sediment production and yield from an alluvial gully in northern queensland australia sediment production transport and yield were quantified over various timescales in response to rainfall and runoff within an alluvial gully 7 8 ha which erodes into dispersible sodic soils of a small floodplain catchment 33 ha along the mitchell river northern australia historical air photographs and recent global positioning system gps surveys and lidar data documented linear increases in gully area and volume indicating that sediment supply has been relatively consistent over the historic period daily time lapse photography of scarp retreat rates and internal erosion processes also demonstrated that erosion from rainfall and runoff consistently supplied fine washload 100 000 mg l and sediment yields 89 to 363 t ha yr were high for both australian and world data total sediment yield estimated from empirical washload and theoretical bed material load was dominated by fine washload 63 m a lack of hysteresis in suspended sediment rating curves scarp retreat and sediment yield correlated to rainfall input and an equilibrium channel outlet slope supported the hypothesis that partially or fully transport limited conditions predominated along the alluvial gully outlet channel this is in contrast to sediment supply limited conditions on uneroded floodplains above gully head scarps while empirical data presented here can support future modelling efforts to predict suspended sediment concentration and yield under the transport limiting situations additional field data will also be needed to better quantify sediment erosion and transport rates and processes in alluvial gullies at a variety of spatial and temporal scales copyright 2013 john wiley sons ltd

the cooling of frontal flow ramps a calorimetric study on the rocche rosse rhyolite flow lipari aeolian islands italy relaxation geospeedometry using differential scanning calorimetry dsc has been applied to quantify the cooling history across the glass transition of flow ramps at the front of the calcalkaline rhyolite rocche rosse flow of lipari aeolian islands italy modelled cooling rates for the obsidian retrieved from two profiles range between 0 2 and 0 03 k min 1 cooling at the flow front appears to be dominated by conductive heat loss of individual flow ramps forming individual cooling units cooling rates of tens of kelvins per day appear to have controlled the last stage of viscous deformation before the entire flow came to rest it is inferred that cooling rates slower than those modelled are required to sustain flow in highly viscous rhyolitic lavas

groundwater pollution by arsenic and other toxic elements in an abandoned silver mine mexico this study evaluated the impact of an abandoned ag mine on the quality of surface and groundwater the mining site of huautla is in south morelos state central mexico ag pb zn and au cu sulfide ores were mined in the district the ores were characterized by the presence of ag galena pbs sphalerite zns and stromeyerite cuags ag was the metal of greater interest other metals included cu pb zn and au mining activities stopped in the early 1990s when the market price of ag decreased the abandoned mines then were flooded by rising groundwater levels because of the urgent demand for water by the inhabitants in the area this water has been used as drinking water and as waterholes for livestock water sampling points included abandoned mines america pajaro santiago tlachichilpa and san francisco dams and dug wells the greatest concentrations of as and other toxic chemical elements fe mn pb cd f were detected in groundwater samples from flooded mines the presence of these elements was related to the rock water interaction process the oxidation of sulfides appears to be the cause of increased metal concentrations in groundwater samples from flooded mine other possible water rock interaction processes that can control the presence of arsenic in groundwater were the adsorption of arsenic in iron oxyhydroxides the adsorption in carbonates and or coprecipitation with calcite in the case of the san francisco and america mines the oxidation conditions low correlation of as with so42 and fe2 and concentrations of silica indicate that the presence of as in the groundwater could be due also to competition for adsorption sites

compositional gradients surrounding spherulites in obsidian and their relationship to spherulite growth and lava cooling spherical masses of crystal fibers spherulites crystalize from rhyolitic melt glass mainly in response to significant undercooling while lava cools spherulite growth should induce compositional gradients in the surrounding glass from expulsion of incompatible constituents and diffusion of those constituents away from the spherulite finite difference numerical modeling of one dimensional diffusion in which diffusivities are allowed to vary with temperature is used to investigate how compositional gradients reflect spherulite growth and lava cooling overall three forms of gradients are identified elements that diffuse quickly are expelled from the spherulite but then migrate away too quickly to become enriched at the boundary of the spherulite elements that diffuse slowly are trapped within the growing spherulite between those endmembers are elements that are not trapped yet diffuse slow enough that they become enriched at the contact their slow diffusion away then elevates their concentrations in the surrounding glass how enriched those elements are at the spherulite matrix interface and how far their enrichments extend outwards into the glass reflect how spherulites grow and thermal conditions during growth concentrations of h2o rb f li cl na k sr cs ba and be were measured in and around spherulites in obsidian from a 4 7 1 km3 rhyolite lava dome erupted from tequila volcano mexico measurable concentration gradients are found for h2o rb and f attributes of those gradients and the behaviors of the other elements are in accord with their experimentally constrained diffusivities spherulites appear to have grown following radial rather than volumetric growth the observed gradients and lack of others are more consistent with growth mainly below the glass transition which would necessitate the dome cooling at ca 10 5 to 10 7 c s 1 such slow cooling is consistent with the relatively large volume of the dome

variolites of the paleoproterozoic hekpoort formation transvaal sub basin kaapvaal craton multistage undercooling textures abstract globular structures in volcanic and hypabyssal rocks called varioles or spherulites have been described from a wide variety of places and rock types including komatiites basalts dacites rhyolites as well as alkaline dykes but the processes leading to their formation are still debated here we present a field petrological and geochemical study of variole bearing lavas variolites in the paleoproterozoic volcanic hekpoort formation in the southern part of the transvaal sub basin of south africa variolites were only observed in a restricted area corresponding to a 3 5 km long basin or depression the variolites are locally up to 50 m thick and correspond to a succession of one meter thick lava flows most of the flows consist of two distinct layers with an upper layer rich in spherical varioles 1 32 cm in diameter overlying a layer generally devoid in varioles but often showing vertical jointing both layers show randomly oriented acicular textures of centimetre sized skeletal clinopyroxene crystals al rich augite that precede the formation of varioles the varioles themselves are mainly composed of radiating acicular augite 1 m in diameter the crystallization of which caused diffusion of incompatible elements which became enriched in the groundmass around the varioles the whole rock geochemistry of the hekpoort variolites is indistinguishable from that of non variolitic hekpoort lavas with the same mgo content this excludes liquid immiscibility as the origin of the varioles the acicular texture of al rich augite in combination with the incompatible element enrichment around the varioles suggests that both skeletal and acicular augite are related to undercooling rather than devitrification sub aqueous deposition is thought to be the cause of undercooling

advances in fourier transform infrared spectroscopy of natural glasses from sample preparation to data analysis fourier transform infrared spectroscopy ftir is an analytical technique utilized to measure the concentrations of h and c species in volcanic glasses water and co2 are the most abundant volatile species in volcanic systems water is present in magmas in higher concentrations than co2 and is also more soluble at lower pressures and therefore it is the dominant volatile forming bubbles during volcanic eruptions dissolved water affects both phase equilibria and melt physical properties such as density and viscosity therefore water is important for understanding magmatic processes additionally quantitative measurements of different volatile species using ftir can be achieved at high spatial resolution recent developments in analytical equipment such as synchrotron light sources and the development of focal plane array fpa detectors allow higher resolution measurements and the acquisition of concentration maps these new capabilities are being used to characterize spatial gradients or lack thereof around bubbles and other textural features which in turn lead to new insights into the behavior of volcanic feeder systems here practical insights about sample preparation and analysis of the distribution and speciation of volatiles in volcanic glasses using ftir spectroscopy are discussed new advances in the field of ftir analysis produce reliable data at high spatial resolution that can be used to produce datasets on the distribution dissolution and diffusion of volatiles in volcanic materials

emplacement of the rocche rosse rhyolite lava flow lipari aeolian islands the rocche rosse lava flow marks the most recent rhyolitic extrusion on lipari island italy and preserves evidence for a multi stage emplacement history due to the viscous nature of the advancing lava 108 to 1010 pa s indicators of complex emplacement processes are preserved in the final flow this study focuses on structural mapping of the flow to highlight the interplay of cooling crust formation and underlying slope in the development of rhyolitic lavas the flow is made up of two prominent lobes small 0 2 m scale folding and a channelled geometry foliations dip at 2 4 over the flatter topography close to the vent and up to 30 50 over steeper mid flow topography brittle faults tension gashes and conjugate fractures are also evident across flow heterogeneous deformation is evident through increasing fold asymmetry from the vent due to downflow cooling and stagnation a steeper underlying topography mid flow led to development of a channelled morphology and compression at topographic breaks resulted in fold superimposition in the channel we propose an emplacement history that involved the evolution through five stages each associated with the following flow regimes 1 initial extrusion crustal development and small scale folding 2 extensional strain stretching lineations and channel development over steeper topography 3 compression at topographic break autobrecciation lobe development and medium scale folding 4 progressive deformation with stagnation large scale folding and re folding and 5 brittle deformation following flow termination the complex array of structural elements observed within the rocche rosse lava flow facilitates comparisons to be made with actively deforming rhyolitic lava flows at the chilean volcanoes of chaiten and cordon caulle offering a fluid dynamic and structural framework within which to evaluate our data

assessment of bedload equations using data obtained with tracers in two coarse bed mountain streams narcea river basin nw spain abstract this paper evaluates the predictive power of nine bedload equations comparing the results provided by the equations with the bedload rates obtained in a previous field based tracer experiment accomplished in river piguena and river coto two coarse bed streams from nw spain rivers from nw spain draining the northern watershed of the cantabrian mountain range flow into the bay of biscay in a short path 50 60 km in this region they are developed forested catchments featured by fluvial networks with relatively steep slopes single thread sinuous channels and where bed sediment is typically coarse cobble and gravel tagged stones were used to trace bed sediment movement during flood events in river piguena and river coto the two main tributaries of the narcea river basin with the tracer results bedload transport rates between 0 2 and 4 0 kg s were estimated for six flood episodes the tracer based bedload discharges were compared with the bedload rates estimated with the bedload formulae duboys straub schoklitsch meyer peter muller bagbold einstein parker klingeman mclean parker klingeman parker and wilcock crowe our assessment shows that all of the bedload equations tend to overestimate when compared with the tracer based results with the wilcock and crowe 2003 equation the only exception in river piguena we linked these results to the particular geomorphology of coarse bed rivers in humid and forested mountain environments within these rivers armored textures and structural arrangements in the bed are ubiquitous these features together with a low sediment supply coming from upstream forested reaches define a supply limited condition for these channels limiting the potential use of bedload equations the wilcock and crowe 2003 equation introduces complex corrections into the hiding function and this could explain why it performs better

