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

unbedded diatreme deposits reveal maar diatreme forming eruptive processes standing rocks west hopi buttes navajo nation usa diatremes provide partial records of how dyke delivered magma periodically interacting with water produces the largest known cylindrical conduit structures of any volcano type we address how pre eruptive country rock is disrupted and redistributed to form the diatreme structure during an eruption by establishing the internal architecture of the unbedded diatreme at standing rocks west and the volumes and sources of wall rock within the diatreme and in a complementary tephra ring at teshim both in the hopi buttes volcanic field the unbedded diatreme is dominantly exposed as a massif comprising multiple subvertical columns interpreted as ephemeral conduit deposits of well mixed poorly sorted composite and juvenile pyroclast rich deposits that truncate marginal layered deposits plus a peripheral heterolithic country rock breccia wall rock clasts are scarce in the massif and were mainly sourced from 75 to 245 m depth below the paleosurface by contrast the tephra ring has abundant wall rock fragments dominantly from the upper 75 m of the pre eruptive sedimentary sequence the diatreme structure is interpreted to have been formed by many small volume explosions fed by a low flux of basaltic magma late stage explosive activity was rooted mostly within pyroclastic debris at shallow to intermediate depths within the diatreme structure this resulted in damped and shifting intra diatreme explosions and jets that facilitated gradual mixing recycling and remobilization of debris in the diatreme with incremental addition of juvenile material and possibly a local rise in the crater floor

models of maar volcanoes lunar crater nevada usa maar volcanoes are generally understood to be the result of highly energetic explosive interaction between magma and water groundwater or surface water two end member conceptual models have been proposed to explain the dimensions diameter depth of maar craters 1 an incremental growth model where a crater grows due to subsidence and ejection of debris over the course of many explosions and the final size is an integrated result of multiple explosive events 2 a model in which the dimensions of a maar crater are the result of the largest single explosion during the lifetime of the maar major explosion dominated model in the latter case the maar size can be used to estimate the energy and depth of the largest explosion which in turn allows estimation of the magma mass involved this paper describes lunar crater maar nevada usa and tests the two models as explanations for the characteristics of the volcano in particular the major explosion dominated model this model implies magma mass and supply rates that are unrealistic and the tephra at the maar do not contain key features observed in the ejecta at large single explosion craters the incremental growth model seems most suitable based upon geological evidence

relative contributions of transient and steady state infiltration during ephemeral streamflow 1 simulations of infiltration during three ephemeral streamflow events in a coarse grained alluvial channel overlying a less permeable basin fill layer were conducted to determine the relative contribution of transient infiltration at the onset of streamflow to cumulative infiltration for the event water content temperature and piezometric measurements from 2 5 m vertical profiles within the alluvial sediments were used to constrain a variably saturated water flow and heat transport model simulated and measured transient infiltration rates at the onset of streamflow were about two to three orders of magnitude greater than steady state infiltration rates the duration of simulated transient infiltration ranged from 1 8 to 20 hours compared with steady state flow periods of 231 to 307 hours cumulative infiltration during the transient period represented 10 to 26 of the total cumulative infiltration with an average contribution of approximately 18 cumulative infiltration error for the simulated streamflow events ranged from 9 to 25 cumulative infiltration error for typical streamflow events of about 8 hours in duration in is about 90 this analysis indicates that when estimating total cumulative infiltration in coarse grained ephemeral stream channels consideration of the transient infiltration at the onset of streamflow will improve predictions of the total volume of infiltration that may become groundwater recharge

explosion depths for phreatomagmatic eruptions subsurface phreatomagmatic explosions can result from the interaction of ascending magma with groundwater experiments over a wide range of energies show that for a given energy there is a depth below which an explosion will be contained within the subsurface not erupt and there is a corresponding shallower depth that will optimize ejecta dispersal we combine these relationships with constraints on the energies of phreatomagmatic explosions at maar diatreme volcanoes and show that most eruptions are likely sourced by explosions in the uppermost 200 m and even shallower ones 100 m are likely to dominate deposition onto tephra rings most explosions below 200 m will not erupt but contribute to formation of and to the vertical mixing of materials within a diatreme vent structure with only rare very high energy explosions between 200 and 500 m erupting similar constraints likely apply at other volcanoes that experience phreatomagmatic explosions

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

subsurface structure of a large basaltic maar volcano examined using geologically constrained potential field modelling lake purrumbete maar newer volcanics province southeastern australia abstract lake purrumbete maar lpm is situated in the late cenozoic intraplate basaltic newer volcanics province southeastern australia it is one of the largest maar volcanoes in the world with a near circular crater that is up to 2800 m in diameter containing a 45 m deep lake gellibrand marl accidental lithics which occurs to a maximum depth of 250 m below lpm represent the deepest excavated host rock unit present in the volcanic succession irregular clast shapes and peperitic textures observed in marl lithics suggest the host rock was poorly consolidated during the eruption high resolution lake and land based gravity and magnetic data were collected to conduct forward and inverse modelling of the subsurface architecture of the maar this is done to test the assumption based on lithics that the diatreme is limited to 250 m depth and identify the reasons behind lpm s large size the collection of gravity data presented a unique challenge due to the nature of measuring small changes in gravitational forces

multiphase flow above explosion sites in debris filled volcanic vents insights from analogue experiments abstract discrete explosive bursts are known from many volcanic eruptions in maar diatreme eruptions they have occurred in debris filled volcanic vents when magma interacted with groundwater implying that material mobilized by such explosions passed through the overlying and enclosing debris to reach the surface although other studies have addressed the form and characteristics of craters formed by discrete explosions in unconsolidated material no details are available regarding the structure of the disturbed debris between the explosion site and the surface field studies of diatreme deposits reveal cross cutting steep sided zones of non bedded volcaniclastic material that have been inferred to result from sedimentation of material transported by debris jets driven by explosions in order to determine the general processes and deposit geometry resulting from discrete explosive injections of entrained particles through a particulate host we ran a series of analogue experiments specific volumes of compressed 0 5 2 5 mpa air were released in bursts that drove gas particle dispersions through a granular host the air expanded into and entrained coloured particles in a small crucible before moving upward into the host white particles each burst drove into the host an expanding cavity containing air and coloured particles total duration of each run recorded with high speed video was approximately 0 5 1 s the coloured beads sedimented into the transient cavity this same behaviour was observed even in runs where there was no breaching of the surface and no coloured beads ejected a steep sided body of coloured beads was left that is similar to the cross cutting pipes observed in deposits filling real volcanic vents in which cavity collapse can result not only from gas escape through a granular host as in the experiments but also through condensation of water vapour a key conclusion from these experiments is that the geometry of cross cutting volcaniclastic deposits in volcanic vents is not directly informative of the geometry of the intrusions that formed them an additional conclusion is that complex structures can form quickly from discrete events

peperite as a record of early stage phreatomagmatic fragmentation processes an example from the hopi buttes volcanic field navajo nation arizona usa abstract peperite is the product of near surface magma wet sediment mingling some peperites are inferred to record frozen coarse mixing stage fuel coolant interactions fcis and hence can be used in attempts to determine specific parameters related to phreatomagmatic fragmentation such as water magma mass ratios confining pressures and vent region hydrology two lamprophyric dikes in the miocene pliocene hopi buttes volcanic field arizona contain ellipsoidal regions of dilation buds associated with peperite these peperite rimmed zones occur within identifiable stratigraphic horizons and at known depths beneath the elevation of the local pre eruptive surface the presence of both fluidal and blocky juvenile clasts combined with physical aspects of the sediment lamprophyre contacts suggest that coarse mixing stage fci was a potential fragmentation mechanism within these systems confining pressures calculated for sites of single buds range from 0 6 to 1 0 mpa average inferred water magma ratios within peperite bodies at the time they formed vary from 0 09 to 0 16 consistent with experimental values for water melt mixing ratios necessary for explosive heat transfer a number of factors may have inhibited further explosive activity in these peperite forming mixing events including anisotropic saturation of the host sediment low confining pressures and water melt ratios and damped explosive interactions resulting from increased density viscosity and surface tension of sediment water coolants nevertheless these deposits record localized thermohydraulic interactions between magma and wet sediment and provide valuable information about the geometry and mixing ratios present during peperite formation

