hydrothermal alteration and fluid circulation related to w au and sb vein mineralization haut allier massif central france w as au and as sb vein mineralizations in the haut allier french massif central are associated with ortho and parametamorphic rocks containing biotite or sillimanite which have been locally intruded by small granitoid bodies hydrothermal alteration is well developed around these veins and has been studied here from a petrographic and a chemical point of view the results are compared with those which were obtained by microthermometric study of fluid inclusions in both mineralized types three superimposed hydrothermal stages are distinguished showing good agreement with the results obtained by other methods in the early stage fluids enriched with co 2 or ch 4 developed a quartz 2m1 muscovite assemblage with 8 paragonitic substitution at temperatures of about 400 degrees c minor chlorite appears replacing biotite near the transition of altered to fresh rocks this early stage is predominant in veins with w as au mineralization in the intermediate stage low salinity aqueous fluids dilute the earlier fluids and control a new alteration with quartz 2m1 phengite or carbonate assemblages at slightly lower temperatures this alteration stage is particularly important in the environment of veins with as sb mineralization in the late stage the last hydrothermal event occurs at a low temperature 200 degrees c and forms a 1m illite kaolinite or quartz assemblage in small alteration veinlets chemically the different phyllosilicates that successively occur in w as au and as sb veins are very similar the differences between these mineralization types can be explained by the proximity of the granitoid bodies to w as au veins and by a greater fluid rock ratio involved in the alteration processes for as sb vein deposition

steep sided domes on venus characteristics geologic setting and eruption conditions from magellan data a survey of more than 95 percent of the venus surface reveals 145 steep sided domes which can be subdivided into a variety of morphologic forms the most common being shaped like inverted bowls or flat topped domes results of a preliminary analysis of the distribution and geologic setting of the domes are presented the relation of the domes to analogous terrestrial features is examined and possible models for their mode of emplacement are outlined

magmatic processes revealed by textural and compositional trends in merapi dome lavas syn eruptive degassing of volcanoes may lead to syn eruptive crystallization of groundmass phases we have investigated this process using textural and compositional analysis of dome material from merapi volcano central java indonesia samples included dome lavas from the 1986 88 1992 93 1994 and 1995 effusive periods as well as pyroclastic material deposited by the november 1994 dome collapse with total crystallinities commonly in excess of 70 phenocrysts 1 microlites the liquids present in merapi andesites are highly evolved rhyolitic at the time of eruption feldspar microlites in dome rocks consist of plagioclase cores ab63an29or8 surrounded by alkali feldspar rims ab53an5or42 compositional pairs which are not in equilibrium a change in the phase relations of the ternary feldspar system caused by degassing best explains the observed transition in feldspar composition a small proportion of highly vesicular airfall tephra grains from the 1994 collapse have less evolved glass compositions than typical dome material and contain rimless plagioclase microlites suggesting that the 1994 collapse event incorporated less degassed partially liquid magma in addition to fully solidified dome rock as decompression drives volatile exsolution rates of degassing and resultant microlite crystallization may be governed by magma ascent rate microlite crystallinity is nearly identical among the 1995 dome samples an indication that similar microlite growth conditions ph2 o and temperature were achieved throughout this extrusive period however microlite number density varied by more than a factor of four in these samples and generally increased with distance from the vent low vent ward microlite number densities and greater microlite concentrations down flow probably reflect progressively decreasing rates of undercooling at the time of crystal nucleation during extrusion of the 1995 dome comparison between dome extrusion episodes indicates a correlation between lava effusion rate and microlite number density suggesting that extrusion slowed during 1995 crystal textures and compositions in the 1992 93 and 1994 domes share the range exhibited by the 1995 dome suggesting that transitions in crystallization conditions i e rates of undercooling determined by effusion rate are cyclic q 2000 elsevier science b v all rights reserved

effects of eruption history and cooling rate on lava dome growth to better understand the factors controlling the shapes of lava domes laboratory simulations measurements from active and prehistoric flows and dimensional analysis were used to explore how effusion history and cooling rate affect the final geometry of a dome fifty experiments were conducted in which a fixed volume of polyethylene glycol wax was injected into a tank of cold sucrose solution either as one continuous event or as a series of shorter pulses separated by repose periods when the wax cooling rates exceeded a critical minimum value the dome aspect ratios height diameter increased steadily with erupted volume over the course of a single experiment and the rate at which height increased with volume depended linearly on the time averaged effusion rate thus the average effusion rate could be estimated from observations of how the dome shape changed with time our experimental results and dimensional analyses were compared with several groups of natural lava flows the recently emplaced mount st helens and soufriere domes which had been carefully monitored while active three sets of prehistoric rhyolite domes that varied in eruptive style and shape and two sets of holocene domes with similar shapes but different compositions geometric measurements suggest that dome morphology can be directly correlated with effusion rate for domes of similar composition from the same locality and that shape alone can be related to a dimensionless number comparing effusion rate and cooling rate extrapolation to the venusian pancake domes suggests that they formed from relatively viscous lavas extruded either episodically or at average effusion rates low enough to allow solidified surface crust to exert a dominating influence on the final morphology

degassing of rhyolitic magma during ascent and emplacement the degassing history of a rhyolitic igneous system was documented from analyses of drill core samples through the extrusive and intrusive portions of obsidian dome and of surface samples of associated tephra the initial volatile composition of the inyo magma was estimated to be 4 0 wt h2o 500 ppm f 800 ppm cl and 80 ppm s retained volatile contents of glassy and crystalline samples reflect the effects of decompression and second boiling decompression is rapid and involves loss of water rich fluid until a close approach to lithostatic equilibrium is achieved second boiling is a slower process and produces a chlorine rich fluid some of which can be trapped during development of extremely fine crystallization textures nearly complete dewatering during decompression of surface extruded magma strongly undercools the system t 175 c suppressing crystallization and yielding glassy rhyolitic lava partial degassing of shallowly intruded magma permits pervasive crystallization even at high cooling rates the subvolcanic intrusive regime is the zone of maximum volatile release because second boiling is incomplete in extrusives and volatile bearing crystalline phases are stable in magma crystallized at greater depth

lidar based quantification of lava flow susceptibility in the city of auckland new zealand abstract lava flows represent one of the most significant volcanic hazards from basaltic monogenetic volcanoes such as spatter cones scoria cones maars and tuff rings they are common features emanating from parasitic vents on the flanks of polygenetic volcanoes and in dominantly flat lying intraplate volcanic fields the auckland volcanic field avf is a volcanic field that has been active for the last ca 250 ka hosting at least 50 monogenetic volcanoes morphometric parameters of lava flows such as volume length thickness and area were used to quantify the potential lava flow inundation susceptibility to new zealand s most densely populated area the city of auckland based on an airborne light detection and ranging lidar digital surface model dsm the morphometric parameters of fifteen studied flows included average length of 2 5 km range 0 7 6 5 km overall average thickness of 14 8 m range 3 4 43 8 m average of maximum thicknesses of 48 2 m range 18 3 180 5 m average area occupied of 5 1 km 2 range 0 4 25 1 km 2 and average volume of 0 12 km 3 range 0 005 1 km 3 based on these parameters and a lidar derived dsm the present topography was classified into sea topographic depressions low lying areas prone to inundation by an average lava flow buffer zones prone to inundation only by extremely thick lava flows and peaks or ridges which are unlikely to be overtopped in monogenetic fields each new vent occurs in a new location creating uncertainty around the spatial location of the volcanic hazard thus this research provides a general vent location independent approach to describe the lava flow susceptibility for a potentially active monogenetic volcanic field what this analysis reveals is that the city of auckland can be divided into two distinct areas with strongly different susceptibility to lava flow inundation the southern part of the city is predominantly flat without hindrance to lava flow whereas the hilly northern and central part has many ridges that can limit or channelise lavas these contrasting properties must be accounted for in scenario based or probabilistic hazard and risk models developed for the avf

