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

melt viscosity temperature and transport processes troodos ophiolite cyprus abstract the lava section in the troodos ophiolite cyprus is chemically stratified and divided into a shallow lava sequence with low tio 2 content and a deeper lava sequence with high tio 2 content we calculate the viscosity at magmatic temperature based on major element chemistry of lavas in cyprus crustal study project ccsp holes cy 1 and 1a we find that typical shallow low ti lavas have a magmatic viscosity that is two to three orders of magnitude lower than that of the deeper high ti lavas this implies that after eruption on axis troodos low ti lavas would have been able to flow down the same slope faster and farther than high ti lavas the calculated lava viscosity increases systematically from the lava sediment interface to the bottom of the composite hole cy 1 1a this suggests that an efficient process of lava segregation by viscosity on the upper flanks of the paleo troodos rise may have been responsible for the chemical stratification in the troodos lava pile calculated magmatic temperature and molar mg mg fe or mg decrease systematically down section while sio 2 content increases correlation of mg in the lavas with mg in the underlying lower crustal plutonic rocks sampled by ccsp hole cy 4 shows that the shallow lavas came from a high temperature lower crustal magma reservoir which is now represented by high mg pyroxenite cumulates while the deeper lavas were erupted from a lower temperature mid crustal reservoir which is now represented by gabbroic cumulates with lower mg

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

yield criteria for glaciotectonically deformed deposits abstract most glaciotectonically deformed deposits including varved clays and glacial tills are characterised by cracks and fissures this paper presents a method for describing the yield criteria for glacitectonically deformed cohesive deposits using a model of cracked geomaterial with isotropic or anisotropic matrix the general representation of the limit conditions for anisotropic materials in plane strain is used to determine the yield criterion the yield criterion represents a convex piece wise surface in the three dimensional stress space revealing explicitly global plastic properties of the materials considered an example of using proposed yield criteria to solve a bearing capacity problem of a strip foundation constructed on a glaciotectonically cracked layer is presented the lower and upper bound estimates of limit loads on the strip footing are given the limit state analysis presented in this paper can be used to solve many other geotechnical engineering problems for example the stability of slopes and reinforced walls or the bearing capacity of pile foundations

accumulations of melt at the base of young oceanic crust as hot solid rock ascends through the mantle the pressure around it decreases while its temperature decreases approximately adiabatically during the ascent the melting temperature decreases faster than the adiabat temperature and if the initial temperature of the rock is high enough the rock will begin to melt at the depth where the adiabat crosses the melting temperature curve models of upwelling beneath mid ocean ridges predict significant melt production over a zone several tens of kilometres wider than the ridge axis region suggesting that some of the ascending melt must arrive away from the ridge axis here i report ocean bottom seismograph observations of an uncommon and previously unreported marine seismic phase which provide strong evidence that melt accumulates in sills at the base of young oceanic crust the phase results from the conversion of compressional p waves to shear s waves on interaction with the crust mantle boundary the moho this phase is rare and very difficult to model but when visible it is strikingly prominent and usually appears at unexpectedly great source to receiver ranges the observations imply the presence of bodies of extremely low rigidity presumably melt sills at the moho close to the receiver melt may be fairly mobile at the base of the crust allowing ridges to accumulate melt through a lateral plumbing system at the locally uplifted boundary between the crust and the underlying thermally expanded mantle escape of this melt to the sea floor may be responsible for the formation of off axis seamounts

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

trace element systematics and ore forming processes in mafic vms deposits evidence from the troodos ophiolite cyprus abstract the volcanogenic massive sulfide vms deposits in the troodos ophiolite cyprus are ancient analogues for modern day seafloor massive sulfide mineralisation formed in a supra subduction zone environment in this study we present the first detailed in situ study of trace elements in sulfides from twenty vms deposits hosted in the troodos ophiolite to better understand factors that influence the distribution enrichment and incorporation of trace elements in different sulfide minerals on a mineral scale trace elements exhibit systematic variations between pyrite chalcopyrite and sphalerite pyrite preferentially incorporates as sb au and te whilst chalcopyrite is enriched in co and se sphalerite is trace element poor with the exception of ag and cd selenium averages 278 ppm n 150 in chalcopyrite but only 42 ppm n 1322 in pyrite bismuth and te in pyrite show a weak positive correlation r2 0 35 in some vms deposits possibly linked with the occurrence of bi telluride inclusions trace element concentrations also vary between colloform and euhedral pyrite with an enrichment of au as sb cu and zn in colloform compared to euhedral pyrite time resolved laser ablation profiles reveal that the trace element distribution on a mineral scale is not uniform and varies with crystallographic effects fluctuating physicochemical fluid conditions such as temperature ph fs2 fo2 and ligand availability during sulfide precipitation incorporation mechanisms in sulfides differ between elements in pyrite ag as se and pb are hosted in solid solution or as nanoscale inclusions whilst au sb and te may form micro scale inclusions on a regional scale 20 km the distribution of trace elements exhibits systematic variations between three major structural domains namely the solea mitsero and larnaca grabens the vms deposits of the magmatic tectonic solea graben are enriched in se co te au and cu relative to mitsero which is a purely extensional feature therefore we hypothesise that a variable magmatic volatile influx related to a magma volume b migration of the magmatic hydrothermal crack front and associated brine liberation or c a variation in protolith metal concentration are responsible for regional scale variations in vms geochemistry this is suggested to be intrinsically linked to the spreading architecture of troodos

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

geology and eruptive history of bogoslof volcano bogoslof volcano is a shallow submarine subaerial volcano in the southern bering sea about 100 km west of the community of dutch harbor alaska the subaerial parts of the volcano consist of two small islands bogoslof island and fire island that together have a total area of about 1 6 km2 bogoslof was first depicted on a russian map in 1772 and since then has been observed and visited occasionally the volcano has had at least nine periods of eruptive activity since 1796 and all of its historical eruptions have been similar in style historical bogoslof eruptions involved the effusion of basalt trachybasalt basaltic trachyandesite and trachyandesite lava domes with above sea level relief of 100 200 m many of the eruptions are accompanied by the formation of tuff rings and ejection of ballistic particles historical observations suggest that eruption clouds are relatively ash poor minor ash fallout has typically occurred within about 100 km of the volcano many of the historical eruptions began at vents that were below sea level and thus seawater has played an important role in the style of eruptive activity exhibited by the volcano at times eruptive activity has been characterized by surtseyan style eruptions and magma interaction with wet vent fill deposits at other times the eruptive style has been more magmatically driven and has resulted in the formation of pyroclastic flows and small ash clouds preliminary studies of the deposits produced during the 2016 2017 eruption indicate vertical sequences of coarse grained horizontally bedded pyroclastic flow and fall deposits with numerous blocks bombs and lapilli of dense juvenile and accidental lithic material these deposits were emplaced by near vent pyroclastic flows surges and explosions some of which originated from shallow highly crystalline cryptodomes

