widespread lavalike silicic volcanic rocks of trans pecos texas several silicic units of the trans pecos volcanic field have outcrop and thin section scale features of lava flows but areal extents and aspect ratios of ignimbrites these voluminous rocks up to hundreds of cubic kilometres per unit are quartz trachytes to low silica rhyolites 68 to 72 sio 2 lava flow features include flow banding and folding elongated vesicles and autobreccias and vitrophyres at the base and top of units pyroclastic flow features include sheetlike geometry lateral extents up to 70 km aspect ratios as low as 1 700 and areal extents up to 3000 km 2 a few of these units are clearly rheomorphic ignimbrites but others show no unambiguous evidence of a primary pyroclastic origin although no adequate explanation currently exists for the origin of the latter we evaluate two end member hypotheses 1 they are ignimbrites in which extreme rheomorphism has obliterated primary internal features and 2 they are highly viscous lavas with unusually high heat retention or effusion rates that allowed them to spread over great areas either origin requires a rock type and eruptive mechanism not commonly recognized

structure and emplacement of a rhyolitic obsidian flow little glass mountain medicine lake highland northern california many rhyolitic obsidian flows show consistent stratigraphic relations among textural units exposed in the flow fronts of undissected flows and in cross sections of older flows the stratigraphy of the holocene little glass mountain rhyolitic obsidian flow consists of from bottom to top air fall tephra deposits basal breccia coarsely vesicular pumice obsidian finely vesicular pumice and surface breccia slightly crystalline rhyolite occurs near the vent areas a model of obsidian flow emplacement based on these textural relations is presented this stratigraphy may reflect the distribution of volatiles within the magma source region with the interlayered contact between coarsely vesicular pumice and overlying obsidian indicating stratification of volatiles in the magma body the distribution of pumiceous and glassy zones along with the orientations of flow banding can be used to map the surface structure of rhyolitic obsidian flows this complex structure as mapped on the little glass mountain flow reflects both the initial flow stratigraphy and subsequent deformational processes during emplacement of the lava three processes disrupt the initial flow configuration the rise of coarse pumice diapirs from the base of the flow the inward propagation of fractures in areas of extension and surface folding in sites of flow parallel compression subvertical flow banding in vent areas indicates that fracturing accompanies the emergence of lava most of the observed upper surface of a dome originates as a fracture plane the structure of domes that form over vent areas may reflect the orientation of dike like conduits as well as the local state of stress during extrusion

volcanology and petrology of volcanic products from the island of milos and neighbouring islets abstract new geochronological and volcanological data from volcanics of the island group of milos south aegean active volcanic arc allow four main cycles of volcanic activity to be distinguished ranging in age between 3 5 and 0 1 m y b p the oldest volcanic activity consists almost totally of pyroclastics and submarine products followed by subaerial ones the eruptive centers migrated in time and concentrated during the most recent volcanic phase in the central part of milos and on the island of antimilos the oldest products are dominated by andesites and dacites while the most recent ones mainly consist of rhyolites the volcanic evolution and the geochemical characters of the erupted products suggest that the feeding system of the oldest volcanism was located in the deep continental crust where contamination and fractional crystallization jointly occurred the younger cycle of volcanic activity was fed from small and relatively shallow magma chambers where contamination processes played a minor role these magma bodies are considered to responsible for the shallow thermal anomaly giving rise to the high enthalpy field on milos

the bad step tuff a lava like rheomorphic ignimbrite in a calc alkaline piecemeal caldera english lake district lava like tuffs are lithologically indistinguishable from lavas and form part of a temperature and composition controlled continuum from low grade tuffs which are non welded or slightly welded through high grade densely welded tuffs extremely high grade tuffs which may be agglutinated right to their upper and lower contacts to spatter fed lava flows in some high grade tuffs a component of nonparticulate flow may postdate emplacement and deposition but in extremely high grade tuffs non particulate deformation normally occurs during emplacement and deposition in such cases syn depositional non particulate deformation previously called primary welding and non particulate slumping previously called secondary flowage processes overlap and are continuous one into the other so that distinction between them and their resultant structures is unrealistic and inapplicable therefore the term rheomorphism should be used to embrace all types of non particulate flow the bad step tuff is the most lava like of a sequence of rheomorphic calc alkaline rhyolitic ignimbrites emplaced during a climactic caldera forming eruption episode in the english lake district it is a ponded sheet 40 to 400 m thick which comprises a basal crudely stratified heterolithic breccia a thick flow laminated and locally vesicular central part which beomes increasingly flow folded upwards and an upper autobreccia despite an absence of vitroclastic textures within the main laminated part field relations show it to be a tuff diagnostic criteria are 1 a gradation within a lithophysal zone from unambiguous vitroclastic matrix of the basal lithic breccia upwards into the central flow laminated tuff 2 only rate autobreccia at the base of the sheet but ubiquitous autobreccia at the top of the sheet and 3 close textural similarity with localized intensely rheomorphic parts of associated ignimbrites that widely display unequivocal vitroclastic textures where their rheomorphism is less marked the extremely high grade character of the bad step tuff may reflect its proximal setting in a piecemeal type caldera high emplacement temperatures resulted from high rate but low velocity vent emission from fissures along numerous cross cutting calderafloor faults producing very low boil over eruption columns and proximal ponding

