the mechanism of surtseyan volcanism during eruption the vent of a surtseyan volcano is occupied by a highly mobile slurry of tephra hyaloclastite and water as magma ascends rapidly through the slurry mixing occurs due to velocity shear acceleration of the fluid fluid contacts and mass incorporation consequent expansion of the admixed water causes the jetting and continuous up rush activity that characterizes surtseyan volcanism there is no evidence of the widely accepted fuel coolant interaction mechanism

high spatial resolution electron probe microanalysis of tephras and melt inclusions without beam induced chemical modification wavelength dispersive analysis of tephra grains and melt inclusions by epma has been carried out using a focused beam of 3 m diameter without detected loss of sodium or potassium in standard glasses including anhydrous basalts a slightly hydrated rhyolite and a sodium rich intermediate composition the ability to make analyses without chemical modification is strongly dependent upon current density at the analysis site analysis with narrow beams requires extremely low beam currents that are normally associated with energy dispersive analysis experiments indicate that a value of 0 1 na m2 must not be exceeded at least for moderately hydrated samples if sodium loss is to be avoided high resolution analysis without beam induced analytical artefacts enables fully quantitative analysis of very distal and or highly vesicular tephras and very small melt inclusions without the need to use post analysis corrections this development has enabled high quality analyses from crypto tephra layers that were pre

eruption processes and deposit characteristics at the monogenetic mt gambier volcanic complex se australia implications for alternating magmatic and phreatomagmatic activity the 5 ka mt gambier volcanic complex in the newer volcanics province australia is an extremely complex monogenetic volcanic system that preserves at least 14 eruption points aligned along a fissure system the complex stratigraphy can be subdivided into six main facies that record alternations between magmatic and phreatomagmatic eruption styles in a random manner the facies are 1 coherent to vesicular fragmental alkali basalt effusive hawaiian spatter and lava flows 2 massive scoriaceous fine lapilli with coarse ash strombolian fallout 3 bedded scoriaceous fine lapilli tuff violent strombolian fallout 4 thin medium bedded undulating very fine lapilli in coarse ash dry phreatomagmatic surge modified fallout 5 palagonite altered cross bedded medium lapilli to fine ash wet phreatomagmatic base surges and 6 massive palagonite altered very poorly sorted tuff breccia and lapilli tuff phreato vulcanian pyroclastic flows since most deposits are lithified to quantify the grain size distributions gsds image analysis was performed the facies are distinct based on their gsds and the fine ash to coarse fine ash ratios these provide insights into the fragmentation intensities and water magma interaction efficiencies for each facies the eruption chronology indicates a random spatial and temporal sequence of occurrence of eruption styles except for a magmatic horizon of effusive activity occurring at both ends of the volcanic complex simultaneously the eruption foci are located along nw se trending lineaments indicating that the complex was fed by multiple dykes following the subsurface structures related to the tartwaup fault system possible factors causing vent migration along these dykes and changes in eruption styles include differences in magma ascent rates viscosity crystallinity degassing and magma discharge rate as well as hydrological parameters

mechanisms of hydrovolcanic pyroclast formation grain size scanning electron microscopy and experimental studies pyroclasts produced by explosive magma water interactions are of various sizes and shapes data from analysis of over 200 samples of hydrovolcanic ash are interpreted by comparison with experimentally produced ash grain size and scanning electron microscopy sem reveal information on the formation of hydrovolcanic pyroclasts strombolian explosions result from limited water interaction with magma and the pyroclasts produced are dominantly centimeter sized with increasing water interaction hydrovolcanism increases in explosivity to surtseyan and vulcanian activity these eruptions produce millimeter to micron sized pyroclasts the abundance of fine ash 63 m diameter increases from 5 to over 30 percent as water interaction reaches an explosive maximum this maximum occurs with interactions of virtually equal volumes of melt and water five dominant pyroclast shape types determined by sem result from hydrovolcanic fragmentation 1 blocky and equant 2 vesicular and irregular with smooth surfaces 3 moss like and convoluted 4 spherical or drop like and 5 plate like types 1 and 2 dominate pyroclasts greater than 100 m in diameter types 3 and 4 are typical of fine ash type 5 pyroclasts characterize ash less than 100 m in diameter resulting from hydrovolcanic fragmentation after strong vesiculation fragmentation mechanisms observed in experimental melt water interactions result from vapor film generation expansion and collapse fragments of congealed melt are products of several alternative mechanisms including stress wave cavitation detonation waves and fluid instability mixing all result in rapid heat transfer these mechanisms can explain the five observed pyroclast shapes stress wave fracturing cavitation of the melt results from high pressure and temperature gradients at the melt water interface simultaneous brittle fracture and quenching produces type 1 pyroclasts type 2 develops smooth fused surfaces due to turbulent mixing with water after fracture and before quenching fluid instabilities promote turbulent mixing of melt and water and produce fine ash this kind of fragmentation occurs during high energy explosions the increased melt surface area due to fine fragmentation promotes high efficiency heat exchange between the melt and water shapes of resulting pyroclasts are determined by maximum surface area type 3 and surface tension effects type 4 type 5 pyroclasts result from nearly simultaneous vesicle burst and melt water fragmentation

faunal turnover and changing oceanography late palaeozoic warm to cool water carbonates sverdrup basin canadian arctic archipelago abstract carbonate sedimentary rocks in the fosheim hamilton sub basin at the eastern end of the sverdrup basin contain a particularly sensitive record of changing palaeoceanography and faunal turnover during the northward movement of pangaea through the late palaeozoic carboniferous and earliest permian inner ramp carbonates are a photozoan association of colonial corals fusulinids algae and ooids with sparse heterozoan elements crinoids brachiopods and bryozoans based on modern analogues these biotic components reflect warm water oligotrophic marine environments contemporaneous mid ramp and basin biotas are dominated by a heterozoan fauna solitary corals crinoids bryozoans and brachiopods that grew in cooler deeper waters that may have been beneath a thermocline initial cooling began in the latest sakmarian with an influx of cool deep water and climatic deterioration allowing cool and warm waters to mix disrupting the thermocline and producing a sub tropical marine biota whose remains were reworked by storms and re deposited as tempestites faunas are heterozoan crinoids bryozoans brachiopods and sponges with photozoan elements colonial corals and symbiont bearing fusulinids prominent only in inner ramp facies by late artinskian these photozoan components are only present in shallowest basin margin environments artinskian subtropical waters contained somewhat elevated trophic resources that together with mixing of photozoan and heterozoan elements in the subtropical inner ramp produced the greatest faunal diversity seen in the sverdrup basin the biota was completely boreal in nature and warm shallow waters with a warm refuge biota were now supplemented by cool waters derived from the panthalassa ocean profound change took place in the kungurian coincident with localised uplift during the melville disturbance and increased siliciclastic input neritic storm dominated sediments are wholly heterozoan brachiopods bryozoans crinoids and sponges with no remaining photozoan elements nutrient levels are elevated but not eutrophic as indicated by phosphate and glauconite extensive bioerosion numerous infaunal foraminifers and an impressive and diverse shelly biota that during times of high siliciclastic input was replaced by a profuse zoophycos infauna brachiopod and bryozoan faunas are distinctly boreal continued cooling relatively high nutrients as indicated by abundant glauconite and phosphate and decreased siliciclastic input eventually resulted in an abundant heterozoan biota brachiopods bryozoans crinoids and sponges with local gigantism but reduced brachiopod and bryozoan diversity these middle permian rocks may be the coolest water deposits in the sverdrup basin the faunas are still part of a boreal arctic province with strong ties to cool water faunas of uralian basin the succession ends with late permian biosiliceous spiculitic cherts containing accessory heterozoan components brachiopods bryozoans and few crinoids these sediments in part coeval with evaporites and terrestrial red beds on the eastern european platform are interpreted to reflect a combination of cool waters elevated nutrients and mid latitude storminess

