massive to horizontally stratified sandstone very similar in texture and composition to the matrix of the debris flow suggesting its formation by surface transformation and elutriation of the latter along the channel margin a basal centimeter scale sandstone layer is virtually unaffected by the megaflute topography and clearly represents a subsequent event it is interpreted as a basal shear carpet driven by the overlying debris flow within the shear carpet a basal friction zone and an overlying collision zone containing a higher concentration of shell hash can be distinguished the overlying debris flow deposit is represented by massive coquina with scattered angular to rounded basement clasts it contains disarticulated bivalves oriented with their concave side up indicating large scale upward fluid escape during deposition a smaller secondary channel shows large rip up rafts of the underlying substrate some rafts appear to have been plucked from the substrate by a process of sand injection from an overriding high density sandy debris flow which probably originated during a tsunami such clasts can climb upward into a laminar flow by down current tilting and tumbling the most distal facies occurs below a second scarp oriented more or less parallel to the present coastline where finer grained turbidites onlap and backlap onto the stoss and lee sides of an obstacle formed by eroded boulder conglomerates the onlap deposits resemble inclined sandy macroforms recently described in submarine canyon settings they are interbedded with diatom containing volcanic ash beds with cross stratification dipping eastwards and containing deepwater microflora typical of continental upwelling zones

shallow water eruption fed mafic pyroclastic deposits along a paleoproterozoic coastline kangerluluk volcano sedimentary sequence southeast greenland abstract the 200 300 m thick volcano sedimentary sequence at kangerluluk is part of the psammite zone one of four major zones which constitute the 1 8 ga ketilidian orogen in south greenland three lithofacies are emphasized in the study 1 the conglomerate sandstone 2 the volcanic and 3 the pyroclastic lithofacies the 2 40 m thick conglomerate sandstone lithofacies represents a subaerial to subaqueous fan delta deposit matrix and clast supported conglomerates are interpreted as debris flow and longitudinal gravel bar deposits erosive based conglomerate channel fills attest to stream incision trough crossbedded sandstone interpreted as lunate megaripples planar bedded sandstone indicative of upper flow regime bar top sands and small scale trough crossbeds reflecting ripples follow up section form collectively with the conglomerate 0 40 2 50 m thick fining upward sequences the sandstone dominated unit up section from the conglomerates and composed of planar and low angle crossbeds minor ripples and graded beds as well as mudstone is indicative of a lower shoreface deposit below normal wave base the clastic sedimentary rocks are suggestive of a fan delta setting the 100 200 m thick volcanic lithofacies composed of pillowed and pillow brecciated lava flows is consistent with shallow water deposition interstratification of lava flows with both conglomerate sandstone and pyroclastic lithofacies intrusion of dykes into volcaniclastic rocks and peperite formation accentuate contemporaneity between volcanism and sedimentation and is a common feature of island arcs the 1 50 m thick pyroclastic lithofacies with sharp depositional contacts to the overlying volcanic and underlying conglomerate sandstone lithofacies was emplaced in a subaqueous setting the lithofacies is divided into a planar to crossbedded tuff lapilli tuff and a bedded lapilli tuff breccia whereby both deposits are inferred to result from shallow water surtseyan type eruptions the 5 15 m thick bedded lapilli tuff breccia with abundant bomb sag structures and graded beds is considered a result of subaqueous eruptions strong enough to form an insulating steam cupola characterized by ballistically emplaced bombs that rapidly collapsed allowing for transport via mass flow processes the deposits are considered proximal to the vent the 2 50 cm thick planar to crossbedded tuff lapilli tuff featuring abundant euhedral and broken crystals of feldspar 2cm and minor pyroxene 1 cm are massive graded crossbedded and stratified the planar but laterally discontinuous beds characterized by abundant low angle scours are interpreted as low to high concentration sediment gravity flows produced directly from subaqueous tephra jets that collapsed due to massive water ingestion local breccia size pyroclasts disrupting beds are interpreted as bomb sags the mafic eruption fed surtseyan type deposits postulated to be a subaqueous counterpart of cold subaerial base surges originate from subaqueous tuff cones formed along a rugged volcanic dominated shoreline featuring high energy fan deltas

rhyolitic phreatomagmatism explored tepexitl tuff ring eastern mexican volcanic belt abstract rhyolitic tepexitl tuff ring in the serdan oriental basin of the eastern trans mexican volcanic belt is a young well preserved circular crater a simple morphology that belies a more complex eruptive history field observations ash studies granulometry componentry and morphology and volatile data loi and xrf allow the broad division of the tepexitl deposits into two sequences a lower sequence characterized by fine grained deposits with abundant bomb and lapilli sags and an upper sequence of coarse grained deposits that show little to no deformation the early eruptions at tepexitl are interpreted to have been dominated by discrete highly efficient phreatomagmatic blasts which caused a progressive deepening of the eruptive center lower sequence followed by a transition to dominantly magmatic behavior in the upper sequence dome growth occurred at the end of the eruption but subsequent retrogressive explosions triggered by external water destroyed all trace of the original dome morphology molten fuel coolant interaction mfci is commonly thought to cause the repetitive water magma interaction in phreatomagmatic eruptions but a process by which magma and viscous felsic magmas can mingle prior to explosive interaction has not been described a viable mechanism for rhyolitic mfci based upon field work at tepexitl and laboratory experiments requires that fluidized sediments intrude marginal fractures in the rhyolite magma creating enough interfacial surface area to initiate phreatomagmatic explosions from within the interior of a rising plug or dome

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

volcano sedimentary characteristics in the abu treifiya basin cairo suez district egypt example of dynamics and fluidization over sedimentary and volcaniclastic beds by emplacement of syn volcanic basaltic rocks abstract this paper describes the neogene lava sediment mingling from the abu treifiya basin cairo suez district egypt the lava sediment interactions as peperites have been identified for the first time at the study area and can be used as paleoenvironmental indicators the identification of peperite reflects contemporaneous time relationship between volcanism and sedimentation and this finding is of primary importance to address the evolutional reconstruction of the abu treifiya basin characterization of the facies architecture and textural framework of peperites was carried out through detailed description and interpretation of their outcrops the peperites and sedimentary rocks are up to 350 m thick and form a distinct stratigraphic framework of diverse lithology that is widespread over several kilometers at the study area lateral and vertical facies of the peperites vary from sediment intercalated with the extrusive intrusive basaltic rocks forming peperitic breccias to lava sediment contacts at a large to small scales respectively peperites encompass five main facies types ascribed to i carbonate sediments hosted fluidal and blocky peperites ii lava flow hosted blocky peperites iii volcaniclastics hosted fluidal and blocky peperites iv sandstone siltstone rocks hosted blocky peperites and iv debris flows hosted blocky peperites soft sediment deformation structures vesiculated sediments sediments filled vesicles and fractures in lava flows indicate that lava flows mingled with unconsolidated wet sediments all the peperites in this study could be described as blocky or fluidal but mixtures of different clast shapes occur regardless of the host sediment the presence of fluidal and blocky juvenile clasts elucidates different eruptive styles reflecting a ductile and brittle fragmentation the gradual variation from fluidal to blocky peperite texture producing the vertical grading is affected by influencing factors e g the viscosity magma temperature confining pressure sediment fluidization and vapor film at the magma sediment interactions peperites in the study area record deposition within a shallow marine and fluvio lacustrine environment accumulated in a rift related basin developed during pre to syn rift phase respectively the facies transitions peperites in this area resulted from the explosive and sediment depositional processes which were mingled separately by volcanism under contrast geological conditions the development of such contrast in the depositional sequences reflects variation in the accommodation to sediment supply in the same accumulation space inside the depocenters during the rifting of the abu treifiya basin hydrothermal mineralizations comprising quartz and carbonate are restricted to peperites and lava flows