pahoehoe and aa in hawaii volumetric flow rate controls the lava structure the historical records of kilauea and mauna loa volcanoes reveal that the rough surfaced variety of basalt lava called aa forms when lava flows at a high volumetric rate 5 10 m3 s and the smooth surfaced variety called pahoehoe forms at a low volumetric rate 5 10 m3 s this relationship is well illustrated by the 1983 1990 and 1969 1974 eruptions of kilauea and the recent eruptions of mauna loa it is also illustrated by the eruptions that produced the remarkable paired flows of mauna loa in which aa formed during an initial short period of high discharge rate associated with high fountaining and was followed by the eruption of pahoehoe over a sustained period at a low discharge rate with little or no fountaining the finest examples of paired lava flows are those of 1859 and 1880 1881 we attribute aa formation to rapid and concentrated flow in open channels there rapid heat loss causes an increase in viscosity to a threshold value that varies depending on the actual flow velocity at which when surface crust is torn by differential flow the underlying lava is unable to move sufficiently fast to heal the tear we attribute pahoehoe formation to the flowage of lava at a low volumetric rate commonly in tubes that minimize heat loss flow units of pahoehoe are small usually 1 m thick move slowly develop a chilled skin and become virtually static before the viscosity has risen to the threshold value we infer that the high discharge rate eruptions that generate aa flows result from the rapid emptying of major or subsidiary magma chambers rapid near surface vesiculation of gas rich magma leads to eruptions with high discharge rates high lava fountains and fast moving channelized flows we also infer that long periods of sustained flow at a low discharge rate which favor pahoehoe result from the development of a free and unimpeded pathway from the deep plumbing system of the volcano and the separation of gases from the magma before eruption achievement of this condition requires one or more episodes of rapid magma excursion through the rift zone to establish a stable magma pathway

the magma system of mount st helens non linear high resolution p wave tomography abstract high resolution three dimensional images of p wave velocity anomalies below mt st helens washington were derived using tomographic inversion the model is a 27 5 21 20 km target volume parameterized by blocks 0 5 km per side the area included 39 stations and 5454 local events leading to 35 475 rays used in the inversion to diminish the effects of noisy data the laplacian was constrained to be small within horizontal layers providing smoothing of the model non linear effects were compensated for by iterating three dimensional ray tracing using pseudo bending between inversions and relocating earthquakes relative to the updated three dimensional model the structural differences between the linear and non linear inversions appear to be insignificant although the amplitudes of the anomalies are larger in the non linear models results indicate a low velocity anomaly 7 approximately 1 km in lateral extent from 1 5 to 3 km depths between 3 and 6 km depth the anomaly appears to spread out below 6 km depth the low velocity feature changes to a higher velocity perturbation with lower velocity perturbations flanking around the perimeter of the volcano the higher velocity material which correlates with the higher seismicity at that depth is interpreted as being a plug capping the low velocity magma chamber which begins below 9 km depth

effects of compressibility on the flow of lava lava contains gas bubbles and hence is a compressible liquid whose density increases as a function of pressure after eruption at the earth s surface it spreads at a rate which is a function of its thickness and it is compressed under its own weight therefore both thickness and spreading rate are determined by a balance between viscous and compressible effects theoretical equations are derived for the shape and velocity of a compressible liquid spreading on a horizontal surface solutions are obtained for a fixed eruption rate q the radial extent of the flow increases proportional to t1 2 a dimensionless number c is defined which characterizes the importance of flow compression c rho 0 beta g left frac mu q rho 0 2 g right 1 mathord left vphantom 1 4 right kern nulldelimiterspace 4 where 0 is bubbly lava density at atmospheric pressure compressibility viscosity and g the acceleration of gravity c can be thought of as the ratio of two characteristic length scales one for compression effects and one for viscous effects the larger c is the more important compressibility effects are as c is increased the flow becomes thinner because the liquid is compressed more and more efficiently compressibility acts to smooth out variations of flow thickness which provide the driving force thus all else being equal a compressible liquid flows less rapidly than an incompressible one when trying to infer the effective viscosity of a flow from its spreading rate the neglect of compressibility leads to an overestimate the various factors which act to determine the distribution of gas bubbles in lava flows are reviewed and discussed quantitatively comparison with data from obsidian dome eastern california shows that disequilibrium effects are important and that bubble resorption during burial in a thick flow is not a pervasive phenomenon the analysis is applied to the 1979 dome of soufriere de saint vincent w i an effective value of compressibility for this 100 m thick dome is 1 5x10 6 pa 1 this implies that all else being equal the viscosity of this lava may be overestimated by a factor of 5 if no account is taken of the compressible nature of the flow

methods of determining the rheological properties of magmas at sub liquidus temperatures non vesicular basaltic melts behave as newtonian fluids at temperatures above their liquidus and their viscosities can be calculated using a method developed by shaw 1972 and bottinga and weill 1972 because many igneous processes involve the flow of silicates at sub liquidus temperatures numerous attempts have been made to calculate the interactive effects of the crystal phase however current methods are appropriate only for relatively low crystal concentrations and they assume newtonian behaviour we argue that this assumption is invalid when bubble or crystal concentrations exceed 30 at higher concentrations factors other than concentration become increasingly important these include particle shape and size distribution a method which takes these factors into account is tested on a range of suspensions including magmas at sub liquidus temperatures and rheological properties calculated using this method agree closely with the measured values we also demonstrate that an equation which was introduced to explain large differences in measured apparent viscosities during the cooling and crystallisation of mount st helens dacite murase et al 1985 and which is currently used to calculate the rheological properties of crystallising lavas generates viscosities which may be in error by several orders of magnitude this difference is argued to be caused by a combination of factors including the ten orders of magnitude range in the strain rates utilised during the mount st helens measurements causing orders of magnitude difference in the resulting apparent viscosities rheological data on crystallising silicic melts are reinterpreted taking into account the non newtonian rheology of the magmas and changes in activation energy of flow during crystallisation

