a volcanologist s review of atmospheric hazards of volcanic activity fuego and mount st helens abstract the large amount of scientific data collected on the mount st helens eruption has resulted in significant changes in thinking about the atmospheric hazards caused by explosive volcanic activity the hazard posed by fine silicate ash with long residence time in the atmosphere is probably much less serious than previously thought the mount st helens eruption released much fine ash in the upper atmosphere these silicates were removed very rapidly due to a process of particle aggregation sorem 1982 carey and sigurdsson 1982 rose and hoffman 1982 there is some evidence to suggest that particle aggregation is particularly successful in removing glass shards with high surface areas mass ratios the primary atmospheric hazard of explosive eruptions is volcanic sulfur which is converted to sulfuric acid and sulfate crystals although the mount st helens dacite magma had a very low sulfur content before eruption the eruptions did make a significant contribution to the stratospheric sulfate layer newell 1982 evidence based on measurements of s and cl in erupted rocks glass inclusions gas samples and atmospheric samples collected for both mount st helens and fuego volcanoes suggests that both volcanoes released substantial contributions of s from intrusive non eruptive magma the amount of sulfur contributed to the atmosphere by an explosive eruption thus depends not only on the volume of magma erupted and its sulfur content but also on the degree of near surface non eruptive magma the data collected to assess atmospheric hazard and to evaluate the processes and mechanisms of explosive volcanic eruptions have helped illuminate our understanding of 1 the dispersion and atmospheric fractionation of volcanic ash and 2 the determination of the size and degassing energetics of shallow magma bodies beneath volcanoes

land sea interaction and morphogenesis of coastal foredunes a modeling case study from the southern baltic sea coast abstract coastal foredunes are developed as a result of the interplay of multi scale land sea processes basic driving mechanisms of coastal foredune morphogenesis as well as natural processes and factors involved in shaping the foredune geometry are quantitatively studied in this paper by a numerical model aeolian sediment transport and vegetation growth on the subaerial part of a beach is simulated by a cellular automata ca approach while the sediment budget in the subaqueous zone serving as a sediment source sink for the foredune ridges is estimated in a process based model the coupled model is applied to a 1 km long section of a barrier coast swina gate in the southern baltic sea for a 61 year 1951 2012 hindcast of its foredune development general consistency is shown between the observational data and simulation results indicating that the formation of an established coastal foredune results from a balance between wind wave impacts and vegetation growth driven by an effective onshore wind and a boundary sediment supply small scale dunes develop on the backshore and migrate landward they are then trapped in a narrow strip characterized by a large density gradient of vegetation cover which separates the hydrodynamically active zone and the vegetated zone continuous accumulation of sediment in this strip induces the development of a foredune ridge according to the simulations the formation of an established coastal foredune has to meet three preconditions 1 an effective onshore aeolian transport 2 a net onshore or lateral sediment supply and 3 a climate favoring vegetation growth the formation of a new foredune ridge in front of an already existing foredune is determined by a combination of the sediment supply rate the extreme wind wave event frequency and the vegetation growth rate simulation results demonstrate a remarkable variability in foredune development even along a small 1 km coast section implying that the medium to long term land sea interaction and foredune morphogenesis is quite sensitive to boundary conditions and various processes acting on multi temporal and spatial scales

geochemistry of the 1989 1990 eruption of redoubt volcano part ii evidence from mineral and glass chemistry abstract early stages december 1989 of the 1989 1990 eruption of redoubt volcano produced two distinct lavas both lavas are high silica andesites with a narrow range of bulk composition 58 64 wt and similar mineralogies phenocrysts of plagioclase hornblende augite hypersthene and fe ti oxides in a groundmass of the same phases plus glass the two lavas are distinguished by groundmass glass compositions one is dacitic and the other rhyolitic sharp boundaries between the two glasses in compositionally banded pumices lack of extensive coronas on hornblende phenocrysts and seismic data suggest that a magma mixing event immediately preceeded the eruption in december 1989 textural disequilibrium in the phenocrysts suggests both magmas dacitic and rhyolitic glasses had a mixing history prior to their interaction and eruption in 1989 sievey plagioclase and overgrowths of magnetite on ilmenite are textures that are at least consistent with magma mixing the presence of two hornblende compositions one a high al pargasitic hornblende and one a low al magnesiohornblende in both the dacitic and rhyolitic groundmasses indicates a mixing event to yield these two amphibole populations prior to the magma mixing in december 1989 the pargasitic hornblende and the presence of ca rich overgrowths in the sievey zones of the plagioclase together indicate at least one component of this earlier mixing event was a mafic magma either a basalt or a basaltic andesite eruptions in 1990 produced only andesite with a rhyolitic groundmass glass glass compositions in the 1990 andesite are identical to the rhyolitic glass in the 1989 andesite cognate xenoliths from the magma chamber or conduit are also found in the 1990 lavas magma mixing probably triggered the eruption in 1989 the eruption ended when this rather viscous rhyolitic groundmass glass magma capable of entraining sidewall xenoliths magma stabalized within the conduit

observations of volcanic clouds in their first few days of atmospheric residence the 1992 eruptions of crater peak mount spurr volcano alaska abstract satellite so2 and ash measurements of mount spurr s three 1992 volcanic clouds are compared with ground based observations to develop an understanding of the physical and chemical evolution of volcanic clouds each of the three eruptions with ratings of volcanic explosivity index three reached the lower stratosphere 14 km asl but the clouds were mainly dispersed at the tropopause by moderate to strong 20 40 m s tropospheric winds three stages of cloud evolution were identified first heavy fallout of large 500 m pyroclasts occurred close to the volcano 25 km from the vent during and immediately after the eruptions and the cloud resembled an advected gravity current second a much larger highly elongated region marked by a secondary mass maximum occurred 150 350 km downwind in at least two of the three events this was the result of aggregate fallout of a bimodal size distribution including fine 25 m ash that quickly depleted the solid fraction of the volcanic cloud for the f

barchan dune mobility in mauritania related to dune and interdune sand fluxes 1 we present a 1 year study of 50 dunes in a small field of barchan dunes in mauritania we documented the morphological evolution of the dunes and their migration rates and measured at 10 min intervals the interdune sand transport the wind strength and its direction for the same interval of time the dune heights h 0 range between 2 and 5 m and their celerity c is found to be well approximated by the standard migration law c q 0 h 0 with q 0 50 m 3 m yr from both the interdune sand flux and the migration rate of the dunes we were able to estimate the spatially averaged sand flux at the dune crest as well as the bulk sand flux associated with the mass of sand transported by the dune we found that the sand flux at the crest was about 3 times greater than the interdune mass transport rate whereas the bulk sand flux was surprisingly of the same order as the interdune flux moreover we analyzed carefully the interdune sand transport data which can be well described by the sorensen law the cumulative mass of sand transported during moderate wind events within 1 year was much greater than that transported during strong wind events