synthetic seismogram and surface wave constraints on crustal models of spitsbergen abstract seismic refraction data collected on spitsbergen in 1978 are used to obtain a crustal model assuming plane horizontal layering the observed travel times and wave forms are compared with those of synthetic seismograms computed for various published crustal models the more detailed models adequately explain some but not all of the features of the synthetics these models are adjusted utilizing travel times and wave form amplitudes until a satisfactory fit is achieved the best fitting model consists of a 4 layer crust having thicknesses of 4 1 10 0 7 4 and 5 8 km and compressional velocities of 4 65 6 21 6 30 and 6 65 km sec with increasing depth the uppermost mantle has a velocity of 7 90 km sec a comparison of observed and synthetic p n waveforms supports the existence of a thin low velocity zone beginning at a depth of about 5 km beneath the moho boundary an inversion of seismic surface wave group velocity data yields a shear wave model which is compatible with the compressional wave model

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

growth and characterization of complex mineral surfaces precipitation of mineral aggregates near the earth s surface or in subsurface fractures and cavities often produces complex microstructures and surface morphologies here we demonstrate how a simple surface normal growth sng process may produce microstructures and surface morphologies very similar to those observed in some natural carbonate systems a simple sng model was used to fit observed surfaces thus providing information about the growth history and also about the frequency and spatial distribution of nucleation events during growth the sng model can be extended to systems in which the symmetry of precipitation is broken for example by fluid flow we show how a simple modification of the sng model in which the local growth rate depends on the distance from a fluid source and the local slope or fluid flow rate produces growth structures with many similarities to natural travertine deposits

revised conceptual model for maar diatremes subsurface processes energetics and eruptive products diatremes are debris filled structures beneath maars that result from many magma water phreatomagmatic explosions during a monogenetic volcano s lifetime a long standing model requires deepening explosions due to water table drawdown that eject progressively deeper seated country rock from the explosion sites while the overlying diatreme and its surface crater widen due to subsidence a revised model is proposed wherein explosions can take place at any level within a diatreme at a given time most effectively venting material from near surface explosions deep seated country rock lithics in tephra deposits record stepwise vertical mixing of material by upward directed debris jets and downward subsidence rather than direct ejection from deep explosions juvenile and lithic clasts erupted during a given explosion may have had a complex history within the diatreme and need not directly reflect fragmentation or brecciation during the explosion that ejects them

helium neon and argon systematics of the european subcontinental mantle implications for its geochemical evolution in this study we present a comprehensive noble gas study of mantle xenoliths from various european cenozoic volcanic provinces the main body of samples is from the massif central france and the eifel germany smaller subsets of samples are from spitsbergen and the graz basin austria in all the helium neon and argon isotopic abundances of a total of forty five mantle xenoliths phenocrysts and xenocrysts were determined the 3he 4he ratios within each volcanic province are very uniform irrespective of the diverse lithologies and p t conditions which are represented by our sample suite mean 3he 4he ratios of the massif central eifel spitsbergen and kapfenstein are 6 53 25 6 03 14 6 65 25 and 6 1 7 1 times the atmospheric ratio ra respectively the strontium and neodymium isotopic composition of some of the cpx separates are highly variable and therefore in contrast to the uniform he signature we thus conclude that he and probably also the other noble gases in the xenoliths are effectively decoupled from the non volatile elements therefore the he signature that is preserved in the xenoliths is actually that of their host magmas published strontium neodymium and lead isotope data of the unevolved host magmas of the xenoliths correlate well with our xenolith he data the position of the fields of the investigated volcanic provinces in hesr hend and hepb variation diagrams depict ternary mixtures between dmm em himu endmembers as the source the host magmas and their volatiles the neon isotopic composition of the gases released from the xenoliths is in most cases atmospheric and probably reflects atmospheric contamination only a few samples reveal indications for morb type ne or evidence of mass fractionation the 40ar 36ar ratios of the xenoliths are mostly radiogenic with the highest ratio being 17 000 however all samples have suffered a certain degree of atmospheric contamination we calculate that the uncontaminated 40ar 36ar ratio of the european subcontinental mantle is 30 000 the4he 40ar ratios have been fractionated since with values around 0 25 they are below both the current production ratio and the morb ratio they cannot be explained by atmospheric contamination as the samples with high 40ar 36ar ratios also have low 4he 40ar ratios helium and ar were probably fractionated during initial closed system degassing of the host magma we calculate the original unfractionated 4he 40ar ratios of the massif central and eifel magma source which are 2 56 18 and 2 71 17 respectively the noble gas systematics are compatible with the notion that addition of crustal material has modified a former morb source signature we use our mean 3he 4 he ratios and reconstructed 4he 40ar ratios to model the time and degree of contamination of a morb like source with crustal material combining our model with evidence from published oxygen strontium neodymium and lead isotope data we judge that the source of the massif central and eifel magmas became enriched in k u and th shortly prior or during the variscan orogeny pre collisional collisional in this case the enrichment in these elements is therefore most probably related to subduction of crustal material in a destructive continental margin setting

sensitivity to long term climate change of subpermafrost groundwater systems in svalbard deep subpermafrost aquifers are highly climate dependent with the permafrost as an aquitard preventing groundwater recharge and discharge a study from the high arctic island of spitsbergen svalbard shows that during a glacial to interglacial phase both the permafrost and the glacier regime will respond to climatic changes and a glacier fed groundwater flow system will vary accordingly a full glaciation results in the melting of permafrost and groundwater can flow through pores and fracture systems in the rocks and sediments below the temperate zones of glaciers these groundwater flow systems will mainly be localized to fjords and valleys and form low lying terrestrial springs when the relative sea level drops during deglaciation due to glacio isostatic rise during an interglaciation permafrost develops and thickens and the groundwater recharge and discharge areas will thereby be gradually reduced to a minimum reached at the warmest part of an interglaciation an already frozen spring system cannot reopen before the permafrost melts only groundwater springs related to permanently warm based glacial ice will persist into the next glaciation during a new glaciation flow systems that terminated during the previous interglaciation may become revitalized if overridden by warm based ice causing permafrost thawing

shallow plumbing systems for small volume basaltic volcanoes 2 evidence from crustal xenoliths at scoria cones and maars abstract monogenetic basaltic volcanoes record complex eruption processes and the relationships between those processes and shallow plumbing are poorly understood this paper explores these relationships building upon earlier studies of exposed shallow plumbing in the upper hundreds of meters of crust beneath volcanoes produced by magmatic eruption processes and those produced by phreatomagmatic processes such as scoria cones and maars respectively eruptive facies and xenolith abundances are described at three scoria cones and at tephra rings around two maars in the san francisco volcanic field arizona usa well constrained subvolcanic sedimentary stratigraphy provides constraints on the depths of origin of xenoliths of different types sedimentary xenolith contents at scoria cones are

