debris jets in continental phreatomagmatic volcanoes a field study of their subterranean deposits in the coombs hills vent complex antarctica abstract the ferrar large igneous province of antarctica contains significant mafic volcaniclastic deposits some of which are interpreted to fill large vent complexes such a complex was re examined at coombs hills to map individual steep sided cross cutting bodies in detail and we found several contrasting types two of which are interpreted to have filled subterranean passageways forcefully opened from below into existing non consolidated debris these transient conduits were opened because of the propagation of debris jets upward moving streams of volcaniclastic debris steam magmatic gases liquid water droplets following explosive magma aquifer interaction some debris jets probably remained wholly subterranean whereas others made it to the surface but the studied outcrops do not allow us to differentiate between these cases the pipes filled with country rock rich lapilli tuff or tuff breccia are interpreted to have formed following phreatomagmatic explosions occurring near the walls or floor of the vent complex causing fragmentation of both magma and abundant country rock material in contrast some of the cross cutting zones filled with basalt rich tuff breccia or lapilli tuff could have been generated following explosions taking place within pre existing basalt bearing debris well away from the complex walls or floor we infer that once focused jets were formed they did not incorporate significant amounts of existing debris while travelling through them instead incorporation of fragments from the granular host took place near explosion sites other basalt rich tuff breccia zones accompanied by domains of in situ peperite and coherent basalt pods are inferred to have originated by less violent processes

drivers of explosivity and elevated hazard in basaltic fissure eruptions the 1913 eruption of ambrym volcano vanuatu sw pacific abstract fissure eruptions along linear structures can extend for several tens of kilometres with distinct separate manifestations of volcanism along their length they typically involve low viscosity mafic magmas forming long lava flows and cinder cones eruptions in 1894 and 1913 on ambrym volcano vanuatu showed how these mildly explosive eruptions can rapidly transform into violent explosive events producing significant hazard and widespread volcanic ash clouds during the 1913 episode a fissure began in the central caldera and basaltic magmas broke out in a series of locations down the island s western flank in all sites over 100 m in elevation fissure outbreaks produced vigorous lava fountains and highly fluid lava flows that travelled rapidly to the shoreline when the outbreaks propagated along the island s axis into coastal plain areas a climactic series of explosive eruptions occurred producing a 1 2 km long by 600 m wide maar and tephra ring a further small tuff ring was formed later creating a temporary island 400 m offshore the onshore tephra ring destroyed a hospital and associated buildings its last evacuating occupants were close witnesses to the eruption processes deposits exposed in the lower portion of the tephra ring show that this part of the eruption began with a mild phreatomagmatic explosive eruption from a narrow vent followed by a magmatic scoria producing phase subsequently a complex sequence of highly explosive phreatomagmatic eruptions occurred producing pyroclastic surges along with repeated distinctive breccia horizons rich in coral and lava country rock these features tally with eye witness accounts to indicate that the main eruption phase was produced by a periodically shifting locus of phreatomagmatic fragmentation and eruption along a single e w fissure the glassy and vesicle poor pyroclasts produced during this eruption phase were dominantly fragmented in a brittle manner by magma water interaction low volatile content of the magma upon fragmentation is confirmed by ftir analysis showing

reconstruction of a kimberlite eruption using an integrated volcanological geochemical and numerical approach a case study of the fox kimberlite nwt canada abstract an integrated approach involving volcanology geochemistry and numerical modelling has enabled the reconstruction of the volcanic history of the fox kimberlite pipe the observed deposits within the vent include a basal massive poorly sorted matrix supported lithic fragment rich eruption column collapse lapilli tuff extensive vent widening during the climactic magmatic phase of the eruption led to overloading of the eruption column with cold dense country rock lithic fragments dense juvenile pyroclasts and olivine crystals triggering column collapse 40 dilution of the kimberlite by granodiorite country rock lithic fragments is observed both in the physical componentry of the rocks and in the geochemical signature where enrichment in al2o3 and na2o compared to average values for coherent kimberlite is seen the wide deep open vent provided a trap for a significant proportion of the collapsing column material preventing large scale run away in the form of pyroclastic flow onto the ground surface although minor flows probably also occurred a massive to diffusely bedded poorly sorted matrix supported accretionary lapilli bearing lithic fragment rich lapilli tuff overlies the column collapse deposit providing evidence for a late phreatomagmatic eruption stage caused by the explosive interaction of external water with residual magma correlation of pipe morphology and internal stratigraphy indicate that widening of the pipe occurred during this latter stage and a thick granodiorite cobble boulder breccia was deposited ash and accretionary lapilli rich tephra deposited on the crater rim during the late phreatomagmatic stage was subsequently resedimented into the vent incompatible elements such as nb are used as indicators of the proportion of the melt fraction or kimberlite ash retained or removed by eruptive processes when compared to average coherent kimberlite the ash rich deposits exhibit 30 loss of fines whereas the column collapse deposit exhibits 50 loss this shows that despite the poorly sorted nature of the column collapse deposit significant elutriation has occurred during the eruption indicating the existence of a high sustained eruption column the deposits within fox record a complex eruption sequence showing a transition from a probable violent sub plinian style eruption driven by instantaneous exsolution of magmatic volatiles to a late phreatomagmatic eruption phase mass eruption rate and duration of the sub plinian phase of the eruption have been determined based on the dimensions of milled country rock boulders found within the intra vent deposits calculations show a short lived eruption of one to eleven days for the sub plinian magmatic phase which is similar in duration to small volume basaltic eruptions this is in general agreement with durations of kimberlite eruptions calculated using entirely different approaches and parameters such as predictions of magma ascent rates in kimberlite dykes

pyroclastic deposits within the east greenland tertiary flood basalts stratigraphic geochemical and mineralogical characterization of pyroclastic deposits on the gronau west nunatak of east greenland indicates that both alkaline and basaltic tephras occurred during the eruption of flood basalts associated with the opening of the north atlantic ocean in the early tertiary within the exposed section of gronau west nunatak c 1 km thick representing c 1 6 ma 17 of the horizons are phreatomagmatic basaltic tephras near the top of the section an alkaline tuff was dated at 53 8 0 3 ma by 40 ar 39 ar method this tuff contains a distinctive mineralogy sanidine mg katophorite aegirine and geochemistry melt inclusion compositions and trace and rare earth element abundances that indicate it was erupted from the gardiner melanephelinite carbonatite volcanic complex located 175 km to the se which was active from c 55 to 52 ma the alkaline tuff can be correlated to age equivalent compositionally similar alkaline pyroclastic horizons reported in north atlantic sediment cores and in outcrops in northern europe making it an important regional time stratigraphic marker this study indicates that north atlantic explosive volcanic events were prevalent regionally widespread and originated in east greenland during continental rifting suggesting the need for reappraisal of the impact of north atlantic volcanism on climate change during the early tertiary

