wave disturbance behind low crested structures diffraction and overtopping effects abstract in recent years the hydraulic performance of low crested structures has been widely studied both theoretically and through experimental analyses but only a few studies have been focused on the combined diffraction overtopping effects on wave transmission and induced wave currents in this paper hydraulic model tests conducted at the 3d wave basin of delft university of technology caceres et al 2008 were used to obtain and discuss two simple methods for predicting the wave height at the lee of a single detached breakwater of finite length and the related current regime for the first time diffraction effects are expressly accounted for the agreement with experimental data is encouraging the main objective of the paper is to aid engineers in the first stage of the design process when using a mathematical model could be unnecessarily excessive

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

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

interpretation of porosity in dacite lava domes as ductile brittle failure textures abstract pores in dacite lavas of three volcanoes in japan unzen yakedake and daisen have characteristics unlike vesicles most of the pores have ragged walls marked by the protrusion of groundmass crystals and phenocrysts the shapes of these pores range from irregular to planar many pores occur in pressure shadows adjacent to phenocrysts some pores are crack like and pass around phenocrysts and have blunt terminations most of the pores are interpreted as the result of failure of the magma by flow during ductile brittle transition this phenomenon known as cavitation is well established in hydraulics and the material sciences cavitation of the lavas in this study was promoted by steep slope of emplacement subtle mechanical contrasts between bands and strain intensification around rigid crystals and lithic fragments cavitation in lava can extract volatiles from magma causing premature embrittlement and weakening cavitation is inferred to be critical to the initiation of lava front pyroclastic flows

ductile brittle transition structures in the basal shear zone of a rhyolite lava flow eastern australia abstract structural and textural evidence indicates that the basal breccia of the tertiary minyon falls rhyolite of tweed volcano eastern australia formed mainly by fragmentation within the basal shear zone of the lava flow cooling at the flow base led to lava behaviour passing through the ductile brittle transition while subjected to progressive deformation the intact lavas of the basal zone record intense ductile shearing in multiple folds rotated phenocrysts strong alignment of crystallites and micro folding of these crystal alignments minor brittle structures including faults on fold limbs and microfaults occur in the intact lava immediately above the basal breccia these structures represent incipient brecciation and complete brecciation has occurred by intense faulting of previously intact lava brittle deformation migrated upward into overlying intact lava as cooling occurred this process is distinctly different from overriding and incorporation of surface breccias which is commonly invoked as the primary process of basal breccia formation

analysis of overtopping flow on sea dikes in oblique and short crested waves abstract dike resilience against wave overtopping has gained more and more attention in recent years due to the effect of expected future climate changes the overtopping flow velocities and flow depths on dikes have recently been studied in 2d small scale experiments this has led to semi empirical formulae for the estimation of flow depths and flow velocities across a dike the results have been coupled to the actual erosion of the landward dike slope determined by full scale 2d tests using the so called overtopping simulator this paper describes the results from 96 small scale tests carried out in a shallow water basin at aalborg university to cover the so far unknown 3d effects from oblique long crested and short crested waves based on results from the laboratory tests expansions are proposed to the existing 2d formulae so as to cover oblique and short crested waves the wave obliquity is seen to significantly reduce the overtopping flow velocities and flow depths on especially the landward slope of a sea dike moreover the tests showed that the average flow directions on the dike crest from oblique long crested and short crested waves correspond approximately to the incident wave direction flow depths and the squared flow velocities on the dike are concluded to be rayleigh distributed in case of both long crested and short crested waves for all considered incident wave obliquities findings in the present paper are needed to obtain more realistic estimates of dike erosion caused by wave overtopping

microstructures and rheology of hydrous synthetic magmatic suspensions deformed in torsion at high pressure the relationship between magma rheology and characteristic magmatic microstructures was investigated by performing high temperature high pressure deformation experiments on hydrous synthetic magmatic suspensions in the range of 0 to 76 solid fraction alumina grains torsion experiments were conducted at 300 mpa confining pressure temperatures ranging from 475 c to 1000 c and shear strain rates ranging from 2 0 10 5 to 2 1 10 3 s 1 up to total strains of 21 3 flow is newtonian for a solid fraction of s 0 0 0 16 with a log dynamic viscosity 10 3 pa s t 500 c a deviation from newtonian behavior is observed for s 0 16 with an increase in apparent viscosity of about 1 order of magnitude between s 0 16 and 0 54 the shape fabric of the solid phase is characterized by a unimodal orientation that is almost stable and nearly parallel to the shear direction both shape fabric and deviation from newtonian behavior originate from the increase in the number of particle clusters in the suspension the apparent viscosity increases by 1 5 orders of magnitude between s 0 54 and 0 65 and extrapolation of the data suggests a very sharp increase in apparent viscosity for s 0 65 at t 550 c and s 0 54 the solid phase forms an almost entirely connected network composed of two alternating orientation domains at t 550 c and s 0 65 intragranular fracturing and tensile fractures result from high local stresses at contact points between neighboring particles the resulting bulk extensional fabric is almost parallel to the shortening direction

strike slip relay ramps areas of reorientated bedding at contractional oversteps between strike slip faults are here called strike slip relay ramps metre scale examples are described from the jurassic sediments at east quantoxhead somerset u k larger strike slip relay ramps occur in the rio de peixe basin ne brazil along the newport inglewood trend california and in the bovey basin sw england although the geometry and development of strike slip relay ramps are similar to those of relay ramps in normal fault systems there are differences in the structures which accommodate the transfer of displacement between the overstepping faults whereas strike slip relay ramps are typically transpressional with pressure solution often occurring relay ramps in normal fault systems are dominated by extension or transtension care needs to be taken when interpreting areas of reorientated bedding between overstepping faults particularly when displacement directions are unknown for example when using seismic data this is because relay ramps can occur in both strike slip and normal fault zones

