azimuth moveout for 3 d prestack imaging we introduce a new partial prestack migration operator called azimuth moveout amo that rotates the azimuth and modifies the offset of 3 d prestack data followed by partial stacking amo can reduce the computational cost of 3 d prestack imaging we have successfully applied amo to the partial stacking of a 3 d marine data set over a range of offsets and azimuths when amo is included in the partial stacking procedure high frequency steeply dipping energy is better preserved than when conventional partial stacking methodologies are used because the test data set requires 3 d prestack depth migration to handle strong lateral variations in velocity the results of our tests support the applicability of amo to prestack depth imaging problems amo is a partial prestack migration operator defined by chaining a 3 d prestack imaging operator with a 3 d prestack modeling operator the analytical expression for the amo impulse response is derived by chaining constant velocity dmo with its inverse equivalentl

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

application of 3d seismic techniques to evaluate ore resources in the west wits line goldfield and portions of the west rand goldfield south africa abstractas expensive as 3d seismic reflection surveys are their high cost is justified by improved imaging of certain ore horizons in some of the witwatersrand basin gold mines the merged historical 3d seismic reflection data acquired for kloof and south deep mines forms an integral part of their ventersdorp contact reef mine planning and development programme the recent advances in 3d seismic technology have motivated the reprocessing and reinterpretation of the old data sets using the latest algorithms therefore significantly increasing the signal to noise ratio of the data in particular the prestack time migration technique has provided better stratigraphic and structural imaging in complex faulted areas such as the witwatersrand basin relative to older poststack migration methods interpretation tools such as seismic attributes have been used to identify a number of subtle geologic structures that have direct impact on ore resource evaluation other improvements include more accurate mapping o

new chronological and geochemical constraints on the genesis and geological evolution of ponza and palmarola volcanic islands tyrrhenian sea italy abstract a new geochronological and geochemical study of the volcanic rocks of the ponza and palmarola islands pontine archipelago has been carried out this archipelago is located along the boundary between the italian continental shelf and the opening tyrrhenian basin it is a key area to study volcanism related to the opening of the tyrrhenian sea ponza is the oldest felsic magmatic manifestation in the central tyrrhenian area previous studies suggested that ponza volcanic activity began before 5 ma twenty five new k ar ages constrain the volcanic activity rhyolitic hyaloclastites and dykes to the last 4 2 ma with two episodes of quiescence between 3 7 and 3 2 ma and between 2 9 and 1 0 ma a new volcanic episode dated at 3 2 2 9 ma has been identified on the central and southern ponza with emplacement of pyroclastic units at 1 0 ma a trachytic episode ended the volcanic activity the near island of palmarola exhibits rhyolitic hyaloclastites and domes dated between 1 6 and 1 5 ma indicating that the island was entirely built during the early pleistocene in a short span of time of ca 120 ka although only 6 8 km apart the two islands display significantly different geochemical signatures ponza rhyolites show major and trace element compositions representative of orogenic magmas of subduction collision zones high k calc alkaline and metaluminous rhyolites agpaitic index ai and alumina saturation index asi 3 and nb ta negative anomalies in palmarola the orogenic character is also present but much less marked than in ponza rhyolites have a peralkaline character ai 1 lower lile hfse th ta 11 15 low lree hfse ratios la nb 1 2 close to those of anorogenic lavas and the nb ta negative anomalies are almost absent y nb ratios indicate different magmatic sources one similar to island arc or active continental margin basalts for ponza rhyolites and the others probably involving an oib type component for palmarola rhyolites and ponza trachytes palmarola volcanics represent a transitional magmatism although a preserved collisional geochemical imprint they show geochemical features approaching those of anorogenic lavas erupted in a within plate context the change of magmatism evidenced in this study can be related to the tectonic evolution of the area indeed hf ta and rb contents suggest that the oldest pliocene rhyolites of ponza would emplace in a syn to late collisional setting while the younger pleistocene rhyolites of palmarola would be emplaced in a post collisional setting in which the orogenic character th ta decreases and mantle influence nb ta increases geochemical modeling strongly suggests that the palmarola rhyolites represent the waning stages of a subduction related magmatism the k ar datings allow us to estimate precisely the transition of magmatism to last less than 1 3 ma the transitional magmas may be the result of the upwelling of asthenospheric mantle inducing melting of a metasomatized lithospheric mantle and the mixing between these two sources this upwelling could occur during the extension of the tyrrhenian basin caused by the slab retreat and steepening or during a process of slab break off starting in the pliocene

subaqueous rhyolite block lavas in the miocene ushikiri formation shimane peninsula sw japan abstract a rhyolite mass of the miocene ushikiri formation in the western part of the shimane peninsula sw japan is a small subaqueous edifice about 600 m high and 4 km wide formed at water depths between 200 and 1000 m it consists mainly of three relatively flat lava flow units 50 300 m in maximum thickness each of which includes lobes and their polyhedral fragments the lava lobes are poorly to well vesiculated glassy to microcrystalline and flow banded and folded compared with mafic pillows they are large having thick quenched and brecciated glassy crusts because of their high viscosity surface tension and thermal conductivity their surfaces disintegrate into polyhedral fragments and grade into massive volcanic breccia the massive volcanic breccia composed of the lobe fragments is poorly sorted and covered with stratified volcanic breccia of the same rock type the rhyolite lavas commonly bifurcate in a manner similar to mafic pillow lavas however they are highly silicic with 1 5 vol phenocrysts and have elongated vesicles and flow folds implying that they were visco plastic during flowage their surface features are similar to those of subaerial block lava with respect to rheological and morphological features they are subaqueous equivalents of block lava

large scale rhyolite peperites jurassic southern chile development of a jurassic volcano tectonic rift basin in the southern andes created a setting in which thick rhyolitic volcaniclastic sequences accumulated in submarine environments and were penetrated by hypabyssal intrusions during or shortly after deposition in the ultima esperanza district of southern chile extensive masses of peperite were produced when rhyolite magma underwent quenching disruption and commingling with wet unconsolidated sediments during intrusion at shallow levels beneath the sea floor the peperite forms discordant intrusive masses with volumes of up to several cubic kilometers in which large widely spaced coherent rhyolite feeder pods are surrounded by and grade into closely packed and dispersed peperite closely packed peperite consists of tightly fitting clasts separated by sediment filled fractures in dispersed peperite the sediment forms a matrix surrounding large masses of fractured rhyolite and smaller more widely separated rhyolite clasts evidence of in situ quench fragmentation is well preserved on both outcrop and thin section scales thin sections show that clast margins and in some cases entire small clasts underwent cooling contraction granulation releasing shards of quenched rhyolite and fragments of phenocrysts into the adjacent sediment interaction between magma and wet sediment was non explosive and involved fluidization of the host sediments creating space for the intruding magma and causing pervasive injection of highly mobile sediment along thermal contraction cracks in quench fragmented rhyolite the ability of the magma to undergo complex intermixing with large volumes of sediment with widespread preservation of in situ fragmentation textures is interpreted to reflect a relatively low magma viscosity presumably caused by retention of volatiles in the magma at the ambient pressures involved beds of redeposited peperite within the rift basin fill indicate that some of the intrusive peperite masses reached the sea floor undergoing slumping and mass flow the peperites were thus an important local source of coarse grained debris during the evolution of the basin

a comparison of miocene and archean rhyolite hyaloclastites evidence for a hot and fluid rhyolite lava abstract in this paper we compare the geology and petrography of miocene and archean submarine rhyolite hyaloclastites the hyaloclastites are sparsely 10 or less plagioclase quartz and pyroxene phyric the hyaloclastites consist of a feeder dyke from which branch lava lobes and irregularly shaped lava pods the lava bodies consist of a holocrystalline core with microlitic texture grading outward into a flow layered rim zone and finally into obsidian the proportion of plagioclase and pyroxene microlites decreases outward some layers of the rim zone may be pumiceous vesicularity up to 50 vesicle size 1 mm or less but most of the lava has less than 5 vesicles one or a few cm long the obsidian shows perlitic fracture patterns the lava bodies grade through an in situ breccia into a hyaloclastite composed of angular obsidian granules and in many cases of fragments of lava lobes evidence for alteration at high temperature is as follows in the archean rhyolite hyaloclastites plagioclase microlites are overgrown by quartz albite spherulites furthermore parts of the miocene and archean hyaloclastite have been cemented and granules have been marginally replaced by quartz and albite hyaloclastite cemented at high temperature locally shows columnar joints at low temperatures obsidian has been hydrated and or has been replaced by clay minerals zeolites chlorite or prehnite chess board albite and fibroradial prehnite in archean hyaloclastite is possibly a pseudomorph after zeolites the sparsely porphyritic nature of the lava and the absence of microlites from the quenched glass suggests that the thyolite hyaloclastites extruded at high near liquidus temperature furthermore pumice is present only locally in the flow layered rim zone and in fragments derived from that zone these features suggest that vesiculation was inhibited by the weight of the water column high temperature and possibly the volatile h 2 o content explain the relatively low viscosity and shear strength of the lava and resulted in the flow morphology particular to this type of hyaloclastic rhyolite flows

