the 25 september 2007 eruption of mount ruapehu new zealand directed ballistics surtseyan jets and ice slurry lahars abstract at 20 26 nzdt on 25 september 2007 a moderate gas driven eruption beneath the summit crater lake of mt ruapehu new zealand generated a directed ballistic fallout apron and surtseyan jet that impacted an area of c 2 5 km2 to the north of the vent two climbers were caught in the blast at a hut 600 m from the vent primary ice slurry lahars were generated in two catchments draining the summit region including a flow that entered a commercial ski field the ejecta consists of blocks of andesitic lava and scoria erupted during previous volcanic episodes a variety of mineral cemented lake floor sediments vent fill debris and rare glassy material interpreted to be derived from a fresh batch of magma a minority of clasts contain formerly molten elemental sulphur indicating that vent temperatures at the base of the lake were in excess of 119 c the ballistic deposit is confined to a c 40 wide swath directed northwards that extends up to 2 km from the inferred vent location beneath crater lake the strong directionality of the jetted deposits and ballistic free shadow zones sheltered by intervening topography all point to a relatively low angle directed blast previous similar eruptions at ruapehu also show preferred deposit orientations that are here correlated with an inclined and off centre vent beneath crater lake the 25 september 2007 eruption appears to have resulted from failure of a relatively shallow hydrothermal seal composed of elemental sulphur and other mineral precipitates developed in the vent breccia which breached following overpressurization by a pulse of magmatic gases released from deeper in the conduit this event highlights the potential hazard from relatively minor gas driven eruptions at ruapehu that can occur with little or no warning improved understanding of the eruption mechanism for this style of eruption will inform future risk assessments for ruapehu and other similar volcanoes worldwide

morphology and dynamics of explosive vents through cohesive rock formations shallow explosive volcanic processes such as kimberlite volcanism and phreatomagmatic and phreatic activity produce volcanic vents exhibiting a wide variety of morphologies including vertical pipes and v shaped vents in this study we report on experimental and numerical models designed to capture a range of vent morphologies in an eruptive system using dimensional analysis we identified key governing dimensionless parameters in particular the gravitational stress to fluid pressure ratio 2 p gh and the fluid pressure to host rock strength ratio 3 p c we used combined experimental and numerical models to test the effects of these parameters the experiments were used to test the effect of 2 on vent morphology and dynamics a phase diagram demonstrates a separation between two distinct morphologies with vertical structures occurring at high values of 2 and diagonal ones at low values of 2 the numerical simulations were used to test the effect of 3 on vent morphology and dynamics in the numerical models we see three distinct morphologies vertical pipes are produced at high values of 3 diagonal pipes at low values of 3 and horizontal sills at intermediate values of 3 our results show that vertical pipes form by plasticity dominated yielding in high energy systems high 2 and 3 whereas diagonal and horizontal vents dominantly form by fracturing in lower energy systems low 2 and 3 although our models are two dimensional they suggest that circular pipes result from plastic yielding of the host rock in a high energy regime whereas v shaped volcanic vents result from fracturing of the host rock in lower energy systems

the palaeolatitude controversy in the silurian of newfoundland resolved new palaeomagnetic results from the central mobile belt abstract a detailed palaeomagnetic investigation of early silurian volcanics of the lawrenceton formation in the central mobile belt cmb of newfoundland reveals primary remanence that places the sampled part of the cmb on the margin of laurentia in low southerly latitudes in the early silurian the palaeomagnetic data indicate large vertical axis net clockwise rotations within the cmb and between the cmb and cratonic laurentia the volcanics were folded faulted and in places cleaved in silurian to early devonian times anisotropy of magnetic susceptibility measurements on lava flow samples verify field observations of a primary flow parallel foliation and confirm field measurements of the palaeo horizontal a well developed tectonic fabric was detected at one sampling site near the reach fault silurian redbeds of the big indian pond formation were also sampled for palaeomagnetic study the sediments are folded faulted and cleaved a statistically sound syn folding remanence was identified which in a tectonically corrected state is consistent in direction with the remanence in the silurian volcanics the sampled sediments are more intensely deformed than the volcanics and magnetic anisotropy measurements indicate both primary layering and secondary cleavage related fabrics in varying degrees from site to site it is possible that cleavage development resulted in remanence reorientation leading to the illusion of syn folding remanence when in fact the remanence was acquired prior to folding previously published palaeomagnetic investigations of silurian rocks in the cmb revealed a discrepancy in palaeofield inclination between volcanic and sedimentary rock types the sedimentary rocks yielded shallow inclinations and the volcanics intermediate inclinations inclination shallowing in the sediments was put forward as an explanation as well as contamination of the sedimentary results with a shallow overprint others argued that the results from the volcanics were in error the present investigation demonstrates that key volcanics carry a shallower remanence than previously thought bringing them into approximate agreement with the majority of the results from sediments steep results from the springdale volcanics and very shallow results from the king george iv lake sediments are at variance with the rest of the data the springdale result has large error limits that overlap with the other data the anomalous king george iv result remains an outlier

a review on phreatic eruptions and their precursors abstract an analysis of the reported information on 132 historical phreatic events indicates that most of these 115 were not followed by a magmatic or phreatomagmatic eruption only in 18 cases precursory phenomena are reportedly lacking whereas they are recognized for the majority 72 of the considered cases these precursors have been analyzed as a function of the kind of phreatic event and of its possible triggering mechanism unfortunately the quality of the available information is poor however data suggest that anomalous seismicity ground deformation changes in the output chemistry and temperature of fumaroles and thermal springs are potential mid term precursors whereas seismic tremor is a common short term precursor

fragmentation efficiency of explosive volcanic eruptions a study of experimentally generated pyroclasts abstract products of magma fragmentation can pose a severe threat to health infrastructure environment and aviation systematic evaluation of the mechanisms and the consequences of volcanic fragmentation is very difficult as the adjacent processes cannot be observed directly and their deposits undergo transport related sorting however enhanced knowledge is required for hazard assessment and risk mitigation laboratory experiments on natural samples allow the precise characterization of the generated pyroclasts and open the possibility for substantial advances in the quantification of fragmentation processes they hold the promise of precise characterization and quantification of fragmentation efficiency and its dependence on changing material properties and the physical conditions at fragmentation we performed a series of rapid decompression experiments on three sets of natural samples from unzen volcano japan the analysis comprised grain size analysis and surface area measurements the grain size analysis is performed by dry sieving for particles larger than 250 m and wet laser refraction for smaller particles for all three sets of samples the grain size of the most abundant fraction decreases and the weight fraction of newly generated ash particles up to 40 wt increases with experimental pressure potential energy for fragmentation this energy can be estimated from the volume of the gas fraction and the applied pressure the surface area was determined through argon adsorption the fragmentation efficiency is described by the degree of fine particle generation results show that the fragmentation efficiency and the generated surface correlate positively with the applied energy