the la belliere gold and antimony district french armorican massif a two stage evolution model controlled by variscan strike slip tectonic the la belliere gold district in the ligerian domain of the armorican massif is the third one of france in term of production it shows gold bearing quartz veins neighboring with a complex network of sb veins hosted in the neoproterozoic metasedimentary basement of the mauges nappe new structural data mineralogical textural and geochemical assays have been carried out to clarify the gold and sb veins relationships our results favor of two step hydrothermal event with an early deep seated gold event followed by a late shallower sb event the gold bearing quartz veins are hosted by n60 e to n90 e sinistral strike slip faults these veins exhibit a four stage paragenesis 1 early barren quartz filling 2 arsenopyrite pyrite with minor scheelite assemblage 3 gold and base metal sulfides crystallized after a ductile brittle deformation stage and 4 pyrite carbonate coeval with a late brittle stage these results support a continuous evolution model from a deep emplacement level 12 to 9 km to a shallow one 7 to 5 km with a brutal change in the p t conditions that triggered the deposition of gold stage 2 controlled by a seismic valve mechanism on the contrary sb veins correspond to open space vein types hosted in a n20 w to n20 e conjugated strike slip fault network arranged in a riedel shear model controlled by n130 140 e regional dextral strike slip shear zone associated to the south armorican shear zone system a two stage evolution is recognized the first one consists of early pyrite arsenopyrite with microcrystalline quartz followed by a late stibnite stage formed after geodic quartz deposition quartz texture argues for a shallow emplacement 5 km of veins controlled by a suction pump mechanism chronologically the gold vein opening complies with a ne sw maximum shortening direction different from the late variscan stress field responsible for the dextral armorican shear zone during late carboniferous and constrains an indirect dating thus a visean age is proposed for the formation of gold veins the overall features of gold deposits are close to those of mesozonal orogenic gold deposits in europe conversely sb bearing veins are generated by a n s shortening during the late carboniferous the sb event distinct in time from the earlier gold one might correspond to a different expression of the so called epizonal type in the orogenic gold model it is proposed that the late carboniferous or 300 event is not the unique metallogenic period for gold deposition in the french variscan domain our study highlights the role of the sinistral strike slip shearing of possible visean age in the gold metallogenesis of the ligerian domain

morphometrical and geochronological constraints on the youngest eruptive activity in east central europe at the ciomadul csomad lava dome complex east carpathians abstract the timing of late pleistocene volcanic activity of the ciomadul csomad dacitic lava dome complex site of the youngest eruptions in the carpathians has been constrained by morphometric analysis and radiometric chronology pelean domes and asymmetric domes coulees built up the volcano including the central edifice that hosts the youngest twin craters of mohos mohos peat bog and lake st ana szent anna a comparative digital elevation model dem based morphometric analysis of lava domes 29 worldwide examples including 5 domes from ciomadul shows that it is the mean slope of the upper dome flank that correlates best with age although the logarithmic relationship is only moderately strong r 0 80 slope characteristics of the ciomadul domes fit to those of 10 100 ka old domes these young ages contradict the previous k ar dates giving as old as 1 ma ages on a number of domes but are supported by ongoing u pb and u th he zircon dating the latter methods constrain the whole volcanic activity to the past 250 ka and the emplacement of most lava domes within the period of 150 100 ka the volcanism at ciomadul produced alternating effusive and explosive eruptions including lava dome collapses and successive crater formations the latest possibly subplinian explosive event formed the well preserved st ana crater radiocarbon dating of organic remains from a sediment core that reached 11 m into the lacustrine infill of st ana suggests that the crater was formed prior to 26 000 years bp

a laboratory analog study of the surface morphology of lava flows extruded from point and line sources abstract in laboratory experiments designed to model lava flow processes liquid polyethylene glycol wax is forced through either a small hole or a long narrow slit onto the base of a tank of cold water where it spreads laterally while cooling and solidifying at its surface we observe the surface structure of the flow and its dependence on the flow rate thermal conditions and basal roughness in each case solidification of a crust during spreading gives rise to a number of different surface morphologies each of which forms under a restricted range of conditions the dominant morphologies referred to as pillows rifts transverse folds and marginal levees correspond to features observed on natural lava flows results for radial spreading over a rough base and for spreading in two directions from the line source over both smooth and rough bases complement those reported earlier for radial spreading of solidifying wax from a point source on a smooth base together they indicate a robust dependence of morphology on the distance from the vent at which solid crust begins to form this distance is in turn determined by the extrusion rate the rheology of the liquid wax or magma the reduced gravity the magnitude of the surface heat flux and the amount of cooling required to solidify the flow surface the results also indicate factors influencing the distribution of crust and its deformation and may provide a means by which observations of surface morphology can be used to place constraints on the emplacement conditions of lava flows

lava flow shapes and dimensions as reflections of magma system conditions abstract the physical conditions producing variation in the shapes of lava extrusions from thin flows to domes to spines are investigated with focus on the influence of the lava thickness the accumulation of lava above the vent acts to increase the vent pressure and reduce the eruption rate in turn the eruption rate determines how the volume and thickness of erupted lava change with time the spreading of lava and the flow of magma from a deep reservoir through a conduit system are therefore coupled a fluid dynamical model is developed to evaluate the influence of magma chamber and conduit dimensions chamber overpressure magma viscosity and dome bulk viscosity on the eruption behaviour and dimensions of the lava flow the model assumes laminar flow of newtonian constant density magma from an overpressured reservoir with elastic walls up a vertical cylindrical conduit and onto a planar horizontal surface on which grows a radially symmetrical lava flow of constant bulk viscosity greater than the magma viscosity two dimensionless numbers characterize the resulting behaviour the ratio of a scale erupted lava volume to the total volume available for eruption and the ratio of the pressure due to a scale thickness of lava to the magma chamber overpressure the dimensionless numbers are used to map the flow behaviours and reveal two extreme extrusion regimes separated by a narrow transition zone one extreme corresponds to thin and voluminous flows and occurs for large deep strongly overpressured magma reservoirs narrow conduits and low viscosity contrasts between the magma and lava the other extreme corresponds to small volume domes and spines and is produced by shallow weakly overpressured chambers wide conduits and high viscosity contrasts near the transitional domain small changes of parameters are sufficient to cause major changes of eruption regime the model provides explanations for the variation in gross morphology of lava extrusions transitions between flow dome and spine eruptions and for long lived episodic extrusion of lava model predictions are consistent with features of the eruptions of montagne pelee 1902 1903 mount st helens 1980 1986 and montserrat 1995 1997 period the model is used to estimate the magnitude of overpressure driving eruptions and suggests that magma buoyancy is important in driving many terrestrial lava dome eruptions the model further suggests that thick extrusions on venus were driven primarily by buoyancy sourced at great depth

