gas evolution in eruptive conduits combining insights from high temperature and pressure decompression experiments with steady state flow modeling abstract in this paper we examine the consequences of bubble nucleation mechanism on eruptive degassing of rhyolite magma we use the results of published high temperature and pressure decompression experiments as input to a modified version of conflow the numerical model of mastin and ghiorso 2000 u s g s open file rep 00 209 53 pp and mastin 2002 geochem geophys geosyst 3 10 1029 2001gc000192 for steady two phase flow in vertical conduits synthesis of the available experimental data shows that heterogeneous nucleation is triggered at p 120 150 mpa and leads to disequilibrium degassing at extreme h2o supersaturation in this latter case nucleation is an ongoing process controlled by changing supersaturation conditions exponential bubble size distributions are often produced with number densities of 106 109 bubbles cm3 our numerical analysis adopts an end member approach that specifically compares equilibrium degassing with delayed disequilibrium degassing characteristic of homogeneously nucleating systems the disequilibrium simulations show that delaying nucleation until p 150 mpa restricts degassing to within 1500 m of the surface fragmentation occurs at similar porosity in both the disequilibrium and equilibrium modes 80 vol but at the distinct depths of 500 m and 2300 m respectively the vesiculation delay leads to higher pressures at equivalent depths in the conduit and the mass flux and exit pressure are each higher by a factor of 2 0 residual water contents in the melt reaching the vent are between 0 5 and 1 0 wt roughly twice that of the equilibrium model

the effects of soluble salts on the threshold shear velocity of fine sand wind tunnel tests were carried out to investigate the effects of two soluble salts nacl and kci on the threshold shear velocity of a well sorted fine sand a sensitive laser monitoring system was used to detect initial grain motion during the tests results indicate that even small amounts of soluble salt can significantly increase the threshold velocity of the sand because of cement like bonds formed between grains that tend to hold individual particles in place however with increasing concentration the salt crystal growth causes the initially smooth sand surface to develop an irregular frothy texture which also directly affects the threshold shear velocity this is indicated by increased scatter of data on the threshold shear velocity surface salt concentration plots for both nacl and kcl although data scatter is evident significant exponential relationships do exist between threshold shear velocity and surface salt concentration in each case

the physics of wind blown loess implications for grain size proxy interpretations in quaternary paleoclimate studies abstract loess deposits are recorders of aeolian activity during past glaciations since the size distribution of loess deposits depends on distance to the dust source and environmental conditions at the source during transport and at deposition loess particle size distributions and derived statistical measures are widely used proxies in quaternary paleoenvironmental studies however the interpretation of these proxies often only considers dust transport processes to move beyond such overly simplistic proxy interpretations and toward proxy interpretations that consider the range of environmental processes that determine loess particle size distribution variations we provide a comprehensive review on the physics of dust particle mobilization and deposition furthermore using high resolution bulk loess and quartz grain size datasets from a last glacial interglacial sequence we show that because grain size distributions are affected by multiple often stochastic processes changes in these distributions over time allow multiple interpretations for the driving processes consequently simplistic interpretations of proxy variations in terms of only one factor e g wind speed are likely to be inaccurate nonetheless using loess proxies to understand temporal changes in the dust cycle and environmental parameters requires i a careful site selection to minimize the effects of topography and source distance and ii the joint use of bulk and quartz grain size proxies together with high resolution mass accumulation rate calculations if possible

textural heterogeneities in pumices from the climactic eruption of mount pinatubo 15 june 1991 and implications for magma ascent dynamics the climactic event of mount pinatubo represents one of the most thoroughly studied eruptions of the century and has provided important insights into the dynamics of explosive volcanism we have performed detailed textural analyses of the white and gray pumices of the plinian and pyroclastic flow deposits and found that differences in color and clast density reflect different crystal and vesicle amounts and size distributions white pumice has higher vesicularity deformed and highly coalesced vesicles with thin walls euhedral phenocrysts and microlite free groundmass gray pumice shows lower vesicularity wider ranges in vesicle number density limited coalescence vesicles with thick walls that are less deformed phenocrysts and microphenocrysts with abundant solution pitting and groundmass containing ubiquitous microlites and crystal fragments the presence of white and gray pumice varieties and the broad range in vesicularity and vesicle number density that characterizes both of them appear to record the complexities of conduit processes such as magma vesiculation and fragmentation and the development of conduit regions marked by different rheological behaviors in particular the results of this study suggest the likely importance of intense shear and viscous dissipation at the conduit walls a mechanism that may be responsible for the creation and discharge of the gray pumice of this eruption along with the dominant white variety

aeolian sand drift from the intertidal zone on a temperate beach potential and actual rates this paper compares the amount of sand that is potentially moved into a coastal dune system from a beach using estimates based solely on the wind force with actual measured amounts the actual sand inputs are determined by measuring sand level changes each month over a period of eighteen months along a 1760 m section of foredune on braunton burrows southwest england the calculation procedure used to estimate the potential rates of sand drift is based on the capacity of the wind force to transport sand using a published transport equation white 1979 the results show that the actual movement rates are markedly smaller than the potential rates for all months except in the summer meteorological variables and the watertable level in the adjacent dune system are used in an empirically derived regression equation in an attempt to explain the large discrepancy between the potential and actual movement rates the resulting corrected potential movement rates are a closer estimate to the actual rates but there remains much unexplained variance

rhyolite magma degassing an experimental study of melt vesiculation the vesiculation of a peralkaline rhyolite melt initially containing 0 14 wt h2o has been investigated at temperatures above the rheological glass transition tg 530 c by a in situ optical observation of individual bubble growth or dissolution and b dilatometric measurements of the volume expansion due to vesiculation the activation energy of the timescale for bubble growth equals the activation energy of viscous flow at relatively low temperatures 650 790 c but decreases and tends towards the value for water diffusion at high temperatures 790 925 c the time dependence of volume expansion follows the avrami equation

the climate archive dune sedimentary record of annual wind intensity active dunes contain an unread high resolution archive of past wind strength variations in the grain size spectrum of an active dune through time are compared with historical time series of wind speed annually averaged sediment derived wind intensity and instrumental records of wind speed show a correlation as high as 0 75 the potential of eolian dunes to gain long term data series of wind intensity in areas and for time periods lacking an instrumental record is demonstrated by reconstructing 20th century wind intensity variations in the southern north sea area the approach can be used in both recent and fossil dune systems potential applications include the validation of climate models the reconstruction of supraregional wind systems and the monitoring of future shifts in the climate system

evolution of melt vapor surface tension in silicic volcanic systems experiments with hydrous melts 1 we evaluate the melt vapor surface tension of natural water saturated dacite melt at 200 mpa 950 1055 c and 4 8 5 7 wt h2o we experimentally determine the critical supersaturation pressure for bubble nucleation as a function of dissolved water and then solve for at those conditions using classical nucleation theory the solutions obtained give dacite melt vapor surface tensions that vary inversely with dissolved water from 0 042 0 003 j m 2 at 5 7 wt h2o to 0 060 0 007 j m 2 at 5 2 wt h2o to 0 073 0 003 j m 2 at 4 8 wt h2o combining our dacite results with data from published hydrous haplogranite and high silica rhyolite experiments reveals that melt vapor surface tension also varies inversely with the concentration of mafic melt components e g cao feototal mgo we develop a thermodynamic context for these observations in which melt vapor surface tension is represented by a balance of work terms controlled by melt structure overall our results suggest that cooling crystallization and vapor exsolution cause systematic changes in that should be considered in dynamic modeling of magmatic processes

sedimentology and development of parabolic dunes grande prairie dune field alberta the grande prairie dune field in northwest central alberta is composed of parabolic and dome dunes presently inactive but which had developed under westerly winds two parabolic dune types are rec

new evidence for degassing processes during explosive eruptions measurements of pore size distribution in five samples of pumice formed during explosive eruptions of plinian type show three distinct sizes of vesicle the characteristics of the three pore size populations are similar in the samples although they were collected from plinian deposits of five different volcanoes we interpret the pore size populations as representing the degassing of silicic magma on three very different time scales the coarse population 60 m represents slow vesiculation within the magma chamber over periods of weeks months or years at small levels of gas supersaturation the medium sized population 50 5 m represents rapid vesiculation of magma on ascent to the surface during the plinian eruption over periods of minutes to hours the fine population 5 0 5 m represents bubbles formed during explosive disruption of magma and rapid ejection into the atmosphere involving short time scales a few seconds but large decompression rates

the evolution of bubble size distributions in volcanic eruptions we review observations of bubble size distributions bsds generated during explosive volcanic eruptions and laboratory explosions as inferred from vesicle size distributions found in the end products unimodal polymodal exponential and power law bsds are common even in the absence of coalescence and both power law and exponential distributions have been generated in the same eruption to date theoretical models have proposed incompatible mechanisms for producing the various distributions we here present a unifying mechanism data from our laboratory analogue experiments suggest that power law distributions are associated with highly non equilibrium degassing a numerical model is developed in which bubbles nucleate repeatedly and grow in the spaces between those of previous generations where in a non equilibrium degassing scenario the volatile concentration remains high this process causes the bsd to evolve from unimodal through exponential into a power law the exponent of the power law is a measure of the number of nucleation events or the duration of the nucleation period compared with the timescale of bubble growth the mathematical inevitability of the evolution from unimodal poissonian to power law is discussed the findings may resolve the apparent contradiction between the equilibrium degassing conduit flow models and the non equilibrium degassing conditions derived from bubble growth models of explosive volcanic eruptions the process of ongoing nucleation is the mechanism whereby the volcanic system maintains near equilibrium in the case of rapid depressurisation and slow volatile diffusion