the lami pyroclastic succession lipari aeolian islands a clue for unravelling the eruptive dynamics of the monte pilato rhyolitic pumice cone abstract the lami succession piled up during the explosive activity of the monte pilato on lipari island central aeolian arc italy it comprises a repetitive alternation of fall and pyroclastic density current beds which define 13 units separated by fine ash beds despite the homogeneous rhyolitic composition the juveniles fragments show strong textural heterogeneities from highly vesicular pumice to dense black obsidians the pumice types are termed i tube which shows elongated tubular vesicles ii frothy highly vesicular with large sub rounded vesicles and iii agglomerated and welded two types of obsidian pyroclasts are recognized i aphyric black obsidian and ii grey obsidian showing spherulitic texture all these pyroclastic types occur throughout the stratigraphic succession and mixed in the same bed they account for changing physical conditions along the conduit which is hypothesised to be filled by a melt plug during the eruption of 12 out of 13 units the strong textural heterogeneities of the juvenile fragments testify for the existence of differential shear rates which can be explained though two shear mechanism i simple shear producing horizontal zonation of texture in the conduit with black obsidians along the conduit walls passing to tube and frothy pumice to the centre and ii pure shear producing a vertical zonation in the conduit with black obsidians at the top overlying frothy pumice agglomerated and welded pumice of unit 13 is thought to represent the portion of magma heated by a batch of hotter melt that entered the magma chamber and triggered the eruption the deposits of unit 13 represent a unique case of spatter accretion due to accumulation of hot low viscous rhyolitic pumice fragments emplaced from pyroclastic fountaining

diagenesis of an upper triassic reef complex wilde kirche northern calcareous alps austria the wilde kirche reef complex early late rhaetian grew as an isolated carbonate structure within the shallow kossen basin at the triassic jurassic boundary a single brief c 10 50 ka period of subaerial exposure occurred the preserved karst profile 70 m thick displays a vadose zone enhanced dissolution at a possible palaeo watertable 5 15 m below the exposure surface and a freshwater phreatic zone karst porosity was predominantly biomouldic primary cavities and biomoulds were enlarged and interconnected in the freshwater phreatic zone cavity networks developed preferentially in patch reef facies resubmergence of the reef complex allowed minor modification of the palaeokarst surface by sea floor dissolution and fe mn crust deposition on a sediment starved passive margin fibrous calcite fc radiaxial fibrous calcite rfc and fascicular optic calcite foc cements preserved as low mg calcite lmc are abundant in primary and karst dissolution cavities fc cement is restricted to primary porosity particularly as a synsedimentary cement at the windward reef margin fc rfc and foc contain microdolomite inclusions and show patchy non bright cathodoluminescence 18o values of non luminescent portions interpreted as near original are 1 16 to 1 82 0 close to the inferred 18o of calcite precipitated from late triassic sea water 13c values are constant 3 to 2 2 0 these observations suggest fc rfc and foc were originally marine high mg calcite hmc precipitates and that the bulk of porosity occlusion occurred not in the karst environment but in the marine environment during and after marine transgression the hmc to lmc transition may have occurred in contact with meteoric water only in the case of fc cement the most altered brightly luminescent portions of rfc foc cements yield 18o 2 44 to 5 8 0 suggesting hmc to lmc alteration at up to 34 c in the shallow burial environment at depths of 180 250 m abundant equant cements with 18o 4 1 to 7 1 0 show crisp uniform or zoned dull luminescence they are interpreted as unaltered cements precipitated at 33 36 c at 200 290 m burial depth from marine derived fluids under a slightly enhanced geothermal gradient fluids carrying the equant cements may have induced the hmc to lmc transition in the fibrous cements

lithofacies association and stratigraphy of the paredao volcano trindade island brazil abstract a stratigraphic framework can be constructed through lithofacies description and interpretation the application of this method in volcanic systems is a fundamental tool for understanding their evolution and eruptive dynamics and for establishing hazard assessments in active volcanic areas this work presents and discusses a detailed stratigraphy of the paredao volcano allowing the understanding of the lateral and vertical distribution of volcanic facies and the 3d geometry of the deposits generating a model for the volcanism and discussing the erupting mechanisms the paredao volcano is a scoria cone located in the southeast of trindade island and represents the youngest subaerial volcanic episode pleistocene recorded in brazil its stratigraphic organization indicates nephelinitic flows alternate with lapillistones and lapilli tuffs of same composition at the base and a pyroclastic cone with nearly 200 m high above the sea level through lithofacies description and association 4 stratigraphic columns were constructed in scale 1 50 three of them at the tartarugas beach trv 01 trv 02 trv 03 and the other one at the pyroclastic cone trv 04 the petrographic characterization of facies was carried through conventional optical microscopy the stratigraphic analysis resulted in the distinction of seven lithofacies divided into two coherent and five volcaniclastic one autoclastic and four pyroclastic the coherent lithofacies are of massive npm and vesicular porphyritic nephelinite npv texturally the lavas are holocrystalline and vesicular composed by olivine phenocrysts in a very fine matrix of clinopyroxene nepheline zeolite and titanomagnetite at times with phlogopite the autoclastic lithofacies of nephelinitic breccia nb is composed of fragments 2 mm 15 cm of vesicular nephelinite and the pyroclastic ones are composed mainly of lapillitic scoria fragments with subordinate bombs and volcanic ash the pyroclastic lithofacies are of massive lapillistone lm and lapilli breccia lbm planar stratified lapilli tuff ltp and reverse grading lapillistone lrg the lithofacies associations characterize a a rubbly pahoehoe and pahoehoe flow types distal and proximal pyroclastic deposits the interpretation of the vertical and lateral facies distribution leads to the conclusion that the volcanism occurred with an initial phreatomagmatic stage succeeded by a strombolian stage pyroclastic with primary fragments domain and a final hawaiian stage lava flows domain

