discrimination of grain size subpopulations in pyroclastic deposits a cubic spline curve fit to histograms of grain size data from pyroclastic deposits affords a rapid means to distinguish subpopulations in complex size distributions using interactive microcomputer graphics the operator can isolate and quantitatively characterize subpopulations by a method of successive gaussian subtractions percentile measures of the entire size range are not needed and segments of the whole size distribution can be analyzed independently recurrent subpopulations with similar means and standard deviations are consistent among the more than 140 samples of pyroclastic surge deposits from sugarloaf in arizona and vulcano in italy the entire data set can be replicated by synthetic curves consisting of various proportions of five identified gaussian components each of which the authors attribute to a specific mechanism of transport and deposition

repeated fracture and healing of silicic magma generate flow banding and earthquakes textures in an exceptionally preserved effusive rhyolite conduit at torfajokull iceland indicate that rising magma repeatedly fractured and healed at shallow levels in the conduit rfh process anastomosing tuffisite veins filled by fine grained juvenile clasts were generated by shear fracture of highly viscous magma in the glass transition interval welding of the particulate material during subsequent deformation led to thorough healing of veins allowing repeated fracture of the same body of magma we propose that the rfh process is a rechargeable trigger mechanism for hybrid seismicity and show that the time scale of the process and the fractures formed by it are consistent with the repeat time and magnitude of hybrid earthquakes during silicic eruptions the rfh process may also form the flow banding that is nearly ubiquitous in obsidian

magma degassing buffered by vapor flow through brecciated conduit margins obsidian pyroclasts a common component of rhyolitic tephra preserve a range of volatile contents which has been used to infer syneruptive conditions of magmatic degassing here we show that the textures of obsidian pyroclasts provide information on physical mechanisms of magma flow and degassing along conduit margins obsidian clasts often contain xenoliths sheared bands of lithic powder and textures consistent with magma autobrecciation these features suggest that pyroclastic obsidian primarily forms near conduit walls where magma fragments and reanneals during ascent we use these observations to develop a degassing model for pyroclastic obsidian from the a d 1340 mono craters california eruptions we suggest that degassing was buffered by continual flux of vapor through highly permeable brecciated magma along conduit walls continuous reequilibration of magma with vapor of relatively constant composition not only explains the co 2 h 2 o and d h 2 o data from mono craters pyroclastic obsidian but also requires much lower magmatic co 2 values than the commonly accepted model of closed system degassing taken together the chemical and physical evidence suggests that magma brecciation along conduit walls aids the degassing of ascending rhyolite

magma composition and viscosity as controls on peperite texture an example from passamaquoddy bay southeastern canada silurian mafic lava flows and rhyolite sills in the passamaquoddy bay area southeastern canada interacted with wet sediment to produce peperitic breccia the pahoehoe type mafic flows flowed over and bulldozed into wet unconsolidated silt and sand to produce both blocky and fluidal peperite at their lower contacts soft sediment deformation structures in the siltstone adjacent to the flows irregularly shaped vesicles in the siltstone and siltstone filled vesicles in the mafic lava all indicate the sediment was wet at the time of flow emplacement sills of rhyolite emplaced into the same sedimentary sequence also interacted with the wet sediment and produced peperite at both the upper and lower contacts evidence for the rhyolitic magma interacting with wet sediment includes sediment filled vesicles in rhyolite quartz filled vesicles in siltstone and angular non vesicular shards of rhyolite in clastic dykes peperite formed by this process is exclusively blocky the rhyolitic magma in the passamaquoddy bay sequence and other examples of high viscosity magma producing only blocky peperite suggest that in contrast to low viscosity magmas which produce a combination of blocky and fluidal peperite only blocky peperite will form associated with highly viscous rhyolite magma the sediment that interacted with both the mafic and rhyolitic lavas is of similar texture indicating that in this case sediment texture did not control peperite type the change from fluidal to blocky peperite texture may also be controlled in part by the magma temperature and or confining pressure and the generation and maintenance of a steam film at the magma wet sediment interface

wet surge deposits at la fossa di vulcano depositional and eruptive mechanisms abstract wet surge deposits of different volcanic cycles of the recent fossa activity at vulcano have been measured on a bed by bed basis with data recorded to millimeter detail the wet surge layers are varicoloured with variable thickness with the most recurrent thickness being about 1 cm the beds consist of fine ash without internal structures textural features include a accretionary lapilli of maximum size of 0 5 cm dispersed thoroughout the layer or forming continuous layers of submillimeter size b vesiculated layers which represent 10 to 65 of the total deposit vesicles have different shapes and smooth walls varying in volume from 1 to 15 20 c soft sediment types of bedding deformation such as gravity flowage ripples load cast and slumps the slope angle has not influenced either the concentration and size of the accretionary lapilli or the shape size and distribution of vesicles only the thickness of the layers decreases with distance from the vent sem investigations show features indicating the hydromagmatic origin of the deposits and stressing the role of the fluid phase noteworthy is the presence of vesiculated grains produced by magmatic exsolution which show chilling effects on the internal walls of the broken bubbles grain size analyses reveal that the layers are not graded and most of the samples have a median size finer than 50 m the grain size distributions are frequently polymodal suggesting several closely timed explosions as all the beds exhibit the same textures grain size and particle morphology a single mechanism can explain their deposition the depositional unit formed at the base of the cloud through the lateral expansion on the ground of a sticky muddy medium consisting of ash and fluid in general each layer lost its plasticity before the deposition of the next layer the deposition occurred in a nearly continuous sequence without periods of rest long enough to permit erosional discontinuity the eruptions are hydromagmatic and occur where magma at least partially fragmented comes into effective contact with subsurface water the process follows a model suggested by wohletz 1983b 1986 developing in more than one fragmentation event in the turbulent surge cloud both severe hydration and alteration of glassy grains and the formation of accretionary lapilli occur