textural development of mixed finely porphyritic silicic volcanic rocks inyo domes eastern california magma mixing in silicic volcanic rocks of the inyo chain is manifest by bimodal mineral chemistry and distinctive banding defined by either variations in 1 glass color and chemistry or 2 microlite abundance within colorless glass bands composed of brown or microlite rich colorless glass are characterized by more mg rich mafic silicates and more calcic plagioclase than microlite poor colorless glass domains a rhyolitic and a dacitic mixing end member can be defined on the basis of this mineral distribution thermobarometry using the mineral assemblages unique to each end member indicates that prior to mixing the dacite was more oxidized and water rich than the rhyolitic magma determined mineral crystallization conditions compare favorably with experimentally derived phase equilibria for melts of broadly similar composition the spatial association of contrasting mineral assemblages with the flow banding in these rocks implies that the banding is a direct product of magma mixing it formed as either 1 volumes of the comingled magmas were immediately vented to the surface and quenched preserving domains of two distinct glass compositions or 2 bands of dacitic magma precipitated microlites when comingled with the rhyolitic magma in an attempt for the two magmas to approach equilibrium

evolution and architecture of a large felsic igneous province in western laurentia the 1 6 ga gawler range volcanics south australia the gawler range volcanics grv of south australia are an intracontinental subaerial large igneous province lip formed during supercontinent assembly in the mesoproterozoic like many lips the grv are intraplate were erupted over a geologically short time 2 million years and are dominated by voluminous lavas however the grv are overwhelmingly dominated by felsic units the grv are essentially undeformed and have not been deeply buried so that their original textures are exceptionally well preserved furthermore they are well exposed in very gently northward and eastward dipping sections up to 4 km thick allowing assessment of the facies architecture and evolution of this felsic volcanic lip the evolution of the grv can be clearly separated into two main stages initial eruptions at numerous volcanic centres produced small to moderate volume geochemically distinct felsic lavas and lava domes together with ignimbrites and minor mafic and intermediate lavas forming a sequence 0 5 to 3 km thick volcanic activity in this lower sequence varied from effusive to explosive and was not much different in style or products from phanerozoic felsic volcanic provinces however the second stage produced at least three voluminous felsic units each of which represents about 1000 3000 km3 of magma moreover the dominance of evenly porphyritic textures and lack of pyroclastic textures pumice shards broken crystal lithic clasts in these units suggests that they were erupted effusively and flowed as lavas each of these felsic lavas are generally dominated by a single uniform composition and commonly mingled with a subordinate and compositionally distinct lava

evidence for a compositional boundary within the lithospheric mantle beneath the kalahari craton from s receiver functions abstract s and p receiver functions from the southern african seismic experiment are analyzed for lithospheric discontinuities beneath the kalahari craton besides the moho the most prominent feature is a discontinuous reduction in seismic velocity of about 4 5 at approximately 150 km depth the discontinuity appears to have a width of about 10 km termed the k discontinuity this feature is restricted to the northern half of the array extending from the zimbabwe craton south to the tml thabazimbi murchison lineament and in the west from botswana to the edge of the kalahari craton in the east it spans several archean sutures and is thus unlikely related to archean tectonics it does however appear to be related to subsequent magmatic episodes the strongest anomaly is coincident with the most intense karoo volcanism and it extends to the northern edge of the bushveld intrusion from mantle xenoliths and xenocrysts the entire lithosphere in this region appears to have experienced a long term infiltration of basaltic melt and metasomatic fluids we propose that the k discontinuity reflects the influence of this melt metasomatic infiltration which has over time intruded and refertilized the lithosphere based on kimberlites pipes that show obvious signs of melt metasomatism and likely karoo influence the observed reduction in seismic velocity is plausibly consistent with the observed major element and volatile enrichment at 150 km depth in such kimberlites if this interpretation is correct then the high temperature kimberlite nodules that most clearly reflect this perturbation likely represent the general state of the lower lithosphere rather than only reflecting local mantle properties in the immediate vicinity of the kimberlite eruption

