decadal scale change of infiltration characteristics of a tephra mantled hillslope at mount st helens washington the cataclysmic 1980 eruption of mount st helens radically reduced the infiltration characteristics of 60 000 ha of rugged terrain and dramatically altered landscape hydrology two decades of erosional biogenic cryogenic and anthropogenic activity have modified the infiltration characteristics of much of that devastated landscape and modulated the hydrological impact of the eruption we assessed infiltration and runoff characteristics of a segment of hillslope thickly mantled with tephra but now revegetated primarily with grasses and other plants to evaluate hydrological modifications due to erosion and natural turbation eruptive disturbance reduced infiltration capacity of the hillslope by as much as 50 fold between 1980 and 2000 apparent infiltration capacities of plots on the hillslope increased as much as ten fold but remain approximately three to five times less than the probable pre eruption capacities common regional rainfall intensities and snowmelt rates presently produce little surface runoff however high magnitude low frequency storms and unusually rapid snowmelt can still induce broad infiltration excess overland flow after 20 years erosion and natural mechanical turbation have modulated but not effaced the hydrological perturbation caused by the cataclysmic eruption copyright 2005 john wiley sons ltd

cirrus mammatus properties derived from an extended remote sensing dataset abstract the first quantitative and statistical evaluation of cirrus mammatus clouds based on wavelet analysis of remote sensing data is made by analyzing the university of utah facility for atmospheric remote sensing fars 10 yr high cloud dataset first a case study of cirrus mammata combining a high resolution lidar system and a w band doppler radar is presented yielding an assessment of the thermodynamic environment and dynamic mechanisms then 25 cirrus mammatus cases selected from the fars lidar dataset are used to disclose their characteristic environmental conditions and vertical and length scales the results show that cirrus mammata occur in the transition zone from moist cloudy to dry air layers with weak wind shear which suggests that cloud induced thermal structures play a key role in their formation their maximum vertical and horizontal length scales vary from 0 3 to 1 1 km and 0 5 to 8 0 km respectively it is also found that small scale structures develop between the large scale

hydrometeor enhanced tephra sedimentation constraints from the 18 may 1980 eruption of mount st helens 1 uncertainty remains on the origin of distal mass deposition maxima observed in many recent tephra fall deposits in this study the link between ash aggregation and the formation of distal mass deposition maxima is investigated through reanalysis of tephra fallout from the mount st helens 18 may 1980 msh80 eruption in addition we collate all the data needed to model distal ash sedimentation from the msh80 eruption cloud four particle size subpopulations were present in distal fallout with modes at 2 2 4 2 5 9 and 8 3 settling rates of the coarsest subpopulation closely matched predicted single particle terminal fall velocities sedimentation of particles 100 m was greatly enhanced predominantly through aggregation of a particle subpopulation with modal diameter 5 9 0 2 19 3 m mammatus on the msh80 cloud provided a mechanism to transport very fine ash particles with predicted atmospheric lifetimes of days to weeks from the upper troposphere to the surface in a matter of hours in this mechanism ash particles initiate ice hydrometeor formation high in the troposphere subsequently the volcanic cloud rapidly subsides as mammatus develop from increased particle loading and cloud base sublimation rapid fallout occurs as the cloud passes through the melting level in a process analogous to snowflake aggregation aggregates sediment en masse and form the distal mass deposition maxima observed in many recent volcanic ash fall deposits this work provides a data resource that will facilitate tephra sedimentation modeling and allow model intercomparisons

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

observations of a squall line and its near environment using high frequency rawinsonde launches during vortex2 abstract rawinsonde data were collected before and during passage of a squall line in oklahoma on 15 may 2009 during the second verification of the origins of rotation in tornadoes experiment vortex2 nine soundings were released within 3 h allowing for unprecedented analysis of the squall line s internal structure and nearby environment four soundings were released in the prestorm environment and they document the following features low level cooling associated with the reduction of solar isolation by a cirrus anvil abrupt warming 1 5 k in 30 min above the boundary layer which is probably attributable to a gravity wave increases in both low level and deep layer vertical wind shear within 100 km of the squall line and evidence of ascent extending at least 75 km ahead of the squall line the next sounding was released 5 km ahead of the squall line s gust front it documented a moist absolutely unstable layer within a 2 km deep layer of ascent with vertical air velocity of approximately 6 m s 1

columnar jointing in vapor phase altered non welded cerro galan ignimbrite paycuqui argentina columnar jointing is thought to occur primarily in lavas and welded pyroclastic flow deposits however the non welded cerro galan ignimbrite at paycuqui argentina contains well developed columnar joints that are instead due to high temperature vapor phase alteration of the deposit where devitrification and vapor phase crystallization have increased the density and cohesion of the upper half of the section thermal remanent magnetization analyses of entrained lithic clasts indicate high emplacement temperatures above 630 c but the lack of welding textures indicates temperatures below the glass transition temperature in order to remain below the glass transition at 630 c the minimum cooling rate prior to deposition was 3 0 10 3 8 5 10 2 c min depending on the experimental data used for comparison alternatively if the deposit was emplaced above the glass transition temperature conductive cooling alone was insufficient to prevent welding crack patterns average 4 5 sides to each polygon and column diameters average 75 cm are consistent with relatively rapid cooling where advective heat loss due to vapor fluxing increases cooling over simple conductive heat transfer the presence of regularly spaced complex radiating joint patterns is consistent with fumarolic gas rise where volatiles originated in the valley confined drainage system below joint spacing is a proxy for cooling rates and is controlled by depositional thickness valley width we suggest that the formation of joints in high temperature non welded deposits is aided by the presence of underlying external water where vapor transfer causes crystallization in pore spaces densifies the deposit and helps prevent welding