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

spatial distribution of wave overtopping water behind coastal structures spatial distribution of random wave overtopping water behind coastal structures was investigated using a numerical model based on reynolds averaged navier stokes solver rans and volume of fluid vof surface capturing scheme rans vof the computed spatial distributions of wave overtopping water behind the structure agree well with the measurements by pullen et al 2008 for a vertical wall and lykke andersen and burcharth 2006 for a 1 2 sea dike a semi analytical model was derived to relate spatial distribution of wave overtopping water behind coastal structures to landward ground level velocity and layer thickness on the crest this semi analytical model agrees reasonably well with both numerical model results and measurements close to coastal structures our numerical model results suggest that the proportion of wave overtopping water passing a landward location increases with a seaward slope when it is less than 1 3 and decreases with a seaward slope when it gets steeper the proportion of wave overtopping water passing a landward location increases with landward ground level and overtopping discharge it also increases with the product of incident wave height and wavelength but decreases with increasing relative structure freeboard and crest width we also found that the extent of hazard area due to wave overtopping is significantly reduced by using a permeable structure crown findings in this study will enable engineers to establish the extent of hazard zones due to wave overtopping behind coastal structures

extreme coastal responses using focused wave groups overtopping and horizontal forces exerted on an inclined seawall abstract total wave group overtopping and maximum horizontal force responses are investigated for an idealised seawall dike on a plane beach subject to compact focused wave attack using both laboratory and numerical wave flumes the wave group interactions have very short durations such that extraneous reflections from the wavemaker arrive long after the main interaction these short test durations facilitate the use of large ensembles of tests to explore the sensitivity of overtopping and force responses to variations in focus location phase angle at focus and linear focus wave amplitude the scope of the laboratory wave flume tests is broadened by accurate numerical simulation based on a 1dh hybrid boussinesq nlsw model for a given focus location and linear focused wave amplitude variations in phase lead to an order of magnitude change in the group overtopping volume substantial increases in overtopping volume owing to the use of linear wavemaker theory compared to second order theory are also observed these observations have implications for phase independent empirical relationships derived using linear paddle signals in physical experiments examination of the incidence of wave groups parametrically optimised for maximum and minimum overtopping volumes indicates that the overtopping volume may be optimised by minimising reflections of pre overtopping waves within the group while maximising the amplitude of the first overtopping bore numerical predictions of horizontal seawall forces are obtained using fluid impulse derivatives and hydrostatic pressures obtained from the shallow water model within the shallow water model framework hydrodynamic force contributions included in the fluid impulse method are observed to be small relative to the hydrostatic pressure force the parametric dependence of the horizontal non impulsive forces on the seawall is very similar to that of the overtopping volumes with clear bands of large values observed as a function of phase and focus location for a given amplitude this suggests that the parametric optimisation of focused wave groups is a robust method for the investigation of multiple coastal responses such as overtopping forces and runup

influence of oblique wave attack on wave overtopping at smooth and rough dikes with a berm abstract oblique wave attack can significantly reduce the amount of wave overtopping at coastal structures compared to perpendicular wave attack guidelines exist to estimate the influence of oblique waves on wave overtopping guidelines to estimate the influence of a berm in the seaward slope are also available these guidelines for the reducing effects of oblique waves and the reducing effects of a berm are frequently applied in combination although limited data is available for this combination of reducing effects to develop a method that can account for the reducing effects of oblique waves and a berm in the seaward slope of a dike physical model tests have been performed in a wave basin on structures with smooth and rough slopes based on these data wave overtopping guidelines for breaking waves on dikes have been validated and one of the guidelines has been extended to take the influence of oblique waves berms and roughness into account

transition from dome forming to plinian eruptive styles controlled by h2o and cl degassing the transition from a plinian pumice to an effusive dome forming eruptive style is frequently observed in volcanic systems and is generally attributed to the progressive loss of volatiles from magma stored in a superficial reservoir this explosive effusive transition has been explained by the evolution from a closed to an open system of degassing1 2 3 4 but in this context an eruption at mt pelee martinique french west indies dated at 650 years ago which exhibited a rarely observed5 6 succession from dome forming to plinian activity in a short interval of time7 is at odds with such an explanation in this eruption near surface explosions of the dome produced two peleean turbulent pyroclastic flows whose deposits are similar to those of the effusive 1902 eruption and then plinian activity produced pumice fallouts and flows the reconstruction of the degassing paths of both eruptive regimes using the densities and the h2o and cl contents of the clasts shows that the interaction of rising magma with hydrothermal fluids at shallow depth may play a critical role in determining eruptive style

