the mechanism of surtseyan volcanism during eruption the vent of a surtseyan volcano is occupied by a highly mobile slurry of tephra hyaloclastite and water as magma ascends rapidly through the slurry mixing occurs due to velocity shear acceleration of the fluid fluid contacts and mass incorporation consequent expansion of the admixed water causes the jetting and continuous up rush activity that characterizes surtseyan volcanism there is no evidence of the widely accepted fuel coolant interaction mechanism

crustal structure evolution and volcanic unrest of the alban hills central italy the alban hills a quaternary volcanic center lying west of the central apennines 15 25 km southeast of rome last erupted 19 ka and has produced approximately 290 km3 of eruptive deposits since the inception of volcanism at 580 ka earthquakes of moderate intensity have been generated there at least since the roman age modern observations show that intermittent periods of swarm activity originate primarily beneath the youngest features the phreatomagmatic craters on the west side of the volcano results from seismic tomography allow identification of a low velocity region perhaps still hot or partially molten more than 6 km beneath the youngest craters and a high velocity region probably a solidified magma body beneath the older central volcanic construct thirty centimeters of uplift measured by releveling supports the contention that high levels of seismicity during the 1980s and 1990s resulted from accumulation of magma beneath these craters the volume of magma accumulation and the amount of maximum uplift was probably at least 40 106 m3 and 40 cm respectively comparison of newer levelings with those completed in 1891 and 1927 suggests earlier episodes of uplift the magma chamber beneath the western alban hills is probably responsible for much of the past 200 ka of eruptive activity is still receiving intermittent batches of magma and is therefore continuing to generate modest levels of volcanic unrest bending of overburden is the most likely cause of the persistent earthquakes which generally have hypocenters above the 6 km deep top of the magma reservoir in this view the most recent uplift and seismicity are probably characteristic and not precursors of more intense activity

the lamongan volcanic field east java indonesia physical volcanology historic activity and hazards abstract the lamongan volcanic field lvf in east java indonesia consists of up to 61 basaltic cinder or spatter cones a minimum of 29 prehistoric maars and a central compound complex comprising three main vents including the historically active lamongan volcano the field lies on the sunda volcanic arc and abuts onto the currently active tengger caldera semeru complex to the west between 1799 and 1898 lamongan was persistently active and discharged up to 15 lava flows of basaltic to basaltic andesite composition which is probably more than what was erupted by any other indonesian volcano in that period several seismic crises have been recorded in the lvf since lamongan s last eruption there are no existing records of cinder cone or maar forming eruptions in the area despite the possible hazards posed by future activity in the lvf the history and structure of the field have been subject to little scrutiny this contribution presents the results of a detailed morphometric and volumetric investigation of prehistoric vents and historic lava flows in the lvf using field data historic records satellite imagery and modelling of erupted volumes a typical lvf maar is characterised as a crater with a diameter of 450 m generated by an eruption with a model ejecta volume of 0 016 km 3 dense rock equivalent dre and a model juvenile component volume of 0 0096 km 3 dre diameter depth ratios of some maars display a morphological freshness comparable to recently formed phreatomagmatic eruption craters elsewhere the median lvf cinder spatter cone has a basal diameter of 600 m a height of 75 m a volume of 0 009 km 3 and a model total erupted volume of 0 014 km 3 variation in cone basal diameter to height ratios indicates that the age of some prehistoric cones may not exceed a few centuries and morphological classification identifies the youngest prehistoric vents on the northern flanks of the central edifice historic activity from vents on the western flanks of lamongan produced at least 0 05 km 3 of basaltic to basaltic andesite lava resurfacing rates at lamongan in the 19th century imply an age of 900 2100 years for the surface of the volcano the steady state eruptive rate from 1843 1898 is evaluated as 0 025 m 3 s 1 which is low in comparison to estimates from many presently active systems using eruptive rate and volume data the age of the lvf is estimated as between 13 and 40 ka placing it among the youngest dated indonesian volcanoes complex regional tectonics in conjunction with magmatic pressure have probably been the major influences on the distribution of volcanism in the lvf the occurrence of maar forming eruptions may have been controlled by a porous substrate possibly a deposit from the holocene iyang argapura complex to the east volcanic hazards from future eruptions in the lvf include base surges ash flows ashfall and ejected blocks associated with phreatomagmatic activity and ashfall and ejecta from magmatic eruptions

the san venanzo maar and tuff ring umbria italy eruptive behaviour of a carbonatite melilitite volcano the late pleistocene san venanzo maar and nearby pian di celle tuff ring in the san venanzo area of umbria central italy appear to represent different aspects of an eruptive cycle accompanied by diatreme formation approximately 6x106 m3 of mostly lapillisized juvenile ejecta with lesser amounts of lithics and 1x106 m3 of lava were erupted the stratigraphy indicates intense explosive activity followed by lava flows and subvolcanic intrusions the pyroclastic material includes lithic breccia derived from vent and diatreme wall erosion roughly stratified lapilli tuff deposited by concentrated pyroclastic surge chaotic scoriaceous pyroclastic flow and inverse graded grain flow deposits the key feature of the pyroclastics is the presence of concentric shelled lapilli generated by accretion around the lithics during magma ascent in the diatreme conduits the rock types range from kalsilite leucite olivine melilitite lavas and subvolcanic intrusions to carbonatite phonolite and calcitic melilitite pyroclasts juvenile ejecta contain essential calcite whose composition and texture indicate a magmatic origin pyroclastic carbonatite activity is also indicated by the presence of carbonatite ash beds the san venanzo maar forming event is believed to have been trigered by fluid rich carbonatite phonolite magma the eruptive centre the moved to the pian di celle tuff ring where the eruption of degassed olivine melilititic magma and late intrusions ended magmatic activity in the area in both volcanoes the absence of phreatomagmatic features together with the presence of large amounts of primary calcite suggests carbonatite segregation and violent exsolution of co2 which flowing through the diatremes produced the peculiar intrusive pyroclastic facies and triggered explosions

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

vent distribution and its relation to regional tectonics cappadocian volcanics turkey abstract the distribution of polygenetic and monogenetic volcanoes of the neogene quaternary cappadocian volcanic province cvp is analyzed to investigate the relationship between vent location and regional tectonic lineaments two fault systems exist in the province one system miocene quaternary tuzgolu ecemis system is oblique whereas the other system late miocene pliocene cvp system is parallel to the long axis of the cvp the polygenetic volcanoes are aligned parallel to the second system but concentrate around the major faults of the first system regional offsets are proposed along the first fault system based on the distribution of the polygenetic volcanoes the monogenetic volcanoes group into five geographically distinct clusters in the western part of the cvp the monogenetic cones are aligned parallel to the cvp system whereas in the central part the cones are fed by dikes injected along the recent fractures of the tuzgolu ecemis system in the eastern part the monogenetic cones form along the radial fractures of the erciyes composite volcano

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

on the first experimental phreatomagmatic explosion of a kimberlite melt abstract detailed field investigations of kimberlite pipes of the upper cretaceous gibeon kimberlite field in southern namibia revealed geologic features which do neither agree with extensive nor with explosive vesiculation in the pipe hanaus 2 vesicle free magmatic kimberlite intruded the diatreme as a plug here we report on the first experiments in which phreatomagmatic explosions were produced with remolten natural non fragmental magmatic kimberlite from the hanaus 2 central plug as a best fit model kimberlite magma system after water was injected into the melt under controlled conditions the resulting water melt mix was triggered by a low energy shock wave 400 m s

