viscosity of microlite bearing rhyolitic obsidians an experimental study to investigate the influence of microlites on lava flow rheology the viscosity of natural microlite bearing rhyolitic obsidians of calc alkaline and peralkaline compositions containing 0 1 0 4 wt water was measured at volcanologically relevant temperatures 650 950 c stresses 103 105 pa and strain rates 10 5 to 10 7 s 1 the glass transition temperatures t g were determined from scanning calorimetric measurements on the melts for a range of cooling heating rates based on the equivalence of enthalpic calorimetric and shear viscosity relaxation we calculated the viscosity of the melt in crystal bearing samples from the t g data the difference between the calculated viscosity of the melt phase and the measured viscosity for the crystal bearing samples is interpreted to be the physical effect of microlites on the measured viscosity the effect of 45 vol crystals for both of these samples a connectivity factor is apparent with for the latter a close packing framework of crystals which is interpreted to influence the apparent viscosity

prevalence of growth twins among anhedral plagioclase microlites crystal textures of volcanic rocks record the processes involved in magma storage and eruptive ascent syn eruptive crystallization in which groundmass crystals form and grow according to environmental factors such as thermodynamic undercooling strongly influences the texture of erupted magma this stage is difficult to isolate for study in natural rocks but well suited for laboratory experiments because the chemical compositions and crystallization timescales of eruptive processes can be emulated this study examines the incipient stages of plagioclase crystallization in hydrous rhyodacite magma undergoing decompression driven degassing experimental samples in which crystal growth at both near equilibrium and far from equilibrium conditions were examined using electron backscatter diffraction ebsd analysis to ascertain crystallographic lattice orientations of individual crystals the crystal orientation investigation affirms a common assumption invoked in textural studies of crystal number density contiguous crystals with parallel faces are crystallographically continuous whereas contiguous crystals with non parallel faces have unrelated crystal lattice orientations and as such represent separate crystals in the highly undercooled sample twinning is identified in 87 of the crystals examined in the near equilibrium sample 38 of the crystals are twinned we find that the observed twinning is unlikely to be the result of deformation transformation or synneusis but rather a result of growth defects introduced during the incipient stages of crystallization we suggest internal structural defects twins control macroscopic morphological defects embayments swallowtails and melt inclusions as a result of the high energy of the twin plane boundary formation of twins during the incipient stages of plagioclase crystallization is the single most important factor contributing to anhedral morphologies of feldspar microlites growing during magma decompression and plays a role in the development of some plagioclase hosted melt inclusions

non newtonian rheology of igneous melts at high stresses and strain rates experimental results for rhyolite andesite basalt and nephelinite the stress strain rate relationships of four silicate melt compositions high silica rhyolite andesite tholeiitic basalt and nephelinite have been studied using the fiber elongation method measurements were conducted in a stress range of 10 400 mpa and a strain rate range of 10 6 to 10 3 s 1 the stress strain rate relationships for all the melts exhibit newtonian behavior at low strain rates but non newtonian nonlinear stress strain rate behavior at higher strain rates with strain rate increasing faster than the applied stress the decrease in calculated shear viscosity with increasing strain rate precedes brittle failure of the fiber as the applied stress approaches the tensile strength of the melt the decrease in viscosity observed at the high strain rates of the present study ranges from 0 25 to 2 54 log10 pa s the shear relaxation times of these melts have been estimated from the low strain rate newtonian shear viscosity using the maxwell relationship s g non newtonian shear viscosity is observed at strain rates time 1 equivalent to time scales that lie 3 log10 units of time above the calculated relaxation time brittle failure of the fibers occurs 2 log10 units of time above the relaxation time this study illustrates that the occurrence of non newtonian viscous flow in geological melts can be predicted to within a log10 unit of strain rate high silica rhyolite melts involved in ash flow eruptions are expected to undergo a non newtonian phase of deformation immediately prior to brittle failure