phreatomagmatic vs magmatic eruptive styles in maar diatremes a case study at twin peaks hopi buttes volcanic field navajo nation arizona the hopi buttes volcanic field hbvf is located on the colorado plateau northern arizona in this miocene volcanic field the erosion level increases southward allowing the study of maar diatreme volcanoes from top posteruptive crater infill and ejecta ring to bottom lower diatreme the twin peaks volcanic complex consists mostly of two hills north peak and south peak with thick lavas at their summits and pyroclastic rocks underneath in the hbvf such volcanic remnants have received little scientific attention so far despite their relative abundance our field observations allow us to interpret the north and south peaks as remnants of two maar diatreme volcanoes which evolved into lava lakes filling the craters within the complex we distinguish four volcanic units from unit 1 at the bottom to unit 4 at the top on the basis of the field description of the deposits and the componentry measurements we suggest that unit 1 is phreatomagmatic unit 2 is phreato strombolian with mixed phreatomagmatic and strombolian characteristics unit 3a is phreato hawaiian with mixed phreatomagmatic and hawaiian characteristics unit 3b is hawaiian formed by lava fountains and unit 4 consists of lava lakes filling the maar craters there is therefore a progressive evolution from a purely phreatomagmatic eruptive style which excavated the craters and diatremes and partly filled them to magmatic explosive to nonexplosive eruptive styles which filled the maar craters up to the pre eruptive surface we discuss traditional criteria used to distinguish phreatomagmatic from magmatic eruptive styles in ultramafic to mafic maar diatreme volcanoes

lidar based quantification of lava flow susceptibility in the city of auckland new zealand abstract lava flows represent one of the most significant volcanic hazards from basaltic monogenetic volcanoes such as spatter cones scoria cones maars and tuff rings they are common features emanating from parasitic vents on the flanks of polygenetic volcanoes and in dominantly flat lying intraplate volcanic fields the auckland volcanic field avf is a volcanic field that has been active for the last ca 250 ka hosting at least 50 monogenetic volcanoes morphometric parameters of lava flows such as volume length thickness and area were used to quantify the potential lava flow inundation susceptibility to new zealand s most densely populated area the city of auckland based on an airborne light detection and ranging lidar digital surface model dsm the morphometric parameters of fifteen studied flows included average length of 2 5 km range 0 7 6 5 km overall average thickness of 14 8 m range 3 4 43 8 m average of maximum thicknesses of 48 2 m range 18 3 180 5 m average area occupied of 5 1 km 2 range 0 4 25 1 km 2 and average volume of 0 12 km 3 range 0 005 1 km 3 based on these parameters and a lidar derived dsm the present topography was classified into sea topographic depressions low lying areas prone to inundation by an average lava flow buffer zones prone to inundation only by extremely thick lava flows and peaks or ridges which are unlikely to be overtopped in monogenetic fields each new vent occurs in a new location creating uncertainty around the spatial location of the volcanic hazard thus this research provides a general vent location independent approach to describe the lava flow susceptibility for a potentially active monogenetic volcanic field what this analysis reveals is that the city of auckland can be divided into two distinct areas with strongly different susceptibility to lava flow inundation the southern part of the city is predominantly flat without hindrance to lava flow whereas the hilly northern and central part has many ridges that can limit or channelise lavas these contrasting properties must be accounted for in scenario based or probabilistic hazard and risk models developed for the avf

pyroclastic kimberlite deposits from the victor northwest pipe ontario canada the transition from phreatomagmatic to magmatic explosivity magmas of all compositions including kimberlites may undergo both magmatic and phreatomagmatic fragmentation during emplacement in this contribution we assess the extent of phreatomagmatism at different stages of the pipe formation for the victor northwest kimberlite northern ontario canada detailed drill core and petrographic observations of all volcanic facies within the pipe suggest emplacement in two volcanic cycles each consisting of three repeated eruptive phases pyroclastic kimberlite formed at the start of both cycles is characterized by the presence of fine grained poorly sorted deposits containing broken olivine crystals angular country rock fragments accretionary lapilli and variably vesicular irregular shaped juvenile pyroclasts all observations indicate these deposits formed by phreatomagmatism subsequent clastogenic coherent kimberlite deposits were formed as a result of hawaiian fire fountaining during the second phase of both cycles these phases were followed by mass wastin

correlation of the triassic jurassic boundary in england and austria an attempt is made to fix the triassic jurassic boundary at the classic locality of st audries bay somerset by reference to biostratigraphy in conjunction with lithostratigraphy utilizing comparison with the more fully marine sequence of the kendelbach gorge in austria the boundary is placed at the base of the blue lias formation

sedimentology eruptive mechanism and facies architecture of basaltic scoria cones from the auckland volcanic field new zealand abstract scoria cones are a common type of basaltic to andesitic small volume volcanoes e g 10 1 10 5 km 3 that results from gas bubble driven explosive eruptive styles although they are small in volume they can produce complex eruptions involving multiple eruptive styles eight scoria cones from the quaternary auckland volcanic field in new zealand were selected to define the eruptive style variability from their volcanic facies architecture the reconstruction of their eruptive and pyroclastic transport mechanisms was established on the basis of study of their volcanic sedimentology stratigraphy and measurement of their pyroclast density porosity scanning electron microscopy 2d particle morphology analysis and visible and near visible infrared spectroscopy collection of these data allowed defining three end member types of scoria cones inferred to be constructed from lava fountaining transitional fountaining and strombolian type and explosive strombolian type using the physical and field based characteristics of scoriaceous samples a simple generalised facies model of basaltic scoria cones for the avf is developed that can be extended to other scoria cones elsewhere the typical avf scoria cone has an initial phreatomagmatic phases that might reduce the volume of magma available for subsequent scoria cone forming eruptions this inferred to have the main reason to have decreased cone volumes recognised from auckland in comparison to other volcanic fields evolved dominantly in dry eruptive condition e g no external water influence it suggests that such subtle eruptive style variations through a scoria cone evolution need to be integrated into the hazard assessment of a potentially active volcanic field such as that in auckland