dynamics of shallow hydrothermal eruptions new insights from vulcano s breccia di commenda eruption understanding the dynamics and effects of hydrothermal eruptions is crucial to the hazard assessment in both volcanic and geothermal areas eruptions from hydrothermal centres may occur associated with magmatic phases but also as isolated events without magmatic input with the most recent examples being those of te maari tongariro new zealand in 2012 and ontake japan in 2014 the most recent caldera of the island of vulcano southern italy hosts in its centre the la fossa cone active since 5 5 ka and now characterised by continuous fumarolic degassing in historical times la fossa cone has experienced several hydrothermal eruptions with the most violent event being the breccia di commenda eruption that occurred during the thirteenth century ad based on analysis of 170 stratigraphic logs we show that the breccia di commenda eruption occurred in three main phases after an opening low intensity ash emission phase phase 1 the eruption energy climaxed during phase 2 when a series of violent explosions produced an asymmetric shower of ballistic blocks and the contemporaneous emplacement of highly dispersed lithic rich blast like pyroclastic density currents pdcs the tephra units emplaced during phase 2 ranging in volume from 0 2 to 2 7 105 m3 were covered in turn by thin ash fall deposits phase 3 the dynamics of the most violent and intense stage of the eruption phase 2 was investigated by numerical simulations a three dimensional numerical model was applied describing the eruptive mixture as a eulerian eulerian two phase non equilibrium gas particle fluid plus a one way coupled lagrangian ballistic block fraction at the initial simulation time a mass of about 109 kg with initial overpressure above 10 mpa and a temperature of 250 c was suddenly ejected from a 200 m long eastward inclined nne ssw trending fissure the mass release formed blast like pdcs on both sides of the fissure and launched ballistic blocks eastwards field investigations and numerical simulations confirm that hydrothermal explosions at la fossa cone include intense ballistic fallout of blocks emission of pdcs potentially travelling beyond the la fossa caldera and significant ash fallout the hazard associated with both ballistic impact and pdc ingress as associated with hydrothermal eruption is significantly larger with respect to that associated with vulcanian type events of la fossa

the volcanic and geothermally active campi flegrei caldera an integrated multidisciplinary image of its buried structure the campi flegrei caldera in southern italy is one of the greatest geohazard areas on earth evidence of an active magmatic and geothermal system is provided by ongoing ground uplift with volcano tectonic and long period lp seismicity the persistent degassing of 1500 tonnes of co2 per day the presence of hot fumaroles at temperatures of 90 150 c brine rich aquifers with total dissolved solids up to 33 g l 1 and high thermal gradients in the crust with temperatures reaching 420 c at 3 050 m b s l since the 1940s more than 100 exploratory boreholes have been drilled in the area to depths of 80 3 100 m by the azienda geologica italiana petroli agip and the societa anonima forze endogene napoletane safen to date however no systematic reanalysis of the drilling data has been carried out and the buried volcanic structure has not been updated using the most recent scientific results and previous findings by integrating unpublished data from the agip and safen reports with published information from geological volcanological petrological petrophysical and geophysical studies this paper presents an improved picture of the campi flegrei caldera that will be useful for volcanic hazard assessment and mitigation in the naples area and for future research planning the results suggest that intra caldera activity has been influenced by how the magmatic system at depths greater than about 4 km has determined the transfer of magma volatiles and heat to the overlying geothermal system and ultimately to the surface in particular intriguing is that the most volcanically active central eastern sector of the caldera which is subject to intense bradyseismic ground movement and gas emission coincides with a structurally delimited subsurface rock volume characterized by an uprising of the 100 c isotherm a deep water supply to the shallower aquifer the early disappearance of secondary calcite lp seismicity and high seismic s wave attenuation in this area we also document evidence of repeated injection at depths of c 1 5 3 0 km of isolated and small volume batches of magma where occurred their crystallization and degassing shallow intrusions and degassing of magma are thus identified as two of the key processes that drive unrest in campi flegrei

using the spatial distribution and lithology of ballistic blocks to interpret eruption sequence and dynamics august 6 2012 upper te maari eruption new zealand the ballistic ejection of blocks during explosive eruptions constitutes a major hazard near active volcanoes fields of ballistic clasts can provide important clues towards quantifying the energy dynamics and directionality of explosive events but detailed datasets are rare during the 6 august 2012 hydrothermal eruption of upper te maari tongariro new zealand three explosions occurred in rapid succession within less than 20 s the first two produced laterally directed pyroclastic density currents pdc and the final vertical explosion generated an ash plume each of these explosions was associated with the ejection of ballistic blocks we present detailed maps of the resulting 5 1 km2 block impact field and the distribution of the 2200 impact craters with diameters 2 5 m there are two distinct regions of high crater concentration where crater densities reach more than six times the average background density these occur at distances of 500 700 m east and 1000 1350 west of a 430 m long fissure that was created during the eruption the high density fields are characterized by a narrow radial spread of 45 and are located along the proximal transport direction of the pyroclastic density currents a provenance analysis of ballistic blocks allowed us to reconstruct two different eruptive vents for the explosions the first two laterally directed explosions were sourced from the fissure while the third explosion occurred through the pre existing upper te maari crater generating a roughly axisymmetric shower of ballistics stratigraphic relationships between impact craters pdc and fall deposits suggest that the ballistic blocks were initially coupled with the rapidly expanding gas particle mixtures that produced the pdcs ballistic trajectory modeling reproducing the lateral extent and main impact density pattern of the western impact field allows estimation of the vertical expansion angle of the second and largest explosion the calculations show that the largest proportion of the explosion energy was strongly focused as a narrow and extremely shallow from 3 to 15 from the horizontal laterally expanding hydrothermal blast the results presented here constitute an important data set for ballistic hazard assessment at tongariro volcano and they can provide further clues towards better understanding highly energetic laterally directed volcanic explosions at similar hydrothermal fields

three fragmentation mechanisms for highly viscous magma under rapid decompression abstract analysis of the behavior of viscous magma under short term tensile stresses demonstrates that the strain rates anticipated in the eruptive stages of collapse related events may be high enough under certain circumstances to provide fragmentation of the melt by brittle failure mechanisms of fragmentation of a highly viscous vesicular magma by the propagation of an unloading wave by layer by layer vesicle bursting fragmentation wave propagation and by rapid filtration gas flow are outlined fragmentation scenarios involving possible combinations of the three fragmentation mechanisms are evaluated for two models of magma with isolated and with connected pores experimental results for magma fragmentation under high decompression rates appear to support such mechanisms these mechanisms of highly viscous magma fragmentation by rapid decompression probably have their greatest application in cases of explosive disintegration of lava domes and cryptodomes caused by collapse events

energy consumption by magmatic fragmentation and pyroclast ejection during vulcanian eruptions abstract magmatic fragmentation during explosive eruptions consumes a significant amount of energy by the creation of new surface via fracturing this process reduces the energy that can be imparted to the resulting pyroclasts in the form of kinetic energy to date models of vulcanian eruptions have neglected the energy balance consumed by magmatic fragmentation we present a 1 d model of vulcanian eruption that considers the energy balance in rapid decompression of a pressurized magma below a caprock followed by fragmentation and acceleration of particles we then tested the model via decompression experiments at different temperatures room vs 850 c and initial pressure different series of experiments involving either fragmentation of an intact volcanic rock followed by ejection of the particles or simply the ejection of loose particles were performed to quantify the energy consumed by fragmentation and the kinetic energy available to eject the particles the ejection velocities observed in the fragmentation experiments of intact rocks are systematically lower than the ones with loose particles due to the energy consumed by fragmentation this energy depends on the fragmentation threshold i e the minimum pressure required to completely fragment the sample and is inversely proportional to the fraction of pressurized pores we propose that the effective pressure propelling the gas particle mixtures after fragmentation corresponds to the pressure initially stored in the pore fraction minus the fragmentation threshold we applied our caprock model to vulcanian eruptions of popocatepetl volcano february 2003 and colima volcano 10 february 1999 and obtained gas pressures at the onset of fragmentation of 11 13 mpa and 19 22 mpa respectively these pressures might correspond to the gas pressures required to disrupt the caprock fragment the underlying porous magma and eject the ballistics at the observed distances the model presented herein may help describe the dynamics of vulcanian eruptions and improve hazard assessment

