temporal evolution of the magmatic system at tungurahua volcano ecuador detected by geodetic observations abstract changes in the pathway and timescale of magma ascent can be responsible for variations in eruptive style during long lived eruptions but are only documented at a small number of well instrumented systems here we integrate ps insar from high resolution terrasar x radar imagery with continuous gps data from 4 sites at tungurahua volcano ecuador our results show long term uplift between 2011 and 2014 associated with a continuously inflating prolate spheroid reservoir at a depth of 10 km beneath the summit comparisons with eruptive flux taking compressibility into account suggest that during this time period slightly over half the magma supplied to the system was erupted the observations span three distinct phases of eruption and in 2012 2013 an increase in eruptive activity was accompanied by uplift on the volcano s western flank similar episodes have previously been observed during large vulcanian eruptions and we attribute them to intrusions into an area of mechanical weakness a localised patch of subsidence mid way up the tungurahua s western flank is co located with a swarm of shallow long period seismicity and may represent a potential site for a development of a lateral vent this study demonstrates that satellite geodetic techniques are capable of characterising the geodetic signature of transitions in eruptive style during long lived andesitic eruptions even where deformation is low magnitude and measurements are challenging

vulcanian explosion at soufriere hills volcano montserrat on march 2004 as revealed by strain data 1 the calipso collaborative volcano monitoring system on the caribbean island of montserrat includes observations of strain at depths 200 m using sacks evertson strainmeters strain data for the march 2004 explosion of the soufriere hills volcano are characterized by large roughly equal but opposite polarity changes at the two near sites and much smaller changes at a more distant site the strain amplitudes eliminate a spherical pressure mogi type source as the sole contributor the initial changes are followed by smaller recoveries but with differing relative recovery magnitudes this dissimilarity requires a minimum of two pressure sources which we model as a deep spherical pressure source and a shallow dike the spherical source is fixed at the location derived from data for the massive dome collapse in july 2003 we solve for the best fitting dike plus sphere source combination the dike geometry is consistent with earlier interpretations of dikes based on gps data and other lines of evidence

geodetic observations during the 2009 eruption of redoubt volcano alaska abstract in march 2009 redoubt volcano about 160 km to the sw of anchorage alaska began its most recent explosive eruption deformation induced by this event was recorded by a gps campaign network consisting of 14 benchmarks which had been established in 1991 after the previous eruption the network was partially reoccupied in 2001 and 2008 and no volcanic deformation was detected during that period in response to precursory unrest starting in january 2009 the alaska volcano observatory temporarily deployed continuously recording gps instruments at four of the campaign benchmarks only days before the onset of explosive activity in march 2009 the only gps instrument recording continuously during the months prior to the eruption was the plate boundary observatory pbo station ac17 about 28 km northeast of the volcano s summit data from this station reveals subtle motion radially outward from the volcano beginning as early as may 2008 which reversed with the onset of explosive activity using simple analytical models we link the precursory activity to a point source intrusion of 0 0194 0 03400 0092 km3 in volume at 13 50 17 3310 17 km below sea level bsl superscripts and subscripts refer to upper and lower ends of confidence intervals at the 95 level during the explosive phase about 0 05 0 10 028 km3 of magma was evacuated from a prolate spheroid with its centroid at 9 17 15 176 92 km bsl a semimajor axis of 4 50 10 001 25 km length and a semi minor axis of 0 475 4 00 3 km the effusive activity is inferred to come from the same source decreasing in volume by 0 0167 0 02280 0106 km3 including observations from seismology and petrology we hypothesize a mid crustal two reservoir system with material sourced from 20 km flowing in at about 13 5 km depth and reheating residual material in the proposed spheroid the mixture migrated to shallower depth 2 4 5 km bsl and reheated material there as this residual magma erupted it was replaced by the material from the spheroidal reservoir at 7 11 5 km depth which renders the shallow source undetectable for geodetic instruments in addition to long term displacements we investigate sub daily kinematic positioning solutions and find that large short term positioning offsets correlate with large explosive events spikes in phase residuals plotted along the sky tracks of individual satellites can be related to individual plumes given favorable satellite station geometry which may be of use in volcano monitoring

martin brasier s contribution to the palaeobiology of the ediacaran cambrian transition martin brasier s work spanned almost the entire geological column but the origin of animals and the nature of the cambrian explosion were areas of particular interest to him martin adopted a holistic approach to the study of these topics that considered the interplay between multiple geological and biological phenomena and he sought to interpret the fossil record within the broad context of geological biogeochemical and ecological changes in the earth system here we summarize martin s main contributions to this area of research and assess the impact of his findings on the development of this field karl popper would have said that palaeontology is not real science because you can t go out and sample it i think absolutely the opposite i think this is actually where science is it s trying to guess what lies over the hill and map terra incognita when people come in and colonize that s just technology martin brasier 2013 excerpt from a phone interview in moor 2016

horizontal layered structure with heterogeneity beneath continents and island arcs from particle orbits of long period p waves we investigate the particle orbits of long period about 20 s p waves observed with the global seismic network by analysing 84 three component seismograms recorded at 25 stations from 60 earthquakes occurring beneath 300 km we quantitatively evaluate the orbits by three sets of eigenvalues and eigenvectors using a covariance matrix method the eigenvalues for p waves recorded at stations located on continents are explained by the standard horizontal layered structure model iasp91 on the other hand the orbits observed at stations close to island arcs are affected not only by the horizontal layered structure but also by heterogeneity due to subducting plates mantle diapirs and so on on the basis of a single scattering model for a plane p wave we quantify the heterogeneities by an isotropic scattering coefficient g0 fitting the theoretical eigenvalues to the observed ones we estimate g0 for the crust and upper mantle beneath continents to be less than 0 0005 km 1 and the mean g0 for the structure beneath island arcs to be about 0 0015 to 0 003 km 1

magmatic conditions and magma ascent as indicated by hornblende phase equilibria and reactions in the 1995 2002 soufriere hills magma a study of amphiboles and associated minerals in samples of soufri ere hills andesite erupted from 1995 to 2002 shows significant compositional variations within hornblende phenocrysts a separate set of small pargasitic crystals in the groundmass and two types of reaction rims on the phenocrysts the composition of the amphiboles and coexisting phases defines the thermal history of the erupting magma as many as seven zones 5200 mm wide in the hornblende phenocrysts begin with a sharp increase in mg and si and then change gradually to a more feand al rich hornblende a transition that is consistent with a temperature rise analyses of the hornblende phenocrysts and associated fe ti oxides verify previous conclusions that the pre eruption magma was at 130mpa and 830 10 c but was variably heated before eruption the heating occurred within 30 days of eruption for all magmas erupted based on the width of ti rich rims on titanomagnetite phenocrysts experimental phase equilibria for the andesite confirm that the natural hornblende phenocrysts would be stable between 825 and 855 c at a ph2o of 130mpa and would be even more al rich if crystallized at higher pressure pargasite is not stable in the andesite and its presence along with high an plagioclase microphenocrysts requires mafic magma mingling and hybridization with pre existing andesite experimental melts of the andesite at 130mpa and 830 and 860 c compare well with melt inclusions in quartz and plagioclase respectively reaction rims on a few hornblende crystals in each andesite sample are rich in high ca pyroxene and are produced experimentally by heating the andesite above the stability limit for hornblende decompression induced breakdown rims occur in some samples and the rate of this reaction has been experimentally calibrated for isothermal andesite magma ascent at 830 860 c the average ascent rate of magma during much of the 1995 2002 eruption has been40 02 m s the rate that allows hornblende to erupt free of decompression induced reaction rims

upper crustal structure of an active volcano from refraction reflection tomography montserrat lesser antilles to better understand the volcanic phenomena acting on montserrat the sea calipso seis mic experiment seismic experiment with airgun source caribbean andesitic lava island precision seismo geodetic observatory was conducted in 2007 december with the aim of imaging the upper crust and the magmatic system feeding the active soufri ere hills volcano the 3 d survey covered an area of about 50 40 km and involved the deployment of 247 land stations and ocean bottom seismometers obss a subset of the data recorded by four obss and four land stations on a southeast to northwest line has been analysed and traveltimes have been inverted to obtain a 2 d seismic velocity model through the island inverted phases include crustal and sediment p waves and wide angle reflections the resulting velocity model reveals the presence of a high velocity body 3 5 5 5 km s 1 beneath the island with highest velocities beneath the soufri ere and centre hills cor responding primarily to the cores of these volcanic edifices built of a pile of andesite lava domes and subsequent intrusions in the off shore region velocities in the surficial sediment layer vary from 1 5 to 3 0 km s 1 consistent with a mainly calcareous and volcaniclastic composition a wide angle reflector is observed at a depth of 1200 m below the seabed and appears to deepen beneath the island the upper crust beneath this reflector has velocities of 4 0 6 0 km s 1 and is infer red to cor respond to plutonic and hypabyssal rocks and sedimentary material of the old arc the high velocity region beneath the island extends into the crust to a depth of at least 5 km and is believed to be caused by an intrusive complex possibly of intermediate composition a low velocity zone as would be expected in the presence of an active magma chamber was not observed perhaps due to the limited resolution beneath 5 km depth our results so far provide the first wide angle seismic constraints on the upper crustal structure of the island to a depth of 10 km and will help understanding the processes that drive volcanism at montserrat and other island arc volcanoes

