a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

volatile characteristics of peralkaline rhyolites from kenya an ion microprobe infrared spectroscopic and hydrogen isotope study peralkaline rhyolites of the greater olkaria volcanic complex kenya rift valley were derived from separated though closely related magma chambers ion microprobe analyses of glass inclusions in quartz phenocrysts show pre eruptive water contents of up to 3 4 wt contrasting with previous estimates that the magmas were anhydrous the values approach predicted solubility levels corresponding to water saturation at low crustal pressures 1 kbar the glass matrices of the rhyolites have low water contents ranging from 0 07 to 0 46 wt suggesting significant degassing during or prior to eruption infrared measurements of the matrix glasses show variation in the relative proportions of the two hydrous species dissolved in the glasses the amount of molecular water determined semi quantitatively apparently increases with increased fluorine content and peralkalinity this suggests a competition between hydroxyl groups and fluoride ions for similar sites within the melt structure the mechanism of degassing has been investigated using hydrogen isotopes the range of d values in most rocks can be produced by varied degrees of open system degassing of rhyolite melt initially in equilibrium with water of a fixed or limited d value there is evidence to suggest that closed system degassing may also have been a significant component in some rhyolites the exact mechanisms of degassing remain uncertain particular problems include the relative contribution of open and closed system degassing during eruption and the initial vapour compositions and solubility relationships

microearthquakes beneath median valley of mid atlantic ridge near 23 n tomography and tectonics data from a microearthquake experiment in the median valley of the mid atlantic ridge near 23 n in 1982 are used to measure earthquake source parameters to determine the laterally heterogeneous seismic velocity structure across the inner floor and to develop a kinematic tectonic model for this portion of the median valley fifty three microearthquakes occurred over a 10 day period beneath the median valley inner floor and eastern rift mountains twenty of 23 well located inner floor epicenters define a line of activity about 17 km long having a strike of n25 e and located near an along axis depression some 300 400 m deeper than surrounding regions earthquakes with well resolved hypocenters generally have focal depths of 4 8 km beneath the seafloor of both the inner floor and the rift mountains the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45 normal faulting mechanisms although poorly constrained are also indicated for the rift mountain microearthquakes seismic moments approximate fault dimensions and average stress drops for the largest events recorded are 1019 1020 dyn cm 200 400 m and 1 70 bars respectively a twodimensional tomographic inversion of p wave travel time residuals from microearthquakes and local shots indicates a well resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor p wave velocities at 1 5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions the most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column by this view the seismic velocity structure at the ridge axis evolves probably by the sealing of cracks and pores within the first few hundred thousand years of crustal accretion consideration of the detailed sea beam bathymetry in this region of the inner floor the characteristics of large earthquakes that the region has experienced during the past 25 years and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years

the variability in seismic structure of layer 2 near the east pacific rise at 12 n seismic refraction data collected by using a large air gun source and oceanbottom hydrophone receivers show that within two areas on the flanks of the east pacific rise the structure of the uppermost 2 km of the crust is laterally homogeneous the two areas are 20 and 40 km in extent on 0 5 and 4 m y old crust respectively and are separated laterally by 110 km a small but significant difference in structure exists between the two areas the younger crust exhibiting the higher velocities ray tracing through laterally heterogenous models of the shallow crust the careful calculation of corrections for seafloor topography and the objective comparison of data sets by using the slant stacking approach suggest that the established apparent variability in shallow crustal structure may in part be an artifact of experimental and interpretative techniques we propose a reasonable model for shallow crustal structure in which the isovelocity contours parallel the seafloor topography this permits determination of lateral homogeneity of the velocity gradient in the shallow crust over the subdued topography of the east pacific rise or in the deeper crust in general where vertical velocity gradients are sufficiently small that topography has negligible effect on the velocity field

chadormalu kiruna type magnetite apatite deposit bafq district iran insights into hydrothermal alteration and petrogenesis from geochemical fluid inclusion and sulfur isotope data abstract the chadormalu is one of the largest known iron deposits in the bafq metallogenic province in the kashmar kerman belt central iran the deposit is hosted in precambrian cambrian igneous rocks represented by rhyolite rhyodacite granite diorite and diabasic dikes as well as metamorphic rocks consisting of various schists the host rocks experienced na albite calcic actinolite and potassic k feldspar and biotite hydrothermal alteration associated with the formation of magnetite apatite bodies which are characteristic of iron oxide copper gold iocg and iron oxide apatite ioa systems iron ores occurring as massive type and vein type bodies consist of three main generations of magnetite including primary secondary and recrystallized which are chemically different apatite occurs as scattered irregular veinlets in various parts of the main massive ore body as well as apatite magnetite veins and disseminated apatite grains in marginal parts of the deposit and in the immediate wall rocks minor pyrite occurs as a late phase in the iron ores chemical composition of magnetite is representative of an ioa or kiruna type deposit which is consistent with other evidence whole rock geochemical data from various host rocks confirm the occurrence of na ca and k alteration consistent with the formation of albite actinolite and k feldspar respectively the geochemical investigation also includes the nature of calc alkaline igneous rocks and helps elaborating on the spatial and temporal association and possible contribution of mafic to felsic magmas to the evolution of ore bearing hydrothermal fluids fluid inclusion studies on apatites from massive and vein type ores show a range of homogenization temperatures from 266 to 580 c and 208 406 c and salinities from 0 5 to 10 7 wt and 0 3 24 4 wt nacl equiv respectively the fluid inclusion data suggest the involvement of evolving fluids from low salinity high temperature to high salinity low temperature in the formation of the massive and vein type ores respectively the 34s values obtained for pyrite from various parts of the deposit range between 8 9 and 14 4 for massive ore and 18 7 to 21 5 for vein type ore a possible source of sulfur for the 34s enriched pyrite would be originated from late precambrian early cambrian marine sulfate or fluids equilibrated with evaporitic sulfates field observations ore mineral and alteration assemblages coupled with lithogeochemical fluid inclusion and sulfur isotopic data suggest that an evolving fluid from magmatic dominated to surficial brine rich fluid has contributed to the formation of the chadormalu deposit in the first stages of mineralization magmatic derived fluids had a dominant role in the formation of the massive type ores whereas a later brine with higher 34s contributed to the formation of the vein type ores

magmatic volatiles in explosive rhyolitic eruptions obsidian clasts in rhyolitic tephra deposits preserve preeruption magmatic volatile contents providing a direct means for determining the volatile content of explosively erupted magmas small to moderate volume plinian eruptions 10 sup 3 to 10 sup 1 km sup 3 appear to be driven by 0 5 1 0 wt volatiles consisting dominantly of h sub 2 o with minor co sub 2 analysis of obsidian from eruptive sequences consisting of tephra and flows indicates that this hydrous magma abruptly overlies magma with only 0 1 0 2 wt h sub 2 o

three dimensional gravity and bathymetry analysis of the mid atlantic ridge between 20 n and 24 n flow geometry and temporal evolution of the segmentation bathymetry and gravity data covering 10 m y on both flanks of the mid atlantic ridge south of the kane fracture zone reveal a complex segmentation pattern well correlated with short scale temporal variations in crustal production segment and paleosegment centers are associated with crustal thickening while their extremities appear as elongated bands of thin crust subsidence rates vary between centers and extremities of a segment with higher than world average values at segmert midpoints and closer to average values at the edges this is consistent with segment centers being the site of focused mantle upwellings with associated thin lithosphere and with thick crust a pronounced circular minimum in the long wavelength of the mantle bouguer anomaly associated to a maximum in the long wavelength of the bathymetry marks the site of a mantle upwelling associated with a particularly robust segment located north of a small fracture zone 21 20 n an offset is often observed between extreme values of topography and crustal thickness resulting in a lack of isostatic equilibrium for off axis short wavelength features an obliquity between maxima of crustal thickening associated with active segments and the topographic axial domain suggests that the shift between topography and crustal structure may arise partly from a decoupling between crust and topography formation processes