d h ratios and h2o contents record degassing and rehydration history of rhyolitic magma and pyroclasts abstract volcanic eruptions of rhyolitic magma often show shifts from powerful vulcanian to plinian explosive episodes to a more gentle effusion of viscous lava forming obsidian flows another prevailing characteristic of these eruptions is the presence of pyroclastic obsidians intermingled with the explosive tephra this dense juvenile product is similar to the tephra and obsidian flow in composition but is generally less degassed than its flow counterpart the formation mechanism s of pyroclastic obsidians and the information they can provide concerning the extent to which magma degassing modulates the eruptive style of rhyolitic eruptions are currently subject to active research porous tephra and pyroclastic and flow obsidians from the 1060ce glass mountain rhyolitic eruption at medicine lake volcano california were analyzed for their porosity water content h2o and hydrogen isotopic composition d h2o in porous pyroclasts is correlated negatively with d and positively with indicating that the samples were affected by post eruptive rehydration numerical modeling suggests that this rehydration occurred at an average rate of 10 23 5 0 5 m2 s 1 during the 960 years since the eruption causing some pyroclasts to gain up to 1 wt of meteoric water pyroclastic and flow obsidians were not affected by rehydration due to their very low porosity comparison between modeled d h2o relationships in degassing magma and values measured in the glass mountain samples supports the idea that rhyolitic magma degasses in closed system until its porosity reaches a value of about 65 5 beyond which degassing occurs in open system until quench during the explosive phase rapidly ascending magma fragments soon after it becomes permeable creating porous lapilli and ash that continue to degas in open system within an expanding gas phase as suggested by recent studies some ash may aggregate and sinter on the conduit sides at different depths above the fragmentation level partly equilibrating with the continuously fluxing heavier magmatic vapor explaining the wide range of h2o contents and high variability in d measured in the pyroclastic obsidians using only h2o and d it is impossible to rule out the possibility that pyroclastic obsidians may also form by permeable foam collapse either syn explosively near the conduit sides below the fragmentation level or during more effusive periods interspersed in the explosive phase during the final effusive phase of the eruption slowly ascending magma degasses in open system until it reaches the surface creating flows with low h2o and d this study shows that h2o measured in highly porous pyroclasts of a few hundred years or more cannot be used to infer syn eruptive magma degassing pathways unless careful assessment of post eruptive rehydration is first carried out if their mechanism of formation can be better understood detailed analysis of the variations in texture and volatile content of pyroclastic obsidians throughout the explosive phase may help decipher the reasons why rhyolitic eruptions commonly shift from explosive to effusive phases

viscosity of the dome of mount st helens abstract a sample of dacite from the dome of mt st helens has been studied experimentally and found to have a viscosity that declines from about 10 15 at 800 c to 10 3 poise at 1500 c its yield strength declines from about 2 10 6 dynes cm 2 at 800 c to less than the precision of our measurements 10 4 dynes cm 2 above 1050 c the geometrical form of the dome is approximately consistent with these properties if the internal temperature of the dome is between 900 and 1000 c the melting behavior of the crystalline dacite has been determined between 1000 and 1225 c data on the composition of the liquid fraction and sizes and proportion of crystals have been used to derive a set of equations for predicting the effective viscosity of crystal bearing magmas below their liquidus temperatures although of the bulk composition of the magma has changed little the increasing crystallinity and viscosity appear to have influenced eruptive behavior of the volcano during its most recent periods of activity

a model for calculating the viscosity of natural iron bearing silicate melts over a wide range of temperatures pressures oxygen fugacites and compositions a new general model that takes into account the pressure and redox state effect is presented to calculate melt viscosities of natural fe bearing melts this new model is applicable to melts that span a wide range of temperatures from 733 to 1873 k pressures 0 001 15 kbar h2o content from 0 to 12 3 wt and compositions from ultramafic mafic to silicic melts the accuracy of the model is calculated to be 0 23 log units of viscosity which is within or close to experimental uncertainty the transport properties including glass transition temperature and melt fragility can also be calculated from this model a spreadsheet to calculate the viscosity is provided in an electronic supplement

methods of determining the rheological properties of magmas at sub liquidus temperatures non vesicular basaltic melts behave as newtonian fluids at temperatures above their liquidus and their viscosities can be calculated using a method developed by shaw 1972 and bottinga and weill 1972 because many igneous processes involve the flow of silicates at sub liquidus temperatures numerous attempts have been made to calculate the interactive effects of the crystal phase however current methods are appropriate only for relatively low crystal concentrations and they assume newtonian behaviour we argue that this assumption is invalid when bubble or crystal concentrations exceed 30 at higher concentrations factors other than concentration become increasingly important these include particle shape and size distribution a method which takes these factors into account is tested on a range of suspensions including magmas at sub liquidus temperatures and rheological properties calculated using this method agree closely with the measured values we also demonstrate that an equation which was introduced to explain large differences in measured apparent viscosities during the cooling and crystallisation of mount st helens dacite murase et al 1985 and which is currently used to calculate the rheological properties of crystallising lavas generates viscosities which may be in error by several orders of magnitude this difference is argued to be caused by a combination of factors including the ten orders of magnitude range in the strain rates utilised during the mount st helens measurements causing orders of magnitude difference in the resulting apparent viscosities rheological data on crystallising silicic melts are reinterpreted taking into account the non newtonian rheology of the magmas and changes in activation energy of flow during crystallisation