growth of megaspherulites in a rhyolitic vitrophyre megaspherulites occur in a rhyolitic vitrophyre near the base of a thick rhyolitic vitrophyre that occupies a late eocene to early oligocene volcanic tectonic basin near silver cliff colorado diameters of the megaspherulites range from 0 3 to over 3 66 m including an alteration layer the megaspherulites are compound spherulites they are holocrystalline very fine to fine grained composed of sanidine and quartz and surrounded by a thin sanidine quartz rind and an alteration layer containing mordenite and 15 a montmorillonite megaspherulite crystallization began soon after the vitrophyre was emplaced as the result of sparse heterogeneous nucleation under highly nonequilibrium conditions spherulite growth proceeded in a diffusion controlled manner under lowered viscosity as a result of a high water content 5 to 7 wt which also contributed to sparse nucleation when nucleation did occur it was at a large t 245 350 c on near critical sized nuclei once the spherulites began to grow continued nucleation was on existing growth cones and new independent spherulites did not form sanidine crystals grew with a fibrous habit which is favored by a large t restricting fibril lengths and diameters between 10 30 and 3 7 m respectively during crystallization these growth cones impinged upon each other resulting in fibril cone free areas these cone free interconal areas consist of coarser fine grained phases dominated by quartz which crystallized from the melt as it accumulated between the crystallizing sanidine fibrils of the cones the anhydrous nature of the sanidine and quartz suggests that their crystallization concentrated a water rich residual fluid enriched in ca mg and fe and depleted in si k and na at the megaspherulite vitrophyre interface this fluid phase enclosed the megaspherulites resulting in the hydrothermal alteration of the vitrophyre forming a hydrothermal alteration layer as indicated by the presence of the minerals mordenite and montmorillonite

evaluating channel response to an extreme sedimentation event in the context of historical range of variability upper colorado river usa in may 2003 a breach in a large irrigation ditch within rocky mountain national park rmnp initiated a debris flow that entered lulu creek and the colorado river where 36 000 m3 of sediment substantially altered channel forms and processes we present a proof of concept to understand whether the 2003 disturbance is within the historical range of variability hrv and whether the recovery potential of the system is sufficient to adapt to the disturbance flow and sediment regimes and channel morphology and stability were monitored on lulu creek and the colorado river from 2004 to 2011 dominant channel response following the debris flow within lulu creek included step development bed armoring and channel widening step height to length ratios h l for three reaches on lulu creek are outside the hrv of reference channels with one reach approaching reference conditions erosion of approximately 23 of the debris fan volume occurred as a result of the long duration 2011 peak flow sediment within the lulu creek fan will persist for 30 190 years assuming current maximum and mean removal rates planform changes on the colorado river since the debris flow include an increase in single thread geometries with braided reaches where bar deposition occurred bedload transport and grain size analysis of bedload indicate translational spreading of a sand wave front with a dispersive component in steeper reaches lulu creek is returning to a condition of natural variability but the colorado river is outside the hrv expected for steep gradient pool riffle channels applying hrv to a situation where management questions require a longer term perspective and pre disturbance baseline data are limited is a useful approach the hrv analysis facilitates a better understanding of site variability and delineates the range of possibilities of channel form and process to achieve management goals copyright 2012 john wiley sons ltd

timescales of texture development in a cooling lava dome abstract crystal growth and crack development in cooling lava domes are both capable of redistributing and mobilizing water cracking and hydration decrease the stability of a dome which may lead to hazards including partial dome collapse and block and ash flows by examining the distribution of water around crystals and cracks we identify and confine temperature and timescales of texture development in glassy rocks of volcanic domes four generations of textures have been identified type a spherulites type b cracks associated with spherulite growth type c perlitic cracks and type d disparate cracks high resolution imaging using fourier transform infrared spectroscopy ftir performed on samples from the ngongotaha dome new zealand show an increase in h2o of up to 450 along gradients of around 100 m up to 300 m in length from perlitic cracks spherulitic cracks and in haloes around spherulites no gradients in water concentrations across the disparate cracks are present water diffusion models show potential timescale temperature couples that coincide with textural observations and previous studies and allow us to develop a conceptual model of spherulite growth and cracking in a cooling lava dome spherulite growth starts around the glass transition temperature t g when the viscous melt cools to a brittle solid and proceeds with cracking related to volume changes at slightly lower temperatures and shorter timescales days to weeks compared to spherulite growth perlitic cracking happens at t t g allowing hydration of a permeable network within weeks to months low temperature 50 c cracks could not be hydrated in the time since eruption 230 ka our data show that textures in cooling glass develop during cooling below t g within days producing cracks and crystals that create inhomogeneities in the spatial distribution of water the lengthscales of water diffusion away from spherulites spherulite cracks and perlite cracks suggest that most of the rehydration of melt glass occurs at relatively high temperatures 400 c lack of evidence for water diffusion around other cracks suggests minor low temperature meteoric water rehydration following emplacement

a comparison between flume and field bed load transport data and consequences for surface based bed load transport prediction 1 the ability of simple equations to predict bed load transport with limited knowledge of the bed surface material was investigated this was done using a data set consisting of 7 636 bed load transport values from the flume 1 317 data and from 84 river reaches 6 319 field data it was possible to collapse field and flume data by correcting the ratio between the shields number and its critical value with a very simple hiding function proposed as a power law of the d84 d50 ratio in so doing a surface based bed load transport formula was proposed it was successfully tested on an independent data set comprising sand and gravel bed rivers with slope in the range 0 0002 0 08 with 86 of the values predicted within a precision of 1 order of magnitude moreover the formula reproduced the low transport rates well contrary to the usual surface based formulas also tested and is particularly well suited for estimating low transport rates associated with near bankfull flow discharge this new formula is neither time consuming no fractionwise calculation nor data consuming the required parameters are the flow discharge the active width the slope and the surface grain diameters d50 and d84

the lami pyroclastic succession lipari aeolian islands a clue for unravelling the eruptive dynamics of the monte pilato rhyolitic pumice cone abstract the lami succession piled up during the explosive activity of the monte pilato on lipari island central aeolian arc italy it comprises a repetitive alternation of fall and pyroclastic density current beds which define 13 units separated by fine ash beds despite the homogeneous rhyolitic composition the juveniles fragments show strong textural heterogeneities from highly vesicular pumice to dense black obsidians the pumice types are termed i tube which shows elongated tubular vesicles ii frothy highly vesicular with large sub rounded vesicles and iii agglomerated and welded two types of obsidian pyroclasts are recognized i aphyric black obsidian and ii grey obsidian showing spherulitic texture all these pyroclastic types occur throughout the stratigraphic succession and mixed in the same bed they account for changing physical conditions along the conduit which is hypothesised to be filled by a melt plug during the eruption of 12 out of 13 units the strong textural heterogeneities of the juvenile fragments testify for the existence of differential shear rates which can be explained though two shear mechanism i simple shear producing horizontal zonation of texture in the conduit with black obsidians along the conduit walls passing to tube and frothy pumice to the centre and ii pure shear producing a vertical zonation in the conduit with black obsidians at the top overlying frothy pumice agglomerated and welded pumice of unit 13 is thought to represent the portion of magma heated by a batch of hotter melt that entered the magma chamber and triggered the eruption the deposits of unit 13 represent a unique case of spatter accretion due to accumulation of hot low viscous rhyolitic pumice fragments emplaced from pyroclastic fountaining

degassing of rhyolitic magma during ascent and emplacement the degassing history of a rhyolitic igneous system was documented from analyses of drill core samples through the extrusive and intrusive portions of obsidian dome and of surface samples of associated tephra the initial volatile composition of the inyo magma was estimated to be 4 0 wt h2o 500 ppm f 800 ppm cl and 80 ppm s retained volatile contents of glassy and crystalline samples reflect the effects of decompression and second boiling decompression is rapid and involves loss of water rich fluid until a close approach to lithostatic equilibrium is achieved second boiling is a slower process and produces a chlorine rich fluid some of which can be trapped during development of extremely fine crystallization textures nearly complete dewatering during decompression of surface extruded magma strongly undercools the system t 175 c suppressing crystallization and yielding glassy rhyolitic lava partial degassing of shallowly intruded magma permits pervasive crystallization even at high cooling rates the subvolcanic intrusive regime is the zone of maximum volatile release because second boiling is incomplete in extrusives and volatile bearing crystalline phases are stable in magma crystallized at greater depth

effective discharge in rocky mountain headwater streams summary whereas effective discharge q eff in mountain streams is commonly associated with a moderate flow such as bankfull discharge q bf this study found that the maximum discharge q max and not bankfull discharge is the channel forming or effective flow for gravel transport in plane bed streams where partial bed mobility causes steep gravel transport rating curves q eff may approach bankfull flow in some step pool channels where gravel moves over a static cobble boulder bed our conclusions are based on magnitude frequency analyses conducted at 41 gauged rocky mountain headwater streams because these gauged streams lacked gravel transport data as is typical comparable streams with measured transport rates were used to develop scaling relations for rating curve exponents with stream and watershed characteristics those scaling relations were then used to estimate the steepness of gravel rating curves at the 41 gauged but unsampled sites the measured flow frequency distributions were characterized by two fitted power functions the steepness of the flow frequency distributions and the estimated steepness of gravel transport relations were combined in magnitude frequency analyses to compute q eff

diffusion controlled spherulite growth in obsidian inferred from h2o concentration profiles spherulites are spherical clusters of radiating crystals that occur naturally in rhyolitic obsidian the growth of spherulites requires diffusion and uptake of crystal forming components from the host rhyolite melt or glass and rejection of non crystal forming components from the crystallizing region water concentration profiles measured by synchrotron source fourier transform spectroscopy reveal that water is expelled into the surrounding matrix during spherulite growth and that it diffuses outward ahead of the advancing crystalline front we compare these profiles to models of water diffusion in rhyolite to estimate timescales for spherulite growth using a diffusion controlled growth law we find that spherulites can grow on the order of days to months at temperatures above the glass transition the diffusion controlled growth law also accounts for spherulite size distribution spherulite growth below the glass transition and why spherulitic glasses are not completely devitrified

devitrification pores and their contribution to volcanic reservoirs a case study in the hailar basin ne china abstract volcanic rocks represent important unconventional hydrocarbon reservoirs however devitrification pores which are ubiquitous in volcanic rocks remain mostly unstudied in this study we use fluorescence image analyzer fia scanning electron microscopy sem electron probe microanalyzer epma and laser scanning confocal microscopy lscm techniques to determine the types characteristics formation mechanisms and contributions to volcanic reservoirs of various devitrification pores in typical oil bearing volcanic rocks primary high temperature devitrification produced clustered and individual spherulites and lithophysae clustered spherulites have small diameters 1 mm and well developed intraspherulitic radiating micropores lithophysae contain spherical cavities and layers comprising skeletal crystallites abundant intercrystallite pores and sieve like intracrystallite micropores occur between and within the crystallites respectively secondary low temperature devitrification generated flow banded crystal fibers within glassy lavas abundant devitrification pores occur between crystal fibers nucleation density and morphology of crystals that formed during devitrification are dependent on the degree of supercooling t which governs the formation of devitrification pores at a constant pressure increasing t results in the systematic formation of lithophysae isolated spherulites clustered spherulites and flow banded crystal fibers each corresponding to distinct devitrification pore types interspherulite pores intercrystallite pores and lithophysa cavities commonly have wide diameters and very good connectivity which define them as good reservoir spaces radiating intracrystallite and flow banded micropores have small diameters and yet they represent significant reservoir spaces due to their high abundance we conclude that devitrified volcanic rocks such as pyromeride lithophysa rhyolite and flow banded glassy lava are favorable targets for volcanic oil and gas exploration