dynamic observations of vesiculation reveal the role of silicate crystals in bubble nucleation and growth in andesitic magmas abstract bubble nucleation and growth control the explosivity of volcanic eruptions and the kinetics of these processes are generally determined from examinations of natural samples and quenched experimental run products these samples however only provide a view of the final state from which the initial conditions of a time evolving magmatic system are then inferred the interpretations that follow are inexact due to the inability of determining the exact conditions of nucleation and the potential detachment of bubbles from their nucleation sites an uncertainty that can obscure their nucleation location either homogeneously within the melt or heterogeneously at the interface between crystals and melts we present results of a series of dynamic real time 4d x ray tomographic microscopy experiments where we observed the development of bubbles in crystal bearing silicate magmas experimentally synthesized andesitic glasses with 0 25 0 5 wt h 2 o and seed silicate crystals were heated at 1 atm to induce bubble nucleation and track bubble growth and movement in contrast to previous studies on natural and experimentally produced samples we found that bubbles readily nucleated on plagioclase and clinopyroxene crystals that their contact angle changes during growth and that they can grow to sizes many times that of the silicate on whose surface they originated the rapid heterogeneous nucleation of bubbles at low degrees of supersaturation in the presence of silicate crystals demonstrates that silicates can affect when vesiculation ensues influencing subsequent permeability development and effusive vs explosive transition in volcanic eruptions

effects of time dependent moisture content of surface sediments on aeolian transport rates across a beach wildwood new jersey u s a a one day field investigation on an unvegetated backbeach documents the importance of surface sediment drying to aeolian transport surface sediments were well sorted fine sand moisture content of samples taken in the moist areas on the backbeach varied from 2 9 to 9 2 per cent lack of dry sediment inhibited transport prior to 08 50 by 09 10 conspicuous streamers of dry sand moved across the moist surface barchan shaped bedforms 30 to 40 mm high and composed of dry sand moisture content 0 10 per cent formed where sand streamers converged the surface composed of dry sand increased from 5 per cent of the area of the backbeach at 09 50 to 90 per cent by 12 50 mean wind speeds were beetween 5 6 and 8 6 m s 1 at 6 m above the backbeach corresponding shear velocities were always above the entrainment threshold for dry sand and below the threshold for the moist sand on the backbeach measured rates of sand trapped by vertical cylindrical traps increased during the day relative to calculated rates the measured rate of sand trapped on the moist foreshore was higher than the rate trapped on the backbeach during the same interval indicating that the moist foreshore moisture content 18 per cent was an efficient transport surface for sediment delivered from the dry portion of the beach upwind measured rates of sand trapped show no clear relationship to shear velocities unless time dependent surface moisture content is considered results document conditions that describe transport across moist surfaces in terms of four stages including 1 entrainment of moist sediment from a moist surface 2 in situ drying of surface grains from a moist surface followed by transport across the surface 3 entrainment and transport of dry sediment from bedforms that have accumulated on the moist surface and 4 entrainment of sand from a dry upwind source and transport across a moist downwind surface 1997 john wiley sons ltd

experimental partial melting of the st severin ll and lost city h chondrites we have performed melting experiments between 1170 1325 c and at an oxygen fugacity of iw 1 on the ordinary chondrites lost city h and st severin ll in order to investigate the origin of diogenites and their possible relationship to eucrites low ca pyroxene disappears from the lost city residuum just above 1220 c at a mg near 66 accordingly lost city does not have a sufficiently large stability field for orthopyroxene to be a likely parent composition for melts which could crystallize diogenites in contrast orthopyroxene in st severin persists up to about 1325 c at which point it has a mg similar to that of typical diogenites 75 this is only marginally compatible with diogenite origins as the most magnesian pyroxenes from diogenites mg 82 and howardites mg 85 are more mgo rich than any pyroxene formed from a st severin melt accordingly if diogenites formed from a source having the bulk composition of an ordinary chondrite than either the fo2 had to be lower than that inferred for eucrite formation or more likely the parent body had a lower fesi ratio than that of an ll chondrite also low temperature melts of st severin are depleted in elements compatible with pyroxene and so while broadly eucritic do not closely match the compositions of eucrites that are thought to be primary partial melts hence it doesn t appear possible to produce both diogenites and eucrites from the same source region composition if eucrites are primary partial melts reconnaiss mce experiments at higher oxygen fugacity iw 2 produce broadly angritic melts for lost city as was observed for murchison and allende in contrast our na2o poor st severin charge produced a broadly eucritic melt at this higher fo2 this difference in melting behavior is probably because ll chondrites have a much lower bulk fesi ratio than the h cm or cv chondrites

temporal and spatial variabilities in the surface moisture content of a fine grained beach abstract this study examined spatial and temporal variations in the surface moisture content of a fine grained beach at padre island texas usa surface moisture measurements were collected on a 27 24 m grid that extended from the dune toe to the upper foreshore the grid was surveyed at 2 to 4 h intervals for two tidal cycles generating 17 maps of the spatial distribution of surface moisture simultaneous measurements of air temperature and humidity wind speed and direction tidal elevation and water table elevation were used to interpret observed changes in surface moisture it was found that the spatial distribution of surface moisture was broadly characterized by a cross shore gradient of high to low content moving landward from the swash zone the distribution of surface moisture was conceptualized in terms of three zones saturated 25 intermediate or transitional 5 25 and dry

the dynamism of salt crust patterns on playas playas are common in arid environments and can be major sources of mineral dust that can influence global climate these landforms typically form crusts that limit evaporation and dust emission modify surface erosivity and erodibility and can lead to over prediction or underprediction of 1 dust emission potential and 2 water and heat fluxes in energybalance modeling through terrestrial laser scanning measurements of part of the makgadikgadi pans of botswana a southern hemisphere playa that emits significant amounts of dust we show that over weeks months and a year the shapes of these surfaces change considerably ridge thrusting of 30 mm week and can switch among continuous ridged and degraded patterns ridged pattern development changes the measured aerodynamic roughness of the surface as much as 3 mm week the dynamic nature of these crusted surfaces must be accounted for in dust entrainment and moisture balance formulae to improve regional and global climate models

wind tunnel modelling of the influence of vegetation structure on saltation threshold the influence of vegetation structure on saltation threshold was investigated using uniformly spaced arrays of non erodible roughness elements on a bed of erodible sand in an wind tunnel structural variables tested using arrays of solid cylinders included element aspect ratio height diameter and lateral cover total frontal silhouette area per unit ground area in agreement with previous studies increase in saltation threshold above the value for bare sand was strongly related to lateral cover increasing aspect ratio from 0 25 to 4 tended to enhance the increase in saltation threshold at a given lateral cover an approximate fit to the results could be obtained using a relation proposed by raupach et al journal of geophysical research 1993 98d 3023 3029 porous elements were constructed as clusters of narrow vertically oriented cylinders model stems forming porous model plant bodies that were cylindrical in overall shape low porosity elements were found to be approximately 50 per cent more effective in increasing saltation threshold than either solid or high porosity elements the ratio of plant body frontal silhouette area based on overall dimensions to total stem frontal silhouette area was found to be a useful measure of plant body porosity for wind erosion studies some conventional measures of vegetation amount fail to account for changes in vegetation structural attributes that may strongly influence aeolian processes

dynamics and energetics of bubble growth in magmas analytical formulation and numerical modeling we have developed a model of diffusive and decompressive growth of a bubble in a finite region of melt which accounts for the energetics of volatile degassing and melt deformation as well as the interactions between magmatic system parameters such as viscosity volatile concentration and diffusivity on the basis of our formulation we constructed a numerical model of bubble growth in volcanic systems we conducted a parametric study in which a saturated magma is instantaneously decompressed to one bar and the sensitivity of the system to variations in various parameters is examined variations of each of seven parameters over practical ranges of magmatic conditions can change bubble growth rates by 2 4 orders of magnitude our numerical formulation allows determination of the relative importance of each parameter controlling bubble growth for a given or evolving set of magmatic conditions an analysis of the modeling results reveals that the commonly invoked parabolic law for bubble growth dynamics r t1 2 is not applicable to magma degassing at low pressures or high water oversaturation but that a logarithmic relationship r log t is more appropriate during active bubble growth under certain conditions a second aspect of our study involved a constant decompression bubble growth model in which an initially saturated magma was subjected to a constant rate of decompression model results for degassing of initially water saturated rhyolitic magma with a constant decompression rate show that oversaturation at the vent depends on the initial depth of magma ascent on the basis of decompression history explosive eruptions of silicic magmas are expected for magmas rising from chambers deeper than 2 km for ascent rates 1 5 m s 1

the glass transition in hydrous granitic melts abstract the influence of water on the glass transition temperature of hydrous granitic melts is estimated on the basis of a variety of available sources of data that provide phenomenological data for the liquid glass transition and for which the relaxation timescale can be accurately estimated these sources include a dilatometric micropenetration viscosity determinations b temperature dependent spectroscopic investigations of hydrous melts and c densities of synthetic fluid inclusions in glasses all available evidence on the glass transition temperature of hydrous granitic melts when corrected for equivalent timescales yields a single consistent trend of decreasing tg with increasing water content that has powerful applications in the rheology and kinetics of processes involving late stage silicic intrusives and eruptive products the brittle ductile transition in hydrous silicic liquids can be accurately predicted using this trend combination of the presently derived relationship for the influence of water on the glass transition of granitic melts with presently available data for the high temperature viscosities of such systems permits the estimation of the temperature dependence of the viscosity of hydrous melts over the entire range of temperature relevant to granite petrogenesis such a comparison reveals that the viscosity temperature relationships of hydrous granitic melts are not arrhenian as is assumed by most existing calculational schemes for the prediction of hydrous melt viscosities the increasing deviation of viscosity temperature relationships of hydrous granitic melts from arrhenian with increasing water content is broadly consistent with similar trends accompanying the addition of excess alkalies to metaluminous granitic melt base compositions in detail however the influence of water on the deviation of viscosity temperature relationships of hydrous granitic melts from arrhenian is not as strong as would be predicted from a molar comparison of the effects of h2o vs alkali oxide or alkaline earth oxide added to these melts the answer to this apparent discrepancy may lie in the incomplete dissociation of water in the melt structure