biogeochemical constraints on the triassic jurassic boundary carbon cycle event 1 the end triassic mass extinctions represent one of the five most severe biotic crises in earth history yet remain one of the most enigmatic ongoing debate concerns the environmental effects of the central atlantic magmatic province camp eruptions and their linkage with the mass extinction event across the triassic jurassic boundary there is conflicting paleo evidence for changes in atmospheric pco2 during the extrusion of the camp basalts studies on sediments from european and pacific localities have however identified a substantial negative isotopic anomaly up to 3 5 across the tr j boundary providing an important indicator of changes in the operation of the ancient global carbon cycle we sought to explain the paleo evidence by utilizing a carbon cycle model for the hothouse world of the end triassic that emphasizes the chemical weathering of silicate and carbonate rocks and the ocean carbonate chemistry we find that volcanic co2 outgassing fails to fully account for either a sufficient rise in atmospheric pco2 indicated by the stomata of fossil leaves or the sedimentary isotopic fingerprint instead the scenario that best fits all of the geologic evidence is a positive feedback loop in which warming due to a buildup of volcanically derived co2 triggers destabilization of seafloor methane hydrates and the catastrophic release of ch4 palfy et al 2001 we calculate that this carbon cycle perturbation was huge involving the release of 8000 9000 gt c as co2 during the camp basaltic eruptions and 5000 gt c as ch4 in the model the initial isotopic excursion is assumed to take place over 70 kyr while complete reequilibration of the ocean atmosphere system with respect to co2 is accomplished over 700 1000 kyr our results thus provide a preliminary theoretical explanation for the bioevents estimated pco2 changes and isotopic excursions observed in marine and continental sediments at this time

the epithermal gold silver deposit of choquelimpie northern chile the choquelimpie gold silver deposit is located at an elevation of 4 860 m above sea level 115 km due east of arica in northern chile a 5 500 metric ton day open pit mine heap leach operation was brought on stream in 1988 by the sociedad contratual minera vilacollo whose shareholders are shell chile westfield minera and citibank the deposit is located near the arica bend of the south american continent where the earth s crust has a thickness of more than 70 km it lies in a belt of calc alkalic to alkali calcic miocene to recent volcanism which runs parallel to the pacific coast and contains a series of volcanic hosted precious metal deposits these have been worked in peru chile and bolivia since the time of the spanish conquest usually for ag but also with locally important au values these deposits are characterized by ag au pb zn the volcano hosting the mineralization is a subalkalic stratovolcano of late miocene age a late magmatic event has been dated at 6 60 or 0 20 ma the volcano is located at the intersection of a major n 60 degrees e trending lineament with a n 35 degrees e fault active from the late miocene to the recent the volcano consists of dacitic and andesitic volcanic flows and breccias and is intruded by dacite domes erosion of its central part has exposed a zone of hydrothermal alteration and mineralization centered upon an andesitic feldspar porphyry which is probably a dome most of the mineralization worked until recently came from narrow argentiferous veins but the present operation is centered on a series of hydrothermal breccia bodies emplaced in a n 60 degrees e trending zone 2 km long peripheral to the mineralized zone propylitic alteration exists grading inward to a sericite pyrite assemblage toward the breccia bodies this assemblage is overprinted by silicification associated with base metals pyrite kaolinite and barite the breccias themselves and some adjacent lithologies contain precious metal species including native au and ag electrum and argentite a final stage of mineralization is represented by base and precious metals in veins which cut the breccias oxidation has produced supergene enrichment of ag but probably not of au limited fluid inclusion data from structures peripheral to the breccia bodies indicate homogenization temperatures ranging from 213 degrees to 305 degrees c salinities are below 5 wt percent nacl equiv samples from one location show evidence of boiling at a minimum depth estimated as 300 m below the paleowater table based on the available evidence choquelimpie is interpreted as a volcanic hosted acid sulfate epithermal deposit it is associated with an ag au pb zn metallogenic province which contrasts with the maricunga and el indio au ag cu as provinces of central chile

variations in eruption style during the 1931 a d eruption of aniakchak volcano alaska abstract the 1931 a d eruption of aniakchak volcano alaska progressed from subplinian to effusive eruptive style and from trachydacite to basaltic andesite composition from multiple vent locations eyewitness accounts and new studies of deposit stratigraphy provide a combined narrative of eruptive events additional field compositional grain size componentry density and grain morphology data document the influences on changing eruptive style as the eruption progressed the eruption began on 1 may 1931 a d when a large subplinian eruption column produced vesicular juvenile rich tephra subsequent activity was more intermittent as magma interacted with groundwater and phreatomagmatic ash and lithic rich tephra was dispersed up to 600 km downwind final erupted products were more mafic in composition and the eruption became more strombolian in style stratigraphic evidence suggests that two trachydacitic lava flows were erupted from separate but adjacent vents before the phreatomagmatic phase concluded and that basaltic andesite lava from a third vent began to effuse near the end of explosive activity the estimated total bulk volume of the eruption is 0 9 km 3 which corresponds to approximately 0 3 km 3 of magma eruption style changes are interpreted as follows 1 a decrease in magma supply rate caused the change from subplinian to phreatomagmatic eruption 2 a subsequent change in magma composition caused the transition from phreatomagmatic to strombolian eruption style additionally the explosion and effusion of a similar magma composition from three separate vents indicates how the pre existing caldera structure controlled the pathway of shallow magma ascent thus influencing eruption style

influences on the variability of eruption sequences and style transitions in the auckland volcanic field new zealand abstract monogenetic basaltic volcanism is characterised by a complex array of eruptive behaviours reflecting spatial and temporal variability of the magmatic properties e g composition eruptive volume magma flux as well as environmental factors at the vent site e g availability of water country rock geology faulting these combine to produce changes in eruption style over brief periods minutes to days in many eruption episodes monogenetic eruptions in some volcanic fields often start with a phreatomagmatic vent opening phase that later transforms into dry magmatic explosive or effusive activity with a strong variation in the duration and importance of this first phase such an eruption sequence pattern occurred in 83 of the known eruption in the 0 25 my old auckland volcanic field avf new zealand in this investigation the eruptive volumes were compared with the sequences of eruption styles preserved in the pyroclastic record at each volcano of the avf as well as environmental influencing factors such as distribution and thickness of water saturated semi to unconsolidated sediments topographic position distances from known fault lines the avf showed that there is no correlation between ejecta ring volumes and environmental influencing factors that is valid for the entire avf in contrary using a set of comparisons of single volcanoes with well known and documented sequences resultant eruption sequences could be explained by predominant patterns of the environment in which these volcanoes were erupted based on the spatial variability of these environmental factors a first order susceptibility hazard map was constructed for the avf that forecasts areas of largest likelihood for phreatomagmatic eruptions by overlaying topographical and shallow geological information combining detailed phase by phase breakdowns of eruptive volumes and the event sequences of the avf along with the new susceptibility map more realistic eruption scenarios can be developed for different parts of the volcanic field this approach can be applied to tailoring field and sub field specific hazard forecasting at similar volcanic fields worldwide

