petrologic determination of ascent rates for the 1995 1997 soufriere hills volcano andesitic magma the recent eruption of the soufriere hills volcano in montserrat july 1995 to present october 1997 has produced a hornblende bearing andesitic lava dome it is possible to petrologically estimate changes in ascent rates of amphibole bearing magmas for certain rates of decompression a breakdown rim of fine grained anhydrous reaction products forms where amphibole is in contact with melt the thickness of the rim varies with ascent rate most of the amphibole phenocrysts in the magma storage region lack breakdown rims about 10 have 200 400 m thick coarse grained breakdown rims that are interpreted to be relicts of a past heating event study of a time series of new dome andesites showed that ascent rate increased from december 1995 0 001 m s to july 1996 0 008 m s while eruptive style remained extrusive ascent rate increased to 0 012 m s in august 1996 and the first major explosive eruption occurred on 17 18 september 1996

the 1995 1998 eruption of the soufriere hills volcano montserrat wi eruption of the soufriere hills volcano began on 18 july 1995 after three years of elevated seismic activity four months of increasingly vigorous phreatic activity culminated in mid november 1995 with the initiation of dome growth growth rates increased unevenly through early march 1996 with fluctuations on time scales from hours to months since march 1996 gravitational collapse of the unstable dome flank has affected an ever increasing area with pyroclastic flows surges and ashfalls major collapse of the eastern flank on 17 september 1996 resulted in a sub plinian explosive eruption later that day by february 1997 the dome had outgrown the confines of the crater and begun to spill into the surrounding valleys a large collapse on 25 june 1997 caused pyroclastic flows and surges on the northern flanks and resulted in the only deaths of the eruption in august september and october 1997 vulcanian explosions followed further collapses on the western and northern flanks the largest event of the eruption occurred on 26 december 1997 with failure of the southwestern flank of the volcano producing a debris avalanche and large dome collapse pyroclastic flows dome growth ceased in early march 1998 but residual volcanic activity has continued and consists of ash venting mild explosions and dome collapse pyroclastic flows

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

patterns of precipitation and mesolow evolution in midlatitude mesoscale convective vortices abstract surface pressure manifestations of mesoscale convective vortices mcvs that traversed oklahoma during the periods may august 2002 05 are studied using the weather surveillance radar 1988 doppler wsr 88d the oklahoma mesonet and the noaa profiler network data forty five mcvs that developed from mesoscale convective systems mcss have been investigated 28 62 of which exhibit mesolows detectable at the surface within this group three distinct patterns of precipitation organization and associated mesolow evolution have been identified the remaining 17 38 of the cases do not contain a surface mesolow two repeating patterns of precipitation organization are identified for the latter group the three categories of mcvs possessing a surface mesolow are as follows nineteen are classified as rear inflow jet mcvs and tend to form within large and intense asymmetric mcss rear inflow into the mcs enhanced by the development of an mcv on the left hand side relative to system motion pro

magma production and growth of the lava dome of the soufriere hills volcano montserrat west indies november 1995 to december 1997 from november 1995 to december 1997 a total volume of 246 106 dre m of andesite magma erupted partitioned into 93 106 m of the dome 125 106 m of pyroclastic flow deposits and 28 106 m of explosive ejecta in the first 11 weeks magma discharge rate was low 0 5 m s from february 1996 to may 1997 discharge rates have averaged 2 1 m s but have fluctuated significantly and have increased with time three pulses lasting a few months can be recognised with discharge rates reaching 3 to 8 m s short term pulsations in growth lasting a few days reach discharge rates of over 10 m s and there are periods of days to a few weeks when dome growth is 0 5 m s discharge rate increased from may 1997 with an average rate of 7 5 m s to december 1997 the observations indicate an open magmatic system

