southern kermadec submarine caldera arc volcanoes sw pacific caldera formation by effusive and pyroclastic eruption abstract three newly identified submarine caldera volcanoes brothers healy and rumble ii west of the southern kermadec arc within water depths 80 and low density 0 3 0 5 g cm 3 erupted pumiceous clasts these observations are interpreted as evidence of subaqueously quenched pyroclastic eruption s sheathed by ambient water flashing to steam discharging some 5 km3 of pyroclastic material from healy volcano caldera collapse is inferred to be penecontemporeous with the pyroclastic eruption s although it is possible formation as an explosive crater occurred but we consider the latter unlikely given the volume of erupted material and water depth extrapolation of these observations to other submarine arcs indicates pyroclastic eruption of silicic magmas and syn eruptive caldera collapse is possible within water depths of 500 1000 m

submarine silicic caldera at the front of the izu bonin arc japan voluminous seafloor eruptions of rhyolite pumice myojin knoll caldera a submarine rhyolitic center 400 km south of tokyo is one of nine silicic calderas along the northern 600 km of the izu bonin ogasawara arc and the first anywhere to receive detailed submersible based study the caldera slightly smaller than the crater lake structure in oregon is 6 7 km in diameter its inner walls are 500 900 m high and it has a remarkably flat floor at 1400 m below sea level mbsl the caldera collapse volume is 18 km 3 suggesting that more than 40 km 3 of pumiceous tephra may have been erupted at the time the caldera formed precaldera seafloor eruptions built a broad volcanic edifice consisting of overlapping composite volcanoes made of rhyolitic lavas shallow intrusions and a variety of volcaniclastic deposits including thick accumulations of rhyolitic pumice erupted at 900 500 mbsl the caldera forming eruption produced a 150 200 m deposit of nonwelded fines depleted pumice that resembles a colossal layer of popcorn at the top of the caldera wall freshly erupted pumice behaved as sinkers or floaters depending on the environment in which it cooled the pumice clasts deposited proximally and exposed in the caldera wall were likely quenched in eruption columns that remained below sea level this pumice ingested seawater and sank as gases filling its vesicles cooled particularly as steam in its vesicles condensed to liquid water some eruption columns may have broken through the sea surface and entered the air especially during vigorous phases of the caldera forming eruption these pumices had the opportunity to ingest air as they cooled becoming floaters as they fell back to the sea these could have been carried distally on the sea surface by the combined effects of ocean currents and wind the age of the caldera is unknown but it may be as young as several thousand years its magmatic system at depth retains sufficient heat to sustain an actively growing intracaldera kuroko type polymetallic sulfide deposit rich in gold and silver and topped by chimneys emitting fluids as hot as 278 c sufficient time has elapsed however for a 250 m high postcaldera dome to grow on the caldera floor and for the caldera rim to be deeply scalloped by slumping

nature of and controls on volcanism in the ca 2 32 2 06ga pretoria group transvaal supergroup kaapvaal craton south africa abstract the time of deposition of the pretoria group between 2 32 and 2 06 ga on south africa s kaapvaal craton was characterized by the first major increase in atmospheric oxygen it was accompanied by the extrusion of significant thicknesses of volcanic deposits namely the bushy bend lavas of the timeball hill formation the hekpoort formation and the machadodorp volcanic member of the silverton formation marking the three major volcanic events within the transvaal supergroup which are thought to be precursors to the succeeding bushveld complex magmatism the bushy bend lava member of the timeball hill formation is characterized by subaqueous basaltic andesitic fissure eruptions with fumarolic activity and probably minor subaerial explosive eruptions in the hinterland of the transvaal supergroup basin the subaerial volcanism of the hekpoort formation appears to have been dominated by fissure eruptions with a preponderance of lava flows and locally important pyroclastic material intermittent hiatuses in volcanism were marked by local lacustrine shale deposition finally the machadodorp lava member can be interpreted as the deposits of several seamounts aligned along a fissure probably within an extensional environment the geodynamic control inferred for the evolution of the pretoria group basin encompasses two cycles of prerift uplift subsequent mechanical rifting and long lived thermal subsidence the limited extent of the bushy bend lavas in the south of the pretoria group depository attests to the likelihood that volcanism accompanying the first rifting event was short lived in contrast the second rifting cycle postulated for the pretoria group basin has a strong association with widespread and large scale volcanism of the hekpoort ongeluk flood basalts which may have been plume related the scale of the second cycle volcanism and its importance in influencing the upper part of the pretoria group basin fill is underlined by the machadodorp volcanism probably related to hot spot volcanism as the ongeluk hekpoort plume waned

a miocene coarse volcaniclastic mass flow deposit in the shimane peninsula sw japan product of a deep submarine eruption a subaqueous volcaniclastic mass flow deposit in the miocene josoji formation shimane peninsula is 15 16 m thick and comprises mainly blocks and lapilli of rhyolite and andesite pumices and non to poorly vesiculated rhyolite it can be divided into four layers in ascending order layer 1 is an inversely to normally graded and poorly sorted lithic breccia 0 3 6 m thick layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6 11 m thick this layer bifurcates laterally into minor depositional units individually composed of a massive lithic rich lower part and a diffusely stratified pumice rich upper part with inverse to normal grading of both lithic and pumice clasts layer 3 is 2 5 3 m thick and consists of interbedded fines depleted pumice rich and pumice poor layers a few centimeters thick layer 4 is a well stratified and well sorted coarse ash bed 1 5 2 m thick the volcaniclastic deposit shows internal features of high density turbidites and contains no evidence for emplacement at a high temperature the mass flow deposit is extremely coarse grained dominated by traction structures and is interpreted as the product of a deep submarine explosive eruption of vesicular magma or explosive collapse of lava

continental growth at convergent margins facing large ocean basins a case study from mesozoic convergent margin basins of baja california mexico abstract mesozoic rocks of the baja california peninsula form one of the most areally extensive best exposed longest lived 160 my least tectonized and least metamorphosed convergent margin basin complexes in the world this convergent margin shows an evolutionary trend that may be typical of arc systems facing large ocean basins a progression from highly extensional phase 1 through mildly extensional phase 2 to compressional phase 3 strain regimes this trend is largely due to the progressively decreasing age of lithosphere that is subducted which causes a gradual decrease in slab dip angle and concomitant increase in coupling between lower and upper plates as well as progressive inboard migration of the arc axis this paper emphasizes the usefulness of sedimentary and volcanic basin analysis for reconstructing the tectonic evolution of a convergent continental margin phase 1 consists of late triassic to late jurassic oceanic intra arc to backarc basins that were isolated from continental sediment sources new progressively widening basins were created by arc rifting and sea floor spreading and these were largely filled with progradational backarc arc apron deposits that record the growth of adjacent volcanoes up to and above sea level inboard migration of the backarc spreading center ultimately results in renewed arc rifting producing an influx of silicic pyroclastics to the backarc basin rifting succeeds in conversion of the active backarc basin into a remnant backarc basin which is blanketed by epiclastic sands phase 1 oceanic arc backarc terranes were amalgamated by late jurassic sinistral strike slip faults they form the forearc substrate for phase 2 indicating inboard migration of the arc axis due to decrease in slab dip phase 2 consists of early cretaceous extensional fringing arc basins adjacent to a continent phase 2 forearc basins consist of grabens that stepped downward toward the trench filled with coarse grained slope apron deposits phase 2 intra arc basins show a cycle of 1 arc extension characterized by intermediate to silicic explosive and effusive volcanism culminating in caldera forming silicic ignimbrite eruptions followed by 2 arc rifting characterized by widespread dike swarms and extensive mafic lavas and hyaloclastites this extensional rifting cycle was followed by mid cretaceous backarc basin closure and thrusting of the fringing arc beneath the edge of the continent caused by a decrease in slab dip as well as a possible increase in convergence rate phase 2 fringing arc terranes form the substrate for phase 3 which consists of a late cretaceous high standing compressional continental arc that migrated inboard with time strongly coupled subduction resulted in accretion of blueschist metamorphic rocks with development of a broad residual forearc basin behind the growing accretionary wedge and development of extensional forearc trench slope basins atop the gravitationally collapsing accretionary wedge inboard of this ongoing phase 3 strongly coupled subduction together with oblique convergence resulted in development of forearc strike slip basins upon arc basement the modern earth is strongly biased toward long lived arc trench systems which are compressional therefore evolutionary models for convergent margins must be constructed from well preserved ancient examples like baja california this convergent margin is typical of many others where the early to middle stages of convergence phases 1 and 2 create nonsubductable arc ophiolite terranes and their basin fills in the upper plate these become accreted to the continental margin in the late stage of convergence phase 3 resulting in significant continental growth

subaqueous explosive eruption and welding of pyroclastic deposits silicic tuffs infilling an ancient submarine caldera at mineral king in california show microscopic fabrics indicative of welding of glass shards and pumice at temperatures 500 degrees c the occurrence indicates that subaqueous explosive eruption and emplacement of pyroclastic materials can occur without substantial admixture of the ambient water which would cause chilling intracaldera progressive aggradation of pumice and ash from a thick fast moving pyroclastic flow occurred during a short lived explosive eruption of approximately 26 cubic kilometers of magma in water 150 meters deep the thickness high velocity and abundant fine material of the erupted gas solids mixture prevented substantial incorporation of ambient water into the flow stripping of pyroclasts from upper surfaces of subaqueous pyroclastic flows in general both above the vent and along any flow path may be the main process giving rise to buoyant convective subaqueous eruption columns and attendant fallout deposits

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

geochemistry of silicic magmas in the macolod corridor sw luzon philippines evidence of distinct mantle derived crustal sources for silicic magmas silicic volcanic deposits 65 wt sio2 which occur as domes lavas and pyroclastic deposits are relatively abundant in the macolod corridor sw luzon philippines at makiling stratovolcano silicic domes occur along the margins of the volcano and are chemically similar to the silicic lavas that comprise part of the volcano pyroclastic flows are associated with the laguna de bay caldera and these are chemically distinct from the domes and lavas at makiling stratovolcano as a whole samples from the laguna de bay caldera contain lower concentrations of mgo and higher concentrations of fe2o3 t than the samples from domes and lavas the laguna de bay samples are more enriched in incompatible trace elements the silicic rocks from the domes makiling volcano and laguna de bay caldera all contain high alkalis and high k2o na2o ratios melting experiments of primitive basalts and andesites demonstrate that it is difficult to produce high k2o na2o silicic magmas by fractional crystallization or partial melting of a low k2o na2o source however recent melting experiments sisson et al contrib mineral petrol 148 635 661 2005 demonstrate that extreme fractional crystallization or partial melting of k rich basalts can produce these silicic magmas our model for the generation of the silicic magmas in the macolod corridor requires partial melting of mantle derived evolved moderate to k rich crystallized calc alkaline magmas that ponded and crystallized in the mid crust major and trace element variations along with oxygen isotopes and ages of the deposits are consistent with this model

substorm wave base felsic hydroclastic deposits in the archean lac des vents volcanic complex abitibi belt canada abstract volcaniclastic deposits of the 2 3 km thick archean lac des vents volcanic complex are an integral part of major submarine volcanic construction the volcanic edifice which formed on a subaqueous basalt plain is comparable to modern seamounts resting on the ocean floor the initial 770 m of the mafic felsic edifice subject of this study is composed of massive brecciated and pillowed basalts massive to brecciated felsic lava flows and abundant felsic fragmental rocks of hydroclastic origin four distinct volcaniclastic lithofacies constitute the latter 1 the pumice lapilli tuff lithofacies 2 the lapilli tuff breccia lithofacies characterized by two sublithofacies 3 the turbidite tuff and tuff breccia lithofacies and 4 the volcanic sandstone and breccia lithofacies these four volcaniclastic lithofacies are considered to be the result of explosive and non explosive hydrovolcanic fragmentation processes operating at depths below storm wave base 200 m primary deposition or limited remobilization of unconsolidated hydroclastic debris is shown by the preservation of delicate clasts and volcanic textures and heat retention structures the principal transport agents are high concentration sediment gravity flows occurring under laminar and turbulent flow conditions high and low density turbiditic tuffs and fine grained tuff fallout deposits are related to either the dissipating stages of volcanic eruptions or slumping of syneruptive volcanic debris on the flanks of a subaqueous volcanic edifice ubiquitous interstratification of volcaniclastic turbidites shale and pillowed basalt flows with the felsic lava flows and fragmental debris favours subaqueous deposition these features combined with the absence of wave induced sedimentary structures imply deposition in water depths in excess of 200 m viscous feldspar phyric massive and brecciated felsic flows and associated volcaniclastics cross cut by felsic dykes suggest vent proximity the abundance of breccia size hydroclastic debris is consistent with this interpretation collectively these criteria argue for subaqueous fragmentation and deposition of volcaniclastics of inferred hydroclastic origin close to the central vent area at depths below storm wave base

thermohydraulic explosions in phreatomagmatic eruptions as evidenced by the comparison between pyroclasts and products from molten fuel coolant interaction experiments 1 thermohydraulic explosions were produced by molten fuel coolant interaction mfci experiments using remelted shoshonitic rocks from vulcano italy the fragmentation history and energy release were recorded the resulting products were recovered and analyzed with the scanning electron microscope fine particles from experiments show shape and surface features that result from melt fragmentation in brittle mode these clasts relate to the thermohydraulic phase of the mfci where most of the mechanical energy is released they are here called active particles the total surface area of such particles is proportional to the energy of the respective explosions other particles from experiments show shape and surface features that result from melt fragmentation in a ductile regime these fragments called passive particles form after the thermohydraulic phase during the expansion phase of the mfci in order to verify thermohydraulic explosions in volcanic eruptions we compared experimental products with samples from phreatomagmatic base surge deposits of vulcano ash particles from the experiments show features similar to those from the deposits suggesting that the experiments reproduced the same fragmentation dynamics to achieve discrimination between active and passive particles we calculated shape parameters from image analysis the mass of active particles in base surge deposits was calculated as the material properties for the natural samples are identical to the experimental ones the energy measurements and calculations of the experiments can be applied for a single phreatomagmatic eruption at vulcano a maximum mechanical energy release of 2 75 1013 j was calculated representing a tnt analogue of 6 5 kt