dike surface lineations as magma flow indicators within the sheeted dike complex of the troodos ophiolite cyprus mesoscopic flow lineations and anisotropy of magnetic susceptibility ams have been measured for dikes within the cretaceous age troodos ophiolite with the goal of comparing the direction of initial magma flow through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth dike margin indicators of flow include cusp axes and elongate vesicles found high in the ophiolite pseudostratigraphy and ridge and groove structures termed hot slickenlines found throughout the complex a unique flow direction is determined where elongate vesicles near dike margins display imbrication with respect to the margin significant changes in vesicle elongation directions across dikes likely indicate either changes in magma flow direction after dike propagation or back flow of magma during the waning stages of intrusion surface lineations generally lie subparallel to the direction of flow inferred from ams determinations on cores within 5 cm of dike margins surface lineations also lie subparallel to the long axis e1 of the orientation ellipsoid defined by long axes of groundmass plagioclase phenocrysts measured in sections from ams cores correlation of surface lineations with interior indicators of flow ams plagioclase trachytic texture indicate that the surface features are good proxies for grain scale magma flow directions during dike propagation in troodos dikes orientations of surface flow features in the dikes of the troodos ophiolite indicate an approximately equal mix of subhorizontal to near vertical magma flow contradicting the paradigm of primarily vertical flow of magma beneath continuous axial magma chambers at oceanic spreading centers our data are consistent with a model of magma emplacement both vertically and horizontally away from isolated magma chambers beneath axial volcanoes spaced along a ridge crest

subaqueous eruption fed density currents and their deposits density currents fed directly from subaqueous eruptions can be divided conceptually into three groups based on modes of fragmentation and transport i explosive fragmentation with deposition from a gas supported current ii explosive fragmentation with deposition from a water supported current iii fragmentation of flowing lava with deposition from a water supported current fig 1 group i products include subaqueously emplaced welded ignimbrite and other high temperature emplaced subaqueous pyroclastic flow deposits group ii products are the most varied and include representatives of both high and low concentration turbidity currents grainflows and debris flows and are termed eruption fed aqueous density currents some clasts in such currents are transported and even deposited at high temperature but the transporting currents ranging from grain flows to dilute turbidity currents are water dominated even though steam may be developed along large clasts margins group iii products formed from lava flow fed density currents tend to be weakly dispersed down gradient along the seafloor and generally consist largely of fragments formed by dynamo thermal quenching and spalling bursting of bubbles formed by vapor expansion probably contributes to some group iii beds distinctive column margin fall deposits may form in water excluded zones that developed very locally around vents in association with group i and ii deposits and are distinguished by heat retention structures indicators of gas phase transport and absence of current formed depositional features 2000 elsevier science b v all rights reserved

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

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

experimental and textural investigation of welding effects of compaction sintering and vapor phase crystallization in the rhyolitic rattlesnake tuff abstract the abrupt changes in character of variably welded pyroclastic deposits have invited decades of investigation and classification we conducted two series of experiments using ash from the nonwelded base of the rhyolitic rattlesnake tuff of oregon usa to examine conditions of welding one series of experiments was conducted at atmospheric pressure 1 at in a muffle furnace with variable run times and temperature and another series was conducted at 5 mpa and 600 c in a cold seal apparatus with variable run times and water contents we compared the results to a suite of incipiently to densely welded natural samples of the rattlesnake tuff experiments at 1 at required a temperature above 900 c to produce welding which is in excess of the estimated pre eruptive magmatic temperature of the tuff the experiments also yielded globular clast textures unlike the natural tuff during the cold seal experiments the gold sample capsules collapsed in response to sample densification textures and densities that closely mimic the natural suite were produced at 5 mpa 600 c and 0 4 wt h2o over run durations of hours to 2 days clast deformation and development of foliation in 2 week runs were greater than in natural samples both more and less water reduced the degree of welding at otherwise constant run conditions for 5 mpa experiments changes in the degree of foliation of shards and of axial ratios of bubble shards and non bubble mainly platy shards are consistent with early densification related to compaction and partial rotation of shards into a foliation subsequent densification was associated with viscous deformation as indicated by more sintered contacts and deformation of shards sintering local fusion of shard shard contacts was increasingly important with longer run times higher temperatures and greater pressures during runs with high water concentrations sintering was rare and adhesion between clasts was dominated by precipitation of sublimates in pore spaces a few tenths wt h2o in the rhyolite glass promote the development of welding by sharp reduction of glass viscosity large amounts of water inhibit welding by creating surface sublimates that interfere with sintering and may exert fluid pressure counter to lithostatic load if sintering and vapor phase sublimates seal permeability in the tuff

the submarine eruption of the bombarda volcano milos island cyclades greece the milos volcanic field includes a well exposed volcaniclastic succession which records a long history of submarine explosive volcanism the bombarda volcano a rhyolitic monogenetic center erupted 1 7 ma at a depth 200 m below sea level the aphyric products are represented by a volcaniclastic apron up to 50 m thick and a lava dome the apron is composed of pale gray juvenile fragments and accessory lithic clasts ranging from ash to blocks the juvenile clasts are highly vesicular to non vesicular the vesicles are dominantly tube vesicles the volcaniclastic apron is made up of three fades massive to normally graded pumice lithic breccia stratified pumice lithic breccia and laminated ash with pumice blocks we interpret the apron beds to be the result of water supported volcaniclastic mass how emplacement derived directly from the collapse of a small volume subaqueous eruption column and from syn eruptive down slope resedimentation of volcaniclastic debris during this eruptive phase the activity could have involved a complex combination of phreatomagmatic explosions and minor submarine effusion the lava dome emplaced later in the source area is made up of flow banded lava and separated from the apron by an obsidian carapace a few meters thick the near vertical orientation of the carapace suggests that the dome was intruded within the apron remobilization of pyroclastic debris could have been triggered by seismic activity and the lava dome emplacement

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

cumulate recycling and crustal evolution in the bay of islands ophiolite the lower crustal section of the north arm mountain massif of the bay of islands ophiolite is composed of interlayered gabbroic and ultramafic cumulates despite pervasive ridge related deformation key contact relationships are preserved in low strain domains or late kinematic intrusions angular and crenulated gabbroic enclaves in ultramafic rocks and ultramafic dikes in gabbroic rocks imply that many of the ultramafic rocks intruded into older gabbroic hosts olivine gabbro gabbro gabbro norite norite and fe oxide amphibole gabbros the ultramafic intrusions are composed of dunite wehrlite harzburgite lherzolite and olivine websterite clino to ortho pyroxenite reaction rims line many peridotitic gabbroic contacts the voluminous pyroxenites of the lower crust are petrologically identical to in situ pyroxenite reaction rims and are thought to represent partially digested gabbros fine grained harzburgitic sills crosscut the foliation of wehrlitic host rocks with orthopyroxenite reaction rims