were intercalated komatiites and dacites at the black swan nickel sulphide mine yilgarn craton western australia emplaced as extrusive lavas or intrusive bodies the significance of breccia textures and contact relationships intercalated archean komatiites and dacites sit above a thick footwall dacite unit in the host rock succession at the black swan nickel mine north of kalgoorlie in the yilgarn craton western australia both lithofacies occur in units that vary in scale from laterally extensive at the scale of the mine lease to localized thin irregular bodies from 100 m thick to only centimetres thick some dacites are only slightly altered and deformed and are interpreted to post date major deformation and alteration late porphyries however the majority of the dacites display evidence of deformation especially at contacts and metamorphism varying from silicification and chlorite alteration at contacts to pervasive low grade regional metamorphic alteration represented by common assemblages of chlorite sericite and albite texturally the dacites vary from entirely massive and coherent to partially brecciated to totally brecciated strangely some dacites are coherent at the margins and brecciated internally breccia textures vary from cryptically defined to blocky closely packed in situ jig saw fit textures with secondary minerals in fractures between clasts to more apparent matrix rich textures with round clast forms giving apparent conglomerate textures some clast zones have multi coloured clasts giving the impression of varied provenance strangely however all these textural variants have gradational relationships with each other and no bedding or depositional structures are present this indicates that all textures have an in situ origin the komatiites are generally altered and pervasively carbonate veined preservation of original textures is patchy and local but includes coarse adcumulate mesocumulate orthocumulate crescumulate harrisite and occasionally spinifex textures where original contacts between komatiites and dacites are preserved intact i e not sheared or overprinted by alteration the komatiites have chilled margins whereas the dacites do not the margins of the dacites are commonly silicified and inclusions of dacite occur in komatiite even at the top contacts of komatiite units but komatiite clasts do not occur in the dacites the komatiites therefore were emplaced as sills into the dacites and the intercalated relationships are interpreted as intrusive the brecciation and alteration in the dacites are interpreted as being largely due to hydraulic fracturing and alteration induced by contact metamorphic effects and hydrothermal alteration deriving from the intrusion of komatiites into the felsic pile the absence of autobreccia and hyaloclastite textures in the dacites suggest that they were emplaced as an earlier intrusive sill complex at a high level in the crust

pillow lavas and the leidenfrost effect field observations of the formation of pillows by hot de gassed lava entering the sea have proved that their generation involves the underwater exposure and movement of incandescent material this is made possible by the leidenfrost effect the phenomenon whereby film boiling replaces the usual nucleated boiling above a certain temperature thereby forming an insulating sheath of vapour around any sufficiently hot body immersed in a liquid only on cooling below a certain temperature defined here as the nukiyama temperature will rapid heat exchange occur between water and hot gas free lava near the surface this could produce phreatic explosions and extensive clouds of steam

facies analyses and volcanic setting of the giant neves corvo massive sulfide deposit iberian pyrite belt portugal in the iberian pyrite belt volcanic rocks are relatively scarce accounting for approximately only 25 of the geologic record with the remaining 75 consisting of sedimentary units this association is very clear in the host succession to the neves corvo massive sulfide deposit in portugal the neves corvo host succession comprises the products of explosive and effusive rhyolitic eruptions intercalated with mudstone that records a submarine below wave base environment and provides precise biostratigraphic age constraints the first and second volcanic events involved eruptions at local intrabasinal vents the first event generated thick beds of fiamme breccia that are late famennian in age the fiamme were originally pumice clasts produced by explosive eruptions and were subsequently compacted the second event was the late strunian latest famennian effusion of rhyolitic lava that was pervasively quench fragmented the third and final event is younger than the massive sulfide deposits poorly represented in the mine area and minor compared with the two other events the integration of biostratigraphic data with the volcanic facies architecture indicates that the neves corvo ore deposits are similar in age to the late strunian rhyolitic lava although regionally the iberian pyrite belt is essentially a sedimentary succession ore formation at neves corvo can be closely linked to discrete volcanic events that produced a relatively narrow range of volcanic facies

facies architecture of a silicic intrusion dominated volcanic centre at highway reward queensland australia abstract the highway reward massive sulphide deposit is hosted by a silicic volcanic succession in the cambro ordovician seventy mile range group northeastern australia three principal lithofacies associations have been identified in the host succession the volcanogenic sedimentary facies association the primary volcanic facies association and the resedimented syn eruptive facies association the volcanogenic sedimentary facies association comprises volcanic and non volcanic siltstone and sandstone turbidites that indicate submarine settings below storm wave base lithofacies of the primary volcanic facies association include coherent rhyolite rhyodacite and dacite and associated non stratified breccia facies autoclastic breccia and peperite the resedimented volcaniclastic facies association contains clasts that were initially formed and deposited by volcanic processes but then redeposited by mass flow processes resedimentation was more or less syn eruptive so that the deposits are essentially monomictic and clast shapes are unmodified this facies association includes monomictic rhyolitic to dacitic breccia resedimented autoclastic facies siltstone matrix rhyolitic to dacitic breccia resedimented intrusive hyaloclastite or resedimented peperite and graded lithic crystal pumice breccia and sandstone pumiceous and crystal rich turbidites the graded lithic crystal pumice breccia and sandstone facies is the submarine record of a volcanic centre s that is not preserved or is located outside the study area pumice shards and crystals are pyroclasts that reflect the importance of explosive magmatic and or phreatomagmatic eruptions and suggest that the source vents were in shallow water or subaerial settings the lithofacies associations at highway reward collectively define a submarine shallow intrusion dominated volcanic centre contact relationships and phenocryst populations indicate the presence of more than 13 distinct porphyritic units with a collective volume of 0 5 km 3 single porphyritic units vary from the size and shape of the lavas and intrusions reflect a restricted supply of magma during eruption intrusion the style of emplacement and the subaqueous emplacement environment due to rapid quenching and mixing with unconsolidated clastic facies the sills and cryptodomes did not spread far from their conduits the shape and distribution of the lavas and intrusions were further influenced by the positions of previously or concurrently emplaced units magma preferentially invaded the sediment avoiding the older units or conforming to their margins large intrusions and their dewatered envelope may have formed a barrier to the lateral progression and ascent of subsequent batches of magma

a model for submarine rhyolite dome growth ponza island central italy abstract the late pliocene rhyolitic submarine volcanic rocks of ponza island italy can be interpreted as the subaqueous equivalent of subaerial dome complexes in terms of geometry and structure three coalescing domes of about 1 km radius and aligned along a ne trending regional fracture have been identified the main difference between subaqueous and subaerial lava domes is that in a subaqueous environment lava is likely to undergo pervasive hyaloclastic brecciation so that domes are mainly composed of variously brecciated in situ to clast rotated hyaloclastite rather than coherent lava we suggest that the shape and rheologic behaviour through time of submarine domes are closely controlled by the development and thickness of the particulate hyaloclastic carapace which assumes the role of the solid crust of domes in subaerial environment the thickness of the hyaloclastic carapace at ponza is greater than 150 m and emplaced during several different pulses or eruptions in the earliest pulses lava is directly extruded on the seafloor and produces hyaloclastite the degree of brecciation of which decreases inward to the coherent flow banded rhyolite lava of the feeder dike once the hyaloclastic carapace is formed further pulses of magma combined with increase in height of the dome result in a local stress pattern characterised by a vertical 1 2 3 producing concentric and radial fractures and normal faults the newly rising magma shielded by the hyaloclastic carapace can intrude along these fault and fracture systems and invade previously emplaced but still water saturated hyaloclastite this produces the characteristic pattern of dikes observed at ponza as a series of concentric dikes that are progressively less inclined outward with respect to the dome centre these late stage dikes in turn produce hyaloclastite at their margins but generally less fragmented than the embedding hyaloclastite probably because the ascending magma is better shielded from direct contact with sea water periodic gravity collapses of the dome maintain the equilibrium between height and radius of the dome as suggested at ponza by the presence of mass flow deposits and the development of small topographic basins on top of the domes probably related to debris removal some of these topographic depressions host subaqueous pyroclastic deposits suggesting that if due to vertical growth of the dome water depth above the dome becomes shallower hydromagmatic explosions may occur