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

new approaches to inferences for steep sided domes on venus abstract new mathematical approaches for the relaxation and emplacement of viscous lava domes are presented and applied to steep sided domes on venus a similarity solution approach is applied to the governing equation for fluid flow in a cylindrical geometry for two distinct scenarios in the first scenario dome relaxation is explored assuming a constant volume of fluid i e lava has been rapidly emplaced onto the surface cooling of lava is represented by a time variable viscosity and singularities inherent in previous models for dome relaxation have been eliminated at the onset of relaxation bulk dynamic viscosities lie in the range between 10 10 10 16 pa s consistent with basaltic andesite to rhyolitic compositions plausible relaxation times range from 5 to 5000 years depending on initial lava viscosity the first scenario however is only valid during the final stages of dome relaxation and does not consider the time taken for lava to be extruded onto the surface in the second scenario emplacement and growth of a steep sided dome is considered when the volume of lava on the surface increases over time i e time variable volume approach the volumetric flowrate may depend on an arbitrary power of the dome thickness thus embracing newtonian as well as other rheologies for describing terrestrial and planetary mass flows the approach can be used to distinguish between basic flowrate models for fluid emplacement the formalism results in radial expansion of a dome proportional to t 1 2 consistent with the diffusive nature of the governing equation the flow at the front is shown to thicken as the front advances for a constant rate of lava supply emplacement times are intimately correlated with the bulk rheology comparison of the theoretical profiles with the shape of a typical dome on venus indicates that a newtonian bulk rheology is most appropriate consistent with prior studies however results here suggest a bulk dynamic viscosity of 10 12 10 13 pa s and emplacement times of approximately 2 16 years both scenarios investigated give emplacement times significantly less than prior estimates and compositions consistent with basaltic andesite

influence of cooling on lava flow dynamics experiments have been carried out to determine the effects of cooling on the flow of fluids with strongly temperature dependent viscosity radial viscous gravity currents of warm glucose syrup were erupted at constant rate into a flat tank filled with a cold aqueous solution cold viscous fluid accumulates at the leading edge altering the flow shape and thickness and slowing the spreading the flows attain constant internal temperature distributions and bulk viscosities the value of the bulk viscosity depends on the peclet number which reflects the advective and diffusive heat transport properties of the flow the flow skin viscosity which reflects cooling and the eruption viscosity our results explain why most lava flows have bulk viscosities much higher than the lava eruption viscosity the results can be applied to understanding dynamic lava features such as flow front thickening front avalanches and welded basal breccias

ages k ar des mineralisations de brioude massiac w au as sb pb zn pontgibaud pb ag sn et labessette as pb sb au place de ces districts dans l evolution geotectonique du massif central francais de nombreuses mineralisations filoniennes polymetalliques recoupent le socle metamorphique et granitique varisque du massif central trois de ces districts notamment brioude massiac w as au as sb au pontgibaud as sn et pb ag et labessette as sb pb au ont recemment fait l objet d etudes sur les condition thermodynamiques du depot des minerais et sur les sources potentielles de leurs elements metalliques une succession d episodes hydrothermaux de haute moyenne et basse temperature developpe une intense alteration phyllosilicatee a muscovite phengite illite et kaolinite dominantes

the ongoing dome emplacement and destruction cyclic process at popocatepetl volcano central mexico the ongoing eruptive activity of popocatepetl volcano has been characterized by emplacement and subsequent destruction of a succession of lava domes between the onset of the current eruption in 1994 and the time of this submission 38 episodes of lava dome formation and removal have been identified each dome has showed particular features related to the magma extrusion process among other manifestations dome emplacement events have been usually accompanied by relatively low intensity protracted explosions referred to as exhalations after variable times of residence emplacements have ended in partial or total destruction of the domes by strong vulcanian explosions that produced sizeable ash plumes with most of them also ejecting incandescent debris onto the volcano flanks here we present a detailed account for the observed activity related to the domes growth and destruction related seismic monitoring signals and morphological features of the domes based on 19 years of visual observations and image analysis we then discuss a model for the process of dome growth and destruction and its hazard implications

periodic behavior in lava dome eruptions lava dome eruptions commonly display fairly regular alternations between periods of high activity and periods of low or no activity the time scale for these alternations is typically months to several years here we develop a generic model of magma discharge through a conduit from an open system magma chamber with continuous replenishment the model takes account of the principal controls on flow namely the replenishment rate magma chamber size elastic deformation of the chamber walls conduit resistance and variations of magma viscosity which are controlled by degassing during ascent and kinetics of crystallization the analysis indicates a rich diversity of behavior with periodic patterns similar to those observed magma chamber size can be estimated from the period with longer periods implying larger chambers many features observed in volcanic eruptions such as alternations between periodic behaviors and continuous discharge sharp changes in discharge rate and transitions from effusive to catastrophic explosive eruption can be understood in terms of the non linear dynamics of conduit flows from open system magma chambers the dynamics of lava dome growth at mount st helens 1980 1987 and santiaguito 1922 2000 was analyzed with the help of the model the best fit models give magma chamber volumes of 0 6 km3 for mount st helens and 65 km3 for santiaguito the larger magma chamber volume is the major factor in explaining why santiaguito is a long lived eruption with a longer periodicity of pulsations in comparison with mount st helens

nd sr pb isotopic constraints on metal and fluid sources in w sb au mineralization at woxi and liaojiaping western hunan china this paper presents nd sr pb isotope data on scheelite inclusion fluids and residues of gangue quartz and sulfides from the w sb au ore deposits at woxi and liaojiaping ljp in the xuefeng uplift belt xub western hunan china sm and nd concentrations in scheelite from woxi are much lower than in scheelite from ljp and can be distinguished by their high 147sm 144nd ratios of 1 25 from the much lower ratios around 0 26 in scheelites from the ljp end values compared to values at 200 ma which is the average timing of granitoid emplacement during the indosinian yanshanian periods in the xub are around 10 for the ljp and compare well with the range of 5 to 11 defined by the granitoids whereas they are around 27 for scheelite from woxi this might indicate that rees in the mineralizing fluids at ljp originated from granitoids that are concentrated along the southern border of the xub whereas in the case of woxi the original fluids might have been masked by rees released during intense high temperature wall rock alteration of unexposed precambrian basement rocks at depth sr isotopes of scheelite from these two deposits show similar relations to host nearby rocks in that 87sr 86sr t 200 ma ratios of 0 721 for ljp scheelite agree with values ranging between 0 718 0 726 for granitoids whereas these ratios are much higher i e 0 745 for scheelite from woxi and correspond to the 87sr 86sr t 200 ma ratio range of 0 743 0 749 for precambrian host slates crushing experiments to release inclusion fluids from gangue quartz and sulfides deposited during later stages of ore deposition in both deposits failed to provide accurate and geologically meaningful two point fluid residue tie lines in rb sr isochron diagrams however sr released from fluid inclusions generally reveals lower initial 87sr 86sr ratios than the respective residues and shows affinities to 87sr 86sr t 200 ma values of indosinian yanshanian granitoids both at woxi and ljp pb stepwise leaching of scheelite and sulfides did not result in sufficient spreads in pb isotope diagrams and therefore no information regarding exact mineralization ages in the two deposits could be deduced overall ore pb isotopes reveal upper crustal signatures and are compatible with pb isotope signatures of the predominant precambrian slates in the woxi area steep trajectories through late stage quartz sulfide mineralization in pb isotope diagrams may hint at mixing scenarios involving pb from the host rocks and a component with lower 207pb 204pb and 208pb 204pb ratios relative to 206pb 204pb ratios which cannot be linked to any known reservoir in the xub mining district sr isotopes of four out of seven residual sulfide samples from woxi plot along a paleomixing line at an age of 199 8 ma supporting a mixing scenario for the fluids indicated by the pb isotopes and pointing to a possible genetic relationship with the emplacement of indosinian yanshanian granitoids the budgets of rees rb sr and pb in the original fluids were severely affected by contamination of these elements apparently during intense wall rock alteration but after sealing of the major pathways the mineralizing fluids tend to have better preserved their original signatures these attest a genetic relationship between the metallogeny in the xub w sb au province and the emplacement of indosinian yanshanian granitoids during mesozoic intracontinental tectonic uplift and thrusting