lineament domain of regional strike slip corridor insight from the neogene transtensional de geer transform fault in nw spitsbergen lineaments on regional scale images represent controversial features in tectonic studies published models explain the presence of the lineament domains in most geodynamic environments as resulting from the enhanced erosion along strikes normal to the upper crustal regional extension despite their success in many tectonic frameworks these models fail to explain the existing lineament domains in the regional strike slip corridors that separate regional blocks including the transform faults the present paper investigates the lineament distribution in such environments and specifically presents the results from a study along the shear corridor of the de geer transform fault in the north atlantic responsible for the separation and drifting away between northern greenland and the svalbard archipelago since oligocene times the study spans from satellite image analysis and outcrop scale investigations to a more regional analysis on a digital bathymetric model of the north atlantic arctic ocean lineaments were automatically detected in the spectral band 8 0 52 0 9 m of a landsat 7 image 15 m pixel resolution a total of 320 image lineaments were extracted from both the regional and the local scale investigations and statistically analyzed results from the multi scalar lineament analyses revealed the existence of a main n s lineament domain regionally persistent from the de geer corridor to the western margin of northern spitsbergen where it relates to the youngest post oligocene tectonics observed onshore this is confirmed by field observations showing that the n s faults represent the youngest brittle deformation system and systematically cut the deformations associated with the building of the tertiary west spitsbergen fold and thrust belt the n s lineament domain is the result of the activity of a larger regional scale tectonic feature nw se oriented and responsible for the localized extension within its deformation corridor the de geer transform fault a model is presented that involves the presence of a thin upper crust with brittle behavior lying above a deeper crustal layer characterized by a more ductile deformation the lower layer suffers more diffuse homogeneous strain this strain is transmitted to the upper brittle layer forming clusters of enhanced fracturing zones aligned following the induced stress trajectories lineaments develop along these weaker fractured zones preferentially etched by erosional processes and align perpendicular to the least horizontal compression sigma3 which in turn forms an angle to the shear kinematics in the western part of spitsbergen this angle is smaller than 45 due to the transtensional tectonic regime the proposed model for lineament domain origin in strike slip environments well integrates the existing models in literature and could be applied to other similar geodynamic contests

isotopic and geochemical investigation of two distinct mars analog environments using evolved gas techniques in svalbard norway the 2010 arctic mars analog svalbard expedition amase investigated two distinct geologic settings on svalbard using methodologies and techniques to be deployed on mars science laboratory msl amase related research comprises both analyses conducted during the expedition and further analyses of collected samples using laboratory facilities at a variety of institutions the sample analysis at mars sam instrument suite on msl includes pyrolysis ovens a gas processing manifold a quadrupole mass spectrometer qms several gas chromatography columns and a tunable laser spectrometer tls an integral part of sam development is the deployment of sam like instrumentation in the field during amase 2010 two parts of sam participated as stand alone instruments a hiden evolved gas analysis mass spectrometer ega qms system represented the ega qms component of sam and a picarro cavity ring down spectrometer ega crds represented the ega tls component of sam a field analog of chemin the xrd xrf on msl was also deployed as part of this field campaign carbon isotopic measurements of co2 evolved during thermal decomposition of carbonates were used together with ega qms geochemical data mineral composition information and contextual observations made during sample collection to distinguish carbonates formation associated with chemosynthetic activity at a fossil methane seep from abiotic processes forming carbonates associated with subglacial basaltic eruptions carbon and oxygen isotopes of the basalt hosted carbonates suggest cryogenic carbonate formation though more research is necessary to clarify the history of these rocks

experimental craters formed by single and multiple buried explosions and implications for volcanic craters with emphasis on maars 1 craters at many volcanoes including most maars are formed by multiple subsurface explosions experiments compared the crater formed by a single large buried explosion with craters formed by multiple explosions with the same cumulative energy explosive charges were detonated in pads composed of layered aggregates in three configurations 1 a single large charge buried near its optimal crater excavation depth 2 three charges each with 1 3 the energy of the first one buried at approximately the same depth with respect to the original pad surface 3 the same three charges buried successively deeper final crater size in the multiple explosion cases is not a good indicator of the energy of individual explosions however crater morphology and ejecta volume and distribution can be good indicators of explosion energy and depth these results directly impact the estimate of the energy released by past maar eruptions and future hazard assessments

the effects of the host substrate properties on maar diatreme volcanoes experimental evidence while the relationship between the host substrate properties and the formation of maar diatreme volcanoes have been investigated in the past it remains poorly understood in order to establish the effects of the qualitative host substrate properties on crater depth diameter morphological features and sub surface structures we present a comparison of four campaigns of experiments that used small chemical explosives buried in various geological media to simulate the formation of maar diatremes previous results from these experiments have shown that primary variations in craters and sub surface structures are related to the scaled depth physical depth divided by cube root of blast energy our study reveals that single explosions at optimal scaled depths in stronger host materials create the largest and deepest craters with steep walls and the highest crater rims for single explosions at deeper than optimal scaled depths the influence of material strength is less obvious and non linear for crater depth and non existent for crater diameter within the range of the experiments for secondary and tertiary blasts there are no apparent relationships between the material properties and the crater parameters instead the presence of pre existing craters influences the crater evolution a general weakening of the materials after successive explosions can be observed suggesting a possible decrease in the host substrate influence even at optimal scaled depth the results suggest that the influence of the host substrate properties is important only in the early stage of a maar diatreme neglecting post eruptive slumping into the open crater and decreases as explosion numbers increase since maar diatremes reflect eruptive histories that involve tens to hundreds of individual explosions the influence of initial substrate properties on initial crater processes could potentially be completely lost in a natural system

a primitive alkali basaltic stratovolcano and associated eruptive centres northwestern spitsbergen volcanology and tectonic significance abstract three quaternary volcanic centres sverrefjell sigurdfjell and halvdanpiggen in northwestern spitsbergen formed by off ridge alkali basaltic volcanism associated with nearby ocean floor spreading and subsequent tectonism the sverrefjell volcano is a unique stratovolcano constructed of primitive alkali basaltic pyroclastics with subordinate lavas about 15 20 vol of this volcanic and the other centres comprises a wide variety of upper mantle and lower crustal xenoliths a new estimate of the age of the sverrefjell volcano places the time of formation at a minimum of 100 000 250 000 years ago earlier than previous estimates the occurrence of interbedded pillow lavas and pahoehoe flows at all levels of sverrefjell suggests that the volcano grew concurrently with the rise of the surrounding glaciers basaltic sequences occurring as nearby mountain cappings represent earlier volcanism 2 5 11 5 ma also probably a result of thermal disturbance related to nearby ocean floor spreading geothermobarometry calculations on xenoliths from the three quaternary localities define a very high geotherm consistent with their location near the yermak hot spot and provide a good datum point for the geothermal gradient adjacent to such a thermal anomaly

unconventional maar diatreme and associated intrusions in the soft sediment hosted mardoux structure gergovie france a miocene age volcanic hypabyssal structure comprising volcaniclastic deposits and mafic intrusions is exposed with vertical relief of 110 m on the side of gergovie plateau auvergne france three main volcaniclastic facies are 1 fluidal tuff breccia composed of juvenile basalt and sediment clasts with dominantly fluidal shapes with several combinations of basalt and sediment within individual clasts 2 thickly bedded lapilli tuff composed of varying proportions of fine grained sediment derived from oligocene miocene lacustrine marls and mudstones and basaltic lapilli blocks and bombs 3 planar bedded tuff forming thin beds of fine to coarse ash size sedimentary material and basalt clasts intrusive bodies in the thickly bedded lapilli tuff range from irregularly shaped and anastomosing dikes and sills of meters to tens of meters in length to a main feeder dike that is up to 20 m wide and that flares into a spoon shaped sill at 100 m in diameter and 10 20 m thick in the eastern part of the structure volcaniclastic deposits and structural features suggest that ascending magma entrained soft saturated sediment host material into the feeder dike and erupted fluidal magma and wet sediment via weak strombolian like explosions host sediment and erupted material subsided to replace the extracted sediments producing the growth subsidence structure that is similar to upper diatreme facies in typical maar diatremes but lacks evidence for explosive disruption of diatreme fill irregularly shaped small intrusions extended from the main feeder dike into the diatreme and many were disaggregated due to shifting and subsidence of diatreme fill and recycled via eruption the mardoux structure is an unconventional maar diatreme in that it was produced mainly by weak explosive activity rather than by violent phreatomagmatic explosions and is an example of complex coupling between soft sediment and ascending magma