may 18 1980 mount st helens deposits in south coldwater creek washington beginning at 0832 pacific daylight time pdt mount st helens washington produced a rockslide debris avalanche and a lateral explosion blast that generated a laterally directed pyroclastic surge called here a blast surge the resulting deposits are thickest in the east west trending valley of north fork toutle river immediately north of the volcano but are almost as thick in south coldwater creek a small tributary of the toutle river in the lee of an east west trending ridge johnston ridge just north of the north fork toutle river the avalanche surmounted johnston ridge 300 m relief and flowed down the valley of south coldwater creek closely following the avalanche and in part contemporaneously with it the blast surge left thin up to 0 5 m deposits on johnston ridge and thick up to 15 m deposits in south coldwater creek these deposits have a lower layer layer a1 which is friable massive poorer in fines particles 1 16 mm and more poorly sorted than the upper layer layer a2 layer a2 is less friable because it contains more silt and clay sized material beneath layer a1 is a zone containing a small percentage of debris from the blast surge mixed with products eroded from the underlying surface this zone designated as layer a0 is a poorly mixed layer containing schlieren of soil shattered volcanic fragments and considerable quantities of organic material layer a1 is strongly sorted and fines depleted and is interpreted to have been deposited from just behind the turbulent head of the blast surge layer a2 is interpreted to have been deposited from the body of the blast surge which is believed to have been less turbulent than the head layer a0 is interpreted to have formed by shearing at the base of the head of the blast surge in the region between the front and the back of the head of the blast surge it is concluded that a strongly fines depleted basal layer of a pyroclastic flow or surge deposit underlain by a thin sheared zone is the signature of an extremely high velocity particulate density current

depositional mechanics and sequences of base surges songaksan tuff ring cheju island korea the songaksan mount in the southwestern part of cheju island korea is a taalian tuff ring produced by phreatomagmatic explosions at an aquifer a detailed analysis of proximal to distal facies changes reveals that the tuff ring sequence can be represented by 21 sedimentary facies one lateral facies sequence lfs and three vertical facies sequences vfs the vfs 1 and 2 are representative of facies relationships in horizontal near vent deposits the vfs 1 comprises scour fill bedded tuff inversely graded tuff massive tuff and laminated tuff from base to top the vfs 2 is a variant of the vfs 1 replaced by an inversely graded lapilli tuff unit at the base the sequences suggest traction carpet suspension and minor traction sedimentation from a high concentration near vent base surge the lfs 1 and the vfs 3 are distilled from outward dipping flank deposits both sequences begin with disorganized lapilli tuff followed successively by stratified lapilli tuff dune bedded lapilli tuff very thinbedded tuff and accretionary lapilli they are suggestive of waning base surge which decreases in particle concentration suspended load fall out rate and flow regimes with an increase in traction and sorting these facies sequences suggest that a base surge experiences flow transformation with its flow characters changing with time and space a near vent base surge is turbulent uniformly mixed and highly concentrated and produces scour fill bedded tuff as capacity decreases the surge transforms into a dense and laminar underflow and a dilute and turbulent upper part gravity transformation depositing inversely graded massive and normally graded lapilli tuff ensuing loss of sediment load and mixing of ambient air result in flow dilution surface transformation stratified and dune bedded units are produced by tractional processes of turbulent and low concentration surge further dilution causes deceleration and cooling and results in precipitation of moistened ash and accretionary lapilli from suspension

deep bedded ultramafic diatremes in the missouri river breaks volcanic field montana usa 1 km of syn eruptive subsidence the ultramafic eocene missouri river breaks volcanic field mrbvf montana usa includes over 50 diatremes emplaced in a mostly soft substrate the current erosion level is 1 3 1 5 km below the pre eruptive surface exposing the deep part of the diatreme structures and some dikes five representative diatremes are described here they are 200 375 m across and have sub vertical walls their infill consists mostly of 55 90 bedded pyroclastic rocks fine tuffs to coarse lapilli tuffs with concave upward bedding and 45 10 non bedded pyroclastic rocks medium lapilli tuffs to tuff breccias the latter zones form steep columns 15 135 m in horizontal dimension which cross cut the bedded pyroclastic rocks megablocks of the host sedimentary formations are also present in the diatremes some being found 1 km or more below their sources the diatreme infill contains abundant lithic clasts and ash sized particles indicating efficient fragmentation of magma and country rocks the spherical to sub spherical juvenile clasts are non vesicular they are accompanied by minor accretionary lapilli and armored lapilli the deposits of dilute pyroclastic density currents are locally observed our main interpretations are as follows 1 the observations strongly support phreatomagmatic explosions as the energy source for fragmentation and diatreme excavation 2 the bedded pyroclastic rocks were deposited on the crater floor and subsided by 1 0 1 3 km to their current location with subsidence taking place mostly during the eruption 3 the observed non bedded pyroclastic columns were created by debris jets that punched through the bedded pyroclastic material the debris jets did not empty the mature diatreme occupying only a fraction of its width and some debris jets probably did not reach the crater floor 4 the mature diatreme was nearly always filled and buttressed by pyroclastic debris at depth there was never a 1 3 1 5 km deep empty hole with sub vertical walls otherwise the soft substrate would have collapsed inward which it only did near the surface to create the megablocks we infer that syn eruptive subsidence shifted down bedded pyroclastic material and shallow sedimentary megablocks by 0 8 1 1 km or more after which limited post eruptive subsidence occurred this makes the mrbvf diatremes an extreme end member case of syn eruptive subsidence in the spectrum of possibilities for maar diatreme volcanoes worldwide

insights into the dynamics of mafic magmatic hydromagmatic eruptions from volatile degassing behaviour the hverfjall fires iceland abstract the style and intensity of hydromagmatic activity is governed by a complex interplay between the relative volumes of magma and water that interact their relative viscosities the depth of subsurface explosions the substrate properties and the vent geometry fundamental questions remain however regarding the role of magmatic vesiculation in determining the dynamics of magma water interaction mwi petrological reconstructions of magmatic degassing histories are commonly employed to interpret the pre and syn eruptive conditions during dry magmatic eruptions but the application of similar techniques to hydromagmatic activity has not yet been fully explored in this study we integrate glass volatile measurements s cl h2o and co2 with field observations and microtextural measurements to examine the relationship between degassing and eruptive style during the hverfjall fires fissure eruption iceland here coeval fissure vents produced both dry magmatic jar ba sholar scoria cone complex and variably wet hydromagmatic hverfjall tuff ring activity generating physically distinct pyroclastic deposits with contrasting volatile signatures matrix glass volatile concentrations in hydromagmatic ash 883 172 1 ppm s 0 45 0 03 1 wt h2o 20 ppm co2 are consistently elevated relative to magmatic ash and scoria lapilli 418 93 1 ppm s 0 12 0 48 1 wt h2o co2 below detection and overlap with the range for co erupted phenocryst hosted melt inclusions 1522 127 1 ppm s 165 27 1 ppm cl measurements of hydromagmatic glasses indicate that the magma has degassed between 17 and 70 of its initial sulfur prior to premature quenching at variably elevated confining pressures by comparing volatile saturation pressures for both magmatic and hydromagmatic glasses and how these vary through the eruptive stratigraphy we place constraints on the conditions of mwi crucially our data demonstrate that the magma was already vesiculating when it encountered groundwater at depths of 100 200 m and that the external water supply was sufficient to maintain mwi throughout the eruption with no significant vertical or lateral migration of the fragmentation surface we propose that development of an in vent water sediment slurry provides a mechanism through which the elevated confining pressures of 1 6 2 6 mpa or up to 6 mpa accounting for uncertainty in co2 below analytical detection could be maintained and buffered throughout the eruption whilst enabling vertical mixing and ejection of fragmented juvenile and lithic material from a range of depths importantly these results demonstrate that the volatile contents of hydromagmatic deposits provide valuable records of 1 the environment of mwi e g groundwater versus surface water vertical migration of the fragmentation level and 2 the state of the magma at the time of fragmentation and quenching we further suggest that the volatile content of tephra glasses provides a reliable alternative or additional indicator of a hydromagmatic origin particularly for reduced ocean island basalts where late stage volatile saturation and degassing s h2o occurs over a pressure range relevant to typical mwi environments