characterization of the natural fracture system of the eagle ford formation val verde county texas the eagle ford formation has attracted considerable industry attention as a self sourced unconventional shale reservoir the productive interval in the eagle ford formation is the transgressive systems tract which contains parasequences whose lithologic content varies upward with increasing proportions of limestones optimum success in both exploration and production depends on the adequate characterization of fracture systems as a function of lithology the outcrops present along us highway 90 in val verde and terrell counties texas provide considerable insight into the regional natural fracture system of the eagle ford formation fracture orientation analysis reveals two sets of conjugate hybrid shear fractures and two sets of regional fractures abutting relationships suggest that hybrid shear fractures formed first followed by the thoroughgoing northeast striking fracture set and finally by a northwest striking set which tends to be confined to individual mechanical units the orientation of these fractures suggests that they formed during post laramide stress relaxation and progressive exhumation spacing frequency distribution analysis of the fracture population reveals a mature hypersaturated fracture system that likely formed at depth by overburden load and or fluid pressure near maximum burial our results indicate that the eagle ford formation displays a well developed fracture network regionally distributed in the val verde basin and likely present in the productive eagle ford play these observations provide evidence for pathways and vertical connectivity for potential fluid pathways throughout the eagle ford formation

scattering matrices of volcanic ash particles of mount st helens redoubt and mount spurr volcanoes 1 we present measurements of the whole scattering matrix as a function of the scattering angle at a wavelength of 632 8 nm in the scattering angle range 3 174 of randomly oriented particles taken from seven samples of volcanic ashes corresponding to four different volcanic eruptions the 18 may 1980 mount st helens eruption the 1989 1990 redoubt eruption and the 18 august and 17 september 1992 mount spurr eruptions the samples were collected at different distances from the vent the samples studied contain large mass fractions of fine particles and were chosen to represent ash that could remain in the atmosphere for at least hours or days they include fine ashfall samples that fell at a variety of distances from the volcano and pyroclastic flows that retained their fine fractions together they represent a range of ashes likely to remain in the atmosphere in volcanic clouds following eruptions from convergent plate boundary volcanoes earth s most important group of explosive sources of ash all measured scattering matrix elements are confined to rather limited domains when plotted as functions of the scattering angle following the general trends presented by irregular mineral particles this similarity in the scattering behavior justifies the construction of an average scattering matrix for volcanic ash particles as a function of the scattering angle to facilitate the use of the average scattering matrix for multiple scattering calculations with polarization included we present a synthetic scattering matrix based on the average scattering matrix for volcanic ashes and the assumption that the diffraction forward scattering peak is the same for randomly oriented nonspherical particles and projected surface area equivalent spheres this synthetic scattering matrix is normalized so that the average of its 1 1 element over all directions equals unity it is available in the full range from 0 to 180 and can be used for example for interpretation of remote sensing data after a volcanic eruption when the actual properties of the volcanic ash are not known the measured results for the mount st helens sample have been compared with results of lorenz mie calculations for projected surface area equivalent spheres with the refractive index of the mount st helens particles we find strong differences between measured and calculated values

geomechanical modeling of hydraulic fracturing why mechanical stratigraphy stress state and pre existing structure matter the increasing exploration and production in unconventional resource plays in the past decade has been accompanied by a greater need for understanding the effectiveness of multistage hydraulic fracturing programs particularly in long 1500 m or 5000 ft subhorizontal boreholes laterals traditional analytical analysis techniques for estimating the size and orientation of fractures induced by fluid injection typically result in predictions of relatively long and planar extension mode i bi wing fractures which may not be representative of natural systems although these traditional approaches offer the advantage of rapid analysis neglect of key features of the natural system e g realistic mechanical stratigraphy pre existing natural faults and fractures and heterogeneity of in situ stresses may render results unrealistic for planning executing and interpreting multimillion dollar hydraulic stimulation programs numerical geomechanical modeling provides a means of including key aspects of natural complexity in simulations of hydraulic fracturing in this study we present the results of two dimensional finite element modeling of fluid injection induced rock deformation that combines a coupled stress pore pressure analysis with a continuum damage mechanics based constitutive relationship the models include both the natural mechanical stratigraphic variability as well as the in situ stress state anisotropy and permit tracking of the temporal and spatial development of shear and tensile permanent strains that develop in response to fluid injection our results show that simple long planar fractures are unlikely to be induced in most mechanically layered natural systems under typical in situ stress conditions analyses that assume this type of fracture geometry may significantly overestimate the reach of hydraulically induced fractures and or effectively stimulated rock volume

the sedimentary structure of linear sand dunes linear sand dunes dunes that extend parallel to each other rather than in star like or crescentic forms are the most abundant type of desert sand dune1 but because their development and their internal structure are poorly understood they are rarely recognized in the rock record2 models of linear dune development2 3 4 5 6 have not been able to take into account the sub surface structure of existing dunes but have relied instead either on the extrapolation of short term measurements of winds and sediment transport or on observations of near surface internal sedimentary structures from such studies it has not been clear if linear dunes can migrate laterally2 7 8 here we present images produced by ground penetrating radar showing the three dimensional sedimentary structure of a linear dune in the namib sand sea where some of the world s largest linear dunes are situated these profiles show clear evidence for lateral migration in a linear dune moreover the migration of a sinuous crest line along the dune produces divergent sets of cross stratification which can become stacked as the dune height increases and large linear dunes can support superimposed dunes that produce stacked sets of trough cross stratification these clear structural signatures of linear dunes should facilitate their recognition in geological records

morphodynamics of barchan and transverse dunes using a cellular automaton model 1 we analyze the morphodynamics of bedforms produced under unidirectional flow conditions by a lattice gas cellular automaton designed to study sediment transport in this model the same instability is responsible for the formation of dunes on flat sand beds and the initiation of superimposed bedforms on dune slopes in transverse dune fields secondary bedforms increase crestline sinuosity and number of defects end of crestlines on the other hand avalanches and lateral grain motions tend to eliminate these defects to produce more regular crestlines lateral fluxes of sediment are also essential for stabilizing the shape of isolated barchan dunes we measure the propagation speed of superimposed bedforms on steady state barchan dunes and show how they contribute to the formation and detachment of smaller barchans along horns the model predicts that barchan dunes are not scale invariant and that their shape varies with respect to the strength of the flow in addition we show that the increase in bed shear stress between the ground and the crest is proportional to the dune aspect ratio finally we present a general methodology for estimating the sediment flux over the brink from dune aspect ratio and flow velocity using these fluxes we rescale the propagation speed of different generations of bedforms to verify that dunes and secondary bedforms are dynamically identical despite more fluctuations than in the case of isolated barchan dunes all these geometric and dynamical relationships hold for a population of dunes with complex dune dune interactions

multi spatial analysis of aeolian dune field patterns abstract aeolian dune fields are composed of different spatial scales of bedform patterns that respond to changes in environmental boundary conditions over a wide range of time scales this study examines how variations in spatial scales of dune and ripple patterns found within dune fields are used in environmental reconstructions on earth mars and titan within a single bedform type different spatial scales of bedforms emerge as a pattern evolves from an initial state into a well organized pattern such as with the transition from protodunes to dunes additionally different types of bedforms such as ripples coarse grained ripples and dunes coexist at different spatial scales within a dune field analysis of dune field patterns at the intersection of different scales and types of bedforms at different stages of development provides a more comprehensive record of sediment supply and wind regime than analysis of a single scale and type of bedform interpretations of environmental conditions from any scale of bedform however are limited to environmental signals associated with the response time of that bedform large scale dune field patterns integrate signals over long term climate cycles and reveal little about short term variations in wind or sediment supply wind ripples respond instantly to changing conditions but reveal little about longer term variations in wind or sediment supply recognizing the response time scales across different spatial scales of bedforms maximizes environmental interpretations from dune field patterns