spatter dike reveals subterranean magma diversions consequences for small multivent basaltic eruptions small volume short lived basaltic volcanoes earth9s most common subaerially are abundant and tractable in size thus offering opportunities for learning how magma behaves in the critical transition from coherent magma in dikes to pyroclastic dispersions spatter dikes at the castle butte trading post volcanic complex hopi buttes volcanic field arizona usa exposed 150 m below the paleosurface present an outstanding record of this transition they record pulsatory magma rise and eruption fissure extension vent stepping and conduit wall rock failure providing a critical link to activity witnessed and geophysically monitored during historic eruptions of well observed and monitored large volume basaltic volcanoes castle butte comprises four linked and closely spaced spatter dikes and maar diatremes the spatter dikes consist of bedded variably welded deposits plus wall rock debris in multiple sequences demarcated by truncation surfaces that indicate progressive southwest northeast emplacement they reveal a shallow plumbing cycle of pulsating weak hot spatter fragmentation concurrent wall rock failure and periodic slips that truncated and downdropped bedded deposits in addition to vertical pulsation we infer from the northeastward younging spatter succession that repeated magma diversions facilitated episodes of magma withdrawal that caused the slips during northeastward extension of the complex toward the large maar diatreme

tephra ring interpretation in light of evolving maar diatreme concepts stracciacappa maar central italy abstract maar diatreme volcanoes in their simplest form are thought to result primarily from discrete subsurface phreatomagmatic explosions tephra ring deposits around maar craters are commonly interpreted in terms of a framework wherein explosions begin at shallow levels and migrate downward ejecting progressively deeper seated lithic clasts that contribute to the tephra rings additionally variations in grain size of the deposits are sometimes inferred to record different degrees of fragmentation which are in turn related to variations in magma water ratios at the explosion sites recent detailed studies of diatremes and of maar tephra rings suggest a different conceptual model wherein explosions can happen at various vertical and lateral locations within a diatreme during its eruptive lifetime rather than being limited to simple downward migration experiments numerical modeling and field data indicate that most explosions deeper than about 200 250 m will not vent to the surface but instead contribute to mixing within the diatremes arrival of deep seated lithics at the surface is related to this mixing process experiments also indicate that explosion phenomena can result in a range of emplacement mechanisms and grain sizes in tephra rings even in the absence of variations in explosion mechanisms fragmentation efficiency here we document the tephra ring of stracciacappa maar central italy and interpret its eruption and emplacement history in terms of evolving conceptual models along with a comparison to more traditional interpretations because most elements of both conceptual models are viable the work points to an important need to develop criteria to distinguish between the various inferred mechanisms

the yermak plateau formed at a triple junction the yermak plateau is an arcuate marginal plateau located seaward of northwestern spitsbergen the plateau is divided into two zones based on bathymetric trends and magnetic characteristics that have roughly coincident boundaries north of latitude 82 n where the bathymetric contours trend ne the magnetic amplitudes are large whereas the southern portion strikes nnw and exhibits a subdued magnetic signature beneath the northern section of the plateau crustal refraction velocities of 5 0 km s 1 underlain by 7 2 km s 1 are recorded while farther south velocities of 4 3 6 0 and 8 0 km s 1 are measured the 6 0 km s 1 velocity is similar to ones reported on the continental shelf south of spitsbergen dredged material gravity heat flow and seismic reflection and refraction data indicate the northern region of the plateau is oceanic and formed in conjunction with the morris jesup rise in contrast the southern section is thinned continental crust formed by the initial rifting process at a triple junction north of greenland plate reconstructions for anomalies 24 and 13 are consistent with this interpretation little space is required for the yermak plateau at anomaly 24 because it is composed of oceanic and thinned continental crust the development of the yermak plateau and the morris jesup rise paired aseismic ridges took place when the north american eurasian and greenland plates were moving independently

complex bombs of phreatomagmatic eruptions role of agglomeration and welding in vents of the 1886 rotomahana eruption tarawera new zealand 1 basaltic bombs ejected from a rotomahana crater during the 1886 tarawera eruption in new zealand are of two fundamental types cored bombs which have single large fragments generally of rhyolite wall rock material as their cores and loaded bombs which contain abundant small fragments dispersed through the bombs similar lithic bearing bombs are common products of other phreatomagmatic eruptions worldwide cored bombs originate when a large wall rock fragment is entrained in basaltic melt and then ejected with an adhering skin of basalt in contrast loaded bombs are constructed by agglomeration of multiple fragments of basalt among which smaller rhyolitic wall rock fragments are dispersed different styles and rates of bomb loading are reflected in the bombs textures and compositions these textures provide insight into the nature of fragmentation during the eruption the processes of recycling that produce multiply ejected clasts versus recapitulated ones those where early formed clasts were reingested in magma and then formed the cores of new bombs the degree and timescale of interaction of magma with vent walls and the timescales over which lithic material from the vent walls is incorporated into complex bombs prior to their ejection because fragments of rhyolitic pumice are readily refused their condition constrains estimates of the timing of fragmentation and agglomeration the presence of nonfused rhyolite pumice within many loaded bombs implies that the processes forming and quenching the bombs cannot have exceeded several seconds in contrast other bombs contain patches of compositionally modified basalt with isolated rhyolite derived feldspar xenocrysts that indicate complete fusing melting and assimilation of entrained rhyolitic debris the bomb population overall indicates that conditions within this rotomahana vent were thermally and physically heterogeneous and that recycling of debris was ubiquitous both as recycling of large individual clasts and by fallback and aggregation of material which was reingested in magma before being reejected as new bombs

dykes cups saucers and sills analogue experiments on magma intrusion into brittle rocks magma is transported in the crust by blade like intrusions such as dykes sills saucers and also collects in thicker laccoliths lopoliths and plutons recently the importance and great number o

maar diatreme phreatomagmatism at hopi buttes navajo nation arizona usa remnants of an extensive maar diatreme volcanic field are magnificently exposed at various depths of erosion in the hopi buttes volcanic field of northeastern arizona field and petrographic studies of both the maar and diatreme elements of a selection of volcanoes within the field show that 1 lower sections of the maar rim sequences are typically rich in sandy mudrock derived from the pre eruptive mio pliocene bidahochi formation and the muddy bidahochi sediment was soft and wet at the time of maar eruptions 2 beds higher within the rims contain generally increased proportions of sandstone clasts from the triassic wingate formation in the diatremes late emplaced breccia has deeper seated lithics than more marginal breccia emplaced earlier and 3 many vents are topped by megacryst enriched scoria and spatter and deep seated xenoliths are known only from upper diatreme and craterfilling tephra these observations show that 1 eruptions at hopi buttes involved interaction of magma with unconsolidated mudrock at shallow levels and the phreatomagmatic processes that provided the bulk of the energy involved in the violent eruptions were driven by the interaction of magma and wet sediment 2 the locus of explosive activity migrated downward as eruptions progressed and 3 the closing stage of many eruptions was characterized by rapid magma rise and relative depletion of water