composite tuff ring cone complexes in jeju island korea possible consequences of substrate collapse and vent migration hydromagmatic volcanoes tuff rings and tuff cones in jeju island korea are divided into holocene volcanoes that were constructed upon rigid plateau forming lavas and early to middle pleistocene volcanoes that underlie the lavas and were constructed upon fragile unconsolidated sediments the older volcanoes are distinguished from the holocene volcanoes in that the former have irregular morphologies the irregular morphologies resulted from superposition or juxtaposition of multiple rim deposits of partial tuff rings and or tuff cones that have contrasting lithofacies characteristics bed attitudes and paleoflow directions the rim deposits are bounded by laterally persistent truncation surfaces which suggest large scale collapse of the substrate together with the overlying tephra piles and or erosion during a break in eruptive activity collapse was probably caused by the instability of the friable sedimentary substrate removal of lateral support because of downward quarrying of volcanic conduits and liquefaction of the water saturated substrate by volcanic seismicity the path of magma supply was diverted in some cases after collapse giving rise to migration of the active vent in other cases the eruption style changed abruptly from cone forming fallout dominated to ring forming surge dominated eruptions after collapse the resultant volcanic edifices thus consist of multiple rim beds of partial tuff rings and or tuff cones commonly with non circular morphologies substrate collapse and lateral vent migration as observed in jeju island appears to be an important phenomenon in hydromagmatic volcanoes because they are commonly constructed upon unstable sedimentary substrates substrate collapse and vent migration needs to be taken into account as an important control on volcano morphology and structures and as a means of lateral enlargement of terrestrial submarine or extraterrestrial volcanoes

lidar observations of stratospheric gravity waves from 2011 to 2015 at mcmurdo 77 84 s 166 69 e antarctica 1 vertical wavelengths periods and frequency and vertical wave number spectra five years of atmospheric temperature data collected with an fe boltzmann lidar by the university of colorado group from 2011 to 2015 at arrival heights are used to characterize the vertical wavelengths periods vertical phase speeds frequency spectra and vertical wave number spectra of stratospheric gravity waves from 30 to 50 km altitudes over 1000 dominant gravity wave events are identified from the data the seasonal spectral distributions of vertical wavelengths periods and vertical phase speeds in summer winter and spring fall are found obeying a lognormal distribution both the downward and upward phase progression gravity waves are observed by the lidar and the fractions of gravity waves with downward phase progression increase from summer 59 to winter 70

the ilopango tierra blanca joven tbj eruption el salvador volcano stratigraphy and physical characterization of the major holocene event of central america abstract the ilopango caldera is the source of the large tierra blanca joven tbj eruption that occurred about 1 5 ka years ago between ca ad270 and ad535 the eruption dispersed volcanic ash over much of the present territory of el salvador and pyroclastic density currents pdcs extended 40 km from the volcano in this study we document the physical characteristics of the deposits from all over el salvador to further constrain the eruption processes and the intensity and magnitude of the different phases of the eruption the succession of deposits generated by the tbj eruption is made of 8 units the eruption started with pdcs of hydromagmatic origin unit a0 followed by fallout deposits units a and b that are

a shallow marine alkali basalt tuff cone in the middle miocene jinzai formation izumo sw japan abstract the jinzai basalt is an alkali basalt tephra unit erupted on the japan sea side of sw japan immediately after the opening of the japan sea ceased 15 ma the jinzai basalt comprises a large volume of glassy alkali basalt clasts of phreatomagmatic origin and is interpreted to have constituted a tuff cone the tuff cone grew to a height of 100 m above the 50 to 150 m deep submarine substrate but the summit and a part of the flank were lost due to syn and post eruptive sliding slumping and erosion the restored tuff cone has a crater 400 500 m wide and 100 m high and the cone rim reaches 1500 m in width with bedding dipping outward at a maximum angle of 20 30 the primary deposits include thinly bedded lapillistone to lapilli tuff and tuff and thickly bedded lapilli tuff and tuff which are thought to be correlated to a small volume phreatomagmatic explosion and more voluminous one respectively the shallow marine jinzai basalt tuff cone is essentially similar in constituents internal structures configuration and size to other tuff cones built in shallow surface water this tuff cone however appears dominated by phreatomagmatic products due to dynamic interaction of rapidly ascending magma and water saturated unconsolidated sediments or vent filling slurry of water and ejecta and has been entirely altered through syn and post eruptive reactions of hot basalt glass and interstitial seawater

tectonic and sedimentary evolution of the korean peninsula a review and new view abstract this review focuses on the tectonics and sedimentation of major sedimentary basins and orogenic belts late proterozoic neogene in the korean peninsula the korean peninsula is part of the amuria plate and represents an important link between continental blocks of north and south china and the island arcs of japan the basement rocks exposed in the kyonggi and yongnam massifs consist of 2 7 to 1 1 ga high grade gneiss and schist these massifs are separated by the okchon fold belt which comprises metasedimentary rocks and bimodal meta volcanic rocks the stratigraphy of the okchon group is unclear at present the okchon basin was probably initiated as an intraplate rift prior to the late proterozoic the hwanggangri formation clast bearing phyllite most likely represents deposition by subaqueous debris flows in slope environments of an enclosed basin the stratigraphic relationship between the okchon group and the choson supergroup cambro ordovician of the taebaeksan basin is poorly constrained the choson supergroup unconformably overlies the yongnam massif and consists mainly of carbonate sequence that formed mostly in shallow marine and tidal environments reflecting numerous sea level fluctuations the sequence is disconformably overlain by siliciclastic sequence of pyongan supergroup carboniferous triassic which formed most likely in shallow marine deltaic and fluvial environments the imjingang belt is an east trending fold and thrust zone and consists of metasedimentary rocks and volcaniclastics devonian carboniferous underlain unconformably by proterozoic basement rocks late proterozoic amphibolites of oceanic affinity were metamorphosed at about 8 13 kbar and 630 740 c during the late permian to the early triassic the south vergent contraction and top down to the north normal faulting are suggestive of a suture belt between the north china block sino korea craton and the south china block yangtze craton an extension of sulu belt across the yellow sea entire peninsula experienced strong deformation and metamorphism during the suturing event namely songrim orogeny during this orogenic event the kyonggi massif and the okchon basin accreted to the yongnam massif and taebaeksan basin along the south korean tectonic line running northeast southwest a series of northward trending thrust formed along the boundary zone to the east kaktong and kongsuwon thrusts and others piggyback basins locally developed along the thrust faults forming the taedong group the crustal deformation resumed in the early to late jurassic daebo event under contractional setting dextral ductile shearing associated with thrusting and folding continued in the mid southern part of the peninsula it was due to orthogonal northwestward subduction of the izanagi plate under the asian continent the yongnam massif experienced continuous dextral offset along the honam shear zone in the early cretaceous the izanagi plate began to subduct northward and caused formation of strike slip basins in retroarc setting i e kyongsang basin in the southeastern part and a number of small scale basins in the mid southwestern part of the peninsula small scale alluvial fans and fluvial channel networks formed in the basin margin and were transitional to ephemeral lacustrine systems under semiarid to arid conditions extensive intrusion of granitoids occurred from triassic to early tertiary with a gap between 160 and 100 ma representing continental magmatic arc in the tertiary the southeastern margin of the korean peninsula experienced back arc opening pull apart basins formed in the miocene bounded by the yangsan and hupo faults the yonil group sedimentary fill of the pohang basin comprises more than 1 km thick siliciclastic sequence which represents deposition in fan delta systems on the hanging wall of the yangsan fault thick more than 10 km thick sediments in the ulleung basin margin were deformed in the late miocene due to the northward movement of kyushu block quarternary volcanic events in cheju island represent intraplate hot spots

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

deposition temperature of the ad 472 pollena pyroclastic density current deposits somma vesuvius italy the deposition temperature of the pyroclastic density current pdc deposits emplaced during the ad 472 pollena eruption somma vesuvius italy has been investigated using the thermal analysis of the magnetic remanence carried by lithic clasts embedded within the deposits a total of 310 lithic clasts were collected from all the pdc units in the pollena stratigraphic succession at different distances from the inferred vent and at different stratigraphic levels the temperature reached by each individual clast during residence in the pdc was estimated through stepwise thermal demagnetization with the values from all clasts collected at each site being used to infer the deposition temperature tdep although the sedimentological features of these pdc deposits show some variation the deposition temperature typically falls in the range 300 to 320 c with a maximum range of 260 to 360 c the fairly uniform temperature observed in both the dune bedded and massive deposits points to homogeneity in attainment of tdep for the different deposits and suggests similarity in the depositional regime of the different pdcs and or in heat transfer to lithic fragments similarity in depositional regime was also favoured by the limited control exerted by topography on the distribution of these pdcs with the northern wall of the somma caldera that did not act as a morphological barrier as a result the currents were capable of moving away from the vent without topographic disturbances and thus significant variations in the cooling regime because the pollena eruption is considered similar to the maximum expected event at somma vesuvius the characteristics of its deposits best simulate the likely maximum hazard for the vesuvius region in this regard pollena produced hot dilute pdcs which were able to travel up to 12 km from the vent maintaining high temperatures across this distance