the udo tuff cone cheju island south korea transformation of pyroclastic fall into debris fall and grain flow on a steep volcanic cone slope the udo tuff cone of cheju island south korea is a middle pleistocene basalt tuff cone that has formed by early surtseyan type eruptions and later drier hydroclastic eruptions the tuff cone comprises steep 20 30 and planar beds of lapillistone lapilli tuff and tuff that can be grouped into seven sedimentary facies a g facies a and b comprise continuous to lenticular layers of grain supported and openwork lapillistone that are inversely graded and coarsen downslope they suggest emplacement by grain flows that are maintained by gravity induced stress and grain collisions facies c includes poorly sorted crudely bedded and locally inversely graded lapilli tuff also suggestive of rapid deposition from highly concentrated grain flows facies d includes thinly stratified and mantle bedded tuff that was probably deposited by fallout of wind borne ash other facies include massive lapilli tuff facies e chaotic lapilli tuff facies f and cross bedded tuffaceous sandstone facies g that were deposited by resedimentation processes such as debris flow slide slump and stream flow respectively the grain flows that produced facies a b and c are interpreted to have originated from falling pyroclasts which initially generated highly dispersed saltating avalanches in which momentum was transferred by the particles themselves this transport mechanism is similar to that of debris fall as the slope gradient was too low to maintain a highly dispersed flow the debris fall decelerated and contracted due to a decrease in dispersive pressure the mode of momentum transfer changed to one of collision because contraction of the debris fall resulted in an increase in particle concentration this transport mechanism is similar to that of common grain flows grain segregation occurred in several ways initial segregation of ash from lapilli occurred due to their differing terminal fall velocities and their contrasting degrees of sliding friction with the bed percolation of ash into interstices of lapilli during flow kinematic sieving augmented further segregation of ash from lapilli the latter process along with a dispersive pressure effect gave rise to vertical inverse size grading downdip inverse grading was produced by particle overpassing

sediment transport and topographic evolution of a coupled river and river plume system an experimental and numerical study sediment transfer from rivers to the ocean is the fundamental driver of continental sedimentation with implications for carbon burial land use dynamics and unraveling global climate change and earth history from sedimentary strata coastal rivers are dynamically coupled to their offshore plumes at the river mouth creating regions of nonuniform flow that can dictate patterns of erosion and deposition both onshore and offshore however there are limited experimental and modeling studies on sediment transport and morphodynamics of coupled river and river plume systems and their response to multiple flood events to address this knowledge gap we developed a quasi 2 d morphodynamic numerical model and conducted exploratory flume experiments in a 7 5 m long flume where a 10 cm wide river channel was connected to a 76 cm wide ocean basin both the numerical model and the flume results demonstrate that 1 during low discharge flows backwater hydrodynamics cause spatial flow deceleration and deposition in the river channel and the offshore plume area and 2 during high flows the water surface is drawn down to sea level resulting in spatial flow acceleration and bed scour during high discharge flows we also found that the offshore river plume self channelized owing to both levee formation and bed scour our study suggests that coastal rivers may be in a perpetual state of morphodynamic adjustment and highlights the need to link rivers and river plumes under a suite of flow discharges to accurately predict fluvio deltaic morphodynamics and connectivity between fluvial sediment sources and marine sinks

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

rhyolitic phreatomagmatism explored tepexitl tuff ring eastern mexican volcanic belt abstract rhyolitic tepexitl tuff ring in the serdan oriental basin of the eastern trans mexican volcanic belt is a young well preserved circular crater a simple morphology that belies a more complex eruptive history field observations ash studies granulometry componentry and morphology and volatile data loi and xrf allow the broad division of the tepexitl deposits into two sequences a lower sequence characterized by fine grained deposits with abundant bomb and lapilli sags and an upper sequence of coarse grained deposits that show little to no deformation the early eruptions at tepexitl are interpreted to have been dominated by discrete highly efficient phreatomagmatic blasts which caused a progressive deepening of the eruptive center lower sequence followed by a transition to dominantly magmatic behavior in the upper sequence dome growth occurred at the end of the eruption but subsequent retrogressive explosions triggered by external water destroyed all trace of the original dome morphology molten fuel coolant interaction mfci is commonly thought to cause the repetitive water magma interaction in phreatomagmatic eruptions but a process by which magma and viscous felsic magmas can mingle prior to explosive interaction has not been described a viable mechanism for rhyolitic mfci based upon field work at tepexitl and laboratory experiments requires that fluidized sediments intrude marginal fractures in the rhyolite magma creating enough interfacial surface area to initiate phreatomagmatic explosions from within the interior of a rising plug or dome

a unified model for subaqueous bed form dynamics 1 bed form evolution remains dynamic even in the special case of steady uniform flow data from the sandy braided north loup river nebraska show that roughness features on the channel bottom display a statistical steady state and robust scaling that are maintained through the collective interactions of transient short lived bed forms motivated by such field data and laboratory observations of bed form growth we develop a nonlinear stochastic surface evolution model for the topography of bed load dominated sandy rivers in which instantaneous sediment flux explicitly depends on local elevation and slope this model quantitatively reproduces laboratory observations of initial growth and saturation of bed forms from a flat surface and also generates long term dynamical behavior characteristic of natural systems we argue that the variability in geometry and kinematics of bed forms in steady flow and the existence of roughness at all wavelengths up to the largest dunes are a consequence of the nonlinear relationship between sediment flux and topography subject to noise

quantitative experiments on phreatomagmatic explosions abstract an experimental set up for controlled generation of thermal explosions fuel coolant interactions has been built by an interdisciplinary research group of volcanologists physicists and engineering scientists the tee haus thermal explosion experiment during the last 3 years more than 500 experimental runs were performed aim of the project was not to build a mini volcano but to produce igniteable mixtures of water and hot melt on a small scale in order to investigate basic physical aspects such small scale mixtures however most probably also occur during phreatomagmatic events in nature mixtures of water and magma on a large scale may be modelled by combination of small elements earlier experimental studies of thermal explosions were carried out with the use of metal melts nearly exclusively therefore a carbonate melt approx 0 3 kg of 1 1 mass ratio na2co3 k2co3 was used for initial experimental series because of easy handling and more magma similarity by injection of water into this melt explosive mixtures were produced and ignited self triggering explosive interactions occurred in a wide melt temperature range 720 to 1040 c at melt temperatures exceeding 1000 c co2 increasingly degassed presence of non condensible gas bubbles in the melt strongly reduced the explosivity the intensity of explosions only slightly increased with higher melt temperatures but strongly increased with higher water injection velocities because of more intensive mixing a minimum injection velocity was found to be approx 0 7 m s 1 experiments with silicate melts derived from diverse remelted volcanic rocks were carried out with a slightly modified set up explosions were ignited by an additional trigger a shock wave of approx 8 j in volcanic systems shock waves of this magnitude are abundant volcanic tremor etc strong explosions with silicate melts were produced at melt temperatures between 1350 and 1750 c the produced explosions exerted repulsion forces of up to 24 000 n with a typical pulse duration of 1 ms carbonate melt resp 1 5 ms silicate melts the ejection velocity was found to be in a range between 200 and 400 m s 1 explosion energy values up to 500 j were calculated the maximum explosion pressure was calculated to be in the range between 10 and 100 mpa only part of the melt so called interactive melt reacts explosively with injected water most of the melt is passively ejected by and after the thermal explosion different fragmentation processes result in specific grain size and grain shape populations that can be distinguished from each other particles resulting from the fine fragmentation process that precedes and causes the thermal explosion are characterized by angular to subrounded shapes and grain sizes ranging from 20 m to 180 m the amount of interactive melt is proportional to the intensity of the explosion the maximum amount of water vapourized at the explosive interaction so called interactive water was estimated thus the water melt mass ratio for explosive interactions could be calculated the values range between 1 25 and 1 35 carbonate melt resp between 1 6 and 1 25 silicate melts

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

testing morphodynamic controls on the location and frequency of river avulsions on fans versus deltas huanghe yellow river china a mechanistic understanding of river avulsion location and frequency is needed to predict the growth of alluvial fans and deltas the huanghe china provides a rare opportunity to test emerging theories because its high sediment load produces regular avulsions at two distinct nodes where the river debouches from the loess plateau avulsions occur at an abrupt decrease in bed slope and reoccur at a time interval 607 yrs consistent with a channel filling timescale set by the superelevation height of the levees downstream natural deltaic avulsions reoccur at a timescale that is fast 7 yrs compared to channel filling timescale due to large stage height variability during floods unlike the upstream node deltaic avulsions cluster at a location influenced by backwater hydrodynamics and show evidence for episodic downstream migration in concert with progradation of the shoreline providing new expectations for the interplay between avulsion location frequency shoreline rugosity and delta morphology

depositional mechanics and sequences of base surges songaksan tuff ring cheju island korea the songaksan mount in the southwestern part of cheju island korea is a taalian tuff ring produced by phreatomagmatic explosions at an aquifer a detailed analysis of proximal to distal facies changes reveals that the tuff ring sequence can be represented by 21 sedimentary facies one lateral facies sequence lfs and three vertical facies sequences vfs the vfs 1 and 2 are representative of facies relationships in horizontal near vent deposits the vfs 1 comprises scour fill bedded tuff inversely graded tuff massive tuff and laminated tuff from base to top the vfs 2 is a variant of the vfs 1 replaced by an inversely graded lapilli tuff unit at the base the sequences suggest traction carpet suspension and minor traction sedimentation from a high concentration near vent base surge the lfs 1 and the vfs 3 are distilled from outward dipping flank deposits both sequences begin with disorganized lapilli tuff followed successively by stratified lapilli tuff dune bedded lapilli tuff very thinbedded tuff and accretionary lapilli they are suggestive of waning base surge which decreases in particle concentration suspended load fall out rate and flow regimes with an increase in traction and sorting these facies sequences suggest that a base surge experiences flow transformation with its flow characters changing with time and space a near vent base surge is turbulent uniformly mixed and highly concentrated and produces scour fill bedded tuff as capacity decreases the surge transforms into a dense and laminar underflow and a dilute and turbulent upper part gravity transformation depositing inversely graded massive and normally graded lapilli tuff ensuing loss of sediment load and mixing of ambient air result in flow dilution surface transformation stratified and dune bedded units are produced by tractional processes of turbulent and low concentration surge further dilution causes deceleration and cooling and results in precipitation of moistened ash and accretionary lapilli from suspension

generation of fine hydromagmatic ash by growth and disintegration of glassy rinds 1 the deposits of mafic hydromagmatic eruptions are more fine grained and variable in vesicularity than dry magmatic deposits blocky equant shapes of many hydromagmatic clasts also contrast with droplet thread and bubble wall morphology of dry magmatic fragments small 180 m blocky hydromagmatic pyroclasts have traditionally been interpreted to result from discrete vapor explosions although such explosions tend to occur only under certain conditions this paper considers a process of hydromagmatic ash formation that involves repeated growth and disintegration of glassy rinds on pyroclast surfaces as they deform within turbulent flows this process termed turbulent shedding may occur during the expansion phase of vapor explosions or during turbulent but nonexplosive mixing of magma with water steam or water sprays the occurrence of turbulent shedding and the resulting fragment sizes depend on the timescale for rind growth and the timescale between disturbances that remove or disintegrate glassy rinds turbulent shedding is directly observable in some small littoral jets at kilauea calculations suggest that in the presence of liquid water or water sprays glassy rinds having a thickness of microns to millimeters should form in milliseconds to seconds this is similar to the timescale between turbulent velocity fluctuations that can shred lava globules and remove such rinds the fraction of a deposit consisting of fine ash should increase with the duration of this process large scale surtseyan jets generate hundreds or thousands of shedding events bubble bursts or tephra jets at kilauea s coast may produce only a few

evidence for the relative depths and energies of phreatomagmatic explosions recorded in tephra rings experimental work and field observations have inspired the revision of conceptual models of how maar diatreme eruptions progress and the effects of variable energy depth and lateral position of explosions during an eruption sequence this study reevaluates natural tephra ring deposits to test these new models against the depositional record two incised tephra rings in the hopi buttes volcanic field are revisited and published tephra ring stratigraphic studies are compared to identify trends within tephra rings five major facies were recognized and interpreted as the result of different transportation and depositional processes and found to be common at these volcanoes tephra rings often contain evidence of repeated discrete phreatomagmatic explosions in the form of couplets of two facies 1 massive lapilli tuffs and tuff breccias and 2 overlying thinly stratified to cross stratified tuffs and lapilli tuffs the occurrence of repeating layers of either facies and the occurrence of couplets are used to interpret major trends in the relative depth of phreatomagmatic explosions that contribute to these eruptions for deposits related to near optimal scaled depth explosions estimates of the mass of ejected material and initial ejection velocity can be used to approximate the explosion energy the 19 stratigraphic sections compared indicate that near optimal scaled depth explosions are common and that the explosion locations can migrate both upward and downward during an eruption suggesting a complex interplay between water availability and magma flux reverse to normal coarse tail graded tuff breccias inferred to be the product of closely timed phreatomagmatic explosions and deposits of magmatic gas driven explosions were observed interspersed with discrete explosion deposits this study not only provides a framework for more detailed interpretations of eruption sequences from tephra rings but also highlights the gap in our understanding of syn eruptive hydrology and variations in magma flux that enables phreatomagmatic explosions