thermodynamics of water permeated unwelded pyroclasts 1 equilibrium properties abstract we report here the results of an investigation of thermodynamic equilibrium properties of systems constituted by phlegraean unwelded pyroclasts dry or saturated with water we have experimentally measured the porosity of samples of phlegraean rocks and determined the values of the heat capacity and of thermal expansion of dry and water perfused tuffs having different texture structures also the energetics of the process of adsorption of water on the large internal surface of these porous materials has been investigated the amount of heat exchanged during the reversible formation of an adsorbed layer has been determined in dependence of its thickness and as a function of temperature the coherent picture emerging from the ensemble of all these measurements is that of a direct connection between the processes of adsorption desorption and the stress of the porous rock in very unsaturated soils these processes may constitute a much more efficient mechanism of transformation of thermal energy into soil deformation compared with that of thermal expansion of the solid matrix

computer simulation models of scoria cone degradation abstract long term erosional modifications of the relatively simple morphology of scoria cinder cones are ideally suited for study by field and computer simulation methods a series of temporally distinct cones in the san francisco and springerville volcanic fields of arizona provides the foundation for documenting the degradational evolution of scoria cones in a semi arid climate progressive changes due to erosion are illustrated by the systematic decrease with increasing age of various morphometric parameters including scoria cone height cone height width ratio hco wco crater depth width ratio and slope angle for example holocene latest pleistocene cones in the san francisco field have a mean hco wco value of 0 178 0 041 a mean maximum slope angle of 29 7 4 2 and a mean average slope angle of 26 4 7 3 whereas the group of pliocene cones have values of 0 077 0 024 20 5 5 8 and 8 7 2 7 respectively comparative morphology of scoria cones is a potentially useful dating tool for mapping volcanic fields in order to better understand the degradational modifications of these volcanic landforms we have developed a numerical approach to simulate the surficial processes responsible for the erosion of a typical scoria cone the simulation algorithm can apply either a linear diffusion equation model or a model with a nonlinear transport law using a finite difference formulation the simulation operates upon a three dimensional scoria cone input as a matrix of elevation values utilizing both field and model results the correlation between changing hco wco value cone age and computer time step was expressed graphically to derive comprehensive values of the transport or diffusion coefficient df for both volcanic fields for the san francisco volcanic field df had a calculated value of 21 4 m2 kyr for the linear model and 5 3 m kyr for the nonlinear model while for the springerville volcanic field df had a calculated value of 24 4 m2 kyr for the linear model and 6 3 m kyr for the nonlinear model

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

the age of the neapolitan yellow tuff caldera forming eruption campi flegrei caldera italy assessed by 40ar 39ar dating method abstract the neapolitan yellow tuff nyt is the product of the largest known trachytic phreatoplinian eruption it covered an area larger than 1000 km2 with an estimated volume of about 40 km3 of erupted magma during the course of the eruption a caldera collapsed within the previously formed campanian ignimbrite caldera the resulting nested structure strongly influenced the following volcanic activity in the campi flegrei caldera as previous dating of the nyt does not converge toward a unique result a new set of 40ar 39ar age determinations has been carried out to better constrain the age of the eruption two variants of the 40ar 39ar dating method were applied to determine the age of the nyt eruption 1 single crystal total fusion sctf on an individual phenocryst of feldspar and 2 laser incremental heating lih on bulk aliquots of feldspar phenocrysts the results of the sctf analyses show that the overall sample weighted mean age derived from the conventional age calculation is 15 6 0 8 ka a weighted mean of the isochron age is 15 3 1 2 ka 2 and has been assumed as the best indicator of age to be derived from the sctf analyses the lih analyses results show that plateau ages vary from 15 4 0 5 to 14 5 0 5 ka the overall weighted mean age of the isochron results is 14 9 0 4 ka 2 this result has been assumed as the reference age for the nyt eruption and agrees with the sctf age the new age obtained for the nyt deposits is of great relevance for the understanding of the evolution and the present state of the campi flegrei caldera and collocates the nyt in a crucial stratigraphical position to date the climatic oscillations that occurred between the late glacial and the holocene

