reconstructing biomes from palaeoecological data a general method and its application to european pollen data at 0 and 6 ka biome models allow the results of experiments with atmospheric general circulation models to be translated into global maps of potential natural vegetation the use of biome models as a diagnostic tool for palaeoclimate simulations can yield maps that are directly comparable with palaeoecological pollen and plant macrofossil records provided these records are biomized i e assigned to biomes in a consistent way this article describes a method for the objective biomization of pollen samples based on fuzzy logic pollen types taxa are assigned to one or more plant functional types pfts then affinity scores are calculated for each biome in turn based on its list of characteristic pfts the pollen sample is assigned to the biome to which it has the highest affinity subject to a tie breaking rule modern pollen data from surface samples reflecting present vegetation across europe are used to validate the method pollen data from dated sediment cores are then used to reconstruct european vegetation patterns for 6 ka the reconstruction shows systematic differences from present that are consistent with previous interpretations the method has proved robust with respect to human impacts on vegetation and provides a rational way to interpret combinations of pollen types that do not have present day analogs the method demands minimal prior information and is therefore equally suitable for use in other regions with richer floras and or lower densities of available modern and fossil pollen samples than europe

origin of the giant eruption cloud of pinatubo june 15 1991 the series of eruptions of june 15 1991 at mt pinatubo philippines were observed hourly by satellite a giant discshaped cloud covering an area of 60 000 km2 appeared in the satellite images at 14 40 philippine time the cloud expanded radially against wind of 20 m s and spread to an area of more than 120 000 km2 within an hour according to eyewitness accounts there was heavy fine ash fall after 14 00 intermittent lapilli fall started at about 14 20 and heavy and continuous lapilli fall widely started at about 15 00 the occurrence of the giant cloud roughly corresponded to the initiation of the intermittent lapilli fall the air fall deposits of the major eruption are widely distributed including upwind from the vent they are composed of 3 units a silt size fine ash layer layer b a lapilli layer commonly including pumice grains of 1 cm in diameter layer c a lapilli bearing volcanic sand layer layer d judging from its wide distribution and depletion of coarse grains most of the fine ash of layer b is not distal deposits of a small eruption but is originated from a large co ignimbrite cloud it is suggested that the major eruption started with the generation of a pyroclastic flow which was subsequently followed by a plinian eruption resulting in the formation of the giant cloud and the lapilli fall the results of calculations on the dynamics of eruption cloud indicate that the dimension and dynamics of the giant eruption cloud is accounted for by a plinian eruption with a magma discharge rate of the order of 109 kg s

dynamics of giant volcanic ash clouds from supervolcanic eruptions 1 the largest explosive volcanic eruptions that have occurred on earth generated giant ash clouds from rising plumes that spread in the stratosphere around a height of neutral buoyancy with estimated supply rates that are in the range 1011 to 1013 m3 s these giant ash clouds are controlled by a balance between gravity and coriolis forces forming spinning bodies of nearly fixed proportions after a few hours and are initially insensitive to stratospheric winds in contrast volcanic plumes from eruptions with small to intermediate magnitude spread as inertial intrusions under the influence of stratospheric winds with the earth s rotation being unimportant in the largest eruptions the giant spinning ash clouds typically develop diameters greater than 600 km and up to 6000 km in the most powerful super eruptions thus explaining why areas of continental size can be covered with volcanic ash the radial expansion and spinning velocities are calculated at tens of metres per second and increase with eruption intensity higher spreading velocities carry larger ash grain sizes to a given radius so that grain size at a given distance from the source increases with eruption intensity consistent with geological observations

modeling ash fall distribution from a yellowstone supereruption we used the volcanic ash transport and dispersion model ash3d to estimate the distribution of ashfall that would result from a modern day plinian supereruption at yellowstone volcano the simulations required modifying ash3d to consider growth of a continent scale umbrella cloud and its interaction with ambient wind fields we simulated eruptions lasting 3 days 1 week and 1 month each producing 330 km3 of volcanic ash dense rock equivalent dre results demonstrate that radial expansion of the umbrella cloud is capable of driving ash upwind westward and crosswind n s in excess of 1500 km producing more or less radially symmetric isopachs that are only secondarily modified by ambient wind deposit thicknesses are decimeters to meters in the northern rocky mountains centimeters to decimeters in the northern midwest and millimeters to centimeters on the east west and gulf coasts umbrella cloud growth may explain the extremely widespread dispersal of the 640 ka and 2 1 ma yellowstone tephra deposits in the eastern pacific northeastern california southern california and south texas