the matagami district abitibi greenstone belt canada volcanic controls on archean volcanogenic massive sulfide deposits associated with voluminous felsic volcanism several volcanogenic massive sulfide vms deposits have been mined in the archean matagami mining district abitibi subprovince quebec canada but the volcanology of their host rocks has been little studied most vms lenses are located along a stratigraphic marker known as the key tuffite footwall rocks belong to the watson lake group tholeiitic dacite and rhyolite whereas hanging wall rocks belong to the wabassee group tholeiitic basalt and tholeiitic to transitional andesite with lesser tholeiitic felsic volcanic rocks all felsic units of the south flank are interpreted as lobe hyaloclastite lava flows for most felsic units many such flows are juxtaposed and stacked a significant proportion of the units is made up of fragmental rocks especially at the top of the watson lake group in some specific areas where the vms deposits sit where hyaloclastite is well developed conditions were favorable for subseafloor replacement style vms deposits as is the case at bracemac mcleod the currently producing mine there sheet shaped ore lenses were emplaced under a capping unit the massive core of a hanging wall lobe hyaloclastite flow the proximal footwall chlorite alteration zone is subconcordant abundant hyaloclastite is also present at the 25 mt mattagami lake deposit and similar ore forming conditions are inferred in other places fragmental rocks were less developed at the top of the watson lake rhyolite and sulfides formed mounds on the sea floor with a discordant chlorite pipe underneath therefore several styles of vms mineralization can be considered in a single mining district even at a specific marker horizon depending on the volcanic facies found in the host sequence

magma flow within dykes in submarine hyaloclastite environments an ams study of the miocene cabo de gata volcanic units this research has been funded by projects cgl2005 03511 bte hi2006 0073 prin2009 prin 2009h37m59 and prin 2010 11 2010tt22sc and 2010tt22sc 003

a unified 3 d seismic workflow a geophysicist who practices seismic data analysis for earth modeling and imaging in depth is overwhelmed by the prolific number of inversion methods to estimate layer velocities and delineate reflector geometries the two constituents of a seismically defined earth model in depth given a specific type of structural play the key to estimating an accurate earth model in depth however is a workflow that is based on a judicious combination of inversion methods appropriately selected for their robustness we present a unified workflow for processing inversion and interpretation of 3 d seismic data that is applicable to low relief structures and complex structures associated with extensional and compressional tectonics with some modifications the workflow also is applicable to complex overburden structures associated with salt and overthrust tectonics although doing it right the first time is most desirable there is never a situation where this is possible when estimating an earth model in depth a fu

late pliocene volcaniclastic products from southern apennines distal witness of early explosive volcanism in the central tyrrhenian sea two volcaniclastic successions intercalated in pliocene basinal clays from the southern apennines have been analysed to determine their provenance and their relationship with the geodynamic evolution of the western mediterranean the studied deposits are exclusively made up of ashy pyroclasts dominated by fresh acidic to intermediate glass mostly in the form of shards pumice fragments and groundmass fragments with vitrophyric texture crystals include pl opx cpx hbl and rare bt sedimentological features suggest that the volcanic material accumulated near the basin margin by primary fallout processes and was later remobilized by density currents 40 ar 39 ar geochronology allowed dating of one succession at 2 24 0 06 ma corresponding to the late pliocene composition of the volcaniclastic material is typical of a transitional high k calc alkaline series the age and chemical composition constrain the provenance of the volcaniclastic rocks from the southern tyrrhenian domain here volcanic centres were active during pliocene time approximately at the northern end of a volcanic arc formed before the opening of the southernmost part of the sea this paper shows that a detailed study of volcaniclastic products from the southern apennines and calabria can be very useful in collecting new pieces of information on the eruption history of the southern tyrrhenian domain since they record additional data not available from the study of exposed volcanic edifices

facies architecture and palaeoenvironmental constraints of subaqueous felsic volcanism in the iberian pyrite belt the paymogo volcano sedimentary alignment abstract the paymogo volcano sedimentary alignment pvsa is an essentially igneous sequence associated with vhms deposits of the iberian pyrite belt unlike other areas of this iberian mining district where mafic igneous rocks are abundant in the pvsa these are essentially felsic two facies associations have been distinguished based on petrographic and chemical features a lower dacite facies association and an upper rhyolite facies association the main ore horizons appear related to volcanic breccias of the rhyolite facies association a detailed petrographic study of these breccias suggests they are resedimented hyaloclastite deposits the analysis of volcanic facies indicates that the volcanism developed at water depths deeper than 500 m consistent with the minimum depth at which vhms deposits can form we conclude that detailed volcanological studies provide effective constraints to palaeoenvironmental interpretations which are relevant in assessing regional stratigraphy and area selection for vhms exploration

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

solidification and morphology of submarine lavas a dependence on extrusion rate the results of recent laboratory experiments with wax extruded beneath relatively cold water may be extrapolated to predict the surface morphology of submarine lavas as a function of the extrusion rate and melt viscosity the experiments with solidifying wax indicated that the surface morphology was controlled by a single parameter the ratio of the time taken for the surface to solidify and a time scale for lateral flow for submarine basalts a solution of the cooling problem which is dominated by conduction in the lava but convective heat transfer in the water and estimates of lava viscosities place this parameter within the empirically determined pillowing regime over a wide range of extrusion rates this result is consistent with the observation that pillow basalts are the most common products of submarine eruptions smoother surfaces corresponding to the various types of submarine sheet flows are predicted for sufficiently rapid extrusions of basaltic magma still higher eruption rates in regions of low topographic relief may produce submarine lava lakes minimum emplacement times can be calculated for submarine volcanic constructs of a single lava flow type

growth destruction and facies architecture of effusive and explosive volcanics in the miocene shama basin southwest of saudi arabia subaqueous subaerial volcanism in a lacustrine setting abstract this paper presents for the first time detailed stratigraphical and petrographical analyses of the harrat shama volcanic basin hsvb of miocene age in sw saudi arabia this basin is filled by abundant volcanics intercalated with fluvio lacustrine sedimentary successions these successions are characterized by a syneruptive intereruptive cycle which results from the intercalation periods of volcanics and volcaniclastic sedimentation along with periods of fluvial incision two depositional sequences showing an overall fining upward trend are preserved within the hsvb the first comprises hyaloclastites zeolite bearing bedded tuffs and bedded accretionary lapilli tuffs interpreted as deep lake whereas the second is made up of extensive pumiceous pyroclastic flows peperitic breccias and basaltic lava flows which are in turn capped by siliciclastic deposits of lacustrine deposits the passage between two sequences is defined by lateral variations of the lithofacies and textural features reflecting a change in paleogeography the fining upward volcaniclastic deposits of the hsvb sequence are interpreted to represent the sedimentation pattern induced by tectonic subsidence with the influence of volcaniclastic sediment input indicating well established proximal distal relationship hsvb was evolved through three stages including pre syn and post caldera stages the first two stages correspond to proximal aggradational syn eruptive event whereas the latter stage refers to distal degradational inter eruptive event these stages overlap in time as shown by the alternation of explosive effusive and their resedimented deposits reflecting diverse eruptive sequences formed by different fragmentation processes the difference in the latter coupled with the variability in the magma water interaction and the nature of substrate are responsible for such diversity resulting in progressive environmental changes from subaqueous to subaerial regime from bottom to top the development of such contrast in the depositional sequences reflects variation in the sediment flux and subsidence rate during the evolution of the hsvb

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

a rheological investigation of vesicular rhyolite abstract the rheology of vesiculating rhyolitic systems exerts a strong control on the transport of silicic magmas in the subvolcanic to volcanic environments we present here an investigation of vesiculating and vesiculated rhyolites using dilatometric methods this study examines the effect of vesicle content on the viscosity of a natural supercooled rhyolitic liquid with 0 70 vesicles the experimental samples of rhyolitic glass are derived from fusion of a natural obsidian from little glass butte oregon crystal free rhyolite glasses of varying porosity were prepared by fusing obsidian powder in a pt crucible differing porosities were obtained by varying the temperature 1300 1650 c and duration 0 5 6 h of the fusions cylindrical samples of the resulting vesiculated rhyolites were cored from the crucible using diamond tools and their ends were ground flat and parallel for dilatometry the porosity of each sample was determined from archimedean buoyancy density determinations and comparison with bubble free rhyolite 2 331 g cm 3 porosity 1 p p o the density of foamed samples was determined using their mass volume and regular geometry viscosities were determined in the parallel plate mode at stresses of 5 10 3 to 10 5 pa the viscosimeter was calibrated using nbs 711 glass the bubble contents were microscopically investigated using a video reflected light system and image analysis software distribution functions of the size orientation aspect ratio and surface porosity were obtained the viscosity of rhyolite decreases with increasing bubble content a general relationship of the form 0 1 c describes the effect of porosity in volume fraction on the viscosity where c is a dimensionless constant 22 4 2 9 and log 10 0 10 94 0 04 pa s at 850 c