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

growth destruction and facies architecture of effusive and explosive volcanics in the miocene shama basin southwest of saudi arabia subaqueous subaerial volcanism in a lacustrine setting abstract this paper presents for the first time detailed stratigraphical and petrographical analyses of the harrat shama volcanic basin hsvb of miocene age in sw saudi arabia this basin is filled by abundant volcanics intercalated with fluvio lacustrine sedimentary successions these successions are characterized by a syneruptive intereruptive cycle which results from the intercalation periods of volcanics and volcaniclastic sedimentation along with periods of fluvial incision two depositional sequences showing an overall fining upward trend are preserved within the hsvb the first comprises hyaloclastites zeolite bearing bedded tuffs and bedded accretionary lapilli tuffs interpreted as deep lake whereas the second is made up of extensive pumiceous pyroclastic flows peperitic breccias and basaltic lava flows which are in turn capped by siliciclastic deposits of lacustrine deposits the passage between two sequences is defined by lateral variations of the lithofacies and textural features reflecting a change in paleogeography the fining upward volcaniclastic deposits of the hsvb sequence are interpreted to represent the sedimentation pattern induced by tectonic subsidence with the influence of volcaniclastic sediment input indicating well established proximal distal relationship hsvb was evolved through three stages including pre syn and post caldera stages the first two stages correspond to proximal aggradational syn eruptive event whereas the latter stage refers to distal degradational inter eruptive event these stages overlap in time as shown by the alternation of explosive effusive and their resedimented deposits reflecting diverse eruptive sequences formed by different fragmentation processes the difference in the latter coupled with the variability in the magma water interaction and the nature of substrate are responsible for such diversity resulting in progressive environmental changes from subaqueous to subaerial regime from bottom to top the development of such contrast in the depositional sequences reflects variation in the sediment flux and subsidence rate during the evolution of the hsvb

petrology and geochemistry of submarine volcanism in the sicily channel rift abstract submarine magmatism in the sicily channel rift began in the early pliocene and lasted until almost 200 yr ago we present here petrological and geochemical data on volcanic rocks dredged from graham and nameless banks and pantelleria seamounts in the pelagian sector of the sicily channel rift petrological evidence suggests that the ascent of magmas to the surface was relatively rapid probably through channels superimposed over the major tectonic discontinuities of the rift major and trace element data indicate an ocean island basalt affinity for graham and nameless bank alkaline lavas and a depleted tholeiitic signature for one pantelleria seamount which had a shallower mantle source sr nd pb isotopic compositions suggest a heterogeneous mantle source involving both focus zone fozo and high himu components lead isotopic compositions are slightly less radiogenic in the tholeiitic basalt and more radiogenic closer to himu in the alkaline lavas we hypothesize that a decompressional me

a similarity theory for unsaturated downdrafts within clouds abstract recent observations of cumulus clouds strongly support the hypothesis of squires 1958 that much of the mixing within such clouds is associated with downward propagating currents initiated near their tops a similarity theory is here proposed to describe the properties of such currents the use of similarity is defended on the basis of the observed and predicted scale of the downdrafts the theory suggests that downward propagating unsaturated thermals are pervasive throughout all but the largest convective clouds and that quasi steady unsaturated downdraft plumes may exist in the lower portions of cumulonimbi in addition to providing a reasonable explanation for the microstructure of and liquid water distribution within cumulus clouds the theory appears to account for certain severe convective phenomena including down bursts a new but related cloud instability is proposed to account for the occurrence of mamma

the influence of water on the strength of neapolitan yellow tuff the most widely used building stone in naples italy neapolitan yellow tuff nyt has been used in construction in naples italy since the greeks founded the city then called neapolis in the sixth century bce we investigate here whether this popular building stone is weaker when saturated with water an issue important for assessments of weathering damage and monument preservation to this end we performed 28 uniaxial compressive strength measurements on dry and water saturated samples cored from a block of the lithified upper member of the nyt our experiments show that the strength of the zeolite rich nyt is systematically reduced when saturated with water the ratio of wet to dry strength is 0 63 complementary experiments show that two other common neapolitan building stones piperno tuff and the grey campanian ignimbrite both facies of the campanian ignimbrite deposit devoid of zeolites do not weaken when wet from these data and previously published data for tuffs around the globe we conclude that the water weakening in nyt is a consequence of the presence of abundant zeolites the block tested herein contains 46 wt of zeolites these data may help explain weathering damage in nyt building stones due to rainfall rising damp and proximity to the sea or water table and the observed link between rainfall and landslides rock falls and sinkhole formation in naples and the weathering of other buildings built from zeolite rich tuffs worldwide

rothenberg scoria cone east eifel a complex strombolian and phreatomagmatic volcano rothenberg scoria cone eifel formed by an alternation of three strombolian and three phreatomagmatic eruptive phases eruptions took place from up to six vents on a 600 m long fissure building an early tuff ring and then two coalescing scoria cones strombolian volcanism dominated volumetrically as the supply of external water was severely limited magma water interaction only occurred during the opening stages of eruption at any vent when discharge rates were low and the fragmentation surface was below the water table the phreatomagmatic deposits consist of relatively well sorted fall beds and only minor surge deposits they contain juvenile clasts with a wide range of vesicularity and grain size implying considerable heterogeneity in the assemblage of material ejected by the phreatomagmatic explosions the transition from phreatomagmatic to strombolian eruption at any vent was rapid and irreversible and strombolian volcanism persisted even when eruption rates are inferred to have waned at the close of each eruptive phase as by then the fragmentation surfaces were high in the growing cones and water was denied access to the magma the strombolian deposits are relatively homogenous consisting of alternating coarser and finer grained well sorted fall beds erupted during periods of open vent eruption and partial blockage of the vent respectively the intervals of strombolian eruption were always a delicate balance between discharge of freely degassing magma and processes such as ponding of degassed magma in the vent collapse of the growing cones and repeated recycling of clasts through the vent clear evidence of the instability of the rothenberg cones is preserved in numerous unconformities within deposits of the inner walls of the cones the close of strombolian phases was probably marked by a decreasing supply of magma to the vents accompanied by ponding and stagnation of lava in the craters