energetics of gas driven limnic and volcanic eruptions abstract this paper lays the foundation for the rigorous treatment of the energetics of gas exsolution from a gas containing liquid which powers gas driven volcanic and limnic eruptions various exsolution processes reversible or irreversible slow or rapid are discussed and the maximum amount of kinetic energy derivable from a reversible gas exsolution process is obtained the concept of dynamic irreversibility is proposed for discussing the kinetic energy available from irreversible gas exsolution processes the changes of thermodynamic properties during gas exsolution processes are derived density pressure relations for gas liquid mixtures are presented including empirical relations for irreversible gas exsolution the energetics of gas driven eruptions through both fluid and rigid media including the role of buoyancy and the role of magma chamber expansion work are investigated for reversible processes the energetics can be used to discuss the dynamics of gas driven eruptions leading to maximum erupting velocities and maximum eruptible fractions for irreversible processes empirical relations and parameters must be employed the exit velocities of the lake nyos eruption and the 18 may 1980 eruption of mount st helens are modeled by incorporating possible irreversibility

the active portion of the campi flegrei caldera structure imaged by 3 d inversion of gravity data 1 we present an improved density model and a new structural map of the neapolitan yellow tuff caldera the active portion of the nested campi flegrei caldera the model was built using a new 3 d inversion of the available high precision gravity data and a new digital terrain and marine model the inversion procedure based on a variable depth lumped assembling of the subsurface gravity distribution via cell aggregation gives better defined insights into the internal caldera architecture that well agree with the available geological geophysical and geochemical data the adopted 3 d gravity method is highly efficient for characterizing the shallow caldera structure down to 3 km depth and defining features related to regional or volcano tectonic lineaments and dynamics in particular the resulting density distribution highlights a pronounced density low in correspondence of the central portion of the caldera with a detail not available till now the joint interpretation of the available data suggests a subsurface structural setting that supports a piecemeal collapse of the caldera and allows the identification of its headwall positive gravity anomalies localize dense intrusions presently covered by late volcanic deposits along the caldera marginal faults and the main structural lineaments both bordering the resurgent block and cutting the caldera floor these results allow us to both refine the current geological structural framework and propose a new structural map that highlights the caldera boundary and its internal setting this map is useful to interpret the phenomena occurring during unrest and to improve both short term and long term volcanic hazards assessment

experimental constraints on phreatic eruption processes at whakaari white island volcano vigorous hydrothermal activity interspersed by sequences of phreatic and phreatomagmatic eruptions occur at whakaari white island volcano new zealand here we investigate the influence of sample type hydrothermally altered cemented ash tuffs and unconsolidated ash lapilli and fragmentation mechanism steam flashing versus gas expansion on fragmentation and ejection velocities as well as on particle size and shape our rapid decompression experiments show that fragmentation and ejection speeds of two ash tuffs cemented by alunite and amorphous opal increase with increasing porosity and that both are significantly enhanced in the presence of steam flashing ejection speeds of unconsolidated samples are higher than ejection speeds of cemented tuffs as less energy is consumed by fragmentation fragmentation dominated by steam flashing results in increased fragmentation energy and a higher proportion of fine particles particle shape analyses before and after fragmentation reveal that both steam flashing and pure gas expansion produce platy or bladed particles from fracturing parallel to the decompression front neither fragmentation mechanisms nor sample type show a significant influence on the shape our results emphasize that under identical pressure and temperature conditions eruptions accompanied by the process of liquid water flashing to steam are significantly more violent than those driven simply by gas expansion therefore phase changes during decompression and cementation are both important considerations for hazard assessment and modeling of eruptions in hydrothermally active environments

how tough is tuff in the event of fire tuff has been extensively used as a building material in volcanically and tectonically active areas over many centuries despite its inherent low strength a common and unfortunate secondary hazard accompanying both major volcanic eruptions and tectonic earthquakes is the initiation of catastrophic fires here we report new experimental results on the influence of high temperatures on the strength of three tuffs that are commonly used for building in the neapolitan region of italy our results show that a reduction in strength was only observed for one tuff the other two were unaffected by high temperatures the cause of this strength discrepancy was found to be a product of the initial mineralogical composition or more specifically the presence of thermally unstable zeolites within the initial rock matrix the implications of these data are that in the event of fire only the stability of buildings or structures built from tuff containing thermally unstable zeolites will be reduced unfortunately this includes the most widespread dimension stone in neapolitan architecture we recommend that this knowledge should be considered during fire hazard mitigation in the neapolitan area and that other tuffs used in construction worldwide should be tested in a similar way to assess their fire resistance

influence of the fragmentation process on the dynamics of vulcanian eruptions an experimental approach abstract the dynamics of magma fragmentation is a controlling factor in the behavior of explosive volcanic eruptions if porous magma is sufficiently decompressed a fragmentation front develops and travels through the magma at a certain speed fragmentation speed while the resulting particles are being ejected to investigate the influence of the fragmentation process on eruption dynamics we have performed fragmentation experiments in a shock tube apparatus using natural volcanic samples with diverse porosities and different applied pressures 4 20 mpa for each experiment we simultaneously measured the fragmentation speed and ejection velocities the results are consistent with a theoretical model based on a 1 d shock tube theory considering the conservation laws across the fragmentation front our results show that a certain pressure threshold has to be exceeded for fragmentation and ejection of the particles to take place and that the fragmentation speed determines the initial conditions of the expansion of the gas particle mixture the fragmentation process has a controlling influence on the velocity density and mass discharge rate per unit area of the gas particle mixture all factors which can affect eruption dynamics significantly the model presented herein may help describe the dynamics of vulcanian eruptions and improve hazard assessment

experimental investigations on the explosivity of steam driven eruptions a case study of solfatara volcano campi flegrei steam driven eruptions both phreatic and hydrothermal expel exclusively fragments of non juvenile rocks disintegrated by the expansion of water as liquid or gas phase as their violence is related to the magnitude of the decompression work that can be performed by fluid expansion these eruptions may occur with variable degrees of explosivity in this study we investigate the influence of liquid fraction and rock petrophysical properties on the steam driven explosive energy a series of fine grained heterogeneous tuffs from the campi flegrei caldera were investigated for their petrophysical properties the rapid depressurization of various amounts of liquid water within the rock pore space can yield highly variable fragmentation and ejection behaviors for the investigated tuffs our results suggest that the pore liquid fraction controls the stored explosive energy with an increasing liquid fraction within the pore space increasing the explosive energy overall the energy released by steam flashing can be estimated to be 1 order of magnitude higher than for simple argon gas expansion and may produce a higher amount of fine material even under partially saturated conditions the energy surplus in the presence of steam flashing leads to a faster fragmentation with respect to gas expansion and to higher ejection velocities imparted to the fragmented particles moreover weak and low permeability rocks yield a maximum fine fraction using experiments to unravel the energetics of steam driven eruptions has yielded estimates for several parameters controlling their explosivity these findings should be considered for both modeling and evaluation of the hazards associated with steam driven eruptions