on the early response of the climate system to a meltwater input from greenland theearlyresponseoftheatmosphere oceansystemtomeltwaterrunofforiginatingfromthegreenlandice sheet is studied using a coupled atmosphere ocean general circulation model aogcm for this purpose aogcm ensemble simulations without and with associated ocean freshening around greenland are compared for freshwater perturbations initiated in northern winter the mean response for the first three months shows the emergence of negative sea surface temperature sst anomalies in the denmark strait in association with enhanced oceanic advection by the east greenland current the response also shows negative sst anomalies in the north atlantic associated with enhanced westerlies at the ocean surface additionally the baroclinic atmospheric cyclonic circulation east of greenland intensifies and anticyclonic circulations with equivalent barotropic structures develop over western europe and the north pacific ocean simulations by the atmospheric component of the aogcm indicate that atmosphere ocean interactions contribute significantly to enhance the response the sensitivity of the coupled system response to the timing of freshwater perturbation is also studied for freshwater perturbations initialized in northern summer the response during the following winter is similar but stronger in magnitude in the northern hemisphere the atmospheric response resembles the arctic oscillation ao mode of variability the association between anomalies in the denmark strait ssts and in the atmosphere east of greenland is consistent with that observedduring previous great salinityanomaly gsa events the results obtainedhighlight the importance ofatmosphere oceaninteractionintheearlyclimateresponsetogreenlandmelting theteleconnectionswith the north pacific and the contribution of gsa events to north atlantic oscillation nao variability

recent remobilisation of shallow level intrusions on montserrat revealed by hydrogen isotope composition of amphiboles abstract ion probe measurements of hydrogen isotopes were made on amphiboles representing different stages of the ongoing eruption of the soufriere hills volcano montserrat the majority 80 of the andesitic amphiboles show relative intra and inter crystal d homogeneity with a mean of 38 12 consistent with known primary magmatic values the remainder 20 of amphiboles show marked d heterogeneity with a mean of 6 30 the heterogeneous amphiboles have 100 m rims with d values similar to the homogeneous crystals but core values which are significantly heavier than primary magmatic values early in the eruption 50 of amphiboles are isotopically heterogeneous but post spring 1996 all amphiboles are homogeneous we interpret these ion probe data in terms of development of a shallow andesitic magma chamber by repeated emplacement of andesitic magma in the shallow crust over at least the last 100 yr we suggest such shallow intrusions emplaced during the volcano seismic crises preceding the ongoing eruption solidified some regions interacted with hydrothermal fluids and isotopic exchange took place generating the heavy hydrogen isotope signatures in the heterogeneous amphiboles on onset of the ongoing eruption such old igneous material was remobilised with intimate mixing between the new magma batch and the old intrusions indicated by the close proximity of homogeneous and heterogeneous crystals the isotopically primary magmatic rims on the heterogeneous crystals indicate hydrogen isotope exchange over a period of a few weeks consistent with timescales of magma ascent at the soufriere hills volcano once the old igneous material was flushed out by spring 1996 the eruption proceeded by extruding material dominated by the new batch of magma this resulted in a marked increase in extrusion rate

fragmentation phenomena in populations of magmatic crystals fragmentation of crystals is an important mechanism and a component of particle dynamics in igneous and metamorphic rocks that has received surprisingly little attention recent advances in textural analysis extraction techniques digital imaging and computer assisted measurements enable rapid accumulation of 3d data on particle shapes and size distributions this paper reviews fragment size distributions fsd that result from fragmentation lognormal fractal loggamma and weibull discusses their genesis mechanisms and presents relevant examples of fragmentation from experimental physics next the paper considers fsds of feldspars on digitized images of thin sections and on published images and quartz extracted from vesicular pumice by acid from eight well known large volume eruptive units the acid solution of pumice enables examination of volume abundance 3d shapes proportions of fragmented crystals and measurements of their csds and fsds fsds were also measured in samples of welded tuff and a granite disaggregated by electric pulse products of syneruptive shock wave fragmentation and fragmentation by an electric pulse are found to be fractal with large breakage probabilities branching ratios and fractal dimensions of 2 to 3 in contrast most quartz fragments in pumice obey a lognormal distribution and fragmentation is driven by a melt inclusion decrepitation mechanism which results in low breakage probability and small number 2 3 of fragments per breakage cycle these results are consistent with one atmosphere heating experiments of quartz phenocrysts that led to melt inclusion decrepitation and caused quartz to break up into several smaller pieces collectively having lognormal fsd measured melt inclusion size distributions suggest decrepitation of outermost melt inclusions and low survival rate for large inclusions and inclusions with large radius crystal size ratio the modeling of periodic fragmentation of crystals with melt inclusions due to overheating and or decompression which may occur many times during the lifetime of a long lived magma body may explain concave down lognormal csds abundant in igneous rocks the genesis of lognormality can be explained by the fragmentation algorithm of kolmogorov 1941 other algorithms may generate lognormal like loggamma distributions fragmentation serves as an important size limiting factor a nucleation aid and it facilitates isotopic and trace elemental exchange

