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

mechanisms of hydrovolcanic pyroclast formation grain size scanning electron microscopy and experimental studies pyroclasts produced by explosive magma water interactions are of various sizes and shapes data from analysis of over 200 samples of hydrovolcanic ash are interpreted by comparison with experimentally produced ash grain size and scanning electron microscopy sem reveal information on the formation of hydrovolcanic pyroclasts strombolian explosions result from limited water interaction with magma and the pyroclasts produced are dominantly centimeter sized with increasing water interaction hydrovolcanism increases in explosivity to surtseyan and vulcanian activity these eruptions produce millimeter to micron sized pyroclasts the abundance of fine ash 63 m diameter increases from 5 to over 30 percent as water interaction reaches an explosive maximum this maximum occurs with interactions of virtually equal volumes of melt and water five dominant pyroclast shape types determined by sem result from hydrovolcanic fragmentation 1 blocky and equant 2 vesicular and irregular with smooth surfaces 3 moss like and convoluted 4 spherical or drop like and 5 plate like types 1 and 2 dominate pyroclasts greater than 100 m in diameter types 3 and 4 are typical of fine ash type 5 pyroclasts characterize ash less than 100 m in diameter resulting from hydrovolcanic fragmentation after strong vesiculation fragmentation mechanisms observed in experimental melt water interactions result from vapor film generation expansion and collapse fragments of congealed melt are products of several alternative mechanisms including stress wave cavitation detonation waves and fluid instability mixing all result in rapid heat transfer these mechanisms can explain the five observed pyroclast shapes stress wave fracturing cavitation of the melt results from high pressure and temperature gradients at the melt water interface simultaneous brittle fracture and quenching produces type 1 pyroclasts type 2 develops smooth fused surfaces due to turbulent mixing with water after fracture and before quenching fluid instabilities promote turbulent mixing of melt and water and produce fine ash this kind of fragmentation occurs during high energy explosions the increased melt surface area due to fine fragmentation promotes high efficiency heat exchange between the melt and water shapes of resulting pyroclasts are determined by maximum surface area type 3 and surface tension effects type 4 type 5 pyroclasts result from nearly simultaneous vesicle burst and melt water fragmentation

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

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

genesis of zeolites in the neapolitan yellow tuff geological volcanological and mineralogical evidence the study proposes a model by which a thick succession of volcanic tuffs can be zeolitized by alteration of pyroclastic material in the presence of sufficient eruptive water and at temperatures close to water vapour condensation in the case of phreatomagmatic products the model simplifies interpretation of problematic deposits that exhibit pronounced vertical and lateral variation in lithification grade a major feature of the model is that thick zeolitized tuffs can be formed during emplacement of pyroclastic products in marked contrast to later alteration in an open hydrologic system geological volcanological and mineralogical data for the neapolitan yellow tuff a widespread trachytic pyroclastic deposit outcropping around campi flegrei southern italy have been used to infer the physico chemical conditions that determined mineral genesis this tuff shows a reduction in lithification grade towards the base top and with distance from the vent and very variable zeolitization within the lithified portion we suggest that during initial emplacement the erupted products chilled against the ground inhibiting zeolite crystallization during rapid deposition of the thick wet succession thermal insulation allowed the persistence of elevated temperatures for a time sufficient for enhancement of hydration dissolution processes in the volcanic glass the highly reactive alkali trachytic glass quickly buffered the acid ph of the system favouring phillipsite crystallization followed by chabazite nucleation the variable zeolite content reflects fluctuating emplacement conditions e g changes in water content and temperature cooling of the upper and relatively thin distal deposits inhibited the zeolitization process thereby preserving the primary unlithified deposit

rock mass properties and edifice strength data from pinnacle ridge mt ruapehu new zealand abstract volcanic edifices exhibit spatially variable physical and mechanical properties magmatic intrusions are common at shallow depths within the volcanic edifice and are a poorly understood contributor to this spatial variability intrusion related alteration has been found to weaken rock mass strength through the development of joints and fractures however there is a paucity of research investigating how intrusions affect rock mass strength specific to the geotechnical units that define the rock masses in this study we employ a range of field techniques field permeametry rock hardness assessment rock mass classification and discontinuity mapping to characterise an exposed fossil geothermal system produced by a shallow intrusion at pinnacle ridge mt ruapehu new zealand we find that intrusions detrimentally affect the rock mass characteristics of altered brecciated lava margins the resulting change in rock mass strength may be offset by an increase in intact rock strength as a product of alteration mineral precipitation in microfractures consequently the final strength of the rock mass of the altered brecciated lava margins has the potential to be lowest of any of the geotechnical units in the volcanic edifice we also conclude that these discontinuities increase permeability of the host rock at distances from the intrusion roughly proportional to 1 2 times the thickness of the intrusion itself under near surface conditions the data and conclusions presented in this study help to bridge the gap between the lab and the field scale and have immediate relevance to engineering geology and geothermal applications worldwide and to rock mass classification assessments in volcanic environments

dynamics of surges generated by hydrothermal blasts during the 6 august 2012 te maari eruption mt tongariro new zealand abstract the 6 august 2012 te maari eruption produced violent and widespread cold pyroclastic density currents pdcs following unroofing of the pressurized hydrothermal system despite an erupted volume of only 5 10 5 m 3 and lacking any juvenile component the 340 000 m 3 of pdcs spread over an area of 6 1 km 2 and had mobilities that were on the order of volcanic blasts or blast like pdcs this great mobility was due to strong lateral focussing of explosion energy producing jets with initial velocities 100 m s we present a type stratigraphy for these hydrothermal derived low temperature pdcs that show a tripartite deposit sequence each of the deposit units dominates respectively three outward gradational sedimentary facies reflecting transitions in the propagating pdc transport and depositional mechanisms the largest pdcs directed west and east of the upper te maari area were generated from outer cone breccias and tuffs that were mostly highly hydrothermally altered landsliding and the geometry of the hydrothermal area led to the directed jetting initial particle laden jets laid sheets grading into lobes of proximal massive sand to gravel rich facies dominated by unit a and extending up to 1 km from the vents as the jets were collapsing a vertically and longitudinally stratified pdc developed within the first few hundred metres from source exponential thinning and coarse tail grading dominated fining with radial distance of massive unit a resulted from fast deposition and progressive depletion of the most concentrated flow region behind the pdc head markedly slower tractional sedimentation from the passing pdc body and tail deposited the highly stratified and ripple bedded fine coarse ash of unit b this formed distinctive dune fields of the medial dune bedded ash rich facies upwards in depositional sequences the waning of the current can be seen by replacement of higher energy bedforms to progressively lower ones downstream progressive waning and further depletion are characterised by the development of the distal wavy to planar bedded ash rich facies this is increasingly dominated by the uppermost deposition unit c of laminated fine med ash deposited by gently turbulent dilute phoenix clouds these high energy pdcs sourced from flank hydrothermal systems should be regarded as a serious threat in any multihazard assessment of a stratovolcano even though they may not be one of the major magmatic vent sites in addition further detailed studies of these hydrothermal jetting events and their deposits should be pursued in order to better understand large volume volcanic blasts which appear to be a larger scale sibling phenomenon