the impact of the kasatochi eruption on the moon s illumination during the august 2008 lunar eclipse 1 the moon s changeable aspect during a lunar eclipse is largely attributable to variations in the refracted unscattered sunlight absorbed by the terrestrial atmosphere that occur as the satellite crosses the earth s shadow the contribution to the moon s aspect from sunlight scattered at the earth s terminator is generally deemed minor however our analysis of a published spectrum of the 16 august 2008 lunar eclipse shows that diffuse sunlight is a major component of the measured spectrum at wavelengths shorter than 600 nm the conclusion is supported by two distinct features namely the spectrum s tail at short wavelengths and the unequal absorption by an oxygen collisional complex at two nearby bands our findings are consistent with the presence of the volcanic cloud reported at high northern latitudes following the 7 8 august 2008 eruption in alaska of the kasatochi volcano the cloud both attenuates the unscattered sunlight and enhances moderately the scattered component thus modifying the contrast between the two contributions

modelling vegetated dune landscapes this letter presents a self organising cellular automaton model capable of simulating the evolution of vegetated dunes with multiple types of plant response in the environment it can successfully replicate hairpin or long walled parabolic dunes with trailing ridges as well as nebkha dunes with distinctive deposition tails quantification of simulated landscapes with eco geomorphic state variables and subsequent cluster analysis and pca yields a phase diagram of different types of coastal dunes developing from blow outs as a function of vegetation vitality this diagram indicates the potential sensitivity of dormant dune fields to reactivation under declining vegetation vitality e g due to climatic changes nebkha simulations with different grid resolutions demonstrate that the interaction between the abiotic geomorphic processes and the biological vegetation component life introduces a characteristic length scale on the resultant landforms that breaks the typical self similar scaling of un vegetated bare sand dunes

the role of seismic attributes in understanding the hydraulically fracturable limits and reservoir performance in shale reservoirs an example from the eagle ford shale south texas as the importance of self sourcing reservoirs continues to increase it is more important than ever to evaluate rock properties that contribute to productive wells it has become increasingly evident that in order to maximize potential returns an integrated approach to shale play characterization is necessary to identify productive areas numerous criteria exist to characterize ultra low permeability shale reservoirs and their associated resource potential these include measures of organic richness thermal maturity lithologic heterogeneity and formation brittleness the latter a descriptor of the geomechanical rock properties can play a significant role in overall well performance and is commonly a key productivity driver thus an understanding of the mechanical properties of the target section is fundamental for high grading prospective areas well placement design and hydraulic stimulation effectiveness observation of geomechanical attributes extracted from seismic data in the eagle ford shale captures changing mechanical properties indicative of strike oriented lithologic facies changes using acoustic logs core and three dimensional seismic data we assess the mechanical contrast between eagle ford facies units and their effect on well performance we use three dimensional seismic data to map the structure and facies distribution in an area where identification of reservoir facies is a major challenge to development drilling in this study we demonstrate how young s modulus and density inverted from three dimensional seismic data prove as effective discriminators for the purpose of identifying and mapping facies changes and establishing the hydraulically fracturable limits in areas where effective stimulation and proppant embedment in the formation during pressure drawdown is a concern the result is an interpretation that identifies and uses the mechanical changes from three dimensional seismic data attributes associated with the brittle carbonate rich eagle ford facies to predict both the reservoirs hydraulically fracturable limits as well as the variability in well performance associated with proppant embedment the changes in mechanical properties of the eagle ford facies are important in high grading productive intervals in these ultra low permeability rocks we believe we can apply this method to other shale reservoirs where rock mechanics may play an important role

character mass distribution and origin of tephra fall deposits of the 1989 1990 eruption of redoubt volcano south central alaska abstract the 1989 1990 eruption of redoubt volcano spawned about 20 areally significant tephra fall deposits between december 14 1989 and april 26 1990 tephra plumes rose to altitudes of 7 to more than 10 km and were carried mainly northward and eastward by prevailing winds where they substantially impacted air travel commerce and other activities in comparison to notable eruptions of the recent past the redoubt events produced a modest amount of tephra fall deposits 6 10 7 to 5 10 10 kg for individual events and a total volume dense rock equivalent of about 3 5 10 7 m 3 of andesite and dacite two contrasting tephra types were generated by these events pumiceous tephra fall deposits of december 14 and 15 were followed on december 16 and all later events by fine grained lithic crystal tephra deposits much of which fell as particle aggregates the change in the character of the tephra fall deposits reflects their fundamentally different modes of origin the pumiceous deposits were produced by magmatically driven explosions the finegrained lithic crystal deposits were generated by two processes hydrovolcanic vent explosions generated tephrafall deposits of december 16 and 19 such explosions continued as a tephra source but apparently with diminishing importance during events of january and february ash clouds of lithic pyroclastic flows generated by collapse of actively growing lava domes probably contributed to tephra fall deposits of all events from january 2 to april 26 and were the sole source of tephra fall for at least the last 4 deposits

relations of sand trapping efficiency and migration speed of transverse dunes to wind velocity a self consistent model which describes transverse dune migration in equilibrium is introduced it shows that an equilibrium expression for dune migration speed cd must take into account sand trapping efficiency te and that te is strongly related to the wind speedup over the windward surface an expression for sand trapping efficiency te is analytically derived from a microscale analysis of sand grain deposition on the slip face sand trapping efficiency te is mainly determined by shear velocity on a level surface u and rapidly decreases as u increases for each dune height h dune migration speed cd first increases and then decreases monotonically after reaching the maximum as the shear velocity on a level surface u increases dune migration speed cd is not inversely proportional to dune height h for low dunes small sand trapping efficiency te suppresses cd whereas for high dunes wind speedup and large te resist the decrease of cd some field data show the same tendency the dune to plane bed transition observed in subaqueous and venusian bedforms could be associated with the decrease of sand trapping efficiency te copyright 2000 john wiley sons ltd

optical modeling of volcanic ash particles using ellipsoids the single scattering properties of volcanic ash particles are modeled here by using ellipsoidal shapes ellipsoids are expected to improve the accuracy of the retrieval of aerosol properties using remote sensing techniques which are currently often based on oversimplified assumptions of spherical ash particles measurements of the single scattering optical properties of ash particles from several volcanoes across the globe including previously unpublished measurements from the eyjafjallajokull and puyehue volcanoes are used to assess the performance of the ellipsoidal particle models these comparisons between the measurements and the ellipsoidal particle model include consideration of the whole scattering matrix as well as sensitivity studies on the point of view of the advanced along track scanning radiometer aatsr instrument aatsr which flew on the envisat satellite offers two viewing directions but no information on polarization so usually only the phase function is relevant for interpreting its measurements as expected ensembles of ellipsoids are able to reproduce the observed scattering matrix more faithfully than spheres performance of ellipsoid ensembles depends on the distribution of particle shapes which we tried to optimize no single specific shape distribution could be found that would perform superiorly in all situations but all of the best fit ellipsoidal distributions as well as the additionally tested equiprobable distribution improved greatly over the performance of spheres we conclude that an equiprobable shape distribution of ellipsoidal model particles is a relatively good yet enticingly simple approach for modeling volcanic ash single scattering optical properties