mixed deposits of complex magmatic and phreatomagmatic volcanism an example from crater hill auckland new zealand a series of alternating phreatomagmatic wet and magmatic dry basaltic pyroclastic deposits forming the crater hill tuff ring in new zealand contains one unit m1 which can only be interpreted as the products of mixing of ejecta from simultaneous wet and dry explosions at different portions of a multiple vent system the principal characteristics of m1 are a rapid lateral changes in the thicknesses of and proportions in juvenile components in individual beds and b wide ranges of juvenile clast densities in every sample m1 appears to have been associated with an elongate source of highly variable and fluctuating magma water ratios and magma discharge rates this contrasts with the only other documented mixed wet and dry basaltic pyroclastic deposits where mixing from two point sources of quite different but stable character has been inferred

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

primary versus secondary and subaerial versus submarine hydrovolcanic deposits in the subsurface of jeju island korea jeju island is a quaternary shield volcano built upon the yellow sea continental shelf off the korean peninsula decades of borehole drilling reveals that the shield forming lavas of the island are underlain by extensive hydrovolcanic deposits the seoguipo formation which are about 100 m thick and show diverse depositional features this study provides criteria for distinguishing between hydrovolcanic deposits formed by primary pyroclastic and secondary resedimentation processes in subaerial and submarine settings based on the observations of several selected cores from the formation five facies associations are identified including i primary hydrovolcanic deposits formed by pyroclastic surges and co surge fallouts in tuff rings facies association phtr ii primary hydrovolcanic deposits formed by surtseyan fallout and related pyroclastic transport processes in tuff cones facies association phtc iii secondary hydrovolcanic deposits formed by debris flows hyperconcentrated flood flows sheet floods and rill flows in subaerial settings facies association rhae iv secondary hydrovolcanic deposits formed in submarine settings under the influence of waves tides and occasional mass flows facies association rhmar and v non volcaniclastic and fine grained deposits formed in nearshore to offshore settings facies association nvmar the primary hydrovolcanic facies associations phtr and phtc are distinguished from one another on the basis of distinct lithofacies characteristics and vertical sequence profiles these facies differ from the secondary hydrovolcanic and non volcaniclastic facies associations rhae rhmar and nvmar because of their distinctive sedimentary structures textures and compositions the depositional processes and settings of some massive and crudely stratified volcaniclastic deposits which occur in many facies associations could not be discriminated unambiguously even with microscopic observations nevertheless these facies associations could generally be distinguished because they occur typically in packets or sequences several metres to tens of metres thick and bounded by distinct stratigraphic discontinuities and comprise generally distinct sets of lithofacies the overall characteristics of the seoguipo formation suggest that it is composed of numerous superposed phreatomagmatic volcanoes intercalated with marine or non marine volcaniclastic or non volcaniclastic deposits widespread and continual hydrovolcanic activity together with volcaniclastic sedimentation is inferred to have persisted for more than a million years in jeju island under the influence of fluctuating quaternary sea levels before effusion of the shield forming lavas extensive distribution of hydrovolcanic deposits in the subsurface of jeju island highlights that there can be significant differences in the eruption style growth history and internal structure between shelfal shield volcanoes and oceanic island volcanoes

mammatus clouds as a response to cloud base radiative heating mammatus clouds are the pouchlike lobes seen hanging from mid to high level clouds they can look quite dramatic but they are also interesting because they provide clues to what controls anvil cirrus dynamic evolution thus far the most commonly accepted explanation for observed subsidence of mammatus lobes is that they are driven by evaporative cooling of precipitation accelerated by mixing with dry subcloud air here an alternative explanation is proposed radiative temperature contrasts between cloud base and the lower troposphere destabilize cloudy air to create a rapidly deepening mixed layer which creates positively buoyant intrusions of dry air into the cloud interior mammatus lobes are just the descending branch of the resulting circulations in this regard mammatus cloud fields might be considered an upside down analog to the radiatively driven formation of cloud holes seen at the tops of stratocumulus layers

distinguishing base surge deposits and volcaniclastic fluviatile sediments an ancient example from the lower devonian snowy river volcanics south eastern australia a 500 m long road cutting in the lower devonian snowy river volcanics srv eastern victoria australia exposes phreatomagmatic units and volcaniclastic sediments based on bed geometry sorting and sedimentary structures it was possible to distinguish base surge deposits from ephemeral fluvial deposits in this relatively well exposed ancient succession where the base surge deposits infill irregular topography bed sets mantle the pre existing surface but thicken into topographic lows in contrast where the fluvial deposits infill topographic depressions beds onlap laterally against channel walls in addition curvi planar slide surfaces within the base surge deposits generated by inter eruptive slumping indicate rapid emplacement as a constructional tuff rampart maar the base surge deposits are always poorly sorted and commonly contain accretionary lapilli reflecting their deposition from turbulent low particle concentration steam rich pyroclastic currents in contrast the fluvial deposits are relatively well sorted reflecting hydraulic sorting and winnowing during tractional transport and deposition there are significant differences in the types of sedimentary structures present 1 bedding in the base surge deposits is entirely tabular and beds can be traced laterally to the limits of the outcrop in contrast the fluvial deposits have abundant internal scour surfaces that result in beds bedding intervals lensing out laterally over intervals of the order of 5 10 m 2 cross beds with relatively high angle foresets are restricted to the fluvial deposits 3 laterally persistent tabular beds that contain abundant densely packed accretionary lapilli are restricted to the base surge deposits in summary although base surge deposits and ephemeral fluvial deposits can appear superficially similar it is possible to apply facies models carefully to distinguish between them even in ancient successions