hydrothermal eruptions at unstable crater lakes insights from the boiling lake dominica lesser antilles abstract the boiling lake a major touristic attraction in the morne trois pitons national park dominica s west indies world heritage site is located in an area of high hydrothermal activity in the southern part of the island this spectacular lake is characterized by unstable behavior which has been interpreted to result from the presence of a water body above a fluctuating gas filled or multi phase vent system the system produces a near boiling lake that occasionally drains and refills with water without any simple periodicity despite the fact that such unstable draining lakes are thought to be non eruptive the boiling lake is known to have sourced occasional small hydrothermal eruptions as well as toxic gas emissions which accompany the refilling phase of these drainage events similar explosive events seem also to be likely after landslide slumped rocks from steep altered crater walls cover the vent area in the lake leading to overpressurized conditions in both cases rock expulsion and gas release continue to pose serious threats for tourists and tour guides while conceptual models exist describing possible mechanisms of the boiling lake drainage itself little information exists to date regarding the trigger mechanisms related to the explosive events further no estimation of the eruption processes and dynamics of these events has previously been attempted to address this knowledge gap we employ here a combined field and laboratory based approach in order to i characterize the properties of the dominant lithologies from the crater walls at boiling lake ii determine how the ongoing alteration may affect their petrophysical properties e g porosity and permeability and iii evaluate how water saturated unconsolidated wall rock materials behave when rapidly decompressed from a slightly elevated pressure and temperature conditions here we propose that it is the petrophysical state of the materials involved e g porosity permeability and alteration related strength which controls the style and energy of these eruptions as well as the potential for landslides from the crater walls the experimentally simulated scenarios included in this study are i decompression from only slightly elevated pressure conditions inferred for small hydrothermal eruptions occurring after lake drainage events and ii decompression from rapidly enhanced pressure conditions produced by the capping and overpressurization of the vent area following landslide episodes ejection velocities obtained from rapid decompression experiments were used as input parameters for ballistic trajectory calculations allowing for estimation of the potential area impacted by ejected debris we anticipate that this study will serve as a contribution to the understanding of the hazard potential of future hydrothermal eruptions at unstable crater lakes in general as well as providing direct insights into the ongoing hydrothermal activity at the boiling lake

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

phreatic activity and hydrothermal alteration in the valley of desolation dominica lesser antilles phreatic eruptions are possibly the most dramatic surface expressions of hydrothermal activity and they remain poorly understood the near absence of precursory signals makes phreatic eruptions unpredictable with respect to both time and magnitude the valley of desolation vod dominica located close to the boiling lake the second largest high temperature volcanic crater lake in the world hosts vigorous hydrothermal activity with hot springs mud pools fumaroles and steaming ground a phreatic or phreatomagmatic eruption from this site is considered to be the most likely scenario for future volcanic activity on dominica yet there is little information regarding the trigger mechanisms and eruption processes of explosive events at this active hydrothermal center and only a very small number of studies have investigated hydrothermal activity in the vod we therefore conducted two field campaigns in the vod to map hydrothermal activity and its surficial phenomena we also investigated alteration processes and their effects on degassing and phreatic eruption processes we collected in situ petrophysical properties of clay rich unconsolidated samples and together with consolidated rock samples we investigated the range of supergene and hydrothermal alteration in the laboratory in addition we performed rapid decompression experiments on unconsolidated soil samples our results show that alteration leads to an increasing abundance of clay minerals and a decrease in both strength and permeability of the rocks in the immediate vicinity of degassing acid sulfate fluids advanced argillic alteration yields a mineral zoning which is influenced by meteoric water the water saturated basal zone is dominated by kaolinite run 0whereas alunite formation is favored at and above the groundwater table where atmospheric oxidation of h2s to h2so4 occurs e g steam heated alteration alteration effects may in turn inhibit degassing at the surface increasing the potential for pressurization in the subsurface and thus lead to phreatic eruptions rapid decompression experiments together with ballistic trajectory calculations constrain estimates of the conditions prior to the 1997 small scale phreatic event in the vod the results presented here may serve as a contribution to the understanding of the hazard potential of ongoing hydrothermal activity within the vod on a broader perspective our results will help evaluate hydrothermal activity in similar areas worldwide which might also have the potential for phreatic eruptions for instance poas costa rica or tongariro and waimangu new zealand

magma fragmentation by rapid decompression the processes leading to magma fragmentation and the generation of pyroclastic debris during explosive volcanic eruptions are of fundamental importance in volcanology observations of explosive eruptions1 3 as well as theoretical analyses of the underlying processes4 have raised the question of whether the rapid decompression of highly viscous vesicular magma that results from the collapse of a lava dome or volcanic edifice can in itself produce explosive magma fragmentation and pyroclast formation here we report the results of a laboratory investigation of pyroclast formation following rapid decompression samples of magma from the 1980 eruption of mount st helens when rapidly depressurized from initial pressures of up to 12 mpa and at temperatures in the range 750 825 c fragmented to form pyroclastic products that are in many respects similar to those formed in real eruptions moreover we observe explosive fragmentation at temperatures well below those normally associated with magmatic processes suggesting that even relatively cool magma bodies can be very hazardous when subjected to rapid unloading events

influence of alteration on the mechanical behaviour and failure mode of andesite implications for shallow seismicity and volcano monitoring volcanoes often host hydrothermal systems that alter the host rock to understand the influence of alteration on mechanical behaviour of edifice forming rock we performed a series of triaxial deformation experiments on variably altered andesite from mt ruapehu new zealand under constant effective pressure under the imposed conditions andesite with intermediate argillic alteration deforms in a brittle manner forming fractures by contrast andesite with advanced argillic alteration deforms in a ductile manner with sample failure driven by distributed cataclastic pore collapse we consider this the result of an increase in porosity and clay content with increasing alteration ancillary experiments highlight that the brittle ductile transition occurs at lower effective pressure i e at shallower depths in andesites with advanced argillic alteration relative to unaltered andesites of comparable porosity we conclude that advanced argillic alteration can create an anomalous shallow ductile zone which has important implications for fluid flow and pre eruptive seismicity

hydrothermal activity and subsoil complexity implication for degassing processes at solfatara crater campi flegrei caldera the solfatara area and its fumaroles are the main surface expression of the vigorous hydrothermal activity within the active campi flegrei caldera system at depth a range of volcanic and structural processes dictate the actual state of the hydrothermal system below the crater the presence of a large variety of volcanic products at shallow depth including pyroclastic fallout ash beds pyroclastic density current deposits breccias and lavas and the existence of a maar related fault system appears to exert major controls on the degassing and alteration behavior adding further to the complexity of this environment variations in permeability and porosity due to subsoil lithology and alteration effects may further influence fluid flow towards the surface here we report results from a field campaign conducted in july 2015 that was designed to characterize the in situ physical temperature humidity and mechanical permeability strength stiffness properties of the solfatara crater subsoil the survey also included a mapping of the surficial hydrothermal features and their distributions finally laboratory measurements porosity granulometry of selected samples were performed our results enable the discrimination of four main subsoils around the crater 1 the fangaia domain located in a topographic low in the southwestern sector 2 the silica flat domain on the western altered side 3 the new crust domain in the central area and 4 the crusted hummocks domain that dominates the north east and south parts these domains are surrounded by encrusted areas reworked material and vegetated soil the distribution of these heterogeneous subsoils suggests that their formation is mostly related to i the presence of the fangaia domain within the crater and ii a system of ring faults bordering it the subsoils show an alternation between very high and very low permeabilities a fact which seems to affect both the temperature distribution and surficial degassing a large range of surface temperatures from 25 up to 95 degrees c has been measured across these surfaces with the hottest spot corresponding to the mud pools the area of new crust formation and the crusted hummocks in the subsoil the distribution of temperature is more complex and controlled by the presence of coarser and more permeable sandy pebbly levels these act as preferential pathways for hot hydrothermal fluid circulation in contrast low permeability fine grained levels act as thermal insulators that remain relatively cold and hinder fluid escape to the surface hot gases reach the surface predominantly along vertical fractures when this occurs mound like structures can be formed by a cracking and healing process associated with significant degassing it is anticipated that the results presented here may contribute to an improved understanding of the hazard potential associated with the ongoing hydrothermal activity within the solfatara crater at this site the permeability of the near surface environment and its changes in space and time can affect the spatial and temporal distribution of gas and heat emission particularly in areas where reduction in permeability occurs it can produce pore pressure augmentation that may result in explosive events

