potential for estimation of snow depth on arctic sea ice from cryosat 2 and saral altika missions abstract the scattering properties of the radar signal at ka and ku band frequencies are investigated using a theoretical model and snow grain observations obtained during previous field campaigns our results show that the combination of radar altimeters operating at these two frequencies should allow for the retrieval of snow depth over arctic sea ice we estimate uncertainties of the ice surface position in relation to crossover observations over sea ice and show that the accuracy of the crossover methodology with short time gap 3 days or less is better than 3 cm comparison of the cryosat 2 altika retrieved snow depth with in situ measurements provided by operation icebridge shows a good agreement with a root mean square error rmse of 5 cm analysis of the cryosat 2 altika retrieved snow depths over three winters 2013 2015 reveals a thinner snow cover on both multi year 32 57 and first year ice 63 75 relative to the 1954 91 warren climatology suggesting the need for more contemporary year round and basin scale snow depth fields

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

parameterization of centimeter scale sea ice surface roughness using terrestrial lidar microwave scattering from sea ice is partially controlled by the ice surface roughness in this paper we propose a technique for calculating 2 d centimeter scale surface roughness parameters including the rms height correlation length and form of autocorrelation function from 3 d terrestrial light detection and ranging data we demonstrate that a single scale of roughness can be extracted from complex sea ice surfaces incorporating multiple scales of topography after sophisticated 2 d detrending and calculate roughness parameters for a wide range of artificial and natural sea ice surface types the 2 d technique is shown to be considerably more precise than standard 1 d profiling techniques and can therefore characterize surface roughness as a stationary single scale process which a 1 d technique typically cannot do sea ice surfaces are generally found to have strongly anisotropic correlation lengths indicating that microwave scattering models for sea ice should include surface spectra that vary as a function of the azimuthal angle of incident radiation however our results demonstrate that there is no fundamental relationship between the rms height and correlation length for sea ice surfaces if the sampling area is above a threshold minimum size

long period earthquakes and co eruptive dome inflation seen with particle image velocimetry dome growth and explosive degassing are fundamental processes in the cycle of continental arc volcanism johnson et al present novel optical geodetic observations to constrain the sources responsible for long period volcanic earthquakes which are coincident with frequent explosive eruptions at santiaguito volcano guatemala they find that acceleration in deformation of the volcanic dome extracted from high resolution optical image processing is coincident with recorded long period seismic sources based on these observations abrupt mass shift of solidified domes conduit magma or magma pads seem to be part of the mechanism responsible for generating long period earthquakes at silicic volcanic systems this study presents optical geodetic observations to constrain the sources responsible for long period volcanic earthquakes which are coincident with frequent explosive eruptions at santiaguito volcano guatemala it is found that acceleration in deformation of the volcanic dome extracted from the high resolution optical image processing is coincident with recorded long period seismic sources on the basis of these observations abrupt mass shift of solidified domes conduit magma or magma pads seem to be part of the mechanism responsible for generating long period earthquakes at silicic volcanic systems dome growth and explosive degassing are fundamental processes in the cycle of continental arc volcanism1 2 because both processes generate seismic energy geophysical field studies of volcanic processes are often grounded in the interpretation of volcanic earthquakes3 although previous seismic studies have provided important constraints on volcano dynamics4 5 6 such inversion results do not uniquely constrain magma source dimension and material properties here we report combined optical geodetic and seismic observations that robustly constrain the sources of long period volcanic earthquakes coincident with frequent explosive eruptions at the volcano santiaguito in guatemala the acceleration of dome deformation extracted from high resolution optical image processing is shown to be associated with recorded long period seismic sources and the frequency content of seismic signals measured across a broadband network these earthquake sources are observed as abrupt subvertical surface displacements of the dome in which 20 50 cm uplift originates at the central vent and propagates at 50 m s 1 towards the 200 m diameter periphery episodic shifts of the 20 80 m thick dome induce peak forces greater than 109 n and reflect surface manifestations of the volcanic long period earthquakes a broad class of volcano seismic activity that is poorly understood and observed at many volcanic centres worldwide7 on the basis of these observations the abrupt mass shift of solidified domes conduit magma or magma pads may play a part in generating long period earthquakes at silicic volcanic systems

volcanic sintering timescales of viscous densification and strength recovery 1 sintering and densification are ubiquitous processes influencing the emplacement of both effusive and explosive products of volcanic eruptions here we sinter ash size fragments of a synthetic national institute of standards and technology viscosity standard glass at temperatures at which the resultant melt has a viscosity of 108 109 pa s at 1bar to assess sintering dynamics under near surface volcanic conditions we track the strength recovery via uniaxial compressive tests we observe that volcanic ash sintering is dominantly time dependent temperature dependent and grain size dependent and may thus be interpreted to be controlled by melt viscosity and surface tension sintering evolves from particle agglutination to viscous pore collapse and is accompanied by a reduction in connected porosity and an increase in isolated pores sintering and densification result in a nonlinear increase in strength micromechanical modeling shows that the pore emanated crack model explains the strength of porous lava as a function of pore fraction and size

a conceptual hydrodynamic model of a geological discontinuity in hard rock aquifers example of a quartz reef in granitic terrain in south india summary the structure and hydrodynamic properties of geological discontinuities and of a deeply weathered granite aquifer near these structures are described on the basis of geological geophysical and hydrodynamic investigations in two sites of south india located along a 20 40 m wide quartz reef intruding a weathered archean biotite granite one of the two sites also comprises a metre wide dolerite dyke weathering processes appear to be at the origin of fissures development and of a related enhanced local hydraulic conductivity both in the quartz reef and in the surrounding granite the weathering profile in the granite saprolite and fissured layer is characterized by an abrupt deepening of the weathered layers in the granite near the contact and in the quartz reef itself therefore the weathering profile shows a u shape geometry with among others the verticalization of the granite s fissured layer the hydraulic conductivity of this verticalized layer is on average 5 10 6 m s and storativity about 10 3 the hydraulic conductivity of the fissured quartz is 4 6 10 6 m s and its storativity about 3 5 10 4 both media are also characterized by a matrix hydraulic conductivity 10 7 10 9 m s and by a significant heterogeneity in hydrodynamic properties that generates preferential flow paths along the sub vertical fissures parallel to the reef axis a special attention has been paid for characterizing this heterogeneity the weathering of the dolerite dyke however results in a local low hydraulic conductivity which consequently does not enhance either the thickness of weathered granite layers or its hydraulic conductivity the obtained results complete the conceptual hydrogeological model developed for weathered granite aquifers in characterizing the relationships between weathering processes and hydrodynamic properties near geological discontinuities

a facet based numerical model for simulating sar altimeter echoes from heterogeneous sea ice surfaces cryosat 2 has provided measurements of pan arctic sea ice thickness since 2010 with unprecedented spatial coverage and frequency however it remains uncertain how the ku band radar interacts with the vast range of scatterers that can be present within the satellite footprint including sea ice with varying physical properties and multiscale roughness snow cover and leads here we present a numerical model designed to simulate delay doppler synthetic aperture radar sar altimeter echoes from snow covered sea ice such as those detected by cryosat 2 backscattered echoes are simulated directly from triangular facet based models of actual sea ice topography generated from operation icebridge airborne topographic mapper data as well as virtual statistical models simulated artificially we use these waveform simulations to investigate the sensitivity of sar altimeter echoes to variations in satellite parameters height pitch and roll and sea ice properties physical properties roughness and presence of water we show that the conventional gaussian assumption for sea ice surface roughness may be introducing significant error into the cryosat 2 waveform retracking process compared to a more representative lognormal surface an echo simulated from a gaussian surface with rms roughness height of 0 2 m underestimates the ice freeboard by 5 cm potentially underestimating sea ice thickness by around 50 cm we present a set of ideal waveform shape parameters simulated for sea ice and leads to inform existing waveform classification techniques this model will ultimately be used to improve retrievals of key sea ice properties including freeboard surface roughness and snow depth from sar altimeter observations

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

heterogeneity of hydrodynamic properties and groundwater circulation of a coastal andesitic volcanic aquifer controlled by tectonic induced faults and rock fracturing martinique island lesser antilles fwi summary we conducted a multidisciplinary study to analyze the structure and the hydrogeological functioning of an andesitic coastal aquifer and to highlight the importance of faults and associated rock fracturing on groundwater flow a helicopter borne geophysical survey with an unprecedented resolution skytem was flown over this aquifer in 2013 tdem resistivity total magnetic intensity geological and hydrogeological data from 30 boreholes and two pumping tests were correlated including one which lasted an exceptional 15 months we demonstrate that heterogeneous hydrodynamic properties and channelized flows result from tectonically controlled aquifer compartmentalization along the structural directions of successive tectonic phases significant fracturing of the central compartment results in enhanced hydrodynamic properties of the aquifer and an inverse relationship between electrical resistivity and transmissivity basalts within the fractured compartment have lower resistivity and higher permeability than basalts outside the compartment pumping tests demonstrate that the key factor is the hydraulic conductivity contrast between compartments rather than the hydrodynamic properties of the fault structure in addition compartmentalization and associated transmissivity contrasts protect the aquifer from seawater intrusion finally unlike basaltic volcanic islands the age of the volcanic formations is not the key factor that determines hydrodynamic properties of andesitic islands basalts that are several million years old 15 ma here have favorable hydrodynamic properties that are generated or maintained by earthquakes faulting that result from active subduction beneath these islands which is superimposed on their primary permeability

sar altimeter backscattered waveform model the backscatters power single look waveform recorded by a synthetic aperture radar altimeter is approximated in a closed form model the model being expressed in terms of parameterless functions allows for efficient computation of the waveform and a clear understanding of how the various sea state and instrument parameters affect the waveform