hydrothermal alteration of surficial rocks at solfatara campi flegrei petrophysical properties and implications for phreatic eruption processes abstract solfatara crater is located within the campi flegrei caldera to the west of naples italy it is one of the largest fumarolic manifestations known and the rocks hosting the hydrothermal system are affected by intense hydrothermal alteration alteration can result in changes of degassing behavior and in the formation of a cap rock thereby increasing the probability of phreatic eruptions here we investigate the effects of alunitic solfataric alteration on the mineralogy the physical properties porosity density permeability and the mechanical properties strength of the rocks involved as well as its influence on fragmentation and ejection behavior our results show that the pristine mineralogy of deposits from the vicinity of the solfatara cryptodome and from pisciarelli is almost completely replaced by amorphous silica and alunite the differences in the degree of alteration among the samples series are reflected in the investigated properties and behavior as well as in the analysis of the experimentally generated particles alunitic alteration increases porosity and permeability whereas it reduces density elastic wave velocity and strength leading to higher fragmentation and ejection speeds for the sample series examined in this study our results also show that alteration results in the generation of a high fraction of fines particle sizes

a fragmentation criterion for highly viscous bubbly magmas estimated from shock tube experiments abstract when a highly viscous bubbly magma is sufficiently decompressed layer by layer fracturing propagates through the magma at a certain speed fragmentation speed on the basis of a recent shock tube theory by koyaguchi and mitani koyaguchi t mitani n k 2005 a theoretical model for fragmentation of viscous bubbly magmas in shock tubes journal of geophysical research 110 b10 b10202 doi 10 1029 2004jb003513 gas overpressures at the fragmentation surface are estimated from experimental data on fragmentation speed in shock tube experiments for natural volcanic rocks with various porosities the results show that gas overpressure at the fragmentation surface increases as initial sample pressure increases and sample porosity decreases we propose a new fragmentation criterion to explain the relationship between the gas overpressure at the fragmentation surface the initial pressure and the porosity our criterion is based on the idea that total fragmentation of highly viscous bubbly magmas occurs when the tensile stress at the midpoint between bubbles exceeds a critical value we obtain satisfactory agreement between our simulation and experiment when we assume that the critical value is inversely proportional to the square root of bubble wall thickness this fragmentation criterion suggests that long micro cracks or equivalent flaws e g irregular shaped bubbles that reach the midpoints between bubbles are a dominant factor to determine the bulk strength of the bubbly magma

experimental estimates of the energy budget of hydrothermal eruptions application to 2012 upper te maari eruption new zealand sudden hydrothermal eruptions occur in many volcanic settings and may include high energy explosive phases ballistics launched by such events together with ash plumes and pyroclastic density currents generate deadly proximal hazards the violence of hydrothermal eruptions or explosive power depends on the energy available within the driving fluids gas or liquid which also influences the explosive mechanisms volumes durations and products of these eruptions experimental studies in addition to analytical modeling were used here to elucidate the fragmentation mechanism and aspects of energy balance within hydrothermal eruptions we present results from a detailed study of recent event that occurred on the 6th of august 2012 at upper te maari within the tongariro volcanic complex new zealand the eruption was triggered by a landslide from this area which set off a rapid stepwise decompression of the hydrothermal system explosive blasts were directed both westward and eastward of the collapsed area with a vertical ash plume sourced from an adjacent existing crater all explosions ejected blocks on ballistic trajectories hundreds of which impacted new zealand s most popular hiking trail and a mountain lodge 1 4 km from the explosion locus we have employed rocks representative of the eruption source area to perform rapid decompression experiments under controlled laboratory conditions that mimic hydrothermal explosions under controlled laboratory conditions an experimental apparatus for 34 by 70 mm cylindrical samples was built to reduce the influence of large lithic enclaves up to 30 mm in diameter within the rock the experiments were conducted in a temperature range of 250 degrees c 300 degrees c and applied pressure between 4 mpa and 6 5 mpa which span the range of expected conditions below the te maari crater within this range we tested rapid decompression of pre saturated samples from both liquid dominated conditions and the vapor dominated field further we tested dry samples at the same pressure and temperature conditions results showed that host rock lithology and state of the interstitial fluid was a major influence on the fragmentation and ejection processes as well as the energy partitioning clasts were ejected with velocities of up to 160 m s as recorded by high speed camera in addition to rare large clasts analogous to ballistics a large amount of fine and very fine 63 mu m ash was produced in all experiments the efficiency of transformation of the total explosive energy into fragmentation energy was estimated between 10 to 15 depending on the host rock lithology while less than 0 1 of this was converted into kinetic energy our results suggest that liquid to vapor flashing expansion provides an order of magnitude higher energy release than steam expansion which best explains the dynamics of the westward and most energetic directed blast at te maari considering the steam flashing as the primary energy source the experiments suggested that a minimum explosive energy of 7 x 10 10 to 2 x 10 12 j was involved in the te maari blast experimental studies under controlled conditions compared closely to a field example are thus highly useful in providing new insights into the energy release and hazards associated with eruptions in hydrothermal areas c 2016 the author s published by elsevier b v

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

subsurface structure of the solfatara volcano campi flegrei caldera italy as deduced from joint seismic noise array volcanological and morphostructural analysis 1 the joint application of different seismological techniques for seismic noise analysis and the results of a volcanological and morphostructural survey have allowed us to obtain a detailed and well constrained image of the shallow crustal structure of the solfatara volcano campi flegrei caldera italy horizontal to vertical spectral ratios inversion of surface wave dispersion curves and polarization analysis provided resonance frequencies and peak amplitudes shear wave velocity profiles and polarization pattern of coherent ambient noise these results combined in a unique framework indicate that the volcanic edifice is characterized by lateral and vertical discontinuities and heterogeneities in terms of shear wave velocity lithological contrasts and structural setting the interpretation of the seismological results with the volcanological and morphostructural constraints supports the hypothesis that the volcano has been characterized by a complex and intense activity with the alternation of constructive and destructive phases during which magmatic and phreatomagmatic explosions built a complex tuff cone later reworked by atmospheric agents and altered by hydrothermal activity the differences in the velocity structure between the central and eastern parts of the crater have been interpreted as resulting from a possible eastward migration of the eruptive vent along the deformational features affecting the area and to the presence of viscous lava and lithified tuff bodies within the feeding conduits which are buried under a covering of reworked materials of variable thickness the observed fault and fracture systems partially inherited from regional structural setting and exhumed during volcanism and ground deformation episodes also seems to strongly control wave propagation affecting the noise polarization properties