on a small scale roughness of the core mantle boundary abstract the roughness of the core mantle boundary on the scale of centimeters to tens of meters is modelled using a cellular automata method square cells of the size of grains of the mantle material on a 2 d grid can be in one of three states corresponding to mantle silicate or oxide core fluid saturated in light element and unsaturated core fluid the dynamical process of evolution is defined as a stationary stochastic process without memory transitions of doublets of cells from one state to another are governed by parameters representing the rates of physical processes dissolution and crystallization at the cmb and diffusion of the light element in the core fluid with reasonable values of the parameters the boundary roughens on the scale of grains and a boundary layer of saturated fluid of a few tens of centimeters thick soon appears at the interface an undulation with dominant wavelength of the order of a few tens of meters eventually appears an interaction of the roughness of the cmb with the fluid flow in the core is considered as possible

barchan dune corridors field characterization and investigation of control parameters 1 the structure of the barchan field located between tarfaya and laayoune atlantic sahara morocco is quantitatively investigated and compared to that in la pampa de la joya arequipa peru on the basis of field measurements we show how the volume the velocity and the output sand flux of a dune can be computed from the value of its body and horn widths the dune size distribution is obtained from the analysis of aerial photographs it shows that these fields are in a statistically homogeneous state along the wind direction and present a corridor structure in the transverse direction in which the dunes have a rather well selected size investigating the possible external parameters controlling these corridors we demonstrate that none among topography granulometry wind and sand flux is relevant we finally discuss the dynamical processes at work in these fields collisions and wind fluctuations and investigate the way they could regulate the size of the dunes furthermore we show that the overall sand flux transported by a dune field is smaller than the maximum transport that could be reached in the absence of dunes i e in saltation over the solid ground

an agent based model of dune interactions produces the emergence of patterns in deserts 1 crescent shaped barchan dunes are highly mobile dunes which are ubiquitous on earth and other solar system bodies although they are unstable when considered separately they form large assemblies in deserts and spatially organize in narrow corridors that extend in the wind direction collision of barchans has been proposed as a mechanism to redistribute sand between dunes and prevent the formation of very large dunes here we use an agent based model with elementary rules of sand redistribution during collisions to access the full dynamics of very large barchan fields we tune the dune field density by changing the sand load lost ratio and follow the transition between dilute fields where barchans barely interact and dense fields where dune collisions control and stabilize the dune field in this dense regime barchans have a small well selected size and form flocks the dune field self organizes in narrow corridors of dunes as it is observed in real dense barchan deserts

development and steady states of transverse dunes a numerical analysis of dune pattern coarsening and giant dunes we investigate the development and steady states of transverse dunes for ranges of flow depths and velocities using a cellular automaton dune model subsequent to the initial bed instability dune pattern coarsening is driven by bed form interactions collisions lead to two types of coalescence associated with upstream or downstream dominant dunes in addition a single collision ejection mechanism enhances the exchange of mass between two adjacent bed forms throughpassing dunes the power law increases in wavelength and amplitude exhibit the same exponents which are independent of flow properties contrary to the wavelength dune height is limited not only by flow depth but also by the strength of the flow superimposed bed forms may propagate and continuously destabilize the largest dunes we identify three classes of steady state transverse dune fields according to the periodicity in crest to crest spacing and the mechanism of size limitation in all cases the steady state is reached and maintained through the dynamic equilibrium between flow strength and dune aspect ratio in the limit of low flow strength where it becomes the primary factor of size limitation the bed shear stress in the dune trough regions is close to its critical value for motion inception comparisons with natural dune fields suggest that many of them may have reached a steady state finally we infer that the sedimentary patterns in the model may be used to bring new constraints on the development of modern and ancient dune fields

reference data set of volcanic ash physicochemical and optical properties uncertainty in the physicochemical and optical properties of volcanic ash particles creates errors in the detection and modeling of volcanic ash clouds and in quantification of their potential impacts in this study we provide a dataset that describes the physicochemical and optical properties of a representative selection of volcanic ash samples from nine different volcanic eruptions covering a wide range of silica contents 50 80 wt sio2 we measured and calculated parameters describing the physical size distribution complex shape and dense rock equivalent mass density chemical bulk and surface composition and optical complex refractive index from ultra violet to near infrared wavelengths properties of the volcanic ash and classified the samples according to their sio2 and total alkali contents into the common igneous rock types basalt to rhyolite we found that the mass density ranges between 2 49 and 2 98 g cm3 for rhyolitic to basaltic ash types and that the particle shape varies with changing particle size d 100 m the complex refractive indices in the wavelength range between 300 nm and 1500 nm depends systematically on the composition of the samples the real part values vary from n 1 38 to 1 66 depending on ash type and wavelength and the imaginary part values from k 0 00027 to 0 00268 we place our results into the context of existing data and thus provide a comprehensive dataset that can be used for future and historic eruptions when only basic information about the magma type producing the ash is known

variations in wind velocity and sand transport on the windward flanks of desert sand dunes the magnitudes of increases in wind velocity or speed up factors have been measured on the windward flanks of transverse and linear dunes of varying height on transverse dunes velocity speed up varied with dune shape and height for linear dunes speed up factors varied principally with wind direction relative to the dune with dune shape and dune height the main effect of velocity speed up on the windward flanks of dunes is to increase potential sand transport rates considerably in crestal areas this is greatest for large dunes with winds of moderate velocity blowing at a large angle to the dune changing ratios of base to crest sand transport rates on transverse dunes tend to reduce dune steepness as overall wind velocities increase on linear dunes the tendency for crestal lowering is counteracted by deposition in this area when winds reverse in a bi directional wind regime

a wind tunnel study of wind erosion and profile reshaping of transverse sand piles in tandem abstract the deformation over time as a result of wind erosion is presented for a single transverse sand pile and also for two sand piles placed in tandem the maximum slope of the two pile geometries studied here is 27 length height 6 and 32 length height 5 respectively the initial contour of each pile is described by a sinusoidal curve and all of them have the same initial height the experimental results were obtained from wind tunnel tests for two selected wind speeds several tests were conducted for each situation in order to obtain the mean longitudinal profile of the pile s at several time instants and to characterize the dispersion range of the results it is shown that the first pile has a strong effect on the downwind one and also vice versa although to a much lesser degree in particular the experimental results indicate that for the lowest test velocity while the first pile remains uneroded the leeward one is severely eroded due to the recirculation bubble between both piles

setting the length and time scales of a cellular automaton dune model from the analysis of superimposed bed forms 1 we present a new 3 d cellular automaton model for bed form dynamics in which individual physical processes such as erosion deposition and transport are implemented by nearest neighbor interactions and a time dependent stochastic process simultaneously a lattice gas cellular automaton model is used to compute the flow and quantify the bed shear stress on the topography local erosion rates are assumed to be proportional to the shear stress in such a way that there is a complete feedback mechanism between flow and bed form dynamics in the numerical simulations of dune fields we observe the formation and the evolution of superimposed bed forms on barchan and transverse dunes using the same model under different initial conditions we perform the linear stability analysis of a flat sand bed disturbed by a small sinusoidal perturbation comparing the most unstable wavelength in the model with the characteristic size of secondary bed forms in nature we determine the length and time scales of our cellular automaton model thus we establish a link between discrete and continuous approaches and open new perspectives for modeling and quantification of complex patterns in dune fields