mixed deposits of complex magmatic and phreatomagmatic volcanism an example from crater hill auckland new zealand a series of alternating phreatomagmatic wet and magmatic dry basaltic pyroclastic deposits forming the crater hill tuff ring in new zealand contains one unit m1 which can only be interpreted as the products of mixing of ejecta from simultaneous wet and dry explosions at different portions of a multiple vent system the principal characteristics of m1 are a rapid lateral changes in the thicknesses of and proportions in juvenile components in individual beds and b wide ranges of juvenile clast densities in every sample m1 appears to have been associated with an elongate source of highly variable and fluctuating magma water ratios and magma discharge rates this contrasts with the only other documented mixed wet and dry basaltic pyroclastic deposits where mixing from two point sources of quite different but stable character has been inferred

paleomagnetic constraints on a time stratigraphic framework for the evolution of ohachidaira volcano and the summit caldera central hokkaido japan we present a revised stratigraphy of the proximal deposits around ohachidaira volcano and their paleomagnetic directions obtained from 21 sites 193 samples we identify four proximal pyroclastic members from ohachidaira volcano that were produced after the early edifice building effusive volcanism from older to younger 1 vent opening mafic pyroclastic density current pdc deposits kobachidaira ignimbrite 2 maar forming mafic tephra ring deposits mamiyadake tephra ring 3 mafic to silicic caldera forming deposits sounkyo member and 4 mafic pdc deposits kumonotaira ignimbrite well grouped paleomagnetic directions obtained from the kobachidaira ignimbrite and the sounkyo member indicate that each deposit was formed over a short time interval compared to geomagnetic secular variation that is less than a century conversely scattered paleomagnetic directions of the mamiyadake tephra ring record the secular variation of the geomagnetic field that took place over a longer time interval that is at least 750 yr probably more the kumonotaira ignimbrite which has a similar paleomagnetic direction to the underlying sounkyo member may have been produced during the sounkyo eruption from comparison between our data and the previously reported paleomagnetic directions of two petrologically distinct distal ignimbrites we conclude that the sounkyo member is a proximal correlative of the distal pyroxene rich ignimbrite and that the distal hornblende rich ignimbrite may not be of an ohachidaira origin the revised stratigraphy suggests that ohachidaira volcano is a maar caldera complex volcano and that the summit caldera is a flaring funnel formed incrementally by explosive erosion and syn eruptive collapse of the vent walls volume estimates imply that the sounkyo eruption fundamentally enlarged the vent and formed the caldera our data confirm that the comparison of paleomagnetic directions can offer a useful identification and correlation tool for the ohachidaira pyroclastic sequences

mass extinctions and sea level changes abstract review of sea level changes during the big five mass extinctions and several lesser extinction events reveals that the majority coincide with large eustatic inflexions the degree of certainty with which these eustatic oscillations are known varies considerably thus the late ordovician and end cretaceous extinctions are associated with unequivocal major regressions demonstrated from numerous widespread regions in contrast the multiple high frequency sea level changes reported for the frasnian famennian crisis based on the supposed depth preferences of conodont taxa have little support from sequence stratigraphic analyses which reveals the interval to be one of highstand the end permian mass extinction has long been related to a severe first order lowstand of sea level newell n d 1967 revolutions in the history of life geol soc am spec pap 89 63 91 based primarily on the widespread absence of latest permian ammonoid markers but field evidence reveals that the interval coincides with a major transgression newell s hypothesis that marine extinctions are related to shelf habitat loss during severe regression remains tenable for the end guadalupian and end triassic extinction events but not for other crises rapid high amplitude regressive transgressive couplets are the most frequently observed eustatic changes at times of mass extinction with the majority of extinctions occurring during the transgressive pulse when anoxic bottom waters often became extensive the ultimate cause of the sea level changes is generally unclear a glacioeustatic driving mechanism can only be convincingly demonstrated for the end ordovician and end devonian events at other times it is speculated that they may relate to the widespread regional doming and subsequent collapse caused by the impingement of superplumes and ultimate eruption on the base of the lithosphere

probabilistic tephra fall simulation for the auckland region new zealand abstract the auckland region new zealand is at significant risk from tephra falls originating both from the local auckland volcanic field avf and several distant large volume centres we use geological data and observations of historical eruptions to develop a catalogue of simulated tephra dispersal patterns for the auckland region using the ashfall model simulated patterns relate to individual eruptive phases and can be combined to simulate events that vary in volume and duration avf eruption parameters were chosen to replicate fine tephra fall dispersed from small volume phreatomagmatic eruptions modelling of eruptions from distant volcanoes was carried out in two stages first the sensitivity of modelled tephra thickness to various model parameters was tested by far the largest thickness variations were the result of varying wind conditions followed by eruption volume the second stage involved modelling volumes that best replicated thicknesses of preserved events within the auckland region allowing the catalogue to be created only about 1 of simulations from distant volcanoes reached the auckland region suggesting that preserved tephra layers are either the result of reasonably long lived eruptions or a number of eruptive phases for eruptions originating within the avf mean modelled tephra thickness on land is small approximately 1 6 mm near the coast and thinning rapidly inland mean andesitic thicknesses are dominated by eruptions from egmont volcano which typically deposit tephra over a narrow section of the auckland region with a maximum thickness of approximately 1 7 mm rhyolitic eruptions are more widespread with mean thickness varying from 30 mm in the south of the region to 9 mm in the north when all centres are combined and weighted by their conditional likelihood of occurrence mean thickness ranges from 7 mm in the south to 1 5 mm in the north however maximum thicknesses of 150 mm for the avf 12 mm for andesitic centres and 830 mm for rhyolitic centres all have the potential to cause much greater damage within parts of the auckland region

palaeoenvironmental interpretation of a triassic jurassic boundary section from western austria based on palaeoecological and geochemical data a section spanning the triassic jurassic boundary is described from near the village of loruns in the vorarlberg region of western austria at loruns the uppermost triassic is characterised by bedded carbonates of the kossen formation supporting a stenotopic fauna indicative of a shallow sub tidal environment of normal marine salinity the triassic jurassic boundary may be represented as a sequence boundary developed on top of a 1 1 m thick red mudstone of the lower schattwald shale which is interpreted to have been deposited in a marginal marine environment possibly a mud flat above the boundary beds the upper schattwald shale is characterised by thin bedded marl and dark limestone beds with an earliest hettangian macrofauna dominated by epifaunal filter feeding bivalves including ostreids mytilids and oxytomids which suggest a shallow subtidal salinity controlled environment typical of an interplatform lagoon carbonate production rejuvenated in the later early hettangian with development of the loruns oolite a shallow subtidal oolitic and oncolitic unit bearing echinoderms indicative of normal marine conditions low thu ratios from the remainder of the section are a result of reduced thorium in carbonate rich sediments and not from authigenic uranium in anoxic sediments in the boundary beds evidence for marine anoxia or dysoxia is absent where thu values determined by gamma ray spectrometry are above 5 the negative excursion in 13c and positive excursion in 18o in the boundary beds may be due to secondary geochemical effects due to organic diagenesis or the precipitation of caliche during paleosol development alternatively the excursions may reflect a primary geochemical signal recording short term decline in primary productivity comparison in 18o and 13c values between the kossen formation and lorfus oolite indicate no significant long term geochemical changes are evident in the section and suggest that any environmental perturbations were restricted to the boundary beds and possible sequence boundary