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

correlation of upper cenozoic tephra layers between sediments of the western united states and eastern pacific ocean and comparison with biostratigraphic and magnetostratigraphic age data five widespread upper cenozoic tephra layers that are found within continental sediments of the western united states have been correlated with tephra layers in marine sediments in the humboldt and ventura basins of coastal california by similarities in major and trace element abundances four of these layers have also been identified in deep ocean sediments at dsdp sites 34 36 173 and 470 in the northeastern pacific ocean these layers erupted from vents in the yellowstone national park area of wyoming and idaho y the cascade range of the pacific northwest c and the long valley area california l are the huckleberry ridge ash bed 2 0 ma y rio dell ash bed ca 1 5 ma c bishop ash bed 0 74 ma l lava creek b ash bed 0 62 ma y and loleta ash bed ca 0 4 ma c the isochronous nature of these beds allows direct comparison of chronologic and climatic data in a variety of depositional environments for example the widespread bishop ash bed is correlated from proximal localities near bishop in east central california where it is interbedded with volcanic and glacial deposits to lacustrine beds near tecopa southeastern california to deformed on shore marine strata near ventura southwestern california to deep ocean sediments at site 470 in the eastern pacific ocean west of northern mexico the correlations allow us to compare isotopic ages determined for the tephra layers with ages of continental and marine biostratigraphic zones determined by magnetostratigraphy and other numerical age control and also provide iterative checks for available age control relative age variations of as much as 0 5 m y exist between marine biostratigraphic datums for example highest occurrence level of discoaster brouweri and calcidiscus tropicus c macintyrei as determined from sedimentation rate curves derived from other age control available at each of several sites these discrepancies may be due to several factors among which are 1 diachronism of the lowest and highest occurrence levels of marine faunal and floral species with latitude because of ecologic thresholds 2 upward reworking of older forms in hemipelagic sections adjacent to the tectonically active coast of the western united states and other similar analytical problems in identification of biostratigraphic and magnetostratigraphic datums 3 dissolution of microfossils or selective diagenesis of some taxa 4 lack of precision in isotopic age calibration of these datums 5 errors in isotopic ages of tephra beds and 6 large variations in sedimentation rates or hiatuses in stratigraphic sections that result in age errors of interpolated datums correlation of tephra layers between on land marine and deep ocean deposits indicates that some biostratigraphic datums diatom and calcareous nannofossil may be truly time transgressive because at some sites they are found above and at other sites below the same tephra layers

ash3d a finite volume conservative numerical model for ash transport and tephra deposition 1 we develop a transient 3 d eulerian model ash3d to predict airborne volcanic ash concentration and tephra deposition during volcanic eruptions this model simulates downwind advection turbulent diffusion and settling of ash injected into the atmosphere by a volcanic eruption column ash advection is calculated using time varying pre existing wind data and a robust high order finite volume method our routine is mass conservative and uses the coordinate system of the wind data either a cartesian system local to the volcano or a global spherical system for the earth volcanic ash is specified with an arbitrary number of grain sizes which affects the fall velocity distribution and duration of transport above the source volcano the vertical mass distribution with elevation is calculated using a suzuki distribution for a given plume height eruptive volume and eruption duration multiple eruptions separated in time may be included in a single simulation we test the model using analytical solutions for transport comparisons of the predicted and observed ash distributions for the 18 august 1992 eruption of mt spurr in alaska demonstrate to the efficacy and efficiency of the routine

modeling tephra sedimentation from a ruapehu weak plume eruption 1 we present a two dimensional model for sedimentation of well mixed weak plumes accounting for lateral spreading of the cloud downwind advection increase of volumetric flux in the rising stage and particle transport during fallout the 17 june 1996 subplinian eruption of ruapehu produced a bent over plume that rose to a height of 8 5 km in a wind field with an average velocity of 24 m s 1 and generated a narrow deposit on land extending up to 200 km from vent the sedimentation from the ruapehu plume was dominated by coarse ash with all the blocks and most of the lapilli falling while the plume was still rising particles with diameter 125 m show three accumulation maxima one coincident with a secondary maximum in the total tephra deposit about 150 km from the vent numerical modeling shows that the plume started spreading horizontally beyond about 30 km from vent investigations also show that fickian diffusion can be a good approximation to the crosswind spreading of both vigorous and low energy weak plumes but a wide range of diffusion coefficients is to be expected with the largest values necessary to describe the most vigorous weak plumes such as ruapehu one dimensional and two dimensional simulations show that tephra sedimentation must have been subvertical this feature also supported by direct observations can be the result of the turbulence structure of the local wind field and convective instabilities which could also have played an important role in the generation of the pronounced sinuosity shown by the ruapehu deposit