a physical appraisal of a new aspect of bradyseism the miniuplifts 1 the campi flegrei caldera is characterized by well known episodes of fast uplift called bradyseism the last of which produced 1 8 m of maximum vertical displacement in the period 1982 1984 less known is a particular aspect of campi flegrei unrests detected only in the last 20 years namely the occurrence of miniuplift episodes characterized by relatively small positive vertical displacements in the range 0 03 0 11 m miniuplift episodes occurred in 1989 1994 and 2000 and probably one or two times in the period 1972 1982 these small ground uplifts were also accompanied by moderate seismicity each of them was followed by a short phase of relatively quick subsidence after which the previous rate of subsidence of the period was resumed an order of magnitude analysis of the forces energy and power that can cause the onset and determine the evolution of these events is derived on the basis of a thermal fluid dynamical approach it allows the quantitative correlation of the power input from the energy source with the rate of energy dissipation in the geologic system during the miniuplifts in order to assess the likelihood of our fluid dynamical model for these episodes theoretical ground displacements have been computed these displacements result from the changes in pressure as a function of depth predicted by the fluid dynamical model for reasonable perturbations of the geothermal system described as changes of the peclet number theoretical results agree extremely well with the observed amounts of the miniuplifts

surge deposit misidentification at spor mountain utah and elsewhere a cautionary message for mars abstract before base surges were described in association with nuclear blasts and explosive volcanic eruptions especially the 1980 eruption of mount st helens washington laminar and cross bedded volcanogenic surge deposits were commonly misinterpreted as being of fluvial or aeolian origin one well documented example involves the water laid tuffs in and near the spor mountain beryllium mine utah other examples abound in light of how frequently volcanogenic surge deposits have been misinterpreted on earth extreme caution is urged for mars studies contrary to what has been claimed the markedly cross bedded salty deposits at meridiani planum on mars need not have been formed by a combination of aeolian and aqueous processes and their contained hematitic spherules need not have formed as aqueous concretions given the lack of indications of volcanism in the vicinity and the planet wide abundance of impact craters deposition by surges associated with distant impact targets consisting of brine soaked locally sulfidic regolith is a reasonable explanation for all features observed especially if diagenesis and weathering are considered the uniformly sized and shaped ni enriched blue gray hematitic spherules would then be some type of vapor condensation spherules including accretionary lapilli a similar interpretation is possible for deposits in the home plate area gusev crater unlike on the dry and atmosphereless moon salty impact surge deposits containing spherules should be common and well preserved on mars

high resolution observations of mammatus in tropical anvils abstract unprecedented high resolution observations of mammatus from a profiling 94 ghz doppler radar during the nasa cirrus regional study of tropical anvils and cirrus layers florida area cirrus experiment crystal face are presented because of its high sensitivity and temporal and spatial resolution the cloud radar used was able to resolve the fine structure of individual mammatus clouds and record significant vertical doppler velocity perturbations 6 to 1 m s 1 strong perturbations of the doppler velocity within the mammatus as it extends below the main cirrus cloud base are captured by the radar observations upward motions in the periphery of descending mammatus cores are documented areas of intense small scale turbulent mixing near the cirrus cloud base are identified using the doppler spectrum width power spectra analysis of the mean doppler velocity field supports the presence of gravity waves and the development of higher frequency structures near the cirrus anvil base where the mamm

giant tuff cone and 12 km wide associated caldera at ambrym volcano vanuatu new hebrides arc abstract ambrym in the new hebrides arc has been considered as an effusive basaltic volcano the present paper discusses the general structure of this edifice which consists of a basal shield volcano topped by an exceptionally large tuff cone surrounding a 12 km wide summit caldera dacitic pyroclastic flow deposits are exposed in the lower part of the tuff series they grade upward into composite sequences of bedded surtseyan type hyaloclastites ash flow deposits and fallout tephra which are essentially basaltic in composition in such a way that the tuff cone may be considered as mainly basaltic the relationship between the eruption of pyroclastics and the collapse event precludes a classical model of caldera formation at a basaltic volcano in which quiet subsidence i e kilauea type is the dominant mechanism interpretation of the tuff series implies intervention of external water and suggests both explosive and collapse mechanisms a model of caldera formation which assumes an enlargement of the ring fracture during a first plinian and dacitic then essentially hydrobasaltic eruption is proposed

doppler radar observations of mammatus abstract observations of mammatus like cloud features associated with a convective rain shower were obtained using a vertically pointing 8 mm wavelength doppler radar the radar s excellent sensitivity and resolution allowed even very weak finescale features of the cloud to be resolved the mammatus were located 3 5 km above ground along the underneath side of an anvil like stratiform region of the storm cloud and preceded the arrival of precipitation both reflectivity and velocity exhibited strongly periodic patterns with larger downward motions and larger reflectivities occurring together the mammatus lobes were separated by about 90 s or approximately 1 km horizontally the mammatus features could be traced more than 500 m upward into the cloud echo interior where the amplitude of the vertical velocity oscillations was greatest the measured vertical velocities of particle motions ranged from 0 5 to 3 0 m s 1 the observed reflectivity and velocity patterns suggest that although evaporation was