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

the role of external water on rapid cooling and fragmentation of magma abstract the cooling rate of magma in the presence of external water during phreatomagmatic and submarine eruptions is one of the key parameters governing non explosive to explosive magma fragmentation and eruption transitions but remains poorly constrained combining results from laboratory experiments with realistic eruptive temperatures of magma cooling in ambient water of variable temperatures and numerical modeling of transient heat transfer we find that magma to water heat flux can be up to 4 10 6 w m 2 the experiments exhibit two distinct water boiling regimes a film boiling regime defined by the presence of a coherent water vapor film between magma surface and ambient water and a nucleate boiling regime below a critical magma surface temperature known as the leidenfrost temperature where the vapor film breaks and numerous bubbles form at the magma water interface in general the vapor film was stable in our experiments for time scales of 5 s indicating that this might be a limiting factor in pre explosion magma water mixing for energetic molten fuel coolant interaction and explosive volcanic eruptions the time scale of vapor film stability increases and the leidenfrost temperature 1223 to 948 k decreases with increasing water temperature 276 to 366 k we show that for the empirically obtained large heat flux to external water the cooling rate of magma can reach up to 106 k s 1 at length scales of few microns thus magma may undergo fine fragmentation due to quench induced large thermal stresses our experimental and modeling results demonstrate that the time scales of various water boiling regimes and erupting magma and ambient water temperatures determine the magma to water heat transfer rates which in turn determine the transition to explosivity under subaqueous eruption conditions

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

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

volcanic glass compositions of the troodos ophiolite cyprus fresh volcanic glass is preserved throughout the extrusive section of the troodos ophiolite which indicates that the lavas have not been pervasively metamorphosed glass compositions reveal the existence of two major magma suites apparently corresponding to distinct stratigraphic intervals the basal 400 500 m of the sequence consists of an andesite dacite rhyolite assemblage containing abundant hyaloclastites the remainder of the section comprises a basalt basaltic andesite assemblage with high mgo and low tio 2 and total iron the lower sequence is interpreted as an evolved arc tholeiite suite the upper has some similarities to boninitic lavas the close association in time and space of these two suites is similar to that observed in the mariana and bonin arcs and suggests that all of the troodos lavas were erupted in a subduction zone environment most probably in an incipient arc or fore arc

effects of thermal quenching on mechanical properties of pyroclasts contact with water can promote magma fragmentation obsidian chips and glass spheres typically crack when quenched vesicular pyroclasts are made of glass so thermal quenching by water may damage them if water enters eruption columns or if pyroclastic density currents interact with water hot pumice can be quenched we performed a set of experiments on air fall pumice from medicine lake california we made quenched samples by heating natural clasts to 600 c quenching them in water at 21 c drying them at 105 c and then cooling them to room temperature we compare these samples with untreated air fall pumice from the same deposit hereafter referred to as regular pumice we tested whether quenched pumice would 1 shatter more easily in collisions and 2 abrade faster we also tested whether individual clasts lose mass upon quenching and whether they increase in effective wet density two measurements which may help characterize the magnitude of clast damage during quenching we also compare pre quenching and post quenching textures using x ray microtomography xrt images results from collision experiments show no clear difference between quenched pumice and regular pumice quenched pumice abraded faster than regular pumice on average 0 3 of mass may have been lost during quenching effective wet density increased 1 5 on average as measured after 5 minutes of immersion in water overall we find modest differences between quenched pumice and regular pumice in experiments and measurements the experimental results imply that quenching may damage small parts of a clast but tends not to cause cracks that propagate easily through the clast post quenching xrt imaging shows no obvious change in clast texture this is in stark contrast to non vesicular glass that develops large cracks on quenching we present four factors that explain why pumice is resistant to damage from thermal quenching thin glass films experience lower transient thermal stresses many internal surfaces are initially vapor cooled vesicles can arrest cracks and cracks from thermal quenching may not occur in the locations most susceptible to fracture in later collisions

grain size distribution of volcaniclastic rocks 2 characterizing grain size and hydraulic sorting quantification of the grain size distribution of sediments allows interpretation of processes of transport and deposition jutzeler et al 2012 developed a technique to determine grain size distribution of consolidated clastic rocks using functional stereology allowing direct comparison between unconsolidated sediments and rocks here we develop this technique to characterize hydraulic sorting and infer transport and deposition processes we compare computed grain size and sorting of volcaniclastic rocks with field based characteristics of volcaniclastic facies for which transport and depositional mechanisms have been inferred we studied pumice rich subaqueous facies of volcaniclastic rocks from the oligocene ohanapecosh formation ancestral cascades washington usa pliocene dogashima formation izu peninsula honshu japan miocene manukau subgroup northland new zealand and the quaternary sierra la primavera caldera jalisco state mexico these sequences differ in bed thickness grading and abundance of matrix we propose to evaluate grain size and sorting of volcaniclastic deposits by values of their modes matrix proportion 2 mm f 1 and d16 instead of median diameter d50 and standard deviation parameters f 1 and d16 can be uniformly used to characterize and compare sieving and functional stereology data volcaniclastic deposits typically consist of mixtures of particles that vary greatly in density and porosity hydraulic sorting ratios can be used to test whether inferred density of mixed clast populations of pumice and dense clasts are hydraulically sorted with each other considering various types of transport under water evaluation of this ratio for our samples shows that most studied volcaniclastic facies are deposited by settling from density currents and that basal dense clast breccia are emplaced by shear rolling these hydraulic sorting ratios can be applied to any type of clastic rocks and indifferently on consolidated and unconsolidated samples

magmatic cycles and formation of the upper oceanic crust at spreading centers geochemical study of a continuous extrusive section in the oman ophiolite 1 the data and analysis presented in this paper provide an assessment of lava morphologies and the geochemistry of lavas from the oman ophiolite in order to provide detailed constraints on the construction of the upper oceanic crust a continuous volcanic transect 300 m thick was sampled at high frequency in the semail ophiolite along wadi shaffan the wadi shaffan section is composed mainly of pillow lavas interbedded with massive flows and occasional hyaloclastites the sampling performed along wadi shaffan implies temporal variations in the activity of the ridge the section is characterized by chemical compositions consistent with those of v1 geotimes volcanism the wadi shaffan transect was built through two main petrological and geochemical sequences of volcanic activity trace element ratios e g zr nb and la yb allow us to distinguish two main sequences with two different parental magmas differences in the degree of partial melting are required to explain these trace element ratio variations beyond these differences in parent melt composition variations in trace element abundances tio2 zr ree involve differentiation processes prior to emplacement in the lower sequence less differentiated lavas are in the upper part of the cycle magma mixing is proposed to explain this reversed geochemical evolution through time in the upper sequence geochemical analysis suggests a different magma chamber process this sequence consists of multiple events of magma emplacement variations in trace element abundance suggest four magmatic cycles each magmatic cycle is characterized by primitive lavas evolving to more differentiated lavas with time the upper sequence lavas appear to be in equilibrium with clinopyroxene and lower sills from the mtz mantle crust transition zone and with lower gabbros we propose a model in which the upper sequence lavas were directly derived from the mtz and lower gabbro sills and then transported to the surface without interaction with higher crustal levels