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

topographic controls on a phreatomagmatic maar diatreme eruption field and numerical results from the holocene dotsero volcano colorado usa we analyze the eruptive products of holocene dotsero volcano colorado usa 4 15 ka which record evidence of a progression from effusive magmatic activity through explosive phreatomagmatic maar forming activity to a final explosive magmatic phase the nature of the deposits suggests that the irregular and mountainous pre eruptive topography strongly influenced the formation of directed pyroclastic density currents pdcs during the phreatomagmatic phase where the topographically high northern crater rim acted as a barrier and promoted transport to the south furthermore the crater shape strongly controlled the grain size of the final deposits providing further evidence that deposit grain size can be a misleading proxy for fragmentation or explosion intensity we test these hypotheses as gleaned from fieldwork by implementing a multiphase numerical model to simulate discrete explosions and their resulting pdcs at maar diatreme volcanoes we confirm that a crater shape where one rim is higher than the other promotes multiple pulses of particle transport in the direction of the lower rim the initial pdcs contain both coarse and fine particles whereas pdcs generated by later pulses contain predominately finer particles this mechanism provides an alternative explanation to the formation of lapilli tuff tuff couplets often found at maar diatreme tephra rings including dotsero volcano additional modeling shows how the crater depth can control grain size of the resulting pdc deposits in the case of a deep crater and a relatively weak explosion it is possible that only the fine particles escape following collapse of the eruptive column resulting in a fine grained deposit this work allows us to improve models developed to describe emplacement of pdcs for different crater geometries and informs on the dynamics of maar diatreme eruptions but also highlights the limitations of our knowledge of key parameters especially those related to the processes of magma water interaction and syn eruptive hydrogeology during such events

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

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

the 1976 1982 strombolian and phreatomagmatic eruptions of white island new zealand eruptive and depositional mechanisms at a wet volcano white island is an active andesitic dacitic composite volcano surrounded by sea yet isolated from sea water by chemically sealed zones that confine a long lived acidic hydrothermal system within a thick sequence of fine grained volcaniclastic sediment and ash the rise of at least 106 m3 of basic andesite magma to shallow levels and its interaction with the hydrothermal system resulted in the longest historical eruption sequence at white island in 1976 1982 about 107 m3 of mixed lithic and juvenile ejecta was erupted accompanied by collapse to form two coalescing maar like craters vent position within the craters changed 5 times during the eruption but the vents were repeatedly re established along a line linking pre 1976 vents the eruption sequence consisted of seven alternating phases of phreatomagmatic and strombolian volcanism strombolian eruptions were preceded and followed by mildly explosive degassing and production of incandescent blocky juvenile ash from the margins of the magma body phreatomagmatic phases contained two styles of activity a near continuous emission of gas and ash and b discrete explosions followed by prolonged quiescence the near continuous activity reculted from streaming of magmatic volatiles and phreatic steam through open conduits frittering juvennile shards from the margins of the magma and eroding loose lithic particles from the unconsolidated wall rock the larger discrete explosions produced ballistic block aprons downwind lobes of fall tephra and cohesive wet surge deposits confined to the main crater the key features of the larger explosions were their shallow focus random occurrence and lack of precursors and the thermal heterogeneity of the ejecta this white island eruption was unusual because of the low discharge rate of magma over an extended time period and because of the influence of a unique physical and hydrological setting the low rate of magma rise led to very effective separation of magmatic volatiles and high fluxes of magmatic gas even during phreatic phases of the eruption while true strombolian phases did occur more frequently the decoupled magmatic gas rose to interact with the conduit walls and hydrothermal system producing phreatomagmatic eruptions the form of these wet explosions was governed by a delicate balance between erosion and collapse of the weak conduit walls if the walls were relatively stable fine ash was slowly eroded and erupted in weak near continous phreatomagmatic events when the walls were unstable wall collapse triggered larger discrete phreatomagmatic explosions

contrasting types of surtseyan tuff cones on marion and prince edward islands southwest indian ocean ten surtseyan tuff cones on marion island 46 54 s 37 46 e and seven on prince edward island 46 38 s 37 57 e were erupted on shallow submerged coastal plains related to normal faulting they range from pleistocene to holocene in age and exhibit a variable degree of erosion by the sea fundamental differences irrespective of age exist between two types type i cones have diameters of 1 1 5 km rim heights of about 200 m and steep 27 outer slopes deposits are plastered against nearby cliffs beds are thin including layers of accretionary lapilli and less than 10 lithic clasts numerous bomb sags soft sediment deformation structures and gravity slides occur on one of these cones mudflows formed small tunnels which resemble lava tubes associated with channels sometimes having oversteepened walls these cones reflect comparatively low energy conditions and probably resulted from extremely wet surges interspersed with mudflows and ballistic falls type ii cones have smaller diameters 0 5 km but widespread fallout surge aprons rim heights are about 100 m and average slope angles are 18 bedding is massive with variable lapilli matrix ratio and more than 10 lithic clasts without bomb sags these cones formed under drier perhaps hotter and more violently explosive conditions than type i though not as energetic as the phreatomagmatic eruptions of terrestrial tuff rings the two types of surtseyan eruptions are explained by invoking not only different water magma ratios in the conduit but also different mechanisms of water magma interaction the slurry model of kokelaar is favoured for type i and a fuel coolant model for type ii the decisive factor is considered to be rate of effusion with rim closure and exclusion of sea water playing a secondary role

depositional processes of the suwolbong tuff ring cheju island korea the suwolbong pyroclastic sequence in the western part of cheju island korea comprises partly preserved rim beds of a quaternary basaltic tuff ring whose vent lies about 1 km seaward of the present shoreline the sequence consists of breccia lapillistone lapilli tuff and tuff eighteen sedimentary facies are established and organized into six lateral facies sequences lfs and seven vertical facies sequences vfs the lfs 1 4 and 5 begin with massive lapilli tuff which transforms downcurrent into either planar bedded lfs 1 undulatory bedded lfs 4 or climbing dune bedded lfs 5 lapilli tuff units they are representative of relatively dry base surge whose particle concentration decreases downcurrent with a progressive increase in both tractional processes and sorting the lfs 2 begins with disorganized and massive lapilli tuff and transforms into crudely stratified units downcurrent it results from relatively wet base surge in which sorting is poor due to the cohesion of damp ash the lfs 3 comprises well sorted lapilli tuff and stratified tuff further downcurrent suggestive of deposition from combined fall and surge of relatively dry hydroclastic eruption all seven vertical facies sequences generally comprise two facies units of coarse grained fines depleted lapilli tuff and an overlying fine grained tuff these sequences are suggestive of deposition from base surge that consists of a turbulent head and a low concentration tail depositional processes in the suwolbong tuff ring were dominated by a relatively dry base surge the base surge comprises turbulent and high concentration suspension near the vent whose deposits are generally unstratified due to the lack of tractional transport as the base surge becomes diluted downcurrent through fallout of clasts and mixing of ambient air it develops large scale turbulent eddies and is segregated into coarse grained bedload and overlying fine grained suspension forming thinly stratified units further downcurrent the base surge may be either cooled and deflated or pushed up into the air depending on its temperature the suwolbong tuff ring comprises an overall wet to dry cycle with several dry to wet cycles in it suggestive of overall decrease in abundance of external water and fluctuation in the rate of magma rise