physical volcanology and eruption dynamics of peralkaline agglutinates from pantelleria peralkaline rhyolitic lava like lobes showing direct evidence for a spatter fed origin are associated with eruptive centres that document a transition from explosive to effusive fountain fed activity owing to the preservation of spatter clasts clast dimensions can be used to model the eruptive dynamics of these deposits we conducted an integrated field experimental and theoretical study of the cala di tramontana centre pantelleria italy with the aim to better understand its eruptive behaviour this centre is composed of four units and a close association of fall lapilli tack welded agglutinate and spatter fed lava like lobes these units were fed initially from a fissure vent and later from a circular vent for the last erupted lobe field evidence indicates that fall out accumulated rapidly coalescing into a 10 m thick deposit that flowed downslope and became lava like high eruption rates and high heat flows are interpreted to promote welding of spatter proximal to the vent overburden pressures and thermal insulation from the accumulating spatter caused complete homogenisation of spatter within the interior of the flow laboratory vesiculation experiments show that peralkaline obsidian can vesiculate at temperatures as low as 620 c over a 4 hour period and indicate that airborne agglutinate can deform and weld rapidly within minutes of landing we measured viscosity on a natural pantellerian sample using the parallel plate method over a temperature range of 560 600 c viscosities of 1010 1011 pa s and an activation energy of 311 kj mol were obtained based on undeformed clast sizes and densities eruption dynamics modelling indicates that gas eruption velocities ranged from 90 to 130 m s 1 erupted mass fluxes were 200 kg s 1 m 1 and 2 104 kg s 1 from the fissure and localised circular vents respectively magmatic viscosities of between 105 and 107 pa s for t 750 850 c precluded intermittent strombolian activity due to gas bubble coalescence in addition the amount of magmatic water available to drive the eruption fountains ranged from 0 4 to 0 6 wt significantly less than the estimated 1 5 wt of exsolved volatiles implying that significant passive degassing occurred

evaluation of a relaxation geospeedometer for volcanic glasses abstract the cooling of silicate melts influences the character of volcanic processes primarily through the effect of temperature on melt viscosity but also through the derivative thermal properties expansivity and heat capacity if the cooling rates of volcanic facies can be established then the undoubtedly complex processes of erupting cooling and degassing magmas can be more readily understood the frozen structure of every glass volcanic or synthetic is influenced by the previous cooling history moreover the relaxation of the glass structure back to equilibrium during reheating across the glass transition is also dependent on the quenched in structure and thus can be used to determine the cooling rate history in this study differential scanning calorimetry dsc measurements have been used to determine the enthalpic relaxation of four volcanic glasses each glass has been heated at known heating rates in the calorimeter to temperatures above the glass transition interval the first heating of the glass involves the relaxation of the structure frozen in during the volcanic cooling subsequent controlled heating and cooling in the calorimeter enables the determination of a set of kinetic parameters which can be used to model the heat capacity curves by modeling the transient values of heat capacity across the glass transition interval the relaxation of enthalpy and the volcanic cooling rate across the glass transition interval can be obtained from our study we can demonstrate four different cooling rates for volcanic glasses over a range of three orders of magnitude from 4 c min 1 to 2 c day 1 we propose that the enthalpic relaxation of silicate melts can be applied as a geospeedometer to the study of volcanic rocks from a variety of occurrences