facies architecture of a continental below wave base volcaniclastic basin the ohanapecosh formation ancestral cascades arc washington usa the 800 m thick oligocene ohanapecosh formation records voluminous sedimentation of volcanic clasts in the ancestral cascades arc washington state usa most volcaniclastic beds are dominated by angular pumice clasts and fiamme of andesitic composition now entirely devitrified and altered all beds are laterally continuous and have uniform thickness fine sandstone and mudstone beds have features typical of deposits from low density turbidity currents and suspension settling erosion surfaces cross beds and evidence of bi directional oscillatory currents i e wave ripples and swaley and hummocky cross stratification are almost entirely absent we infer that the setting was subaqueous and below wave base the abundance of angular pumice clasts crystals and dense volcanic clasts and the extreme thickness of several facies suggest they were derived from magmatic volatile driven explosive eruptions the extremely thick beds are ungraded or weakly graded and lack evidence of hot emplacement suggesting deposition from subaqueous water supported high concentration volcaniclastic density currents some of the thickest beds contain coarse rounded dense clasts at their base and are interbedded with accretionary lapilli bearing mudstone these beds are interpreted to be deposits from subaqueous density currents fed by subaerial pyroclastic flows that crossed the shoreline shallow basaltic intrusions and mafic volcanic breccia composed of scoria lapilli indicate the presence of intra basinal scoria cones that may have been partly subaerial the range in facies in the ohanapecosh formation is typical of below wave base continental lacustrine basins that form in proximity to active volcanic arcs and includes eruption fed and resedimented facies extreme instantaneous aggradation rates are related directly to explosive eruptions and sediment pathways reflect the locations of active volcanoes in contrast to conventional sedimentation processes acting in non volcanic environments

pumice rafting and faunal dispersion during 2001 2002 in the southwest pacific record of a dacitic submarine explosive eruption from tonga a new influx of sea rafted pumice reached the eastern coast of australia in october 2002 approximately 1 year after a felsic shallow marine explosive eruption at a previously unknown volcano 0403 091 along the tofua volcanic arc tonga the eruption produced floating pumice rafts that first became stranded in fiji in november 2001 approximately i month after the eruption strandings of sea rafted pumice along shorelines have been the only record of products from this submarine explosive eruption at the remote submerged volcano computed drift trajectories of the sea rafted pumice using numerical models of southwest pacific surface wind fields and ocean currents indicate two cyclonic systems disturbed the drift of pumice to eastern australia as well as the importance of the combined wave and direct wind effect on pumice trajectory pumice became stranded along at least two thirds 2000 km of the coastline of eastern australia being deposited on beaches during a sustained period of fresh onshore winds typical amounts of pumice initially stranded on beaches were 500 4000 individual clasts per in and a minimum volume estimate of pumice that arrived to eastern australia is 1 25 x 10 5 m 3 pumice was beached below maximum tidal storm surge levels and was quickly reworked back into the ocean such that the concentration of beached pumice rapidly dissipated within weeks of the initial stranding and little record of this stranding event now exists most stranded pumice clasts ranged in size from 2 to 5 cm in diameter the largest measured clasts were 10 cm in australia and 20 cm in fiji the pumice has a low phenocryst content 3500 km and period of pumice floatation greater than or equal to1 year confirm the importance of sea rafted pumice as a long distance dispersal mechanism for marine organisms including marine pests and harmful invasive species billions of individual rafting pumice clasts can be generated in a single small volume eruption such as observed here and the geological implications for the transport of sessile taxa over large distances are significant an avenue for future research is to examine whether speciation events and volcanicity are linked the periodic development of globalism for some taxa e g corals gastropods bryozoa may correlate in time and or space with voluminous silicic igneous events capable of producing 10 6 km 3 of silicic pumice rich pyroclastic material and emplaced into ocean basins c 2004 elsevier b v all rights reserved

magmas and magma chamber evolution troodos ophiolite cyprus the troodos ophiolite traditionally has been considered to have formed by constructive processes at an oceanic spreading center new information on the phase layering and cryptic variation in gabbros and ultramafic cumulates overlying tectonized harzburgites however does not support the existence of a large steady state frequently replenished axial magma chamber compositional evidence suggests that the troodos ophiolite was formed by two distinct and chemically unrelated magmatic systems a stratigraphically lower websteritic and gabbroic cumulate sequence exhibits only one major magma chamber replenishment suggesting a nonspreading environment an upper gabbroic cumulate sequence suggests frequent magma chamber replenishments pointing to a relatively fast spreading environment this suggests that the alleged troodos axial magma chamber formed in an off axis nonspreading tectonic environment probably by magmatic underplating of a thin oceanic crust spreading related magmatic activity in the troodos ophiolite was confined to the upper gabbros and dike complex

the early development of an archean submarine caldera complex with emphasis on the mattabi ash flow tuff and its relationship to the mattabi massive sulfide deposit the south sturgeon lake area of northwestern ontario is underlain by an archean submarine caldera complex which is host to six massive sulfide deposits the complex is approximately 30 km in strike length and is partly filled by mesobreccias subaqueous debris flow deposits and other epiclastic rocks as well as five major ash flow tuff units the most voluminous of these the mattabi ash flow tuff is up to 800 m in thickness and can be traced for more than 15 km across the caldera complex based on flow morphology and composition the mattabi ash flow tuff is divisible into two distinct units a bedded quartz crystal and pumice rich unit 150 650 m thick and a massive to poorly bedded ash unit 20 150 m thick the bedded unit is host to the mattabi massive sulfide deposit as well as numerous other massive sulfide occurrences in addition this unit exhibits a pronounced stratigraphic and geochemical cyclicity individual beds flow units can be subdivided into basal quartz and pumice rich lower sections which are overlain by bedded ash tops thicknesses of individual beds vary from 14 to 155 m each bed also exhibits a pronounced zonation of trace elements with high concentrations of zr y and nb at and or near the base gradually decreasing upward into the ash tops the overlying massive ash unit contains no known massive sulfide mineralization and lacks any pronounced geochemical zoning the bedded nature and geochemical zonation of individual flow units can be explained by periodic eruptions from a zoned or layered recharged siliceous magma chamber the difference in flow morphology between the bedded unit and the overlying massive ash flow tuff most likely reflects the deeper submarine nature of the eruptive vents associated with the bedded deposits the periodic and subaqueous nature of the mattabi eruptions is indicated by the presence of exhalites and massive sulfide lenses between flow units the ore lenses which comprise the mattabi deposit formed between eruptive events from high temperature fluids whose origin may be related to processes within the magma chamber