eruptive conditions and depositional processes of narbona pass maar volcano navajo volcanic field navajo nation new mexico usa phreatomagmatic deposits at narbona pass a mid tertiary maar in the navajo volcanic field nvf new mexico usa were characterized in order to reconstruct the evolution and dynamic conditions of the eruption our findings shed light on the temporal evolution of the eruption dominant depositional mechanisms influence of liquid water on deposit characteristics geometry and evolution of the vent efficiency of fragmentation and the relative importance of magmatic and external volatiles the basal deposits form a thick 5 20 m massive lapilli tuff to tuff breccia deposit this is overlain by alternating bedded sequences of symmetrical to antidune cross stratified tuff and lapilli tuff and diffusely stratified clast supported reversely graded lapilli tuffs that pinch and swell laterally this sequence is interpreted to reflect an initial vent clearing phase that produced concentrated pyroclastic density currents followed by a pulsating eruption that produced multiple density currents with varying particle concentrations and flow conditions to yield the well stratified deposits only minor localized soft sediment deformation was observed no accretionary lapilli were found and grain accretion occurs on the lee side of dunes this suggests that little to no liquid water existed in the density currents during deposition juvenile material is dominantly present as blocky fine ash and finely vesiculated fine to coarse lapilli pumice this indicates that phreatomagmatic fragmentation was predominant but also that the magma was volatile rich and vesiculating at the time of eruption this is the first study to document a significant magmatic volatile component in an nvf maar diatreme eruption the top of the phreatomagmatic sequence abruptly contacts the overlying minette lava flows indicating no gradual drying out period between the explosive and effusive phases the lithology of the accidental clasts is consistent throughout the vertical pyroclastic stratigraphy suggesting that the diatreme eruption did not penetrate below the base of the uppermost country rock unit a sandstone aquifer 360 m thick by comparison other nvf diatremes several tens of kilometers away were excavated to depths of 1 000 m beneath the paleosurface e g delaney pt ship rock new mexico the vent of a violent volcanic eruption in beus ss ed geological society of america centennial field guide rocky mountain section 2 411 415 1987 this can be accounted for by structurally controlled variations in aquifer thickness beneath different regions of the volcanic field variations in accidental clast composition and bedding style around the edifice are indicative of a laterally migrating or widening vent that encountered lateral variations in subsurface geology we offer reasonable evidence that this subsurface lithology controlled the availability of external water to the magma which in turn controlled characteristics of deposits and their distribution around the vent

dynamics stratigraphy and proximal dispersal of supraglacial tephra during the ice confined 2004 eruption at grimsvotn volcano iceland the basaltic phreatomagmatic eruption of grimsvotn volcano iceland in november 2004 g2004 lasted for 5 days during which time two separate vents were active significant deposition of tephra occurred in the first 45 h only we have subdivided the deposit into seven units a g on the basis of differences in texture grain size and componentry and the presence of sharp contacts between the layers the distribution of tephra lobes was used to infer the vent of origin for each unit the g2004 deposit is poorly sorted overall and consists of non vesicular to highly vesicular juvenile components units a and b comprise almost exclusively non to poorly vesicular glass fragments whereas units c g contain at least 30 vol highly vesicular pumice the proportion of non juvenile fragments increases significantly in the final unit unit f of the main phase non juvenile fragments are restricted to the coarse 0 fraction of the deposit main phase units c and e account for 80 of the total deposit volume including the entire distal portion and are interpreted to represent a mixture of 1 a widely dispersed component that fell from the upper margins of a strongly inclined 45 6 10 km high plume and 2 a locally dispersed 3 km from source component originating from pyroclastic density currents and minor tephra jets

contrasting mechanisms of magma fragmentation during coeval magmatic and hydromagmatic activity the hverfjall fires fissure eruption iceland growing evidence for significant magmatic vesiculation prior to magma water interaction mwi has brought into question the use of diagnostic features such as low vesicularities and blocky morphologies to identify hydromagmatic pyroclasts we address this question by quantifying co variations in particle size shape and texture in both magmatic and hydromagmatic deposits from the hverfjall fires fissure eruption iceland overlapping vesicularity and bubble number density distributions measured in rapidly quenched magmatic and hydromagmatic pyroclasts indicate a shared initial history of bubble nucleation and growth with substantial vesiculation prior to mwi hydromagmatic fragmentation occurred principally by brittle mechanisms where the length scale and geometry of fracturing was controlled by the bubble population this suggests that the elevated fragmentation efficiency of hydromagmatic deposits is driven at least in part by brittle disintegration of vesicular pyroclasts due to high thermal stress generated during rapid cooling in this way the shape and size distributions of hydromagmatic pyroclasts both critical input parameters for ash dispersion models are strongly influenced by the dynamics of vesiculation prior to mwi this result underlines the need to analyse multiple grain size fractions to characterise the balance between magmatic and hydromagmatic processes during the hverfjall fires eruption the external water supply was sufficient to maintain mwi throughout the eruption with no evidence for progressive exhaustion of a water reservoir we suggest that both the longevity and the spatial distribution of mwi were determined by the pre existing regional hydrology and represent continuous interaction between a propagating dike and a strong groundwater flow system hosted within permeable basalt lavas

submarine basaltic fountain eruptions in a back arc basin during the opening of the japan sea abstract basaltic rock generated during the middle miocene opening of the japan sea is widely distributed on the back arc side of the japanese archipelago few studies have investigated on submarine volcanism related to opening of the japan sea the present study aimed to reconstruct details of the subaqueous volcanism that formed the back arc basin basalts babb during this event and to discuss the relationship between volcanism and the tectonics of back arc opening using facies analyses based on field investigation the study area of the southern dewa hills contains well exposed basalt related to the opening of the japan sea five types of basaltic rock facies are recognized 1 coherent basalt 2 massive platy basalt 3 jigsaw fit monomictic basaltic breccia 4 massive or stratified coarse monomictic basaltic breccia with fluidal clasts and 5 massive or stratified fine monomictic basaltic breccia the basaltic rocks are mainly hyaloclastite based on facies distributions we infer that volcanism occurred along fissures developed mainly at the center of the study area given that the rocks contain many fluidal clasts submarine lava fountaining is inferred to have been the dominant eruption style the basaltic rocks are interpreted as the products of back arc volcanism that occurred by tensional stress related to opening of the japan sea which drove strong tectonic subsidence and active lava fountain volcanism

secondary hydroeruptions in pyroclastic flow deposits examples from mount st helens abstract secondary hydroeruptions occur in pyroclastic flow deposits when water or ice is trapped beneath hot pyroclastic debris and rapidly heated to steam these eruptions display various styles of activity including fumarolic degassing tephra fountaining and explosive cratering the deposits which occupy the layer 3 stratigraphic position on the top of pyroclastic flow units can be distinguished from ash cloud material by lateral thickness variation clast composition and other sedimentary features the ejecta of secondary hydroeruptions comprise a subset of hydrovolcanic pyroclastic deposits a small secondary hydroeruption observed on the mount st helens pumice plain in 1981 produced tephra that was emplaced ballistically by deposition from base surges and by fallout from an eruption column stratigraphic descriptions and grain size analysis of the ejecta from several secondary craters on the pumice plain demonstrate that the bedforms produced by a hydroeruption change with crater diameter in particular craters of small diameter are surrounded by interbedded ripple laminated ash horizons and nonstratified fines depleted units large craters have ejecta ramparts comprised of coarse dunes and antidunes these bedform changes are related to a progressive increase in eruptive energy which produces base surges of greater power and eruptive columns of greater height we suggest that the style of activity displayed during a secondary hydroeruption is controlled by both the total thermal energy of the system and the permeability of the pyroclastic overburden