multidisciplinary constraints of hydrothermal explosions based on the 2013 gengissig lake events kverkfjoll volcano iceland hydrothermal explosions frequently occur in geothermal areas showing various mechanisms and energies of explosivity their deposits though generally hardly recognised or badly preserved provide important insights to quantify the dynamics and energy of these poorly understood explosive events furthermore the host rock lithology of the geothermal system adds a control on the efficiency in the energy release during an explosion we present results from a detailed study of recent hydrothermal explosion deposits within an active geothermal area at kverkfjoll a central volcano at the northern edge of vatnajokull on august 15th 2013 a small jokulhlaup occurred when the gengissig ice dammed lake drained at kverkfjoll the lake level dropped by approximately 30 m decreasing pressure on the lake bed and triggering several hydrothermal explosions on the 16th here a multidisciplinary approach combining detailed field work laboratory studies and models of the energetics of explosions with information on duration and amplitudes of seismic signals has been used to analyse the mechanisms and characteristics of these hydrothermal explosions field and laboratory studies were also carried out to help constrain the sedimentary sequence involved in the event the explosions lasted for 40 50 s and involved the surficial part of an unconsolidated and hydrothermally altered glacio lacustrine deposit composed of pyroclasts lavas scoriaceous fragments and fine grained welded or loosely consolidated aggregates interbedded with clay rich levels several small fans of ejecta were formed reaching a distance of 1 km north of the lake and covering an area of approximately 0 3 km2 with a maximum thickness of 40 cm at the crater walls the material volume of approximately 104 m3 has been ejected by the expanding boiling fluid generated by a pressure failure affecting the surficial geothermal reservoir the maximum thermal craterisation and ejection energies calculated for the explosion areas are on the order of 1011 1010 and 109 j respectively comparison of these with those estimated by the volume of the ejecta and the crater sizes yields good agreement we estimate that approximately 30 of the available thermal energy was converted into mechanical energy during this event the residual energy was largely dissipated as heat while only a small portion was converted into seismic energy estimation of the amount of freshly fragmented clasts in the ejected material obtained from sem morphological analyses reveals that a low but significant energy consumption by fragmentation occurred decompression experiments were performed in the laboratory mimicking the conditions due to the drainage of the lake experimental results confirm that only a minor amount of energy is consumed by the creation of new surfaces in fragmentation whereas most of the fresh fragments derive from the disaggregation of aggregates furthermore ejection velocities of the particles 40 50 m s measured via high speed videos are consistent with those estimated from the field the multidisciplinary approach used here to investigate hydrothermal explosions has proven to be a valuable tool which can provide robust constraints on energy release and partitioning for such small size yet hazardous steam explosion events

experimental modelling of fragmentation applied to volcanic explosions abstract explosions during volcanic eruptions cause fragmentation of magma and host rock resulting in fragments with sizes ranging from boulders to fine ash the products can be described by fragment size distributions fsd which commonly follow power laws with exponent d the processes that lead to power law distributions and the physical parameters that control d remain unknown we developed a quantitative experimental procedure to study the physics of the fragmentation process through time the apparatus consists of a hele shaw cell containing a layer of cohesive silica flour that is fragmented by a rapid injection of pressurized air the evolving fragmentation of the flour is monitored with a high speed camera and the images are analysed to obtain the evolution of the number of fragments n their average size a and the fsd using the results from our image analysis procedure we find transient empirical laws for n a and the exponent d of the power law fsd as functions of the initial air pressure we show that our experimental procedure is a promising tool for unravelling the complex physics of fragmentation during phreatomagmatic and phreatic eruptions

geochemical evidences of magma dynamics at campi flegrei italy abstract campi flegrei caldera within the neapolitan area of italy is potentially one of the most dangerous volcanoes in the world and during the last decade it has shown clear signs of reactivation marked by the onset of uplift and changes in the geochemistry of gas emissions we describe a 30 year long data set of the co2 he ar n2 compositions of fumarolic emissions from la solfatara crater which is located in the center of the caldera the data display continuous decreases in both the n2 he and n2 co2 ratios since 1985 paralleled by an increase in he co2 these variations cannot be explained by either processes of boiling condensation in the local hydrothermal system or with changes in the mixing proportions between a magmatic vapor and hydrothermal fluids we applied the magma degassing model of nuccio and paonita 2001 earth planet sci lett 193 467 481 using the most recent inert gas solubilities in order to interpret these peculiar features in accordance with petrologic constraints derived from the ranges of the melt compositions and reservoir pressures at campi flegrei the model simulations for mafic melts trachybasalt and shoshonite show a remarkably good agreement with the measured data both decompressive degassing of an ascending magma and mixing between magmatic fluids exsolved at various levels along the ascent path can explain the long term geochemical changes recalling that i a sill like reservoir of gases at a depth of 3 4 km seems to be the main source of ground inflation and ii there is petrologic and geophysical evidence for a reservoir of magma at about 8 km below campi flegrei we suggest that the most intense episodes of inflation occur when the gas supply to the sill like reservoir comes from the 8 km deep magma although fluids exsolved by magma bodies at shallower depths also contribute to the gas budget our work highlights that in caldera systems where the presence of hydrothermal aquifers commonly masks the magmatic signature of reactive volatiles inert gases are the preferred species to use when seeking information on the melt composition dynamics and structure of the plumbing systems

dynamics of explosive volcanism at unzen volcano an experimental contribution knowledge of the dynamics of magma fragmentation is necessary for a better understanding of the explosive behaviour of silicic volcanoes here we have measured the fragmentation speed and the fragmentation threshold of five dacitic samples 6 7 53 5 vol open porosity from unzen volcano kyushu japan the measurements were carried out using a shock tube based fragmentation apparatus modified after alidibirov and dingwell 1996a b the results of the experimental work confirm the dominant influence of porosity on fragmentation dynamics the velocity of the fragmentation front increases and the value of the fragmentation threshold decreases with increasing porosity further we observe that the fragmentation speed is strongly influenced by the initial pressure difference and the texture of the dacite at an initial pressure difference of 30 mpa the fragmentation speed varies from 34 m s for the least porous sample to 100 m s for the most porous sample these results are evaluated by applying them to the 1990 1995 eruptive activity of unzen volcano emplacements of layered lava dome lobes merapi type pyroclastic flows and minor explosive events dominated this eruption the influence of the fragmentation dynamics on dome collapse and vulcanian events is discussed

history of earthquakes and vertical ground movement in campi flegrei caldera southern italy comparison of precursory events to the a d 1538 eruption of monte nuovo and of activity since 1968 abstract the record of felt earthquakes around naples bay in southern italy is probably complete since the mid 15th century according to this record intense earthquake swarms originating beneath campi flegrei an explosive caldera located along the north coast of naples bay have occurred only twice 1 before the only historical eruption in campi flegrei in 1538 and 2 from mid 1983 to december 1984 earthquake activity during the earlier period which began at least a few years and possibly as many as 30 years before the 1538 eruption damaged many buildings in the city of pozzuoli located near the center of campi flegrei minor seismic activity which consisted of only a few felt earthquakes occurred from 1970 to 1971 the second period of intense earthquake swarms lasted from mid 1983 to 1984 again damaging many buildings in pozzuoli two periods of uplift along the shoreline within campi flegrei have also been noted since the mid 15th century 1 during the few decades before the 1538 eruption and 2 as two distinct episodes since 1968 uplift of a few meters probably occurred a few decades before the 1538 eruption uplift of as much as 3 0 m has occurred in pozzuoli since 1968 these similarities strongly suggest that for the first time in 440 years the same process that caused intense local earthquake swarms and uplift in the early 1500 s and led to an eruption in 1538 has again occurred beneath campi flegrei though no major seismicity or uplift has occurred since december 1984 because of the large amount of extensional strain accumulated during the past two decades if a third episode of seismicity and rapid uplift occurs it may lead to an eruption within several months after the resumption of activity