experimental birth of a maar diatreme volcano abstract maar diatreme eruptions are hazardous to people and infrastructure and are also linked to the formation of the kimberlitic variety of diatremes which is important economically processes occurring in the subsurface diatreme and their relation to surface eruptions are not yet well understood we conducted field scale experiments using analog materials to shed more light on these processes especially the formation of the proto diatreme during the first explosions of a maar eruption specifically a series of buried explosions in a prepared layered substrate pad produced craters extra crater deposits and sub crater deposits analogous to volcanic maar craters tephra rings and incipient diatremes post explosion substrate excavation revealed that single large explosions produce sub crater deposits extending nearly to the crater rim crest the same energy divided into three blasts either co located or at different depths with the same epicenter produced narrower and sometimes deeper sub crater deposits even though the final sizes of the craters were similar to that produced by the single large blast the sub crater deposits have an upper zone with domains from different substrate depths and an underlying zone distinguished primarily by being more loosely packed than the original substrate videos show surface motion extending beyond the post shot crater rim and largely vertical ejection and fallback of material into the footprint of these deposits especially for the explosions that occurred below optimal depth of burial we infer that much of the loosely packed material was disassembled vertically transported to different heights during the explosions then fell back without significant relative lateral movement of grains however subvertical fallback did produce apparent cross cutting structures in shallow sub crater deposits one explosion ejected material from the deepest substrate horizon but it was redeposited only within the crater and is unrepresented in the ejecta rim implications of the experiments for maar diatreme volcanoes including some kimberlite pipes are as follows 1 vertical focusing of deep explosions in the diatreme explains the deficit of deep wallrock lithics observed at maar volcanoes 2 direct vertical fallback is possibly an important process forming diatreme deposits especially during the earliest stages and 3 even in our limited simulation the number and scaled depth of explosions clearly affect proto diatreme size and structure

peperite a review of magma sediment mingling the study of peperite is important for understanding magma water interaction and explosive hydrovolcanic hazards this paper reviews the processes and products of peperite genesis peperite is common in arc related and other volcano sedimentary sequences where it can be voluminous and dispersed widely from the parent intrusions it also occurs in phreatomagmatic vent filling deposits and along contacts between sediment and intrusions lavas and hot volcaniclastic deposits in many environments peperite can often be described on the basis of juvenile clast morphology as blocky or fluidal but other shapes occur and mixtures of different clast shapes are also found magma is dominantly fragmented by quenching hydromagmatic explosions magma sediment density contrasts and mechanical stress as a consequence of inflation or movement of magma or lava magma fragmentation by fluid fluid shearing and surface tension effects is probably also important in fluidal peperite fluidisation of host sediment hydromagmatic explosions forceful intrusion of magma and sediment liquefaction and shear liquification are probably the most important mechanisms by which juvenile clasts and host sediment are mingled and dispersed factors which could influence fragmentation and mingling processes include magma host sediment and peperite rheologies magma injection velocity volatile content of magma total volumes of magma and sediment involved total volume of pore water heated presence or absence of shock waves confining pressure and the nature of local and regional stress fields sediment rheology may be affected by dewatering compaction cementation vesiculation fracturing fragmentation fluidisation liquefaction shear liquification and melting during magma intrusion and peperite formation the presence of peperite intraclasts within peperite and single juvenile clasts with both sub planar and fluidal margins imply that peperite formation can be a multi stage process that varies both spatially and temporally mingling of juvenile clast populations formed under different thermal and mechanical conditions complicates the interpretation of magma fragmentation and mingling mechanisms

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

evolution morphostructurale du plateau volcano sedimentaire de gergovie au miocene inferieur implications geodynamiques sur la phase tardi tectonique du rift de limagne massif central france the gergovie plateau is a lower miocene topographically inverted volcano sedimentary system located in the monogenetic volcanic field of the limagne rift tertiary basin it is composed of three east west aligned maars partly covered by a basaltic lava flow the eruption of the central maar maar 1 occurred at the oligocene miocene transition during the first volcanic phase this phreatomagmatic structure was almost totally cut through by the opening of a second maar maar 2 during the next eruptive phase the basaltic lava flow at the summit and the eastern maar maar 3 were placed during a third and last eruptive phase during the middle or upper burdigalian 19 16 ma between these periods of volcanism three fluvial to fluviolacustrine sedimentation episodes separated by two erosive stages followed one another a bedrock thickness of 100 300 m was eroded from maar 2 during the upper aquitanian and or the lower burdigalian 22 19 ma this erosion is partly due to a volcano tectonic uplift in

maar diatreme geometry and deposits subsurface blast experiments with variable explosion depth basaltic maar diatreme volcanoes which have craters cut into preeruption landscapes maars underlain by downward tapering bodies of fragmental material commonly cut by hypabyssal intrusions diatremes are produced by multiple subsurface phreatomagmatic explosions although many maar diatremes have been studied the link between explosion dynamics and the resulting deposit architecture is still poorly understood scaled experiments employed multiple buried explosions of known energies and depths within layered aggregates in order to assess the effects of explosion depth and the morphology and compaction of the host on the distribution of host materials in resulting ejecta the development of subcrater structures and deposits and the relationships between them experimental craters were 1 2 m wide analysis of high speed video shows that explosion jets had heights and shapes that were strongly influenced by scaled depth physical depth scaled against explosion energy and by the presence or absence of a crater jet properties in turn controlled the distribution of ejecta deposits outside the craters and we infer that this is also reflected in the diverse range of deposit types at natural maars ejecta were dominated by material that originated above the explosion site and the shallowest material was dispersed the farthest subcrater deposits illustrate progressive vertical mixing of host materials through successive explosions we conclude that the progressive appearance of deeper seated material stratigraphically upward in deposits of natural maars probably records the length and time scale for upward mixing through multiple explosions with ejection by shallow blasts rather than progressive deepening of explosion sites in response to draw down of aquifers

lateral vent migration during phreatomagmatic and magmatic eruptions at tecuitlapa maar east central mexico abstract tecuitlapa maar located in the eastern central volcanic belt of mexico is a late quaternary 100 m deep 1 km diameter crater with an alignment of scoria cones building in height to the east within it analysis of stratigraphic sections around the maar indicates that the phreatomagmatic eruption began in the eastern part of the crater with interaction between the basaltic magma and liquefied tuffaceous sediments the explosion locus gradually moved westward as evidenced by bomb sag trajectories duneform axes and facies changes producing an elliptical crater the eruption then dried out and began to produce scoria spatter cones with nested craters that young in age and climb in height to the east along the same alignment exhibited by the phreatomagmatic eruption the lithic lapilli and ash in the deposits throughout the eruption are from the 10 40 m thick water saturated tuffaceous sediments partially exposed in the maar crater walls rather than the underlying lavas and limestones this implies that the vent migrated laterally rather than vertically during the eruption and this lack of deepening of the explosion loci may be related to the high lithologic contrast between the non consolidated sediments and the underlying fractured bedrock water movement in the bedrock would have been by fracture flow whereas movement within the sediments was through liquefaction failure and fluidization of the deposit low magma supply rates would allow collapse of the non lithified walls of the dike producing mingling and phreatomagmatic explosions whereas the same magma supply rate in fractured bedrock may seal off water access to the magma the alignment of the vents is parallel to regional structural trends so was probably set in the underlying bedrock and the distance that the vent migrated is likely related to the overall dike length the lack of fluidized sediment coating the scoria cone clasts both in the cones and in the deposits in the walls of the maar implies that the phreatomagmatic eruptions used up most of the liquefied sediment which ended the phreatomagmatic portion of the eruption the migration of the scoria cone vent loci can be explained by an actual migration of the vents or because the evidence on their locations is based on surficial crater overlap relations by a fissure eruption that closed up from the west to the east producing later activity at the taller eastern vents