concordant pulse in mn y and hrees concentrations during uhp eclogitic garnet growth transient rock dynamics along a cold subduction plate interface abstract we herein report strikingly concordant mn y and hree garnet zoning in ultrahigh pressure low temperature oceanic eclogites found in the fossil devonian carboniferous subduction plate interface in sw tianshan zr in rutile thermometry thermodynamic modeling and diffusion controlled garnet growth modeling show that this type of zoning can be explained by a short lived transient most likely a 80 c thermal pulse from 490 to 570 c this event could be as transient as 300 ka assuming a prograde to peak burial duration of 5 ma such garnet zoning is observed for grains sampled meters apart fig 1d and was detected in other eclogites in the region this transient event could reflect the short lived movement of uhp oceanic slices once detached towards the hotter mantle wedge and or deformation induced remobilization of fluids enhancing diffusion controlled garnet growth

microtexture development during rapid cooling in three rhyolitic lava flows the effects of water concentration and degassing history on the development of spherulites and flow banding were examined in three middle tertiary rhyolitic lava flows from the atascosa mountains of southern arizona the hell s gate lava flow and the atascosa lava flow host spherulites of strongly contrasting texture and neither are flow banded the sycamore canyon lava flow is a flow banded rhyolite that hosts two populations of spherulites spherulites in the hell s gate lava flow consist of two to four generations of bladed radiating alkali feldspar crystals that increase in water concentration along their length needle like radiating feldspar crystals in spherulites in the atascosa and sycamore canyon lava flows are in some cases punctuated by concentric rinds of glass that are reservoirs for water rejected by the feldspar crystals the differences in spherulite crystal morphology between the sycamore canyon and atasocosa flows both needle like and the hell s gate flow bladed may reflect a more rapid cooling rate of the sycamore canyon and atascosa flows thick gray flow bands in the sycamore canyon lava flow host higher water concentrations than thin orange flow bands suggesting that flow bands are zones of greater and lesser volatile concentration deformed by stretching of the flowing magma temperature was uniform across the light and dark flow bands of the sycamore canyon flow indicating that water concentration one of the variables that controls diffusion coefficients rather than temperature controlled spherulite size in this case phenocrysts in the hell s gate lava flow are strongly resorbed probably as a result of magma ascent along a near adiabatic gradient that resulted in exsolution of water from the melt and subsequent dissolution of existing quartz phenocrysts by the water rich melt resumption of crystallization of anhydrous phases such as quartz and feldspar would have further enriched the melt in water facilitating the growth of spherulites spherulites in two of the lava flows sycamore canyon and hell s gate increase in water concentration from core to rim as would be expected in spherulites growing in melt enriched in water rejected by the growing feldspar crystals spherulites in the atascosa rhyolite flow decrease slightly in water concentration from core to rim possibly because the magma degassed during spherulite growth calculation of water concentration profiles in spherulites from all three rhyolite flows on the basis of rayleigh fractionation of water between sanidine and rhyolitic melt shows that the very high water concentrations in spherulites typically 0 6 water concentration in surrounding glass cannot be accounted for by rayleigh fractionation instead it is likely that sanidine incorporated water as fluid inclusions and or as water clusters during rapid crystal growth water concentration profiles in the glass surrounding spherulites do not preserve the high concentration zone at the spherulite boundary that has been observed in younger lava flows so that spherulite growth rates cannot be calculated on the basis of mass balance calculations of distribution of water during spherulite growth rather the water concentration profile in the surrounding glass is a half plateau the height of which is approximately equivalent to the far field water concentration in the surrounding glass indicating that water that accumulated at the spherulite magma boundary diffused sufficiently rapidly to equilibrate with the surrounding magma as the lava flow cooled

a general power equation for predicting bed load transport rates in gravel bed rivers 1 a variety of formulae has been developed to predict bed load transport in gravel bed rivers ranging from simple regressions to complex multiparameter formulations the ability to test these formulae across numerous field sites has until recently been hampered by a paucity of bed load transport data for gravel bed rivers we use 2104 bed load transport observations in 24 gravel bed rivers in idaho to assess the performance of eight different formulations of four bed load transport equations results show substantial differences in performance but no consistent relationship between formula performance and degree of calibration or complexity however formulae containing a transport threshold typically exhibit poor performance furthermore we find that the transport data are best described by a simple power function of discharge from this we propose a new bed load transport equation and identify channel and watershed characteristics that control the exponent and coefficient of the proposed power function we find that the exponent is principally a factor of supply related channel armoring transport capacity in excess of sediment supply whereas the coefficient is related to drainage area a surrogate for absolute sediment supply we evaluate the accuracy of the proposed power function at 17 independent test sites

spherulites and thundereggs from pitchstones of the deccan traps geology petrochemistry and emplacement environments spherulites and thundereggs are rounded typically spherical polycrystalline objects found in glassy silicic rocks spherulites are dominantly made up of radiating microscopic fibers of alkali feldspar and a silica mineral commonly quartz they form due to heterogeneous nucleation in highly supercooled rhyolitic melts or by devitrification of glass associated features are lithophysae stone bubbles which have an exterior sometimes concentric shells of fine quartz and feldspar and internal cavities left by escaping gas when filled by secondary silica these are termed thundereggs here we describe four distinct occurrences of spherulites and thundereggs in pitchstones mostly rhyolitic some trachytic of the deccan traps india the thundereggs at one locality were previously misidentified as rhyolitic lava bombs and products of pyroclastic extrusive activity we have character ized the thundereggs petrographically and geochemically and have determined low contents of magmatic water 0 21 0 38 wt in them using fourier transform infrared spectros copy we consider that the spherulite bearing outcrops at one of the localities are of lava flows but the other three represent subvolcanic intrusions based on the structural disposition of the deccan sheet intrusions studied here and considerations of regional geology we suggest that they are cone sheets emplaced from a plutonic center now submerged beneath the

crystallization of mount st helens 1980 1986 dacite a quantitative textural approach quantitative measurements of crystal size distributions csds have been used to obtain kinetic information on crystallization of industrial compounds randolph and larson 1971 and more recently on hawaiian basalts cashman and marsh 1988 the technique is based on a population balance resulting in a differential equation relating the population densityn of crystals to crystal sizel i e at steady staten no exp l itg whereno is nucleation density g is the average crystal growth rate is the average growth time and the nucleation ratej nog csd inn vsl plots of plagioclase phenocrysts in 12 samples of mount st helens blast dacite and 14 samples of dacite from the 1980 1986 mount st helens dome are similar and averageg 9 6 1 1 10 3 cm andno 1 2 106 cm 4 reproducibility of the measurements was tested by measuring csds of 12 sections cut from a single sample in three mutually perpendicular directions precision of the size distributions is good in terms of relative but not necessarily absolute values 10 growth and nucleation rates for plagioclase have been calculated from these measurements using time brackets of 30 150 years growth ratesg are 3 10 10 12cm s and nucleation ratesj are 5 21 10 6 cm3 s g andj for fe ti oxides calculated from csd data areg 2 13 10 13 cm sec andj 7 33 10 5 cm3 s respectively the higher nucleation rate and lower growth rate of oxides resulted in a smaller average crystal size than for plagioclase sizes of plagioclase microlites 0 01 mm in the blast dacite groundmass have been measured from backscatter sem photographs nucleation of these microlites was probably triggered by intrusion of material into the cone of mount st helens in spring 1980 this residence time of 52 days gives minimum crystallization estimates ofg 1 3 10 11 cm s andj 9 16 1o3 cm3 s the skeletal form of the microlites provides evidence for nucleation and growth at high values of undercooling t relative to the phenocrysts a comparison of nucleation and growth rates for the two crystal populations phenocrysts vs microlites suggests that while growth rate seems to be only slightly affected by changes in t nucleation rate is a very strong function of undercooling a comparison of plagioclase nucleation and growth rates measured in the mount st helens dacite and in basalt from makaopuhi lava lake in hawaii suggests that plagioclase nucleation rates are not as dependent on composition groundmass textures suggest that plagioclase microphenocrysts crystallized at depth rather than in the conduit in the dome or after extrusion onto the surface most of this crystallization appears to be in the form of crystal growth coarsening of groundmass microphenocrysts at small degrees of undercooling rather than extensive nucleation of new crystals this continuous crystallization in a shallow magmatic reservoir may provide the overpressurization needed to drive the continuing periodic domebuilding extrusions which have been the pattern of activity at mount st helens since december 1980

structural variation and the development of thick rhyolite lava a case study of the sanukayama rhyolite lava on kozushima island japan abstract geological studies of ancient rhyolite lavas are required for understanding flow dynamics because rhyolite lava eruption is an infrequent event the pleistocene sanukayama rhyolite lava is about 130 m thick and located on kozushima island japan vertical sections of the lava are well exposed in the sea cliff and in a dissected valley providing a good opportunity for the elucidation of the internal structural variations the vertical lithofacies variations are classified into three zones of pumiceous obsidian and crystalline from the top to the base respectively the exposed area is considered to correspond to the upper half of the lava in the boundaries between each zone the lithofacies are mingled rather than exhibiting gradual change this indicates that rheological contrasts are arisen within the lava due to differences of vesicularity and or crystallinity various heterogeneous structures are recognized throughout the lava such as obsidian and reddish layers in the pumiceous zone strongly deformed white colored bands and a clastic layer in the obsidian zone and dark colored bands in the crystalline zone we demonstrate on the basis of geological evidences and magnetic studies that these heterogeneous structures originated mostly from shear induced fracturing within the advancing lava this study indicates that brittle lava fracturing during ductile brittle transition likely governs the acceleration of rhyolite lava heterogeneity

analysis and classification of bedload transport events with variable process characteristics abstract knowledge about the magnitude of bedload fluxes at given hydraulic conditions in natural streams is essential for improved process understanding for the application calibration and validation of bedload transport formulas and for numerical sediment transport models nonetheless extensive field measurements of bedload transport are challenging and therefore data from such efforts are rare bedload transport has been measured comprehensively at the downstream section of the urslau torrent in salzburg austria since 2011 we used an integrative monitoring system that combines direct mobile basket sampler slot sampler and indirect measuring devices geophone plates continuous information about the intensity and distribution of bedload transport within the channel cross section is available in high spatial and temporal resolution seven geophone plates at a stream width of 8 m are part of a measurement system that delivers data in 1 min intervals these geophone data are calibrated using results of direct bedload measurements providing an opportunity to calculate bedload rates and bedload yields in selected time periods continuous data on the bedload transport process over three years enabled assessing several bedload transport events the investigation of bedload transport rate discharge relationships reveals order of magnitude changes for individual events we observed shifts in the data reflecting different bedload rates at comparable hydraulic conditions this study reveals that variable sediment supply conditions affect the prevailing bedload transport rates at the urslau stream calculating the bedload transport efficiency enables comparing bedload transport events that exhibit similar process characteristics finally we provide a conceptual model of bedload transport process types as a function of bedload transport efficiency and dimensionless stream power

vesiculation rates of obsidian domes inferred from h2o concentration profiles 1 explosivity of rhyolite domes depends on bubble nucleation and growth to understand these processes we measure millimeter scale variations in h2o by synchrotron fourier transform infrared ftir spectromicroscopy on interlayered obsidian pumice samples the h2o contents of all samples are above the 1 atm solubility value 0 10 wt and decrease systematically towards vesicular zones indicating that gas bubbles were growing and that degassing of melt to atmospheric pressure was incomplete h2o profiles are compared with models for water diffusion in order to constrain the temporal scale of vesiculation diffusion timescales range from 0 4 to 15 days and represent the time between bubble nucleation and quenching we use these results to estimate the duration of volatile exsolution and evaluate the timing of the formation of explosion pits frequently found on the surfaces of obsidian domes though never witnessed