permeability control on magma fragmentation fragmentation of porous magma that is subject to gas overpressure is considered to be a crucial process in the generation of explosive volcanic eruptions a decompressive event e g rapid magma ascent landslide dome collapse disrupts the stress equilibrium between the gas phase and the surrounding melt when the gas in the pores is exposed to a pressure gradient it may either fragment the surrounding magma or escape from the magma along an existing pathway of cracks and interconnected bubbles therefore magma permeability can be a decisive parameter in determining if an eruption experiences fragmentation i e whether it is explosive or effusive or exhibits a temporal transition between the two eruptive styles despite the central role that gas permeability may play in the fragmentation of volcanic rocks previous studies have not experimentally verified or quantified this influence based on a comprehensive database of combined permeability and fragmentation experiments we show that high permeability substantially increases the overpressure required to fragment porous volcanic rocks our results allow us to deduce a fragmentation criterion that incorporates gas permeability as well as porosity and internal overpressure this criterion implies that the energy required for fragmentation is less dependent on the actual pore geometry than on the way the void space is interconnected and thus on the contribution of permeable gas flow to decompression

evidence of thermal driven processes triggering the 2005 2014 unrest at campi flegrei caldera abstract an accelerating process of ground deformation that began 10 years ago is currently affecting the campi flegrei caldera the deformation pattern is here explained with the overlapping of two processes short time pulses that are caused by injection of magmatic fluids into the hydrothermal system and a long time process of heating of the rock the short pulses are highlighted by comparison of the residuals of ground deformation fitted with an accelerating polynomial function with the fumarolic co 2 ch 4 and he ch 4 ratios which are good geochemical indicators of the arrival of magmatic gases the two independent datasets show the same sequence of five peaks with a delay of 200 days of the geochemical signal with respect to the geodetic signal the heating of the hydrothermal system which parallels the long period accelerating curve is inferred by temperature pressure gas geoindicators referring to a recent interpretation that relates variations in the fumarolic inert gas species to open system magma degassing we infer that the heating is caused by enrichment in water of the magmatic fluids and by an increment in their flux heating of the rock caused by magmatic fluids can be a central factor in triggering unrest at calderas

depositional mechanisms and alteration processes in different types of pyroclastic deposits from campi flegrei volcanic field southern italy the campi flegrei volcanic field southern italy is composed of a large variety of pyroclastic deposits related to both magmatic and hydromagmatic activity in spite of similarities of composition grain size age and pre eruptive environment the various deposits exhibit great differences in terms of degree and type of alteration of the juvenile components in particular detailed analyses of pyroclasts collected in the numerous monogenetic volcanic cones show the broad range of alteration from fresh glass to complete replacement of the glass by the zeolites the comparative analysis of 17 different deposits has allowed us to conclude that the eruptive and emplacement conditions were the major controls on the zeolitization processes the relationships between volcanological features of deposits and secondary mineralization are consistent with the glass fluid phase reaction in a near closed post depositional system eruptions characterised by separation of water vapour from the pyroclastic particles produce unaltered or poorly altered deposits in contrast eruptions characterised by mechanisms that permit the capture and retention of pore water and water vapour produce zeolitized deposits according to these lines of evidence the zeolitization of pyroclastic products of campi flegrei took place immediately after emplacement during the cooling of wet deposits

experimental volcanology on eruptive products of unzen volcano abstract protracted dome building eruptions may be profitably investigated using laboratory based experiments density distribution studies on the pyroclastic flow deposits of unzen 1990 1995 allow us to apply the results of experimental investigations on unzen samples to the interpretation of the last eruption of unzen here primary laboratory experiments have focused on several aspects of the degassing permeability and the fragmentation behavior threshold speed and efficiency those investigations have been flanked by analyses of flexural strength fracture toughness and seismic velocities to provide new insights into eruption related processes here we present a review of these results and their application to the eruption dynamics of unzen volcano we propose that efforts be made to incorporate routinely such comprehensive experimental analyses into the response to emerging volcanic crises in future

native sulfur sulfates and sulfides from the active campi flegrei volcano southern italy genetic environments and degassing dynamics revealed by mineralogy and isotope geochemistry abstract we investigated sulfur bearing minerals from the campi flegrei caldera southern italy in relation to the increase of hydrothermal activity phenomena since 2006 aimed at providing insights into the volcanic system dynamics mineral encrustations and muds were sampled between 2013 and 2015 at the long standing degassing crater of the solfatara tuff cone and its recently restless north eastern pisciarelli slope deep seated sulfides were further separated from two drill cores agip s mofete boreholes 1500 m and 2695 m depth the mineral assemblage and texture of sampled encrustations were determined by x ray diffraction optical and scanning electron microscopy and x ray microanalysis by energy dispersive spectrometry native sulfur and alunite dominate among the newly formed mineral phases other minerals are mostly alunogen and locally pickeringite potassium alum hematite and pyrite mereiterite and amarillite sporadically occur the mud pools are rich in gypsum potassium alum and pyrite quartz and argillic phases locally with analcime are dispersed in the outcropping rocks 34 s values were determined for shallow subsurface native sulfur 5 5 to 0 0 and alunite 1 7 to 0 2 as well as for the deep seated pyrite 3 3 to 7 4 in the depth range 1500 2695 m 18 o values were measured for shallow native alunite 4 2 to 7 0 pisciarelli alunite was finally analyzed for its 87 sr 86 sr ratio and 143 nd 144 nd ratios 0 707517 6 and 0 512459 6 respectively textural and isotopic data constrain the genesis of alunite at the expense of k feldspars through rock alteration by hydrothermal fluids we suggest that the caldera is a low sulfidation system hosting acid sulfate deposits in its active degassing area the acid sulfate environment developed on an argillitic facies that thins outwards and is characteristic for steam heated and magmatic steam environments these environments developed in relation to the fractured settings that facilitates the gas emissions from deeper levels and the meteoric infiltration from surface to depth newly determined 34 s data are lower than previously published values suggesting a progressive decrease in the sulfur isotopic composition through time at least since 1956 the isotopic variation is associated with a lowering of the so 2 h 2 s ratio and the widening of the surface of co 2 degassing in the absence of significant changes in the average co 2 flux values and in the carbon and helium isotope compositions native sulfur and sulfates move from sulfur isotope equilibrium to disequilibrium we ascribe this transition to enhanced so 2 scrubbing due to a high rock permeability that facilitates a faster gas ascent rate in the shallow crust