magma ascent rates from amphibole breakdown an experimental study applied to the 1980 1986 mount st helens eruptions recent 1980 1986 mount st helens dacites contain the phenocryst assemblage plagioclase amphibole low ca pyroxene magnetite ilmenite and rare high ca pyroxene which indicates that they all originated from an 8 km deep reservoir at 900 20 c with xh2o 0 67 in fluid according to experimental data iron titanium oxide phenocryst compositions indicate that all post may 18 dacitic magmas erupted at 900 20 c except for the final lava extrusion in october 1986 the magma reservoir may have cooled to 866 c by october 1986 amphiboles in the post may 18 1980 magma contain one or more amphibole populations characterized by reaction rims of different thicknesses the development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt during an approximately adiabatic ascent from a deep reservoir constant p and t and isothermal decompression experiments show that during a 900 c constant rate decompression from 8 km to the surface no reaction rim develops on amphibole in 4 days a 10 m rim develops in 10 days and a 35 m rim develops in 20 days these experimental data and histograms of rim widths in 1980 1986 mount st helens dacites show that post may 18 eruptions are composed in large part of magma represented by a population of thin rimmed amphiboles magma which ascended from the deep 8 km reservoir in 6 to 10 days the remainder of each sample consists of magma containing amphiboles with reaction rims ranging from 14 to 60 m magma which apparently spent from 8 to 25 days along the conduit margins before being mixed thoroughly millimeter scale into the erupting magma the mixing in a viscous slowly ascending dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicle population the experimental calibration of amphibole reaction rim width versus decompression time yields average ascent velocities for post may 18 dacites of about 15 30 m h for magma represented by the thick rimmed amphiboles and from 35 to 50 m hr for magma represented by the thin rimmed crystals an ascent rate of 66 m h is indicated for the may 18 1980 eruption which contains amphiboles with no reaction rims the volume of endogenous dome growth which preceded extrusion of magma newly derived from the deep source region suggests that the effective conduit volume beneath mount st helens in 1981 1982 was equivalent to a cylinder 8 km long and 8 9 m in radius

the role of magma mixing in triggering the current eruption at the soufriere hills volcano montserrat west indies the andesite lava currently erupting at the soufriere hills volcano montserrat contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesite the andesite phenocrysts have a range of textures and zonation patterns which suggest that non uniform reheating of the magma occurred shortly before the current eruption reheating resulted in remobilisation of the resident magma and may have induced eruption

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

amphibole bearing inclusions from boisa island papua new guinea evaluation of the role of fractional crystallization in an andesitic volcano boisa island is the tip of a quaternary island arc volcano it consists of an older cone made up of porphyritic amphibole free mafic rocks and two younger cumulodomes of hornblende bearing high silica andesite one of the cumulodomes contains coarse grained igneous inclusions consisting of amphibole plagioclase clinopyroxene orthopyroxene olivine and spinel some of these minerals in the andesites themselves have both cognate and xenocrystal components both the anhydrous phenocryst assemblage in the basalts and the amphibple bearing assemblage of the inclusions provide low residuals in least squares linear mixing calculations suggesting that fractionation of either assemblage in the basalts could have given rise to the andesites however this is not supported by the results of rayleigh fractionation calculations for several incompatible elements four possible interpretations are 1 the rayleigh fractionation results are inappropriate and misleading 2 the andesites were formed by crystal f

composite analysis of long lived mesoscale vortices over the middle reaches of the yangtze river valley octant features and evolution mechanisms abstracta 14 yr climatology is presented of the mesoscale vortices generated in the vicinity of the dabie mountains dabie vortices dbvs in the yangtze river valley analyzing these vortices using the climate forecast system reanalysis cfsr dbvs were found to be a frequent type of summer mesoscale weather system with a mean monthly frequency of 12 2 dbvs were mainly located in the middle and lower troposphere and 92 of them triggered precipitation most dbvs were short lived and only 19 5 persisted for more than 12 h latent heat release associated with precipitation is a dominant factor for the dbv s three dimensional geometry features life span and intensity the long lived dbvs all of which triggered torrential rainfall were analyzed using a composite analysis under the normalized polar coordinates results indicate that these vortices generally moved eastward and northeastward which corresponded to the vortices orientation divergence vorticity budget and kinetic energy budget the

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

formation of long lived summertime mesoscale vortices over central east china semi idealized simulations based on a 14 year vortex statistic abstractregions around dabie mountain dbm in the yangtze river basin yrb are the source of a mesoscale vortex the dabie vortex dbv based on a 14 yr statistical study 11 long lived heavy rain producing dbvs were composited for convection permitting semi idealized simulations a control simulation initialized 12 h before the composite vortex formation successfully reproduced a dbv with all the salient characteristics of the 11 events sensitivity experiments were designed to understand the impacts of large scale environmental conditions regional topography and latent heating on dbv formation the main results were as follows i supposition of a 500 hpa shortwave trough with an east west oriented lower level transversal trough around the dbm is crucial for the formation of vortices a nocturnal lower level jet on the southern side of the transversal trough accelerates dbv formation by enhancing convergence triggering sustaining convection and producing cyclonic vorticity ii during the si