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

hydrodynamic aspects of caldera forming eruptions numerical models comparison of results from a two dimensional numerical eruption simulation kachina to calculations based upon a shock tube analog supports the conclusion that the hydrodynamics during the initial minutes of large caldera forming ash flow eruptions may be dominated by blast wave phenomena field evidence for this phenomenology is pyroclastic surge deposits commonly occurring both directly below caldera related ash flow sheets on top of a preceding plinian fall deposit ground surge and separating individual ash flow units we model the eruption of the tshirege member of the bandelier tuff 1 1 ma b p from the valles caldera new mexico in the model a magma chamber at 100 mpa 1 kbar and 800 c is volatile rich with an average h2o abundance above saturation greater than 8 7 wt increasing to nearly 100 wt near the very top of the chamber using a shock tube analogy decompression of the chamber through a wide open dikelike vent 0 1 km wide and 1 to 5 km long forms a shock wave of 3 mpa 3o atm with a velocity greater than 1 0 km s 1 steady flow of material erupted from the vent begins after 20 to 100 s based upon a 7 km depth from the ground surface to a reflective density boundary in the chamber and a rarefaction wave velocity of 100 to 600 m s 1 the velocity of the ash front behind the shock wave is 300 to 500 m s 1 the shock tube model serves as a basis to evaluate the consistency of the kachina code results which are similar to a one dimensional problem along the symmetry axis the results of the kachina simulation show in some detail the effect of multiple reservoir rarefaction reflections and possibly prandtl meyer expansion in generating compressive wave fronts following the initial shock the rarefaction resonance not only prolongs unsteady flow in the vent but tends to promote surging flow of ash behind the leading shock furthermore these results are consistent with a blast wave characterized as a shock front followed by one or more pulses of entrained ash the blast wave shocks ambient air to higher pressures and temperatures the magnitudes of which depend strongly on the initial chamber overpressure distance and direction from the vent in consideration of volcanic hazards our numerical model shows that a shock wave compressed the atmosphere to pressures of 0 2 to 0 7 mpa 2 7 atm and temperatures of 200 to 300 c for distances to 10 km from the bandelier vent s

three fragmentation mechanisms for highly viscous magma under rapid decompression abstract analysis of the behavior of viscous magma under short term tensile stresses demonstrates that the strain rates anticipated in the eruptive stages of collapse related events may be high enough under certain circumstances to provide fragmentation of the melt by brittle failure mechanisms of fragmentation of a highly viscous vesicular magma by the propagation of an unloading wave by layer by layer vesicle bursting fragmentation wave propagation and by rapid filtration gas flow are outlined fragmentation scenarios involving possible combinations of the three fragmentation mechanisms are evaluated for two models of magma with isolated and with connected pores experimental results for magma fragmentation under high decompression rates appear to support such mechanisms these mechanisms of highly viscous magma fragmentation by rapid decompression probably have their greatest application in cases of explosive disintegration of lava domes and cryptodomes caused by collapse events

magmatic model for the mount st helens blast of may 18 1980 analytical and numerical solutions to the hydrodynamic equations of motion constrained by physical properties of juvenile ejecta in the mount st helens blast deposit were used to investigate magmatic conditions required to produce the initial devastating blast phase of the eruption of may 18 1980 evidence that the blast was magmatic includes equivalence in volume of juvenile blast ejecta to preemption inflation of the cone substantial vesicularity of this ejecta and continued vesiculation of large juvenile clasts after eruption observed or inferred ejecta velocities of 100 to 250 m s are shown to require 0 2 to 0 7 wt water vapor preexisting in magma unloaded by a landslide 200 to 900 m thick these conditions imply total magmatic water contents of 0 7 to 1 7 wt respectively such low required water content suggests that volcanic blasts may be regarded as a normal consequence of magma intrusion into an unstable edifice

a seismically constrained mass discharge rate for the initiation of the may 18 1980 mount st helens eruption we calculate the vertical mass discharge rate from mount st helens for the first few minutes of the may 18 1980 cataclysmic eruption using a new method based on seismic constraints the observed seismic waves indicate that the seismic source is a series of single forces we model these forces as thrusts due to a combination of the momentum flux of the erupted products and the pressure of the eruptive jet the momentum discharge rate is converted to a mass discharge rate based on estimates of the velocity and jet pressure as constrained by a simple fluid dynamical model only two parameters are necessary for the calculation mach number and sound velocity the calculated mass ejected in the first 100 s is 1 6 10 11 4 6 10 11 kg since the total blast deposit is 3 2 10 11 4 1 10 11 kg one possible interpretation is that the directed blast had a significant 30 vertical component

a magmatic geohygrometer application to mount st helens and other dacitic magmas compositions of andesitic to dacitic melts in equilibrium with plagioclase orthopyroxene and clinopyroxene have been experimentally calibrated as a geohygrometer application to melson s data on melt inclusions and matrix glasses from the 1980 1982 cycle of eruptions of mount st helens reveals systematic dewatering trends ca 4 1980 to 1 1982 h 20 in subliquidus intratelluric melt in pyroclastics and 2 3 1980 to 1 1982 in subliquidus melt in dome building dacites before each pyroclastic event intratelluric crystallization achieved melt h 20 contents of ca 6 at a depth of at least 8 km a necessary condition for crystallization of amphibole dewatering trends indicate that individual magma blobs or ribbons then ascended to successively higher levels over the 3 yr cycle from a minimum depth of 4 km in 1980 to 0 35 km in 1982 and pyroclastically erupted comparison of mount st helens compositions to worldwide andesites and dacites suggests that near liquidus arc magmas that crystallize plagioclase and pyroxenes contain l 2 h 2 0 on average whereas magmas in continental margins contain appreciably more h2o

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

thermal properties of vesicular rhyolite abstract thermal diffusivity of rhyolite melt and rhyolite foam 70 80 porosity has been measured using the radial heat transfer method cylindrical samples length 50 55 mm diameter 22 mm of rhyolite melt and foam have been derived by heating samples of little glass mountain obsidian using available data on heat capacity and density of rhyolite melt the thermal conductivity of samples has been determined the difference in thermal conductivity between rhyolite melt and foam at igneous temperatures 1000 c is about one order of magnitude the effect of thermal insulation of magmas due to vesiculation and foaming of the top layer is discussed in terms of the data obtained using a simple illustrative model of magma chamber convection

metabasalts axial hot springs and the structure of hydrothermal systems at mid ocean ridges knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid ocean ridge studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the mid atlantic ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy mineral abundances and mineral compositions especially for chlorite these relationships can be interpreted in terms of the distribution of alteration with respect to time temperature and water rock ratio in young crust assemblages of chl ab ep act chl ab ep act qtz chl ab qtz and chl qtz are produced at successively higher effective sea water to rock ratios within the temperature range of the greenschist facies 250 to 450 c toward higher ratios chlorites tend to become more mg rich pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending rather than the ascending limb of a convection system this type of alteration which reflects moderately high water rock ratios may characteristically occur above a still partially molten magma chamber as the magma chamber solidifies and is then penetrated more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water rock ratios producing solutions such as those recovered from springs on the galapagos rift and the east pacific rise at 21 n localized upwelling limbs of the convection system can produce veins filled with quartz sulfides and fe rich chlorite

the east pacific rise near 21 n 13 n and 20 s inferences for along strike variability of axial processes of the mid ocean ridge abstract data obtained along various segments of the mid ocean ridge mor are used to construct an idealized model for crustal accretion the model seeks to predict the topographic volcanic tectonic and hydrothermal characteristics of any given spreading segment of the mor as a function of distance away from the bounding transform faults this model is based on a series of detailed mapping efforts carried out on segments of the mor having a broad range of spreading rates between 2 and 16 cm yr this paper includes the results of the french searise program carried out during the summer of 1980 aboard the n o jean charcot and two american cruises conducted in 1981 aboard the r v melville

petrology of crystal clots in the pumice of mount st helens march 19 1982 eruption significant role of fe ti oxide crystallization abstract a petrographic mineralogical and chemical description of the dacite pumice and of the contained mafic crystal clots from the march 19 1982 eruption of mount st helens is presented the crystal clots are interpreted as cumulates and the phenocryst phases in the pumice are not fully representative of phases fractionating at depth the crystallization sequence is inferred to be olivine plus plagioclase followed by pyroxenes and late stage amphibole magnetite and ilmenite crystallized early providing a mechanism for minimal iron enrichment as estimated from fe ti oxide and two pyroxene geothermometers crystallization temperatures were approximately 1000 c at oxygen fugacities approximately one order of magnitude higher than the nno buffer the parental magma was more mafic than dacite and had a comparatively low water content as the magma evolved to dacite the water content increased to a value in excess of 4 wt and amphibole began to crystallize

crystallization of mount st helens 1980 1986 dacite a quantitative textural approach quantitative measurements of crystal size distributions csds have been used to obtain kinetic information on crystallization of industrial compounds randolph and larson 1971 and more recently on hawaiian basalts cashman and marsh 1988 the technique is based on a population balance resulting in a differential equation relating the population densityn of crystals to crystal sizel i e at steady staten no exp l itg whereno is nucleation density g is the average crystal growth rate is the average growth time and the nucleation ratej nog csd inn vsl plots of plagioclase phenocrysts in 12 samples of mount st helens blast dacite and 14 samples of dacite from the 1980 1986 mount st helens dome are similar and averageg 9 6 1 1 10 3 cm andno 1 2 106 cm 4 reproducibility of the measurements was tested by measuring csds of 12 sections cut from a single sample in three mutually perpendicular directions precision of the size distributions is good in terms of relative but not necessarily absolute values 10 growth and nucleation rates for plagioclase have been calculated from these measurements using time brackets of 30 150 years growth ratesg are 3 10 10 12cm s and nucleation ratesj are 5 21 10 6 cm3 s g andj for fe ti oxides calculated from csd data areg 2 13 10 13 cm sec andj 7 33 10 5 cm3 s respectively the higher nucleation rate and lower growth rate of oxides resulted in a smaller average crystal size than for plagioclase sizes of plagioclase microlites 0 01 mm in the blast dacite groundmass have been measured from backscatter sem photographs nucleation of these microlites was probably triggered by intrusion of material into the cone of mount st helens in spring 1980 this residence time of 52 days gives minimum crystallization estimates ofg 1 3 10 11 cm s andj 9 16 1o3 cm3 s the skeletal form of the microlites provides evidence for nucleation and growth at high values of undercooling t relative to the phenocrysts a comparison of nucleation and growth rates for the two crystal populations phenocrysts vs microlites suggests that while growth rate seems to be only slightly affected by changes in t nucleation rate is a very strong function of undercooling a comparison of plagioclase nucleation and growth rates measured in the mount st helens dacite and in basalt from makaopuhi lava lake in hawaii suggests that plagioclase nucleation rates are not as dependent on composition groundmass textures suggest that plagioclase microphenocrysts crystallized at depth rather than in the conduit in the dome or after extrusion onto the surface most of this crystallization appears to be in the form of crystal growth coarsening of groundmass microphenocrysts at small degrees of undercooling rather than extensive nucleation of new crystals this continuous crystallization in a shallow magmatic reservoir may provide the overpressurization needed to drive the continuing periodic domebuilding extrusions which have been the pattern of activity at mount st helens since december 1980