la fossa caldera breaching and submarine erosion vulcano island italy abstract la fossa caldera lfc on vulcano island in the aeolian archipelago italy is a multi collapse caldera whose north eastern part lies underwater through the integration of marine geophysical surveys and sampling carried out in the last decade its submarine extension and offshore areas have been reconstructed in unprecedented detail strong erosive processes breached the submarine portion of lfc and gradually dismantled the caldera rims and infill leading to the emplacement of a wide volcaniclastic fan from the base of lfc down to over 1000 m the bathy morphologic setting seismoacoustic facies and analyses of sediment samples from the fan together with volumetric considerations suggest that this feature is the pathway deposit of submarine unconfined gravity flows and turbidity currents deriving from the gradual dismantling of lfc submarine erosive processes are still active and the progressive erosion of the northern caldera sector threatens the stability of the active la fossa cone and the nearby coastal settlements

the 1891 submarine eruption offshore pantelleria island sicily channel italy identification of the vent and characterization of products and eruptive style high resolution bathymetry and seafloor sampling have been used to characterize the 1891 submarine eruption of the pantelleria volcanic complex this submarine eruption has been documented mainly by historical reports describing basaltic scoria bombs floating on the sea surface i e lava balloons in this study the 1891 eruptive vent has been identified as a small cone volume of 700 000 m3 rising 90 m from 350 m w d and located within a newly discovered submarine volcanic field covering a wide area offshore from the nw coast of pantelleria recently kelly et al 2012 confirmed this location by a multibeam and rov survey pyroclasts from the 1891 eruption crop out directly on the seafloor and are fresh scoria clasts i e small bombs bomb fragments and lapilli and glass ash sized grains both have been characterized in their morphology textures and geochemistry the distinctive vesicularity and crystallization characteristics displayed by the scoriaceous pyroclasts reflect modes of degassing in both syn and posteruptive regimes these characteristics along with the distribution of deposits suggest for the strongest eruptive phase of the 1891 eruption a style analogous to hawaiian fountaining glass grains from a buoyant plume were dispersed northward from the vent up to distances of 1 5 km redirected by the levantine intermediate water the identification of the 1891 submarine eruptive vent offshore pantelleria as well as the features of erupted pyroclasts improve our knowledge of submarine explosive eruptions that occur at shallow intermediate depths and among these of the rare eruptions producing lava balloons

on the first experimental phreatomagmatic explosion of a kimberlite melt abstract detailed field investigations of kimberlite pipes of the upper cretaceous gibeon kimberlite field in southern namibia revealed geologic features which do neither agree with extensive nor with explosive vesiculation in the pipe hanaus 2 vesicle free magmatic kimberlite intruded the diatreme as a plug here we report on the first experiments in which phreatomagmatic explosions were produced with remolten natural non fragmental magmatic kimberlite from the hanaus 2 central plug as a best fit model kimberlite magma system after water was injected into the melt under controlled conditions the resulting water melt mix was triggered by a low energy shock wave 400 m s

a kuroko type polymetallic sulfide deposit in a submarine silicic caldera manned submersible studies have delineated a large and actively growing kuroko type volcanogenic massive sulfide deposit 400 kilometers south of tokyo in myojin knoll submarine caldera the sulfide body is located on the caldera floor at a depth of 1210 to 1360 meters has an area of 400 by 400 by 30 meters and is notably rich in gold and silver the discovery of a large kuroko type polymetallic sulfide deposit in this arc front caldera raises the possibility that the numerous unexplored submarine silicic calderas elsewhere might have similar deposits

comparison of lake and land tephra records from the 2015 eruption of calbuco volcano chile tephra layers in lake sediment cores are regularly used for tephrostratigraphy as isochronous features for dating and recording eruption frequencies however their value for determining volcanic eruption size and style may be complicated by processes occurring in the lake that modify the thickness and grain size distributions of the deposit to assess the reliability of data from lake cores we compare tephra deposited on land during the 2015 eruption of calbuco volcano in chile to records in sediment cores from three lakes of different sizes that are known to have received primary fall deposits in general the thickness and granulometry of the deposit in lake cores and nearby terrestrial sections are very similar as anticipated however cores sampled close to here within 300 m of fluvial inflows were affected by sediment deposition from the lake s catchment they differed from primary deposits not only in their greater thickness and organic content but also in poor sorting and lack of grading cores 850 m away from the inlet were not affected we consider our results in the context of the particle settling regime as well as each lake s location bathymetry and catchment area we find that the particle settling regime is important in more distal settings where the ash particles are small and particle settling occurs in density plumes rather than as individual particles we conclude that lake cores can be useful for physical volcanology providing consideration is given to eruption parameters such as particle size and mass flux as well as lake features such as bathymetry and catchment area

syn eruptive soft sediment deformation structures in a deep submarine caldera havre 2012 the 2012 rhyolitic eruption of havre submarine caldera in the kermadec arc generated an assemblage of new soft sediment deformation structures on the caldera floor these structures are the first documented examples of syn eruptive soft sediment deformation structures within a submarine caldera they are exquisitely portrayed on a high resolution 1 m bathymetric map made in 2015 their timing has been resolved by means of stratigraphic relationships with an overlying giant pumice deposit that correlates with the pumice raft first observed on 18th july 2012 utc weeks to months earlier in 2012 the largest lava was erupted from a vent on the southwestern caldera wall flowed down to the caldera floor and advanced almost 1 km farther a surficial layer of the caldera floor sediment surrounding the leading edge of the lava failed most likely in response to load and heat transfer from the lava failure produced a slump sheet comprising folds with arcuate crests at the toe an undeformed domain in the middle and a basin and ridge domain at the head adjacent to the lava subsequently 1 the lava ruptured on two main faults that propagated across the caldera floor and 2 strike slip on the faults moved the lava that was in between forward simultaneously offsetting segments of the slump sheet these two events were probably triggered by the earthquake swarm recorded on the 17th july 2012 utc a day before the pumice raft appeared the leading edge of the lava then sank slowly into the underlying sediment sinking was accommodated by sideways expulsion of sediment from beneath the lava modifying the topography around the edge of the lava the caldera floor sediment was also disturbed when part of the southwestern caldera wall collapsed generating a small debris avalanche which is represented by a cluster of hummocks on the caldera floor collapse was a response to shallow intrusion of a rhyolite dyke into the southwestern caldera wall

recent slowing of atlantic overturning circulation as a recovery from earlier strengthening the atlantic meridional overturning circulation has weakened over the past decade examination of a global reanalysis that matches independent observations shows that the decline is consistent with recovery from an earlier invigoration

volcanic and hydrothermal processes in submarine calderas the kulo lasi example sw pacific abstract the study area is located at the transition between the northern end of the tonga trench and the north fiji fracture zone where tectonic movements are reputed to be the fastest in the world to the southeast of futuna island a broad area of volcanism occurs within a region characterized by a change in the tectonic fabric between a ne sw oriented volcanic graben and the n s oriented alofi ridge in 2010 the active volcano kulo lasi which represents the most recent volcanic episode in the futuna area was discovered in the center of this extensive volcanic zone kulo lasi is a 20 km diameter shield volcano that rises 400 m above the seafloor it is composed of basaltic to trachy andesitic lava with no obvious geochemical affinity with the tonga subduction that occurs 500 km to the east the central caldera is 5 km in diameter and 300 m deep and is located at a water depth of 1500 m diving operations with the submersible nautile and high resolution auv mapping have revealed the presence of numerous active and inactive hydrothermal fields on the floor and the walls of the caldera four tectono volcanic stages can be distinguished at kulo lasi caldera in stage 1 the shield volcano is built annular reverse faults develop at the summit and control circulation of water rock dominated hydrothermal fluids and high temperature alteration of rocks along the nascent normal faults mixing of hydrothermal fluids with seawater is favored along normal superficial faults leading to the formation of low temperature fe mn mineralization at the summit of the volcano during stage 2 the caldera collapse gradually revealing outcrops of the altered and mineralized zones formed during stage 1 as the magma chamber cools and collapses less heat is available as a result medium to low temperature

large volume rhyolite ash flow eruptions and submarine caldera collapse in the lower mesozoic sierra nevada california the mineral king roof pendant in the southern sierra nevada california provides a view of a triassic and early jurassic submarine caldera complex which was tilted to vertical and intruded by middle cretaceous granitoids an estimated 90 km3 of rhyolitic and 10 km3 of andesitic volcanic rocks are interstratified with marine sandstones siltstones limestones and conglomerates periods of volcanotectonic activity alternated with periods of marine sedimentation volcanic eruptions at mineral king resulted in 1 deposition of widespread very thick sheets of rhyolite ash flow tuff and 2 construction of small 1 8 km3 andesitic stratocones growth faults fault talus aprons slide blocks slumps and debris flows found in association with these volcanic deposits indicate that the eruptive phases represent periods of great seismic activity interuptive phases were periods of volcanic and tectonic quiescence during which sequences of shallow to deep marine sediments were deposited four different times at mineral king eruption of large volumes of rhyolite ash flow tuff resulted in catastrophic caldera collapse and ponding of single ash flow tuffs to thicknesses greater than 0 5 km only minimum volumes can be estimated for three of these intracaldera tuffs because they are truncated by younger granites volumes are estimated to be at least 25 km3 each placing these tuffs in the lower size range of epicontinental ring structures the fourth caldera the vandever mountain caldera is entirely preserved in the roof pendant the vertically dipping strata of the vandever mountain caldera provide an unusual opportunity to examine a submarine caldera in cross section evidence for caldera subsidence during the eruption of the vandever mountain rhyolite ash flow tuff includes the following 1 the great thickness of the ash flow tuff and the rapid accumulation of the tuff during progressive draw down of a compositionally zoned magma chamber 2 high angle faults which pond the ash flow tuff and offset underlying but not overlying stratigraphy and 3 megabreccia and slide blocks up to 0 5 km long that were derived from the caldera wall and floor and incorporated into the ash flow tuff as it filled the caldera disruption of the caldera floor was intense near the caldera margin but negligible along most of its length owing to pistonlike subsidence of an intact cylinder of crust small volume ash flow eruptions occurred along an incipient leaky ring fracture zone prior to the caldera forming eruption of the vandever mountain ash flow tuff the vandever mountain caldera was not affected by resurgent doming or late stage rhyolite magmatism in the cross sectional view provided by the mineral king roof pendant

the saturation behaviour of pumice and some sedimentological implications abstract caldera forming explosive eruptions from rhyolitic volcanoes typically produce huge quantities of pyroclastic material blanketing the landscape and triggering long term voluminous and widespread sedimentary responses pumice is a major component of such eruptions but its behaviour in the sedimentary environment is little understood its most notable feature is its very low density which frequently enables pumice to float on water for long periods of time before eventually waterlogging enough to sink once fully saturated pumice behaves in a broadly similar way to normal clastic material in terms of its hydrodynamic behaviour modified by its lower density however relatively little is known about its earlier behaviour in the sedimentary environment and in particular the manner and rate by which initially dry pumice absorbs water in this paper we develop a mathematical model which predicts that the time required for a pumice clast to saturate is proportional to the square of its radius tests on samples of real pumice accord with predicted trends and match lacustrine pumiceous facies observed in the field the ability to predict the time required for large pumice clasts to sink may provide a tool for estimating the timescales over which certain rapid post eruptive sedimentary processes operate

quenching of steam charged pumice implications for submarine pyroclastic volcanism abstract huge quantities of silicic pumice have been deposited in intra oceanic convergent margin settings throughout earth s history the association of submarine silicic calderas with thick proximal accumulations of pumice lapilli suggests that these pyroclasts were deposited as a direct result of submarine eruptions yet when first erupted these highly vesicular gas filled clasts had densities significantly less than seawater experiments carried out 1 atm on heated pumice samples whose vesicles were charged with steam the dominant component of magmatic volatiles show that buoyancy of freshly erupted submarine pumice is transient upon quenching the phase change of steam to liquid water creates strong negative pore pressures within the pumice vesicles that accelerate the absorption of surrounding water generating high density pumice and promoting rapid clast sinking variations in the physical properties of steam with temperature and pressure have important implications for submarine pyroclastic eruptions firstly highly vesicular pumice can be deposited on the seafloor at temperatures elevated significantly above ambient if they are erupted at sufficient depths to remain wholly submarine 200 m and either the fluid in which they cool contains heated water and or they only absorb sufficient water to sink secondly the rapid increase in density of the eruption column caused by condensation and the transition from buoyant gas filled to denser water saturated pumice lapilli together with turbulent mixing with the surrounding seawater favour collapse and transport of pyroclasts in water supported gravity currents finally this mixing of the ejecta with seawater and the ease of water ingestion into permeable pumice clasts suggest that water supported transport mechanisms can operate as primary dispersal processes in explosive submarine eruptions