discriminating secondary from magmatic water in rhyolitic matrix glass of volcanic pyroclasts using thermogravimetric analysis abstract pyroclasts from explosive eruptions such as the 1060 ce explosive glass mountain eruption of medicine lake volcano california contain large amounts of water this may be the consequence of diffusive rehydration of the volcanic glass by meteoric secondary water after the eruption discriminating between magmatic and secondary water in the matrix glass of pyroclasts is important because the degassing of magmatic water affects the intensity of volcanic eruptions such discrimination has remained a challenging problem especially because some aspects of water diffusion in silicate glasses at low temperatures and atmospheric pressure remain poorly constrained we used thermogravimetry to analyze the loss of water from natural volcanic glasses and glasses that were hydrated in the laboratory at magmatic temperatures and pressures numerical modeling of diffusive water loss during thermogravimetric analyses accounted for the interconversion of molecular water h 2 o m and hydroxyls groups oh and indicates that glass mountain pumices contain 0 2 0 5 wt primary water but gained 1 2 wt of meteoric water by diffusive rehydration during the past 950 years these results confirm that the majority of magmatic water is lost from the magma during explosive eruptions furthermore the integration of thermogravimetric analysis and numerical modeling facilitates discrimination between the magmatic and secondary water content of volcanic glasses

rheology of crystal bearing silicate melts an experimental study at high viscosities the viscosity of partially crystallized mg3al2si3o12 melts has been measured under uniaxial compression in the interval 1010 1013 poise as a function of the volume fraction of crystals these inclusions are well rounded spherulites of aluminous enstatite having the same composition as the melt and whose growth rate is negligible at the temperature of the measurements the viscosity increases by less than 1 order of magnitude for crystal fractions of 40 vol and remains newtonian up to the maximum stress exerted namely 1 kbar the einstein roscoe equation 0 1 m n provides very good fits to the measurements only if either the m or n parameter is allowed to depend on temperature for modeling of magmatic processes however the widely recommended constant values m 0 6 and n 2 5 should be adequate the rheology changes abruptly when the clustered spherulites begin to oppose shear deformation at a crystal fraction of about 40 vol the viscosity becomes non newtonian with yield strengths of a few tens of bars at temperatures at which the viscosity of the melt is higher than 1010 poise as long as the crystal fraction remains lower than 70 vol the deformation proceeds in an irregular manner with a nonuniform distribution of crystals and melt the deformation becomes again regular at low stresses with lower melt fractions but samples undergo extensive fracturation along the direction of uniaxial stress similar rheology changes have been observed during the isothermal crystallization of li2si2o5 melts which produces small ellipsoidal inclusions these results suggest that the influence of solid suspensions on the rheology of magmas is primarily determined by the crystal fraction even though additional measurements would be useful to determine the possible influence of other factors such as the size distribution or the shape of the inclusions

a rheological investigation of vesicular rhyolite abstract the rheology of vesiculating rhyolitic systems exerts a strong control on the transport of silicic magmas in the subvolcanic to volcanic environments we present here an investigation of vesiculating and vesiculated rhyolites using dilatometric methods this study examines the effect of vesicle content on the viscosity of a natural supercooled rhyolitic liquid with 0 70 vesicles the experimental samples of rhyolitic glass are derived from fusion of a natural obsidian from little glass butte oregon crystal free rhyolite glasses of varying porosity were prepared by fusing obsidian powder in a pt crucible differing porosities were obtained by varying the temperature 1300 1650 c and duration 0 5 6 h of the fusions cylindrical samples of the resulting vesiculated rhyolites were cored from the crucible using diamond tools and their ends were ground flat and parallel for dilatometry the porosity of each sample was determined from archimedean buoyancy density determinations and comparison with bubble free rhyolite 2 331 g cm 3 porosity 1 p p o the density of foamed samples was determined using their mass volume and regular geometry viscosities were determined in the parallel plate mode at stresses of 5 10 3 to 10 5 pa the viscosimeter was calibrated using nbs 711 glass the bubble contents were microscopically investigated using a video reflected light system and image analysis software distribution functions of the size orientation aspect ratio and surface porosity were obtained the viscosity of rhyolite decreases with increasing bubble content a general relationship of the form 0 1 c describes the effect of porosity in volume fraction on the viscosity where c is a dimensionless constant 22 4 2 9 and log 10 0 10 94 0 04 pa s at 850 c