the genesis of diogenites and hed parent body petrogenesis abstract samples of clasts and whole rocks from eighteen of the twenty four known diogenites orthopyroxenite breccia meteorites have been studied by combined inaa empa in order to elucidate their petrogenetic history the incompatible elements a12o3 and tio2 are positively correlated in diogenite orthopyroxenes these elements determined by empa are also positively correlated with incompatible trace elements such as yb determined by inaa on bulk samples these correlations indicate that most diogenites may form a single igneous fractionation sequence the incompatible minor and trace elements are decoupled from the major elements for essentially constant mg tio2 varies by about 4 times and yb by about 8 times hence there is no strong evidence for igneous fractionation preserved in the major element composition of orthopyroxenes geothermometry calculations using two pyroxene and spinelorthopyroxene thermometers show that these phases in diogenites maintained equilibrium to temperatures in the range 650 880 c far below likely magmatic temperatures in some diogenites orthopyroxenes of differing composition are in breccia contact therefore the low equilibration temperatures were achieved in the diogenites prior to brecciation decoupling of major and trace elements was most likely caused by slow subsolidus cooling in the diogenite pluton which allowed ca fe and mg to reach diffusive equilibration incompatible trace elements likely have much lower diffusivities in orthopyroxene and therefore may have largely preserved their magmatic distributions the extensive variation in incompatible trace elements indicates that either the diogenites were formed through an extensive crystallization interval or that partition coefficients greatly increased during crystallization if the former were true one would expect to find substantial amounts of plagioclase in the diogenites if they are comagmatic with eucrites this is contrary to observations a model in which diogenites crystallized from one or more melts of approximately orthopyroxenitic composition can satisfy both the mineralogie and trace element constraints

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

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

the dynamic effects of moisture on the entrainment and transport of sand by wind abstract there is an increasing awareness of the influence of surface moisture on aeolian entrainment and transport of sediment existing wind tunnel studies have shown the impact of a limited range of moisture contents on entrainment thresholds but similar investigations are lacking in the field the research reported here investigated the influence of changes in surface moisture content on sand entrainment and transport on a meso tidal beach in anglesey north wales high frequency 1 hz wind velocities measured with hot wire anemometers were combined with grain impact data from a sensit monitor and mass flux measurements from a standard sand trap surface and near surface moisture contents were assessed gravimetrically from surface sand scrapes and also directly by using a thetaprobe critical threshold values for entrainment were specified using a modified form of the time fraction equivalence method stout j e zobeck t m 1996a establishing the threshold condition for soil movement in wind eroding fields proceedings of the international conference on air pollution from agricultural operations mwps c 3 kansas city 7 9 february 1996 pp 65 71 results indicate a time dependent change in dominant control of the sand transport system from moisture to wind speed dependent upon the moisture content of the surface sediment this interchange between controlling parameters on both entrainment and transport was very sensitive to prevailing moisture conditions and took place over a period of minutes to hours under conditions experienced in the experiments presented here the critical moisture threshold for sediment entrainment was determined to be between 4 and 6 higher than the 1 4 specified in previous wind tunnel experiments furthermore a moisture content of nearly 2 where moisture was adhered to transported sediment appeared to have little or no impact on the rate of sand flux

a simple method to estimate threshold friction velocity of wind erosion in the field 1 this study provides a fast and easy to apply method to estimate threshold friction velocity tfv of wind erosion in the field wind tunnel experiments and a variety of ground measurements including air gun pocket penetrometer torvane and roughness chain were conducted in moab utah and cross validated in the mojave desert california patterns between tfv and ground measurements were examined to identify the optimum method for estimating tfv the results show that tfvs were best predicted using the air gun and penetrometer measurements in the moab sites this empirical method however systematically underestimated tfvs in the mojave desert sites further analysis showed that tfvs in the mojave sites can be satisfactorily estimated with a correction for rock cover which is presumably the main cause of the underestimation of tfvs the proposed method may be also applied to estimate tfvs in environments where other non erodible elements such as postharvest residuals are found

the initiation of particle movement by wind when air blows across the surface of dry loose sand a critical shear velocity fluid threshold ut must be achieved to initiate motion however since most natural sediments consist of a range of grain sizes fluid threshold for any sediment cannot be defined by a finite value but should be viewed as a threshold range which is a function of the size shape sorting and packing of the surface sediment in order to investigate the initiation of particle movement by wind a series of wind tunnel tests was carried out on a range of pre screened fluvial sands and commercially available glass beads with differing mean sizes and sorting characteristics a sensitive laser monitoring system was used in conjunction with a high speed counter to detect initial grain motion and to count individual grain movements test results indicate that when velocity is slowly increased over the sediment surface the smaller or more exposed grains are first entrained by the fluid drag and lift forces either in surface creep rolling or in saltation bouncing or hopping downwind as velocity continues to rise larger or less exposed grains may also be moved by fluid drag on striking the surface saltating grains impart momentum to stationary grains this impact may result in the rebound of the original grain as well as the ejection of one or more stationary grains into the air stream at shear velocities lower than that required to entrain a stationary particle by direct fluid pressure as a result there is a cascade effect with a few grains of varying size initially moving over a range of shear velocities the fluid threshold range and setting in motion a rapidly increasing number of grains results of the tests showed that the progression from fluid to dynamic threshold based on grain movement can be characterized by a power function the coefficients of which are directly related to the mean size and sorting characteristics of the sediment

geomorphology and drift potential of major aeolian sand deposits in egypt abstract aeolian sand deposits cover a significant area of the egyptian deserts they are mostly found in the western desert and northern sinai in order to understand the distribution pattern and forms of sand dunes in these dune fields it is crucial to analyze the wind regimes throughout the sandy deserts of the country therefore a set of wind data acquired from twelve meteorological stations were processed in order to determine the drift potential dp the resultant drift potential rdp and the resultant drift direction rdd of sand in each dune field the study showed that the significant aeolian sand deposits occur in low energy wind environments with the dominance of linear and transverse dunes regions of high energy wind environments occur in the south of the country and exhibit evidence of deflation rather than accumulation with the occurrence of migratory crescentic dunes analysis of the sand drift potentials and their directions help us to interpret the formation of major sand seas in egypt the pattern of sand drift potential direction suggests that the sands in these seas might be inherited from exogenous sources

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

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

the influence of porosity and vesicle size on the brittle strength of volcanic rocks and magma volcanic rocks and magma display a wide range of porosity and vesicle size a result of their complex genesis while the role of porosity is known to exert a fundamental control on strength in the brittle field less is known as to the influence of vesicle size to help resolve this issue here we lean on a combination of micromechanical sammis and ashby s pore emanating crack model and stochastic rock failure and process analysis code modelling the models show for a homogenous vesicle size that an increase in porosity in the form of circular vesicles from 0 to 40 and or vesicle diameter from 0 1 to 2 0 mm results in a dramatic reduction in strength for example uniaxial compressive strength can be reduced by about a factor of 5 as porosity is increased from 0 to 40 the presence of vesicles locally amplifies the stress within the groundmass and promotes the nucleation of vesicle emanating microcracks that grow in the direction of the applied macroscopic stress as strain increases these microcracks continue to grow and eventually coalesce leading to macroscopic failure vesicle clustering which promotes the overlap and interaction of the tensile stress lobes at the north and south poles of neighbouring vesicles and the increased ease of microcrack interaction is encouraged at higher porosity and reduces sample strength once a microcrack nucleates at the vesicle wall larger vesicles impart higher stress intensities at the crack tips allowing microcracks to propagate at a lower applied macroscopic stress larger vesicles also permit a shorter route through the groundmass for the macroscopic shear fracture this explains the reduction in strength at higher vesicle diameters at a constant porosity the modelling highlights that the reduction in strength as porosity or vesicle size increases is nonlinear the largest reductions are observed at low porosity and small vesicle diameters in detail we find that vesicle diameter can play an important role in dictating strength at low porosity but is largely inconsequential above 15 porosity vesicle clustering and stress lobe interaction are implicit at high porosity regardless of the vesicle diameter in the case of an inhomogeneous vesicle size the microcracks grow from the largest vesicles and brittle strength is closer to that of the largest vesicle end member the results of this study highlight the important role of vesicle size and the complex interplay between porosity and vesicle size in controlling the brittle strength of volcanic rocks and magma

homogeneous bubble nucleation in rhyolitic melt experiments and nonclassical theory 1 the transfer of volatiles from the earth s interior to the atmosphere occurs through degassing of magma the dynamics of which assert a significant control on volcanic eruptions the first and most critical step in degassing is the nucleation of gas bubbles which requires that a sufficient number of volatile molecules cluster together to overcome the free energy associated with the formation of a new interface between nucleus and surrounding melt this free energy is a function of surface tension typically assumed to equate to the macroscopically measurable value surface tension estimates inferred from bubble nucleation experiments in silicate melts are however lower than direct macroscopic measurements making it difficult to accurately predict magma ascent and decompression rates from measured bubble number densities in pyroclasts we provide a potential resolution to this problem through an integrated study of bubble nucleation experiments and modeling thereof based on nonclassical nucleation theory we find that surface tension between critical bubble nuclei and the surrounding melt depends on the degree of supersaturation and is lower than the macroscopically measured value this is consistent with the view that far from equilibrium the interface between a nucleus and surrounding metastable bulk phase is diffuse instead of sharp as a consequence the increase in nucleation rate with supersaturation is significantly larger at high supersaturations than predicted by classical nucleation theory

gum rosin acetone system as an analogue to the degassing behaviour of hydrated magmas solutions of gum rosin and acetone reproduce the volatile and temperature dependent viscosity together with the phase behaviour of hydrated magmas a range of experimental exsolution conditions was investigated including the variation of supersaturation rate of decompression solution temperature and volatile content degassing processese were controlled by the formation of an exsolution interface above a supersaturated liquid the end products ranged from a mildly degassed liquid to a solid foam which preserved strained vesicles solutions of gum rosin and acetone are proposed as a suitable analogue system with which to study magma degassing processes