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

evolution of submarine eruptive activity during the 2011 2012 el hierro event as documented by hydroacoustic images and remotely operated vehicle observations submarine volcanic eruptions are frequent and important events yet they are rarely observed here we relate bathymetric and hydroacoustic images from the 2011 to 2012 el hierro eruption with surface observations and deposits imaged and sampled by rov as a result of the shallow submarine eruption a new volcano named tagoro grew from 375 to 89 m depth the eruption consisted of two main phases of edifice construction intercalated with collapse events hydroacoustic images show that the eruptions ranged from explosive to effusive with variable plume types and resulting deposits even over short time intervals at the base of the edifice rov observations show large accumulations of lava balloons changing in size and type downslope coinciding with the area where floating lava balloon fallout was observed peaks in eruption intensity during explosive phases generated vigorous bubbling at the surface extensive ash vesicular lapilli and formed high density currents which together with periods of edifice gravitational collapse produced extensive deep volcaniclastic aprons secondary cones developed in the last stages and show evidence for effusive activity with lava ponds and lava flows that cover deposits of stacked lava balloons chaotic masses of heterometric boulders around the summit of the principal cone are related to progressive sealing of the vent with decreasing or variable magma supply hornitos represent the final eruptive activity with hydrothermal alteration and bacterial mats at the summit our study documents the distinct evolution of a submarine volcano and highlights the range of deposit types that may form and be rapidly destroyed in such eruptions

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

in situ granulation by thermal stress during subaqueous volcanic eruptions some of the most complex volcanic thermodynamic processes occur when erupting magma interacts with water in shallow water surtseyan eruptions are spectacular and they efficiently fragment magma into fine ash particles the aviation hazard from these eruptions depends the amount of transportable fine ash that is generated and whether it is aggregated into particle coatings or accretions to investigate both mechanisms we analyzed ash encased lapilli from the surtseyan eruptions of capelinhos azores 1957 1958 and hunga tonga hunga ha apai tonga 2014 2015 using x ray computed microtomography and electron microscopy we discovered pyroclasts that were not coated sensu stricto but had enveloping ash produced by in situ granulation of the particle surface this was caused by thermal stress as pyroclasts briefly traveled through water and were quenched during eruption in situ granulation is thus an important secondary disruption process in shallow subaqueous eruptions our results imply that ash encasement is not always evidence of particle aggregation and accretion but it may also result from new ash formation shallow water conditions produce the most efficient ash generation conditions leading to the greatest hazard to downwind populations and air traffic

basaltic maar diatreme volcanism in the lower carboniferous of the limerick basin sw ireland lead zinc exploration drilling within the limerick basin sw ireland has revealed the deep internal architecture and extra crater deposits of five alkali basaltic maar diatremes these were emplaced as part of a regional north east south west tectonomagmatic trend during the lower carboniferous period field relationships and textural observations suggest that the diatremes erupted into a shallow submarine environment limerick trace element data indicates a genetic relationship between the diatremes and extra crater successions of the knockroe formation which records multiple diatreme filling and emptying cycles deposition was controlled largely by bathymetry defined by the surrounding waulsortian carbonate mounds an initial non diatreme forming eruption stage occurred at the water sediment interface with magma water interaction prevented by high magma ascent rates this was followed by seawater incursion and the onset of phreatomagmatic activity magma water interaction generated poorly vesicular blocky clasts although the co occurrence of plastically deformed and highly vesicular clasts indicate that phreatomagmatic and magmatic processes were not mutually exclusive at a later stage the diatreme filled with a slurry of juvenile lapilli and country rock lithic clasts homogenised by the action of debris jets the resulting extra crater deposits eventually emerged above sea level so that water ingress significantly declined and late stage magmatic processes became dominant these deposits largely confined to the deep vents incorporate high concentrations of partially sintered globular and large raggy lapilli showing evidence for heat retention our study provides new insights into the dynamics and evolution of basaltic diatremes erupting into a shallow water 20 120 m submarine environment

the lyngen gabbro the lower crust of an ordovician incipient arc we present evidence for the origin of the lyngen gabbro of the ordovician lyngen magmatic complex in troms northern norway the two magmatic suites of the lyngen gabbro strike parallel nne ssw and have distinct magmatic signatures we define these signatures by using major and trace element analyses together with selected major and trace element mineral analyses and 143nd 144nd isotope whole rock analyses of gabbroic to tonalitic plutonic rocks from seven detailed cross sections from this large gabbro complex the western suite of the lyngen gabbro precipitated from magma that may have been derived from the same system as the associated volcanic rocks the gabbros have high an content an 90 of their plagioclases relative to co existing mafic minerals together with somewhat high nd t values 6 this implies that the parental magmas were hydrous tholeiites similar to those found in back arc basins today the eastern suite on the other hand consist of cumulates that were precipitated from melts resembling those of ultra depleted high ca boninitic magmas found in fore arcs extremely high an plagioclases an 95 co exist with evolved mafic minerals and oxides and the nd t values are lower 4 than in the western suite the eastern suite has no volcanic counterpart but dikes intersecting the suites have compositions that possibly represent its parental magma the oceanic rypdalen shear zone generally separates the two suites in the north but several non tectonic transitions from boninitic to tholeiitic signatures southwards advocate that the magmatism happened concurrently the magmatic proximity between the suites the hydrous magmatism and the absence of a silicic or calc alkaline mature arc section suggests that the lyngen gabbro formed in the iapetus ocean under conditions presently found in incipient arcs later emplaced as outer arc highs

facies architecture and late pliocene pleistocene evolution of a felsic volcanic island milos greece the volcanic island of milos greece comprises an upper pliocene pleistocene thick up to 700 m compositionally and texturally diverse succession of calc alkaline volcanic and sedimentary rocks that record a transition from a relatively shallow but dominantly below wave base submarine setting to a subaerial one the volcanic activity began at 2 66 0 07 ma and has been more or less continuous since then subaerial emergence probably occurred at 1 44 0 08 ma in response to a combination of volcanic constructional processes and fault controlled volcano tectonic uplift the architecture of the dominantly felsic intermediate volcanic succession reflects contrasts in eruption style proximity to source depositional environment and emplacement processes the juxtaposition of submarine and subaerial facies indicates that for part of the volcanic history below wave base to above wave base and shoaling to subaerial depositional environments coexisted in most areas the volcanic facies architecture comprises interfingering proximal near vent medial and distal facies associations related to five main volcano types 1 submarine felsic cryptodome pumice cone volcanoes 2 submarine dacitic and andesitic lava domes 3 submarine to subaerial scoria cones 4 submarine to subaerial dacitic and andesitic lava domes and 5 subaerial lava pumice cone volcanoes the volcanic facies are interbedded with a sedimentary facies association comprising sandstone and or fossiliferous mudstone mainly derived from erosion of pre existing volcanic deposits the main facies associations are interpreted to have conformable disconformable and interfingering contacts and there are no mappable angular unconformities or disconformities within the volcanic succession