multiphase flow numerical modeling of the 18 may 1980 lateral blast at mount st helens usa volcanic lateral blasts are among the most spectacular and devastating of natural phenomena but their dynamics are still poorly understood here we investigate the best documented and most controversial blast at mount st helens washington state united states on 18 may 1980 by means of three dimensional multiphase numerical simulations we demonstrate that the blast front propagation final runout and damage can be explained by the emplacement of an unsteady stratified pyroclastic density current controlled by gravity and terrain morphology such an interpretation is quantitatively supported by large scale observations at mount st helens and will influence the definition and predictive mapping of hazards on blast dangerous volcanoes worldwide

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

the volcanic tectonic cycle of the famous and amar valleys mid atlantic ridge 36 47 n evidence from basalt glass and phenocryst compositional variations for a steady state magma chamber beneath the valley midsections amar 3 the 1978 amar expedition extended the investigation of the mid atlantic ridge that was begun by the 1973 1974 famous expedition from the northern end of the famous rift the original famous area to the narrow central and southern end of the famous rift narrowgate region and into the broad amar valley south of fracture zone b available field and geochemical data allow us to characterize in detail the volcanic tectonic cycle for these two segments of the mid ocean ridge system a dynamic model of the crust forming processes within these two rift valley segments is deduced from 1 variations in the chemical composition of the erupted basaltic liquids as preserved in chilled glassy margins 2 composition and petrographie relationships of megacrysts and phenocrysts that represent cumulus crystallization onto the floor of a shallow magma chamber of a primitive magma that has reached pyroxene saturation early in its evolution and 3 detailed stratigraphie and regional observations that characterize the periodicity of volcanic construction and superimposed tectonism related to the ongoing extension of the valley floor each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition the decreasing extrustion rates at the end of an eruptive event are associated with more evolved liquid compositions that reflect fractionation within the shallow conduits intermixing of magmas is indicated by the relatively limited variability in glass composition not explicable in terms of simple fractionation processes the common evidence of crystal resorption and the apparent disequilibrium between megacrysts phenocrysts and their enclosing glass this indicates control by a crustal magma chamber liquid and crystal compositions and textural relations can best be explained by a three stage crystallization history 1 crystallization of olivine plagioclase and clinopyroxene onto the floor of a magma chamber 2 partial resorption of mineral phases by a superheated undersaturated liquid above the floor 3 intratelluric crystallization of olivine and plagioclase clinopyroxene during rise of melt to the surface through the conduit system above the magma chamber a magma chamber model is suggested for the amar famous rift valleys that satisfies both seismic and geological constraints we propose that a small steady state magma chamber is maintained beneath the topographically higher midsections of each valley segment amar and narrowgate regions but that these thin and terminate near the intersecting fracture zones expansion and contracton of the central magma body is controlled by the fluctuating imbalance between magma supply and chamber crystallization the famous narrowgate rift is currently in a contraction period so that in the northern famous region primitive liquid has erupted at mount venus and mount pluto without intercepting and mixing into the steady state chamber that supplies the mixed magmas to the narrowgate region

directed blasts and blast generated pyroclastic density currents a comparison of the bezymianny 1956 mount st helens 1980 and soufriere hills montserrat 1997 eruptions and deposits we compare eruptive dynamics effects and deposits of the bezymianny 1956 bz mount st helens 1980 msh and soufriere hills volcano montserrat 1997 shv eruptions the key events of which included powerful directed blasts each blast subsequently generated a high energy stratified pyroclastic density current pdc with a high speed at onset the blasts were triggered by rapid unloading of an extruding or intruding shallow magma body lava dome and or cryptodome of andesitic or dacitic composition the unloading was caused by sector failures of the volcanic edifices with respective volumes for bz msh and shv c 0 5 2 5 and 0 05 km3 the blasts devastated approximately elliptical areas axial directions of which coincided with the directions of sector failures we separate the transient directed blast phenomenon into three main parts the burst phase the collapse phase and the pdc phase in the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into the atmosphere with much of the expansion having an initially lateral component the erupted material fails to mix with sufficient air to form a buoyant column but in the collapse phase falls beyond the source as an inclined fountain and thereafter generates a pdc moving parallel to the ground surface it is possible for the burst phase to comprise an overpressured jet which requires injection of momentum from an orifice however some exploding sources may have different geometry and a jet is not necessarily formed a major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be attributed to shock waves within an overpressured jet or alternatively to dynamic pressures and shocks within the energetic collapse and pdc phases internal shock structures related to unsteady flow and compressibility effects can occur in each phase we withhold judgment about published shock models as a primary explanation for the damage sustained at msh until modern 3d numerical modeling is accomplished but argue that much of the damage observed in directed blasts can be reasonably interpreted to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified pdc this view is reinforced by recent modeling cited for shv in distal and peripheral regions solids concentration maximum particle size current speed and dynamic pressure are diminished resulting in lesser damage and enhanced influence by local topography on the pdc despite the different scales of the blasts devastated areas were respectively 500 600 and 10 km2 for bz msh and shv and some complexity involving retrogressive slide blocks and clusters of explosions their pyroclastic deposits demonstrate strong similarity juvenile material composes 50 of the deposits implying for the blasts a dominantly magmatic mechanism although hydrothermal explosions also occurred the character of the magma fragmented by explosions highly viscous phenocryst rich variable microlite content determined the bimodal distributions of juvenile clast density and vesicularity thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater and laterally from the axial region the proximal stratigraphy of the blast deposits comprises four layers named a b c d from bottom to top layer a is represented by very poorly sorted debris with admixtures of vegetation and soil with a strongly erosive ground contact its appearance varies at different sites due to different ground conditions at the time of the blasts the layer reflects intense turbulent boundary shear between the basal part of the energetic head of the pdc and the substrate layer b exhibits relatively well sorted fines depleted debris with some charred plant fragments its deposition occurred by rapid suspension sedimentation in rapidly waning high concentration conditions layer c is mainly a poorly sorted massive layer enriched by fines with its uppermost part laminated created by rapid sedimentation under moderate concentration weakly tractive conditions with the uppermost laminated part reflecting a dilute depositional regime with grain by grain traction deposition by analogy to laboratory experiments mixing at the flow head of the pdc created a turbulent dilute wake above the body of a gravity current with layer b deposited by the flow body and layer c by the wake the uppermost layer d of fines and accretionary lapilli is an ash fallout deposit of the finest particles from the high rising buoyant thermal plume derived from the sediment depleted pyroclastic density current the strong similarity among these eruptions and their deposits suggests that these cases represent similar source transport and depositional phenomena

depressurization of fine powders in a shock tube and dynamics of fragmented magma in volcanic conduits abstract samples of fine glass beads mean grain size equal to 38 and 95 m have been depressurized within a vertical shock tube these short lived rapid decompressions resemble discrete cannon like vulcanian explosions and produce two phase flows that are inhomogeneous in density in both vertical and horizontal directions because of the presence of bubble like heterogeneities we suggest that also volcanic flows may present similar inhomogeneities in density in the experimental apparatus the flow velocities increase from approximately 1 to 13 m s when the pressure drop increases from approximately 200 to 900 mbar a physical model of the initial velocities of expansions in the shock tube has been applied to a range of volcanic overpressures between 0 1 and 20 mpa suggesting initial velocities of volcanic flows caused by the removal of a rock plug in volcanic conduits between 25 and 400 m s during the experiments at large pressure drops as the mixture expands and moves up the tube the flow front becomes highly irregular and bubble like heterogeneities form the shape of these bubbles becomes distorted and stretched in the turbulent flow during the experiments at relatively small pressure drops the sample oscillates when the particles after the expansion flow back and bounce upward again jets with diameter smaller than that of the tube are ejected from the oscillating samples generating independent pulses large bubble like heterogeneities whose diameter is a significant fraction of the tube diameter can also discretize the flows similar mechanisms in real volcanoes may produce pulse like ejections of gas particle mixtures out of the vent