a combined field and numerical approach to understanding dilute pyroclastic density current dynamics and hazard potential auckland volcanic field new zealand abstract the most dangerous and deadly hazards associated with phreatomagmatic eruptions in the auckland volcanic field avf auckland new zealand are those related to volcanic base surges dilute ground hugging particle laden currents with dynamic pressures capable of severe to complete structural damage we use the well exposed base surge deposits of the maungataketake tuff ring manukau coast auckland to reconstruct flow dynamics and destructive potential of base surges produced during the eruption the initial base surge s snapped trees up to 0 5 m in diameter near their base as far as 0 7 0 9 km from the vent beyond this distance the trees were encapsulated and buried by the surge in growth position using the tree diameter and yield strength of the wood we calculate that dynamic pressures p dyn in excess of 12 35 kpa are necessary to cause the observed damage next we develop a quantitative model for flow of and sedimentation from a radially spreading dilute pyroclastic density currents pdcs to determine the damage potential of the base surges produced during the early phases of the eruption and explore the implications of this potential on future eruptions in the region we find that initial conditions with velocities on the order of 65 m s 1 bulk density of 38 kg m 3 and initial near vent current thicknesses of 60 m reproduce the field based p dyn estimates and runout distances a sensitivity analysis revealed that lower initial bulk densities result in shorter run out distances more rapid deceleration of the current and lower dynamic pressures initial velocity does not have a strong influence on run out distance although higher initial velocity and slope slightly decrease runout distance due to higher rates of atmospheric entrainment using this model we determine that for base surges with runout distances of up to 4 km complete destruction can be expected within 0 5 km from the vent moderate destruction can be expected up to 2 km but much less damage is expected up to the final runout distance of 4 km for larger eruptions base surge runout distance 4 6 km p dyn of 35 kpa can be expected up to 2 5 km from source ensuring complete destruction within this area moderate damage to reinforced structures and damage to weaker structures can be expected up to 6 km from source in both cases hot ash may still cause damage due to igniting flammable materials in the distal most regions of a base surge this work illustrates our ability to combine field observations and numerical models to explore the depositional mechanisms macroscale current dynamics and potential impact of dilute pdcs thus this approach may serve as a tool to understand the damage potential and extent of previous and potential future eruptions in the avf

geology of the orion south kimberlite fort a la corne canada abstract a wide variety of primary pyroclastic volcaniclastic and re sedimented volcaniclastic deposits derived from multiple kimberlite eruptions are exceptionally well preserved at the orion south kimberlite body in the fort a la corne field in central saskatchewan construction of this kimberlite complex involved episodic kimberlite volcanic events punctuated by periods of volcanic quiescence erosion and sedimentary deposition within the western canadian sedimentary basin the orion south kimberlite complex erupted into deltaic coastal plain transitional estuarine mannville group cantuar and pense formations and shallow marine lower colorado group joli fou viking and westgate formations environments at orion south eight distinct kimberlite phases erupted over 6 to 7 million years from ca 106 ma cantuar formation equivalent to ca 99 ma viking formation equivalent detailed core logging geochemistry chronostratigraphy petrography and geophysics were undertaken to define distinct eruptive phases and reconstruct the depositional environment volcanic styles and 3 d architecture of the complex the eight main kimberlite deposits record eruptive styles ranging from magmatic to wet and dry phreatomagmatic to submarine eruptions a consequence of the varying interaction between kimberlite magma and seawater and or groundwater

origins and energetics of maar volcanoes examples from the ultrapotassic sabatini volcanic district roman province central italy maar volcanoes represent a common volcano type which is produced by the explosive interaction of magma with external water here we provide information on a number of maars in the ultrapotassic sabatini volcanic district svd roman province as young as 90 ka the svd maars are characterised in terms of crater and ejecta ring morphologies eruptive successions and magma compositions in light of the local substrate settings with the aim of assessing magma water interaction conditions eruption energetics and genetic mechanisms feeder magmas spanned the whole svd differentiation trend from trachybasalts shoshonites to phonolites from the ejected lithic fragments from aquifer rocks the range of depth of magma water explosive interaction is estimated to have been mostly at 400 600 m below ground level with a single occurrence of surficial interaction in palustrine lacustrine environment in particular the interaction with external water may have triggered the explosive behaviour of poorly differentiated magmas whereas it may have acted only as a late controlling factor of the degree of fragmentation and eruption style for the most differentiated magma batches during low flux ascent in an incipiently fragmented state crater sizes ejecta volumes and ballistic data allow a reconstruction of the energy budget of svd maar forming eruptions erupted tephra volumes from either monogenetic or polygenetic maars ranged 0 004 0 07 km3 during individual maar forming eruptions with corresponding total magma thermal energies of 8 1015 4 1017 j based on energy partitioning and volume balance of erupted magmas and lithic fractions vs crater holes we consider the different contributions of explosive excavation of the substrate vs subsidence in forming the svd maar craters following available models based on crater sizes highly variable fractions 5 50 of the magma thermal energies would have been required for crater excavation it appears that subsidence may have played a major role in some svd maars characterised by low lithic contents whilst substrate excavation became increasingly significant with increasing degrees of aquifer fragmentation

carbon isotope anomaly and other geochemical changes at the triassic jurassic boundary from a marine section in hungary most mass extinctions are linked with carbon isotope excursions implying that biotic crises are coupled with changes in the global carbon cycle the isotopic evolution during the end triassic extinction is far less documented than that for the other major phanerozoic extinctions here we report a sharp and short lived 23 5 excursion in carbon isotope values for carbonate d 13 ccarb corresponding to a 22 excursion in the isotopic composition of marine organic matter d 13 corg and other geochemical