pattern analysis of dune field parameters as a basic form of pattern analysis the parameters of dune spacing defect density crest orientation and crest length are measured from remote images and treated statistically for dunes at white sands in new mexico the algodones in california the agneitir in mauritania and the namib in namibia statistical populations are identified from frequency plots of dune spacing and crest length field scale calculations of defect density and rose diagrams of crest orientation single populations characterize simple dune fields white sands whereas multiple populations characterize compound complex dunes algodones namib and complex dune fields agneitir as time increases dune fields show an increase in dune spacing and crest length a decrease in defect density more tightly clustered crest orientation and a reduction in the variance associated with measurements of these parameters the results are consistent with models of dune fields as self organizing complex systems in which a characteristic pattern emerges as a function of constructional time because pattern evolution is a function of time it may be possible to use pattern analysis to augment current methods of age determination statistically defined populations can be used in geomorphic backstripping to unstack generations of simple patterns that give rise to complex patterns and to reconstruct each generation in terms of construction time and palaeo wind regime copyright 2006 john wiley sons ltd

quantitative shape measurements of distal volcanic ash 1 large scale volcanic eruptions produce fine ash 200 m which has a long atmospheric residence time 1 hour or more and can be transported great distances from the volcanic source thus becoming a hazard to aircraft and public health ash particles have irregular shapes so data on particle shape size and terminal velocities are needed to understand how the irregular shaped particles affect transport processes and radiative transfer measurements in this study a methodology was developed to characterize particle shapes sizes and terminal velocities for three ash samples of different compositions the shape and size of 2500 particles from 1 distal fallout 100 km of the 14 october 1974 fuego eruption basaltic 2 the secondary maxima 250 km of the 18 august 1992 spurr eruption andesitic and 3 the miocene ash hollow member nebraska rhyolitic were measured using image analysis techniques samples were sorted into 10 to 19 terminal velocity groups 0 6 59 0 cm s using an air elutriation device grain size distributions for the samples were measured using laser diffraction aspect ratio feret diameter and perimeter measurements were found to be the most useful descriptors of how particle shape affects terminal velocity these measurement values show particle shape differs greatly from a sphere commonly used in models and algorithms the diameters of ash particles were 10 120 larger than ideal spheres at the same terminal velocity indicating that irregular particle shape greatly increases drag gas adsorption derived surface areas are 1 to 2 orders of magnitude higher than calculated surface areas based on measured dimensions and simple geometry indicating that particle shapes are highly irregular correction factors for surface area were derived from the ash sample measurements so that surface areas calculated by assuming spherical particle shapes can be corrected to reflect more realistic values

wind velocity and sand transport on a barchan dune we present measurements of wind velocity and sand flux performed on the windward side of a large barchan dune in jericoacoara northeastern brazil from the measured profile we calculate the air shear stress using an analytical approximation and treat the problem of flow separation by an heuristic model we find that the results from this approach agree well with our field data moreover using the calculated shear velocity we predict the sand flux according to well known equilibrium relations and with a phenomenological continuum saltation model that includes saturation transients and thus allows for nonequilibrium conditions based on the field data and theoretical predicted results we indicate the principal differences between saturated and nonsaturated sand flux models finally we show that the measured dune moves with invariant shape and predict its velocity from our data and calculations

formation of the highest sand dunes on earth characterization of dune morphology has historically been based on relationships between dune forms and wind regimes with dune height shown to be sensitive to atmospheric boundary layer depth sand availability and sediment properties while these parameters have been used in numerical simulations to model the occurrences of some types of dunes they cannot alone explain the great diversity in form and size seen on earth and on other solar system bodies here we present results from our studies of dune formation in the badain jaran desert in western china where earth s tallest dunes occur we measured the variability of the dune morphology in this desert on the basis of landsat etm data and we detected the bedrock landforms beneath the aeolian sands by applying gravity methods wind records from stations at the periphery of the desert and srtm topographical data were examined also to augment the interpretation our studies demonstrate that in addition to average wind parameters dune height is highly sensitive to local geology subsurface characteristics and topography and interactions between changing climate conditions and aeolian and fluvial processes these additional factors need to be considered in the interpretation and simulation of dunes on earth we anticipate that analysis of anomalous dune heights like those seen in the badain jaran may also provide critical information on subsurface characteristics and environmental conditions on earth and on other planetary bodies

characterization and tectonic evolution of a mesoproterozoic island arc in the southern grenville orogen llano uplift central texas abstract integrated field and u pb geochronologic data document the presence and tectonic evolution of a distinct lithotectonic terrane in the southeastern llano uplift here termed the coal creek domain ccd the ccd consists of tonalitic and amphibolitic gneisses foliated gabbroic tonalitic and granodioritic intrusions and the structurally emplaced coal creek serpentinite the oldest rocks in the ccd consist of tonalitic gneisses with protolith crystallization ages of 1326 ma and 1301 ma and pre 1292 ma amphibolitic gneisses all ages are u pb zircon they were intruded by dominantly mafic and intermediate with subordinate amounts of felsic low k plutonic rocks between 1286 ma and 1275 ma high temperature metamorphism and local anatexis that resulted in the formation of gneisses and migmatites occurred between 1301 ma and 1286 ma and is suggested to have occurred at 1292 ma metamorphism at 1256 ma resulted in the growth of zircons in mafic rocks the regional significance of this metamorphic event is unclear but it may be related to dynamothermal metamorphism of the ccd the contemporaneous emplacement of low k gabbro tonalite and granodiorite and their spatial association with serpentinite combined with published geochemical data strongly suggest that the ccd represents an ensimatic arc the coal creek serpentinite is interpreted as a fragment of oceanic crust that formed either basement to the arc or in a related back arc or intra arc setting combined field u pb geochronologic isotopic and geochemical data indicate that the magmatic metamorphic and structural history of the ccd differs from that of coeval rocks elsewhere in the llano uplift the boundary between the ccd and deformed and metamorphosed granitic and supracrustal rocks to the north is a zone of intense mylonitization and polyphase deformation this zone is interpreted as the structural boundary between the arc terrane and units to the north

faulting and fracturing in shale and self sourced reservoirs introduction production from self sourced reservoirs relies on natural and induced fracturing for permeability and conductance of hydrocarbons to the producing wellbores thus natural or induced fracturing is often a key to success in unconventional reservoir plays on the other hand fractures may compromise seals and large or well connected fractures or faults may cause undesirable complications for unconventional reservoirs natural and induced fractures are influenced by 1 mechanical stratigraphy 2 pre existing natural deformation such as faults fractures and folds and 3 in situ stress conditions both natural and as modified by stimulation and pressure depletion this special issue of the aapg bulletin elucidates some of these structural geologic and geomechanical controls understanding the occurrence and controls on natural and induced faulting and fracturing in self sourced reservoirs is a key component for developing effective approaches for exploiting self sourced reservoirs

collision of barchan dunes as a mechanism of size regulation 1 barchans are propagating crescentic dunes that form in arid regions where strong unidirectional winds blow across a firm soil lightly covered with sand recently solitary barchan dunes have been shown to be unstable towards sand flux variation either growing to form mega dunes or shrinking and disappearing when perturbed from equilibrium however observations of large corridors of barchan dunes in the field suggest that these bedforms are stable over long periods of time since dunes migration rates are inversely proportional to their size barchans of different size must collide and these collisions may be crucial in maintaining the stability of dunes in nature here we first explain the unstable behavior of solitary barchans and then illustrate using down scaled physical experiments the qualitative dynamics of binary collisions a stability analysis inspired by these experiments suggests that collisions may indeed regulate the size of barchans migrating in a corridor by redistributing sand from large dunes to smaller ones