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

eruptive response of oceanic islands to giant landslides new insights from the geomorphologic evolution of the teide pico viejo volcanic complex tenerife canary large sector collapses are a major component of oceanic islands evolution here we show that voluminous events such as the icod landslide on tenerife canary islands cause dramatic changes on the magma feeding system and control the subsequent volcanic and geomorphologic evolution of the eruptive complex over a period of more than 150 kyr instantaneous unloading by the icod landslide is marked by the development of a large phonolitic explosive eruption dated at 175 3 ka and interpreted as reflecting the immediate emptying of a shallow pre existing magma chamber geochronological geomorphological and geochemical analyses carried out on the post landslide volcanic succession sampled in a 4 4 km long underground water recovery gallery provide further evidence for an enhanced extrusion of primitive lavas starting in the 10 kyr time interval following the failure rapid construction 40 kyr of a thick basaltic volcano in the landslide scar at high eruptive rates up to 8 km3 kyr 1 increased the lithostatic pressure which then favored the intermittent storage of basic magma under the edifice this resulted in more episodic construction evidenced by a significant decrease in output rates and the increasing occurrence of lavas with intermediate composition from 117 7 to 52 7 ka an apparent volcanic gap is observed between 52 7 and 18 1 ka after which highly differentiated lavas have been dominantly erupted we propose that part of the gap can be explained by the individualization of a shallow magma reservoir a few kilometers below the base of the teide volcano during recent periods vertical and lateral extrusions of trachytic and phonolitic viscous bodies from this storage area contributed to increase the slope of the main edifice up to 35 overall favoring its present day instability

the oceanographer fracture zone i crustal structure from seismic refraction studies summary this paper presents the results of four lookm long seismic refraction lines carried out in and around the active transform of the oceanographer fracture zone which offsets the mid atlantic ridge rightlaterally by 130 km near 35 n the seismic data are interpreted using standard travel time inversions the time term method and two dimensional ray tracing techniques the results show that beneath the oceanographer fracture zone the crust is typically only 4 5 km thick it consists of a thin upper crustal layer in which the p wave velocity increases rapidly with depth and a main crustal layer with velocity increasing from 6 4 km s l at the top to 6 8 km s at the base of the crust some degree of crustal thinning is found to extend for distances of 30 40 km on either side of the fracture zone axis in addition to this a small area 10 15 km long of crust less than 2 5 km thick is found beneath the centre of the transform valley crust less than 4 km thick is also observed on a smaller offset fracture zone 85 km to the south of the oceanographer but no evidence is found for significantly thin crust beneath another small offset fracture zone 65 km to the north of it we conclude that the moderate thnning of the crust at the fracture zone is due to a progressive reduction in the crustal production rates of the oceanic spreading centres as they approach the transform fault however we attribute the occurrence locally of extremely thin crust to a different process in which a thin lid of mafic and ultramafic rocks overlying unaltered upper mantle material is accreted at the junction of a spreading centre and a transform fault this may happen if the accretion rate at the end of the spreading centre becomes so slow that the usual processes which give rise to normal oceanic crust can no longer operate it therefore seems that there are two distinct types of crust either of which may be found at different points along a fracture zone this might account for the extreme variability in crustal thickness found in different fracture zones by previous studies

water contents of silicic to intermediate magmas silicic to intermediate magmas have water contents that range from 1 wt to around 7 wt depending on the character and depth of the magma source type of melting reaction and the degree of differentiation the condition of water saturation is rare implying that crustal magma genesis generally takes place under fluid absent conditions or that any fluid present must contain volatile species other than h2o the source and quantity of water present during partial melting profoundly affect the types of melting reactions residues and magmas as well as subsequent magma evolution for s type granitic magmas melting reactions involving muscovite breakdown in pelites yield low temperature wet melts that form veins pockets and small plutons in regional metamorphic terranes biotite breakdown in pelites near the amphibolite granulite facies transition yields hotter magmas with variable water contents these are emplaced at medium to high levels under granulite facies conditions biotite breakdown in quartzofeld spathic rocks yields high temperature dry magmas emplaced as subvolcanic batholiths and volcanics

ion microprobe studies of water in silicate melts concentration dependent water diffusion in obsidian abstract the temperature dependence of water diffusivity in rhyolite melts over the range 650 950 c and pt h2o 700 bars is evaluated from water concentration distance profiles measured in glass with an ion microprobe diffusivities are exponentially dependent on concentration over this temperature range and vary from about 10 8 cm2 s at 650 c to about 10 7 cm2 s at 950 c at 2 wt water water solubility also varies with temperature at a rate of 0 14 wt per 100 c increase the avtivation energy ea appears to be constant at 19 1kal mole for 1 2 and 3 wt h2o comparison of these data with results for cation diffusion indicates that this value is a minimum ea for diffusion of any species in a rhyolite melt compensation plots of log10d0 the frequency factor versus ea indicate that hydrous rhyolite melts follow the same trend as anhydrous basalts d0 increases for h2o and ca2 1 as ea decreases this suggests that these molecules may diffuse by different mechanisms than do monovalent cations and that hydration of the melt affects diffusion of ca2 and h2o differently than it does monovalent cation diffusion the results imply that dramatic increases in cation diffusivities by hydration 1 may occur with additions of less than 1 wt h2o

the initial giant umbrella cloud of the may 18th 1980 explosive eruption of mount st helens abstract the initial eruption column of may 18th 1980 reached nearly 30 km altitude and released 10 17 joules of thermal energy into the atmosphere in only a few minutes ascent of the cloud resulted in forced intrusion of a giant umbrella shaped cloud between altitudes of 10 and 20 km at radial horizontal velocities initially in excess of 50 m s the mushroom cloud expanded 15 km upwind forming a stagnation point where the radial expansion velocity and wind velocity were equal the cloud was initiated when the pyroclastic blast flow became buoyant the flow reduced its density as it moved away from the volcano by decompression by sedimentation and by mixing with and heating the surrounding air observations indicate that much of the flow covering an area of 600 km 2 became buoyant within 1 5 minutes and abruptly ascended to form the giant cloud calculations are presented for the amount of air that must have been entrained into the flow to make it buoyant assuming an initial temperature of 450 c and a magmatic origin for the explosion these calculations indicate that the flow became buoyant when its temperature was approximately 150 c and the flow consisted of a mixture of 3 25 10 11 kg of pyroclasts and 5 0 10 11 kg of air if sedimentation is considered these figures reduce to 1 1 10 11 kg of pyroclasts and 1 0 10 11 kg of air

an example of a recent accretion on the mid atlantic ridge the snake pit neovolcanic ridge mark area 23 22 n abstract the inner rift vailey of the mark area mid atlantic ridge at the kane fracture zone was investigated with the submersible nautile at the latitude of the snake pit hydrothermal field 23 22 n a transect across the axial valley shows a stepwise increase in the sediment thickness away from the axis these steps probably reflect major magmatic and tectonic events in the evolution of the inner floor dives on the summit of the snake pit neovolcanic ridge constrain a generalized geological map and document relations between hydrothermal activity magmatism and faulting the neovolcanic ridge was constructed by the build up of several hundred metres of pillow lava flows its summit is intensely fissured and cut by an axial graben about 100 m wide three hydrothermal sites were recognized within the axial graben the eastern site the most active is structurally related to the eastern fault scarp the central site is located within the graben the western site associated with the western fault scarp is more complex and was constructed by two successive hydrothermal events the new data suggest a model of evolution for the inner floor of the rift valley involving alternating stages of magmatism and volcanism which produce ridges and troughs preserved outside of the axial domain these volcano tectonic cycles have an approximate duration of 100 000 years we discuss variations in the morphology of the mar along strike in the light of this model of evolution

the ocean continent boundary off the western continental margin of iberia ii crustal structure in the tagus abyssal plain summary an 80 km long reversed seismic refraction line line 5 was shot over the tagus abyssal plain off portugal the main p wave reflected and refracted phases were modelled both for traveltime and amplitude the resulting p wave velocity depth model has the following features a an extremely thin crust of about 2 km b the absence of oceanic layer 3 and c very low upper mantle velocities between 7 6 and 7 9 km s this very unusual seismic velocity crustal structure is quite unlike thinned continental crust but is remarkably similar to the seismic crustal structures found at atlantic fracture zones and in particular to the structures found in profiles shot along the transform valley and near ridge transform intersections a magnetic anomaly chart seems to allow the possibility of several fracture zones one of which could intersect the centre of line 5 as an alternative to the fracture zone hypothesis we show that if the oceancontinent transition in the tagus abyssal plain is located at about 11 30 w in a symmetric position with respect to the ocean continent transition in the conjugate south newfoundland basin then magnetic anomalies can be modelled simply by assuming sea floor spreading west of 11 45 w at 10 mm yr beginning at m11 time 133 myr bp and blocks of rifted continental crust to the east the location of the proposed ocean continent transition in the tagus abyssal plain is marked by a well defined n s linear magnetic anomaly which is adjacent to the oldest sea floor spreading block east of the proposed ocean continent transition there is an increase in the depth to basement similar to that found east of the ocean continent transition in the iberia abyssal plain and elsewhere this model also allows us to explain why purdy s 1975 seismic refraction line a ar in the tagus abyssal plain cannot be interpreted as a conventional reversed pair because most of line a was shot over the ocean continent transition zone and most of line ar over thinned continental crust remarkably similar velocity depth structures to that under line 5 are found close to the ocean continent transition zone off the whole of western iberia in areas which show no clear evidence of fracture zones therefore it appears more likely that the seismic structure of line 5 is due to its proximity to the ocean continent transition than to a local association with a fracture zone and further that its structure is typical of this transition off the western margin of iberia we also suspect that the low upper mantle velocities associated with the ocean continent transition indicate the widespread occurrence of serpentinized peridotite