subaqueous eruption and emplacement of ot2 in the middle miocene iizuka formation noto peninsula japan abstract ot2 okada tuff 2 is one of six rhyolitic tuff beds that occur in a middle miocene marine diatomite lower iizuka formation and are known as the okada tuff ot2 is well sorted and thins and thickens infilling channels 1 3 m across and 0 30 cm deep with a thickness up to 30 cm small channels 10 30 cm across and 1 10 cm deep are commonly superimposed within wider channels a thin diatomite layer is ripped from the substrate up to a level immediately above the small channels the basal part beneath the ripped up diatomite layer is dominated by ash components with vague wavy stratifications above the ripped up diatomite layer diatomite clasts are dispersed and pumice lapilli increase upward in amount and size with local interruption at a middle lower level by a poorly defined pumice lapilli poor thin layer many glass and pumice shards are blocky with curviplanar surfaces and jigsaw cracks presumably produced by phreatomagmatic explosions the poorly defined two layers of ot2 are interpreted to be ash turbidites individually produced by discrete phreatomagmatic explosions lenticular bedforms and channels indicate that the currents were fully turbulent and unsteady fed by the submarine eruptions upward coarsening of pumice probably represents settling lag of hot pumice ot2 perhaps provides potential diagnostic features to identify eruption fed turbidites

effects of water depth on pumice formation in submarine domes at sumisu izu bonin arc western pacific domes of the sumisu volcanic complex western pacific having summits at ocean depths of 1100 600 245 and 95 m are mantled with compositionally identical rhyolitic pumice that has similar vesicularity but that varies systematically in distribution size and surface texture suggesting that facies and morphology can serve as useful indicators of eruption depth at depths 500 m the pumice formed a thick carapace on dense rhyolite and disintegrated by quench fracture and mechanical failure into a jumble of giant meters to tens of meters polyhedral blocks with smooth curviplanar surfaces that display a single quenched margin vesiculation was arrested on eruption in seawater in all but the interior of the thickest carapace at 500 m depths the pumice occurs as an apron of blocky giant and smaller rough textured clasts enclosed by quenched margins and pockmarked by coarse cm vesicles no carapace pumice occurs and the summit is composed of craggy dense dome rock these shallower water pumice clasts resemble those spalled from historic submarine dome forming eruptions that buoyed to the sea surface we interpret spalling to result from vent derived weak volatile driven explosions that take place at water depths 500 m our study shows that an increase in hydrostatic pressures over a range of 12 mpa reduces volatile driven explosivity for subaqueous rhyolitic dome forming eruptions but does not affect vesicularity we conclude that meter size highly vesicular pumice is diagnostic of subaqueous dome eruptions in water depths of at least 1300 m and its morphology can be used to distinguish between explosive and effusive origins

formation of volcanogenic massive sulfide deposits the kuroko perspective abstract the main objective of this paper is to identify the geochemical hydrological igneous and tectonic processes that led to the variations in the physical size geometry and chemical mineralogy metal ratios and zoning characteristics of volcanogenic massive sulfide deposits with respect to space from a scale of mining district size area to a global scale and time from a all volcanogenic massive sulfide deposits vmsds appear to have formed in extensional tectonic settings such as at mid ocean spreading centers backarc spreading centers and intracontinental rifts and failed rifts all vmsds appear to have formed in submarine depressions by seawater that became ore forming fluids through interactions with the heated upper crustal rocks submarine depressions especially those created by submarine caldera formation and or by large scale tectonic activities e g rifting become most favorable sites for the formation of large vmsds because of hydrological physical and chemical reasons the fundamental processes leading to the formation of vmsds include the following six processes 1 1 intrusion of a heat source typically a 10 3 km size pluton into an oceanic crust or a submarine continental crust causes deep convective circulation of seawater around the pluton the radius of a circulation cell is typically 5 km the temperature of fluids that discharge on the seafloor increases with time from the ambient temperature to a typical maximum of 350 c and then decreases gradually to the ambient temperatures in a time scale of 100 to 10 000 years the majority of sulfide and sulfate mineralization occurs during the waxing stage of hydrothermal activity 2 2 reactions between low temperature t 2 4 as disseminated gypsum and anhydrite in the country rocks 3 3 reactions of the modified seawater with higher temperature rocks at depths during the waxing stage cause the transformation of the seawater to metal and h 2 s rich ore forming fluids the metals and sulfide sulfur are leached from the county rocks the previously formed gypsum and anhydrite are reduced by fe 2 bearing minerals and organic matter providing additional h 2 s the mass of high temperature rocks that provide the metals and reduced sulfur is typically 10 11 tons 40 km 3 in volume the roles of magmatic fluids or gases are minor in most massive sulfide systems except for so 2 to produce acid type alteration in some systems 4 4 reactions between the ore forming fluids and cooler rocks in the discharge zone cause alteration of rocks and precipitation of some ore minerals in the stockwork ores 5 5 mixing of the ore forming fluids with local seawater within unconsolidated sediments and or on the seafloor causes precipitation of primitive ores with the black ore mineralogy sphalerite galena pyrite barite anhydrite 6 6 reactions between the primitive ores with later and hotter hydrothermal fluids cause transformation of primitive ores to matured ores that are enriched in chalcopyrite and pyrite variations in the mineralogical and elemental characteristics the geometry and the size of submarine hydrothermal deposits are controlled by the following four parameters 1 a the chemical and physical characteristics of seawater composition temperature density which depend largely on the geographical settings e g equatorial evaporating basins 2 b the chemical and physical characteristics of the plumbing system lithology fractures 3 c the thermal structure of the plumbing system which is determined largely by the ambient geothermal gradient and the size and temperature of the intrusive and 4 d the physical characteristics of the seafloor depth basin topography for example the submarine hydrothermal deposits developed in basaltic plumbing systems are generally poor in pb and ba compared to those developed in felsic plumbing systems the lower temperature systems are generally poorer in sulfides but richer in iron oxides and sulfates the higher temperature and larger hydrothermal systems tend to produce chalcopyrite and pyrite rich ores contrasts in the metal ratios between the noranda type archean vmsds and the younger vmsds reflect the differences in the geothermal gradient of the plumbing systems the submarine hydrothermal deposits developed in the near equatorial regions tend to form large continuous bedded type ores because of the likeliness of creating large stratified basins the basic processes of submarine hydrothermal mineralization have remained essentially the same throughout the geologic history from at least 3 5 billion year ago to the present

nature and tectonic setting of the southern kermadec submarine arc volcanoes an overview abstract the 1000 km long kermadec subduction system northern new zealand and adjacent havre trough back arc basin associated with the pacific australian plate boundary is an archetypal example of an active island arc convergent margin although the position and tectonic setting of the frontal arc is critical to understanding tectonism and magmatism within an arc back arc system this has not been unequivocally established for the kermadec subduction system swath surveys from the 35 37 s segment of the kermadec system reveal that the frontal arc comprises seven large arc stratovolcanoes rumble ii iii iv silent ii rumble v tangaroa and clark the frontal arc continues south with the whakatane volcano sited at the northern extremity of the taupo volcanic zone of these eight volcanoes the latter four are newly identified and all have constructional volumes varying between 50 and 260 km 3 all of these arc edifices are sited 15 25 km west of the kermadec frontal ridge bathymetric data similarly indicate that other kermadec arc volcanoes between 32 and 35 s also lie west of the ridge axis within the back arc region while north of 32 s the arc volcanoes lie along the frontal ridge this apparent variation in the tectonic position of the frontal arc coincides with other changes in the morphology of the subduction system north and south of 32 s and as suggested by pelletier and dupont 1990 may reflect differences in the length of the underlying subducted slab the 35 37 s frontal arc segment is sited within 35 45 km of the present day back arc rift axis and may control the initial loci of rift graben evolution sidescan imagery and seafloor photography of the southern kermadec arc volcanoes reveal a complex volcanic stratigraphy with intertwined pillow lavas proximal pillow breccias highly vesicular scoriaceous lapilli and pervasively mass wasted volcaniclastic and epiclastic detritus all volcanic deposits interdigitate with pelagic sediments a single k ar dated andesite from whakatane volcano suggests that the history of the present frontal arc may extend back to at least 0 77 ma while rumble iii and v volcanoes are the major although not exclusive sites of geologically recent edifice construction the historical acoustic record of southern kermadec submarine eruptions since 1958 is now regarded as equivocal with the july 1986 eruption of rumble iii the only unambiguous volcanic activity recorded in modern times

subaqueous calderas in the archean abitibi greenstone belt an overview and new ideas abstract the 300 700 km abitibi greenstone belt in canada contains numerous world class archean volcanogenic massive sulfide vms deposits yet documentation of archean subaqueous calderas hosting such deposits is lacking the modern sunrise deposit in the myojin knoll caldera of the izu bonin arc shows that submarine calderas are first order sites for vms the hunter mine and normetal calderas as well as the blake river megacaldera complex are reviewed with respect to geometry regional geology physical volcanology dyke emplacement and hydrothermal carbonate alteration these subaqueous calderas are placed into the geodynamic context of the abitibi greenstone belt the abitibi belt displays a complex history of oceanic arc formation evolution collision and fragmentation with numerous types of volcanic edifices and sedimentary basins controlled by oblique subduction and mantle plumes analogue models yielding new insights into the sequence of caldera forming events and their synvolcanic fault patterns are applied to explain archean examples underpressure experiments develop a consistent sequence of fault patterns that compare favourably with natural caldera events during subsidence two sets of faults propagate from the magma chamber margin 1 early reverse outward dipping faults and 2 normal inward dipping faults the largest massive sulfide deposits form preferentially at the caldera margin based on experimental studies and detailed facies mapping the hunter mine caldera is of the segmented piecemeal variety and the normetal caldera is akin to a piston structure the blake river megacaldera contains 1 the e w trending misema 2 the nw trending new senator and 3 the ne trending noranda calderas with the latter two being nested graben type calderas consistent with oblique archean subduction the poorly documented hydrothermal carbonate alteration is discussed and a new exploration model for calderas is presented the alteration study was evaluated at three levels microprobe analysis geochemical staining of drill core and whole rock analyses there were two principal stages of alteration related to vms formation an early silicification phase developed either at depth and or at the seawater interface whereby silicification was bedding parallel in fine grained volcaniclastic deposits near the water surface a pervasive and extensive semi conformable carbonate alteration zone with a discordant focused root zone along faults evolved subsequently three distinct carbonate pairings were observed 1 proximal siderite sideroplesite fe ankerite next to the vms deposit 2 an intermediate ankerite fe dolomite zone and 3 a distal calcite dolomite zone the carbonate phase is associated with the mineralizing event transitions between carbonate species are subtle and changes are indicated by mineral assemblage overlap the silica carbonate hydrothermal alteration pattern is a replacement product and is far more widespread than has been documented this alteration can be easily confused with primary carbonates and chemical precipitation deposits e g banded iron formation if detailed volcano sedimentary facies mapping with alteration has not been conducted and if outcrop is insufficient

subaqueous eruption and emplacement of the shinjima pumice shinjima moeshima island kagoshima bay sw japan abstract the shinjima pumice is a fines depleted pumice lapilli tuff emplaced several thousands years ago at about 100 140 m below sea level this 40 m thick deposit comprises many poorly defined flow units which are 1 10 m thick diffusely stratified and showing upward coarsening of pumice clasts with a sharp to transitional base parallel to wavy diffuse stratifications are commonly represented by alignment of pumice clasts especially in the lower half of the flow units pumice clasts of block to coarse lapilli size commonly have thermal contraction cracks best developed on the surfaces demonstrating that they were hot but cooled down to the ambient temperatures prior to their emplacement these features are suggestive of the direct origin of the shinjima pumice from subaqueous eruptions a theoretical consideration on the behavior of subaqueous eruption plumes and hot and cold pumice clasts suggests that subaqueous eruption plumes commonly collapse by turbulent mixing with the ambient water and are transformed into water logged mass flows

influence of shock propagation on lightning evidence in volcanic ashfall deposits abstract lightning induced textures in volcanic ashfall deposits can take several forms including lightning induced volcanic spherules livs spherule aggregates and pumiceous particles here two newly identified textures are described in the products of current impulse experiments conducted on ash samples spanning a range of bulk compositions from basic 49 wt sio2 to intermediate 53 58 wt sio2 to acidic 72 76 wt sio2 at peak currents of 7 ka 25 ka and 100 ka due to the location of subjected samples on a solid flat surface in the experimental apparatus both hair like particles 1 10 m in diameter and plate like particles 100 m in diameter are produced these textures result from not only the high temperatures generated by the discharge 1500 c but also by the expansion of a shock front with a radius twice the size of the discharge channel the pressure behind this shock front ranges from 0 41 mpa for the 7 ka impulse currents to 0 77 mpa for the 100 ka impulse currents and the velocity of the shock ranges from 617 m s to 846 m s the combined high temperatures and shock wave expansion simultaneously melts the volcanic ash particles fuses them together and stretches them out which alters the very small