the porosity of pyroclasts as an indicator of volcanic explosivity abstract the porosity of pyroclasts is a valuable indicator of the character of a volcanic eruption here we analyse the porosity distribution of deposits of 17 different volcanic eruptions covering a broad range of inferred magma properties and observed eruption styles histograms of porosity versus pyroclast frequency show distinctive features in terms of the mean variance modality and skewness we present characteristic porosity distributions for five different eruption style categories dome forming explosive basaltic cryptodome forming subplinian plinian ultraplinian and phreatomagmatic we find that the mean porosity values of the deposits correlate positively with the volcanic explosivity index vei of newhall and self 1982 regardless of the degree of complexity of the individual distributions we then propose causes of the deviations from a general trend in terms of magma viscosity degassing efficiency and cap rock confinement finally a generalised correlation between clast porosity and explosivity of an eruption is developed that should help together with conventional field techniques facilitate the interpretation of pyroclastic sequences even in cases where no direct observation of the eruption exists

syn and post eruptive erosion gully formation and morphological evolution of a tephra ring in tropical climate erupted in 1913 in west ambrym vanuatu abstract syn and post eruptive erosion of volcanic cones plays an important role in mass redistribution of tephra over short periods descriptions of the early stages of erosion of tephra from monogenetic volcanic cones are rare particularly those with a well constrained timing of events in spite of this lack of data cone morphologies and erosion features are commonly used for long term erosion rate calculations and relative age determinations in volcanic fields this paper offers new observations which suggest differing constraints on the timing of erosion of a tephra ring may be operating than those conventionally cited in 1913 a tephra ring was formed as part of an eruption in west ambrym island vanuatu and is now exposed along a continuous 2 5 km long coastal section the ring surrounds an oval shaped depression filled by water it is composed of a succession of a phreatomagmatic fall and base surge beds interbedded with thin scoriaceous lapilli units toward the outer edges of the ring base surge beds are gradually replaced in the succession by fine ash dominated debris flows and hyperconcentrated flow deposits the inter fingering of phreatomagmatic deposits with syn volcanic reworked volcaniclastic sediments indicates that an ongoing remobilisation of freshly deposited tephra was already occurring during the eruption gullies cut into the un weathered tephra are up to 4 m deep and commonly have c 1 m of debris flow deposit fill in their bases there is no indication of weathering vegetation fragments or soil development between the gully bases and the basal debris flow fills gully walls are steep and superficial fans of collapsed sediment are common most gullies are heavily vegetated although some active ephemeral channels occur these observations suggest that the majority of the erosion of such tephra rings in tropical climates takes place directly during eruption and possibly for only a period of days to weeks afterward after establishment of the gully network tephra remobilisation is concentrated only within them therefore the shape of the erosion modified volcanic landform is predominantly developed shortly after the eruption ceases this observation indicates that gully erosion morphology may not necessarily relate to age of such a landform different intensities of erosion during eruption related to water supply or rainfall are probably the major influence on gully spacing modal depth and form longer term post eruption processes that could be indicators of relative age may include internal gully deepening below basal debris flow fill sediments and possibly widening and side slope lowering due to undercutting and side collapse

syn and posteruptive hazards of maar diatreme volcanoes abstract maar diatreme volcanoes represent the second most common volcano type on continents and islands this study presents a first review of syn and posteruptive volcanic and related hazards and intends to stimulate future research in this field maar diatreme volcanoes are phreatomagmatic monogenetic volcanoes they may erupt explosively for days to 15 years above the preeruptive surface a relatively flat tephra ring forms below the preeruptive surface the maar crater is incised because of formation and downward penetration of a cone shaped diatreme and its root zone during activity both the maar crater and the diatreme grow in depth and diameter inside the diatreme which may penetrate downwards for up to 2 5 km fragmented country rocks and juvenile pyroclasts accumulate in primary pyroclastic deposits but to a large extent also as reworked deposits ejection of large volumes of country rocks results in a mass deficiency in the root zone of the diatreme and causes the diatreme fill to subside thus the diatreme represents a kind of growing sinkhole due to the subsidence of the diatreme underneath the maar crater is a subsidence crater and also grows in depth and diameter with ongoing activity as long as phreatomagmatic eruptions continue the tephra ring grows in thickness and outer slope angle syneruptive hazards of maar diatreme volcanoes are earthquakes eruption clouds tephra fall base surges ballistic blocks and bombs lahars volcanic gases cutting of the growing maar crater into the preeruptive ground formation of a tephra ring fragmentation of country rocks thus destruction of area and ground changes in groundwater table and potential renewal of eruptions the main hazards mostly affect an area 3 to possibly 5 km in radius distal effects are comparable to those of small eruption clouds from polygenetic volcanoes syneruptive effects on infrastructure people animals vegetation agricultural land and drainage are pointed out posteruptive hazards concern erosion and formation of lahars inside the crater a lake usually forms and diverse types of sediments accumulate in the crater volcanic gases may be released in the crater compaction and other diagenetic processes within the diatreme fill result in its subsidence this posteruptive subsidence of the diatreme fill and thus crater floor is relatively large initially but will decrease with time it may last millions of years various studies and monitoring are suggested for syn and posteruptive activities of maar diatreme volcanoes erupting in the future the recently formed maar diatreme volcanoes should be investigated repeatedly to understand more about their syneruptive behaviour and hazards and also their posteruptive topographic limnic and biologic evolution and potential posteruptive hazards for future maar diatreme eruptions a hazard map with four principal hazard zones is suggested with the two innermost ones having a joint radius of up to 5 km areas that are potentially endangered by maar diatreme eruptions in the future are pointed out

recycling of magmatic clasts during explosive eruptions estimating the true juvenile content of phreatomagmatic volcanic deposits the juvenile content of phreatomagmatic deposits contains both first cycle juvenile clasts derived from magma at the instant of eruption and recycled juvenile clasts which were fragmented and first ejected by earlier explosions during the eruption but fell back or collapsed into the vent recycled juvenile clasts are similar to accessory and accidental lithics in that they contribute no heat to further magma water interaction but previously no effective criteria have been defined to separate them from first cycle juvenile clasts we have investigated componentry parameters vesicularity clast morphology and extent of mud coating which in specific circumstances can distinguish between first cycle juvenile clasts involved in only one explosion and such recycled juvenile clasts phreatomagmatic fall deposits commonly show gross grainsize and sorting characteristics identical to deposits of purely dry or magmatic eruptions however the abundance of non juvenile clasts in pyroclastic deposits is a sensitive indicator of the involvement of external water if this component is calculated including recycled juvenile clasts with accidental and accessory clasts the contrast is even more striking data from a holocene maar deposit in taupo volcanic zone new zealand suggest that the first cycle juvenile component of the deposits is less than one third of that determined by simple juvenile lithic crystal componentry