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

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

the stabilizing role of bonding agents on the entrainment of sediment by wind well sorted fine sand pretreated with varying concentrations of two soluble salts mgcl2 and cacl2 was tested for threshold shear velocity in a recirculating wind tunnel these tests demonstrate that even relatively low surface salt concentrations can significantly increase threshold shear velocity however the relationships between surface salt concentration and threshold shear velocity for the two salts were not significantly different which is similar to the results for monovalent chlorides presented by nickling ecclestone a general relationship between threshold shear velocity and total surface soluble salt content is presented

aeolian processes across transverse dunes ii modelling the sediment transport and profile development this paper discusses a model which simulates dune development resulting from aeolian saltation transport the model was developed for application to coastal foredunes but is also applicable to sandy deserts with transverse dunes sediment transport is calculated using published deterministic and empirical relationships describing the influence of meteorological conditions topography sediment characteristics and vegetation a so called adaptation length is incorporated to calculate the development of transport equilibrium along the profile changes in topography are derived from the predicted transport using the continuity equation vegetation height is incorporated in the model as a dynamic variable vegetation can be buried during transport events which results in important changes in the sediment transport rates the sediment transport model is dynamically linked to a second order closure air flow model which predicts friction velocities over the profile influenced by topography and surface roughness modelling results are shown for a the growth and migration of bare initially sine shaped dunes and b dune building on a partly vegetated and initially flat surface results show that the bare symmetrical dunes change into asymmetric shapes with a slipface on the lee side this result could only be achieved in combination with the secondorder closure model for the calculation of air flow the simulations with the partly vegetated surfaces reveal that the resulting dune morphology strongly depends on the value of the adaptation length parameter and on the vegetation height the latter result implies that the dynamical interaction between aeolian activity and vegetation reaction to burial growth rates is highly relevant in dune geomorphology and deserves much attention in future studies copyright 1999 john wiley sons ltd

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

on the origin and age of the great sand dunes colorado abstract over the past 100 yr several hypotheses have been proposed for the origin and age of the great sand dunes these hypotheses differ widely in the descriptions of dune morphometry the immediate source of eolian sand and when sand transport occurred the primary purpose of this paper is to evaluate these hypotheses and where warranted to present new ideas about the origin and age of the great sand dunes to evaluate the previous hypotheses we had to develop more detailed information about the surficial geology of the northern san luis valley thus we mapped the surficial geology of an area extending several tens of kilometers north south and west of the great sand dunes and examined subsurface stratigraphy in more than 200 wells and borings in addition we used relative dating criteria and several radiocarbon and osl ages to establish the chronology of surficial deposits and we determined the u pb ages of detrital zircons to obtain information about the sources of the sand in the great sand dunes the first principal finding of this study is that the lower part of the closed basin north of the rio grande referred to here as the sump is the immediate source of the sand in the great sand dunes rather than the late pleistocene flood plain of the rio grande the most widely accepted hypothesis a second principal finding is that the great sand dunes are older than late pleistocene they postdate the draining of lake alamosa which began 440 ka and predate the time when streams draining the west flank of the sangre de cristo mountains were deflected by incipient dunes that formed near the mountain front geomorphic and stratigraphic evidence indicate that this deflection occurred prior to the end of the next to last glaciation bull lake i e prior to 130 ka

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

in situ high pressure and high temperature bubble growth in silicic melts we present the first investigation of in situ high pressure and high temperature bubble growth in silicic melts in a hydrothermal diamond anvil cell a haplogranite melt 79 wt sio2 is hydrated then subjected to cooling and decompression with decreasing pressure water exsolves from the melt and bubbles grow the whole experiment is observed through an optical microscope and video recorded so that bubble nucleation bubble growth and the glass transition are directly monitored bubbles nucleate and expand in melt globules having radii from 15 to 70 m bubbles reached 3 6 9 1 m in radius within 6 1 11 7 s until the glass transition is attained while temperature decreases from 709 879 c to 482 524 c corresponding to decompressions from 7 0 21 9 to 3 4 15 2 kbar bubbles nucleated either in a single event occurring within the first second or in successive pulses over a period of up to 7 s when the melt globules are in contact with a diamond culet of the cell in these experiments bubble growth can be fitted to the cube root or a logarithm of time mainly ascribable to the combination of large water oversaturations due to rapid cooling and decompression at pressures of 3 4 15 2 kbar we measure glass transition temperatures that are 20 80 c higher than those calculated at atmospheric pressure

complete miscibility between silicate melts and hydrous fluids in the upper mantle experimental evidence and geochemical implications the phase relationships between silicate melts and hydrous fluids were studied by direct visual observation in an externally heated diamond anvil cell complete miscibility of silicate melt and water was observed for a wide range of melt compositions including nepheline jadeite dacite haplogranite and ca bearing granite some evidence for complete miscibility was also observed in the system basalt h2o the critical temperatures in all systems decrease rapidly with pressure at 15 kbar the critical temperature for nepheline is around 550 c for jadeite around 800 c and for granitic compositions it is close to 900 c in general the critical temperatures appear to increase with silica content in the system our results suggest that there is complete miscibility between silicate melts and water in most of the upper mantle except at very shallow depths this means that a water saturated solidus cannot be defined any more in the deeper parts of the upper mantle very silica rich melt inclusions found in spinel lherzolites associated with fluid inclusion are probably the result of the unmixing of a supercritical fluid containing comparable amounts of water and silicate components the decomposition of amphibole in a subducted slab occurs at conditions where the miscibility gap between fluid and silicate melt is not yet closed while the decomposition of phengite and lawsonite occurs far beyond the critical curve accordingly the fluids released by the breakdown of these minerals should have very different properties highly mobile hydrous fluids containing little dissolved silicate should be produced by amphibole breakdown while the decomposition of lawsonite and phengite will lead to much more silicate rich and less mobile fluid phases

wind tunnel investigation of the influence of surface moisture content on the entrainment and erosion of beach sand by wind using sands from tropical humid coastal southern china abstract wind erosion has major impacts on dune growth desertification and architecture on sea coasts the deflation threshold shear velocity is a crucial parameter in predicting erosion and surface moisture greatly affects this threshold and thus sand stability wind tunnel studies have shown that reduced moisture contents decrease entrainment thresholds and increase wind erosion but field and wind tunnel test data is lacking for tropical humid coastal areas in this study we investigated the influence of surface moisture contents at 1 mm depth on sand entrainment and erosion using tropical humid coastal sands from southern china shear velocities were deduced from velocity profiles above the sand the threshold shear velocity increased linearly with increasing ln100 m m gravimetric moisture content the increase was steepest below a moisture content of 0 0124 i e at m 1 5 the moisture content in the sand at a matric potential of 1 5 mpa we compared several popular models that predict threshold shear velocity of moisture sediment and found substantial differences between their predicted results at a surface moisture content of 0 0124 the predicted increase in the wet threshold shear velocity compared with the dry threshold shear velocity ranged from 34 to 195 the empirical model of chepil and selah simulated the data well for m m 1 5 whereas belly s empirical model simulated the data best for m 0 0062 wind erosion modulus increased with increasing effective wind velocity following a power function with a positive exponent at all moisture contents but decreased with increasing surface moisture content following a power function with a negative exponent when wind speed and moisture content varied simultaneously wind erosion modulus was proportional to the 0 73 power of effective wind velocity but inversely proportional to the 1 48 power of m the increase in resistance to erosion at low moisture contents probably results from cohesive forces in the water films surrounding the sand particles at a moisture content near m 1 5 wind erosion ceases nearly for all wind velocities that we tested

threshold friction velocities and rupture moduli for crusted desert soils for the input of soil particles into the air desert soils having clay crusts mostly from the mojave desert were tested for threshold friction velocity the friction velocity at which soil erosion begins with an open bottomed wind tunnel the soils were also tested for content of clay water soluble material calcium carbonate organic material mineralogy of clay and of salts soil moisture modulus of rupture and crust thickness if no loose material existed on the soil surface crusts having modulus of rupture greater than 0 7 bar and crust thickness of 0 7 cm to 0 3 cm were effective in protecting against wind erosion disturbed clay crusts having modulus of rupture before disturbance greater than 2 bar with thickness less than 1 9 cm did not experience significant wind erosion modulus of rupture was related to composition of soil but was shown to depend mostly on clay content soil composition is related to modulus of rupture in an empirical equation

the composition of the mantle of the eucrite parent body and the origin of eucrites abstract previous estimates of the composition of the epb have tacitly assumed chondritic abundances of refractory lil elements a model is presented here which utilizes refractory element ratios instead consideration of these ratios and of known olivine liquid partition coefficients for sc mg and si imply that the epb mantle may be accurately modeled as a simple mixture of 25 eucrite 75 olivine the model also implies that eucrites such as juvinas and sioux county were derived by large degrees of partial melting 20 30 comparison of the model epb mantle to known meteorite classes indicates a similarity between the element abundances of the epb and c3 chondrites