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

large phreatomagmatic vent complex at coombs hills antarctica wet explosive initiation of flood basalt volcanism in the ferrar karoo lip the mawson formation and correlatives in the transantarctic mountains and south africa record an early eruption episode related to the onset of ferrar karoo flood basalt volcanism mawson formation rocks at coombs hills comprise mainly 80 vol structureless tuff breccia and coarse lapilli tuff cut by irregular dikes and sills within a large vent complex 30 km2 quenched juvenile fragments of generally low but variable vesicularity accretionary lapilli and country rock clasts within vent fill and pyroclastic density current deposits point to explosive interaction of basalt with groundwater in porous country rock and wet vent filling debris metre scale dikes and pods of coherent basalt in places merge imperceptibly into peperite and then into surrounding breccia steeply dipping to sub vertical depositional contacts juxtapose volcaniclastic rocks of contrasting componentry and grainsize these sub vertical tuff breccia zones are inferred to have formed when jets of debris steam water passed through unconsolidated vent filling deposits these jets of debris may have sometimes breached the surface to form subaerial tephra jets which fed subaerial pyroclastic density currents and fall deposits others however probably died out within vent fill before reaching the surface allowing mixing and recycling of clasts which never reached the atmosphere most of the ejecta that did escape the debris filled vents was rapidly recycled as vents broadened via lateral quarrying of country rock and bedded pyroclastic vent rim deposits which collapsed along the margins into individual vents the unstratified poorly sorted deposits comprising most of the complex are capped by tuff lapilli tuff and tuff breccia beds inferred to have been deposited on the floor of the vent complex by pyroclastic density currents development of the extensive coombs hills vent complex involved interaction of large volumes of magma and water we infer that recycling of water as well as recycling of pyroclasts was important in maintaining water supply for phreatomagmatic interactions even when aquifer rock in the vent walls lay far from eruption sites as a consequence of vent complex widening the proportion of recycled water increased with vent complex size in the same way that the proportion of recycled tephra did though water recycling leaves no direct rock record the volcaniclastic deposits within the vent complex show through their lithofacies structural architecture lithofacies characteristics and particle properties clear evidence for extensive and varied recycling of material as the complex evolved

monogenetic volcanism personal views and discussion monogenetic volcanism produces small volume volcanoes with a wide range of eruptive styles lithological features and geomorphic architectures they are classified as spatter cones scoria or cinder cones tuff rings maars maar diatremes and tuff cones based on the magma water ratio dominant eruption styles and their typical surface morphotypes the common interplay between internal such as the physical chemical characteristics of magma and external parameters such as groundwater flow substrate characteristics or topography plays an important role in creating small volume volcanoes with diverse architectures which can give the impression of complexity and of similarities to large volume polygenetic volcanoes in spite of this volcanic facies complexity we defend the term monogenetic volcano and highlight the term s value especially to express volcano morphotypes this study defines a monogenetic volcano a volcanic edifice with a small cumulative volume typically 1 km3 that has been built up by one continuous or many discontinuous small eruptions fed from one or multiple magma batches this definition provides a reasonable explanation of the recently recognized chemical diversities of this type of volcanism

recycling in debris filled volcanic vents eruptions through debris filled vents produce deposits containing magmatic juvenile lithic and recycled clasts recycled clasts are exposed to multiple transportation and fragmentation events we used experiments with multiple subsurface explosions to track clasts and highlight dominant recycling processes in eruptions through analog debris filled vents recycled clasts include those that fall back into and reside in the vent for extended time periods and those that return to the vent through crater growth or collapse clasts are recycled by any combination of lofting and fallback of material in the crater by explosion jets mixing and churning of material at depth in the debris fill and redistribution of extra crater deposits by explosion induced excavation or slumping we compare experimental processes with natural deposits that preserve recycling signatures from discrete explosions through debris filled vents such as maar diatremes strombolian vents and hydrothermal craters clasts may not preserve textures diagnostic of their complete recycling histories but can be used to infer if that history occurred in part in the vent debris or in the eruptive jet experiment results and natural deposits suggest that for volcanic craters that undergo multiple explosions clasts likely undergo some form of recycling before final deposition outside the craters the underestimation of recycled clast contributions to deposits can lead to inaccurate estimates of thermal budgets and eruption processes

the importance of the transport system in shaping the growth and form of kimberlite volcanoes transport of kimberlite magma from the mantle to the surface is a complex process that involves many often sudden and dramatic changes in transport style the pace and magnitude of changes in the transport system acclerate as the magma nears the surface culminating in eruptions during which multiple transport events take place simultaneously as well as single transport events that change significantly from their point of origin to final deposition understanding the range of transport styles recorded by kimberlite deposits is key to describing the type and style of eruptions the growth of cones and craters and the distribution geometry and structure of the deposits that make up kimberlite volcanoes in an economic context building a clear picture of the processes that shape deposits is essential for selecting exploration targets and evaluating the grade and value of diamond bearing kimberlites variations in diamond grade reflect differences in the diamond content of different magma parcels erupted during the lifetime of the kimberlite volcano sorting during transport of eruption products or re working of diamonds during crater growth cone collapse and erosion deep transport systems control delivery of magma and diamonds to eruption sites the initial stages of kimberlite transport involve rise of magma in narrow interconnected networks of fluidfilled fractures there is compelling evidence that kimberlite magmas are extracted from their source region and erupted without significant residence times in the crust many single kimberlite volcanoes have significant variations in diamond grade between different kimberlite phases suggesting eruption of distinct perhaps unrelated magma parcels at the same site

the effect of pre existing craters on the initial development of explosive volcanic eruptions an experimental investigation most volcanic eruptions occur in craters formed by previous activity the presence of a crater implies specific confinement geometries variably filled by loose fragmental deposits which are expected to exert a strong yet poorly studied control on the violent gas expansion that drives the eruption here we analyze patterns of ejection from buried explosions in analog experiments in order to investigate how the presence of a crater and changes in explosion depth and intensity may affect the formation of eruptive ejecta jets results show that scaled depth charge burial depth divided by the cubic root of charge energy controls the velocity and partly spread angle of eruptive jets independently of the presence of a pre existing crater conversely for a fixed scaled depth the presence of a pre existing crater limits the development of a laterally expanding annulus of the jet these results are directly applicable to interpretation of volcanic explosions key points analysis of ejection patterns from intra and extra crater buried explosions velocity and spread angle of ejecta jet are sensitive to explosion scaled depth radially expanding annulus of the jet is sensitive to pre existing crater

root zone processes in the phreatomagmatic pipe emplacement model and consequences for the evolution of maar diatreme volcanoes abstract the understanding of processes within the root zone of maar diatreme volcanoes is important for the interpretation of the geology volcanology and even hazard assessment of these volcanoes in the phreatomagmatic model of pipe formation the irregularly shaped root zone is the site of the phreatomagmatic explosions and thus functions as the engine for pipe formation in this model the root zone grows over a period of time in a series of many single thermohydraulic i e phreatomagmatic explosions the explosions initially occur close to the surface and with ongoing explosive activity penetrate towards deeper levels the ejection of country rock clasts from the root zone results in a mass deficiency in the root zone that causes the overlying tephra and the adjacent country rocks to subside passively in a sinkhole like fashion into the root zone many phreatomagmatic eruptions consequently result in the formation of a cone shaped diatreme thus with ongoing eruptions the cone shaped diatreme has to grow systematically both in depth and diameter during its growth processes in the lower diatreme levels successively destroy the upper levels of the evolving root zone at the surface the maar crater in turn reacts to the underlying subsidence processes and also grows both in depth and diameter thermohydraulic explosions which fragment both magma and the surrounding country rocks mostly occur within the bottom part of the root zone violent explosions in small pipes may clear the overlying diatreme for a short period of time before tephra fall and collapse of the walls of the new crater refill the small initial diatreme in larger pipes via expansion of the mixture of highly pressurized water vapor juvenile gas phases and explosively produced tephra the confined and expanding eruption cloud has to pierce through the diatreme fill in a feeder conduit in order to erupt diatreme clearing events in large pipes are difficult or impossible to maintain since the explosive force in the root zone is only in exceptional instances strong enough to lift or entrain the entire diatreme tephra knowledge of the genetic relationships between root zones and diatremes is critical to understand pipe growth processes the combination of such processes can lead to substantial variation in volcanic behavior and thus produce fundamentally different volcano and rock types it is the purpose of this paper to outline important features of root zones and suggest their significance for the genesis and evolution of maar diatreme and related volcanoes