submarine deposits from pumiceous pyroclastic density currents traveling over water an outstanding example from offshore montserrat iodp 340 pyroclastic density currents have been observed to both enter the sea and to travel over water for tens of kilometers here we identified a 1 2 m thick stratified pumice lapilli ash cored at site u1396 offshore montserrat integrated ocean drilling program iodp expedition 340 as being the first deposit to provide evidence that it was formed by submarine deposition from pumice rich pyroclastic density currents that traveled above the water surface the age of the submarine deposit is ca 4 ma and its magma source is similar to those for much younger soufriere hills deposits indicating that the island experienced large magnitude subaerial caldera forming explosive eruptions much earlier than recorded in land deposits the deposit s combined sedimentological characteristics are incompatible with deposition from a submarine eruption pyroclastic fall over water or a submarine seafloor hugging turbidity current derived from a subaerial pyroclastic density current that entered water at the shoreline the stratified pumice lapilli ash unit can be subdivided into at least three depositional units with the lowermost one being clast supported the unit contains grains in five separate size modes and has a 12 phi range particles are chiefly subrounded pumice clasts lithic clasts crystal fragments and glass shards pumice clasts are very poorly segregated from other particle types and lithic clasts occur throughout the deposit fine particles are weakly density graded we interpret the unit to record multiple closely spaced 2 d hot pyroclastic density currents that flowed over the ocean releasing pyroclasts onto the water surface and settling of the various pyroclasts into the water column our settling and hot and cold flotation experiments show that waterlogging of pumice clasts at the water surface would have been immediate the overall poor hydraulic sorting of the deposit resulted from mixing of particles from multiple pulses of vertical settling in the water column attesting to complex sedimentation slow settling particles were deposited on the seafloor together with faster descending particles that were delivered at the water surface by subsequent pyroclastic flows the final sediment pulses were eventually deflected upon their arrival on the seafloor and were deposited in laterally continuous facies this study emphasizes the interaction between products of explosive volcanism and the ocean and discusses sedimentological complexities and hydrodynamics associated with particle delivery to water

tephra dispersal from myojinsho japan during its shallow submarine eruption of 1952 1953 a new and detailed bathymetric map of the myojinsho shallow submarine volcano provides a framework to interpret the physical volcanology of its 1952 1953 eruption especially how the silicic pyroclasts both primary and reworked enlarged the volcano and were dispersed into the surrounding marine environment myojinsho 420 km south of tokyo along the izu ogasawara arc was the site of approximately 1000 phreatomagmatic explosions during the 12 5 month eruption these explosions shattered growing dacite domes producing dense clasts that immediately sank into the sea minor amounts of pumice floated on the sea surface after some of these events the myojinsho cone has slopes of almost precisely 21 in the depth range 300 700 m we interpret this to be the result of angle of repose deposition of submarine pyroclastic gravity flows that traveled downslope in all directions many of these gravity flows resulted from explosions and associated dome collapse but others were likely triggered by the remobilization of debris temporarily deposited on the summit and steep upper slopes of the cone tephra was repeatedly carried into air in subaerial eruption columns and fell into the sea within 1 2 km of the volcano s summit entering water as deep as 400 m because the fall velocity of single particles decreased by a factor of 30 in passing from air into the sea we expect that the upper part of the water column was repeatedly choked with hyperconcentrations of fallout tephra gravitational instabilities within these tephra choked regions could have formed vertical density currents that descended at velocities greater than those of the individual particles they contained upon reaching the sea floor many of these currents probably continued to move downslope along myojinsho s submarine slopes fine tephra was elutriated from the rubbly summit of the volcano by upwelling plumes of heated seawater that persisted for the entire duration of the eruption ocean currents carried this tephra to distal areas where it presumably forms a pyroclastic component of deep sea sediment

extensive young silicic volcanism produces large deep submarine lava flows in the ne lau basin new field observations reveal that extensive up to 402 km2 aphyric glassy dacite lavas were erupted at multiple sites in the recent past in the ne lau basin located about 200 km southwest of samoa this discovery of volumetrically significant and widespread submarine dacite lava flows extends the domain for siliceous effusive volcanism into the deep seafloor although several lava flow fields were discovered on the flank of a large silicic seamount niuatahi two of the largest lava fields and several smaller ones northern lava flow fields were found well north of the seamount the most distal portion of the northernmost of these fields is 60 km north of the center of niuatahi caldera we estimate that lava flow lengths from probable eruptive vents to the distal ends of flows range from a few km to more than 10 km camera tows on the shallower near vent areas show complex lava morphology that includes anastomosing tube like pillow flows and ropey surfaces endogenous domes and or ridges some with crease like extrusion ridges and inflated lobes with extrusion structures a 2 1 5 km 30 m deep depression could be an eruption center for one of the lava flow fields the lau lava flow fields appear to have erupted at presumptive high effusion rates and possibly reduced viscosity induced by presumptive high magmatic water content and or a high eruption temperature consistent with both erupted composition 66 sio2 and glassy low crystallinity groundmass textures the large areal extent 236 km2 and relatively small range of compositional variation 0 60 for wt si02 within the northern lava flow fields imply the existence of large eruptible batches of differentiated melt in the upper mantle or lower crust of the ne lau basin at this site the volcanism could be controlled by deep crustal fractures caused by the long term extension in this rear arc region submarine dacite flows exhibiting similar morphology have been described in ancient sequences from the archaean through the miocene and in small batches on present day seafloor spreading centers this study shows that extensive siliceous lavas can erupt on the modern seafloor under the right conditions

caldera forming processes and the origin of submarine volcanogenic massive sulfide deposits certain volcanogenic massive sulfide vms ore deposits form in submarine calderas this association is well known but the link between caldera formation and the origins of the deposits remains poorly understood here we show that the size and location of a vms deposit within a submarine caldera may be determined by how and when the caldera formed these spatial temporal conditions control development of the hydrothermal system associated with the vms deposit we propose that caldera opening along outward dipping faults promotes magma degassing seawater influx and high temperature leaching resulting in a metal rich hydrothermal fluid these outward dipping faults are considered to provide critical pathways for ore forming fluids responsible for some caldera hosted vms deposits and may also be fundamentally important for the formation of many other caldera hosted ore deposit types

emplacement processes of submarine volcaniclastic deposits iodp site c0011 nankai trough during iodp nantroseize expedition 322 four packages of tuffaceous sandstones tst 1 2 3a 3b were recovered within a moderately lithified and bioturbated silty claystone succession in the late miocene 7 6 to 9 1 ma upper part of the middle shikoku basin deposits to assess the emplacement processes of the tuffaceous sandstones we investigate modal and geochemical compositions of 24 thin sections that reveal systematic vertical changes within each bed tst 1 2 and 3b are single beds whereas tst 3a is composed of at least two beds suggesting several rapidly succeeding sedimentation events the beds are density graded such that low density pyroclasts including pumice lapilli are enriched at the top whereas dense lithic components and minerals are enriched at the bottom the volcanic glass particles pumice and shards that are the dominant modal constituents of each sandstone bed have homogeneous geochemical compositions in each bed moreover tst 1 2 and 3a glass compositions overlap completely but tst 3b glass has a different composition as is analogously observed for the mineral compositions this unique multistage approach of sedimentological and geochemical methods facilitated the detailed investigation of distal volcano derived probably tsunamogenic turbidites in order to contribute to the distinction between primary and secondary induced mass flows we interpret that all tuffaceous sandstones were emplaced by turbidity currents that were formed during major explosive volcanic eruptions however while tst 1 2 and 3a turbidity currents formed by the entry into the ocean of voluminous pyroclastic flows erupted at a volcano on mainland japan tst 3b was emplaced by a turbidity current formed by a shallow submarine or subaerial volcanic eruption at the izu bonin arc where it collided with japan these results regarding distal turbidites encourage the revisiting of older marine deposits in the scope of hazard evaluation through past events especially in regions near to volcanic sources

submarine hydrothermal mineralization in the okinawa trough sw of japan an overview abstract the okinawa trough is a heavily sedimented rifted back arc basin formed in an intracontinental rift zone submarine hydrothermal activity is located within the six back arc rifts located in the middle and southern okinawa trough and its distribution is controlled principally by tectonic factors subduction of the daito and gagua ridges beneath the ryukyu arc has resulted in fracturing of the brittle lithosphere beneath the okinawa trough hydrothermal activity is strongest in the volcanic arc rift migration phenomenon vamp area plus the jade site and southernmost part of the okinawa trough spot area which form the prolongation of these two ridges these areas are characterized by extremely high heat flow locally submarine hydrothermal fluids from the okinawa trough tend to be strongly influenced by interaction of the hydrothermal fluids with organic matter in the sediment resulting in high alkalinity and nh 4 concentrations of the fluids the fluids also contain high concentrations of co 2 of magmatic origin submarine hydrothermal mineralization in the trough is diverse the clam site consists principally of carbonate chimneys interaction of the hydrothermal fluid with organic matter in the sediment is particularly strong at this site this is most probably a sediment hosted deposit in which sulphide minerals have deposited within the sediment column leaving spent ore fluids to emerge at the seafloor the jade site consists of active and inactive sulphide sulphate chimneys and mounds the zn pb rich sulphides at this site contain the highest concentrations of pb ag and au so far recorded in submarine hydrothermal sulphide deposits at minami ensei knoll and hatoma knoll active and inactive chimneys consist principally of anhydrite and barite as a result of phase separation of the hydrothermal fluids beneath the seafloor an intense black smoker has recently been discovered at yonaguni knoll in the spot area if it is confirmed that sulphide mineralization is dominant at this site this could be a highly prospective area the most prospective areas for economic grade minerals in the okinawa trough appear to be the jade site and the spot area

growth and mass wasting of volcanic centers in the northern south sandwich arc south atlantic revealed by new multibeam mapping new multibeam swath bathymetric sonar data acquired using an em120 system on the rrs james clark ross supplemented by sub bottom profiling reveals the underwater morphology of a not vert similar 12 000 km2 area in the northern part of the mainly submarine south sandwich volcanic arc the new data extend between 55 45 s and 57 20 s and include protector shoal and the areas around zavodovski visokoi and the candlemas islands groups each of these areas is a discrete volcanic center the entirely submarine protector shoal area close to the northern limit of the arc forms a 55 km long east west trending seamount chain that is at least partly of silicic composition the seamounts are comparable to small subaerial stratovolcanoes in size with volumes up to 83 km3 indicating that they are the product of multiple eruptions over extended periods zavodovski visokoi and the candlemas island group are the summits of three 3 3 5 km high volcanic edifices the bathymetric data show evidence for relationships between constructional volcanic features including migrating volcanic centers structurally controlled constructional ridges satellite lava flows and domes and mass wasting of the edifices mass wasting takes place mainly by strong erosion at sea level and dispersal of this material along chutes probably as turbidity currents and other mass flows that deposit in extensive sediment wave fields large scale mass wasting structures include movement of unconsolidated debris in slides slumps and debris avalanches volcanism is migrating westward relative to the underlying plate and major volcanoes are asymmetrical being steep with abundant recent volcanism on their western flanks and gently sloping with extinct eroded volcanic sequences to their east this is consistent with the calculated rate of subduction erosion of the fore arc

pele s hairs and tears natural probe of volcanic plume we present a detailed petrographic study of pele s hairs and tears sampled from masaya volcano nicaragua this study provides new observations of these little known pyroclastic objects using both secondary electron images sei and back scattering electron images bsei our work shows that pele s tears can be associated with pele s hairs after their formation tears can be trapped on the walls and or in the cavities of pele s hairs moreover chemical investigations of the pele s hairs and tears highlight the presence of a chemical zonation the edge of these tears and hairs show a siliceous enrichment allowing us to quantify the interaction time of the silicate glass with acid gases in the volcanic plume this study confirms the syneruptive and post eruptive volatile exsolution from pele s hairs and tears

physical parameters governing the formation of pele s hair and tears the physical parameters that affect the formation of pele s hair and pele s tears during lava fountaining are discussed experiments on ink jets produced from a nozzle under different weber number we and reynolds number re show the following results if re is relatively large compared with we an ink droplet is produced however if re is relatively small and we is large the spurting ink becomes thread like i define the pele number pe as we re which is expressed as v 0 where v is the spuring velocity from an erupting vent and are viscosity and interfacial tension of the erupting magma and and 0 are density of air and magma the experimental results from ink jets suggest that pele s hair will be produced for larger pe while pele s tears are very likely produced for relatively small pe i conclude that pele s hair could be produced when the spurting velocity of erupting magmas is high and pele s tears when it is relatively low as an additional point of interest the similarity of sem photographs of the characteristic shape of pele s hair to those of the failed products of commercial glass fibre are shown

timing and characteristics of the archean subaqueous blake river megacaldera complex abitibi greenstone belt canada abstract the archean blake river group of the abitibi greenstone belt represents a megacaldera complex which evolved over 8 11 m y from approximately 2704 to 2696 ma the early misema caldera developed from a series of amalgamated shield volcano complexes identified by the remnants of mafic dyke and sill systems these remnants are found in the jevis clericy montsabrais renault clifford tannahill and colnet regions where summit calderas are delineated by circular ring dyke structures the secondary 330 trending new senator caldera formed within the envelope of the misema caldera and exhibits a box work graben type structure finally the felsic dominated 070 striking noranda caldera well known for its vms endowment represents the final collapse of the megacaldera complex deformation has overprinted the calderas but structural patterns can be used in order to reconstruct the pre existing volcanic architecture structures such as mafic ring dyke complexes and rhyolitic dome flow complexes have nucleated fold geometries and synvolcanic fractures were reactivated within zones of ductile deformation precise u pb geochronological analyses were conducted in selected areas with specific emphasis on the misema and new senator calderas felsic volcanism progressed throughout the evolution of the megacaldera complex and intermediate to felsic units have been dated to limit this evolution the misema caldera formed between 2704 and 2702 ma via the amalgamation of shield volcanoes that were probably active prior to 2704 ma the new senator caldera was generated between 2702 and 2700 ma during paroxysmal felsic volcanism followed by the collapse of the noranda caldera culminating between 2700 2696 ma the misema caldera was generated by a gravitational stress field consistent with the formation of ring and radial dyke architecture whereas the se trending new senator caldera is more compatible with a sw trending principal compression direction related to oblique convergence in the abitibi belt the noranda caldera is interpreted as a ne rift structure that formed in the final stages of megacaldera evolution