genesis of megaspherulites in el viejo rhyolitic coulee pleistocene southern puna argentina crystalline domains in rhyolitic domes and coulees are commonly characterized by spherulites and lithophysae spherulites typically range from microscopic to a few centimeters in diameter larger spherulites termed megaspherulites are rare but have been reported in the usa and mexico the uncommon nature of such structures supports the need for a study to explore the factors and special conditions that allow them to reach such a large size in the southern puna of argentina el viejo coulee includes megaspherulites up to 4 m in diameter we present observations from field work petrography scanning electron microscopy bulk geochemistry and fourier transform infrared spectroscopy the megaspherulites occur in obsidian lenses that differ in phenocryst content and composition from the foliated coherent facies that forms the rest of the coulee the obsidian lacks vesicles and microlites and is unaltered the megaspherulites comprise growth cones consisting of micropoikilitic texture where quartz encloses potassium feldspar the growth cones are separated by interconal areas composed of lithophysae we propose that the megaspherulites were formed above the glass transition temperature tg and are the product of primary crystallization of rhyolitic melt the exceptionally large size of the megaspherulites implies high diffusion rates which are favored by temperatures above tg during crystallization the large size also suggests scarcity of nucleation sites which is consistent with the megaspherulites being hosted by unaltered microlite and vesicle free glass the position of the obsidian lens at the base of the coulee may have played a critical role in maintaining the temperature above the tg long enough to allow the crystallization of the megaspherulites these conditions also favored crystallization in the most advanced stage where micropoikilitic texture replaced the fans of crystal fibers typical of spherulites crystallization of anhydrous quartz and feldspar in the growth cones led to the concentration of volatiles in the melt in the interconal areas resulting in volatile exsolution and formation of vesicles that became nucleation sites for lithophysae the study advances our understanding of some of the special processes that are involved in the cooling and solidification of rhyolitic magmas fundamentally we find that the position of the obsidian at the base of the coulee was critically important because this position favored maintenance of the temperature above the tg which in turn favored high diffusion rates also the scarcity of nucleation sites in the obsidian melt allowed only a small number of spherulites to nucleate those that nucleated therefore grew very large the meter scale megaspherulites may have taken 55 years to create

unravelling textural heterogeneity in obsidian shear induced outgassing in the rocche rosse flow abstract obsidian flow emplacement is a complex and understudied aspect of silicic volcanism of particular importance is the question of how highly viscous magma can lose sufficient gas in order to erupt effusively as a lava flow using an array of methods we study the extreme textural heterogeneity of the rocche rosse obsidian flow in lipari a 2 km long 100 m thick 800 year old lava flow with respect to outgassing and emplacement mechanisms 2d and 3d vesicle analyses and density measurements are used to classify the lava into four textural types glassy obsidian 40 vesicles high aspect ratio shear banded lava 20 40 vesicles and low aspect ratio frothy obsidian with 30 60 vesicles textural heterogeneity is observed on all scales m to m and occurs as the result of strongly localised strain magnetic fabric described by oblate and prolate susceptibility ellipsoids records high and variable degrees of shearing throughout the flow total water contents are derived using both thermogravimetry and infrared spectroscopy to quantify primary magmatic and secondary meteoric water glass water contents are between 0 08 0 25 wt water analysis also reveals an increase in water content from glassy obsidian bands towards frothy bands of 0 06 0 08 wt reflecting preferential vesiculation of higher water bands and an extreme sensitivity of obsidian degassing to water content we present an outgassing model that reconciles textural volatile and magnetic data to indicate that obsidian is generated from multiple shear induced outgassing cycles whereby vesicular magma outgasses and densifies through bubble collapse and fracture healing to form obsidian which then re vesiculates to produce dry vesicular magma repetition of this cycle throughout magma ascent results in the low water contents of the rocche rosse lavas and the final stage in the degassing cycle determines final lava porosity heterogeneities in lava rheology vesicularity water content microlite content viscosity play a vital role in the structural evolution of an obsidian flow and overprint flow scale morphology post emplacement hydration also depends heavily on local strain whereby connectivity of vesicles as a result of shear deformation governs sample rehydration by meteoric water a process previously correlated to lava vesicularity alone

scales of columnar jointing in igneous rocks field measurements and controlling factors columnar jointing is a common feature of solidified lavas sills and dikes but the factors controlling the characteristic stoutness of columns remain debated and quantitative field observations are few in number in this paper we provide quantitative measurements on sizing of columnar joint sets and our assessment of the principal factors controlling it we focus on 1 chemistry as it is the major determinant of the physical mechanical and thermal properties of the lava and 2 geology as it influences the style of emplacement and lava geometry setting boundary conditions for the cooling process and the rate of heat loss in our analysis we cover lavas with a broad range of chemical compositions from basanite to phonolite for six of which we provide new geochemical analyses and of geological settings our field measurements cover 50 columnar jointing sites in three countries we provide reliable manually digitized data on the size of individual columns and focus the mathematical analysis on their geometry 23 889 data on side length of which 17 312 are from full column sections and 3 033 data on cross sectional area and order of polygonality the geometrical observations show that the variation in characteristic size of columns between different sites exceeds one order of magnitude side length ranging from 8 to 338 cm and that the column bounding polygons average order is less than 6 the network of fractures is found to be longer than required by a minimum energy hexagonal configuration indicating a non equilibrium geologically quick process in terms of the development and characteristic sizing of columnar joint sets our observations suggest that columns are the result of an interplay between the geological setting of emplacement and magma chemistry when the geological setting constrains the geometry of the emplaced body it exerts a stronger control on characteristic column stoutness at unconstrained geometries e g unconfined lava flows chemistry plays the major role resulting in stouter columns in felsic lavas and slenderer columns in mafic lavas

lightning induced weathering of cascadian volcanic peaks abstract the process of meteorological lightning induced modification of coherent volcanic rocks is examined by geochemical textural and experimental analysis of fulgurites from south sister volcano oregon cascades usa lightning s effects on volcanic target rocks was simulated with an arc welding device in order to reproduce the geochemical and textural features of natural fulgurites and to constrain temperatures of melting and devolatilization behavior during lightning strikes melting of volcanic target rocks produces melts of exceptional compositional diversity ranging from those with pure mineral compositions e g diopside and plagioclase resulting from congruent melting reactions to those that are highly mixed and compositionally identical to the target rock geochemical and textural observations on fulgurites suggest that melting proceeds rapidly by an in situ modal batch process whereby individual crystals melt and contribute small aliquots to a larger batch that is then mixed by viscous flow hydrous species remaining in the glasses are trace oh 0 1 wt and consistent with partial equilibration to high temperature 2000 c supra liquidus 1 atm conditions lightning also induces physical weathering in that the target rocks may be vapourized to form channels through the rock representing injection of elemental components to the atmosphere finally lightning acts as a multi faceted producer of dry chemically equivalent volcanic glass which may be subject to long term hydration by environmental waters at surface conditions these re hydration signatures contrast those of volcanic glasses which have both magmatic and meteoric components and are used herein to examine the post lightning environmental history of these glasses including age dating lightning events by diffusion chronometry our study of volcanic fulgurites illustrates the potential to use their hydration signatures to date paleoweather events at decadal to centuries scale because the lightning strike is in and of itself so effective at devolatilizing melts in an instant the resultant fulgurites are a unique earth material that record individual weather events i e a thunderstorm and also longer term paleoweather intervals

quaternary geochemical evolution of the salars of uyuni and coipasa central altiplano bolivia risacher f and fritz b i99 i quaternary geochemical evolution ofthe salars of uyuni and coipasa central altiplano bolivia chem geol 90 21 1 231 the central trough of the bolivian altiplano is occupied by two wide salt crusts the salar of uyuni which is probably the largest salt pan in the world 10 000 km and the salar of coipasa 2 500 km both crusts are essentially made of porous halite filled with an interstitial brine very rich in li k mg b up to 4 7 g l li 4 3 g l b 30 g l k and 75 g l mg lithium reserves are the highest known in the world around 9x io6 tons potassium magnesium and boron reserves in brines are also important around 194x io6 tons k 8x 10 tons b and 21 1 x io6 tons mg the cnists are the remnant of saline lake tauca 13 000 10 000 yr bp its salinity was estimated approximately at 80 g l its paleochemistry was derived in two ways 1 by dissolving the present amounts of all chemical components in the former lake volume and 2 by simulating the evaporation of the major inflows to the basin the resulting chemical compositions are quite different the dissolution derived one is 5 to 50 times less concentrated in li k mg b than the evaporation simulated ones however all compositions present the same na and ci contents this suggests either a removal of bittern salts or an enrichment of the former lake water in na and ci the most probable interpretation is that lake tauca redissolved a salt crust akin to that existing today several older lakes have been detected on the altiplano nevertheless such an explanation only pushes the problem back it is likely that the anomaly was transferred from one lake to an other three hypotheses may be put forward 1 bittern seepage through bottom sediments 2 uptake of the missing components by minerals and 3 leaching of ancient evaporites from the catchment area at the beginning of the lacustrine history of the basin the excess halite could have been recycled from lake to lake this latter process seems to be the most effective to explain the large excess of na and ci over the bittern solutes li k mg and b the occurrence of almost pure na ci saline springs flowing out from a gypsum diapir in the northern altiplano gives substantial support to this hypothesis

the emplacement of an obsidian dyke through thin ice hrafntinnuhryggur krafla iceland an eruption along a 2 5 km long rhyolitic dyke at krafla volcano northern iceland during the last glacial period formed a ridge of obsidian hrafntinnuhryggur the ridge rises up to 80 m above the surrounding land and is composed of a number of small volume lava bodies with minor fragmental material the total volume is 0 05 km3 the lava bodies are flow or dome like in morphology and many display columnar jointed sides typical of magma ice interaction quench fragmented lower margins indicative of interaction with meltwater and pumiceous upper surfaces typical of subaerial obsidian flows the fragmental material compromises poorly sorted perlitic quench hyaloclastites and poorly exposed pumiceous tuffs lava bodies on the western ridge flanks are columnar jointed and extensively hydrothermally altered at the southern end of the ridge the feeder dyke is exposed at an elevation 95 m beneath the ridge crest and flares upwards into a lava body using the distribution of lithofacies we interpret that the eruption melted through ice only 35 55 m thick which is likely to have been dominated by firn hrafntinnuhryggur is therefore the first documented example of a rhyolitic fissure eruption beneath thin ice firn the eruption breached the ice leading to subaerial but ice firn contact lava effusion and only minor explosive activity occurred the ridge appears to have been well drained during the eruption aided by the high permeability of the thin ice firn which appears not to have greatly affected the eruption mechanisms we estimate that the eruption lasted between 2 and 20 months and would not have generated a significant jokulhlaup 70 m3s 1