measurements of dielectric constant of volcanic ash erupted from five volcanoes in japan the dielectric properties of volcanic ash from five volcanoes in japan were measured by using the transmission reflection coaxial line method of a network analyzer in a frequency range from 3 to 13 ghz the measured permittivity of powdered ash was then converted to that of solid ash dense rock equivalent with the aid of effective medium theory the values of solid ash relative permittivities for the five volcanoes revealed that they were in a relatively narrow range their real part was generally in a range from five to six in magnitude while their imaginary part was in a range from 0 08 to 0 18

sediment flux and airflow on the stoss slope of a barchan dune abstract measurements of sediment flux on the windward slope of an isolated barchan using an array of 30 sand traps provide new data that can constrain models of dune dynamics the data show that at low wind incident speeds flux increases up the dune exponentially whereas at higher wind speeds the increase with distance approaches linearity wind profile measurements conducted at the same time as the flux measurements indicate that although wind speed at a given height increases by 1 2 times from dune toe to brinkline wind shear velocity derived from the profile data decreases up the dune and is in many cases below transport threshold values this demonstrates that conventional wind profiles derived from anemometry on dunes do not measure the part of the boundary layer that is significant for sediment transport

modeling emergent large scale structures of barchan dune fields in nature barchan dunes typically exist as members of larger fi elds that display striking enigmatic structures that cannot be readily explained by examining the dynamics at the scale of single dunes or by appealing to patterns in external forcing to explore the possibility that observed structures emerge spontaneously as a collective result of many dunes interacting with each other we built a numerical model that treats barchans as discrete entities that interact with one another according to simplifi ed rules derived from theoretical and numerical work and from fi eld observations 1 dunes exchange sand through the fl uxes that leak from the downwind side of each dune and are captured on their upstream sides 2 when dunes become suffi ciently large small dunes are born on their downwind sides calving and 3 when dunes collide directly enough they merge results show that these relatively simple interactions provide potential explanations for a range of fi eld scale phenomena including isolated patches of dunes and heterogeneous arrangements of similarly sized dunes in denser fi elds the results also suggest that 1 dune fi eld characteristics depend on the sand fl ux fed into the upwind boundary although 2 moving downwind the system approaches a common attracting state in which the memory of the upwind conditions vanishes this work supports the hypothesis that calving exerts a fi rst order control on fi eld scale phenomena it prevents individual dunes from growing without bound as single dune analyses suggest and allows the formation of roughly realistic persistent dune fi eld patterns

control of mechanical stratigraphy on bed restricted jointing and normal faulting eagle ford formation south central texas outcrops of the middle eagle ford formation in south central texas reveal well developed joint networks in subhorizontal competent carbonate chalk beds and less well developed networks in interlayered incompetent calcareous mudrock beds northeast striking bed perpendicular joints in competent beds have the longest trace lengths and are abutted by northwest striking joints all observed joints terminate vertically in incompetent beds normal faults are common but less abundant than joints dominantly dip north northwest or southeast and are abutted by the joint sets and thus predated jointing the faults cut multiple competent and incompetent beds providing vertical connectivity across mechanical layering products of hybrid and shear failure the dip of these faults is steep through competent beds and moderate through incompetent beds resulting in refracted fault profiles with dilation and calcite precipitation along steep segments fluid inclusions in fault zone calcite commonly contain liquid hydrocarbons rare two phase fluid inclusions homogenized between about 1 40 and 58 c and 2 90 and 100 c suggesting trapping of aqueous fluids at elevated temperatures and depths on the order of 2 km 6562 ft fluid inclusion and stable isotope geochemistry analyses suggest that faults transmitted externally derived fluids these faults likely formed at depths equivalent to portions of the present day oil and gas production from the eagle ford play in south texas faults connect across layering and provide pathways for vertical fluid movement within the eagle ford formation in contrast to vertically restricted joints that produce bed parallel fracture permeability these observations elucidate natural fractures and induced hydraulic fracturing within the eagle ford formation

comparison of laser grain size analysis with pipette and sieve analysis a solution for the underestimation of the clay fraction classically the grain size of soil and sediment samples is determined by the sieve method for the coarse fractions and by the pipette method based on the stokes sedimentation rates for the fine fractions results from the two methods are compared with results from laser diffraction size analysis which is based on the forward scattering of monochromatic coherent light from a point of view of laboratory efficiency the laser sizing technique is far superior accuracy and reproducibility are shown by measurements on certified materials it appears that laser grain size measurements of certified materials correspond very well with the certificated measurements tests were also done on a set of randomly selected sediments of fluvial aeolian and lacustrine origin except for the 2 m clay fraction there is a coarsening of the mean diameter of one to two size classes 0 25 caused by the non sphericity of the particles the platy form of the clay particles induces considerable differences eight size classes between pipette and laser measurements the 2 m grain size defined by the pipette method corresponds with a grain size of 8 m defined by the laser particle sizer for the studied sediments using a higher grain size level for the clay fraction when laser analysis is applied enables workers in the geological and environmental field to compare classical pipette analysis with a laser sizing technique

fault structure stress or pressure control of the seismicity in shale insights from a controlled experiment of fluid induced fault reactivation clay formations are present in reservoirs and earthquake faults but questions remain on their mechanical behavior as they can vary from ductile aseismic to brittle seismic an experiment at a scale of 10 m aims to reactivate a natural fault by fluid pressure in shale materials the injection area was surrounded by a dense monitoring network comprising pressure deformation and seismicity sensors in a well characterized geological setting thirty two microseismic events were recorded during several injection phases in five different locations within the fault zone their computed magnitude ranged between 4 3 and 3 7 their spatiotemporal distribution compared with the measured displacement at the injection points shows that most of the deformation induced by the injection is aseismic whether the seismicity is controlled by the fault architecture mineralogy of fracture filling fluid and or stress state is then discussed the fault damage zone architecture and mineralogy are of crucial importance as seismic slip mainly localizes on the sealed with calcite fractures which predominate in the fault damage zone as no seismicity is observed in the close vicinity of the injection areas the presence of fluid seems to prevent seismic slips the fault core acts as an impermeable hydraulic barrier that favors fluid confinement and pressurization therefore the seismic behavior seems to be strongly sensitive to the structural heterogeneity including permeability of the fault zone which leads to a heterogeneous stress response to the pressurized volume

aeolian dune field self organization implications for the formation of simple versus complex dune field patterns abstract the interpretation of aeolian dune field patterns as self organizing complex systems is a new paradigm in which pattern evolution may be addressed computer simulations supported by field and experimental data indicate that a given wind regime produces a simple dune field pattern dune type and crest orientation are determined by wind regime and pattern ordering occurs through dune dune interactions over time because dunes reorient only at their crest terminations with a change in wind regime the rate of formation of a new pattern of small dunes is typically faster than the rate of reorientation of the existing pattern resulting in the superposition of simple patterns to give rise to complex patterns complex patterns are distinct from spatial changes in a simple pattern and from the type of superposition that characterizes compound complex dunes complex patterns necessarily indicate a rate of pattern formation that is rapid compared to the rate of sediment accumulation on the depositional surface