the fracture behaviour of volcanic glass and relevance to quench fragmentation during formation of hyaloclastite and phreatomagmatism quench fragmentation is a non explosive process that occurs when molten magma is super cooled to glass upon contact with ambient water this occurs when coherent lavas are erupted subaqueously when they flow into water when magma intrudes into water saturated sediments and when magma and water interact explosively during phreatomagmatism quench fragmentation also occurs alongside explosive phreatomagmatic activity although products of quench fragmentation hyaloclastite sensu stricto have been discussed qualitatively in the volcanological literature compared to explosive fragmentation processes very little is known about the exact dynamics of quench fragmentation of magma and how this relates to the rheology and physical properties of volcanic glass based on literature from materials engineering we present a detailed review of the processes by which glass forms the properties of glass and the fracture mechanics that cause it to fragment non explosively we also consider how this can be applied to understanding the dynamics behind the volcanological processes of in situ glass fragmentation during quenching in wet environments and phreatomagmatism important parameters for the occurrence of quench fragmentation are the temperature difference between the magma and the ambient water and how much the ambient water is superheated above its leidenfrost temperature the geometry of the lava or magma intrusion the thermal conductivity and the thermal expansion are also of great importance the resistance of the magma against fragmentation can be increased with the presence of crystals provided the thermal expansion of the crystals does not greatly exceed that of the glass vesicles have the opposite effect unless the magma is highly vesicular this overview then provides a solid basis for further quantitative study of quench fragmentation and hyaloclastite formation

subaqueous rhyolitic dome top tuff cones a model based on the devonian bunga beds southeastern australia and a modern analogue the relics of two late devonian subaqueous rhyolitic dome top tuff and pumice cone successions are preserved in the bunga beds outlier of the boyd volcanic complex southeastern australia these cone successions and other rhyolitic volcanics of the bunga beds are associated with turbidite and other deep water massflow sedimentary rocks the two cone successions have a generally similar stratigraphy at the base flow banded variably autobrecciated and quench fragmented rhyolite representing an intrusive to extrusive dome is overlain by rhyolitesediment breccia representing extrusion of the dome through the deep water sediment pile and resedimentation down its flanks in the northern cone succession an overlying succession of bedded pumiceous crystal rich to crystal poor tuffs represents the onset of pyroclastic activity and growth of a tuff cone an overyling debris flow deposit represents degradation of part of the cone the topmost unit a stratified pumice succession is thought to represent another cone building eruptive phase and is separated from the underlying strata by a major slide surface the southern cone succession contains less tuff and abundant pumice and is also terminated by a debris flow deposit indicating cone degradation a modern analogue for the inferred eruptive style and sequence is the 1953 1957 rhyolite eruption that formed the tuluman island lava tuff cone complex in the bismarck sea the eruptions were often cyclical consisting of an initial inferred submarine lava forming stage passing into a pumicecone forming stage in some cases a subaeriallava forming stage and a final stage following the cessation of volcanism during which the cones collapsed gravitationally or were destroyed by wave erosion using observations from both the tuluman island eruptions and the preserved stratigraphies of the devonian tuff cones a dynamic model is proposed for the formation of subaqueous rhyolitic dome top tuff and pumice cones

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

facies architecture and origin of a submarine rhyolitic lava flow dome complex ponza italy the islands of ponza and gavi western pontine archipelago italy preserve parts of a subaqueously emplaced high silica 75 77 sio2 rhyolitic lava flow that overlies the margins of three older domes exposures of the lava alone cover 24 km2 and with a thickness of 150 m exposed in the cliffs the preserved volume of the lava flow is 3 6 km3 and the original volume is likely to have exceeded 7 km3 the lava flow consists of coherent obsidian and several hyaloclastite facies including in situ and clast rotated breccias and sandstones which all exhibit gradational relationships the coherent obsidian occurs as separate domains and is often flow banded although flow folds occur the banding is generally subhorizontal dipping gently to the nne the hyaloclastite representing 90 of the lava by volume has a diffuse layering varying from 0 5 to 2 m in thickness this layering also dips gently to the nne and is marked in places by alternation of some coherent horizons coarse and finely fragmented in situ hyaloclastite pumiceous hyaloclastite and clast rotated hyaloclastite the layering is laterally continuous and consistent in orientation over most of the 13 km length of ponza the subhorizontal orientation of the flow banding and presence of resedimented deposits in the upper parts of the sequence indicate that the unit is a lava flow the resedimented deposits comprise both proximal and distal deposits of debris flows and turbidity currents initiated by gravitational instability of the upper and marginal parts of the lava flow and include stratified monomictic obsidian breccias and bedded vitric siltstones and sandstones the lava flow has been intruded and crosscut by rhyolitic bodies of various ages including discordant coherent to internally fractured contemporaneous rhyolite bodies and late penecontemporaneous dikes with glassy margins and alteration haloes

morphology and growth style of a miocene submarine dacite lava dome at atsumi northeast japan abstract a miocene submarine lava dome at tate iwa atsumi yamagata japan displays well preserved primary morphological features the dome currently rises above a wave cut coastal platform and is 55 m high and 90 180 m across it comprises a massive core and a lava lobe hyaloclastite rim both of which are composed of compositionally uniform feldspar phyric dacite sio2 64 wt the boundary between these two zones is distinct but gradational the massive core consists of homogeneous coherent dacite and is characterized by flow banding along the margin and by columnar joints radiating from the centre to the margin the lava lobe hyaloclastite rim encircles the massive core and consists of a complex of 80 90 dacitic lava lobes and 10 20 hyaloclastite the lava lobes are 1 6 m thick 3 12 m wide and more than 5 m long each lobe consists of a radially columnar jointed core and a glassy rim 10 30 cm thick the hyaloclastite comprises polyhedral dacite clasts 5 100 cm across in a matrix of dacite fragments up to 5 mm across the lava lobes and hyaloclastite have gradational contacts in places paleobathymetric studies based on foraminifera in sediment beneath the dome and the k ar age of the dome suggest that it was extruded in a middle bathyal environment 1500 2000 m below sea level at 12 9 0 6 ma the internal structures of the dome suggest that it formed by a combination of exogenous growth involving extrusions of small dacite lobes and endogenous growth involving a continuous magma supply and simple expansion from the interior the presence of small lava lobes along the rim suggests that the magma had relatively low viscosity at the time of extrusion in spite of its high silica content magma temperatures calculated by two pyroxene and fe ti oxide geothermometers were 999 1042 and 957 1005 c respectively the inferred low viscosity may be attributed to the high temperature of the magma and or a high confining pressure resulting from the deep sea environment

three dimensional lithofacies variations in hyaloclastite deposits analysis of the spatial lithofacies variability within lava fed delta formations in southern iceland has revealed complex three dimensional volcanic architectures in hyaloclastite deposits in non glacial settings two depositional environments are studied a lava entering a marine embayment stori nupur and b lava advancing into a body of water of the flanks of a surtseyan cone hjorleifshof i interaction between environmental factors such as shoreline geomorphology water depth wave energy levels the nature of the lava transport system lava supply rate all affect the resulting lava deltas creating complex lithofacies arrangements and stacking patterns recognised here are two types of hyaloclastite deltas one of syn sedimentary origin hjorleifshof i and one derived from primary fragmentation processes stori nupur syn sedimentary systems are dominated by destabilisation of the hyaloclastite pile leading to reworking downslope and share similarities to alluvial delta systems conversely primary fragmented systems are controlled by waxing and waning cycles in volcanic effusivity whereby the hyaloclastite unit recorded is not the product of one lava flow rather than one eruptive event

hydrothermal breccia textures and processes lisca bianca islet panarea volcano aeolian islands italy panarea is a largely submarine to partly subaerial quaternary lava dome complex stratovolcano with a long lived active shallow hydrothermal system located in the aeolian islands volcanic arc of southern italy the emergent top of the volcano forms a small archipelago made up of calc alkaline basaltic andesite to rhyolite lava domes ca 150 20 ka we document the facies outcropping on lisca bianca islet panarea archipelago based on grain size clast fabric and degree of hydrothermal alteration identifying coherent facies boulder breccia facies cobble breccia facies pebble breccia facies and pervasively altered andesite facies alunite marcasite sulfur the breccias all have ubiquitous jigsaw fit clast textures and are variably hydrothermally altered the breccias are interpreted as hydrothermal breccias and are distinguished from primary volcanic facies based on their distinguishing characteristics the breccias formed through a cyclical process involving the following stage 1 progressive build up of fluid pressure toward the level of the tensile strength of the host andesite stage 2 incipient fracturing of the andesite when fluid pressure approaches and then exceeds the tensile strength of the andesite under critical fracturing conditions stage 3 pervasive fracturing of the host andesite leading to an increase in permeability as a network of fractures develops stage 4 declining pressure with fluid flow rates that lead to infilling and sealing of fractures by natroalunite thereby reducing permeability leading to progressive build up of fluid pressure again and the beginning of a new cycle

subaqueous cryptodome eruption hydrothermal activity and related seafloor morphologies on the andesitic north su volcano abstract north su is a double peaked active andesite submarine volcano located in the eastern manus basin of the bismarck sea that reaches a depth of 1154 m it hosts a vigorous and varied hydrothermal system with black and white smoker vents along with several areas of diffuse venting and deposits of native sulfur geologic mapping based on rov observations from 2006 and 2011 combined with morphologic features identified from repeated bathymetric surveys in 2002 and 2011 documents the emplacement of a volcanic cryptodome between 2006 and 2011 we use our observations and rock analyses to interpret an eruption scenario where highly viscous crystal rich andesitic magma erupted slowly into the water saturated gravel dominated slope of north su an intense fragmentation process produced abundant blocky clasts of a heterogeneous magma olivine crystals within a rhyolitic groundmass that only rarely breached through the clastic cover onto the seafloor phreatic and phreatomagmatic explosions beneath the seafloor cause mixing of juvenile and pre existing lithic clasts and produce a volcaniclastic deposit this volcaniclastic deposit consists of blocky non altered clasts next variably 1 100 altered clasts hydrothermal precipitates and crystal fragments the usually applied parameters to identify juvenile subaqueous lava fragments i e fluidal shape or chilled margin were not applicable to distinguish between pre existing non altered clasts and juvenile clasts this deposit is updomed during further injection of magma and mechanical disruption gas propelled turbulent clast recycling causes clasts to develop variably rounded shapes an abundance of blocky clasts and the lack of clasts typical for the contact of liquid lava with water is interpreted to be the result of a cooled high viscosity crystal rich magma that failed as a brittle solid upon stress the high viscosity allows the lava to form blocky and short lobes the pervasive volcaniclastic cover on north su is partly cemented by hydrothermal precipitates these hydrothermally cemented breccias crusts and single pillars show that hydrothermal circulation through a thick layer of volcaniclastic deposits can temporarily increase slope stability through precipitation and cementation

internal structure and emplacement of an upper pliocene dacite cryptodome milos island greece the upper pliocene kalogeros cryptodome is exposed on the northeastern part of milos greece the dacite cryptodome is 800 1300 m across and at least 120 m high it is inferred to have intruded wet unconsolidated pumiceous sediments in a shallow marine environment the kalogeros cryptodome includes five facies the coherent dacite facies 80 vol consists of massive non vesicular dacite and is characterised by radial columnar joints 20 250 cm across the banded dacite facies 15 vol encircles the coherent core and is up to 40 m thick it comprises alternating bands 0 5 4 m thick of pale grey and black dacite the fractured dacite facies less than 1 vol forms an irregular zone 1 3 m thick at the outer margin of the cryptodome the outermost massive dacite breccia facies less than 2 vol consists of blocky to polyhedral dacite clasts 1 40 cm in diameter and is characterised by domains of jigsawfit and clast rotated breccia the stratified dacite breccia 2vol is clast to matrix supported monomictic poorly sorted and composed of dacite clasts up to several metres in diameter the facies association collectively records endogenous growth of a cryptodome that involved a continuous magma supply during a single intrusive phase and simple expansion inflation during emplacement the margins of the kalogeros cryptodome were quench fragmented forming an outer domain of intrusive hyaloclastite and intensely fractured dacite the near solid outer carapace insulated the hotter less viscous interior laminar shear accompanied inflation generating large scale flow banding around the outer part of the core once stagnant concentric isotherms were established within the cryptodome and controlled the orientation of columnar joints cryptodomes are characterised by a well developed internal concentric distribution of distinctive textural domains modest autoclastic breccia and the absence of redeposited autoclastic facies

hyaloclastites peperites and soft sediment deformation textures of a shallow subaqueous miocene rhyolitic dome cryptodome complex palhaza hungary abstract the ne tokaj mountains at palhaza in ne hungary are made up of a complex association of miocene rhyolitic shallow intrusions cryptodomes and endogenous lava domes emplaced into and onto soft wet pelitic sediment in a shallow submarine environment the intrusive extrusive complex shows a range of interaction textures with the host muddy sediment ranging from blocky peperites formed on a 0 1 m scale through to irregular contacts closely resembling globular mega peperites on a 10 m scale the over 200 m thick igneous succession is interpreted to result from the pulsatory growth of shallow cryptodomes through muddy saturated host sediment the intrusions eventually breached the sedimentary cover to build up thick in situ hyaloclastite piles in the shallow subaqueous environment the coherent rhyolitic cryptodome facies is surrounded by intrusive hyaloclastite in the contact zone to the pelitic host sediment in the upper level of the complex rhyolitic dome rock is capped and surrounded by hyaloclastite formed due to quench fragmentation upon contact of the lava surface with sea water

spherulitic aphyric pillow lobe metatholeiitic dacite lava of the timmins area ontario canada a new archean facies formed from superheated melts fragmental rocks of the v10 units of the vipond formation of the tisdale assemblage previously have been identified as pillow basalts but many samples are shown to be intermediate to felsic in character likely tholeiitic dacite in composition specifically the v10b unit is mapped as a pillow lobe dacite aside from being more geochemically evolved in terms of their immobile trace elements these rocks differ from typical pillow basalts in that they have more abundant primary breccia and hyaloclastite the pillow lobes are contorted having been folded in a plastic state and are zoned typically having a spherulite rich core moreover the flows are aphyric interpreted to mean that they were erupted in a superheated state this along with their pillow lobe nature demonstrates that they were erupted as relatively low viscosity melts for such silicic compositions interaction with water quenched the outer pillow lobe and contributed to the formation of the abundant breccia the fact that the melt was crystal and microlite free inhibited crystal growth such that the bulk of the lobes were quenched to crystal free glass nucleation occurred only in the cores where cooling rates were lower in comparison to the medial and exterior areas of the pillow lobes although in the cores crystal growth rates were high so that abundant spherulite formation took place the flows are exposed over a strike length of more than 10 km and are interpreted to be continuous and to have resulted from fissure eruptions the resulting porous permeable high fe mg high surface area glassy rocks may have been ideal for channeling and reacting with gold bearing hydrothermal solutions descriptions of most other archean subaqueous felsic lobes document that the lavas were phyric and that spherulites formed near the external lobe margin the distinctly different zoning of the v10b pillow lobes suggests that they constitute a new archean facies formed from superheated melts

hyaloclastite fragmentation below the glass transition an example from el barronal submarine volcanic complex spain this research has been funded by projects cgl2005 03511 bte hi2006 0073 prin2009 prin 2009h37m59 and prin 2010 11 2010tt22sc and 2010tt22sc 003

facies reconstruction of a hydrothermally altered dacite extrusive sequence evidence from geophysical downhole logging data odp leg 193 odp leg 193 drilled into the pacmanus hydrothermal field papua new guinea which is an activehydrothermal vent field associated with felsic magmatism in a convergent geodynamic setting thepacmanus hydrothermal field is part of the eastern manus basin and is located near the crest of pualridge a 500 to 700 meters high felsic neo volcanic ridge two sites snowcap site 1188 and romanruins site 1189 were drilled approximately 800 m apart although low core recovery limited theexamination of the subsurface geology geophysical downhole measurements provided continuous recordsof in situ physical properties with depth and were used to reconstruct lithostratigraphic profiles downholelogging operations included standard wire line logging and logging while drilling lwd measurements electrical resistivity images of the borehole wall were examined to find characteristic textures and threedifferent volcanic facies were distinguished coherent dacite volcaniclastic dacite and brecciated dacite inaddition intervals with sulfide mineralization were identified based on characteristic responses of thephotoelectric factor log a comparison of the reconstructed profiles from both sites shows considerabledifferences in the proportions of facies and the thickness of individual layers thick units of coherent faciesare predominant at snowcap and indicate a proximal position of the site within the volcanic system of pualridge at roman ruins thin individual layers with rapid changes in volcanic facies and a higherproportion of volcaniclastic and brecciated dacite suggest a medial position these differences in volcanicfacies show that the volcanic architecture at pual ridge is characterized by small scale facies changes emplacement of small volume individual lava flows and complex geometries of individual emplacementunits geophysical logging data suggest that subseafloor hydrothermal activity is focused to the areaaround one hole at roman ruins downhole logs from this hole show characteristic cyclic patterns in theelectrical resistivity and photoelectric factor logs indicating that hydrothermal sulfide mineralization isconcentrated at the tops of individual lava flows representing paleo seafloor positions

migration of common shot gathers three depth migration methods which operate on common shot data are presented the first migration method maps digitized horizons from the x t domain to the x z domain because this method is based on ray tracing its computation time is short it is suggested for iterative velocity analysis the other two migration methods map the entire common shot gather into a depth section the common shot migration requires calculation of the arrival time of the direct wave from the source to all the depth points and is done through a direct solution of the eikonal equation all three methods are suitable for areas with both lateral and vertical velocity variation

drill core based facies reconstruction of a deep marine felsic volcano hosting an active hydrothermal system pual ridge papau new guinea odp leg 193 abstract pual ridge is a deep marine felsic volcanic edifice in the eastern manus back arc basin papua new guinea with an estimated volume of 6 to 9 km 3 it is 1 1 5 km wide 20 km long and rises 500 600 m above the surrounding ocean floor the active pacmanus hydrothermal field on the crest of pual ridge at 1640 1690 m below sea level was the target of ocean drilling program leg 193 variably altered dacite lavas have been recovered from the subsurface of a low t discharge site snowcap and a high t black smoker site roman ruins reaching a maximum depth of 380 m below seafloor mbsf volcanic facies interpretation of these cores is difficult due to incomplete recovery and widespread pseudoclastic textures generated by fracturing and multi phase incomplete fluid dacite interaction however distinction of genuine volcaniclastic facies and facies with alteration related clastic appearance is important in order to define paleo seafloor positions within the volcanic stratigraphy that may be prospective for massive sulfide mineralization this has been accomplished using remnant primary characteristics indicative of transportation such as polymictic composition grading or textural evidence for differential movement of individual clasts three phases of volcanic activity can be distinguished and a proximal facies association dominated by coherent facies of dacite lavas exists below snowcap at roman ruins a medial facies association consists of lava flows with about equal proportions of coherent and volcaniclastic facies endogenous growth was an important process during lava flow emplacement and groundmass textures such as perlite flow banding and spherulites indicate that cooling rates were variable locally allowing for high temperature devitrification a tube pumice breccia unit is interpreted as the resedimented facies of a quench fragmented highly vesicular dacite lava carapace sulfide accumulations in the subsurface are restricted to roman ruins suggesting that the abundance of volcaniclastic facies favored ascent of metal bearing fluids due to the inherent high permeability evidence for mineralization at paleo seafloor positions at 110 and 195 mbsf indicate a complex interrelationship between volcanic and hydrothermal activity a facies model has been developed which may help to locate subaqueous felsic lava dominated volcanic centers and associated sulfide deposits in ancient volcano sedimentary successions

archean subqueous high silica rhyolite coulees examples from the kidd munro assemblage in the abitibi subprovince abstract subaqueous high silica rhyolite in prosser township southern abitibi subprovince ontario consists of intercalated thin mass balance calculations and textural analysis demonstrate that the high silica aphyric rhyolites were originally glassy having been recrystallized to a pseudo spherulitic quartz albite mixture which was subsequently sericitized and chloritized the ree were moderately mobile 10 40 whereas the high field strength elements were less mobile the viscosity of the aphyric lava protolith was estimated to be 7 8 10 6 pa s based on an eruption temperature of 850 c calculated from measurements using the ti in zircon saturation thermometer this viscosity estimate is similar to the calculated viscosity 4 10 7 pa s of the subaerial namshraun rhyolite flow iceland the prosser and namshraun rhyolites have comparable dimensions and eruption temperature it is concluded that the low aspect ratio of the prosser township flows may have been controlled by a number of environmental factors including paleo slope magma rheology eruption rate and the insulative effects of the flow breccia

giant rafted pumice blocks from the most recent eruption of taupo volcano new zealand insights from palaeomagnetic and textural data abstract giant blocks of pumice lie strewn along a former shoreline of intracaldera lake taupo new zealand and are the sole subaerial evidence of the most recent volcanism at the taupo supervolcano geochemically they are identical to material erupted during the complex and multiphase 1 8 ka taupo eruption which they post date by one to two decades the blocks some of which are 10 m long show complex jointing patterns indicative of both surface chilling and continued interior expansion as well as heterogeneous vesicularity with dense rims mean density 917 kg m3 grading via an intervening transition zone mean density 844 kg m3 into a more highly vesicular interior mean density 815 kg m3 analysis of thermal demagnetisation data indicates significant reorientation of the blocks as they cooled through a series of blocking temperatures some parts of block rims cooled to below 580 c well before emplacement on the shore whereas other parts in the interior and transition zones which cooled more slowly acquired different orientations before stranding some block interiors cooled after blocks were finally deposited and record the direction of the 1 8 ka field the blocks are believed to be derived from one or both of a pair of rhyolitic lava domes that developed on the bed of lake taupo several decades after the climactic taupo eruption over the inferred vent area these and similar giant rafted pumice blocks in other marine and lacustrine settings raise a number of questions about how volatile rich felsic magma can be erupted underwater with only limited thermal fragmentation furthermore the prolonged flotation of out sized fragments of vesiculated magma formed during subaqueous dome growth contrasts with the rapid sinking of smaller pieces of hot plinian pumice under laboratory conditions the genesis of pumice forming the blocks is not entirely clear most simply the blocks may represent part of a vesiculated carapace of a growing lava dome broken loose as the dome grew and deformed then rising buoyantly to the surface parts of the carapace could also be released by local magma water explosions some textures of the pumice however suggest fresher magma released from beneath the carapace this may suggest that silicic dikes and pillows pods intruded into a growing mound of silicic hyaloclastite itself formed by quench fragmentation and thermal granulation of the dike margins this fragmental cover would have inhibited cooling of a still hot and actively vesiculating interior which was then released to float to the surface by gravitational destabilisation and collapse of the growing pile following their formation the large fragments of pumice floated to the lake s surface where they were blown ashore to become embedded in accumulating transgressive shoreface sediments and continue cooling

pumiceous rhyolitic peperite in ancient submarine volcanic successions pumiceous peperite is associated with a rhyolitic sill that intruded wet unconsolidated submarine stratified pumice breccia in the cambrian mount read volcanics australia other examples of pumiceous peperite described in this paper occur at the pumiceous base of a rhyolitic lava and the margins of a rhyolitic cryptodome complex in the miocene green tuff belt japan intervals of pumiceous peperite are thin less than 15 m laterally discontinuous massive poorly sorted and the clast to matrix ratio varies significantly over short distances they are composed of feldsparphyric tube pumice clasts and domains of clastic sediment that include stratified pumice breccia pumiceous siltstone and bioturbated mudstone the sediment domains may be massive or may display relic depositional structures sediment adjacent to pumice clasts is silicified possibly reflecting induration on contact with hot pumice pumiceous peperite has gradational contacts with in situ and clast rotated pumiceous hyaloclastite and coherent pumiceous rhyolite gradational relationships between pumiceous peperite and coherent pumiceous rhyolite pumice clasts with chilled margins and curviplanar surfaces and abundant jigsaw fit texture imply that quench fragmentation and autobrecciation were the dominant mechanisms of clast formation the formation of pumiceous peperite is favoured by a combination of low confining pressure and delayed quenching allowing vesiculation pumiceous peperite may easily be misinterpreted or overlooked as it resembles other pumice rich facies that are common in submarine volcanic successions the correct identification of pumiceous peperite can provide evidence to constrain the timing of intrusive episodes in volcanic successions

volcanic architecture in the odiel river area and the volcanic environment in the riotinto nerva unit iberian pyrite belt spain abstract in order to assess the volcanic architecture of the vsc in the iberian pyrite belt spain a study of the volcanic and sedimentary lithofacies has been undertaken on a section up to 1000 m thick in the odiel river considered to be also representative of other areas of the riotinto nerva unit among other mineral deposits this unit includes the world class volcanic hosted massive sulfide deposits of the riotinto mining district several conclusions have been obtained a a complex thick volcanic succession as well as mafic and felsic sills and thin sedimentary intervals predated the genesis of the sulfide deposits which did not form in a dominantly sedimentary environment b two successive felsic volcanic sequences can be distinguished showing textural and chemical variations from bottom to top c mafic lithofacies decrease upwards in the sequence and d volcanic structures eruption style and internal organization suggest a deep subaqueous depositional environment which agrees with the depositional setting for massive sulfide deposits