submarine silicic volcanism of the healy caldera southern kermadec arc sw pacific i volcanology and eruption mechanisms the submarine healy volcano southern kermadec arc with a 2 2 5 km wide caldera is pervasively mantled with highly vesicular silicic pumice within a water depth of 1 150 1 800 m pumices comprise type 1 white light grey pumice with 30 mm vesicles and weak moderate foliation type 2 grey pumice with millimetre scale laminae flow banded foliation including stretched vesicles 55 mm in length and a minor finely vesicular type 3 pumice all types are sparsely porphyritic with undevitrified glassy groundmass 68 70 sio2 which is microlite and lithic free coexisting pyroxenes yield magma temperatures of 950 c pumice density is 0 5 g cm 3 and vesicularity is 78 83 vesicle size distributions for types 1 and 2 pumice range from 20 m to 20 mm with a strong power law relation with d 2 5 0 4 for vesicles 9 mpa will limit vesicularity to less than the observed 78 83 whereas pressure 6 mpa require a more shallower reconstruction of the edifice and larger volume syn eruptive collapse uniformly high vesicularity is interpreted as evidence of insulation within an eruption column comprising steam and hot pyroclasts most pyroclasts cool condensing and ingesting water into steam inflated vesicles and then sink progression into pyroclastic mode would expand the eruption column displace ambient water reduce the hydrostatic load and further promote vesiculation and fragmentation pyroclasts within the column would quench at these reduced pressures we argue that healy eruptions deeper than 1 000 m cannot be pyroclastic volumes for the lower and upper bounds of edifice size are 2 36 and 3 58 km3 respectively but do not account for intra caldera pumice fill these volumes are considered to be predominantly primary eruption output as shown by a dearth of accessory lithics in all pumice yielding at an average 81 vesicularity eruptive pumice volumes of between 10 and 15 km3 some pyroclasts may have risen to the sea surface and be a correlative of the sea rafted loisels pumice the latter occurs in some new zealand holocene beach sequences and has a estimated age of 590 80 calendar years

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

explosive destruction of a pliocene hot lava dome underwater dogashima japan transition from effusive to explosive volcanism is common during subaerial eruptions and here we demonstrate that this behavior is also possible underwater the pyroclastic facies produced underwater are distinctive and can be used to distinguish subaqueous from subaerial eruptions and depositional settings the pliocene dogashima formation izu peninsula japan is a pumice rich succession originally deposited in an open marine below wave base setting jutzeler et al 2014a a thick clast supported gray andesite breccia composed of very coarse dense andesite clasts with quenched margins was sourced from disintegration of an active lava dome overall the gray andesite breccia is gradationally to sharply overlain by thick graded clast supported white pumice breccia chiefly composed of angular pumice clasts and free broken crystals regional setting and distinctive facies show that this succession was produced by a fully underwater magmatic volatile driven pumice forming explosive eruption the gradational contact between the two breccias compositional similarities rare mingled clasts and fluidal textures in the gray andesite clasts suggest that the explosive eruption destroyed a hot lava dome and generated an eruption fed high concentration density current in most places the coarsest hot lava dome fragments were deposited first followed by the lower density white pumice clasts the low amount of fine 2 mm components well developed hydraulically controlled grading and sorting clast angularity and very coarse dome derived clasts some including well defined quenched margins and common fluidal textures distinguish the products of subaqueous effusive to explosive eruptions from their subaerial counterparts

mass transfer during sub seafloor alteration of the upper troodos crust cyprus five major alteration zones in the extrusive series and the sheeted dike complex of the troodos ophiolite are each characterized by a distinct elemental changes compared to the original composition and b secondary mineralogy the upper ca 300 m of the extrusive crust the highly oxidatedcold seawater alteration zone csa is strongly enriched in k2o and depleted in na2o it is followed downwards by alow temperature alteration zone 170 c which is most widespread in the troodos extrusives and where na2o and k2o are enriched the latter less strongly than in the csa zone three types ofhigh temperature alteration zones 440 c hta i iii are found in the sheeted dike complex all are marked by thorough leaching of k2o while the behavior of na2o e g unchanged in type iii and cao depleted in type i enriched in types ii iii is variable mass budgets of elemental changes are quantified by calibration of whole rock analyses via systematic stable element variations of fresh glasses found throughout the extrusive section the troodos extrusive crust and upper sheeted dike complex are a major sink for mgo k2o and na2o and a source for cao the overall scale of fluxes drastically exceeds estimates based on fresh basalt compositions from present ocean crust

fallout of pyroclastic debris from submarine volcanic eruptions volcanic fallout deposits on land being widespread and accessible for study have received much attention and have revealed a great deal about subaerial eruption mechanisms in contrast virtually nothing is known about equivalent deposits produced by submarine volcanoes despite the probable abundance of such material in today s oceans and in accreted volcanic arc terrains many submarine deposits may form by the fallout of debris to the sea floor downcurrent from the umbrella region of submarine eruption columns experiments on water saturated pumice and pieces of rock lithics show that particles settling to the sea floor at terminal velocities of 10 to 50 centimeters per second will display conspicuous bimodality of particle diameters pieces of pumice may be five to ten times as large as codeposited lithic fragments similar material erupted into the air and deposited on land displays less well developed bimodality pumice diameters are generally two to three times as large as associated lithics submarine fallout deposits are therefore distinctive and may be used to indicate a subaqueous origin for some of the great thicknesses of nonfossiliferous volcanic debris contained in ancient volcanic terrains worldwide whose environment of deposition has been uncertain

contrasting pyroclast density spectra from subaerial and submarine silicic eruptions in the kermadec arc implications for eruption processes and dredge sampling pyroclastic deposits from four caldera volcanoes in the kermadec arc have been sampled from subaerial sections raoul and macauley and by dredging from the submerged volcano flanks macauley healy and the newly discovered raoul sw suites of 16 32 mm sized clasts have been analyzed for density and shape and larger clasts have been analyzed for major element compositions density spectra for subaerial dry type eruptions on raoul island have narrow unimodal distributions peaking at vesicularities of 80 85 whereas ingress of external water wet type eruption or extended timescales for degassing generate broader distributions including denser clasts submarine erupted pyroclasts show two different patterns healy and raoul sw dredge samples and macauley island subaerial emplaced samples are dominated by modes at 80 85 implying that submarine explosive volcanism at high eruption rates can generate clasts with similar vesicularities to their subaerial counterparts a minor proportion of healy and raoul sw clasts also show a pink oxidation color suggesting that hot clasts met air despite 0 5 to 1 km of intervening water in contrast macauley dredged samples have a bimodal density spectrum dominated by clasts formed in a submarine eruptive style that is not highly explosive macauley dredged pyroclasts are also the mixed products of multiple eruptions as shown by pumice major element chemistry and the sea floor deposits reflect complex volcanic and sedimentation histories the kermadec calderas are composite features and wide dispersal of pumice does not require large single eruptions when coupled with chemical constraints and textural observations density spectra are useful for interpreting both eruptive style and the diversity of samples collected from the submarine environment