geology of the gahcho kue kimberlite pipes nwt canada root to diatreme magmatic transition zones abstract the cambrian gahcho kue kimberlite cluster includes four main pipes that have been emplaced into the archaean basement granitoids of the slave craton each of the steep sided pipes were formed by the intrusion of several distinct phases of kimberlite in which the textures vary from hypabyssal kimberlite hk to diatreme facies tuffisitic kimberlite breccia tkb the tkb displays many diagnostic features including abundant unaltered country rock xenoliths pelletal lapilli serpentinised olivines and a matrix composed of microlitic phlogopite and serpentine without carbonate the hk contains common fresh olivine set in a groundmass composed of monticellite phlogopite perovskite serpentine and carbonate a number of separate phases of kimberlite display a magmatic textural gradation from tkb to hk which is characterised by a decrease in the proportion of pelletal lapilli and country rock xenoliths and an increase in groundmass crystallinity proportion of fresh olivine and the degree of xenolith digestion the pipe shapes and infills of the gahcho kue kimberlites are similar to those of the classic south african pipes particularly those of the kimberley area similar intrusive magmatic emplacement processes are proposed in which the diatreme zone results from the degassing after breakthrough of the intruding magma column the transition zones represent frozen degassing fronts the style of emplacement of the gahcho kue kimberlites is very different from that of many other pipes in canada such as at lac de gras fort a la corne or attawapiskat

large scale phreatomagmatic silicic volcanism a case study from new zealand abstract pyroclastic deposits of the 20 000 year old wairakei formation occur in the region surrounding lake taupo in the central north island of new zealand recent advances in the correlation of both proximal and distal equivalents of the wairakei deposits show that they were the result of one large eruption the ash layers once covered most of new zealand they are also present in deep sea cores from the sw pacific and on the chatham islands the dispersal and grain size parameters of the ash fall members indicate that their vent area was within the taupo volcano tectonic depression which for a long period has contained a large lake the wairakei eruption is interpreted to have been phreatomagmatic throughout each phase of the eruption sequence generated its own characteristic deposits the sequence of events can be summarized as follows 1 eruption and deposition of a white fine grained bedded fall deposit 2 eruption and deposition of a medium grained pumice fall deposit 3 generation of ultra fine wet ash clouds with a fall deposit accumulating largely as aggregates and accretionary lapilli accompanied by base surges 4 formation of water cooled pyroclastic flows and deposition of an extensive ignimbrite 5 return to a dominantly vertical eruption column and deposition of a medium grained bedded ash fall immediately followed by another wet ultra fine fall deposit charged with accretionary lapilli finally 6 an ignimbrite less widespread than that of phase 4 was produced co ignimbrite fall deposits of fine vitric dust and medium grained pumice were laid down over a huge area there may have been a short hiatus in activity between phases 4 and 5

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

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

structures and grain size distribution in surge deposits as a tool for modelling the dynamics of dilute pyroclastic density currents at la fossa di vulcano aeolian islands italy abstract turbulent dilute pyroclastic density currents surges represent the most common and hazardous type of eruptive event at la fossa di vulcano during the volcanic history these currents have frequently passed over the topographic barrier of la fossa caldera maintaining a high velocity we interpret lateral facies variation in terms of fluid dynamic behaviour by analyses of structures thickness and grain size of correlated surge layers from the palizzi succession using these data we validate a sedimentation model of traction fed by a low concentration turbulent and stratified current by the use of a sedimentological model we have then reconstructed the fluid dynamic parameters of the current and calculated the maximum horizontal distance that solid particles travelled before deposition results have been checked against actual deposit features consistency of data indicates that the method used in this study even if it is approximate and needs further refinement by specific experimental approaches and numerical simulations gives a reasonable indication of surge characters such as velocity and density which are relevant for hazard calculation and risk assessment

the 1989 submarine eruption off eastern izu peninsula japan ejecta and eruption mechanisms the submarine eruption of a new small knoll which was named teishi knoll off eastern izu peninsula behind the izu mariana arc occurred in the evening of 13 july 1989 this is the first historic eruption of the higashi izu monogenetic volcano group the eruption of 13 july followed an earthquake swarm near ito city starting on 30 june there were subsequent volcanic tremors on 11 and 12 july and the formation of the teishi knoll on the 100 m deep insular shelf 4 km northeast of ito city there were five submarine explosions which were characterized by intermittent domelike bulges of water and black tephra jets which occurred within 10 min on 13 july ejecta of the eruption was small in volume and composed of highly crystalline basalt scoria highly vesiculated pumice and lithic material petrographical features suggest that the pumice was produced by vesiculation of reheated wet felsic tuff of an older formation the teishi knoll before the eruption was a circular dome 450 m across and 25 m high with steep sides and a flat summit considerations of submarine topographic change indicate the knoll was raised by sill like intrusion of 106 m3 of magma beneath a 30 m thick sediment blanket this shallow intrusion is assumed to have started on 11 july when volcanic tremors were observed for the first time but there was no indications of violent interaction between wet host sediments and intruding magma the submarine eruption of 13 july appears to have been friggered by a major lowering of the magma column the basalt scoria having crystal contents of more than 60 is assumed to be derived from the cooled plastic margin of the shallow intrusive body however glassy scoria which would indicate the interaction between hot fluidal magma and external water was not observed a scenario for the 1989 submarine eruption is as follows when rapid subsidence of the hot interior of the intrusive magma occurred reduced pressure caused the implosion of cooled plastic magma adjacent pressurized hot host material and wet sediment the mixing of these materials triggered the vigorous vapor explosions

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

external ballistic of volcanic explosions in order to determine the kinetic energy of an explosion it is necessary to know the initial velocities of ejected fragments calculations of initial velocities made earlier with few exceptions did not take into account the resistance of the air and therefore greatly underestimated the initial velocities and consequently the energy of the explosions a solution of the inverse problem of ballistics which takes into account air resistance makes it possible to find precise values of initial velocities of ejection determined from the distance of ejected fragments and their ballistic coefficients the method makes it possible to determine the kinetic energy of explosions for eruptions which were not directly observed it is shown that the velocity field in volcanic explosions is not spherical i e the velocities of ejection of fragments differ markedly in different directions accordingly the maximum distance of ejection of fragments depends not only on the initial angle of ejection but also on the character of distribution of initial velocities and the ballistic coefficients of the fragments for fragments with diameters of 40 cm to 2 m the maximum distance of ejection is realized in the case of angles of ejection of 30 36 for smaller fragments the optimal angles of ejection are still smaller the ballistics of explosive eruptions of the volcanoes asama arenal and shiveluch are considered

pre emergent construction of a lacustrine basaltic volcano pahvant butte utah usa the subaqueous phases of an eruption initiated approximately 85 m beneath the surface of pleistocene lake bonneville produced a broad mound of tephra a variety of distinctive lithofacies allows reconstruction of the eruptive and depositional processes active prior to emergence of the volcano above lake level at the base of the volcano and very near inferred vent sites are fines poor well bedded broadly scoured beds of sideromelane tephra having local very low angle cross stratification m1 lithofacies these beds grade upward into lithofacies m3 which shows progressively better developed dunes and cross stratification upsection to its uppermost exposure approximately 10 m below syneruptive lake level both lithofacies were emplaced largely by traction from relatively dilute sediment gravity flows generated during eruption intercalated lithofacies are weakly bedded tuff and breccia m2 and nearly structureless units with coarse basal layers above strongly erosional contacts m4 the former combines products of deposition from direct fall and moderate concentration sediment gravity flows and the latter from progressively aggrading high concentration sediment gravity flows early in the eruption subaqueous tephra jetting from phreatomagmatic explosions discontinuously fed inhomogeneous unsteady dilute density currents which produced the m1 lithofacies near the vent dunes and crossbeds which are better developed upward in m3 resulted from interaction between sediment gravity flows and surface waves triggered as the explosion generated pressure waves and eruption jets impinged upon and occasionally breached the surface intermingling of a tephra emplaced after brief transport by tephra jets within a gaseous milieu and b laterally flowing tephra formed lithofacies m2 along vent margins during parts of the eruption in which episodes of continuous uprush produced localized water exclusion zones above a vent m4 comprises mass flow deposits formed by disruption and remobilization of mound tephra intermittent explosive magma water interactions occurred from the outset of the pahvant eruption with condensation entrainment of water and lateral flow marking the transformation from eruptive to sedimentary processes leading to deposition of the mound lithofacies