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

high resolution ammonite and carbon isotope stratigraphy across the triassic jurassic boundary at new york canyon nevada the triassic jurassic boundary is generally considered as one of the major extinctions in the history of phanerozoic the high resolution ammonite correlations and carbon isotope marine record in the new york canyon area allow to distinguish two negative carbon excursions across this boundary with different paleoenvironmental meanings the late rhaetian negative excursion is related to the extinction and regressive phase the early hettangian 13corg negative excursion is associated with a major floristic turnover and major ammonite and radiolarian radiation the end triassic extinction early jurassic recovery is fully compatible with a volcanism triggered crisis probably related to the central atlantic magmatic province the main environmental stress might have been generated by repeated release of so2 gas heavy metals emissions darkening and subsequent cooling this phase was followed by a major long term co2 accumulation during the early hettangian with development of nutrient rich marine waters favouring the recovery of productivity and deposition of black shales

sub surface dynamics and eruptive styles of maars in the colli albani volcanic district central italy protezione civile italy 2004 2006 agreement with istituto nazionale di geofisica e vulcanologia ingv project v3 1

reconstructing eruption processes of a miocene monogenetic volcanic field from vent remnants waipiata volcanic field south island new zealand the miocene waipiata volcanic field new zealand is an eroded phreatomagmatic intracontinental volcanic field formed during a period of weak lithospheric extension the field includes remnants of at least 55 volcanoes in an area of 5000 km2 vent filling deposits comprising predominantly lava e g plugs necks lava flows or dykes often associated with thin basal phreatomagmatic pyroclastic deposits were classified as type 1 vents and are inferred to be the remnants of scoria cones vents represented by predominantly pyroclastic infill are classified as type 2 vents and are inferred to have been the substructures of phreatomagmatic tuff ring and or maar volcanoes type 3 vent complexes are groups of closely spaced or overlapping vents with voluminous preserved lava flows they are inferred to be the remnants of volcanoes comprising adjoining to coalescing maars and tuff rings with magmatic explosive and effusive products pyroclastic rocks of most of the waipiata vents record initial phreatomagmatic explosive activity fuelled by groundwater followed by strombolian style eruptions aligned and clustered vents are accommodated to structural features of the regional basement rock otago schist

carbon isotope stratigraphy and palynostratigraphy of the triassic jurassic transition in the tiefengraben section northern calcareous alps austria abstract the end triassic is characterized by enhanced rates of biotic turnover in both the marine and terrestrial realms carbon isotope anomalies have been reported worldwide that may indicate coeval global disturbances in the biogeochemical cycles here we present the results of an integrated study of the palynomorph assemblages and the carbon isotope composition of organic matter in the tiefengraben section near st wolfgang austria carbon isotope values from bulk organic matter show significant fluctuations through the triassic jurassic t j boundary interval with two prominent negative 13 c org excursions an initial negative carbon isotope excursion occurs at the base of the kendlbach formation tiefengraben member pre planorbis beds although triassic ammonoids and conodonts have their highest occurrence at the top of the underlying kossen formation the palynomorph and calcareous microfossil assemblages indicate still a rhaetian age for the lower part of the tiefengraben member high abundance of prasinophytes during the interval of the initial carbon isotope excursion may represent a bloom of green algae that flourished as a result of the disturbance of the marine ecosystems increased spore abundance together with the disappearance of distinctive late triassic pollen types e g ovalipollis rhaetipollis indicate vegetation changes on land that are contemporaneous with the marine biotic crisis a second negative carbon isotope excursion occurs in the middle part of the tiefengraben member based on palynological evidence the t j boundary may be drawn at about this level by the lowest occurrence of cerebropollenites thiergartii the established carbon isotope curve as well as palynostratigraphic and other biostratigraphic events can be correlated with st audrie s bay england

dyke diatreme transition in monogenetic volcanoes insights from the hillier bay volcanic complex western australia the factors controlling phreatomagmatism and diatreme formation are still poorly constrained and understood here we describe the field relationships between mafic intrusions and volcaniclastic deposits observed at hillier bay western australia and discuss the implications for the formation of monogenetic basaltic volcanoes involving both phreatomagmatic and magmatic eruption phases the hillier bay volcanic complex consists of a series of basaltic sheeted dykelets and larger dykes injected within the metamorphic basement volcaniclastic lithologies also occur usually trapped between basaltic dykes and the basement these vary between mixtures of juvenile basaltic fragments metamorphic basement fragments and quartz feldspar sand in different relative amounts based on the textures of the clastic lithologies we argue that initial phreatomagmatic phases resulted from sequential injections of thin dykelets due to ascending magma struggling to open a path to the surface the relatively lower magma water ratio maximises the efficiency of premixing leading to phreatomagmatic explosions later as the main body of magma reaches the shallow conduit through wider dykes magma overcomes water availability and the eruption style switches to magmatic effusive this implies that magma flux may be a determining factor controlling eruption style in at least some monogenetic volcanoes

shallow seated controls on styles of explosive basaltic volcanism a case study from new zealand abstract the pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles e g hawaiian versus strombolian or dry magmatic versus wet phreatomagmatic these transitions are controlled dominantly by variations in degassing patterns magma ascent rates and degrees of interaction with external water we use crater hill a 29 ka explosive effusive monogenetic centre in the auckland volcanic field new zealand as a case study of the transitions between these end member eruptive styles the crater hill eruption took place from at least 4 vents spaced along a nne trending 600 m long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases early explosive phases at crater hill were characterised by eruption from multiple unstable and short lived vents later dominantly extrusive volcanism took place from a more stable point source most of the crater hill pyroclastic deposits were formed in 3 phreatomagmatic p and 4 dry magmatic m episodes forming in turn the outer tuff ring and maar crater p1 m1 p2 and scoria cone 1 m2 m4 this activity was followed by formation of a lava shield and scoria cone 2 purely wet activity is represented by the bulk of p1 and p2 and purely dry activity by much of m2 m4 however m1 and parts of m2 and m4 show evidence for simultaneous eruptions of differing style from adjacent vents and rapid variations in the extent and timing of magma water interaction at each vent the nature of the wall rock lithics and these rapid variations in inferred water magma ratios imply interaction was occurring mostly at depths of 80 m and the vesicularity patterns in juvenile clasts from these and the p beds imply that rapid degassing occurred at these shallow levels we suggest that abrupt transitions between eruptive styles in time and space at crater hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent