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

large scale experiments on the mechanics of pyroclastic flows design engineering and first results 1 a newly designed apparatus for experimental studies of pyroclastic flows consists of a cylindrical conduit that is filled with samples of natural volcanic products tephra blowing nozzles in the base plate of the conduit are connected to a volume of highly pressurized gas opening of fast solenoid valves results in impulse like coupling of the released gas to the sample the system was designed so that the range of mechanical energy transferred to the particle mass in the conduit reflects the mechanical energy observed and measured during fragmentation experiments with melts of similar composition depending on the specific mechanical energy sme of the system which results from pv m where p is gas overpressure i e pressure atmospheric v is gas volume and m is sample mass different behaviors are observed if sme 2 6 kj kg a dilute plume develops and particles are sedimented by fallout exclusively if sme 106 implying that flows are fully turbulent and that particle coupling to gas turbulence of natural pyroclastic flows is replicated by the experiments the measured shear current velocities are proportional to the impact mass flow rate i e the product of mixture density and impact velocity experimental data and grain size analysis of the produced particle deposits suggest that the scale of the experiment is large enough to reproduce the transport dynamics of natural pyroclastic flows

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

transport and mixing dynamics from explosions in debris filled volcanic conduits numerical results and implications for maar diatreme volcanoes abstract most volcanoes experience some degree of phreatomagmatism during their lifetime however the current understanding of such processes remains limited relative to their magmatic counterparts maar diatremes are a common volcano type that form primarily from phreatomagmatic explosions and are an ideal candidate to further our knowledge of deposits and processes resulting from explosive magma water interaction due to their abundance as well as their variable levels of field exposure which allows for detailed mapping and componentry two conceptual models of maar diatreme volcanoes explain the growth and evolution of the crater maar and subsurface vent diatreme through repeated explosions caused by the interaction of magma and groundwater one model predicts progressively deepening explosions as water is used up by phreatomagmatic explosions while the other allows for explosions at any level in the diatreme provided adequate hydrologic conditions are present in the former deep seated lithics in the diatreme are directly ejected and their presence in tephra rings is often taken as a proxy for the depth at which that particular explosion occurred in the latter deep seated lithics are incrementally transported toward the surface via upward directed debris jets here we present a novel application of multiphase numerical modeling to assess the controls on length scales of debris jets and their role in upward transport of intra diatreme material to determine the validity of the two models the volume of gas generated during a phreatomagmatic explosion is a first order control on the vertical distance a debris jet travels unless extremely large amounts of magma and water are involved it is unlikely that most explosions deeper than 250 m breach the surface other factors such as pressure and temperature have lesser effects on the length scales assuming they are within realistic ranges redistribution of material within a diatreme is primarily driven by subsidence following debris jet passage natural debris jet deposits often contain wall rock material that originated above the explosion site likely this material was first transported downward by subsidence associated with preceding explosions and debris jets the length scales of debris jets decrease with increasing depth all other factors staying equal the characteristics of the domains formed from just one explosion along with the difficulty of erupting deep seated material indicate that erupted material at maar diatreme volcanoes has a complex history before it is erupted and is mostly sourced from the upper most part of the diatreme

facies distribution of ejecta in analog tephra rings from experiments with single and multiple subsurface explosions the volume grain size and depositional facies of material deposited outside an explosion crater ejecta are sensitive to the depth of the explosion the explosion energy and the presence or absence of a crater before the explosion we detonate buried chemical explosives as an analog for discrete volcanic explosions in experiments to identify unique characteristics of proximal medial and distal ejecta facies and their distribution from a range of scaled depths in undisturbed and cratered ground ejecta are here discussed in terms of three facies 1 proximal ejecta which form a constructional landform around a crater 2 medial ejecta which form a continuous sheet deposit that thins much more gradually with distance and 3 distal ejecta that are deposited as isolated clasts the extent of proximal ejecta away from the crater relative to crater size is not sensitive to scaled depth but the volume proportion of proximal ejecta to the total ejecta deposit is sensitive to the presence of a crater and scaled depth medial ejecta distribution and volume contributions are both sensitive to the presence of a crater and to scaled depth distal ejecta distance is dependent on scaled depth and the presence of a crater while the volume proportion of distal ejecta is less sensitive to scaled depth or presence of a crater experimental facies and deposit structures inferred from observations of jet dynamics are used to suggest facies associations anticipated from eruptions dominated by explosions of different scaled depth configurations we emphasize that significant differences in tephra ring deposits can result from the effects of scaled depth and preexisting craters on ejecta dynamics and are not necessarily related to fundamental differences in explosion mechanisms or degree of magma fragmentation