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

experimental simulation of magma mixing at high pressure abstract magma mixing features are observed in many plutonic and volcanic environments they result from the juxtaposition of two chemically contrasted magmas usually not only during the replenishment of a magmatic reservoir but also syn eruptively within the conduit despite its ubiquity only a few experimental studies have explored mixing between magmas existing data have been mostly acquired at atmospheric pressure and high shear rates 10 1 s 1 which differ from those accompanying magma mixing in reservoirs to fill this gap we performed high pressure mixing experiments at strain rates ranging from 4 10 4 to 1 10 3 s 1 layers of a synthetic crystal free haplotonalite and a natural partially molten basalt were juxtaposed in a paterson apparatus at 300 mpa and deformed between 900 and 1200 c the experiments shed light on the first stages of magma mixing and illustrate the role and behaviour of crystals either pre existing or newly grown experiments evidence a rheological threshold for mafic material disruption which sets in abruptly as its melt fraction exceeds 50 which in the experiments occurs in the narrow temperature interval 1160 1170 c below this threshold plagioclase crystals in the mafic magma form a rigid touching network and all the deformations are accommodated by the less viscous felsic layer above it the crystal network collapses allowing typical mingling mixing features to appear altogether such as enclaves melt filaments and single xenocrysts isolated into the felsic end member coexisting with newly grown phases plagioclase and pyroxene whose compositions spread out over considerable ranges the pre existing fabric of the mafic magma is only slightly affected by deformation altogether providing few clues on either the regime or geometry of applied deformation during the magmatic stage

rheological properties of dome lavas case study of unzen volcano abstract transitions between effusive and explosive styles of lava dome eruptions are likely accompanied by changes in lava rheology the common presence of crystals in dome lavas produces a complex non newtonian rheology thus models of such complex rheology are essential for volcanic eruption models here we have measured the rheology of natural unzen lavas with a compressive uniaxial press operating at stresses between 1 and 70 mpa and temperatures between 940 and 1010 c crystal rich unzen lavas are characterised by two essential rheological features which produce non newtonian effects the first is an instantaneous response of the apparent viscosity to applied stress which requires that the magma be described as a visco elastic fluid that exhibits shear thinning the second effect takes the form of a time dependence of the viscosity at moderate to high stress 10 mpa in this regime the apparent viscosity slowly decreases as increasing fracturing of the phenocrysts and the groundmass occurs fragmentation of crystals and alignment of crystal fragments are observed to produce flow banding effects which in turn lower the apparent viscosity of natural dome lavas ultimately deformation may lead to complete rupture of the lava if the stress is sufficient cracking thus stands as an important process in natural dome lava rheology the ubiquitous non newtonian rheology of dome lavas observed experimentally here needs to be adequately treated in order to generate appropriate eruption models

experimental investigation of magma rheology at 300 mpa from pure hydrous melt to 76 vol of crystals the rheological behaviour of synthetic crystal bearing magmas containing up to 75 vol of crystals 0 s 0 75 has been investigated experimentally at a confining pressure of 300 mpa and temperatures between 475 and 600 c at shear rates between 10 4 and 2x10 3 s 1 starting hydrated crystal bearing glasses were synthesized from a dry haplogranitic glass qz36ab39or29 and 2 5 wt water mixed with 0 pure hydrous melt 16 34 54 65 or 75 vol of al2o3 sieved 45 o 90 m crystals shear viscosity measurements were performed in torsion in a paterson gas medium apparatus for each composition the rheological data were fitted to a power law of the form 1 where is shear rate is shear stress t is temperature r is the gas constant and a n and q are empirical parameters defined as the preexponential term the stress exponent and the activation energy respectively the rheology is found to be newtonian n 1 for pure hydrated melt and 16 vol of crystals at higher crystal contents the magmas behave as pseudoplastic materials and exhibit shear thinning behaviour with stress exponents of 1 3 1 9 and 2 8 for 34 54 and 65 vol of crystals respectively the activation energy seems to be nearly independent of crystal fraction and has a mean value of 231 kj mol 1 the einstein roscoe equation adequately estimates viscosities of the crystal bearing magmas at low crystal contents s 0 25 but progressively deviates from the measured viscosities with increasing crystal content as the rheological behaviour becomes non newtonian a modified power law formulation incorporating crystal content as a variable allows a fit to the experimental data set over the whole range of conditions

in situ confirmation of permeability development in shearing bubble bearing melts and implications for volcanic outgassing the ferocity of volcanic eruptions their penchant for either effusive or explosive behaviour is to a large extent a matter of the ease with which volatiles are able to escape the volcanic system of particular importance are the mechanisms by which permeable networks within magma are fabricated and how they permit gas escape thereby diffusing possibly calamitous explosions here we present a series of experiments that confirms sample scale fracture propagation and permeability development during shearing viscous flow of initially impermeable bubble bearing 2 10 4 s 1 at moderate shear strain rates 2 10 4 s 1 3 via en echelon mode i fractures produced by repeated fracture events at high shear strain rates 4 5 10 4 s 1 permeability develops shortly after the onset of inelastic deformation and is again established through a series of en echelon mode i fractures critically strain is not immediately localized on mode i fractures making them long lived and efficient outgassing channels that are ideally oriented for directing volatiles from the central conduit upward and outward toward the conduit rim indeed mode i fracture arrays may prove necessary for dissipating gas overpressures in the central regions of the magma column which are considered difficult to outgas these experiments highlight mechanisms that are likely active along conduit margins and constrain previously postulated processes under truly applicable conditions