vesiculation in rhyolite at low h2o contents a thermodynamic model we present experimental data on the thermodynamics and kinetics of bubble nucleation and growth in weakly h2o oversaturated rhyolitic melts the high temperature 900 1100 c experiments involve heating of rhyolitic obsidian from hrafntinnuhryggur krafla iceland to above their glass transition temperature tg 690 c at 0 1 mpa for times of 0 25 24 h during experiments the rhyolite cores increase in volume as h2o vapor filled bubbles nucleate and expand the extent of vesiculation as tracked by porosity is mapped in temperature time t t space at constant temperature and for a characteristic dwell time the rhyolite cores achieve a maximum volume where the t t conditions reach thermochemical equilibrium for each t t snapshot of vesiculation we use 3 d analysis of x ray computed tomographic xct images of the quenched cores to obtain the bubble number density bnd and bubble size distribution bsd bnds for the experimental cores are insensitive to t and t indicating a single nucleation event all bsds converge to a common distribution independent of t melt viscosity or initial degree of saturation suggesting a common growth process we use these data to calibrate an empirical model for predicting the rates and amounts of vesiculation in rhyolitic melts as a function of and thermochemical affinity a two computable parameters that are dependent on t pressure and h2o content the model reproduces the experimental data set and data from the literature to within experimental error and has application to natural volcanic systems where bubble formation and growth are not diffusion limited e g lavas domes ignimbrites conduit infill

textural characterization major and volatile element quantification and ar ar systematics of spherulites in the rocche rosse obsidian flow lipari aeolian islands a temperature continuum growth model spherulitic textures in the rocche rosse obsidian flow lipari aeolian islands italy have been characterized through petrographic crystal size distribution csd and in situ major and volatile elemental analyses to assess the mode temperature and timescales of spherulite formation bulk glass chemistry and spherulite chemistry analyzed along transects across the spherulite growth front glass boundary reveal major oxide and volatile h2o co2 f cl and s chemical variations and heterogeneities at a 5 m scale numerous bulk volatile data in non vesicular glass spatially removed from spherulitic textures reveal homogenous distributions of volatile concentrations h2o 0 089 0 012 wt f 950 40 ppm and cl 4 100 330 ppm with co2 and s consistently below detection limits suggesting either complete degassing of these volatiles or an originally volatile poor melt volatile concentrations across the spherulite boundary and within the spherulitic textures are highly variable these observations are consistent with diffusive expulsion of volatiles into melt leaving a volatile poor rim advancing ahead of anhydrous crystallite growth which is envisaged to have had a pronounced effect on spherulite crystallization dynamics argon concentrations dissolved in the glass and spherulites differ by a factor of 20 with ar sequestered preferentially in the glass phase petrographic observation csd analysis volatile and ar data as well as diffusion modeling support continuous spherulite nucleation and growth starting at magmatic emplacement temperatures of 790 825 c and progressing through the glass transition temperature range tg 750 620 c being further modified in the solid state we propose that nucleation and growth rate are isothermally constant but vary between differing stages of spherulite growth with continued cooling from magmatic temperatures such that there is an evolution from a high to a low rate of crystallization and low to high crystal nucleation based on the diffusion of h2o across these temperature ranges 800 300 c timescales of spherulite crystallization occur on a timescale of 4 days with further modification up to 400 years growth is prohibitively slow 400 c and would become diffusion reliant selective deformation of spherulites supports a down temperature continuum of spherulite formation in the rocche rosse obsidian indeed petrographic evidence suggests that high strain zones may have catalyzed progressive nucleation and growth of further generations of spherulites during syn and post emplacement cooling

coarse bed material patch evolution in low order ephemeral channels abstract in river channel beds composed of a wide range of grain sizes the bed material is often arranged in discrete patches discernable by relative texture these bed material patches are the primary source of entrainable coarse sediment within the channel system and their composition and size have been found to influence the composition and rate of sediment transport twelve coarse gravel cobble sediment patches distributed throughout the channel network within a 4 53 ha watershed in southeastern arizona were monitored for 2 years changes in patch area and grain size were measured and painted patch grains were monitored to confirm that patch grains were mobilized during flow individual coarse bed material patches exhibited variable persistence during flows with return frequencies ranging from approximately 1 year to 4 6 years while no patch fully dispersed during the study period two new patches formed most coarse patches remained relatively stable in area and grain size distribution despite the entrainment of patch grains as lost grains were sufficiently replaced with sediment from upstream because of the grain replacement process and the effect of other sediment supply dynamics the changes in patch area and grain size distribution display a complex relationship with the magnitude of predicted grain mobilization within each patch results indicate that relative stability varies from patch to patch influenced by the balance of patch grains transported out of the patch and the deposition of new grains into the patch predictive models of coarse sediment transport and yield that assume the channel bed is a fixed source of sediment supply may not adequately capture the sediment dynamics within patchy channel beds and should be used with caution when applied to these environments due to the possibility of patch instability as documented in this study

hydrogeochemical processes governing the origin transport and fate of major and trace elements from mine wastes and mineralized rock to surface waters abstract the formation of acid mine drainage from metals extraction or natural acid rock drainage and its mixing with surface waters is a complex process that depends on petrology and mineralogy structural geology geomorphology surface water hydrology hydrogeology climatology microbiology chemistry and mining and mineral processing history the concentrations of metals metalloids acidity alkalinity cl f and so 4 2 found in receiving streams rivers and lakes are affected by all of these factors and their interactions remediation of mine sites is an engineering concern but to design a remediation plan without understanding the hydrogeochemical processes of contaminant mobilization can lead to ineffective and excessively costly remediation furthermore remediation needs a goal commensurate with natural background conditions rather than water quality standards that might bear little relation to conditions of a highly mineralized terrain this paper reviews hydrogeochemical generalizations primarily from us geological survey research that enhance our understanding of the origin transport and fate of contaminants released from mined and mineralized areas mobility of potential or actual contaminants from mining and mineral processing activities depends on 1 occurrence is the mineral source of the contaminant actually present 2 abundance is the mineral present in sufficient quantity to make a difference 3 reactivity what are the energetics rates and mechanisms of sorption and mineral dissolution and precipitation relative to the flow rate of the water and 4 hydrology what are the main flow paths for contaminated water estimates of relative proportions of minerals dissolved and precipitated can be made with mass balance calculations if minerals and water compositions along a flow path are known combined with discharge these mass balance estimates quantify the actual weathering rate of pyrite mineralization in the environment and compare reasonably well with laboratory rates of pyrite oxidation except when large quantities of soluble salts and evaporated mine waters have accumulated underground quantitative mineralogy with trace element compositions can substantially improve the identification of source minerals for specific trace elements through mass balances post dissolution sorption and precipitation attenuation reactions depend on the chemical behavior of each element solution composition and ph aqueous speciation temperature and contact time with mineral surfaces for example little metal attenuation occurs in waters of low ph

supercooled melt inclusions in lower crustal granulites as a consequence of rapid exhumation by channel flow abstract here we report on the unexpected occurrence of felsic granitic inclusions with quench textures such as spherulite and dendrite hereafter referred to as felsite inclusions similar to some volcanic rocks within garnet in presumably slowly cooled lower crustal granulites of various geologic ages ranging from early proterozoic to middle paleozoic and wide global distribution the limpopo belt the grenville province the lutzow holm complex of east antarctica the highland complex of sri lanka and the southern bohemian massif the well preserved textures of felsite inclusions are indicative of melts formed by anatexis during high pressure and high temperature metamorphism crystallization under far from equilibrium conditions at 50 c undercooling and subsequent rapid cooling the occurrence of felsite inclusions in granulites in restricted tectonostratigraphic zones in sri lanka among other examples may be the first geologic evidence for fast exhumation of lower crustal rocks to andalusite stable upper crustal conditions by channel flow in a continental collision orogen we hypothesize that granulites ascend episodically along discrete high strain zones and cool as fast as some felsic magmas this conclusion sheds new light on the debate regarding the deep crustal processes and necessitates changes to fundamental beliefs about exhumation rates based on rates of plate convergence 1 10 cm year

clay clast aggregates in gouges new textural evidence for seismic faulting 1 spherical aggregates named clay clast aggregates ccas have been reported from recent investigations on retrieved clay bearing fault gouges from shallow depth seismogenic faults and rotary shear experiments conducted on clay bearing gouge at seismic slip rates the formation of ccas appears to be related to the shearing of a smectite rich granular material that expands and becomes fluidized we have conducted additional high velocity rotary shear experiments and low velocity double shear experiments we demonstrate that a critical temperature depending on dynamic pressure temperature conditions is needed for the formation of ccas this temperature corresponds to the phase transition of pore water from liquid to vapor or to critical which induced gouge pore fluid expansion and therefore a thermal pressurization of the fault a detailed examination by energy dispersive x ray spectrometry edx sem element mapping sem and transmission electron microscopy tem shows strong similar characteristics of experimental and natural ccas with a concentric well organized fabric of the cortex and reveals that their development may result from the combination of electrostatic and capillary forces in a critical reactive medium during the dynamic slip weakening accordingly the occurrence of ccas in natural clay rich fault gouges constitutes new unequivocal textural evidence for shallow depth thermal pressurization and consequently for past seismic faulting

melt fracturing and healing a mechanism for degassing and origin of silicic obsidian we present water content transects across a healed fault in pyroclastic obsidian from lami pumice cone lipari italy using synchrotron fourier transform infrared spectroscopy results indicate that rhyolite melt degassed through the fault surface transects define a trough of low water content coincident with the fault trace surrounded on either side by high water content plateaus plateaus indicate that obsidian on either side of the fault equilibrated at different pressure temperature p t conditions before being juxtaposed the curves into the troughs indicate disequilibrium and water loss through diffusion if we assume constant t melt equilibrated at pressures differing by 0 74 mpa before juxtaposition and the fault acted as a low p permeable path for h2o that diffused from the glass within time scales of 10 and 30 min assuming constant p instead melt on either side could have equilibrated at temperatures differing by as much as 100 c before being brought together water content on the fault trace is particularly sensitive to post healing diffusion its preserved value indicates either higher temperature or lower pressure than the surroundings indicative of shear heating and dynamic decompression our results reveal that water contents of obsidian on either side of the faults equilibrated under different p t conditions and were out of equilibrium with each other when they were juxtaposed due to faulting immediately before the system was quenched degassing due to faulting could be linked to cyclical seismic activity and general degassing during silicic volcanic activity and could be an efficient mechanism of producing low water content obsidian