the 1732 surtseyan eruption of eggoya jan mayen north atlantic deposits distribution chemistry and chronology the island of jan mayen in the north atlantic ocean is home to the world s northernmost active subaerial volcano beerenberg of the five known historical eruptions on the island the locations of two eruptions 1732 and 1818 have not been accurately located it is known that the 1732 and 1818 eruptions occurred on the south flank of beerenberg and several eruption sites have been proposed for these events here we show that the tuff cone of eggoya on the sw flank of beerenberg was the site of the 1732 eruption based on interpretation of the deposits field relations and historical sources we further describe the deposits from the eruption and show that eggoya is the largest explosive eruption described from jan mayen emplacing at least at least 0 3 0 4 km3 vei 4 of basanitic tephra up to distances of at least 111 km from jan mayen and covering a minimum area of around 500 km2 within the 2 cm isopach we also present our eruption scenario and show that this was an emergent surtseyan eruption with activity shifting between tephra jetting continuous uprush and more magmatic phases

hydrovolcanic explosions the systematics of water pyroclast equilibration the initial contact of external water with erupting magma and the mass ratio of water to magma govern the development of hydrovolcanic phenomena the phase relations of water within the runout system and the separation of vapor or liquid from the pyroclasts explain gradational transitions between some pyroclastic flows base surges mud hurricanes mudflows and sheetfloods

modeling tephra dispersal in absence of wind insights from the climactic phase of the 2450 bp plinian eruption of pululagua volcano ecuador the determination of eruptive parameters is crucial in volcanology not only to document past eruptions but also for tephra fallout hazard assessments in most tephra fallout studies eruptive parameters have been determined either by empirical techniques or analytical models but the uncertainty of such parameters is usually not well described we have applied both empirical and analytical models to characterize the climactic phase of the 2450 bp plinian eruption of pululagua bf2 layer and explore the variations in the total erupted mass column height and total grain size distribution both approaches yield comparable results in the total mass of tephra erupted 4 5 1 5 1011 kg while they show some discrepancies for the determination of the column height 36 20 km from empirical techniques and 30 20 km from analytical techniques the total grain size distribution of the bf2 layers varies with the different techniques used for the calculation and significantly affects the outputs of analytical models furthermore the determination of the total grain size distribution depends strongly on the number and spatial distribution of the sample location inverting tephra fallout deposits on the total accumulation or thickness gives a good constraint on the total mass erupted but not on the column height however inverting on individual grain size classes better constrains the possible range of column heights but cannot resolve particle release height results from the inversion on individual grain size classes show that large diffusion coefficients are necessary to model the bf2 layer and might be required to model proximal tephra deposits in order not to overestimate the total erupted mass additionally we used a statistical method smoothed bootstrap approach to quantify the uncertainty in eruptive parameters such as column height and total erupted mass our uncertainty analysis yields a mean total erupted mass of 4 5 0 3 1011 kg and a mean column height of 30 3 km results from the uncertainty analysis compare well with results from other approaches finally although the climactic phase of the 2450 bp plinian eruption of pululagua occurred in relatively calm atmospheric conditions our results show that the dispersion of the bf2 layer was influenced by slight northeasterly wind conditions

depositional processes of the suwolbong tuff ring cheju island korea the suwolbong pyroclastic sequence in the western part of cheju island korea comprises partly preserved rim beds of a quaternary basaltic tuff ring whose vent lies about 1 km seaward of the present shoreline the sequence consists of breccia lapillistone lapilli tuff and tuff eighteen sedimentary facies are established and organized into six lateral facies sequences lfs and seven vertical facies sequences vfs the lfs 1 4 and 5 begin with massive lapilli tuff which transforms downcurrent into either planar bedded lfs 1 undulatory bedded lfs 4 or climbing dune bedded lfs 5 lapilli tuff units they are representative of relatively dry base surge whose particle concentration decreases downcurrent with a progressive increase in both tractional processes and sorting the lfs 2 begins with disorganized and massive lapilli tuff and transforms into crudely stratified units downcurrent it results from relatively wet base surge in which sorting is poor due to the cohesion of damp ash the lfs 3 comprises well sorted lapilli tuff and stratified tuff further downcurrent suggestive of deposition from combined fall and surge of relatively dry hydroclastic eruption all seven vertical facies sequences generally comprise two facies units of coarse grained fines depleted lapilli tuff and an overlying fine grained tuff these sequences are suggestive of deposition from base surge that consists of a turbulent head and a low concentration tail depositional processes in the suwolbong tuff ring were dominated by a relatively dry base surge the base surge comprises turbulent and high concentration suspension near the vent whose deposits are generally unstratified due to the lack of tractional transport as the base surge becomes diluted downcurrent through fallout of clasts and mixing of ambient air it develops large scale turbulent eddies and is segregated into coarse grained bedload and overlying fine grained suspension forming thinly stratified units further downcurrent the base surge may be either cooled and deflated or pushed up into the air depending on its temperature the suwolbong tuff ring comprises an overall wet to dry cycle with several dry to wet cycles in it suggestive of overall decrease in abundance of external water and fluctuation in the rate of magma rise

blackriveran carbonates from the subsurface of the lake simcoe area southern ontario stratigraphy and sedimentology of a low energy carbonate ramp the lowest upper ordovician blackriveran strata in the lake simcoe area represent the base of a major transgressive sequence of peritidal carbonates and siliciclastics from detailed petrographic

mosaic of environments recorded by bryozoan faunas from the middle miocene of hungary the marine sediments of the badenian middle miocene of hungary pannonian basin central paratethys are composed of abundant bryozoan skeletal grains seventy one bulk samples collected at 18 localities outcrops and boreholes yielded a total number of 238 bryozoan species in order to reconstruct the badenian palaeoenvironments the present study investigates the composition of this very diverse fauna using a combination of statistical and palaeoecological methods the statistical analyses make use of cluster analysis and non metric multi dimensional scaling the palaeoecological approaches are based on the known ecological requirements of the bryozoan colonial growth forms and of the numerous extant species five facies have first been differentiated on the basis of sedimentological and palaeontological features coral buildups coralline algal limestones biocalcarenites sands and marls each of them is characterized by the abundance the diversity and the types of growth forms of the bryozoans the palaeoecological statistical and facies analyses further permitted to identify four depositional settings carbonate platform distal and proximal terrigenous platform slope and basin these environments developed at depths between 0 to about 300 m in a mixed carbonate siliciclastic depositional system skeletal production and spatial distribution of carbonate factories were predominantly controlled by terrigenous input this resulted in a complex mosaic of facies habitats where rich bryozoan faunas could thrive many warm water organisms among them foraminifers zooxanthellate corals molluscs bryozoans and echinoids were recorded from the study sites the coexistence of coralgal and bryomol carbonate skeletal assemblages in this subtropical setting is explained mostly by variations in the productivity of surface waters periods of nutrient enrichment and increased benthic eutrophication affected coral diversity and abundance in favour of bryozoans

pliocene seasonality across the north atlantic inferred from cheilostome bryozoans abstract previous studies have shown an inverse correlation between zooid size in cheilostome bryozoans and ambient water temperature this relationship underlies the mart technique which uses intracolonial variation in zooid size to predict mean annual range in temperature experienced by bryozoan colonies during their life here we apply the mart technique to study early and mid pliocene bryozoans from central america panama costa rica the usa florida south carolina north carolina virginia and the uk suffolk to reconstruct palaeoseasonality across a range of latitudes for the north atlantic during the pliocene epoch compared to the present day our analyses suggest greater seasonality ca 4 5 c in the southern caribbean at the time of cayo agua formation deposition ca 4 25 ma in keeping with inferred upwelling prior to the closure of the isthmian barrier at 2 7 ma bryozoans also indicate seasonal upwelling on the gulf coast of florida in a similar manner to the present day because upwelling can be highly localised and prone to spatial and temporal variation in the gulf of mexico today it contributes little to a broad understanding of pliocene north atlantic waters however mart estimates for the coastal plain region indicate a general reduction in the annual range in temperature relative to the present suggesting that the colder surface waters that today reach south to cape hatteras had less influence in early to mid pliocene times these results along with evidence from other proxies strongly support reduced seasonality and warmer conditions along the eastern seaboard of the usa in the early to mid pliocene finally the mart estimates amongst coralline crag localities provide evidence for an increased annual range in temperature in the southern north sea than at present our study shows that bryozoan mart estimates provide a powerful independent proxy for palaeoseasonality and is the first to demonstrate that the mart technique can be applied to infer palaeoclimates across a wide range of latitudes focusing on a variety of geological formations and geographical regions