pressure induced forces and shear stresses on rubble mound breakwater armour layers in regular waves abstract this paper presents the results from an experimental investigation of the pressure induced forces in the core material below the main armour layer and shear stresses on the armour layer for a porous breakwater structure two parallel experiments were performed which both involved pore pressure measurements in the core material 1 core material with an idealized armour layer made out of spherical objects that also allowed for detailed velocity measurements between and above the armour and 2 core material with real rock armour stones the same core material was applied through the entire structure i e no additional filter layers were applied for both experiments high speed video recordings were synchronised with the pressure measurements for a detailed investigation of the coupling between the run up and run down flow processes and the measured pressure variations outward directed pressure gradients were found which exerted a lift force up to 60 of the submerged weight of the core material these maximum outward directed pressure gradients were linked to the maximum run down event and were in general situated at or slightly below the maximum run down level detailed velocity and turbulence measurements showed that the large outward directed pressure gradients in general coincide both in time and space with the maximum bed shear stresses on the armour layer based on the reynolds stresses the bed shear stresses were found to result in a shields parameter in the same order of magnitude as the critical value for movement of the armour stones

the effects of heterogeneity in magma water concentration on the development of flow banding and spherulites in rhyolitic lava abstract this study focuses on the origin of flow banded rhyolites that consist of compositionally similar darker and lighter flow bands of contrasting texture and color infrared radiation was used to obtain fourier transform infrared ftir spectra from which water concentrations were calculated and to map variations in water concentrations across zones of spherulites and glass from the 23 million year old sycamore canyon lava flow of southern arizona lighter colored thicker flow bands consist of gray glass fine grained quartz and large 1 0 to 1 5 mm spherulites darker colored thinner flow bands consist of orange glass and smaller 0 1 to 0 2 mm spherulites the centers of both large and small spherulites are occupied by either 1 a quartz or sanidine crystal 2 a granophyric intergrowth or 3 a vesicle mapping of water concentration dominantly oh in glass and oh and h2o in sanidine crystals illustrates fluctuating water availability during quenching of the host melt textures of large spherulites in the lighter gray bands in some cases indicate complex quenching histories that suggest that local water concentration controlled the generation of glass versus crystals small spherulites in darker orange bands have only one generation of radiating crystal growth both the glass surrounding spherulites and the crystals in the spherulites contain more water in the gray flow bands than in the orange flow bands flow banding in the sycamore canyon lava flow may have originated by the stretching of a magma that contained pre existing zones vesicles or proto vesicles of contrasting water concentration as the magma flowed in the conduit and on the surface variation in the original water concentration in the alternating layers is interpreted to have resulted in differences in undercooling textures in spherulites in the lighter compared to the darker flow bands

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

porosity redistribution enhanced by strain localization in crystal rich magmas magma degassing characterized by changes in permeability and porosity distribution has a crucial control on the style of eruption during ascent magma might develop large porosities and crystallize while it is subjected to shear shear in turn enhances complex fabrics that result from the reorganization of the different phases crystals gas melt such fabrics have not yet been evaluated experimentally on a three phase system we performed torsion experiments on a synthetic crystal rich hydrous magma at subsolidus conditions with 11 vol porosity to establish a link between strain partitioning and porosity redistribution crystals induce non newtonian deformation resulting in localization of the shear strain three dimensional microtomography and two dimensional scanning electron microscope imaging show gas accumulation in local microstructures caused by shear induced crystal fabric our data show that strain localization is a mechanism that could enable magma degassing at very low vesicularity

origin of broken phenocrysts in ash flow tuffs surprisingly little attention has been devoted to the textural nature of phenocrysts of feldspar and quartz in tuff although many geologists have briefly alluded to broken phenocrysts none have addressed their origin in any detail petrographic study of 117 cooling units in the middle tertiary ash flow province of the great basin united states provides a basis for characterization of the shapes and for interpretation of the origin of felsic phenocrysts in ash flow tuffs although not proven to be wholly ineffective breakage of phenocrysts by mutual impact in the erupting magma and pyroclastic flow is doubtful for at least four reasons first the statistical probability of mutual collision between phenocrysts diminishes exponentially as their proportion to vitroclasts diminishes e g only 1 probability for 10 phenocrysts collision is less likely if pyroclasts move by laminar rather than turbulent flow second the coating of glass and or melt on the phenocrysts provides a cushion that absorbs the impact force third plagioclases broken by impact in the laboratory have unusual shapes unlike those seen in great basin tuffs fourth euhedral phenocrysts of feldspar are commonplace in many great basin tuffs and in some they constitute a significant proportion of the phenocrysts indicating that mutual impact does not modify all intratelluric crystals during explosive eruption the two most populated categories of phenocryst shape in great basin tuffs probably correspond to what has been previously called broken phenocrysts somewhat less than half of the plagioclase and many sanidine phenocrysts are subhedral to anhedral these are similar in shape size and composition to grains in polycrystalline aggregates within the same thin section kindred aggregates and discrete phenocrysts could have been derived from holocrystalline to partly crystalline material in the magma chamber that was disaggregated to varying extents during explosive eruption more than half of the plagioclase and all of the quartz phenocrysts in great basin tuffs consist of irregularly shaped fragments with cuspate embayed outlines resembling pieces of a jigsaw puzzle which we call phenoclasts inclusions of glass are common and are especially evident in larger more or less whole crystals textural features of some phenocrysts in cognate pumice clasts in the tuffs reveal that they broke apart while still in the vesiculating but unfragmented magma as the erupting magma decompressed vesiculation of the melt that was entrapped at higher pressures as inclusions within the phenocrysts blew them apart forming the phenoclasts shapes of felsic phenocrysts in volcanic rocks provide insight into their mode of emplacement euhedral phenocrysts are common in ash flow tuffs as well as lava flows phenoclasts however are diagnostic of ash flow tuffs because they do not occur in plinian ash fall deposits and are rare in lava flows these textural contrasts are useful for interpretation of generally older but in any case altered and recrystallized volcanic rocks in such rocks critical groundmass features and field relations that could provide clues to their origin have been obscured but the shapes of relict phenocrysts are commonly well preserved