the eucarro rhyolite gawler range volcanics south australia a 675 km3 compositionally zoned lava of mesoproterozoic age the eucarro rhyolite is remarkable in being far more voluminous 675 km3 and extensive than most felsic lavas 1 km3 its dimensions are comparable to the largest known volcanic units felsic ignimbrites and flood basalts it is one of several voluminous felsic units within the mesoprot erozoic ca 1592 ma gawler range volcanics an intracontinental intraplate volcanic province in south australia and is typical of the voluminous felsic magmatism associated with laurentia cf belt supergroup midcontinent united states the lava is 300 m thick with groundmass textures and lithofacies characteristics consistent with cooling and degassing as a single unit there are gradational compositional and textural variations throughout its highly elongate distribution 225 km east west 74 wt sio2 pyroxene is the dominant ferromagnesian phase but at the base in the west primary amphibole and olivine are present granitic and mafic clasts are ubiquitous but most abundant 5 vol in western and eastern parts of the unit the main eucarro magma was compositionally zoned and several smaller batches of compositionally distinct magma were tapped during the eruption concentrations of granitic clasts coincide with changes in bulk composition and may record the rupture of wall rock between nearby but separate magmas the distribution subtle lateral variations in composition and texture and local presence of compositional flow banding along the northern margins suggest that the eucarro rhyolite was derived from several source vents located along an east trending fissure system although very widespread outflow of the eucarro lava was probably no more than a few tens of kilometers from source vents

partially melted lithic megablocks in the yardea dacite gawler range volcanics australia implications for eruption and emplacement mechanisms lithic megablocks ranging from 200 m felsic volcanic unit in the mesoproterozoic gawler range volcanic province grv of south australia throughout its vast extent the yardea dacite shows typical lava like features in that it is massive columnar jointed and evenly porphyritic with 30 40 crystals in a spherulitic and granophyric groundmass in addition flow banding is present at many locations the megablocks are abundant at two sites 50 km apart but isolated megablocks and smaller 6 cm lithic clasts are also scattered throughout the unit at both sites the megablocks are matrix supported non graded randomly oriented and show no evidence of being confined to a particular stratigraphic level in the dacite the most abundant and largest megablocks are granitoids derived from older basement and from early crystallised plutons of the hiltaba suite which is broadly coeval and comagmatic with the grv the granitoid megablocks have been partially melted most likely prior to eruption when resident in the thermal aureole of the yardea dacite magma chamber the lithic megablock occurrences are unlike coarse pyroclastic breccias but are similar in distribution and abundance to xenoliths in lavas consistent with the lava like character of the host dacite using reasonable estimates of megablock density magma density and magma viscosity we show that the rise rate of the dacitic magma exceeded the settling velocity of the megablocks implying that they could have been entrained and erupted effusively all but the largest and least melted megablocks would have remained suspended or else settled very slowly in the dacitic lava during outflow the rapid rate of magma withdrawal required to produce such an extensive felsic sheet could have also triggered disintegration of the thermally stressed wallrock surrounding the magma chamber dislodging megablocks that were later entrained and effusively erupted

in situ formation of welded tuff like textures in the carapace of a voluminous silicic lava flow owyhee county sw idaho the badlands rhyolite on the owyhee plateau of southwestern idaho can be demonstrated to be a large lava flow on the basis of its geometry of large and small flow lobes its well exposed near vent features and its response to pre existing topography however samples of the dense upper vitrophyre of the unit reveal a range of annealed fragmental textures including material which closely resembles the compressed welded glass shards which are characteristic of ignimbrites formation of these tuff like textures involved processes probably common to emplacement of most silicic lava flow units decompression upon extrusion causes inflation of pumice at the surface of the lava flow some of this pumice is subsequently comminuted producing loose bubble wall shards bits of pumice chips of dense glass and fragments of phenocrysts this debris sifts down around loose blocks and into open fractures deeper in the flow where it can be reheated compressed and annealed to varying degrees the end result is a dense vitrophyre layer beneath the true upper non welded carapace breccia which can be extremely texturally heterogeneous with areas of flow foliated lava occurring very near lava which in many aspects looks like welded ignimbrite complete with flattened pumices identical textures in other silicic units have been cited by previous workers as evidence that those units erupted as pyroclastic flows which then underwent sufficient rheomorphism to create a flow foliated rock which otherwise appears to be lava the textures described herein indicate that lava flows can come to mimic rheomorphic ignimbrites at least at scales ranging from thin sections to outcrops voluminous silicic units with scattered fragmental textures but with otherwise lava like features are probably true effusive lava flows