origin of the fluids associated with granodiorite hosted sb as au w mineralisation at dubrava nizke tatry mts western carpathians mineral parageneses of the polymetallic sbrich deposit at dubrava has been formed during five separated stages a fluid inclusion study demonstrates that the earliest stages with scheelite molybdenite and arsenopyrite have been associated with immiscible co2 ch4 rich low saline fluids at temperatures between 300 and 400 c and pressures as much as 2 kbar deposition of the main superimposed ores stibnite and zinckenite has been intimately connected with circulation of aqeuous moderately saline fluids 15 5 23 5 wt nacl equiv upon epithermal conditions salinity of the aqueous fluids associated with tetrahedrite is clustered around 10 wt nacl equiv quartz from the latest barite stage has precipitated from aqueous fluids enriched in divalent cations these fluids are believed to be genetically linked with triassic evaporite formations preserved in the region temperature salinity diagrams constructed from microthermometry data indicate influx of diluted meteorite water in the stibnite tetrahedrite and barite stages isotopic data are in accordance with model the 18o values between 9 3 and 1 5 have been derived for water in equilibrium with quartz coexisting with sphalerite tetrahedrite and barite thus confirming the participation of isotopically lighter meteoric waters in the mineral forming solutions the 18o values between 3 3 and 8 5 estimated for the water associated with the scheelite and arsenopyrite stages are suggestive for the majority of metamorphic and or magmatic water in the mineral forming co2 rich solutions

internal fabric development in complex lava domes abstract viscous lava extrusions were modeled using plaster of paris with admixed magnetite dust which served as a tracer of the internal anisotropy of magnetic susceptibility fabric in model lava domes used analogue material showed pseudoplastic behavior and yield strength level proportional to increasing mixing ratio of plaster powder and water a series of models ranging from simple gravity flows to complex lava domes showing combined endogenous and exogenous growth were created by intrusion of plaster into a sandbox the similarity of model bodies is compared with natural lava domes on the basis of dynamic scaling analysis growth dynamics exogenous growth and internal fabric development in natural lava domes is critically discussed using the experimental results

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

tracing metal sources in peribatholitic hydrothermal w deposits based on the chemical composition of wolframite the example of the variscan french massif central abstract we present a complete dataset for major minor and trace elements in wolframite fe mn wo4 based on the example of several peribatholitic hydrothermal w deposits located in the variscan french massif central fmc the wolframite samples were characterized down to the micrometer scale by combining scanning electron microscopy observations and in situ chemical analysis by electron probe microanalyzer and laser ablation inductively coupled plasma mass spectrometry most samples are fe dominated wolframites and are characterized by variable fe fe mn values for the studied deposits from 0 36 to 0 86 without significant alteration features wolframites present intracrystalline variations for concentrations of several minor and trace elements principally for nb ta and sn which reflect growth zoning the minor and trace element compositions of the different wolframite samples show at first order similar enrichments in ta nb sn zn and in as well as depletions in lree li th ti zr hf pb v co and mg compared to the upper continental crust second order compositional variations are observed between the studied deposits which can be separated in four main regional geochemical groups this work highlights that the chemical composition of wolframite is controlled by both the crystallochemical parameters and the composition of the primary hydrothermal fluid the regional variations observed in the geochemical signatures of wolframite reflect local variations in the composition of the hydrothermal fluid i e variable metal sources therefore this suggests that the minor and trace element contents in wolframite likely represent direct markers of the source of the hydrothermal fluid and ultimately of the metals the comparison between the trace element compositions of wolframite with the compositions of regional granitic rocks and worldwide shales suggests that evolved peraluminous granites represent the main source of metals in peribatholitic hydrothermal w deposits from the fmc whereas metasedimentary rocks have a limited to no influence on the metal budget

quartz veins with associated sb pb ag au mineralization in the schwarzwald sw germany a record of metamorphic cooling tectonic rifting and element remobilization processes in the variscan belt a combination of textural observations from 38 permian sb pb ag au bearing quartz veins in the schwarzwald sw germany with isotopic fluid inclusion and geochemical data allows to refine genetic models for this common type of mineralization to understand the origin of mineralogical diversity and the correlation with large scale tectonic events textures record four main mineralization stages i fe as sb au ii pb zn cu iii pb sb and iv ag sb stage i sulfides dominated by pyrite and arsenopyrite formed due to cooling of low salinity metamorphic fluids during the permian with maximum homogenization temperatures of 400 c invisible gold in arsenopyrite pyrite marcasite and stibnite was remobilized and locally precipitated as electrum at the end of stage i minerals of stages ii and iii comprise a rich diversity of sb bearing sulfosalts including e g bournonite zinkenite and jamesonite this assemblage formed during jurassic times due to mixing of high salinity mid and upper crustal fluids up to 27 wt nacl cacl2 with homogenization temperatures between 50 and 250 c stage iii is marked by a rich variety of pb sb sulfosalts its local abundance is directly related to the presence of stage i mineralization and its extent of remobilization during a significant influx of pb in the jurassic cretaceous the formation of the upper rhine graben during the tertiary reactivated especially ne sw oriented veins percolating ag rich fluids reacted with earlier stage iii pb sb sulfosalts forming ag rich minerals stage iv such as miargyrite pyrargyrite and stephanite the transition from metamorphic fluids to basinal saline e g pb bearing brines over hundreds of millions of years as is shown in this study and present in other variscan occurrences indicates a possibly typical poly stage characteristic of sb deposits worldwide which has however not been investigated in detail so far

an experimental study of the surface thermal signature of hot subaerial isoviscous gravity currents implications for thermal monitoring of lava flows and domes received 22 july 2011 revised 23 november 2011 accepted 23 november 2011 published 7 february 2012 1 management of eruptions requires a knowledge of lava effusion rates for which a safe thermal proxy is often used however this thermal proxy does not take into account the flow dynamics and is basically time independent in order to establish a more robust framework that can link eruption rates and surface thermal signals of lavas measured remotely we investigate the spreading of a hot isoviscous axisymmetric subaerial gravity current injected at constant rate from a point source onto a horizontal substrate we performed laboratory experiments and found that the surface thermal structure became steady after an initial transient we develop a theoretical model for a spreading fluid cooled by radiation and convection at its surface that also predicts a steady thermal regime we show that despite the model s simplicity relative to lava flows it yields the correct order of magnitude for the effusion rate required to produce the radiant flux measured on natural lava flows for typical thermal lava properties and an effusion rate between 0 1 and 10 m 3 s 1 the model predicts a steady radiated heat flux ranging from 10 8 to 10 10 w the assessed effusion rate varies quasi linearly with the steady heat flux with much weaker dependence on the flow viscosity this relationship is valid only after a transient time which scales as the diffusive time ranging from a few days for small basaltic flows to several years for lava domes the thermal proxy appears thus less reliable to follow sharp variations of the effusion rate during an eruption