ediacaran remains from intertillite beds in northwestern canada a turbidite sequence within the twitya formation windermere supergroup in the ackenzie mountains northwestern canada has yielded an assemblage composed of simple centimetric annuli and discs which are interpreted as biogenic and referred to nimbia occlusa fedonkin vendetta sp and irridinitus sp they occur below a glaciomarine diamictite about 2 5 km below the base of the cambrian and they may represent the oldest ediacaran fossils now known the find has implications for models that relate the evolution of the ediacara fauna to the waning of the varanger glaciation

using dual well seismic measurements to infer the mechanical properties of a hot dry rock geothermal system high frequency 8 15 khz seismic waves that travel between two boreholes are used to probe the detailed structure of rock in the los alamos national laboratory fenton hill hot dry rock geothermal energy reservoir we study travel times of direct p waves and spectral characteristics of waveforms in an effort to determine whether or not large fluid filled fractures are present in the rock between the two boreholes data from two experiments separated in time by 1 year are studied we find dramatic differences between the two experiments in p wave velocity and in spectral characteristics of waveforms that traversed the same region we also find variations in p wave velocity and spectra as a function of location during one experiment loss of high frequency content of direct arriving p waves during the second dual well seismic experiment is compared with q found by using the coda wave technique aki and chouet 1975 and also with theoretical results on frequency dependence of transmission through a water filled fracture to determine the minimum number of large fractures in the system two to four fractures with thickness about 4 mm and vertical extent of as much as 200 m are required to explain observations during the second experiment efficient transmission of high frequencies observed during the first experiment is interpreted as meaning that fracture thickness was less than 1 mm at that time increased fracture thickness during the second experiment is probably due to a 75 day period of heat extraction during which in situ temperature dropped by as much as 100 below the virgin temperature since the coefficient of linear thermal expansion of granite is approximately 10 5 c a decrease in temperature of 100 c in a region on the order of 1 m around the fracture is required to explain the increased fracture thickness

synthesizing multi sensor multi satellite multi decadal datasets for global volcano monitoring abstract owing to practical limitations less than half of earth s 1400 subaerial volcanoes have no ground monitoring and few are monitored consistently earth observing satellite missions provide global and frequent measurements of volcanic activity that are closing these gaps in coverage we compare databases of global satellite detections of ground deformation 1992 2016 so2 emissions 1978 2016 and thermal features 2000 2016 that together include 306 volcanoes each database has limitations in terms of spatial and temporal resolution but each technique contributed 45 86 unique detections of activity that were not detected by other techniques integration of these three databases shows that satellites detected 102 volcanic activities per year before the year 2000 and 103 activities per year after the year 2000 we find that most of the 54 erupting volcanoes without satellite detections are associated with low volcano explosivity index eruptions and note that many of these eruptions 71 97 135 occurred in the earliest decades of remote sensing pre 2000 when detection thresholds were high from 1978 to 2016 we conduct a preliminary analysis of the timing between the onset of satellite detections of deformation n 154 episodes n 71 volcanoes thermal features n 16 544 episodes n 99 volcanoes and so2 emissions n 1495 episodes n 116 volcanoes to eruption start dates we analyze these data in two ways first including all satellite detected volcanic activities associated with an eruption and second by considering only the first satellite detected activity related to eruption in both scenarios we find that deformation is dominantly pre eruptive 47 and 57 whereas available databases of thermal features and so2 emissions utilizing mainly low resolution sensors are dominantly co eruptive 88 and 76 for thermal features 97 and 96 for so2 emissions

magma emplacement and remobilization timescales beneath montserrat insights from sr and ba zonation in plagioclase phenocrysts received november 22 2001 accepted december 12 2002since 1995 the soufri ere hills volcano on montserrat lesserantilles island arc has erupted crystal rich andesite magmathat is geochemically very similar to the volcanic products of atleast the last 174 3kyr 1s nomarski images of plagio clase phenocrysts from six andesites erupted between 151 4ka 1s and

remobilization of andesite magma by intrusion of mafic magma at the soufriere hills volcano montserrat west indies the 1995 1999 eruption of the soufriere hills volcano montintroduction serrat has produced a crystal rich andesite containing quenchthe soufriere hills volcano montserrat located in the textured mafic inclusions which show evidence of having been molten lesser antilles island arc in the west indies began to when incorporated into the host magma individual crystals in the erupt on 18 july 1995 after about 350 years of dormancy andesite record diverse histories amphibole phenocrysts vary from young et al 1998b the eruption was preceded by a pristine and unaltered to strongly oxidized and pseudomorphed by period of seismic swarms which began in january 1992 anhydrous reaction products plagioclase phenocrysts are commonly previous earthquake crises have occurred on montserrat reverse zoned often with dusty sieve textures reverse zoned rims are in 1896 1897 1932 1935 and 1966 1967 powell 1938 also common on orthopyroxene phenocrysts pyroxene geothermometry shepherd et al 1971 initial phreatic and phregives an average temperature of 858 20 c for orthopyroxene atomagmatic activity was followed by extrusion of an phenocryst cores whereas reverse zoned rims record temperatures andesitic lava dome in november 1995 pyroclastic flows from about 880 to 1050 c the heterogeneity in mineral rim generated by gravitational collapse of the unstable dome compositions zoning patterns and textures is interpreted as reflecting have been the dominant form of activity throughout the non uniform reheating and remobilization of the resident magma eruption but periods of explosive activity producing body by intrusion of hotter mafic magma convective remobilization pumice flows and falls have also occurred robertson et results in mixing together of phenocrysts that have experienced al 1998 young et al 1998b dome growth ceased different thermal histories depending on proximity to the intruding suddenly in march 1998 and further activity up to may mafic magma the low temperature and high crystallinity are 1999 has mainly involved gravitational collapses of the interpreted as reflecting the presence of a cool highly crystalline dome and periods of ash venting with some small scale magma body beneath the soufriere hills volcano the petrological vulcanian explosive events observations in combination with data on seismicity extrusion rate the soufriere hills volcano is the only active centre and so2 fluxes indicate that the current eruption was triggered by on montserrat the volcano is a composite of at least recent influx of hot mafic magma five andesitic lava domes flanked by pyroclastic deposits rea 1974 identified four other volcanic centres all

algal mats load structures and synsedimentary sulfides in revett quartzites of montana and idaho carbonate rich patches in quartzites of the revett formation are due to algal mats in marine arenites of about a 1 3 b y age some occur in growth positions on bedding planes some on slumped foresets but mostly they were disrupted by faster currents depositing heavy minerals as lag sediments discovery of stromatolites in the revett formation confirms the algal growths algal debris provided the food source for anaerobes which produced h 2 s for sulfide precipitation of copper silver lead zinc and iron in the zonal sequence typical of stratiform syngenetic sulfides at the troy mine these sulfide zones cross the strata sulfides vary in grain size and aggregation and are classified into original fine disseminations reworked aggregates formed during dewatering and infilling of fractures and vein openings by lateral secretion the relationship of fine sulfide disseminations to heavy minerals and to algal carbonate indicates deposition of minute sulfide grains with the sediment a quartzite pillow which contained 24 sagged and folded layers each layer with three sublayers of first heavy minerals and then clean quartzite both overlain by sulfide rich quartzite was examined and revealed the previously poorly understood mechanism of pillow growth contemporaneous with sedimentation and in this example also with sulfide precipitation better mineralization in arenites is ascribed not to permeability but to stronger currents depositing sand and causing increased mixing of the upper metal bearing waters with the lower anoxic water in which hydrogen sulfide precipitates selectively first copper and silver as minute sulfide grains the sulfides are incorporated into the suspended sediment which is then sorted by waxing and waning currents according to grain size and specific gravity into the varying proportions especially observed in the tripartite laminae of the 24 layers of the quartzite pillow described in detail

strain field analysis on montserrat w i as tool for assessing permeable flow paths in the magmatic system of soufriere hills volcano strain dilatometers have been operated on the volcanic island of montserrat west indies for more than a decade and have proven to be a powerful technique to approach short term dynamics in the deformational field in response to pressure changes in the magmatic system of the andesitic dome building soufriere hills volcano shv we here demonstrate that magmatic activity in each of the different segments of the shv magmatic system shallow dyke conduit upper and lower magma chambers generates a characteristic strain pattern that allows the identification of operating sources in the plumbing system based on a simple scheme of amplitude ratios we use this method to evaluate strain data from selected vulcanian explosions and gas emission events that occurred at shv between 2003 and 2012 our results show that the events were initiated by a short phase of contraction of either one or both magma chambers and a simultaneous inflation of the shallow feeder system the initial phase of the events usually lasted only tens to hundreds of seconds before the explosion gas emission started and the system recovered the short duration of this process points at rapid transport of fluids rather than magma ascent to generate the pressure changes we suggest the propagation of tensile hydraulic fractures as viable mechanism to provide a pathway for fluid migration in the magmatic system at the observed time scale fluid mobilization was initiated by a sudden destabilization of large pockets of already segregated fluid in the magma chambers our study demonstrates that geodetic observables can provide unprecedented insights into complex dynamic processes within a magmatic system commonly assessed by theoretical modeling and petrologic observations