pyroclastic stratigraphy of the soufriere hills volcano montserrat implications for the present eruption the stratigraphy of the soufriere hills volcano is dominated by dense andesite block and ash flow deposits forty one radiocarbon ages permit a reconstruction of the eruptive history over the past 31 000 years a single stratigraphic member yielding radiocarbon ages of 22 000 to 20 000 years b p is composed of block and ash flow deposits with semi vesiculated clasts concentrated in the upper third of each deposit the present eruption is also producing block and ash flow deposits with both dense and semi vesiculated clasts it marks a return to a more explosive driven type of activity as occurred 22 000 to 20 000 years ago deposits of this member have a broad sheet like form and volumes in excess of any produced so far by the present eruption

the evolution of aqueous vapor from silicate melts effect on oxygen fugacity abstract the simplest possible aqueous vapor evolved from an iron bearing magma under normal conditions is an h 2 o h 2 mixture hydrogen is produced by the oxidation of fe ii to fe iii by water in the magma yielding an increase in f o 2 equations are derived which model this mass transfer process the resulting differential equation expresses the change in magmatic f o 2 as a function of a change in n 1 h 2 o the vapor evolution progress variable the calculations demonstrate that in this simple scheme very iron poor magmas h 2 o feo are required to produce significant increases in magmatic f o 2 the incremental increase in o 2 during vapor evolution becomes progressively smaller as o 2 increases with this process becoming highly inefficient as the hematite stability field is approached other more complete processes involving meteoric water assimilation of altered rocks and subsolidus processes may produce the observed increase in f o 2 inferred to have occurred in systems of normal iron contents in low iron systems such as the high silica rhyolitic magmas associated with climax type porphyry molybdenum systems high oxygen fugacities may be obtained by this mechanism

impacts of convective activity over the tibetan plateau on plateau vortex southwest vortex and downstream precipitation abstractin summer convective activity over the tibetan plateau tp is vigorous with some of it moving eastward and vacating the plateau defined as the eastward moving type emt although the

numerical simulation of the 1981 sichuan flood part i evolution of a mesoscale southwest vortex abstract during the period 11 15 july 1981 heavy rainfall occurred over the sichuan basin in china resulting in severe floods that took a large toll in human life and property damage mesoscale analyses by kuo cheng and anthes have shown that the flood was directly related to the development of a long lived mesoscale southwest sw vortex over the basin in this paper we present the results of numerical experiments aimed at 1 testing the capability of a limited area mesoscale model to predict the evolution of the sw vortex and the accompanying heavy precipitation 2 examining the structure of the simulated vortex using the model data and 3 elucidating the role of various physical processes in the evolution of the sw vortex principal findings are 1 the control experiment which utilized an 80 km grid spacing and simple physical parameterizations was able to simulate the evolution of the mesoscale sw vortex and the accompanying heavy precipitation the simulation captured many observed features a

a numerical study of deep convection in tropical cyclones idealized numerical model simulations are used to investigate the generation and evolution of vertical vorticity by deep convection in a warm cored vortex of near tropical storm strength deep convective updraughts are initiated by thermal perturbations located at different radii from the vortex axis it is found that as the location of the thermal perturbation is moved away from the axis of rotation the updraught that results becomes stronger the cyclonic vorticity anomaly generated by the updraught becomes weaker the structure of the vorticity anomaly changes markedly and the depth of the anomaly increases for an updraught along or near the vortex axis the vorticity anomaly has the structure of a monopole and little or no anticyclonic vorticity is generated in the core vorticity dipoles are generated in updraughts near or beyond the radius of maximum tangential wind speed and this structure reverses in sign with height in all cases the vorticity anomalies persist long after the initial updraught has decayed implications of the results for understanding the vorticity consolidation during tropical cyclogenesis are discussed the effects of eddy momentum fluxes associated with a single updraught on the tangential mean velocity tendency are investigated and a conceptual framework for the interpretation of these eddy fluxes is given the simulations are used to appraise long standing ideas suggesting that latent heat release in deep convection occurring in the high inertial stability region of a vortex core is more efficient than deep convection outside the core in producing temperature rise in the updraught