geothermal energy release at the solfatara of pozzuoli phlegraean fields phreatic and phreatomagmatic explosion risk implications the h2o co2 and h2s outputs at the solfatara of pozzuoli have been measured and a map of the exhaling areas has also been made the energy released at the surface by the fluids has been estimated to be 1019 ergs day

the stability of sulfate and hydrated sulfate minerals near ambient conditions and their significance in environmental and planetary sciences abstract sulfate and hydrated sulfate minerals are abundant and ubiquitous on the surface of the earth and also on other planets and their satellites the humidity buffer technique has been applied to study the stability of some of these minerals at 0 1 mpa in terms of temperature relative humidity space on the basis of hydration dehydration reversal experiments updated phase relations in the binary system mgso 4 h 2 o are presented as an example to show how reliable thermodynamic data for these minerals could be obtained based on these experimental results and thermodynamic principles this approach has been applied to sulfate and hydrated sulfate minerals of other metals including fe both ferrous and ferric zn ni co cd and cu metal sulfate salts play important roles in the cycling of metals and sulfate in terrestrial systems and the number of phases extends well beyond the simple sulfate salts that have thus far been investigated experimentally the oxidation of sulfide minerals particularly pyrite is a common process that initiates the formation of efflorescent metal sulfate minerals also the overall abundance of iron bearing sulfate salts in nature reflects the fact that the weathering of pyrite or pyrrhotite is the ultimate source for many of these phases many aspects of their environmental significance are reviewed particularly in acute effects to aquatic ecosystems related to the dissolution of sulfate salts during rain storms or snow melt events hydrous mg ca and fe sulfates were identified on mars with wide distribution and very large quantities at many locations on the basis of spectroscopic observations from orbital remote sensing and surface explorations by rovers however many of these findings do not reveal the detailed information on the degree of hydration that is essential for rigorous interpretation of the hydrologic history of mars laboratory experiments on stability fields reactions pathways and reaction rates of hydrous sulfates likely to be found on mars enhance our understanding of the degrees of hydration of various sulfates that should currently exist on mars at various seasons and locations and during various atmospheric pressure and obliquity periods two sets of systematic experiments were described one on hydrous mg sulfates and the other on hydrous fe 3 sulfates also their implications to mars sulfates mineralogy were discussed

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

chemical and isotopic compositions of minerals and waters from the campi flegrei volcanic system naples italy abstract based on their 34 s signature sulfate minerals and native sulfur around fumaroles and hot water pools from the campi flegrei volcanic area derive from supergenic oxidation of volcanic h 2 s their mean 34 s value 0 2 1 7 matches with that of fumarolic h 2 s at solfatara 0 3 0 3 as well as with the 34 s of 1 4 obtained for total sulfur in fresh trachyte from the area all 34 s values indicate a mostly deep seated origin for sulfur thermal waters were analysed for major and minor chemistry and for oxygen hydrogen and sulfur isotope compositions pools at pisciarelli are filled with evaporated meteoric water heated by rising magmatic gases the water 18 o 3 8 1 3 and 2 h 6 5 2 2 values in these steam heated waters are controlled by mixing and evaporation effects and the 34 s value of dissolved sulfate 1 3 0 3 basically agrees with supergenic oxidation of deep seated h 2 s as the major source of sulfur instead water from thermal springs and wells elsewhere in the campi flegrei appears to be a mixture between dilute meteoric and saline marine components the latter may be local seawater from the bay of pozzuoli the 18 o and 2 h values of waters sampled during 1993 1994 range from 5 6 to 0 3 and from 33 to 3 4 respectively the 34 s values of dissolved sulfate range between 0 1 and 19 5 in general sulfate is probably derived essentially from two sources both within the volcanic cover i e oxidation dissolution of pyrite and anhydrite and marine water an occasional source of water and sulfate is represented by magmatic gases which directly interact with shallow meteoric water as in the case of the hotel tennis well yielding steam heated water with 18 o 1 5 0 2 2 h 17 1 and 34 s 0 1

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

microcrystalline textures resulting from rapid crystallization in a pseudotachylite melt in a meta anorthosite the very fine grained 1 m polygonal microcrystalline texture occurring in the groundmass of pseudotachylite veins in the harris meta anorthosite and its variation with position are described and an origin by crystallization from a melt at very large undercooling suggested the intrusive nature of the veins is shown by their geometry and internal structures clasts which are almost always only plagioclase are generally concentrated towards the centres of veins flow of tens of millimetres can account for this concentration in millimetre thick veins as a result of the bagnold effect the veins are generally thin 5 m to 5 mm or more are frequently zoned and always contain transparent granules of high relief probably al rich pyroxene and opaque granules of magnetite up to a few micrometres in size the granules are either uniformly distributed in microcrystalline textures or concentrated locally giving cellular textures in some veins spherulitic or bow tie textures occur the coarsest textures are found in the centres of the thickest veins the groundmass of the pseudotachylite is never completely isotropic but consists of a mosaic of transparent plagioclase crystals decreasing in size from the centres of thick veins to less than 1 m in thin veins or in the margins of thicker veins this fine microcrystalline texture was studied by both scanning and transmission electron microscopy and consists of polyhedral crystals of regular size in the range 0 2 1 5 m which show little sign of deformation the local composition of the pseudotachylites varies little from the average compositions of the rocks in optically homogeneous veins the variation being within the compositional space defined by the minerals of the host rock this shows that homogenization of the pseudotachylite has occurred in cellular veinsdifferentiation has occurred as the compositions of the cell centres lie outside those of the minerals of the host rock this was produced by segregation of the granules pyroxene being absent from the host rock the plagioclase in the pseudotachylite is more disordered than that in the host rock all the microtextures described are absent from the associated cataclasites and cannot be due to recrystallization of a fine grained and intensely strained rock powder the physical state on and after intrusion was that of a melt and injection was followed by crystallization the melt was produced by more or less total fusion of the host rock minerals at shallow depth by heat produced during local faulting and perhaps during crack propagation the fine microcrystalline texture very closely resembles that produced during hypercooling of molten metals and alloys it thus possibly formed not by devitrification but from a melt at much greater degrees of undercooling than the spherulitic and bow tie textures