the mysteries of mammatus clouds observations and formation mechanisms mammatus clouds are an intriguing enigma of atmospheric fluid dynamics and cloud physics most commonly observed on the underside of cumulonimbus anvils mammatus also occur on the underside of cirrus cirrocumulus altocumulus altostratus and stratocumulus as well as in contrails from jet aircraft and pyrocumulus ash clouds from volcanic eruptions despite their aesthetic appearance mammatus have been the subject of few quantitative research studies observations of mammatus have been obtained largely through serendipitous opportunities with a single observing system e g aircraft penetrations visual observations lidar radar or tangential observations from field programs with other objectives theories describing mammatus remain untested as adequate measurements for validation do not exist because of the small distance scales and short time scales of mammatus modeling studies of mammatus are virtually nonexistent as a result relatively little is known about the environment formation mechanisms properties microphysics and dynamics of mammatus this paper presents a review of mammatus clouds that addresses these mysteries previous observations of mammatus and proposed formation mechanisms are discussed these hypothesized mechanisms are anvil subsidence subcloud evaporation sublimation melting hydrometeor fallout cloud base detrainment instability radiative effects gravity waves kelvin helmholtz instability rayleigh taylor instability and rayleigh benard like convection other issues addressed in this paper include whether mammatus are composed of ice or liquid water hydrometeors why mammatus are smooth what controls the temporal and spatial scales and organization of individual mammatus lobes and what are the properties of volcanic ash clouds that produce mammatus the similarities and differences between mammatus virga stalactites and reticular clouds are also discussed finally because much still remains to be learned research opportunities are described for using mammatus as a window into the microphysical turbulent and dynamical processes occurring on the underside of clouds

sedimentation history of the palaeoproterozoic dhanjori formation singhbhum eastern india abstract the palaeoproterozoic dhanjori formation eastern india is a terrestrial dominantly fluvial volcano sedimentary succession basal conglomerate coarse grained sandstone and shale rests directly on the granite basement and represents the distal fringe of an alluvial fan complex sieve sheet flood mass flow and alluvial channel and overbank deposits comprise the fan segment the rest of the formation is entirely constituted by fining upward fluvial cycles an event of basin tilting and volcanic eruption intervened and resulted a second phase of dhanjori sedimentation although the general fluvial depositional framework remained unaltered the two members of the dhanjori formation display different paleocurrent trends related to fluvial response to basin tilting in the lower member spatial variability in paleocurrent directions occurs as a consequence of fan development unlike the first phase the second phase of fluvial deposition incorporates profuse volcanics and accompanying volcaniclastic deposits these volcanic and volcaniclastic rocks might have locally blocked the river courses resulting in short lived lacustrine deposition composition of the volcanics varies upward from ultramafic to mafic the dhanjori volcanism took place in an intracontinental rift setting as is evident from the interbedded terrestrial deposits interbedded volcanic and volcaniclastic rocks in different stratigraphic levels suggest episodic volcanic eruption a semiarid paleoclimate is proposed and is consistent with development of an alluvial fan comprised of coarse clastic deposits

the restless resurgent campi flegrei nested caldera italy constraints on its evolution and configuration abstract we have collected new stratigraphic data in the campi flegrei caldera through surface geological mapping and study of cores drilled to various depths we integrate these data with geological geochronological and geophysical constraints available in the literature to reconstruct the volcanic and deformational history of the caldera the caldera is a resurgent nested structure resulting from two main collapses related to the campanian ignimbrite 37 ka and the neapolitan yellow tuff 12 ka eruptions while the structure is affected by a broad subsidence ongoing local resurgence and unrest occur inside the younger nested caldera structure although most of the caldera floor consists of multiple structural blocks due to long term deformations magmas were able to erupt only in one sector of the resurgent part the caldera has shown signs of unrest during the past 25 years with short term deformations that have generated a net uplift of 3 5 m the short term deformations are related to the neapolitan yellow tuff caldera resurgence and interpreted as the net result of brittle and ductile components the long term deformations likely represent the sum of the permanent brittle component of each short term deformational event

mu radar and lidar observations of clear air turbulence underneath cirrus abstract turbulence generation mechanisms prevalent in the atmosphere are mainly shear instabilities breaking of internal buoyancy waves and convective instabilities such as thermal convection due to heating of the ground in the present work clear air turbulence underneath a cirrus cloud base is described owing to coincident observations from the vhf 46 5 mhz middle and upper atmosphere mu radar a rayleigh mie raman rmr lidar and a balloon radiosonde on 7 8 june 2006 at shigaraki japan 34 85 n 136 10 e time height cross section of lidar backscatter ratio obtained at 2206 lt 7 june 2006 showed the presence of a cirrus layer between 8 0 and 12 5 km msl downward penetrating structures of ice crystals with horizontal and vertical extents of 1 0 4 0 km and 200 800 m respectively have been detected at the cirrus cloud base for about 35 min at the same time the mu radar data revealed clear air turbulence layers developing downward from the cloud base in the environment of the protuberances