antimoniferous vein type mineralization of the berga antiform eastern thuringia germany a fluid inclusion study abstract the antimoniferous vein mineralization in the schleiz and greiz area eastern thuringia are bound to ne sw striking variscan fault zones along the margins of the berga antiform the sb pb zn ores form massive sulfide sulfosalt quartz carbonate veins breccias or stockworks within ordovician and silurian metasedimentary units devonian metabasalts and metagranitoids mineralogical investigations from several sb mines and hard rock quarries near schleiz and greiz germany show that the sb pb zn sulfides are texturally in close association with quartz and carbonates stibnite is the major sb bearing mineral minerals of the sb pb zn sulfide assemblage replace an earlier zn fe as sulfide quartz assemblage quartz i named the ore stage i mineralization stibnite and quartz ii overgrow these early sulfides accounting for the ore stage ii lead dominated sb sulfides e g zinkenite plagionite semseyite represent the ore stage iii phase followed by sphalerite ii galena and quartz iii accounting for ore stage iv fluid inclusion studies on ore and gangue minerals of the different ore stages give insights into the composition and temperature of the mineralizing fluid s primary fluid inclusion assemblages in ore stage ii stibnite and co genetic quartz ii show variable vapor fractions within fluid inclusion assemblages heterogeneous trapping of co2 at constant salinities of fluid inclusions of the same assemblage indicates the influx of a buoyant pure co2 fluid into an aqueous fluid phase the presented data demonstrates the first example of co2 influx into an aqueous fluid identified by fluid inclusion analyses during this ore stage ii a h2o co2 nacl fluid with eq w nacl of 1 to 2 7 at 220 226 c prevails in contrast fluid inclusions in ore stage iii minerals boulangerite contain a homogeneously trapped h2o nacl cacl2 fluid with eq w nacl up to 8 1 at temperatures 146 c ore stage iv minerals sphalerite ii quartz iii contain a homogeneously trapped h2o nacl cacl2 fluid with eq w nacl of 20 and homogenization into the liquid phase at around 120 c fluid inclusion data suggest p t conditions at the onset of mineralization at around 220 c at 0 70 to 0 75 kbar fluid pressure the mechanisms of sb ore precipitation were induced by the influx of co2 fluid cooling quartz ii stibnite boulangerite and late stage mixing with a high salinity fluid resulting in the precipitation of sphalerite ii galena and quartz iii the unique opportunity to analyze fluid inclusions in many different subsequently precipitating ore and gangue minerals enables the reconstruction of the temporal evolution of the sb veins the distinct change of the ore forming processes from the influx of co2 into an aqueous fluid and cooling ore stage i ii to mixing ore stage iii iv as the dominant precipitation mechanisms are also accompanied by a drastic change in the mineralogy from sb sulfides to pb zn sulfides late orogenic granitic intrusions served as a potential heat metal and sulfur source variscan wrench and thrust tectonics and associated metamorphism account for the ore stage i mineralization hydrothermal sb pb zn assemblages ore stages ii iii and iv precipitated during the latest part of the variscan orogeny most likely around the carboniferous permian boundary although fluid composition and formation temperatures of the greiz and schleiz mineralization are on the first view quite similar to those of other variscan sb mineralization the observation of co2 influx and fluid mixing with a high salinity fluid is unique for european sb occurrences

on the relationship between dike width and magma viscosity on the basis of measurements of bulk rock and phenocryst chemical compositions phenocryst content and estimation of water content and magma temperature the magma viscosity of 44 dikes from oki dozen and tango southwest japan and ocros peruvian andes was estimated to examine a relationship between viscosity and dike width the results show that magma viscosity increases with dike width mafic magmas with viscosities of 101 102 pa s form dikes l m wide while felsic magmas with viscosities of 106 107 pa s form dikes 100 m wide this indicates that magma viscosity as well as magma driving pressure and host rock stiffness are important factors in explaining the variation of dike width because magma viscosity is a measure of internal friction the magmatic force required to widen the fracture during hydrofracturing is proportional to viscosity on the basis of the range of chemical composition of the studied samples dike widths formed by highly viscous felsic magma should be of the order of 100 m maximum furthermore the observed basaltic dike width variations from various tectonic settings indicate that the effective factor on driving pressure of dike magma may be not magma pressure but compressive stress on the other hand dikes derived from low viscosity basaltic magma and exceeding 100 m width may be formed by extremely high magma pressure associated with flood basalt volcanism this is supported by 1 the observation that a rifting event with decreased compressive stress was not always associated with the volcanism and 2 the inferred effusion rates for this type of volcanism are anomalously high suggesting increased magma pressure in addition the active magma generation and the decreased host rock stiffness by locally anomalous thermal conditions like a hotspot phenomenon may assist flood basalt volcanism or emplacement of anomalously wide mafic dikes

on the variations of flow rate in non explosive lava eruptions abstract a physical model is developed to determine factors which influence the dynamics of non explosive lava eruptions in the model magma rises in a laminar regime from an overpressured chamber surrounded by elastic rock and erupts at the surface where the accumulation of lava may alter the vent pressure in a flow rate versus volume diagram an eruption follows a path which is sensitive to relative changes in flow variables such as viscosity conduit dimensions and thickness of lava over the vent the path does not depend on the unknown elastic properties of the country rock or the chamber dimensions with appropriate volcanological data an observed flow rate versus volume path can be interpreted the approach is applied to three well documented eruptions in each case the eruption rate was partially influenced by decreasing chamber pressure the 1988 1990 eruption of lonquimay chile exhibits an early phase of conduit shrinkage which lasted about 100 days probably due to magma solidification against country rock the 1979 eruption of la soufriere de saint vincent west indies was initially affected by lava dome growth and later passed through a phase possibly caused by constriction of the conduit the 1943 1952 eruption of paricutin mexico was affected by changes of lava level in the cinder cone and magma properties in these three eruptions the pre eruptive chamber overpressures were similar 10 20 mpa even though the erupted volumes differed by as much as two orders of magnitude

crystallization history of the 1984 mauna loa lava flow during a 3 week eruption in 1984 mauna loa produced vent lavas that increased in crystallinity from 1 to 30 and 27 km long flows that increased in crystallinity as they moved downstream we examined the crystallization history of these lavas using crystal size distribution csd analysis to study the rates of crystallization viscosity increase and latent heating typical average growth and nucleation rates were 5 10 9 cm s 1 and 5 cm 3 s 1 for microphenocrysts 20 to 500 m size crystals nucleated in the rift zone and 5 10 8 cm s 1 and 5 104 cm 3 s 1 for microlites 1 to 20 m size crystals nucleated in the channel these crystallization rates are high compared with those found in other csd studies of igneous rocks probably due to highly nonequilibrium conditions brought on by rapid degassing in the rift zone and cooling in the lava channel growth and nucleation rates decreased with time at the vent and with distance downstream the maximum downstream total crystallinity measured is 39 25 microlites 14 microphenocrysts in a quenched sample 14 km from the vent growth and nucleation rates cannot be calculated for postemplacement samples but they place upper limits of 53 58 on the amount of crystallization in the channel 9 20 km from the vent crystallization could have been mostly responsible for the 105 fold downstream increase in apparent viscosity although degassing and increasing incorporation of solid lava fragments also contributed another effect of crystallization on the lava flow was the sizeable latent heating 0 01 j g 1 s 1 over the first half of the flow length if the crystallinity of downstream quench samples is representative of the hot fluid core which may have been counteracted by entrainment of cooler material measurements of crystallization are shown to be crucial in the study of lava flow emplacement dynamics