excitation mechanism of atmospheric pressure waves from the 1980 mount st helens eruption summary atmospheric pressure waves from the mount st helens eruption 1980 may 18 have been clearly recorded by a sensitive microbarograph at berkeley california the record shows three types of waves with different group velocities the pressure waves can be interpreted in terms of direct waves a1 antipodean travelling waves a2 and circumnavigating waves a3 all of which are composed of several acoustic gravity modes propagated in the lower atmosphere synthetic barograms appropriate to the berkeley station have been calculated on the basis of the dynamic response of the lower atmospheric structure together with various assumptions of source properties comparisons between synthetic and observed barograms provide estimates for ranges of the time history of upward particle velocity at the source source dimensions and the velocity of the source spreading over the blast zone as well as for the average dissipation effects over the circumferential path the results suggest that two major compression pulses on the a1 record correlate with the arrival of pressure waves from the first lateral blast and second vertical blast although the inferred interblast time interval is not consistent with that estimated from seismic observations

crustal structure and isostatic compensation near the kane fracture zone from topography and gravity measurements i spectral analysis approach summary six gravity and bathymetry profiles perpendicular to the kane fracture zone each more than 300 km long were gathered to study the variation in crustal structure in the vicinity of a major fracture zone and the gravitational edge effect at the contact between lithosphere of two different ages a spectral analysis of the gravity and bathymetric series as a function of wavelength shows that the gravitational edge effect is only significant at the longest wavelengths for remaining wavelengths the admittance the ratio of the amplitude of the gravity anomaly to the amplitude of the bathymetry is best explained by a model of isostasy in which topographic loads are partially supported by the flexural rigidity of an elastic plate about 6 km in thickness after subtracting the gravitational attraction of the bathymetry and its compensation substantial isostatic anomalies remain we interpret these anomalies as being caused by variations in crustal thickness which have little correlation with surface topography except at very long wavelengths the apparent crustal thickness varies by as much as a factor of 2 but there is no evidence indicating systematic thinning of the crust beneath the fracture zone our data do suggest that such density variations within the plate are also compensated by the isostatic response of an elastic plate but with very different effect from those at the surface this indicates that there are two different modes of crustal formation with different gravity and topographic signatures effusive volcanism which loads the surface of the elastic plate producing both topographic relief and coherent gravity anomalies and intrusive volcanism or underplating producing gravity anomalies but little topographic relief

structure and variability of oceanic crust on the flanks of the east pacific rise between 11 and 13 n we use over 500 seismograms collected by five ocean bottom seismometers to examine the structure and variability of 0 5 m y old crust in the rivera ocean seismic experiment rose area of the east pacific rise between 11 and 13 n there are no significant differences among the first arrival travel times for 10 independent refraction line segments along the isochron amplitude patterns recorded on the individual lines however suggest that structural boundaries within the upper crust vary by hundreds of meters in depth over lateral distances of tens of kilometers within this 200 km segment of lithosphere on the basis of the modeling of amplitude patterns at ranges of 5 10 km and of comparisons with the upper crust sampled at deep sea drilling project hole 504b and in ophiolite complexes layer 2 can be divided into three units 2a corresponds to extrusive volcanics with a steep velocity gradient 2b to extrusives with a near zero velocity gradient and 2c to a complex region of transition from extrusives to sheeted dikes the total thickness of layer 2 varies by about 0 7 km over the length of the line another amplitude peak which varies in range between 14 and 21 km may be related to the downward transition from sheeted dikes to isotropic gabbros within layer 3 the variability of these two amplitude patterns provides direct evidence that the along strike structure of oceanic crust in the rose area is definably heterogeneous and that the process of crustal accretion is likewise variable on the scale resolved by amplitude and travel time information

a model for the mechanism of the may 18 1980 mount st helens blast abstract a model is suggested for the mechanism of the blast that accompanied the may 18 1980 mount st helens eruption the model explains the large duration of the blast process and accounts for several explosive episodes recognized during the blast the mechanism is based on the idea of destruction of the near solidus porous cryptodome by a fragmentation wave termed the f wave originating from cryptodome decompression induced by a landslide the f wave propagates from a free surface to the interior of the cryptodome at velocity n the products of cryptodome fragmentation a gas pyroclast mixture are ejected into the atmosphere at velocity u calculations show that for pressure of gas in the cryptodome pores of 20 mpa and porosity 0 4 0 7 the f wave travels at 5 m s and velocity of pyroclast ejection is 100 250 m s for a cryptodome with diameter of 700 m the duration of blast fragmentation is estimated at 140 s energy released during the blast is estimated as 4 8 7 2 1015 j blast power is 3 4 5 1 1013 wt gas pyroclastic mixture mass flux is 5 0 7 5 103kg m2 s the maximum discharge is 1 9 2 9 sx 109 kg s and the weight fraction of water vapor in the cryptodome is 0 006 0 04 several explosive episodes during the blast are accounted for by fragmentation wave stoppage due to blockage of the exploding cryptodome by landslide material

a model for viscous magma fragmentation during volcanic blasts fragmentation of magma containing gas bubbles is of great interest in connection with developing models for the formation of pyroclastics and for volcanic blasts explosions this paper considers the problem of fragmentation of highly viscous 108 pa s or solidified magma containing bubbles with excess gas pressure it is suggested that the fragmentation of magma be considered on the basis of the fragmentation wave theory proposed by nikolsky and khristianovich which is generally applicable to gas dynamic phenomena occurring in mines then it becomes possible to derive the equations of conservation for the fragmentation wave front which moves into a body of magma from its free surface as a result the velocity n of magma fragmentation and the velocity u of the movement of the gas pyroclastic mixture behind the fragmentation wave front are determined calculations show that n can reach 5 m s therefore the duration of the fragmentation of the magma body blast duration proves to be long the suggested model explains the possibility of several explosions during the blast as a result of the fragmentation wave stopping and accounts for the angular shape of pyroclasts by the brittle disruption of interbubble partitions during fragmentation wave propagation through the porous magma body the initiation and cessation of fragmentation are defined by magma porosity magma tensile strength and the pressure differential between gas pressure in pores and the atmospheric pressure the physical model of magma fragmentation developed explains the mechanism of energy release during volcanic blasts of the vulcanian or pelean types

influence of pre eruptive degassing and crystallization on the juvenile products of laterally directed volcanic explosions abstract strikingly similar examples of edifice collapse and directed blast are the 18 may 1980 eruption of mount st helens msh washington usa and the 30 march 1956 eruption of bezymianny volcano bz kamchatka russia in these cases flank failures led to near instantaneous decompression and fragmentation of intra edifice cryptodome magma which produced catastrophic laterally directed blasts in both instances the blast products consisted of juvenile material with bimodal density vesicularity distributions low and high density modes at 1900 and 2400 kg m 3 for bz 1600 and 2300 kg m 3 for msh although the proportion of high density material is greater at bz blast materials also exhibit striking variety in groundmass crystallinity 90 vol despite having fairly uniform pheno crystallinities suggesting that degassing driven groundmass crystallization occurred to varying extents within cryptodome magma at both volcanoes new bulk rock h2o and d measurements confirm that progressive open system outgassing occurred prior to both blasts the correlations between crystallinity clast density and bulk h2o contents suggest that syn blast magma expansion was modulated both by non uniform volatile distribution within the cryptodome and rheological controls associated with non uniform crystal content spatial heterogeneities in volatiles and crystallinity within a given cryptodome are attributed to distance from the wallrock margin which probably correlates with timing of magma injection the greater proportion of high density material at bz is speculatively related to lower blast energy compared with msh

multiphase flow dynamics of pyroclastic density currents during the may 18 1980 lateral blast of mount st helens 1 the dynamics of the may 18 1980 lateral blast at mount st helens washington usa were studied by means of a three dimensional multiphase flow model numerical simulations describe the blast flow as a high velocity pyroclastic density current generated by a rapid expansion burst phase lasting less than 20 s of a pressurized polydisperse mixture of gas and particles and its subsequent gravitational collapse and propagation over a rugged topography model results show good agreement with the observed large scale behavior of the blast and in particular reproduce reasonably well the front advancement velocity and the extent of the inundated area detailed analysis of modeled transient and local flow properties supports the view of a blast flow led by a high speed front with velocities between 100 and 170 m s with a turbulent head relatively depleted in fine particles and a trailing sedimenting body in valleys and topographic lows pyroclasts accumulate progressively at the base of the current body after the passage of the head forming a dense basal flow depleted in fines less than 5 wt with total particle volume fraction exceeding 10 1 in most of the sampled locations blocking and diversion of this basal flow by topographic ridges provides the mechanism for progressive current unloading on ridges sedimentation occurs in the flow body just behind the current head but the sedimenting basal flow is progressively more dilute and enriched in fine particles up to 40 wt in most of the sampled locations in the regions of intense sedimentation topographic blocking triggers the elutriation of fine particles through the rise of convective instabilities although the model formulation and the numerical vertical accuracy do not allow the direct simulation of the actual deposit compaction present results provide a consistent quantitative model able to interpret the observed stratigraphic sequence