magnetostratigraphy and tephrochronology of an upper pliocene to holocene record in lake sediments at tulelake northern california combined paleomagnetic and tephra chronologies of one of the most complete middle pliocene through holocene stratigraphic records yet recovered in western north america provide a reference section for much of northwestern north america and adjacent pacific ocean five long drill cores of lacustrine sediments at tulelake northern california recovered a nearly continuous 331 m thick record spanning the past 3 m y the brunhes normal polarity matuyama reversed polarity and gauss normal polarity chronozones are recognized within these the jaramillo olduvai reunion and kaena subchronozones are present six short stratigraphic intervals exhibit anomalous remanent inclinations that may record excursions and brief subchrons within the brunhes and matuyama chronozones age estimates suggest correlation of five of the anomalous intervals with 1 one of the biwa excursions at about 18 000 yr b p 2 the mono lake excursion at about 27 000 yr b p 3 the blake reversed polarity subchron at about 114 000 yr b p 4 the kamikatsura normal polarity subchron at about 850 000 yr b p and 5 the cobb mountain normal polarity subchron at about 1 10 ma age of the sixth interval of anomalous inclination is broadly constrained between 117 000 and 180 000 yr b p sixty three individual tephra layers were characterized by electron microprobe and x ray fluorescence analyses of volcanic glass shards identified tephra of relatively well known age include 1 the basal airfall pumice at llao rock 7015 yr b p 2 the trego hot springs bed 23 400 yr b p 3 the olema ash bed between 55 000 and 75 000 yr b p 4 the airfall pumice at cloudcap road pumice castle like tephra 2 about 120 000 yr b p 5 the rockland ash bed about 410 000 yr b p 6 the lava creek b ash bed 620 000 yr b p 7 the rio dell ash bed about 1 45 ma and 8 the bear gulch ash bed about 1 9 ma a sedimentation rate curve based on independently dated tephra and polarity reversals is used to infer age estimates of undated or previously unidentified ash beds some of these ash beds are found over large areas of the western united states and eastern pacific ocean basin and provide widespread horizons for correlation most of the tephra at tulelake records eruptions from the nearby southern and central cascade range of oregon and northern california and the medicine lake highland of northern california deposition took place during most of the past 3 m y within the tulelake basin notable periods of slow or sporadic accumulation or erosion occurred between about 620 000 and 200 000 yr b p and between about 2 5 and 2 1 ma rapid deposition occurred during marine oxygen isotope stage 6 between about 170 and 125 ka regional volcanism during the past 3 m y was markedly episodic with notable volcanic activity from about 2 1 to 1 9 ma and from 0 4 ma to the present

computer simulation of transport and deposition of the campanian y 5 ash analyses of grain size and modal composition of the campanian tuff ash layer y 5 from 11 deep sea cores have been carried out this layer represents ash fall that has been correlated with the 38 000 y b p campanian ignimbrite thunell et al 1979 a deposit formed by the largest eruption documented in the mediterranean region during the late pleistocene barberi et al 1978 the bulk deposit is bimodal in grain size and dominated by glass shards the calculated mean grain size of the coarse mode of the individual size distributions decreases with distance from the source and progressively approaches a near constant fine mode of approximately 13 microns distal samples are unimodal in grain size these data combined with a set of vertical profiles of wind 10 year average have been used as input to a computer model that simulates fallout of tephra modelling indicates that the downwind variation of grain size of the coarse mode can be accurately reproduced with transport of ash between 5 and 35 km the observed fine mode of the deposit cannot however be generated by transport of ash as individual particles at these elevations such transport would result in deposition of virtually all of the fine ash beyond the studied area deposition of fine ash within the studied distance of 1600 km from source can only occur by fallout as particle aggregates from a high eruption plume or as individual particles from co ignimbrite ash clouds with a maximum elevation of 3 km the large volume of ash in the fine mode 70 wt and the irregularity in azimuth of low level winds argue against major low level transport of co ignimbrite ash rather the ash may have been derived from both a plinian eruption column and high altitude clouds of co ignimbrite ash with settling of fine ash as particle aggregates