evolution and hydrological conditions of a maar volcano atexcac crater eastern mexico abstract the atexcac maar is located in the central part of the serdan oriental lacustrine playa basin in the eastern mexican volcanic belt it is part of a dispersed and isolated monogenetic field consisting of maar volcanoes basaltic cinder cones and rhyolitic domes atexac is a maar volcano excavated into pyroclastic deposits basaltic lava flows and the flanks of a cinder cone cluster which itself was built on a topographic high consisting of limestone it has an ene trending elliptical shape with beds mostly unconsolidated deposits that dip outward at 16 22 the atexcac crater was formed from vigorous phreatomagmatic explosions in which fluctuations in the availability of external water temporal migration of the locus of the explosion and periodic injection of new magma were important controls on the evolution of the maar crater variations in grain sizes and component proportions of correlated deposits from the different sections suggest a migration of the locus of explosions producing different eruptive conditions with fluctuating water magma interactions deposits rich in large intrusive and limestone blocks are associated with a matrix enriched in small andesitic lapilli this could suggest differential degrees of fragmentation due to inherited previously acquired fragmentation and or relative distance to the locus of explosions initial short lived phreatic explosions started at the southwest part of the crater and were followed by an ephemeral vertical column and the influx of external water that led to relatively shallow explosive interactions with the ascending basaltic magma drier explosions progressed downward and or laterally northward sampling subsurface rock types particularly intrusive limestone and andesitic zones as well as localized altered zones n ne caused by repetitive injection of basaltic magma a final explosive phase involved a new injection of magma and a new influx of external water producing wetter conditions at the end of the maar formation we infer the aquifer was formed by fractured rocks predominantly andesitic lava flows and limestone rocks andesitic accessory clasts dominate in all stratigraphic levels but these rocks are not exposed in the nearby area these local hydrogeological conditions contrast with those at nearby maar volcanoes where the water for the magma water interactions apparently mostly came from a dominantly unconsolidated tuffaceous aquifer producing tuff rings with a much lower profile than atexcac

maar diatreme volcanoes a review maar diatreme volcanoes are produced by explosive eruptions that cut deeply into the country rock a maar is the crater cut into the ground and surrounded by an ejecta ring while the diatreme structure continues downward and encloses diatreme and root zone deposits here we attempt an evenhanded review of maar diatreme volcanology that extends from mafic to kimberlitic varieties and from historical maar eruptions to deeply eroded or mined diatreme structures we conclude that maar diatreme eruptions are episodic ejecta rings provide invaluable insight into eruption processes and sequence but are incomplete records of diatreme formation deposits within the diatreme structure include in varying proportions lower unbedded deposits sometimes typified by subvertical contacts among domains of debris emplaced sequentially and upper bedded deposits formed by sedimentation on surfaces open to the atmosphere a basal root zone comprises the transition from coherent magmatic feeder dike to clastic deposits formed by fragmentation of magma and enclosing country rock root zones are irregular in form and the clastic deposits are typically intruded by contorted dikes irregular root zone like chaotic breccias cut by contorted dikes are also present within diatreme deposits where they represent intra diatreme fragmentation zones and record changes in the location of the explosion locus during eruption

on the formation of deep seated subterranean peperite like magma sediment mixtures abstract in addition to large amounts of fragmented country rocks maar diatreme volcanoes commonly eject juvenile lapilli and cauliflower bombs containing many dispersed xenoliths and or xenocrysts derived from the country rocks surrounding the diatremes formation of these lapilli and bombs is attributed to thermohydraulic explosions in the root zones of the diatremes and their consequences the thermohydraulic explosions release shock waves which are capable of intensely fragmenting the surrounding country rocks vaporisation of water superheated during thermohydraulic explosions and its expansion to lower pressure causes ejection of major parts of the fragmented country rocks surviving sections of fragmented country rocks may collapse or slide into the partially evacuated root zone and thus form a mass flow deposit subsequent to each explosion when prior to a new explosion magma rises from the underlying feeder dyke into the root zone it intrudes this clastic debris inflates it and may mingle with the debris to form a peperite subsequent explosions can fragment and eject fragments of this peperite in the form of juvenile ash grains juvenile lapilli and bombs containing xenoliths or xenocrysts from the country rock debris lack of further explosions but continued rise and intrusion of magma will cause emplacement of such inflated peperite masses as plugs

discrete blasts in granular material yield two stage process of cavitation and granular fountaining a discrete blast within granular material such as a single subterranean explosion within a debris filled diatreme structure is typically considered to produce a single uprush of material our experiments demonstrate that apparent debris jet deposits can be formed by a two stage process of cavitation and subsequent granular fountaining bench scale experiments reported here demonstrate that for a range of overpressures and depths individual discrete buried gas blasts open space and expel particles from the blast site in two largely decoupled stages expanding gas initially pierces material nearest the blast source to open a cavity above it then a fountain of grains rises from the source into the cavity this staged motion dynamically segregates source grains from host material grains and the rates of cavity opening versus fountain rise show a power law decay relationship with initial pressure our experimental analysis has implications for maar diatreme systems field scale detonation experiments and underground nuclear testing

a note on maar eruption energetics current models and their application hydromagmatic eruptions convert thermal into mechanical energy via the expansion of ground and or surface water several models address the energetics of these eruptions based on different physical volcanological approaches here we test different models with two case studies in the colli albani volcanic district central italy the monogenetic prata porci and the polygenetic albano maars test results are mutually consistent and show cumulative mechanical energy releases on the order of 1015 1017 j for single maars the fraction of thermal energy turned into mechanical ranges from 0 45 as calculated from the theoretical maximum mechanical energy through 0 1 calculated from country rock fragmentation crater formation and ballistic ejection to 0 03 derived from magma fragmentation by thermohydraulic explosions it appears that the energy released during the most intense hydromagmatic events may account for a dominant fraction of the total mechanical energy released during the whole maar eruptive histories finally we consider the role of magmatic explosive activity intervening during maar eruptions in causing departures from predictions of the models evaluated

entrainment of country rock during basaltic eruptions of the lucero volcanic field new mexico as magma rises through the lithosphere it may entrain wall rock debris the entrainment process depends on the local hydrodynamic regime of the magma e g velocity temperature bulk density the extent of interaction of magma with groundwater and the mechanical properties of the wall rocks wall rock entrainment results in local flaring of dikes and conduits which in turn affects the hydrodynamics of magma ascent and eruption we studied upper crustal xenoliths erupted from small volume basaltic volcanoes of the lucero volcanic field west central new mexico in order to assess the relative importance of various entrainment mechanisms during a range of eruptive styles including strongly hydrovolcanic strombolian and effusive processes total xenolith volume fractions ranged from 0 3 0 9 in hydrovolcanic facies to 10 4 10 2 in strombolian facies the volcanoes erupted through a thick well characterized sequence of paleozoic and lower mesozoic sedimentary rocks so that erupted xenolith

rapid injection of particles and gas into non fluidized granular material and some volcanological implications in diatremes and other volcanic vents steep bodies of volcaniclastic material having differing properties particle size distribution proportion of lithic fragments etc from those of the surrounding vent filling volcaniclastic material are often found it has been proposed that cylindrical or cone shaped bodies result from the passage of debris jets generated after phreatomagmatic explosions or other discrete subterranean bursts to learn more about such phenomena we model experimentally the injection of gas particulate dispersions through other particles analogue materials glass beads or sand and a finite amount of compressed air are used in the laboratory the gas is made available by rapidly opening a valve therefore the injection of gas and coloured particles into a granular host is a brief 1 s discrete event comparable to what occurs in nature following subterranean explosions the injection assumes a bubble shape while expanding and propagating upwards in reaction the upper part of the clastic host moves upward and outward above the bubble forming a dome the doming effect is much more pronounced for shallow injection depths thin hosts with dome angles reaching more than 45 significant surface doming is also observed for some full scale subterranean blasts e g buried nuclear explosions so it is not an artefact of our setup what happens next in the experiments depends on the depth of injection and the nature of the host material with shallow injection into a permeable host glass beads the compressed air in the bubble is able to diffuse rapidly through the roof meanwhile the coloured beads sediment into the transient cavity which is also closing laterally because of inward directed granular flow of the host depending on the initial gas pressure in the reservoir the two phase flow can erupt or not non erupting injections produce cylindrical bodies of coloured beads whereas erupting runs produce flaring upward or conical deposits changing the particle size of the host glass beads does not have a large effect under the size range investigated 100 200 to 300 400 m doubling the host thickness injection depth requires a doubling of the initial gas pressure to produce similar phenomena such injections whether erupting or wholly subterranean provide a compelling explanation for the origin and characteristics of multiple cross cutting bodies that have been documented for diatreme and other vent deposits