large volume phreatomagmatic ignimbrites from the colli albani volcano middle pleistocene italy abstract in this paper we describe four large volume ash and accretionary lapilli rich phreatomagmatic compound ignimbrite units mafic in composition from colli albani volcano south of rome the four units that form the pisolitic tuffs succession are separated by paleosols and represent the earliest explosive large volume eruptive episodes from the quaternary alban hills volcano the occurrence of large volume phreatomagmatic phreatoplinian eruptions implies the availability of large quantities of water interacting with the rising magma the paleogeography of the area below the volcano has been reconstructed by the analysis of stratigraphic data from more than a thousand bore holes distributed around the volcano that allowed to identify nw trending and ne trending paleotopographic lows that underlie the central area of the volcano and interpreted as extensional tectonic basins these lows are filled with lower to middle pleistocene pre volcanic lacustrine and fluvial deposits and suggest that at least the central part of the colli albani volcanic area hosted a large lake or lagoon the absence of sedimentary xenoliths in the pisolitic tuffs and the low vesicularity of scoria and shards suggest that water interacted with a poorly fragmented magma at very shallow level triggering the large explosivity of the eruptions we suggest that water interacting with magma was mostly surficial water related to the presence of the lagoon or lake in this environment it is likely that vents were subaqueous allowing a continuous access of water to the conduit considering that the minimum calculated volume of products for the pisolitic tuffs succession is 37 km3 and each eruption unit averages approximately around minimum volume of 10 km3 we suggest that each eruption was related to a caldera collapse which would have allowed the persistence of a taupo like lake in the central area and of phreatoplinian activity after the last phreatomagmatic eruption however the fragmentation style of large volume ignimbrites from colli albani became magmatic suggesting the extinction of the lake after that date each of the four units show a basal phreatoplinian fallout level overlain by a complex association of low aspect ratio surge deposits and ignimbrites phreatomagmatic pyroclastic flow deposits are found at distances of more than 40 km from the central area of the volcano and show important facies variation according to the paleotopography to the west of the volcano pyroclastic flows reached the tyrrhenian coast and emplaced mostly stratified facies on a flat topography interpreted to reflect both the spreading of pyroclastic flows on an unconfined topography and their interaction with lacustrine and lagoon areas by contrast to the east of the volcano where pyroclastic flows were confined within paleovalleys the main facies is thick and massive

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence structures at calderas may form as a result of precursory tumescence subsidence due withdrawal of magmatic support resurgence and regional tectonism structural reactivation and overprinting are common to explore which types of structures may derive directly from subsidence without other factors evidence is reviewed from pits caused by the melting of buried ice blocks mining subsidence scaled subsidence models and from over 50 calderas this review suggests that complex patterns of peripheral deformation with multiple ring and arcuate fractures both inside and outside caldera rims topographic embayments arcuate graben and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history downsag is a feature of many calderas and it does not indicate subsidence on an inward dipping ring fault as has been inferred previously where magmatic inflation is absent or slight initial arcuate faults formed during collapse are likely to be multiple and dip outwards to vertical associated downsag causes the peripheries of calderas undergo radial centripetal extension and this accounts for some of the complex peripheral fractures arcuate crevasses graben and some topographic moats the structural boundary of a caldera defined here as the outermost limits of subsidence and related deformation including downsag commonly lies outside ring faults and outside the embayed topographic wall it is likely to be funnel shaped i e inward dipping even though ring and arcuate fractures within it may dip outward inward dipping arcuate normal faults at shallow levels and steep inward dipping contacts between a caldera s fill and walls may both occur at a caldera that has initially subsided on outward dipping ring faults they arise due to peripheral surficial extension gravitational spreading and scarp collapse topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag respectively these processes may occur in addition to surficial degradation of oversteep ring fault scarps

geology of the victor kimberlite attawapiskat northern ontario canada cross cutting and nested craters abstract the pipe shapes infill and emplacement processes of the attawapiskat kimberlites including victor contrast with most of the southern african kimberlite pipes the attawapiskat kimberlite pipes are formed by an overall two stage process of 1 pipe excavation without the development of a diatreme sensu stricto and 2 subsequent pipe infilling the victor kimberlite comprises two adjacent but separate pipes victor south and victor north the pipes are infilled with two contrasting textural types of kimberlite pyroclastic and hypabyssal like kimberlite victor south and much of victor north are composed of pyroclastic spinel carbonate kimberlites the main features of which are similar clast supported discrete macrocrystal and phenocrystal olivine grains pyroclastic juvenile lapilli mantle derived xenocrysts and minor country rock xenoliths are set in serpentine and carbonate matrices these partly bedded juvenile lapilli bearing olivine tuffs appear to have been formed by subaerial fire fountaining airfall processes the victor south pipe has a simple bowl like shape that flares from just below the basal sandstone of the sediments that overlie the basement the sandstone is a known aquifer suggesting that the crater excavation process was possibly phreatomagmatic in contrast the pipe shape and internal geology of victor north are more complex the northwestern part of the pipe is dominated by dark competent rocks which resemble fresh hypabyssal kimberlite but have unusual textures and are closely associated with pyroclastic juvenile lapilli tuffs and country rock breccias volcaniclastic kimberlite current evidence suggests that the hypabyssal like kimberlite is in fact not intrusive and that the northwestern part of victor north represents an early formed crater infilled with contrasting extrusive kimberlites and associated breccias the remaining main part of victor north consists of two macroscopically similar but petrographically distinct pyroclastic kimberlites that have contrasting macrodiamond sample grades the juvenile lapilli of each pyroclastic kimberlite can be distinguished only microscopically the nature and relative modal proportion of primary olivine phenocrysts in the juvenile lapilli are different indicating that they derive from different magma pulses or phases of kimberlite and thus represent separate eruptions the initial excavation of a crater cross cutting the earlier northwestern crater was followed by emplacement of phase i a low grade olivine phenocryst rich pyroclastic kimberlite and the subsequent eruption of phase ii a high grade olivine phenocryst poor pyroclastic kimberlite as two separate vents nested within the original phase i crater the second eruption was accompanied by the formation of an intermediate mixed zone with moderate grade thus the final pyroclastic pipe infill of the main part of the victor north pipe appears to consist of at least three geological macrodiamond grade zones in conclusion the victor kimberlite was formed by several eruptive events resulting in adjacent and cross cutting craters that were infilled with either pyroclastic kimberlite or hypabyssal like kimberlite which is now interpreted to be of probable extrusive origin within the pyroclastic kimberlites of victor north there are two nested vents a feature seldom documented in kimberlites elsewhere this study highlights the meaningful role of kimberlite petrography in the evaluation of diamond deposits and provides further insight into kimberlite emplacement and volcanism

the potential for catastrophic dam failure at lake nyos maar cameroon the upper 40 m of lake nyos is bounded on the north by a narrow dam of poorly consolidated pyroclastic rocks emplaced during the eruptive formation of the lake nyos maar a few hundred years ago this 50 m wide natural dam is structurally weak and is being eroded at an uncertain but geologically alarming rate the eventual failure of the dam could cause a major flood estimated peak discharge 17000 m3 s that would have a tragic impact on downstream areas as far as nigeria 108 km away this serious hazard could be eliminated by lowering the lake level either by controlled removal of the dam or by construction of a 680 m long drainage tunnel about 65 m below the present lake surface either strategy would also lessen the lethal effects of future massive co2 gas releases such as the one that occurred in august 1986