a fifty year perspective of magmatic evolution on ruapehu volcano new zealand verification of open system behaviour in an arc volcano geochemical and petrological data for samples from well documented eruptions that occurred at ruapehu volcano over the period 1945 1996 can be used to illustrate the complexity of short term geochemical variation in an arc type volcano collectively data from ruapehu volcano show trends with time of increasing sio2 abundance and rising 87sr 86sr ratios consistent with broad control by assimilation and crystal fractionation processes afc however the magmas emplaced during the past fifty years show geochemical variability that spans most of the range shown by lavas erupted over the entire history of the volcano magma compositions fluctuate through wide ranges over relatively short time intervals reflecting the effects of processes associated with magma recharge events within the volcano these complex trends are also manifested when the geochemistry and petrography of sequences of prehistoric lavas are examined in detail and they arise from short term effects that are imposed during recharge on the overall afc trend we show that the temporal geochemical and petrographic variations among erupted magmas are modulated by processes of mixing and mingling between fresh magma from below and stagnant melt and entrained crystals from earlier events remaining in the volcanic edifice probably in dikes and sills these processes are probably replicated over longer time periods hundreds to thousands of years as melts arrested at different levels in the near surface conduit system are progressively displaced by new magma batches arc type volcanoes such as ruapehu are characterised by pulsatory growth in which bursts of high magma production are superimposed on a background of subdued but more or less continuous activity this style of activity is difficult to predict through the usual seismicity ground deformation lake water geochemistry volcano monitoring techniques and petrology and geochemistry may provide the basis for an alternative strategy

false pyroclastic textures in altered silicic lavas with implications for volcanic associated mineralization there is strong emphasis in recent literature that zn cu pb massive sulfide deposits associated with felsic volcanism the kuroko type deposits are directly related to submarine calderas formed by large submarine pyroclastic eruptions altered silicic volcanic rocks with the appearance of welded and nonwelded pyroclastic flow deposits volcaniclastic debris flows and massive to thin bedded tuffaceous rocks form a major component of a silurian succession that hosts zn cu pb massive sulfide deposits at benambra southeastern australia however critical evaluation of rock textures indicates that these silicic volcanic rocks are mainly lavas and associated autoclastic facies with remarkably deceptive false pyroclastic and volcaniclastic textures including apparent pumice fiamme and glass shards intercalated sediments indicate that most of the silicic volcanics were emplaced in subaqueous settings ranging from relatively shallow water to deep marine the false textures were produced mainly in originally glassy lava by the combined effects of devitrification perlitic fracture and pervasive hydrothermal alteration these processes are related to the emplacement and cooling history of individual lavas and to a regional essentially synvolcanic hydrothermal system tectonic foliation of the more altered and mechanically weakened rocks has led to partial dismembering of phenocrysts and groundmass fabrics thereby further enhancing clastic appearance subsequent to recognition of widespread false pyroclastic textures at benambra similar features have been identified in three other australian massive sulfide districts these results suggest that a reinvestigation of host rocks is warranted at other deposits where submarine pyroclastics are documented noncaldera settings combining subsidence in an extensional volcano tectonic basin passive submarine lava effusion within the basin and ambient sedimentation derived from basin margins including resedimented pyroclastic debris may be more relevant models in a number of massive sulfide districts

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

phenocryst fragments in rhyolitic lavas and lava domes although rhyolitic lavas and lava domes are characterised by evenly porphyritic textures not all the phenocrysts are whole euhedra we undertook image analysis of 46 rhyolitic lava and lava dome samples to determine the abundance and shape of quartz and feldspar phenocryst fragments phenocryst fragments were identified in nearly all samples on average fragments amount to approximately 5 of the total phenocryst population or approximately 0 5 modal the abundance of fragments in lavas and lava domes is not related to the groundmass texture whether vesicular flow banded massive glassy or crystalline nor to distance from source fragments are however more abundant in samples with higher phenocryst contents the phenocryst fragments in rhyolitic lavas and lava domes are mainly medium to large 0 5 3 5 mm almost euhedral crystals with only a small portion removed or chunky equant subhedral fragments and occur in near jigsaw fit or clast rotated pairs or groups the fragments probably formed in response to decompression of large melt inclusions shear during laminar flow then dismembered the phenocrysts continued laminar shear separated and rotated the fragments fractures probably formed preferentially along weaknesses in the phenocrysts such as zones of melt inclusions cleavage planes and twin composition planes rare splintery fragments are also present especially within devitrified domains splinters are attributed to comminution of solid lava adjacent to fractures that were later healed for comparison we measured crystal abundance in a further 12 rhyolite samples that include block and ash flow deposits and ignimbrite phenocryst fragments within clasts in the block and ash flow samples showed similar shapes and abundances to those fragments within the lava and lava domes crystal fragments are much more abundant in ignimbrite exceeding 67 of the crystal population however and dominated by small equant anhedral chunks or splinters the larger crystals in the ignimbrite are subrounded the phenocrysts within ignimbrite pumice lapilli are also more intensely fractured than those in lavas and lava domes thus in deformed and altered volcanic successions data on crystal fragment abundance and shape can help discriminate lavas from pyroclastic facies