magma sponge hypothesis and stratovolcanoes case for a compressible reservoir and quasi steady deep influx at soufriere hills volcano montserrat 1 we use well documented time histories of episodic gps surface deformation and efflux of compressible magma to resolve apparent magma budget anomalies at soufriere hills volcano shv on montserrat wi we focus on data from 2003 to 2007 for an inflation succeeded by an episode of eruption plus deflation we examine mogi type and vertical prolate ellipsoidal chamber geometries to accommodate both mineralogical constraints indicating a relatively shallow pre eruption storage and geodetic constraints inferring a deeper mean pressure source an exsolved phase involving several gas species greatly increases andesite magma compressibility to depths 10 km i e for water content 4 wt crystallinity 40 and this property supports the concept that much of the magma transferred into or out of the crustal reservoir could be accommodated by compression or decompression of stored reservoir magma i e the magma sponge our results suggest quasi steady deep mainly mafic magma influx of the order of 2 m3s 1 and we conclude that magma released in eruptive episodes is approximately balanced by cumulative deep influx during the eruptive episode and the preceding inflation our magma sponge model predicts that between 2003 and 2007 there was no evident depletion of magma reservoir volume at shv which comprises tens of km3 with radial dimensions of order 1 2 km in turn implying a long lived eruption

the evolution of volcanic systems following sector collapse abstract sector collapses affect volcanic edifices across all tectonic settings and involve a rapid redistribution of mass comparable in scale to the largest magmatic eruptions the eruptive behaviour of a volcano following sector collapse provides a test of theoretical relationships between surface loading and magma storage which imply that collapse driven unloading may lead to changes in eruption rate and erupted magma compositions large sector collapses are infrequent events globally with all historical examples being relatively small in comparison to many of the events documented in the geological record as a result exploration of the impacts of sector collapse on eruptive behaviour requires detailed investigation of prehistoric collapses but this is often hindered by poorly resolved stratigraphic relationships and dating uncertainties nevertheless observations from a number of volcanoes indicate sharp changes in activity following sector collapse here a global synthesis of studies from individual volcanoes in both arc and intraplate settings is used to demonstrate a number of common processes in post collapse volcanism multiple examples from large 5 km3 sector collapses in arc settings show that collapse may be followed by compositionally anomalous large volume and often effusive eruptions interpreted to originate via disruption of a previously stable upper crustal reservoir these anomalous eruptions highlight that magma compositions erupted during periods of typical i e unperturbed by sector collapse volcanism may not be representative of the range of compositions stored within a vertically extensive crustal reservoir if eruptible magma is not present upper crustal reservoirs may rapidly solidify following collapse without further eruption allowing more mafic compositions to ascend to the surface with only limited upper crustal modification resulting in edifice regrowth at temporarily elevated eruption rates subsequent re establishment of an upper crustal reservoir further supports a relationship between surface loading and crustal storage but long term chemical and mineralogical differences between pre and post collapse evolved magmas imply that a newly developed reservoir can overprint the influence of a preceding reservoir forming a spatially and compositionally distinct plumbing system these broad patterns are replicated in intraplate settings despite differences in scale and melting processes current evidence suggests that post collapse evolution of intraplate volcanoes can be explained by unloading induced destabilisation of the magma plumbing system rather than increased melt production what emerges from an apparently diverse set of observations is a systematic behaviour that strongly supports a coupling between edifice growth and magma ascent storage and pressurisation eruption rates erupted compositions and the style of volcanism at any particular system may thus be modulated from the surface and long term shifts in surface behaviour may occur without any changes in the deep parts of magmatic systems observations of sharp post collapse changes in erupted compositions including the ascent of primitive mafic magmas also require a crystal dominated mid to upper crustal reservoir consistent with recent models of crustal magmatic systems

three dimensional seismic velocity tomography of montserrat from the sea calipso offshore onshore experiment 1 the sea calipso experiment in december 2007 incorporated a sea based airgun source and seismic recorders both on montserrat and on the adjacent sea floor a high quality subset of the data was used for a first arrival p wave velocity tomographic study a total of more than 115 000 traveltime data from 4413 airgun shots and 58 recording stations were used in this high resolution tomographic inversion the experiment geometry limited the depth of well resolved structures to about 5 km the most striking features of the tomography are three relatively high velocity zones below each of the main volcanic centers on montserrat and three low velocity zones flanking centre hills we suggest that the high velocity zones represent the solid andesitic cores of the volcano complexes characterized by wave speeds faster than adjacent volcaniclastic material the low velocity zones may reflect porous volcaniclastic material and or alteration by formerly active hydrothermal systems

laboratory study on scattering characteristics of shear waves in rock samples to understand the scattering characteristics of seismic shear waves in the earth a laboratory physical model experiment in ultrasonic frequency range was performed by analyzing waveform envelopes and particle motions of shear waves in media with different correlation distance and fluctuation intensity we studied the strength of scattered wave excitation and the distortion of shear wave polarization as a function of ka wavenumber times the correlation distance we used gabbro and granite as media having different small scale random heterogeneities the granite we used contained preferred oriented thin microcracks the correlation distance a and the standard deviation of the fractional fluctuation of the shear wave velocity were estimated by fitting exponential type autocorrelation functions the estimated values were a 0 84 mm and 8 1 for the gabbro and a 0 39 mm and 17 0 for the granite waveform measurements were performed in the frequency range 0 25 1 mhz which roughly corresponds to ka as 0 25 1 envelope broadening appeared when ka exceeds 0 4 for the granite and 1 4 for the gabbro shear wave polarizations were distorted by scattering when ka exceeds 0 2 for the granite and 0 7 for the gabbro the results on envelope broadening suggest the importance of large angle scattering due to small scale random heterogeneities and cracks in the envelope broadening in addition to diffraction effects due to large scale heterogeneities the effect of aligned microcracks on scattering was also examined it was found that scattering was prominent when shear wave propagates perpendicular to the aligned crack plane manuscript received 1 march 2002

gps measurement of surface deformation around soufriere hills volcano montserrat from october 1995 to july 1996 global positioning system geodesy was used to measure surface deformation on soufriere hills volcano montserrat from october 6 1995 to july 1 1997 during initial dome growth and gravitational collapse our data from this period show non axially symmetric horizontal displacements and decreasing subsidence as a function of radial distance from the former topographic high of the volcanic edifice forward modeling suggests that surface deformation is caused by a shallow vertical dike 3 km which expanded approximately 1 m coupled with a deflating mogi source at about 6 km depth these inferred source parameters are in good agreement with independent observations of regional dike widths and preemption magma storage depth

lava production at soufriere hills volcano montserrat 1995 2009 1 we estimate that about 1 km3 of andesitic lava has been produced at soufriere hills volcano montserrat from 1995 to 2009 there were three major episodes of extrusion each lasting about 2 to 3 5 years and producing about 280 to 340 m m3 of lava and one minor episode our estimates account for the dense rock equivalent volumetric contributions from the core and talus components of the lava dome pyroclastic flow deposits and air fall deposits by 2005 at least two thirds of the erupted mass has already entered the sea the average lava flux across the major extrusion episodes has been 3 5 m3s 1 with short period 10 15 days pulses up to 10 20 m3s 1 the first and third episodes of extrusion show similar flux histories suggesting similar behaviour of the system ten years apart waning flux towards the end of each episode may be caused by declining overpressure in the magma reservoir

possible indicators of microbial mat deposits in shales and sandstones examples from the mid proterozoic belt supergroup montana u s a abstract it has been suspected for some time that microbial mats probably colonized sediment surfaces in many terrigenous clastic sedimentary environments during the proterozoic however domination of mat morphology by depositional processes post depositional compaction and poor potential for cellular preservation of mat building organisms make their positive identification a formidable challenge within terrigenous clastics of the mid proterozoic belt supergroup a variety of sedimentary structures and textural features have been observed that can be interpreted as the result of microbial colonization of sediment surfaces among these are a domal buildups resembling stromatolites in carbonates b cohesive behaviour of laminae during soft sediment deformation erosion and transport c wavy crinkly character of laminae d bed surfaces with pustular wrinkled appearance e rippled patches on otherwise smooth surfaces f laminae with mica enrichment and or randomly oriented micas g irregular curved wrinkled impressions on bedding planes h uparched laminae near mud cracks resembling growth ridges of polygonal stromatolites and i lamina specific distribution of certain early diagenetic minerals dolomite ferroan carbonates pyrite although in none of the described examples can it irrefutably be proven that they are microbial mat deposits the observed features are consistent with such an interpretation and should be considered indicators of possible microbial mat presence in other proterozoic sequences

estimating volcanic deformation source parameters with a finite element inversion the 2001 2002 unrest at cotopaxi volcano ecuador deformation at cotopaxi was observed between 2001 and 2002 along with recorded seismicity beneath the northeast ne flank despite the fact that the last eruption occurred in 1942 we use electronic distance meter deformation data along with the patterns of recorded seismicity to constrain the cause of this unrest episode to solve for the optimum deformation source parameters we employ inverse finite element fe models that account for material heterogeneities and surface topography for a range of source shapes the models converge on a shallow reservoir beneath the southwest sw flank the individual best fit model is a small oblate shaped source approximately 4 5 km beneath the summit with a volume increase of roughly 20 10 6 m 3 this sw source location contrasts with the ne seismicity locations subsequently further fe models that additionally account for temperature dependent viscoelasticity are used to reconcile the deformation and seismicity simultaneously comparisons of elastic and viscous timescales allude to aseismic pressurization of a small magma reservoir in the sw seismicity in the ne is then explained through a mechanism of fluid migration from the sw to the ne along fault systems we extend our analyses to further show that if future unrest crises are accompanied by measurable seismicity around the deformation source this could indicate a higher magma supply rate and increased likelihood of a forthcoming eruption