identifying magma water interaction from the surface features of ash particles the deposits from explosive volcanic eruptions those eruptions that release mechanical energy over a short time span1 are characterized by an abundance of volcanic ash2 3 this ash is produced by fragmentation of the magma driving the eruption and by fragmenting and ejecting parts of the pre existing crust host rocks interactions between rising magma and the hydrosphere oceans lakes and ground water play an important role in explosive volcanism4 5 because of the unique thermodynamic properties of water that allow it to very effectively convert thermal into mechanical energy although the relative proportion of magma to host rock fragments is well preserved in the pyroclastic rocks deposited by such eruptions it has remained difficult to quantitatively assess the interaction of magma with liquid water from the analysis of pyroclastic deposits2 3 4 5 here we report the results of a study of natural pyroclastic sequences combined with scaled laboratory experiments we find that surface features of ash grains can be used to identify the dynamic contact of magma with liquid water the abundance of such ash grains can then be related to the water magma mass ratios during their interaction

contrasting mechanisms of magma fragmentation during coeval magmatic and hydromagmatic activity the hverfjall fires fissure eruption iceland growing evidence for significant magmatic vesiculation prior to magma water interaction mwi has brought into question the use of diagnostic features such as low vesicularities and blocky morphologies to identify hydromagmatic pyroclasts we address this question by quantifying co variations in particle size shape and texture in both magmatic and hydromagmatic deposits from the hverfjall fires fissure eruption iceland overlapping vesicularity and bubble number density distributions measured in rapidly quenched magmatic and hydromagmatic pyroclasts indicate a shared initial history of bubble nucleation and growth with substantial vesiculation prior to mwi hydromagmatic fragmentation occurred principally by brittle mechanisms where the length scale and geometry of fracturing was controlled by the bubble population this suggests that the elevated fragmentation efficiency of hydromagmatic deposits is driven at least in part by brittle disintegration of vesicular pyroclasts due to high thermal stress generated during rapid cooling in this way the shape and size distributions of hydromagmatic pyroclasts both critical input parameters for ash dispersion models are strongly influenced by the dynamics of vesiculation prior to mwi this result underlines the need to analyse multiple grain size fractions to characterise the balance between magmatic and hydromagmatic processes during the hverfjall fires eruption the external water supply was sufficient to maintain mwi throughout the eruption with no evidence for progressive exhaustion of a water reservoir we suggest that both the longevity and the spatial distribution of mwi were determined by the pre existing regional hydrology and represent continuous interaction between a propagating dike and a strong groundwater flow system hosted within permeable basalt lavas

growth of an emergent tuff cone fragmentation and depositional processes recorded in the capelas tuff cone sao miguel azores abstract the capelas tuff cone is an emergent surtseyan type tuff cone that erupted in shallow seawater off the coast of sao miguel azores in this paper we present a detailed stratigraphic study which is used to infer depositional processes and modes of fragmentation for the capelas tuff cone deposits the growth of the tuff cone can be divided into three stages based on variations in depositional processes that are probably related to differences in water magma w m ratios the first stage corresponds well to wet surtseyan type activity where wet fallout is the dominant depositional process with only minor representation of pyroclastic surge deposits the second stage of the eruption is suggested to be the result of alternating wet and slightly drier periods of surtseyan activity with an overall lower w m ratio compared to the first stage the drier surtseyan periods are characterized by the presence of minor grain flow deposits and undulating pyroclastic surge deposits that occasionally display relatively dry structures such as strongly grain segregated layers and brittle behavior when impacted by ballistic ejecta the first deposits of the second stage show an intense activity of pyroclastic surges but fallout commonly modified by surges is still the dominant depositional process during the second stage the third stage represents a final effusive period with the build up of a scoria cone and ponded lava flows inside the tuff cone crater phreatomagmatic fragmentation as seen by studies of the fine ash fraction

the late ordovician mass extinction abstract near the end of the late ordovician in the first of five mass extinctions in the phanerozoic about 85 of marine species died the cause was a brief glacial interval that produced two pulses of extinction the first pulse was at the beginning of the glaciation when sea level decline drained epicontinental seaways produced a harsh climate in low and mid latitudes and initiated active deep oceanic currents that aerated the deep oceans and brought nutrients and possibly toxic material up from oceanic depths following that initial pulse of extinction surviving faunas adapted to the new ecologic setting the glaciation ended suddenly and as sea level rose the climate moderated and oceanic circulation stagnated another pulse of extinction occurred the second extinction marked the end of a long interval of ecologic stasis an ecologic evolutionary unit recovery from the event took several million years but the resulting fauna had ecologic patterns similar to the fauna that had become e

pre emergent construction of a lacustrine basaltic volcano pahvant butte utah usa the subaqueous phases of an eruption initiated approximately 85 m beneath the surface of pleistocene lake bonneville produced a broad mound of tephra a variety of distinctive lithofacies allows reconstruction of the eruptive and depositional processes active prior to emergence of the volcano above lake level at the base of the volcano and very near inferred vent sites are fines poor well bedded broadly scoured beds of sideromelane tephra having local very low angle cross stratification m1 lithofacies these beds grade upward into lithofacies m3 which shows progressively better developed dunes and cross stratification upsection to its uppermost exposure approximately 10 m below syneruptive lake level both lithofacies were emplaced largely by traction from relatively dilute sediment gravity flows generated during eruption intercalated lithofacies are weakly bedded tuff and breccia m2 and nearly structureless units with coarse basal layers above strongly erosional contacts m4 the former combines products of deposition from direct fall and moderate concentration sediment gravity flows and the latter from progressively aggrading high concentration sediment gravity flows early in the eruption subaqueous tephra jetting from phreatomagmatic explosions discontinuously fed inhomogeneous unsteady dilute density currents which produced the m1 lithofacies near the vent dunes and crossbeds which are better developed upward in m3 resulted from interaction between sediment gravity flows and surface waves triggered as the explosion generated pressure waves and eruption jets impinged upon and occasionally breached the surface intermingling of a tephra emplaced after brief transport by tephra jets within a gaseous milieu and b laterally flowing tephra formed lithofacies m2 along vent margins during parts of the eruption in which episodes of continuous uprush produced localized water exclusion zones above a vent m4 comprises mass flow deposits formed by disruption and remobilization of mound tephra intermittent explosive magma water interactions occurred from the outset of the pahvant eruption with condensation entrainment of water and lateral flow marking the transformation from eruptive to sedimentary processes leading to deposition of the mound lithofacies

a pre hirnantian late ordovician interval of global cooling the boda event re assessed facies analysis suggests that the early mid ashgill late ordovician boda event of fortey and cocks fortey r a cocks l r m 2005 late ordovician global warming the boda event geology 33 5 405 408 was an episode of global cooling not of warming as they proposed limestones are widely developed at this level from mid latitude to high latitude commonly interrupting thick ordovician successions of fine clastic sediments bryozoan carbonate mud mounds in a laterally extensive cool water carbonate belt along the high palaeolatitude margin of w gondwana are associated with glacial diamictites and grew during glacial lowstand with sea ice prior to the hirnantian glaciation from high to mid palaeolatitudes contemporaneous cool water carbonates with mud mounds including stromatactis both in gondwana and in baltica developed in mid outer ramp settings during regression in the dominantly clastic welsh basin of avalonia phosphatic facies on an iapetus facing carbonate ramp indicate upwelling cold waters down ramp yet up ramp coralliferous algal limestones suggest warm surface waters we propose that the late ordovician cool water carbonates developed in response to global cooling and expansion of gondwanan terrestrial and sea ice that lowered eustatic sea level and on the w gondwanan shelf reduced or shut down fluvial clastic input previously poorly circulated salinity stratified oceans that had restricted benthic dispersal in mid low latitudes were replaced by thermohaline ocean circulation that oxygenated bottom waters and dispersed benthic faunas an earlier caradoc carbonate interval in low mid palaeolatitudes that shares many facies characteristics with the ashgill cool water limestones and correlates with a widely recognised isotope excursion is interpreted as indicating an earlier global cooling episode