shear thickening dilatancy in crystal rich flows abstract rheological models of molten rock are mostly based on behaviours in which viscosity is constant with respect to strain rate e g newtonian bingham plastic or viscosity decreases with increasing strain rate pseudo plastic or shear thinning rheological models in which viscosity increases with increasing strain rate shear thickening or dilatant models have rarely been used yet such behaviour is common in particulate systems whereas magmas with low crystal content may be appropriately modelled by the former behaviours greater crystal content may lead to particle interactions resulting in shear thickening behaviour the effect is illustrated using a powdered rhyolitic rock mixed with water which exhibits extreme shear thickening behaviour caused by particles moving apart as they move past each other the expansion of the grain fabric during flow is opposed by the applied loads such that dilatancy is invariably associated with shear thickening rheology crystal rich lava may exhibit shear thickening dilatancy a proposition supported by observations of glass rich microscopic shear zones disrupting crystal fabrics plutonic crystal mush may also exhibit shear thickening dilatancy which can explain synplutonic intrusion involving brittle fracture followed by ductile disruption in general strain rate is an important contributor to the transition from ductile to brittle behaviour in crystal rich igneous rocks

the equivalence of enthalpy and shear stress relaxation in rhyolitic obsidians and quantification of the liquid glass transition in volcanic processes abstract the relaxation of enthalpy and shear stress has been investigated for six silicic volcanic obsidians calc alkaline rhyolitic obsidians from ben lomond dome new zealand erevan dry fountain armenia and little glass butte usa peralkaline obsidians from mayor island new zealand and eburru kenya and a macusanite obsidian from se peru the temperature dependences of enthalpy and shear stress relaxation are obtained from the dependence of the calorimetric heat capacity peak temperature on heating rate and the dependence of shear viscosity on temperature respectively both processes enthalpy relaxation and shear stress relaxation can be approximated to be arrhenian in the investigated temperature ranges relevant to volcanological processes activation energies derived for enthalpy and shear stress relaxation for each sample are equal this equality permits the calculation of viscosity at the glass transition as a function of cooling rate of a volcanic melt the relationship between viscosity at the glass transition and the cooling rate is given by log10 s at tg k log10 q where s the shear viscosity at the glass transition q is the cooling rate in c s 1 and k is a constant the six melt compositions investigated here exhibit a value of k 10 49 0 31 for the modeling of volcanic processes this equation allows the prediction of the viscosity at the glass transition temperature for a given value of cooling rate taken together with the maxwell relation an effective relaxation time can be obtained for the cooling rate prediction of the glass transition temperature permits the allotment of temperature ranges for the liquid and glassy segments of the cooling history of the volcanic melt and thus for the correct assignment of glassy and liquid values of the derivative thermodynamic properties such as expansivity and heat capacity in thermodynamic modeling of late stage volcanic processes

experimental generation of volcanic pseudotachylytes constraining rheology volcanic systems are highly dynamic environments that comprise rocks and magmas which in the process of strain localisation e g in catastrophic flank collapse or dome building events are candidate materials for the occurrence of frictional melting and the formation of pseudotachylytes we evaluate the frictional behaviour of a plagioclase two pyroxene and glass bearing andesite and introduce an approach to constrain the rheology of frictional melts frictional slip at a rate of 1 3 m s under an axial stress of 1 5 mpa induces heating at a rate approximating 130 c per meter of slip and frictional melting of the andesite occurs at 1000 c which corresponds to a peak in shear stress with continuing displacement the shear stress decreases by 25 from the peak strength and stabilises while the temperature equilibrates to 1230 1290 c the shear stress for the system is congruent with a non arrhenian temperature dependent frictional melt rheology exhibiting a non newtonian viscosity evolving from approximately 104 2 to 103 4 pa s post experiment analysis show a gradation from 1 the host rock to 2 an extraordinary zone of unmelted yet viscously deformed material to 3 a thin outer region of chemically heterogeneous protomelts and to 4 an inner region of chemically homogeneous frictionally generated modified melt in the core of the slip zone we discuss the role and identification of frictional melting in volcanic systems and the implication of viscous remobilisation of magmatic plugs during frictional slip of glass bearing volcanic materials