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

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

deep structure of the scottish caledonides revealed by the moist reflection profile the moine and outer isles seismic traverse moist is a deep crustal reflection profile shot at sea off the north coast of scotland in 1981 spectacular reflections are observed from the moho and from thrust zones within the caledonian fold belt and foreland deep reflection profiling of the continental crust of the united states under the direction of the cocorp group has amply demonstrated the value of this technique when applied to suitable geological problems see for example refs 1 2 a similar group the british institutes reflection profiling syndicate birps has now been set up in the uk to organize such projects funds are provided by the natural environment research council nerc through the core group based at cambridge university however a preliminary experiment of which moist is the result was organized through the institute of geological sciences igs

the deep magmatic system of the campi flegrei caldera italy 1 melt inclusions in phenocrysts of the minopoli2 shoshonite and fondo riccio latite eruptive products campi flegrei caldera 10 2 to 9 5 ka bp constrain the nature of deep phlegraean magmatic fluids their role in volcanism and give new insights into the magmatic plumbing system the analyzed melt inclusions show that co2 is an abundant gaseous species confirming the results of geochemical studies on the magmatic gas fraction involved in present day fumarolic emissions volatile concentrations within the melt inclusions require gas melt equilibria between 8 and 9 km depth and closed system degassing in presence of ascending co2 rich fluids of deep provenance magmas later re equilibrate at depths up to 3 2 km i e above the top of the carbonate basement these observations correlate well with independent geophysical and geochemical evidence and are useful constraints for volcanic hazards assessment at campi flegrei

the averno 2 fissure eruption a recent small size explosive event at the campi flegrei caldera italy the averno 2 eruption 3 700 50 a b p was an explosive low magnitude event characterized by magmatic and phreatomagmatic explosions generating mainly fall and surge beds respectively it occurred in the western sector of the campi flegrei caldera campanian region south italy at the intersection of two active fault systems oriented ne and nw the morphologically complex crater area largely filled by the averno lake resulted from vent activation and migration along the ne trending fault system the eruption generated a complex sequence of pyroclastic deposits including pumice fall deposits in the lower portion and prevailing surge beds in the intermediate upper portion the pyroclastic sequence has been studied through stratigraphical morphostructural and petrological investigations and subdivided into three members named a through c member a was emplaced during the first phase of the eruption mainly by magmatic explosions which generated columns reaching a maximum height of 10 km during this phase the eruption reached its climax with a mass discharge rate of 3 2 106 kg s intense fracturing and fault activation favored entry of a significant amount of water into the system which produced explosions driven by variably efficient water magma interaction these explosions generated wet to dry surge deposits that emplaced member b and c respectively isopachs and isopleths maps as well as areal distribution of ballistic fragments and facies variation of surge deposits allow definition of four vents that opened along a ne oriented 2 km long fissure the total volume of magma extruded during the eruption has been estimated at about 0 07 km3 dre the erupted products range in composition from initial weakly peralkaline alkali trachyte to last emplaced alkali trachyte isotopic data and modeling suggest that mixing occurred during the averno 2 eruption between a more evolved less radiogenic stored magma and a less evolved more radiogenic magma that entered the shallow reservoir to trigger the eruption the early phases of the eruption during which the vent migrated from sw to the center of the present lake were fed by the more evolved uppermost magma while the following phases extruded the less evolved lowermost magma integration of the geological and petrological results suggests that the averno 2 complex eruption was fed from a dyke shaped shallow reservoir intruded into the ne sw fault system bordering to the west the la starza resurgent block within the caldera floor

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

genesis of acid sulfate alteration and au cu ag mineralization at summitville colorado the summitville au cu ag deposit occurs within a porphyritic quartz latite primarily in a zone of vuggy silica alteration characterized by leaching of all major elements except silica and iron vuggy silica is best developed between an elevation of 3 700 and 3 500 m where it occurs in irregular pipes and lenticular pods up to 70 m wide and is enclosed by up to 30 m of intense quartz alunite alteration the vuggy silica zones diminish in size with decreasing elevation and rarely exceed a width of 2 m below an elevation of 3 400 m these deep vuggy silica zones are enclosed by quartz kaolinite alteration with little or no alunite sulfide mineralization occurs primarily within the vuggy silica the sulfide assemblage changes with decreasing elevation from covellite luzonite or native gold to chalcopyrite tennantite sulfides occur lining voids or in microveinlets and are accompanied by minor kaolinitc but not by alunite gold also occurs in a near surface barite goethite jarosite assemblage that crosscuts the vuggy silica consideration of alunite kaolinitc and pyrite stability relations and of aluminum speciation at 250 degrees c indicates that a ph of 2 or below and a log f sub o 2 of 31 or 1 at 250 degrees c were necessary to produce vuggy silica alteration the chalcopyrite tennantite mineralization occurring at depth in the vuggy silica zone is indicative of less oxidizing and less acidic conditions the covellite luzonite or gold assemblage requires conditions intermediate between those necessary for vuggy silica alteration and for deep sulfide mineralization barite gold mineralization is interpreted as a near surface assemblage produced under highly acidic oxidizing conditions these textural and geochemical relationships indicate that sulfide and gold mineralization postdate the formation of vuggy silica alteration in the deposit and are not directly related to the highly acidic solutions necessary for this early acid leaching event the change from extreme acid leaching to copper sulfide and precious metal deposition may represent a transition from a magmatic vapor dominated to a liquid dominated system leading to more reducing and less acidic conditions during sulfide deposition

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

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

fracture geometry of pseudotachylyte generation zones a study of shear fractures formed during seismic events abstract the orientation and movement sense of shear fractures in pseudotachylyte generation zones developed in the ikertoq shear belt west greenland are described and the fracturing sequence is inferred the generation zones occur in strongly foliated gneisses and consist of concordant usually paired principal displacement shear fractures paired shears on which most melt was generated and a system of minor shear fractures minor shear fracturing was most intense between principal displacement shear fractures and most melt intruded from these paired shears inwards with respect to the zone intrusion of injection veins of pseudotachylyte off the principal displacement shear fractures is believed to be contemporaneous with minor shear fracturing in the generation zones generation zones affect intact rocks and individual principal displacement shear fractures have a maximum displacement of 4 6 m paired principal displacement shear fractures are up to 3 m apart and have a maximum observed length of 1 km in all the zones described the fracturing appears to be the result of a single episode of slip both dextral and sinistral generation zones occur within the study area and have different geometries in each case the displacement is strike slip

monitoring and modelling hydrothermal fluid emission at la solfatara phlegrean fields italy an interdisciplinary approach to the study of diffuse degassing discharge of hydrothermal fluids at la solfatara is one of the many pieces of evidence testifying to the level of activity and the potential danger characterizing the volcanic complex of the phlegrean fields recent measurements of diffuse degassing shed light on the unexpected magnitude of this process and prompted further research to improve our understanding of the hydrothermal system feeding the surface phenomenon at la solfatara in this work we first present new data on co2 diffuse degassing that were collected during a campaign in july 2000 and confirm previous measurements of the degassing rate these data together with other information on subsurface structures and conditions provided the necessary constraints to perform physical modelling of the hydrothermal fluid circulation at la solfatara numerical simulations describe the ascent and evolution of a hot multi phase and multi component fluid composed of water and carbon dioxide through a homogeneous porous medium when certain system properties and conditions are used this model reproduces some of the main features which characterize the natural system including the energy budget associated with the ascent and condensation of hot fluids and the development of a single phase gas region whose existence was inferred from independent chemical data further simulations investigated the effects associated with changes in rock properties and fluid injection rates results showed that an increase in the permeability at shallow depth may have an important influence on the system conditions and fluid discharge they imply that surface phenomena accompanied by very high temperatures at shallow depths may survive long after deep feeding has ceased

towards more realistic values of elastic moduli for volcano modelling abstract the accuracy of elastic analytical solutions and numerical models widely used in volcanology to interpret surface ground deformation depends heavily on the young s modulus chosen to represent the medium the paucity of laboratory studies that provide young s moduli for volcanic rocks and studies that tackle the topic of upscaling these values to the relevant lengthscale has left volcano modellers ill equipped to select appropriate young s moduli for their models here we present a wealth of laboratory data and suggest tools widely used in geotechnics but adapted here to better suit volcanic rocks to upscale these values to the scale of a volcanic rock mass we provide the means to estimate upscaled values of young s modulus poisson s ratio shear modulus and bulk modulus for a volcanic rock mass that can be improved with laboratory measurements and or structural assessments of the studied area but do not rely on them in the absence of information we estimate upscaled values of young s modulus poisson s ratio shear modulus and bulk modulus for volcanic rock with an average porosity and an average fracture density quality to be 5 4 gpa 0 3 2 1 gpa and 4 5 gpa respectively the proposed young s modulus for a typical volcanic rock mass of 5 4 gpa is much lower than the values typically used in volcano modelling we also offer two methods to estimate depth dependent rock mass young s moduli and provide two examples using published data from boreholes within k lauea volcano usa and mt unzen japan to demonstrate how to apply our approach to real datasets it is our hope that the data and analysis presented herein will assist in the selection of elastic moduli for volcano modelling to this end we provide a microsoft excel spreadsheet containing the data and necessary equations to calculate rock mass elastic moduli that can be updated when new data become available the selection of the most appropriate elastic moduli will provide the most accurate model predictions and therefore the most reliable information regarding the unrest of a particular volcano or volcanic terrain

fragmentation of foamed silicic melts an experimental study we present the first experimental investigation of the fragmentation behavior of two phase melt gas rhyolitic systems under rapid decompression two phase samples have been generated by foaming water oversaturated rhyolitic melts up to 900 c and up to 18 mpa prior to rapid decompression in a fragmentation bomb the fragmented particles or experimental pyroclasts were recovered for analysis several features of naturally foamed pumices have been reproduced including the generation of both isotropic and tube pumices we focus here on the fragmentation behavior fragmentation occurred through a layer by layer process in the brittle regime of melt response we investigated the influence of the magnitude of the decompression 4 to 18 mpa the porosity 0 to 85 vol and the pore morphology tube versus isotropic on the fragment size distribution less vesicular samples porosity 50 vol yield coarser fragments when decompressed below 15 mpa and finer fragments when decompressed above 15 mpa increasing decompression of the vesicular samples results in a decrease in fragment size of 0 2 unit mpa the presence of tubes instead of isotropic pores in vesicular samples generates finer fragments under decompression implications for dome eruptions are discussed

heat source of the 2014 phreatic eruption of mount ontake japan we present petrological evidence of the shallow magmatic processes that may have supplied heat and gas to the eruption of mount ontake japan on 27 september 2014 which resulted in 63 fatalities ash from the eruption comprises primarily hydrothermally altered white toned rock fragments however the ash contains trace amounts 0 7 wt of vitreous less altered particles laps which are only altered on their surfaces suggesting rapid ascent through the hydrothermal system the laps are classified into two categories glassy and crystalline glassy laps comprise high silica rhyolitic glass 74 83 wt sio2 with rounded quartz chalcedony free vesicles and reversely zoned plagioclase cores 47 mol an rims 70 mol an indicating magma re heating crystalline laps have a groundmass like texture that suggests eutectic crystallization at shallow depths thermodynamic calculations indicate that the pre eruptive temperatures of the glassy and crystalline laps were 700 1300 c and 700 c respectively and that the storage depth was 4 km pressure 100 mpa the observed petrological features suggest that the laps were sourced from a magma recently intruded at shallow depths although crustal deformation i e the volume change associated with magmatic intrusion was insignificant prior to the 2014 eruption a clear signature of crustal deformation was observed in 2007 the source of which was located 3 km below the summit we suggest that the magma that was intruded 3 km below the summit in 2007 supplied the heat and gas for the 2014 phreatic eruption

mechanical behaviour and failure modes in the whakaari white island volcano hydrothermal system new zealand abstract volcanic hydrothermal systems host a prodigious variety of physico chemical conditions the physico chemical state and mechanical behaviour of rocks within is correspondingly complex and often characterised by vast heterogeneity here we present uniaxial and triaxial compression experiments designed to investigate the breadth of mechanical behaviour and failure modes dilatant or compactant for hydrothermally altered lava and ash tuff deposits from whakaari white island volcano in new zealand a volcano with a well documented and very active hydrothermal system our deformation experiments show that the failure mode of low porosity lava remains dilatant over a range of depths up to pressures corresponding to depths of about 2 km upon failure shear fractures the result of the coalescence of dilatational microcracks are universally present the high porosity ash tuffs switch however from a dilatant to a compactant failure mode driven by progressive distributed pore collapse at relatively low pressure corresponding to a depth of about 250 m we capture the salient features of the dynamic conditions e g differential stress effective pressure in a schematic cross section for the whakaari hydrothermal system and map for the different lithologies areas susceptible to either dilatant vs compactive modes of failure the failure mode will impact for example the evolution of rock physical properties e g porosity permeability and elastic wave velocity and the nature of the seismicity accompanying periods of unrest we outline accordingly the potential implications for the interpretation of seismic signals outgassing ground deformation and the volcanic structural stability for whakaari and similar hydrothermally active volcanoes worldwide

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

electromagnetic outline of the solfatara pisciarelli hydrothermal system campi flegrei southern italy abstract we describe the results from a combined controlled source audio magnetotelluric csamt and natural source magnetotelluric mt survey carried out in the solfatara pisciarelli s p area located in the central part of the campi flegrei cf composite caldera west of naples southern italy the s p area represents the most active zone within the cf caldera in terms of hydrothermal manifestations and local seismicity since 1969 the cf caldera is experiencing ground deformation seismicity and geochemical fluid changes which are particularly evident in the s p area a 1 km long nearly w e directed csamt mt profile crossing the fumarole field was carried out in the s p area with the aim of deducting a resistivity model of the structural setting of the hydrothermal system in the first 3 km depth an interpretation of the modelled section across the profile is given in this paper taking advantage from already existing seismic gravity and geochemical data in the same area three well distinct zones have been outlined the first zone is a very shallow electrically conductive body localized beneath the westernmost segment of the profile which within a short distance of about 100 m dips westwards from near surface down to some hundred metres in depth mostly accounting for the very low resistivity 1 10 m and the exceedingly high values of v p v s 4 this shallow zone has been ascribed to a water saturated high pressurized geothermal reservoir the second zone which has been localized below the west central portion of the csamt mt transect appears as a composite body made up of a nearly vertical plumelike structure that escapes at about 2 25 km depth from the top edge of the east side of a presumably horizontal platelike body the plumelike structure rises up to the free surface in correspondence of the fumarole field whereas the platelike structure deepens at least down to the 3 km of maximum exploration depth the combined interpretation of resistivity 50 100 m body wave velocity ratio v p v s 3 and geochemical data indicates that the plumelike portion can likely be associated with a steam gas saturated column and the platelike portion with a high temperature 300 c over pressurized gas saturated reservoir finally the third zone which has been localized beneath the eastern half of the transect from about 1 2 km down to about 3 km of depth is also characterized by very low resistivity values 1 10 m jointly interpreted with seismic v p v s 0 g cm 3 data this last electrically conductive structure appears to be associated with a hydrothermally mineralized clay rich body

geothermometry and geobarometry of a cummingtonite bearing dacite from martinique lesser antilles a dacite sample from a nuee ardente deposit at pitons du carbet martinique contained cummingtonite orthopyroxene quartz magnetite and ilmenite as phenocrysts and thus was suitable for evaluation of the t f text o text 2 p text total p text h text 2 text o conditions of crystallization of nuee ardente materials according to the method of ewart et al 1971 the estimations were obtained with the aid of a fortran iv program which permitted the operation of all the arrangements between the available microprobe analyses the results indicated that the phenocrystic association crystallized at moderate temperatures ca 750 c under high oxygen fugacities nno and total pressures 2 3 4 5 kb with a maximal density of points at about 3 5 kb and water pressures approximating total pressures the groundmass fe ti oxides equilibrated at lower temperatures and under relatively increasing oxygen fugacities the variation of which is tentatively ascribed to the aerial emplacement of the nuee ardente materials the phenocryst data imply that in the case studied the high fluid pressures characterizing the nuee ardente eruptions were not acquired at a shallow level because of the deep intratelluric stage of crystallization of these differentiated calcalkalic magmas

a way to hydrothermal paroxysm colli albani volcano italy the main issue addressed in this work is the process leading to fluid subsurface entrapment and pressure increase up to hydrofracturing and possibly to paroxysm in a hydrothermal setting in order to envisage such processes and mitigate their effects in the volcanically active study area and elsewhere a field and laboratory multidisciplinary approach is used in the fossil late pleistocene portion of an active hydrothermal system colli albani volcano rome italy in this area sulfate and sulfide mineralizations and strongly altered ignimbrites are exposed the alteration acme occurs on top of a buried normal fault where abundant degassing is still active and fades away in 2 3 km based on pervasive versus discrete alteration styles mineral assemblages and further evidence proximal and distal alteration domains are recognized both domains underwent steam heated advanced argillic alteration with likely temperatures up to 400 c in the proximal domain and less than 150 c in the distal domain the process of hydrothermal alteration progressively and severely depleted many elements from the most permeable rock units whereas the lowest permeable unit tufo lionato underwent fracture and porosity healing accompanied by both mass and volume gain in the proximal domain the advanced argillic hydrothermal alteration eventually formed a substantial barrier to fluids the hydrothermal fluids accumulated in and below this barrier which was then suddenly hydrofractured when heat driven hydraulic pressure overcame the effective stress thus possibly leading to hydrothermal paroxysm the decompression associated with hydrofracturing enhanced gas exsolution and mineral precipitation from the entrapped overpressured fluids mineral precipitation contributed in turn to fracture healing and to reinitiation of a new cycle of hydrothermal fluid entrapment the key preconditions for the occurrence of the inferred processes are the contrasting compositions of k alkaline host rocks and acidic alteration fluids as also previously documented in other similar settings elsewhere

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

cyclic processes and factors leading to phreatic eruption events insights from the 25 september 2007 eruption through ruapehu crater lake new zealand abstract a dangerous block ash eruption occurred without warning from ruapehu crater lake on 25 september 2007 ending a 6 month period of temperature decline in the lake the northerly directed blast sent ballistics of up to 2 m diameter more than 2 km from the source the ejecta comprised a variety of volcano lacustrine lithologies many showing evidence of intense hydrothermal alteration petrographic studies of the ballistics reveal numerous examples of pores vesicles being filled with assemblages of elemental sulphur anhydrite pyrite and natroalunite and many ballistics exuded liquid sulphur from their pores during cooling at atmospheric pressure up to 5 of the ash was comprised of fine grained juvenile glass material confirming the presence of magma in the shallow vent environment a long recognised characteristic of this volcano is the thermal cycling behaviour of the crater lake behaviour which has remained effectively unchanged through two magmatic eruption events in 1945 and in 1995 96 airborne measurements of co2 and so2 flux from the volcano since 2003 show a strong positive correspondence with temperature cycling in the lake pointing to a definitive link between heat and mass discharge through the system low so2 emission rates and c s mole ratios 30 were typical of the period 2003 to early 2007 and appear to reflect vigorous scrubbing of s gases in the hydrothermal system during this time to better understand the chemical and physical processes operating in this very dynamic volcanic hydrothermal system and its propensity to develop potentially dangerous hydrothermal seals we have developed an integrated finite difference tough2 heat mass transport simulation of the hydrothermal environment the model which is constrained by observed heat and mass flux characteristics of the volcano demonstrates the transient nature of the heating cycles on the vent lake system of particular interest is the role of co2 as a relatively inert pressure transmitting medium in the hydrothermal environment seal formation was simulated using the reactive transport code x1t which showed that condensation of magmatic vapour into a highly porous medium of andesitic composition leads to rapid formation of an elemental sulphur anhydrite natroalunite mineral assemblage and a drastic reduction in permeability we suggest that since the 1995 96 vent clearing eruptions a shroud of elemental sulphur and other alteration phases formed along the margins of the 2 phase liquid vapour region enclosing the conduit this effectively seals the upper portion of the vent from the adjacent hydrostatically controlled environment and allows for development of pressurised vapour static gas columns beneath the northern and central vents

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

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

escalating co2 degassing at the pisciarelli fumarolic system and implications for the ongoing campi flegrei unrest abstract this short communication aims at providing an updated report on degassing activity and ground deformation variations observed during the ongoing 2012 2019 campi flegrei caldera unrest with a particular focus on pisciarelli currently its most active fumarolic field we show that the co2 flux from the main pisciarelli fumarolic vent referred as soffione has increased by a factor 3 since 2012 reaching in 2018 2019 levels 600 tons day that are comparable to those typical of a medium sized erupting arc volcano a substantial widening of the degassing vents and bubbling pools and a further increase in co2 concentrations in ambient air up to 6000 ppm have also been detected since mid 2018 we interpret this escalating co2 degassing activity using a multidisciplinary dataset that includes thermodynamically estimated pressures for the source hydrothermal system seismic and ground deformation data from this analysis we show that degassing deformation and seismicity have all reached in 2018 2019 levels never observed since the onset of the unrest in 2005 with an overall uplift of 57 cm and 448 seismic events in the last year the calculated pressure of the campi flegrei hydrothermal system has reached 44 bar and is rapidly increasing our results raise concern on the possible evolution of the campi flegrei unrest and reinforce the need for careful monitoring of the degassing activity at pisciarelli hopefully with the deployment of additional permanent gas monitoring units

fluids in early stage hydrothermal alteration of high sulfidation epithermal systems a view from the vulcano active hydrothermal system aeolian island italy abstract high sulfidation hs epithermal systems have elements in common with passively degassing volcanoes associated with high t acid fumarole fields or acid crater lakes they are considered to form in two stages the first of which involves advanced argillic alteration resulting from intense strongly acidic fluid rock interaction the la fossa hydrothermal system vulcano island represents a classic example of such an active hs system and can be considered as a modern analogue of this early stage of alteration resulting in a core of intense silicic 90 95 pure sio2 alteration surrounded by alunitic alteration zones this paper focuses on a geochemical and stable isotope study of the surficial alteration facies of vulcano particularly the horizon characterized by strong silicic alteration and on deep seated xenoliths ejected during the last eruption of la fossa volcano 1888 90 that can be considered as representative of fragments of the deep conduit system of la fossa volcano using directly measured temperatures at the sites of sampling we have calculated fluid composition in isotopic equilibrium with the alteration products the large range of measured silica 18o 12 3 to 29 reflects the wide range of formation temperatures 80 240 c the fluid compositions calculated for intense silicic alteration vary from 0 9 to 6 5 these are significantly heavier than local meteoric water 6 and are consistent with derivation from the condensation of high temperature fumarolic gases dominated by magmatic fluids and rich in acid gases so2 h2s hcl hf into shallow groundwaters of meteoric origin with dynamically variable ratios of fumarolic steam meteoric water the calculated 18o and d of water in equilibrium with alunite also suggest the mixing of magmatic and meteoric waters for the fluids involved in the genesis of advanced argillic alteration facies the calculated 18o of water in equilibrium with hedenbergitic clinopyroxene found in a veinlet in a metasomatized xenolith is 8 9 this value cannot reasonably result from water rock interaction between the host volcanic rocks and surface water instead it most likely represents a fluid brine exsolved from magma which was responsible for high temperature metasomatism in the deep conduit system

juvenile magma recognition and eruptive dynamics inferred from the analysis of ash time series the 2015 reawakening of cotopaxi volcano forecasting future activity and performing hazard assessments during the reactivation of volcanoes remain great challenges for the volcanological community on august 14 2015 cotopaxi volcano erupted for the first time in 73 years after approximately four months of precursory activity which included an increase in seismicity gas emissions and minor ground deformation here we discuss the use of near real time petrological monitoring of ash samples as a complementary aid to geophysical monitoring in order to infer eruption dynamics and evaluate possible future eruptive activity at cotopaxi twenty ash samples were collected between august 14 and november 23 2015 from a monitoring site on the west flank of the volcano these samples contain a range of grain types that we classified as hydrothermal altered lithic juvenile and free crystals the relative proportions of theses grains evolved as the eruption progressed with increasing amounts of juvenile material and a decrease in hydrothermally altered material in samples from the initial explosion juvenile grains are glassy microlite poor and contain hydrothermal minerals opal and alunite the rising magma came in contact with the hydro thermal system under confinement causing hydro magmatic explosions that cleared the upper part of the plumbing system subsequently the magmatic column produced a thermal aureole in the conduit and dried out the hydrothermal system allowing for dry eruptions magma ascent rates were low enough to allow for efficient outgassing and microlite growth constant supply of magma from below caused quasi continuous disruption of the uppermost magma volume through a combination of shear deformation and gas expansion the combination of increasing crystallinity of juvenile grains and high measured so2 flux indicate decreasing integrated magma ascent rates and clearing of the hydrothermal system along transport pathways in a system open to gas loss the near real time monitoring of ash samples combined with traditional geophysical monitoring techniques during the reawakening of cotopaxi allowed us to gain a much clearer understanding of events than when using traditional geophysical monitoring alone c 2016 elsevier b v all rights reserved

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

experimentally produced pseudotachylite like veins in gabbro abstract thin veins of mobile microcrystalline particles 50 500 a in size have been produced experimentally during catastrophic failure of gabbroic rocks under rapidly applied stresses in the range 2 0 4 0 gpa optically the veins contain crushed fragments of minerals in an isotropic groundmass and are locally brown in color particularly where they cross pyroxene crystals in appearance they resemble some pseudotachylite veins transmission electron microscopy has been used to characterize the microstructure of the grains and document their crystalline structure

melt origin of fault generated pseudotachylytes demonstrated by textures a petrographic and electron microprobe study of fault generated pseudotachylytes from the outer hebrides thrust zone scotland demonstrates that the textures have resulted from the primary crystallization of a clast laden melt rather than the devitrification of a glass or by crushing and cataclasis the observations are consistent with these pseudotachylytes having formed by frictional fusion followed by rapid quenching implying faulting at shallow crustal depths

transcrystalline shear fracturing and pseudotachylite generation in a meta anorthosite harris scotland abstract the meta anorthosite is locally deformed by brittle shear fracturing which progressively increases from isolated fractures with little cataclasite to many generations of closely spaced fractures the intervening rock being highly deformed in both a plastic and brittle way in most cases an e w compression on gently dipping to steep reverse shear planes occurs which we relate to a caledonian thrust zone in places the highly deformed rock is cut by pseudotachylite veins which locally form networks the pseudotachylite is generally intrusive but does not appear to be related to movement on major slip surfaces very locally it may have formed in situ pseudotachylite only occurs in highly deformed rock is only very occasionally deformed itself and thus generally represents at each locality the last stage of a complex deformation history as if its presence welded the rock and prevented further deformation these striking differences from the country rock gneisses in which pseudotachylite occurs on well developed fractures in very slightly deformed rock are considered to be due to the low anisotropy of the meta anorthosite to its lower shear strength and to the easy propagation and branching of the shear fractures in plagioclase the source of the heat necessary to generate the pseudotachylite melt is not clear it may come from crack propagation as well as frictional sliding

predicting rock type and detecting hydrothermal alteration using machine learning and petrophysical properties of the canadian malartic ore and host rocks pontiac subprovince quebec canada abstract the canadian malartic deposit is a world class intrusion related archean gold deposit hosted in the pontiac subprovince superior province in quebec canada laboratory petrophysical properties measurements were performed on 824 rock samples collected from the various rock types observed within the ore body and peripheral host rocks the various rock types present in the malartic district mainly meta sedimentary rocks felsic intermediate intrusive rocks and mafic dykes have contrasting grain densities and magnetic susceptibilities using support vector machines it is shown that these two physical properties can be used to predict the rock type of a sample with an average precision and recall rate of 89 within the meta sedimentary rocks class variations in magnetic susceptibility are due to the changes in mineralogy associated with hydrothermal alteration these are caused by the destruction of iron bearing silicate minerals and magnetite in unaltered rocks 10 4 to 10 3 si to form pyrite carbonates k feldspar and fe depleted hydrothermal biotite in altered rocks 10 5 to 10 4 si within the felsic intermediate intrusive rocks grain densities below 2 7 g cm3 and magnetic susceptibilities in the 10 6 to 10 4 si range yield the highest probabilities that a rock has been submitted to carbonate and pyrite alteration however magnetic susceptibility and grain density of these intrusive rocks are also dependent on their fe2o3 al2o3 and tio2 al2o3 ratios which are not related to the hydrothermal alteration footprint but are rather due to distinct protolith compositions within the mafic dykes grain density is the best indicator of hydrothermal alteration unaltered mafic dykes 2 95 to 3 10 g cm3 are mostly composed of amphibole whereas altered mafic dykes 2 70 to 2 95 g cm3 have reduced amphibole contents and higher abundances of carbonates pyrite quartz and biotite alteration the support vector machine classifier is extended to predict if meta sedimentary rocks felsic intermediate intrusive rocks and mafic dykes have undergone hydrothermal alteration with average f1 scores of 73 69 and 93 respectively in altered meta sedimentary rocks the integration of grain density and magnetic susceptibility allows the identification of altered but unmineralized samples the classifier is further extended to predict if the gold content of meta sedimentary rocks is above or below threshold values of 0 01 0 1 and 1 ppm with average f1 scores of 83 80 and 76 respectively using conceptual models of the rock physical properties at the malartic district scale it is shown that ground magnetic surveys are the most promising geophysical tool for early stage greenfield exploration of this type of deposit however depending on the scale at which the surveys are conducted magnetic susceptibility contrasts between the various investigated rock types can overshadow the specific signatures of hydrothermally altered rocks this in part explains why past airborne geophysical exploration campaigns for this type of deposit in the malartic district were inconclusive finally the machine learning process used in this case study can be applied in advanced exploration stages during which drilling and either subsequent laboratory petrophysical analyses of core samples or downhole geophysical surveys produce large amounts of data that can be used to train prediction models

the respiratory health hazards of volcanic ash a review for volcanic risk mitigation studies of the respiratory health effects of different types of volcanic ash have been undertaken only in the last 40 years and mostly since the eruption of mt st helens in 1980 this review of all published clinical epidemiological and toxicological studies and other work known to the authors up to and including 2005 highlights the sparseness of studies on acute health effects after eruptions and the complexity of evaluating the long term health risk silicosis non specific pneumoconiosis and chronic obstructive pulmonary disease in populations from prolonged exposure to ash due to persistent eruptive activity the acute and chronic health effects of volcanic ash depend upon particle size particularly the proportion of respirable sized material mineralogical composition including the crystalline silica content and the physico chemical properties of the surfaces of the ash particles all of which vary between volcanoes and even eruptions of the same volcano but adequate information on these key characteristics is not reported for most eruptions the incidence of acute respiratory symptoms e g asthma bronchitis varies greatly after ashfalls from very few if any reported cases to population outbreaks of asthma the studies are inadequate for excluding increases in acute respiratory mortality after eruptions individuals with pre existing lung disease including asthma can be at increased risk of their symptoms being exacerbated after falls of fine ash a comprehensive risk assessment including toxicological studies to determine the long term risk of silicosis from chronic exposure to volcanic ash has been undertaken only in the eruptions of mt st helens 1980 usa and soufriere hills montserrat 1995 onwards in the soufriere hills eruption a long term silicosis hazard has been identified and sufficient exposure and toxicological information obtained to make a probabilistic risk assessment for the development of silicosis in outdoor workers and the general population a more systematic approach to multi disciplinary studies in future eruptions is recommended including establishing an archive of ash samples and a website containing health advice for the public together with scientific and medical study guidelines for volcanologists and health care workers

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

three dimensional electrical resistivity tomography of the solfatara crater italy implication for the multiphase flow structure of the shallow hydrothermal system the solfatara volcano is the main degassing area of the campi flegrei caldera characterized by 60 years of unrest assessing such renewal activity is a challenging task because hydrothermal interactions with magmatic gases remain poorly understood in this study we decipher the complex structure of the shallow solfatara hydrothermal system by performing the first 3 d high resolution electrical resistivity tomography ert of the volcano the 3 d resistivity model was obtained from the inversion of 43 432 resistance measurements performed on an area of 0 68 km2 the proposed interpretation of the multiphase hydrothermal structures is based on the resistivity model a high resolution infrared surface temperature image and 1 136 soil co2 flux measurements in addition we realized 27 soil cation exchange capacity cec and ph measurements demonstrating a negligible contribution of surface conductivity to the shallow bulk electrical conductivity hence we show that the resistivity changes are mainly controlled by fluid content and temperature the high resolution tomograms identify for the first time the structure of the gas dominated reservoir at 50 m depth that feeds the bocca grande fumarole through a 10 m thick channel in addition the resistivity model reveals a channel like conductive structure where the liquid produced by steam condensation around the main fumaroles flows down to the fangaia area within a buried fault the model delineates the emplacement of the main geological structures mt olibano solfatara crypto dome and tephra deposits it also reveals the anatomy of the hydrothermal system especially two liquid dominated plumes the fangaia mud pool and the pisciarelli fumarole respectively

seismic imaging of the solfatara crater campi flegrei caldera southern italy new evidence of the fluids migration pathways in the shallow structures abstract seismic imaging is used in this work to reconstruct the shallow structure of the solfatara crater a surface marker of deep magmatic activity inside campi flegrei caldera southern italy we analyze the 1d orthogonal seismic arrays deployed in nne ssw directions for 400 m through the solfatara crater during the recent active experiment ricen repeated induced earthquake and noise performed in the framework of the eu project medsuv seismic data have been edited in the pre stack stage by removing the noisy traces low frequency noise and reduce the ground roll phases then a velocity analysis has been carried out starting from the velocity spectrum to obtain a rms velocity model which has been followed by nmo residual statics and dmo corrections we used the post stack kirchhoff migration technique before the extraction of the energy root mean square envelope and sweetness attributes defining the highest amplitude zones the time gain attribute in order to interpret the deep reflectors and the variance attribute to define the faults discontinuities and chaotic zones amplitude versus offset avo analysis has been performed to identify the gas saturated zone using partial angle stacks which is a benchmark to verify the presence of gas the main contribution of this work is to provide a more detailed and improved seismic reflection image of the shallow depth of the solfatara crater the reliability of our velocity model has been assessed through the use of the vp velocity model previously obtained by the non linear bayesian technique this provides a final stack showing that the shallower minor faults and the extension of the major faults represent the main pathways for the uprising fluids we present the visual evidence for the fluids trapped in the shallower part of the crater at depths between 20 and 50 m below the surface of the crater

an experimental facility for the investigation of magma fragmentation by rapid decompression a special experimental facility has been developed to investigate the fragmentation of vesicular magma undergoing rapid decompression the facility operates in a regime similar to that of shock tubes and at temperatures up to 950 c and pressures up to 200 bar cylindrical samples diameter ca 17 mm length ca 50 mm undergo rapid decompression in a high temperature high pressure section of the facility following the disruption of a diaphragm separating that section from a low pressure low temperature section actual vesicular magma samples have been experimentally fragmented at elevated temperatures and pressures corresponding to those observed during explosive volcanic eruptions and the resulting pyroclastics have been photographically resolved in flight and collected for physical characterization the results of these experiments show that the rapid decompression of highly viscous vesicular magma can generate pyroclastic ejecta via rapid and complete fragmentation of magma at high temperature this new fragmentation facility is presented as a tool for experimental volcanology under well constrained conditions