an archean megacaldera complex the blake river group abitibi greenstone belt abstract the archean blake river group of the southern abitibi greenstone belt is defined as a subaqueous megacaldera compelling evidence include 1 a radial and concentric organization of synvolcanic mafic to intermediate dykes 2 an overall dome geometry defined by the volcanic strata 3 a peripheral distribution of subaqueous volcaniclastic units 4 a zonal distribution of carbonate alteration and 5 a distinct annular synvolcanic inner and outer ring fault pattern three caldera forming events have been identified 1 the early misema caldera 2 the new senator caldera and 3 the well known noranda caldera the multi vent brg mafic volcanic complex developed on a monotonous sequence of tholeiitic basalts forming a submarine plain and experienced a first major collapse that created the giant misema caldera 80 km in diameter an endogenic dyke swarm intruded the synvolcanic fractures and an underlying magma chamber developed major volcaniclastic units were generated by local volcanic centers and summit calderas formed along the outer and inner ring faults this fault system was used as a conduit for co2 rich hydrothermal activity renewed volcanic activity was associated with a resurgent central dome inside the misema caldera this second collapse event created the 35 km by 14 km nw trending new senator caldera this caldera was produced by multi step sagging after the underlying magmatic chamber migrated to the se and formed the flavrian powel plutons the final collapse resulted in the formation of the noranda caldera that generated a well developed 070 trending fracture pattern associated with several vms deposits the multiple caldera setting provides an effective model to explain the presence of vms mineralization along the synvolcanic fractures associated with the three collapse episodes however the importance of the new senator caldera fracture pattern is emphasized because of its role in the positioning of the giant horne mine deposit

a hot miocene subaqueous scoria flow deposit in the shimane peninsula sw japan abstract a subaqueous scoria flow deposit occurs in the miocene takashibiyama formation in sw japan faunal assemblages in associated sediments suggest an outer shelf to upper bathyal environment with water depth likely between 200 and 1500 m the deposit 5 7 m thick is similar to subaerial scoria flow deposits containing mainly non to moderately vesiculated hyalopilitic basaltic andesite lapilli and coarse ash the essential basaltic andesite clasts are variably vesiculated even within single clasts and are commonly cracked with curviplanar surfaces whereas the inner parts are cracked in a pattern similar to perlitic cracks the clasts are blocky with curviplanar surfaces if non to poorly vesiculated and are fibrous with irregular shapes and flattened by compaction if more vesiculated at one proximal location fines depleted rootless pipes 5 90 cm wide and 1 3 m long occur and spherical to amoebic vesicles 0 01 0 3 mm in diameter and filled with stilbite quartz or greenish brown clay are common in the fine ash matrix and in siltstone clasts contained in the deposit the vesicles represent open spaces once supported by gas and the rootless pipes are interpreted as gas escape pipes magnetization vectors of essential clasts are dispersed not at random but around a single direction suggesting that the clasts gained thermoremanent magnetism mainly during cooling in the flow or during post emplacement cooling and then rotated to some extent by differential movement of the flow or by gas escape in the deposit the vesicularity shape and texture of essential clasts suggest a phreatomagmatic eruption involving magmatic vesiculation and dynamic interaction of magma and ambient water the perlitic cracks in the inside of the essential clasts and the paleomagnetic vectors of the essential clasts indicate that the flow was initially hot and that the deposit cooled slowly internal depositional features change rapidly over only 500 m rootless pipes and vesicles in the fine ash matrix and siltstone clasts are therefore absent at distal locations indicating that an initially cohesionless debris flow quickly became sufficiently turbulent to mix with ambient seawater

exogenous and endogenous construction of the subaqueous glenwood felsic lava flow complex abitibi greenstone belt quebec canada abstract both intrusive and extrusive volcanic facies of a felsic lava flow complex are exposed as the outcrops of the glenwood rhyolite within the blake river group of the abitibi greenstone belt quebec canada sub vertically dipping volcanic facies allows for the identification of the internal architecture of an archean subaqueous felsic flow complex detailed facies analyses show volcanic facies and volcanic textures and structures atypical of subaqueous pyroclastic deposits or deposits of dome complexes additionally a complex network of mafic dykes cross cut the felsic facies when integrated with facies specific geochemical analyses a coherent model for all volcanic and intrusive facies and supplemental information regarding the host volcanic complex has been developed detailed mapping reveals extrusive or exogenous aphanitic felsic volcanic facies and intrusive or endogenous quartz and feldspar phyric felsic volcanic facies exogenous facies are comprised of massive lobate in situ brecciated flow breccia and flow front breccia facies whereas the only visible endogenous facies is a quartz and feldspar phyric unit that intrudes the brecciated facies on the eastern side of the complex all felsic facies demonstrate a west to east flow direction both exogenous and endogenous facies have similar chemical compositions ranging from dacite to rhyodacite with a transitional affinity mafic to intermediate dykes predominantly strike north south and east west with occasional east northeast trending units wispy terminations at some dyke margins and dismembered dykes indicate they were intruded into a still cooling viscoelastic medium geochemical analyses further defines that most east west trending dykes have a tholeiitic affinity whereas north south trending dykes are dominantly calc alkaline and east northeast trending dykes are transitional the subaqueous glenwood felsic flow complex was emplaced in a fault bounded depression and consists of stacked exogenous dacite rhyodacite lobes and intruded by a late quartz and feldspar phyric endogenous lobe the intrusion of mafic dykes occurred relatively soon after the cessation of felsic activity as evidenced by primary textures consistent with intrusion into a viscoelastic medium combined the extrusive and intrusive facies of the complex attests to the complexity of the host volcanic environment

petrogenesis of dacite in an oceanic subduction environment raoul island kermadec arc abstract raoul volcano in the northern kermadec arc is typical of volcanoes in oceanic subduction systems in that it is composed mainly of low k high al basalts and basaltic andesite however during the last 4 ka raoul volcano has produced mainly dacite magma in pyroclastic eruptions associated with caldera formation the rocks produced in these episodes are almost aphyric containing only sparse crystals of plagioclase clinopyroxene orthopyroxene and magnetite these apparent phenocrysts have chemical compositions that suggest that they did not crystallise from melts with the chemical composition of their host rocks rather they are xenocrysts and only their rims show evidence for crystallisation from their host melt chemical compositions of samples of the dacites show that each eruption has tapped a distinct magma batch compositional variations through the analysed suite cannot be accommodated in any reasonable model of fractional crystallisation from likely parental magma compositions the hypothesis that best fits the petrology of raoul island dacites is one of crustal anatexis this model requires heating of the lower crust by a magma flux to the point where dehydration melting associated with amphibole breakdown produces magma from a preconditioned source it is suggested that raoul is passing through an adolescent stage of development in which siliceous melts are part of an open system in which felsic and mafic magmas coexist

volcaniclastic processes on modern submarine arc stratovolcanoes sidescan and photographic evidence from the rumble iv and v volcanoes southern kermadec arc sw pacific abstract studies of volcaniclastic processes on modern submarine arc stratovolcanoes are rare facies models of submarine volcanism and associated volcaniclastic deposits have to date emphasized general and or regional processes two andesite alkali basaltic volcanoes rumble iv and v volcanoes from the southern kermadec arc are mapped with swath bathymetry and sidescan imagery and photographic data to characterise volcaniclastic processes on these modern submarine arc stratovolcanoes and their interaction with ambient marine processes the principal observation from the rumble iv and v data is the fine scale spatial variability in the distribution of effusive lavas various volcaniclastic deposits and hemipelagic pelagic sediments mass movement of volcaniclastic deposits is common and includes 1 epiclastic debris flows 2 pillow breccia talus deposition penecontemporaneous with lava eruption 3 traction particulate gravity flows and possibly 4 sector collapse epiclastic redeposition is inferred to be mobilised by episodic but frequent localised small volume flows which largely redeposit debris on the edifice winnowed and rippled volcaniclastic sediment indicates bottom current activity is common but localised over the entire height of both volcanoes down to a water depth of 2200 m these currents also possibly entrain detritus for subsequent mobilisation by particulate gravity flows irregular and ragged spatter agglutinates on rumble iv interpreted as evidence of lava fountaining and wellsorted fines poor scoriaceous material on rumble v interpreted as evidence of explosive eruption indicate the transition between effusive and explosive volcanism is between 650 and 700 m water depth other southern kermadec volcanoes e g tangaroa have scoriaceous detritus at 960 970 m water depth positions of these effusive explosive transition zones are likely to oscillate up and down the volcano during glacioeustatic fluctuations

felsic fire fountaining beneath archean seas pyroclastic deposits of the 2730 ma hunter mine group quebec canada abstract the lapilli tuff breccias ltb 1 and ltb 2 of the archean hunter mine group in the south central part of the abitibi greenstone belt are inferred to be the product of subaqueous lava fountaining intercalated sub wave base iron formations interstratified turbiditic tuffs the absence of wave induced sedimentary structures and the stratigraphic position of lapilli tuff breccias beneath basaltic komatiites support this contention a complete eruptive sequence shows a tripartite division into a massive breccia b stratified lapilli tuff and c turbiditic tuff lapilli tuff division the massive breccia division is characterized by clusters of isolated and compressed irregular shaped clasts inferred to be deposited directly from the hot magmatic lava fountain abundant vesicular pyroclasts with a vesicle content of up to 60 exhibit locally coalescing vesicles indicating bubble nucleation prior to eruption the prevalence of irregular to amoeboid clast shapes suggests transport from the vent in a steamy rich high density current to the site under a self generated steam cupola ubiquitous subequant lapilli size pyroclasts of the stratified lapilli tuff division suggest that significant ingress of water into the fountain changed the prevalent fragmentation process from magmatic to hydrovolcanic the turbiditic tuff lapilli tuff division composed of pumice lithic fragments and vitric ash is envisaged to have formed by gravitational collapse of a subaqueous turbulent eruptive plume this type of eruptive mechanism constituted a minor but important process of volcanic construction on the ocean floor during the archean and possibly during incipient arc and backarc formation in modern day settings

water settling and resedimentation of submarine rhyolitic pumice at yali eastern aegean greece abstract the yali pumice breccia is a very thick 150 m quaternary succession of submarine pumice that has been uplifted and exposed in the southern part of yali island in the eastern aegean greece the pumice breccia comprises moderately to well sorted 0 03 3 m thick beds of loose pumice clasts that are poor in fine matrix predominantly the larger pumice clasts cobbles and boulders are prismatic with quenched margins and internal polyhedral joints in comparison the smaller pumice clasts are polyhedral angular to subrounded blocky pebbles and granules these smaller clasts lack quenched margins and have sharp curviplanar surfaces that cross cut vesicle boundaries we interpret the larger pumice clasts to be the products of spalling and explosive fragmentation of a small extrusion of submarine pumiceous lava the smaller pumice clasts were generated by a combination of 1 passive and explosive disintegration of the larger pumice clasts and 2 phreatomagmatic explosions phreatomagmatic explosions also generated a minor component of non or poorly vesicular juvenile and basement derived lithic clasts the yali pumice breccia includes four major facies cobble boulder pebble mixed cobble dominant and mixed pebble dominant the cobble boulder facies occurs in well sorted massive tabular beds of large 64 mm 1 5 m pumice up to 3 m thick that lack both lithic clasts and fine matrix this facies has textural and lithological features consistent with deposition of large pumice clasts by means of water settling from suspension the three remaining facies are comparatively less well sorted although still fines poor and include rare lithic clasts they exhibit massive or internally diffusely stratified wedging 0 1 2 m thick beds of coarser grained medium pebble to cobble pumice clasts that onlap or are separated by wedge shaped thinner

high silica rhyolites of niijima volcano in the northern izu bonin arc japan petrological and geochemical constraints on magma generation and supply abstract niijima volcano in the northern part of the quaternary izu bonin volcanic arc japan consists predominantly of rhyolitic volcanic units associated with minor basaltic and andesitic units most of the rhyolites are characterized by a phenocryst assemblage of orthopyroxene opx cummingtonite cum biotite bt and hornblende which are classified into four types opx cum cum cum bt and bt in eruption order the major element compositions of the rhyolites are typified by high sio2 73 78 wt and k2o 1 5 3 3 wt contents and the trace elements are indicative of a highly differentiated nature which differs significantly from rhyolites at other sites in the izu bonin arc the presence of cummingtonite phenocrysts and mineralogical and whole rock chemical compositions of the rhyolite lavas clearly indicate that these magmas were produced under low temperatures 13 units in a shallow magma system at the same time and at deeper levels basaltic and rhyolitic parental magmas mixed to produce andesitic magmas between the basaltic and rhyolitic parental magmas in some cases basaltic and andesitic magmas in deeper level ascended and mingled with shallow rhyolitic magmas before eruption the eruption of rhyolitic magmas from shallow levels might have been triggered by the injection of basaltic and andesitic magmas from deeper levels the complex magmatic system beneath niijima volcano formed within a relatively thick crustal sequence on the rear arc side of the izu bonin volcanic arc