hornblende dehydration reactions during magma ascent at soufriere hills volcano montserrat hornblende phenocrysts in recent andesites of the soufriere hills volcano display reaction rims of microcrystalline plagioclase pyroxene fe oxides and interstitial glass formed by decompression during magma ascent mass balance calculations give the following reactions with mineral proportions in agreement with modal abundances begin aligned 100 0 text hbl to 49 8 text cpx 27 6 text opx 18 2 text plag 4 5 text ti mag 100 0 text hbl to 53 9 text cpx 24 4 text opx 18 1 text plag 1 9 text ilm 1 7 text mag end aligned these reactions require an open chemical system with exchange of selected components with surrounding melt volatiles tio2 and alkalis are expelled and sio2 and feot are consumed matrix glasses fall into two compositional groups glasses in pumice are relatively rich in cao and poor in k2o and na2o compared to glasses in dome samples the former glasses formed by moderate amounts of groundmass crystallisation of plagioclase associated with rapid magma ascent in explosive eruptions the later glasses evolved in response to hornblende breakdown groundmass crystallisation and mixing of melts from different levels during slow magma ascent and extended residence time in the dome interstitial glass compositions in reaction rims reflect the compositions of the surrounding matrix glasses but show variable compositional differences mostly consistent with the proposed open system reactions

a complex magma mixing origin for rocks erupted in 1915 lassen peak california the eruption of lassen peak in may 1915 produced four volcanic introduction rock types within 3 days and in the following order 1 hybrid magma mixing is an important process among interblack dacite lava containing 2 undercooled andesitic inclusions mediate magmas in volcanic arcs e g turner 3 compositionally banded pumice with dark andesite and light campbell 1986 philpotts 1990 and references therein dacite bands and 4 unbanded light dacite all types represent laboratory studies of synthetic analogs of silicate magma stages of a complex mixing process between basaltic andesite systems and mathematical modeling of mixing viscous and dacite that was interrupted by the eruption they contain liquids have suggested a range of possible mixing mechdisequilibrium phenocryst assemblages characterized by the coanisms e g cashman bergantz 1991 however use existence of magnesian olivine and quartz and by reacted and of these results to interpret magmatic systems is limited unreacted phenocrysts derived from the dacite the petrography and by the applicability of synthetic analogs to magmatic crystal chemistry of the phenocrysts and the variation in rock conditions and by the imprecise knowledge of the effective compositions indicate that basaltic andesite intruded dacite magma viscosity of silicate liquid crystal mixtures at magmatic and partially hybridized with it phenocrysts from the dacite magma temperatures and pressures if magmas mix incompletely were reacted cooling crystallization and vesiculation of the hybrid i e mingle features such as compositional banding or andesite magma converted it to a layer of mafic foam the decreased undercooled inclusions are usually apparent but if they density of the andesite magma destabilized and disrupted the foam mix completely mineralogical disequilibrium may be the blobs of foam rose into and were further cooled by the overlying dacite magma forming the andesitic inclusions disaggregation of only direct evidence for a mixing origin andesitic inclusions in the host dacite produced the black dacite and if the thermal and compositional contrasts between light dacite magmas formation of foam was a dynamic process two magmas are great and the ratio of silicic to mafic removal of foam propagated the foam layer downward into the magma is large there is little interaction between them hybrid andesite magma eventually the thermal and compositional and the mafic magma is undercooled e g bacon 1986 contrasts between the hybrid andesite and black dacite magmas were many undercooled inclusions contain reacted phereduced then they mixed directly forming the dark andesite magma nocrysts inherited from their host silicic magma heiken about 40 50 andesitic inclusions were disaggregated into the eichelberger 1980 and bacon 1986 pointed out host dacite to produce the hybrid black dacite thus disaggregation that undercooled inclusions are typically formed from of inclusions into small fragments and individual crystals can be hybrid magmas in general the formation of undercooled an efficient magma mixing process disaggregation of undercooled inclusions retards further mixing sakuyama 1984 inclusions carrying reacted host magma phenocrysts produces cothompson dungan 1985 sparks marshall 1986 existing reacted and unreacted phenocryst populations koyaguchi blake 1991 however fragmentation and or disaggregation of undercooled inclusions plays an important role in hybridization in some magma systems thompson dungan 1985 coulon et al 1986 clynne christiansen 1987 clynne 1989 linneman myers