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

the influence of palaeotopography on facies architecture and pyroclastic flow processes of a lithic rich ignimbrite in a high gradient setting the abrigo ignimbrite tenerife canary islands abstract the late pleistocene 188 19 ka abrigo ignimbrite represents the last major explosive caldera forming eruption on tenerife it consists almost entirely of a moderate volume 1 8 km 3 light grey phonolitic lithic rich ignimbrite deposit although a possible abrigo member thin fall deposit is locally preserved beneath the ignimbrite suggesting an initial short lived stable plinian eruption column the ignimbrite deposit is dispersed radially around the lower flanks of the las canadas edifice and correlates to a clast supported lithic breccia on the eastern and northern caldera wall much of the ignimbrite was probably deposited offshore or within the caldera now buried by more recent lavas two major depositional units separated by a co ignimbrite ash cloud deposit and which display dramatic variations in thickness pyroclastic flows were channelised along and deposited within the same valleys that exist today and were commonly continuous and sheet like across the interfluves the basal zones of pyroclastic flows are envisaged as a bed load of coarse lithic lapilli and blocks transported under their own momentum and with the support of the surrounding fine ash gas granular flow

growth and evolution of an emergent tuff cone considerations from structural geology geomorphology and facies analysis of sao roque volcano sao miguel azores abstract the syn eruptive and post eruptive history of sao roque tuff cone its geological setting and volcanological features were studied in detail to understand the role played by the different factors that contributed to the morphological evolution of this relatively simple and small volcanic edifice in addition attention was also focused on the series of natural changes that affected the tuff cone during the course of the years and that finally led to its structural disassembly a novel model is proposed to explain this process the sao roque volcanic centre located on the island of sao miguel azores consists of two well consolidated bodies and numerous small islets that formed more than 4700 years ago during the hydromagmatic activity that took place along an intruding dyke whose nnw sse trend is in agreement with the regional tectonic pattern the eruptive vents probably migrated progressively from sse to nnw forming small edifices through the rapid accumulation of sediments during alternating phases of dry and wet magmatic emissions syn eruptive partial collapses greatly modified the original morphological structure of these edifices probably allowing sea water to continuously flow into the vents the complex interaction of these factors controlled the depth of magma fragmentation producing different types of deposits in which the ash lapilli ratio varies considerably the high water saturation degree of these deposits caused syn eruptive and post eruptive remobilization which resulted in collapses and some small scale landslides post eruptive wnw ese trending transtensional and extensional tectonic activities operated during the initial dissection of the cone generating instability furthermore the rapid accumulation of wet tephra and its following consolidation caused selective collapses that favoured the fragmentation of the deposit and caused the formation of numerous islets separated by radially arranged channels collapses also involved the lava units emplaced in more recent times around the tuff cone which show that brittle deformation has been significant in the area for a prolonged period

a model for generation of ash clouds by pyroclastic flows with application to the 1980 eruptions at mount st helens washington pyroclastic flows are defined as consisting of a dense basal avalanche underlying a turbulent ash cloud the generation of ash clouds by the basal avalanche is commonly assumed to be closely related either to fluidization mechanisms within the avalanche or to airflow flow front interactions these mechanisms carry ash upward from the basal avalanche into the ash cloud in currents of gas or air but so far no mechanisms have been proposed to keep the ash in turbulent suspension in the ash cloud here i consider the ash cloud turbulence that is generated in two ways by temperature differences between the basal avalanche and overriding air and by the motions of the blocks lapilli and ash on the outer surface of the basal avalanche this turbulence would carry ash from saltation into turbulent suspension at the base of the ash cloud and this transition would simultaneously occur from the upper surface and front of the basal avalanche this approach quantifies one type of airflow flow front interaction the analysis incorporates studies of heat and mass transfer from rough surfaces during high reynolds number flow equations for conservation of mass momentum heat energy and ash particle concentration are formulated and presented in integral form model results show that for typical avalanche velocities and temperatues temperature differences between the basal avalanche and the surrounding air are insignificant in generating the initial turbulence within the ash cloud instead the initial distribution of particle sizes in the ash cloud is controlled by the turbulence generated by the saltating cascading particles in the basal avalanche study of the morphology of observed ash clouds suggests that thermally driven convection often becomes significant hundreds of meters or more behind the flow front and this may result from deposition of significant quantities of ash from the ash cloud onto the basal avalanche the variations in ash cloud heights morphologies and particle sizes that are predicted by this model are consistent with published observations of small pyroclastic flows at mount st helens in 1980

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

the 1982 eruptions of el chichon volcano mexico 3 physical properties of pyroclastic surges two major pyroclastic surges generated during the 4 april 1982 eruption of el chichon devastated an area of 153 km2 with a quasi radial distribution around the volcano the hot surge clouds carbonized wood throughout their extent and were too hot to allow accretionary lapilli formation by vapor condensation field evidence indicates voidage fraction of 0 99 in the surge cloud with extensive entrainment of air thermal calculations indicate that heat content of pyroclasts can heat entrained air and maintain high temperatures in the surge cloud the dominant bed form of the surge deposits are sand waves shaped in dune forms with vertical form index of 10 20 characterized by stoss side erosion and lee side deposition of 1 10 cm reversely graded laminae a systematic decrease in maximum lithic diameter with distance from source is accompanied by decrease in wavelength and amplitude modal analysis indicates fractionation of glass and pumice from the surge cloud relative to crystals resulting in loss of at least 10 25 of the cloud mass due to winnowing out of fines during surge emplacement greatest fractionation from the 1 0 0 0 grain sizes reflects relatively lower pumice particle density in this range and segregation in the formative stages of the surge cloud extensive pumice rounding indicates abrasion during bed load transport flow of pyroclastic debris in the turbulent surge cloud was by combination of bed load and suspended load transport the surges are viewed as expanding pyroclastic gravity flows which entrain and mix with air during transport the balance between sedimentation at the base of the surge cloud and expansion due to entrainment of air contributed to low cloud density and internal turbulence which persisted to the distal edge of the surge zone

a late ordovician phreatomagmatic complex in marine soft substrate environment the crozon volcanic system armorican massif france the mafic lavas and the diabases of crozon armorican massif france belong to an anorogenic ordovician volcanic complex emplaced on a rifted passive margin in north gondwana magma passed through syn volcanic soft sedimentary substrate which is today mostly composed of alternating sandstones and mudstones from llanvirn to ashgill in age field observations together with microscopic studies and geochemical analyses of magmatic rocks lead us to propose a model of volcano formation which combines hydromagmatic processes peperitic intrusions a shallow submarine tephra settling eruption fed turbidity currents and a pillow lava effusion the crozon outcrops can be used to reconstruct a complete cross section from the root of the volcanic complex to the lavas and breccias emplaced on the sea floor the sites expose i a hypabyssal breccia containing mud chunks and coarse grained diabase clasts with amoeboidal fine grained magmatic material ii bulbous peperitic sills and pillow like lobes bearing a great quantity of sediment derived enclaves of fluidal morphology iii volcaniclastic breccias containing near spherical magmatic clasts that resulted from the complete fragmentation of sills in the ductile regime iv a rhythmic peperitic breccia interpreted as the product of mingling between thin lava flows and soft calcareous sediment the crozon volcanic form resulting from explosive interaction with subsurface surface water was probably a subaqueous collapsed tuff cone this upper part of the system is synchronous with an ashgill carbonate sedimentation which overlies an ordovician siliciclastic succession deposited in shelf environments

late paleozoic glacially related sandstone reservoirs in the parana basin brazil upper paleozoic glacially related sandstones of the itarar group constitute thick and complex hydrocarbon reservoirs in the intracratonic paran basin of southern brazil because these sandstones were deposited in both glacial and nonglacial environments they show a high degree of variability in facies in this article we analyze outcrop and shallow well data to describe facies heterogeneities and their reservoir quality implications a regional subsurface sequence stratigraphic framework was built based on both strike and dip oriented well to well correlations a well exposed sandstone succession correlative with the main reservoir interval in the subsurface was selected in the eastern part of the basin the vila velha sandstone comprises an up to 100 m 328 ft thick unconformity bounded sand rich succession composed of four stacked facies assemblages vertical facies succession shows a transition from glacial and glaciomarine to periglacial shallow marine and nearshore settings which define a retrogradational progradational succession channel mouth bars and wave modified shoreline sandstones in the progradational section contain stratigraphically predictable reservoir quality lithofacies in subsurface gas bearing sandstone intervals with the same well log response occur below unconformities which control the overall reservoir geometry