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

controls on ers altimeter measurements over ice sheets footprint scale topography backscatter fluctuations and the dependence of microwave penetration depth on satellite orientation we consider the reliability of radar altimeter measurements of ice sheet elevation and snowpack properties in the presence of surface undulations we demonstrate that over ice sheets the common practice of averaging echoes by aligning the first return from the surface at the origin can result in a redistribution of power to later times in the average echo mimicking the effects of microwave penetration into the snowpack algorithms that assume the topography affects the radar echo shape in the same way that waves affect altimeter echoes over the ocean will therefore lead to biased estimates of elevation this assumption will also cause errors in the retrieval of echo shape parameters intended to quantify the penetration of the microwave pulse into the snowpack using numerical simulations we estimate the errors in retrievals of extinction coefficient surface backscatter and volume backscatter for various undulating topographies in the flatter portions of the antarctic plateau useful estimates of these parameters may be recovered by averaging altimeter echoes recorded by the european remote sensing satellite ers 1 by numerical deconvolution of the average echoes we resolve the depths in the snowpack at which temporal changes and satellite travel direction effects occur both of which have the potential to corrupt measurements of ice sheet elevation change the temporal changes are isolated in the surface backscatter cross section while directional effects are confined to the extinction coefficient and are stable from year to year this allows the removal of the directional effect from measurement of ice sheet elevation change

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

fault textures in volcanic conduits evidence for seismic trigger mechanisms during silicic eruptions it is proposed that fault textures in two dissected rhyolitic conduits in iceland preserve evidence for shallow seismogenic faulting within rising magma during the emplacement of highly viscous lava flows detailed field and petrographic analysis of such textures may shed light on the origin of long period and hybrid volcanic earthquakes at active volcanoes there is evidence at each conduit investigated for multiple seismogenic cycles each of which involved four distinct evolutionary phases in phase 1 shear fracture of unrelaxed magma was triggered by shear stress accumulation during viscous flow forming the angular fracture networks that initiated faulting cycles transient pressure gradients were generated as the fractures opened which led to fluidisation and clastic deposition of fine grained particles that were derived from the fracture walls by abrasion fracture networks then progressively coalesced and rotated during subsequent slip phase 2 developing into cataclasite zones with evidence for multiple localised slip events fluidisation and grain size reduction phase 2 textures closely resemble those formed on seismogenic tectonic faults characterised by friction controlled stick slip behaviour increasing cohesion of cataclasites then led to aseismic distributed ductile deformation phase 3 and generated deformed cataclasite zones which are enriched in metallic oxide microlites and resemble glassy pseudotachylite continued annealing and deformation eventually erased all structures in the cataclasite and formed microlite rich flow bands in obsidian phase 4 overall the mixed brittle ductile textures formed in the magma appear similar to those formed in lower crustal rocks close to the brittle ductile transition with the rheological response mediated by strain rate variations and frictional heating fault processes in highly viscous magma are compared with those elsewhere in the crust and this comparison is used to appraise existing models of volcano seismic activity based on the textures observed it is suggested that patterns of long period and hybrid earthquakes at silicic lava domes reflect friction controlled stick slip movement and eventual healing of fault zones in magma which are an accelerated and smaller scale analogue of tectonic faults

greenland 2012 melt event effects on cryosat 2 radar altimetry cryosat 2 data are used to study elevation changes over an area in the interior part of the greenland ice sheet during the extreme melt event in july 2012 the penetration of the radar signal into dry snow depends heavily on the snow stratigraphy and the rapid formation of refrozen ice layers can bias the surface elevations obtained from radar altimetry we investigate the change in cryosat 2 waveforms and elevation estimates over the melt event and interpret the findings by comparing in situ surface and snow pit observations from the north greenland eemian ice drilling project camp the investigation shows a major transition of scattering properties around the area and an apparent elevation increase of 56 26 cm is observed in reprocessed cryosat 2 data we suggest that this jump in elevation can be explained by the formation of a refrozen melt layer that raised the reflective surface introducing a positive elevation bias

the 2017 reversal of the beaufort gyre can dynamic thickening of a seasonal ice cover during a reversal limit summer ice melt in the beaufort sea during winter 2017 the semi permanent beaufort high collapsed and the anticyclonic beaufort gyre reversed the reversal drove eastward ice motion through the western arctic causing sea ice to converge against banks island and halted the circulation of multiyear sea ice via the gyre preventing its replenishment in the beaufort sea prior to the reversal an anomalously thin seasonal ice cover had formed in the beaufort following ice free conditions during september 2016 with the onset of the reversal in january 2017 convergence drove uncharacteristic dynamic thickening during winter by the end of march despite seasonal ice comprising 97 of the ice cover the reversal created the thickest roughest and most voluminous regional ice cover of the cryosat 2 record within the beaufort sea previous work has shown that winter ice export can precondition the region for increased summer ice melt but that a short reversal during april 2013 contributed to a reduction in summer ice loss hence the deformed ice cover at the end of winter 2017 could be expected to limit summer melt in spite of this the beaufort ice cover fell to its fourth lowest september area as the gyre re established during april and divergent ice drift broke up the pack negating the reversal s earlier preconditioning our work highlights that dynamic winter thickening of a regional sea ice cover for instance during a gyre reversal offers the potential to limit summer ice loss but that dynamic forcing during spring dictates whether this conditioning carries through to the melt season

structure and hydrogeological functioning of an insular tropical humid andesitic volcanic watershed a multi disciplinary experimental approach summary the aim of the study is to establish an hydrogeological scheme of the structure and the functioning of andesitic volcanic systems in the insular and humid tropical context a watershed has been instrumented at fefe in guadeloupe french west indies in the lesser antilles to measure rainfall runoff and piezometric fluctuations between 2003 and 2006 given the heterogeneity of the volcanic deposits a multi disciplinary approach has been used a geological and an hydrogeological survey an hydrodynamic approach hydraulic tests and signal processing of hydrological time series and finally an hydrogeochemical approach major elements and some trace elements two main superposed aquifer respectively in aerial deposits and in recent nuees ardentes glowing ash and lava flows as well as the underlying aquiclude the highly weathered older substratum were identified at a local scale some hydrogeological heterogeneities internal to the aquifers were highlighted and related to a polyphased building up of the geological formations hydraulic relationships between the multi layered aquifers such as the confinement of the deep aquifer upstream of the basin were identified groundwater stream exchanges were characterised detailing various contributions of the shallow and the deep aquifers to stream and spring waters significant river losses were also observed at a global scale within such relatively permeable volcanic formations the aquifer recharge is about 85 of the annual effective rainfall for an average rainfall year the hydrogeological scheme appears to be coherent with a geological structure of cut and fill paleovalleys frequently observed in andesitic areas consequently groundwater drainage axis are mainly localised in recent nuees ardentes deposits whereas the inversion of relief characteristic of volcanic areas explains the non superposition of surface water and groundwater divides and both surface water losses and springs finally this work shows the highly heterogeneous hydrogeological processes occurring in volcanic formations of andesitic type it also shows the relationships between the geological structure and the hydrogeological functioning of such basins and proposes an affordable methodological approach to investigate such a hydrogeological context in depth

high velocity frictional properties of gabbro high velocity friction experiments have been performed on a pair of hollow cylindrical specimens of gabbro initially at room temperature at slip rates from 7 5 mm s to 1 8 m s with total circumferential displacements of 125 to 174 m and at normal stresses to 5 mpa using a rotary shear high speed friction testing machine steady state friction increases slightly with increasing slip rate at slip rates to about 100 mm s velocity strengthening and it decreases markedly with increasing slip rate at higher velocities velocity weakening steady state friction in the velocity weakening regime is lower for the non melting case than the frictional melting case due perhaps to severe thermal fracturing a very large peak friction is always recognized upon the initiation of visible frictional melting presumably owing to the welding of fault surfaces upon the solidification of melt patches frictional properties thus change dramatically with increasing displacement at high velocities and such a non linear effect must be incorporated into the analysis of earthquake initiation processes

on the spacing and draft distributions for pressure ridge keels the negative exponential distribution is reconsidered as a description of pressure ridge spacings and drafts it is found to give an excellent fit for drafts but not for spacings the spacing distribution is found to be fitted much better by a three parameter lognormal distribution of which the third parameter the threshold is dependent on the beam width of the instrument used to collect the ice bottom profile

fracture healing in a magma an experimental approach and implications for volcanic seismicity and degassing 1 the healing of magmatic fractures is considered essential to repetitive seismicity and the closure of degassing paths during emplacement of lavas to estimate the healing time of magmatic fractures we performed healing experiments on rhyolitic melts at 850 1000 c and 1 6 3 2 mpa for 0 5 94 h two cylindrical obsidian cores were juxtaposed on surfaces prepared by cutting the cores both with and without polishing these were annealed in an open system cell the contact interface became coherent and finally disappeared the water content across the contact initially decreased toward the interface via diffusive dehydration but later homogenized this change was interpreted to reflect atomic scale closure of the interface probably by chemical bonding we defined this closure interval as microscopic healing time and determined this by fitting the measured profiles with a diffusion model the microscopic healing time was strongly dependent on temperature and roughness of the interface and was for the nonpolished interfaces 67 74 4 0 4 9 and 0 36 0 38 h at 850 900 and 950 c respectively whereas for the polished examples it was 1 3 and 0 5 0 6 h at 850 and 900 c respectively this microscopic healing time is consistent with the period of actual seismicity and is prolonged sufficiently to permit the formation of millimeter thick bubble free obsidian layers along fractures in vesicular lavas through bubble resorption due to diffusive degassing

magma degassing buffered by vapor flow through brecciated conduit margins obsidian pyroclasts a common component of rhyolitic tephra preserve a range of volatile contents which has been used to infer syneruptive conditions of magmatic degassing here we show that the textures of obsidian pyroclasts provide information on physical mechanisms of magma flow and degassing along conduit margins obsidian clasts often contain xenoliths sheared bands of lithic powder and textures consistent with magma autobrecciation these features suggest that pyroclastic obsidian primarily forms near conduit walls where magma fragments and reanneals during ascent we use these observations to develop a degassing model for pyroclastic obsidian from the a d 1340 mono craters california eruptions we suggest that degassing was buffered by continual flux of vapor through highly permeable brecciated magma along conduit walls continuous reequilibration of magma with vapor of relatively constant composition not only explains the co 2 h 2 o and d h 2 o data from mono craters pyroclastic obsidian but also requires much lower magmatic co 2 values than the commonly accepted model of closed system degassing taken together the chemical and physical evidence suggests that magma brecciation along conduit walls aids the degassing of ascending rhyolite