ascent driven crystallisation of dacite magmas at mount st helens 1980 1986 we introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system qz ab or h2o in order to relate variations in volcanic glass chemistry to changing pressure p and temperature t conditions in the sub volcanic magma system by this means an important distinction can be made between ascent driven and cooling driven crystallisation under water saturated or undersaturated conditions in samples containing feldspar and a silica phase quartz or tridymite quantitative p t estimates of the conditions of last equilibrium between crystals and melt can be made formation of highly silicic melts i e 77 wt sio2 is a simple consequence of the contraction of the silica phase volume with decreasing pressure such that high silica glasses can only form by crystallisation at low pressure resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation groundmass and inclusion glasses in dacites from the 1980 1986 eruption of mount st helens volcano wa span a wide range in sio2 68 80 wt anhydrous the compositions of the least evolved sio2 poor inclusions in amphibole phenocrysts record entrapment of silicic liquids with 5 4 wt water corresponding to a water saturation pressure of 200 mpa at 900 c the compositions of more evolved higher sio2 plagioclase hosted inclusions and groundmass glasses are consistent with extensive ascent driven fractional crystallisation of plagioclase oxide and orthopyroxene phenocrysts and microlites to low pressures during this polybaric crystallisation plagioclase phenocrysts trapped melts with a wide range of dissolved water contents 3 5 5 7 wt magmas erupted during the plinian phase of the 18 may 1980 eruption were derived from a large reservoir at depths of 6 km subsequent magmas ascended to varying depths within the sub volcanic system prior to extraction from glass chemistry and groundmass texture two arrest levels have been identified at depths of 0 5 1 and 2 4 km a single dome sample from february 1983 contains groundmass plagioclase tridymite and quartz testifying to temperatures of at least 885 c at 11 mpa these shallow storage conditions are comparable to those in the cryptodome formed during spring 1980 the corresponding thermal gradient 0 2 c mpa 1 is consistent with near adiabatic magma ascent from 8 km we argue that the crystallisation history of mount st helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation this challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980 1986 eruption because the crystallisation process is both polybaric and fractional it cannot be simulated directly using isobaric equilibrium crystallisation experiments however calculation of the phase proportions in water saturated 910 15 c experiments by rutherford et al 1985 over the pressure range 220 125 mpa reproduces the crystallisation sequence and phenocryst modes of mount st helens dacites from 18 may 1980 by allowing for the effects of fractional versus equilibrium crystallisation entrained residual source material and small temperature differences between nature and experiment phase compositions can also be matched to the natural samples we conclude that decompression of water saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres

mineral disequilibrium in lavas explained by convective self mixing in open magma chambers characteristic features of many porphyritic andesite and dacite lavas are that they are rich in crystals and display a range of disequilibrium features including reversely zoned crystals resorption surfaces wide ranges of mineral compositions and minerals which are not in equilibrium with the surrounding rock matrix these features are often interpreted as evidence of the mixing of magmas of contrasting composition temperature and origin1 2 here however we propose that such features can also be caused by convection within a magma body with a single composition that is heated from below and cooled from above we describe petrological observations of andesite lava erupted at the soufriere hills volcano montserrat which indicate a heating event and the intermingling of crystals that have very different thermal histories we present experimental data on a representative groundmass composition of this lava which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferred average temperature the concept of convective self mixing allows us to explain the occurrence of compositions of minerals that apparently cannot coexist under equilibrium conditions

petrologic evidence for pre eruptive pressure temperature conditions and recent reheating of andesitic magma erupting at the soufriere hills volcano montserrat w i the recent eruption of the soufriere hills volcano in montserrat july 1995 to present september 1997 has produced an andesitic dome sio 2 59 61 wt the eruption has been caused by invasion of mafic magma into a preexisting andesitic magma storage region p 130 mpa 5 km depth the composition of the andesite has remained essentially constant throughout the eruption but heating by the mafic magma increased the andesite temperature from 830 c to 880 c prior to being heated the stable mineral assemblage in the andesite was plagioclase amphibole orthopyroxene titanomagnetite ilmenite quartz the rise in temperature from 830 c to 880 c fo 2 1 log unit above nno has caused quartz to become unstable and has also caused changes in silicate and fe ti oxide mineral compositions the andesitic magma is likely saturated with an h 2 o rich vapor phase in the upper part of the magma storage region melt h 2 o content is 4 7 wt