magma water interactions in subaqueous and emergent basaltic in the subaqueous growth and emergence of a basaltic volcano clasts are formed by one or a combination of 1 explosive release of magmatic volatiles 2 explosive expansion and collapse of steam formed at magma water contact surfaces 3 explosive expansion of steam following enclosure of water in magma or entrapment of water close to magma and 4 cooling contraction these processes named respectivelymagmatic explosivity contact surface steam explosivity bulk interaction steam explosivity andcooling contraction granulation can be enhanced by mutual interaction and feedback the first three explosive processes are limited at certain water depths hydrostatic pressures and become increasingly vigorous at shallower levels the depth of onset of magmatic explosivity depends largely on juvenile volatile content it is up to 200 m for tholeiitic magmas and up to 1 km for alkalic magmas at the depth where formation of clastic deposits becomes predominant over effusion of lavas magmatic explosivity is subordinate to steam explosivity as a clast forming process the upward transition to accumulation of dominantly clastic deposits is not simply related to the onset of substantial exsolution of magmatic volatiles and can occur without it contact surface explosivity commonly requires initiation by a vigorous impact between magma and water and although no certain depth limit is known likelihood of such explosivity decreases rapidly with depth clast generation by bulk interaction explosivity appears to be restricted to depths much shallower than that of the critical pressure of water which in sea water is at about 3 km cooling contraction granulation can occur in any depth of water but at shallow levels may be replaced by contact surface explosivity during continuous eruption under water tephra can be ejected and deposited within a cupola of steam such that rapid quenching does not occur emergent volcanoes are characterized by distinctive steam explosive activity that results primarily from a bulk interaction between rapidly ascending magma and a highly mobile slurry of clastic material water and steam the water gets into the vent by flooding across or through the top of the tephra pile and violent explosions cease when this access is sealed the eruptions during emergence of surtsey and capelinhos typify the distinctive explosive activity the style and controls of which are different from those of maar volcanoes

subaqueous strombolian eruptions and eruption fed pyroclastic deposits in a paleoproterozoic rifted arc hidden formation flin flon canada abstract the hidden formation of the paleoproterozoic flin flon arc assemblage comprises the stratigraphic hanging wall to the volcanogenic massive sulfide vms deposits at flin flon manitoba canada it represents a basaltic volcanic edifice erupted within a large subsidence structure developed in a rifted arc environment it is predominately pillow lavas with subordinated amounts volcaniclastic rocks rocks that stratigraphically overly the schist and mandy vms deposits 5 km southeast of the main flin flon deposits have been correlated through litho and chemo stratigraphy with the hidden formation they display evidence of pyroclastic volcanism and concomitant subsidence pillow lavas and hyaloclastite are unequivocal evidence of subaqueous eruption the hidden formation in the schist mandy area comprises mafic tuff mafic megabreccia basalt lava fluidal clast breccia and plagioclase crystal rich volcaniclastic lithofacies intruded by synvolcanic dikes and sills the megabreccia lithofacies is characterized by angular basalt clasts 1 20 m in a matrix of finer basalt clasts and is interpreted to have been deposited by mass flows due to slumping of the caldera wall a within flow vertical transition in the lavas from massive to pillowed flows suggests a slowing of emplacement velocity decrease in temperature and an increase in the viscosity during eruption based on the monolithic nature of the fluidal clast breccias and the fluidal shapes of clasts with delicate tails and intact glassy margins they are interpreted to be the result of strombolian eruptions an increase in the ingestion of water in the vent s lead to a change from magmatic to phreatomagmatic fragmentation planar and cross laminations graded beds and the abundance of broken plagioclase crystals in combination with scoria and lithic clasts suggest the mafic tuff and plagioclase crystal rich volcaniclastic lithofacies were deposited by subaqueous eruption fed density currents derived from a phreatomagmatic eruption detailed mapping of the volcanic lithofacies in the schist mandy area has facilitated the identification of synvolcanic faults defining inner caldera margins and volcanic vents these structures when traced downwards through stratigraphy define structural corridors corresponding with the schist and mandy vms deposits indicating they represent long lived structures that also acted as magmas and fluid pathways

stratigraphy petrography and dispersion of the lower permian syn eruptive deposits in the viar basin spain abstract this paper presents a stratigraphic petrographic and sedimentologic analysis of felsic syn eruptive deposits of the southernmost permian basin in europe viar basin an inner orogenic transtensional half graben located between two major geotectonic zones of the iberian variscan belt sw spain the syn eruptive deposits of the viar basin represent a mappable lithostratigraphic unit that is herein named the los canchales formation this unit forms a wedge shaped body bounded by alluvial fluvial deposits red layers that are in gradational contact the stratigraphic architecture of the los canchales formation is complex with frequent facies changes and interfingering among pyroclastic rocks resedimented volcaniclastic rocks and sedimentary rocks abundant fossil flora is also recorded a wax wane cycle of volcanic activity is recognized during the syn eruptive period two short lived volcanic episodes were recorded including ignimbrites and a possible phreatomagmatic event explosive volcanism generated pyroclastic flow surge and ash fall deposits in cases collapsing the drainage network they also modified the vegetation and created a new depositional setting with four successive sedimentary environments vegetated floodplain ephemeral sandy braidplain palustrine environment and again vegetated floodplain input of pyroclasts followed a nw to se longitudinal axial dispersal pattern

syn and post eruptive erosion gully formation and morphological evolution of a tephra ring in tropical climate erupted in 1913 in west ambrym vanuatu abstract syn and post eruptive erosion of volcanic cones plays an important role in mass redistribution of tephra over short periods descriptions of the early stages of erosion of tephra from monogenetic volcanic cones are rare particularly those with a well constrained timing of events in spite of this lack of data cone morphologies and erosion features are commonly used for long term erosion rate calculations and relative age determinations in volcanic fields this paper offers new observations which suggest differing constraints on the timing of erosion of a tephra ring may be operating than those conventionally cited in 1913 a tephra ring was formed as part of an eruption in west ambrym island vanuatu and is now exposed along a continuous 2 5 km long coastal section the ring surrounds an oval shaped depression filled by water it is composed of a succession of a phreatomagmatic fall and base surge beds interbedded with thin scoriaceous lapilli units toward the outer edges of the ring base surge beds are gradually replaced in the succession by fine ash dominated debris flows and hyperconcentrated flow deposits the inter fingering of phreatomagmatic deposits with syn volcanic reworked volcaniclastic sediments indicates that an ongoing remobilisation of freshly deposited tephra was already occurring during the eruption gullies cut into the un weathered tephra are up to 4 m deep and commonly have c 1 m of debris flow deposit fill in their bases there is no indication of weathering vegetation fragments or soil development between the gully bases and the basal debris flow fills gully walls are steep and superficial fans of collapsed sediment are common most gullies are heavily vegetated although some active ephemeral channels occur these observations suggest that the majority of the erosion of such tephra rings in tropical climates takes place directly during eruption and possibly for only a period of days to weeks afterward after establishment of the gully network tephra remobilisation is concentrated only within them therefore the shape of the erosion modified volcanic landform is predominantly developed shortly after the eruption ceases this observation indicates that gully erosion morphology may not necessarily relate to age of such a landform different intensities of erosion during eruption related to water supply or rainfall are probably the major influence on gully spacing modal depth and form longer term post eruption processes that could be indicators of relative age may include internal gully deepening below basal debris flow fill sediments and possibly widening and side slope lowering due to undercutting and side collapse

maar diatreme volcanoes a review maar diatreme volcanoes are produced by explosive eruptions that cut deeply into the country rock a maar is the crater cut into the ground and surrounded by an ejecta ring while the diatreme structure continues downward and encloses diatreme and root zone deposits here we attempt an evenhanded review of maar diatreme volcanology that extends from mafic to kimberlitic varieties and from historical maar eruptions to deeply eroded or mined diatreme structures we conclude that maar diatreme eruptions are episodic ejecta rings provide invaluable insight into eruption processes and sequence but are incomplete records of diatreme formation deposits within the diatreme structure include in varying proportions lower unbedded deposits sometimes typified by subvertical contacts among domains of debris emplaced sequentially and upper bedded deposits formed by sedimentation on surfaces open to the atmosphere a basal root zone comprises the transition from coherent magmatic feeder dike to clastic deposits formed by fragmentation of magma and enclosing country rock root zones are irregular in form and the clastic deposits are typically intruded by contorted dikes irregular root zone like chaotic breccias cut by contorted dikes are also present within diatreme deposits where they represent intra diatreme fragmentation zones and record changes in the location of the explosion locus during eruption