vesiculation and fragmentation history in a submarine scoria cone forming eruption an example from nishiizu izu peninsula japan an uplifted 50 m thick half dissected submarine emplaced below wave base scoria cone occurs as dipping beds in coastal outcrops at nishiizu on the izu peninsula in japan concentrically outward dipping weakly stratified ungraded framework supported thin to very thick beds consist of brown coarse tuff to scoria lapilli tuff with outsized fluidal bombs throughout accessory lithic clasts chiefly occur in the lowermost visible beds scoria bombs have quenched margins weak bread crust textures and their vesicle number densities decrease inward which is indicative of fast surface cooling composite textures in the scoria bombs indicate recycling and agglutination of quenched and semi molten pyroclasts at the submarine vent in contrast to weak concentric gradations in vesicle size distribution in the bombs lapilli have asymmetrical gradients in vesicle size distribution indicating that they are fragments of coarser quenched lumps three grain size modes characterise the nishiizu brown scoria with coarse magma lumps ejected during magmatic fragmentation and quench jointed upon contact with seawater to be subsequently fragmented into lapilli and coarse ash by various styles of fragmentation where seawater plays a critical role the cone was constructed by slow moving fallout fed granular flow creep fed directly by suspension settling focused at the crater rim but extending onto the cone flanks with only minor resedimentation by granular flows nishiizu deposits yield an exceptional record of eruption and sedimentation dynamics during submarine cone building activity and in this study we compare their vesiculation and fragmentation mechanisms with those of potential subaerial analogues

pyroclastic flows and surges over water an example from the 1883 krakatau eruption pyroclastic deposits from the 1883 eruption of krakatau are described from areas northeast of the volcano on the islands of sebesi sebuku and lagoendi and the southeast coast of sumatra massive and poorly stratified units formed predominantly from pyroclastic flows and surges that traveled over the sea for distances up to 80 km granulometric and lithologic characteristics of the deposits indicate that they represent the complement of proximal subaerial and submarine pyroclastic flow deposits laid down on and close to the krakatau islands the distal deposits exhibit a decrease in sorting coefficient median grain size and thickness with increasing distance from krakatau crystal fractionation is consistent with the distal facies being derived from the upper part of gravitationally segregated pyroclastic flows in which the relative amount of crystal enrichment and abundance of dense lithic clasts diminished upwards the deposits are correlated to a major pyroclastic flow phase that occurred on the morning of 27 august at approximately 10 a m energetic flows spread out away from the volcano at speeds in excess of 100 km h and traveled up to 80 km from source the flows retained temperatures high enough to burn victims on the sw coast of sumatra historical accounts from ships in the sunda straits constrain the area affected by the flows to a minimum of 4x103 km2 at the distal edge of this area the flows were relatively dilute and turbulent yet carried enough material to deposit several tens of centimeters of tephra the great mobility of the krakatau flows from the 10 a m activity may be the result of enhanced runout over the sea it is proposed that the generation of steam at the flow seawater interface may have led to a reduction in the sedimentation of particles and consequently a delay in the time before the flows ceased lateral motion and became buoyantly convective the buoyant distal edge of these ash and steam laden clouds lifted off into the atmosphere leading to cooling condensation and mud rain

tavui volcano neighbour of rabaul and likely source of the middle holocene penultimate major eruption in the rabaul area a major geologically youthful submarine caldera volcano tavui was discovered in the rabaul area of papua new guinea in 1986 tavui volcano has lateral dimensions of 9 to 10 km slightly smaller than those of rabaul volcano but tavui s caldera is much deeper than the nested caldera complex at rabaul and in general its escarpment walls are very steep the two caldera systems are essentially silicic and are separated by a zone of dominantly basalt andesite stratovolcanoes rock samples dredged from tavui have low to medium k contents in contrast to the medium to high k rocks of rabaul indicating that the two systems have evolved separately and that the chemical and perhaps physical conditions within these neighbouring systems are different tavui is the likely source of the 6 9 ka bp raluan ignimbrite the penultimate major eruption deposit in the rabaul area the raluan ignimbrite is rhyolitic and has geochemical characteristics incompatible with those of products from rabaul volcano on the other hand there is a close match between the geochemistry of the raluan ignimbrite and that of rhyolitic samples dredged from tavui caldera the much older 79 ka tokudukudu ignimbrite which is also rhyolitic is slightly more k rich than both the raluan ignimbrite and rhyolites dredged from tavui caldera but in general its geochemical characteristics are similar to those of tavui rhyolites and therefore is considered to be a possible product of tavui the recognition that tavui was the likely source of the penultimate major eruption and of at least one other significant eruption in the rabaul area markedly changes the perceptions of local volcanic hazard in addition tavui s potential for generation of tsunami is acknowledged not just in association with volcanic eruptions but also from earthquake related and possibly spontaneous collapse of parts of the steep caldera walls the presence of tavui greatly increases the net geologic hazard in the rabaul area

petrography and geochemistry of achnelithic tephra from las herrerias volcano calatrava volcanic field spain formation of nephelinitic achneliths and post depositional glass alteration abstract we present the results of a petrographic and geochemical study carried out on a layer of achnelithic tephra outcropping at the base of the volcanic cone of las herrerias miocene quaternary volcanic region of campo de calatrava spain the tephra with a composition of nephelinite and ash 3 of all major elements and notable increase in h 2 o content the elements liberated by this process formed smectite with an average structural formula comprised between those of beidellite and nontronite end terms na 0 01 k 0 03 ca 0 18 mg 0 22 fe 0 16 2 fe 0 48 al 1 02 3 ti 0 18 4 si 3 58 al 0 42 o 10 oh 2 the degree of palagonitization in each achnelith was likely related to the amount of water incorporated by individual clasts at the moment of their deposition in a volcanic maar lake afterwards there was no more water circulation through the achnelithic tephra which was sealed from water by overlying hydrovolcanic tuff deposits it was this isolation that made possible the preservation of glass to the present day

type of volcanoes hosting the massive sulfide deposits of the iberian pyrite belt the volcanic sedimentary complex vsc of the iberian pyrite belt ipb in southern portugal and spain comprises an upper devonian to lower carboniferous submarine succession with a variety of felsic volcanic lithofacies the architecture of the felsic volcanic centres includes felsic lavas domes pyroclastic units intrusions and minor mafic units that define lava cryptodome pumice cone volcanoes the diversity of volcanic lithofacies recognized in different areas of the ipb mainly reflects variations in proximity to source but also differences in the eruption style the ipb volcanoes are intrabasinal range in length from 2 km to 8 km and their thickest sections vary from 400 m to 800 m these volcanoes are dominated by felsic lavas domes that occur at several stratigraphic positions within the volcanic centre however the pyroclastic units are also abundant and are spatially related to the lavas domes the intrusions are minor and define cryptodomes and partly extrusive cryptodomes the hydrothermal systems that formed the neves corvo and lousal massive sulfide ore deposits are associated with effusive units of felsic volcanic centres at neves corvo the massive sulfide orebodies are associated to rhyolitic lavas that overlie relatively thick fiamme rich pyroclastic unit in several other locations within the belt pyroclastic units contain sulfide clasts that may have been derived from yet to be discovered coeval massive sulfide deposits at or below the sea floor which enhances the exploration potential of these pyroclastic units and demonstrates the need for volcanic facies analysis in exploration

differential preservation in the geologic record of intraoceanic arc sedimentary and tectonic processes abstract records of ancient intraoceanic arc activity now preserved in continental suture zones are commonly used to reconstruct paleogeography and plate motion and to understand how continental crust is formed recycled and maintained through time however interpreting tectonic and sedimentary records from ancient terranes after arc continent collision is complicated by preferential preservation of evidence for some arc processes and loss of evidence for others in this synthesis we examine what is lost and what is preserved in the translation from modern processes to the ancient record of intraoceanic arcs composition of accreted arc terranes differs as a function of arc continent collision geometry forward facing collision can accrete an oceanic arc on to either a passive or an active continental margin with the arc facing the continent and colliding trench and forearc side first in a backward facing collision involving two subduction zones with similar polarity the arc collides backarc first with an active continental margin the preservation of evidence for contemporary sedimentary and tectonic arc processes in the geologic record depends greatly on how well the various parts of the arc survive collision and orogeny in each case preservation of arc terranes likely is biased towards those that were in a state of tectonic accretion for tens of millions of years before collision rather than tectonic erosion the prevalence of tectonic erosion in modern intraoceanic arcs implies that valuable records of arc processes are commonly destroyed even before the arc collides with a continent arc systems are most likely to undergo tectonic accretion shortly before forward facing collision with a continent and thus most forearc and accretionary prism material in ancient arc terranes likely is temporally biased toward the final stages of arc activity when sediment flux to the trench was greatest and tectonic accretion prevailed collision geometry and tectonic erosion vs accretion are important controls on the ultimate survival of material from the trench forearc arc massif intra arc basins and backarc basins and thus on how well an ancient arc terrane preserves evidence for tectonic processes such as subduction of aseismic ridges and seamounts oblique plate convergence and arc rifting forward facing collision involves substantial recycling melting and fractionation of continent derived material during and after collision and so produces melts rich in silica and incompatible trace elements as a result forward facing collision can drive the composition of accreted arc crust toward that of average continental crust

cryptoachneliths hidden glassy ash in composite spheroidal lapilli abstract cryptoachneliths perceptible by means of electron microscopy but unresolved under the optical microscope occur unnoticed inside spheroidal lapilli of ultrabasic composition of the cabezo segura volcano calatrava volcanic province spain the cryptoachneliths are glassy spherical particles that have compositions of al rich silicate with minor amounts of fe ca and other elements the smallest cryptoachneliths of 1 m microachneliths and homogeneous less regular masses of similar composition nano and microachneliths welded each other or to other types of volcanic particles crystals crystal fragments spinning droplets cognate lithic clasts etc to form spheroidal lapilli and even bomb size clasts within proximal fall deposits of the cabezo segura volcano the welding processes took place inside the eruptive column previous to the fall of the spheroidal lapilli on top of the volcanic cone the presence of the cryptoachneliths implies that lapilli and even bomb size tephra within deposits formed during explosive eruptions of low viscosity basic to ultrabasic magmas should be carefully examined in order to establish key parameters of eruption dynamics like size amount and distribution of juvenile fine particles

mechanism of formation of volcanic bombs insights from a pilot study of anisotropy of magnetic susceptibility and preliminary assessment of analytical models volcanic bombs and achneliths are a special type of pyroclastic fragments formed by mildly explosive volcanic eruptions models explaining the general shapes of those particles can be divided in two broad categories the most popular envisages the acquisition of shapes of volcanic bombs as the result of the rush of air acting on a fluid clot during flight and it includes many variants the less commonly quoted model envisages their shapes as the result of forces acting at the moment of ejection of liquid from the magma pool in the conduit experiencing an almost negligible modification through its travel through air quantitative evidence supporting either of those two models is limited in this work i explore the extent to which the anisotropy of magnetic susceptibility ams might be useful in the study of mechanisms of formation of volcanic bombs by comparing measurements made on two spindle and two bread crusted bombs the results of this pilot study reveal that the degree of anisotropy of spindle bombs is larger and their principal susceptibility axes are better clustered than on bread crusted bombs also the orientation of the principal susceptibility axes is consistent with two specific models one of the in flight variants and the general ejection model consequently the reported ams measurements albeit limited in number indicate that it is reasonable to focus attention on only two specific models to explain the acquisition of the shapes of volcanic bombs based on a parallel theoretical assessment of analytical models a third alternative is outlined envisaging volcanic bomb formation as a two stage process that involves the bursting of large m gas bubbles on the surface of a magma pond the new model advanced here is also consistent with the reported ams results and constitutes a working hypothesis that should be tested by future studies richer in data fortunately since this work also establishes that ams can be used to determine magnetic fabric in small size samples the possibility to expand this study to a larger collection of bombs is granted

evidence for voluminous bimodal pyroclastic volcanism during rifting of a paleoproterozoic arc at snow lake manitoba the thrust bounded mcleod road birch lake mb sequence occurs within the paleoproterozoic snow lake arc sla assemblage of the flin flon belt stratigraphic correlation of volcanic strata of the mb sequence with strata of the thrust bounded chisel sequence indicates that distinctive submarine eruption fed pyroclastic flow deposits are more extensive and voluminous than previously recognized 10 km3 these voluminous felsic pyroclastic deposits define a distinct magmatic and explosive volcanic event during bimodal volcanism that accompanied rifting of the sla the felsic pyroclastic deposits define the remnants of a basin or of nested basins that formed during arc rifting and subsidence and their eruption immediately preceded formation of the chisel sequence volcanogenic massive sulfide vms deposits although the chisel sequence ore interval is recognized in the mb sequence the lack of vms related alteration indicates that vms hydrothermal activity was restricted to the chisel portion of the

hybrid fall deposits in the bishop tuff california a novel pyroclastic depositional mechanism hybrid fall deposits in the bishop tuff show features common to both archetypal fall and surge deposits like normal fall deposits they have an overall plane parallel bedding and flat lying pumice clasts but also like surge deposits they show variable development of cross bedding some crystal and pumice sorting and some rounding of pumice clasts all variations exist from normal fall deposits through streaky material with incipient development of cross bedding to the hybrid fall deposits with well developed cross bedding the streaky and hybrid deposits are interpreted as fall material contemporaneously redeposited by strong up to 40 m s swirling winds comparable to firestorm whirlwinds generated by air currents associated with coeval emplacement of pyroclastic flows recognition of hybrid fall deposits is important in interpreting the dynamics of explosive eruptions and correctly assessing volcanic hazards however although such deposits may be commonly produced by explosive eruptions especially where pyroclastic flows accompany fall activity they are likely to be overlooked or wrongly interpreted as surge deposits or secondary reworked material

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

deep submarine pyroclastic eruptions theory and predicted landforms and deposits submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows however that significant pyroclastic activity can occur even at depths in excess of 3000 m in order to document and illustrate the full range of submarine eruption styles we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions 1 no gas exsolution 2 gas exsolution but no magma disruption 3 gas exsolution magma disruption and hawaiian style fountaining 4 volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse 5 magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity and 6 volatile content builds up in the top of a dike leading to vulcanian eruptions we also examine the role of bulk interaction steam explosivity and contact surface steam explosivity as processes contributing to volcaniclastic formation in these environments we concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities the most likely circumstance is the quiet effusion of magma with minor gas exsolution and the production of somewhat vesicular pillow lavas or sheet flows depending on effusion rate the amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption the most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity due to the localized concentration of volatiles in magma that has a low rise rate magmatic gas collects by bubble coalescence and ascends in large isolated bubbles which disrupt the magma surface in the vent producing localized blocks bombs and pyroclastic deposits another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions these deposits being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater should be easily distinguishable from strombolian and hawaiian eruptions however we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles in this case a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian style activity the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile induced hawaiian eruption specifically typical pyroclast sizes might be smaller fountain heights may exceed those expected for the purely magmatic hawaiian case cooling of descending pyroclasts would be more efficient leading to different types of proximal deposits and runout distances for density flows would be greater potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer in addition flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles and thus extremely poor in vesicles relative to typical flows associated with hawaiian style eruptions in the primary magmatic gas case we examine several cases of reported submarine volcaniclastic deposits found at depths as great as 3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid ocean ridges furthermore the latter stages of development of volcanic edifices seamounts formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth but also to 1 the presence of a summit magma reservoir which favors the buildup of magmatic foams enhancing hawaiian style activity and episodic dike emplacement which favors strombolian style eruptions and 2 the common occurrence of alkalic basalts the co2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts these models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles

subaqueous strombolian eruptions and eruption fed pyroclastic deposits in a paleoproterozoic rifted arc hidden formation flin flon canada abstract the hidden formation of the paleoproterozoic flin flon arc assemblage comprises the stratigraphic hanging wall to the volcanogenic massive sulfide vms deposits at flin flon manitoba canada it represents a basaltic volcanic edifice erupted within a large subsidence structure developed in a rifted arc environment it is predominately pillow lavas with subordinated amounts volcaniclastic rocks rocks that stratigraphically overly the schist and mandy vms deposits 5 km southeast of the main flin flon deposits have been correlated through litho and chemo stratigraphy with the hidden formation they display evidence of pyroclastic volcanism and concomitant subsidence pillow lavas and hyaloclastite are unequivocal evidence of subaqueous eruption the hidden formation in the schist mandy area comprises mafic tuff mafic megabreccia basalt lava fluidal clast breccia and plagioclase crystal rich volcaniclastic lithofacies intruded by synvolcanic dikes and sills the megabreccia lithofacies is characterized by angular basalt clasts 1 20 m in a matrix of finer basalt clasts and is interpreted to have been deposited by mass flows due to slumping of the caldera wall a within flow vertical transition in the lavas from massive to pillowed flows suggests a slowing of emplacement velocity decrease in temperature and an increase in the viscosity during eruption based on the monolithic nature of the fluidal clast breccias and the fluidal shapes of clasts with delicate tails and intact glassy margins they are interpreted to be the result of strombolian eruptions an increase in the ingestion of water in the vent s lead to a change from magmatic to phreatomagmatic fragmentation planar and cross laminations graded beds and the abundance of broken plagioclase crystals in combination with scoria and lithic clasts suggest the mafic tuff and plagioclase crystal rich volcaniclastic lithofacies were deposited by subaqueous eruption fed density currents derived from a phreatomagmatic eruption detailed mapping of the volcanic lithofacies in the schist mandy area has facilitated the identification of synvolcanic faults defining inner caldera margins and volcanic vents these structures when traced downwards through stratigraphy define structural corridors corresponding with the schist and mandy vms deposits indicating they represent long lived structures that also acted as magmas and fluid pathways

mfci experiments on the influence of nacl saturated water on phreatomagmatic explosions abstract molten fuel coolant interaction mfci experiments were performed using remelted foiditic rock samples from the west eifel volcanic field germany two experimental series were carried out with one magmatic melt and two water compositions bi distilled water was used in the first series dw 1 to dw 5 in the second series sw 1 to sw 5 the bi distilled water was saturated 350 g l 1 with sodium chloride nacl for both experimental series the fragmentation history and the energy release were recorded and compared the smallest particles 125 m were studied using scanning electron microscopy sem most mfci experiments with bi distilled water reached higher explosion intensities than the experiments with the saline water this was accompanied by higher particle ejection velocities as well as the formation of more fine grained and more interactive particles of angular shape additionally the smallest artificial pyroclasts were examined by evolved gas analyses ega the particles from the mfci experiments with salt solutions are found to contain more sodium hydroxide naoh these observations can be explained by thermodynamic arguments in contrast to the mfci experiments with pure water an additional reaction occurs with saline water that results in evolution of hydrogen chloride hcl gas and leaves a residue of sodium hydroxide the mfci process with saline water consumes more enthalpy and gibbs free energy so that less energy is available for the explosion with other sodium halides dissolved in the water naf nabr or nai the additional reaction can be predicted to have greater or lesser effects on phreatomagmatic explosions

submarine eruption fed and resedimented pumice rich facies the dogashima formation izu peninsula japan in the izu peninsula japan the pliocene pumice rich dogashima formation 4 55 0 87 ma displays exceptional preservation of volcaniclastic facies that were erupted and deposited in a below wave base marine setting it includes high concentration density current deposits that contain clasts that were emplaced hot indicating an eruption fed origin the lower part of the dogashima 2 unit consists of a very thick sequence 1 m pumice and ash rich bed overlying the breccias may be derived from delayed settling of pyroclasts from suspension in dogashima 1 and 2 thick cross and planar bedded facies composed of sub rounded pumice clasts are intercalated with eruption fed facies implying inter eruptive mass wasting on the flank of a submarine volcano and reworking and resedimentation by high energy tractional currents in a below wave base environment

dynamics of deep submarine silicic explosive eruptions in the kermadec arc as reflected in pumice vesicularity textures abstract despite increasing recognition of silicic pumice bearing deposits in the deep marine environment the processes involved in explosive silicic submarine eruptions remain in question here we present data on bubble sizes and number densities number of bubbles per unit of melt matrix for deep submarine erupted pumices from three volcanoes healy raoul sw and havre along the kermadec arc sw pacific to investigate the effects of a significant 1 km overlying water column and the associated increased hydrostatic pressure on magma vesiculation and fragmentation we compare these textural data with those from chemically similar subaerially erupted pyroclasts from nearby raoul volcano as well as submarine erupted tangaroan fragments derived by non explosive buoyant detachment of foaming magma from macauley volcano also along the kermadec arc deep submarine erupted pumices are macroscopically similar colour density texture to subaerial or shallow submarine erupted pumices but show contrasting microscopic bubble textures deep submarine erupted pyroclasts have fewer small

volcanic evolution of the south sandwich volcanic arc south atlantic from multibeam bathymetry abstract new multibeam bathymetry data are presented for the south sandwich intra oceanic arc which occupies the small sandwich plate in the south atlantic and is widely considered to be a simple end member in the range of intra oceanic arc types the images show for the first time the distribution of submarine volcanic tectonic and erosional depositional features along the whole length of the 540 km long volcanic arc allowing systematic investigation of along arc variations the data confirm that the volcanic arc has a simple structure composed of large volcanoes which form a well defined volcanic front but with three parallel cross cutting seamount chains extending 38 60 km from near the volcanic front into the rear arc there is no evidence for intra arc rifting or extinct volcanic lines topographic evidence for faulting is generally absent except near the northern and southern plate boundaries most of the volcanic arc appears to be built on ocean crust formed at the associated back arc spreading centre as previously proposed from magnetic data but the southern part of the arc appears to be underlain by older arc or continental crust whose west facing rifted margin facing the back arc basin is defined by the new bathymetry the new survey shows nine main volcanic edifices along the volcanic front and ca 20 main seamounts the main volcanoes form largely glaciated islands with summits 3 0 3 5 km above base levels which are 2500 3000 m deep in the north and shallower at 2000 2500 m deep in the south some of the component seamounts are interpreted to have been active since the last glacial maximum and so are approximately contemporaneous with the volcanic front volcanism seven calderas all either submarine or ice filled have been identified adventure volcano a newly discovered submarine volcanic front caldera volcano is described for the first time all but one of the calderas are situated on summits of large volcanoes in the southern part of the arc and most are associated with current or historic volcanic or hydrothermal activity shallow shelves around the islands are generally 1 10 km wide submerged banks up to 1100 m deep are interpreted as subsided erosional surfaces seamounts and emergent volcanoes experienced a range of mass wasting processes including by landsliding and smaller mass flows

development of spheroidal composite bombs by welding of juvenile spinning and isotropic droplets inside a mafic eruption column abstract it is generally assumed that spheroidal volcanic bombs are formed during cooling of molten clots pulled up into spheres by the surface tension of the magma also volcanic bombs are regarded as non fragmental structures i e made of coherent material we describe non coherent spheroidal bombs of mafic composition from the tuff cinder cone of the cabezo segura volcano calatrava volcanic province spain that exhibit features consistent with welding of essentially non vesicular juvenile spherical droplets it is assumed that the eruption of a low viscosity magma containing suspended phenocrysts mantle clasts and cognate lithic clasts resulted in the efficient separation of volatiles and melt and the explosive fragmentation of the magma into different types of pyroclasts the finest magma fragments would yield melt blobs achneliths or lava spray ca 1 5 m small droplets largely made up also of melt and isolated solid clasts phenocryst microphenocryst and lithic the largest lava fragments incorporated phenocrysts microphenocrysts and lithic clasts all these types of clasts would be erupted in a rapid gas thrust that prevented rapid collision among particles massive aggregation and premature collapse by defluidization the rotation of the largest magma droplets during ascent in a fluidized system produced spinning droplets similar to pelletal lapilli the collision and aggregation of the fluidal clasts might start as soon as the pyroclasts gas mixture was thrust out and progressively decelerated a model is proposed for the formation of spheroidal composite bombs assuming fluidal clasts sintering either by coalescence or agglutination and welding of constituent pyroclasts inside the eruption column prior to bomb accumulation on the field

variability of ash deposits at piton de la fournaise la reunion island insights into fragmentation processes at basaltic shield volcanoes 25 it is commonly accepted that effusive activity emplaces the main emitted magmatic volume in basaltic 26 shield volcanoes at piton de la fournaise la reunion island france eruptive activity occurs mostly within the 27 non populated enclos fouque caldera and generally does not pose any risk to the population however historical 28 observations recent monitoring data and field work on tephra deposits suggest that some eruptions have produced 29 unusual and unexpected explosive phases a comprehensive sampling of tephra from major historical and recent 30 eruptions on this volcano allowed us to perform systematic componentry grain size chemical and morphological 31 analyses in order to characterize the eruptive dynamics involved in these explosive basaltic eruptions this 32 integrative approach reveals highly variable characteristics of the studied tephra reflecting different fragmentation 33 efficiencies and multiple associated mechanisms primary ductile and partially brittle fragmentation of various 34 intensity of juvenile magma emitted during hawaiian fountaining or mild strombolian explosions were identified 35 as the most common fragmentation mechanisms in particular during the 2014 2018 dominantly effusive eruptive 36 sequence in parallel we distinguished more efficient short lived fragmentation events related to i plug 37 pressurization and brittle fragmentation enhanced by syn eruptive crystallization ii magma water interactions 38 iii rare phreatic explosions and iv secondary fragmentation producing fine ash during caldera collapse we 39 conclude that textural geochemical and morphological analysis make it possible to identify and characterize the 40 variability in eruptive processes with the grain size and the componentry of the ash particles being probably the 41 most important parameters to quantify both the efficiency and the nature of the fragmentation 42 43

silicate glass micro and nanospherules generated in explosive eruptions of ultrabasic magmas implications for the origin of pelletal lapilli abstract the genesis of spherical ash to lapilli sized clasts with a central phenocryst or lithic fragment mantled by a rim of fine grained juvenile material that includes abundant concentrically arranged prismatic crystals is interpreted as either i the result of the spinning of a magma bleb with a crystalline kernel in a fluidized system or ii the accretion of small melt droplets to a previously crystallized nucleus in a gas jet we demonstrate that the rims of pelletal lapilli within tephras of the cabezo segura volcano calatrava spain are clastic and were formed by the progressive welding of juvenile crystals and silicate glass droplets and to a lesser extent filaments both melt in origin around a large crystalline nucleus our results support the accretion hypothesis and offer explicit and new images of the melt droplets that were so far considered hypothetical particles these results indicate also that nanometre scale juvenile pyroclasts melt droplets and crystals can be generated in explosive eruptions of ultrabasic magmas those pyroclasts can be subsequently welded together inside a dense gas jet generating pelletal lapilli and ash

tephrostratigraphy and provenance from iodp expedition 352 izu bonin arc tracing tephra sources and volumes from the oligocene to recent provenance studies of widely distributed tephras integrated within a well defined temporal framework are important to deduce systematic changes in the source scale distribution and changes in regional explosive volcanism here we establish a robust tephro chronostratigraphy for a total of 157 marine tephra layers collected during iodp expedition 352 we infer at least three major phases of highly explosive volcanism during oligocene to pleistocene time provenance analysis based on glass composition assigns 56 of the tephras to a japan source including correlations with 12 major and widespread tephra layers resulting from individual eruptions in kyushu central japan and north japan between 115 ka and 3 5 ma the remaining 101 tephras are assigned to four source regions along the izu bonin arc one of exclusively oligocene age is proximal to the bonin ridge islands two reflect eruptions within the volcanic front and back arc of the central izu bonin arc and a fourth region corresponds to the northern izu bonin arc source first order volume estimates imply eruptive magnitudes ranging from 6 3 to 7 6 for japan related eruptions and between 5 5 and 6 5 for ibm eruptions our results suggest tephras between 30 and 22 ma that show a subtly different izu bonin chemical signature compared to the recent arc after a 11 m y gap in eruption tephra supply from the izu bonin arc predominates from 15 to 5 ma and finally a subequal mixture of tephra sources from the palaeo honshu and izu bonin arcs occurs within the last 5 ma