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

rheology of bubble bearing magmas the rheology of bubble bearing suspensions is investigated through a series of three dimensional boundary integral calculations in which the effects of bubble deformation volume fraction and shear rate are considered the behaviour of bubbles in viscous flows is characterized by the capillary number ca the ratio of viscous shear stresses that promote deformation to surface tension stresses that resist bubble deformation estimates of ca in natural lava flows are highly variable reflecting variations in shear rate and melt viscosity in the low capillary number limit e g in carbonatite flows bubbles remain spherical and may contribute greater shear stress to the suspension than in high capillary number flows in which bubble deformation is significant at higher ca deformed bubbles become aligned in the direction of flow and as a result contribute less shear stress to the suspension calculations indicate that the effective shear viscosity of bubbly suspensions at least for ca 0 5 is a weakly increasing function of volume fraction and that suspensions of bubbles are shear thinning field observations and qualitative arguments however suggest that for sufficiently large ca ca greater than about 1 the effective shear viscosity may be less than that of the suspending liquid bubbles reach their quasi steady deformed shapes after strains of order one for shorter times the continuous deformation of the bubbles results in continual changes of rheological properties in particular for small strains the effective shear viscosity of the suspension may be less than that of the liquid phase even for small ca results of this study may help explain previous experimental theoretical and field based observations regarding the effects of bubbles on flow rheology

ku band radar penetration into snow cover on arctic sea ice using airborne data satellite radar altimetry provides data to monitor winter arctic sea ice thickness variability on interannual basin wide scales when using this technique an assumption is made that the peak of the radar return originates from the snow ice interface this has been shown to be true in the laboratory for cold dry snow as is the case on arctic sea ice during winter however this assumption has not been tested in the field we use data from an airborne normal incidence ku band radar altimeter and in situ field measurements collected during the cryosat validation experiment cryovex bay of bothnia 2006 and 2008 field campaigns to determine the dominant scattering surface for arctic snow covered sea ice in 2006 when the snow temperatures were close to freezing the dominant scattering surface in 25 of the radar returns appeared closer to the snow ice interface than the air snow interface however in 2008 when temperatures were lower the dominant scattering surface appeared closer to the snow ice interface than the air snow interface in 80 of the returns

volcanic drumbeat seismicity caused by stick slip motion and magmatic frictional melting during volcanic eruptions solidifying magma ascends through the volcanic conduit often accompanied by repetitive drum beat seismicity laboratory experiments on magma samples from soufriere hills volcano montserrat and mount st helens volcano usa show that viscous melt formed at the surface between the rising magma and conduit walls can temporarily halt magma ascent accentuating the cyclical seismicity

petrology of the 1991 1995 eruption at unzen effusion pulsation and groundmass crystallization abstract effusive eruption of dacite magma 2 1 108 m3 during 1991 1995 formed a lava dome at the summit of unzen volcano japan the effusion rate was highest at the beginning 4 0 105 m3 day 4 6 m3 s and decreased roughly with time to almost zero before this pattern was repeated with a second pulse of magma supply the whole rock chemistry of lavas shows significant variation attributable to variations in phenocryst abundance the more mafic the more abundant the phenocrysts the pattern of chemical variation with time shows some difference from that of the effusion rate all phenocrysts in dacite plagioclase hornblende biotite quartz and magnetite show evidence of disequilibrium with melt although a glomerophyric aggregation of phenocrysts suggests coexistence with each other phenocrysts are isotopically heterogeneous from species to species the calculated initial melt composition was rhyodacite and was nearly constant throughout the activity in contrast the bulk phenocryst population is andesite a model explaining the textures and the isotopic heterogeneity is the capture of diorite fragments or xenocrysts by parental rhyodacite magma it is suggested that when effusion rate was high less viscous crystal poor magma exited from the reservoir groundmass glass and plagioclase microlite rims show temporal chemical variations correlating with the effusion rate the higher the effusion rate the more evolved the compositions groundmass crystallinity increased with decreasing effusion rate from 33 to 50 textures in dome lavas suggest that groundmass crystallization had been mostly completed when magma reached the conduit top the fe ti oxide temperature 880 780 c was low when the crystallinity was high micropumice erupted before dome growth provided a sample recording magmatic foam in the conduit porosity of dome lavas was lower at lower effusion rates collapse of foam magma and simultaneous escape of volatiles through the conduit top were probably responsible for the accompanying low frequency earthquakes phenocrysts were broken and the breakdown rims on hornblende phenocrysts were torn off during collapse and successive compaction when effusion waned degassing and the resultant crystallization proceeded more completely so that the magma became too viscous to flow in the conduit top and behaved as a plug resulting in a temporary halt of effusion in turn groundmass crystallization in magma below the plug increased excess pressure in the upper parts of conduit due to slow cooling the plug was scavenged when rising excess pressure overcame its effective strength then the second pulse of magma supply began strong endogenous growth and extrusion of a lava spine in the later stage probably occurred for the same reason

increased ice losses from antarctica detected by cryosat 2 we use 3 years of cryosat 2 radar altimeter data to develop the first comprehensive assessment of antarctic ice sheet elevation change this new data set provides near continuous 96 coverage of the entire continent extending to within 215 km of the south pole and leading to a fivefold increase in the sampling of coastal regions where the vast majority of all ice losses occur between 2010 and 2013 west antarctica east antarctica and the antarctic peninsula changed in mass by 134 27 3 36 and 23 18 gt yr 1 respectively in west antarctica signals of imbalance are present in areas that were poorly surveyed by past missions contributing additional losses that bring altimeter observations closer to estimates based on other geodetic techniques however the average rate of ice thinning in west antarctica has also continued to rise and mass losses from this sector are now 31 greater than over the period 2005 2010

silicic lava effusion controlled by the transition from viscous magma flow to friction controlled flow silicic magma forms a vertical fault in volcanic conduits via the brittle ductile transition the formation of the fault changes flow type from viscous flow to friction controlled flow here we investigate flow dynamics of magma by coupling a one dimensional conduit flow model with an experimentally calibrated brittle ductile transition the numerical simulation demonstrates that the length of magma plug at which friction controlled flow becomes the main flow type depends on magma flux because of crystallization kinetics and the ductile brittle transition at high flux a short plug forms because nonequilibrium crystallization inhibits an increase of magma viscosity this results in the effusion of less viscous lava and large shear stress at the shallow part of the conduit in contrast the long plug that forms under low flux cannot maintain large shear stress due to weakness of the magmatic fault which causes the extrusion of a solidified lava spine

coastal sea level from inland cryosat 2 interferometric sar altimetry the european space agency s cryosat 2 satellite can operate in a novel synthetic aperture radar interferometric sarin mode where its nominal footprint swath is observed by two antennas and the phase difference between the signals is used to determination the exact location of the scatterer through an off nadir correction the potential of sarin for sea level determination is investigated over the fjords of eastern greenland in principle the satellite should only track sea level within its nominal footprint of 7 km across track but we observe that scattering targets fjords within twice its nominal footprint are frequently observed but mislocated in cryosat 2 due to phase wrapping we devised a way to relocate the observations and correct the range accordingly when cryosat 2 is flying inland we consequently observed that the satellite occasionally provide valid sea level in fjords up to 13 km away in the across track direction

cyclic shear zone cataclasis and sintering during lava dome extrusion insights from chaos crags lassen volcanic center usa abstract the ascent and extrusion of crystal rich magma is commonly facilitated by deformation partitioned within annular conduit parallel shear zones the physical properties and textures of the shear zone materials where exposed at surface provide a record of ascent and eruption dynamics we describe the shear zone developed in dome c part of chaos crags in the lassen volcanic center california usa the extruded shear zone comprises volcanic fault gouge and variably densified cataclasites the competent cataclasites evidence deep seated gouge production followed by gouge densification within the conduit on the timescale of lava dome ascent textural geochemical and mineralogical data identify solid state sintering as the densification mechanism at the temperatures and pressures in the volcanic conduit solid state sintering causes rapid porosity and permeability loss within the gouge and concomitant material strengthening longer dwell times i e slower ascent allow for more sintering producing stronger denser and less permeable cataclasites at chaos crags we use the extent of sintering quantified by residual porosity to recover minimum in conduit dwell times necessary to produce the observed cataclasites our analysis of the dome c cataclasites suggests a maximum linear ascent rate of 10 m day and a minimum ascent time of 100 days we evaluate the consequences of shear zone lithification by solid state sintering for the eruption of other crystal rich glass poor magmas chaos crags cataclasites preserve evidence of multiple cycles of fracturing cataclasis and re sintering suggesting a mechanism for transitions between effusive and explosive phases of dome building eruptions

using the interferometric capabilities of the esa cryosat 2 mission to improve the accuracy of sea ice freeboard retrievals a significant source of error in the retrieval of sea ice freeboard from pulse limited radar altimeters arises when scattering from off nadir leads dominates the power echo causing the onboard tracker to snag and overestimate the two way travel time this range overestimate translates into an ocean elevation underestimate relative to the ice surface and an overestimate of the sea ice freeboard we demonstrate using interferometric cryosat 2 data that it is possible to infer the across track angle of return to off nadir leads and their location in the cryosat 2 footprint and hence correct for the associated range error for the first time it is found that specular scattering from leads can dominate the radar echo some 1530 m off nadir over the region studied the mean ocean elevation bias is closely associated with the pulse peakiness pp parameter used for identifying specular waveforms considering only the most specular waveforms the elevation bias was measured to be 1 21 0 93 cm however lowering the pp threshold includes lower power waveforms originating from higher off nadir angles and the elevation bias becomes 4 06 1 66 cm unaccounted for these biases represent an 10 40 cm overestimate in ice thickness despite the relatively large error on the mean bias correcting for off nadir ranging contributes only a small amount to the elevation uncertainty when combined with range and orbit determination uncertainties we found that making use of cryosat 2 s interferometric mode over sea ice ultimately decreases the uncertainty on the area averaged ocean elevation by allowing the inclusion of more waveforms in the analysis

numerical and experimental evaluation of terrestrial lidar for parameterizing centimeter scale sea ice surface roughness terrestrial light detection and ranging lidar offers significant advantages over conventional techniques for measuring the centimeter scale surface roughness of natural surfaces such as sea ice however the laser scanning technique is inherently limited principally by the following 1 the high inclination scanning angle of the sensor with respect to nadir 2 the precision of the laser ranging estimate and 3 the beam divergence of the laser in this paper we introduce a numerical model that has been designed to simulate the acquisition of lidar data over a regular rough surface results from the model compare well r 2 0 97 with lidar observations collected over two experimental surfaces of known roughness that were constructed from medium density fibreboard using a computer numerical control three axis router the model demonstrates that surface roughness parameters are not sensitive to minor variations in the lidar sensor s range and laser beam divergence but are slightly sensitive to the precision of the ranging estimate the model also demonstrates that surface roughness parameters are particularly sensitive to the inclination angle of the lidar sensor the surface rms height is underestimated and the correlation length is overestimated as either the inclination angle of the sensor or the true roughness of the surface increases an isotropic surface is also increasingly observed as an anisotropic surface as either the inclination angle or the true surface roughness increases based on the model results we propose a set of calibration functions that can be used to correct in situ lidar measurements of surface roughness

three dimensional imaging of fracture propagation in tight sandstones of the upper triassic chang 7 member ordos basin northern china abstract the characteristics of fracture propagation in heterogeneous tight sandstones are critical to volumetric fracturing which is the key to unlocking unconventional resources in tight sandstones quantification of the influence of pre existing pore systems and particle arrangements on the propagation of fractures is challenging due to inadequate imaging of the internal void systems in tight sandstones from three dimensional 3d aspect in this study the 3d geometry of tight sandstones from the chang 7 member in the ordos basin is continuously imaged under different loading stresses and the voxel resolution of the x ray computed tomography is 2 5 m the data set captured in this process shows the changes in the samples at the microstructural level as they approach fracturing the data are stored as a time series of 3d images the results demonstrate that i fractures propagate progressively and gradually link with pre existing pores resulting in macroscopic fractures with a maximum width of 250 m while newly generated fractures could break up particles and may not follow the line of pre existing fractures and ii three stages were identified in the failure process of tight sandstones with new fractures running at an angle of about 30 to the general direction of the stress of compression the total volumes of both the sample and pore fractures and the damage index which were extracted from the 3d images all increased when approaching fracturing the final volume of pore fracture systems could be 11 times that of the initial pore volume all of these observations provide valuable insights and design guidelines for hydraulic fracturing in unconventional tight sandstones and a quantified model of the dynamics and the morphology of fracture propagation with increasing stress approaching failure which may shed light on dynamic critical transitions in the earth s crust

the volcanic evolution of martinique island insights from k ar dating into the lesser antilles arc migration since the oligocene abstract the lesser antilles island arc bifurcates into two lines in its northern part with an old branch to the east and a recent active branch to the west martinique is located at the southern tip of the separation the two arcs diverge northward and at maximum divergence are separated by a 50 km wide depression despite this separation which suggests a jump in volcanism activity has been almost continuous in martinique island with a slow displacement of the eruptive centers to the west considering timing of emplacement previous authors defined three cycles of activity the old intermediate and recent arcs of late oligocene early miocene mid miocene and late miocene to present ages respectively the present study investigates the timing of emplacement of the volcanic units in martinique island in order to constrain the activity of the old and intermediate lesser antilles arcs as recorded on this island unspiked k ar age determinations on groundmass and plagioclase separates cassignol gillot technique were conducted on 20 samples from the old and intermediate volcanic chains martinique has evolved as eight distinct volcanic centers 1 basal complex and sainte anne series 24 8 0 4 20 8 0 4 ma for the old arc 2 vauclin pitault chain 16 1 0 2 8 44 0 12 ma and 3 south western volcanism 9 18 0 16 7 10 0 10 ma for the intermediate arc and 4 morne jacob volcano 5 14 0 07 1 54 0 03 ma 5 trois ilets volcanism 2 358 0 034 ma and 346 27 ka 6 carbet complex 998 14 to 322 6 ka 7 mount conil 543 8 to 127 2 ka and 8 mount pelee 126 2 ka to present for the recent arc germa et al 2010 germa et al 2011a we propose migration rates of 1 1 1 4 km myr westward toward the back arc region throughout the whole volcanic history of martinique island these rates together with geochemical evidence for a more enriched signature in the youngest magmas are consistent with a geodynamic evolution involving the migration of the northern volcanic front away from the trench in response to the subduction of an aseismic ridge possibly influenced by convergence between north american and south american plates

quasi 3d modeling of airborne tem data by spatially constrained inversion we present a new methodology spatially constrained inversion sci that produces quasi 3d conductivity modeling of electromagnetic em data using a 1d forward solution spatial constraints are set between the model parameters of nearest neighboring soundings data sets models and spatial constraints are inverted as one system the constraints are built using delaunay triangulation which ensures automatic adaptation to data density variations model parameter information migrates horizontally through spatial constraints increasing the resolution of layers that would be poorly resolved locally sci produces laterally smooth results with sharp layer boundaries that respect the 3d geological variations of sedimentary settings sci also suppresses the elongated artifacts commonly seen in interpretation results of profile oriented data sets in this study sci is applied to airborne time domain em data but it can also be implemented with other ground based or airborne data types

sea ice surface roughness estimates from airborne laser scanner and laser altimeter observations in fram strait and north of svalbard we present sea ice surface roughness estimates i e the standard deviation of relative surface elevation in the arctic regions of fram strait and the nansen basin north of svalbard acquired by an airborne laser scanner and a single beam laser altimeter in 2010 we compare the scanner to the altimeter and compare the differences between the two survey regions we estimate and correct sensor roll from the scanner data using the hyperbolic response of the scanner over a flat surface measurement surveys had to be longer than 5 km north of svalbard and longer than 15 km in fram strait before the statistical distribution in surface roughness from the scanner and altimeter became similar the shape of the surface roughness probability distributions agrees with those of airborne electromagnetic induction measurements of ice thickness the ice in fram strait had a greater mean surface roughness 0 16m vs 0 09m and a wider distribution in roughness values than the ice in the nansen basin an increase in surface roughness with increasing ice thickness was observed over fast ice found in fram strait near the coast of greenland but not for the drift ice

a frictional law for volcanic ash gouge abstract volcanic provinces are structurally active regions undergoing continual deformation along faults within such fault structures volcanic ash gouge containing both crystalline and glassy material may act as a potential fault plane lubricant here we investigate the frictional properties of volcanic ash gouges with varying glass fractions using a rotary shear apparatus at a range of slip rates 1 3 1300 mm s and axial stresses 0 5 2 5 mpa we show that the frictional behaviour of volcanic ash is in agreement with byerlee s friction law at low slip velocities irrespective of glass content the results reveal a common non linear reduction of the friction coefficient with slip velocity and yield a frictional law for fault zones containing volcanic ash gouge textural analysis reveals that strain localisation and the development of shear bands are more prominent at higher slip velocities 10 mm s the textures observed here are similar to those recorded in ash gouge at the surface of extrusive spines at mount st helens usa we use the rate weakening component of the frictional law to estimate shear stress resistance reductions associated with episodic seismogenic slip events that accompany magma ascent pulses we conclude that the internal structure of volcanic ash gouge may act as a kinematic marker of exogenic dome growth

influence of successive phases of volcanic construction and erosion on mayotte island s hydrogeological functioning as determined from a helicopter borne resistivity survey correlated with borehole geological and permeability data summary the purpose of this study is to show how a multidisciplinary approach that combines geophysics geology and hydrogeology has made it possible to a significantly improve our understanding of the hydrogeological regime of the volcanic island of mayotte and b provide a new set of geophysical measurement calibration data in 2010 a helicopter borne geophysical survey skytem was flown over the entire island 374 km2 with a measurement density hitherto unheard of in a volcanic environment in addition a database was compiled containing the geological logs of 55 boreholes 52 of these boreholes have hydrogeological information like aquifer position and piezometric level 21 of the boreholes have transmissivity values correlations were made between the inverted resistivities as obtained from the helicopter borne tdem profiles and the nature age and hydrodynamic properties of the formations as obtained from the borehole data five hydrogeological units were mapped these are characterized by an alternation between phases of dominant volcanic construction with the emplacement of basaltic lavas phonolite massifs and pyroclastic deposits and phases of dominant erosion with the deposition of volcaniclastic material colluvium breccias basaltic lavas and phonolite blocks and all materials resulting from slope slides along the slopes and in the topographic depressions it has also been possible to assign resistivity and permeability ranges to four of these units ranges that are also dependent on the age of the deposits the younger the formation is the greater its resistivity and the higher its permeability the hydrogeological regime is marked by the phases of volcanic construction and erosion that succeeded one another during the geological history of mayotte over the last 10 ma a conceptual model adapted to the specific geological context of this island and differing from the canarian and hawaiian models is also put forward this model is marked by the island s fragmented character resulting from its geological history several volcanic edifices several phases of construction and erosion and is applicable to an old volcanic island in an advanced stage of erosion and weathering with a volcanic history similar to that of mayotte i e with climate variations and erosion periods long enough to register volcaniclastic deposits

repeated fracture and healing of silicic magma generate flow banding and earthquakes textures in an exceptionally preserved effusive rhyolite conduit at torfajokull iceland indicate that rising magma repeatedly fractured and healed at shallow levels in the conduit rfh process anastomosing tuffisite veins filled by fine grained juvenile clasts were generated by shear fracture of highly viscous magma in the glass transition interval welding of the particulate material during subsequent deformation led to thorough healing of veins allowing repeated fracture of the same body of magma we propose that the rfh process is a rechargeable trigger mechanism for hybrid seismicity and show that the time scale of the process and the fractures formed by it are consistent with the repeat time and magnitude of hybrid earthquakes during silicic eruptions the rfh process may also form the flow banding that is nearly ubiquitous in obsidian

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

evaluating scattering contributions to c band radar backscatter from snow covered first year sea ice at the winter spring transition through measurement and modeling in this paper we present model and measurement results for time series angular dependencies of c band hh and vv normalized radar cross sections nrcs over first year snow covered sea ice during a winter spring transition period experimental scatterometer and physical data were collected near cambridge bay nunavut canada between may 20 and may 28 2014 covering a severe storm event on may 25 we use the small perturbation scattering theory to model small scale surface scattering the mie scattering theory to estimate the level of volume scattering in snow and the kirchhoff physical optics model to compute the large scale surface scattering component we observed good agreement between the model and experimental hh and vv nrcs before the storm r 2 between model and experimental nrcs was 0 88 and 0 82 for vv and hh respectively after the storm r 2 was 0 81 and 0 78 for vv and hh respectively our model results suggest an overall increase in surface roughness after the storm event supported by lidar measurements of the snow surface topography before the storm the large scale and small scale surface scattering from the air snow interface as well as volume scattering components dominated after the storm the large and small scale scattering contributions increased while the volume scattering component considerably dropped we attribute these effects to the increase in surface roughness and snow moisture content during the poststorm period our results could aid in interpretation of time series synthetic aperture radar images with respect to physical properties of snow and ice during the winter spring transition period

toward 1 mgal accuracy in global marine gravity from cryosat 2 envisat and jason 1 more than 60 of the earth s land and shallow marine areas are covered by 2 km of sediments and sedimentary rocks with the thickest accumulations on rifted continental margins figure 1 free air marine gravity anomalies derived from geosat and ers 1 satellite altimetry fairhead et al 2001 sandwell and smith 2009 andersen et al 2009 outline most of these major basins with remarkable precision moreover gravity and bathymetry data derived from altimetry are used to identify current and paleo submarine canyons faults and local recent uplifts these geomorphic features provide clues to where to look for large deposits of sediments while current altimeter data delineate large offshore basins and major structures they do not resolve some of the smaller geomorphic features and basins yale et al 1998 fairhead et al 2001 improved accuracy and resolution is desirable to facilitate comparisons between continental margins as an exploration tool and to permit extrapolation of known structu

sea ice thickness in the eastern canadian arctic hudson bay complex baffin bay abstract past observations of sea ice thickness in the eastern canadian arctic eca have generally been restricted to drill hole measurements at a few local sites on landfast ice here we use data from the laser altimeter icesat and the radar altimeter cryosat 2 to present a 14 year record 2003 2016 of high resolution and spatially extensive ice thickness observations for the eca and identify 12 sub regions with distinct patterns the mean sea ice growth rate within the seasonally ice covered eca from november to april is 23 cm mo 1 565 km 3 mo 1 with the fastest increase in thickness occurring through strong ice convergence and deformation in eastern hudson bay and foxe basin our results demonstrate characteristically asymmetrical distributions of sea ice thickness in both hudson bay and baffin bay but in opposing directions in hudson bay the spring ice cover is 40 cm thicker in the eastern region compared to the northwestern region whereas in baffin bay the ice is 20 cm thicker in the western half of the bay compared to the eastern half in hudson bay we find that years with strong and positive ice drift vorticity i e cyclonic and convergent conditions correlate with increasingly asymmetrical sea ice covers with the level of west east asymmetry varying from 2 to 11 cm per 100 km however in baffin bay the ice drift vorticity is typically negative i e anticyclonic and divergent with no obvious link to the asymmetry of the spring ice cover finally we estimate that large interannual variations in spring sea ice volume within the eca lead to 15 variations in the volume of freshwater available at the ocean surface during summer

the possible role of thermal feedback in the eruption of siliceous magmas abstract using values of viscosity and its temperature dependence density heat capacity and thermal conductivity appropriate to rhyolitic liquids an analysis of the macroscopic heat balances of these liquids flowing under constant stress has been carried out the analysis shows that for shear couples and shear stresses expected in the flow of silicic lava on the earth s surface thermal feedback may occur after time periods ranging from a few seconds to several days and may cause rapid increases in temperature for silicic magmas flowing in a dike or circular conduit under a constant pressure gradient these thermal instabilities may occur after a few days to several weeks for dike half thicknesses or conduit radii ranging from a few centimeters to a few meters for silicic lava flows this phenomenon may be important in producing commonly observed flow structures such as pumiceous banding and color banding local temperature increases would have the effect of reducing local water solubility increasing diffusion rates and increasing nucleation and growth rates of vapor bubbles thus causing highly vesicular bands that parallel the shear planes temperature increases could also cause increases in f o 2 within a fluid layer resulting in oxidation and red color banding for silicic lavas flowing in dikes or conduits the possible thermal instabilities are accompanied by instabilities in volume rate of flow which may contribute to rapid vesiculation of upward moving magma and result in explosive volcanism

equations for determining the gas and brine volumes in sea ice samples on a etabli des equations utilisables pour determiner le taux de gaz inclus dans la glace de mer a partir de mesures de densite de salinite et de temperature dans des gammes de temperatures entre 2 c et 30 c inversement ces relations peuvent etre utilisees pour calculer la densite de la glace de mer en fonction de la temperature et de la salinite en prenant en compte a la fois la presence de gaz et de cristaux de sel dans la glace on donne aussi des equations qui permettent le calcul du volume de gaz et d eau salee dans la glace a des temperatures autres que celle pour laquelle la densite a ete determinee

shear heating in granular layers heat flow measurements imply that the san andreas fault operates at lower shear stresses than generally predicted from laboratory friction data this suggests that a dramatic weakening effect or reduced heat production occur during dynamic slip numerical studies intimate that grain rolling or localization may cause weakening or reduced heating however laboratory evidence for these effects are sparse we directly measure frictional resistance m shear heating and microstructural evolution with accumulated strain in layers of quartz powder sheared at a range of effective stresses sn5 70 mpa and sliding velocities v0 01 10 mm s tests conducted at sn 25 mpa show strong evidence for shear localization due to intense grain fracture in contrast tests conducted at low effective stress sn 5 mpa show no preferential fabric development and minimal grain fracture hence we conclude that non destructive processes such as grain rolling sliding distributed throughout the layer dominate deformation temperature measured close to the fault increases systematically with sn and v consistent with a one dimensional heat flow solution for frictional heating in a finite width layer mechanical results indicate stable sliding m0 6 for all tests irrespective of deformation regime and show no evidence for reduced frictional resistance at rapid slip or high effective stresses our measurements verify that the heat production equation qmsnv holds regardless of localization state or fracture regime thus for quasistatic velocities v510 mm s and effective stresses relevant to earthquake rupture neither grain rolling sliding or shear localization appear to be a viable mechanism for the dramatic weakening or reduced heating required to explain the heat flow paradox

major natural hazards in a tropical volcanic island a review for mayotte island comoros archipelago indian ocean abstract mayotte is a french overseas territory of the comoros archipelago since the 1980 mayotte has developed quickly its population has multiplied by four in less than 30 years infrastructure expanded rapidly in lowlands and slums developed in steep areas through deforestation or close to riverbeds this development urgently required steps to ensure the safety of urban areas from 2002 to 2006 a major programme of natural hazards evaluation and zonation was conducted by the french geological survey in the 17 districts of mayotte the programme resulted in the publication of the first single hazard zonation maps for mayotte at 1 25 000 scale in rural areas and at 1 10 000 scale in urban areas hazard maps regarding landslides and rock falls stream floods seismic lithological and topographical amplification potential liquefaction potential and cyclonic marine submersion were produced for mayors and land planners these atlases also included prevention guidelines to minimize risks associated with land use or urbanization within hazard prone areas this article presents the main achievements of the programme whose main problems were the absence of archives of previous studies and the original volcanic geology of the island of mayotte the atlases highlight areas with numerous landslides zones susceptible to flash floods sectors likely to be submerged by storm surges and areas subject to seismic site effects due to thick volcanic weathered formations this programme emphasizes the fundamental role in such tropical volcanic islands of a good knowledge of the weathered formations for the assessment of most natural hazards this initial hazard mapping represented the first step to achieve specific regulatory risk prevention plans for an efficient land use risk mitigation and reduction policy in mayotte

rheological behavior of bubble bearing magmas abstract the vesiculation of the rising magma in a volcanic conduit results in a large change in the viscosity of magma understanding the effect of bubbles on the viscous behavior of bubble melt mixtures over a wide range of bubble volume fractions is essential for accurate modeling of the volcanic processes in this study new equations for the relative viscosity of concentrated bubble bearing suspensions are developed using a differential scheme along with the solution of an infinitely dilute suspension of bubbles out of the four models developed in the paper two models predict the relative viscosity to be a function of only two variables capillary number and volume fraction of bubbles there are no adjustable parameters in these models the remaining two models include an additional parameter that is the maximum packing volume fraction of bubbles the proposed models are evaluated using published numerical and experimental data on the relative viscosity of concentrated suspensions of bubbles the viscosity models for bubble bearing suspensions developed in this study are useful in modeling of the volcanic processes

shear viscosity of rhyolite vapor emulsions at magmatic temperatures by concentric cylinder rheometry abstract the viscosity of natural rhyolitic melt from lipari aeolian islands and melt bubble emulsions 30 50 vol porosity generated from lipari rhyolite have been measured in a concentric cylinder rheometer at temperatures and shear rates in the range 925 1150 c and 10 3 10 1 2 s 1 respectively in order to better understand the dependence of emulsion shear viscosity on temperature and shear rate in natural systems bubble free melt exhibits newtonian arrhenian behavior in the temperature range 950 1150 c with an activation energy of 395 30 kj mol the shear viscosity is given by log m 8 320 20624 t suspensions were prepared from natural rhyolite glass to which small amounts of na2so4 were added as a foaming agent reasonably homogeneous magmatic mixtures with an approximate log normal distribution of bubbles were generated by this technique suspension viscosity varied from 106 1 to 108 37 pa s and systematically correlates with temperature and porosity in the shear stress range 104 26 105 46 pa of the experiments the viscosity of melt bubble emulsions is described in terms of the relative viscosity r e m where e is the emulsion viscosity and m is the viscosity of melt of the same composition and temperature the dependence of relative viscosity on porosity for magmatic emulsions depends on the magnitude of the capillary number ca g rb 1 m 1 the ratio of viscous forces acting to deform bubbles to interfacial forces resisting bubble deformation for inviscid bubbles in magmatic flows three regimes may be identified for ca 10 an asymptotic regime is reached in which relative viscosity decreases with increasing porosity and is independent of ca our experiments were carried out for 30

a semianalytical model of the synthetic aperture interferometric radar altimeter mean echo and echo cross product and its statistical fluctuations this paper develops a model of the synthetic aperture interferometric satellite radar altimeter echo power and echo cross product the model uses the smallness of the satellite pitch and roll angles and the limited range of satellite altitude to provide a semianalytical echo model whose numerical dimensions are limited to two in the synthetic aperture case or three in the interferometric case at large roll angles making its application to extensive data practical by not implementing the synthetic beam formation it is demonstrated that the model recovers the conventional case of a pulse limited altimeter a theoretical description of the fluctuations in the multilooked interferometric cross product and the synthetic aperture and pulse limited powers is given by extending the model to describe correlations between individual looks in each case the model offers the potential to retrieve ocean surface parameters from synthetic aperture and pulse limited altimeter data simultaneously with obvious application to the new generation of altimeters onboard cryosat 2 sentinel 3 and sentinel 6 during the transition to operational synthetic aperture radar altimetry

variability of arctic sea ice thickness and volume from cryosat 2 we present our estimates of the thickness and volume of the arctic ocean ice cover from cryosat 2 data acquired between october 2010 and may 2014 average ice thickness and draft differences are within 0 16 m of measurements from other sources moorings submarine electromagnetic sensors icebridge the choice of parameters that affect the conversion of ice freeboard to thickness is discussed estimates between 2011 and 2013 suggest moderate decreases in volume followed by a notable increase of more than 2500 km 3 or 0 34 m of thickness over the basin in 2014 which could be attributed to not only a cooler summer in 2013 but also to large scale ice convergence just west of the canadian arctic archipelago due to wind driven onshore drift variability of volume and thickness in the multiyear ice zone underscores the importance of dynamics in maintaining the thickness of the arctic ice cover volume estimates are compared with those from icesat as well as the trends in ice thickness derived from submarine ice draft between 1980 and 2004 the combined icesat and cryosat 2 record yields reduced trends in volume loss compared with the 5 year icesat record which was weighted by the record setting ice extent after the summer of 2007

fragility and an extremely low shear modulus of high porosity silicic magma abstract the rheology and strength of bubbly magma govern eruption dynamics by determining the possibility of fragmentation of ascending magmas they are also required parameters for understanding seismic monitoring we measured the rheology and strength of high porosity rhyolitic magma at 500 950 c the measured shear modulus and strength are several orders of magnitude lower than bubble free rhyolite melt implying that high porosity magma cannot avoid fracturing during magma ascent the occurrence of fractures is observed in the low temperature magma 800 c in this temperature range the measured attenuation is low that is the elastic energy originated by deformations avoids attenuation and is stored in the bubbly magma until released by fracturing q 1 the newly found porosity dependent strength based on our measurements comprehensively explains three different fragmentation criteria that have been previously proposed independently our measurements also show that the shear modulus becomes lower by increasing porosity which can slow the shear wave velocity these results suggest that knowing the attenuation of the seismic wave is useful to evaluate magma temperature and the possibility of a fragmentation event that may determine subsequent volcanic activities

time scales of compaction in volcanic systems we used 12 high temperature deformation experiments on fabricated cores of rhyolite ash to explore the effect of porosity on the rheology of pyroclastic deposits during deformation the cores of ash accommodate strain mainly by shortening and reduction of porosity porosity loss creates a strain dependent rheology expressed by a marked and continuous increase in effective viscosity the effective viscosity graphic 1 of pyroclastic materials is predicted as a function of porosity graphic 2 and melt viscosity graphic 3 o by graphic 4 the optimal value for the parameter based on the experimental data is 0 78 0 15 these experiments provide constraints on the time scales of compaction and flow processes in volcanic systems under variable conditions i e temperature load stress our results indicate that welding and compaction processes in pyroclastic deposits e g ignimbrites conduits can occur on time scales of tens of minutes to hours at these time scales welding is fully decoupled from cooling of the deposits and may be coupled to depositional processes within volcanic conduits welding processes operate on fragmented magma rapidly enough to eliminate porosity in a matter of hours thereby sealing off permeability and supporting cyclical vulcanian style explosive eruptions 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif 3 embed inline graphic 3 gif 4 embed inline graphic 4 gif

coupled effect of magma degassing and rheology on silicic volcanism abstract explosive volcanism such as the 1991 mt pinatubo philippines and the 2008 mt chaiten chile eruptions is caused by violent vesiculation of hydrous magma however gas may efficiently separate from magma owing to the enhancement of gas permeability by shear deformation of magma flowing in a volcanic conduit this makes it difficult to maintain the driving force of explosive volcanism although explosive volcanism is actually common here we propose that shear localization in a volcanic conduit controls the eruption style and explosivity based on deformation experiments of vesicular magma linked with synchrotron radiation x ray radiography and computed tomography we observed for the first time in situ that the shear localization caused magma fracturing and formed a slip plane and thus inhibited deformation and outgassing elsewhere we also observed the compaction of vesicular magma into a dense lava as a result of outgassing when shear localization did not occur in a natural setting shear localizes along the edges of a volcanic conduit where the strain rate is high causing a highly permeable fracturing layer to form at the conduit s edge and leaving less sheared and less outgassed magma at its center the less outgassed magma in the center may ascend rapidly and cause explosive volcanism non explosive lava effusion may occur only when shear localization does not occur effectively this new view explains the rapid ascent of viscous magma and the formation of pyroclasts with contrasting vesicularity pyroclastic obsidian and highly vesiculated pumice

tube pumices as strain markers of the ductile brittle transition during magma fragmentation magma fragmentation the process by which relatively slow moving magma transforms into a violent gas flow carrying fragments of magma is the defining feature of explosive volcanism yet of all the processes involved in explosively erupting systems fragmentation is possibly the least understood1 2 several theoretical and laboratory studies on magma degassing3 4 5 6 7 and fragmentation8 9 10 11 have produced a general picture of the sequence of events leading to the fragmentation of silicic magma12 13 14 but there remains a debate2 over whether magma fragmentation is a consequence of the textural evolution of magma to a foamed state where disintegration of walls separating bubbles becomes inevitable due to a foam collapse criterion or whether magma is fragmented purely by stresses that exceed its tensile strength here we show that tube pumice where extreme bubble elongation is observed is a well preserved magmatic strain marker of the stress state immediately before and during fragmentation structural elements in the pumice record the evolution of the magma s mechanical response from viscous behaviour foaming and foam elongation through the plastic or viscoelastic stage and finally to brittle behaviour these observations directly support the hypothesis that fragmentation occurs when magma undergoes a ductile brittle transition and stresses exceed the magma s tensile strength

sea ice roughness from airborne lidar profiles light detection and ranging lidar data acquired during the amsr ice03 arctic flights are used to observe and characterize the spectral behavior of sea ice roughness height distributions and roughness spectral densities are derived from surface profiles measured over a variety of sea ice types the spectral densities are fit to lorentzian curves to obtain the correlation length and root mean square height of roughness within the sampling bandpass of the lidar profiler the utilization of these spectral parameters outside of their bandpass is also discussed

arctic sea ice freeboard retrieval with waveform characteristics for nasa s airborne topographic mapper atm and land vegetation and ice sensor lvis data from an icebridge arctic campaign on april 20 2010 with the airborne topographic mapper atm and the land vegetation and ice sensor lvis in operation on the same airplane were used in this study atm and lvis lidar waveforms were fitted with gaussian curves to calculate pulsewidth peak location pulse amplitude and signal baseline for each waveform the centroid skewness kurtosis and pulse area were also calculated received waveform parameters such as pulsewidth pulse amplitude pulse area skewness and kurtosis show coherent response to variations of geophysical features along an atm or lvis profile these parameters combined with elevation were used to identify leads in sea ice freeboard calculation the relationship between these parameters and sea ice freeboard and surface features were studied by comparing the parameters with atm and lvis derived freeboard and coincident digital mapping system images which have been used to classify sea ice surface types such as leads thin ice gray ice and thick ice an elevation bias of more than 16 cm peak to peak as a function of laser scanner azimuth was found in the atm data and an empirical correction was applied this correction will improve the atm shot to shot freeboard significantly the newly derived atm freeboard was compared with the current icebridge idcsi2 freeboard product at national snow and ice data center nsidc and the freeboard derived from lvis data over the studied area the mean freeboard is 0 540 pm 0 091 m for the idcsi2 at nsidc 0 496 pm 0 062 m for the atm after empirical elevation correction and 0 509 pm 0 048 m for the lvis

rheological transitions in high temperature volcanic fault zones silicic magma experiences shear induced brittle fracturing during its ascent resulting in the formation of a magmatic fault at the conduit margin once the fault is formed frictional behavior of the fault controls the magma ascent process we observed torsional deformation of a magmatic fault gouge in situ at temperatures of 800 and 900 c using synchrotron radiation x ray radiography the torsional deformation rate was set at 0 1 10 rpm corresponding to equivalent slip velocities of 2 27 10 5 1 74 10 3 m s 1 and shear strain rates of 0 014 1 16 s 1 the normal stresses used were 1 5 and 10 mpa the magmatic fault showed frictional sliding as well as viscous flow even above the glass transition temperature the transition between frictional sliding and viscous flow depends on temperature deformation rate and normal stress on the fault at 900 c the fault showed viscous deformation at a normal stress of 10 mpa while frictional sliding was predominant at 800 c we propose the ratio of timescales of fault healing and deformation as a criterion for transition between frictional sliding and viscous flow the experimentally calibrated criterion infers that frictional sliding is predominant from 500 m in depth during explosive eruption this may explain rapid magma ascent without efficient outgassing frictional heating would in turn enhance fault healing resulting in the reverse transition from frictional sliding to viscous flow followed by deceleration of magma ascent therefore cyclic transitions between frictional sliding and viscous flow are a possible explanation for the cyclic behavior of lava effusion

combined airborne laser and radar altimeter measurements over the fram strait in may 2002 knowledge of sea ice thickness is critical for the prediction of future climate and for assessing the significance of changes in thickness sea ice thickness can be calculated from radar or laser satellite altimetry measurements of freeboard however a lack of knowledge of snow depth introduces significant uncertainties into these calculations this paper compares the first coincident airborne laser and radar altimetry data over sea ice collected during the laser radar altimetry lara field campaign lara was a flight of opportunity that provided valuable data to explore techniques to validate satellite measurements of ice freeboard and the possibility of combining laser and radar measurements over snow covered sea ice to calculate the snow depth two new methods were created to analyse these data sets a new radar retracker and a radar power simulator which models radar returns from the laser data we present the first quantitative analysis of data from the lara laser and radar altimeters and demonstrate the potential of combining laser and radar altimetry to estimate snow depth lara elevation estimates compare well with elevations from the radar altimeter onboard ers 2 at the sub meter level and the study provides lessons for future validation of satellite altimetry data over sea ice laser elevations are consistently higher than the radar elevations over snow covered sea ice as lara was a flight of opportunity no coincident in situ measurements were available nevertheless the difference between the reflecting surface of the laser and radar is consistent with snow depth from climatology and the analysis techniques developed in this paper will be useful for future radar and laser altimetry comparisons

strain localization in vesicular magma implications for rheology and fragmentation the rheology of two or three phase magmas has been the focus of much interest because it controls magma ascent and eruption behavior research on magma rheology has typically considered homogeneous fl ow here we demonstrate based on natural examples that strain resulting from viscous fl ow preceding explosive fragmentation localizes into shear zones at a microscopic scale strain localization affects the rheological behavior of magmas modifying predictions based on experiments localization leads to high strain rate shear zones where elongated sheared vesicles and shear heating have a weakening effect surrounding regions of relatively low strain rate where subequant vesicles have a strengthening effect thus energy is dissipated more effi ciently into localized bands where strain rate increases through feedback effects and can lead to melt fragmentation

dynamics of seismogenic volcanic extrusion at mount st helens in 2004 05 the 2004 05 eruption of mount st helens exhibited sustained near equilibrium behaviour characterized by relatively steady extrusion of a solid dacite plug and nearly periodic shallow earthquakes here we present a diverse data set to support our hypothesis that these earthquakes resulted from stick slip motion along the margins of the plug as it was forced incrementally upwards by ascending solidifying gas poor magma we formalize this hypothesis with a dynamical model that reveals a strong analogy between behaviour of the magma plug system and that of a variably damped oscillator modelled stick slip oscillations have properties that help constrain the balance of forces governing the earthquakes and eruption and they imply that magma pressure never deviated much from the steady equilibrium pressure we infer that the volcano was probably poised in a near eruptive equilibrium state long before the onset of the 2004 05 eruption

extensive perched aquifer and structural implications revealed by 3d resistivity mapping in a galapagos volcano abstract due to the complexity of geological formations and limited subsurface data in volcanic islands hydrogeological conceptual models can differ from one island to another and for the same island the galapagos islands like most inhabited volcanic islands face important water resource problems which might have a major impact on their unique and pristine ecosystems endanger world heritage list june 2007 the scarcity of geological and hydrological data combined with the difficulty of access for field measurements lead to a poor understanding of the island hydrogeology and unconvincing interpretation of traditional geophysical data here we present three dimensional 3 d resistivity maps for the supposedly waterless santa cruz island obtained by using the skytem device a helicopter borne transient electromagnetic method the latter is non invasive and measurements over inaccessible terrain were of vital importance we show that even in high relief terrain with extreme subsurface resistivity contrasts 1 6 000 ohm m the method is sensitive to low resistivity layers of hydrogeological interest 50 200 ohm m to a depth of approximately 300 m the unique spatial resolution and 3 d view of the subsurface resistivity structures allow identification of two zones of hydrogeological importance a previously unknown extensive perched aquifer 50 km 2 on the southern mountain side and the geometry of the salt water wedge in the basal aquifer located with an accuracy of a few meters this finding supports the existence of hidden perched aquifers on basaltic islands until now only inferred from hydrogeological studies it is seen to be affected by faulting beyond the vital implications for water resource and ecosystem management in the unique yet severely threatened galapagos islands some conceptual flow models of volcanic islands have to be reconsidered the presence of such structures in a volcanic edifice recognized as low strength layer may help to explain the origin of potentially catastrophic landslides

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

shear deformation experiments on vesicular rhyolite implications for brittle fracturing degassing and compaction of magmas in volcanic conduits 1 shear induced brittle fractures in vesicular magmas are thought to be possible pathways for open system degassing that controls the explosivity of volcanic eruptions to investigate the detailed processes involved in the shear induced degassing we performed torsional deformation experiments on columnar rhyolites with water contents and averaged vesicularities of 0 4 0 5 wt and 20 41 vol respectively at temperatures of 780 930 c and strain rates of 0 03 s 1 the experimental conditions such as viscosity and vesicularity of the sample strain rate and total strain simulated those of typical natural dehydrated rhyolites in shallow volcanic conduits at relatively high temperatures 855 c the columnar samples were homogeneously twisted and deformed on the other hand at temperatures of 830 c the deformation was localized and finally resulted in brittle failure into a column and a disk followed by a slip at the fractured interface this slip prevented further brittle failure and shear induced bubble coalescence in the remaining parts of the sample a permeable fragmented zone was formed near the interface while in the rest of the sample the bubbles remained isolated and permeability did not increase from this we infer that a single event of magma fracturing may enhance open system degassing locally near a fracture but repeated fracturing and healing processes are necessary for the effective degassing of the entire magma that leads to nonexplosive eruptions

interdecadal changes in snow depth on arctic sea ice snow plays a key role in the growth and decay of arctic sea ice in winter it insulates sea ice from cold air temperatures slowing sea ice growth from spring to summer the albedo of snow determines how much insolation is absorbed by the sea ice and underlying ocean impacting ice melt processes knowledge of the contemporary snow depth distribution is essential for estimating sea ice thickness and volume and for understanding and modeling sea ice thermodynamics in the changing arctic this study assesses spring snow depth distribution on arctic sea ice using airborne radar observations from operation icebridge for 2009 2013 data were validated using coordinated in situ measurements taken in march 2012 during the bromine ozone and mercury experiment bromex field campaign we find a correlation of 0 59 and root mean square error of 5 8 cm between the airborne and in situ data using this relationship and icebridge snow thickness products we compared the recent results with data from the 1937 1954 1991 soviet drifting ice stations the comparison shows thinning of the snowpack from 35 1 9 4 to 22 2 1 9 cm in the western arctic and from 32 8 9 4 to 14 5 1 9 cm in the beaufort and chukchi seas these changes suggest a snow depth decline of 37 29 in the western arctic and 56 33 in the beaufort and chukchi seas thinning is negatively correlated with the delayed onset of sea ice freezeup during autumn

calibration of the cryosat 2 interferometer and measurement of across track ocean slope this paper describes the calibration of the cryosat 2 interferometer whose principal purpose is to accurately measure the height of the antarctic and greenland ice sheets a sequence of cryosat 2 data acquisitions over the tropical and midlatitude oceans were obtained between june and september 2010 from the siral a and redundant siral b radars operating in their sarin mode during a sequence of satellite rolls between 0 6 and 0 4 using the arrival angle of the echo relative to the interferometer baseline the attitude of the satellite determined by the star trackers and estimates of the ocean surface across track slope from the egm08 geoid we determined the errors in the interferometer estimate of surface slope as functions of the roll angle and ocean surface waveheight these were found to be in close agreement with the theoretical description the scale factor of the interferometric measurement of angle was determined to be 0 973 0 002 we estimate the accuracy of the across track slope measurement of the interferometer by applying this scale factor to the measured phase in applying this scale factor to the measurements the across track slope of the marine geoid was obtained with an accuracy of 26 rad at 10 km and 10 rad at 1000 km we conclude that the instrument performance considerably exceeds that needed for the accurate determination of height over the sloping surfaces of the continental ice sheets the results also demonstrate that cryosat 2 provides the first observations of the instantaneous vector gradient of the ocean surface and that the normal incidence interferometric configuration has a greater potential for the measurement of the ocean across track slope than has been previously recognized

extreme frictional processes in the volcanic conduit of mount st helens usa during the 2004 2008 eruption abstract the 2004 2008 eruption of mount st helens saw the extrusion of seven high viscosity spines and formation of discrete shear zones along the conduit margin at spine 7 this shear zone consists of four structurally distinct layers the outer surface gouge l1 crosscuts a dark banded layer l2 which grades into a moderately sheared layer l3 and undeformed rock l4 inside the spine field observations porosity measurements geochemistry mineralogy microstructure crystal size and shape distribution kinetic properties and magnetic analyses chart the evolution of deformation processes and products throughout the eruption gouge formation was concomitant with characteristic microseismic drumbeats at depths 0 5 1 km in addition the seismic record shows two larger earthquakes with similar seismic signatures in august 2006 which we conclude represent larger slip amounts along the conduit margin of spine 7 extensive slip resulted in frictional heating on the order of several hundreds of degrees melting the highly viscous crystalline ascending magma plug and forming a pseudotachylyte high ambient temperatures in the conduit resulted in near equilibrium melting and slow recrystallisation thus impeding the development of signature pseudotachylyte characteristics and hindering identification thus frictional melting and recrystallisation in ascending magma plugs may be a common but unidentified phenomena at composite volcanoes worldwide

arctic sea ice freeboard from altika and comparison with cryosat 2 and operation icebridge satellite radar altimeters have improved our knowledge of arctic sea ice thickness over the past decade the main sources of uncertainty in sea ice thickness retrievals are associated with inadequate knowledge of the snow layer depth and the radar interaction with the snow pack here we adapt a method of deriving sea ice freeboard from cryosat 2 to data from the altika ka band radar altimeter over the 2013 14 arctic sea ice growth season altika measures basin averaged freeboards between 4 4 cm and 6 9 cm larger than cryosat 2 in october and march respectively using airborne laser and radar measurements from spring 2013 and 2014 we estimate the effective scattering horizon for each sensor while cryosat 2 echoes penetrate to the ice surface over first year ice and penetrate the majority 82 3 of the snow layer over multiyear ice altika echoes are scattered from roughly the midpoint 46 5 of the snow layer over both ice types

fully focused sar altimetry theory and applications in this paper we introduce the concept develop the theory and demonstrate the advantages of fully focused coherent processing of pulse echoes from a nadir looking pulse limited radar altimeter this process similar to synthetic aperture radar sar imaging systems reduces the along track resolution down to the theoretical limit equal to half the antenna length we call this the fully focused sar ff sar altimetry processing the technique is directly applicable to sar altimetry missions such as cryosat 2 sentinel 3 or sentinel 6 jason cs the footprint of an ff sar altimeter measurement is a narrow strip on the surface which is pulse limited across track and sar focused along track despite the asymmetry of the altimeter footprint the fully focused technique may be useful for applications in which one needs to separate specific targets within highly heterogeneous scenes such as in the case of sea ice lead detection hydrology and coastal altimetry applications in addition over rough homogeneous surfaces such as the ocean or ice sheets the improved multilooking capability of ff sar leads to a significant increase in the effective number of looks with respect to the delay doppler processing resulting in better geophysical parameter estimation the proposed processing technique was verified by processing full bit rate cryosat 2 sar mode data over the svalbard transponder hydrology sea ice and open ocean applications are also demonstrated in this paper showing the improvement of this technique with respect to conventional and delay doppler altimetry