mafic plinian volcanism and ignimbrite emplacement at tofua volcano tonga tofua island is the largest emergent mafic volcano within the tofua arc tonga southwest pacific the volcano is dominated by a distinctive caldera averaging 4 km in diameter containing a freshwater lake in the south and east the latest paroxysmal vei 5 6 explosive volcanism includes two phases of activity each emplacing a high grade ignimbrite the products are basaltic andesites with between 52 wt and 57 wt sio2 the first and largest eruption caused the inward collapse of a stratovolcano and produced the tofua ignimbrite and a sub circular caldera located slightly northwest of the island s centre this ignimbrite was deposited in a radial fashion over the entire island with associated plinian fall deposits up to 0 5 m thick on islands 40 km away common sub rounded and frequently cauliform scoria bombs throughout the ignimbrite attest to a small degree of marginal magma water interaction the common intense welding of the coarse grained eruptive products however suggests that the majority of the erupted magma was hot water undersaturated and supplied at high rates with moderately low fragmentation efficiency and low levels of interaction with external water we propose that the development of a water saturated dacite body at shallow 6 km depth resulted in failure of the chamber roof to cause sudden evacuation of material producing a plinian eruption column following a brief period of quiescence large scale faulting in the southeast of the island produced a second explosive phase believed to result from recharge of a chemically distinct magma depleted in incompatible elements this similar but smaller eruption emplaced the hokula ignimbrite sheet in the northeast of the island a maximum total volume of 8 km3 of juvenile material was erupted by these events the main eruption column is estimated to have reached a height of 12 km and to have produced a major atmospheric injection of gas and tephra recorded in the widespread series of fall deposits found on coral islands 40 80 km to the east in the direction of regional upper tropospheric winds radiocarbon dating of charcoal below the tofua ignimbrite and organic material below the related fall units imply this eruption sequence occurred post 1 000 years bp we estimate an eruption magnitude of 2 24 1013 kg sulphur release of 12 tg and tentatively assign this eruption to the ad 1030 volcanic sulphate spike recorded in antarctic ice sheet records

the influence of magma plumbing complexity on monogenetic eruptions jeju island korea terra nova 23 70 75 2011 abstract continuous pyroclastic successions from monogenetic volcanoes in the jeju island volcanic field korea were targeted for detailed geochemical investigations of juvenile ejecta to show that multiple distinct magma pulses may be erupted during a single monogenetic eruption deeply derived and possibly multiply sourced evolving magma pulses may erupt sequentially from the same vent and form generally uninterrupted depositional sequences with continuous chemical trends alternatively pulses can rise independently resulting in breccia horizons and truncation surfaces in depositional sequences and in stepped and mixed chemical trends we infer that uninterrupted eruptions result from clear plumbing systems in which a single large dyke system feeds an eruption and where differently sourced magmas are erupted independently congested plumbing systems consisting of dyke complexes give rise to vent shifts asymmetrical eruptive centres and composite depositional sequences integration of chemical variation and eruptive dynamics provides a powerful means of understanding monogenetic volcanoes

new age constraints for hominid footprints found on jeju island south korea abstract in 2004 numerous hominid footprints along with diverse animal footprints were found in the late quaternary strata of jeju island south korea however the age of the sediments in which the footprints were found is still controversial previous age estimates included radiocarbon ages of ca 15 000 yr bp late pleistocene and quartz optically stimulated luminescence ages of ca 7000 yr bp mid holocene in this study we report on 11 ams 14 c dating results from a new set of samples collected from the footprint bearing strata and from associated sediments despite some variations and age reversal all samples collected from the footprint bearing strata yielded 14 c ages of late pleistocene these ages are comparable with previous radiocarbon dating results furthermore the presence of the proboscidean footprints attributable to woolly mammoths in the footprint bearing strata supports the radiocarbon dating results based on the new radiocarbon dates and the presence of the alleged mammoth footprints the age of the hominid footprints found at jeju island is thought to be late pleistocene about 19 000 25 000 cal yr bp therefore this is the second discovery of hominid footprints dated to the pleistocene age in asia and the first to be discovered in korea

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

long runout pyroclastic surge on a cretaceous alluvial plain republic of korea pyroclastic surge is a dilute and turbulent flow of volcanic gas and tephra that is commonly generated during explosive volcanic eruptions and can threaten lives along its flow paths assessing its travel distance and delineating future volcanic hazards have therefore been major concerns of volcanologists historical eruptions show that most pyroclastic surges travel a few tens of kilometres or less from their sources aeolian or aquagene processes have therefore been evoked for the emplacement of supposed surge deposits much beyond this distance here we show that a cretaceous tuff bed in korea was emplaced by an exceptionally powerful pyroclastic surge that flowed as far as the most powerful pyroclastic flows that formed the low aspect ratio ignimbrites lari this has significant implications for interpreting ancient volcanic eruptions and delineating volcanic hazards by pyroclastic surges and casts intriguing questions on the eruption dynamics and physics of long runout pyroclastic surges and their distinction from lari forming pyroclastic flows

the 1245 yr bp asososca maar new advances on recent volcanic stratigraphy of managua nicaragua and hazard implications abstract asososca maar is located at the western outskirts of managua nicaragua in the central part of the active n s trending and right lateral nejapa miraflores fault that marks an offset of the middle america volcanic arc it constitutes one of the 21 vents aligned along the fault between the chiltepe volcanic complex to the north and ticomo vents to the south asososca consists of an east west elongated crater filled by a lake which is currently used for supplying part of managua with drinking water 10 of the capital city demand the crater excavated the previous topography allowing the observation of a detailed holocene stratigraphic record that should be taken into account for future hazard analyses we present a geological map together with the detailed stratigraphy exposed inside and around asososca crater aided by radiocarbon dating of paleosols the pre existing volcanic sequence excavated by asososca is younger than 12 730 255 250 yr bp and is capped by the phreato plinian masaya tuff 2 are dominated by accidental clasts including basaltic to basaltic andesitic olivine bearing scoriae lapilli porphyritic and hypocrystalline andesite blocks and lapilli altered pumice lapilli and ash and ignimbrite fragments juvenile fragments were only identified in size fractions smaller than 1 in proportions lower than 25 and consist of black moss like fused shape and poorly vesiculated fresh glass fragments of basaltic composition sio2 48 the asososca tephra is interpreted as due to the emplacement of several pyroclastic surges originated by phreatomagmatic eruptions from asososca crater as suggested by impact sags geometry and dune crest migration structures according to radiocarbon dating these deposits were emplaced at 1245 125 120 yr bp the stratigraphic position of the asososca tephra and the well preserved morphology of the crater indicate that asososca is the youngest vent along the nejapa miraflores fault explosive eruptions might therefore occur again along this fault at the western outskirts of managua such kind of activity together with the strong seismicity associated to the active fault represents a serious hazard to urban infrastructure and a population of ca 1 3 million inhabitants

lithofacies and architecture of a basinwide tuff unit in the miocene eoil basin se korea modes of pyroclastic sedimentation changes in eruption style and implications for basin configuration the miocene eoil basin se korea is a small half graben basin that was rifted by both extensional and dextral strike slip deformations during backarc opening of the east sea sea of japan the basin was filled by fluviolacustrine sediments and abundant basaltic and dacitic volcanic deposits the paljori tuff is a 2 20 m thick basinwide dacitic volcaniclastic unit that is intercalated in the topmost part of a fluviolacustrine formation the tuff consists of two pyroclastic units units ii and iii which are underlain and overlain by two resedimented volcaniclastic units units i and iv unit i at the base is composed mainly of fine grained resedimented tephra unit ii is a massive and stratified tuff produced by a dense pyroclastic density current that was ponded in the southwestern part of the basin unit iii is a massive lapilli tuff emplaced by a voluminous and turbulent pyroclastic density current that swept across the basin toward the northeast without ponding or blocking by intrabasinal highs and basin margin relief and unit iv at the top is composed of coarse grained resedimented tephra the general coarsening up of these units the lack of nonvolcaniclastic sedimentary interbeds and the offset stacking of the units toward the northeast suggest an episodic short duration eruption from the southwest that waxed in mass eruption rate and changed eruption style from phreatomagmatic to magmatic these units show basin scale variations in geometry including 1 thick accumulations of unit i in the northeastern and southwestern extremities of the basin 2 ponding of unit ii in the southwestern part of the basin 3 truncation of unit iii in the same area and 4 thickening of unit iv toward the northeastern basin margin these provide important clues to the distribution of and temporal changes in accommodation space and hence the configuration and structural evolution of the eoil basin it is inferred that the greatest subsidence occurred at the southwestern and northeastern corners of the basin the southwest northeast trending axes of the basin plunge gently toward the northeast and the major intrabasinal faults of the basin were produced before the eruption of the paljori tuff the basinwide preservation of the tuff and its fairly uniform thickness suggest that the basin was undergoing rapid extension and subsidence because of the onset of the rift climax the paljori tuff shows that important clues to and high resolution records of the large scale configuration and structural evolution of a sedimentary basin can be obtained from the study of lithofacies variations and the architecture of a single basinwide volcaniclastic unit