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

changes in whangaehu river lahar characteristics during the 1995 eruption sequence ruapehu volcano new zealand abstract during the 1995 ruapehu eruptive sequence multiple lahars occurred in the whangaehu river which drains ruapehu s crater lake during the earlier phreatic and phreatomagmatic eruptions lahars were generated by expulsion of waters from the lake but once the lake had emptied lahars were formed by remobilisation of seasonal snowpack laden with saturated freshly erupted tephra four types of lahars occurred during the eruptive sequence 1 initial snow slurry lahars composed of granular snow and ice incorporated by eruptively expelled crater lake waters which left behind frozen deposits with 2 5 20 clastic sediment 2 large dilute lahars generated as the volumes of ejected lake water increased and removed much of the readily available snow at least one third of the pre eruption crater lake volume was expelled during one day producing the largest lahars of the series these lahars were hyperconcentrated flows for up to 84 km from source leaving extensive deposits along the channel margins 3 concentrated lahars smaller volume lahars generated as the frequency of eruptions and volumes of expelled water declined these lahars were able to maintain high sediment concentrations measured at 46 52 by volume suspended sediment at 42 km from source their high sediment concentrations were maintained by erosion and incorporation of sand from the deposits of earlier flows which were lining the channel margins 4 remobilised tephra lahars generated following the two largest tephra eruptions of the sequence seasonal snowpack was covered by water saturated tephra warmer spring temperatures and heavy rainfall events caused collapse and remobilisation of snow and tephra producing several lahars in catchments draining eastern ruapehu

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

the strait of sicily continental rift systems physiography and petrochemistry of the submarine volcanic centres abstract new morphological geophysical mineralogical and petrochemical data on the submarine volcanic centres of the strait of sicily continental rift systems lead to the following conclusions magma types both in the rift valleys pantelleria and linosa and plateau areas adventure bank suggest mantle sources and fractionation processes comparable to those of the ethiopian afar section of the east african rift despite their different tectonic regimes however the volume of the erupted magmas less than 2000 3000 km 3 indicates the similarity with low volcanicity dry rifts this is probably related to the peculiar tectonic setting of the pelagian block african foreland dominated by extensional transcurrent motions connected with a strong neogene compressive dextral e w megashear along the sicilian collisional front in the plateau regions the most evolved rocks are hawaiites graham bank which formed by fractionation in the uppermost mantle probably at the base of the crust magma types vary from quartz tholeiites to basanites in the rift valleys the range of magma types is more restricted transitional to alkali basalts and a considerable volume of highly evolved magmas particularly in the axial volcanic complex of the pantelleria rift was produced in crustal magma chambers isotopic trace element evidence suggests the involvement of transitional to enriched morb and oib type mantle sources in the genesis of the quartz tholeiites alkali basalts and basanites throughout the strait of sicily continental rift system including the iblean plateau

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

numerical simulation of mammatus abstract mammatus are hanging lobes on the underside of clouds although many different mechanisms have been proposed for their formation none have been rigorously tested in this study three dimensional numerical simulations of mammatus on a portion of a cumulonimbus cirruslike anvil are performed to explore some of the dynamic and microphysical factors that affect mammatus formation and evolution initial conditions for the simulations are derived from observed thermodynamic soundings five observed soundings are chosen four were associated with visually observed mammatus and one was not initial microphysical conditions in the simulations are consistent with in situ observations of cumulonimbus anvil and mammatus mammatus form in the four model simulations initialized with the soundings for which mammatus were observed whereas mammatus do not form in the model simulation initialized with the no mammatus sounding characteristics of the modeled mammatus compare favorably to previously published mamm

experimental determination of permeability of neapolitan yellow tuff abstract this paper presents and discusses the measurement of permeability of neapolitan yellow tuff nyt samples obtained in the framework of a study concerning the phenomenon of bradyseism i e the slow vertical movement of soil in the campi flegrei caldera campania italy measurements have been performed under isothermal non isothermal and transient non isothermal conditions using a specifically designed apparatus results of measurements of porosity of different samples are also reported experimental results in isothermal conditions show that the volume flux through the samples changes linearly with applied pressure the values of permeability obtained turn out to be independent of the temperature and pressure gradients applied to the samples this result is consistent with the fact that the permeability is a characteristic of the porous medium and as such is not affected by temperature and pressure variation at least in the range examined the permeability values measured in our laboratories agree quite well with the ones measured in situ by the agenzia generale italiana petroli agip during a geothermal exploration of the campi flegrei area in 1980 an interesting still unexplained phenomenon has been detected during transient phases when both pressure and temperature gradients were applied to the samples the phenomenon consists in an enhancement of volume flux due to heat flux in the transient phase the extra volume flux disappears once the steady temperature gradient is reached

lateral facies variations in hydromagmatic pyroclastic deposits at linosa italy abstract the linosa volcano less than a million years old located in the sicilian channel is formed by a series of coalescing tuff cones tuff rings spatter cones and lava flows among which the fossa cappellano produced a succession of surges the fossa cappellano 2 deposits which travelled up to 1225 m the lapilli and coarse ashes in the deposits are generally mud coated accretionary lapilli are common and the tuffs are frequently vesiculated suggesting that the surges responsible for the deposits were wet the continuous lateral exposures from the interior of the fossa to the extreme edge of the island show facies evolving in the direction of transport from a massive in the most proximal section to one dominated by cross laminated beds in the most distal section passing by a parallel laminated plane bed facies in each facies mean bed thickness and grain size decrease regularly down current in the cross laminated facies mega ripple wavelength and amplitude decrease in the direction of transport in the coarser sizes there is a significant increase in the direction of transport of the lower density scoriae coupled with a decrease of the non vesiculated lithics in the finer sizes there is a significant increase in relative abundance of the lower density feldspar crystals relative to the fe mg crystals the composition texture and primary structure variations of the fossa deposits are identical to those observed in turbidite sequences and are similar to the characteristics observed in dry surge deposits recently described from korea they are interpreted similarly in terms of decreasing flow power in the direction of transport