a review of the source behaviour and distribution of arsenic in natural waters the range of as concentrations found in natural waters is large ranging from less than 0 5 g l 1 to more than 5000 g l 1 typical concentrations in freshwater are less than 10 g l 1 and frequently less than 1 g l 1 rarely much higher concentrations are found particularly in groundwater in such areas more than 10 of wells may be affected defined as those exceeding 50 g l 1 and in the worst cases this figure may exceed 90 well known high as groundwater areas have been found in argentina chile mexico china and hungary and more recently in west bengal india bangladesh and vietnam the scale of the problem in terms of population exposed to high as concentrations is greatest in the bengal basin with more than 40 million people drinking water containing excessive as these large scale natural as groundwater problem areas tend to be found in two types of environment firstly inland or closed basins in arid or semi arid areas and secondly strongly reducing aquifers often derived from alluvium both environments tend to contain geologically young sediments and to be in flat low lying areas where groundwater flow is sluggish historically these are poorly flushed aquifers and any as released from the sediments following burial has been able to accumulate in the groundwater arsenic rich groundwaters are also found in geothermal areas and on a more localised scale in areas of mining activity and where oxidation of sulphide minerals has occurred the as content of the aquifer materials in major problem aquifers does not appear to be exceptionally high being normally in the range 1 20 mg kg 1 there appear to be two distinct triggers that can lead to the release of as on a large scale the first is the development of high ph 8 5 conditions in semi arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates this ph change leads either to the desorption of adsorbed as especially as v species and a range of other anion forming elements v b f mo se and u from mineral oxides especially fe oxides or it prevents them from being adsorbed the second trigger is the development of strongly reducing conditions at near neutral ph values leading to the desorption of as from mineral oxides and to the reductive dissolution of fe and mn oxides also leading to as release iron ii and as iii are relatively abundant in these groundwaters and so4 concentrations are small typically 1 mg l 1 or less large concentrations of phosphate bicarbonate silicate and possibly organic matter can enhance the desorption of as because of competition for adsorption sites a characteristic feature of high groundwater as areas is the large degree of spatial variability in as concentrations in the groundwaters this means that it may be difficult or impossible to predict reliably the likely concentration of as in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well arsenic affected aquifers are restricted to certain environments and appear to be the exception rather than the rule in most aquifers the majority of wells are likely to be unaffected even when for example they contain high concentrations of dissolved fe

channel morphology and bedload pulses in braided rivers a laboratory study bedload pulses in gravel bed rivers have been widely reported in recent years and attempts have been made to relate them to channel morphology bedload transport and channel morphology were measured in a small scale generic model of braided gravel bed streams two experiments are described in which braided channels developed in a 14 m 3 m sand tray total bedload output from the tray was weighed every 15 minutes stream bed geometry was surveyed every four hours pulses were observed in the bedload output time series and were qualitatively related to the channel morphology immediately upstream of the measuring section the bagnold 1980 bedload equation generally overpredicts measured bedload transport rates when applied to channels that were in equilibrium or aggrading underprediction occurred when applied to degrading channels aggradation was associated with channel multiplication and bar deposition channel pattern simplification occurred when degradation took place and bars emerged from the water flow development of phases of aggradation and degradation is dependent upon the three dimensional geometry of the stream beds spatial and temporal feedback loops can be identified enabling links between channel morphology and bedload transport rate to be directly identified

geochemical processes controlling mobilization of arsenic and trace elements in shallow aquifers and surface waters in the antequera and poopo mining regions bolivian altiplano a geochemical approach was applied to understand the factors controlling the mobilization of as and trace elements tes in mining areas of the poopo and antequera river sub basins on the bolivian altiplano a total of 52 samples surface groundwater and geothermal water were collected during the rainy season 2009 arsenic cd and mn concentrations exceed world health organization who drinking water guidelines and bolivian regulations for drinking water in 28 groundwater samples but cu ni pb and zn do not arsenic cd mn pb and zn concentrations exceed world health organization guidelines for drinking water and bolivian regulations class a standard for discharge to water bodies in 20 surface water samples whereas levels of cu do not and ni and fe rarely exceed regulation and guideline values factor analysis was applied to 18 hydrochemical parameters of 52 samples five factors for groundwater plagioclase weathering dissolution of gypsum and halite tes mobilization at acidic ph sulfide oxidation and release of as account for 86 5 of the total variance for antequera and 83 9 for poopo sub basins four factors for surface water data weathering and mobilization of tes influenced by ph dissolution of evaporate salts neutralization of acid mine drainage and as release due to dissolution of mn and fe oxides account for 91 of the total variance in antequera and 96 in poopo sub basins the as and tes mobilized in these regions could affect the local water sources which is a prevalent concern with respect to water resource management in this semi arid altiplano region presence of both natural and anthropogenic sources of contamination requires careful monitoring of water quality

sources and behavior of arsenic and trace elements in groundwater and surface water in the poopo lake basin bolivian altiplano water management in semiarid and arid catchments such as the poopo lake basin requires improved understanding of the complex behavior of the various contaminants which affect the drinking water quality and considered as crucial for sustainable development of the region mechanisms of arsenic as release in the surface and groundwater were studied hydrochemical data for surface water 4 samples and groundwater 28 samples were collected in a small watershed in the poopo catchment at the highland of the bolivian andes altiplano all of them show high electrical conductivity values and moderately oxidizing conditions the surface water contains high concentration of sulfate and the trace elements as zn and pb in the zone affected by acid mine drainage there is a large variability of the concentration of as and of the trace elements in the groundwater in the five different regions within the poopo catchment the metal concentrations sensitive to changes of redox state and results of speciation modeling suggest that as v is a predominant aqueous species which conforms to the prevailing oxidizing conditions in the shallow groundwater environment two generalized trends for as distribution were identified in groundwater a high concentrations are found in the arid zone 100 250 g l in the southern region iii and in the northwestern region v regions and b low concentrations 50 g l are found in the remaining part of the basin region i ii and iv however the spatial distribution within these regions needs to be investigated further a conclusion from the present study is that there are multiple sources of as as well as other trace elements such as cd mn and zn in the poopo lake basin among the sources and the processes which led to the mobility of as and other trace metals in the region are a weathering of sulfide minerals b oxidation of pyrite and or arsenopyrite in mineralized areas and c desorption from hydrous ferric oxide hfo surfaces in non mining areas volcanic ash is suggested to be a significant source of as

spherulites and lithophysae 200 years of investigation on high temperature crystallization domains in silica rich volcanic rocks high temperature crystallization domains htcds including spherulites and lithophysae form during cooling of silica rich lava and welded ignimbrites spherulites grow in silicate melts or hot glass and they display a radiating or microcrystalline texture typically consisting of cristobalite tridymite and sanidine lithophysae are htcds comprising one or more cavities this contribution reviews the research and discussions on htcds carried out over the last 200 years the emphasis here is on lithophysae and summarizes current knowledge of their formation a number of parameters influence the initiation and growth of lithophysae as well as their shapes and internal textures the most likely cause of cavity formation is transient tensional stress that produces a mechanical opening and widening at the interface between the crystallization front and the host melt e g where t tg cavity growth and expansion forced by rising vapor pressure is considered less important in some cases further growth of htcd cavities results from vapor phase corrosion and brecciation

near bed sediment concentration distribution and basic probability of sediment movement sediment concentration distribution and the basic probability of sediment movement near the channel bed are two of the most important and fundamental issues in the study of sediments based on statistical analysis and considering the transport mechanisms the rules of sediment concentration distribution near a channel bed are studied analytical expressions for the near bed sediment concentration distribution and mean sediment concentration are derived and the expression for the mean sediment concentration near the bed is verified by measured data which were obtained from previous experiments with the help of statistical theory the expressions of basic probabilities i e rolling saltating and suspending probabilities for sediment movement near the bed are also derived the expression for starting probability is verified by the measured data the verification shows that the results from the proposed expression agree well with the measured data this research has both theoretical and practical significance for further investigation concerning rules of bed load and suspended sediment transport

bedload transport in alluvial channels hydraulic sediment land use and rock erosivity data of 22 alluvial streams were used to evaluate conditions of bedload transport and the performance of selected bedload transport equations transport categories of transport limited tl partially transport limited ptl and supply limited sl were identified by a semiquantitative approach that considers hydraulic constraints on sediment movement and the processes that control sediment availability at the basin scale equations by parker et al in 1982 schoklitsch in 1962 and meyer peter and muller in 1948 adequately predicted sediment transport in channels with tl condition whereas the equations of bagnold in 1980 and schoklitsch in 1962 performed well for ptl and sl conditions overall the equation of schoklitsch predicted well the measured bedload data for eight of 22 streams and the bagnold equation predicted the measured data in seven streams

redox characteristics and zonation of arsenic affected multi layers aquifers in the choushui river alluvial fan taiwan summary in the light of the redox characteristics of groundwater in the arsenic affected multi layers aquifers in the choushui river alluvial fan an investigation on major redox sensitive species were carried out to explore their spatial patterns the controlling factors and the redox zonations factor analysis was applied to 16 hydrochemical parameters of 152 groundwater samples five factors including seawater salinization arsenic pollutant nitrate contamination alkaline and fe mn accounted for 86 4 of the total variance identified among them arsenic pollutant nitrate contamination and fe mn were redox sensitive factors arsenic pollutant factor has strong loading on as and toc moderate loading on nh 4 and low loading on alkalinity and fe these hydrochemical parameters closely relate to the reductive dissolution processes of arsenic release to groundwater discriminant analysis was adopted to delineate three redox zones oxidative transitional and reductive zones in 4 layers aquifers and yielded 90 4 correctness on groundwater quality data the spatial distributions of as and nh 4 were similar but showed a reverse pattern with no 3 arsenic and nh 4 were mainly distributed in the reductive zone whereas no 3 was majorly distributed in the oxidative zone suggesting the denitrification is partially responsible to the reductive dissolution of as in the groundwater

response of a channel with alternate bars to a decrease in supply of mixed size bed load a flume experiment the response of a channel with a topography and modeled bed material size typical of gravel bed rivers to reductions in sediment supply was investigated in a laboratory flume filled and fed with a sand gravel mixture after a series of quasi stationary alternate bars were formed under equilibrium sediment transport feed rate was reduced in two steps to one third and one tenth the initial rate as discharge was held constant the primary response following both reductions was an increase in bed surface particle size as a corridor of intensive bed load transport contracted and local transport rates decreased after the first feed rate reduction the channel incised by twice the mean water depth on average and caused distal bar surfaces to emerge as terracelike features bar roughness decreased and mean boundary shear stress exerted on bed surface particles increased little incision occurred after the second feed rate reduction

near liquidus growth of feldspar spherulites in trachytic melts 3d morphologies and implications in crystallization mechanisms the nucleation and growth processes of spherulitic alkali feldspar have been investigated in this study through x ray microtomography and electron backscatter diffraction ebsd data here we present the first data on shape preferred orientation spo and crystal preferred orientation cpo of alkali feldspar within spherulites the analysis of synchrotron x ray microtomography and ebsd datasets allowed us to study the morphometric characteristics of spherulites in trachytic melts in quantitative fashion highlighting the three dimensional shape preferred orientation branching of lamellae and crystal twinning providing insights about the nucleation mechanism involved in the crystallization of the spherulites the nucleation starts with a heterogeneous nucleus pre existing crystal or bubble and subsequently it evolves forming bow tie morphologies reaching radially spherulitic shapes in few hours since each lamella within spherulite is also twinned these synthetic spherulites cannot be considered as single nuclei but crystal aggregates originated by heterogeneous nucleation a twin boundary may have a lower energy than general crystal crystal boundaries and many of the twinned grains show evidence of strong local bending which combined with twin plane creates local sites for heterogeneous nucleation this study shows that the growth rates of the lamellae 10 6 10 7 cm s in spherulites are either similar or slightly higher than that for single crystals by up to one order of magnitude furthermore the highest volumetric growth rates 10 11 10 12 cm3 s show that the alkali feldspar within spherulites can grow fast reaching a volumetric size of 10 m3 in 1 s