experiments with vertically and laterally migrating subsurface explosions with applications to the geology of phreatomagmatic and hydrothermal explosion craters and diatremes we present results of experiments that use small chemical explosive charges buried in layered aggregates to simulate the effects of subsurface hydrothermal and phreatomagmatic explosions at varying depths and lateral locations extending earlier experimental results that changed explosion locations only along a vertical axis the focus is on the resulting crater size and shape and subcrater structures final crater shapes tend to be roughly circular if subsurface explosion epicenters occur within each other s footprints defined as the plan view area of reference crater produced by a single explosion of a given energy as predicted by an empirical relationship craters are elongate if an epicenter lies somewhat beyond the footprint of the previous explosion such that their footprints overlap but if epicenters are too far apart the footprints do not overlap and separate craters result explosions beneath crater walls formed by previous blasts tend to produce inclined laterally directed ejecta jets while those beneath crater centers are vertically focused lateral shifting of explosion sites results in mixing of subcrater materials by development of multiple subvertical domains of otherwise pure materials which progressively break down with repeated blasts and by ejection and fallback of deeper seated material that had experienced net upward displacement to very shallow levels by previous explosions a variably developed collar of material that experienced net downward displacement surrounds the subvertical domains the results demonstrate key processes related to mixing and ejection of materials from different depths during an eruptive episode at a maar diatreme volcano as well as at other phreatomagmatic and hydrothermal explosion sites

deep bedded ultramafic diatremes in the missouri river breaks volcanic field montana usa 1 km of syn eruptive subsidence the ultramafic eocene missouri river breaks volcanic field mrbvf montana usa includes over 50 diatremes emplaced in a mostly soft substrate the current erosion level is 1 3 1 5 km below the pre eruptive surface exposing the deep part of the diatreme structures and some dikes five representative diatremes are described here they are 200 375 m across and have sub vertical walls their infill consists mostly of 55 90 bedded pyroclastic rocks fine tuffs to coarse lapilli tuffs with concave upward bedding and 45 10 non bedded pyroclastic rocks medium lapilli tuffs to tuff breccias the latter zones form steep columns 15 135 m in horizontal dimension which cross cut the bedded pyroclastic rocks megablocks of the host sedimentary formations are also present in the diatremes some being found 1 km or more below their sources the diatreme infill contains abundant lithic clasts and ash sized particles indicating efficient fragmentation of magma and country rocks the spherical to sub spherical juvenile clasts are non vesicular they are accompanied by minor accretionary lapilli and armored lapilli the deposits of dilute pyroclastic density currents are locally observed our main interpretations are as follows 1 the observations strongly support phreatomagmatic explosions as the energy source for fragmentation and diatreme excavation 2 the bedded pyroclastic rocks were deposited on the crater floor and subsided by 1 0 1 3 km to their current location with subsidence taking place mostly during the eruption 3 the observed non bedded pyroclastic columns were created by debris jets that punched through the bedded pyroclastic material the debris jets did not empty the mature diatreme occupying only a fraction of its width and some debris jets probably did not reach the crater floor 4 the mature diatreme was nearly always filled and buttressed by pyroclastic debris at depth there was never a 1 3 1 5 km deep empty hole with sub vertical walls otherwise the soft substrate would have collapsed inward which it only did near the surface to create the megablocks we infer that syn eruptive subsidence shifted down bedded pyroclastic material and shallow sedimentary megablocks by 0 8 1 1 km or more after which limited post eruptive subsidence occurred this makes the mrbvf diatremes an extreme end member case of syn eruptive subsidence in the spectrum of possibilities for maar diatreme volcanoes worldwide

transient multiphase processes during the explosive eruption of basalt through a geothermal borehole namafjall iceland 1977 and implications for natural volcanic flows abstract experimental and numerical studies have shown that vertical flows of gas particle mixtures are characterized by transient behavior with development of waves of high particle concentration separated by regions of relatively clean gas in contrast most models of explosive flow in volcanic conduits either treat the multiphase mixture as a single fluid pseudo fluid approximation and or assume steady flow thereby eliminating the potential for time dependent effects related to multiphase dynamics the 8 september 1977 explosive eruption of basaltic tephra through a geothermal borehole namafjall iceland provides a unique test case for multiphase volcanic processes given that its vertical extent 1 km is similar to that of natural volcanic conduits and its geometry is exactly known we model this eruption by solving separate time dependent governing equations for conservation of mass momentum and energy of the gas and particle phases allowing for drag and heat transfer between the phases model results are consistent with the development of transient waves of high particle concentration that propagate up the borehole resulting in complex compressible flow phenomena along with ejection of particles in pulses in a manner that is consistent with observations at namafjall these transient processes occur even though the influx of gas and particles at the base of the borehole is treated as constant our results indicate that transient multiphase behavior is likely to be common in volcanic conduit flows and that a key topic of future research is quantifying the types of time dependent behaviors and their impacts on eruption column dynamics

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

seismo acoustic evidence for an avalanche driven phreatic eruption through a beheaded hydrothermal system an example from the 2012 tongariro eruption abstract the 6 august 2012 te maari eruption comprises a complex eruption sequence including multiple eruption pulses a debris avalanche that propagated 2 km from the vent and the formation of a 500 m long arcuate chasm located 300 m from the main eruption vent the eruption included 6 distinct impulses that were coherent across a local infrasound network marking the eruption onset at 11 52 18 all times utc an eruption energy release of 3 1012 j was calculated using a body wave equation for radiated seismic energy a similar calculation based on the infrasound record shows that 90 of the acoustic energy was released from three impulses at onset times 11 52 20 20 of total eruption energy 11 52 27 50 and 11 52 31 20 these energy impulses may coincide with eyewitness accounts describing an initial eastward directed blast followed by a westward directed blast and a final vertical blast pre eruption seismic activity includes numerous small unlocatable micro earthquakes that began at 11 46 50 two larger high frequency earthquakes were recorded at 11 49 06 and 11 49 21 followed directly by a third earthquake at 11 50 17 the first event was located within the scarp based on an arrival time location from good first p arrival times and probably represents the onset of the debris avalanche the third event was a tornillo characterised by a 0 8 hz single frequency resonance and has a resonator attenuation factor of q 40 consistent with a bubbly fluid filled resonator this contrasts with a similar tornillo event occurring 2 5 weeks earlier having q 250 1000 consistent with a dusty gas charged resonator we surmise from pre eruption seismicity and the observed attenuation change that the debris avalanche resulted from the influx of fluids into the hydrothermal system causing destabilisation and failure the beheaded hydrothermal system may have then caused depressurisation frothing of the remaining gas charged system leading to the onset of explosive activity

comparative crystal chemistry of orthosilicate minerals the earth s average mantle composition is presumed to lie between an si o atom ratio of 1 3 and 1 4 e g ita and stixrude 1992 the orthosilicate group which comprises minerals that contain isolated sio4 tetrahedra has thus been the subject of considerable structural investigation at elevated temperature or pressure the group includes olivines silicate spinels garnets the aluminosilicates zircon and a few minor mineral groups such as humites and datolites deer et al 1997 in addition the silicate spinelloids are typically included here but are not strictly orthosilicates as they contain si2o7 dimers titanite catisio5 though technically an orthosilicate has a framework structure and is considered in a chapter by ross this volume which also examines aspects of the garnet framework structure not reviewed here interestingly no natural members of the group contain major amounts of monovalent alkali cations although liscsio4 has been synthesized with the olivine structure the most geologically significant members of the group are those minerals of formula x 2sio4 where x is a divalent cation typically mg fe mn ca and more rarely co and ni these groups include the olivines the silicate spinels and wadsleyite type spinelloids plus some minor phases such as phenakite be2sio4 willemite zn2sio4 and cadmium and chromous orthosilicates the structures of most of the major members of the group have been studied at elevated temperatures or at elevated pressures but there have not yet been many studies of structures at simultaneously elevated temperature and pressure other members of the orthosilicate group that are of major geological and geophysical significance are the al2sio5 aluminosilicate polymorphs sillimanite andalusite and kyanite zircon zrsio4 and the garnets which are abundant in high pressure metamorphic rocks the structures of these

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

carbonate cement fabrics displayed a traverse across the margin of the bahamas platform near lee stocking island in the exuma cays abstract consolidated to friable carbonate rocks found in the lee stocking island area in the exuma cays include 1 reef rock 2 channel stromatolites 3 shallow water hardgrounds 4 beachrock rimming the islands and 5 pleistocene bedrock the most common cement fabrics observed are aragonitic fibers which include acicular fan druse and square tipped coarse fibers cementing beachrock and stromatolites and an isopachous needle fiber rim cementing hardgrounds and stromatolites less common are high mg calcite bladed textures of the reef rock and stromatolites two types of blades are present the more common stubby variety of either high mg or low mg calcite and an elongated variety of high mg calcite which was found in only three beachrock samples aragonitic micrite envelopes usually surround grains in beachrock hardgrounds and stromatolites but only in association with fibrous cement an aragonitic crust cements the surfaces of lime mud beds of the tidal channel while a high mg calcite cryptocrystalline cement occurs in all the rock types calcified algal filaments of high mg calcite from the abundant green and blue green algae in the area are a primary cement in stromatolites and a secondary cement in hardgrounds and beachrock a low mg calcite equant spar cements the pleistocene samples and is associated with meteoric diagenesis and cementation of the pleistocene surface cement precipitation coincides with the path of the cool oceanic water from exuma sound as it warms and loses co 2 and moves up onto the bank near lee stocking with the incoming tide cryptocrystalline cement is the first and commonest cement forming to the seaward while platformward fibrous cements become predominant as suggested by their crystal size and location on the shelf margin we think that the reef rock cryptocrystalline material are the fastest forming of the cements where the incoming oceanic water is more saturated with respect to calcium carbonate and undergoes the most significant warming the rate of the warming and degassing process is thought to increase in the tidal channel though the cementation rate is thought to fall slightly in response to a reduced availability of calcium carbonate on the platform interior further warming and degassing are believed to cause cement precipitation and the development of hardgrounds but these may form at a slower rate than that of the margin though this rate is still quite high cementation gradients occur from the tidal channel to the intertidal zones of 1 west norman s pond cay where cement fabric suggests a reduced calcium carbonate availability and 2 west lee stocking island where a change in mineralogy suggests a change in water chemistry thus a sequence of cement fabrics and mineralogies can be traced micritic textures occur in a more seaward position fine fibrous aragonite fibers in a more lagoonal and levee position and coarser aragonite fibers and mg calcite cements in the intertidal and supratidal position this sequence is thought to track the evolution of the water mass

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

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

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

multiphase magmatic flows at yucca mountain nevada 1 the proposed yucca mountain radioactive waste repository is sited in southern nevada in a region that has experienced sporadic basaltic volcanism since the late miocene volcanic risk assessment for the proposed repository requires estimating the consequences of a new monogenetic volcano intersecting the underground facility during its 104 106 year performance period we report numerical studies aimed at understanding the range of processes and dynamic parameter values that could accompany intersection of an open repository drift by a volatile rich trachybasaltic magma as it ascends in a dike we focus on one end member type of magmatic behavior namely a fragmented magmatic mixture under pressure interacting with an underground cavity initial and boundary conditions are based upon field data and previous modeling studies of the interaction between vertically propagating dikes and a repository opening the calculations are two dimensional and time dependent and are conducted with the multiphase hydrodynamics code gmfix calculations indicate that gas particle mixtures as they rise from below and interact with horizontal openings form complex flow patterns involving varying degrees of recirculation and deposition of pyroclasts dynamic pressures are up to 106 pa but are more typically on the order of 103 to 104 pa the geometry and number of outlets play a key role in determining the types of flow patterns as do volatile contents and the degree of fragmentation the detailed numerical simulations provide information that will be used to confirm the adequacy of simplified probabilistic consequence models used in risk assessments

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

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

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

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

global imaging of polar cap patches with dual airglow imagers during a 2 h interval from 2240 to 2440 ut on 12 november 2012 regions of increased 630 0 nm airglow emissions were simultaneously detected by dual all sky imagers in the polar cap one at longyearbyen norway 78 1 n 15 5 e and the other at resolute bay canada 74 7 n 265 1 e the resolute bay incoherent scatter radar observed clear enhancements of the f region electron density up to 1012 m 3 within these airglow structures which indicates that these are optical manifestations of polar cap patches propagating across the polar cap during this interval of simultaneous airglow imaging the nightside dawnside dayside duskside half of the patches was captured by the imager at longyearbyen resolute bay this unique situation enabled us to estimate the dawn dusk extent of the patches to be around 1500 km which was at least 60 70 of the width of the antisunward plasma stream seen in the super dual auroral radar network convection maps in contrast to the large extent in the dawn dusk direction the noon midnight thickness of each patch was less than 500 km these observations demonstrate that there exists a class of patches showing cigar shaped structures such patches could be produced in a wide range of local time on the dayside nearly simultaneously and spread across many hours of local time soon after their generation