magma water interactions in subaqueous and emergent basaltic in the subaqueous growth and emergence of a basaltic volcano clasts are formed by one or a combination of 1 explosive release of magmatic volatiles 2 explosive expansion and collapse of steam formed at magma water contact surfaces 3 explosive expansion of steam following enclosure of water in magma or entrapment of water close to magma and 4 cooling contraction these processes named respectivelymagmatic explosivity contact surface steam explosivity bulk interaction steam explosivity andcooling contraction granulation can be enhanced by mutual interaction and feedback the first three explosive processes are limited at certain water depths hydrostatic pressures and become increasingly vigorous at shallower levels the depth of onset of magmatic explosivity depends largely on juvenile volatile content it is up to 200 m for tholeiitic magmas and up to 1 km for alkalic magmas at the depth where formation of clastic deposits becomes predominant over effusion of lavas magmatic explosivity is subordinate to steam explosivity as a clast forming process the upward transition to accumulation of dominantly clastic deposits is not simply related to the onset of substantial exsolution of magmatic volatiles and can occur without it contact surface explosivity commonly requires initiation by a vigorous impact between magma and water and although no certain depth limit is known likelihood of such explosivity decreases rapidly with depth clast generation by bulk interaction explosivity appears to be restricted to depths much shallower than that of the critical pressure of water which in sea water is at about 3 km cooling contraction granulation can occur in any depth of water but at shallow levels may be replaced by contact surface explosivity during continuous eruption under water tephra can be ejected and deposited within a cupola of steam such that rapid quenching does not occur emergent volcanoes are characterized by distinctive steam explosive activity that results primarily from a bulk interaction between rapidly ascending magma and a highly mobile slurry of clastic material water and steam the water gets into the vent by flooding across or through the top of the tephra pile and violent explosions cease when this access is sealed the eruptions during emergence of surtsey and capelinhos typify the distinctive explosive activity the style and controls of which are different from those of maar volcanoes

hydrothermal alteration of a rhyolitic hyaloclastite from ponza island italy abstract a rhyolitic hyaloclastite from ponza island italy has been hydrothermally altered producing four distinct alteration zones based on xrd and field textures 1 non pervasive argillic zone 2 propylitic zone 3 silicic zone and 4 sericitic zone the unaltered hyaloclastite is a volcanic breccia with clasts of vesiculated obsidian in a matrix of predominantly pumice lapilli incomplete alteration of the hyaloclastite resulted in the non pervasive argillic zone characterized by smectite and disordered opal ct obsidian clasts some pumice lapilli and pyrogenic plagioclase and biotite are unaltered smectite has an irregular flakey morphology although euhedral particles are occasionally observed the propylitic zone is characterized by mixed layer illite smectite i s with 10 to 85 illite i mordenite opal c and authigenic k feldspar akspar the matrix of the hyaloclastite is completely altered and obsidian clasts are silicified however plagioclase and biotite phenocrysts remain unaltered flakey i s replaces pumice and mordenite akspar and silica line and fill pores i s particles are composed predominantly of subequant plates and euhedral laths the silicic zone is characterized by highly illitic i s with 90 i quartz akspar and occasional albite in this zone the matrix and clasts are completely altered and pyrogenic plagioclase shows significant alteration illitic i s has a euhedral lath like morphology in the sericitic zone the hyaloclastite altered primarily to illitic i s with 66 i quartz and minor akspar and pyrite clay minerals completely replace pyrogenic feldspars and little evidence remains of the original hyaloclastite texture unlike other zones illitic i s is fibrous and pure illite samples are composed of euhedral laths and hexagonal plates the temperatures of hydrothermal alteration likely ranged from 30 to 90 c for the argillic zone from 110 to 160 c for the propylitic zone from 160 to 270 c for the silicic zone and were possibly as high as 300 c for the sericitic zone the four zones occur as linear bands that increase in intensity north of the bentonite mine at cala dell acqua the alteration zones have two orientations and may be structurally controlled by e w and ne sw trending faulting which is consistent with the dominant structural trends of the pontine archipelago finally hydrothermal alteration most likely involved seawater based on the geologic evolution of ponza

fluidization of wet sediments during the emplacement and cooling of various igneous bodies during the emplacement and cooling of igneous bodies fluidization of wet sediment by water vapour may occur by heating or by pressure relief during the opening of fractures momentary fluidization causes sediment reconstitution and local transport continuous fluidization can result in substantial sediment displacement theoretical considerations show that fluidization due to heating is not likely to occur at depths where pressure is much greater than 312 bars studies of the emplacement of sills of andesite ayrshire fluid rhyolite ramsey island and viscous rhyolite moelwyn hills recognize a wide range of phenomena attributable to wet sediment fluidization and in places modify existing interpretations the transgressive bases of the subaqueously welded ash flow tuffs of the capel curig volcanic formation are consistent with emplacement attended by fluidization of the subjacent sediment evidently fluidization is a means by which large volumes of wet sediment can be replaced by igneous material with minimal disturbance of the remaining host also fluidization can result in exceptionally complex contact relationships between sedimentary host and intrusive rocks and can cause fractures formed during cooling to be thoroughly pervaded by sediment fluidization is commonly associated with the formation of peperites

maximum energy traveltimes calculated in the seismic frequency band prestack kirchhoff migration using first arrival traveltimes has been shown to produce poor images in areas of complex structure to avoid this problem i propose a new method for calculating traveltimes that estimates the traveltime of the maximum energy arrival rather than the first arrival this method estimates a traveltime that is valid in the seismic frequency band not the usual high frequency approximation instead of solving the eikonal equation for the traveltime i solve the helmholtz equation to estimate the wavefield for a few frequencies i then perform a parametric fit to the wavefield to estimate a traveltime amplitude and phase the images created by using these parameters in a kirchhoff imaging algorithm are comparable in quality to those created using full wavefield finite difference shot profile migration

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

improved tests reveal that the accelerating moment release hypothesis is statistically insignificant 1 we test the hypothesis that accelerating moment release amr is a precursor to large earthquakes using data from california nevada and sumatra spurious cases of amr can arise from data fitting because the time period area and sometimes magnitude range analyzed before each main shock are often optimized to produce the strongest amr signal optimizing the search criteria can identify apparent amr even if no robust signal exists for both 1950 2006 california nevada m 6 5 earthquakes and the 2004 m9 3 sumatra earthquake we can find two contradictory patterns in the pre main shock earthquakes by data fitting amr and decelerating moment release we compare the apparent amr found in the real data to the apparent amr found in four types of synthetic catalogs with no inherent amr when spatiotemporal clustering is included in the simulations similar amr signals are found by data fitting in both the real and synthetic data sets even though the synthetic data sets contain no real amr these tests demonstrate that apparent amr may arise from a combination of data fitting and normal foreshock and aftershock activity in principle data fitting artifacts could be avoided if the free parameters were determined from scaling relationships between the duration and spatial extent of the amr pattern and the magnitude of the earthquake that follows it however we demonstrate that previously proposed scaling relationships are unstable statistical artifacts caused by the use of a minimum magnitude for the earthquake catalog that scales with the main shock magnitude some recent amr studies have used spatial regions based on hypothetical stress loading patterns rather than circles to select the data we show that previous tests were biased and that unbiased tests do not find this change to the method to be an improvement the use of declustered catalogs has also been proposed to eliminate the effect of clustering but we demonstrate that this does not increase the statistical significance of amr given the ease with which data fitting can find desired patterns in seismicity future studies of amr like observations must include complete tests against synthetic catalogs that include spatiotemporal clustering

a robust approach to time to depth conversion and interval velocity estimation from time migration in the presence of lateral velocity variations the problem of conversion from time migration velocity to an interval velocity in depth in the presence of lateral velocity variations can be reduced to solving a system of partial differential equations in this paper we formulate the problem as a non linear least squares optimization for seismic interval velocity and seek its solution iteratively the input for the inversion is the dix velocity which also serves as an initial guess the inversion gradually updates the interval velocity in order to account for lateral velocity variations that are neglected in the dix inversion the algorithm has a moderate cost thanks to regularization that speeds up convergence while ensuring a smooth output the proposed method should be numerically robust compared to the previous approaches which amount to extrapolation in depth monotonically for a successful time to depth conversion image ray caustics should be either nonexistent or excluded from the computational domain the resulting velocity can be used in subsequent depth imaging model building both synthetic and field data examples demonstrate the applicability of the proposed approach

gas geochemistry applied to earthquake prediction an overview terrestrial gases in groundwater and soil air have been extensively studied in recent years in seismically active areas especially in ussr china japan and united states in search of premonitory changes that might be useful for earthquake prediction concentrations of radon helium hydrogen mercury carbon dioxide and several other volatiles have been found generally to be anomalously high along active faults suggesting that the faults may be paths of least resistance for the terrestrial gases generated or stored in the earth to escape to the atmosphere temporal gas concentration changes with durations of a few hours to many months have been observed before many large earthquakes at a relatively small number of favorably situated stations at epicentral distances of up to several hundreds of kilometers these sensitive stations are generally located along active faults especially at intersections or bends of faults or some other structurally weak zones possibly because of tectonic strain concentration at such places however gas concentrations especially those measured at or near the ground surface may also be significantly affected by meteorological hydrological and other nontectonic changes in the environment the nontectonically induced variations must be carefully eliminated or recognized in the search of true earthquake related anomalies