the volcanic eruption on jan mayen january 1985 interaction between a volcanic island and a fracture zone abstract an eruption started on jan mayen on 6 january 1985 it lasted only 35 40 hours and discharged about 7 million m3 of magma an earthquake swarm accompanied the eruption with individual quakes reaching magnitude 5 the volcanic fissure that opened strikes about midway between the strike of the jan mayen fracture zone 25 n of w and the general trend of the island and its volcanic features sw ne this fact the strong earthquakes and compositional features all suggest that it was an eruption from a leaky fracture zone rather than from the jan mayen magma system proper

geochemistry of lavas from mohns ridge norwegian greenland sea implications for melting conditions and magma sources near jan mayen mohns ridge lavas between 71 and 72 30 n 360 km have heterogeneous compositions varying between alkali basalts and incompatible element depleted tholeiites on a large scale there is a continuity of incompatible element and isotopic compositions between the alkali basalts from the island jan mayen and mohns ridge tholeiites the variation in isotopes suggests a heterogeneous mantle which appears to be tapped preferentially by low degree melts 5 close to jan mayen but also shows its signature much further north on mohns ridge three lava types with different incompatible element compositions e g chondrite normalized la sm n 2 occur in the area at 72 n and were generated from this heterogeneous mantle the relatively depleted tholeiitic melts were mixed with a small degree melt from an enriched source the elements ba rb and k of the enriched melt were probably buffered in the mantle by residual amphibole or phlogopite that such a residual phase is stable in this region of oceanic mantle suggests both high water contents and low mantle temperatures at odds with a hotspot origin for jan mayen instead we suggest that the melting may be induced by the lowered solidus temperature of a wet mantle mohns morb mid ocean ridge basalt and jan mayen area alkali basalts have high contents of ba and rb compared to other incompatible elements e g ba la 10 these ratios reflect the signature of the mantle source ratios of ce pb and rb cs are normal morb mantle ratios of 25 and 80 respectively thus the enrichments of ba and rb are not indicative of a sedimentary component added to the mantle source but were probably generated by the influence of a metasomatizing fluid as supported by the presence of hydrous phases during the petrogenesis of the alkali basalts geophysical and petrological models suggest that jan mayen is not the product of hotspot activity above a mantle plume and suggest instead that it owes its existence to the unique juxtaposition of a continental fragment a fracture zone and a spreading axis in this part of the north atlantic

chemical composition distribution and origin of silicic volcanic ash layers in the greenland iceland norwegian sea explosive volcanism from 10 to 300 ka as recorded in deep sea sediments explosive ocean island volcanism in the greenland iceland norwegian sea gin sea is indicated by marine tephra layers at 10 300 ka peaks of explosive volcanism occurred in oxygen isotope stages 8 7 5 and 1 the depositional age of the tephra was estimated using the oxygen isotope stratigraphy and dating of marine records geochemical analyses of the tephra layers show that all originate from iceland here we report the characteristics of tephra from these major icelandic events in 30 deep sea cores from the gin sea our findings provide constraints on the distribution of tephra from the eruption source for the vedde ash oxygen isotope stage 1 we estimate a minimum fallout area of 2 105 km2 stretching from central greenland in the west and southern sweden in the east to 71 n in the gin sea the magnitude of the eruption and the regional wind conditions controlled the extent and concentrations of these ash fallout events oceanic circulation and differential settling may have affected the distribution and final deposition of ash particles such as bubble wall shards

grain shape of basaltic ash populations implications for fragmentation here we introduce a new quantitative method to produce grain shape data of bulk samples of volcanic ash and we correlate the bulk average grain shape with magma fragmentation mechanisms the method is based on automatic shape analysis of 2d projection ash grains in the size range 125 63 m loose bulk samples from the deposits of six different basaltic eruptions were analyzed and 20 000 shape measurements for each were obtained within 45 min using the particle insight dynamic shape analyzer pidsa we used principal component analysis on a reference grain dataset to show that circularity rectangularity form factor and elongation best discriminate between the grain shapes when combined the grain population data show that the studied eruptive environments produce nearly the same range of grain shapes although to different extents our new shape index the regularity index ri places an eruption on a spectrum between phreatomagmatic and dry magmatic fragmentation almost vesicle free surtseyan ash has an ri of 0 207 0 002 2 whereas vesiculated hawaiian ash has an ri of 0 134 0 001 2 these two samples define the end member ri while two subglacial one lacustrine and another submarine ash sample show intermediate ris of 0 168 0 002 2 0 175 0 002 2 0 187 0 002 2 and 0 191 0 002 2 respectively the systematic change in ri between wet and dry eruptions suggests that the ri can be used to assess the relative roles of magmatic vs phreatomagmatic fragmentation we infer that both magmatic and phreatomagmatic fragmentation processes played a role in the subglacial eruptions

quantitative allochem compositional analysis of lochkovian pragian boundary sections in the prague basin czech republic abstract quantitative allochem compositional trends across the lochkovian pragian boundary event were examined at three sections recording the proximal to more distal carbonate ramp environment of the prague basin multivariate statistical methods principal component analysis correspondence analysis cluster analysis of point counted thin section data were used to reconstruct facies stacking patterns and sea level history both the closed nature allochem percentages and their centred log ratio clr coordinates were used both these approaches allow for distinguishing of lowstand transgressive and highstand system tracts within the praha formation which show gradual transition from crinoid dominated facies deposited above the storm wave base to dacryoconarid dominated facies of deep water environment below the storm wave base quantitative compositional data also indicate progradative retrogradative trends in the macrolithologically monotonous shallow water succession and enable its stratigraphic correlation with successions from deeper water environments generally the stratigraphic trends of the clr data are more sensitive to subtle changes in allochem composition in comparison to the results based on raw data a heterozoan dominated allochem association in shallow water environments of the praha formation supports the carbonate ramp environment assumed by previous authors

pyroclast textures of the ilchulbong wet tuff cone jeju island south korea abstract jeju island is a quaternary shield volcano situated approximately 95 km south of the korean peninsula comprising holocene late pleistocene phreatomagmatic deposits and shield forming basaltic to trachytic lavas that overlie early mid pleistocene marine sedimentary and hydrovolcanic deposits ilchulbong tuff cone is among the youngest hydrovolcanic centres on the island formed along the shoreline from a surtseyan style eruption involving interaction of magma with external water we have analysed lapilli from selected sites on the volcanic edifice in terms of vesicle population texture shape and size and quantitatively with reference to bulk vesicularity vesicle number density vesicle dimensions and distributions the lapilli range in total vesicularity from low 15 35 to high 50 80 values and show a modest range of vesicle number densities n v number of vesicles per unit rock volume between 2 26 10 3 and 6 97 10 3 vesicles mm 3 such vesicle number densities are higher than those typical of strombolian n v between 9 3 10 1 and 1 3 10 3 vesicles mm 3 stromboli 2002 lautze and houghton 2007 and hawaiian eruptions and more similar to those of plinian n v between 1 5 and 2 5 10 3 vesicles mm 3 tarawera sable et al 2009 eruptions the lapilli studied were not strongly quenched and we infer that the high vesicle number densities reflect a late nucleated bubble population related in origin and formed in response to high levels of microlite growth in the ilchulbong magma prior to final ascent

bryozoans in transition the depauperate and patchy jurassic biota bryozoans were profoundly affected by the end permian and probably also by the end triassic mass extinction events their recovery in terms of diversity and disparity during the jurassic was slow and geographically patchy critical compilation of data from the published literature reveals only 172 valid species 160 cyclostomes 10 ctenostomes and 2 cheilostomes some as yet not formally named however the rate of description of new jurassic bryozoan species during the last two hundred years implies that many more species remain to be discovered and described the jurassic is the only geological period during which cyclostomes were the dominant order and is also important in having the oldest cheilostome bryozoan the first representative of the order that dominates the modern bryozoan biota species diversity peaked in the bathonian where local assemblages may contain up to 33 species encrusters account for 124 73 of the 172 jurassic species among which small sheet like forms especially those assigned to the form genus berenicea are the most numerous the overwhelming predominance in the jurassic of weeds with runner ribbon or sheet colony forms is striking unlike the triassic and cretaceous large tree like or frondose erect colonies are uncommon and species with fenestrate articulated and free living colonies are unknown of 92 geographically categorized entries for jurassic bryozoans in the zoological record 1937 2003 80 87 are european records outside europe diversity appears to have remained low throughout the jurassic and non weedy species are particularly scarce compared with the cretaceous jurassic bryozoans show no adaptations that can be interpreted as related to the increased predation pressure associated with the mesozoic marine revolution the jurassic bryozoan biota shares characteristics with both the stenolaemate dominated palaeozoic triassic biota and the cheilostome dominated cretaceous recent biotas