three dimensional investigations of wave overtopping on rubble mound structures to study the influence of wave obliquity and directional spreading on wave overtopping of rubble mound breakwaters a total of 736 three dimensional model tests were carried out at aalborg university the results of these tests are presented and analysed in this paper yielding a new empirical reduction factor to describe the influence of wave obliquity and directional spreading on the average wave overtopping discharges the study shows that perpendicularly incident long crested waves result in conservative values of the overtopping discharge for the tested cross section

oblique wave attack on rubble mound breakwaters abstract stability formulae for armour layers of rubble mound breakwaters are usually being applied assuming perpendicular wave attack often the effects of oblique waves are neglected this is however a conservative assumption since the stability of armour slopes generally increases for oblique waves new wave basin tests have been performed to assess the effects of oblique waves on the stability of rock slopes and the stability of cube armoured rubble mound breakwaters the physical model tests were focussed on wave directions between perpendicular 0 and parallel 90 the test programme included tests with long crested waves and tests with short crested waves the results show that for rock slopes the influence of oblique waves is larger for long crested waves based on the test results a design guideline is provided to account for effects of oblique waves on the stability of rock slopes armour layers with a double layer of cubes and armour layers with a single layer of cubes

pre explosive conduit conditions of the 1997 vulcanian explosions at soufriere hills volcano montserrat i pressure and vesicularity distributions an authoritative case of vulcanian eruptive dynamics is the series of 88 explosions that occurred between august and october 1997 at soufriere hills volcano on montserrat island the state of the magmatic column just before a vulcanian explosion is still poorly understood but conditions the eruptive style this study establishes such a pre explosive stratigraphy by 1 documenting the textures covering the range of the 1997 products 2 quantitative analysis of h2o content in interstitial glass measured by karl fischer titration and 3 combining these data with a simple model linking pre and post explosive vesicularities the model shows that syn explosive degassing affects greatly the way porosity evolves by decompression during an explosion the stratigraphy reconstruction shows a three part vertical layering of the conduit prior to explosion with overall denser values than those previously suggested a dense and strongly degassed plug caps the column it is underlain by a shallow transition zone featuring complex mingling between vesicular and dense magma up to 10 mpa at higher pressure up to 80 mpa lies a more homogeneous zone of relatively dense 10 20 vol magma which was emplaced under partly open system degassing this conduit stratigraphy gives the vision of a strongly heterogeneous magma column immediately prior to its disruption our analysis suggests that fragmenting such a composite magma cannot happen in a single coherent pulse but rather as stages the transition zone contains heterogeneous amounts of exsolved gas that could explain the pulsatory nature of the vulcanian jets at the beginning of the explosions this contrasts with the nearly constant vesicularities of the deeper part of the pre explosive magma column which are propitious to a general short lived disruption

strain induced outgassing of three phase magmas during simple shear a major factor determining the explosivity of silicic eruptions is the removal of volatiles from magma through permeability controlled outgassing we studied the microstructural development of permeability during deformation of highly viscous magma by performing simple shear experiments on bubble 0 12 0 36 volume fraction and crystal bearing 0 0 42 volume fraction silicate melts experiments were performed under torsion at high temperature and pressure 723 873 k and 150 200 mpa in a paterson deformation apparatus at bulk shear strains between 0 and 10 the experimental setup allows for gas escape if bubble connectivity is reached on the sample periphery three dimensional imaging and analysis of deformed bubbles was performed using x ray tomography the development of localized deformation in all samples enhanced by crystal content leads to brittle fracture at bulk strains 2 and sample wide fracturing in samples deformed to strains 5 a decrease in both bubble fraction and dissolved volatile content with increasing strain along with strain hardening rheological behavior suggests significant shear induced outgassing through the fracture networks applicable to shallow conduit degassing in magmas containing crystal fractions of 0 0 42 this study contributes to our understanding of highly viscous magma outgassing and processes governing the effusive explosive transition