blast dynamics at mount st helens on 18 may 1980 at 8 32 a m on 18 may 1980 failure of the upper part of the north slope of mount st helens triggered a lateral eruption the blast that devastated the conifer forests in a sector covering 500 km2 north of the volcano i present here a steady flow model for the blast dynamics and propose that through much of the devastated area the blast was a supersonic flow of a complex multiphase solid liquid vapour mixture the shape of the blast zone pressure temperature velocity mach number and density distributions within the flow positions of weak and strong internal shocks and mass flux energy flux and total energy are calculated the shape of blast zone was determined by the initial areal expansion from the reservoir by internal expansion and compression waves including shocks and by the density of the expanding mixture the pressure within the flow dropped rapidly away from the source of the blast until at a distance of 11 km the flow became underpressured relative to the surrounding atmosphere weak shocks within the flow subparallel to the east and west margins coalesced at about this distance into a strong mach disk shock across which the flow velocities would have dropped from supersonic to subsonic as the pressure rose back towards ambient the positions of the shocks may be reflected in differences in the patterns of felled trees at the limits of the devastated area the temperature had dropped only 20 from the reservoir temperature because the entrained solids thermally buffered the flow the dynamic and thermodynamic effects of the admixture of the surrounding atmosphere and the uprooted forest and soils into the flow are not considered the density of the flow decreased with distance until at the limits of the blast zone 20 25 km from the volcano the density became comparable with that of the surrounding dirty atmosphere and the flow became buoyant and ramped up into the atmosphere according to the model the mass flux per unit area at the source was 0 6 104 g s 1 cm 2 and the energy flux per unit area was 2 5 mw cm 2 from the measured total ejected mass 0 25 1015 g the total energy released during the eruption was 1024 erg or 24 megatons the model triggering of the eruption and the transition from unsteady to steady flow and applications to eyewitness observations and atmospheric effects are discussed in ref 1

axial intrusion zone beneath the median valley of the mid atlantic ridge at 37 n detected by explosion seismology summary four seismic refraction lines three of which had shots every 250 m were shot across along and parallel to the median valley of the mid atlantic ridge at 37 n a method has been developed for calculating the effect on the travel times of the rough sea floor relief beneath the profiles and has been used to correct all the travel times for this effect most arrivals were from a main refractor of apparent velocity 5 4 to 6 3 km s 1 only beyond 35 km were faster arrivals observed from an 8 09 36 km s 1 refractor the main refractor corresponds in depth at least approximately to the top of layer 3 of the ocean basins but its velocity is significantly less than normal for layer 3 perhaps due to dip a study of time residuals along two profiles across the median valley indicates the presence of a 2 to 3 km wide low velocity zone about 3 2 km s 1 beneath the median valley floor this zone extends over the upper 2 5 km of the crust and is believed to represent a zone of intrusion through which magma passes on its way to the sea floor

the memory of the accreting plate boundary and the continuity of fracture zones a detailed aeromagnetic anomaly map of the mesozoic seafloor spreading lineations southwest of bermuda reveals the dominant magnetic grain of the oceanic crust and the character of the accreting boundary at the time of crustal formation the magnetic anomaly pattern is that of a series of elongate lobes perpendicular to the fracture zone flowline trends the linear sets of magnetic anomaly peaks and troughs have narrow regions of reduced amplitude anomalies associated with the fracture zones during the period of mesozoic geomagnetic polarity reversals when 1200 km of central north atlantic seafloor formed the atlantic accreting boundary consisted of stationary elongate spreading center cells that maintained their independence even though sometimes only minor spatial offsets existed between cells normal oceanic crustal structure was formed in the spreading center cells but structural anomalies and discontinuities characteristic of fracture zones were formed at their boundaries which parallel flowlines of mesozoic relative plate motion in the central north atlantic we suggest that the memory for a stationary pattern of independent spreading center cells resides in the young brittle lithosphere at the accreting boundary where the lithosphere is weakest here each spreading center cell independently goes through its cylce of stress buildup stress release and crustal accretion after which its memory is refreshed the temporal offset between the peaks of the accretionary activity that takes place within each cell may provide the mechanism for maintaining the independence of adjacent spreading center cells through times when no spatial offset between the cells exists

experimental volcanology on eruptive products of unzen volcano abstract protracted dome building eruptions may be profitably investigated using laboratory based experiments density distribution studies on the pyroclastic flow deposits of unzen 1990 1995 allow us to apply the results of experimental investigations on unzen samples to the interpretation of the last eruption of unzen here primary laboratory experiments have focused on several aspects of the degassing permeability and the fragmentation behavior threshold speed and efficiency those investigations have been flanked by analyses of flexural strength fracture toughness and seismic velocities to provide new insights into eruption related processes here we present a review of these results and their application to the eruption dynamics of unzen volcano we propose that efforts be made to incorporate routinely such comprehensive experimental analyses into the response to emerging volcanic crises in future

monitoring the 1980 1982 eruptions of mount st helens compositions and abundances of glass the mount st helens eruptive sequence of 1980 through 1982 reflects the tapping of successively less water rich more highly crystallized and more viscous highly phyric dacitic magmas these changes reflect both syn and preeruption processes the decreasing water content points to a continued decline in the volume and intensity of explosive pyroclastic activity this decreasing water content appears to be composed of a long term trend established during a long period of repose about 130 years imposed on short term trends established during short periods about 7 to 100 days of repose between eruptions in the present eruptive cycle the last two eruptive cycles of this volcano the t a d 1800 and w cycles about a d 1500 exhibited similar trends these changes are inferred from a combination of petrographic bulk chemical and electron and ion microprobe analyses of matrix and melt inclusion glasses

the southeast indian ridge between 88 e and 118 e gravity anomalies and crustal accretion at intermediate spreading rates although slow spreading ridges characterized by a deep axial valley and fast spreading ridges characterized by an axial bathymetric high have been extensively studied the transition between these two modes of axial morphology is not well understood we conducted a geophysical survey of the intermediate spreading rate southeast indian ridge between 88 e and 118 e a 2300 km long section of the ridge located between the amsterdam hot spot and the australian antarctic discordance where satellite gravity data suggest that the southeast indian ridge seir undergoes a change from an axial high in the west to an axial valley in the east a basic change in axial morphology is found near 103 30 e in the shipboard data the axis to the west is marked by an axial high while a valley is found to the east although a well developed axial high characteristic of the east pacific rise epr is occasionally present the more common observation is a rifted high that is lower and pervasively faulted sometimes with significant 50 m throw faults within a kilometer of the axis a shallow axial valley 1200 m deep valley across a transform at 114 e the changes in axial morphology along the seir are accompanied by a 500 m increase in near axis ridge flank depth from 2800 m near 88 e to 3300 m near 114 e and by a 50 mgal increase in the regional level of mantle bouguer gravity anomalies over the same distance the regional changes in depth and mantle bouguer anomaly mba gravity can be both explained by a 1 7 2 4 km change in crustal thickness or by a mantle temperature change of 50 c 90 c in reality melt supply crustal thickness and mantle temperature are linked so that changes in both may occur simultaneously and these estimates serve as upper bounds the along axis mba gradient is not uniform pronounced steps in the regional level of the mba gravity occur at 103 30 e 104 e and at 114 e 116 e and correspond to the changes in the nature of the axial morphology and in the amplitude of abyssal hill morphology suggesting that the different forms of morphology do not grade into each other but rather represent distinctly different forms of axial structure and tectonics with a sharp transition between them the change from an axial high to an axial valley requires a threshold effect in which the strength of the lithosphere changes quickly the presence or absence of a quasi steady state magma chamber may provide such a mechanism the different forms of axial morphology are also associated with different intrasegment mba gravity patterns segments with an axial high have an mba low located at a depth minimum near the center of the segment at epr like segments the mba low is about 10 mgal with along axis gradients of 0 15 0 25 mgal km similar to those observed at the epr rifted highs have a shallower low and lower gradients suggesting an attenuated composite magma chamber and a reduced and perhaps episodic melt supply segments with a shallow axial valley have very flat along axis mba profiles with little correspondence between axial depth and axial mba gravity

the seismic structure of 140 myr old crust in the western central atlantic ocean summary a detailed seismic refraction experiment using explosive sources and ocean bottom hydrophone obh receivers was carried out over mesozoic magnetic anomaly m17 about 300 km south west of bermuda amplitude and travel time interpretations show this 140myr old crust to be uniform on the seismic scale over a lateral distance of almost 1ookm the best estimate of the velocity structure consists of an average 700 m of sediment overlying a total thickness of igneous crust of 7 2 km the principle components beginning with a 5 km s l velocity at the top of layer 2 are a 0 4 km thickness with gradient 1 1 s a 1 9km thickness with gradient 0 64s 1 7 km thickness with gradient 0 1 s a 3 2 km thickness with zero gradient and constant velocity of 7 km s below which lies a 0 5 km thick moho transition zone the uppermost few kilometres of the upper mantle apparently has little or no velocity gradient the normal incidence two way reflection time through this structure agrees to better than 0 1 s with the location of the moho reflection seen on the ipod usgs multichannel reflection profile that passes within 1 km of this experiment the travel times of the converted shear wave arrivals constrain poisson s ratio in the igneous crust to 0 28 k 0 01 a delay time study of a 20km radius circle of shots fired around a small 6 km five element array of ocean bottom hydrophone receivers confirms the uniformity of structure of this old crust on the scale of a few kilometres the observed differences in shallow crustal delay time between the five obh instruments can be wholly attributed to differences in sediment thickness beneath the receivers as determined by a deep towed hydrophone seismic reflection profile these experiments were located clear of fracture zones or other structural anomalies in a region of well defined linear constant amplitude magnetic anomalies we propose that this was the primary cause of our result of lateral uniformity of structure on the scale of a few kilometres we suggest that the reason that such a simple uniform structural model for oceanic crust is not supported by the historical seismic refraction dataset is that a large proportion of the old experiments were poorly located relative to fracture zones and other structural anomalies unknown at the time and the