statistical analysis of textural data from complex pyroclastic sequences implications for fragmentation processes of the agnano monte spina tephra 4 1 ka phlegraean fields southern italy the agnano monte spina eruption 4 1 ka generated pyroclastic deposits with complex internal stratigraphy that reflects variations in eruptive style to obtain constraints on the fragmentation processes we analysed grain size and scanning electron microscopy sem data by means of multivariate statistical techniques of the cluster and factor analysis types the results allow the identification of end member deposits related to either pure magmatic activity or pure phreatomagmatic activity in addition some deposits show evidence of the contemporaneous operation of both end member fragmentation modes the presence of the two contrasting fragmentation processes during the various phases of explosive activity is tentatively interpreted to be a function in inhomogeneities in gas exsolution processes within the magma column that feeds the eruption

growth and evolution of an emergent tuff cone considerations from structural geology geomorphology and facies analysis of sao roque volcano sao miguel azores abstract the syn eruptive and post eruptive history of sao roque tuff cone its geological setting and volcanological features were studied in detail to understand the role played by the different factors that contributed to the morphological evolution of this relatively simple and small volcanic edifice in addition attention was also focused on the series of natural changes that affected the tuff cone during the course of the years and that finally led to its structural disassembly a novel model is proposed to explain this process the sao roque volcanic centre located on the island of sao miguel azores consists of two well consolidated bodies and numerous small islets that formed more than 4700 years ago during the hydromagmatic activity that took place along an intruding dyke whose nnw sse trend is in agreement with the regional tectonic pattern the eruptive vents probably migrated progressively from sse to nnw forming small edifices through the rapid accumulation of sediments during alternating phases of dry and wet magmatic emissions syn eruptive partial collapses greatly modified the original morphological structure of these edifices probably allowing sea water to continuously flow into the vents the complex interaction of these factors controlled the depth of magma fragmentation producing different types of deposits in which the ash lapilli ratio varies considerably the high water saturation degree of these deposits caused syn eruptive and post eruptive remobilization which resulted in collapses and some small scale landslides post eruptive wnw ese trending transtensional and extensional tectonic activities operated during the initial dissection of the cone generating instability furthermore the rapid accumulation of wet tephra and its following consolidation caused selective collapses that favoured the fragmentation of the deposit and caused the formation of numerous islets separated by radially arranged channels collapses also involved the lava units emplaced in more recent times around the tuff cone which show that brittle deformation has been significant in the area for a prolonged period

estimating the volume of tephra deposits a new simple strategy volume determination of tephra deposits is necessary for the assessment of the dynamics and hazards of explosive volcanoes several methods have been proposed during the past 40 years that include the analysis of crystal concentration of large pumices integrations of various thinning relationships and the inversion of field observations using analytical and computational models regardless of their strong dependence on tephra deposit exposure and distribution of isomass isopach contours empirical integrations of deposit thinning trends still represent the most widely adopted strategy due to their practical and fast application the most recent methods involve the best fitting of thinning data using various exponential segments or a power law curve on semilog plots of thickness or mass area versus square root of isopach area the exponential method is mainly sensitive to the number and the choice of straight segments whereas the power law method can better reproduce the natural thinning of tephra deposits but is strongly sensitive to the proximal or distal extreme of integration we analyze a large data set of tephra deposits and propose a new empirical method for the determination of tephra deposit volumes that is based on the integration of the weibull function the new method shows a better agreement with observed data reconciling the debate on the use of the exponential versus power law method in fact the weibull best fitting only depends on three free parameters can well reproduce the gradual thinning of tephra deposits and does not depend on the choice of arbitrary segments or of arbitrary extremes of integration

total grain size distribution and volume of tephra fall deposits on 17 june 1996 ruapehu volcano new zealand produced a sustained andesitic sub plinian eruption which generated a narrow tephra fall deposit extending more than 200 km from the volcano the extremely detailed data set from this eruption allowed methods for the determination of total grain size distribution and volume of tephra fall deposits to be critically investigated calculated total grain size distributions of tephra fall deposits depend strongly on the method used and on the availability of data across the entire dispersal area the voronoi tessellation method was tested for the ruapehu deposit and gave the best results when applied to a data set extending out to isomass values of 1 g m 2 the total grain size distribution of a deposit is also strongly influenced by the very proximal samples and this can be shown by artificially constructing subsets from the ruapehu database unless the available data set is large all existing techniques for calculations of total grain size distribution give only apparent distributions the tephra fall deposit from ruapehu does not show a simple exponential thinning but can be approximated well by at least three straight line segments or by a power law fit on semi log plots of thickness vs area 1 2 integrations of both fits give similar volumes of about 4 106 m3 integration of at least three exponential segments and of a power law fit with at least ten isopach contours available can be considered as a good estimate of the actual volume of tephra fall integrations of smaller data sets are more problematic

the physical volcanology of the 1600 eruption of huaynaputina southern peru volcan huaynaputina is a group of four vents located at 16 36 s 70 51 w in southern peru that produced one of the largest eruptions of historical times when 11 km3 of magma was erupted during the period 19 february to 6 march 1600 the main eruptive vents are located at 4200 m within an erosion modified amphitheater of a significantly older stratovolcano the eruption proceeded in three stages stage i was an 20 h sustained plinian eruption on 19 20 february that produced an extensive dacite pumice fall deposit magma volume 2 6 km3 throughout medial distal and distal parts of the dispersal area a fine grained plinian ashfall unit overlies the pumice fall deposit this very widespread ash magma volume 6 2 km3 has been recognized in antarctic ice cores a short period of quiescence allowed local erosion of the uppermost stage i deposits and was followed by renewed but intermittent explosive activity between 22 and 26 february stage ii this activity resulted in intercalated pyroclastic flow and pumice fall deposits 1 km3 the flow deposits are valley confined whereas associated co ignimbrite ash fall is found overlying the plinian ash deposit following another period of quiescence vulcanian type explosions of stage iii commenced on 28 february and produced crudely bedded ash lapilli and bombs of dense dacite 1 km3 activity ceased on 6 march compositions erupted are predominantly high k dacites with a phenocryst assemblage of plagioclase hornblende biotite fe ti oxides apatite major elements are broadly similar in all three stages but there are a few important differences stage i pumice has less evolved glass compositions 73 sio2 lower crystal contents 17 20 lower density 1 0 1 3 g cm3 and phase equilibria suggest higher temperature and volatile contents stage ii and stage iii juvenile clasts have more evolved glass 76 sio2 compositions higher crystal contents 25 35 higher densities up to 2 2 g cm3 and lower temperature and volatile contents all juvenile clasts show mineralogical evidence for thermal disequilibrium inflections on a plot of log thickness vs area1 2 for the fall deposits suggest that the pumice fall and the plinian ash fall were dispersed under different conditions and may have been derived from different parts of the eruption column system the ash appears to have been dispersed mainly from the uppermost parts of the umbrella cloud by upper level winds whereas the pumice fall may have been derived from the lower parts of the umbrella cloud and vertical part of the eruption column and transported by a lower altitude wind field thickness half distances and clast half distances for the pumice fall deposit suggests a column neutral buoyancy height of 24 32 km and a total column height of 34 46 km the estimated mass discharge rate for the 20 h long stage i eruption is 2 4 108 kg s and the volumetric discharge rate is 3 6 105 m3 s the pumice fall deposit has a dispersal index hildreth and drake 1992 of 4 4 and its index of fragmentation is at least 89 reflecting the dominant volume of fines produced of the 11 km3 total volume of dacite magma erupted in 1600 approximately 85 was evacuated during stage 1 the three main vents range in size from 70 to 400 m alignment of these vents and a late stage dyke parallel to the nnw sse trend defined by older volcanics suggest that the eruption initiated along a fissure that developed along pre existing weaknesses during stage i this fissure evolved into a large flared vent vent 2 with a diameter of approximately 400 m this vent was active throughout stage ii at the end of which a dome was emplaced within it during stage iii this dome was eviscerated forming the youngest vent in the group vent 3 a minor extra amphitheater vent was produced during the final event of the eruptive sequence recharge may have induced magma to rise away from a deep zone of magma generation and storage subsequently vesiculation in the rising magma batch possibly enhanced by interaction with an ancient hydrothermal system triggered and fueled the sustained plinian eruption of stage i a lower volatile content in the stage ii and stage iii magma led to transitional column behavior and pyroclastic flow generation in stage ii continued magma uprise led to emplacement of a dome which was subsequently destroyed during stage iii no caldera collapse occurred because no shallow magma chamber developed beneath this volcano