magma origin and evolution of white island whakaari volcano bay of plenty new zealand white island is an active composite stratovolcano in the bay of of the north island the pacific plate is being subducted plenty new zealand that comprises many small volume westwards under the australian plate at a rate of 50 mm yr to form the taupo hikurangi arc trench with phenocryst contents of 15 44 minor high mg basaltic system volcanic activity is now concentrated in the andesite explosive eruptions such as those of 1976 1992 may taupo volcanic zone tvz the youngest expression have occurred at intervals throughout the history of white island of 22 ma cenozoic continental margin arc volcanism in but are rarely preserved these alternate with major episodes of the north island of new zealand cole 1990 tvz andesite dacite lava extrusion the high mg magmas form by fig 1 inset is at the southern end of the 2800 km hydrous melting of mantle metasomatized by fluids from the tonga kermadec taupo volcanic arc system and the dehydrating slab at the slab mantle wedge interface that rise lau basin havre trough ngatoro basin back arc basin rapidly to shallow magma chambers 2 7 km where limited system southern havre trough and tvz are offset mixing and contamination occurs before eruption some of this sinistrally by 40 50 km and wright 1992 believes the magma remains in the magma chamber where it interacts with the offset to be accommodated by a series of dextrally oblique crystal mush from which it inherits phenocrysts to form so called and en echelon bookshelf faults the southwestern limit dirty lavas total phenocryst content of these lavas is correspondingly of this fault system is placed 20 km north of white higher as more magma is intruded into the chamber the heat flux island and has nw se orientation fig 1 inset will increase and melt fraction will eventually rise to the surface to tvz extends offshore into the bay of plenty as a form high silica andesite dacite magma clean lavas with fewer graben structure of 45 km width fig 1 bounded by the inherited phenocrysts similar multi magma chamber plumbing tauranga fault zone to the west and the white island systems with complex evolution involving fractionation and confault zone to the east wright 1992 the eastern side tamination probably occur in most andesite dacite arc volcanoes of tvz coincides with the current active volcanic front and a broadly linear alignment of andesitic volcanoes extends from ruapehu in the southwest to whakatane

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

internal textures of rhyolite flows as revealed by research drilling recent drilling of 550 and 140 000 yr old rhyolite lava flows in the inyo dome chain california and valles caldera new mexico and field inspection of 8 ma flows in western arizona reveal a more detailed vertical zonation of lava textures in glassy rhyolite flows than has previously been recognized the upper vitrophyre of a flow can usually be subdivided into a surficial finely vesicular pumice underlain by obsidian coarsely vesicular pumice and a second obsidian layer beneath these layers is the crystalline center of the flow which in turn is underlain by a basal obsidian layer lower vitrophyre differences in lava vesicularity and crystallinity between zones in a flow result from primary effervescence devitrification lava rheology and migration of water vapor

extended cooling and viscous flow of large hot rhyolite lavas implications of numerical modeling results abstract a common misconception about rhyolite lava flows is that they cannot advance far from their vents and are constrained to be small due to their high viscosities although a high viscosity lava will indeed flow quite slowly it may advance a great distance if the lava efficiently retains heat and thus mobility and if the available magma supply is large to evaluate this possibility a finite difference numerical model was used to determine the rate of heat loss from units interpreted to be large rhyolite lavas cooling of such 100 300 m thick units is very slow and is further retarded by a vesicular carapace and by evolution of latent heat thermal history alone suggests that large thick lava flows could remain active for several decades to constrain the duration of flow advance of these units their inferred apparent viscosities are compared with those of active lava flows mean flow velocity of some large rhyolites in southwestern idaho usa was estimated based on the observation that the units must still have been advancing by viscous flow at the start of spherulite crystallization indicating perhaps a 10 to 18 year flow duration based on studies of obsidian dome california mean flow velocities would have been roughly 0 59 to 2 5 km yr and calculated average apparent viscosities of the flows ranged from 1 8 to 3 6 orders of magnitude greater than the viscosity of the lava itself this relationship also holds for active andesite and basaltic andesite block lava flows which have apparent viscosities 2 7 to 4 orders of magnitude greater than lava viscosities this implies that large rhyolites may indeed have been emplaced in the same manner as less silicic block lavas and that long distance viscous flow is not unrealistic for rhyolite lavas a good example of a large volume lava is the 15 km 3 rhyolite of the badlands sw idaho where vent relations show that lava effused from a fissure and advanced up to 9 km more extensive rhyolites could also be emplaced solely by effusion extended flow of these lavas may lead to the remelting and removal of basal crumble breccias and to the formation of welded shard textures in their margins both processes tending to obscure the lava flow origin of the units the great dimensions of lava like rhyolitic units with volumes of 10 to at least 200 km 3 are not a sufficient reason to assume they erupted as ignimbrites

