an experimental study of the kinetics of decompression induced crystallization in silicic melt 1 experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression pinatubo dacite was held at 780 c 220 mpa fo2 nno 2 h2o saturated conditions for an equilibration period decompressed to final pressures pf ranging from 175 to 5 mpa and then held for 0 3 931 hours according to the plagioclase liquidus curve in ph2o t space for the relevant melt composition these decompressions impose effective undercoolings teff of 34 266 c growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 pf 150 mpa teff 34 93 c and equilibrium crystal modes are achieved in 168 hours microlite nucleation is the dominant transformation process for 10 pf 50 mpa teff 125 241 c and chemical equilibrium is not attained by 168 hours under these conditions slow steady decompressions typically produced normally zoned euhedral and planar faceted feldspar crystals although anhedral morphologies were produced at very low pf contrary to expectation slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure although counterintuitive these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth controlled by teff and melt viscosity experienced during each decompression path analysis of liquid to solid transformation kinetics using ttt style diagrams shows that crystallization occurs most rapidly at 100 mpa by a crystal growth mechanism the next most efficient crystallization conditions are at 25 mpa in a crystal nucleation dominated regime

global correlation of the radiolarian faunal change across the triassic jurassic boundary precise comparison of the change in radiolarian faunas 3 5 m above a u pb zircon dated 199 6 0 3 ma tuff and approximately coincident with a negative 13 c anomaly in the queen charlotte islands b c canada with inuyama japan sequences indicates that major global changes occurred across the triassic jurassic t j boundary nearly 20 genera and over 130 rhaetian species disappeared at the end of the triassic the index genera betraccium and risella disappear and the final appearance of globolaxtorum tozeri livarella valida and pseudohagiastrum giganteum sp nov are also diagnostic for the end of the triassic the low diversity hettangian survival fauna immediately above the boundary is composed mainly of small primitive spumellarians with spongy or irregularly latticed meshwork and rod like spines and new genera charlottea udalia and parahsuum s l first appear in the lowest hettangian in both localities irrespective of different sedimentation rates and sedimentary environments such as shelf to upper slope queen charlotte islands and deep sea below carbonate compensation depth ccd inuyama radiolarians show a similar turnover pattern at the t j boundary

volcanic ash clusters tephra rafts and scavengers abstract much of the volcanic ash deposited in the vicinity of pullman washington from the may 18th eruption of mt st helens in 1980 fell in the form of composite ash clusters 0 25 to 0 5 mm in diameter particles in the clusters ranged from sub micron to more than 40 microns in size the porous clusters rafted large ash particles great distances and scavenged particles of all sizes as the wind blew them eastward the result is poorly sorted ash deposits as far as 644 km east missoula within individual clusters textural relationships suggest that both mechanical interlocking and electrostatic attraction played a role in cluster formation the clusters are very fragile and may never be preserved in the stratigraphic record but it is suggested that this kind of ash transport may well have occurred in the geologic past documentation of this ash cluster fall will promote a better understanding of the physical aspects of the downwind distribution of fine grained volcanic ash the poor size sorting in some ancient ash deposits may thus be explained

two dimensional kinematic and rheological modeling of the 1912 pyroclastic flow katmai alaska pyroclastic flow emplacement is strongly influenced by eruption column height a surface along which kinetic energy is zero theoretically connects the loci of eruption column collapse with all coeval ignimbrite termini this surface is reconstructed as a two dimensional energy line for the 1912 katmai pyroclastic flow in the valley of ten thousand smokes from mapped flow termini and the runup of the ignimbrite onto obstructions and through passes extrapolation of the energy line to the vicinity of the source vent at novarupta suggests the eruption column which generated the ignimbrite eruption was approximately 425 m high the 1912 pyroclastic flow travelled about 25 km downvalley empirical velocity data calculated from runup elevations and surveyed centrifugal superelevations indicate initial velocities near novarupta were greater than 79 88 m s 1 the flow progressively decelerated and was travelling only 2 8 m s 1 when it crossed a moraine 16 km downvalley the constant slope of the energy line away from novarupta suggests the flow was systematically slowed by internal and basal friction using a simple physical model to calculate flow velocities and a constant kinetic friction coefficient heim coefficient of 0 04 derived from the reconstructed energy line the flow is estimated to have decelerated at an average rate of 0 16 m s 2 and to have taken approximately 9 5 minutes to travel 25 km down the valley of ten thousand smokes the shear strength of the flowing ignimbrite at the moraine was approximately 0 5 kpa and its bingham viscosity when it crossed the moraine was 3 5 103p if the flow was newtonian its viscosity was 4 2 103p reynolds and froude numbers at the moraine were only 41 62 and 0 84 1 04 respectively indicating laminar subcritical flow

mechanisms for transition in eruptive style at a monogenetic scoria cone revealed by microtextural analyses lathrop wells volcano nevada u s a explosive activity at lathrop wells volcano nevada u s a originated with weak strombolian ws eruptions along a short fissure and transitioned to violent strombolian vs activity from a central vent with lava effusion during both stages the cause for this transition is unknown it does not reflect a compositional change as evidenced by the consistent bulk geochemistry of all the eruptive products however comparison of agglutinate samples from the early ws events with samples of scoria from the later vs events reveal differences in the abundance and morphology of groundmass phases and variable textures in the rims of olivine phenocrysts scanning electron microscope sem examination of thin sections from the ws samples show euhedral magnetite microlites in the groundmass glass and olivine phenocrysts show symplectite lamellae in their rims secondary ion mass spectrometry sims depth profiles of these symplectites indicate they are diffusion controlled the calculated dfe mg allows an estimation of the oxygen fugacity fo2 and indicates an increased fo2 during eruption of the ws products conversely the vs samples show virtually no magnetite microlites in the groundmass glass a lack of symplectites in the olivines and a lower calculated fo2 these microtextural features suggest that the lathrop wells trachybasalt experienced increased oxidation during ws activity as magma ascended through the original fissure exsolved bubbles were concentrated in the wider part s the protoconduit and this bubble flux drove convective circulation that oxidized the magma through exposure to atmosphere and recirculation this oxidation resulted in groundmass crystallization of magnetite within the melt and formation of symplectites within the olivine phenocrysts bubble driven convection mixed magma vertically within the protoconduit keeping it fluid and driving strombolian bursts while microlite crystallization in narrower parts of the fissure helped to focus flow development of a central conduit increased the magma ascent velocity due to a greater product volume in the later eruptive stages and caused the shift in eruption intensity consequently variations in microtextures of the lathrop wells products reveal how a combination of fluid dynamic and crystallization processes in the ascending magma resulted in different styles of activity while the products maintained a consistent bulk composition

character mass distribution and origin of tephra fall deposits from the 2009 eruption of redoubt volcano alaska highlighting the significance of particle aggregation abstract the 2009 eruption of redoubt volcano included 20 tephra producing explosions between march 15 2009 and april 4 2009 utc next generation radar nexrad data show that plumes reached heights between 4 6 km and 19 km asl and were distributed downwind along nearly all azimuths of the volcano explosions lasted between 0 8 mm thick including communities along the kenai peninsula 80 100 km and the city of anchorage 170 km trace ash on march 15 a small 4 6 km asl phreatic explosion containing minor non juvenile ash erupted through the summit ice cap the first five magmatic explosions events 1 5 occurred within a 6 hour period on march 23 plumes rose to heights between 5 5 km and 14 9 km asl during 2 to 20 minute duration explosions and were dispersed mainly along a nne trajectory trace ash fall was reported as far as fairbanks owing to a shift in wind direction and heavy snowfall during these events field discrimination among many of these layers was possible all deposits comprise a volumetrically significant amount of particle aggregates yet only event 5 deposits contain coarse clasts including glacier ice the most voluminous tephra fall was deposited on march 24 event 6 from a 15 minute explosion that sent a plume to 18 3 km asl and dispersed tephra to the wnw within 10 km of the vent this deposit contains 1 11 cm pumice clasts in a matrix of 1 2 mm aggregate lapilli a small dome was presumably emplaced between march 23 and march 26 and was subsequently destroyed during 1 14 minute magmatic explosions of events 7 8 march 26 that sent plumes between 8 2 km and 19 km asl ash fell along a broad swath to the ese covering communities along the kenai peninsula with up to 1 mm of ash proximal deposits are largely composed of aggregate lapilli of 1 2 mm with very little coarse juvenile material events 9 18 march 27 sent plumes between 5 2 km and 15 5 km asl during particle size data showing a preponderance of fine ash even in the most proximal locations along with the abundance of aggregate lapilli documented in most samples confirms that particle aggregation played a significant role in the 2009 eruption and induced premature fallout of fine ash

observations of volcanic lightning during the 2009 eruption of redoubt volcano abstract observations of volcanic lightning during the eruption of redoubt volcano in march and april 2009 were made with the lightning mapping array during the eruption twenty three distinct episodes of volcanic lightning were observed electrical activity occurred as either a volcanic lightning storm with up to thousands of lightning discharges or as a weak electrical event with only a handful of lightning discharges during the volcanic lightning storms we observed two phases of electrical activity the explosive phase and the plume phase the explosive phase consisted of very small discharges on the order of 10 100 m occurring directly above the vent while an explosive eruption was ongoing whereas the plume phase was comprised of discharges occurring throughout the plume subsequent to the explosive eruption the area of discharges during the explosive phase ranged from less than 1 km2 to 50 km2 or more the electrical activity at the beginning of the plume phase was dominated by small discharges over time the horizontal extent of the flashes increased with the largest flashes occurring at the end of the plume phase the distribution of the horizontal size of the discharges over the lifetime of the storm indicate that the charge structure of the plume evolved from a complex and clumpy structure to a more simple horizontally stratified structure plume height was shown to be a key factor in the quantity of lightning in a storm the volcanic lightning storms occurred in plumes with column heights greater than 10 km the tall plumes may contribute to the efficiency of charge generation through ice collisions by providing strong updrafts from the large thermal energy input from the eruption

coarse tail vertical and lateral grading in pyroclastic flow deposits of the latera volcanic complex vulsini central italy origin and implications for flow dynamics abstract coarse tail vertical and lateral grading are common features of pyroclastic flow deposits of the latera volcanic complex vulsini volcanoes central italy and are described for four representative flow units lithic clasts show normal vertical and lateral grading in all four units pumice clasts show reverse vertical and lateral grading within three flow units normal vertical and lateral grading of scoria clasts has been observed in one flow unit the origin of vertical and lateral grading of lithic and pumice clasts is related to mechanisms operating during the transport process within the high particle concentration basal avalanche of pyroclastic flows vertical grading results from the balance between gravitational and dispersive forces and is transferred to a lateral grading by vertical velocity gradients within a nonturbulent flow zone of pyroclastic flows the pyroclastic flows are modeled as bingham type fluids a framework that explains some of the basic deposit features even though it is a highly simplified treatment of the flows plug flow zones in the flows were a relatively minor part of their thicknesses evidence for the flows having had high densities probably within a factor of two of the final deposit density is presented finally their origin by single pulse or progressive formation is discussed

lithic rich and lithic poor ignimbrites and their basal deposits sovana and sorano formations latera caldera italy conceptual models explaining the characteristics of ignimbrites and their basal deposits have mainly focused on variations in particle concentration and speed of the parent pyroclastic currents here we focus on the effects of relative proportions of clasts that are well coupled to the carrier gas phase and those that are poorly coupled i e the proportions of fine and or low density clasts compared to coarse and or dense clasts we document facies and macroscopic clast populations of two ignimbrite producing eruptive sequences of similar volume and composition both erupted from latera caldera central italy the sovana ignimbrite has 1 15 vol dense lithic clasts and locally up to 70 vol in lithic rich domains it preserves evidence of emplacement of multiple overlapping flow units with local internal channeling the ignimbrite is underlain by two basal deposits 1 a radially distributed 20 80 cm thick vitric tuff referred to as bus basal unit sovana that thins gradually with distance and shows evidence of emplacement by lateral currents and 2 a fines depleted lenticular massive lapilli tuff up to 20 cm thick with abundant lithic clasts the sorano ignimbrite in contrast has 1 vol dense lithic clasts it contains multiple reverse coarse tail graded emplacement units but we observed no internal channeling in most places its basal deposits are 20 100 cm of dominantly planar parallel 5 10 cm thick reverse coarse tail graded tuff beds however in local depressions near the tops of paleocanyons that were filled by sorano ignimbrite the basal deposits are up to 2 m thick and are clearly cross stratified we interpret the different deposit characteristics in terms of recent modeling and experimental results the sovana eruption involved major caldera collapse that introduced abundant dense lithic clasts into the erupting mixtures bus may have partly resulted from expulsion of a dilute fine grained mixture at the site where the pyroclastic fountain s impacted the ground which then propagated outward as a dilute pyroclastic current ahead of the main ignimbrite producing currents the expulsion process is promoted by collapsing mixtures rich in poorly coupled clasts the fines depleted layer overlying bus was deposited by rapid sedimentation of abundant dense clasts from the head of the ash cloud surge which accompanied the concentrated underflow that deposited the ignimbrite spatial and temporal variations in dense clast abundance and likely other parameters during eruption resulted in multiple flow units the sorano eruption lacked poorly coupled clasts and did not produce an expulsion driven current its ash cloud head deposited ash and small pumice lapilli that were able to move in traction carpets prior to being overridden by a concentrated underflow the overriding ash cloud bodies locally deposited cross stratified horizons in depressions high on the canyon walls before they were inundated as the concentrated underflows filled the canyons with ignimbrite to summarize the differences between the ignimbrites and their basal deposits can be largely attributed to the differences in proportions of poorly coupled relative to well coupled particles in the collapsing eruptive mixtures the resulting conceptual model simplifies the previous range of interpretations for basal deposits and is consistent with recent modeling and experimental results

simply pyroclastic currents gravity driven ground hugging gas pyroclast mixtures produced during explosive volcanic eruptions define a full spectrum of particle concentration flow regime and particle support mechanisms to describe these phenomena the term pyroclastic density current pdc has become increasingly popular in the last few tens of years here i question the general application of the term pdc to the whole flow spectrum and instead i propose the simpler term pyroclastic current

influence of shock propagation on lightning evidence in volcanic ashfall deposits abstract lightning induced textures in volcanic ashfall deposits can take several forms including lightning induced volcanic spherules livs spherule aggregates and pumiceous particles here two newly identified textures are described in the products of current impulse experiments conducted on ash samples spanning a range of bulk compositions from basic 49 wt sio2 to intermediate 53 58 wt sio2 to acidic 72 76 wt sio2 at peak currents of 7 ka 25 ka and 100 ka due to the location of subjected samples on a solid flat surface in the experimental apparatus both hair like particles 1 10 m in diameter and plate like particles 100 m in diameter are produced these textures result from not only the high temperatures generated by the discharge 1500 c but also by the expansion of a shock front with a radius twice the size of the discharge channel the pressure behind this shock front ranges from 0 41 mpa for the 7 ka impulse currents to 0 77 mpa for the 100 ka impulse currents and the velocity of the shock ranges from 617 m s to 846 m s the combined high temperatures and shock wave expansion simultaneously melts the volcanic ash particles fuses them together and stretches them out which alters the very small

tectonic and paleobiogeographic significance of radiolarian microfaunas in the permian to mesozoic basement rocks of the north island new zealand abstract from the terranes sutured to the new zealand gondwana margin in mid cretaceous time the murihiku waipapa torlesse and mata river terranes have yielded tectonically and paleographically significant radiolarian faunas a late jurassic fauna from the murihiku forearc terrane is more cosmopolitan than coeval faunas from the waipapa faunas from the terranes east of the murihiku which all show evidence of accretion tectonics can be subdivided into ocean floor assemblages and assemblages of terrigenous clastic deposits waipapa ocean floor assemblages are typically tethyan while the terrigenous clastic assemblages are non tethyan possibly of southern hemisphere high latitude origin ocean floor assemblages show the same eastward younging trends opposed to the westward sedimentological younging due to imbrication as do the assemblages of the terrigenous clastics but are always a few tens of million years older radiolarians from ophiolites tangihua volcanics and matakaoa volcanics indicate that significant portions of the ocean floor obducted onto new zealand in late oligocene to earliest miocene time were of late cretaceous to early tertiary age

lightning induced weathering of cascadian volcanic peaks abstract the process of meteorological lightning induced modification of coherent volcanic rocks is examined by geochemical textural and experimental analysis of fulgurites from south sister volcano oregon cascades usa lightning s effects on volcanic target rocks was simulated with an arc welding device in order to reproduce the geochemical and textural features of natural fulgurites and to constrain temperatures of melting and devolatilization behavior during lightning strikes melting of volcanic target rocks produces melts of exceptional compositional diversity ranging from those with pure mineral compositions e g diopside and plagioclase resulting from congruent melting reactions to those that are highly mixed and compositionally identical to the target rock geochemical and textural observations on fulgurites suggest that melting proceeds rapidly by an in situ modal batch process whereby individual crystals melt and contribute small aliquots to a larger batch that is then mixed by viscous flow hydrous species remaining in the glasses are trace oh 0 1 wt and consistent with partial equilibration to high temperature 2000 c supra liquidus 1 atm conditions lightning also induces physical weathering in that the target rocks may be vapourized to form channels through the rock representing injection of elemental components to the atmosphere finally lightning acts as a multi faceted producer of dry chemically equivalent volcanic glass which may be subject to long term hydration by environmental waters at surface conditions these re hydration signatures contrast those of volcanic glasses which have both magmatic and meteoric components and are used herein to examine the post lightning environmental history of these glasses including age dating lightning events by diffusion chronometry our study of volcanic fulgurites illustrates the potential to use their hydration signatures to date paleoweather events at decadal to centuries scale because the lightning strike is in and of itself so effective at devolatilizing melts in an instant the resultant fulgurites are a unique earth material that record individual weather events i e a thunderstorm and also longer term paleoweather intervals

the eruptive and magmatic history of the youngest pulse of volcanism at the valles caldera implications for successfully dating late quaternary eruptions abstract new 40ar 39ar and u th ages provide insight into the youngest eruptions at the valles caldera and also reveal previously unknown pulses of magmatism the youngest eruptive units collectively termed the east fork member of the valles rhyolite include the el cajete pyroclastic beds and co erupted battleship rock ignimbrite and the disconformably overlying banco bonito lava flow previous attempts to date these units using a variety of techniques yielded ages ranging from 1 ma new 40ar 39ar ages were generated using the high sensitivity multi collector argus vi mass spectrometer which provides more than an order of magnitude increase in precision compared to most single detector mass spectrometers 40ar 39ar dating of single crystals yields a range of ages of which the youngest populations are interpreted to represent the eruption age sanidine ages indicate that the el cajete pyroclastic beds and battleship rock ignimbrite erupted at 74 4 1 3 ka whereas the banco bonito lava erupted at 68 3 1 5 ka populations of older crystals represent variably degassed xenocrysts explaining why previous 40ar 39ar bulk step heating analyses yielded spuriously old and irreproducible results u th dating of unpolished zircon surfaces also yield multiple age populations which range from the eruption age to 350 ka indicating protracted magmatism during the 453 ka long eruptive hiatus prior to the eruption of the east fork member 40ar 39ar and u th ages indicate that the east fork member represents a short 6 1 2 8 ka eruptive cycle from a longer lived magmatic system beneath the southern caldera equally short repose periods similar to the interval between the el cajete battleship rock and banco bonito eruptions are possible during future volcanism at the valles caldera results demonstrate that detailed geochronology using single crystal and in situ techniques is necessary for understanding the eruptive history and magmatic evolution at some young volcanic systems

bnd bubble number density decompression rate meter for explosive volcanic eruptions abstract this decade has been witness to substantial progress in the understanding of bubble dynamics and its role in volcanic eruptions a comparison between the results of experiments and a model for bubble nucleation shows that bubble number density bnd can be calculated as a function of decompression rate for given physical properties such as the diffusivity of water in a silicate melt and interfacial tension within an acceptable margin of error conversely in this paper we propose a method hereafter referred to as bnd decompression rate meter to estimate the decompression rate by using bnd data of natural pumice samples from volcanic eruptions as a result of the application of the bnd decompression rate meter to pumice and scoria from explosive eruptions it is found that the decompression rates increase in the range from 10 6 to 10 8 pa s with the eruption column heights these rates are proportional to the 4th power of the eruption column heights and are linear to the discharge rates the absolute values of the estimated decompression rates are very high and cannot be explained by the existing models of conduit flows in order to explain such high rates of decompression and the correlation with discharge rate we propose a possible model according to which the bubble nucleation propagates downward as a rarefaction shock wave in the conduit

volcanic tremor and plume height hysteresis from pavlof volcano alaska the march 2016 eruption of pavlof volcano alaska produced an ash plume that caused the cancellation of more than 100 flights in north america the eruption generated strong tremor that was recorded by seismic and remote low frequency acoustic infrasound stations including the earthscope transportable array the relationship between the tremor amplitudes and plume height changes considerably between the waxing and waning portions of the eruption similar hysteresis has been observed between seismic river noise and discharge during storms suggesting that flow and erosional processes in both rivers and volcanoes can produce irreversible structural changes that are detectable in geophysical data we propose that the time varying relationship at pavlof arose from changes in the tremor source related to volcanic vent erosion this relationship may improve estimates of volcanic emissions and characterization of eruption size and intensity