submarine volcanism eruption styles products and relevance to understanding the host rock successions to volcanic hosted massive sulfide deposits the increasing hydrostatic pressure of the water column with increasing water depth in subaqueous environments limits the ability of superheated volatiles to expand instantaneously against the ambient pressure explosive submarine eruptions are only likely in water depths less than 1 km and generally less than 500 m thus refuting models of highly explosive very deep water calderas popularly proposed as the host volcanic centers for the kuroko volcanic hosted massive sulfide vhms deposits of japan large volumes of blocky breccias can be produced by quench fragmentation during submarine lava eruptions and such deposits may have been mistaken for pyroclastic deposits in many cases in the past pyroclastic debris may occur in the host rock successions of some vhms deposits and may even be very voluminous however facies characteristics and theoretical constraints indicate that these are usually the products of mass flow deposition from shallow water or basin margin volcanic centers the maximum water depths for submarine explosive eruptions coincide approximately with the minimum pressures and water depths required to prevent boiling of mineralizing hydrothermal fluids in the stockwork before the fluids reach the sea floor the key elements in evaluating the prospectivity of ancient volcanic successions for vhms deposits appear to be deep water sediments and lavas or shallow intrusions in an extensional basin setting pyroclastic debris in many cases at least appears to be an accidental externally introduced component there is little evidence that explosive submarine calderas are essential as host volcanic centers to vhms deposits

emplacement of the final lava dome of the 2009 eruption of redoubt volcano alaska abstract after more than 8 months of precursory activity and over 20 explosions in 12 days redoubt volcano alaska began to extrude the fourth and final lava dome of the 2009 eruption on april 4 by july 1 the dome had filled the pre 2009 summit crater and ceased to grow by means of analysis and annotations of time lapse webcam imagery oblique image photogrammetry techniques and capture and analysis of forward looking infrared flir images we tracked the volume textural effusive style and temperature changes in near real time over the entire growth period of the dome the first month of growth april 4 may 4 produced blocky intermediate to high silica andesite lava 59 62 3 wt sio 2 that initially formed a round dome expanding by endogenous growth breaking the surface crust in radial fractures and annealing them with warmer fresh lava on or around may 1 more finely fragmented and scoriaceous andesite lava 59 8 62 2 wt sio 2 began to appear at the top of the dome coincident with increased seismicity and gas emissions the more scoriaceous lava spread radially over the dome surface while the dome continued to expand from endogenous growth and blocky lava was exposed on the margins and south side of the dome by mid june the upper scoriaceous lava had covered 36 of the dome surface area vesicularity of the upper scoriaceous lava range from 55 to 66 some of the highest vesicularity measurements recorded from a lava dome we suggest that the stability of the final lava dome primarily resulted from sufficient fracturing and clearing of the conduit by preceding explosions that allowed efficient degassing of the magma during effusion the dome was thus able to grow until it was large enough to exceed the magmastatic pressure in the chamber effectively shutting off the eruption

quenching and hydroclastic disruption of andesitic to rhyolitic intrusions in a submarine island arc sequence northern sierra nevada california devonian submarine island arc deposits of the sierra buttes formation in the northern sierra nevada are penetrated by numerous penecontemporaneous hypabyssal intrusions but related extrusive lavas are absent significant parts of the arc succession are occupied by andesitic to rhyolitic intrusive complexes that record the quenching and disruption of large volumes of magma injected into wet sediments at shallow levels beneath the sea floor extensive masses of intrusive hydroclastic breccia are characterized by in situ fragmentation textures and developed by progressive brittle disintegration of quenched magma pillows dispersed within the breccia formed as a series of interconnected tubes that supplied magma to the growing intrusive masses beds of redeposited hydroclastic breccia indicate that the intrusions locally reached the sea floor to undergo slumping and debris flow large scale steam explosions did not occur during magma wet sediment interaction probably due at least in part to the lithostatic and hydrostatic pressure acting on the intrusions low viscosity rhyolitic magma developed intrusive pillows and in situ hydroclastic breccia in the same fashion as quenched andesitic magma this fluid behavior was favored by retention of volatiles in the rhyolitic magma under relatively deep marine conditions quenched and disrupted hypabyssal complexes of the type described herein may be common elements of the submarine parts of island arcs and other volcanic environments where uprise of magma through thick sequences of wet sediment is impeded due to buoyancy constraints and rapid chilling

facies architecture emplacement mechanisms and eruption style of the submarine andesite el barronal complex cabo de gata se spain abstract el barronal complex consists of a succession of andesite lavas and andesite volcaniclastic facies interbedded with carbonate and siliciclastic sedimentary rocks carbonate and siliciclastic rocks were deposited in a shallow marine environment during periods of volcanic quiescence lavas consist of an inner coherent core grading outward into hyaloclastite breccia made of dense clasts that in turn grade into hyaloclastite breccia made of vesicular clasts in massive to layered zones volcaniclastic facies contain clasts produced during explosive eruptions and reworked clasts from sources above wave base volcaniclastic facies were deposited from cold granular flows with different grain size populations stratigraphy and facies architecture at el barronal suggest that a succession of several discrete eruptive events occurred with a similar cyclic pattern made of an initial explosive phase followed by effusive emplacement of lavas in turn followed by a period of quiescence of volcanic activity hyaloclastic fragmentation of magma took place in the final stages of lava emplacement allowing only for local disorganization of the jigsaw fit texture

chilling and brecciation of a devonian rhyolite sill intruded into wet sediments northern sierra nevada california localized pockets of peperite breccia occur near the base of a well exposed hypabyssal rhyolite sill within the upper devonian volcanic sequence of the northern sierra nevada the peperites resulted from chilling fragmentation and commingling of rhyolite with wet poorly consolidated sediment zones of hyaloclastite breccia extend into the sill above the peperites and represent areas where chilled rhyolite underwent fragmentation but was not intermixed with sediment a dense spherulitic intergrowth developed rapidly within the chilled rhyolite immediately prior to its fragmentation formation of this quench texture probably reflects a reduction in viscosity of the magma due to retention of volatiles in the melt under conditions of relatively high confining pressure

an upper pliocene coarse pumice breccia generated by a shallow submarine explosive eruption milos greece the filakopi pumice breccia fpb is a very well exposed pliocene volcaniclastic unit on milos greece and has a minimum bulk volume of 1 km3 it consists of three main units a basal lithic breccia 4 8 m mainly composed of angular to subangular andesitic and dacitic clasts up to 2 6 m in diameter b very thickly bedded poorly sorted pumice breccia 16 17 m and c very thick reversely graded grain supported coarse pumice breccia 6 5 20 m at the top the depositional setting is well constrained as shallow marine up to a few hundred metres by overlying fossiliferous and bioturbated mudstone this large volume of fine pumice clasts is interpreted to be the product of an explosive eruption from a submarine vent because 1 pumice clasts are the dominant component 2 the coarse pumice clasts 64 mm have complete quenched margins 3 very large 1 m pumice clasts are common 4 overall the formation shows good hydraulic sorting and 5 a significant volume of ash was deposited together with the coarsest pyroclasts

products of an effusive subglacial rhyolite eruption blahnukur torfajokull iceland we present field observations from blahnukur a small volume 400 m thick lithofacies associations attest to complex patterns of volcano ice interaction erosive channels at the base of the subglacial sequence are filled by both eruption derived material and subglacial till which show evidence for deposition by flowing meltwater this suggests that meltwater was able to drain away from the vent area during the eruption much of the subglacial volcanic deposits consist of conical to irregularly shaped lava lobes typically 5 10 m long set in poorly sorted breccias with an ash grade matrix a gradational lavabreccia contact at the base of lava lobes represents a fossilised fragmentation interface driven by magma water interaction as the lava flowed over poorly consolidated waterlogged debris sets of columnar joints on the upper surfaces of lobes are interpreted as ice contact features the morphology of the lobes suggests that they chilled within conically shaped subglacial cavities 2 5 m high avalanche deposits mantling the flanks of blahnukur appear to have been generated by the collapse of lava lobes and surrounding breccia a variety of deposit characteristics suggests that this occurred both prior to and after quenching of the lava lobes collapse events may have occurred when the supporting ice walls were melted back from around the cooling lava lobes and breccias much larger lava flows were emplaced in the latter stages of the eruption columnar joint patterns suggest that these flowed and chilled within subglacial cavities 20 m high and 100 200 m in length there is little evidence for magma water interaction at lava flow margins which suggests that these larger cavities were drained of meltwater as rhyolite magma rose to the base of the glacier the nature of the subglacial cavity system played an important role in governing the style of eruption and the volcanic facies generated we present evidence that the cavity system evolved during the eruption reflecting variations in both melting rate and edifice growth that are best explained by a fluctuating eruption rate