the role of oxides in the shallow vesiculation of ascending magmas abstract despite their generally low volume fraction fe ti oxides have the potential to greatly influence the eruptive style because they lower the supersaturation pressure for heterogeneous bubble nucleation once nucleated bubbles respond fast to pressure changes fostering rapid expansion and explosive behavior yet oxide microlite quantifications are often absent from data of explosive products we used new re analyzed and published data to build a compilation of oxide number densities onds and vesicle number densities vnds of trachytic and calc alkaline products four eruptive styles were selected 1 vulcanian explosions from soufriere hills volcano montserrat lascar volcano chile and kilian volcano france 2 blasts from mt pelee volcano lesser antilles mount st helens usa and merapi volcano indonesia 3 a sub plinian explosion from merapi volcano and 4 lava dome effusions with intermittent collapse from soufriere hills and mt pelee volcanoes natural samples were separated into two groups according to the dominant texture of the products of each event 1 vesicular pumice clasts from explosions with a strong vertical component and 2 dense clasts with diktytaxitic textures from dome collapse event and lateral blast group 1 samples are either distributed alongside the 1 1 trend between vnd and ond that spans from 1015 to 1017 m 3 or have a constant vnd of 1016 16 5 m 3 regardless of ond a large proportion of oxides 55 100 touch vesicles a more variable proportion of vesicles 16 72 are in contact with oxides because of syn explosive growth and coalescence group 2 samples have onds in the same broad range as group 1 samples we also used new and published data to build a compilation of onds and vnds of five series of experimental decompression of rhyolitic and phonolitic melts in samples with ond vnd most bubbles are in contact with more than one oxide and 64 88 of the oxides are in contact with bubbles such high levels of connectivity suggest that the role of oxides in controlling bubble nucleation has been underestimated when vnd ond nucleation densities of experimental vesicles can be reproduced by heterogeneous nucleation models which we used to calculate syn explosive decompression rates from vnds at merapi soufriere hills and kilian these rates and textural evidence suggest that the decompression front accompanying these vulcanian and sub plinian explosions is responsible for syn explosive bubble nucleation we calculated the average pre explosive ascent rates necessary to yield the observed onds at soufriere hills and merapi volcanoes the resulting rates 0 005 20 m s overlap considerably with the range of critical ascent rate inferred for the effusive explosive transition supporting the hypothesis that this transition is primarily controlled by oxide microlites in the conduit because oxides are a primer for explosive behavior when present in sufficiently high number densities focusing on the older eruption of kilian for which no observation is available we infer that pre explosive ascent rates of 7 10 3 m s were necessary for explosive behavior to occur

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

rheological properties of magma from the 1538 eruption of monte nuovo phlegrean fields italy an experimental study abstract the rheology of natural partially crystallized samples from monte nuovo 1538 ad eruption have been investigated using a high temperature high pressure internally heated pressure vessel within the temperature interval of investigation the samples exhibited newtonian rheology at deformation strain rates up to 10 4 s 1 at higher strain rates a decrease of viscosity with increasing strain rate shear thinning eventually terminating in a sudden viscosity drop stress weakening was observed stress weakening occurred at higher applied strain rates and stresses with increasing temperature the microstructural analyses of the samples revealed that stress weakening resulted from strain localization which occurred in brittle and viscous regimes at the lowest and highest temperatures investigated respectively the different rheological behaviors of the investigated material is likely responsible for the changes in the dynamics of magma ascent within the volcanic conduit during the eruption of monte nuovo volcano we infer that the occurrence of syn eruptive decompression driven crystallization observed in previous studies induced strong variations in the rheological properties of magma which in turn was responsible for the changes of eruptive style observed during the monte nuovo eruption

strain localization and fluid pathways in mylonite inferences from in situ deformation of a water bearing quartz analogue norcamphor simple shearing of polycrystalline norcamphor containing 10 15 vol water at near atmospheric pore fluid pressure and a range of constant temperatures 3 35 c and shear strain rates 5 10 5 4 10 4 s 1 induces localization of both strain and fluid flow prior to deformation the water is located at grain triple junctions and pockets along grain boundaries it forms an average dihedral angle of 46 with the surrounding norcamphor grains during initial shearing c1 grain boundaries oriented subparallel to the principal shortening direction dilate and fill with water at 1 15 c the tips of the shear surfaces propagate alternately by intracrystalline plasticity and subcritical fracturing concomitant with dynamic recrystallization in the rest of the sample the episodic interconnection of dilatant shear surfaces is associated with short term increases in displacement rate parallel to the surfaces these surfaces coalesce to form a high strain fluid filled network subparallel to the experimental shear zone however this network never spans the entire length of the shear zone at any given time even after shearing to c 8 5 the deformation is more homogeneous and fewer dilatant shear surfaces develop at higher deformational temperatures and or lower strain rates due to the increased activity of dislocation creep prolonged stress annealing removes most microstructural evidence of the syntectonic fluid pathways dilatant shear surfaces in norcamphor resemble relics of mica filled synmylonitic fractures in dynamically recrystallized quartz from greenschist facies mylonite suggesting that fluid played a similar mechanical role in nature and experiment the coalescence of dilatant fluid filled shear surfaces represents a strain dependent increase in pore connectivity within mylonitic shear zones the experiments indicate that fluid flow along deep crustal mylonitic shear zones is probably limited by the rate at which the tips of the dilatant shear surfaces propagate subparallel to the shearing plane 2000 elsevier science b v all rights reserved