the rheology of two phase magmas a review and analysis abstract we consider the current state of our understanding of the rheology of two phase magmas that is suspensions of either bubbles or crystals in a viscous silicate melt the discussion is restricted to strain rates at which the suspending melt can be considered newtonian we start by considering the range of textures found in magmas and the bubble deformation and particle motions caused by shearing we then review proposed models for suspensions focussing on those functions of the form r f or f that have been most widely used to describe magmatic systems r is the relative apparent viscosity of the suspension is the volume fraction of the suspended phase is the driving stress and is the strain rate both theoretical and empirical methods are presented and then compared against the available analogue i e non magmatic and magmatic data the paper contains new data and significant re analysis of previously published data we present a new semi empirical constitutive model for bubble bearing magmas that is valid for steady and unsteady flow and large strains and strain rates this equation utilises a new parameter the capillarity cx that encapsulates the combined effect of shearing and unsteadiness on bubble suspensions we also present a new scheme for dealing with polydispersivity of bubble suspensions new data on the rheology of particle suspensions undergoing forced oscillations are presented these data show that the cox merz rule only holds for dilute particle suspensions 0 25 a re analysis of all available experimental data that relate rheology to particle aspect ratio provides distinct curves of maximum packing as a function of aspect ratio for smooth and rough particles with magmatic data lying on the curve appropriate for rough particles we analyse several rheological datasets of crystal bearing basaltic magmas and find that they are in good agreement with the constitutive equations derived from analogue data by contrast the same equations do not agree well with data for high viscosity haplogranitic melts this may be an effect of fracturing or viscous dissipation within these samples the paper concludes with a practical rheological recipes section giving a step by step method for calculating a constitutive equation for a two phase magmatic suspension and assessing its likely accuracy

temperature dependent thermal expansivities of silicate melts the system anorthite diopside abstract the temperature dependent thermal expansivities of melts along the join anorthite diopside have been determined on glassy and liquid samples using a combination of calorimetry dilatometry and pt double bob archimedean densitometry supercooled liquid volumes and molar thermal expansivities were determined using scanning calorimetric and dilatometric measurements of properties in the glass region and their behavior at the glass transition the extraction of low temperature liquid molar expansivities from dilatometry calorimetry is based on an assumed equivalence of the relaxation of volume and enthalpy at the glass transition using a method developed and tested by webb et al 1992 this method corrects for transient effects at the glass transition which can lead to serious overestimates of liquid thermal expansivity from peak values superliquidus volumes were determined using double pt bob archimedean densitometry at temperatures up to 1650 c the resulting data for liquid volumes near glass transition temperatures 810 920 c and at superliquidus temperatures 1400 1650 c are combined to yield thermal expansivities over the entire supercooled and stable liquid range the molar expansivities are in general temperature dependent the temperature dependence of thermal expansivity increases from anorthite to diopside composition the thermal expansivity of anorthite is essentially temperature independent whereas that of diopside decreases by 50 between 800 and 1500 c with the consequence that the thermal expansivities of the liquids in the anorthite diopside system converge at high temperature

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

coupled conduit and atmospheric dispersal dynamics of the ad 79 plinian eruption of vesuvius abstract the ad 79 eruption of vesuvius is certainly one of the most investigated explosive eruptions in the world this makes it particularly suitable for the application of numerical models since we can be quite confident about input data and the model predictions can be compared with field based reconstruction of the eruption dynamics magma ascent along the volcanic conduit and the dispersal of pyroclasts in the atmosphere were simulated the conduit and atmospheric domain were coupled through the flow conditions computed at the conduit exit we simulated two different peak phases of the eruption which correspond to the emplacement of the white and gray magma types that produced plinian fallout deposits with interlayered pyroclastic flow units during the gray phase the input data independently constrained and representative of each of the two eruptive phases consist of liquid magma composition crystal and water content mass flow rate and pressure temperature depth of the magma at the conduit entrance a parametric study was performed on the less constrained variables such as microlite content of magma pressure at the conduit entrance and particle size representative of the eruptive mixture numerical results are substantially consistent with the reconstructed eruptive dynamics in particular the white eruption phase is found to lead to a fully buoyant eruption plume in all cases investigated whereas the gray phase shows a more transitional character i e the simultaneous production of a buoyant convective plume and pyroclastic surges with a significant influence of the microlite content of magma in determining the partition of pyroclast mass between convective plumes and pyroclastic flows