geology of the mwadui kimberlite shinyanga district tanzania abstract the mwadui pipe represents the largest diamondiferous kimberlite ever mined and is an almost perfectly preserved example of a kimberlitic crater in fill albeit without the tuff ring the geology of mwadui can be subdivided into five geological units viz the primary pyroclastic kimberlite pk re sedimented volcaniclastic kimberlite deposits rvk granite breccias subdivided into two units the turbidite deposits and the yellow shales listed in approximate order of formation the pk can be further subdivided into two units lithic rich ash and lapilli tuffs which dominate the succession and lithic poor juvenile rich ash and lapilli tuffs the lower crater is well bedded down to at least 684 m from present surface extent of current drill data the bedding is defined by the presence of juvenile rich lapilli tuffs vs lithic rich lapilli tuffs and the systematic variation in granite content and clast size within much of the lithic rich lapilli tuffs four distinct types of bedding have been identified in the pyroclastic deposits diffuse zones characterised by increased granite abundance and size and upward fining units represent the dominant types throughout the deposit lateral heterogeneity was observed in addition to the vertical changes suggesting that the eruption was quite heterogeneous or that more than one vent may have been present the continuous nature of the bedding in the pyroclastic material and the lack of ash partings suggest deposition from a high concentration ejecta sustained eruption column at times e g the massive very diffusely stratified deposits the paucity of tractional bed forms suggest near vertical particle trajectories i e a clear air fall component but the poorly sorted matrix supported nature of the deposits suggest that pyroclastic flow and or surge processes may also have been active during the eruption available diamond sampling data were examined and correlated with the geology data derive from the old 120 37 m 200 61 m 300 92 m and 1200 ft 366 m levels pits sunk during historical mining operations drill logs as well as more recent bench mapping correlating macro diamond sample data and geology shows a clear relationship between diamond grade and lithology localised enrichment and dilution of the primary diamond grade has taken place in the upper reworked volcaniclastic deposits due to post eruptive sedimentary in fill processes clear distinction can be drawn between upper re sedimented and lower pyroclastic crater deposits at mwadui both from a geological and diamond grade perspective finally an emplacement model for the mwadui kimberlite is proposed geological evidence suggests that little or no sedimentary cover existed at the time of emplacement the nature of the bedding within the pyroclastic deposits and the continuity of the bedding in the vertical dimension suggest that the eruption was continuous but that the eruption column may have been heterogeneous both petrologically as well as geometrically volcanic activity appears to have ceased thereafter and the crater was gradually filled with granite debris from the unstable crater walls and re sedimented volcaniclastic material derived from the tuff ring the mwadui kimberlite exhibits marked similarities compared to the orapa kimberlite in botswana

distinct kimberlite pipe classes with contrasting eruption processes abstract field and scott smith field m scott smith b h 1999 contrasting geology and near surface emplacement of kimberlite pipes in southern africa and canada proc 7th int kimb conf eds gurney et al 1 214 237 propose that kimberlite pipes can be grouped into three types or classes classical or class 1 pipes are the only class with characteristic low temperature diatreme facies kimberlite in addition to hypabyssal and crater facies kimberlite class 2 and 3 pipes are characterized only by hypabyssal and crater facies kimberlite in an increasing number of class 1 pipes a new kimberlite facies transitional facies kimberlite is being found in most cases this facies forms a zone several metres wide at the interface between the hypabyssal and diatreme facies the transitional facies exhibits textural and mineralogical features which are continuously gradational between the hypabyssal and the diatreme types the textural gradations are from a coherent magmatic texture to one where the rock becomes increasingly magmaclastic and this is accompanied by concomitant mineralogical gradations involving the decline and eventual elimination of primary calcite at the expense of microlitic diopside both transitional and diatreme facies kimberlites are considered to have formed in situ from intruding hypabyssal kimberlite magma as a consequence of exsolution of initially co 2 rich volatiles from the volatile rich kimberlite magma the transitional facies is initiated by volatile exsolution at depths of about 3 km below the original surface with subsequent cracking through to the surface and resultant rapid decompression the further catastrophic exsolution of volatiles and their expansion leads to the formation of the diatreme facies thus diatreme facies kimberlite and class 1 pipes are emplaced by essentially magmatic processes rather than by phreatomagmatism distinctly different petrographic features characterize crater facies kimberlite in each of the three pipe classes in crater facies kimberlites of class 1 pipes small pelletal magmaclasts and abundant microlitic diopside are characteristic these features appear to reflect the derivation of the crater facies material from the underlying diatreme zone most class 2 pipes have shallow craters and the crater facies rocks are predominantly pyroclastic kimberlites with diagnostic amoeboid lapilli which are sometimes welded and have vesicles as well as glass possible kimberlite lava also occurs at two class 2 pipes in n angola the possible presence of lava as well as the features of the pyroclastic kimberlite is indicative of hot kimberlite magma being able to rise to levels close to the surface to form class 2 pipes most class 3 kimberlites have very steep craters and crater facies rocks are predominantly resedimented volcaniclastic kimberlites in some cases characterized by the presence of abundant angular magmaclasts which are petrographically very similar to typical hypabyssal facies kimberlite found in class 1 pipes the differences in crater facies kimberlite of the three classes of pipe reflect different formation and depositional processes as well as differences in kimberlite composition specifically volatile composition kimberlite forming pipe classes 1 and 3 is thought to be relatively water rich and is emplaced by processes involving magmatic exsolution of volatiles the kimberlite magma forming class 2 pipes is co 2 rich can rise to shallow levels and can initiate phreatomagmatic emplacement processes

types of phreatomagmatic volcanoes in the western snake river plain idaho usa abstract the western snake river plain graben in southwestern idaho includes a large hydrovolcanic field which was produced in late miocene to pleistocene time by the interaction of rising basaltic magmas with the waters and water saturated deposits of an enormous freshwater lake lake idaho the phreatomagmatic volcanoes in this field may be grouped into three types emergent subaqueous and subaerial emergent volcanoes which began erupting under water and built up above the lake level are relatively large and symmetrical are dominated by bedded tuffs and late magmatic deposits and are excellent indicators of water depth at the time of the eruption subaqueous volcanoes which never built up above the lake level are relatively small and asymmetrical are dominated by basal massive deposits and are potentially useful in discriminating between deep and very deep water settings subaerial volcanoes which were formed when magmas intercepted buried aquifers and interacted explosively with water are small tuff rings and maars with variable shapes are composed of subequal although variable proportions of basal massive deposits bedded tuffs and late magmatic deposits and are useful in determining the stratigraphic successions underlying them

post eruptive gravity changes from 1990 to 1996 at krafla volcano iceland the 1975 1984 krafla rifting episode was a major lava and dyke producing event associated with the release of extensional strain accumulated over more than 200 years at the divergent plate boundary in north iceland the present work provides a unique example of gravity decreases and increases sustained over a long period following a major eruptive episode at a rift volcano after height correction persistent net gravity decreases over the source of observed mogi type deflation occur with gravity increases occurring further away from this centre of deformation gravity decreases are interpreted in terms of drainage from a shallow magma chamber the net gravity decreases require that at least 4 1010 kg of magma must have been drained during the 6 year observation period assuming a density of 2700 kg m 3 this magma would occupy 1 5 107 m3 and by analogy with results obtained for kilauea this implies a magma chamber volume change of 4 1 106 m3 this is consistent with the chamber volume change deduced from ground deformation data assuming a poisson s ratio of 0 25 and a mogi source net gravity increases are more spatially extensive and are most likely caused by migration of the steam water interface and or closure of micro fractures in lavas above the magma chamber during post eruptive cooling and contraction we present a model for the krafla magma chamber in which a cooling contracting and draining magma body causes subsidence at the surface these results contrast with observations from the askja caldera iceland where post eruptive deflation has been shown to be accompanied by negligible net gravity changes above the mogi type source in the caldera long term post eruptive deflation and magma drainage have not been observed at subduction related volcanoes this may be a function of a difference in magma viscosity