experimental study of bubble coalescence in rhyolitic and phonolitic melts abstract we have experimentally studied the process of bubble coalescence in rhyolite and phonolite melts of natural composition the experiments involved decompression of water saturated melts equilibrated at pressures and temperatures from 100 to 150 mpa and 775 to 840 c in vertically oriented rapid quench capable cold seal pressure vessels one type of experiments rhyolite mcr 100 120 150 and phonolite lsp 120 series approximates a static bubble coalescence case where we held the decompressed samples for 5 seconds to 4320 minutes 3 days before quenching the second type rhyolite lpc 100 series replicates an expanding bubble coalescence environment where we continually decompressed the experiments at a rate of 0 5 mpa s examining samples quenched at ending pressures between 10 and 80 mpa our static case mcr 100 120 and 150 and lsp 120 results show significant increases in the modal bubble sizes and in the sizes of the largest bubbles corresponding to measurable broadening in the size distributions their bubble number densities nv decrease as a function of hold time at their final pressures pf and can be fit well by power law functions our expanding case experiments lpc 100 show a significant drop in nv during the duration of the experiments that can be fit by an exponential equation as nv vs either time or pf average estimates of bulk coalescence rates indicate a 1 order of magnitude drop in nv for static case rhyolites in a 2 3 day period and 2 orders of magnitude for phonolites within a 3 day period despite a 2 order of magnitude difference in viscosity coalescene in the phonolite is only slightly faster than the rhyolite the expanding case experiments show a 1 order of magnitude drop in nv over 180 seconds thus nv s decrease 4 orders of magnitude faster in expanding vs static bubbly rhyolite melts our results imply that significant bubble coalescence can occur in rhyolite magmas at relatively fast 20 m s ascent rates in the conduit thus bubble interconnectivity leading to high permeability is possible during ascent bubble coalescence may occur during second boiling in magma bodies that are stalled in the crust the timescales over which this occurs is much faster than the estimated rise rates for bubbles to reach the top of the magma chamber

climate induced sediment palaeosol cycles in a late cretaceous dry aeolian sand sheet marilia formation north west bauru basin brazil aeolian sand sheets which are characterized by low relief surfaces that lack dunes are common in arid and semi arid climatic settings the surface of an aeolian sand sheet can either be stable and subject to pedogenetic effects or unstable such that it is affected by deflation or sedimentation the maro lia formation late cretaceous may be interpreted as an ancient aeolian sand sheet area where alternating phases of stability and instability of the accumulation surface have been recorded detailed field studies were carried out in several sections of the maro lia formation where cyclic alternations of palaeosols and aeolian deposits were evident using palaeopedological and facies analysis methods supported in the laboratory by the analysis of rock samples cut and polished in slabs thin sections scanning electron microscope images and x ray diffraction data from the clay minerals the deposits comprise three lithofacies that in order of abundance are characterized by i translatent wind ripple strata ii flood deposits and iii ephemeral river channel deposits palaeosols constitute on average 65 of the vertical succession three types of palaeosols pedotypes are recognized i aridisols ii entisols and iii vertisols erosional surfaces due to aeolian deflation divide the top of the palaeosol profiles from the overlying aeolian deposits the palaeoenvironmental interpretation of the deposits and the palaeosols allows the depositional system of the maro lia formation to be defined as a flat area dominated by aeolian sedimentation with subordinate ephemeral river sedimentation and characterized by a dry climatic setting with occasional rainfall the climate is the main forcing factor controlling the alternation between episodes of active sedimentation and periods of palaeosol development a climate controlled model is proposed in which i the palaeosols are indicative of a stable surface that is developed during the more humid climatic phases and ii the erosional surfaces and the overlying aeolian sediments attest to periods of deflation and subsequent sedimentation thereby increasing the availability of sediment during the drier climatic phases the ephemeral fluvial deposits mark the more humid climatic conditions and contribute to the lagged sediment influx caused during the drier periods by the erosion of previously stored sediment

aeolian system sediment state theory and mojave desert kelso dune field example the sediment state of aeolian dune fields and sand seas at a basinal scale is defined by the separate components of sediment supply sediment availability and the transport capacity of the wind the sediment supply for aeolian systems is the sediment that contemporaneously or at some later point serves as the source material for the aeolian system numerous factors impact the susceptibility of grains on a surface to transport but these are cumulatively manifested by the actual transport rate which serves as a proxy for sediment availability transport capacity is the potential sediment transport rate of the wind because the three aspects of sediment state can be given as a volumetric rate they are directly comparable plotted simultaneously against time the generated curves define nine possible classes of sediment state sediment supply that is stored occurs because it is transport or availability limited or generated at a rate greater than the potential or actual transport rates respectively contemporaneous or lagged influx to an aeolian system may be limited by sediment availability but cannot exceed the transport capacity of the wind for the kelso dune field in the mojave desert of california a variety of stratigraphic and geomorphic evidence is used to approximate the sediment state of the system the sediment supply was generated during the latest pleistocene and earliest holocene during humid periods of enhanced discharge by the mojave river to form the lake mojave fan delta or terminal fan and has been calculated over time from the sedimentation rate and the frequency of floods estimation of transport capacity over time was based upon modern wind data with an allowance for greater winds during the pleistocene based upon climatic models sediment availability was approximated by calculation of a modern dune mobility index with variation over time based upon climatic inferences while quantifying the kelso or any natural system is subject to numerous uncertainties the sediment state approach reflects the temporal and spatial disjointed nature of accumulations at kelso as well as illuminating questions for future research

h2o in rhyolitic glasses and melts measurement speciation solubility and diffusion dissolved h2o in silicate melts and glasses plays a crucial role in volcanic eruptions on terrestrial planets and affects glass properties and magma evolution in this paper major progress on several aspects of the h2o melt or glass system is reviewed consistency among a variety of data is investigated discrepancies are evaluated and confusion is clarified on the infrared measurement of total h2o and species concentrations calibration for a variety of glasses has been carried out at room temperature the measurements for h2o in rhyolitic glasses have undergone the most scrutiny resulting in the realization that absorptivities for the near infrared bands depend on total h2o content although the variation of the absorptivities does not seem to significantly affect the determination of total h2o it does affect the determination of molecular h2o and oh species concentrations calibration of the infrared technique for h2o in rhyolitic glasses still needs much improvement especially at high total h2o furthermore it is now almost certain that the molar absorptivities also depend on the measurement temperature in in situ studies hence it will be necessary to carry out calibrations in situ at high temperatures on h2o speciation results from two experimental approaches the quench technique and the in situ technique are very different leading to controversy in our understanding of true speciation a solution is presented to reconcile this controversy it is almost certain that the quench technique does not suffer from a quench problem but interpretation of in situ results suffered from ignoring the dependence of the molar absorptivities on measurement temperature accurate calibration at high temperatures is necessary for the quantitative application of the in situ technique to h2o speciation in silicate melts and glasses on h2o solubility in silicate melts recent experimental work has significantly expanded the t p range of solubility measurements and recent solubility models fill a gap for predicting solubility for a wide range of melt compositions i present a solubility model for rhyolitic and quasi rhyolitic melts over a wide range of t and p 500 1350 c 0 8 kbar by incorporating the role of speciation the solubility model is able to recover the experimental solubility data and has extrapolative value although the partial molar volume of h2o derived from the solubility model differs from that derived from density measurements on h2o diffusion recent studies on h2o diffusion in a quasi rhyolitic melt at 800 1200 c 0 5 5 kbar and up to 7 total h2o not only provide important new diffusion data but are also challenging earlier understanding of h2o diffusion based on data in rhyolitic glasses at 400 550 c 1 bar and 0 2 1 8 total h2o a comparison between the earlier model and recent data is made the recent high temperature diffusivities at total h2o 2 can be predicted by the earlier model however at higher total h2o the earlier model fails new work is under way to understand the diffusion mechanisms at high h2o contents

climate surface pore water interactions on a salt crusted playa implications for crust pattern and surface roughness development measured using terrestrial laser scanning sodium accumulating playas also termed sodic or natric playas are typically covered by polygonal crusts with different pattern characteristics but little is known about the short term hours dynamics of these patterns or how pore water may respond to or drive changing salt crust patterning and surface roughness it is important to understand these interactions because playa crust surface pore water and roughness both influence wind erosion and dust emission through controlling erodibility and erosivity here we present the first high resolution 10 3m hours co located measurements of changing moisture and salt crust topography using terrestrial laser scanning tls and infra red imagery for sua pan botswana maximum nocturnal moisture pattern change was found on the crests of ridged surfaces during periods of low temperature and high relative humidity these peaks experienced non elastic expansion overnight of up to 30 mm and up to an average of 1 5 mm night during the 39 day measurement period continuous crusts on the other hand showed little nocturnal change in moisture or elevation the dynamic nature of salt crusts and the complex feedback patterns identified emphasise how processes both above and below the surface may govern the response of playa surfaces to microclimate diurnal cycles

wind initiation thresholds of the moistened sands 1 the widely accepted bagnold type function for calculating threshold wind velocity or shear velocity was developed for dry sands but surface moisture is an extremely important variable controlling the entrainment process of sands by wind because the tensile force between the water molecules and sand grains produces cohesion here we report detailed wind tunnel experimental results on the threshold shear velocity of moistened sand of different sizes the results show that relative threshold shear velocity which is the ratio of threshold shear velocity of sand in the moistened state to that in the dry state is better related to moisture content than threshold shear velocity itself function modified after the bagnold equation has been developed to estimate the threshold shear velocities of moistened sands for a given grain size the threshold shear velocity is proportional to 1 km 1 2 where m is the moisture content and k is a coefficient depending on grain size

rates of aeolian transport on a beach in a temperate humid climate short term field measurements of aeolian transport using a saltiphone indicate that actual rates of transport deviate considerably from the potential rates predicted by transport equations when examined in detail however potential rates are approached during special conditions mostly when the wind blows offshore parallel or slightly oblique to the beach and relative humidity is below 85 also the potential rates of transport may be approached when wind speeds considerably exceed threshold speed even during showers the main cause of the large deviation of actual from potential rates is in the absence of transport during very wet conditions prolonged rainfall regardless of wind speed and the variation of threshold velocity with time the threshold velocity increases during wet conditions with onshore winds or because of the presence of algae therefore potential transport is defined as the transport optimum to predict the actual rates of transport accurately insight into the variation of the threshold velocity is essential aeolian transport of sand seems to be related to moderate events and not to extreme events conforming to the principle of magnitude and frequency