antimony quartz and antimony gold quartz veins from northern portugal abstract antimony and pb sb quartz veins from the braganca district portugal are mainly hosted by silurian phyllites antimony au quartz veins from the durico beira region are mainly hosted by a cambrian schist metagraywacke complex as well as ordovician phyllites and quartzites the deposits were mostly exploited in the late 19th century mineralogical characteristics and chemical compositions of individual ore minerals are similar in the two areas first and second generations of arsenopyrite precipitated at 390 and 300 c respectively berthierite and stibnite are the most abundant sb bearing minerals and precipitated between 225 and 128 c native antimony at

the biards sb au bearing shear zone massif central france an indicator of crustal scale transcurrent tectonics guiding late variscan collapse structural textural mineralogic microthermometric and isotopic data obtained on the biards sb au district located in the south limousin area massif central france provide new evidence for the tectonic setting of stibnite and gold deposition during the late orogenic variscan collapse in western europe a variety of geologic and geophysical data define a major crustal scale left lateral strike slip shear zone that acted as a trap for sb au mineralization during these movements fault fill veins and then extensional veins of limited extent were created along rheologic heterogeneities that focused brittle deformation geometry and orientation of both vein sets indicate two main episodes of emplacement of ore related to a progressive wrench movement in response to an counterclockwise rotation of the regional stress axes both veins sets are filled by quartz and sulfides in three stages an early arsenopyrite pyrite stage with fine grained quartz a sphalerite pb sb sulfosalt native gold stage with comb quartz and a late stibnite berthierite stage with comb quartz combined fluid inclusion and stable isotope studies on quartz muscovite and sulfides reveal that the three stages result from the continuous evolution of a deep seated hydrothermal system in which the latest stages mixed with meteoric water these data argue for cooling and a pressure drop during ore deposition and indicate a shallow level of emplacement for sb au veins a comparison of the biards deposits with other upper paleozoic gold and antimony deposits in the massif central the western variscides and elsewhere in the world illustrates their common association with crustal scale strike slip shear zones the upper carboniferous age 305 300 ma of the sb au deposits indicates that they formed during the early stages of the postthickening extension of the variscan lithosphere sb au bearing strike slip shear zones resulted from the late variscan crustal collapse guided by the very large scale dextral shearing between central europe northern africa and the northern appalachians and associated with the opening of the paleo tethys and the closing of the theic ocean during the late paleozoic gondwana laurasia translation

the massive stibnite veins of the french palaeozoic basement a metallogenic marker of late variscan brittle extension the stibnite ore deposits of the french palaeozoic basement are spatially related to major late variscan strike slip faults they occur as small discontinuous veins mostly hosted in epizonal or catazonal siliciclastics stibnite crystallizes in the final stage of a polymetallic paragenesis from an antimony bearing solution which deposits a first discrete fe as assemblage at 300 400 c and 0 5 0 8 kbar experimental data have shown that antimony solubility drastically decreases on cooling characterization of the late variscan antimony bearing fluids enables us to predict the temperature range 270 150 c under which stibnite was precipitated since the latter coincides with the fluid inclusion homogenization temperature range the pressure at which stibnite crystallized can be estimated from vapour pressure data to have been around 0 1 kbar extensional brittle shear zones developed at the end of the late variscan orogeny probably at the westphalian stephanian boundary drained the hydrothermal fluids near to the surface c 1000 m reaching the critical p t conditions for stibnite crystallization

morphological structural and textural variations in the 1988 1990 andesite lava of lonquimay volcano chile the 1988 1990 eruption of lonquimay volcano chile 38 s formed a 10 2 km long andesite lava with a volume of 0 23 km 3 over a period of 13 months the lava extrusion rate decreased with time as chamber pressure and vent dimensions decreased the velocity of the flow front decreased exponentially with distance from vent as a consequence of cooling and the increase of apparent viscosity at the flow front the lava developed a central channel which decreased in width and depth with time three prominent lava levees were formed on each margin and resulted from abandonment as the channel decreased in width as a result of a rapid decrease of flow rate over the first 100 days of activity a fourth major levee developed in february during a brief period of flow rate increase down the main channel but its walls were gradually exposed as the lava depth again decreased due to declining flow rate the structure of lava levees depended on their age and longevity of the flow in the adjacent channel initial levees were formed in the first few days as the lava spread laterally and then retreated leaving levees of massive lava more mature rubble levees were formed during the next month by the lava pushing and then shearing aa and blocky breccia which formed on the cooling flow margin fragmentation and abrasion formed a characteristic zonation in the levees a basal zone consists of very poorly sorted matrix rich breccia with very rounded vesicular clasts and bimodal grain size distribution the basal breccia zone strongly resembles block and ash flow deposits this zone passes up into a zone of clast supported clinker breccia which becomes increasingly matrix poor and coarser with clasts becoming more angular upwards the crest of the levee is composed of large 10 100 cm angular to subangular blocks with no matrix the zoned levees form after the active lava channel suddenly narrows lava depth initially increases and breccias are deposited on the channel margins and acquire the zoned structure by progressive shearing and accretion of clinkery aa breccia the lava level then drops exposing the steep inner scarp of a levee the most mature levee type formed in a long lived channel over several months the outer wall of the levee consists of zoned breccia but the inner wall consists of a massive curving wall of strongly foliated lava with well developed horizontal striations and ductile reidel shears the massive foliated facies is a consequence of prolonged flow which coats strongly sheared lava onto the inner levee wall scanning electron microscopy shows that the aa clinker clasts and foliated lava from the levee walls form at low melt fractions 15 in the last three months of the eruption the flow front ceased to advance but thickened as lava drained from proximal regions and intruded into the interior of the distal lava the last stages of lava movement were characterized by updoming in the central channel a lava surface feature named here armadillo structure was formed by deformation of the cooler but still ductile lava crust the deformation caused by underflow produced reidel shears dipping upstream and doming of the lava due to rotation of the shear planes the study demonstrates that lava morphology structure and texture are strongly influenced by variations of effusion rate local flow rate channel topography and thermal maturity of the lava which is reflected in downstream changes in viscosity

ice rafting history from the spitsbergen ice cap over the last 200 kyr abstract three piston cores taken from the mid to upper continental slope of the spitsbergen margin have been used to elucidate the calving history of the spitsbergen ice mass for the last 200 kyr the abundance of clasts larger than 2 mm was used as a measure of ice rafted detritus ird in the cores these clasts were counted from x ray photographs the fraction coarser than 63 m was also measured and compared to the clast counts to assess the validity of this parameter as a measure of ird content the correlation between clast counts and the fraction 63 m was poor and it was concluded that the fraction 63 m was a poor measure of ird in the spitsbergen cores in cores close to a major continental landmass or from an area of steep submarine relief the fraction 63 m is likely to be affected by factors other than ice rafting alone the ird peaks identified in this study were thought to be produced by the following advance of the spitsbergen ice cap during periods of increased moisture supply during interstadial periods mid stage 5 and stage 3 disintegration of a large ice mass on the continental shelf during deglaciations stages 6 5e 5b 5a and 2 1 boundaries summer melting of a large ice mass during glacial periods stage 6 a good correlation was found between interpreted ice advances from ird input to the spitsbergen margin and postulated ice advances from terrestrial evidence the ird peaks found from the spitsbergen margin appear to be produced by a different mechanism from that proposed for the production of the heinrich layers of the north atlantic these are thought to be produced by the collapse of the laurentide ice sheet once it reaches a threshold size correlation of heinrich layers produced from a mid latitude ice sheet with the ird events of the norwegian greenland sea is not possible due to their different mechanism of formation in fact there is possibly an antiphase relationship as heinrich layers are formed mainly during glacial periods whereas ird peaks in the greenland sea are produced during periods of rapid ice advance when moisture supply is high when relatively warm atlantic waters enter the norwegian greenland sea during interstadial periods as well as during melting events