analytical solutions for analysing pumping tests in a sub vertical and anisotropic fault zone draining shallow aquifers summary we present new analytical solutions for examining the influence during a pumping test in a well of an infinite linear and anisotropic strip aquifer that drains shallow aquifers of different diffusivity and thickness the whole system is confined and the aquifer geometry can be represented by a t an aquifer geometry resembling a sub vertical fault or a sub vertical vein cross cutting shallower aquifers the proposed solutions are based upon an unconventional application of well image theory without limitation of the diffusivity contrast between the three domains solutions for drawdown were developed for the three domains i e the strip aquifer and the two shallow compartments and flow signatures are discussed in detail and compared to numerical modelling the proposed solutions are not shown to be exact solutions to the appropriate partial differential equation but very good and useful approximations the solutions were applied to a 63 day pumping test in a steep fault zone in crystalline aquifer rock of brittany france after that the flow contributions of the fault zone and of the shallow aquifers deduced from groundwater dating were compared to analytical solutions the solutions and theoretical type curve examples can help in understanding flow processes from tests conducted in settings that are similar to such a conceptual model

viscous heating in rhyolite an in situ experimental determination viscous heating during magmatic flow may play a major role in eruption dynamics in order to document viscous heating during deformation of magma we have conducted a series of rheological experiments where viscous heating is directly monitored via thermocouples in high viscosity magmas we observe experimentally the strain rate dependence of viscous heating viscous heating becomes rheologically significant in the highly viscous lavas investigated at strain rates above ca 10 3 s 1 a simple analysis shows that the temperature increase generated through viscous heating during deformation of melts with viscosities ranging from 108 to 1012 pa s can account for their apparent non newtonian rheology in these experiments this thermal correction transforms apparent non newtonian strain rate dependent rheology of magma to a newtonian behavior over the range of conditions accessed in this work in this manner this study provides an experimental basis for separating the relative roles of structural relaxation and viscous heating in the generation of apparent non newtonian rheology at high strain rates here viscous heating dominates and the observation of the structural onset of non newtonian behavior is precluded by a viscous heating induced lowering of the newtonian viscosity the interplay of viscous heating and structural relaxation in melts in nature is discussed briefly

extrusion cycles during dome building eruptions abstract we identify and quantify controls on the timescales and magnitudes of cyclic periodic volcanic eruptions using the numerical model domeflow de michieli vitturi et al 2010 which was developed by the authors for magma systems of intermediate composition domeflow treats the magma mixture as a liquid continuum with dispersed gas bubbles and crystals in thermodynamic equilibrium with the melt and assumes a modified poiseuille form of the viscous term for fully developed laminar flow in a conduit of cylindrical cross section during ascent magma pressure decreases and water vapor exsolves and partially degasses from the melt as the melt simultaneously crystallizes causing changes in mixture density and viscosity two mechanisms previously proposed to cause periodic eruption behavior have been implemented in the model and their corresponding timescales explored the first applies a stick slip model in which motion of a shallow solid plug is resisted by static dynamic friction as described in iverson et al 2006 for a constant magma supply rate at depth this mechanism yields cyclic extrusion with timescales of seconds to tens of seconds with values generally depending on assumed friction coefficients the second mechanism does not consider friction but treats the plug as a high viscosity newtonian fluid during viscous resistance pressure beneath the degassed plug can increase sufficiently to overcome dome overburden plug weight and viscous forces and ultimately drive the plug from the conduit in this second model cycle periods are on the order of hours and decrease with increasing magma supply rate until a threshold is reached at which point periodicity disappears and extrusion rate becomes steady vanishingly short periods magma volatile content for fixed chamber pressure has little effect on cycle timescales but increasing volatile content increases mass flow rate and cycle magnitude as defined by the difference between maximum and minimum extrusion rates increasing mass flow rate also increases the magnitude of predicted deformation cycles both models have been applied to dome building eruptions using conditions appropriate for the soufriere hills volcano montserrat results suggest that the modeled stick slip mechanism cannot explain the cycles of extrusion and explosion well documented at montserrat timescales of hours however a subset of the simulations for the viscous plug model are consistent with the montserrat data and therefore demonstrate the feasibility of this second formulation

rheology of bubble bearing magmas abstract the physical effects of air or argon bubbles on the rheology of a calcium aluminosilicate melt have been measured at temperatures ranging from 830 to 960 c at 1 bar pressure the melt composition is sio 2 64 al 2 o 3 23 and cao 13 wt while bubble volume fractions are 0 0 06 0 13 0 32 0 41 and 0 47 measured newtonian viscosities range from 10 10 to 10 14 dpa s melts with bubble fractions of 0 06 and 0 13 show with increasing temperature t an increasing relative viscosity for t t 850 c for all bubble fractions the viscosity decreases markedly with temperature the observed maximum decrease of the relative viscosity is 75 for a bubble fraction of 0 47 at 907 c at all bubble fractions the viscosity is independent of the applied stress which ranged from 11 to 677 bars no clear indications were observed of non newtonian rheological behavior under our experimental conditions the relative viscosity of the two phase liquid depends primarily on the bubble fraction physical and volcanological implications of these measurements are discussed

permeability reduction of fractured rhyolite in volcanic conduits and its control on eruption cyclicity repetitive explosions of silicic volcanoes are thought to be triggered by volcanic gases in the shallow parts of the conduit to trap the volcanic gases in the conduit the lava dome or plug must be impermeable however silicic magma undergoes brittle fracturing during ascent because its viscosity increases via dehydration and groundmass crystallization since fractured magma has high permeability the reduction of permeability is a key process that allows gases to accumulate leading to an explosion we performed uniaxial compression experiments on rhyolite fragments at temperatures of 700 900 c to investigate the permeability reduction of fractured rhyolite experimental results show that permeability is controlled by porosity and permeability reduction can be caused by compaction of fractured magma based on the observed relationship between permeability and porosity we calculate the time scale of permeability reduction i e the transition from permeable to impermeable magma the calculated time scale 100 1000 s is much shorter than the eruption repose 1 10 h observed at sakurajima volcano japan this result implies that an impermeable lava dome and plug formed during the eruption repose in contrast the repose time 20 40 min of santiaguito volcano guatemala is shorter than the time scale of permeability reduction 10 h this indicates that gases could not have accumulated before volcanic explosions and that other mechanisms could control eruption cyclicity

volcanic dilemma flow or blow volcanic eruptions can be either slow outpourings of lava or explosive ejections of fragmented rock in his perspective dingwell discusses the physical and chemical properties of magma that lead to these two styles of eruption new experiments and computer simulations are pointing to the involvement of solidlike behavior of liquid magma when it makes a transition to a glass

simulated effects of a snow layer on retrieval of cryosat 2 sea ice freeboard the impact of a snow layer on the location of the tracking point rp for ranging to the sea ice surface in cryosat 2 synthetic aperture interferometric radar altimeter waveforms is simulated with a range resolution of 47 cm the response of the air snow a s interface broadens the response of the snow ice s i interface and displaces the rp toward the altimeter this effect is largest when the strengths of their returns are comparable and when snow thicknesses are 20 cm on the other hand the rp is displaced away from the altimeter when the reduced propagation speed in the snow layer is not accounted for this analysis examines the dependence of these competing corrections on snow thickness and the relative scattering strengths of the interfaces and the sensitivity of two different tracking approaches leading edge and centroid to these snow parameters expected errors depend on a better understanding of the relative scattering strengths of the interfaces and snow layer

conceptual hydrogeological model and aquifer system classification of a small volcanic island la gomera canary islands abstract conceptual groundwater models in volcanic island aquifers remain an issue in many regions of the world this study takes advantage of the large number of outcrops exposing the inner structure of la gomera canary islands due to its inactivity during the last 2 5 ma to develop a conceptual groundwater model that helps improving groundwater resource management in the island and increasing knowledge in volcanic aquifers based on a volcanostratigraphic model five hydrostratigraphic units have been defined that include two main aquifers and volcaniclastic deposits located between them that control the vertical movement of the groundwater the island has been classified into four aquifer systems or water management units based on their hydrogeological features that permit a sustainable and independent management of the resources of one aquifer system in relationship with the other ones the consideration of the complexity and heterogeneity of la gomera geology has allowed to develop a conceptual groundwater flow model and to identify four general groundwater situations 1 groundwater in the upper aquifer discharging to the sea 2 groundwater in the upper aquifer springing out along the main ravines 3 groundwater in the basal aquifer impounded by dikes and 4 groundwater in the basal aquifer discharging to the sea

origins of mount st helens cataclasites experimental insights the 2004 2006 eruption of mount st helens produced a sequence of lava domes characterized by a 1 3 m thick outer carapace of highly brecciated and comminuted dacite fault rocks this outer layer of fault rocks is proposed to be a physical manifestation of the drumbeat microseismicity such that magma extrusion occurred via integration of rapid co seismic slip along small displacement 5 mm faults a suite of deformation experiments performed on samples of mount st helens msh dacite under confining pressures of 0 25 50 and 75 mpa at room temperature and an average displacement rate of 10 4 s were run to reproduce fault textures found in nature the msh dacite starting material has low porosity 7 8 and a microcrystalline groundmass with little glass a subsidiary set of deformation experiments on dacite samples from the 2006 augustine eruption was run under identical experimental conditions to evaluate the effect of increased porosity 20 24 on failure mechanisms the msh dacite samples show a progressive increase in peak strength with increasing confining pressure are strong peak stress at 75 mpa is 700 mpa and failed by localized brittle behavior characterized by macroscopic fractures and rapid stress drops in contrast augustine dacite deformed by distributed cataclastic flow and is much weaker peak stress at 75 mpa is 220 mpa we propose that the generation of low porosity dacite was an important variable in promoting wholesale localized faulting and the attendant drumbeat microseismicity at mount st helens microstructures of gouge developed experimentally at room temperature are remarkably similar to those developed at 730 c at msh mode i microcracking shear fracture of grains and grain size comminution occurred in both natural and experimental fault rocks laser grain size particle analyses show peaks at 1 5 m for experimental run products vs peaks at 1 9 and 4 m for natural msh gouge we conclude that because the msh lava had solidified prior to faulting temperature is secondary in importance in the formation of the gouge material based on the amount of fault displacement per microseismic event the number of drumbeats and the aggregate radial thickness 1 3 m of the gouge we calculate that the 3 6 m day eruption rate allows for gouge filled slip surfaces having thicknesses of 0 8 5 mm and strike lengths of 98 190 m per seismic event