fault zone processes and fluid history in austin chalk southwest texas faults are commonly the largest fractures in a fracture network forming the backbone of the permeability system in low permeability rocks faults in outcrops of the austin chalk an important oil and gas reservoir in south texas louisiana and westernmost mississippi exemplify deformation and fluid histories of subseismic scale faults in mechanically layered chalk dominated strata exposures of near horizontal austin chalk in southwest texas contain northwest and southeast dipping normal faults with exposed heights of meters to tens of meters and displacements of centimeters to decimeters local fault dips are steep to vertical through chalk and limestone beds and moderate through marl mud rock and clay rich ash beds producing refracted fault profiles failure surface characteristics indicate various failure modes including tensile hybrid shear and locally compactive shear behavior depending on the host lithology dilational zones associated with steep fault segments host calcite veins crack seal textures in fault zone veins record repeated reactivation of refracted fault profiles fluid inclusion and stable isotope geochemistry analyses of fault veins indicate formation at depths of 1 4 to 2 9 km 4593 to 9514 ft and possibly of 4 2 km 13 780 ft fluids include locally sourced saline waters and externally sourced waters and oil in rocks that did not reach oil maturation conditions inclusions of 30 api to greater than 50 api oil are found in veins 30 km 19 mi updip from the eagle ford formation oil zone these results illustrate the migration of oil and other fluids within subseismic scale austin chalk faults and fundamental control of lithology on fault zone architecture

giant aeolian dune size determined by the average depth of the atmospheric boundary layer sand dunes common on earth and other planetary bodies still hold some mysteries the ways that small dunes form those with lengths of tens of metres are well understood but much less clear is how giant kilometre scale dunes come into existence what dictates their shapes and what limits their size andreotti et al use a combination of field measurements and aerodynamic calculations to show that the growth of giant dunes is limited by the confinement of the sand transport flow over the average depth of the atmospheric boundary layer the new findings can explain the mean spacing of wind generated giant dunes up to 3 5 km in length this paper shows that the growth of aeolian giant dunes ascribed to the non linear interaction between small scale superimposed dunes is limited by the confinement of the flow over the average depth of the atmospheric boundary layer the findings explain the mean spacing of aeolian giant dunes ranging from 300 metres in coastal deserts to 3 5 kilometres depending on the wind regime1 2 sand dunes exhibit linear3 4 crescent shaped or star like forms5 resulting from the interaction between dune morphology and sand transport6 7 8 small scale dunes form by destabilization of the sand bed9 10 11 with a wavelength a few tens of metres determined by the sand transport saturation length11 12 13 the mechanisms controlling the formation of giant dunes and in particular accounting for their typical time and length scales have remained unknown using a combination of field measurements and aerodynamic calculations we show here that the growth of aeolian giant dunes ascribed to the nonlinear interaction between small scale superimposed dunes4 10 14 15 is limited by the confinement of the flow within the atmospheric boundary layer16 17 aeolian giant dunes and river dunes form by similar processes with the thermal inversion layer that caps the convective boundary layer in the atmosphere18 acting analogously to the water surface in rivers in both cases the bed topography excites surface waves on the interface that in turn modify the near bed flow velocity this mechanism is a stabilizing process that prevents the scale of the pattern from coarsening beyond the resonant condition our results can explain the mean spacing of aeolian giant dunes ranging from 300 m in coastal terrestrial deserts to 3 5 km we propose that our findings could serve as a starting point for the modelling of long term evolution of desert landscapes under specific wind regimes

detection of gas in sandstone reservoirs using avo analysis a 3 d seismic case history using the geostack technique the geostack technique is a method of analyzing seismic amplitude variation with offset avo information one of the outputs of the analysis is a set of direct hydrocarbon indicator traces called fluid factor traces the fluid factor trace is designed to be low amplitude for all reflectors in a clastic sedimentary sequence except for rocks that lie off the mudrock line the mudrock line is the line on a crossplot of p wave velocity against s wave velocity on which water saturated sandstones shales and siltstones lie some of the rock types that lie off the mudrock line are gas saturated sandstones carbonates and igneous rocks in the absence of carbonates and igneous rocks high amplitude reflections on fluid factor traces would be expected to represent gas saturated sandstones of course this relationship does not apply exactly in nature and the extent to which the mudrock line model applies varies from area to area however it is a useful model in many basins of the world including the one

collision behaviors of barchans in aeolian dune fields in this paper the probability of the collision behaviors of barchans in aeolian dune fields and the processes of three collision types i e coalescence breeding and solitary wave like behavior are analyzed by the scale coupled model of dune fields moreover the influence of wind speed sand diameter sand supply the height ratio between two barchans and the angle between the dune trend and wind direction as well as the distance between the centerlines of two barchans on the collision results of barchans is demonstrated

the role of streamline curvature in sand dune dynamics evidence from field and wind tunnel measurements abstract field measurements on an unvegetated 10 m high barchan dune in oman are compared with measurements over a 1 200 scale fixed model in a wind tunnel both the field and wind tunnel data demonstrate similar patterns of wind and shear velocity over the dune confirming significant flow deceleration upwind of and at the toe of the dune acceleration of flow up the windward slope and deceleration between the crest and brink this pattern including the widely reported upwind reduction in shear velocity reflects observations of previous studies such a reduction in shear velocity upwind of the dune should result in a reduction in sand transport and subsequent sand deposition this is not observed in the field wind tunnel modelling using a near surface pulse wire probe suggests that the field method of shear velocity derivation is inadequate the wind tunnel results exhibit no reduction in shear velocity upwind of or at the toe of the dune evidence provided by reynolds stress profiles and turbulence intensities measured in the wind tunnel suggest that this maintenance of upwind shear stress may be a result of concave unstable streamline curvature these additional surface stresses are not recorded by the techniques used in the field measurements using the occurrence of streamline curvature as a starting point a new 2 d model of dune dynamics is deduced this model relies on the establishment of an equilibrium between windward slope morphology surface stresses induced by streamline curvature and streamwise acceleration adopting the criteria that concave streamline curvature and streamwise acceleration both increase surface shear stress whereas convex streamline curvature and deceleration have the opposite effect the relationships between form and process are investigated in each of three morphologically distinct zones the upwind interdune and concave toe region of the dune the convex portion of the windward slope and the crest brink region the applicability of the model is supported by measurements of the rate of sand transport and the change of the dune surface in the field