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

on the effect of moisture bonding forces in air dry soils on threshold friction velocity of wind erosion wind erosion is a dominant geomorphological process in arid and semi arid regions with major impacts on regional climate and desertification the erosion process occurs when the wind speed exceeds a certain threshold value which depends on a number of factors including surface soil moisture the understanding and modelling of aeolian erosion requires a better understanding of the soil erodibility associated with different moisture conditions in arid regions during the dry season the atmospheric humidity plays an important role in determining the surface moisture content and the threshold shear velocity by a series of wind tunnel tests and theoretical analyses this dependence of threshold velocity on near surface air humidity is shown for three soils of different textures sand sandy loam and clay loam the results show that the threshold shear velocity decreases with increasing values of relative humidity for values of relative humidity between about 40 and 65 while above and below this range the threshold shear velocity increases with air humidity a theoretical framework is developed to explain these dependencies assuming an equilibrium between the surface soil moisture and the humidity of the overlying atmosphere the conditions under which soil atmosphere equilibrium occurs were tested experimentally in the laboratory for different soils in order to determine the effect of grain surface area and texture on the time required to reach equilibrium starting from different initial conditions

wind driven particle mobility on mars insights from mars exploration rover observations at el dorado and surroundings at gusev crater 1 the ripple field known as el dorado was a unique stop on spirit s traverse where dust raising active mafic sand ripples and larger inactive coarse grained ripples interact illuminating several long standing issues of martian dust mobility sand mobility and the origin of transverse aeolian ridges strong regional wind events endured by spirit caused perceptible migration of ripple crests in deposits sse of el dorado erasure of tracks in sandy areas and changes to dust mantling the site localized thermal vortices swept across el dorado leaving paths of reduced dust but without perceptibly damaging nearly cohesionless sandy ripple crests from orbit winds responsible for frequently raising clay sized dust into the atmosphere do not seem to significantly affect dunes composed of more easily entrained sand sized particles a long standing paradox this disparity between dust mobilization and sand mobilization on mars is due largely to two factors 1 dust occurs on the surface as fragile low density sand sized aggregates that are easily entrained and disrupted compared with clay sized air fall particles and 2 induration of regolith is pervasive light toned bed forms investigated at gusev are coarse grained ripples an interpretation we propose for many of the smallest linear light toned bed forms of uncertain origin seen in high resolution orbital images across mars on earth wind can organize bimodal or poorly sorted loose sediment into coarse grained ripples coarse grained ripples could be relatively common on mars because development of durable well sorted sediments analogous to terrestrial aeolian quartz sand deposits is restricted by the lack of free quartz and limited hydraulic sediment processing

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

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

experimental constraints on degassing of magma isothermal bubble growth during continuous decompression from high pressure numerical models predict that rapid ascent of hydrous magma can lead to supersaturation of dissolved volatile constituents possibly leading to explosive eruption we have performed controlled decompression experiments to investigate the ascent rates required to maintain bubble melt equilibrium high silica rhyolitic melts were saturated with water at 200 mpa and 825 c decompressed to lower pressures at constant rates of 0 025 0 25 0 5 and 1 0 mpa s 1 and then rapidly quenched isobarically other samples were saturated with water over the pressure range investigated to determine equilibrium water solubility in order to quantify degassing efficiency during decompression at a decompression rate of 0 025 mpa s 1 melt vapor equilibrium was maintained over the entire pressure range examined 200 to 17 5 mpa a single bubble nucleation event occurred in response to decompression and quenched bubble sizes can be modeled by a equilibrium bubble growth model that takes into account the number density of bubbles at decompression rates of 0 25 0 5 and 1 0 mpa s 1 rhyolitic melts could not degas in equilibrium when pressure decreased from 200 mpa to 140 mpa and water supersaturation p in the melt reached up to 60 mpa with higher values at faster decompression rates further pressure release resulted in near equilibrium degassing and p dropped significantly in each case p decreased when bubbles exceeded 10 vol a single heterogeneous bubble nucleation event occurred in each experiment when p 20 mpa no other bubbles nucleated despite p reaching 60 mpa which is probably too low to trigger homogeneous nucleation compared to estimates for magma decompression rates during lava dome eruptions our results indicate that magmas can degas efficiently throughout their ascent to the surface in explosive eruptions decompression rates may exceed those of this study and hence melts may become supersaturated with water such fast decompressions are expected however only when magma is highly vesicular which would aid approach to equilibrium degassing

timescales of bubble coalescence outgassing and foam collapse in decompressed rhyolitic melts abstract the timescale of degassing and outgassing in hydrous rhyolitic melts is investigated in a wide range of conditions by means of decompression experiments the evolution of vesicularity bubble diameter and number density is characterized as a function of time either of decompression or spent at final pressure in order to determine the effect of final pressure temperature syn versus post decompression degassing melt composition and microlites on the timescale of bubble growth coalescence and outgassing the result suggests that different bubble evolution and degassing outgassing timescale corresponding to explosive and effusive eruption regimes can be cast in bulk viscosity melt bubbles bulk versus decompression time rather than path space the bulk time relationship defines three domains of i bubble nucleation and growth restricted to short durations and high bulk 0 03 h for bulk 10 5 6 pa s ii equilibrium degassing with coalescence increasing from negligible permeability 10 13 m 2 to extensive permeability 10 11 12 m 2 and iii outgassing restricted to long durations and low bulk 10 h for bulk 10 6 pa s permeability 10 10 m 2 that eventually leads to foam collapse these findings are applied to the case studies of mt pelee and mt pinatubo to infer the transition from pumice to dense pyroclasts in volcanic eruptions and the possibility of evolving from an explosive plinian eruption to an effusive dome growth event by giving the vesicular magma enough time to outgas and collapse i e hundreds to tens of hours for bulk 10 5 to 10 4 pa s respectively we also show the drastic effect of microlites on re arranging preexistent bubbles and potentially triggering a late nucleation event

the mechanics and geological implications of dust transport and deposition in deserts with particular reference to loess formation and dune sand diagenesis in the northern negev israel

aeolian sediment supply at a mega nourishment abstract mega nourishments are intended to enhance growth and resilience of coastal dunes on medium to long time scales by stimulation of natural sediment transport processes the growth and resilience of coastal dunes largely depends on the presence of a continuous supply of aeolian sediment a recent example of a mega nourishment is the 21 mm 3 mega nourishment known as the sand motor the sand motor is intended to nourish the entire holland coast over a period of two decades four years of bi monthly topographic measurements of the sand motor domain provide an opportunity to analyze spatiotemporal variations in aeolian sediment supply using an aeolian sediment budget analysis it appears that more than 58 of all aeolian sediment deposits originate from the low lying beaches that are regularly reworked by waves aeolian sediment supply from higher beaches diminished after half a year after construction of the sand motor likely due to the formation of deflation lag deposits that constitute a beach armor layer the compartmentalization of the sand motor in armored and unarmored surfaces suggests that the construction height is an important design criterion that influences the lifetime and region of influence for any mega nourishment

nearest neighbor analysis of mega barchanoid dunes ar rub al khali sand sea the application of geographical indices to the understanding of dune field self organization maturity and environmental change abstract the geographic signature of dune distribution and self organization as measured by the r statistic offers a viewpoint on the geography of crescentic aeolian systems and proposes an index from which to determine the degree of self organization maturity and possibly relative age across a variety of spatial scales and planetary contexts this analogue study examines and compares the geography of crescentic megadune patterns for the al liwa basin of the ar rub al khali sand sea on the arabian peninsula fields of mega barchanoid forms have shown the degree of self organization to be largely an association between morphological diversity and dimensional variability in short fields of transitional megadunes barchanoid seif are comparatively less regular in distribution than are those fields or part thereof that consist of ubiquitous compound barchanoid morphologies whose patterns are more highly regular the change in dune morphology dimensions and spatial signature can for the most part be associated with the directional differences and varying intensities of the north westerly shamal and reversing south westerly kharif wind regimes such associations have significant bearing on the maturity of dunefields and their response to sediment supply transport capacity and sediment availability within an aeolian system this study provides an analogue on which to base further comparative studies with other terrestrial dune fields as well as the ergs and intracrater fields of mars