structures within large volume rhyolite lava flows of the devonian comerong volcanics southeastern australia and the pleistocene ngongotaha lava dome new zealand abstract many rhyolitic units within the late devonian comerong volcanics erupted as lava flows and domes some up to 18 km long and 350 m thick the textural and structural characteristics that distinguish the flows and domes as lava rather than rheomorphic ignimbrite include unbroken phenocrysts zones of autobrecciation and finely developed flow layering with individual layers continuous for several metres the flow layering is typically contorted into isoclinal folds with forms suggesting fluidal deformation and is interlayered with and gradational into restricted zones of pumice rich lapilli tuff and zones of lenticulite breccia the lenticulite breccia comprises discontinuous lenticular rhyolite fragments the long axes of which define a foliation parallel to flow layering lenticles in the breccia vary from elongate layers up to 1 m long and several millimetres thick to short fragments less than 10 cm long and several centimetres wide similar zones of lenticulite breccia consisting of glassy lenticular clasts in a devitrified spherulitic matrix of cristobalite and albite exist within the late pleistocene ngongotaha dome near rotorua new zealand the lenticulite breccia is considered to form by aqueous diffusion and selective devitrification of the rhyolite along anastomosing fluid paths and to be modified by mechanical fracturing of the lava in a zone of high shear stress geochemically the rhyolites are high si and a type with high zr and y contents indicating that they formed from high temperature relatively anhydrous f rich melts a type granitoids crop out intermittently along the length of and adjacent to the volcanic complex and are comagmatic with the rhyolite the high temperature low bubble and phenocryst content and a high eruptive rate of the rhyolite likely resulted in a low effective viscosity and extensive flow units

magma dynamics beneath kameni volcano thera greece as revealed by crystal size and shape measurements size distributions of plagioclase crystals in series of recent porphyritic dacite lavas from kameni volcano greece can be modelled by mixing two populations of crystals each with overlapping linear crystal size distributions csd termed microlites and megacrysts the magmas bearing the microlites and megacrysts started to crystallise 6 13 and 24 96 years respectively before each eruption the dates of initiation of crystallisation of the megacrysts indicate that they are left overs of earlier injections of new magma into a shallow chamber some magma remains after each eruption and continues to crystallise new magma with few or no crystals is then introduced and the microlites crystallise from the mixed magma eruption followed 6 13 years after mixing such a model would suggest that some porphyritic magmas are products of a shallow magma chamber that is never completely emptied just topped up from time to time

experimental constraints on degassing of magma isothermal bubble growth during continuous decompression from high pressure numerical models predict that rapid ascent of hydrous magma can lead to supersaturation of dissolved volatile constituents possibly leading to explosive eruption we have performed controlled decompression experiments to investigate the ascent rates required to maintain bubble melt equilibrium high silica rhyolitic melts were saturated with water at 200 mpa and 825 c decompressed to lower pressures at constant rates of 0 025 0 25 0 5 and 1 0 mpa s 1 and then rapidly quenched isobarically other samples were saturated with water over the pressure range investigated to determine equilibrium water solubility in order to quantify degassing efficiency during decompression at a decompression rate of 0 025 mpa s 1 melt vapor equilibrium was maintained over the entire pressure range examined 200 to 17 5 mpa a single bubble nucleation event occurred in response to decompression and quenched bubble sizes can be modeled by a equilibrium bubble growth model that takes into account the number density of bubbles at decompression rates of 0 25 0 5 and 1 0 mpa s 1 rhyolitic melts could not degas in equilibrium when pressure decreased from 200 mpa to 140 mpa and water supersaturation p in the melt reached up to 60 mpa with higher values at faster decompression rates further pressure release resulted in near equilibrium degassing and p dropped significantly in each case p decreased when bubbles exceeded 10 vol a single heterogeneous bubble nucleation event occurred in each experiment when p 20 mpa no other bubbles nucleated despite p reaching 60 mpa which is probably too low to trigger homogeneous nucleation compared to estimates for magma decompression rates during lava dome eruptions our results indicate that magmas can degas efficiently throughout their ascent to the surface in explosive eruptions decompression rates may exceed those of this study and hence melts may become supersaturated with water such fast decompressions are expected however only when magma is highly vesicular which would aid approach to equilibrium degassing