early mississippian climate based on oxygen isotope compositions of brachiopods alamogordo member of the lake valley formation south central new mexico stable isotope compositions of brachiopod shells from the alamogordo member of the lake valley formation south central new mexico provide information about early mississippian paleoclimate the alamogordo member was deposited on a ramp at the southwestern edge of the north american craton at lat 20 s it consists of cherty lime mudstone to lime packstone predominantly mudstone and wackestone strata and scattered mud cement waulsortian mounds the brachiopods analyzed lived at water depths estimated to have been between 100 and 300 m the 1 8 o values of well preserved shells of a single species of brachiopod average 2 3 relative to pdb the peedee belemnite isotope standard this average represents a temperature of 21 26 c if the isotopic composition of the seawater 1 8 o s w was between 0 and 1 as inferred from pleistocene and holocene 1 8 o s w and other considerations sea surface temperature would have been 26 31 c after we applied a 5 c correction for water depth this tropical climate for an early mississippian low latitude site agrees with previous isotopic and climate modeling results but contrasts with a temperate climate inferred from comparison of the lithology and biota of the alamogordo member with those of modern temperate carbonates

pyroclast characteristics of a subaqueous to emergent surtseyan eruption black point volcano california abstract we present the first ever quantitative analysis of pyroclast textures from subaqueous deposits of a surtseyan emergent volcano the volcano black point formed in mono lake s pleistocene precursor lake russell at 13 300 500 bp from an explosive basaltic eruption involving the interaction of shallow lake water 105 m with vesiculating magma the bulk of the edifice comprises unconsolidated tephra and represents submerged stages of eruption thin summit deposits formed during a final phase of emergent eruption and produced a group of low tuff rings lapillus clasts 16 mm to 32 mm and fine ash grains 4 0 0625 mm from the subaqueous deposits have textures that reveal a conduit history of bubble nucleation growth collapse intense microlite crystallisation and magma water interaction lapilli vesicularity ranges broadly to high values up to 89 while vesicle number densities 5 4 102 4 8 104 vesicles per mm3 are comparable to those published for strombolian and plinian basaltic eruptions 1 104 vesicles per mm3 vesicle size and volume distributions suggest continuous bubble nucleation but the suite of data as a whole indicates that a late stage nucleation event played a significant role in shaping the eruption ash grain morphology and surface features indicative of magma water interactions attest to water s role in fragmentation overall the pyroclast textures are consistent with water modification of a rather intense basaltic eruption which might have been of a violent strombolian or plinian style if the eruption had been dry and occurred subaerially

did lingering ice sheets moderate anoxia in the early palaeozoic of libya the hirnantian glaciation of west gondwana produced a glacially sculpted topography which is draped by organic rich latest ordovician and early silurian hot shales although these are the most important early palaeozoic source rock in north africa organic enrichment is distributed unevenly for example in al kufrah basin libya hot shales are elusive but outcrop analysis at the western basin demonstrates why this is the case the topmost mamuniyat formation of hirnantian age comprises glaciogenic sandstones passing upward into mixed facies of the tanezzuft formation which has a latest ordovician early silurian age the basal tanezzuft formation contains a shelly carbonate cool water deposits accumulated under oxygenating conditions and bioturbated sandstone succession above hummocky cross bedded and graded sandstone intervals are intercalated with shale and siltstone storm influx onto a muddy shelf these are interrupted by several lonestone bearing intervals ice rafted debris a striated pavement of subglacial origin and manganese oxide crusts and concretions the concretions and bioturbation imply oxygenation of the sea floor during transgression these putative glacial deposits were deposited following the main phase of the hirnantian glaciation at the same stratigraphic level as hot shales elsewhere in northern gondwana lingering ice caps may have produced well oxygenated marine waters precluding hot shale deposition

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

textural variation in juvenile pyroclasts from an emergent surtseyan type volcanic eruption the capelas tuff cone sao miguel azores abstract here i present textural data i e vesicularity vesicle size distributions vsd plagioclase crystallinity crystal size distributions csd combined with fractal analyses of particle outlines from a natural succession of alternating fall and surge deposits in the emergent capelas tuff cone azores the textural variation in the capelas succession is surprisingly small considering the wide variety of fragmentation processes vent activity and emplacement mechanisms that are characteristic of emergent eruptions the plagioclase crystal content varies between 24 and 33 vol csd analyses of plagioclase show near linear trends with a slight increase in time for the smallest crystal sizes with surge deposits having more groundmass plagioclase when compared with fall deposits this is consistent with crystallization induced by degassing and decompression at lower eruption rates the vesicularities of the capelas pyroclasts are more variable 18 to 59 vol with vsds displaying kinked trends characteristic of coalescence this is especially evident in the fall deposits and consistent with being formed in continuous uprush jetting with an overall shallow fragmentation level within the conduit bubble coalescence can also be identified in the surge deposits although to a much lesser extent the amount of bubble coalescence is negatively correlated with the amount of groundmass crystallization i e plagioclase in the capelas deposits a relatively broad range of fractal dimensions with average dbox 1 744 and 0 032 for the outlines of pyroclastic fragments emplaced by fall or as surges indicate that there is little difference in the fragmentation process itself at capelas in addition to this the fact that the fractal dimensions for both the fall and surge end members completely overlap suggests that shape modification due to abrasion and chipping of grain edges was minor during emplacement of base surges these results are consistent with emergent eruptions building tuff cones to be a relatively low energy phreatomagmatic landform e g at least when compared with more energetic phreatomagmatic eruptions producing tuff rings and maar volcanoes

paleoclimatic significance of phanerozoic reefs a database of pre quaternary phanerozoic reefs is used to test the significance of ancient reefs as paleoclimatic tracers the compilation of reef paleolatitudes through time and comparison with published paleoclimate curves shows that neither the width of the tropical reef zone nor the total latitudinal range of reefs is correlated with published estimates of paleotemperature however reefs trace paleoclimate indirectly algal reefs tend to prevail during icehouse climatic intervals and distinct high latitude reefs are only developed in cold intervals whereas during greenhouse episodes the reef zone usually ended abruptly at a particular latitudinal boundary in the subtropics additionally different biotic reef types tended to be concentrated in different latitudes the lowest latitudes have usually been occupied by coralline sponge or microbial reefs coral reefs tended to grow in intermediate latitudes and bryozoan reefs constantly occupied the highest latitudinal position

bryozoan distribution and growth form associations as a tool in environmental interpretation tasmania australia abstract the cool temperate carbonate sediments of eastern and western tasmania and bass strait composed of 29 50 of bryozoan skeletal grains two hundred and eighty three bryozoan bulk sediments around tasmania were studied to determine the relationship between bryozoan zoarial growth form association and their depth zone bryozoans skeletons are grouped into 11 morphotypes according to architecture and style of disarticulation fragmentation encrusting forms erect flexible cellariiform and erect rigid robust branching adeoniform cheilostomes are consistently the most widespread with delicate branching cyclostomes the next most common growth form associations were grouped into four depth related settings strand line shallow shelf middle shelf and deep shelf this study demonstrates that bryozoan morphotype associations occupying different depth zones are valuable for providing fundamental paleo environmental information morphotype associations are related mainly to water depth water energy sedimentation rate and substrate in the strand line setting encrusting unilaminar and erect flexible articulated branching are abundant while in the shallow shelf erect flexible encrusting multilaminar erect rigid robust fenestrate and delicate branching are major growth forms the above associations indicate high to moderate energy levels moderate to high sedimentation rates and flexible to hard substrates in middle shelf areas erect rigid fenestrate robust and delicate branching cheilostomes and cyclostomes are the most common growth forms these associations indicate decreasing water energy lower sedimentation rate and presence of stable hard to soft substrates in the deep shelf setting the dominance of erect rigid robust fenestrate and foliose forms indicate deep quiet water low sedimentation rate and hard substrate