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

pyroclastic phases of a rhyolitic dome building eruption puketarata tuff ring taupo volcanic zone new zealand the 14 ka puketarata eruption of maroa caldera in taupo volcanic zone was a dome related event in which the bulk of the 0 25 km3 of eruption products were emplaced as phreatomagmatic fall and surge deposits a rhyolitic dike encountered shallow groundwater during emplacement along a ne trending normal fault leading to shallow seated explosions characterised by low to moderate water magma ratios the eruption products consist of two lava domes a proximal tuff ring three phreatic collapse craters and a widespread fall deposit the pyroclastic deposits contain dominantly dense juvenile clasts and few foreign lithics and relate to very shallow level disruption of the growing dome and its feeder dike with relatively little involvement of country rock the distal fall deposit representing 88 of the eruption products is despite its uniform appearance and apparently subplinian dispersal a composite feature equivalent to numerous discrete proximal phreatomagmatic lapilli fall layers each deposited from a short lived eruption column the puketarata products are subdivided into four units related to successive phases of a shallow lava intrusion and initial dome growth b rapid growth and destruction of dome lobes c slower sustained dome growth and restriction of explosive disruption to the dome margins and d post dome withdrawal of magma and crater collapse phase d was phreatic phases a and c had moderate water magma ratios and phase b a low water magma ratio dome extrusion was most rapid during phase b but so was destruction and hence dome growth was largely accomplished during phase c the puketarata eruption illustrates how vent geometry and the presence of groundwater may control the style of silicic volcanism early activity was dominated by these external influences and sustained dome growth only followed after effective exclusion of external water from newly emplaced magma

felsic maar diatreme volcanoes a review felsic maar diatreme volcanoes host major ore deposits but have been largely ignored in the volcanology literature especially for the diatreme portion of the system here we use two mexican tuff rings as analogs for the maar ejecta ring new observations from one diatreme and the economic geology literature on four other mineralized felsic maar diatremes to produce an integrated picture of this type of volcano the ejecta rings are up to 50 m thick and extend laterally up to 1 5 km from the crater edge in two mexican examples the lower part of the ejecta ring is dominated by pyroclastic surge deposits with abundant lithic clasts up to 80 at hoya de estrada these deposits display low angle cross bedding dune bedforms undulating beds channels bomb sags and accretionary lapilli and are interpreted as phreatomagmatic rhyolitic juvenile clasts at tepexitl have only 0 25 vesicles in this portion of the ring the upper parts of the ejecta ring sequences in the mexican examples have a different character lithic clasts can be less abundant the grain size is typically coarser and the juvenile clasts can be different in character with some more vesicular fragments fragmentation was probably shallower at this stage the post eruptive maar crater infill is known at wau and consists of reworked pyroclastic deposits as well as lacustrine and other sediments underneath are bedded upper diatreme deposits interpreted as pyroclastic surge and fall deposits the upper diatreme and post eruptive crater deposits have dips larger than 30 at wau with approximately centroclinal attitudes at still lower structural levels the diatreme pyroclastic infill is largely unbedded montana tunnels and kelian are good examples of this at cerro de pasco the pyroclastic infill seems bedded despite about 500 m of post eruptive erosion relative to the pre eruptive surface the contact between the country rocks and the diatreme is sometimes characterized by country rock breccias kelian mt rawdon pyroclastic rocks in the diatreme are typically poorly sorted and ash rich they contain a heterolithic mix of juvenile clasts and lithic clasts from various stratigraphic levels megablocks derived from the ejecta ring or the country rocks are often found in the diatremes evidence for multiple explosions is in the form of steep crosscutting pyroclastic bodies within some diatremes and fragments of pyroclastic rocks within other pyroclastic facies pyroclastic rocks are cut by coherent felsic dikes and plugs which may have been feeders to lava domes at the surface allowing for the difference in magma composition felsic maar diatreme volcanoes have many similarities with their ultramafic to mafic equivalents differences include a common association with felsic domes inside the crater or just outside wau although the domes within the crater may be destroyed during the eruption hoya de estrada tepexitl the dikes and plugs feeding and invading felsic diatremes seem larger the processes of phreatomagmatic explosions involving felsic magmas may be different

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

phreatomagmatic explosions of rhyolitic magma experimental and field evidence 1 experimental studies on explosive molten fuel coolant interaction mfci using basaltic melt compositions and water as the coolant have provided insight into the physical processes of basaltic and andesitic phreatomagmatic volcanism abundant field evidence indicates that rhyolitic and dacitic phreatomagmatism occurs in nature but it has not been possible until now to generate laboratory mfci explosions from the interaction between high silica melts and water under laboratory conditions the high viscosity of these melts apparently prevents formation of an effective hydrodynamic premix of melt and water the documented precursor of experimental explosive mfci caused by mafic melts our new experiments utilized samples from a rhyolitic tuff ring volcano in mexico tepexitl an experimental approach was developed in which premixing conditions were generated by mechanical deformation of the melt leading to brittle type fragmentation at the melt water interface physical measurements recorded during laboratory explosion provide quantitative evidence for rhyolitic explosive mfci additionally a comparison of experimentally produced particles with natural ones from tepexitl deposits show nearly identical chemical mineralogical composition grain size and grain morphology detailed textural analysis confirmed the presence of phreatomagmatically produced particles in both experimental and natural analog particles the results from this series of experiments indicate that under natural conditions stress induced magma fracturing can lead to a critical magma water interface growths and trigger phreatomagmatic explosions of high silica magma the water source for these eruptions may include shallow aquifers surface water bodies strong precipitation and intrusion into ice or wet unconsolidated sediments

interaction between particles transported by fallout and surge in the deposits of the agnano monte spina eruption campi flegrei southern italy abstract the agnano monte spina tephra amst is a complex sequence of beds generated by contrasting fragmentation and transportation dynamics the 4 1 ka eruption was accompanied by a volcano tectonic collapse part of which is the present agnano plain the pyroclastic sequence is subdivided into members and submembers each characterized by different lithological and sedimentological features plinian subplinian fallout deposits frequently alternate with base surge beds of phreatomagmatic origin analysis of lateral facies variations and vertical facies associations of correlated layers of submembers b2 d2 and e2 reveals that during some eruption phases the contrasting eruptive dynamics were almost contemporaneous base surge deposits of submember b2 formed during the declining stage of a plinian column they resulted from highly energetic and steady pyroclastic density currents that traveled long distances from the vent area and surmounted topographic obstacles such as the posillipo hill 150 m and the northern sector of the camaldoli hill 250 m submember d2 pyroclastic density currents formed when contemporaneous fallout from an eruptive column was present coarse particles from the column settled throughout the pyroclastic density current determining a significant increase of the solid load of the base surge the consequent increase of supply rate from the transportation to the depositional zone of the base surge led to the formation of unsteady flows that could not efficiently transport the solid load and did not have the ability to travel long distances base surge deposits of submember e2 were fed by a pulsating phreatomagmatic activity which was punctuated by a short lived fallout phase fallout material which was incorporated as an additional load to the base surges was partially transported by low energy steady pyroclastic density currents that traveled over the agnano plain but did not surmount either the camaldoli or the posillipo hills only the very fine material in continuous suspension in the upper no shearing part of the base surge cloud was dispersed higher in the atmosphere and quietly settled over a large area outside the caldera rim the phreatomagmatic origin of base surges which contrasts with the magmatic origin of fallout activity suggests that the pyroclastic density currents of the agnano monte spina eruption did not result from eruption column collapses they were most likely related to radially spreading clouds which were contemporaneous with fallout activity but issued from distinct zones in the vent area the turbulent nature and the high expansion of base surges made them capable under certain conditions of passing over high topographic obstacles with hazardous effects in distal areas

causes of complexity in a fallout dominated plinian eruption sequence 312 ka fasnia member diego hernandez formation tenerife spain the 312 ka fasnia eruption from the las canadas caldera on tenerife canary islands spain produced a complex sequence of twenty two intercalated units including 7 pumice fall 7 ignimbrite and 8 ash surge and fall deposits that define two distinct eruption sequences lower and upper fasnia sequences the fallout units themselves are internally complex reflecting waxing and waning of the eruption column while many of the ignimbrites reflect multiple intra plinian partial column collapse events associated with the injection of lithic clasts into the eruption column the lower and upper fasnia eruption phases were each terminated by caldera collapse and complete column collapse events probable blockage of the conduit and vent system during lower fasnia caldera collapse event briefly terminated the eruption resulting in a short lived period of erosion and sedimentation prior to the onset of the upper fasnia phase the transition to the upper fasnia eruption phase coincided with the eruption of more geochemically homogeneous pyroclasts in total 62 km3 of tephra were erupted including 49 km3 of juvenile clasts and n12 km3 of lithic clasts the dre volume of magma erupted was 13 km3 lower fasnia n 5 km3 upper fasnia n 8 km3 two thirds of which 9 10 km3 was deposited purely by fallout the fasnia member is one of the most complex plinian sequences known

transition zone between the upper diatreme and lower diatreme origin and significance at round butte hopi buttes volcanic field navajo nation arizona round butte is a small but complex miocene diatreme that crops out 190 m below the pre eruptive surface in the southeastern part of the hopi buttes volcanic field erosional remnants consist of a diatreme 170 190 m in diameter of which the central 130 150 m is well exposed in a massif featuring 20 30 m high sub vertical cliffs and a 50 cm thick basanite dike field mapping allowed us to define three main groups of pyroclastic rocks in the diatreme undisturbed beds disturbed beds and non bedded rocks pyroclastic rocks range in grain size from coarse tuff to tuff breccia and in componentry from juvenile rich to lithic rich with a dominance of heterolithic lapilli tuffs rocks from the undisturbed bedded pyroclastic group are present above an unconformity found all around the massif whereas the disturbed bedded and the non bedded pyroclastic groups are always found below it this unconformity was previously understood as the contact between the upper and the lower diatremes the undisturbed beds above the unconformity indeed compose the upper diatreme but the assemblage of non bedded rocks invasive columns and disturbed beds residual columns below it is not typical of the lower diatreme instead they represent a transition zone between the upper and lower diatremes such a transition zone also occurs in other diatremes it is important genetically and we propose to add it to the general model of maar diatreme volcanoes

the lowermost mississippi river a mixed bedrock alluvial channel in this study the distribution of channel bed sediment facies in the lowermost mississippi river is analysed using multibeam data complemented by sidescan sonar and compressed high intensity radar pulse seismic data as well as grab and core samples of bed material the channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls the consolidated substratum is made up of latest pleistocene and early holocene fluvio deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends the exposed substratum commonly displays a suite of erosional features including flutes that are quantitatively similar in form to those produced under known laboratory conditions a total of five bed facies are mapped three of which include modern alluvial deposits and two facies that are associated with the relict substratum a radius of curvature analysis applied to the mississippi river centreline demonstrates that the reach scale distribution of channel bed facies is related to river planform from a broader perspective the distribution of channel bed facies is related to channel sinuosity higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment for example the ratio of alluvial cover to substratum is ca 1 5 1 for a 45 km segment of the river that has a sinuosity of 1 76 and this ratio increases to ca 3 1 for a 120 km segment of the river that has a sinuosity of 1 21 the exposed substratum is interpreted as bedrock and given the relative coverage of alluvial sediment in the channel the lowermost mississippi river can be classified as a mixed bedrock alluvial channel the analyses demonstrate that a mixed bedrock alluvial channel boundary can be associated with low gradient and sand bed rivers near their marine outlet

strombolian and effusive activity as precursors to phreatomagmatism eruptive sequence at maars of the pinacate volcanic field sonora mexico the pinacate volcanic field sonora mexico contains 400 cinder cones and eight maars it is noteworthy that most of the maar forming phreatomagmatic eruptions were immediately preceded by effusive and strombolian activity rather than occurring when magma first approached the surface the strombolian activity may have facilitated access of groundwater to the conduits in this arid region the field evidence suggests that phreatomagmatism is inhibited unless the magma flux is low relative to the rate of water supply and unless the top of the magma column has subsided probably below the water table the latter condition is difficult to prove in the absence of direct observation and alternative hypotheses involving disturbance of the conduit system are considered the spatial distribution of maars in the pinacate and the lithology of their associated tuffaceous ejecta both may reflect the course of an ancient river channel whose permeable gravels were pierced by the magmatic conduits

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

morphodynamic response of a variable width channel to changes in sediment supply river channels commonly exhibit downstream variations in channel width which can lead to the development of alternating shallow and deep areas known as riffle pool sequences the response of these channels to variations in sediment supply remains largely unexplored here we investigate the morphodynamic response of a variable width channel to changes in sediment supply through laboratory experiments conducted in a straight flume in which we imposed sinusoidal variations in width we first developed equilibrium conditions under a constant sediment supply and then eliminated the sediment feed to create a degraded armored bed this sediment starved bed was subjected to two types of sediment supply increases a return to the initial constant supply and the introduction of a well sorted sediment pulse analogous to gravel augmentation riffles and pools formed in wide and narrow areas respectively and the location of and relief between riffles and pools remained the same throughout all experimental runs regardless of the sediment supply the primary channel response to changes in supply was adjustment of the overall slope the sediment pulse evolved primarily through dispersion rather than translation which contrasts with prior gravel augmentation experiments conducted in constant width channels and suggests that width variation and resulting riffle pool topography enhances pulse dispersion our results indicate that width variation is a primary control on the location and relief of riffles and pools in straight channels and sediment supply changes are unlikely to affect riffle pool morphology when bank geometry is fixed and water discharge is steady

explosive tephra emissions at mount st helens 1989 1991 the violent escape of magmatic gas following storms from 24 august 1989 until 18 june 1991 mount st helens produced at least 28 shallow explosion like seismic events with signatures similar to those produced by gas explosions on the dome during the mid 1980s at least six were accompanied by violent emission off non juvenile tephra ejection of blocks of rock nearly 1 km from the vent and avalanching of debris off the north side of the dome four produced no emission of tephra and the remainder occurred when the crater could not be observed although later crater visits found no new tephra deposits all six confirmed emissions and most although not all other seismic events took place hours to days after storms over time periods ranging from 2 to 15 days statistical goodness of fit tests give a probability of 1 x 10 2 to 1 x 10 5 that the precipitation prior to emissions was fortuitously greater than normal similar tests for precipitation prior to all 28 events give probabilities of 3 x 10 2 to 1 x 10 3 the short delay between storms and emissions suggests that the events that follow storms originate at very shallow depth probably within the dome itself although the exact causal mechanism is not known it is speculated that slope instability or accelerated growth of cooling fractures following storms may have released gas trapped within or at the base of the dome of the events that did not follow storms three were located seismically between 1 4 and 2 1 km depth thus the events as a group are not confined to shallow depth and probably record gas transport from a deeper magmatic source a deep source is also suggested by an unprecedented 4 yr long increase in background seismicity at 2 to 9 km depth that peaked in 1989 and 1990 calculations indicate that the energy content of gas exsolved from the crystallizing conduit was probably sufficient to power the emissions

facies variation of eruption units produced by the passage of single pyroclastic surge currents hopi buttes volcanic field usa abstract base surges are a significant volcanic hazard associated with phreatomagmatism but their transport and depositional dynamics are incompletely understood stratigraphic analysis and lateral tracing of surge eruption units produced by the passage of single base surges from haskie tuff ring hopi buttes volcanic field usa extend to 1 km from the vent and show similar downcurrent changes in lithofacies within 200 m of the vent eruption units are primarily disorganized lapilli tuff that is rich in juvenile pyroclasts at distances 200 m from the vent eruption units transform into a couplet of juvenile rich lapilli that grades upward into bedform bearing ash tuff at distances 900 m upper and lower beds of the eruption unit couplet transform into planar stratified beds at distances 1100 m couplets composing different eruption units are almost entirely composed of planar beds and amalgamate to form almost identical planar bedded ash tuff the collapse of dense eruption columns following explosive magma wet sediment interaction generated turbulent surge currents as the surges traveled away from the haskie vent they expanded and partitioned into a basal juvenile rich traction carpet and overlying accidental rich ash cloud surge near the limit of their runout the surges cooled and lost their pyroclastic load leaving planar beds of accidental rich ash the deposition of multiple surge eruption units with similar lateral and vertical facies transitions and a paucity of interbedded fallout suggests an eruption characterized by recurring phreatomagmatic explosions of similar size and the generation of surges with similar flow characteristics

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

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

fragmentation of a porous viscoelastic material implications to magma fragmentation fragmentation of vesicular magma by rapid decompression is one of the most likely triggers for explosive eruptions in this phenomenon the decompression rate and the viscoelastic nature of magma are considered to be key factors in order to obtain a clear idea on the effects of these two factors controlled fragmentation experiments have been conducted these experiments have three advantages first the specimen is made of a viscoelastic material with controlled porosity and geometry second the fragmentation process is directly monitored finally both the magnitude and rate of decompression are controlled brittle fragmentation and ductile expansion were both observed in the same porous material at different timescales the various mechanical responses of the specimen elastic flow and fragmentation were correlated with the pressure profile measured at the base of the specimen fragmentation was noted to occur when the decompression exceeded a critical value within a critical time two relevant timescales are discussed in terms of physical mechanisms of relaxation the first is the measured glass transition time the second is the estimated timescale for the onset of viscous bubble expansion the observed phenomena bear several similarities with natural magma fragmentation it is thus considered that the present results are a useful step toward constructing a model for magma fragmentation