emplacement history of a low viscosity fountain fed pantelleritic lava flow abstract the strongly peralkaline 8 ka flow mayor island new zealand was emplaced as a low viscosity fountain fed lava flow the flow stratigraphy comprises from top to base a finely vesicular pumiceous carapace an upper obsidian layer central crystalline rhyolite a lower obsidian layer incorporating a crumble breccia above a fall lapilli unit rare relict clasts welded within the lowermost 0 5 m of the lower obsidian layer support a fountain fed origin for the 8 ka flow five physical parameters density porosity microlite size void and autoclast aspect ratios and water content were measured throughout the thickness of the lava flow and show that prolonged vesiculation occurred in the upper 33 of the flow thickness and that autoclast and pyroclast shapes and microlite lengths were influenced by the thermal conditions of the lava after eruption crystallisation of the flow interior commenced before the flow stopped moving compared with calc alkaline rhyolites the high flow temperature 950 c and peralkaline chemistry contributed to a less polymerised low viscosity melt log 6 4 pa s these data and structural observations of the 8 ka flow suggest that near complete homogenisation and remobilisation of spatter from a lava fountain occurred on the caldera rim producing a distally tapering lava sheet that also spilled back down the caldera wall towards the vent

progressive growth of the cerro bayo cryptodome chachahuen volcano argentina implications for viscous magma emplacement cryptodome and dome collapse is associated with volcanic hazards such as explosive eruptions pyroclastic flows and volcanic edifice collapse study of the growth and evolution of volcanic domes

deformation of foamed rhyolites under internal and external stresses an experimental investigation the style of magma eruption depends strongly on the character of melt degassing and foaming depending on the kinetics of these processes the result can be either explosive or effusive volcanism in this study the kinetics of foaming due to the internal stresses of gas expansion of two types of obsidian have been investigated in time series experiments 2 min 24 h followed by quenching the samples the volumetric gas melt ratio has been estimated through the density measurements of foamed samples

geoid height age relation from seasat altimeter profiles across the mendocino fracture zone twenty eight seasat altimeter profiles crossing the mendocino fracture zone are used together with seafloor ages determined from magnetic lineations to estimate the change in oceanic geoid height with age between ages of 15 and 135 m y an unbiased estimate of the overall geoid offset along each profile is determined from a least squares fit of the along track derivative of the geoid to the geoid slope predicted from a simple two layer gravitational edge effect model uncertainties based upon the statistical properties of each profile are also determined a geoid slope age relation is constructed by normalizing the geoid offsets and uncertainties by the age offsets the results are in agreement with geoid slope age relations determined from symmetrically spreading ridges sandwell and schubert 1980 however the fracture zone estimates have smaller uncertainties and show less scatter a comparison of these results with the geoid slope age prediction of the boundary layer cooling model shows that the thermal structure begins to deviate from this model at an early age 20 40 m y a plate cooling model with a thickness of 125 km is most compatible with the geoid slope age estimates although significant deviations occur these may indicate that the lithospheric thermal structure is not entirely age dependent

groundmass crystallisation and cooling rates of lava like ignimbrites the grey s landing ignimbrite southern idaho usa constraining magmatic and eruptive processes is key to understanding how volcanoes operate however reconstructing eruptive and pre eruptive processes requires the ability to see through any post eruptive modification of the deposit the well preserved grey s landing ignimbrite from the central snake river plain provides an opportunity to systematically investigate the post eruptive processes occurring through a single deposit sheet despite overall compositional homogeneity in both bulk and glass compositions the grey s landing ignimbrite does preserve differences in the abundance of li in plagioclase crystals which are strongly associated with the host lithology li abundances in plagioclase from the quickly cooled upper and basal vitrophyres are typically low average 5 ppm n 262 while plagioclase from the microcrystalline interior of the deposit has higher li contents average 33 ppm n 773 given that no other trace elemental parameter in plagioclase varies we interpret the variability in li to reflect a post depositional process groundmass crystallisation of a rhyolite like grey s landing requires 50 crystallisation of sanidine and variable amounts of a silica rich phase quartz tridymite cristobalite and plagioclase to satisfy mass balance we suggest the low affinity of li for sanidine causes migration of groundmass li into plagioclase during crystallisation even within the microcrystalline interior of the deposit the morphology of the groundmass varies the more marginal finer grained regions are dominated by cristobalite as the sio2 rich phase while tridymite and quartz are additionally found in the more slowly cooled coarser grained portions of thick sections of the ignimbrite numerical models of cooling and crystallisation tested against field observations indicate that the groundmass crystallisation occurred relatively rapidly following emplacement a maximum of a few years where the ignimbrite is thickest these numerical models also illustrate that the time at which the potential for rheomorphism ceases either the ignimbrite is a solid glass or 50 crystalline is diachronous across the deposit