bryozoan carbonates through time and space the calcareous skeletons of modern bryozoans are major contributors to sediments only outside the tropics and many temperate limestones are dominated by bryozoans often in conjunction with mollusks in the so called bryomol grain association can the presence of rock forming abundances of bryozoans be used to recognize nontropical sediments in the geologic past we have addressed this question by assembling a database of 176 ordovician pleistocene bryozoan rich deposits and plotting them on paleogeographical maps to obtain their latitudes of formation the results reveal a striking difference between post paleozoic and paleozoic latitudinal distributions whereas the great majority 94 of post paleozoic bryozoan rich deposits formed outside the tropics paleozoic bryozoan rich deposits have a more variable latitudinal distribution most 68 having formed within the tropics the presence of rock forming abundances of bryozoans in paleozoic sediments should not therefore be used to infer nontropical carbonate deposition the change in latitudinal distribution between the paleozoic and post paleozoic corresponds with a major taxonomic turnover in bryozoan faunas biotic interactions especially with predators may have led to the exclusion of sediment producing bryozoans from the post paleozoic tropics these evolutionary changes have had a more profound effect on carbonate platforms than has been previously appreciated

black point pyroclasts of a surtseyan eruption show no change during edifice growth to the surface from 100 m water depth abstract surtseyan eruptions have long been studied from their edifice deposits emplaced subaqueously or subaerially here we characterize a surtseyan eruption from both edifice and ash sheet deposits using a combination of techniques the volcano studied is black point which erupted into lake russell about 13 000 years ago from a water depth of 105 m producing a total of 0 8 km3 of tephra the techniques applied to samples acquired by targeted sampling based on geological observations in the field include geochemical characterization of the eruption products grain size analysis of the ash size fraction at proximal and medial sites three dimensional characterization of vesicles in lapilli and particle shape analysis for ash grains using the recently introduced freeware partisan the aim of this combined approach is to determine how the eruption evolved through its course from before magma fragmentation to the mechanisms of formation transport and deposition of the tephra field observations are consistent with the ash sheet deposits forming from eruption fed density currents and or fallout through the water column from a subaerial plume umbrella cloud geochemical correlation links the mound deposits as well as the tuff rings with a stratigraphic sequence at about 2 km west from the edifice both show non linear but broadly correlative geochemical variations in deposits from the subaqueous stage of the eruption through the subaerial one pyroclast vesicle textures indicate that volatile expansion played only a minor role in magma fragmentation and that there was a high level of bubble coalescence and hence outgassing during the eruption we infer that fragmentation was mostly phreatomagmatic resulting from explosive interactions between rising magma and water saturated sediments two dimensional particle shape analysis of ash grains shows a large variability in the shape parameters of convexity and solidity this tells us little about fragmentation style but serves as a proxy for magma vesicularity the most interesting aspect of this eruption is that none of the shape parameters or qualitative observations show any sort of systematic change during the course of the eruption despite the eruption having begun in 100 m of water and ended up subaerial

evolution of a miocene carbonate shelf northern apennines italy revealed through a quantitative compositional study abstract the evolution of the miocene san marino carbonate shelf torriana outcrop developed on the accretionary prism of the northern apennines has been interpreted through a stratigraphic and compositional study modal analysis allowed to quantify the framework components and to identify four microfacies through which the main steps of the carbonate ecosystem were traced the healthy phase of the carbonate shelf dominated by bryozoans and echinoids originated in a high energy transgressive setting and evolved during a warm period characterized by a progressive increase of nutrients the transitional stage is marked by a reduction of carbonate productivity and by terrigenous intermittent pulses associated with bioclast fragmentation the drowning succession corresponds to deepening upward facies formed by fine grained hybrid arenites to sandy marls with abundant planktonic foraminifera glauconitic grains and clay matrix the demise of the carbonate shelf might have resulted from a combination of regional and global factors that interplayed controlling the detrital input the nutrient budget and the deepening of the basin synsedimentary tectonics triggered subsidence of the basin and enhanced terrigenous discharge moreover the superposition of paleoclimatic and paleoceanographic events monterey and middle miocene climate optimum could have contributed with the intense weathering and remarkable detrital and nutrients supply

capelinhos 1957 1958 faial azores deposits formed by an emergent surtseyan eruption the 1957 1958 eruption of capelinhos faial island azores involved three periods of surtseyan hydromagmatic activity two in 1957 and one in 1958 deposits from this eruption are exposed both in sea cliffs cut into the flanks of the tuff cone and more distally 1 km from the vent five lithofacies are identified lithofacies i is composed of even thickness beds with laterally continuous internal stratigraphy and is interpreted to have been formed by fallout lithofacies ii consists of beds with internally discontinuous lenses and has sand wave structures that increase in abundance toward the outer margins of the tuff cone this lithofacies is interpreted as having been deposited from pyroclastic surges lithofacies iii is composed of mantle bedded deposits with laterally discontinuous internal stratigraphy this lithofacies is interpreted to have been formed by hybrid processes where fallout of tephra occurred simultaneously with pyroclastic surges in the outer flanks of the tuff cone lithofacies iii grades laterally into fallout beds of lithofacies i lithofacies iv consists of alternating beds of coarse ash aggregates and non aggregated fine ash and is particularly well developed in distal regions some of this facies was formed by fallout alternating beds also occur plastered against obstacles up to 2 km from the vent indicating an origin from wet pyroclastic surges the orientation of plastered tephra indicates that the surges were deflected by topography as they decelerated the distinction between surge and fallout in distal regions is uncertain because wind drifted fallout and decelerating surge clouds can generate similar deposits lithofacies v consists of scoria lapilli beds interpreted to be fallout from hawaiian style fire fountaining in the later stages of the eruption juvenile pyroclasts within hydromagmatic deposits are predominantly poorly vesicular 25 60 of clasts 30 vesicles however on both micro and macroscopic scales there is a wide range in clast vesicularity up to 70 vesicles indicating that although fragmentation was predominantly hydromagmatic vesiculation and magmatic volatile driven fragmentation operated simultaneously

secular changes in colony forms and bryozoan carbonate sediments through geological history ever since their first radiation in the ordovician bryozoans have contributed significantly to carbonate sedimentation most of the numerous colony forms developed by bryozoans have evolved repeatedly in different taxonomic groups and vary in their sediment producing potential there are nine basic bryozoan colony forms encrusting dome shaped palmate foliose fenestrate robust branching delicate branching articulated and free living the proportion of these morphotypes in bryozoan faunas period by period is shown to change significantly through the phanerozoic notable patterns include i steady increase in the number and proportion of encrusting species through time interrupted by a transient drop in the late palaeozoic ii post triassic decrease in robust branching colonies iii rise in the proportion of fenestrate colonies through the palaeozoic followed by their absence in the triassic and jurassic rarity in the cretaceous and reappearance in smaller proportions in the cenozoic and iv scarcity of articulated colonies and absence of free living colonies until the cretaceous most palaeozoic bryozoan sediments come from two architecturally distinct groups of colonies i domal delicate branching robust branching and palmate and ii fenestrate the former generate coarse particles both as sediment and components of stromatoporoid coral reefs in the early and mid palaeozoic whereas the delicate lacy fans of the latter create both prolific coarse sediment and form the cores of late palaeozoic deep water sub photic biogenic mounds nearly all post palaeozoic bryozoan sediments comprise cyclostomes and cheilostomes with many of the same growth forms but with the addition of free living colonies and significant numbers of articulated colonies the latter produced sand and mud sized bryozoan sediment via disarticulation for the first time in contrast to the palaeozoic post palaeozoic bryozoans generated sediment varying more widely across the grain size spectrum from mud to sand to gravel this article highlights the need to consider evolutionary changes in carbonate producing organisms when interpreting facies changes through time