reconstructing eruption processes of a miocene monogenetic volcanic field from vent remnants waipiata volcanic field south island new zealand the miocene waipiata volcanic field new zealand is an eroded phreatomagmatic intracontinental volcanic field formed during a period of weak lithospheric extension the field includes remnants of at least 55 volcanoes in an area of 5000 km2 vent filling deposits comprising predominantly lava e g plugs necks lava flows or dykes often associated with thin basal phreatomagmatic pyroclastic deposits were classified as type 1 vents and are inferred to be the remnants of scoria cones vents represented by predominantly pyroclastic infill are classified as type 2 vents and are inferred to have been the substructures of phreatomagmatic tuff ring and or maar volcanoes type 3 vent complexes are groups of closely spaced or overlapping vents with voluminous preserved lava flows they are inferred to be the remnants of volcanoes comprising adjoining to coalescing maars and tuff rings with magmatic explosive and effusive products pyroclastic rocks of most of the waipiata vents record initial phreatomagmatic explosive activity fuelled by groundwater followed by strombolian style eruptions aligned and clustered vents are accommodated to structural features of the regional basement rock otago schist

characteristics of dune bedded pyroclastic surge bedsets abstract the grainsize characteristics of dune bedded pyroclastic surge bedsets are surveyed the variance between coarsest and finest beds ranges from 1 to 6 phi in different surge bedsets and it increases as the grainsize of the coarsest bed increases reflecting an increasing velocity of emplacement deposits of wet surges identified as those which contain accretionary and ash coated lapilli tend to be finer and show less variance this partly because wet ash is cohesive but mainly because wet surges tend to be weaker dry surge bedsets are strongly fines depleted wet ones less so the lack of erosion of underlying ash layers shows that the environment is a strongly depositional one individual bedsets are demarcated by thin intervening fine ash fall layers which are the complementary ash cloud deposits settled or flushed out after the passage and decay of each turbulent surge surge deposits are generally less coarse than the coarsest associated airfall deposits which shows that they are formed by generally weaker events this study helps interpret the dune bedded parts of the landscape mantling may 18th 1980 blast deposit of mount st helens the blast was a very violent event but the variance and the grainsize of the coarsest bed are those of a relatively weak surge this suggests that the dune bedding was produced by a weak effect such as minor turbulence in a thin pyroclastic flow coming to rest in a mountainous terrain roughened by tree stumps and fallen logs

hazard of pyroclastic density currents at the campi flegrei caldera southern italy as deduced from the combined use of facies architecture physical modeling and statistics of the impact parameters abstract pyroclastic density currents of the recent eruptions at campi flegrei caldera cfc southern italy have been studied with the aim of assessing the potential impact of similar events in the future eruptions of different scales have been investigated by means of the combined use of facies architecture laboratory analysis and physical modeling both in the small averno 2 and intermediate astroni scales facies analysis indicates that deposits result from the emplacement of pyroclastic density currents like base surge formed by multiple closely timed impulses of phreatomagmatic origin in the large scale event agnano monte spina the facies architecture suggests that the currents started as concentrated flows near the vent as originating from the collapse of a dense eruptive column and evolved laterally into expanded flows by the propagation of the basal shear current laboratory analyses on samples from the main layers of deposits allowed obtaining the input data for the pyflow code which was used for reconstructing the flow dynamic characteristics of the currents the expected damage is discussed in terms of the probability density function of dynamic pressure and particle volumetric concentration in this way the range of potential impact that similar pyroclastic density currents could cause to buildings infrastructures and population is defined in the large scale event the dynamic pressure ranges from 9 38 to 1 00 kpa integrating the basal 10 m of the current at distances of 1 5 and 4 0 km from the vent respectively the values are highly influenced by the local topography in the intermediate scale event the dynamic pressure ranges from 2 43 to 0 26 kpa at distances of 1 1 and 1 4 km from the vent respectively in the small scale event the dynamic pressure ranges from 1 49 to 0 39 kpa at distances of 0 5 and 1 1 km from the vent respectively the particle volumetric concentration at a height of 2 m within the current is always lower than 0 01 but typically it is higher than 0 001 inside the caldera and could have severe effects on the unsheltered population also at locations topographic highs where velocity and dynamic pressures are low in distal reaches outside the caldera the widespread fine ash originated by buoyancy from the currents which was transported by lower atmosphere winds is to be considered as a fallout load slowly accumulating on the ground surface and building roofs during the waning stage of the current

the influence of variable topography on the depositional behaviour of pyroclastic density currents the examples of the upper pollara eruption salina island southern italy abstract stratigraphic reconstruction of the upper pollara eruption has allowed for the inference of eruptive mechanisms and the distillation of a sedimentological model for pyroclastic density currents pdcs moving across variable topography the pre eruptive topography in the study area was characterised by a tuff ring like morphology with both inward and outward dipping slopes highly viscous moderately porphyritic dacitic to rhyolitic magmas fed the eruption which was characterised by a vulcanian eruptive style the stratigraphic succession was divided into five eruption units eus which result from different phases of the eruption separated by stases sustained columns occurred only during eu1 while pdc generation dominates eu2 5 lithofacies analysis of the pdc deposits indicates the prevalence of massive coarse grained deposits on the inner slopes of the pollara crater which are interpreted as the deposits of a flow boundary zone dominated by granular flow or fluid escape regimes dune bedded massive to stratified lithofacies dominate the outer slopes of the pollara crater and are interpreted as the deposits of pdcs with flow boundary zones in which traction played a major role thin massive pdc deposits are exposed on the sub horizontal malfa terrace and are interpreted as representative of flow boundary zones dominated by a granular flow regime the occurrence of stacked deposits indicates that most of the pdcs were characterised by unsteady pulsatory behaviour with development of trains of pulses during their transport the downcurrent lithofacies transitions observed for the upper pollara deposits have finally been compared with other similar lithofacies associations which have been described for short lived pdcs at tuff rings in order to discuss the influence of pre eruptive topography on lithofacies association

origin of the joya honda maar san luis potosi mexico abstract joya honda is a quaternary maar of unusual type from the mexican basin and range province its 300 m deep crater is excavated in cretaceous limestones the surrounding tephra deposit which in places is 100 m thick begins with a series of weakly indurated pyroclastic surge and fall layers that we interpret as dry surge deposits these are overlain by the main sequence of strongly indurated massive tuff breccias that we interpret as wet surge deposits joya honda formed subaerially from the interaction of groundwater with rapidly ascending intraplate type basanitic magma carrying peridotitic mantle xenoliths local aquifer characteristics controlled the style of eruption and the nature of the deposits groundwater in the limestone hosted aquifer beneath joya honda was apparently contained within solution enhanced fractures at the onset of the eruption magma began to interact with a moderate amount of groundwater producing the dry surge deposits which are typical of deposits found at many maars and tuff rings as the eruption continued the crater grew and the hydromagmatic blasts fractured the limestones around the explosion foci a marked increase in the water magma ratio of the system followed when a large fracture or a portion of the limestone with enhanced secondary permeability was intersected by the expanding crater subsequent phreatomagmatic explosions occurred in a system with groundwater flow rates several orders of magnitude larger than in the initial dry surge stage at the maar rim these wet eruptions led to the emplacement of massive tuff breccias through a combination of fallout surges and mudflows these steeply dipping tuff breccias are similar to deposits found at many tuff cones juvenile clasts in the near vent deposits show marked upward increases in both hydration palagonitization and vesicularity the increased palagonitization with height in the section appears to be a consequence of the overall increased wetness of the eruption with time correlating with greater carbonate cementation and lithification in the upper part of the deposit the transition toward higher vesicularity is interpreted as evidence of a gradual reduction in the confining pressure for the ascending magma prior to explosive fragmentation perhaps related to unroofing during progressive excavation of the overlying maar crater thus joya honda does not support maar formation models that invoke downward displacement of explosion foci caused by formation of a cone of depression in the aquifer in order to maintain the confining pressure for the hydromagmatic blasts