petrological architecture of a magmatic shear zone a multidisciplinary investigation of strain localisation during magma ascent at unzen volcano japan shearing of magma during ascent can promote strain localisation near the conduit margins anymechanical and thermal discontinuities associated with such events may alter the chemical physicaland rheological stability of the magma and thus its propensity to erupt lava spines can record suchprocesses preserving a range of macroscopic and microscopic deformation textures attributed toshearing and friction as magma ascends through the viscous brittle transition here we use a multi disciplinary approach combining petrology microstructures crystallography magnetics and experi mentation to assess the evidence role and extent of shearing across a marginal shear zone of the1994 1995 lava spine at unzen volcano japan our results show that crystals can effectively moni tor stress conditions during magma ascent with viscous remobilisation crystal plasticity and com minution all systematically increasing towards the spine margin accompanying this we find anincrease in mineral destabilisation in the form of pargasitic amphibole breakdown displaying tex tural variations across the shear zone from symplectitic to granular rims towards the spine margin in addition the compaction of pores chemical and textural alteration of interstitial glass and mag netic variations all change systematically with shear intensity the strong correlation between thedegree of shearing crystal deformation and disequilibrium features together with distinct magneticproperties implies a localised thermal input due to shear and frictional processes near the conduitmargin during magma ascent this was accompanied by late stage or post emplacement fluid andgas induced alteration of the gouge as well as oxidation and glass devitrification understandingand recognising evidence for strain localisation during magma ascent may therefore be vital whenassessing factors that regulate the style of volcanic eruptions which may provide insights into thecryptic shifts from effusive to explosive activity as observed at many active lava domes

gas storage transport and pressure changes in an evolving permeable volcanic edifice abstract the total volume of gas in a magma dissolved and subsequently exsolved greatly influences the degree of explosiveness of a volcanic system there is a marked contrast between the behaviour of a volcano in an open system compared to one which is closed it is therefore essential to understand the entire degassing process gas transport storage and loss a particular focus of this study is the effect different permeabilities and pressure gradients within a volcanic edifice have on the degree and pattern of the gas velocity gas loss is modelled numerically in two dimensions using a finite element approach which allows the specification of boundary conditions with respect to pressure and different permeability domains within the volcanic edifice by combining the time dependent continuity equation and darcy s law a partial differential equation is derived and solved for the pressure the associated pressure gradient is then used within darcy s law to determine the corresponding gas velocity distribution this method is used not only for stationary systems in equilibrium but also as a time dependent progression it permits the modelling of different situations to study how various volcanic characteristics affect the gas loss the model is used to investigate the change in pressure and gas in response to time dependent scenarios these are a dome collapse or sudden increase in permeability by magma rupture at the conduit margin the formation of cracks within the lava dome and sealing by crystallisation our results show that a combination of high and low permeability regions is required for effective gas storage high permeability allows the gas to enter the system but impermeable areas act to confine the gas thereby increasing its pressure and consequently increasing the amount of gas which may be dissolved in the melt furthermore our results show that permeability is an essential factor influencing the response time to system changes which could be linked in future to deformation and other geophysical observations our model is highly versatile and sheds new light on the understanding of gas storage and transport in a permeable volcanic edifice

reconstructing magma failure and the degassing network of dome building eruptions volcanic eruptions are regulated by the rheology of magmas and their ability to degas both detail the evolution of stresses within ascending subvolcanic magma but as magma is forced through the ductile brittle transition new pathways emerge as cracks nucleate propagate and coalesce constructing a permeable network current analyses of magma dynamics center on models of the glass transition neglecting important aspects such as incremental strain accommodation and the key monitoring tool of seismicity here in a combined methods study we report the first high resolution 20 m neutron computed tomography and microseismic monitoring of magma failure under controlled experimental conditions the data reconstruction reveals that a competition between extensional and shear fracturing modes controls the total magnitude of strain to failure and importantly the geometry and efficiency of the permeable fracture network that regulates degassing events extrapolation of our findings yields magma ascent via strain localization along conduit margins thereby providing an explanation for gas and ash explosions along arcuate fractures at active lava domes we conclude that a coupled deformation seismicity analysis holds a derivation of fracture mechanisms and network and thus holds potential application in forecasting technologies

influence of oblique wave attack on wave overtopping and forces on rubble mound breakwater crest walls abstract at the crest of rubble mound structures a crest wall is often present to reduce the consumption of material compared to breakwaters without a crest wall to design the crest wall several methods based on estimates of the hydraulic wave loads are available these methods are focussed on waves that approach breakwaters perpendicular in practice the dominant wave conditions are often not perpendicular but approach the breakwater under an angle oblique waves can significantly reduce the amount of wave overtopping and reduce the wave loads on coastal structures compared to perpendicular waves therefore physical model tests have been performed in a wave basin where the wave loads on crest walls and the wave overtopping discharges have been measured under oblique wave attack design guidelines have been developed to take the reduction due to oblique waves into account together with available methods to predict the wave forces and wave overtopping for perpendicular wave attack the obtained results enable a more advanced design of crest walls

wave reflection in 3d conditions based on recent experiments carried out in wave basin on breakwaters with armour layer of rocks and cubes this paper examines the dependence of the reflection coefficient on wave directional spreading and obliquity results suggest that long crested and short crested waves give similar reflection the reflection coefficient is markedly dependent on the wave angle of incidence the performance of formulae available in the literature is checked against the new dataset and a significant improvement is proposed by including the wave obliquity factor that appears in the traditional expression for the overtopping discharge