new multibeam mapping and geochemistry of the 30 35 s sector and overview of southern kermadec arc volcanism new multibeam mapping and whole rock geochemistry establish the first order definition of the modem submarine kermadec arc between 30 degrees and 35 degrees s twenty two volcanoes with basal diameters 5 km are newly discovered or fully mapped for the first time giggenbacb macauley havre haungaroa kuiwai ngatoroirangi sonne kibblewhite and yokosuka for each large volcano edifice morphology and structure surficial deposits lava fields distribution of sector collapses and lava compositions are determined macauley and havre are large silicic intra oceanic caldera complexes for both concentric ridges on the outer flanks are interpreted as recording mega bedforms associated with pyroclastic density flows and edifice foundering other stratovolcanoes reveal complex histories with repeated cycles of tectonically controlled construction and sector collapse extensive basaltic flow fields and the development of summit craters and or small nested calderas combined with existing data for the southernmost arc segment we provide an overview of the spatial distribution and magmatic heterogeneity along similar to 780 km of the kermadec arc at 30 degrees 36 degrees 30 s coincident changes in arc elevation and lava composition define three volcano tectonic segments a central deeper segment at 32 degrees 20 34 degrees 10 s has basement elevations of 3200 m water depth and relatively simple stratovolcanoes dominated by low k series basalt basaltic andesite in contrast the adjoining arc segments have higher basement elevations typically 3 km coincide with the shallower arc segments the dominant mode of large caldera formation is interpreted as mass discharge pyroclastic eruption with syn eruptive collapse hence the shallower arc segments are characterized by both the generation of volatile enriched magmas from crustal melting and a reduced hydrostatic load allowing magma vesiculation and fragmentation to initiate and sustain pyroclastic eruptions proposed initiation parameters for submarine pyroclastic eruptions are water depths 70 wt sio2 and a high discharge rate c 2005 elsevier b v all rights reserved

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

characteristics of submarine pumice rich density current deposits sourced from turbulent mixing of subaerial pyroclastic flows at the shoreline field and experimental assessment this study investigates the types of subaqueous deposits that occur when hot pyroclastic flows turbulently mix with water at the shoreline through field studies of the znp marine tephra in japan and flume experiments where hot tephra sample interacted with water the znp is a very thick pumice rich density current deposit that was sourced from subaerial pyroclastic flows entering the japan sea in the pliocene notable characteristics are well developed grain size and density grading lithic rich base pumice rich middle and ash rich top preponderance of sedimentary lithic clasts picked up from the seafloor during transport fine ash depletion in coarse facies and presence of curviplanar pumice clasts flume experiments provide a framework for interpreting the origin and proximity to source of the znp tephra on contact of hot tephra sample with water steam explosions produced a gas supported pyroclastic density current that advanced over the water while a water supported density current was produced on the tank floor from the base of a turbulent mixing zone experimental deposits comprise proximal lithic breccia medial pumice breccia and distal fine ash experiments undertaken with cold water saturated slurries of tephra sample and water did not produce proximal lithic breccias but a medial basal lithic breccia beneath an upper pumice breccia results suggest the characteristics and variations in znp facies were strongly controlled by turbulent mixing and quenching proximity to the shoreline and depositional setting within the basin presence of abundant curviplanar pumice clasts in submarine breccias reflects brittle fracture and dismembering that can occur during fragmentation at the vent or during quenching subsequent transport in water supported pumiceous density currents preserves the fragmental textures careful study is needed to distinguish the products of subaerial versus subaqueous eruptions

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

seismological evidence for three dimensional melt migration beneath the east pacific rise the extent to which crustal processes along mid ocean ridges are controlled by either the pattern of mantle upwelling or the mode of magma injection into the crust is not known models of mantle upwelling vary from two dimensional passive flow1 to three dimensional diapiric flow2 3 4 similarly beneath a ridge segment bounded by tectonic offsets crustal magma chambers may be replenished continuously along the ridge5 6 7 or at a central injection zone2 3 4 from which magma migrates towards the segment s ends here we present tomographic images that reveal the seismic structure and anisotropy of the uppermost mantle beneath the east pacific rise the anisotropy is consistent with two dimensional mantle flow diverging from the rise whereas the anomalous isotropic structure requires a three dimensional but continuous distribution of melt near the crust mantle interface our results indicate that crustal magma chambers are replenished at closely spaced intervals along axis and that crustal systems inherit characteristics of scale from melt transport processes originating in the mantle

sumisu volcano izu bonin arc japan site of a silicic caldera forming eruption from a small open ocean island sumisu volcano was the site of an eruption during 30 60 ka that introduced 48 50 km3 of rhyolite tephra into the open ocean environment at the front of the izu bonin arc the resulting caldera is 8 10 km in diameter has steep inner walls 550 780 m high and a floor averaging 900 m below sea level in the course of five research cruises to the sumisu area a manned submersible two rovs a deep tow camera sled and dredge samples were used to study the caldera and surrounding areas these studies were augmented by newly acquired single channel seismic profiles and multi beam seafloor swath mapping caldera wall traverses show that pre caldera eruptions built a complex of overlapping dacitic and basaltic edifices that eventually grew above sea level to form an island about 200 m high the caldera forming eruption began on the island and probably produced a large eruption column we interpret that prodigious rates of tephra fallback overwhelmed the sumisu area forming huge rafts of floating pumice choking the nearby water column with hyperconcentrations of slowly settling tephra and generating pyroclastic gravity currents of water saturated pumice that traveled downslope along the sea floor thick compositionally similar pumice deposits encountered in odp leg 126 cores 70 km to the south could have been deposited by these gravity currents the caldera rim presently at ocean depths of 100 400 m is mantled by an extensive layer of coarse dense lithic clasts but syn caldera pumice deposits are only thin and locally preserved the paucity of syn caldera pumice could be due to the combined effects of proximal non deposition and later erosion by strong ocean currents post caldera edifice instability resulted in the collapse of a 15 sector of the eastern caldera rim and the formation of bathymetrically conspicuous wavy slump structures that disturb much of the volcano s surface

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

petrological and chemical evolution of the northeastern troodos extrusive series cyprus four magma series are distinguished in the northeastern troodos extrusive series a a lower low ti series lo lts of basaltic andesites b a high ti series hts of basaltic andesites to rhyodacites c a low ti series lts and d a depleted low ti series dlts the last two being of basaltic andesite trace element characteristics vary systematically from series a to d and are interpreted in terms of a variable contribution of three major source components scs lile enriched water rich fluids sci derived from dehydration of a subducted lithosphere slab were continuously added to the overlying mantle wedge increasing lile hfse and lile ree ratios and decreasing absolute hfse and ree concentrations from series a to d indicate progressive depletion of the actual mantle source scii a negative ta anomaly in the lavas decreases from series a to d and is interpreted to have resulted from partial melting of the lower crust scii where ta nb ti may be fractionated by ti rich accessory phases the contribution of sciii decreases when the eruptive sites successively move away from the central axial zone and the temperature of the lower crust decreases preventing partial melting of the lower crust chemical compositions of fresh glass separates and phenocrysts indicate a change of major petrogenetic processes from series a to d lo lts and hts lavas are intrepreted to be directly related by open system fractional crystallization in crustal magma chambers removal of observed phenocryst phases clinopyroxene orthopyroxene plagioclase and magnetite and repeated subsequent mixing of dacitic to rhyodacitic magmas with batches of replenishing basaltic andesites are the major processes possibly induced by vesicle formation in the mafic layer after a period of some crystallization lts and dlts magmas were directly fed to the surface without stagnating at crustal levels with feeder dykes positioned marginal to the central rift zone and thus by passing the central magma chambers these magmas apparently experienced only limited fractional crystallization of 10 15 wt olivine clinopyroxene chromite probably at the mantle crust boundary