magnesium oxide iron oxide mass balance constraints and a more detailed model for the relationship between eucrites and diogenites according to a currently popular model for petrogenesis on the howardite eucrite and diogenite hed parent asteroid the diogenites are not comagmatic with most eucrites but instead formed in separate orthopyroxenite dominated plutons this model can be tested for consistency with mass balance for mgo and feo assuming the overall diogenite diogenite eucrite ratio d of the parent asteroid is at least comparable to the average d for the eucrite diogenite dominated howardite regolith breccias average mg mgo mgo feo is much lower for eucrites especially noncumulate eucrites than for diogenites unless the diogenite parent magmas eventually produced a large proportion of low mg residual basalt and gabbro rbg the implied initial magma s mg is vastly higher than that of any noncumulate eucrite starting from a source previously depleted by putative primary eucrite genesis melt mg can be estimated as a function of the exchange reaction kd and degree of melting using several very conservative assumptions e g assuming that the total mgo feo concentration is nearly the same in the nascent melt as in the residual solids the degree of melting required to yield a melt with mg high enough to satisfy mass balance without implying an rbg component that accounts for 50 of all eucrites is an implausibly high 60 80 wt the separate orthopyroxenitic plutons sop model also seems inconsistent with the uniform density of melts across the diogenite eucrite compositional spectrum 2 77 2 82 g cm3 which implies that diogenitic magmas should have been as capable as eucrites of extruding to form lavas this difficulty cannot be reduced by simply assuming that later formed magmas were systematically both more plutonic and more mgo rich than earlier ones because the plutonic cumulate eucrites equilibrated with melts systematically lower in mg than noncumulate eucrites conceivably the bulk mg of the asteroid s silicate system was increased between primary melt eucrite genesis and sop diogenite genesis by graphite fueled reduction of feo however the graphite oxidation process generates a huge proportion of gas which would have enhanced the buoyancy of the nascent diogenite parent magmas thus exacerbating the difficulty of achieving the implied high degrees of partial melting to avoid these difficulties but still form most eucrites as rapidly cooled extrusives i propose the nerd noncumulate eucrites as extruded residua of diogenites model in this model the diogenites form as early cumulates from a large magma system probably a global magma ocean that yields a large proportion of eucritic melt as residuum this residual melt zone undergoes relatively little crystallization during a period when it is episodically tapped to produce extrusions dikes and sills of rapidly cooled noncumulate eucrites slight 5 10 porosity in the nascent eucritic crust keeps it marginally buoyant over the residual melt zone the common thermal metamorphism of noncumulate eucrites results from baking by superjacent flows during the episodic venting of the melt zone the nerd model s greatest advantage is that it does not require implausibly high degrees of localized melting in the mature stages of igneous evolution of the hed asteroid

aeolian transport of sediment on a beach during and after rainfall wildwood nj usa abstract a field investigation was conducted 9 march 1994 on a dissipative beach to compare wind characteristics and aeolian transport during and after a light rain winds blew alongshore during the monitoring period moisture content of surface sediment on the backbeach was above 7 during rain and 4 four hours after rainfall ceased trapping rates measured using a cylindrical vertical trap were 14 1 kg m 1 h 1 during rainfall and 140 2 kg m 1 h 1 for a period beginning 3 h 33 min after rainfall ceased results indicate that light rain resulting in bulk surface moisture values over 7 is not sufficient to eliminate transport but the rate of transport is lower than can occur on the beach three to four hours following rain the amount of sand trapped at wildwood during rainfall is relatively high when compared to many previous studies on beaches in the absence of rain the great length of the beach as a source area combined with parallel or oblique winds are significant in increasing rates of sediment transport

aeolian slipface dynamics and grainflow morphologies on earth and mars abstract in 2015 an active dune field on mars was visited up close by the curiosity rover in gale crater providing the first high resolution ground images of fine scale windblown features not previously resolved from orbital based imagery for the first time these images allow for direct comparison with terrestrial aeolian dynamics and provide critical ground truth data to bridge the gap between model predictions and satellite observations the image data from the slipface on the namib dune within the bagnold dune field shows grainflow morphologies that are similar to dunes on earth quantitative estimates of flow thickness based on shadow length are presented for the grainflows on the namib dune slipface and compared to grainflow characteristics measured by terrestrial laser scans from the maspalomas dune field located in gran canaria spain using observations from maspalomas to support interpretations of martian slipface dynamics we discuss implications for the local wind regime style of grainflow seasonal activity and dune migration the presence of multiple large magnitude grainflows on the namib slipface suggests an active aeolian environment capable of delivering enough sediment to the slipface to initiate these flows and transport sediment to the bottom of the lee slope however the thinness of grainflows on the namib dune the formation of smaller grainflows directly below the dune brink and limited grainfall suggest a lower wind energy environment at least for the most recent slipface activity large actively migrating stoss ripples obliquely oriented to the dune crest regularly deposit sediment on to the upper slipface and may be a mechanism in which larger grainflow occur during seemingly low energy wind events this mechanism of sediment delivery may also explain the existence of a variety of slipface morphologies both young and old which are otherwise quickly erased on earth due to sediment redistribution and grainfall

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

in situ x ray tomographic microscopy observations of vesiculation of bubble free and bubble bearing magmas magma degassing is thought to play a major role in magma fractionation transport storage and volcanic eruption dynamics however the conditions that determine when and how magma degassing operates prior to and during an eruption remain poorly constrained we performed experiments to explore if the initial presence of gas bubbles in magma influences the capability of gas to escape from the magma vesiculation of natural h2o poor 1 wt silicic obsidian glasses was investigated by in situ high temperature above the glass transition experiments using synchrotron based x ray tomographic microscopy with high spatial 3 m pixel and temporal resolution 1 second per 3d dataset as a validation a second set of experiments was performed on identical starting materials using a karl fisher titration setup to quantify the amount of extracted gas that escapes via volatile diffusion and or bubble coalescence during vesiculation in both sets of experiments vesiculation was triggered by heating the samples at room pressure our results suggest that the presence of pre existing gas bubbles during a nucleation event significantly decreases the tendency of bubbles to coalesce and inhibits magma outgassing in contrast in initially bubble free samples the nucleation and growth of bubbles is accompanied by significant coalescence and outgassing we infer that volatile undersaturated i e bubble free magmas in the reservoirs are more likely to erupt effusively while the presence of excess gas already at depth i e bubble bearing systems increases the likelihood of explosive eruptions

the role of wind direction in eolian transport on a narrow sandy beach comparison of eolian transport during five high velocity wind events over a 29 day period on a narrow estuarine beach in delaware bay new jersey usa reveals the temporal variability of transport due to changes in direction of wind approach mean wind speed measured 6 m above the dune crest for the five events ranged from 8 5 to 15 9 ms 1 mean wind direction was oblique to the shoreline 63 from shore normal during one event but was within 14 of shore normal during the other events eolian transport is greatest during low tide and rising tide when the beach source area is widest and when drying of surface sediments occurs the quantity of sediment caught in a vertical trap for the five events varied from a total of 0 07 to 113 73 kgm 1 differences in temperature relative humidity and moisture and salt content of surficial sediments were slight mean grain sizes ranged from 0 33 to 0 58 mm causing slight differences in threshold shear velocity but shear velocities exceeded the threshold required for transport during all events beach width measured normal to the shoreline varied from 15 5 to 18 0 m beach slope differed by 0 5 the oblique wind during one event created a source width nearly double the width during other days beach slope measured in the direction of the wind was less than half as steep as the slope measured normal to the shoreline the amount of sand trapped during the oblique wind was over 20 times greater than any other event even those with higher shear velocities the ability of the beach surface to supply grains to the air stream is limited on narrow beaches but increased source width due to oblique wind approach can partially overcome limitations of surface conditions on the beach

comment on experimental partial melting of the st severin ll and lost city h chondrites by a j g jurewicz d w mittlefehldt and j h jones

decompression induced crystallization in hydrated silica rich melts empirical models of experimental plagioclase nucleation and growth kinetics isothermal and isobaric crystallization of plagioclase in a water saturated synthetic rhyolitic melt is investigated through time series of decompression experiments the experimental variables are the rate at which samples are initially decompressed 30 150 and 1200 mpa h from 200 mpa and 875 c final pressure 25 to 160 mpa and holding time at final pressure up to 17 days through textural measurements of the crystals plagioclase crystallization kinetics has been characterized in terms of nucleation lag and rates of nucleation and growth plagioclase crystallization is markedly dependent on effective undercooling teff and holding time at crystallization pressure with teff increasing from 55 to 110 c i nucleation lag decreases from 1 2 days to 15 min ii maximum nucleation rates increase from 10 3 to 10 2 mm 2 s 1 and iii maximum growth rates decrease from 10 6 to 5x10 7 mm s 1 the initial decompression rate 30 150 and 1200 mpa h has no systematic control on crystallization at final pressure except for the 1200 mpa h series in which samples show nucleation difficulties from the experimental data of teff constrained plagioclase number density proportion and morphology we provide means to assess conditions of nucleation and growth of natural plagioclase microlites from rapidly ascended rhyolitic melts through the determination of plagioclase liquidus curve and teff prevailing during crystallization

the strength of heterogeneous volcanic rocks a 2d approximation abstract volcanic rocks typically contain heterogeneities in the form of crystals and pores we investigate here the influence of such heterogeneity on the strength of volcanic rocks using an elastic damage mechanics model in which we numerically deform two dimensional samples comprising low strength elements representing crystals and zero strength elements representing pores these circular elements are stochastically generated so that there is no overlap in a medium representing the groundmass our modelling indicates that increasing the fraction of pores and or crystals reduces the strength of volcanic rocks and that increasing the pore fraction results in larger strength reductions than increasing the crystal fraction the model also highlights an important weakening role for pore diameter but finds that crystal diameter has a less significant influence for strength to account for heterogeneity pores and crystals we propose an effective medium approach where we define an effective pore fraction p v p v p v g where v p and v g are the pore and groundmass fractions respectively highly heterogeneous samples containing high pore and or crystal fractions will therefore have high values of p and vice versa when we express our numerical samples more than 200 simulations spanning a wide range of crystal and pore fractions in terms of p we find that their strengths can be described by a single curve for a given pore diameter to provide a predictive tool for the strength of heterogeneous volcanic rocks we propose a modified version of 2d solution for the sammis and ashby 1986 pore emanating crack model a micromechanical model designed to estimate strength using microstructural attributes such as porosity pore radius and fracture toughness the model reformulated to include p and therefore crystal fraction captures the strength curves for our numerical simulations over a sample heterogeneity range relevant to volcanic systems we find that published experimental data for which the porosity and crystal fraction is known correlate well with our modelled curves adding validity to our approach we present herein a 2d analytical tool to estimate the strength of volcanic rock that can be used when the porosity crystal fraction and maximum pore diameter are known