geochemistry of the 1989 1990 eruption of redoubt volcano part i whole rock major and trace element chemistry the 1989 1990 eruption of redoubt volcano produced medium k calc alkaline andesite and dacite of limited compositional range 58 2 63 4 sio2 and entrained quenched andesitic inclusions 55 sio2 which bear chemical similarities to the rest of the ejecta the earliest december 15 magmas are pumiceous often compositionally banded and the majority is relatively mafic 59 sio2 the most silicic magmas of the eruption are the late december to early january domes up to 63 4 sio2 subsequent magmas formed domes and rare pumices which converge on 60 sio2 chemical variations among ejecta comprise tight linear two component arrays for all elements for which the analytical uncertainty is much less than the compositional range the two component arrays are interpreted as mixing arrays between unrelated magmas because several of the arrays are at steep angles to the normal liquid line of descent additionally the felsic endmember cannot be easily related to the mafic endmember by normal high temperature igneous processes e g the silicic endmember has higher zr yet lower hf than the mafic endmember also relative enrichments of highly incompatible elements are dramatically different across the arrays the mixing event must have preceded eruption by a significant yet unspecified amount of time because groundmass glass compositions are homogeneous for all post december samples swanson et al 1994 this volume in spite of the whole rock chemical diversity this implies time for additional crystallization after the mixing event swanson et al 1994 this volume discuss evidence for a potentially different mixing event recorded only in december 15 magmas cognate cumulate xenoliths composed of pl cpx opx hb mt melt were recovered from january and april deposits these blocks differ from local batholithic country rock in their low concentrations of incompatible elements e g rb 60 wt they have mg cr ni sc and v contents higher than the andesites but lower than redoubt basalts and basaltic andesites thus they may be crystallization products of andesites but do not represent the cumulate residue of basalt fractionation the xenoliths were probably derived from a shallow or intermediate crustal chamber

dynamics of sediment storage and release on aeolian dune slip faces a field study in jericoacoara brazil sediment transport on the lee sides of aeolian dunes involves a combination of grain fall deposition on the upper portion of the slip face until a critical angle is exceeded transport of a portion of those sediments down the slip face by grain flows and finally deposition at an angle of repose we measured the mean critical and repose angles and the rate of slip face avalanching using terrestrial laser scanning tls on two barchans of different size in jericoacoara brazil wind speeds and sand fluxes were measured simultaneously at the dune crests we found that the mean critical and repose angles decrease with increasing wind speed we attribute this to turbulent shear stresses the magnitude of which are predicted using 3d large eddy simulation les modeling that episodically act down the slip face i e in the direction of gravity to trigger grain flows at lower angles than with gravity stresses alone the rate of avalanching is a maximum in the morning at our study site and coincides with the maximum rate of increase in wind speed and not with the maximum rate of sediment supply to the slip face we developed and tested a new predictive model for the rate of avalanching that includes both sediment flux delivered to the slip face and the derivative of the critical angle with time our data also suggest that the mean critical angle varies inversely with slip face height these results have important implications for aeolian dune evolution interpretations of aeolian stratigraphy and granular mechanics

eolian dunes computer simulations and attractor interpretation a simple computer simulation algorithm for the transport of sand by wind produces forms resembling barchan crescentic ridge linear and star natural dune classes sand is moved as slabs composed of many grains that are picked up at random transported in a specified direction and deposited 1 with a probability that depends on the local presence or absence of sand or 2 in shadow zones in the lee of dunes the simulated dune fields are interpreted as complex systems with sand dune classes being dynamical attractors of these systems the evolution of dunes once formed becomes decoupled from the details of eolian sand transport

fault zone deformation controlled by carbonate mechanical stratigraphy balcones fault system texas normal faults in cretaceous carbonates in the balcones fault system provide important analogs for fault zone architecture and deformation in carbonate reservoirs worldwide mechanical layering is a fundamental control on carbonate fault zones relatively planar faults with low displacement gradients develop in massive strong clay poor limestones and dolomites in less competent clay rich strata shale beds impede fault propagation resulting in fault related folding and locally steep bedding dips faults in clay poor massive limestones and dolomites tend to be steep 70 or more whereas weaker clay rich limestones develop faults with shallower dips 60 or less fault zone rocks show evidence of cataclasis cementation deformation of cement by mechanical twinning and pressure solution and multiple generations of cement with differing degrees of deformation indicating contemporaneous cementation and fault slip in stratigraphic sequences consisting of both competent and incompetent strata the ratio of incompetent to competent strata by thickness is a useful guide for inferring the relative rates of fault displacement and propagation low displacement to propagation ratios associated with competent strata generate low displacement gradients inhibiting fault related folding conversely high displacement to propagation ratios associated with incompetent strata promote high displacement gradients and fault related folding

a scaling law for aeolian dunes on mars venus earth and for subaqueous ripples abstract the linear stability analysis of the equations governing the evolution of a flat sand bed submitted to a turbulent shear flow predicts that the wavelength at which the bed destabilises to form dunes should scale with the drag length l drag s f d this scaling law is tested using existing and new measurements performed in water subaqueous ripples in air aeolian dunes and fresh snow dunes in a high pressure co2 wind tunnel reproducing conditions close to venus atmosphere and in the low pressure co2 martian atmosphere martian dunes the difficulty is to determine the diameter of saltating grains on mars a first estimate comes from photographs of aeolian ripples taken by the rovers opportunity and spirit showing grains whose diameters are smaller than on earth dunes in addition we calculate the effect of cohesion and viscosity on the dynamic and static transport thresholds it confirms that the small grains visualised by the rovers should be grains experiencing saltation finally we show that within error bars the scaling of with ldrag holds over almost five decades we conclude with a discussion on the time scales and velocities at which these bed instabilities develop and propagate on mars

relations between dune generations in the gran desierto of mexico multiple generations or genetic units of dunes characterized by different morphologies sand grain size and sorting and degree of post depositional alteration of sediments occur in the gran desierto sand sea of mexico dune generations are separated by super bounding surfaces episodic accumulation of dunes is a result of a complex response of different sediment sources to regional climatic and eustatic changes based on examples from three parts of the sand sea relations between different generations of dunes suggest that there are two end member modes of sand sea accumulation 1 stacking or superposition of dune generations and 2 accumulating mosaics of dunes of different morphological units the mode of accumulation is determined by sand supply and accumulation rate stacking occurs where sand supply is high and or the sand sea area is restricted by topography accumulation of mosaics is the preferred mode of accumulation where sand supply is low

field evidence for surface wave induced instability of sand dunes barchans are crescent shaped dunes that move faster over desert surfaces than most other dune types a three year field study shows how they do it collisions and changes in wind direction destabilize the dunes generating surface waves that form new smaller barchans this prevents the formation of a single giant dune so plays a fundamental role in the development of dune patterns field studies of barchans crescent shaped dunes that propagate over solid ground under conditions of unidirectional wind1 have long focused on the investigation of an equilibrium between sand transport by wind and the control of air flow by dune topography2 3 4 which are thought to control dune morphology and kinematics5 6 7 because of the long timescale involved however the underlying dynamic processes responsible for the evolution of dune fields remain poorly understood8 here we combine data from a three year field study in the moroccan sahara with a model study to show that barchans are fundamentally unstable and do not necessarily behave like stable solitary waves as suggested previously9 10 11 12 we find that dune collisions and changes in wind direction destabilize the dunes and generate surface waves on the barchans because the resulting surface waves propagate at a higher speed than the dunes themselves they can produce a series of new barchans of elementary size by breaking the horns of large dunes the creation of these new dunes provides a mechanism for sand loss that prevents dune fields from merging into a single giant dune and therefore plays a fundamental role in the control of size selection and the development of dune patterns

emergence of oblique dunes in a landscape scale experiment the dynamics of dune evolution under bimodal wind regimes are poorly understood owing to a lack of long term wind records and the limitations of most experimental set ups a 4 year landscape scale experiment in the tengger desert mongolia demonstrates that the orientation of oblique dune crests is controlled by the wind regime