fluidization of wet sediments during the emplacement and cooling of various igneous bodies during the emplacement and cooling of igneous bodies fluidization of wet sediment by water vapour may occur by heating or by pressure relief during the opening of fractures momentary fluidization causes sediment reconstitution and local transport continuous fluidization can result in substantial sediment displacement theoretical considerations show that fluidization due to heating is not likely to occur at depths where pressure is much greater than 312 bars studies of the emplacement of sills of andesite ayrshire fluid rhyolite ramsey island and viscous rhyolite moelwyn hills recognize a wide range of phenomena attributable to wet sediment fluidization and in places modify existing interpretations the transgressive bases of the subaqueously welded ash flow tuffs of the capel curig volcanic formation are consistent with emplacement attended by fluidization of the subjacent sediment evidently fluidization is a means by which large volumes of wet sediment can be replaced by igneous material with minimal disturbance of the remaining host also fluidization can result in exceptionally complex contact relationships between sedimentary host and intrusive rocks and can cause fractures formed during cooling to be thoroughly pervaded by sediment fluidization is commonly associated with the formation of peperites

internal textures of rhyolite flows as revealed by research drilling recent drilling of 550 and 140 000 yr old rhyolite lava flows in the inyo dome chain california and valles caldera new mexico and field inspection of 8 ma flows in western arizona reveal a more detailed vertical zonation of lava textures in glassy rhyolite flows than has previously been recognized the upper vitrophyre of a flow can usually be subdivided into a surficial finely vesicular pumice underlain by obsidian coarsely vesicular pumice and a second obsidian layer beneath these layers is the crystalline center of the flow which in turn is underlain by a basal obsidian layer lower vitrophyre differences in lava vesicularity and crystallinity between zones in a flow result from primary effervescence devitrification lava rheology and migration of water vapor

volatile contents of obsidian clasts in tephra from the taupo volcanic zone new zealand implications to eruptive processes obsidian clasts in 6500 20 000 year old rhyolitic tephra deposits from the taupo volcanic zone new zealand contain variable h2o 0 2 2 5 wt and ci 0 12 0 18 wt contents these are elevated compared to degassed obsidian from rhyolitic domes 0 1 wt h2o2 0 1 wt cl which quenched at the surface major and trace element compositions of obsidian from tephra deposits suggest that they are co genetic with associated pumice the clear glassy appearance of the obsidian their high h2o release temperatures and 18o of 7 show that the water is magmatic and was not absorbed into the obsidian after tephra deposition based on analysis of melt inclusions recent primary taupo volcanic zone rhyolitic magmas contain 4 3 wt h2o and 0 18 wt cl the obsidian in the tephra deposits therefore represents partially degassed quenched primary magma water and cl contents correlate in the obsidian suggesting that the magmatic cl partitioned into a h2o rich fluid phase during eruptive degassing the partitioning of cl between the water rich vapor phase and the residual magma can be modelled as open system rayleigh fractionation using a pre eruptive h2o content of 4 3 wt and burnham s 1979 water solubility model the pressure and depth of initial vesiculation of the taupo magma is calculated to occur at about 0 9 kbar or 3 5 km depth initial fragmentation assumed to occur at a vapor melt ratio of 3 1 began at 0 1 kbar or 500 m depth obsidian in most tephra deposits contains less than 1 wt h2o and are calculated to have formed at 0 1 kbar or 500 m depth therefore some obsidian in these tephra deposits may have formed from fragmented magma the variable water contents of obsidian fragments in a single deposit indicates that quenching occurred over a range of depths this and the strong alignment of microphenocrysts in obsidian suggests that fragmented magma was welded onto the conduit walls during eruption cooled to form obsidian then incorporated into the tephra the 2000 year old hatepe phreatoplinian tephra contains many obsidian fragments with 2 wt h2o the meteoric water which caused this eruption to be phreatomagmatic may have caused obsidian to quench at unusually great depths resulting in higher h2o contents

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

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

thermal hydrologic mechanism for rainfall triggered collapse of lava domes hazardous gravitational collapses involving hot lava domes can be triggered by intense rainfall both in periods of active dome growth and volcanic repose the collapses can evolve into energetic failures involving as much as 90 of the dome or 100 3 10 6 m 3 of dome lava retrogressively removed over several hours under standing such potentially lethal phenomena is vital but traditional explanations for rain induced slope failure are problematic for rainfall on hot typically 400 8c crystalline lava in this paper we quantitatively develop a new thermal hydrologic mechanism that can cause such failures pressure buildup within fissures due to effusive gas trapped by a rain saturated dome carapace results in increased destabilizing forces and the loss of mass strength and ultimately results in failure of the dome our mechanistic models are consistent with field observations and provide a quantification of threshold rainfall intensities and durations required to trigger failure

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

surface and subsurface facies architecture of a small hydroexplosive rhyolitic centre in the mesoproterozoic gawler range volcanics south australia abstract at menninnie dam south australia a drilling program has revealed a complete section through the subsurface feeder system and erupted products of a small hydroexplosive rhyolitic centre within the mesoproterozoic gawler range volcanics porphyritic rhyolite intruded near vertical faults in the palaeoproterozoic basement and at less than a few hundred metres depth interacted with fault hosted hot groundwater hydrofracturing of the wall rock occurred in advance of and at the margins of the rhyolitic intrusions the rhyolitic intrusions have peperitic margins and grade into discordant lithic rich pb facies the advancing fragmentation front intersected the palaeosurface triggering phreatic eruptions that deposited a poorly sorted lithic rich explosion breccia rhyolite then rose to the surface through the intrusive breccias and shallow seated magma water interaction occurred in the conduit within 800 m from the inferred source the products include muddy sandstone and pumice breccia at the end of the eruption rhyolitic lava was extruded in the form of a small dome the presence of contemporaneous pb zn ag mineralisation in the wall rocks suggests that an active hydrothermal system may have been involved in the formation of the menninnie dam hydroexplosive volcanic centre