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

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

late pleistocene alberca de guadalupe maar volcano zacapu basin michoacan stratigraphy tectonic setting and paleo hydrogeological environment abstract the late pleistocene 21 000 yr bp alberca de guadalupe maar is one of the few phreatomagmatic volcanoes occurring within the scoria cone dominated plio quaternary michoacan guanajuato volcanic field the scarcity of this type of volcano implies that conditions favoring their formation are rarely met in this region we identify these factors by implementing current methods of investigation with emphasis on hydrogeological conditions we present the stratigraphy including a set of 40 ar 39 ar and 14 c dates of the se margin of the zacapu intermontane tectonic basin where the maar just forms a hole 1 km in diameter 150 m deep bearing a 9 m deep lake in the otherwise planar topography of the underlying early pleistocene lava flows of the cerro pelon scoria cone the maar is composed of typical phreatomagmatic surge deposits that are poorly sorted and rich in accidental lithics 60 wt of the deposit with few juveniles basaltic andesite sio 2 54 58 wt the entire structure is cut by an ene wsw trending normal fault and is underlain by andesite lavas and silicic ignimbrites partly inferred from xenoliths encountered in the maar deposits that are miocene to early pleistocene in age the crater is at the center of a n s elongated drainage basin surrounded by topographic highlands that channel water with high hydraulic pressure from most directions towards the location of the maar this geometric configuration was already in existence at the time of the maar forming eruption but the climate was different colder and more humid conditions during the last glacial maximum cw2 climate type annual precipitation of 1000 mm favored the saturation of the fractured shallow aquifer system hydraulic conductivity of 10 8 to 10 7 m s that supplied sufficient groundwater at a high flow rate directed towards the center of the basin upon contact near surface 150 m deep crater with steep inner walls and an underlying shallow diatreme

short time electrical effects during volcanic eruption experiments and field measurements laboratory experiments on the fragmentation and expansion of magmatic melt have been performed using remelted volcanic rock at magmatic temperatures as magma simulant a specially designed dc amplifier in combination with high speed data recording was used to detect short time electrostatic field effects related to the fragmentation and expansion history of the experimental system as documented by simultaneous force and pressure recording as well as by high speed cinematography it was found that 1 the voltage time ratio of electrostatic field gradients 100 to 104 v s reflects different physical mechanisms of fragmentation and expansion and 2 the maximum voltage measured in 1 m distance 0 1 to 180 v can be correlated with the intensity of the respective processes based on these experimental results a field method was developed and tested at stromboli volcano in italy a 0 8 m rod antenna was used to detect the dc voltage against local ground i e the electrostatic field gradient at a distance of 60 to 260 m from the respective vent upwind position of the detection site was chosen to prevent interference caused by contact of charged ash particles with the antenna a standard 8 hz geophone was used to detect the accompanying seismicity three types of volcanic activity occurred during the surveillance operation two of these could be clearly related to specific electrical and seismical signals a typical delay time was found between the electrical and the seismical signal corresponding to the seismic velocity within the crater deposits using a simple first order electrostatic model the field measurements were recalibrated to the laboratory scale comparison of field and laboratory data at first approximation revealed striking similarities thus encouraging the further development of this technique for real time surveillance operation at active volcanoes

the submarine eruption and erosion of surtla surtsey iceland abstract surtla is the site of a short lived submarine vent which built basaltic elastic deposits almost to sea level in 1963 early in the eruption of surtsey since then wave and current activity have eroded the volcanic pile such that in july 1981 its top was a fairly level plateau 45 m below sea level and its surface comprised a lag deposit of sparse blocks of lava in a bed mainly of glass granules this winnowed layer was underlain by a nonreworked poorly sorted and finer deposit of glassy clasts formed by a combination of disruption by magmatic volatiles steam explosions and quench brecciation during the eruption the explosion violence and associated comminution increased as the pile built up to shallower water depths it is argued that at times of continuous effusion a cupola of steam was situated over the vent as indicated by scoriaceous spatter which shows agglutination and bread crust features that can only have developed in conditions more akin to subaerial than hitherto envisaged in a subaqueous eruption

crustal xenoliths in the 6220 bp saefell tuff cone south iceland evidence for a deep diatreme forming surtseyan eruption abstract three types of xenoliths were collected from a 80 m high vertical section of the saefell tuff cone on heimaey iceland these types are alkali basaltic type i sedimentary type ii and cpx bearing basalts type iii they are distinct in hand specimen thin section and geochemistry near the base of the eruptive succession only juvenile material and type i xenoliths are present the middle section has type ii xenoliths in addition to type i xenoliths of type iii first appear in the uppermost xenolith rich horizons where they are found together with all other types juvenile saefell clasts are present in various amounts throughout the investigated section a downward migration of explosion foci during the eruption is established based on the distribution of the different types of xenoliths in the tuff cone i e the appearance of type i type ii type iii xenoliths with increasing stratigraphic height in combination with the observed relations in basement stratigraphy beneath heimaey in the beginning of the eruption a combination of high eruption rate and abundant seawater resulted in shallow seated explosions depositing only juvenile material and type i xenoliths the downward migration of explosion foci is suggested to be the result of lowered influx of seawater due to the formation of a cone that reached 50 75 m a s l in combination with lowered eruption rate as the basement stratigraphy beneath heimaey is well documented by a 1565 m deep drill hole the depth of the explosions can be constrained to vary between less than 170 m in the beginning of the eruption and exceeding 820 m in the final phases this is much deeper than previously reported for any surtseyan tuff cone forming eruption and consistent with the downward penetration of a diatreme

the geology of kimberlite pipes of the ekati property northwest territories canada abstract this paper reviews key characteristics of kimberlites on the ekati property nwt canada to date 150 kimberlites have been discovered on the property five of which are mined for diamonds the kimberlites intrude archean basement of the central slave craton numerous proterozoic diabase dykes intrude the area the precambrian rocks are overlain by quaternary glacial sediments no phanerozoic rocks are present however mudstone xenoliths and disaggregated sediment within the kimberlites indicate that late cretaceous and tertiary cover likely

how much water swept over the lake nyos dam during the 1986 gas disaster the toxic aerosol of water and carbon dioxide which was released from lake nyos in august 1986 passed over the natural dam which confines the lake and devastated the valleys to the north the gas release generated a wave of water which swept across the lake surface and damaged vegetation on the eastern and southern shore however the vegetation to either side of the spillway was undamaged and therefore despite a recent suggestion to the contrary any wave of water which passed over the dam could have been no more than a few centimetres high