internal textures of rhyolite flows as revealed by research drilling recent drilling of 550 and 140 000 yr old rhyolite lava flows in the inyo dome chain california and valles caldera new mexico and field inspection of 8 ma flows in western arizona reveal a more detailed vertical zonation of lava textures in glassy rhyolite flows than has previously been recognized the upper vitrophyre of a flow can usually be subdivided into a surficial finely vesicular pumice underlain by obsidian coarsely vesicular pumice and a second obsidian layer beneath these layers is the crystalline center of the flow which in turn is underlain by a basal obsidian layer lower vitrophyre differences in lava vesicularity and crystallinity between zones in a flow result from primary effervescence devitrification lava rheology and migration of water vapor

crystallization kinetics of rhyolitic melts using oxygen isotope ratios crystals provide themeans to understand igneous systems but natural constraints on crystallization kinetics are rare because thermal conditions and crystallization timescales are typically unknown oxygen isotope ratios in quartz and alkali feldspar crystals in spherulites provide a natural record of the temperature interval of crystallization and crystal growth rates in rhyolitic melts oxygen isotope compositions in both phases change progressively with position from the spherulite core to rim quartz o increases from 5 0 0 3 in the core to 5 6 0 3 at the rims whereas alkali feldspar decreases from 3 7 0 4 in the core to 2 7 0 9 at the rims fractionation therefore increases from 1 3 0 7 in the cores to 2 9 1 1 at the rims oxygen isotope thermometry tracks crystallization temperature with position spherulites nucleate at 578 160 c and continue to grow until 301 88 c the in situ analyses demonstrate that spherulites self contain a record of their thermal history and that of the host lava

modelling suspended sediment concentration and load in a transport limited alluvial gully in northern queensland australia alluvial gullies are often formed in dispersible sodic soils along steep banks of incised river channels field data collected by shellberg et al earth surface processes and landforms 38 1765 1778 2013 from a gully outlet in northern australia showed little hysteresis between water discharge and fine 63 m suspended sediment indicating transport limited rather than source limited conditions the major source of the fine silt clay component was the sodic soils of upstream gully scarps and the coarser sand component was sourced locally from channel bed material in this companion paper at the same study site a new method was developed for combining the settling velocity characteristics of these two sediment source components to estimate the average settling velocity of the total suspended sediment this was compared to the analysis of limited sediment samples collected during flood conditions these settling velocity data were used in the steady state transport limit theory of hairsine and rose water resources research 28 237 243 245 250 1992 that successfully predicted field data of concentrations and loads at a cross section regardless of the complexity of transport limited upstream sources sheet erosion scalds rills gullies mass failure bank and bed erosion other disturbed areas the analysis required calibration of a key model parameter the fraction of total stream power f 0 025 that is effective in re entraining sediment practical recommendations are provided for the prediction of sediment loads from other alluvial gullies in the region with similar hydrogeomorphic conditions using average stream power efficiency factors for suspended silt clay fw 0 016 and sand fs 0 038 respectively but with no requirement for field data on sediment concentrations only basic field data on settling velocity characteristics from soil samples channel geometry measurements estimates of water velocity and discharge and associated error margins are needed for transport limit theory predictions of concentration and load this theory is simpler than that required in source limited situations copyright 2015 john wiley sons ltd

correction to a general power equation for predicting bed load transport rates in gravel bed rivers 1 in the paper a general power equation for predicting bed load transport rates in gravel bed rivers by jeffrey j barry et al water resources research 40 w10401 doi 10 1029 2004wr003190 2004 the y axis for figures 5 and 10 was incorrectly labeled and should have read log10 predicted transport log10 observed transport in addition flow depth d is incorrectly shown in the denominator of equation a9 of barry et al 2004 and should be replaced by di the mean particle diameter for the ith size class as shown below

spherulite crystallization induces fe redox redistribution in silicic melt rhyolitic obsidians from krafla volcano iceland record the interaction between mobile hydrous species liberated during crystal growth and the reduction of ferric iron in the silicate melt we performed synchrotron ftir and xanes measurements along a transect extending from a spherulite into optically distinct colorless and brown glass zones measurements show that the colorless glass is enriched in oh groups and depleted in ferric iron while the brown glass shows the opposite relationship the color shift between brown and clear glass is sharp suggesting that the colorless glass zone was produced by a redox front that originated from the spherulite margin and moved through surrounding melt during crystallization we conclude that the most likely reducing agent is hydrogen produced by magnetite crystallization within the spherulite the krafla obsidians dramatically capture redox disequilibrium on the micoscale and highlight the importance of hydrous fluid liberation and late stage crystallization to the redox signature of glassy lavas

extended cooling and viscous flow of large hot rhyolite lavas implications of numerical modeling results abstract a common misconception about rhyolite lava flows is that they cannot advance far from their vents and are constrained to be small due to their high viscosities although a high viscosity lava will indeed flow quite slowly it may advance a great distance if the lava efficiently retains heat and thus mobility and if the available magma supply is large to evaluate this possibility a finite difference numerical model was used to determine the rate of heat loss from units interpreted to be large rhyolite lavas cooling of such 100 300 m thick units is very slow and is further retarded by a vesicular carapace and by evolution of latent heat thermal history alone suggests that large thick lava flows could remain active for several decades to constrain the duration of flow advance of these units their inferred apparent viscosities are compared with those of active lava flows mean flow velocity of some large rhyolites in southwestern idaho usa was estimated based on the observation that the units must still have been advancing by viscous flow at the start of spherulite crystallization indicating perhaps a 10 to 18 year flow duration based on studies of obsidian dome california mean flow velocities would have been roughly 0 59 to 2 5 km yr and calculated average apparent viscosities of the flows ranged from 1 8 to 3 6 orders of magnitude greater than the viscosity of the lava itself this relationship also holds for active andesite and basaltic andesite block lava flows which have apparent viscosities 2 7 to 4 orders of magnitude greater than lava viscosities this implies that large rhyolites may indeed have been emplaced in the same manner as less silicic block lavas and that long distance viscous flow is not unrealistic for rhyolite lavas a good example of a large volume lava is the 15 km 3 rhyolite of the badlands sw idaho where vent relations show that lava effused from a fissure and advanced up to 9 km more extensive rhyolites could also be emplaced solely by effusion extended flow of these lavas may lead to the remelting and removal of basal crumble breccias and to the formation of welded shard textures in their margins both processes tending to obscure the lava flow origin of the units the great dimensions of lava like rhyolitic units with volumes of 10 to at least 200 km 3 are not a sufficient reason to assume they erupted as ignimbrites

models for viscosity and shear localization in bubble rich magmas abstract bubble content influences magma rheology and thus styles of volcanic eruption increasing magma vesicularity affects the bulk viscosity of the bubble melt suspension and has the potential to promote non newtonian behavior in the form of shear localization or brittle failure here we present a series of high temperature uniaxial deformation experiments designed to investigate the effect of bubbles on the magma bulk viscosity the starting materials are cores of natural rhyolitic obsidian synthesized to have variable vesicularity 0 66 the foamed cores were deformed isothermally t 750 c at atmospheric conditions using a high temperature uniaxial press under constant displacement rates strain rates between 0 5 1 10 4 s 1 and to total strains of 10 40 the viscosity of the bubble free melt 0 was measured by micropenetration and parallel plate methods to establish a baseline for experiments on the vesicle rich cores at the experimental conditions rising vesicle content produces a marked decrease in bulk viscosity that is best described by a two parameter empirical equation log 10 bulk log 10 0 1 47 1 0 48 our parameterization of the bubble melt rheology is combined with maxwell relaxation theory to map the potential onset of non newtonian behavior shear localization in magmas as a function of melt viscosity vesicularity and strain rate for low degrees of strain i e as in our study the rheological properties of vesicular magmas under different flow types pure vs simple shear are indistinguishable for high strain or strain rates where simple and pure shear viscosity values may diverge our model represents a maximum boundary condition vesicular magmas can behave as non newtonian fluids at lower strain rates than unvesiculated melts thereby promoting shear localization and explosive or non explosive magma fragmentation the extent of shear localization in magma influences outgassing efficiency thereby affecting magma ascent and the potential for explosivity

sediment supply and the development of the coarse surface layer in gravel bedded rivers the bed surface of most gravel rivers is considerably coarser than the sub surface or the gravel load transported over it a phenomenon affecting river dynamics morphology and ecology the coarse surface layer often called an armour or pavement has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1 2 and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3 4 where bedload supply is cut off below dams selective erosion causes coarsening5 6 using a simple quantitative model we propose that surface coarsening develops in gravel bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load we present laboratory results which support this hypothesis and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse less active bed it may therefore be possible to relate the degree of river bed surface coarsening to sediment supply resulting from land use

spherulites as in situ recorders of thermal history in lava flows spherulites in rhyolitic obsidian provide a record of the thermal history of their host lava during the interval of spherulite growth we use trace element concentration profiles across spherulites and into the obsidian host from yellowstone national park usa to reconstruct the conditions that existed during spherulite formation the measured transects reveal three behaviors expulsion of the most diffusively mobile elements from spherulites with no concentration gradients in the surrounding glass type 1 enrichment of slower diffusing elements around spherulites with concentration gradients extending outward into the glass type 2 and complete entrapment of the slowest diffusing elements by the spherulite type 3 we compare the concentration profiles measured by laser ablation inductively coupled plasma mass spectrometry and fourier transform infrared spectroscopy to the output of a spherulite growth model that incorporates known diffusion parameters the temperature interval of spherulite growth the cooling rate of the lava and data on the temporal evolution of spherulite radius our results constrain spherulite nucleation to the temperature interval 700 550 c and spherulite growth to 700 400 c in a portion of lava that cooled at 10 5 2 0 3 c s 1 which matches an independent experimental estimate of 10 5 3 c s 1 measured using differential scanning calorimetry maximum spherulite growth rates at nucleation are on the order of 1 m hr 1 and are inferred to decrease exponentially with time hence spherulites may serve as valuable in situ recorders of the thermal history of lava flows