stratified flow in pyroclastic surges stratified flow theory is applied to pyroclatic surges in an effort to gain insight into transport dynamics during explosive eruptions particle transport is assumed to be by turbulent suspension and calculations contained herein show that this is likely for many cases including the 18 may 1980 blast at mount st helens the discussion centers on the rouse number pn which represents a ratio of particle settling velocity to scale of turbulence the brunt vaisala frequency n which is the maximum possible frequency of internal waves the froude number fr representing the ratio of inertial forces to gravitational forces and the richardson number ri a ratio of buoyant restoring forces to turbulent mixing forces the velocity or flow power dependence of bed form wavelength in surge deposits is related to a velocity dependence of wavelength of internal waves in the turbulent surge this produces a decrease in dune wavelength with increasing distance from vent migration direction of bed forms is related tofr as it is defined for a continuously stratified flow proximal to distal facies variations in surge deposits reflect increasingpn andri as the flows move away from their sources this produces the progression from sandwave to massive to planar facies with increasing distance from vent where the long axis of topography is at low angles to the flow direction massive facies in topographic lows may from concurrently with sandwave facies on highs due to the higher particle concentration in the lows where long axis of topography is at high angles to flow direction denser lower parts of the surge may be dammed or blocked blocked material tends to form massive flows that may move down slope independent of the overriding surge a model incorporating turbulent transport stratified flow and time evolution of pyroclastic surges is proposed for deposits which have been attributed to both pyroclastic flow and pyroclastic surge transport by various workers during the initial high energy waxing phase of the eruptive event pn is sufficiently low that only coarse but poorly sorted material is deposited to form relatively coarse bottom layers as the event wanes remaining finer material is deposited through a thin bed load to produce overlying bedded and cross bedded veneer deposits throughout most of the event blocking occurs to produce relatively thick and massive deposits in valley bottoms

early hettangian ammonites and radiolarians from the queen charlotte islands british columbia and their bearing on the definition of the triassic jurassic boundary abstract the triassic jurassic boundary t j boundary interval is one of the most important but poorly understood intervals in the phanerozoic here we fully document the ammonite and radiolarian successions at several sites in the queen charlotte islands qci british columbia eleven ammonite taxa are described and eighteen radiolarian taxa are discussed with the radiolarian genus tipperella described as new type species t kennecottensis n sp there are two continuous t j boundary sections in the qci one is located at kennecott point on graham island section i the other on the southeastern shore of kunga island section iii a second section at kennecott point contains fossils of early hettangian age only section ii radiolarian faunas include taxa from the upper rhaetian globolaxtorum tozeri zone in the two continuous sections and the lower hettangian canoptum merum zone in all three sections in each section the lowest hettangian fossils are radiolarians and a few collections contain rare rhaetian holdovers these mixed faunas may equate with the lowest hettangian ammonite zone the spelae zone of the north american succession the upper portion of section i contains a sparse ammonite fauna which indicates an early hettangian age this section has also provided carbon isotope data with a negative excursion spanning the t j boundary section ii contains a more diverse early hettangian ammonite assemblage that permits correlation of the upper portion of this section with the lower hettangian minutus zone section iii has been previously proposed as a potential basal hettangian global stratotype section and point gssp in addition to an outstanding radiolarian fauna the section contains moderately diverse early hettangian ammonites that permit correlation of the lower portion of the section with the minutus and pacificum zones and the upper portion with the lower hettangian polymorphum zone this section has previously provided a u pb date of 199 6 0 3 ma from 3 6 m below the t j boundary currently there are four candidates and no ideal gssp for the basal hettangian a section in new york canyon nevada has also been proposed as a potential gssp this section is easily accessible and provides a virtually complete ammonoid succession but lacks geochronology and microfossils the section on kunga island in the qci provides geochronologic data and excellent radiolarian constraints in essence the sections at kunga island in the qci and new york canyon in nevada are complementary close correlations between the two sections are possible using ammonite faunas we suggest the section from nevada be designated as holostratotype for the basal hettangian and the qci section be designated as a parastratotype to improve recognition of this interval

the contribution of experimental volcanology to the study of the physics of eruptive processes and related scaling issues a review abstract the experimental approach has become a major tool increasingly used by volcanologists in recent decades to investigate the physics of eruptive processes in complement to field and theoretical works researchers have developed various methodologies to study volcanic phenomena at reduced length scale the works involve natural or analogue materials and their types range from first order tests to identify fundamental processes and make qualitative comparison with field observations to more sophisticated experiments in which precise data obtained in a controlled environment can be used to validate outputs of theoretical models scaling is a central issue to ensure dynamic similarity between the small scale experiments and the large scale volcanic phenomena when natural conditions cannot be simulated due to inherent length scale difference and or technical limitations in this respect dimensionless numbers are used to map physical regimes and to define scaling laws which allow experimental results to be extrapolated to natural scale this review presents the variety of experimental studies conducted to investigate subterraneous and aerial volcanic phenomena involving in particular fluid particle mixtures we focus on the major scaling issues and the physical regimes investigated and we also highlight in a historical perspective some of the major advances achieved through experimental studies finally we conclude on some perspectives for future works

the influence of palaeotopography on facies architecture and pyroclastic flow processes of a lithic rich ignimbrite in a high gradient setting the abrigo ignimbrite tenerife canary islands abstract the late pleistocene 188 19 ka abrigo ignimbrite represents the last major explosive caldera forming eruption on tenerife it consists almost entirely of a moderate volume 1 8 km 3 light grey phonolitic lithic rich ignimbrite deposit although a possible abrigo member thin fall deposit is locally preserved beneath the ignimbrite suggesting an initial short lived stable plinian eruption column the ignimbrite deposit is dispersed radially around the lower flanks of the las canadas edifice and correlates to a clast supported lithic breccia on the eastern and northern caldera wall much of the ignimbrite was probably deposited offshore or within the caldera now buried by more recent lavas two major depositional units separated by a co ignimbrite ash cloud deposit and which display dramatic variations in thickness pyroclastic flows were channelised along and deposited within the same valleys that exist today and were commonly continuous and sheet like across the interfluves the basal zones of pyroclastic flows are envisaged as a bed load of coarse lithic lapilli and blocks transported under their own momentum and with the support of the surrounding fine ash gas granular flow

volcanic lightning and plume behavior reveal evolving hazards during the april 2015 eruption of calbuco volcano chile soon after the onset of an eruption model forecasts of ash dispersal are used to mitigate the hazards to aircraft infrastructure and communities downwind however it is a significant challenge to constrain the model inputs during an evolving eruption here we demonstrate that volcanic lightning may be used in tandem with satellite detection to recognize and quantify changes in eruption style and intensity using the eruption of calbuco volcano in southern chile on 22 and 23 april 2015 we investigate rates of umbrella cloud expansion from satellite observations occurrence of lightning and mapped characteristics of the fall deposits our remote sensing analysis gives a total erupted volume that is within uncertainty of the mapped volume 0 56 0 28 km3 bulk observations and volcanic plume modeling further suggest that electrical activity was enhanced both by ice formation in the ash clouds 10 km above sea level and development of a low level charge layer from ground hugging currents

doppler weather radar observations of the 2009 eruption of redoubt volcano alaska abstract the u s geological survey usgs deployed a transportable doppler c band radar during the precursory stage of the 2009 eruption of redoubt volcano alaska that provided valuable information during subsequent explosive events we describe the capabilities of this new monitoring tool and present data captured during the redoubt eruption the minimax 250 c mm 250c radar detected seventeen of the nineteen largest explosive events between march 23 and april 4 2009 sixteen of these events reached the stratosphere above 10 km within 2 5 min of explosion onset high column and proximal cloud reflectivity values 50 to 60 dbz were observed from many of these events and were likely due to the formation of mm sized accretionary tephra ice pellets reflectivity data suggest that these pellets formed within the first few minutes of explosion onset rapid sedimentation of the mm sized pellets was observed as a decrease in maximum detection cloud height the volcanic cloud from the april 4 explosive event showed lower reflectivity values due to finer particle sizes related to dome collapse and related pyroclastic flows and lack of significant pellet formation eruption durations determined by the radar were within a factor of two compared to seismic and pressure sensor derived estimates and were not well correlated ash dispersion observed by the radar was primarily in the upper troposphere below 10 km but satellite observations indicate the presence of volcanogenic clouds in the stratosphere this study suggests that radar is a valuable complement to traditional seismic and satellite monitoring of explosive eruptions

lightning induced volcanic spherules glass spherules have been documented in many geologic deposits and are formed during high temperature processes that include cloud to ground lightning strikes volcanic eruptions of low viscosity magmas and meteorite impacts this study reviews the known glass spherule forming processes and proposes for the first time a mechanism induced through the heat generated by volcanic lightning in eruptive columns and plumes laterally spreading clouds during explosive eruptions ash fall samples were collected from two eruptions where volcanic lightning was extensively documented the a d 2009 eruption of mount redoubt alaska usa and the 2010 eruption of eyjafjallajokull iceland these samples reveal individual glass spherules 50 mm in average diameter that compose 5 of the examined portion of the deposit textures include smooth hollow or cracked spherules as well as aggregates which suggest melting of ash particles as a result of proximity to the electrical discharge channel and subsequent re solidification of the particles into spherical morphologies the natural ash fall samples are compared with pseudo ash samples collected from high voltage insulator experiments in order to test our hypothesis that volcanic ash particles can be transformed into glass spherules through the heat generated by electrical discharge we refer to this new morphological classification of ash grains as lightning induced volcanic spherules and hypothesize that this texture not only provides direct physical evidence of lightning occurrence during explosive eruptions but will also increase settling velocities and reduce aggregation of these particles affecting ash transport dynamics

new biostratigraphic elements for dating upper norian strata from the sandilands formation queen charlotte islands british columbia canada abstract upper norian radiolarians have been recovered from the basal sandilands formation on northwest graham island kennecott point kunga island and louise island queen charlotte islands rare conodonts and ammonoids are associated with the radiolarians at several levels at kennecott point over 90 m of the lower sandilands formation is coarser and less tuffaceous than is typical of the formation elsewhere and appears to represent continuous sedimentation diverse well preserved radiolarians have been found in limestone concretions in the lower 75 m of section which has also yielded conodonts from the epigondolella bidentata zone approximates cordilleranus to basal crickmayi ammonoid zones and ammonoids from the crickmayi zone at kunga island in the southern queen charlotte islands excellent radiolarian faunas have also been recovered from a 30 m interval beginning 80 m above the final occurrence of monotis cordilleranus zone and from a second 90 m interval lying structurally above the former that contains the upper norian conodont epigondolella bidentata at the base three preliminary radiolarian assemblages are recognized for upper norian strata lying above the monotis beds assemblage 1 the lower one is tentatively correlated with the upper part of the cordilleranus zone assemblage 2 approximates the amoenum zone and assemblage 3 is correlated with the crickmayi zone the age of the sandilands formation is now confirmed to be late norian and hettangian as well as sinemurian considering the record of apparently uninterrupted sedimentation the section thickness and the excellent fossil recovery the sandilands formation has been shown to contain an unusually thick upper norian sequence and probably the triassic jurassic boundary

measurements of electric charge distribution in volcanic plumes at sakurajima volcano japan measurements of perturbations in the atmospheric potential gradient around volcanic plumes at multiple from two to five sites and measurements of the charge mass ratio of ash particles falling from volcanic plumes were carried out at sakurajima volcano japan results from 28 and 29 october 1995 show that the nature of the perturbations depends on the intensity of plume activity although plume activity was vigorous on 28 october negative perturbations were predominant as plume activity peaked the magnitude of negative perturbations decreased just below the plume and increased at an off axis site during the peak period positively charged ash particles fell out from the plume this suggests that the active plume dominantly contained negatively charged materials and that positively and negatively charged materials were added to the lower and upper parts of the plume respectively during the peak period on the other hand as plume activity became less vigorous on 29 october the perturbations were characterized by a positive anomaly followed by a negative anomaly because wind velocity increased with altitude that day we infer that positive and negative charges were distributed in the upper and lower parts of the plume respectively the differences in perturbations observed on 28 and 29 october suggest that volcanic plumes are generally composed of three parts an upper part with positively charged gas and aerosol a middle part with negatively charged fine ash particles and a lower part with positively charged coarse ash particles the compilation of present and previous results from sakurajima and other volcanoes indicates that the effect of the negative charge in the middle part was predominant in most cases although positive perturbations caused by the upper part were observed around some weak plumes the effect of positively charged particles in the lower part was observable only when plume activity was sufficiently strong because positively charged coarse particles tended to fall out near the vent

three types of pyroclastic currents and their deposits examples from the vulsini volcanoes italy abstract this paper deals with ground hugging gas pyroclast currents from explosive volcanic eruptions and their deposits key field observations and laboratory determinations are proposed to relate specific deposit types with flow regimes and particle concentration in the transport and depositional systems three relevant flow scenarios and corresponding deposit types have been recognized from a survey of pyroclastic successions of the vulsini volcanic district central italy 1 dilute turbulent pyroclastic currents producing normally or multiply graded beds by direct suspension sedimentation 2 concentrated bedload regions beneath suspension currents depositing inversely graded beds by traction carpet sedimentation 3 self sustained high particle concentration laminar mass flows developing massive poorly sorted bodies with opposite grading of coarse lithic and pumice clasts overlying fine grained inversely graded basal layers main distinguishing criteria include the occurrence and pattern of clast grading clast thickness relationships grain size ash matrix componentry and pyroclast size density relationships downcurrent and temporal transitions among identified flow scenarios are likely to occur for changing energy conditions and gas pyroclast ratio both on regional and local scales the nature and efficiency of magma fragmentation volatile content conduit geometry which determine the characteristics of the erupted mixture and possible lateral blast component at the vent and the angle of incidence of the column collapse are suggested as the main factors controlling the generation of one type over the other at flow inception dilute fine grained overpressured eruption clouds are thought to favor the formation of low particle concentration turbulent currents column collapse over slightly inclined volcano slopes causing a high degree of compression of the collapsing mixture and of gas expulsion would favor the generation of high particle concentration pyroclastic currents

eruption and emplacement of a laterally extensive crystal rich and pumice free ignimbrite the cretaceous kusandong tuff korea abstract the cretaceous kusandong tuff korea is a thin 1 5 m thick but laterally extensive 200 km silicic ignimbrite emplaced in a fluviolacustrine basin adjacent to a continental volcanic arc the tuff has been used as an excellent key bed because of its great lateral continuity and unique lithology characterized by the virtual absence of juvenile clasts and an abundance of quartz and feldspar crystals up to 55 73 vol the tuff is mostly massive and ungraded and locally shows crude internal layering basal inverse grading and near top normal grading of crystals either erosional or non erosional lower surfaces and flat lying to imbricated grain fabrics fragile intraformational clasts of mudstone and tuff are also included these features provide only ambiguous information on the properties of the responsible pyroclastic density currents i e whether they were dense and laminar or dilute and turbulent the overall lateral continuity and sheet like geometry of the tuff suggests however that the transport system of the currents was highly expanded dilute and turbulent a plug flow or slab flow model cannot explain the origin of crude internal layering imbricated grain fabrics and the high crystal content which is most likely the result of vigorous sorting processes within a dilute and turbulent current features indicative of deposition from a dense and laminar transporting medium are locally present suggesting that a dense and laminar depositional system could develop locally at the base of the dilute and turbulent transport system the virtual absence of juvenile clasts in the tuff is interpreted to be due to rapid ascent sudden decompression and full fragmentation of silicic magma into fine glass shards and crystals scarcity of basement derived accidental components together with the absence of pumiceous fallout deposits beneath the tuff is interpreted to be due to shallow level fragmentation of magma followed by immediate generation of pyroclastic density currents from shallow level blasts at the onset of eruption the eruption occurred through multiple vent sites in a short period of time producing a seemingly single but actually composite ignimbrite unit such an eruption was probably possible because of a regional tectonic event within the basin or in its vicinity it is proposed that a composite ignimbrite with the characteristics of the kusandong tuff can be an exemplary product of syntectonic volcanism that can provide an insight into the interpretation of structural and stratigraphic evolution of a sedimentary basin

a two layer model for the evolution and propagation of dense and dilute regions of pyroclastic currents we present calculations of a simplified two layer pyroclastic flow model which incorporates different physics to represent the dense and dilute regions of column collapse pyroclastic density currents the two layers evolve separately but are coupled through mass exchange as suspended ash in the dilute cloud settles into the underlying dense basal layer the runout distance of the upper dilute current and the associated runout time are found to increase with increasing initial column height and decreasing particle size likewise steeper slopes increase the propagation velocity and thus the runout distance the independent runout distance of the basal flow exhibits opposite behaviour to that of the parent dilute current increasing with decreasing initial column height and increasing particle size observed runout distances can be calculated for basal flows using a coulomb friction law but deposit morphology is not well reproduced and is more realistic when an empirical slope dependent sedimentation rate is included dominant flow behaviour is controlled by the rate of mass transfer from the parent suspension current into the dense underflow tall column collapses which contain fine grained particles transfer their mass slowly to the dense basal flow and are well described by dilute cloud assumptions however for short columns containing coarse grains the particulate mass is rapidly transferred from the collapsing dilute current into the basal flow the bulk of the pyroclastic current material propagates as a concentrated suspension for the majority of its travel distance and is better described by the physics of granular avalanches

influence of particle aggregation on deposition of distal tephra from the may 18 1980 eruption of mount st helens volcano the may 18 1980 eruption of mount st helens msh produced an extensive ashfall deposit in washington idaho and montana with a minumum volume of 0 55 km3 tephra an unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ene of msh near ritzville washington grain size and component abundance analysis of samples along the main dispersal axis indicates that ash in this region is very fine grained mean size 22 m poorly sorted polymodal and rich in glass shards and pumice fragments a computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics particularly the thickness versus distance relationship the model was constrained by observations of the eruption column height elevation of major ash transport lateral spreading of the eruption plume and atmospheric wind structure in the vicinity of msh results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause for the model to fit the observed characteristics of the deposit significant particle aggregation of ash finer than 63 m must be invoked the best fit occurs when ash less than 63 m is aggregated into particles several hundred microns in diameter with a settling velocity of 0 35 m s support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at pullman washington during the may 18 eruption sorem 1982 the premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics variations in component abundances and thickness versus distance relationship of the may 18 msh ash fall deposit

a two layer depth averaged model for both the dilute and the concentrated parts of pyroclastic currents pyroclastic currents are very destructive and their complex behavior makes the related hazards difficult to predict a new numerical model has been developed to simulate the emplacement of both the concentrated and the dilute parts of pyroclastic currents using two coupled depth averaged approaches interaction laws allow the concentrated current pyroclastic flow to generate a dilute current pyroclastic surge and inversely the dilute current to form a concentrated current or a deposit the density of the concentrated current is assumed to be constant during emplacement whereas the density of the dilute current changes depending on the particle supply from the concentrated current and the mass lost through sedimentation the model is explored theoretically using simplified geometries as a proxies for natural source conditions and topographies it reproduces the relationships observed in the field between the surge genesis and the topography the increase in surge production in constricted valleys the decoupling between the concentrated and the dilute currents and the formation of surge derived concentrated flows the strong non linear link between the surge genesis and the velocity of the concentrated flow beneath it could explain the sudden occurrence of powerful and destructive surges and the difficulty of predicting this occurrence a companion paper compares the results of the model with the field data for the eruption of merapi in 2010 kelfoun et al submitted and demonstrates that the approach is able to reproduce the natural emplacement of the concentrated and the dilute pyroclastic currents studied with good accuracy

size limits for rounding of volcanic ash particles heated by lightning volcanic ash particles can be remelted by the high temperatures induced in volcanic lightning discharges the molten particles can round under surface tension then quench to produce glass spheres melting and rounding timescales for volcanic materials are strongly dependent on heating duration and peak temperature and are shorter for small particles than for large particles therefore the size distribution of glass spheres recovered from ash deposits potentially record the short duration high temperature conditions of volcanic lightning discharges which are hard to measure directly we use a 1 d numerical solution to the heat equation to determine the timescales of heating and cooling of volcanic particles during and after rapid heating and compare these with the capillary timescale for rounding an angular particle we define dimensionless parameters capillary fourier stark biot and peclet numbers to characterize the competition between heat transfer within the particle heat transfer at the particle rim and capillary motion for particles of different sizes we apply this framework to the lightning case and constrain a maximum size for ash particles susceptible to surface tension driven rounding as a function of lightning temperature and duration and ash properties the size limit agrees well with maximum sizes of glass spheres found in volcanic ash that has been subjected to lightning or experimental discharges demonstrating that the approach that we develop can be used to obtain a first order estimate of lightning conditions in volcanic plumes

particle fabric in a small type 2 ignimbrite flow unit laacher see germany and implications for emplacement dynamics ignimbrites of the 13 ka upper laacher see tephra were deposited from small highly concentrated moderately fluidized pyroclastic flows their unconsolidated nature and the prominence of accidental devonian slate fragments make these ignimbrites ideal for clast fabric studies the upper flow unit of ignimbrite m14 has characteristics typical of a type 2 ignimbrite layer 2a and the lower part of layer 2b of the flow unit have strong upstream inclined a p fabrics a p means long particle axes parallel to flow direction only clasts with a b axial ratios of 2 5 or greater preserve good a p fabrics whereas the a b planes of flat fragments dip upstream irrespective of axial ratio the a axis fabric becomes weaker flatter and more girdle like in the upper half of layer 2b at one locality the a axis fabric appears to rotate 40 up through the flow unit suggesting either shear decoupling of different levels in the moving flow or unsteadiness effects in a flow depositing progressively at its base the existence of similarly strong a p fabrics in layer 2a and the lower half of layer 2b appears inconsistent with the common interpretation that ignimbrite flow units are emplaced as a plug of essentially non shearing material layer 2b on a thin shear layer layer 2a and that the entire flow freezes en masse to form the deposit the data suggest that if the flow froze en masse it was shearing pervasively through at least half its thickness another possibility is that the flow unit aggraded progressively from the base up and that the fabrics record the integrated history of shear directions and intensities immediately above the bed throughout the duration of deposition

origin of layer 1 deposits in ignimbrites types of layer 1 deposits in ignimbrite sequences include ground surge ground layer and jetted deposits ground surge deposits are generally thought to represent deposition from turbulent low concentration pyroclastic density currents resulting from ash hurricane initial stages of plinian eruption column collapse or blast wave phenomena ground layers form by sedimentation from the fluidized head of a pyroclastic flow and jetted deposits form by explosive jetting from the flow head as a result of heating of ingested air theoretical evidence suggests some layer 1 deposits may result from turbulent boundary layers at the base of pyroclastic flows deposits formed from this process will have characteristics that depend on local flow conditions and thus may be largely independent of distance from vent the turbulenit boundary layer model may explain the occurrence of surgelike deposits at distances from sources that appear too far for transportation by a low concentration turbulent pyroclastic surge

correlating the electrification of volcanic plumes with ashfall textures at sakurajima volcano japan abstract volcanic lightning detection has become a useful resource for monitoring remote under instrumented volcanoes previous studies have shown that the behavior of volcanic plume electrification responds to changes in the eruptive processes and products however there has not yet been a study to quantify the links between ash textures and plume electrification during an actively monitored eruption in this study we examine a sequence of vulcanian eruptions from sakurajima volcano in japan to compare ash textural properties grain size shape componentry and groundmass crystallinity to plume electrification using a lightning mapping array and other monitoring data we show that the presence of the continual radio frequency crf signal is more likely to occur during eruptions that produce large seismic amplitudes 7 m and glass rich volcanic ash with more equant particle shapes we show that crf is generated during energetic impulsive eruptions where charge buildup is enhanced by secondary fragmentation milling as particles travel out of the conduit and into the gas thrust region of the plume we show that the crf signal is influenced by a different electrification process than later volcanic lightning by using volcanic crf and lightning to better understand the eruptive event and its products these key observations will help the monitoring community better utilize volcanic electrification as a method for monitoring and understanding ongoing explosive eruptions

the aerodynamic behaviour of volcanic aggregates a large proportion of solid material transported within the atmosphere during volcanic eruptions consists of particles less than 500 m in diameter the majority of these particles become incorporated into a wide range of aggregate types the aerodynamic behaviour of which has not been determined by either direct observation or in the laboratory in the absence of such data theoretical models of fallout from volcanic plumes make necessarily crude assumptions about aggregate densities and fall velocities larger volcanic ejecta often consists of pumice of lower than bulk density experimental data are presented for the fall velocities of porous aggregates and single particles determined in systems analogous to that of ejecta falling from a volcanic plume it is demonstrated that the fall of aggregates may be modelled in identical fashion to single particles by using a reduced aggregate density dependent on the porosity and a size corresponding to an enclosing sphere particles incorporated into aggregates attain a substantially higher fall velocity than single particles this is due to the larger physical dimensions of the aggregate which overcomes the effect of lower aggregate density additionally the internal porosity of the aggregate allows some flow of fluid through the aggregate and this results in a small increase in fall velocity the increase in fall velocity of particles incorporated into aggregates rather than falling individually results in the enhanced removal of fine material from volcanic plumes

physical properties of volcanic lightning constraints from magnetotelluric and video observations at sakurajima volcano japan abstract the lightning generated by explosive volcanic eruptions is of interest not only as a promising technique for monitoring volcanic activity but also for its broader implications and possible role in the origin of life on earth and its impact on the atmosphere and biosphere of the planet however at present the genetic mechanisms and physical properties of volcanic lightning remain poorly understood as compared to our understanding of thundercloud lightning here we present joint magnetotelluric mt data and video imagery that were used to investigate the physical properties of electrical discharges generated during explosive activity at sakurajima volcano japan and we compare these data with the characteristics of thundercloud lightning using two weeks of high sensitivity high sample rate mt data recorded in 2013 we detected weak electromagnetic signals radiated by volcanic lightning close to the crater by carefully inspecting all mt waveforms that synchronized with visible flashes and comparing with high speed 3000 frame s and normal speed 30 frame s videos we identified two types of discharges the first type consists of impulses type a and is interpreted as cloud to ground cg lightning the second type is characterized by weak electromagnetic variations with multiple peaks type b and is interpreted as intra cloud ic lightning in addition we observed a hybrid mt event wherein a continuous weak current accompanied type a discharge the observed features of volcanic lightning are similar to thunderstorm lightning and the physical characteristics show that volcanic lightning can be treated as a miniature version of thunderstorm lightning in many respects the overall duration length inter stroke interval peak current and charge transfer all exhibit values 1 2 orders of magnitude smaller than those of thunderstorm lightning thus suggesting a scaling relation between volcanic and thunderstorm lightning parameters that is independent of the type of charged particles on the other hand the polarities which are estimated by long time 3 4 yrs mt 32 samples s and video 30 frame s observations are different than those of normal thunderstorm lightning these observations are consistent with the notion that charge structures in volcanic ash plumes are highly disordered and are characterized by numerous small charged regions with high charge density

triassic radiolarian biostratigraphy this paper summarizes 30 years of research on the biostratigraphy of triassic radio larians and presents a correlation of currently used radiolarian zonations established in north america europe japan and far east russia an up to date stratigraphic distribution of all hitherto described and still valid triassic genera is provided this new range chart consists of 282 genera and allows an accurate dating to substage level it also clearly manifests general trends in radiolar ian evolution through the triassic the end permian extinction the most severe extinction in the history of radiolarians was followed by a long recovery until the early anisian the middle and late anisian were then characterized by a rapid explosion of new morphologies maximum generic diversity was attained during the early carnian but the first severe extinctions also occurred in the carnian a progressive decline of diversity took place through the norian and rhaetian and ended in a mass extinction around the triassic jurassic boundary since the revolution that signified the use of the hydrofluoric acid method in extracting radiolarians from hard siliceous rocks radiolarians have proven to be of great importance in reconstructing the stratigraphy of the mesozoic particularly the triassic system this methodology discovered independently by dumitrica 1970 and pessagno newport 1972 together with the building of a new taxonomic system for mesozoic radiolarians

basal layered deposits of the peach springs tuff northwestern arizona usa basal layered deposits of the large volume peach springs tuff occur beneath the main pyroclastic flow deposit over a minimum lateral distance of 70 km in northwestern arizona usa the basal deposits are interpreted to record initial blasting and pyroclastic surge events at the beginning of the eruption the pyroclastic surges traveled a minimum of 100 km from the as yet unknown source changes in bedding structures with increasing flow distance are related to the decreasing sediment load of the surges some bed forms in the most proximal part of the study area kingman arizona can be interpreted as being shock induced reflecting a blast origin for the surges component analyses support a hydrovolcanic origin for some of the blasting and subsequent pyroclastic surges the eruption apparently began with magmatic blasts which were replaced by hydrovolcanic blasts hydrovolcanic activity may be partially related to failure of the conduit walls that temporarily plugged the vent a single large volume pyroclastic flow immediately followed the blast phase and no evidence has been observed for a plinian eruption column the stratigraphic sequence indicates that powerful hydrovolcanic blasting rapidly widened the vent thus bypassing a plinian fallout phase and causing rapid evolution to a collapsing eruption column similar processes may occur in other large volume ignimbrite eruptions which commonly lack significant plinian fallout deposits

long runout pyroclastic surge on a cretaceous alluvial plain republic of korea pyroclastic surge is a dilute and turbulent flow of volcanic gas and tephra that is commonly generated during explosive volcanic eruptions and can threaten lives along its flow paths assessing its travel distance and delineating future volcanic hazards have therefore been major concerns of volcanologists historical eruptions show that most pyroclastic surges travel a few tens of kilometres or less from their sources aeolian or aquagene processes have therefore been evoked for the emplacement of supposed surge deposits much beyond this distance here we show that a cretaceous tuff bed in korea was emplaced by an exceptionally powerful pyroclastic surge that flowed as far as the most powerful pyroclastic flows that formed the low aspect ratio ignimbrites lari this has significant implications for interpreting ancient volcanic eruptions and delineating volcanic hazards by pyroclastic surges and casts intriguing questions on the eruption dynamics and physics of long runout pyroclastic surges and their distinction from lari forming pyroclastic flows

two different supply styles of crater outflow materials on venus inferred from numerical simulations over dems abstract the three dimensional numerical simulation code for nonisothermal bingham fluid was applied to four typical crater outflows on venus to estimate emplacement styles of outflow materials we have succeeded in reproducing emplacement features qualitatively over digital elevation models dems obtained by the magellan spacecraft and we have shown that outflow materials can be formed by melts of basaltic crust therefore the presence of crater outflows may not be evidence of the presence of previously suggested fluid materials such as komatiite or carbonatite although we failed to model the very low viscosity flow numerically estimated supply rates of flow materials are categorized into two types a catastrophic style and a gentle style markham and isabella crater outflows are examples of catastrophic styles and the flow materials required to form them were probably supplied at higher rates than 1010 m3 s and the durations of material supply were less than 100 s this very high supply rate is difficult to attribute to any volcanic process because typical eruption rates of terrestrial volcanic flows are less than 8 or 9 orders of magnitude lower than that such a style of emplacement is more plausibly considered as a result of the impact itself for example a catastrophic debris flow or a vapor cloud triggered by the impact on the other hand supply rates of willard and xantippe outflows which are considered gentle style flows were estimated to be about 104 m3 s and the durations were more than 105 s such a gentle style of outflow emplacement is probably not a direct result of the impact but is rather due to a secondary process possibly segregation and drainage of impact melt materials from within previously emplaced ejecta materials or post impact volcanism

development of lithic breccias in the 1982 pyroclastic flow deposits of el chichon volcano mexico abstract pyroclastic flow deposit f1 volume 0 02 km 3 produced during phase iii of the 1982 eruption of el chichon volcano mexico contains basal lithic breccias the breccia layers are well exposed in el platanar gully between 2 and 4 km east of the volcano crater the lithic breccias are inversely graded as defined by lithics dense juvenile blocks and pumice supported in a coarse sandy matrix composed of the same constituents the contact between the main body of the pyroclastic flow deposit and the lithic breccias is generally sharp and planar but not erosive in some outcrops it is gradational and is only shown by an alignment of lithic clasts the origin of these beds is interpreted to be due to a hydraulic jump in the moving pyroclastic flow formed after a pronounced slope break from 11 to 3 at the site where the flow began to be funnelled into the el platanar gully we have investigated the possible modes of formation of the lithic breccias with analog laboratory experiments the experiments show that coarse localized segregations could form through a number of mechanisms the field observations interpreted with the assistance of the laboratory results suggest that pyroclastic flow 1 which produced deposit f1 moved as a kind of density stratified flow with a basal lithic rich zone transporting larger particles and an upper less dense zone transporting smaller particles in suspension at the slope break flow 1 lost competence and dumped the largest particles forming a piled up breccia downstream somewhat smaller lithic particles may have been deposited as ballistics from a low angle jetlike structure comprising a hydraulic jump this deposit thins with distance downstream once the deposit was sufficiently thick on its upstream end particles may have been re entrained into the jet to be deposited further along the flow in a low angle downstream prograding bouldery dune reverse grading in the dune beds may result from kinetic sieving we suggest that planar contacts between the lithic breccias and the associated pyroclastic flow deposit may be the result of strong density and velocity gradients between the two parts of the flow

on pyroclastic flow emplacement in this note i investigate some theoretical characteristics of pyroclastic flow deposits assuming that these flows are bingham fluids probably the simplest non newtonian fluids pyroclastic flows are modeled as laminar debris flows moving on an inclined plane and their physics is discussed within the classical framework of lubrication theory using general hydrodynamics methods i show that the arrestment and emplacement of pyroclastic flows may be seen as the time asymptotic limit of their equations of motion this limit is found to be a nonlinear ordinary differential equation whose solution gives the shape of pyroclastic flow deposits the model suggests that these flows stop when the supply of material from the source is depleted deposit thickness is controlled principally by the flow yield stress z a parameter characteristic of bingham fluids while deposit length a measure of flow mobility depends on z on the source flux q0 and on the slope of the solid substrate even in this simple model theoretical analysis shows a complex correlation between flow parameters and deposit profiles

basaltic ignimbrites in monogenetic volcanism the example of la garrotxa volcanic field ignimbrites are pyroclastic density current deposits common in explosive volcanism involving intermediate and silicic magmas and in less abundance in eruptions of basaltic central and shield volcanoes however they are not widely described in association with monogenetic volcanism where typical products include lava flows scoria and lapilli fall deposits as well as various kinds of pyroclastic density current deposits and explosion breccias in la garrotxa basaltic monogenetic volcanic field part of the neogene quaternary european rift system located in the northeast of the iberian peninsula we have identified a particular group of pyroclastic density current deposits that show similar textural characteristics to silicic ignimbrites indicating an overlap in transport and depositional processes these deposits can be clearly distinguished from other pyroclastic density current deposits generated during phreatomagmatic phases that typically correspond to thinly laminated units with planar to cross bedded stratification the monogenetic ignimbrite deposits correspond to a few meters to several tens of meters thick units rich in lithic and lapilli scoria fragments with an abundant ash matrix and internally massive structure emplaced along valleys and gullies with run out distances up to 6 km and individual volumes ranging from 106 to 1 5 107 m3 the presence of flattened scoria and columnar jointing in some of these deposits suggests relatively high emplacement temperatures coinciding with available paleomagnetic data that suggests an emplacement temperature around 450 500 c in this work we describe the main characteristics of these pyroclastic deposits that were generated by a number of phreatomagmatic episodes comparison with similar deposits from silicic eruptions and previous examples of ignimbrites associated with basaltic volcanism allows us to classify them as basaltic ignimbrites the recognition in monogenetic volcanism of such pyroclastic products which may extend several kilometres from source has an important consequence for hazard assessment in these volcanic fields which previously have been considered to present only minor hazards and risks

directional fabric measurements an investigative approach to transport and depositional mechanisms in pyroclastic flows abstract computer assisted image analysis data of rock fabrics from two quaternary ignimbrites in the vulsini and cimini volcanic districts of central italy are interpreted in terms of transport and depositional mechanisms samples were collected vertically at m spaces up two sections through each deposit the orvieto bagnoregio ignimbrite obi is a non welded ignimbrite that shows both fluctuations in the mean particle orientation values of up to approximately 60 and large variations in the strength of particle iso orientation with height the circular frequency distributions of particle orientations are almost always anisotropic and unimodal in line with a theoretical von mises distribution the circular equivalent of a unimodal log normal distribution in contrast the welded cimina ignimbrite ci shows vertical homogeneities in mean orientation values with height and generally lower degrees of anisotropy such differences are interpreted as being the results of different depositional mechanisms incremental deposition at the base of a density stratified partially turbulent flow for the obi deposition of a laminar mass flow for the ci in the former case during transport particles under solidus temperature are subjected to a frictional regime particles gliding and dispersive pressures which finally produce size inverse grading and variable fabric development depending on the residence time of particles at the basal shear conditions in the latter case elongated particles supported in a laminar flowing viscous matrix undergo periodic motions which tend to develop parallel to flow iso orientation fabric data in the deposit suggest vertical constancy in the rheological properties of the flow absence of rheological decoupling and shearing pervasively during transport a minor importance of plug horizons