fracturing of volcanic systems experimental insights into pre eruptive conditions conditions for fracturing are a primary control on the behaviour of volcanic systems especially during the approach to eruption we here present the results of deformation experiments under simulated volcanic conditions on a porhyritic andesite from ancestral mount shasta andesite was chosen as a representative material because it is common at subduction zone volcanoes among both erupted products and country rock we deformed the lava in tension and triaxial compression tests at strain rates of 10 5 s 1 confining pressures from 0 to 50 mpa and temperatures up to 900 degrees c we also concurrently recorded acoustic emissions ae in order to monitor cracking activity the results show that deformation behaviour changes significantly in the temperature range 600 750 degrees c thus as temperatures increased across this interval the tensile fracture toughness increased from 2 5 0 5 mpa m 1 2 to 3 5 1 mpa m 1 2 the compressive strength decreased from 110 30 mpa to 55 35 mpa at 900 degrees c and the corresponding young s modulus decreased from 20 4 gpa to 6 4 gpa the changes occur when the deformation of the sample changes from elastic brittle to brittle ductile behaviour which we attribute to the blunting of crack tips due to melting of the glass phase and enhanced crystal plasticity at high temperature ae activity was observed in all experiments indicating that earthquakes can be generated not only in country rock but also in hot magma such as may be found in lava domes and at the margins of magma conduits in addition the trends in accelerating ae event rates before sample failure were comparable to those seen in earthquakes before some volcanic eruptions and a minimum in the seismic b value coincided with sample failure applied to volcanic systems the results suggest that 1 andesite strength and elasticity will not be affected by temperature or pressure beyond similar to 10 100 m from active magma 2 before eruptions fractures propagate preferentially through weaker horizons in a mechanically heterogeneous volcano and 3 volcanic rocks have characteristic seismic b values that are perturbed during the approach to bulk failure each of these conditions provides quantitative constraints on models for seismic precursors to eruption or intrusion c 2009 elsevier b v all rights reserved

magmatic metering controls the stopping and restarting of eruptions 1 we use time series of magma efflux and gps derived surface deformation observations to constrain the transfer of compressible magma within the crustal plumbing of the soufriere hills volcano for three cycles of effusion and pause our system model has two vertically stacked spherical chambers deep melt supply to the system is constrained to be continuous and steady yielding a rate of 1 2 m3 s which fixes the geometry of the dual interconnected chambers to depths of about 5 and 19 km the eruptive volume change of the shallow chamber is in phase and an order of magnitude smaller than the deep chamber significantly the shallow chamber seems to control the periodic system behavior surface magma efflux resumes when the shallow chamber reinflates to its initial threshold pre eruptive volume triggering re opening of an eruptive feeder dike and ceases when it has lost 14 22 mm3 106 m3 of its volume sealing the conduit and staunching magma flow these observations are consistent with eruption re initiation and re cessation controlled by magma overpressure thresholds

thermomechanics of shallow magma chamber pressurization implications for the assessment of ground deformation data at active volcanoes abstract in this study we utilize thermomechanical models to investigate how magma chambers overpressurize as the result of either magmatic recharge or volatile exsolution by implementing an adaptive reservoir boundary condition we are able to track how overpressure dissipates as the magma chamber expands to accommodate internal volume changes we find that the size of the reservoir greatly impacts the resultant magma chamber overpressure in particular overpressure estimates for small to moderate sized reservoirs 1 10 km 3 are up to 70 lower than previous analytical predictions we apply our models to santorini volcano in greece where recent seismic activity and ground deformation observations suggested the potential for eruption the incorporation of an adaptive boundary condition reproduces mogi flux estimates and suggests that the magma reservoir present at santorini may be quite large furthermore model results suggest that if the magma chamber is 100 km 3 overpressures generated due to the high magma flux may not exceed the strength of the host rock thus requiring an additional triggering mechanism for eruption although the adaptive boundary condition approach does not calculate the internal evolution of the magma reservoir it represents a fundamental step forward from elastic mogi models and fixed boundary solutions on which future investigations of the evolution of the magma can be built

crustal structure beneath the montserrat region of the lesser antilles island arc 1 we use receiver function analysis to estimate the first order subsurface seismic structure in the vicinity of the island of montserrat located in the northern lesser antilles arc and home of the active soufriere hills volcano near surface complexity in the island structure inhibits our ability to resolve lateral variations in seismic structure so we average the signals from eight stations in two directions to produce a first order regional seismic velocity model although we are unable to resolve a precise sharpness of the crust mantle transition we can limit its thickness to less than 4 km lateral variations in shallow structure complicate a receiver function based estimate of poisson s ratio but the data indicate that in both directions sampled by the receiver functions the lower crustal poisson s ratio is greater than 0 27 and more likely in the range 0 29 0 30 we estimate a mean crustal thickness of 30 km and the observations suggest that the crust may be slightly thinner northwest of the island 26 30 km than it is to the south 30 34 km the high p wave speed and poisson s ratio indicate a generally mafic lower crust with rocks of intermediate composition not precluded in the upper part of the lower crust we did not find evidence for a thick high speed lower crust 7 4 km s as has been inferred in some other arcs

temperature dependent viscoelastic modeling of ground deformation application to etna volcano during the 1993 1997 inflation period abstract we used the finite element method fem for modeling time dependent ground deformation due to volcanic pressure sources embedded in a viscoelastic medium especially in volcanic areas the presence of heterogeneous materials and high temperatures produce a lower effective viscosity of the earth s crust that calls for considering the thermal regime of crustal volume surrounding the magmatic sources we propose a thermo mechanical numerical model for evaluating the temperature dependency of the viscoelastic solution both temperature distributions and ground deformation are evaluated by solving an axi symmetric problem to estimate the effects of thermo viscoelastic response of the medium the thermo mechanical model permits to evidence that viscoelastic relaxation is responsible for significant time dependent variations in long term deformation these effects may be relevant for the interpretation and quantitative assessments of the pressure changes within magmatic sources with this in mind we reviewed the ground deformation observed on etna volcano during the 1993 1997 inflation period by setting up a fully 3d temperature dependent viscoelastic model since 1993 different geodetic measurements edm gps sar and leveling data identified an inflationary phase characterized by a uniform and continuous expansion of the overall volcano edifice that was not perturbed by eruptive activity the numerical model including significant viscoelastic material and reduced crustal rigidity around the magmatic source enables to produce deformation comparable with those obtained from elastic model requiring a significantly lower pressure for a purely elastic model with the same geometry and rigidity the pressure change necessary to describe the 1993 through 1997 inflation is around 320 mpa whereas for the viscoelastic model a pressure increase of about 200 mpa is required

on the geophysical fingerprint of vulcanian explosions abstract extrusion of viscous magma and the subsequent formation of a lava dome is often interspersed by short lived vigorous vulcanian explosions the causes for and the timing of the transition from effusive to explosive activity during dome formation are poorly understood and forecasting this transition remains a challenge here we describe and interpret a robust and unique multi parameter data set documenting the subsurface processes associated with vulcanian explosions at soufriere hills volcano montserrat w i in july and december 2008 we quantify explosion priming by processes in either the shallow

implications of magma transfer between multiple reservoirs on eruption cycling volcanic eruptions are episodic despite being supplied by melt at a nearly constant rate we used histories of magma efflux and surface deformation to geodetically image magma transfer within the deep crustal plumbing of the soufriere hills volcano on montserrat west indies for three cycles of effusion followed by discrete pauses supply of the system from the deep crust and mantle was continuous during periods of reinitiated high surface efflux magma rose quickly and synchronously from a deflating mid crustal reservoir at about 12 kilometers augmented from depth during repose the lower reservoir refilled from the deep supply with only minor discharge transiting the upper chamber to surface these observations are consistent with a model involving the continuous supply of magma from the deep crust and mantle into a voluminous and compliant mid crustal reservoir episodically valved below a shallow reservoir at about 6 kilometers

on the influence of magma chambers in controlling the evolution of explosive volcanic eruptions abstract the influence of magma chambers on the evolution of explosive volcanic eruptions is examined by coupling a model of an elastic chamber containing compressible magma bower s m woods a w 1997 the control of magma volatile content and chamber depth on the mass erupted during explosive volcanic eruptions j geophys res 102 10273 10290 with a model of conduit flow wilson l sparks r s j walker g p l 1980 explosive volcanic eruptions iv the control of magma properties and conduit geometry on eruption column behaviour geophys j r astr soc 63 117 148 it is found that for a given magma volatile content and conduit geometry variations in the chamber volume depth and aspect ratio have a significant impact on the mass erupted and the eruption duration shallow volatile saturated chambers of large cross sectional area tend to erupt a much larger fraction of the chamber volume 10 than deep volatile unsaturated chambers of the same magma 0 1 1 0 however in either case the steady decrease in chamber pressure over the course of an eruption only leads to a decrease in eruption rate of 10 50 therefore for a given chamber volume shallow chambers of large cross sectional area produce much more long lived and voluminous explosive eruptions in contrast we show that for a given chamber geometry variations in magma volatile content can lead to significant changes in both the eruption rate and the erupted volume in turn this also affects the eruption duration finally we note that as described by wilson et al increases in conduit radius lead to very large increases in eruption rate and hence a decrease in eruption duration