formation of composite pyroclasts by welding inside a lithic rich mafic eruption column los marteles caldera canary islands abstract the tuff ring of the los marteles phreatomagmatic maar gran canaria canary islands includes a particular unit that comprises abundant composite spheroidal clasts however these clasts differ from cored clasts elsewhere that consist of erupted lithic pyroclasts with an adhering rim of coherent chilled magma the composite clasts here studied range from lapilli to bombs and are cored or concentrically banded among the welded components several types of primary clasts are discerned that include i juvenile clasts resultant of the magma inertial fragmentation and ii cognate or accessory lithic clasts juvenile clasts include glass spheres nano and microachneliths and crystals lithic clasts include lava fragments scoriae and olivine xenocrysts primary clasts are interpreted to have been produced by the explosion of a partially crystallized mafic sio 2 50 55 magma that erupted into a maar filled up with scoriae plus lava fragments the explosion would have originated as a dense and turbulent gas thrust into which the repeated collisions of melt droplets with solid clasts generated droplet coated crystals lithic clasts and single glass spheres we interpret the cored and concentrically banded pyroclasts to be made of coalesced and or agglutinated particles coated with a melt rim to the rest of the components these composite clasts developed spheroidal shapes during transport within a violent strombolian eruption column due to spin effect and finally landed in solid state

mineralogy and composition of kuroko deposits from northeastern honshu and their possible modern analogues from the izu ogasawara bonin arc south of japan implications for mode of formation the kuroko deposits of ne honshu are a key type deposit for the study of volcanogenic massive sulfide deposits however these deposits have not been studied in detail since the early 1980 s and knowledge of their mode of formation is now dated in this study we present the analysis of 12 samples of the kuroko deposits 12 samples of submarine hydrothermal minerals from the sunrise deposit and 6 samples from suiyo seamount both of which are located on the izu ogasawara bonin arc for 27 elements for the kuroko deposit cd sb ag pb hg as zn cu are highly enriched au te bi ba mo are moderately enriched in tl are somewhat enriched and fe is not significantly enriched relative to the average continental crust within each of these deposits a similar pattern of element associations is apparent zn pb with as sb cd ag hg tl and au fe cu ba with as sb ag tl mo te and au si ba with ag and au caso4 the enrichment of the chalcophilic elements in these deposits is consistent with hydrothermal leaching of these elements from the host rocks which are dominantly rhyolite dacite in the case of the kuroko deposits rhyolite in the case of the sunrise deposit and dacite rhyolite in the case of the suiyo seamount deposit however this pattern of element enrichment is also similar to that observed in fumarolic gas condensates from andesitic volcanoes this suggests that there may be a significant magmatic contribution to the composition of the hydrothermal fluids responsible for the formation of the kuroko deposits although it is not yet possible to quantify the relative contributions of these two sources of elements the compositional data show that sunrise and suiyo seamount deposits are much closer compositionally to the kuroko deposits from ne honshu than are the submarine hydrothermal deposits from the jade site in the okinawa trough which contain on average significantly higher concentrations of pb zn sb as and ag than each of these deposits in spite of the greater similarity in tectonic setting of the hokuroku basin in which the kuroko deposits formed to the okinawa trough intracontinental rifted back arc basin compared to myojin knoll and suiyo seamount active arc volcanoes it appears that submarine hydrothermal deposits from myojin knoll and suiyo seamount are closer analogues of the kuroko deposit than are those from the okinawa trough the present data are consistent with the magmatic hydrothermal model for the formation of kuroko type deposits as formulated by urabe and marumo urabe t marumo k 1991 a new model for kuroko type deposits of japan episodes 14 246 251

discovery of hydrothermal sulfide mineralization from southern kermadec arc volcanoes sw pacific abstract we report the discovery of hydrothermal sulfide mineralization within the summit caldera of two southern kermadec frontal arc volcanoes brothers and rumble ii west of the 1200 km long kermadec havre arc back arc system the brothers and rumble ii west calderas both with re surgent domes and comprising mostly dacite and basalt andesite host rocks respectively rise to water depths of 1500 m these calderas comprise mostly effusive lavas and volcaniclastic deposits including talus breccias along their inner caldera walls these two southern kermadec hydrothermal sites have similar geological settings and mineralogy to other arc front vent sites two principle ore assemblages are identified comprising i chalcopyrite pyrite barite and ii sphalerite marcasite barite pyrite the most prevalent ore texture consists of massive chalcopyrite pyrite sphalerite commonly overprinting barite early marcasite cores within large euhedral pyrites and trails of pyrite inclusions within chalcopyrite are indicative of recrystallization the ore mineralogy is consistent with postulated venting temperatures of 300 c sulfide geochemistry is consistent with the ore mineralogy with concentrations of cu fe and zn up to 15 3 19 1 and 18 8 wt respectively the sulfide geochemistry is distinct however from comparable western pacific vent sites and may reflect source heterogeneity due to proximity to continental new zealand and sediment recycling along the southern kermadec trench arc system the recovery of two partial caridean vent shrimps suggests that present day hydrothermal venting is occurring within the brothers caldera

transport of reef corals into the great barrier reef major submarine pumice eruptions have occurred repeatedly during the past 25 years in the tonga kermadec region1 3 pumice from this area drifts through fiji and new caledonia reaching australia roughly one year after an eruption1 2 coral larvae settle and grow on the floating pumice fragments fig 1 the geographic pattern of abundance and size distribution of rafted corals found on drift pumice in relation to the pumice source area suggests massive transport of corals into the great barrier reef from regions of lower diversity lying far to the east and southeast dispersal range is not limited by longevity of larvae and centres of diversity may be sites of species accumulation rather than sites of species origin

subglacial intermediate volcanism at kerlingarfjoll iceland magma water interactions beneath thick ice the products of andesite and dacite glaciovolcanism at kerlingarfjoll are unlike others previously described in the literature three sequences of lithofacies are described and interpreted here the andesitic deposits at campsite gully are divided into massive vitriclastic lapilli tuff with intrusions fluidal clast bearing vitriclastic lapilli tuff and stratified pumice rich vitriclastic lapilli tuff at tindur andesitic eruptions produced contorted pillow fragment breccia and the clast supported lithic breccia finally pillow lava with intrusions and crudely bedded vitriclastic lapilli tuff of dacitic composition are described at haraldur abundant vitriclasts the presence of pillow lavas hackly fracturing of lava bodies and lack of oxidation of clasts demonstrate that each of these lithofacies formed in the presence of abundant water this contrasts with all other descriptions of subglacially erupted intermediate magmas which are characterised by jointed and glassy lava flows and domes with a marked scarcity of fragmental material the kerlingarfjoll sequences therefore demonstrate that it is possible for intermediate magmas to generate and interact with significant volumes of water at the base of a glacier preliminary estimates of volatile contents in glassy clasts correspond to quenching pressures equivalent to 500 m water or 550 m ice this is consistent with eruption beneath an ice sheet that was thick enough to overwhelm the underlying topography and where meltwater drainage was controlled by the morphology of the glacier surface it is argued that the drainage of water due to steep topography and or thin and fractured ice as opposed to thermodynamic considerations is the most likely explanation for the absence of evidence for significant magma water interaction in previously described instances of intermediate glaciovolcanism the apparent low viscosity of the kerlingarfjoll magmas may relate to relatively high eruption temperatures and or the inhibition of degassing of the magma due to high ambient pressures and the consequent limitation of groundmass crystallisation

observations of eruptive plume dynamics and pyroclastic deposits from submarine explosive eruptions at nw rota 1 mariana arc abstract observations at the submarine volcano nw rota 1 mariana arc have provided a unique opportunity to study basaltic explosive eruptions at 550 m below sea level in particular during one week of observations in 2006 the active vent evolved from a region of diffuse gas venting and slow lava effusion to a focused vent that emitted a nearly continuous stream of gas with entrained juvenile pyroclasts the eruptions were small strombolian bursts that ejected bomb sized pyroclasts meters above the vent and produced buoyant plumes that rose tens of meters above the vent and contained variable amounts of ash sampled pyroclasts are dominantly coarse ash and lapilli with fluidal textures that provide evidence of magmatic fragmentation however the blocky morphologies of some clasts combined with visual evidence of secondary fragmentation suggest that quench granulation due to rapid cooling was also important in pyroclast formation rapid cooling is also suggested by the slow rise of buoyant plumes which indicates a low heat content and small density contrast with surrounding seawater rapid plume deceleration in seawater limited deposition of coarse ash lapilli and bombs to within a few meters of the vent and allowed recycling of 15 of the ejected clasts petrologic evidence for pyroclast recycling is provided by numerous microcrystalline inclusions incorporated within microlite poor matrix glass the matrix glass within these inclusions is strongly enriched in chlorine

the fukuyama volcanic rocks submarine composite volcano in the late miocene to early pliocene akita yamagata back arc basin northeast honshu japan abstract the fukuyama volcanic rocks are composed of pyroxene andesite fkv 1 hornblende pyroxene andesite fkv 2 biotite hornblende dacite fkv 3 and volcaniclastic debris flow deposits and or turbidites fkv 1 fkv 2 and fkv 3 are medium k calc alkaline rocks depleted in nd similar to other back arc volcanic rocks of the northeast japan arc and constitute a dome cluster at fukuyama volcaniclastic beds surround the dome cluster and thin and fine upwards the predominant clast type in the volcaniclastic beds changes upwards from pyroxene andesite through hornblende pyroxene andesite to biotite hornblende dacite consistent with the stratigraphic relationships of fkv 1 fkv 2 and fkv 3 lavas all the siltstones inter bedded with the volcaniclastic beds and overlying the whole succession contain diatom fossils indicative of the lower part of the thalassionema schraderi zone 7 8 ma to 8 5 ma compatible with the isotopic ages of fkv 1 fkv 2 and fkv 3 the fukuyama volcano has a total eruption volume of 60 100 km3 with a lifetime of the order of 105 years as typically observed for volcanoes in the present back arc region of northeast honshu fkv 1 erupted in deep water and partly disintegrated into hyaloclastite breccias due to direct contact with water fkv 2 lava repeatedly effused over the fkv 1 lava and produced a volcanic apron of breccias that eventually grew above wave base and was eroded by wave action the magma of fkv 3 was probably hydrous as it contains biotite and hornblende the fkv 3 magma could have explosively erupted from a shallow water dome or vent emergent above the wave base followed by growth of a degassing lava dome repose between eruptions allowed accumulation of silt and after the fukuyama eruptions ceased silt entirely mantled the volcano a small magma supply rate perhaps allowed a relatively long period of quiescence between eruptions of fkv 1 fkv 2 and fkv 3 magmas resulting in abrasion and reworking of volcanic fragments and accumulation of non volcanic sediments that constitute part of the volcaniclastic apron oil generated in the contemporaneous or underlying sediments migrated to the upper levels of the volcaniclastic apron and accumulated there under the thick cover of fine grained sediments that mantle the volcano

experimental interaction of magma and dirty coolants the presence of water at volcanic vents can have dramatic effects on fragmentation and eruption dynamics but little is known about how the presence of particulate matter in external water will further alter eruptions volcanic edifices are inherently dirty places where particulate matter of multiple origins and grainsizes typically abounds we present the results of experiments designed to simulate non explosive interactions between molten basalt and various coolants ranging from homogeneous suspensions of 0 to 30 mass bentonite clay in pure water to heterogeneous and or stratified suspensions including bentonite sand synthetic glass beads and or naturally sorted pumice four types of data are used to characterise the interactions 1 visual video observations 2 grainsize and morphology of resulting particles 3 heat transfer data from a network of eight thermocouples and 4 acoustic data from three force sensors in homogeneous coolants with 20 sediment heat transfer is by forced convection and conduction and thermal granulation is less efficient resulting in fewer blocky particles larger grainsizes and weaker acoustic signals many particles are droplet shaped or and vesicular containing bubbles filled with coolant both of these particle types indicate significant hydrodynamic magma coolant mingling and many of them are rewelded into compound particles the addition of coarse material to heterogeneous suspensions further slows heat transfer thus reducing thermal granulation and variable interlocking of large particles prevents efficient hydrodynamic mingling this results primarily in rewelded melt piles and inefficient distribution of melt and heat throughout the coolant volume our results indicate that even modest concentrations of sediment in water will significantly limit heat transfer during non explosive magma water interactions at high concentrations the dramatic reduction in cooling efficiency and increase in mingling help to explain globular peperite and provide information relevant to analyses of premixing associated with highly explosive molten fuel coolant interactions in debris filled volcanic vents

pele s tears and spheres examples from kilauea iki abstract pele s tears are a well known curiosity commonly associated with low viscosity basaltic explosive eruptions however detailed studies of these pyroclasts are rare and thus there is no full explanation for their formation these intriguing pyroclasts have smooth glassy surfaces vesiculated interiors 30 and fluidal morphologies trending towards teardrops and then spheres as they decrease in size to