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

dissolved gases in hydrothermal phreatic and geyser eruptions at yellowstone national park usa multiphase and multicomponent fluid flow in the shallow continental crust plays a significant role in a variety of processes over a broad range of temperatures and pressures the presence of dissolved gases in aqueous fluids reduces the liquid stability field toward lower temperatures and enhances the explosivity potential with respect to pure water therefore in areas where magma is actively degassing into a hydrothermal system gas rich aqueous fluids can exert a major control on geothermal energy production can be propellants in hazardous hydrothermal phreatic eruptions and can modulate the dynamics of geyser eruptions we collected pressurized samples of thermal water that preserved dissolved gases in conjunction with precise temperature measurements with depth in research well y 7 maximum depth of 70 1 m casing to 31 m and five thermal pools maximum depth of 11 3 m in the upper geyser basin of yellowstone national park usa based on the dissolved gas concentrations we demonstrate that co 2 mainly derived from magma and n 2 from air saturated meteoric water reduce the near surface saturation temperature consistent with some previous observations in geyser conduits thermodynamic calculations suggest that the dissolved co 2 and n 2 modulate the dynamics of geyser eruptions and are likely triggers of hydrothermal eruptions when recharged into shallow reservoirs at high concentrations therefore monitoring changes in gas emission rate and composition in areas with neutral and alkaline chlorine thermal features could provide important information on the natural resources geysers and hazards eruptions in these areas

the porosity of pyroclasts as an indicator of volcanic explosivity abstract the porosity of pyroclasts is a valuable indicator of the character of a volcanic eruption here we analyse the porosity distribution of deposits of 17 different volcanic eruptions covering a broad range of inferred magma properties and observed eruption styles histograms of porosity versus pyroclast frequency show distinctive features in terms of the mean variance modality and skewness we present characteristic porosity distributions for five different eruption style categories dome forming explosive basaltic cryptodome forming subplinian plinian ultraplinian and phreatomagmatic we find that the mean porosity values of the deposits correlate positively with the volcanic explosivity index vei of newhall and self 1982 regardless of the degree of complexity of the individual distributions we then propose causes of the deviations from a general trend in terms of magma viscosity degassing efficiency and cap rock confinement finally a generalised correlation between clast porosity and explosivity of an eruption is developed that should help together with conventional field techniques facilitate the interpretation of pyroclastic sequences even in cases where no direct observation of the eruption exists

the phlegraean fields structural evolution volcanic history and eruptive mechanisms abstract the main event within the volcanic history of the phlegraean fields was the eruption about 35 000 years ago of a huge alkali trachytic ignimbrite 80 km 3 dre followed by caldera collapse the pre caldera activity evidence from geothermal wells and surface outcrops changed from submarine to subaerial shortly before the ignimbrite eruption the caldera was subsequently invaded by the sea and became progressively filled with tuffites and subordinate submarine flows in its northern half the last submarine explosive activity dates back to 12000 10500 yr with the eruption of mostly hydroplinian yellow tuffs recent mostly subaerial activity consists of a series of explosive eruptions whose volume decreases with time until the last eruption of mt nuovo in 1538 a d two peaks in recent activity occur at 10000 8000 and 4700 3000 years ago during post caldera activity eruptive vents migrated from the caldera rim toward the center where there are two distinct zones of recent vents minor collapses occurred within the caldera after the main eruptions often they are eccentric to the vent and displaced toward the caldera center phlegraean explosive activity is characterized by water magma interaction eruptive events without a hydromagmatic component are extremely rare in most cases the evolved trachytic magmas interacted with surface water either the sea or intracaldera lakes however there is evidence of interaction of magma with deep seated aquifers for some plinian events explosivity i e the efficiency of the transformation of thermal into kinetic energy during magma water interaction is largely controlled by the primary fragmentation of the magmatic melt

hydrothermal eruption mechanisms and hot spring gold deposits episodes of gold mineralization in the shallow hot spring environment are related in time to hydrothermal eruption events and in space to the resulting vent breccias and peripheral stockwork zones it is proposed that large but short lived overpressures in a geothermal reservoir probably triggered by sudden magmatic heat fluxes induce hydraulic fracturing which then evolves into hydrothermal eruptions if driven through to the surface the maximum available energy in hot shallow reservoirs appears easily sufficient to drive such eruptions particularly if co 2 rich fluids are involved in mineralized systems gold bearing fluids are subsequently channeled into the outflow conduit where they flood the permeable vent breccia and peripheral stockwork gold is lifted into this hot spring environment above a boiling level that is elevated by high flow rates to within several hundred meters of the surface and is precipitated with abundant quartz pyrite and adularia along with a distinctive and steeply zoned trace element suite these sequential events probably occur as a continuum which if repeated cyclically in a single vent zone can result in ore grade mineralization

the origin of the fumaroles of la solfatara campi flegrei south italy abstract the analysis of gaseous compositions from solfatara campi flegrei south italy fumaroles since the early 1980s clearly reveals a double thermobarometric signature a first signature at temperatures of about 360 c was inferred by methane based chemical isotopic geoindicators and by the h2 ar geothermometer these high temperatures close to the critical point of water are representative of a deep zone where magmatic gases flash the hydrothermal liquid forming a gas plume a second signature was found to be at around 200 240 c at these temperatures the kinetically fast reactive species h2 and co re equilibrate in a pure vapor phase during the rise of the plume a combination of these observations with an original interpretation of the oxygen isotopic composition of the two dominant species i e h2o and co2 shed light on the origin of fumarolic fluids by showing that effluents are mixture between fluids degassed from a magma body and the vapor generated at about 360 c by the vaporization of hydrothermal liquids a typical andesitic water type d 20 18o 10 and a co2 rich composition x co 2 0 4 has been inferred for the magmatic fluids while for the hydrothermal component a meteoric origin and a co2 fugacity fixed by fluid rock reaction at high temperatures have been estimated in the time the fraction of magmatic fluids in the fumaroles increased up to 0 5 at each seismic and ground uplift crisis bradyseism which occurred at campi flegrei suggesting that bradyseismic crises are triggered by periodic injections of co2 rich magmatic fluids at the bottom of the hydrothermal system

geophysical and hydrogeological experiments from a shallow hydrothermal system at solfatara volcano campi flegrei italy response to caldera unrest 1 integration of high resolution geophysical and hydrogeological investigations at solfatara volcano campi flegrei italy allowed us to 1 image the shallow and intermediate subsurface to the crater 2 elucidate patterns in the shallow subsurface degassing and 3 refine and upgrade volcano monitoring strategies for this dynamic area our results show that the subsurface to the crater can be divided roughly into two zones a dry outcropping layer overlying a horizon saturated by hydrothermal fluids within this saturated zone intersections of dominant nw and ene striking structural lineaments act as preferential escape conduits for the fluids which generate high microseismic noise amplitudes in the southeastern part of the crater hydrogeological data suggest an uprising of the isotherms below solfatara crater and a marked increment of fluid degassing over the last 40 years sudden variations of both seismic noise level and noise cycling are positively correlated with early stages of ground inflation during the ad 2000 uplift we believe therefore that monitoring of seismic noise can be used for upgrading early warning strategies in this sector of the campi flegrei volcanic system