fragmentation of a porous viscoelastic material implications to magma fragmentation fragmentation of vesicular magma by rapid decompression is one of the most likely triggers for explosive eruptions in this phenomenon the decompression rate and the viscoelastic nature of magma are considered to be key factors in order to obtain a clear idea on the effects of these two factors controlled fragmentation experiments have been conducted these experiments have three advantages first the specimen is made of a viscoelastic material with controlled porosity and geometry second the fragmentation process is directly monitored finally both the magnitude and rate of decompression are controlled brittle fragmentation and ductile expansion were both observed in the same porous material at different timescales the various mechanical responses of the specimen elastic flow and fragmentation were correlated with the pressure profile measured at the base of the specimen fragmentation was noted to occur when the decompression exceeded a critical value within a critical time two relevant timescales are discussed in terms of physical mechanisms of relaxation the first is the measured glass transition time the second is the estimated timescale for the onset of viscous bubble expansion the observed phenomena bear several similarities with natural magma fragmentation it is thus considered that the present results are a useful step toward constructing a model for magma fragmentation

geometrical constraints of the 1980 mount st helens intrusion from analogue models the 1980 mount st helens cryptodome intrusion has been simulated by scaled experiments silicone used as a magma analogue was intruded into a cohesive cone that models the edifice measuring surface deformation features associated with variations of injection rate and depth of cryptodome initiation revealed linear relationships with the intrusion characteristics scaled to mount st helens dimensions the 1980 cryptodome is found to have had a maximum e w width of 850 m an initial depth of 680 to 835 m it had an ascending velocity of 5 7 7 3 m day that corresponds to a vertical displacement of 330 380 m toward the surface within the two months of deformation this brought the intrusion to less than 250 m below the crater floor at the point when the north flank failed on may 18 1980

pressure wave generated by the mount st helens eruption histories of the air pressure wave radiated from the eruption of mount st helens on may 18 1980 were calculated for two models of the eruption cloud expansion the first considered the wave radiated from an accelerated plane surface while the second examined the wave radiated from an expanding hemisphere two histories of eruption cloud motion based on photographs were used peak positive overpressures were about the same for these cloud motion histories of expansion into a hemisphere was assumed if an accelerated planar source model was used the peak positive pressures have again about the same value in east and west direction but values are about half in the north and south direction observed peak overpressures at microbarograph stations are somewhat higher than the calculated with the most marked departures at the greater surface ranges these observed overpressures may have been about half the correct values however microbarograph records show a weaker rarefaction than calculated histories or none at all this can be explained in part by a lack of areal motion coherence in the slowing eruption cloud it is also possible the net ash cloud volume increased considerably after its vertical growth ceased and weakened the negative phase as well as lengthening the positive phase

crustal velocity structure on the flanks of the mid atlantic ridge at 24 n summary a detailed seismic refraction survey was made over a 70 35 km area of young 9 ma crust at 24 n 45 w to investigate the detailed velocity structure of a portion of typical oceanic crust an array of eight ocean bottom hydrophones was used to monitor 358 explosive charges and over 750 airgun shots the crust exhibits normal oceanic structure with a steep vertical velocity gradient through layer 2 and almost 3 km of layer 3 material with a mean velocity of 6 65 km s 1 overlying probably anisotropic mantle ray tracing shows that the major amplitude variations observed in the upper crustal returns are governed more strongly by the seafloor topography than by the seismic velocity structure within the crust a delay time function method was used to invert the 1613 well determined first arrival travel times from the explosives we find that the delay time vanes by only 0 05 0 10s over most of the survey area but that there are rather larger delay times in the north west corner which may be associated with crust formed in the vicinity of a ridge jump we also find a systematic correlation between variations in delay time and in the seafloor depth at the ray entry point with smallest delay times where the seafloor is deepest this is consistent with a model of the crust in which the topography of the base of layer 2 and the moho is a subdued copy of the upper basement surface

experiments on the indentation process during cryptodome intrusions new insights into mount st helens deformation scaled experiments were carried out to study the deformation of a volcanic edifice by forcible intrusion of a cryptodome as the magma analogue is injected vertically into a sand cone asymmetric deformation is caused by the formation of a curved major shear fault which dips inward from one side of the cone to the opposite edge of the intrusion the path of the ascending silicone deviates to follow the trajectory of the fault and a lateral bulge grows slowly from the footwall of the fault the oblique push makes the bulge migrate outward causing extension upslope to form an asymmetric graben in the hanging wall of the major shear fault we suggest that the pattern of internal deformation within mount st helens prior to the may 18 1980 eruption was similar to that observed in our scaled models

a model of hydrothermal circulation in fault zones at mid ocean ridge crests summary a model of hydrothermal circulation close to mid ocean ridge crests has been developed the flow is modelled as a transient open loop thermosyphon restricted to a single fracture parallel to the spreading centre circulation is to a depth of 500 1000m and heat exchange occurs at the fault walls conduction is the sole heat transfer mechanism within the rock the aim is to model the conditions under which deposition of ore deposits such as those in the cyprus ophiolites could occur hot springs with the characteristics of the black smokers at 21 n on the east pacific rise are obtained from the model but only when the vents are at the unrealistically large spacing of lokm apart and then water at over 350 c is vented for a maximum of only 175 yr the model demonstrates that heat transfer through the rock by conduction alone is too slow to maintain the power output of a group of black smokers the skin depth of the temperature field limits the total heat available to the system but even without this constraint insufficient high grade heat would be available from the volume of rock within the likely field of influence of a vent it would take about 2500yr to accumulate a 3 million ton ore deposit from 1 vent or 250yr from a group of 10 vents the rate of heat extraction and the duration of the vents must both be at least an order of magnitude greater than those obtained from this model thus requiring a much greater source of heat than in the rocks alone the only source likely to be great enough is the magma chamber itself

crustal structure of the mid atlantic ridge at 23 n from seismic refraction studies an explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120 km long section of the mid atlantic ridge mar median valley at latitude 23 n immediately south of its intersection with the kane fracture zone the data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques these solutions are combined using two dimensional ray tracing to produce a single model defining the major along axis structural changes occurring beneath this section of the median valley with the exception of slightly low layer 3 velocities the crust beneath several tens of kilometers of the mar median valley has all the characteristics of simple mature ocean crust including a well defined moho transition zone 8 km s upper mantle velocities and a total crustal thickness of 6 7 km no evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along axis we infer from these observations that the basic seismic characteristics of the ocean crust are frozen in in a time period less than that separating the major volcanic injection events 10 000 50 000 years by hydrothermal circulation penetrating to the base of the crust along the 100 km long ridge segment studied here two major along axis structural changes occur the first is a 10 to 15 km wide along axis zone of lowered velocities in the lower crust centered beneath a major along axis topographic high this is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities the second occurs abruptly at latitude 23 15 n and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly north of this boundary the 30 km long ridge segment that abuts the kane fracture zone has a crustal thickness of 4 5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust interpretation of the cause of this structural change is uncertain but may be related to local tectonic events perhaps a recent 10 to 20 km ridge jump to the east

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

anomalously thin crust in oceanic fracture zones new seismic constraints from the kane fracture zone three detailed and carefully positioned seismic refraction experiments have recently been carried out along the kane fracture zone near 24 n 44 w in order to place better geophysical constraints on the extent and origin of the anomalously thin crust reported from parts of this and other large atlantic fracture zones these three experiments when combined with earlier studies result in reversed structural control along about a 300 km long segment of the kane fracture zone extending from the ridge transform intersection out onto crust about 25 m y old an analysis of refraction lines obtained along the fracture zone trough by using both travel time modeling and delay time function techniques reveals large variations in crustal thickness and or velocity however along most of the eastern fracture zone trough upper mantle type velocities are found at depths of only 2 3 km below the seafloor less than half the typical depth to moho in the ocean basins this anomalous fracture zone crust is generally characterized by unusually low compressional wave velocities relatively high velocity gradients throughout most of the thinned crustal section about 1 2 s 1 and a distinct crust mantle boundary the very thin crust appears to be confined to the deepest parts of the kane fracture zone trough 10 km wide although a more gradual crustal thinning may extend up to several tens of kilometers from the fracture zone a specially designed delay time experiment at the intersection of the kane fracture zone and the mid atlantic ridge also revealed the presence of a thinned crustal section of a relatively uniform thickness about 3 km except beneath the nodal basin where the crust may be less than 1 km thick the anomalous crust in the intersection area appears to extend at least part way down the median valley we interpret this anomalous fracture zone crust as a primary accretionary feature that forms as a result of a restricted magma supply near ridge transform intersections although tectonic processes may also play a part at least locally in thinning the crust our results support a ribbon model of the gross seismic structure of the ocean basins in which bands of relatively homogeneous so called normal crust 50 100 km wide extend away from spreading centers each formed at a discrete ridge crest segment and each separated by the thin crust underlying the fracture zone trough