shallow crustal mechanics from volumetric strain data insights from soufriere hills volcano montserrat volumetric strain data from the 29 july 2008 vulcanian explosion of the soufriere hills volcano provide an excellent opportunity to probe the mechanical properties of the volcanic edifice and the shallow crust beneath montserrat we use finite element analysis to constrain the geometry of the shallow plumbing system as well as edifice and crustal mechanical properties for mechanically plausible pressure drops associated with dilatational volumetric strains recorded during the explosive activity our results from both forward and inverse models indicate a conduit radius of 40 m and a length of 1500 m and hence much larger conduit dimensions than previously suggested in order to fit the syneruptive volumetric strain data for a conduit pressure drop of 10 mpa the conduit needs to be surrounded by a halo of mechanically compliant rocks for which the best fit models indicate a width of 600 m and a young s modulus of 1 gpa young s moduli for the main edifice and the shallow crust up to 1000 m below sea level are found to be 10 gpa our best fit inverse model predicts a syneruptive radial conduit contraction by 0 24 m with a corresponding volume loss of 0 1 mm3 implying only partial emptying of the conduit upon the explosion this study demonstrates the critical role of edifice and shallow crustal mechanics for strain partitioning on montserrat our findings may have implications for the assessment of conduit processes at other dome building volcanoes

temporal change in attenuation intensity before and after the eastern yamanashi earthquake of 1983 in central japan a crutal earthquake of magnitude 6 0 took place on august 8 1983 in the middle of the kanto tokai microearthquake observation network of the national research center for disaster prevention in central japan by analyzing vertical seismograms of microearthquakes occurring in a small volume in the aftershock region we measured temporal change in attenuation intensity for a period from 22 months before to 9 months after the main shock attenuation change was estimated by using two different kinds of analyses in the first analysis station magnitude calculated from the maximum amplitude at each station was compared with the average of station magnitudes deviations of station magnitude from the average magnitude at five stations in the vicinity of the aftershock area are found to have increased after the main shock compared with that before in the second analysis s coda duration from the origin time was measured and was compared with the average magnitude the increase of s coda duration for the same average magnitude is found after the main shock at two stations close to the main shock epicenter the simplest interpretation of the results is that the attenuation intensity in the upper crust in the close vicinity of the focal region decreased after the main shock for frequencies f from 2 to 20 hz qs 1 1 4 10 2f 1 since other precursors appeared in the preparation stage it is possible to interpret the change in qs 1 as a precursor the attenuation intensity would have been anomalously large before the main shock

remarkable preservation of microbial mats in neoproterozoic siliciclastic settings implications for ediacaran taphonomic models abstract it is beyond doubt that the appearance of infaunal bioturbation and metazoan biomineralization across the ediacaran cambrian transition irreversibly affected the nature of marine sediment architecture and biogeochemistry here we review those changes in relation to their likely effect upon the processes of fossil preservation especially within siliciclastic sediments processes of soft tissue preservation in siliciclastic settings from the ediacaran period including microbes and microbial mats as well as ediacaran macrofossils are here reviewed within this context highlighted examples include the exceptional preservation of microbes found in association with wrinkle structures and ediacaran macrofossils in england and newfoundland replicated by silicate minerals and in the white sea region of russia replicated by iron sulphide these occurrences show that soft tissue preservation in siliciclastic settings went well beyond that typical for ediacaran macrofossils alone and also extended to similar modes of preservation in associated microbes using these new observations it can be argued that several existing explanations for ediacaran fossil preservation can be united within a biogeochemical model that involves evolution of the sediment mixed layer across this transition

catastrophic lava dome failure at soufriere hills volcano montserrat 12 13 july 2003 the lava dome collapse of 12 13 july 2003 was the largest of the soufriere hills volcano eruption thus far 1995 2005 and the largest recorded in historical times from any volcano 210 million m3 of dome material collapsed over 18 h and formed large pyroclastic flows which reached the sea the evolution of the collapse can be interpreted with reference to the complex structure of the lava dome which comprised discrete spines and shear lobes and an apron of talus progressive slumping of talus for 10 h at the beginning of the collapse generated low volume pyroclastic flows it undermined the massive part of the lava dome and eventually prompted catastrophic failure from 02 00 to 04 40 13 july 2003 large pyroclastic flows were generated these reached their largest magnitude at 03 35 when the volume flux of material lost from the lava dome probably approached 16 million m3 over two minutes the high flux of pyroclastic flows into the sea caused a tsunami and a hydrovolcanic explosion with an associated pyroclastic surge which flowed inland a vulcanian explosion occurred during or immediately after the largest pyroclastic flows at 03 35 13 july and four further explosions occurred at progressively longer intervals during 13 15 july 2003 the dome collapse lasted approximately 18 h but 170 of the total 210 million m3 was removed in only 2 6 h during the most intense stage of the collapse

the effects of thermomechanical heterogeneities in island arc crust on time dependent preeruptive stresses and the failure of an andesitic reservoir using ground deformation data from soufriere hills volcano shv we present results from numerical modeling of the temperature and time dependent stress evolution in a mechanically heterogeneous crust prior to reservoir failure and renewed eruptive activity the best fit models do not allow us to discriminate between a magmatic plumbing system consisting of either a single vertically elongated reservoir or a series of stacked reservoirs a prolate reservoir geometry with volumes between 50 and 100 km3 reservoir pressure changes between 4 and 7 mpa and reservoir volume changes between 0 03 and 0 04 km3 with magma compressibility between 4 10 11 and 1 10 9 pa 1 provide plausible thermomechanical model parameters to explain the deformation time series around an order of magnitude less overpressure than is generally inferred from homogeneous elastic crustal models reservoir failure is predicted to occur at the crest of the reservoir except for reservoirs with highly compressible magma 4 10 9 pa for which subhorizontal sill formation is predicted upon reservoir failure introducing a deep crustal hot zone modulates the partitioning of strains into the hotter underlying crust and results in a further reduction in overpressure estimates to values of around 1 2 mpa upon reservoir failure deduced volume fluxes are consistent with constraints from thermal modeling of active subvolcanic systems and imply dynamic failure of a compressible magma mush column feeding eruptions at shv our interpretation of the results is that the combined thermomechanical effects of a deep crustal hot zone and hot encasing rocks around a midcrustal andesitic reservoir fundamentally alter the time dependent subsurface stress and strain partitioning upon reservoir priming these effects substantially influence surface strains recorded by volcano geodetic monitoring

interpretation of seismic data from hydraulic fracturing experiments at the fenton hill new mexico hot dry rock geothermal site an attempt was made to synthesize the results of active seismic experiments carried out by the los alamos national laboratory s hot dry rock project staff for determining the geometrical and physical properties of the fracture system produced by hydraulic fracturing in a hot low permeability rock interpretation of data from several reflection transmission and attenuation experiments using seismic probes in the frequency range from a few hundred to more than 10 sup 4 hz led us to postulate the existence of a highly complex fracture system consisting of major discrete vertical cracks intersected by several inclined joints which are surrounded by a large volume of rock containing small scale cracks we found an interesting coincidence between the mean square fluctuation of p arrival time and q sup 1 of coda waves both are increased by nearly the same amount 35approx 40 when fluid pressure in the reservoir is increased another coincidence is the scale length of the inhomogeneity of roughly 3 m obtained from the frequency dependence of attenuation and also from the spatial variation of p arrival time these results suggest that the seismic attenuation in the fractured region is caused primarily by scattering

fossils and matgrounds from the neoproterozoic longmyndian supergroup shropshire uk body and trace fossils of ediacaran affinity are described from strata of the late neoproterozoic longmyndian supergroup exposed near church stretton shropshire uk the almost spherical soft bodied ediacaran fossil beltanelliformis brunsae menner occurs rarely in the burway formation but much smaller simpler discoidal structures are common in both the burway and synalds formations and are referred to beltanelliformis minutae sp nov similar discoidal structures but with a distinct central depression are included in intrites punctatus fedonkin and are common at several horizons two blocks with numerous examples of medusinites aff asteroides sprigg glaessner wade were recovered from the burway formation the purported ediacaran body fossil arumberia glaessner walter is common at several horizons but its biogenicity is not accepted herein arumberia is thus treated along with evidence for microbially bound sediment surfaces or matgrounds that have been suggested by several authors to be necessary for some types of ediacaran preservation the assemblage of simple trace and body fossils along with matgrounds is typical of latest neoproterozoic time though some elements range into the phanerozoic