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

thermal implications for the evolution of the spitsbergen transform fault abstract heat flow taken between svalbard and greenland reveal three thermal provinces 1 1 the molloy ridge within the spitsbergen transform 2 2 the yermak plateau 3 3 the northeastern margin of svalbard nordaustlandet the molloy ridge is a short spreading segment and the average heat flow is much above the sclater et al 1971 cooling curve but agrees with values from the norwegian greenland sea an additional zone of intrusion identified by heat flow lies to the northwest of the molloy ridge it straddles both the visible fracture zone and part of the yermak plateau a thermal boundary lies between the warm western segment of the yermak plateau and the shelf off nordaustlandet if the thermal subsidence of the western yermak plateau can be traced to the latest heating episode then it is likely that the crust is similar to oceanic in composition and not older than 13 m y approximately 20 m y younger than the northeastern segment of the plateau plate rotation shows that there was no room for the western segment of the plateau prior to anomaly 7 we postulate that the original transform is associated with the hornsund fault zone in response to deviatoric stress across the oblique ridge transform system the nansen ridge propagated southwestward aborting the old transform trace and shifted to its present position it is suggested that this propagation and migration of the ridge transform system across a zone of extensional deviatoric stress allowed the massive intrusion of basalt forming the western yermak plateau the propagation phenomenon coincides with large scale tertiary volcanic activity on svalbard readjustment and migration of the oblique transform is still taking place as the transform ridge system is liberated from continental constraints the migration rate will diminish as orthogonality is approached

reconstructing the eruption magnitude and energy budgets for the pre historic eruption of the monogenetic 5 ka mt gambier volcanic complex south eastern australia understanding explosive volcanic eruptions especially phreatomagmatic eruptions their intensities and energy budgets is of major importance when it comes to risk and hazard studies with only a few historic occurrences of phreatomagmatic activity a large amount of our understanding comes from the study of pre historic volcanic centres which causes issues when it comes to preservation and vegetation in this research we show that using 3d geometrical modelling it is possible to obtain volume estimates for different deposits of a pre historic complex monogenetic centre the mt gambier volcanic complex south eastern australia using these volumes we further explore the energy budgets and the magnitude of this eruption vei 4 including dispersal patterns eruption columns varying between 5 and 10 km dispersed towards north east to south to further our understanding of intraplate monogenetic eruptions involving phreatomagmatic activity we also compare which thermodynamic model fits best in the creation of the maar crater of mt gambier the major explosion dominated model or the incremental growth model in this case the formation of most of the craters can best be explained by the latter model

immense vent complex marks flood basalt eruption in a wet failed rift coombs hills antarctica large lava effusions can have impressive explosive antecedents although our picture of flood basalt is overwhelmingly effusive phreatomagmatic eruptions have preceded quiet effusion of some flood basalts and reflect the same influence of vent architecture and hydrology on eruptive style as seen for small volume eruptions the scale of phreatomagmatic deposits associated with flood basalts can be huge at coombs hills a vast but otherwise typical phreatomagmatic vent complex is exposed over more than 25 km 2 and its features are interpreted to reflect processes of tephra jet eruptions with diatreme development similar vent complexes are probably the source of laharic deposits reported elsewhere in the transantarctic mountains and in the karoo province of south africa

physical characteristics of kimberlite and basaltic intraplate volcanism and implications of a biased kimberlite record we assess bias in the record of kimberlite volcanism by using newly acquired size data on more than 900 kimberlite bodies from 12 kimberlite fields eroded to depths of between 0 m and 1200 m and by a comparison with intraplate monogenetic basaltic volcanic fields eroded kimberlite fields are composed of pipes or diatremes and dikes and within any one kimberlite field regardless of erosion level kimberlite bodies vary in area at earth s surface over 2 3 orders of magnitude typically 60 70 of the bodies are

basaltic phreatomagmatic fissure at 71 gulch part 2 unusual pyroclasts from sediment magma mingling and melting the behavior of magma as it encounters unconsolidated sediment can be studied in fortuitous exposures of incised volcanic systems to help determine the conditions that control the transport arrest or mingling of that magma in the sediment the pliocene subaqueous basaltic fissure at 71 gulch idaho usa contains unusual light colored glassy mingled clasts that were produced through the incorporation of domains of siliciclastic sands and silts into basalt these clasts contain textures reflective of melting and mechanical mixing of these sediments with the rising basalt along intrusions at depths 30 m beneath the pre eruptive surface and transport into and through a shallow subsurface debris filled vent mingled clasts comprised colorless glass with locally high silica contents and visible swirls of sideromelane along with crystals derived from both the host sediment and the 71 gulch basalt the unique combination of textures minerals plus major and trace element geochemistry within the geologic context of 71 gulch points to a two phase mixing that did not contribute directly to nor impede the phreatomagmatic explosions that erupted the mingled clasts

eocene to miocene igneous activity in ne greenland northward younging of magmatism along the east greenland margin radiometric dating by the 40 ar 39 ar incremental heating method was carried out on lavas sills dykes and a central intrusion from ne greenland eighteen samples gave acceptable crystallization ages lavas of both lower and upper plateau lava series gave ages in the range 55 5 53 5 ma and cannot be constrained to better than 56 53 ma sills and dykes from traill o to shannon with compositions fairly similar to those of the lavas gave ages of 55 1 51 3 ma contemporaneous with and slightly younger than the lavas alkaline lavas on inland nunataks have ages of 53 50 ma and the kap broer ruys intrusive centre has an age of 48 7 0 5 ma an alkaline sill on hvalroso is much younger at 20 3 0 1 ma there are no pre breakup lavas onshore ne greenland we surmise that the hot mantle of the iceland plume arrived and melted extensively beneath the northern basins only at the time of breakup around 55 ma post breakup intrusive events in ne greenland coincided with plate tectonic events such as reorganization uplift and opening in the north the hvalroso sill represents a local small melting event that may be related to coeval opening of the lena trough supplementary materials details of the source data results and the compositions and locations of the dated samples are available at www geolsoc org uk sup18738

multiple origins of xenoliths and xenocrysts in the elephant moraine 79001 lithology a olivine phyric shergottite

preliminary interpretation of aeromagnetic data from spitsbergen svalbard archipelago 76 79 n implications for structure of the basement abstract new arctic aeromagnetic data from spitsbergen svalbard archipelago have been interpreted the depths to the top of the magnetic source rocks have been interpreted using profile data and the interpretation shows that the magnetic basement is deeper than 3 km below sea level over large areas a picture of basement horsts and grabens emerges from this map a deep n s trending trough more than 8 km deep exists beneath isfjorden it has a graben structure and is bordered to the east by a southward extension of the billefjorden fault zone model calculations show that steep east dipping structures in the pre devonian hecla hoek basement rocks can explain anomalies above the billefjorden fault zone the billefjorden fault zone is interpreted to extend from the shelf north of spitsbergen to south of isfjorden and may extend southwards into the barents sea a new aeromagnetic map covering spitsbergen and the adjacent northern barents sea has been compiled which shows that structures on spitsbergen continue far into the barents sea some of the anomalies that originate within the crystalline basement occur near the boundaries to younger sedimentary basins and suggest that basement tectonics have influenced basin formation