microlites and nanolites in rhyolitic glass microstructural and chemical characterization we used transmission electron microscopy tem and scanning electron microscopy sem to study magmatic crystals in the ben lomond rhyolite lava dome taupo volcanic center new zealand using tem and sem to investigate the size distributions of these crystals we identified three size populations microphenocrysts 1 2 m wide microlites 0 6 m wide and smaller crystals 0 6 m wide which we term nanolites the predominant mineral phases of the microlites and nanolites are augites pigeonites and hypersthenes the compositions and microstructures within these pyroxenes indicate disequilibrium crystallization at approximately 850 900 c and undercoolings as high as 300 c from equilibrium crystallization temperatures complex microstructures resulting from subsolidus reactions in augite and pigeonite are consistent with moderate cooling rates within the upper obsidian layer of the ben lomond rhyolite dome this study demonstrates the existence of sub micron magmatic crystals in a rhyolite and illustrates the potential of tem to provide unique information about the crystallization and cooling histories of glassy volcanic rocks

spatter stability constraining accumulation rates and temperature conditions with experimental bomb morphology we have developed the first experimental methodology to create a volcanic spatter pile using molten basalt this method permits reproduction of thermal conditions that yield the wide variety of spatter morphologies observed in nature the morphology of the clasts is most strongly controlled by the time the clast spends above the glass transition temperature which is in turn affected by the rate of accumulation and cooling of the deposit also spatter piles that remain hotter over longer durations experience increased fusion between clasts less void space between clasts and generally larger aspect ratios our experimental method successfully replicated natural microcrystal textures rheology and clast size work is still therefore required to achieve realistic vesicle distribution and deposit void space based on presented experimental work we estimate emplacement conditions of southern idaho spatter vents to have been 850 900 c with eruption temperatures closer to 1000 1100 c the rapid decrease from eruption temperature to effective emplacement temperature is the result of clast flight as well as equilibrating with the cooler surrounding material the morphology of the natural clasts matches experiments that have accumulation rates of 2 5 4 5 m h which also is consistent with the few measurements made at active eruptions finally we provide a constraint on the temperatures and accumulation rates that can lead to the construction of fused spatter features as well as provide the steps for future experiments to investigate other aspects such as compression impact and larger sizes of spatter formation by adapting our methodology

rheology of melts from the colli albani volcanic district italy a case study in this study the first viscosity measurements in the glass transition range of melts from highly explosive large volume eruptions from the colli albani volcanic district cavd are presented the magmas are ultrapotassic rich in iron and cao and characterised by a low silica content 45 wt melt compositions range from tephri phonolitic to foiditic the colli albani eruptions appear anomalous since they produced a large volume of erupted material in spite of their silica undersaturated compositions the viscosity of the colli albani melt changes as the melt composition evolves from the original melt to a country rock contaminated melt to a crystal bearing melt with a permanent decrease in liquid viscosity conventional estimations of viscosities assume these magmas to have a low viscosity the presented data show that the melt viscosities are higher than expected taking into account further chemical or rheological features of a melt the investigated cavd melts are not that striking as assumed in comparison with other large volume eruptions consequently considering the alkaline earth to alkaline ratio together with the sio2 content could provide an alternative when comparing large volume eruptions

constraints on interpretation of the eltanin impact from numerical simulations the results of numerical simulations of the eltanin impact are combined with the available geological data in order to reconstruct the impact dynamics and to get some constraints on the impact parameters numerical simulations show that the eltanin projectile size should be less than 2 km for a 45 oblique impact and less than 1 5 km for a vertical impact on the other hand we demonstrate that the projectile diameter cannot be considerably smaller than 1 km otherwise the impact induced water flow cannot transport eroded sediments across large distances the maximum displacement approximately equals the water crater radius and rapidly decreases with increasing distances numerical simulations also show that ejecta deposits strongly depend on impact angle and projectile size and therefore cannot be used for reliable estimates of the initial projectile mass the initial amplitudes of tsunami like waves are estimated the presence of clay rich sediments typical for the abyssal basins in cores ps2709 and ps2708 on the freeden seamounts bellingshausen sea southern ocean combined with numerical data allow us to suggest a probable point of impact to the east of the seamounts the results do not exclude the possibility that a crater in the ocean bottom may exist but such a structure has not been found yet