the dynamic influence of microbial mats on sediments fluid escape and pseudofossil formation in the ediacaran longmyndian supergroup uk microbial mats are thought to have been widespread in marine settings before the advent of bioturbation and the range of their influence on sediments is gradually becoming recognized we propose that mat sealing can dynamically affect porewater conditions and allow the build up of overpressure that can drive dewatering and degassing to produce a suite of atypical fluid escape features finely bedded silty and sandy laminae from the c 560 ma burway formation of the longmyndian supergroup shropshire england reveal evidence for sediment injection including disrupted bedding clastic injections sill like features and sediment volcanoes at sub millimetre scale these features are associated with crinkly laminae diagnostic of microbial matgrounds matground associated sediment injection can explain the formation of several types of enigmatic discoidal impressions common in rocks of this age which have previously been attributed to the ediacaran macrobiota serial grinding of longmyndian forms previously described as medusinites aff asteroides and beltanelliformis demonstrates that such discoidal features can be fully explained by fluid escape and associated load structures our observations emphasize the non actualistic nature of shallow marine ediacaran sediments matground associated sediment injection features provide a new insight into the interpretation of proterozoic rocks and the biogenicity of their enigmatic discoidal markings supplementary materials a document containing further images of fluid escape and loading features observed in the upper burway formation at ashes hollow together with an annotated diagram of features appearing in one typical vertical cross section is available at www geolsoc org uk sup18870

deposit loading and its effect on co eruptive volcano deformation abstract erupting volcanoes commonly exhibit characteristic ground deformation that is typically interpreted in terms of pressure changes of magma reservoirs within the crust however other processes may also be significant since 1995 the soufriere hills volcano montserrat has erupted about 1 km3 of magma over five discrete extrusive phases with clear cycles of associated ground deformation recorded by gps here we consider the contribution to deformation by loading of the ground surface with erupted deposits we estimate topographic change and net deformation for the whole eruption to date between november 1995 and february 2010 we derive the surface load distribution using differenced digital elevation data which additionally enables us to constrain the budget of erupted lava about a third of the lava erupted from soufriere hills since 1995 remains in subaerial onshore deposits more than previously thought another third is emplaced immediately offshore and the remaining third has been transported further afield we combine the deposit thickness map with representative deposit densities to calculate surface load and model the deformation response using finite elements our results show that net displacements accumulated over 14 years on montserrat tens of centimetres could be explained by loading of erupted deposits on the flanks the proportion of the observed deformation that can be explained by loading alone depends on crustal rheology using rheology structures favoured in the literature our forward modelled displacements are remarkably similar to long term observations down to detail that we ascribe to localised load topography interaction results suggest that the shallow crust beneath montserrat is more compliant than usually assumed in geodetic models with more rigid rheology at depth loading is largely accommodated by elastic strain in the shallow crust top few kilometres with negligible contribution from intra crustal viscous flow over the time period investigated we thus infer that the role of stress transfer from the surface load in metering magma reservoir behaviour must be negligible but may influence degassing in the shallow conduit for example our findings suggest that when volcanic ground deformation accompanies a voluminous eruption geodetic model inversions will be misled if data are not appropriately corrected for the surface loading effect

long term surface deformation of soufriere hills volcano montserrat from gps geodesy inferences from simple elastic inverse models 1 campaign and continuous gps geodetic measurements on soufriere hills volcano montserrat are reported from 1995 to 2009 spanning three dome growth and repose episodes uniform elastic half space inversions were used to examine how crustal pressure sources evolved by inverting subsets of all available 3d site data for any given episode using a single mogi source changes in network topology were also examined the average best fitting single mogi model yields x 0 3 0 5 km y 0 8 0 4 km and a depth of z 10 4 2 1 km 1 z positive down relative to the center of the model domain the mean mogi depth for effusive deflation versus repose inflation is different and significant at 90 confidence yielding z 11 4 2 0 km and 9 3 1 6 km respectively a vertical prolate ellipsoid improves the fit at 95 confidence over a single mogi source or stacked two source model for the 2003 2005 repose and 2005 2007 dome growth episodes

unprecedented pressure increase in deep magma reservoir triggered by lava dome collapse 1 the collapse of the soufriere hills volcano lava dome on montserrat in july 2003 is the largest such event worldwide in the historical record here we report on borehole dilatometer data recording a remarkable and unprecedented rapid 600s pressurisation of a magma chamber triggered by this surface collapse the chamber expansion is indicated by an expansive offset at the near dilatometer sites coupled with contraction at the far site by analyzing the strain data and using added constraints from experimental petrology and long term edifice deformation from gps geodesy we prefer a source centered at approximately 6 km depth below the crater for an oblate spheroid with overpressure increase of order 1 mpa and average radius 1 km pressurisation is attributed to growth of 1 3 of gas bubbles in supersaturated magma triggered by the dynamics of surface unloading recent simulations demonstrate that pressure recovery from bubble growth can exceed initial pressure drop by nearly an order of magnitude

quantification of the intrusive magma fluxes during magma chamber growth at soufriere hills volcano montserrat lesser antilles magma fluxes in the crust control the thermal viability and mechanical stability of magma chambers we estimated the magma fluxes required to generate the negative seismic velocity anomaly observed below soufriere hills volcano montserrat growth of a magma body by accretion of andesitic sills was simulated numerically and the resulting temperatures and melt fractions were used to calculate a synthetic anomaly of seismic wave velocity which was filtered to be comparable with the velocity anomaly obtained from a tomographic experiment petrology indicates that before it was reheated remobilized and erupted the temperature of the magma residing in the chamber was about 850 c we ran simulations where convection is assumed to be low and heat transfer is mostly by conduction and simulations where convection is assumed to be vigorous enough to rapidly cool the magma chamber to 850 c in both cases magma chamber growth over the last 350 years results in tomography anomalies that are too strong unless the magma was emplaced at an unlikely low melt fraction 0 5 good fits between the modelled and the observed velocity anomaly were obtained with sills 2 5 km in radius emplaced over 6000 150 000 years depending on the temperature and melt fraction of the emplaced magma because of a trade off between intrusion dimensions and emplacement durations the volumetric magma fluxes are restricted to 7 10 4 and 5 10 3 km3 a 1 the velocity anomaly can be reproduced with a chamber containing high melt fraction magma or with a mush of crystals and melt the range of magma ages in the modelled magma chamber is much wider than the crystal residence time of the erupted andesite this suggests that the eruption taps small pockets of recently assembled magma and that the velocity anomaly is mostly due to a non eruptible mush

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

cambrian burgess shale animals replicated in clay minerals although the evolutionary importance of the burgess shale is universally acknowledged there is disagreement on the mode of preservation of the fossils after burial elemental mapping demonstrates that the relative abundance of elements varies between different anatomical features of the specimens these differences reflect the compositions of the minerals that replicated the decaying organism which were controlled by contrasts in tissue chemistry delicate morphological details are replicated in the elemental maps showing that authigenic mineralization was fundamental to preserving these fossils even though some organic remains are also present

volcanism and the dynamics of open magma chambers the repeated eruption of many volcanoes is now accounted for by events taking place in a subterranean magma chamber which is open in the sense that the inflow of material to the chamber from below leads to a magma pressure p which exceeds the lithostatic pressure pl by an amount greater than the tensile strength of the chamber walls before the contents of the magma chamber have reached thermodynamic and geochemical equilibrium this is the condition which must be satisfied when an eruption occurs we have now derived a relationship giving the critical volume of added material v required to cause an eruption as a function of magma compressibility and the volume by which the chamber expands

microbial mats implicated in the generation of intrastratal shrinkage synaeresis cracks intrastratal shrinkage often termed synaeresis cracks are commonly employed as diagnostic environmental indicators for ancient salinity stressed transitional fluvial marine or marginal marine depositional environments despite their abundance and use in facies interpretations the mechanism of synaeresis crack formation remains controversial and widely accepted explanations for their formation have hitherto been lacking sedimentological ichnological petrographic and geochemical study of shallow marine mudstone beds from the ordovician beach formation of bell island newfoundland has revealed that crack development cf synaeresis cracks on the upper surface of mudstone beds is correlated with specific organic geochemical and sedimentological parameters contorted sinuous sand filled cracks are common at contacts between unbioturbated mudstone and overlying sandstone beds cracks are absent in highly bioturbated mudstone and are considered to pre date firmground assemblages of trace fossils that include planolites and trichophycus the tops of cracked mudstone beds contain up to 2 1 wt total organic carbon relative to underlying mudstone beds that contain around 0 5 wt total organic carbon high resolution carbon isotope analyses reveal low 13corg values 27 6 on bed tops compared with sandy intervals lacking cracks 24 4 to 24 9 cracked mudstone facies show evidence for microbial matgrounds including microbially induced sedimentary structures on bedding planes and carbonaceous laminae and tubular carbonaceous microfossils in thin section non cracked mudstone lacks evidence for development of microbial mats microbial mat development is proposed as an important prerequisite for intrastratal shrinkage crack formation both microbial mats and intrastratal shrinkage cracks have broad palaeoenvironmental distributions in the precambrian and early phanerozoic in later phanerozoic strata matgrounds are restricted to depositional environments that are inhospitable to burrowing and surface grazing macrofauna unless evidence of synaeresis i e contraction of clay mineral lattices in response to salinity change can be independently demonstrated the general term intrastratal shrinkage crack is proposed to describe sinuous and tapering cracks in mudstone beds