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

the rheological transition in plagioclase bearing magmas 1 volcanological processes such as melt segregation ascent and eruption are directly dependent on the rheological behavior of magmatic suspensions an increase of the crystal fraction of the suspension leads to the formation of a solid particle network which abruptly increases magma viscosity the crystal fraction at which this rheological transition occurs depends on parameters such as the strain rate and the size shape and sorting of particles to determine the influence of the crystal shape on the rheological transition suspensions of plagioclase a representative anisometric crystal have been investigated at high temperatures and pressures synthetic suspensions with crystal fractions s ranging from 0 38 to 0 75 were deformed both in compression and torsion in a paterson apparatus at 300 mpa 900 c and 800 c and for strain rates between 1 0 10 5 and 1 0 10 3 s 1 all suspensions exhibit a non newtonian shear thinning rheological behavior the experimental results coupled with existing data and models at low crystal fractions s 0 3 allow several rheological domains to be identified from steady state flow to strain weakening each characterized by a specific microstructure in particular a progressive evolution from a pervasive to a strain partitioning fabric is found when increasing the crystal fraction our results highlight the influence of both the strain rate and the shape of crystals on the rheological behavior of magmas during crystallization magmatic suspensions of anisometric minerals such as plagioclase would develop a solid particle network earlier s 0 3 than suspensions of isometric minerals s 0 5 our study shows that localization of strain early in the crystallization history of mushy zones in the magma chamber near the conduit margins and at the base of lava flows would facilitate the mobilization the transfer and the final emplacement at the surface of highly viscous feldspar rich magmas

the late paleozoic felsic lava dominated large igneous province in northeast germany volcanic facies analysis based on drill cores in the deep subsurface of the northeast german basin products of extensive volcanism are present that formed during the waning phase of the variscan orogeny permian carboniferous boundary large volumes 48 000 km3 of dominantly felsic magma were emplaced in the northern foreland of the variscan mountains in a terrestrial semi arid environment most of these units were inferred by previous authors to represent the products of explosive volcanic eruptions such as ignimbrites however a detailed analysis of the volcanic texture and lithofacies association shows that most units are best interpreted as coherent lavas emplaced either as extrusive units or as shallow intrusions with important implications for the reconstruction of the paleo environment this study shows that detailed textural examination of drill cores provide important observations which can be applied as criteria for the interpretation of the mode of volcanic eruptions furthermore phenocryst logging and geochemical fingerprinting based on immobile element ratios can be employed for the classification and discrimination of individual emplacement units

a geophysical study of the taupo volcanic zone new zealand the taupo volcanic zone is a region of intense volcanic activity from which over 16 000 km3 of lavas and pyroclastics have been erupted since the late pliocene analysis of gravity and magnetic anomalies over the zone has revealed the presence of five major centers each associated with an approximately circular negative gravity anomaly and a positive pseudogravity anomaly these centers are interpreted as collapse features resulting from large scale withdrawal of magma from the upper crust four of the depressions occur beneath recognized volcanic centers or calderas the fifth kapenga is inferred to underlie an extinct hidden volcanic center three dimensional models of the thicknesses of low density and magnetized rocks in the taupo volcanic zone have been determined by inversion of gravity and magnetic anomalies beneath three of the centers the thicknesses of low density and magnetized rocks are approximately equal 2 3 km that is the collapse structures are infilled by magnetized volcanics however there is a significant discrepancy between the thicknesses of the gravity 5 km and magnetic models 1 5 km in the region of the okataina volcanic centre it is therefore suggested that a significant volume of low density nonmagnetic rock possibly acidic magma underlies okataina

crystallization history of obsidian dome inyo domes california samples obtained by u s department of energy research drilling at the 600 year old obsidian dome volcano provide the rare opportunity to examine the transition from volcanic dome to plutonic intrusion textures in a silicic magma system textures in the lavas from obsidian dome record multiple periods of crystallization initiated in response to changes in undercooling t related to variable degassing in the mag ma phenocr ysts formed first at low t a drastic increase in t related to loss of a vapor phase during initial stages of eruption caused nucleation of microlites all of the lavas thus contain phenocrysts and microlites extrusion and subsequent devitrification of the dry 0 1 wt h2o magma crystallized spherulites and fine grained rhyolite at high t a granophyric texture representing crystallization at a moderate t formed in the intrusions beneath obsidian dome textures in the intrusion apparently represent crystallization of hydrous 1 2 wt h2o rhyolitic magma at shallow depths