high resolution airborne radar observations of mammatus abstract high resolution doppler radar observations of mammatus clouds coupled with soundings of the preanvil and anvil environments provide a unique opportunity to examine previously reported observations of and evaluate various hypotheses of mammatus formation these observations confirm the general hypothesis for mammatus formation advanced by ludlam and scorer and provide detail of the cloud interior structure specifically the radar observations indicate that mammatus elements are reminiscent of eddy circulations with a weak downdraft core flanked by horizontal convergence and divergence at the top and base of the cloud respectively doppler spectral width measurements however yielded values of only 2 3 m s 1 indicating only weak turbulent motions within individual mammatus elements reflectivity analyses of mammatus elements indicate a firm link to the parent anvil a dual doppler analysis of the parent anvil indicates that the larger scale environment where the mammatus exist is characteriz

the neapolitan yellow tuff a large magnitude trachytic phreatoplinian eruption eruptive dynamics magma withdrawal and caldera collapse the results of a detailed stratigraphical and volcanological study has allowed the definition of the neapolitan yellow tuff nyt as the largest known trachytic phreatoplinian eruption with an estimated volume of not less than 30 km3 dre the nyt sequence can be subdivided in a lower and an upper member sedimentological and textural variations in time and space indicate that different eruptive and depositional mechanisms operated during the eruption the lower member eruptive phase was characterized by alternating phreatoplinian and magmatic explosions while the upper member eruptive phase was characterized by a random sequence of phreatomagmatic and magmatic explosions particles produced by the same explosion were deposited contemporaneously by fallout and surge mechanisms either at different or at the same distance from the vent compositional variations show that the nyt sequence cannot be modeled as an inverted compositionally zoned magma body with magma becoming more basic in the course of the eruption the chamber composed of three geochemically distinct magma layers was tapped at different depths according to the eruption dynamics each phreatoplinian explosion tapped concurrently all the magma layers while magmatic explosions tapped only one of them the onset of the nyt caldera collapse is well constrained between lower and upper member eruptive phases the collapse which is only gravimetrically detectable occurred inside the larger phlegrean fields caldera which therefore can be defined as a nested caldera

genesis and evolution of unrest episodes at campi flegrei caldera the role of thermal fluid dynamical processes in the geothermal system we develop a model for describing water flow in a porous medium under the effect of thermal and pressure gradients the model simulates geothermal systems in calderas given the boundary conditions and the fluid dynamical properties of the medium the model allows computation in fluid dynamical stationary states of parameters characterizing the flow such as flow velocity and temperature and pressure distributions at depth the model is applied to investigate the effects of the local geothermal system on the unrest episodes at campi flegrei caldera using experimentally determined fluid dynamical parameters for the caldera rocks we show that changes of water flow in shallow aquifers under the effect of pressure and or temperature variations within the geothermal system can be very important in the genesis and evolution of unrest crises in particular they can strongly amplify the effect of pressure increase in the magma chamber on ground uplift they can also explain the timescales of evolution of ground movements in terms of transit times of the water front and of the connected temperature fronts due to advective transport on such grounds an integrated mechanic thermal fluid dynamical model was built allowing us to give a semiquantitative global explanation to the genesis and evolution of unrest phenomena results obtained here can be generalized to other similar calderas

shallow seated controls on styles of explosive basaltic volcanism a case study from new zealand abstract the pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles e g hawaiian versus strombolian or dry magmatic versus wet phreatomagmatic these transitions are controlled dominantly by variations in degassing patterns magma ascent rates and degrees of interaction with external water we use crater hill a 29 ka explosive effusive monogenetic centre in the auckland volcanic field new zealand as a case study of the transitions between these end member eruptive styles the crater hill eruption took place from at least 4 vents spaced along a nne trending 600 m long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases early explosive phases at crater hill were characterised by eruption from multiple unstable and short lived vents later dominantly extrusive volcanism took place from a more stable point source most of the crater hill pyroclastic deposits were formed in 3 phreatomagmatic p and 4 dry magmatic m episodes forming in turn the outer tuff ring and maar crater p1 m1 p2 and scoria cone 1 m2 m4 this activity was followed by formation of a lava shield and scoria cone 2 purely wet activity is represented by the bulk of p1 and p2 and purely dry activity by much of m2 m4 however m1 and parts of m2 and m4 show evidence for simultaneous eruptions of differing style from adjacent vents and rapid variations in the extent and timing of magma water interaction at each vent the nature of the wall rock lithics and these rapid variations in inferred water magma ratios imply interaction was occurring mostly at depths of 80 m and the vesicularity patterns in juvenile clasts from these and the p beds imply that rapid degassing occurred at these shallow levels we suggest that abrupt transitions between eruptive styles in time and space at crater hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent