physical models of volcanic eruptions abstract our present understanding of the dynamics of volcanic eruptions is summarized a few important facts about volcanic eruptions are reviewed and outstanding problems are identified current research efforts are aimed at explaining why an eruption may alternate between and pass through different dynamical regimes the basic physics of eruptions are explained from a dynamical point of view with emphasis on the processes acting to fix the gas content of the erupted material the coupling between flow processes and pressure release leads to large changes of decompression rate during ascent in a volcanic conduit the decompression rate may reach very high values which suggests that the kinetics of degassing play an important role accurate values of the solubility of the major magmatic volatile species are required at pressures of

distribution and properties of soft weathered bedrock at 1 m depth in the contiguous united states the weathered bedrock zone is increasingly recognized as an important part of ecological and hydrologic systems but its distribution is poorly known in the contiguous united states we used spatial and laboratory characterization data from the us department of agriculture usda natural resources conservation service to assess the distribution and soil like properties of soft weathered bedrock saprock and saprolite within the 48 contiguous united states because usda soil inventories generally do not extend below 2 m and because the upper 1 m is clearly involved in ecosystem function and vadose zone hydrology we restricted our inquiry to soft weathered bedrock within 1 m of the land surface soft weathered bedrock within 1 m of the land surface is widespread throughout the contiguous united states underlying at least 6 of the land area in depth analysis of three states showed that soft weathered bedrock at the 1 m depth underlies 22 of the total land area in california 33 in wyoming and 18 in north carolina soft weathered bedrock hosts pedogenic activity as indicated by morphological features such as roots clay films and iron fe manganese mn oxide concretions recorded in pedon descriptions in the database the physical and chemical properties of soft weathered bedrock are often similar to those of the overlying soil suggesting that in many respects soft weathered bedrock behaves like soil it supplies water and nutrients to plants whose roots penetrate into it and it modulates throughflow runoff to streams for a more complete understanding of soft weathered bedrock systematic data are needed on its thickness across landscapes and a consistent terminology for its various forms needs to be adopted and widely used copyright 2012 john wiley sons ltd

column collapse and generation of pyroclastic density currents during the a d 79 eruption of vesuvius the role of pyroclast density the plinian columns formed during the magmatic phase of the a d 79 eruption of vesuvius alternated several times between fully stable buoyantly rising regimes and unstable regimes of partial or total collapse six pyroclastic density currents pdcs were produced during unstable regimes and ultimately caused the destruction of roman towns around the volcano through new measurements of juvenile clast density and estimations of ascent parameters we show that four partial collapses were likely triggered by increases in the abundance of dense juvenile clasts within the eruptive column in contrast the total collapse probably occurred in response to an increase in the wall rock content injected into the plume during a progressive widening of the conduit a sixth low energy small collapse resulted from high abundances in both dense juvenile clasts and wall rock material simulations of eruption column behavior already account for the effects of variations in conduit radius mass discharge rate and particle size but have yet to include variable clast density and wall rock abundance that cause temporal variations in plume density our results suggest that both parameters can exert a significant control on the potential for generation of pdcs

shallow vent architecture during hybrid explosive effusive activity at cordon caulle chile 2011 12 evidence from direct observations and pyroclast textures in june 2011 an eruption of rhyolite magma began at the puyehue cordon caulle volcanic complex southern chile by january 2012 explosive activity had declined from sustained pyroclastic plinian to sub plinian fountaining to mixed gas and ash jetting punctuated by vulcanian blasts this explosive activity was accompanied by synchronous effusion of obsidian lava in a hybrid explosive effusive eruption fortuitous climatic conditions permitted ground based observation and video recording of transient vent dynamics as well as real time collection of proximal juvenile ash as it sedimented from the active plume the main eruptive vent complex and site of lava effusion was represented by two loci of vulcanian blasts within a single tephra cone containing a pancake shaped proto lava dome these blast loci each consisted of clusters of sub vents that expressed correlated shifts in eruption intensity indicating the presence of partially connected and or branching zones of high permeability within the upper conduit pyroclast textures were examined by x ray computed microtomography and their permeability was modeled by lattice boltzmann simulations the porosity 39 to 67 and darcian permeability 3 1x10 15 m2 perpendicular to fabric to 3 8x10 11 m2 parallel to fabric of fine ash emitted during ash jetting indicates that the permeable zones comprised highly sheared tube like bubbly magma and contrast with the low porosity 17 and nul permeability of bombs ejected to hundreds of metres from the vent in vulcanian blasts residual h2o content of ash 0 14 wt and two bombs 0 2 0 25 wt determined by karl fischer titration indicate degassing of this pyroclastic material to near atmospheric pressures ash textures and simple degassing vesiculation models indicate the onset of permeability by ductile processes of shear enhanced bubble coalescence in the upper 1 to 1 5 km of the conduit repeated ash jetting and vulcanian blasts indicate that such ductile processes were not sufficiently effective to accommodate all degassing requirements and additional brittle e g fragmentation events transiently assisted in maintaining an open system degassing regime that permitted long lived hybrid explosive effusive activity the eruption at cordon caulle has provided an excellent opportunity to closely observe key processes of silicic volcanism

assessing lithologic discontinuities and parent material uniformity within the texas sandy mantle and implications for archaeological burial and preservation potential in upland settings alfisols within the texas gulf coast plain commonly exhibit textural contrasts between sandy artifact bearing a e horizons i e sandy mantle and artifact sterile clay rich bt argillic horizons this has invoked debate about parent material uniformity and pedogenic versus geomorphic sandy mantle origins which has implications for the scientific value of buried archaeological sites to improve our understanding of archaeological burial in upland settings we evaluated parent material uniformity within five pedons to distinguish pedogenically derived textural changes from geomorphologically created lithologic discontinuities depth trends in clay free particle size classes and stable immobile ti and zr constituents failed to reveal lithologic discontinuities between the sandy mantle and bt horizons and the observed textural differences are interpreted to have resulted from pedogenic processes this interpretation is supported by clay skins fine clay increases in bt horizons and micromorphological observations consequently artifacts buried in upland summits have likely moved down the soil profile due to biomantle processes deep sandy mantle sites non parallel contacts between the sandy mantle and bt horizons and paleogullies incised into eocene bedrock are better explained by colluvial soil creep processes adjacent to summits where archaeological materials may exhibit preservation potential no single explanation can account for sandy mantle origins and we advocate a case by case approach

field based density measurements as tool to identify preeruption dome structure set up and first results from unzen volcano japan for an improvement in the quality of conduit flow and dome related explosive eruption models knowledge of the preeruption or precollapse density of the rocks involved is necessary as close investigation is impossible during eruption the best substitute comes from quantitative investigation of the eruption deposits the porosity of volcanic rocks is of primary importance for the eruptive behaviour and accordingly a key parameter for realistic models of dome stability and conduit flow fortunately this physical property may be accurately determined via density measurements we developed a robust battery powered device for rapid and reliable density measurements of dry rock samples in the field the density of the samples sealed in plastic bags at 250 mbar is determined using the archimedean principle we have tested the device on the deposits of the 1990 1995 eruption of unzen volcano japan short setup and operation times allow up to 60 measurements per day under fieldwork conditions the rapid accumulation of correspondingly large data sets has allowed us to acquire the first statistically significant data set of clast density distribution in block and ash flow deposits more than 1100 samples with a total weight of 2 2 tons were measured the data set demonstrates that the deposits of the last eruptive episode at unzen display a bimodal density distribution with peaks at 2 0 0 1 and 2 3 0 1 g cm3 corresponding to open porosity values of 20 and 8 vol respectively we use this data set to link the results of laboratory based fragmentation experiments to field studies at recently active lava domes

geophysical imaging of the inner structure of a lava dome and its environment through gravimetry and magnetism volcanic lava domes are compound edifices resulting from complex growth processes including intrusion and extrusion phases explosions and collapses here we present the study of a complex volcanic system located in the chaine des puys volcanic field french massif central france and centred on the puy de dome volcano an 11 000 years old volcano our approach is based on a morpho structural analysis of a high resolution dtm 0 5 m and geophysical imaging methods both gravity and magnetic high resolution surveys have been carried out on the lava dome and the nearby volcanic structures we computed 3d inverse and 2d forwards models based on our current knowledges about volcanic dome structure the geophysical models allow us to propose a synthetic geological model of the inner structure of the puy de dome and surrounding areas this model suggests a scenario for the formation of the lava dome and the inferred intrusions located on both sides the puy de dome could possibly be the southern tip of the northern intrusion

dynamics of an unusual cone building trachyte eruption at pu u wa awa a hualālai volcano hawai i the pu u wa awa a pyroclastic cone and pu u anahulu lava flow are two prominent monogenetic eruptive features assumed to result from a single eruption during the trachyte dominated early post shield stage of hual lai volcano hawai i pu u wa awa a is composed of complex repetitions of crudely cross stratified units rich in dark dense clasts which reversely grade into coarser pumice rich units pyroclasts from the cone are extremely diverse texturally ranging from glassy obsidian to vesicular scoria or pumice in addition to fully crystalline end members the 100 m thick pu u anahulu flow is in contrast entirely holocrystalline using field observations coupled with whole rock analyses this study aimed to test whether the pu u wa awa a tephra and pu u anahulu lava flows originated from the same eruption as had been previously assumed crystal and vesicle textures are characterized along with the volatile contents of interstitial glasses to determine the origin of textural variability within pu u wa awa a trachytes e g magma mixing vs degassing origin we find that 1 the two eruptions likely originated from distinct vents and magma reservoirs despite their proximity and similar age 2 the textural diversity of pyroclasts forming pu u wa awa a can be fully explained by variable magma degassing and outgassing within the conduit 3 the pu u wa awa a cone was constructed during explosions transitional in style between violent strombolian and vulcanian involving the formation of a large cone and with repeated disruption of conduit plugs but without production of large pyroclastic density currents pdcs and 4 the contrasting eruption styles of hawaiian trachytes flow cone and pdc forming are probably related to differences in the outgassing capacity of the magmas prior to reaching the surface and not in intrinsic compositional or temperature properties these results further highlight that trachytes are kinetically faster magmas compared to dacites or rhyolites likely degassing and crystallizing more rapidly

kinematics of fault related folding derived from a sandbox experiment we analyze the kinematics of fault tip folding at the front of a fold and thrust wedge using a sandbox experiment the analog model consists of sand layers intercalated with low friction glass bead layers deposited in a glass sided experimental device and with a total thickness h 4 8 cm a computerized mobile backstop induces progressive horizontal shortening of the sand layers and therefore thrust fault propagation active deformation at the tip of the forward propagating basal decollement is monitored along the cross section with a high resolution ccd camera and the displacement field between pairs of images is measured from the optical flow technique in the early stage when cumulative shortening is less than about h 10 slip along the decollement tapers gradually to zero and the displacement gradient is absorbed by distributed deformation of the overlying medium in this stage of detachment tip folding horizontal displacements decrease linearly with distance toward the foreland vertical displacements reflect a nearly symmetrical mode of folding with displacements varying linearly between relatively well defined axial surfaces when the cumulative slip on the decollement exceeds about h 10 deformation tends to localize on a few discrete shear bands at the front of the system until shortening exceeds h 8 and deformation gets fully localized on a single emergent frontal ramp the fault geometry subsequently evolves to a sigmoid shape and the hanging wall deforms by simple shear as it overthrusts the flat ramp system as long as strain localization is not fully established the sand layers experience a combination of thickening and horizontal shortening which induces gradual limb rotation the observed kinematics can be reduced to simple analytical expressions that can be used to restore fault tip folds relate finite deformation to incremental folding and derive shortening rates from deformed geomorphic markers or growth strata

explosive to effusive transition during the largest volcanic eruption of the 20th century novarupta 1912 alaska silicic volcanic eruptions commonly show abrupt shifts between powerful and dangerous plinian explosive episodes and gentle effusion of lava whether the onset of magma permeability and ensuing gas loss controls these transitions has been a subject of debate we measured porosities and permeabilities in samples from the a d 1912 eruption of novarupta volcano alaska and analyzed them within the context of a well constrained eruptive sequence that encompasses sustained explosive and effusive activity for the explosive samples we find that the degree of vesicle interconnectivity measured as the ratio of connected to total porosity decreases with phenocryst content and with increasing eruption intensity permeabilities of explosive samples show a weak dependence on porosity dome samples are not significantly different in permeability but are of lower porosity which together with abundant flattened vesicles is consistent with bubble collapse by permeable outgassing quantitative analysis indicates that outgassing alone was insufficient to affect the transition to effusive activity rather the change from explosive to effusive activity was probably a consequence of high versus low magma ascent rates

petrologic constraints on the decompression history of magma prior to vulcanian explosions at the soufriere hills volcano montserrat abstract a series of 88 vulcanian explosions occurred at the soufriere hills volcano montserrat between august and october 1997 conduit conditions conducive to creating these and other vulcanian explosions were explored via analysis of eruptive products and one dimensional numerical modeling of magma ascent through a cylindrical conduit the number densities and textures of plagioclase microlites were documented for twenty three samples from the events the natural samples all show very high number densities of microlites and 50 by number of microlites have areas 2 pre explosion conduit conditions and decompression history have been inferred from these data by comparison with experimental decompressions of similar groundmass compositions our comparisons suggest quench pressures 13 75 mpa each during ascent from chamber to surface values are consistent with field studies of the same events and statistical analysis of explosion time series data the microlite volume number density trend with depth reveals an apparent transition from growth dominated crystallization to nucleation dominated crystallization at pressures of 7 mpa and lower a concurrent sharp increase in bulk density marks the onset of significant open system degassing apparently due to a large increase in system permeability above 70 vesicularity this open system degassing results in a dense plug which eventually seals the conduit and forms conditions favorable to vulcanian explosions the corresponding inferred depth of overpressure at 250 700 m near the base of the dense plug is consistent with depth to center of pressure estimated from deformation measurements here we also illustrate that one dimensional models representing ascent of a degassing crystal rich magma are broadly consistent with conduit profiles constructed via our petrologic analysis the comparison between models and petrologic data suggests that the dense conduit plug forms as a result of high overpressure and open system degassing through conduit walls

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

abrupt shifts between wet and dry phases of the 1875 eruption of askja volcano microscopic evidence for macroscopic dynamics abstract the eruption of askja in 1875 was a powerful eruption with intervals of sustained activity yet also with abrupt shifts in eruption style shifts from dry to wet fall from buoyant to collapsing column conditions and sharp fluctuations of eruption intensity occurred throughout the duration of the eruption deep ascent processes including decompression bubble nucleation and early growth of bubbles were similar among the four main phases regardless of style or column conditions uniformly high vesicle number densities 8 0 108 to 2 109 cm 3 suggest that magma degassing throughout the eruption was a disequilibrium process with very high nucleation rates power law distributions of the subpopulations of small bubbles on cumulative number density plots suggest continuous nucleation of bubbles shallow processes of bubble growth and magma ascent were at times decoupled from deep ascent processes at the onset of each phase the erupted magma appears to have a mature vesicle signature suggesting extended residence times in the shallow conduit irrespective of decompression or magma ascent rate during each phase the textural and density data suggest more uniform conditions of ascent and shallow degassing two pauses in the eruption were accompanied by shifts in vent position and we suggest that magma during this eruption was intruded in an elongated dike like geometry and vent migrations between phases led to the eruption of magma that had slightly greater residence time and hence more mature textures the major yet reversible shifts between dry or magmatic and wet or phreatomagmatic eruption styles were driven by changing external conditions accompanying the shifts in vent position the migration of the vent into and out of water sources rather than by changes of mass flux the shift between buoyant phreatoplinian wet plume conditions to the production of wet density currents is inferred to be a function of vent widening during the high intensity phreatoplinian phase thereby decreasing the exit velocity of the wet mixture in terms of the role of external water in the phreatoplinian phase our qualitative textural observations and quantitative data suggest that the magma was a foam prior to fragmentation however presently we cannot resolve the exact role and timing of the interaction with the external water

the generation of gas overpressure in volcanic eruptions abstract observations of natural eruption products show that different parts of a single magma batch may experience different degassing histories during ascent in a volcanic conduit in non explosive eruptions lava issuing from a volcanic vent may contain overpressured gas bubbles these important features of volcanic eruptions cannot be accounted for by existing flow models which rely on simplifying hypotheses for the relationship between pressures in the gas phase and in the bulk flow volcanic flows involve highly compressible material which undergoes large viscosity variations as degassing proceeds we show that these properties may lead to large gas overpressures in erupting lava the magnitude of this overpressure depends on the initial volatile content of magma and is largest for relatively volatile poor magmas due to the extreme viscosity variations at water contents less than 1 wt we develop a simple analytical model to illustrate the main features of compressible viscous flows 1 at any level gas pressure is larger near the conduit axis than at the walls 2 gas overpressure is an increasing function of mass discharge rate

hydrology of a forested hillslope during storm events abstract subsurface flow from a 0 47 ha hillslope subcatchment at the upper most reach of the west fork 38 ha of walker branch watershed was monitored throughout 1988 stormflow in the first order stream draining the west fork responded simultaneously with the initiation of subsurface flow through 0 to 2 5 m depth from the hillslope subsurface flow from the upper hillslope was only 1 3 of total precipitation for 1988 however individual storms yielded as much as 12 5 of precipitation as subsurface flow the west fork had 10 4 of precipitation observed as storm flow with an additional 29 as baseflow individual storms exhibited up to 71 of precipitation as storm flow in the west fork illustrating the significant increase in percent rain converted to subsurface flow in lower landscape positions as compared to upper hillslopes peak subsurface flow preceded peak streamflow and occurred primarily through the 1 0 2 5 m depth bt2 horizons due to perched water tables preferential flow from hillslopes through macro and mesopores was concluded to be the predominant stormflow mechanism the contributions of the various size pores was reasoned to be dependent upon the position within the soil profile and the stage of the hydrograph preferential flow occurred at extreme soil water conditions with the response due to perched water tables under wet conditions and believed to be due to hydrophobic conditions under extremely dry conditions intermediate soil water conditions resulted in infiltration into soil matrix pores precluding subsurface stormflow

a nanolite record of eruption style transition microlite characteristics have been successfully used as indicators of magma decompression rate however microlites do not record the explosive effusive transitions in sub plinian eruptions when such transitions are governed by the decrease of exsolved volatile pressure in the shallow conduit rather than by the magma ascent rate to overcome this limitation we studied the nanolites in the quenched products of the 2011 shinmoedake japan eruption nanolites are nanometer scale components of the groundmass minerals and exhibit a steeper slope of crystal size distribution than that of microlites in the 2011 eruption the style of activity had undergone transformation from sub plinian eruption to vulcanian explosion and intermittent effusion of lava we found that although the products formed by different eruptive activities have similar microlite characteristics such products can be distinguished clearly by their mineral assemblage of nanolites the samples of pumice of sub plinian eruptions and vulcanian explosions and the dense juvenile fragments of lava in descending order of explosivity contained respectively nanolites of low ca pyroxene low ca pyroxene plagioclase and low ca pyroxene plagioclase fe ti oxides nanolites crystallize when undercooling of the magma due primarily to dehydration increases rapidly near the surface the water contents of the interstitial glass indicate that the quenched depths did not differ greatly between eruption styles hence the different nanolite assemblages of each eruption style are assumed to have resulted from differences in magma residence time near the surface therefore we propose that nanolites have the potential to indicate the physicochemical conditions of magma at the transition points of eruption styles

fossil rootlet biopores as conduits for contaminant transport through clay horizons a case study of dnapl behaviour in severn alluvium uk this paper presents a case study of several dnapl contaminated sites around the severn estuary uk where a combination of detailed observations core dissections and physicochemical characterisation of alluvial clay silt horizons have revealed the presence of fossil rootlet biopores which act and have the potential to act as conduits for contaminant migration through up to 13 m of clay silt the biopores are shown to penetrate the low permeability k 10 10 m s clay silt matrix throughout its entire depth up to 13 m and provide a preferential transport pathway for dnapls from near surface to the underlying aquifer with particularly high concentrations measured in the biopores themselves capillary rise experiments with coal tar distillate demonstrate that dnapls are drawn into biopores with values of surface interfacial tension for the system calculated as 8 10 2 j m2 wicking by residual plant fibres was demonstrated qualitatively and is thought to be an important additional transport mechanism the dnapl contamination below and throughout the severn alluvial clay silt horizons demonstrates that the assumption that these horizons act as an effective seal protecting underlying aquifers from severe pollution from the legacy sites around the severn is flawed and highlights the failure of current protocols for sampling of clay horizons for hydraulic conductivity assessments since current methods can destroy delicate in situ biopore structures the study demonstrates that the interplay between ecological and the geological depositional environment can at a regional scale result in a network of biopores that can potentially act as conduits for contaminant transport

rheological change and degassing during a trachytic vulcanian eruption at kilian volcano chaine des puys france magma ascent during silicic dome forming eruptions is characterized by significant changes in magma viscosity permeability and gas overpressure in the conduit these changes depend on a set of parameters such as ascent rate outgassing and crystallization efficiency and magma viscosity which in turn may influence the prevailing conditions for effusive versus explosive activity here we combine chemical and textural analyses of tephra with viscosity models to provide a better understanding of the effusive explosive transitions during vulcanian phases of the 9 4 ka eruption of kilian volcano chaine des puys france our results suggest that effusive activity at the onset of vulcanian episodes at kilian volcano was promoted by i rapid ascent of initially crystal poor and volatile rich trachytic magma ii a substantial bulk and melt viscosity increase driven by extensive volatile loss and crystallization and iii efficient degassing outgassing in a crystal rich magma at shallow depths trachytic magma repeatedly replenished the upper conduit and variations in the amount of decompression and cooling caused vertical textural stratification leading to variable degrees of crystallization and outgassing outgassing promoted effusive dome growth and occurred via gas percolation through large interconnected vesicles fractures and tuffisite veins fostering the formation of cristobalite in the carapace and talus regions build up of overpressure was likely caused by closing of pore space bubbles and fractures in the dome through a combination of pore collapse cristobalite formation sintering in tuffisite veins and limited pre fragmentation coalescence in the dome or underlying hot vesicular magma sealing of the carapace may have caused a transition from open to closed system degassing and to renewed explosive activity we generalize our findings to propose that the broad spectrum of eruptive styles for trachytic magmas may be inherited from a combination of characteristics of trachytic melts that include high water solubility and diffusivity rapid microlite growth and low melt viscosity compared to their more evolved subalkaline dacitic and rhyolitic equivalents we show that trachytes may erupt with a similar style e g vulcanian but at significantly higher ascent rates than their andesitic dacitic and rhyolitic counterparts this suggests that the periodicity of effusive explosive transitions at trachytic volcanoes may differ from that observed at the well monitored andesitic dacitic and rhyolitic volcanoes which has implications for hazard assessment associated with trachytic eruptions

delayed disequilibrium degassing in rhyolite magma decompression experiments and implications for explosive volcanism recent numerical models and analog shock tube experiments show that disequilibrium degassing during magma ascent may lead to violent vesiculation very near the surface in this study a series of decompression experiments using crystal free rhyolite melt were conducted to examine the development of large supersaturations due to delayed homogenous spontaneous bubble nucleation melts were saturated at 900 c and 200 mpa with either 5 2 wt dissolved h2o or with 4 2 wt h2o and 640 ppm co2 and isothermally decompressed at linear rates of either 0 003 0 025 or 8 5 mpa s to final pressures between 25 and 175 mpa additional isobaric saturation experiments 900 c 200 25 mpa using pure h2o or mixed h2o co2 fluids establish reference equilibrium solubility curves values homogenous nucleation is triggered in both h2o only and h2o co2 experiments once the supersaturation pressure pss reaches 120 150 mpa and the melt contains two times its equilibrium water contents bubble number density and nucleation rate depend on the supersaturation pressure with values on the order of 102 cm3 and 1 cm3 s for pss 120 mpa 106 cm3 and 103 105 cm3 s for pss 130 150 mpa and 107 cm3 and 106 cm3 s for pss 160 175 mpa nucleation rates are consistent with classical nucleation theory and infer an activation energy for nucleation of 1 5 10 18 j nucleus a critical bubble radius of 2 10 9 m and an effective surface tension for rhyolite at 5 2 wt h2o and 900 c of 0 10 0 11 n m the long nucleation delay limits the time available for subsequent diffusion such that disequilibrium dissolved h2o and co2 contents persist to the end of our runs the disequilibrium degassing paths inferred from our experiments contrast markedly with the equilibrium or quasi equilibrium paths found in other studies where bubble nucleation occurs heterogenously on crystals or other discontinuities in the melt at low pss homogenous and heterogenous nucleation rates are comparable however as are bubble number densities so that at a given decompression rate it appears that nucleation mechanism rather than nucleation rate determines degassing efficiency by fixing the pressure depth at which vesiculation commences and hence the time available for equilibration prior to eruption although real systems are probably never truly crystal free our results show that rhyolitic magmas containing up to 104 crystals cm3 and perhaps as high as 106 crystals cm3 are controlled by homogenous rather than heterogenous nucleation during ascent

directed blasts and blast generated pyroclastic density currents a comparison of the bezymianny 1956 mount st helens 1980 and soufriere hills montserrat 1997 eruptions and deposits we compare eruptive dynamics effects and deposits of the bezymianny 1956 bz mount st helens 1980 msh and soufriere hills volcano montserrat 1997 shv eruptions the key events of which included powerful directed blasts each blast subsequently generated a high energy stratified pyroclastic density current pdc with a high speed at onset the blasts were triggered by rapid unloading of an extruding or intruding shallow magma body lava dome and or cryptodome of andesitic or dacitic composition the unloading was caused by sector failures of the volcanic edifices with respective volumes for bz msh and shv c 0 5 2 5 and 0 05 km3 the blasts devastated approximately elliptical areas axial directions of which coincided with the directions of sector failures we separate the transient directed blast phenomenon into three main parts the burst phase the collapse phase and the pdc phase in the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into the atmosphere with much of the expansion having an initially lateral component the erupted material fails to mix with sufficient air to form a buoyant column but in the collapse phase falls beyond the source as an inclined fountain and thereafter generates a pdc moving parallel to the ground surface it is possible for the burst phase to comprise an overpressured jet which requires injection of momentum from an orifice however some exploding sources may have different geometry and a jet is not necessarily formed a major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be attributed to shock waves within an overpressured jet or alternatively to dynamic pressures and shocks within the energetic collapse and pdc phases internal shock structures related to unsteady flow and compressibility effects can occur in each phase we withhold judgment about published shock models as a primary explanation for the damage sustained at msh until modern 3d numerical modeling is accomplished but argue that much of the damage observed in directed blasts can be reasonably interpreted to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified pdc this view is reinforced by recent modeling cited for shv in distal and peripheral regions solids concentration maximum particle size current speed and dynamic pressure are diminished resulting in lesser damage and enhanced influence by local topography on the pdc despite the different scales of the blasts devastated areas were respectively 500 600 and 10 km2 for bz msh and shv and some complexity involving retrogressive slide blocks and clusters of explosions their pyroclastic deposits demonstrate strong similarity juvenile material composes 50 of the deposits implying for the blasts a dominantly magmatic mechanism although hydrothermal explosions also occurred the character of the magma fragmented by explosions highly viscous phenocryst rich variable microlite content determined the bimodal distributions of juvenile clast density and vesicularity thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater and laterally from the axial region the proximal stratigraphy of the blast deposits comprises four layers named a b c d from bottom to top layer a is represented by very poorly sorted debris with admixtures of vegetation and soil with a strongly erosive ground contact its appearance varies at different sites due to different ground conditions at the time of the blasts the layer reflects intense turbulent boundary shear between the basal part of the energetic head of the pdc and the substrate layer b exhibits relatively well sorted fines depleted debris with some charred plant fragments its deposition occurred by rapid suspension sedimentation in rapidly waning high concentration conditions layer c is mainly a poorly sorted massive layer enriched by fines with its uppermost part laminated created by rapid sedimentation under moderate concentration weakly tractive conditions with the uppermost laminated part reflecting a dilute depositional regime with grain by grain traction deposition by analogy to laboratory experiments mixing at the flow head of the pdc created a turbulent dilute wake above the body of a gravity current with layer b deposited by the flow body and layer c by the wake the uppermost layer d of fines and accretionary lapilli is an ash fallout deposit of the finest particles from the high rising buoyant thermal plume derived from the sediment depleted pyroclastic density current the strong similarity among these eruptions and their deposits suggests that these cases represent similar source transport and depositional phenomena

bnd bubble number density decompression rate meter for explosive volcanic eruptions abstract this decade has been witness to substantial progress in the understanding of bubble dynamics and its role in volcanic eruptions a comparison between the results of experiments and a model for bubble nucleation shows that bubble number density bnd can be calculated as a function of decompression rate for given physical properties such as the diffusivity of water in a silicate melt and interfacial tension within an acceptable margin of error conversely in this paper we propose a method hereafter referred to as bnd decompression rate meter to estimate the decompression rate by using bnd data of natural pumice samples from volcanic eruptions as a result of the application of the bnd decompression rate meter to pumice and scoria from explosive eruptions it is found that the decompression rates increase in the range from 10 6 to 10 8 pa s with the eruption column heights these rates are proportional to the 4th power of the eruption column heights and are linear to the discharge rates the absolute values of the estimated decompression rates are very high and cannot be explained by the existing models of conduit flows in order to explain such high rates of decompression and the correlation with discharge rate we propose a possible model according to which the bubble nucleation propagates downward as a rarefaction shock wave in the conduit

breadcrust bombs as indicators of vulcanian eruption dynamics at guagua pichincha volcano ecuador vulcanian eruptions are common at many volcanoes around the world vulcanian activity occurs as either isolated sequences of eruptions or as precursors to sustained explosive events and is interpreted as clearing of shallow plugs from volcanic conduits breadcrust bombs characteristic of vulcanian eruptions represent samples of different parts of these plugs and preserve information that can be used to infer parameters of pre eruption magma ascent the morphology and preserved volatile contents of breadcrust bombs erupted in 1999 from guagua pichincha volcano ecuador thus allow us to constrain the physical processes responsible for vulcanian eruption sequences of this volcano morphologically breadcrust bombs differ in the thickness of glassy surface rinds and in the orientation and density of crack networks thick rinds fracture to create deep widely spaced cracks that form large rectangular domains of surface crust in contrast thin rinds form polygonal networks of closely spaced shallow cracks rind thickness in turn is inversely correlated with matrix glass water content in the rind assuming that all rinds cooled at the same rate this correlation suggests increasing bubble nucleation delay times with decreasing pre fragmentation water content of the melt a critical bubble nucleation threshold of 0 4 0 9 wt water exists below which bubble nucleation does not occur and resultant bombs are dense at pre fragmentation melt h2o contents of 0 9 wt only glassy rinds are dense and bomb interiors vesiculate after fragmentation for matrix glass h2o contents of 1 4 wt rinds are thin and vesicular instead of thick and non vesicular a maximum measured h2o content of 3 1 wt establishes the maxi mum pressure 63 mpa and depth 2 5 km of magma that may have been tapped during a single eruptive event more common h2o contents of 1 5 wt suggest that most eruptions involved evacuation of 1 5 km of the conduit as we expect that substantial overpressures existed in the conduit prior to eruption these depth estimates based on magmastatic pressure are maxima moreover the presence of measurable co2 17 ppm in quenched glass of highly degassed magma is inconsistent with simple models of either open or closed system degassing and leads us instead to suggest re equilibration of the melt with gas derived from a deeper magmatic source together these observations suggest a model for the repeated vulcanian eruptions that includes 1 evacuation of the shallow conduit during an individual eruption 2 depressurization of magma remaining in the conduit accompanied by open system degassing through permeable bubble networks 3 rapid conduit re filling and 4 dome formation prior to the subsequent explosion an important part of this process is densification of upper conduit magma to allow repressur ization between explosions at a critical overpressure trapped pressurized gas fragments the nascent impermeable cap to repeat the process

significance of 87 sr 86 sr ratios in the merensky cyclic unit of the bushveld complex in a number of mafic layered intrusions layers with platinum group element pge mineralization coincide with breaks in cumulus mineral composition and isotopic variation trends through the layered sequence1 2 for example in the bushveld complex in south africa abrupt increases in initial 87sr 86sr ratios occur at the ug 2 and merensky pegmatoid or reef horizons both of which are exploited for pges we report here the results of a detailed sr isotope study across the merensky unit and its immediate foot and hangingwall rocks which when considered together with new ideas on processes operating in magma chambers3 5 suggest that magma mixing and post cumulus infiltration of liquids displaced from below were important during crystallization of the merensky pegmatoid

the plinian phase of the campanian ignimbrite eruption phlegrean fields italy evidence from density measurements and textural characterization of pumice textural characterization of pumice clasts from explosive volcanic eruptions provides constraints on magmatic processes through the quantification of crystal and vesicle content size shape vesicle wall thickness and the degree of interconnectivity the plinian fallout deposit directly underlying the campanian ignimbrite ci eruption represents a suitable case to investigate pumice products with different textural characteristics and to link the findings to processes accompanying conduit magma ascent to the crater the deposit consists of a lower lfu and upper ufu pumice lapilli bed generated by the sub steady eruption of trachytic magma with 0 90 with vesicle number density ranging from 107 108 cm 3 the degree of vesicle coalescence is high for all pumice types with interconnected vesicles generally representing more than 90 of the bulk vesicle population the results show a high degree of heterogeneous textures among pumice clasts from both phases of the eruption and within each eruption phase the different pumice types and also within each single pumice type fragment the origin of pumice clasts with different textural characteristics is ascribed to the development of conduit regions marked by different rheological behavior the conclusions of this study are that vesicle deformation degree of coalescence and intense shear at the conduit walls play a major role on the degassing process hence affecting the entire conduit dynamics

the mechanism of volcanic eruptions a steady state approach a theory of a volcanic eruption as a steady magma flow from a magma chamber to the surface is described magma is considered as a two component two phase medium and magma flow is steady one dimensional and isothermal mass and momentum exchange between condensed and gaseous phases is assumed to be in equilibrium with nucleation of bubbles as a fast process at small oversaturations three basic flow structures in a conduit are recognized and three resulting types of eruption are described 1 in the discrete gas separation dgs flow melt with discrete gas bubbles erupts this type includes lava eruptions and strombolian activity 2 in the dispersion regime there is continuous eruption of a gas pyroclastic suspension catastrophic explosive eruptions are included in this type 3 here a partly destroyed foam is erupted with permeable structure that allows gas escape near the surface this structure evolves with an increase in permeability and a decrease in the total porosity this type corresponds to lava dome extrusions a criterion dividing dgs flow from the other two regimes is obtained di u n1 3a c0 where u is the magma ascent velocity is magma viscosity n is number of bubble nuclei in a unit mass of magma a is the solubility coefficient for volatiles in magma and c0 is the volatile content the boundary between the dispersion regime and the partly destroyed foam regime depends on u c0 and particle size dp for the dispersion regime the basic governing parameters are magma chamber depth h conduit conductivity b2 b is the characteristic cross dimension of the conduit and excess pressure in the magma chamber instability of the dispersion regime is investigated numerically the dependence of magma discharge rate on the governing parameters is described as a combination of equilibrium surfaces with cusp catastrophes magma chamber depth is a splitting parameter in most cusps and controls the existence of unstable magma flow with sharp regime changes catastrophic rise of eruption intensity is possible if h hcr the critical chamber depth hcr depends approximately linearly on c0 and is about 15 km at c0 0 05 some scenarios of eruption evolution are described in a steady state approximation and the mechanism of catastrophic explosive eruptions is proposed the theory is applied to well documented eruptions tolbachik 1975 1976 and mount st helens 1980 the theory can also describe changes in a volcanic system such as destruction of the conduit or chamber walls destruction of the edifice changes in magma composition and caldera subsidence due to magma evacuation

grand sarcoui volcano chaine des puys massif central france a case study for monogenetic trachytic lava domes abstract the grand sarcoui is a prominent trachytic volcano of the intraplate quaternary volcanic field of chaine des puys massif central france which fulfills basic requirements for being qualified as monogenetic grand sarcoui looks like a simple axisymmetric lava dome but close observation reveals a complex and dissymmetric structure and composition the construction of the dome about 12 5 ka ago combined both endogenous and exogenous growth which resulted in variable modes of emplacement and textures of the lava one of its most interesting features is a large 0 29 106 m2 fan of deposits bearing hummocks and secondary hydro eruption craters cross sections of these deposits demonstrate that they originated from a sector collapse accompanied by a blast like event the dome is covered by a thin layer of lapilli and ash attributed to a delayed summit eruption which occurred about 10 6 ka ago surprisingly late after its construction so this volcano has at a reduced scale features that are more usually observed in large composite volcanoes however some of these features differ slightly from those that have been documented to date and they remain partly unexplained this shows that monogenetic well preserved trachytic lava domes are uncommon and poorly known unlike rhyolitic andesitic and dacitic domes

soil physicochemical properties and moisture dynamics of a large soil profile in a tropical monsoon forest abstract evergreen forests in tropical monsoon asia experience a severe dry season lasting several months deep soil moisture is responsible for evergreen forest transpiration during this season this study provides preliminary information on soil physicochemical properties in a large soil profile 800 cm deep and evaluated the influences of deep soil moisture dynamics on the transpiration of evergreen forests in cambodia using a water flow model in general changes in soil physicochemical properties with depth were small excluding topsoil the soil texture in the large soil profile was characterized by very little gravel and a large sand fraction the soil carbon stock was 71 7 and 130 1 mg c ha 1 at depths of 0 150 and 0 800 cm respectively the coarse pore volume 100 k s clearly decreased with depth from 100 to 600 cm and were extremely low at 600 700 cm in the water flow simulation a rooting depth deeper than 400 cm was necessary for the evergreen forest to maintain transpiration during the dry season on the other hand when the k s of deep soil 550 750 cm depth was assumed to be higher than that estimated by the cylinder sample a rooting depth of 800 cm was necessary since most of the soil water that was replenished during the rainy season was rapidly lost during the dry season also it was necessary for evergreen forests to effectively use soil water within the deep root zone 400 800 cm depth the compacted deep soil horizons played an important role in maintaining the moist conditions in the root zone of an evergreen tropical monsoon forest in asia that experienced a severe dry season

quantifying diffusive mass transfer in fractured shale bedrock a significant limitation in defining remediation needs at contaminated sites often results from an insufficient understanding of the transport processes that control contaminant migration the objectives of this research were to help resolve this dilemma by providing an improved understanding of contaminant transport processes in highly structured heterogeneous subsurface environments that are complicated by fracture flow and matrix diffusion our approach involved a unique long term steady state natural gradient injection of multiple tracers with different diffusion coefficients br he ne into a fracture zone of a contaminated shale bedrock the spatial and temporal distribution of the tracers was monitored for 550 days using an array of groundwater sampling wells instrumented within a fast flowing fracture regime and a slow flowing matrix regime the tracers were transported preferentially along strike parallel fractures with a significant portion of the tracer plumes migrating slowly into the bedrock matrix movement into the matrix was controlled by concentration gradients established between preferential flow paths and the adjacent rock matrix observed differences in tracer mobility into the matrix were found to be a function of their free water molecular diffusion coefficients the multiple tracer technique confirmed that matrix diffusion was a significant process that contributed to the overall physical nonequilibrium that controlled contaminant transport in the shale bedrock the experimental observations were consistent with numerical simulations of the multitracer breakthrough curves using a simple fracture flow model the simulated results also demonstrated the significance of contaminant diffusion into the bedrock matrix the multiple tracer technique and ability to monitor the fracture and matrix regimes provided the necessary experimental constraints for the accurate numerical quantification of the diffusive mass transfer process the experimental and numerical results of the tracer study were also consistent with indigenous contaminant discharge concentrations within the fracture and matrix regimes of the field site these findings suggest that the secondary source contribution of the bedrock matrix to the total off site transport of contaminants is relatively large and potentially long lived

magma degassing during the plinian eruption of novarupta alaska 1912 1 we have modeled the nucleation and isothermal growth of bubbles in dacite from the 1912 plinian eruption of novarupta alaska bubble growth calculations account for the exsolution of h2o and co2 beginning with bubble nucleation and ending when bubble sizes reproduced the observed size distribution of vesicles in novarupta pumice clasts assuming classical nucleation theory bubbles nucleated with a diameter of the order of 10 8 m and grew to sizes ranging from 10 6 m to greater than 10 3 m the typical range of vesicle sizes found in novarupta pumice the smallest vesicles in novarupta pumices are also the most abundant and bubbles with radii of 10 6 m to 10 5 m comprise almost 90 of the entire bubble population we find that these bubbles must have nucleated and grown to their final size within a few 100 milliseconds despite these extremely fast growth rates the pressures of exsolved volatiles contained within the bubbles remained high up to about 107pa in excess of ambient pressure assuming a closed system the potential energy of these compressed volatiles was sufficient to cause magma fragmentation even though only a fraction of the pre eruptive volatiles had exsolved unless the matrix glasses of novarupta pyroclasts retains a large fraction of pre eruptive volatiles the majority of magmatic volatiles 80 90 was likely lost by open system degassing between magma fragmentation and quenching

kinetics of bubble nucleation in a rhyolitic melt an experimental study of the effect of ascent rate in order to characterize the effect of ascent rate on the kinetics of bubble nucleation in a rhyolitic magma we performed three series of experiments decompressed at rates of either 1000 167 or 27 8 kpa s the experiments were carried out in an externally heated pressure vessel at 800 c and in the pressure range 260 59 mpa the starting material was a crystal free and bubble free rhyolitic glass containing 7 0 wt dissolved h2o in all the decompression experiments homogeneous bubble nucleation began at 90 2 mpa that is 150 mpa below the water saturation pressure of the silicate liquid 240 mpa the degree of supersaturation phon required to trigger homogeneous bubble nucleation was almost independent of decompression rate phon is the difference between the saturation pressure and the nucleation pressure nucleation pressure decreased by 3 mpa for a 36 fold increase in decompression rate these results are in good agreement with the classical theory of nucleation assuming a rhyolite h2o surface tension of 0 106 n m 1 our major experimental finding is that after a short nucleation event the nucleation rate dropped and the bubble number density n reached a stationary value that was strongly sensitive to decompression rate 6 8 mm 3 at 27 8 kpa s 470 mm 3 at 167 kpa s and 5800 mm 3 at 1000 kpa s the smaller value of n at low decompression rate was compensated by a larger mean bubble size so that at a given pressure vesicularity was almost independent of decompression rate the experimental values of n can be reproduced within a factor 0 3 1 4 using a relationship derived from published numerical simulations of vesiculation in ascending magmas the nucleation behavior in our experiments is dictated by a competition between bubble nucleation and diffusive bubble growth which depletes in water the surrounding liquid and therefore reduces the degree of volatile supersaturation once a critical value of n is attained diffusive bubble growth can keep pace with decompression and prevent the degree of volatile supersaturation in the liquid to increase with decreasing pressure the strong correlation between bubble number density and decompression rate has fundamental volcanological implications if we extrapolate the experimental data to the typical ascent rates of silicic magmas we obtain bubble number densities 10 3 101 mm 3 for homogeneous nucleation 10 1 103 mm 3 for heterogeneous nucleation that are orders of magnitude smaller than those measured in most natural pumices we therefore propose that the large values of n in silicic pumices may be due to two successive nucleation events 1 a first event which occurs relatively deep in the volcanic conduit and which yields a moderate number of bubbles and 2 a second nucleation event yielding a very large number of small bubbles and presumably related to the dramatic increase of decompression rate that precedes fragmentation the small bubble number densities associated with homogeneous nucleation suggest that a strong departure from equilibrium degassing should be the rule even at slow ascent rates

abrupt transitions during sustained explosive eruptions examples from the 1912 eruption of novarupta alaska plinian ignimbrite activity stopped briefly and abruptly 16 and 45 h after commencement of the 1912 novarupta eruption defining three episodes of explosive volcanism before finally giving way after 60 h to effusion of lava domes we focus here on the processes leading to the termination of the second and third of these three episodes early erupted pumice from both episodes show a very similar range in bulk vesicularity but the modal values markedly decrease and the vesicularity range widens toward the end of episode iii clasts erupted at the end of each episode represent textural extremes at the end of episode ii clasts have very thin glass walls and a predominance of large bubbles whereas at the end of episode iii clasts have thick interstices and more small bubbles quantitatively all clasts have very similar vesicle size distributions which show a division in the bubble population at 30 m vesicle diameter and cumulative number densities ranging from 107 109 cm 3 patterns seen in histograms of volume fraction and the trends in the vesicle size data can be explained by coalescence signatures superimposed on an interval of prolonged nucleation and free growth of bubbles compared to experimental data for bubble growth in silicic melts the high 1912 number densities suggest homogeneous nucleation was a significant if not dominant mechanism of bubble nucleation in the dacitic magma the most distinct clast populations occurred toward the end of plinian activity preceding effusive dome growth distributions skewed toward small sizes thick walls and teardrop vesicle shapes are indicative of bubble wall collapse marking maturation of the melt and onset of processes of outgassing the data suggest that the superficially similar pauses in the 1912 eruption which marked the ends of episodes ii and iii had very different causes through episode iii the trend in vesicle size data reflects a progressive shift in the degassing process from rapid magma ascent and coupled gas exsolution to slower ascent with partial open system outgassing as a precursor to effusive dome growth no such trend is visible in the episode ii clast assemblages we suggest that external changes involving failure of the conduit vent walls are more likely to have effected the break in explosive activity at 45 h

eruption dynamics inferred from microlite crystallization experiments application to plinian and dome forming eruptions of mt pelee martinique lesser antilles decompression experiments have been conducted to simulate syn eruptive crystallization in volcanic conduits to infer magma ascent rates and conditions during dome forming and plinian eruptions of silicic arc volcanoes the experiments were carried out on starting material with the composition of mt pele e rhyolitic interstitial melt 76 wt sio2 and consisted of three consecutive steps hydration decompression and annealing hydration saturated and undersaturated was performed at 8508c and 200mpa and was followed by isothermal decompression either linearly or stepwise to a final pressure pf of 30 or 5 10mpa decompression rates range from 0 003 to 25mpamin 1 decompression durations of 15 min to 40 days two samples were cooled by 258c and 508c during a 3 day step at pf subsequent to decompression the samples were held for up to 15 days at pf the experiments generated three types of crystals as products of pre syn and post decompression crystallization the experiments basically differ from previous studies in that they are specifically designed to discriminate crystal nucleation from growth and to evaluate the influence of pre decompression crystals on the decompression induced crystallization the effects of pre decompression crystals decompression rate undercooling pf and terminal cooling have been determined on plagioclase nucleation growth morphology and composition the main results suggest a positive correlation between decompression rate and the number density of plagioclase crystals nucleated at pf and highlight the effect of pre decompression crystals in further decompression induced crystallization the relations between the decompression conditions and the plagioclase characteristics have been used to infer mt pele e eruption dynamics suggesting that 1 plinian magmas ascend from the reservoir within less than 1h 1 10 m s 2 dome and block and ash flow magmas ascend within more than 2 5 days giving time for syn decompression crystallization around pre existent microlites 3 dome magmas provide evidence for long stagnation and cooling at low pressure and 4 surge magmas ascend without significant crystallization within less than 4 days and massively nucleate plagioclase at very low pressure the extent and violence of dome destruction may depend on the size or age of the dome with large or old domes favouring mildly explosive block and ash flows whereas small or young protodomes may generate highly explosive surges

influence of pre eruptive degassing and crystallization on the juvenile products of laterally directed volcanic explosions abstract strikingly similar examples of edifice collapse and directed blast are the 18 may 1980 eruption of mount st helens msh washington usa and the 30 march 1956 eruption of bezymianny volcano bz kamchatka russia in these cases flank failures led to near instantaneous decompression and fragmentation of intra edifice cryptodome magma which produced catastrophic laterally directed blasts in both instances the blast products consisted of juvenile material with bimodal density vesicularity distributions low and high density modes at 1900 and 2400 kg m 3 for bz 1600 and 2300 kg m 3 for msh although the proportion of high density material is greater at bz blast materials also exhibit striking variety in groundmass crystallinity 90 vol despite having fairly uniform pheno crystallinities suggesting that degassing driven groundmass crystallization occurred to varying extents within cryptodome magma at both volcanoes new bulk rock h2o and d measurements confirm that progressive open system outgassing occurred prior to both blasts the correlations between crystallinity clast density and bulk h2o contents suggest that syn blast magma expansion was modulated both by non uniform volatile distribution within the cryptodome and rheological controls associated with non uniform crystal content spatial heterogeneities in volatiles and crystallinity within a given cryptodome are attributed to distance from the wallrock margin which probably correlates with timing of magma injection the greater proportion of high density material at bz is speculatively related to lower blast energy compared with msh

bubble nucleation growth and coalescence during the 1997 vulcanian explosions of soufriere hills volcano montserrat abstract soufriere hills volcano had two periods of repetitive vulcanian activity in 1997 each explosion discharged the contents of the upper 0 5 2 km of the conduit as pyroclastic flows and fallout frothy pumices from a deep gas rich zone lava and breadcrust bombs from a degassed lava plug and dense pumices from a transition zone vesicles constitute 1 66 vol of breadcrust bombs and 24 79 of pumices all those larger than a few tens of m being interconnected small vesicles few hundreds of m in pumices are interpreted as pre dating explosion implying pre explosive conduit porosities up to 55 about a sixth of large vesicles in pumices and all those in breadcrust bombs are angular voids formed by syn explosive fracturing of amphibole phenocrysts an intermediate sized vesicle population formed by coalescence of the small syn explosive bubbles bubble nucleation took place heterogeneously on titanomagnetite number densities of which greatly exceed those of vesicles and growth took place mainly by decompression development of pyroclast vesicle textures was controlled by the time interval between the onset of explosion decompression and surface quench in contact with air lava plug fragments entered the air quickly after fragmentation 10 s so the interiors continued to vesiculate once the rinds had quenched forming breadcrust bombs deeper gas rich magma took longer 50 s to reach the surface and vesiculation of resulting pumice clasts was essentially complete prior to surface quench this accounts for the absence of breadcrusting on pumice clasts and for the textural similarity between pyroclastic flow and fallout pumices despite different thermal histories after leaving the vent it also allowed syn explosive coalescence to proceed further in the pumices than in the breadcrust bombs uniaxial boudinage of amphibole phenocrysts in pumices implies significant syn explosive vesiculation even prior to magma fragmentation probably in a zone of steep pressure gradient beneath the descending fragmentation front syn explosive decompression rates estimated from vesicle number densities 0 3 6 5 mpa s 1 are consistent with those predicted by previously published numerical models

petrophysical properties of saprolites from the oak ridge integrated field research challenge site tennessee at the oak ridge integrated field research challenge site near oak ridge tennessee the shallow saprolitic aquifer is contaminated by nitric acid uranium and metals originating from the former s3 settling ponds to interpret low frequency geophysical methods used to image contaminant plumes we have characterized the petrophysical properties of three representative saprolite core samples their hydraulic conductivity ranges from 10 7 to 10 6 ms 1 in agreement with field data complex conductivity measurements in the frequency range of 1 mhz to 45 khz were performed with nacl solutions with electrical conductivities in the range 5 10 3 to 2 35 sm 1 a range representative of field conditions the electrical conductivity data were well reproduced with a simple linear conductivity model between the saprolite conductivity and the pore water conductivity the conductivity plots were used to estimate the formation factor the cementation exponent was about 2 2 0 3 and the surface conductivity 0 007 0 040 sm 1 the magnitude of the surface conductivity depended on the degree of weathering and therefore on the amount of smectite and mixed layer illite smectite clays present in the saprolite the chargeability of the core samples was in the range of 20to800 mvv 1 and is strongly dependent on the salinity we also performed streaming potential measurements with the same pore fluid composition as that used for the complex conductivity measurements we found an excess of movable electrical charges on the order of 100 to 500 cm 3 in agreement with previous investigations connecting the movable excess charge density to permeability the zeta potential was in the range of 10 to 20mv independent on the salinity the electrical measurements were consistent with an average cation exchange capacity in the range of 1 4 to 11 cmolkg 1 and a specific surface area on the order of 4000 to about 30 000 m 2 kg 1

30 years in the life of an active submarine volcano a time lapse bathymetry study of the kick em jenny volcano lesser antilles effective monitoring is an essential part of identifying and mitigating volcanic hazards in the submarine environment this is more difficult than onshore because observations are typically limited to land based seismic networks and infrequent shipboard surveys since the first recorded eruption in 1939 the kick em jenny kej volcano located 8km off northern grenada has been the source of 13 episodes of t phase signals these distinctive seismic signals often coincident with heightened body wave seismicity are interpreted as extrusive eruptions they have occurred with a recurrence interval of around a decade yet direct confirmation of volcanism has been rare by conducting new bathymetric surveys in 2016 and 2017 and reprocessing 4 legacy datasets spanning 30 years we present a clearer picture of the development of kej through time processed grids with a cell size of 5m and vertical precision on the order of 1 4m allow us to correlate t phase episodes with morphological changes at the volcano s edifice in the time period of observation 7 09x106 m3 of material has been added through constructive volcanism yet 5 times this amount has been lost through landslides limited recent magma production suggests that kej may be susceptible to larger eruptions with longer repeat times than have occurred during the study interval behavior more similar to sub aerial volcanism in the arc than previously thought t phase signals at kej have a varied origin and are unlikely to be solely the result of extrusive submarine eruptions our results confirm the value of repeat swath bathymetry surveys in assessing submarine volcanic hazards

lithologic and pedogenic influences on porosity distribution and groundwater flow in fractured sedimentary saprolite a new application of environmental sedimentology abstract soil and fractured saprolite formed from weathered middle cambrian sedimentary rocks were analyzed from two shallow 2 5 m deep pits located at the oak ridge reservation in eastern tennessee the effects of both parent heterogeneity and pedogenic processes on fracture and matrix porosity were examined using thin section petrography and geochemistry the a and bw horizons of the soil profiles comprise thin 20 60 cm weakly developed loamy inceptisols characterized by high 40 50 matrix porosity subjacent saprolite with pore occluding pedogenic clay crt horizon occurs beneath a saprolite without pedogenic clay cr horizon and varies lithologically depending on parent material limestone siltstone and shale or sandstone the soil and saprolite commonly contain root pores the saprolite additionally contains bedding parallel and bedding perpendicular orthogonal fracture sets the fractures and root holes were initially expected to control bulk or mesoscale hydraulic conductivity of the materials this study indicates however that extensive pedogenic clay and fe mn oxide accumulations in the pores and fractures in the saprolite at depths of 100 250 cm cause porosity reduction that is so extensive it serves to create a low hydraulic conductivity barrier within the upper vadose zone this barrier is known from prior work to cause formation of perched water table conditions and rapid downslope flow during rainfall events but this is the first study to investigate the origin of the low conductivity barrier geochemical mass balance using ti as an immobile index also demonstrates that three distinct weathering environments defined by element translocation patterns occur in each pit which correspond to the soil zone a and bw horizons shallow saprolite cr horizon and deep saprolite crt horizon the geochemical zonation is also consistent with the pedogenic and hydraulic interpretation of the system the potential environmental significance of the zone of pore occlusion is also discussed from the perspective of runoff recharge and contaminant migration

multi stage growth of the trachytic lava dome of the puy de dome chaine des puys france field geomorphological and petro geochemical evidence abstract understanding lava dome eruptions is a major concern in volcanology regarding the assessment of associated hazards this question is usually addressed through the study of active calc alkaline domes however alkaline domes remaining poorly known the puy de dome is the most prominent volcano of the chaine des puys quaternary intraplate volcanic field in the french massif central it is a complex trachytic lava dome ca 11 ka old field investigations geomorphological and petro geochemical studies were performed to understand the magmatic system of this dome and reconstruct its volcanic evolution the different lava facies were fully characterised and their relationships with the volcano building stages have been established a new four step evolution is proposed for the growth of this edifice mineralogical and geochemical variations document magmatic differentiation with self mixing and minor crustal contamination in a zoned reservoir within the upper crust magma mixing and crustal contamination through fluids interactions during magma ascent and the involvement of fumarolic activity throughout the edifice building are also documented further this work highlights the specific features of the puy de dome compared to calc alkaline domes lack of lava flows and almost complete magma degassing during ascent resulting in the lack of significant explosive activity until the last eruption step

micro porous pyroclasts reflecting multi vent basaltic andesite plinian eruptions at mt tongariro new zealand abstract mt tongariro is a multi vent volcanic complex in the central north island of new zealand constructed along the axis of an active graben its five largest known explosive eruption episodes occurred 11 000 years ago during a 200 year long paroxysm during these stable plinian and collapsing sub plinian columns erupted from multiple sequential and synchronous vents along the graben axis we examined the spatially varying eruption dynamics during the 200 year paroxysm through the lens of pyroclastic textures each eruption episode produced micro vesicular basaltic andesitic juvenile lapilli with high vesicle number densities and high groundmass crystallinity textural characteristics indicate that pressurisation via second boiling drove eruptions and rhyolite like melt viscosities promoted brittle failure highly variable and mingled textures within individual juvenile lapilli evidence diverse conditions developed across the conduit during the eruption foamy magma rose rapidly through the conduit core driving explosive vigour and dragged and sheared gas poor crystallised magma from the conduit margins consistent with the northward widening graben the most stable plinian eruptions occurred at northern vents under highest extensional stresses and possibly widest most stable conduits by contrast collapsing unstable eruption columns occurred at southern vents where mainly degassed and crystalline magma erupted at lower ejection rates through narrower unstable and shifting conduits these data collectively show how spatial variations in structure and tectonics may strongly influence explosive eruption style

sequence stratigraphy of an intrashelf basin carbonate ramp to rimmed platform transition maryville limestone middle cambrian southern appalachians middle and upper cambrian strata in the southern appalachians consist of a sequence of six alternating limestone and shale formations conasauga group the conasauga group constitutes part of a thick pericratonic cambro ordovician passive margin sequence along the eastern north american continent the cambrian carbonate platform was flanked by a high relief shelf margin toward the east facing the open ocean while to the west the carbonate platform sloped into an intrashelf basin it is this western shelf margin that is the topic of the present study detailed lithofacies analysis of the middle cambrian maryville limestone along a shelf to basin depositional transect reveals that the shelf evolved from a gently basinward sloping ramp to a rimmed platform fringed with steeper slopes cyanobacterial buildups renalcis girvanella dominated the platform margin environments progradation of the platform occurred toward the craton a process oriented approach has allowed us to define the sequences sequence boundaries and the stacking pattern of the maryville limestone the maryville limestone sequence consists of two depositional subsequences the boundary between the two subsequences is not a sequence boundary because it does not separate rocks deposited in different environmental regimes the two subsequences within the maryville sequence consist of a combination of aggradational retrogradational and progradational units with respect to the platform interior the stacking pattern recognized is the result of variations in sedimentation rate subsidence and eustatic sea level change each of the dominantly carbonate units within the conasauga represents this gradual transition from a ramp like platform to basin transition into a rimmed platform the transition between the maryville limestone middle cambrian and the overlying nolichucky shale late cambrian is a sequence boundary this sequence boundary is both an exposure surface and a drowning unconformity and it marks a distinct shift in the pattern of sedimentation shallow water carbonate deposition terminated at the boundary followed by the onlap of deeper water basinal siliciclastics over shallow carbonates the approach presented here can serve as a useful analogue to other lower paleozoic and possibly younger passive margin sequences

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

mind the gap a classification system for integrating the subsolum into soil surveys abstract as soil surveys were traditionally conducted in support of agriculture soil classification tended to focus on upper soil horizons and their characteristics however when dealing with environmental issues such as vegetation ecology or water quality an integrated knowledge of the soil soil to substratum and deeper substratum continuum is required in both world reference base for soil resources wrb and soil taxonomy st the lower boundary for soil classification is arbitrarily set at 2 m including weathered and continuous rock however as soil classification hinges on diagnostic horizons and characteristics which often occur within the first 100 cm collecting data on the subsolum is often neglected we propose a classification system of the subsolum the structure of which is inspired by wrb we define regolite saprolite saprock and bedrock as four subsolum reference groups corresponding to different weathering stages intergrades of these reference groups can be qualified with principal qualifiers while morphologic and lithologic characteristics can be presented with supplementary qualifiers the proposed subsolum classification system is not intended to substitute geological surveys but rather to complement existing soil classification systems such that at least the whole 2 m can be categorized still whenever desired the system can also be used for deeper materials

contrasting pyroclast density spectra from subaerial and submarine silicic eruptions in the kermadec arc implications for eruption processes and dredge sampling pyroclastic deposits from four caldera volcanoes in the kermadec arc have been sampled from subaerial sections raoul and macauley and by dredging from the submerged volcano flanks macauley healy and the newly discovered raoul sw suites of 16 32 mm sized clasts have been analyzed for density and shape and larger clasts have been analyzed for major element compositions density spectra for subaerial dry type eruptions on raoul island have narrow unimodal distributions peaking at vesicularities of 80 85 whereas ingress of external water wet type eruption or extended timescales for degassing generate broader distributions including denser clasts submarine erupted pyroclasts show two different patterns healy and raoul sw dredge samples and macauley island subaerial emplaced samples are dominated by modes at 80 85 implying that submarine explosive volcanism at high eruption rates can generate clasts with similar vesicularities to their subaerial counterparts a minor proportion of healy and raoul sw clasts also show a pink oxidation color suggesting that hot clasts met air despite 0 5 to 1 km of intervening water in contrast macauley dredged samples have a bimodal density spectrum dominated by clasts formed in a submarine eruptive style that is not highly explosive macauley dredged pyroclasts are also the mixed products of multiple eruptions as shown by pumice major element chemistry and the sea floor deposits reflect complex volcanic and sedimentation histories the kermadec calderas are composite features and wide dispersal of pumice does not require large single eruptions when coupled with chemical constraints and textural observations density spectra are useful for interpreting both eruptive style and the diversity of samples collected from the submarine environment

the influence of conduit processes on changes in style of basaltic plinian eruptions tarawera 1886 and etna 122 bc basaltic volcanism is most typically thought to produce effusion of lava with the most explosive manifestations ranging from mild strombolian activity to more energetic fire fountain eruptions however some basaltic eruptions are now recognized as extremely violent i e generating widespread phreatomagmatic subplinian and plinian fall deposits we focus here on the influence of conduit processes especially partial open system degassing in triggering abrupt changes in style and intensity that occurred during two examples of basaltic plinian volcanism we use the 1886 eruption of tarawera new zealand the youngest known basaltic plinian eruption and the only one for which there are detailed written eyewitness accounts and the well documented 122 bc eruption of mount etna italy and present new grain size and vesicularity data from the proximal deposits these data show that even during extremely powerful basaltic eruptions conduit processes play a critical role in modifying the form of the eruptions even with very high discharge and presumably ascent rates partial open system behaviour of basaltic melts becomes a critical factor that leads to development of domains of largely stagnant and outgassed melt that restricts the effective radius of the conduit the exact path taken in the waning stages of the eruptions varied in response to factors which included conduit geometry efficiency and extent of outgassing and availability of ground water but a relatively abrupt cessation to sustained high intensity discharge was an inevitable consequence of the degassing processes

pre explosive conduit conditions of the 1997 vulcanian explosions at soufriere hills volcano montserrat i pressure and vesicularity distributions an authoritative case of vulcanian eruptive dynamics is the series of 88 explosions that occurred between august and october 1997 at soufriere hills volcano on montserrat island the state of the magmatic column just before a vulcanian explosion is still poorly understood but conditions the eruptive style this study establishes such a pre explosive stratigraphy by 1 documenting the textures covering the range of the 1997 products 2 quantitative analysis of h2o content in interstitial glass measured by karl fischer titration and 3 combining these data with a simple model linking pre and post explosive vesicularities the model shows that syn explosive degassing affects greatly the way porosity evolves by decompression during an explosion the stratigraphy reconstruction shows a three part vertical layering of the conduit prior to explosion with overall denser values than those previously suggested a dense and strongly degassed plug caps the column it is underlain by a shallow transition zone featuring complex mingling between vesicular and dense magma up to 10 mpa at higher pressure up to 80 mpa lies a more homogeneous zone of relatively dense 10 20 vol magma which was emplaced under partly open system degassing this conduit stratigraphy gives the vision of a strongly heterogeneous magma column immediately prior to its disruption our analysis suggests that fragmenting such a composite magma cannot happen in a single coherent pulse but rather as stages the transition zone contains heterogeneous amounts of exsolved gas that could explain the pulsatory nature of the vulcanian jets at the beginning of the explosions this contrasts with the nearly constant vesicularities of the deeper part of the pre explosive magma column which are propitious to a general short lived disruption

dynamics of an open basaltic magma system the 2008 activity of the halema uma u overlook vent kīlauea caldera on march 19 2008 a small explosive event accompanied the opening of a 35 m wide vent overlook vent on the southeast wall of halema uma u crater in k lauea caldera initiating an eruptive period that extends to the time of writing the peak of activity in 2008 consisted of alternating background open system outgassing and spattering punctuated by sudden short lived weak explosions triggered by collapses of the walls of the vent and conduit near daily sampling of the tephra from this open system along with exceptionally detailed observations allow us to study the dynamics of the activity during two eruptive sequences in late 2008 each sequence includes background activity preceding and following one or more explosions in september and october 2008 respectively componentry analyses were performed for daily samples to characterise the diversity of the ejecta nine categories of pyroclasts were identified in all the samples including wall rock fragments the six categories of juvenile clasts can be grouped in three classes based on vesicularity 1 poorly 2 uniformly highly to extremely and 3 heterogeneously highly vesicular the wall rock and juvenile clasts show dissimilar grainsize distributions reflecting different fragmentation mechanisms the wall rock particles formed by failure of the vent and conduit walls above the magma free surface and were then passively entrained in the eruptive plume the juvenile componentry reveals consistent contrasts in degassing and fragmentation processes before during and after the explosive events we infer a crude layering developed in the shallow melt in terms of both rheology and bubble and volatile contents beneath a convecting free surface during background activity a tens of centimetres thick viscoelastic surface layer was effectively outgassed and relatively cool while at depths of less than 100 m the melt remained slightly supersaturated in volatiles and actively vesiculating decoupled metre sized bubbles rising through the column burst through the free surface frequently ejecting fragments of the outgassed upper layer when the surface was abruptly perturbed by the rock falls existing mm sized bubbles expanded leading to the acceleration of adjacent melt upward and consecutive explosions while renewed nucleation created a minor population of 10 micron sized bubbles after each explosive event in september october 2008 this layering was re established but with decreasing vigour suggesting that the magma batch as a whole was becoming progressively depleted in dissolved volatiles

bubble nucleation in rhyolitic melts experiments at high pressure temperature and water content abstract we report the first measurement of bubble nucleation in hydrated rhyolitic melts in response to pressure release two rhyolitic obsidians one containing less than 1 of microlites of fe ti oxides and the other about 20 of various crystals were hydrated at 150 mpa and 780 850 c after saturation was reached 5 3 5 5 wt water pressure was lowered and the samples were allowed to nucleate and grow bubbles for various amounts of time before the final rapid quenching of the experiments the results demonstrate the importance of heterogeneous nucleation microlites of fe ti oxides are very efficient as sites for bubble nucleation in their presence modest nucleation was observed even after decompression by 6 10 8 bubbles cm 3 in the absence of microlites no nucleation occurred at p mpa at p 10 mpa bubbles also nucleated on crystals of biotite zircon and apatite modest nucleation 10 3 10 5 cm 3 took place even in crystal free samples but it was still heterogeneous when p exceeded 80 mpa nucleation in crystal free samples became extensive 10 5 10 7 cm 3 the lack of correlation of bubble density with either time or decompression suggests that nucleation was still heterogeneous nucleation rates were controlled mainly by the availability of sites rates were faster than 10 6 cm 3 s 1 when microlites were present and faster than 10 5 cm 3 s 1 in the absence of microlites at p 70 mpa narrow size distributions in most samples suggests that nucleation took place immediately after the pressure drop the experimental data we present here indicate that the presence or absence of efficient nucleation sites can lead to two distinct modes of bubble formation when a large number of efficient sites e g fe ti oxide are present bubble nucleation requires very little supersaturation and to a good approximation gas and magma are in equilibrium in magmas that are crystal free or contain crystals that are inefficient at nucleating bubbles very high degrees of supersaturation are required in order to initiate nucleation these two modes of exsolution may lead to contrasting styles of convection pressure build up and eruption

the role of oxides in the shallow vesiculation of ascending magmas abstract despite their generally low volume fraction fe ti oxides have the potential to greatly influence the eruptive style because they lower the supersaturation pressure for heterogeneous bubble nucleation once nucleated bubbles respond fast to pressure changes fostering rapid expansion and explosive behavior yet oxide microlite quantifications are often absent from data of explosive products we used new re analyzed and published data to build a compilation of oxide number densities onds and vesicle number densities vnds of trachytic and calc alkaline products four eruptive styles were selected 1 vulcanian explosions from soufriere hills volcano montserrat lascar volcano chile and kilian volcano france 2 blasts from mt pelee volcano lesser antilles mount st helens usa and merapi volcano indonesia 3 a sub plinian explosion from merapi volcano and 4 lava dome effusions with intermittent collapse from soufriere hills and mt pelee volcanoes natural samples were separated into two groups according to the dominant texture of the products of each event 1 vesicular pumice clasts from explosions with a strong vertical component and 2 dense clasts with diktytaxitic textures from dome collapse event and lateral blast group 1 samples are either distributed alongside the 1 1 trend between vnd and ond that spans from 1015 to 1017 m 3 or have a constant vnd of 1016 16 5 m 3 regardless of ond a large proportion of oxides 55 100 touch vesicles a more variable proportion of vesicles 16 72 are in contact with oxides because of syn explosive growth and coalescence group 2 samples have onds in the same broad range as group 1 samples we also used new and published data to build a compilation of onds and vnds of five series of experimental decompression of rhyolitic and phonolitic melts in samples with ond vnd most bubbles are in contact with more than one oxide and 64 88 of the oxides are in contact with bubbles such high levels of connectivity suggest that the role of oxides in controlling bubble nucleation has been underestimated when vnd ond nucleation densities of experimental vesicles can be reproduced by heterogeneous nucleation models which we used to calculate syn explosive decompression rates from vnds at merapi soufriere hills and kilian these rates and textural evidence suggest that the decompression front accompanying these vulcanian and sub plinian explosions is responsible for syn explosive bubble nucleation we calculated the average pre explosive ascent rates necessary to yield the observed onds at soufriere hills and merapi volcanoes the resulting rates 0 005 20 m s overlap considerably with the range of critical ascent rate inferred for the effusive explosive transition supporting the hypothesis that this transition is primarily controlled by oxide microlites in the conduit because oxides are a primer for explosive behavior when present in sufficiently high number densities focusing on the older eruption of kilian for which no observation is available we infer that pre explosive ascent rates of 7 10 3 m s were necessary for explosive behavior to occur

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

bubble nucleation in magmas a dominantly heterogeneous process abstract bubble nucleation is at the heart of magma ascent and degassing nucleation kinetics control how easily a magma exsolves its volatiles and thereby its explosivity close to the surface homogeneous nucleation i e without assistance from crystal substrates or other phases often requires attainment of large supersaturation pressures p n 100 mpa the change in pressure required for onset of nucleation while heterogeneous nucleation on pre existing crystals can occur at significantly lower values p n p n given experimental constraints on pressure conditions at the onset of nucleation decompression rate meters e g toramaru 2006 also relate the number density of bubbles in pyroclasts a readily obtained textural characteristic to rates of pressure change dp dt during ascent all these treatments require an explicit or implicit decision as to whether magma degassing was dominated by homogeneous or heterogeneous nucleation which is guided by the choice of surface tension values this contribution exploits the current textural datasets available for eruptions involving a variety of magma compositions to test the assumptions that homogeneous and or heterogeneous nucleation dominate bubble vesiculation in natural melts the comparison between pyroclast textures and those obtained via decompression experiments in the laboratory indicates that supersaturation pressures and decompression rates required for homogeneous nucleation are unrealistically high for natural magmas i review the arguments for and against homogenous nucleation and defend the alternate view that heterogeneous nucleation may dominate in all magma types including rhyolites in order to solve this central debate our focus needs to be shifted towards understanding the pre decompression phase equilibria and textural characteristics of magnetite in addition to the state of the melt structure

crystallization of microlites and degassing during magma ascent constraints on the fluid mechanical behavior of magma during the tenjo eruption on kozu island japan bubble and crystal textures provide information with regard to the kinetics of the vesiculation and crystallization processes they also provide insights into the fluid mechanical behavior of magma in a conduit we performed textural bubble and crystal and compositional analyses of pyroclasts that were obtained from the tenjo pyroclastic flow which resulted on account of the eruption in 838 a d on kozu island about 200 km south of tokyo japan pyroclasts in one flow unit 300 2 060 kg m3 average density 1330 kg m3 can be classified into three types on the basis of vesicle textures type i pyroclasts have small isolated spherical bubbles with higher vesicularities 67 77 vol and number density 10 8 11 7 log m 3 type ii pyroclasts have vesicularities similar to type i 61 69 vol but most bubbles exhibit evidences of bubble coalescence and lower number densities than type i 8 9 9 5 log m 3 type iii pyroclasts contain highly deformed bubbles with lower vesicularities 16 34 vol and number densities 8 2 9 0 log m 3 the microlite volume fraction dre converted also changes consistently across type i type ii and type iii as 0 06 0 08 and 0 10 0 15 respectively however the number density of the microlites remains nearly invariant in all the pyroclast types these facts indicate that the variation in the microlite volume fraction is controlled not by the number density i e nucleation process but by the size i e growth process the growth history of each type of microlite was different water content determinations show that the three types of pumices have similar h2o contents 2 6 0 2 wt this fact implies that all three types were quenched at nearly the same depth 35 5 mpa assuming that the magma was water saturated in the conduit if the crystal sizes are limited only by growth time a variation in this parameter can be related to the residence time which is attributed to the flow heterogeneity in the conduit by assuming a laminar poiseuille type flow these textural observations can be explained by the difference in ascent velocity and shearing motion across the conduit which in turn results in the differences in growth times of crystals degrees of deformation and bubble coalescence consequently for crystals in the inner part of the conduit the crystal growth time from nucleation to quenching is shorter than that near the conduit wall the vesicle texture variation of bubbles in types i ii and iii results from the difference in the deformation history implying that the effect of degassing occurred primarily towards the conduit wall

textural studies of vesicles in volcanic rocks an integrated methodology abstract vesicles in volcanic rocks are frozen records of degassing processes in magmas for this reason their sizes spatial arrangements numbers and shapes can be linked to physical processes that drive magma ascent and eruption although numerous techniques have been derived to describe vesicle textures there is no standard approach for collecting analyzing and interpreting vesicular samples here we describe a methodology for techniques that encompass the entire data acquisition process from sample collection to quantitative analysis of vesicle size and number carefully chosen samples from the lower mean and higher density vesicularity endmembers are characterized using image nesting strategies we show that the texture of even microvesicular samples can be fully described using less than 20 images acquired at several magnifications to cover efficiently the range of existing vesicle sizes a new program foams was designed to perform the quantification stage from vesicle measurement to distribution plots altogether this approach allows substantial reduction of image acquisition and processing time while preserving enough user control to ensure the validity of obtained results we present three cameo investigations on basaltic lava flows scoria deposits and pumice layers to show that this methodology can be used to quantify a wide range of vesicle textures which preserve information on a wide range of eruptive conditions

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

long term isovolumetric leaching of aluminum from rocks during weathering implications for the genesis of saprolite abstract isovolumetric geochemical analyses of 26 weathering profiles saprolites on crystalline rocks in brazil and south carolina indicate that on average si and al are leached from rocks in a long terms molar ratio of 8 3 hewever with the exception of the rio negro river the molar ratio of si to al in rivers draining these regions is about 100 if saprolites are still forming the large discrepancy in al mobilization between the saprolite record and modern river chemistry may be due to degassing of dissolved co2 from soil water entering streams with attendant increase in ph and al precipitation if saprolites are relic the rio negro basin may be a model for the environmental conditions under which they form these conditions are 1 low relief 2 abundant rainfall and intense leaching and 3 organic rich soils which produce large concentrations of co2 and dissolved organic acids in the soil abundant rainfall maintains dilute soil water large co2 pressure lowers ph dissolved organic matter complexes al and fe and thus enhances the solubility of secondary clays all of these effects hinder the precipitation of secondary weathering products and thus promote rock dissolution that is more nearly congruent the problem with the rio negro model is to reconcile the low relief of the basin which inhibits rapid groundwater movement with the short contact time indicated by the diluteness of its chemistry

types and mechanisms of strombolian explosions characterization of a gas dominated explosion at stromboli textural and chemical analyses of bombs quenched directly from a normal explosion at stromboli volcano italy were integrated with coincident seismic acoustic and thermal data the data set defines a new gas dominated type of strombolian eruption named type 0 these events are characterized by high velocity emission 150 250 m s 1 of a few relatively small juvenile particles with entrained non juvenile clasts that previously fell back into the vent to be re erupted for the studied event the explosion depth was more than 250 m deep and the particles showed little residence time in the shallow system slug ascent velocities over the final 20 35 m of magma filled conduit and the low viscosity of the resident magma are all consistent with simple bubble burst in a clean conduit this conduit type and eruption style likely fit popular slug ascent and burst models used to explain strombolian eruptions in contrast the ballistic dominated type of explosions type 1 are associated with larger proportions of stagnant material in the shallow system magma mix we argue that the additional volume of this stagnant material pushes the free surface upward because of the larger volume of material available for entrainment into a type 1 slug burst which has to fragment through a thick cap of degassed material type 1 events tend to be rich in particles in contrast the less spectacular gas rich type 0 events have little material to entrain thus being poor in lapilli and bombs

textural and geophysical characterization of explosive basaltic activity at villarrica volcano 1 villarrica volcano chile is one of the most active volcanoes in south america its activity is currently characterized by continuous degassing from a summit lava lake vent punctuated by explosive events during november 2004 a multidisciplinary experiment was deployed for a 10 d period to define the style of emission and infer shallow conduit dynamics at this basaltic center this involved collection of thermal seismic and infrasonic data to describe the background activity confined inside the crater and use of samples to texturally and chemically characterize the ejecta from more energetic explosions able to attain the crater rim the background activity was characterized by gas bursting with a frequency of 9 events per minute this involved emission of gas puffs fed by bubble bursting with larger bursts emplacing sheets of magma onto the lower crater walls the ejecta population from the more energetic events was characterized by the coexistence of both scoriae and golden pumice these two types of clasts have different textures but identical glass compositions suggesting that they underwent different conduit histories the golden pumice is interpreted as the expanding inner part of a short lived jet fed by a rapidly ascending magma batch the scoria forms the outer portion of the jet and comprises degassed material entrained during passage of the fresh batch through material residing in the upper most portion of the conduit we thus have a largely degassed upper column that feeds persistent bubble bursting through which fresh batches occasionally rise to feed events of relatively higher energy

insights into eruption dynamics from textural analysis the case of the may 2008 chaiten eruption the may 2008 chaiten southern chile eruption was characterized by several explosive events each associated with plumes which reached up to about 19 km above sea level on may 6 a study of the textural and physical features of the juvenile clasts erupted during the climactic phase of the 2008 eruption of chaiten is presented pumice clasts show unimodal density distribution main mode at 600 kg m3 average vesicularity of about 69 a glassy groundmass with no microcrystals and vesicles with dimension between 1 m and 2 mm they also show a unimodal vesicle size distribution with most frequent vesicle size in the range 0 05 0 08 mm and an estimated vesicle number density of 1 3 0 5 105 mm 3 related to a rapid nucleation event produced during the late phases of magma rise this is confirmed by the absence of microcrystals that could otherwise have delayed vesicle formation and allowed the magma to maintain a low viscosity and a supersaturation in volatiles vesiculation and fragmentation were triggered by a sudden decompression of the melt associated with the opening of the volcanic conduit 10 mpa s 1

kinetics of heterogeneous bubble nucleation in rhyolitic melts implications for the number density of bubbles in volcanic conduits and for pumice textures we performed decompression experiments to simulate the ascent of a phenocryst bearing rhyolitic magma in a volcanic conduit the starting materials were bubble free rhyolites water saturated at 200 mpa 800 c under oxidizing conditions they contained 6 0 wt dissolved h2o and a dense population of hematite crystals 8 7 2 105 mm 3 pressure was decreased from the saturation value to a final value ranging from 99 to 20 mpa at constant temperature 800 c the rate of decompression was either 1 000 or 27 8 kpa s in all experiments we observed a single event of heterogeneous bubble nucleation beginning at a pressure pn equal to 63 3 mpa in the 1 000 kpa s series and to 69 1 mpa in the 27 8 kpa s series below pn the degree of water supersaturation in the liquid rapidly decreased to a few 0 1 wt the nucleation rate dropped and the bubble number density bnd stabilized to a value strongly sensitive to decompression rate 80 mm 3 at 27 8 kpa s vs 5 900 mm 3 at 1 000 kpa s this behaviour is like the behavior formerly described in the case of homogeneous bubble nucleation in the rhyolite h2o system and in numerical simulations of vesiculation in ascending magmas similar degrees of water supersaturation were measured at 27 8 and 1 000 kpa s implying that a faster decompression rate does not result in a larger departure from equilibrium our experimental results imply that bnds in acid to intermediate magmas ascending in volcanic conduits will depend on both the decompression rate left left text d p text d t right right and the number density of phenocrysts especially the number density of magnetite microphenocrysts 1 100 mm 3 which is the only mineral species able to reduce significantly the degree of water supersaturation required for bubble nucleation very low bnds 1 mm 3 are predicted in the case of effusive eruptions left left text d p text d t right right 0 1 kpa s high bnds up to 107 mm 3 and bimodal bubble size distributions are expected in the case of explosive eruptions 1 a relatively small number density of bubbles 1 100 mm 3 will first nucleate in the lower part of the conduit left left text d p text d t right right 10 kpa s either at high pressure on magnetite or at lower pressure on quartz and feldspar or by homogeneous nucleation in the liquid and 2 then extreme decompression rates near the fragmentation level left left text d p text d t right right 103 kpa s will trigger a major nucleation event leading to the multitude of small bubbles typically a few micrometers to a few tens of micrometers in diameter which characterizes most silicic pumices

vesicle connectivity in pyroclasts and implications for the fluidisation of fountain collapse pyroclastic flows montserrat west indies abstract pyroclastic flows are noted for their highly mobile behaviour being able to flow on slopes as low as a few degrees and this has been attributed to fluidisation or partial fluidisation by escaping gases the three types of pyroclastic flows generated during the eruption of soufriere hills volcano on montserrat dome collapse fountain collapse and surge derived differ in their apparent mobility suggesting different degrees of fluidisation and friction reduction in the flowing granular materials several possible sources of fluidising gas operate in pyroclastic flows the purpose of this paper is to assess the feasibility of one mechanism the release during flow of pressurised gases trapped in the vesicles of juvenile clasts measurements with a helium pycnometer show that all vesicles in dense lava blocks from dome collapse pyroclastic flows are connected so that the mechanism is probably not viable for such flows on the other hand pumices from fountain collapse pyroclastic flows from soufriere hills and other volcanoes contain up to 10 vol of isolated vesicles this is confirmed by sem imagery which shows that soufriere hills pumices contain two vesicle populations with a bimodal size distribution the large vesicles are mostly interconnected whereas about two thirds of the smaller population

hydrogeology and pedology of saprolite formed from sedimentary rock eastern tennessee usa abstract groundwater flow in sedimentary rock saprolite and in soils derived from this material is strongly influenced by sedimentary layering and fractures inherited from the parent bedrock the main objectives of this study were to determine whether parent bedrock lithology and infilling of fractures and other macropores with pedogenically derived clays and fe mn oxides also play major roles in controlling hydraulic conductivity and groundwater flow the study was carried out by measuring profiles of saturated and unsaturated hydraulic conductivity ksat and k and comparing them to soil and saprolite pedology and lithology in a 3 4 m deep pit excavated in interbedded limestone and shale saprolite in the depth interval of 50 to 100 cm there was an abrupt decline in ksat and k 0 by a factor of up to 250 this corresponds to the occurrence of a zone where almost all of the fractures and other macropores are occluded with pedogenic clays and fe mn oxides below a depth of 100 cm both degree of pore infilling and hydraulic conductivity tend to vary with a spatial regularity that is similar to the thickness of sedimentary layering in the saprolite however variations in hydraulic conductivity do not always correspond to changes in lithology suggesting that hydraulic conductivity is a result of the complex interaction of several factors including parent bedrock lithology and the degree of infilling of the macropores similarities between hydrogeologic conditions at this site and at research sites in weathered sedimentary shale siltstone and carbonate rock settings at the oak ridge reservation in tennessee and in weathered crystalline rock in north carolina indicate that macropore infilling plays an important role in controlling hydraulic conductivity and groundwater flow for a variety of different types of parent bedrock

convective fractionation a mechanism to provide cryptic zoning macrosegregation layering crescumulates banded tuffs and explosive volcanism in igneous processes a large array of igneous and volcanic features has characteristics that are recognized in other disciplines as conclusive and direct evidence of convection in stratified and or solidifying melts e g macroscopic segregation cryptic variation zoning in magma chambers mineral layering in mafic intrusives crescumulates fingering in the vertical and horizontal banding in pyroclastics rollover with attendant flashing of volatiles explosive volcanism etc some quantitative and qualitative aspects of convection in solidifying and or stratified melts e g mineral layer widths such as are observed in the skaergaards are examined to show consistency with field evidence convective fractionation does not possess the physical implausibilities of gravitational segregation crystal settling neither is the field evidence as ambiguous if interpreted in terms of convective fractionation which can explain amongst other things heavier material overlying lighter convective fractionation may operate on larger scales in the interior of planets

textural evolution of magma during the 9 4 ka trachytic explosive eruption at kilian volcano chaine des puys france textural parameters such as density porosity pore connectivity permeability and vesicle size distributions of vesiculated and dense pyroclasts from the 9 4 ka eruption of kilian volcano were quantified to constrain conduit and eruptive processes the eruption generated a sequence of five vertical explosions of decreasing intensity producing pyroclastic density currents and tephra fallout the initial and final phases of the eruption correspond to the fragmentation of a degassed plug as suggested by the increase of dense juvenile clasts bimodal density distributions as well as non juvenile clasts resulting from the reaming of a crater in contrast the intermediate eruptive phases were the results of more open conduit conditions unimodal density distributions decreases in dense juvenile pyroclasts and non juvenile clasts vesicles within the pyroclasts are almost fully connected however there are a wide range of permeabilities especially for the dense juvenile clasts textural analysis of the juvenile clasts reveals two vesiculation events 1 an early nucleation event at low decompression rates during slow magma ascent producing a population of large bubbles 1 mm and 2 a syn explosive nucleation event followed by growth and coalescence of small bubbles controlled by high decompression rates immediately prior to or during explosive fragmentation the similarities in pyroclast textures between the kilian explosions and those at soufriere hills volcano on montserrat in 1997 imply that eruptive processes in the two systems were rather similar and probably common to vulcanian eruptions in general

diverse patterns of ascent degassing and eruption of rhyolite magma during the 1 8 ka taupo eruption new zealand evidence from clast vesicularity k lauea hawaii like vesuvius and etna is a frequently active volcano associated with a national park which is a focus for volcano tourism two contrasting summit eruptions at k lauea in 2008 2009 and 1959 emphasize how even small eruptions on populous volcanoes have significant impact on both hawaii volcanoes national park hvnp and the surrounding communities the effects are typically more indirect and social than direct physical but have significant fiscal repercussions the former include both positive benefits and negative impacts as an illustration direct consequences of the 1959 eruption were the rerouting of the major park access road and destruction of a summer camp but benefits included an unprecedented surge in visitor numbers and construction of nature trails that are now the most visited asset in the park both positive and negative impacts have increased in intensity with time due to growth in the resident population and visitor numbers for example the entire park has been evacuated twice during the ongoing 2008 2009 eruption

craters of elevation revisited forced folds bulging and uplift of volcanoes the eighteenth nineteenth century craters of elevation theory required magma to uplift strata doming the surface and creating a central down fallen crater or graben exponents of craters of elevation attempted to apply it to explain the origin of all volcanoes and rapidly the contemporary competing craters of eruption theory replaced it as the paradigm for volcano construction several historic examples have shown that intrusions can cause uplift termed bulges and can create features like those proposed for craters of elevation e g at usu 1944 bezymianny 1955 and mt st helens 1980 work on sedimentary basins that have had igneous activity has shown that intrusions create forced folds that uplift and deform strata in a similar way to that originally proposed for craters of elevation in view of the above we investigate large scale intrusion related topographic changes at two sites where the craters of elevation theory was developed the monogenetic volcanoes of the chaine des puys france and the teide stratovolcano tenerife we combine observations of such features with examples of forced folding to integrate the two fields of research our observations in the chaine des puys show that 1 the petit puy de dome has a bulge of up to 150 m uplift the uplift has a central depressed area a graben a dense network of normal faults basal thrusts and an aborted landslide 2 the grosmanaux volcano is a forced fold created by uplift of a previously flat lying area and has dense faulting and a graben on the resultant topographic bulge it was the site also of a major vulcanian eruption from the associated kilian crater 3 the gouttes volcano was uplifted by an intrusion like the petit puy de dome but then collapsed to generate a landslide and lateral blast 4 excavation in the lemptegy volcano exposes intra eruption intrusions with associated uplift providing examples in cross section of the internal deformation likely to be found inside other chaine des puys uplifted bulges on teide a bulge near the summit shows similar structures and surface tilting as seen on the petit puy de dome and this bulging may have formed during the eruption of the lavas negras the most recent activity on the summit area fault scarps on teide also expose small cryptodomes like those seen at lemptegy these examples coupled with field studies on eroded intrusions data on forced folds in basins and analogue models show how large scale topographic remodelling and structural change can be created by intrusions these can rapidly and significantly change the volcanic edifice a crater of elevation bulge or forced fold that is stabilised by the cooling of the intrusion will remain an important structural element in a volcano this process starts even at the small scale of monogenetic volcanoes and could occur through the lifetime of any growing stratovolcano such activity may be commonplace but may be masked by concomitant eruption or removed by subsequent collapse monitoring and hazard strategies should be ready to deal with such large scale events that will seriously modify the eruptive activity and stability of a volcano within days or weeks

bubble development in explosive silicic eruptions insights from pyroclast vesicularity textures from raoul volcano kermadec arc critical to understanding explosive eruptions is establishing how accurately representative pyroclasts are of processes during magma vesiculation and fragmentation here we present data on densities and vesicle size and number characteristics for representative pyroclasts from six silicic eruptions of contrasting size and style from raoul volcano kermadec arc we use these data to evaluate histories of bubble nucleation coalescence and growth in explosive eruptions and to provide comparisons with pumiceous dome carapace material density vesicularity distributions show a scarcity of pyroclasts with 65 75 vesicularity however pyroclasts closest to this vesicularity range have the highest bubble number density bnd values regardless of eruptive intensity or style clasts with vesicularities greater than this 65 75 pivotal vesicularity range have decreasing bnds with increasing vesicularities interpreted to reflect continuing bubble growth and coalescence clasts with vesicularities less than the pivotal range have bnds that decrease with decreasing vesicularity and preserve textures indicative of processes such as stalling and open system degassing prior to vesiculation in a microlite rich magma or vesiculation during slow ascent of degassing magma bubble size distributions bsds and bnds show variations consistent with 65 75 representing the vesicularity at which vesiculating magma is most likely to undergo fragmentation consistent with the closest packing of spheres we consider that the observed vesicularity range may reflect the development of permeability in the magma through shearing as it flows through the conduit these processes can act in concert with multiple nucleation events generating a situation of heterogeneous bubble populations that permit some regions of the magma to expand and bubbles to coalesce with other regions in which permeable networks are formed fragmentation preserves the range in vesicularity seen as well as any post fragmentation pre quenching expansion which may have occurred we demonstrate that differing density pyroclasts from a single eruption interval can have widely varying bnd values corresponding to the degree of bubble maturation that has occurred the modal density clasts the usual targets for vesicularity studies have likely undergone some degree of bubble maturation and are therefore may not be representative of the magma at the onset of fragmentation

the role of melt fracture degassing in defusing explosive rhyolite eruptions at volcan chaiten explosive volcanic eruptions of silicic magma often evolve towards non explosive emissions of lava the mechanisms underlying this transition remain unclear however a widely cited idea holds that shear induced magma fragmentation plays a critical role by fostering volatile loss from fragmentary magma and through ash filled cracks termed tuffisite we test this hypothesis by measuring h2o concentrations within glassy tuffisite from the 2008 2011 rhyolitic eruption at volcan chaiten chile we show that while h2o concentrations decrease next to tuffisite veins and at the margins of obsidian fragments the depletions cannot account for the disparity in h2o between explosively and effusively erupted rhyolite tuffisite vein lifetimes derived from diffusion modeling min to h imply degassing rates that are too slow to effectively degas magma unless the magma was entirely fragmented to mm or smaller particles this level of brecciation may locally develop near conduit margins but is unrealistic for entire conduits the primary role of melt fracturing may therefore be to provide gas escape pathways for more efficient degassing of permeable vesicular magma in the conduit interior

vesiculation path of ascending magma in the 1983 and the 2000 eruptions of miyakejima volcano japan the vesiculation of magma during the 1983 eruption of miyakejima volcano japan is discussed based on systematic investigations of water content vesicularity and bubble size distribution for the products the eruption is characterized by simultaneous lava effusion and explosive sub plinian dry eruptions with phreatomagmatic wet explosions the magmas are homogeneous in composition basaltic andesite and in initial water content h2o 3 9 0 9 wt and residual groundmass water contents for all eruption styles are low h2o 0 4 wt suggestive of extensive dehydration of magma