nonisothermal viscous sintering of volcanic ash volcanic ash is often deposited in a hot state volcanic ash containing glass deposited above the glass transition interval has the potential to sinter viscously both to itself particle particle and to exposed surfaces here we constrain the kinetics of this process experimentally under nonisothermal conditions using standard glasses in the absence of external load this process is dominantly driven by surface relaxation in such cases the sintering process is rate limited by the melt viscosity the size of the particles and the melt vapor interfacial tension we propose a polydisperse continuum model that describes the transition from a packing of particles to a dense pore free melt and evaluate its efficacy in describing the kinetics of volcanic viscous sintering we apply our model to viscous sintering scenarios for cooling crystal poor rhyolitic ash using the 2008 eruption of chaiten volcano as a case example we predict that moderate linear cooling rates of 0 1 c min 1 can result in the common observation of incomplete sintering and the preservation of pore networks

tracking the permeable porous network during strain dependent magmatic flow abstract rheological variations have been postulated as the cause of transitions from effusive to explosive volcanic eruption style rheology is integrally linked to the composition and textural state porosity crystallinity of magma as well as the stress temperature and strain rate operative during flow this study characterises the rheological behaviour and importantly the evolution of physical properties of two magmas with different crystallinity and porosity from volcan de colima mexico a volcanic system known for its rapid fluctuations in eruption style magma samples deformed in a uniaxial press at a constant stress of 2 8 12 or 24 mpa a constant temperature of 940 945 c comparable to upper conduit or lava dome conditions to strains of 20 or 30 displayed different mechanical behaviour and significant differences in measured strain rates 10 2 10 5 s 1 the evolution of porosity permeability dynamic young s modulus and dynamic poisson s ratio illustrate a complex evolution of the samples manifested as strain hardening visco elastic constant rate and strain weakening deformation both magmas behave as shear thinning non newtonian liquids and viscosity decreases as a function of strain we find that strain localisation during deformation leads to the rearrangement and closure of void space a combination of pores and cracks followed by preferentially aligned fracturing in the direction of the maximum principal stress to form damage zones as well as densification of other areas in a dome setting highly viscous low permeability magmas carry the potential to block volcanic conduits with a magma plug resulting in the build up of pressures in the conduit above a certain threshold of strain dependent upon stress strain rate the initiation propagation and coalescence of fractures leads to mechanical degradation of the magma samples which then supersedes magmatic flow and crystal rearrangement as the dominant form of deformation this results in lower apparent viscosities than those anticipated for magma of such crystallinity especially at high strain rates in a lava dome this could result in dome collapse and the concomitant depressurisation could trigger an explosive eruption

structural analysis of flow related textures in lavas abstract the textures of coherent volcanic rocks including lavas and volcanic intrusives commonly contain features that are attributed to flowage previous applications of structural analysis to volcanic rocks are expanded here to provide a framework for analysis textures defined as the crystallinity granularity and shapes and arrangements of the components crystals glass and voids of a rock together with structures defined as individual features composed of the disposition attitude arrangement or relative positions of the components of a rock are first described second the spatial fabrics shapes and arrangements of the components of a rock and the orientation of textures and structures are identified third textures structures and fabrics are placed in the spatial and temporal geological context finally detailed interpretations of the kinematics and rheology of structures and fabrics is made leading to an integrated flow history of the rock illustrative case studies include rhyolite from the basal part of the tertiary minyon falls dome northeastern new south wales australia which has a texture comprising planar domains of differing crystal abundance flow bands multiple folds of these domains relatively homogeneous crystal alignment parallel to the fold axes and microfolding of these domains in the zone of interaction between phenocrysts and matrix including retrorotation of phenocrysts on short limbs of inequant folds trachyte dykes on fraser island queensland australia have a texture comprising crystal alignment planar concentration domains banding two planar domains of crystal alignment interpreted to be conjugate shear zones phenocrysts influence the spacing and distribution of the domains and interacted with shear zones by undergoing small amounts of rotation the shear zones overprinted a homogeneous crystal alignment during the last stage of flow before solidification as a result of dilatant granular interactions lava from miocene submarine banded silicic mafic domes on katsurajima on the coast of the shimane peninsula southwestern japan has a texture comprising crystal alignment irregular domains interpreted as the result of quenching local internal deformation of crystals including brittle boudinage and elongate vesicles and amygdales dacite from the late stages of the 1991 1995 eruption of mount unzen in western kyushu japan has a texture comprising crystal alignment deformation of phenocrysts and their surrounding matrix producing voids by mechanical means with the possible involvement of vapour phases the pervasive mechanical failure of the flowing magma may have triggered frontal collapse and the ensuing pyroclastic eruptions a spectrum of increasingly intense flow related textures can be recognised from simple planar alignments of crystals and vesicles to overprinting effects and domainal textures such as banding and shear zones and deformed domains pervasive ductile tearing and ultimately brittle fracture record flow during the transition from ductile magma to brittle rock the development of these flow related textures is controlled by the interplay of driving forces magma pressure gravity and resisting forces viscosity where driving forces are strong flow related textures are well developed when viscous resistance exceeds the driving forces flow and the development of flow related textures ceases