impact of long term climate change and sea level fluctuation on mississippian to permian mid oceanic atoll sedimentation akiyoshi limestone group japan abstract to understand the impacts of long term climatic episodes and sea level changes on mid oceanic atoll sedimentation the stratigraphic changes of litho and bio facies and stratigraphic levels of freshwater diagenetic alteration and biostratigraphic discontinuity of the akiyoshi limestone group upper visean to upper capitanian were reviewed this group records approximately 70 million years of atoll sedimentation in a mid oceanic realm of the panthalassa ocean which developed nearly synchronously with the late paleozoic gondwana glaciation the entire succession of the akiyoshi limestone group was divided into four facies associations skeletal oolitic grainstone muddy limestone muddy limestone skeletal grainstone and reefal limestone associations skeletal oolitic grainstone and reefal limestone associations upper visean to bashkirian are characterized by an abundance of oolite and crinoidal grainstone and reef facies of warm adapted metazoan reef builders corals chaetetes and bryozoans respectively characteristics of skeletal oolitic grainstone and reefal limestone associations indicate their deposition in a warm climate with elevated sea level muddy limestone association moscovian to kasimovian records the demise of the warm adapted metazoan reef builders and frequent emergence events of the buildup many geochemical data support the significant drop in temperature during moscovian through kasimovian time these depositional and biotic events of muddy limestone association indicate that its sedimentation was strongly affected by cool climate and generally low sea level muddy limestone skeletal grainstone association is characterized by an upward increasing significance of calcimicrobes calcareous algae and calcisponges the succession records less frequent emergence events than that of muddy limestone association ii these characters reflect a warming climate and sea level rise in gzhelian to capitanian time the climate episodes and sea level changes interpreted from the akiyoshi limestone group generally correspond to the global trend of mississippian to permian long term climatic change and sea level fluctuation related to the major glacial events of gondwana glaciation the late visean to bashkirian warm climate and elevated sea levels moscovian to kasimovian cooling and sea level lowering and gzhelian to capitanian warming and sea level rise recorded in the akiyoshi buildup are comparable to the global climate model and sea level fluctuation which contain early to middle mississippian pre glacial warm climate and highstands middle to late pennsylvanian cooling and related glacio eustatic sea level fluctuation at the culmination of gondwana glaciation and early to middle permian global warming and sea level rise due to the retreat of gondwana ice sheets akiyoshi atoll sedimentation was most significantly affected pennsylvanian global cooling and related sea level lowering but their impact was short lived the onset of the cooling and frequent emergence events in the akiyoshi buildup occurred delayed approximately 10 millions years later than that of comparable episodes on the pangean shelves the post glacial warming in the akiyoshi buildup started at least several millions of years earlier than in epicontinental seas of the pangea this phenomenon may be due to the palaeoposition of the akiyoshi buildup in a mid oceanic realm of the panthalassa ocean

contemporaneous phreatomagmatic and effusive activity along the hverfjall eruptive fissure north iceland eruption chronology and resulting deposits abstract the 2500 bp hverfjall eruption in northern iceland produced pyroclastic and effusive deposits of widely different characteristics along the length of the eruptive fissure the southern half of the fissure was located in a shallow lake whereas the northern part of the fissure extended onto dry land this specific setting with overlapping periods of activity at the different vents resulted in various mingling features between fine grained phreatomagmatic deposits and lava flows here we reconstruct the course of events during this eruption based on field observations and granulometric analyses of the resulting deposits the eruption can be divided into three main phases depending on the main depositional characteristics and vent locations the initial phase is dominated by phreatomagmatic fall deposits which are attributed to an overall high eruption rate in a shallow lacustrine setting the second phases involved opening of two new vents on dry ground and deposition of scoria and lava flows the third and final phase of the eruption is associated with a lowering of the eruption rate in the southernmost vent with a shift in the activity from continuous uprush and fall deposits to discrete explosions and emplacement of base surges these surges display features consistent with drying up with increasing distance from the vent suggesting that their dynamics changed during emplacement most wet surges were channelized within a preexisting graben structure close to the vent but some more dilute i e dry surges were able to flow over this obstacle and continued to flow for more than 5 km away from the vent and 100 m uphill before stopping

holocene explosive volcanism of the jan mayen island volcanic province north atlantic abstract the volcanic island jan mayen located in the norwegian greenland sea hosts the active stratovolcano of beerenberg the northernmost active subaerial volcano in the world at least five eruptions are known from the island following its discovery in the 17th century but its eruptive history prior to this is basically unknown in this paper two sediment cores retrieved close to jan mayen have been studied in detail to shed light on the holocene history of explosive volcanism from the jan mayen volcanic province horizons with elevated tephra concentrations were identified and tephra from these was analysed to determine major element chemistry of the tephra the tephra chemistry was used to provide a link between the two cores and the land based tephra records from jan mayen island we managed to link two well developed tephra peaks in the cores by their geochemical composition and age to jan mayen one of these peaks represents the 1732 ad eruption of eggoya while the other peak represents a previously undescribed eruption dated to around 10 3 ka bp two less prominent tephra peaks one in each core dated to approximately 2 3 and 3 0 ka bp also have a distinct geochemical character linking them to jan mayen volcanism however the most prominent tephra layer in the cores located close to jan mayen and numerous other cores along the jan mayen ridge is the 12 1 ka bp vedde ash originating from the iceland volcanic province we find that the holocene volcanism on jan mayen is much less explosive than volcanism in iceland and propose that either low amounts of explosive volcanic activity from the summit region of beerenberg or small to absent glacier cover on beerenberg is responsible for this

characteristics of dune bedded pyroclastic surge bedsets abstract the grainsize characteristics of dune bedded pyroclastic surge bedsets are surveyed the variance between coarsest and finest beds ranges from 1 to 6 phi in different surge bedsets and it increases as the grainsize of the coarsest bed increases reflecting an increasing velocity of emplacement deposits of wet surges identified as those which contain accretionary and ash coated lapilli tend to be finer and show less variance this partly because wet ash is cohesive but mainly because wet surges tend to be weaker dry surge bedsets are strongly fines depleted wet ones less so the lack of erosion of underlying ash layers shows that the environment is a strongly depositional one individual bedsets are demarcated by thin intervening fine ash fall layers which are the complementary ash cloud deposits settled or flushed out after the passage and decay of each turbulent surge surge deposits are generally less coarse than the coarsest associated airfall deposits which shows that they are formed by generally weaker events this study helps interpret the dune bedded parts of the landscape mantling may 18th 1980 blast deposit of mount st helens the blast was a very violent event but the variance and the grainsize of the coarsest bed are those of a relatively weak surge this suggests that the dune bedding was produced by a weak effect such as minor turbulence in a thin pyroclastic flow coming to rest in a mountainous terrain roughened by tree stumps and fallen logs

stratigraphy sedimentology and eruptive mechanisms in the tuff cone of el golfo lanzarote canary islands the tuff cone of el golfo on the western coast of lanzarote canary islands is a typical hydrovolcanic edifice along with other edifices of the same age it was constructed along a fracture oriented nee sww that coincides with the main structural trend of recent volcanism in this part of the island we conducted a detailed stratigraphic study of the succession of deposits present in this tuff cone and here interpret them in light of the depositional processes and eruptive dynamics that we were able to infer the eruptive sequence is represented by a succession of pyroclastic deposits most of which were emplaced by flow plus a number of air fall deposits and ballistic blocks and bombs we distinguished five different eruptive depositional stages on the basis of differences in inferred current flow regimes and fragmentation efficiencies represented by the resulting deposits the different stages may be related to variations in the explosive energy eight lithofacies were identified based on sedimentary discontinuities grain size components variations in primary laminations and bedforms the volcanic edifice was constructed very rapidly around the vent and this is inferred to have controlled the amount of water that was able to enter the eruption conduit the sedimentological characteristics of the deposits and the nature and distribution of palagonitic alteration suggest that most of the pyroclastic succession in el golfo was deposited in a subaerial environment this type of hydrovolcanic explosive activity is common in the coastal zones of lanzarote and the other canary islands and is one of the main potential hazards that could threaten the human population of this archipelago detailed studies of these hydrovolcanic eruptions such as the one we present here can help volcanologists understand the hazards that this type of eruption can generate and provide essential information for undertaking risk assessment in similar volcanic environments