crustal accretion and tectonic setting of the troodos ophiolite cyprus abstract the troodos ophiolite is composed of two distinct magmatic suites an early relatively high tio 2 basaltic suite constitutes the upper gabbros the sheeted dike complex and the andesitic to rhyodacitic lower pillow lavas a late low tio 2 basaltic andesite suite constitutes the lower ultramafic to gabbroic cumulates and the upper pillow lavas parental magmas to the early suite originated as partial melts from hydrous relatively undepleted peridotite diapirs rising in a mantle wedge in response to dehydration of a subducting crust the late basaltic andesite suite is highly depleted in incompatible elements and show similarities with second stage or refractory basalts from oceanic spreading centers the troodos basaltic andesites and associated cumulates may thus have formed in response to continued adiabatic upwelling as second stage melts from the hydrous water undersaturated peridotite diapirs from which the earlier spreading related magmas were extracted the basaltic andesites formed in a relatively slow spreading or non spreading tectonic environment and dominantly crystallized in magma chambers underplating the troodos ophiolite the extrusion of the basaltic andesites of the upper pillow lavas occurred simultaneously with still active evolving volcanic centers belonging to the earlier magmatic suite an alternative possibility would be that the basaltic andesites separated from renewed diapirs rising as a consequence of re initiation of subduction and liberation of fluids from the subducting slab into the overlying depleted peridotite the existence of lree and lil enriched lavas among the upper pillow lavas support this possibility the apparent conflict between a spreading related geology and the arc petrology and geochemistry of the troodos ophiolite may be an artifact of lack of detailed studies of the geology and chemistry of oceanic and back arc rifting environments however it is proposed that the available data for the troodos ophiolite are consistent with its formation in a short lived spreading basin formed by splitting of an immature submarine arc

troodos extrusive series of cyprus a comparison with oceanic crust the extrusive section on the northern flank of the troodos ophiolite is 1 000 to 1 500 m thick and consists of about 12 lithologic units of pillowed and massive lava and breccia some pillowed units which average 100 to 200 m in thickness vary systematically from lava tubes up to 6 m in diameter at the base through common pillows less than 1 m in diameter to small pillows and extrusive breccias at the top these assemblages appear to be individual volcanoes suggesting episodic volcanic accretion the upper 100 to 200 m of the section is pervasively altered and oxidized but fresh volcanic glass is preserved throughout most of the extrusive section the lavas are subdivided on geochemical evidence into a lower differentiated andesite to dacitic andesite series resembling evolved island arc tholeiites and an upper series of picrite mafic basalt and basaltic andesite very low in tio2 k2o p2o5 nb rb and ba the highly depleted basalt and basaltic andesite resemble lavas from ocean crust near island arcs lateral continuity of lithologic units interpreted as individual volcanoes and the relatively constant dips of the extrusive rocks suggest that tilting and rotation have not played major roles in the accretion process the occurrence of fractionated lavas at the base and primitive lavas at the top suggests a broad zone of accretion located above a subduction zone

possible distinguishing characteristics of very deepwater explosive and effusive silicic volcanism recent seafloor exploration has shown that volcanic hosted massive sulfides vhms occur in modern silicic calderas formed by highly explosive eruptions at substantial water depths sampling of these has so far been restricted to surficial deposits ancient analogs provide a time integrated view of the structure and fill of deepwater calderas but constraints on paleo water depths have previously been lacking the ordovician bald mountain vhms northern maine usa is unique in its degree of preservation not only permitting detailed textural and structural analysis of the enclosing volcanic rocks but also allowing fluid inclusion analysis to determine a paleo water depth of eruption of greater than 1 45 km the deposits that result from very deepwater explosive eruptions have distinctive textural characteristics recognized in other ancient examples where paleo water depths are less well constrained i propose that hydrostatic pressure suppresses the explosivity of the eruption sufficiently to retard the formation of fine ash producing ignimbrite composed largely of pumice lapilli it also inhibits exsolution of dissolved water in the magma producing fluidal rhyolite eruptions and intrusions these include silicic fire fountaining to produce proximal cones of hyalotuff or spatter accumulations and intrusions of globular peperite

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

products of neptunian eruptions a common pyroclastic facies in subaqueous volcanic successions comprises massive to graded very thick several to tens of meters laterally extensive several kilometers beds of nonwelded pumice lapilli with volumes ranging to tens of cubic kilometers this facies may be overlain by laminated ash or bimodal ash and giant 1 m pumice clasts and underlain by coarse lithic breccia the association is inferred to be the typical product of sustained magmatic volatile driven explosive eruptions from vents at water depths of 1300 200 m we propose the term neptunian for such eruptions and their products the eruption column rapidly mixes with the surrounding water cools increases in density and collapses while remaining under water lithic clasts that are too heavy to be entrained in the column are deposited close to the source forming a neptunian lithic breccia pumice lapilli are rapidly waterlogged and form the dominant component in the collapsing column and in eruption fed water supported density currents neptunian density currents hot buoyant giant pumice clasts continue to rise and may reach the water surface before being waterlogged and settling along with temporarily suspended ash forming neptunian suspension deposits eruption magnitude fragmentation mechanisms and juvenile pyroclast characteristics especially vesicularity are very similar in neptunian and plinian style eruptions but column behavior differs primarily because of the contrasting physical properties of the ambient fluid water versus air

dynamics of a powerful deep submarine eruption recorded in h2o contents and speciation in rhyolitic glass the 2012 havre eruption constraining the syn eruptive volatile contents of magmatic melt is critical to understanding the intensities and styles of deep submarine volcanic eruptions for which direct observations are scarce quantifying residual magmatic water contents in volcanic glass is complicated by rehydration i e late stage addition of molecular water the 2012 deep submarine silicic eruption of havre volcano provides an unusual opportunity to quantify glass water contents from a recent well sampled stratigraphic sequence fourier transform infrared and microraman spectroscopy measurements of water concentration and water speciation across the havre 2012 eruptive sequence reveal an unanticipated range of excess molecular water within pumice this excess water requires rapid timescales of diffusion that are inconsistent with our current understanding of low temperature secondary rehydration in both subaerial and subaqueous eruptive products diffusion models applied to enrichment profiles at vesicle edges confirm that low temperature rehydration is an unlikely cause we instead support higher temperature syn eruptive pumice rehydration by condensed magmatic water and seawater in a submarine plume hydroxyl concentrations suggest shallow quenching depths of havre pumice hundreds of meters above the 900 meter deep main vent our data also support the presence of a vapor rich plume and consequent modification of ocean pressure above the vent we combine this novel volatile data with textural information and cooling rate calculations to explore the conditions that would cause slower shallow cooling of clasts from a deep submarine eruption by exploring the physical conditions for the interaction between pumice and submarine plumes we emphasize fundamental differences between subaerial and submarine clast producing eruptions