microearthquakes beneath the median valley of the mid atlantic ridge near 23 n hypocenters and focal mechanisms we report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the mid atlantic ridge near 23 n during a 3 week period in early 1982 the network consisted of seven ocean bottom hydrophones and three three component ocean bottom seismometers the instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence the hypocentral parameters of the 26 largest microearthquakes are reported 18 of these events have epicenters arid focal depths which are resolvable to within 1 km formal error at the 95 confidence level microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30 or more these solutions are similar to the mechanisms of nearby large earthquakes microearthquakes also occur beneath the steep eastern inner rift mountains the rift mountain earthquakes have nominal focal depths of 5 7 km and epicenters as distant as 10 15 km from the center of the median valley but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure we interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7 8 km we infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment

crustal structure of the mid atlantic ridge south of the kane fracture zone from seafloor and sea surface gravity data seafloor and sea surface gravity data are inverted together to construct a model for the near axis crustal structure of a slow spreading ridge the seafloor data set offers two main advantages it allows us to recover shorter wavelengths signal and to constrain the value of a potential field at two different levels the model we propose here would not have been derived from sea surface data alone it is based on a dense sea surface gravity coverage and on 121 sea bottom gravity measurements collected in the mid atlantic ridge at kane mark area during the hydrosnake 1988 and gravinaute 1993 cruises the primary goal of the seafloor surveys was to test for the presence of a magma reservoir beneath the axial neovolcanic ridge first a forward two dimensional 2 d model of the crustal structure across the axis is fit to observed gravity anomalies using constraints from geological and structural observations bouguer anomalies computed from sea bottom measurements and downward continuation of sea surface measurements both constrain the forward modeling this forward model is the starting point of a 2 d monte carlo inversion of seafloor and sea surface data in addition to the crustal thickness variations along axis our data document the amplitude variations of the crustal thickness and or its density in the across axis direction the model resulting from our inversion exhibits several features of the crustal structure in the mark area 1 the presence of a low density ap 300 50 kg m 3 body beneath the neovolcanic ridge is suggested and could correspond to a magma chamber or more probably to a highly hydrothermally fissured zone 2 both long and short wavelength gravity signals exhibit a difference between the western and eastern sides of the axial domain the mean value and the amplitude of bouguer anomalies are higher on the western part this difference suggests that axial processes in this area are very asymmetric 3 abyssal hills are not associated with a single gravity signature for instance on the west side of the axis one of the explored hills has no bouguer anomaly and is interpreted as a neovolcanic ridge whereas the others are associated with a shifted bouguer anomaly high and are interpreted as having thinner magmatic crust 4 the last feature of the crustal fabric we document here is the asymmetric emplacement of some deep rocks outcrops in the mark area we find that pink hill a topographic high where serpentinized peridotites are outcropping is much more serpentinized on its east flank toward the axial valley than on its west flank alteration occurring mainly by fluid circulation through faulted zones the asymmetric serpentinization suggests that deep origin rocks have outcropped by means of a main fault zone and are not emplaced by diapirism

a numerical model for simulating deformation of mount st helens volcano previous work on surface deformation associated with volcanic activity has revealed significant departures from ideal elastic and homogeneous behavior to account for the highly heterogeneous and discontinuous mechanical behavior of volcanic edifices a numerical model named bloc was used to model surface deformation data the basic assumption of this model is that the volcanic edifice behaves mechanically as an assemblage of rock blocks that can move in relation to one another the well documented eruption of mount st helens in 1980 was used to test this model and to compare the fit between the observed and computed surface deformation from the north south cross section of the volcano several two dimensional models were developed to study the effect of various parameters geometry boundary conditions coefficient of friction etc the models were loaded with a step by step increasing internal pressure simulating the intrusion of magma within the volcano and with a horizontal acceleration simulating an earthquake the deformation and failure patterns are highly dependent on the coefficient of friction on block boundaries for the lower value of this coefficient 0 5 a good fit is found between the observed and computed surface displacements the failure mode by gravitational sliding of the north flank is obtained with internal pressure loading alone as well as with internal pressure and an earthquake trigger however the second kind of loading gives a better fit with the particular deformation pattern observed

the correction for the travel time effects of seafloor topography in the interpretation of marine seismic data problems associated with the application of topographic corrections to deep ocean marine seismic data are described synthetic travel time data are used to show that corrections for the delay time introduced by the water column are inferior to corrections for the true water path water path corrections place the shots at the ray entry points on the seafloor require no assumptions about the structure under investigation and maintain the correct distribution of data with range the water path correction can be most accurately applied during the stacking operation that transposes the observed data into intercept time slowness space there is then no requirement to make determinations of slowness and for experiments involving seafloor receiver and surface shots the data may be transposed to the seafloor with complete objectivity water path corrected stacks from different locations may then be compared in the sure knowledge that any observed differences are due to real structural changes and not to topographic correction errors or to invalid assumptions of lateral homogeneity

upper crustal velocity structure in the rose area of the east pacific rise we describe a method to constrain the seismic velocity structure of the upper 500 800 m of young oceanic crust conventional interpretations of seismic refraction profiles provide little or no information about this shallowmost crust the method described here requires the assumption of a linear velocity gradient and utilizes measured values of range travel time and slowness of the observed refracted waves results are reported from several refraction profiles that sample crust between 0 and 4 m y in age on the flanks of the east pacific rise at 12 n these data give an average gradient in the uppermost 800 m of the crust of 3 5 s 1 and a velocity at the seafloor of 2 5 km s ray path and travel time modeling show that the refracted waves sampling the uppermost part of the crust are effectively masked by reflections from the seafloor

grain size characteristics of experimental pyroclasts of 1980 mount st helens cryptodome dacite effects of pressure drop and temperature using the fragmentation bomb we analysed the effects of temperature and pressure drop on the grain size characteristics of experimentally produced pyroclasts experiments performed on vesicular samples of grey dacite of the 1980 mount st helens cryptodome at t 20 900 c and initial pressure differential up to 18 5 mpa provide clear evidence of the influence of these physical conditions upon fragment size and character cylindrical dacite samples diameter 17 mm length 50 mm are placed in the high pressure temperature section of the apparatus heated and saturated by argon gas the disruption of a diaphragm located between the high and low pressure sections of the apparatus initiates the rapid depressurisation of the sample the main results may be summarised as follows 1 increasing tempera ture from 20 to 900 c results in a decrease in the fragmentation threshold value from 9 to 3 mpa and an increase in the median diameter of the experimental pyroclasts these observations imply a decrease in the dynamic tensile strength of dacite at higher temperatures which in turn influences the characteristic size of fragments 2 increasing initial pressure differential yields a decrease of the median diameter thus a higher initial elastic potential energy in the magma generates a higher degree of fragmentation 3 fragments of angular shape are observed from experiments at all investigated temperatures 20 900 c including thereby temperatures significantly higher than the classical dilatometrically or calorimetrically determined glass transition temperature determined for this dacite of 810 c thus brittle response of the dacite is observed under rapid decom pression 4 fragment size distributions do not corre spond to log normal distributions and are more closely described by rosin rammler distributions with a de crease of temperature and increase of the initial pressure differential fragment size distributions approach a rosin rammler distribution dependence 5 microscopic and grain size comparison of particle characteristics of ex perimental and natural fragments of the 1980 mount st helens cryptodome dacite demonstrate similar median diameters and shapes

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

physical processes in volcanic eruptions mechanisms are modeled in terms of dynamic physico chemical processes in this respect volcanology shares with much of earth science in general the benefits that accrue from the application of the methods and concepts of other disciplines to its problems basic concepts from the fields of thermo dynamics fluid mechanics and rheology have been particularly impor tant in fostering new insights recent developments have been so numerous as to make it necessary to limit the scope of this review and thus we concentrate here on the basic physical processes involved in the rise of magma to the surface and the distribution of cruption products on thc ground or in the atmosphere the perspective of planetary volcanism wilson hcad 1983 underlines the fact that volcanoes can be regarded as probes of the composition and structure of a planetary interior as a function of position depth and time

magma fragmentation dynamics experiments with analogue porous low strength material experiments were conducted on the fragmentation of analogue low strength porous material plastiprin by rapid decompression in a shock tube type apparatus the porous samples length 365 mm cross section dimensions 40 40 mm pressurized by air to pressures up to 0 9 mpa were rapidly decompressed to 0 1 mpa rapid decompression of samples caused fragmentation and ejection of the fragmentation products into a large volume tank the process of analogue material fragmentation was documented using high speed cinematography and dynamic pressure measurements the duration of the fragmentation event is significantly shorter than that of the ejection event the fragmentation of material precedes the acceleration of fragments as a result of fragmentation sub parallel fractures are generated the characteristic fragment size decreases as the initial pressure differential increases the ejected fragments obtain velocities of 60 m s the mechanisms of material fragmentation during unloading and fragmentation wave propagation are discussed the experimental results provide insight into the fragmentation dynamics of highly viscous magmas in which brittle failure at high strain rate is possible