the effects of water vaporization on rock fragmentation during rapid decompression implications for the formation of fluidized ejecta on mars abstract crater and ejecta morphology provide insight into the composition and structure of the target material fluidized ejecta surrounding martian rampart craters are thought to result from the addition of water to the ejecta during impact into a water rich ice or liquid regolith here we test experimentally an alternate hypothesis we propose that the decompression of a rock water mixture across the water vaporization curve during the excavation stage of impact cratering results in an increased proportion of fines in the ejecta this enables the ejecta to flow with little or no liquid water present to test this hypothesis fragmentation experiments were conducted on sandstone 28 vol open porosity from the northern eldorado mountains nevada using a shock tube apparatus at the lmu munich germany rock samples with 0 92 of their open pore space filled with water were pressurized to 15 mpa at 177 c or 300 c and rapidly decompressed as the water vaporization curve is crossed the water in the pore space rapidly flashes to steam causing together with the expanding gas in the water free pore space the sample to fragment the presence of water has a significant effect on the grain size distribution and grain shape of the fragmented rock samples in comparison with dry control samples samples with water with 15 50 open pore space exhibit much smaller grain sizes the predominant grain shape of dry as well as partially water saturated samples is bladed reflecting fracturing parallel to the decompression front samples with 80 water in open pore space had an increase in fines and larger particles but less intermediate sized particles fragments from experiments with 80 water in open pore space also displayed a more equant grain shape indicating that the decompression of water caused fracturing independent of the orientation of the decompression front these results may provide insight into the morphology of martian rampart craters we propose here that even relatively low water contents in the target 16 may be sufficient to produce a significant increased proportion of fines allowing the ejecta to flow with little or no water present

the fragmentation threshold of pyroclastic rocks in response to rapid decompression porous magma may fragment explosively this occurs when the melt can no longer withstand forces exerted upon it due to the overpressure in included bubbles this occurs at a critical pressure difference between the bubbles and the surrounding magma in this study we have investigated this pressure threshold necessary for the fragmentation of magma here we present the first comprehensive high temperature experimental quantification of the fragmentation threshold of volcanic rocks varying widely in porosity permeability crystallinity and chemical composition we exposed samples to increasing pressure differentials in a high temperature shock tube apparatus until fragmentation was initiated experimentally we define the fragmentation threshold as the minimum pressure differential that leads to complete fragmentation of the pressurized porous rock sample our results show that the fragmentation threshold is strongly dependent on porosity high porosity samples fragment at lower pressure differentials than low porosity samples the fragmentation threshold is inversely proportional to the porosity of the other factors permeability likely affects the fragmentation threshold at high porosity values whereas chemical composition crystallinity and bubble size distribution appear to have minor effects the relationship for fragmentation threshold presented here can be used to predict the minimum pressure differential necessary for the initiation or cessation of the explosive fragmentation of porous magma

mineralogical geochemical and isotopic features of tuffs from the cfddp drill hole hydrothermal activity in the eastern side of the campi flegrei volcano southern italy abstract a 506 m drill hole has been recently drilled in the framework of the campi flegrei deep drilling project cfddp and the international continental scientific drilling program icdp with the intention of coring the subsurface in the eastern sector of the campi flegrei caldera the borehole located in the western district of the neapolitan city bagnoli plain 3 km to the east of the most active volcanic area and about 5 m above sea level is now targeted for monitoring purposes this paper reports the results obtained from the analysis of two short cores collected at depths of 443 and 506 m below the ground level the cores sampled two pre caldera tuffs observations performed by optical and scanning electron microscopy energy dispersive spectroscopy and powder x ray diffraction were used to achieve data on the primary lithology both primary and secondary mineralogical assemblages and the relationship between texture and secondary mineralization sr isotope ratios were determined on selected primary feldspars whereas 13c and 18o analyses were performed on carbonates from veins and filled voids in tuffs our results provide information on the hydrothermal system in the eastern sector of the caldera that was not among the goals in the previous drilling programs secondary mineralization suggests a saline hydrothermal environment characterized by fluids that progressively evolved from boiling toward more alkaline and cooler conditions a paleo temperature of ca 160 c has been inferred from authigenic mineral occurrences and calculated on the basis of equilibria between cored calcites and fluids presently emitted at the surface by using carbon and oxygen isotope data the temperature measured at the bottom of the drilling is about 80 c

grain size characteristics of experimental pyroclasts of 1980 mount st helens cryptodome dacite effects of pressure drop and temperature using the fragmentation bomb we analysed the effects of temperature and pressure drop on the grain size characteristics of experimentally produced pyroclasts experiments performed on vesicular samples of grey dacite of the 1980 mount st helens cryptodome at t 20 900 c and initial pressure differential up to 18 5 mpa provide clear evidence of the influence of these physical conditions upon fragment size and character cylindrical dacite samples diameter 17 mm length 50 mm are placed in the high pressure temperature section of the apparatus heated and saturated by argon gas the disruption of a diaphragm located between the high and low pressure sections of the apparatus initiates the rapid depressurisation of the sample the main results may be summarised as follows 1 increasing tempera ture from 20 to 900 c results in a decrease in the fragmentation threshold value from 9 to 3 mpa and an increase in the median diameter of the experimental pyroclasts these observations imply a decrease in the dynamic tensile strength of dacite at higher temperatures which in turn influences the characteristic size of fragments 2 increasing initial pressure differential yields a decrease of the median diameter thus a higher initial elastic potential energy in the magma generates a higher degree of fragmentation 3 fragments of angular shape are observed from experiments at all investigated temperatures 20 900 c including thereby temperatures significantly higher than the classical dilatometrically or calorimetrically determined glass transition temperature determined for this dacite of 810 c thus brittle response of the dacite is observed under rapid decom pression 4 fragment size distributions do not corre spond to log normal distributions and are more closely described by rosin rammler distributions with a de crease of temperature and increase of the initial pressure differential fragment size distributions approach a rosin rammler distribution dependence 5 microscopic and grain size comparison of particle characteristics of ex perimental and natural fragments of the 1980 mount st helens cryptodome dacite demonstrate similar median diameters and shapes

thermodynamics of gas and steam blast eruptions eruptions of gas or steam and non juvenile debris are common in volcanic and hydrothermal areas from reports of non juvenile eruptions or eruptive sequences world wide at least three types or end members can be identified 1 those involving rock and liquid water initially at boiling point temperatures boiling point eruptions 2 those powered by gas primarily water vapor at initial temperatures approaching magmatic gas eruptions and 3 those caused by rapid mixing of hot rock and ground or surface water mixing eruptions for these eruption types the mechanical energy released final temperatures liquid water contents and maximum theoretical velocities are compared by assuming that the erupting mixtures of rock and fluid thermally equilibrate then decompress isentropically from initial near surface pressure 10 mpa to atmospheric pressure maximum mechanical energy release is by far greatest for gas eruptions 1 3 mj kg of fluid rock mixture about one half that of an equivalent mass of gunpowder and one fourth that of tnt it is somewhat less for mixing eruptions 0 4 mj kg and least for boiling point eruptions 0 25 mj kg the final water contents of crupted boiling point mixtures are usually high producing wet sloppy deposits final erupted mixtures from gas eruptions are nearly always dry whereas those from mixing eruptions vary from wet to dry if all the enthalpy released in the eruptions were converted to kinetic energy the final velocity v max of these mixtures could range up to 670 m s for boiling point eruptions and 1820 m s for gas eruptions highest for high initial pressure and mass fractions of rock m r near zero for mixing eruptions v max ranges up to 1150 m s all observed eruption velocities are less than 400 m s largely because 1 most solid material is expelled when m r is high hence v max is low 2 observations are made of large blocks the velocities of which may be less than the average for the mixture 3 heat from solid particles is not efficiently transferred to the fluid during the eruptions and 4 maximum velocities are reduced by choked flow or friction in the conduit