time dependent failure of rock under cyclic loading abstract a simple model which describes the failure of brittle rock under cyclic compressive loading is presented it is assumed that the damage incurred by the material during each stress cycle is due to the extension of tensile microcracks and that the specimen will fail when the microcrack damage reaches a critical level two mechanisms for crack extension stress corrosion and cyclic fatigue are assumed to act simultaneously and independently the rate of crack growth due to stress corrosion was determined from an empirical relationship for the static fatigue of glass the equation describing the rate of crack growth due to cyclic fatigue was taken to be similar to those derived for structural metals by combining the crack extension rates due to these two processes the time to failure for a specimen subjected to a given cyclic loading program was computed the time to failure was found to depend strongly on the mean differential stress and the magnitude of the stress change during each cycle at high mean stress and low cycle amplitude stress corrosion dominates whereas at low mean stress and high cycle amplitude cyclic fatigue dominates the predictions of the model are in good agreement with existing experimental data further the model explains the observed breakdown of discrete memory in dilatant rock under certain stress conditions

the relationship between microseismicity and high pore pressures during hydraulic stimulation experiments in low permeability granitic rocks several hundred microearthquakes with local magnitudes ranging between 4 and 2 were recorded during hydraulic stimulation experiments at the hot dry rock demonstration site fenton hill new mexico these events are probably caused by shear failure induced by high pore fluid pressures since the event locations seem to cluster in a narrow band near the hydraulic fracture we were able to use micro seismic techniques to locate the hydraulic fracture and monitor its growth we calculate the minimum pore pressure increase necessary to induce rock failure using a simple model of slippage on preexisting fractures by comparing a pore pressure distribution calculated using a one dimensional diffusion model with the distribution of event foci we demonstrate that high pore pressures are a probable cause for these microearthquakes stress drops for these microearthquakes calculated from spectral parameters increased by a factor of 3 during the course of the experiment the stress drop increase correlates with a migration of seismic activity away from the injection well bore which suggests relatively low in situ shear stresses near the well bore caused by strain release in this region during previous pressurizations

constraints on magma flux from displacements data at merapi volcano java indonesia the displacement field has been monitored in the vicinity of the crater rim at mount merapi indonesia from 1993 to 1997 during this period the volcanic activity has been quasicontinuous with dome growth explosions and pyroclastic flows we measured a nine point network every year with the global positioning system static method interpretation of results is conducted with a three dimensional elastostatic boundary elements code that takes into account topography fractures and complex magma source geometry the inversion technique yields an estimate of the variation with time of the boundary conditions at the magma duct interface together with the probability associated with the best model the young s modulus of the equivalent continuum is found to be very low of the order of 1 gpa a feature which suggests that a viscoelastic behavior may be more appropriate for this rock mass given the observed seismic velocities for the domain of interest a striking compatibility is outlined between observed deformations and the rate of occurrence of multiphase seismic events once the main fractures of the structure have been taken into consideration this suggests that the summit elastic or viscoelastic deformation field is controlled by the magma flux within the duct rather than by magma pressure variations in addition a nonelastic displacement has been identified at the westernmost point of the network this was considered critical for the stability of the summit structure a concern whose validity has been verified a posteriori by the july 1998 explosion

textural and mineral chemistry constraints on evolution of merapi volcano indonesia abstract we analyze and compare the textures of merapi lavas basalts and basaltic andesites ranging in age from proto merapi through modern activity with the goal of gaining insights on the temporal evolution of merapi s magmatic system analysis of textural parameters such as phenocryst and microphenocryst crystallinity coupled with crystal size distribution theory provides information about the storage and transport of magmas we combine textural analyses with geochemical investigations for a comprehensive comparison of erupted lavas over time the chemical analyses identify crystal growth processes in magma chambers and underline differences between sample groups our work suggests the occurrence of two distinct histories presumably associated with at least two generally distinct types of rheological behaviors and storage transport systems these behaviors are associated with different plagioclase growth patterns with both groups influenced by late stage shallow decompression degassing induced microlite crystallization both groups contain amphibole crystals that indicate an early period of mid crustal to deep crustal storage of water rich magmas dome lavas from the 20th century eruptive activity indicate quasi steady state nucleation and growth evolution interspersed with episodes of reheating and textural coarsening suggesting residence in magma storage at multiple depths both 10 km and

properties of seismic wave scattering around water injection well at fenton hill hot dry rock geothermal site to determine the scattering properties of microcracks created by water injection in hot dry rock a method for analyzing trajectory ellipsoids of seismic particle motions has been applied lengths and directions of three principal axes of trajectory ellipsoids were calculated and a distribution of the shape of these ellipsoids suggests existence of a strong scattering region around the injection point this strong scattering has an anisotropic property presumably due to microcrack orientation in the original rock

sedimentology structural evolution and tectonic setting of the late precambrian longmyndian supergroup of the welsh borderland uk abstract the longmyndian supergroup is an approximately 6500 m thick sedimentary sequence of late precambrian age major unconformities within this sequence are lacking the longmyndian supergroup can be divided into four facies associations 1 turbidite 2 subaqueous delta 3 alluvial floodplain and 4 braided alluvial these are organized into a single broadly upwards coarsening progradational sequence with the turbidite association at the base and the braided alluvial association towards the top the sediments are interpreted to have been deposited in a forearc or possibly interarc basin which lay to the north of a contemporaneous magmatic arc of which the now adjacent uriconian volcanic complex is considered to have been a part the longmyndian supergroup has been deformed into a tight nne ssw trending syncline fault patterns and strain markers within the longmyndian indicate that it was deformed during sinistral transpression the adjacent pontesford linley and church stretton fault systems are considered to be major strike slip fault systems which were initiated at about the same time as the deformation of the longmyndian supergroup strike slip movements along these fault systems resulted in the juxtaposition of the uriconian volcanic complex with the longmyndian supergroup late precambrian early cambrian deformation is indicated by some stratigraphic and radiometric age dating evidence since there is evidence that the fault systems have suffered strike slip displacements during the palaeozoic it is difficult to be certain of the magnitude and style of the late precambrian early cambrian deformation however the recognition of late precambrian early cambrian sinistral transpressional deformation in northwest wales suggests that the precambrian of the welsh borderland was similarly deformed during the late precambrian early cambrian

magma tectonic interaction and the eruption of silicic batholiths abstract due to its unfavorable rheology magma with crystallinity exceeding about 50 vol and effective viscosity 106 pa s is generally perceived to stall in the earth s crust rather than to erupt there is however irrefutable evidence for colossal eruption of batholithic magma bodies and here we analyze four examples from spain mexico usa and the central andes these silicic caldera forming eruptions generated deposits characterized by i ignimbrites containing crystal rich pumice ii co ignimbritic lag breccias and iii the absence of initial fall out the field evidence is inconsistent with most caldera forming deposits which are underlain by initial fall out indicating deposition from a sustained eruption column before the actual collapse sequence in contrast the documented examples suggest early deep level fragmentation at the onset of eruption and repeated column collapse generating eruption volumes on the order of hundreds of cubic kilometers almost exclusively in the form of ignimbrites these examples challenge our understanding of magma eruptability and eruption initiation processes in this paper we present an analysis of eruption promoters from geologic theoretical and experimental considerations assessing relevant dynamics and timescales for failure of crystal melt mush we propose a framework to explain eruption of batholithic magma bodies that primarily involves an external trigger by near field seismicity and crustal failure strain rate analysis for dynamic and static stressing chamber roof collapse and rapid decompression indicates that large solid like silicic reservoirs may undergo catastrophic failure leading to deep level fragmentation of batholithic magma at approximately 2 orders of magnitude lower strain rates than those characteristic for failure of crystal poor magmas or pure melt eruption triggers can thus include either amplified pressure transients in the liquid phase during seismic shaking of a crystal melt mush decompression by block subsidence or a combination of both we find that the window of opportunity for the eruption of large silicic bodies may thus extent to crystallinities beyond 50 vol for strain rates on the order of 10 3 to 10 4 s 1

dynamics of magma storage in the summit reservoir of kilauea volcano hawaii an eruption of kilauea volcano hawaii normally results in a decrease in the internal pressure of a summit magma reservoir as it partly drains the interior of the volcano responses to the decrease in pressure 1 by elastic expansion of remaining magma and gas 2 additional exsolution of gas dissolved in the magma and 3 by contraction of the volcanic edifice this process is modeled here using gravity geodetic and observational data for recent events at kilauea during magma removal the ratio of pressure change in the magma reservoir to the volume of associated summit subsidence ranges from less than 0 22 pa m3 to more than 0 43 pa m3 lower ratios occur during large subsidences because of a large effective volume of the reservoir small events may involve source volumes of only 2 km3 in contrast a volume of near 13 km3 is associated with larger events because the boundary between plastic and elastic behavior may migrate outward with increased time and strain the aggregate compressibility of kilauea s reservoir is significantly increased by volumetric changes of the co2 gas phase by bulk compression and solution with pressure this effect is more pronounced for relatively shallow sources because the bulk modulus of gas is proportional to pressure the average reservoir co2 weight fraction may be near 0 0005 to 0 0009 co2 appears to accumulate in the upper portion of the reservoir where the concentration reaches 0 0030 weight fraction gradual long term subsidences are different from the short term deflations associated with specific eruptions during long term subsidence no net change in mass or pressure occurs within the summit reservoir rifting of the summit area which may tend to shorten the reservoir as it widens or slow degassing and contraction of stored magma may cause long term subsidence