effusive eruption of viscous silicic magma triggered and driven by recharge a case study of the cerro chascon runtu jarita dome complex in southwest bolivia the cerro chascon runtu jarita complex is a group of ten late pleistocene 85 ka lava domes located in the andean central volcanic zone of bolivia these domes display considerable macroscopic and microscopic evidence of magma mixing two groups of domes are defined chemically and geographically a northern group the chascon consists of four lava bodies of dominantly rhyodacite composition these bodies contain 43 48 phenocrysts of plagioclase quartz sanidine biotite and amphibole in a microlite poor rhyolitic glass rare mafic enclaves and selvages are present mineral equilibria yield temperatures from 640 to 750 c and log o2 of 16 geochemical data indicate that the pre eruption magma chamber was zoned from a dominant volume of 68 to minor amounts of 76 sio2 this zonation is best explained by fractional crystallization and some mixing between rhyodacite and more evolved compositions the mafic enclaves represent magma that intruded but did not chemically interact much with the evolved magmas a southern group the runtu jarita is a linear chain of six small domes 1 km3 total volume that probably is the surface expression of a dike the five most northerly domes are composites of dacitic and rhyolitic compositions the southernmost dome is dominantly rhyolite with rare mafic enclaves the composite domes have lower flanks of porphyritic dacite with 35 vol phenocrysts of plagioclase orthopyroxene and hornblende in a microlite rich rhyodacitic glass sieve textured plagioclase mixed populations of disequilibrium plagioclase compositions xenocrystic quartz and sanidine with ternary composition reaction rims indicate that the dacite is a hybrid the central cores of the composite domes are rhyolitic and contain up to 48 vol phenocrysts of plagioclase quartz sanidine biotite and amphibole this is separated from the dacitic flanks by a banded zone of mingled lava macroscopic microscopic and petrologic evidence suggest scavenging of phenocrysts from the silicic lava mineral equilibria yield temperatures of 625 727 c and log o2 of 16 for the rhyolite and 926 1000 c and log o2 of 9 5 for the dacite the rhyolite is zoned from 73 to 76 sio2 and fractionation within the rhyolite composition produced this variation most of the 63 73 sio2 compositional range of the lava in this group is the result of mixing between the hybrid dacite and the rhyolite eruption of both groups of lavas apparently was triggered by mafic recharge a paucity of explosive activity suggests that volatile and thermal exchanges between reservoir and recharge magmas were less important than volume increase and the lubricating effects of recharge by mafic magmas for the runtu jarita group the eruption is best explained by intrusion of a dike of dacite into a chamber of crystal rich rhyolite close to its solidus the rhyolite was encapsulated and transported to the surface by the less viscous dacite magma which also acted as a lubricant simultaneous effusion of the lavas produced the composite domes and their zonation reflects the subsurface zonation the role of recharge by hotter more fluid mafic magma appears to be critical to the eruption of some highly viscous silicic magmas

on lava dome growth with application to the 1979 lava extrusion of the soufriere of st vincent abstract a theoretical analysis is presented for the spread of a viscous liquid flowing under its own hydrostatic pressure on a horizontal surface in order to model lava dome formation two situations are considered in detail the spreading of a constant volume of liquid and the case where the amount of liquid is continually increased experiments with silicone liquids show close agreement with theory the formation of a basaltic andesite lava extrusion in 1979 on the crater floor of the soufriere of st vincent west indies provided the motivation for and an application of the model the extrusion reached a diameter of 868 m and a height of 133 m over a period of 150 days over the first 90 days the growth relationships were consistent with those predicted by theory application of the theory to the soufriere dome suggests an effective viscosity of 2 x 10 12 poise for the basaltic andesite lava the large effective viscosity calculated for the lava may be attributed to the dominant influence of a high viscosity skin which forms at the margins of the flow as it cools after 70 days the rate of growth of the extrusion markedly decreased because a substantial collar of rubble accumulated at the flow front due to this collar the growth of the extrusion ceased after 150 days from approximately two weeks after the initiation of the extrusion the discharge rate of lava decreased approximately linearly with increasing dome height this observation suggests that the lava ascended under a decreasing hydrostatic driving pressure and that extrusion ceased when the lava column reached hydrostatic equilibrium

the spreading of variable viscosity axisymmetric radial gravity currents applications to the emplacement of venusian pancake domes the magellan images of venus have revealed a number of intriguing volcanic features including the steep sided or pancake domes these volcanic domes or flows have morphologies that suggest formation by a single continuous emplacement of lava with a higher viscosity than that of the surrounding basaltic plains numerous investigators have suggested that such high viscosity is due to high silica content leading to the conclusion that the domes are evidence of evolved magmatic products on venus however viscosity depends on crystallinity as well as on silica content high viscosity could therefore also be due to a cooler and therefore higher crystal content lava models of dome emplacement which include both cooling and composition factors are thus necessary in order to determine the ranges of crystallinity and silica content which might lead to the observed gross dome morphologies accordingly in this study domes are modelled as radial viscous gravity currents with an assumed cooling induced viscosity increase to include both effects analytical and numerical results indicate that pancake dome formation is feasible with compositions ranging from basaltic to rhyolitic therefore observations of gross dome morphology alone are insufficient for determining composition and the domes do not necessarily represent strong evidence for evolved magmatism on venus

dynamics of eruptive phenomena the scope of this chapter is limited to the dynamics of lava and gas rising in a volcanic conduit no mention will be made of the behaviour of eruption columns in the atmosphere which would deserve a chapter on their own some basic knowledge of the main principles of fluid dynamics is assumed the text contains no complex mathematical derivation and save for a few simple points details all the steps required to arrive at a given solution the aim of this chapter is to highlight a few key physical principles using simple problems of relevance to volcanic eruptions true eruptions are of course more complex than the examples given and involve various interactions between the variables

high effusion rates of the cordon caulle 2011 2012 eruption southern andes and their relation with the quasi harmonic tremor high effusion rates of intermediate to high silica lavas seem to be less uncommon than previously thought in particular during their initial eruptive stages in this study we report satellite based time averaged discharge rates for the 2011 2012 effusive phase at cordon caulle which are well correlated with the evolution of the quasi harmonic tremor the most significant seismic signal after the initial explosive stage such a correspondence could become a key method for detection of the onset of effusive phases especially in remote and or very cloudy areas supplying an additional tool for effective warnings and near real time hazards appraisal