in situ bubble vesiculation in silicic magmas volatile degassing is a major process driving volcanic eruptions therefore a full understanding of mechanisms ranging from bubble nucleation growth coalescence to magma fragmentation is required we have simulated magma degassing during ascent in the volcanic conduit by depressurizing hydrated haplogranite melts in high pressure and high temperature optical cells a hydrothermal diamond anvil cell and an internally heated pressure vessel fitted with sapphire windows this allowed the whole process of bubble nucleation growth and coalescence to be directly observed in situ through images captured from the recording videos bubble nucleation pressures number densities growth laws and characteristics of coalescence were estimated as a function of melt water content decompression rate and temperature melt vapor surface tension during bubble nucleation and coalescence was calculated our data show good agreement with those previously obtained in classical vessels methodological improvements are proposed for the experimental simulation of magma degassing in volcanic conduits

quantifying dust emissions from desert landforms eastern mojave desert usa abstract the measurement of natural dust emissions from desert landforms is crucial in environmental hazard assessment and field checking the accuracy of global dust models more than 500 individual dust measurements from eight common desert landforms in southern california were collected using the pi swerl portable in situ wind erosion lab the largest emitters of dust are dry washes 13 787 to 0 007 mg m 2 s 1 dunes playa margins distal alluvial fans and lacustrine beaches low emitters include salt crusted playas 0 692 to 0 002 mg m 2 s 1 silt clay crusted playas and desert pavements high emissions are a function of saltating sand that bombards the surface liberating dust sized particles for entrainment low dust emissions are primarily a function of surface crusting gravel armoring and vegetation density pi swerl measurements reveal that emission rates can vary by at least three orders of magnitude reflecting local variability in soil texture and continuity of surface crusts shear stress partitioning models can be applied to dust data measured by the pi swerl to account for large surface roughness features such as vegetation the results presented here give an approximation of the contributions to atmospheric dust loading by landforms in the mojave desert and can potentially be used to improve atmospheric dust models

the effect of surface moisture on the entrainment of dune sand by wind an evaluation of selected models reliable predictions of wind erosion depend on the accuracy of determining whether erosion occurs or not among the several factors that govern the initiation of soil movement by wind surface moisture is one of the most significant some widely used models that predict the threshold shear velocity for particle detachment of wet soils by wind were critically reviewed and evaluated wind tunnel experiments were conducted on pre wetted dune sand with moisture contents ranging from 0 00 to 0 04 kg kg 1 sand samples were exposed to different wind speeds for 2 min moisture content was determined gravimetrically before and after each experiment and the saltation of sand particles was recorded electronically with a saltiphone shear velocities were deduced from the wind speed profiles for each moisture content the experiments were repeated at different shear velocities with the threshold shear velocity being determined by least squares analysis of the relationships between particle number rates and shear velocity within the 2 min test runs temporal changes in particle number rates and moisture contents were detected a steep increase in the threshold shear velocity with moisture content was observed when comparing the models large differences between the predicted results became apparent at a moisture content of 0 007 kg kg 1 which is half the moisture content retained to the soil matrix at a water tension or matric potential of 1 5 mpa the increase in wet threshold shear velocity predicted with the different models relative to the dry threshold shear velocity ranged from 117 to 171 the highest care should therefore be taken when using current models to predict the threshold shear velocity of wet sediment nevertheless the models of chepil 1956 proc soil sci soc am 20 288 292 and saleh fryrear 1995 soil sci 160 304 309 are the best alternatives available

nucleation and growth of bubbles on plagioclase crystals during experimental decompression degassing of andesitic melts abstract the nucleation growth and attachment detachment of gas bubbles on crystals in silicate melts is one of the key drivers of volcanic eruptions and can greatly influence their explosivity formerly oxides were considered the best candidates for heterogeneous bubble nucleation but recent studies showed that silicate crystals can also be bubble nucleation sites at 1 atmosphere atm this study examines whether bubbles can nucleate on plagioclase crystal surfaces during decompression which is a more common natural situation than 1 atm degassing experimental samples were synthesised from andesitic rock powder h2o and plagioclase seed crystals and melted and decompressed in a piston cylinder apparatus the products were imaged with 3d x ray computed tomography due to partial melting the plagioclase crystals developed sieve rims and rough crystal melt interfaces all plagioclase crystals in all samples were covered with bubbles bubble size distribution comparisons between bubbles in contact with plagioclase and bubbles in the melt show that bubbles belong to two populations generated by two different events one of which is heterogeneous nucleation on plagioclase crystal surfaces and the other is homogeneous nucleation within the melt the latter potentially accompanied by heterogeneous nucleation on oxide microlite surfaces the calculated attachment force between the bubbles and the plagioclase surface is stronger than the detachment force hence such bubbles would remain attached during crystal movements in our experiments the net buoyancy of the bubble crystal aggregates indicates that they could sink in an andesitic melt our findings highlight the need to reconsider the role of plagioclase crystals in magmatic degassing and shows interesting possibilities for magma mixing triggered degassing

in situ observations of bubble growth in basaltic andesitic and rhyodacitic melts bubble growth strongly affects the physical properties of degassing magmas and their eruption dynamics natural samples and products from quench experiments provide only a snapshot of the final state of volatile exsolution leaving the processes occurring during its early stages unconstrained in order to fill this gap we present in situ high temperature observations of bubble growth in magmas of different compositions basalt andesite and rhyodacite at 1 100 to 1 240 c and 0 1 mpa 1 bar obtained using a moissanite cell apparatus the data show that nucleation occurs at very small degrees of supersaturaturation 60 mpa in basalt and andesite 200 mpa in rhyodacite probably due to heterogeneous nucleation of bubbles occurring simultaneously with the nucleation of crystals during the early stages of exsolution melt degassing is the driving mechanism of bubble growth with coalescence becoming increasingly important as exsolution progresses ostwald ripening occurs only at the end of the process and only in basaltic melt the average bubble growth rate g r ranges from 3 4 10 6 to 5 2 10 7 mm s with basalt and andesite showing faster growth rates than rhyodacite the bubble number density n b at nucleation ranges from 7 9 104 mm 3 to 1 8 105 mm 3 and decreases exponentially over time while the rhyodacite melt maintained a well sorted bubble size distribution bsd through time the bsds of basalt and andesite are much more inhomogeneous our experimental observations demonstrate that bubble growth cannot be ascribed to a single mechanism but is rather a combination of many processes which depend on the physical properties of the melt depending on coalescence rate annealing of bubbles following a single nucleation event can produce complex bubble size distributions in natural samples such bsds may be misinterpreted as resulting from several separate nucleation events incipient crystallization upon cooling of a magma may allow bubble nucleation already at very small degrees of supersaturation and could therefore be an important trigger for volatile release and explosive eruptions

simulating bubble number density of rhyolitic pumices from plinian eruptions constraints from fast decompression experiments decompression experiments of a crystal free rhyolitic liquid with 6 6 wt h2o were carried out at a pressure range from 250 mpa to 30 75 mpa in order to characterize effects of magma ascent rate and temperature on bubble nucleation kinetics especially on the bubble number density bnd the number of bubbles produced per unit volume of liquid a first series of experiments at 800 c and fast decompression rates 10 90 mpa s produced huge bnds 2 1014 m 3 at 10 mpa s 2 1015 m 3 at 90 mpa s comparable to those in natural silicic pumices from plinian eruptions 1015 1016 m 3 a second series of experiments at 700 c and 1 mpa s produced bnds 9 1012 m 3 close to those observed at 800 c and 1 mpa s 6 1012 m 3 showing that temperature has an insignificant effect on bnds at a given decompression rate our study strengthens the theory that the bnds are good markers of the decompression rate of magmas in volcanic conduits irrespective of temperature huge number densities of small bubbles in natural silicic pumices from plinian eruptions imply that a major nucleation event occurs just below the fragmentation level at which the decompression rate of ascending magmas is a maximum 1 mpa s

origin of howardites diogenites and eucrites a mass balance constraint abstract howardite meteorites are regolith breccias impact produced mixtures that consist of two main rock types basalts that strongly resemble eucrite meteorites and pyroxene cumulates that strongly resemble diogenite meteorites there are basically two petrogenetic models for the noncumulate basaltic parts of howardites 1 1 genesis as residual liquids of the same magma s that had earlier crystallized the pyroxene cumulates and 2 2 genesis as primary unmodified partial melts from the interior of the parent asteroid a mass balance constraint based on al indicates that unless the ratio degree of melting during genesis of melts parental to pyroxene cumulates degree of melting during genesis of primary basalts was on average greater than about 2 8 a scenario that implies an implausible bimodality of degree of melting on the howardite parent body an average howardite must contain more residual liquid basalt than primary basalt if as seems likely eucrites per se are identical to the howardite basaltic components and diogenites per se are the pyroxene cumulate components then more than half of the eucrites probably formed from residual liquids of magmas that had earlier produced the diogenites considering all the potential complexities of petrpgenesis on the howardite parent body neither the partial melting model nor the fractional crystallization residual liquid model is likely correct for all noncumulate eucrites