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

experiments and models on h2o retrograde solubility in volcanic systems we present a suite of 36 high temperature 900 1100 c experiments performed on 10 10 mm unjacketed cores of rhyolitic obsidian from hrafntinnuhryggur krafla iceland under atmospheric pressure the obsidian is bubble and crystal free with an h2o content of 0 11 4 wt the obsidian cores were heated above the glass transition temperature tg held for 0 25 24 h then quenched during each experiment the volume of the samples increased as h2o vapor filled bubbles nucleated and expanded uniquely the bubbles did not nucleate on the surface of the core nor escape conserving mass during all experiments within each isothermal experimental suite the cores increased in volume with time until they reached a maximum after which continued heating caused no change in volume measured by he pycnometry we interpret these t t conditions as representing thermochemical equilibrium between the melt and exsolved vapor these experiments are modeled to recover the 1 atm temperature dependent solubility of water in the rhyolite melt our results define the magnitude of retrograde solubility 7 1 10 3 wt h2o per 100 c and provide estimates of the enthalpy and entropy of the h2o exsolution reaction h 17 8 kj mol s 107 j k mol we conclude by modeling the implications of retrograde solubility for the glass transition temperatures tg of cooling volcanic systems at pressures relevant to volcanic conduits and the earth s surface all volcanic systems cool the effects of retrograde solubility are to allow melts to rehydrate by h2o dissolution as they cool isobarically thereby depressing tg and expanding the melt window ultimately the melt is quenched at higher h2o contents and lower temperatures where the isobaric retrograde solubility curve catches the evolving tg

cristobalite in the 2011 2012 cordon caulle eruption chile cristobalite is a low pressure high temperature polymorph of sio2 found in many volcanic rocks its volcanogenic formation has received attention because 1 pure particulate cristobalite can be toxic when inhaled and its dispersal in volcanic ash is therefore a potential hazard and 2 its nominal stability field is at temperatures higher than those of magmatic systems making it an interesting example of metastable crystallization we present analyses by xrd sem epma laser raman and synchrotron ct of representative rhyolitic pyroclasts and of samples from different facies of the compound lava flow from the 2011 2012 eruption of cordon caulle chile cristobalite was not detected in pyroclasts negating any concern for respiratory hazards but it makes up 0 23 wt of lava samples occurring as prismatic vapour deposited crystals in vesicles and or as a groundmass phase in microcrystalline samples textures of lava collected near the vent which best represent those generated in the conduit indicate that pore isolation promotes vapour deposition of cristobalite mass balance shows that the sio2 deposited in isolated pore space can have originated from corrosion of the adjacent groundmass textures of lava collected down flow were modified during transport in the insulated interior of the flow where protracted cooling additional vesiculation events and shearing overprint original textures in the most slowly cooled and intensely sheared samples from the core of the flow nearly all original pore space is lost and vapour deposited cristobalite crystals are crushed and incorporated into the groundmass as the vesicles in which they formed collapse by strain and compaction of the surrounding matrix holocrystalline lava from the core of the flow achieves high mass concentrations of cristobalite as slow cooling allows extensive microlite crystallization and devitrification to form groundmass cristobalite vapour deposition and devitrification act concurrently but semi independently both are promoted by slow cooling and it is ultimately devitrification that most strongly contributes to total cristobalite content in a given flow facies our findings provide a new field context in which to address questions that have arisen from the study of cristobalite in dome eruptions with insight afforded by the fundamentally different emplacement geometries of flows and domes

causes and consequences of pressurisation in lava dome eruptions abstract high total and fluid pressures develop in the interior of high viscosity lava domes and in the uppermost parts of the feeding conduit system as a consequence of degassing two effects are recognised and are modelled quantitatively first large increases in magma viscosity result from degassing during magma ascent strong vertical gradients in viscosity result and large excess pressures and pressure gradients develop at the top of the conduit and in the dome calculations of conduit flow show that almost all the excess pressure drop from the chamber in an andesitic dome eruption occurs during the last several hundred metres of ascent second microlites grow in the melt phase as a consequence of undercooling caused by gas loss rapid microlite growth can cause large excess fluid pressures to develop at shallow levels theoretically closed system microlite crystallization can increase local pressure by a few tens of mpa although build up of pressure will be countered by gas loss through permeable flow and expansion by viscous flow microlite crystallization is most effective in causing excess gas pressures at depths of a few hundred metres in the uppermost parts of the conduit and dome interior some of the major phenomena of lava dome eruptions can be attributed to these pressurisation effects including spurts of growth cycles of dome growth and subsidence sudden onset of violent explosive activity and disintegration of lava during formation of pyroclastic flows the characteristic shallow level long period and hybrid seismicity characteristic of dome eruptions is attributed to the excess fluid pressures which are maintained close to the fracture strength of the dome and wallrock resulting in fluid movement during formation of tensile and shear fractures within the dome and upper conduit

degassing driven crystallisation in basalts abstract syn eruptive crystallisation can drastically increase magma viscosity with profound implications for conduit dynamics lava emplacement and volcanic hazards there is growing evidence that crystallisation is not only cooling driven but can also occur almost isothermally during decompression induced degassing on ascent from depth here we review field and experimental evidence for degassing driven crystallisation in a range of magma compositions we then present new results showing for the first time experimental evidence for this process in basaltic magma our experiments use simultaneous thermogravimetric analysis and differential scanning calorimetry coupled with mass spectrometry tga dsc ms to monitor degassing patterns and thermal events during heating and cooling of porphyritic basaltic samples from mt etna italy the partly degassed samples which contained 0 39 0 81 wt total volatiles in the glass fraction were subjected to two cycles of heating from ambient to 1250 c on the first heating tga data show that 30 60 of the total volatiles degassed slowly at

timescales of spherulite crystallization in obsidian inferred from water concentration profiles we determined the kinetics of spherulite growth in obsidians from krafla volcano iceland we measured water concentration profiles around spherulites in obsidian by synchrotron fourier transform infrared spectroscopy the distribution of oh groups surrounding spherulites decreases exponentially away from the spherulite glass border reflecting expulsion of water during crystallization of an anhydrous paragenesis plagioclase sio2 clinopyroxene magnetite this pattern is controlled by a balance between the growth rate of the spherulites and the diffusivity of hydrous solute in the rhyolitic melt we modeled advective and diffusive transport of the water away from the growing spherulites by numerically solving the diffusion equation with a moving boundary numerical models fit the natural data best when a small amount of post growth diffusion is incorporated in the model comparisons between models and data constrain the average spherulite growth rates for different temperatures and highlight size dependent growth among a small population of spherulites

crystallization history of obsidian dome inyo domes california samples obtained by u s department of energy research drilling at the 600 year old obsidian dome volcano provide the rare opportunity to examine the transition from volcanic dome to plutonic intrusion textures in a silicic magma system textures in the lavas from obsidian dome record multiple periods of crystallization initiated in response to changes in undercooling t related to variable degassing in the mag ma phenocr ysts formed first at low t a drastic increase in t related to loss of a vapor phase during initial stages of eruption caused nucleation of microlites all of the lavas thus contain phenocrysts and microlites extrusion and subsequent devitrification of the dry 0 1 wt h2o magma crystallized spherulites and fine grained rhyolite at high t a granophyric texture representing crystallization at a moderate t formed in the intrusions beneath obsidian dome textures in the intrusion apparently represent crystallization of hydrous 1 2 wt h2o rhyolitic magma at shallow depths

cooling rates of mid ocean ridge lava deduced from clinopyroxene spherulites abstract we present a compositional analysis of spherulites and their surrounding groundmass in basaltic andesite lava samples collected from the pacific antarctic ridge the spherulites consist of radiating dendritic branches of clinopyroxene and contain abundant vesicles that are stretched parallel to the branches stretched vesicles require that the spherulites grew when the groundmass was liquid and thus while it was hotter than its glass transition although chemical components incompatible in clinopyroxene should have been enriched in the melt by spherulite growth no compositional variations occur at a scale larger than the 10 m wide slit aperture used for laser ablation icp ms the absence of large scale gradients implies that the spherulites grew faster than the elements could diffuse 10 m because diffusion is temperature controlled cooling must have been rapid enough to prevent movement if the spherulites nucleated when the melt was at between 1380 and 1223 k i e undercooled by 0 to 157 k then the constant concentrations of li na and k the fastest diffusing elements analyzed require temperature to have cooled faster than 10 6 7 k min 1 if instead the spherulites nucleated at undercoolings of 357 to 465 k or as cold as 915 k equal to the glass transition temperature then the cooling rate could have been as slow as those inferred for glassy rinds on submarine lavas by relaxation geospeedometry at all possible cooling rates however the spherulites must have grown as fast as 10 3 5 m s 1 to reach their final sizes of 1 4 to 3 2 mm we speculate that the rapid growth resulted from low kinetic barriers to clinopyroxene growth in the basaltic andesite melt

entrainment of gravel from naturally sorted riverbed material the critical dimensionless shear stress ci required to entrain a given size particle di was computed from extensive bedload transport measurements in three self formed rivers that have naturally sorted gravel and cobble bed material the value of ci was determined to be significantly affected by the size distribution of the riverbed material for bed particles between 0 3 to 4 2 times the median diameter of the subsurface bed material d50 the average critical dimensionless shear stress ci is equal to ci 0 0834 di d50 0 872 with a coefficient of determination of 0 980 for bed particles larger than 4 2 times d50 ci appears to approach a constant value of approximately 0 020 in a noneroding channel thus ci varies from 0 25 to 0 020 for a given particle depending on the ratio of di to d50 previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0 25 to 0 020 for a given particle size this analysis indicates that virtually all of the variation is due to differences in the subsurface bed material size distribution this analysis shows that ci varies almost inversely proportional to the particle diameter for a nonuniform bed material therefore bed particles between 0 3 to 4 2 times d50 are entrained at nearly the same discharge an investigation of the hydraulic characteristics and bed material mobility of 24 self formed gravel bed rivers in colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less particles as large as the 90th percentile fraction were entrained by the bankfull discharge in nine of the rivers studied consequently in these rivers a significant fraction of the riverbed was entrained rather frequently over a period of years although the transport rate was very small

arsenic associations in sediments from the loess aquifer of la pampa argentina groundwater from the quaternary loess aquifer of la pampa central argentina has significant problems with high concentrations of as up to 5300 g l 1 as well as other potentially toxic trace elements such as f b mo u se and v total as concentrations in 45 loess samples collected from the aquifer have a range of 3 18 mg kg 1 with a mean of 8 mg kg 1 these values are comparable to world average sediment as concentrations five samples of rhyolitic ash from the area have as concentrations of 7 12 mg kg 1 chemical analysis included loess sediments and extracted porewaters from two specially cored boreholes results reveal a large range of porewater as concentrations being generally higher in the horizons with highest sediment as concentrations the displaced porewaters have as concentrations ranging up to 7500 g l 1 as well as exceptionally high concentrations of some other oxyanion species including v up to 12 mg l 1 the highest concentrations are found in a borehole located in a topographic depression which is a zone of likely groundwater discharge and enhanced residence time comparison of sediment and porewater data does not reveal unequivocally the sources of the as but selective extract data acid ammonium oxalate and hydroxylamine hydrochloride suggest that much of the as and v is associated with fe oxides primary oxides such as magnetite and ilmenite may be partial sources but given the weathered nature of many of the sediments secondary oxide minerals are probably more important extract compositions also suggest that mn oxide may be an as source the groundwaters of the region are oxidising with dissolved o2 no3 and so4 normally present and as v usually the dominant dissolved as species under such conditions the solubility of fe and mn oxides is low and as mobilisation is strongly controlled by sorption desorption reactions desorption may be facilitated by the relatively high ph conditions of the groundwaters in the region 7 0 8 8 and high concentrations of potential competitors e g v p hco3 phreeqc modelling suggests that the presence of v at the concentrations observed in the pampean porewaters can suppress the sorption of as to hydrous fe iii oxide hfo by up to an order of magnitude bicarbonate had a comparatively small competitive effect oxalate extract concentrations have been used to provide an upper estimate of the amount of labile as in the sediments a near linear correlation between oxalate extractable and porewater as in one of the cored boreholes investigated has been used to estimate an approximate kd value for the sediments of 0 94 l kg 1 this low value indicates that the sediments have an unusually low affinity for as