the role of volatiles in magma chamber dynamics many andesitic volcanoes exhibit effusive eruption activity1 with magma volumes as large as 107 109 m3 erupted at rates of 1 10 m3 s 1 over periods of years or decades during such eruptions many complex cycles in eruption rates have been observed with periods ranging from hours to years2 3 4 5 6 7 longer term trends have also been observed and are thought to be associated with the continuing recharge of magma from deep in the crust and with waning of overpressure in the magma reservoir here we present a model which incorporates effects due to compressibility of gas in magma we show that the eruption duration and volume of erupted magma may increase by up to two orders of magnitude if the stored internal energy associated with dissolved volatiles can be released into the magma chamber this mechanism would be favoured in shallow chambers or volatile rich magmas and the cooling of magma by country rock may enhance this release of energy leading to substantial increases in eruption rate and duration

insights into cyanobacterial fossilization in ediacaran siliciclastic environments ediacaran sedimentary successions are noted for the preservation of microbes and microbial textures on the surfaces of sandstones and siltstones although microorganisms have been preserved in coarse grained siliciclastic sand throughout geologic history the exceptional preservation of microbes in ediacaran sediments suggests the potential for a unique taphonomic window here we identify conditions conducive to the fossilization of filamentous cyanobacteria growing in the presence of siliciclastic sand and demonstrate that the sheaths of filamentous cyanobacteria can become coated by clay minerals within days under oxic conditions smooth extensive mineral coatings develop in the presence of 5 6 to 55 6 mg l of suspended clay and 0 1 mm or greater concentrations of dissolved silica thus elevated concentrations of seawater silica and the delivery of suspended clays promote microbial preservation on sandy and silty surfaces these factors likely facilitated microbial fossilization in coarse grained siliciclastic sand throughout the ediacaran period and may have also contributed to microbial fossilization in siliciclastic deposits at other times throughout earth s history

control of magma flow in dykes on cyclic lava dome extrusion 1 lava dome eruptions are commonly characterized by large fluctuations in discharge rate with cyclic behaviour on time scalesranging from hours to decades examples include bezymianny volcano russia merapi java santiaguito guatemala mt st helens usa mt unzen japan and soufriere hills volcano montserrat previous models have assumed simple cylindrical conduits for magma transport but extrusions are mainly fed by dykes with cylindrical geometries developing only at shallow levels the widths of dykes embedded in an elastic medium are influenced by local magma pressure affecting flow rates and system dynamics strongly we develop a model for magma flow in dykes which predicts intense pulsations of magma extrusion for the case of a constant source pressure the period time scale is determined by the elastic deformation of the dyke walls and the length to width ratio of the dyke the dyke acts like a volumetric capacitor storing magma as pressure increases and then releasing magma in a pulse of extrusion for the soufriere hills volcano cyclic extrusions with time scales of a few weeks are predicted for dykes 300 500 m long and 3 6 m wide matching observations the model explains the sharp onset of tilt pulsations and seismic swarms citation costa a o melnik r s j sparks and b voight 2007 control of magma flow in dykes on cyclic lava dome extrusion geophys res lett 34 l02303 doi 10 1029 2006gl027466

magma chamber properties from integrated seismic tomography and thermal modeling at montserrat it is widely believed that andesitic magmas erupted at arc volcanoes are stored in shallow reservoirs prior to eruption but high resolution images of focused regions of magma in the shallow crust are rare we integrate seismic tomography with numerical models of magma chamber growth to constrain the magma chamber beneath soufriere hills volcano montserrat our approach reveals the characteristics and dynamics of the magmatic system with a level of detail that no single method has yet achieved the integrated analysis suggests that a magma chamber of 13 km3 with over 30 melt fraction formed between 5 5 and at least 7 5 km depth a significantly higher melt fraction than inferred from the seismic data alone the magma chamber may have formed by incremental sill intrusion over a few thousand years and is likely to be a transient geologically short lived feature these volume and geometry estimates are critical parameters to model eruption dynamics which in turn are key to hazard assessment and eruption forecasting

diverse microbially induced sedimentary structures from 1 ga lakes of the diabaig formation torridon group northwest scotland abstract the siliciclastic lacustrine rocks of the 1000 ma diabaig formation northwest scotland contain a remarkable diversity of macroscopic structures on bedding planes that can be compared with various kinds of microbially induced sedimentary structures miss field sedimentological investigations combined with laboratory analysis of bedding planes and petrographic study of thin sections have allowed us to characterise a range of depositional environments and document the spectrum of biological structures miss are reported from frequently subaerial environments through commonly submerged facies and down to permanently sub wavebase settings palaeoenvironmental conditions water depth exposure hydrodynamic energy control the distribution of miss within these facies this demonstrates that mat forming microbial communities were arguably well adapted to low light levels or periodic exposure some miss from the diabaig formation are typical of precambrian microbial mats including reticulate fabrics and old elephant skin textures in addition to these a number of new and unusual fabrics of putative microbial origin are described including linear arrays of ridges and grooves cf arumberia and discoidal structures that are comparable with younger ediacaran fossils such as beltanelliformis these observations indicate that benthic microbial ecosystems were thriving in freshwater lake systems 1000 ma and indicate how microbially induced sedimentary structures may be applied as facies indicators for proterozoic lacustrine environments the discovery of structures closely resembling ediacaran fossils cf beltanelliformis also serves to highlight the difficulty of interpreting simple discoidal bedding plane structures as metazoan fossils

effect of mechanical heterogeneity in arc crust on volcano deformation with application to soufriere hills volcano montserrat west indies 1 analyses of volcano surface deformation are commonly based on models that assume mechanical homogeneity of rocks surrounding the causative pressure source here we present a detailed study that shows the differences in deduced surface deformation caused by source pressurization accounting for either mechanical homogeneity or mechanical heterogeneity of encasing rocks in a volcanic arc setting using finite element models accounting for crustal heterogeneity from seismic data we test for a range of source geometries and intermediate crustal depths and explore the misfits of deduced source parameters from the two families of models in the second part of this study we test the results from the generic study against cgps data from two deformation periods the 2003 2005 ground inflation and the 2005 2007 ground deflation at soufriere hills volcano montserrat west indies to inform on source parameters accounting for a variable crustal rigidity with depth as deduced by seismic analysis beneath montserrat we find the data to be best explained by pressurization and depressurization of a slightly prolate midcrustal magma chamber that is centered between 11 5 and 13 km below sea level about 640 m ne of the active vent considering source dimension and source pressure changes we demonstrate that magma compressibility and viscoelasticity of host rocks considerably affect dynamics in the midcrustal magmatic system of soufriere hills volcano and need to be accounted for as first order effects in geodetic data analyses and modeling

arumberia glaessner walter a review of its potential for correlation in the region of the precambrian cambrian boundary arumberia is a large impression fossil which may be allied to the ediacaran fossils ernietta and pteridinium new finds and published evidence suggest that it was restricted to very shallow possibly non marine environments and to rocks of latest precambrian to lower cambrian age well preserved material from the longmyndian shows that it was associated with numerous small spherical structures at present it is only of broad stratigraphic use but as the wide range of characters observed becomes better understood it may prove more valuable especially as it occurs in otherwise poorly fossiliferous environments

constraints on an intrusive system beneath the soufriere hills volcano montserrat from finite difference modeling of a controlled source seismic experiment 1 the sea calipso wide angle seismic experiment revealed a high velocity region beneath the island of montserrat field recordings show a decrease in the amplitude of seismic signals crossing this high velocity region we constrain the geometry and nature of this attenuating body by forward modeling of the seismic wave field with a viscoelastic finite difference method we interpret the attenuation observed as caused by a scattering region which we model as a stochastic perturbation of the velocity field with characteristic length scale of 0 4 km the scattering region approximately coincides with the top part of the high velocity region and is estimated to have a volume of about 800 km3 we argue that the scattering is caused by geological heterogeneities corresponding to a system of dikes and sills and to the complex structure of the volcanic edifice

identification of p s wave successions for application in microseismicity interpretation ofp s wave successions is used in induced or passive microseismicity it makes the location of microseismic events possible when the triangulation technique cannot be used to improve the reliability of the method we propose a technique that identifies thep s wave successions among recorded wave successions a polarization software is used to verify the orthogonality between thep ands polarization axes the polarization parameters are computed all along the 3 component acoustic signal then the algorithm detects time windows within which the signal polarization axis is perpendicular to the polarization axis of the wave in the reference time window representative of thep wave the technique is demonstrated for a synthetic event and three application cases are presented the first one corresponds to a calibration shot within which the arrivals of perpendicularly polarized waves are correctly detected in spite of their moderate amplitude the second example presents a microseismic event recorded during gas withdrawal from an underground gas storage reservoir the last example is chosen as a counter example concerning a microseismic event recorded during a hydraulic fracturing job the detection algorithm reveals that in this case the wave succession does not correspond to ap s one this implies that such an event must not be located by the method based on the interpretation of ap s wave succession as no such a succession is confirmed