magma water interactions in subaqueous and emergent basaltic in the subaqueous growth and emergence of a basaltic volcano clasts are formed by one or a combination of 1 explosive release of magmatic volatiles 2 explosive expansion and collapse of steam formed at magma water contact surfaces 3 explosive expansion of steam following enclosure of water in magma or entrapment of water close to magma and 4 cooling contraction these processes named respectivelymagmatic explosivity contact surface steam explosivity bulk interaction steam explosivity andcooling contraction granulation can be enhanced by mutual interaction and feedback the first three explosive processes are limited at certain water depths hydrostatic pressures and become increasingly vigorous at shallower levels the depth of onset of magmatic explosivity depends largely on juvenile volatile content it is up to 200 m for tholeiitic magmas and up to 1 km for alkalic magmas at the depth where formation of clastic deposits becomes predominant over effusion of lavas magmatic explosivity is subordinate to steam explosivity as a clast forming process the upward transition to accumulation of dominantly clastic deposits is not simply related to the onset of substantial exsolution of magmatic volatiles and can occur without it contact surface explosivity commonly requires initiation by a vigorous impact between magma and water and although no certain depth limit is known likelihood of such explosivity decreases rapidly with depth clast generation by bulk interaction explosivity appears to be restricted to depths much shallower than that of the critical pressure of water which in sea water is at about 3 km cooling contraction granulation can occur in any depth of water but at shallow levels may be replaced by contact surface explosivity during continuous eruption under water tephra can be ejected and deposited within a cupola of steam such that rapid quenching does not occur emergent volcanoes are characterized by distinctive steam explosive activity that results primarily from a bulk interaction between rapidly ascending magma and a highly mobile slurry of clastic material water and steam the water gets into the vent by flooding across or through the top of the tephra pile and violent explosions cease when this access is sealed the eruptions during emergence of surtsey and capelinhos typify the distinctive explosive activity the style and controls of which are different from those of maar volcanoes

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

hurricane intensity and eyewall replacement observations made during the historic 2005 hurricane season document a case of eyewall replacement clouds outside the hurricane eyewall coalesce to form a new eyewall at a greater radius from the storm center and the old eyewall dies the winds in the new eyewall are initially weaker than those in the original eyewall but as the new eyewall contracts the storm reintensifies understanding this replacement mechanism is vital to forecasting variations in hurricane intensity processes in the moat region between the new and old eyewall have been particularly unclear aircraft data now show that the moat becomes dynamically similar to the eye and thus is converted into a region inimical to survival of the inner eyewall we suggest that targeting aircraft to key parts of the storm to gain crucial input to high resolution numerical models can lead to improvements in forecasting hurricane intensity

recent advances in the geology of koffiefontein mine free state province south africa abstract the complex internal geology of the koffiefontein pipe has contributed to the marginal nature of the mine the key to this is the presence of a large zone dominated by down rafted country rock karoo sediment and dolerite xenoliths recent work indicates that the kimberlite pipe at koffiefontein consists of precursor dykes the west and east fissures and the main pipe in which two main eruptive phases have been recognized groundmass spinel compositions have been used to provide a chemical fingerprint of each lithology there is evidence for at least three magma batches each with its own chemical signature cross cutting contact relationships were used to determine the emplacement sequence the characterization of the different internal geological units permitted the development of a three dimensional 3d model of the pipe both main eruptive phases viz the speckled west kimberlite and the speckled east kimberlite comprise volcaniclastic kimberlite they are separated by a large irregular mass of kimberlite that contains abundant country rock xenoliths comprising varying proportions of karoo mudstone and dolerite as well as probable bedded crater facies fragments this zone of contamination dilutes the grade of the kimberlites affects the geotechnical stability and adversely affects the economics of the mine

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

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

capelinhos 1957 1958 faial azores deposits formed by an emergent surtseyan eruption the 1957 1958 eruption of capelinhos faial island azores involved three periods of surtseyan hydromagmatic activity two in 1957 and one in 1958 deposits from this eruption are exposed both in sea cliffs cut into the flanks of the tuff cone and more distally 1 km from the vent five lithofacies are identified lithofacies i is composed of even thickness beds with laterally continuous internal stratigraphy and is interpreted to have been formed by fallout lithofacies ii consists of beds with internally discontinuous lenses and has sand wave structures that increase in abundance toward the outer margins of the tuff cone this lithofacies is interpreted as having been deposited from pyroclastic surges lithofacies iii is composed of mantle bedded deposits with laterally discontinuous internal stratigraphy this lithofacies is interpreted to have been formed by hybrid processes where fallout of tephra occurred simultaneously with pyroclastic surges in the outer flanks of the tuff cone lithofacies iii grades laterally into fallout beds of lithofacies i lithofacies iv consists of alternating beds of coarse ash aggregates and non aggregated fine ash and is particularly well developed in distal regions some of this facies was formed by fallout alternating beds also occur plastered against obstacles up to 2 km from the vent indicating an origin from wet pyroclastic surges the orientation of plastered tephra indicates that the surges were deflected by topography as they decelerated the distinction between surge and fallout in distal regions is uncertain because wind drifted fallout and decelerating surge clouds can generate similar deposits lithofacies v consists of scoria lapilli beds interpreted to be fallout from hawaiian style fire fountaining in the later stages of the eruption juvenile pyroclasts within hydromagmatic deposits are predominantly poorly vesicular 25 60 of clasts 30 vesicles however on both micro and macroscopic scales there is a wide range in clast vesicularity up to 70 vesicles indicating that although fragmentation was predominantly hydromagmatic vesiculation and magmatic volatile driven fragmentation operated simultaneously

stratigraphy petrochemistry and quaternary depositional record of the songaksan tuff ring jeju island korea abstract the songaksan tuff ring str is one of several recent hydrovolcanic centers on jeju island korea which provides an excellent example of proximal to distal facies changes in wet pyroclastic surge deposits a multidisciplinary study has been carried out on the str and adjacent lithostratigraphic units to constrain absolute age geochemical characteristics and quaternary depositional history a number of rock units were identified inside the crater of the str including scoria deposit i trachybasalt lava scoria deposit ii and a late stage basaltic tuff indicative of a rather complex sequence of magmatic and phreatomagmatic eruptions after the construction of the tuff ring petrochemical analysis shows that the str was generated from different magma batches that fractionated from a homogenous magma chamber and the early erupted magma was more evolved and volatile rich reworking of the str commenced shortly after the hydromagmatic eruption in a high energy nearshore environment resulting in deposition of the hamori formation the formation is composed of planar stratified and low to high angle cross stratified tuffaceous pebbly sandstones and occurs up to an altitude of about 4 m above present sea level 14c dating of molluscan shells beneath the formation indicates that it began to be deposited after about 4000 yr bp detailed sedimentary logging reveals that the formation consists of several stratal packages bounded by laterally persistent and distinct lithologic boundaries probably formed by millennial scale sea level fluctuations occurrence of another hydrovolcanic sequence the sinyangri formation on the opposite side of jeju island having similar sedimentary characteristics and ages suggests that the sea level fluctuations as seen in the hamori formation have affected a wide area of jeju island probably related to the high frequency sea level oscillations during the post 6 ka bp regression period in the east asian region it can be concluded that the formation of the str was possible because of the holocene transgression which made the present coastal areas water saturated and adequate for hydrovolcanic eruptions the str in turn contributed to record high frequency sea level fluctuations during the holocene via acting as a local and short lived but affluent source of loose sediment

on the volcanology of the gibeon kimberlite field namibia kimberlite volcanism in the upper cretaceous gibeon kimberlite field southern namibia consisting of at least 42 diatremes and a number of associated dykes is closely related to carbonatitic and ultrabasic volcanic and intrusive activity which occurred at the margin of the field the volcanology of the diatremes and dykes as well as their structural setting is reported here because of the paleohydrogeological setting and since juvenile kimberlite occurring in dykes intrusive plugs and spherical lapilli is devoid of vesicles a phreatomagmatic eruption mechanism is proposed for the genesis of the kimberlite diatremes karoo dolerite basalt and sediment xenoliths in the diatremes provide evidence for the former extent of karoo strata at the time of eruption

post eruptive development of the ukinrek east maar since its eruption in 1977 a d in the periglacial area of south west alaska maar eruptions form small initially steep walled basins that contain important archives for the climatic and palaeoenvironmental history in continental areas the two ukinrek maars in south western alaska erupted between 30 march and 9 april 1977 and are the best documented maars that have erupted in historical time this study presents a preliminary analysis of geomorphology hydrology magnetic susceptibility geochemistry and sedimentology data of a field study in august 2004 these results photographs and topographic surveys are combined for reconstructing the post eruptive evolution of ukinrek east maar within less than 30 years the initially polygonal shape of the crater with nearly vertical crater walls has developed into an almost elliptic form with slopes of 35 inclination on debris fans between a few escarpments the water table and the crater floor have risen significantly and the crater diameter depth ratio increased from 3 4 to 5 7 whereas the average height of the crater rim remained almost constant the main sub aerial resedimentation process is formation of rock falls rock slumps and scree resulting in debris flows and turbidites within the lake that is ice covered throughout about half of the year distal lake sediments consist of laminated minerogenic clayey sandy silts that document frequent turbidity currents from the linear sedimentation rate of only ca 5 mm year 1 in 17 cm long cores it may be concluded that the largest portion of the crater sediments formed within the first few months of the maar history however this has to be confirmed by future studies

deaths and injuries in the eruption of galeras volcano colombia 14 january 1993 abstract six volcanologists and three tourists were killed in the crater of galeras volcano colombia when it erupted without warning the scientists were attending the united nations international decade for natural disaster reduction workshop which had been convened to improve monitoring research and disaster mitigation at galeras at the time the most active and one of the most hazardous volcanoes in south america information on the events surrounding the eruption was obtained by sending a questionnaire to twelve scientists who had been inside the caldera at the time of the eruption or who had assisted in the search and rescue operation the autopsy reports on the five corpses and the few pieces of equipment and clothing retrieved from the crater area were also studied the main causes of death and injury were the forces at the eruptive vent and the bombardment by hot rocks ejected in the first 15 min of the eruption ranging from blocks over 1 m in size to pea sized lapilli which fell last some conclusions can be drawn for the future safety of volcanologists working in craters at high altitude hard hats would protect against concussion from blows to the head during escape from the danger area and a lightweight heat resistant and water repellent coverall would limit the skin burns and the risk of clothing being ignited from contact with incandescent falling ejecta the coverall could also be life saving by protecting immobilised casualties from hypothermia due to the rain and wind whilst waiting to be rescued especially as the volcanic activity cloud cover or nightfall could curtail rescue efforts work in hazardous craters should be strictly limited to essential tasks and periods of good visibility and a climbing team should leave the area at least four hours before nightfall in case rescue is needed tourists must be warned against visiting active crater areas

the neapolitan yellow tuff a large magnitude trachytic phreatoplinian eruption eruptive dynamics magma withdrawal and caldera collapse the results of a detailed stratigraphical and volcanological study has allowed the definition of the neapolitan yellow tuff nyt as the largest known trachytic phreatoplinian eruption with an estimated volume of not less than 30 km3 dre the nyt sequence can be subdivided in a lower and an upper member sedimentological and textural variations in time and space indicate that different eruptive and depositional mechanisms operated during the eruption the lower member eruptive phase was characterized by alternating phreatoplinian and magmatic explosions while the upper member eruptive phase was characterized by a random sequence of phreatomagmatic and magmatic explosions particles produced by the same explosion were deposited contemporaneously by fallout and surge mechanisms either at different or at the same distance from the vent compositional variations show that the nyt sequence cannot be modeled as an inverted compositionally zoned magma body with magma becoming more basic in the course of the eruption the chamber composed of three geochemically distinct magma layers was tapped at different depths according to the eruption dynamics each phreatoplinian explosion tapped concurrently all the magma layers while magmatic explosions tapped only one of them the onset of the nyt caldera collapse is well constrained between lower and upper member eruptive phases the collapse which is only gravimetrically detectable occurred inside the larger phlegrean fields caldera which therefore can be defined as a nested caldera

patterns of quartz sand shape variation long island littoral and shelf abstract fourier analysis of quartz grain shapes across the shelf and adjacent beach littoral system off long island reveals a pattern of variation that distinguished a beach littoral zone an inner zone inner shelf and an outer zone mid and outer shelf the basis of this distinction is the relative proportion of type i abraded and type 2 irregular sand present irregularly shaped sands type 2 of glaciofluvial origin 70 100 type 2 off long island first became abraded in the beach environment 50 type 2 sand the inner shelf averages 45 percent type 2 sands similar to the beach sands the outer zone mid and outer shelf contains the lowest proportions averaging 15 of type 2 sand suggesting a multicycle abrasion history the boundary between the inner and outer zones is sharp and may represent the seaward limit of dominantly glaciofluvial sediment the beach littoral zone off eastern long island has the widest variation in relative proportions of shape types 35 95 type 2 sand within this zone two sources of sand are indicated 1 the partially abraded sands 50 type 2 associated with the beach and 2 highly irregular glaciofluvial sands 95 type 2 of glaciofluvial substrate underlying the beach facies the crest of the longshore bar contains samples containing a wide range in proportions of these sand types implying episodes of both seaward transport onto the bar crest as well as landward transport from a glaciofluvial substrate exposed just seaward of the beach littoral bar approximately 10 m water depth in a similar fashion as the nearshore zone off cape cod massachusetts

characteristics of dune bedded pyroclastic surge bedsets abstract the grainsize characteristics of dune bedded pyroclastic surge bedsets are surveyed the variance between coarsest and finest beds ranges from 1 to 6 phi in different surge bedsets and it increases as the grainsize of the coarsest bed increases reflecting an increasing velocity of emplacement deposits of wet surges identified as those which contain accretionary and ash coated lapilli tend to be finer and show less variance this partly because wet ash is cohesive but mainly because wet surges tend to be weaker dry surge bedsets are strongly fines depleted wet ones less so the lack of erosion of underlying ash layers shows that the environment is a strongly depositional one individual bedsets are demarcated by thin intervening fine ash fall layers which are the complementary ash cloud deposits settled or flushed out after the passage and decay of each turbulent surge surge deposits are generally less coarse than the coarsest associated airfall deposits which shows that they are formed by generally weaker events this study helps interpret the dune bedded parts of the landscape mantling may 18th 1980 blast deposit of mount st helens the blast was a very violent event but the variance and the grainsize of the coarsest bed are those of a relatively weak surge this suggests that the dune bedding was produced by a weak effect such as minor turbulence in a thin pyroclastic flow coming to rest in a mountainous terrain roughened by tree stumps and fallen logs

stress induced brittle fragmentation of magmatic melts theory and experiments 1 the release of kinetic energy during explosive volcanic eruptions is a key parameter for hazard assessment and civil defense the explosive production of volcanic ash by intensive fragmentation of magma and host rocks represents a substantial part of this energy for cases of explosive eruption where predominantly host rock was fragmented phreatomagmatic eruptions to form the major part of volcanic ash rock mechanical parameters could be measured and fragmentation energies assigned in cases where most of the produced ash is of juvenile origin magmatic eruptions a general method for the determination of fragmentation energy is still lacking in this article we introduce a thermodynamic approach that relates grain size data of the produced ash deposits to shear rates acting during the deformation of magma with the use of a standardized fragmentation experiment the physical parameters needed to determine the specific fragmentation energy and deformation history were measured the experiment was calibrated and tested with two case histories of the campi flegrei volcanic field southern italy both eruptions are classified as most probable worst case scenarios during the next period of activity to be expected within the next 10 100 years using the experimentally determined specific fragmentation energies the total mass of produced ash of each eruption and assuming an energy dissipation as observed in the experiments the total kinetic energy release of the worst case campi flegrei eruptive events to come were calculated with 25 and 40 kt tnt equivalent