flow banding in obsidian a record of evolving textural heterogeneity during magma deformation we perform a quantitative textural analysis of banding in obsidian from big glass mountain bgm medicine lake volcano california and from mayor island mi new zealand the samples are compositionally homogeneous with banding defined by variable microlite content bgm or vesicularity mi the spatial distribution of alternating light and dark bands has a 1 wavenumber power spectral density as well as multifractal characteristics through the use of simple mathematical models we demonstrate that formation of the observed banding structure is consistent with a continuous deformational reworking of magma our results indicate that changes in crystallinity vesicularity were concurrent with magma deformation and are random multiples of the total amount of crystallinity vesicularity that was already present at any given time banded obsidian from bgm and mi can be found in close spatial proximity to brittle deformational textures we therefore propose that a key element of banding formation in our samples may be magma brecciation or fragmentation fragmentation can result in variable rates of degassing of fragments which in turn can lead to nonuniform crystal vesicle nucleation and growth rates repeated fragmentation welding and subsequent viscous deformation can thus lead to banding formation

overview and plumbing system implications of monogenetic volcanism in the northernmost andes volcanic province abstract monogenetic volcanic fields are commonly related to rifts and or intraplate tectonic settings however they are also less commonly recognised in subduction zones including both front and back arc volcanoes e g monogenetic volcanoes associated with the llaima volcano in chile or the san agustin volcanic field in colombia here we describe monogenetic volcanic fields associated with subduction related polygenetic volcanism in the northernmost part of the andes northern volcanic zone nvz 2 s to 4 30 n these fields are associated with the main axis of the quaternary volcanoes they are linked to the polygenetic san diego cerro machin volcano tectonic province 140 km long scvtp in colombia the chain that hosts the iconic nevado del ruiz volcano presently three monogenetic volcanic fields with a typical calc alkaline signature have been identified on both sides of this province from south to north they are 1 pijaos monogenetic volcanic field pmvf 2 villamaria termales monogenetic volcanic field vtmvf and 3 samana monogenetic volcanic field smvf pmvf is located 25 km south of cerro machin volcano the southernmost active volcano of the scvtp this field was formed by at least two eruptions with both effusive and explosive eruptive styles three cones and a maar are recognised previous work has defined the volcanoes as basaltic andesitic in composition which we recognised as the most mafic expression mgo 10 11 wt in the whole scvtp its source is related to the same but deeper magma that feeds the volcanoes in the scvtp stratigraphic relationships show that the volcanoes are younger than the underlying alluvial and volcaniclastic ibague fan 5 km of the axis of the province this field is made up of at least 14 volcanoes aligned with the active villamaria termales fault system as previous work has recognised the volcanism has been mainly effusive represented by lava domes and some lava flows since the 1980s studies have framed these andesitic to dacitic volcanoes in the context of the history of the nevado del ruiz volcano it is inferred that the magmatic source is a shallow magma reservoir underneath the scvtp stratigraphic and field relationships have shown that the last eruption occurred

szabolcs harangi vulkanok a karpat pannon terseg tűzhanyoi volcanoes the volcanoes of the carpathian pannonian region hardcover pp 440 in hungarian

the transition from explosive to effusive eruptive regime the example of the 1912 novarupta eruption alaska the shift from explosive to effusive silicic volcanism seen in many historical eruptions reflects a change in the style of degassing of erupted magma this paper focuses on such a transition during the largest eruption of the twentieth century the 1912 eruption of novarupta the transition is recorded in a dacite block bed which covers an elliptical area of 4 km 2 around the vent approximately 700 studied blocks fall into four main lithologic categories 1 pumiceous 2 dense 3 flow banded dacites and 4 welded breccias textural analyses of the blocks indicate portions of the melt underwent highly variable degrees of outgassing vesicle populations show features characteristic of bubble coalescence and collapse a decrease in measured vesicularity and increased evidence for bubble collapse compared with pumice from earlier plinian episodes mark the transition from closed to open system degassing block morphology and textures strongly suggest the magma was first erupted as a relatively gas rich lava dome plug but incomplete out gassing led to explosive disruption heterogeneous degassing of ascending magma began in plinian episode iii and resulted in instability during episode iv dome growth and a series of vulcanian explosion s modeling of the dynamics of explosion initiation and ejecta dispersal indicates that a significant concentration in gas is required to produce the explosions responsible for the observed block field dispersal the amount of gas available in the hot pumiceous dome material appears to have been inadequate to drive the explosion s therefore external water most likely contributed to the destruction

el escondido a newly identified silicic quaternary volcano in the ne region of the northern volcanic segment central cordillera of colombia abstract el escondido 1650 m a s l is a silicic quaternary volcano recently identified on the central cordillera of colombian andes it is located about 50 km ne from the northern segment of active volcanoes it has been recognized as a polygenetic pyroclastic dome ring complex structure whose eruptive products derived from a crater like semi circular depression of 1 7 km in diameter this crater formed at the contact of the granitic florencia stock 54 9 1 8 my with the paleozoic metamorphic basement and at the intersection of the nnessw trending palestina fault system with an ew trending fault the preserved volcano morphology and the volcanic succession associated with it were defined as the el escondido volcanic center the eruptive products are alternating silicic mostly dacitic pyroclastic density current deposits pumice fall deposits and ballistic projectiles which suggest high explosiveness two 14c radiogenic dates obtained on carbonized wood fragments of the middle part of the eruptive sequence displayed ages of 36 030 380 bp and 33 550 280 bp suggesting two major eruptive episodes of the volcano about 2500 years apart the latest eruptive activity is inferred to be the emplacement of lava domes to the south of the crater like structure which are responsible for the partial destruction of the pyroclastic ring in that sector the products emitted by el escondido correspond to medium k andesites to dacites of the calc alkaline series and petrographically to amphibolic andesites towards the western and eastern sector of the volcano there are chlorinated thermal waters with temperatures up to 45 c the first monitoring of the volcanic activity carried out by the colombian geological survey cgs through the volcanological and seismological observatory of manizales vsom has not detected so far unusual volcano seismic activity the previously unreported el escondido volcano represents one of the northernmost occurrence of quaternary volcanism in the whole andean cordillera thus its study may have wide implications regarding the geodynamics of the region and subduction zone the correct assessment of geologic hazards and the identification of natural resources like geothermal reservoirs and mineral deposits of economic interest

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