influence of water and sediment supply on the stratigraphic record of alluvial fans and deltas process controls on stratigraphic completeness 1 stratigraphy contains the most complete record of information necessary to quantitatively reconstruct paleolandscape dynamics but this record contains significant gaps over a range of time and space scales these gaps result from stasis on geomorphic surfaces and erosional events that remove previously deposited sediment building on earlier statistical studies we examine stratigraphic completeness in three laboratory experiments where the topography of aggrading deltas was monitored at high temporal and spatial scales the three experiments cover unique combinations in the absolute magnitudes of sediment and water discharge in addition to generation of accommodation space through base level rise this analysis centers on three time scales 1 the time at which a record is discretized t 2 the time necessary to build a deposit with mean thickness equivalent to the maximum roughness on a surface tc and 3 the time necessary for channelized flow to migrate over all locations in a basin tch these time scales incorporate information pertaining to the time variant topography of actively changing surfaces kinematics by which the surfaces are changing and net deposition rate we find that stratigraphic completeness increases as a function of t tc but decreases as a function of tc tch over the parameter space covered in the experiments our results suggest that environmental signals disconnected from a sediment routing system are best preserved in systems with low tc values nondimensionalizing t by tc however shows that preservation of information characterizing system morphodynamics is best preserved in stratigraphy constructed by systems with low water to sediment flux ratios

the initial giant umbrella cloud of the may 18th 1980 explosive eruption of mount st helens abstract the initial eruption column of may 18th 1980 reached nearly 30 km altitude and released 10 17 joules of thermal energy into the atmosphere in only a few minutes ascent of the cloud resulted in forced intrusion of a giant umbrella shaped cloud between altitudes of 10 and 20 km at radial horizontal velocities initially in excess of 50 m s the mushroom cloud expanded 15 km upwind forming a stagnation point where the radial expansion velocity and wind velocity were equal the cloud was initiated when the pyroclastic blast flow became buoyant the flow reduced its density as it moved away from the volcano by decompression by sedimentation and by mixing with and heating the surrounding air observations indicate that much of the flow covering an area of 600 km 2 became buoyant within 1 5 minutes and abruptly ascended to form the giant cloud calculations are presented for the amount of air that must have been entrained into the flow to make it buoyant assuming an initial temperature of 450 c and a magmatic origin for the explosion these calculations indicate that the flow became buoyant when its temperature was approximately 150 c and the flow consisted of a mixture of 3 25 10 11 kg of pyroclasts and 5 0 10 11 kg of air if sedimentation is considered these figures reduce to 1 1 10 11 kg of pyroclasts and 1 0 10 11 kg of air

the bishop tuff new insights from eruptive stratigraphy the 0 76 ma bishop tuff from long valley caldera in eastern california consists of a widespread fall deposit and voluminous partly welded ignimbrite the fall deposit f exposed over an easterly sector below and adjacent to the ignimbrite is divided into nine units f1 f9 with no significant time breaks except possibly between f8 and f9 maximum clast sizes are compared with other deposits where accumulation rates are known or inferred to estimate an accumulation time for f1 f8 as ca 90 hrs the ignimbrite ig is divided into chronologically and or geographically distinct packages of material earlier packages ig1 were emplaced mostly eastward are wholly intraplinian coeval with fall units f2 f8 lack phenocrystic pyroxenes and contain few or no glass mountain derived rhyolite lithic fragments earlier packages ig2 were erupted mostly to the north and east are at least partly intraplinian interbedded with fall unit f9 to the east contain pyroxenes and have lithic fractions rich in g

density driven transport in the umbrella region of volcanic clouds implications for tephra dispersion models this work has benefited from funding provided by the italian presidenza del consiglio dei ministri dipartimento della protezione civile dpc agreement ingv dpc 2012 2013 this paper does not necessarily represent dpc official opinion and policies a f acknowledges funding by the spanish project atmost cgl2009 10244

volcanological study of the great tambora eruption of 1815 the 1815 eruption of tambora is one of the largest explosive volcanic events of the past 10 000 yr by a conservative estimate 175 km3 of nepheline normative trachyandesitic pyroclastic material equivalent to about 50 km3 of dense rock was erupted in 24 h major activity began with a brief plinian phase followed by pyroclastic flows much of the ignimbrite that was produced shows evidence of deposition by turbulent flows a 6 7 km caldera about 1 km deep formed at the end of the pyroclastic flow phase caldera volume closely matches that of magma ejected plinian and co ignimbrite ash fall 1 cm thick covered 500 000 km2 of the java sea and surrounding islands accounting for more than two thirds of the magma volume erupted this distal tephra is known only from 1815 reports indicating that some great eruptions especially those from island volcanoes may be virtually undetectable in the geologic record without supporting information such as deep sea core data land based volume estimates may be insufficient to characterize eruption magnitude in many cases

simulating the dispersal of tephra from the 1991 pinatubo eruption implications for the formation of widespread ash layers abstract puff and hazmap two tephra dispersal models developed for volcanic hazard mitigation are used to simulate the climatic 1991 eruption of mt pinatubo puff simulations indicate that the majority of ash was advected away from the source at the level of the tropopause 17 km several eruptive pulses injected ash and so2 gas to higher altitudes 25 km but these pulses represent only a small fraction 1 of the total erupted material released during the simulation comparison with toms images of the so2 cloud after 71 and 93 h indicate that the so2 gas originated at an altitude of 25 km near the source and descended to an altitude of 22 km as the cloud moved across the indian ocean hazmap simulations indicate that the pinatubo tephra fall deposit in the south china sea was formed by an eruption cloud with the majority of the ash concentrated at a height of 16 18 km results of this study demonstrate that the largest concentration of distal ash was transported at a level significantly below the maximum eruption column height 40 km and at a level below the calculated height of neutral buoyancy 25 km simulations showed that distal ash transport was dominated by atmospheric circulation patterns near the regional tropopause in contrast the movement of the so2 cloud occurred at higher levels along slightly different trajectories and may have resulted from gas particle segregations that took place during intrusion of the pinatubo umbrella cloud as it moved away from source

injection transport and deposition of tephra during event 5 at redoubt volcano 23 march 2009 abstract among the events of the 2009 eruption at redoubt volcano alaska event 5 was the best documented by radar satellite imagery and deposit mapping we use the new eulerian tephra transport model ash3d to simulate transport and deposition of event 5 tephra at distances up to 350 km the eruption which started at about 1230 utc on 23 march 2009 sent a plume from the vent elevation estimated at 2 3 0 1 km above sea level or a s l to about 16 2 km above sea level in 5 min the plume was a few kilometers higher than would be expected for the estimated average mass eruption rate and atmospheric conditions possibly due to release of most of the eruptive mass in the first half of the 20 minute event the eruption injected tephra into a wind field of high shear with weak easterly winds below 3 km elevation strong southerly winds at 6 10 km and weak westerlies above 16 km model simulations in this wind field predicted development of a northward migrating inverted v shaped cloud with a southwest trending arm at a few kilometers elevation which was not visible in ir satellite images due to cloud cover and a southeast trending arm at 10 km elevation that was clearly visible simulations also predicted a deposit distribution that strongly depended on plume height a plume height below 15 km predicted ash deposits that were located west of those mapped whereas good agreement was reached with a modeled plume height of 15 18 km field sampling of the deposit found it to contain abundant tephra aggregates which accelerated the removal of tephra from the atmosphere we were able to reasonably approximate the effect of aggregation on the deposit mass distribution by two methods 1 adjusting the grain size distribution taking the erupted mass 2 isomass lines that was within a few tens of percent of mapped area however they under predicted deposit mass at very proximal 250 km locations modeled grain size distributions at sample locations are also generally coarser than observed the mismatch may result from a combination of limitations in field sampling approximations inherent in the model errors in the numerical wind field and aggregation of particles larger than 0 063 mm

satellite observations and interpretation of the 1991 mount pinatubo eruption plumes we demonstrate the use of hourly geostationary meteorological satellite gms and noaa polar orbiting advanced very high resolution radiometer avhrr satellite images of the june 1991 mount pinatubo volcanic eruption plumes in providing details of the timing changing eruptive style and umbrella plume movement and dynamics a chronology of explosive events which culminated in a 9 hour long climactic phase on june 15 has been determined from satellite data and favorably compared to ground based and other ancillary information maximum eruption column altitudes of 40 km and column top undercooling in excess of 55 c below the ambient atmosphere occurred during the climactic phase phases dominated by plinian versus coignimbrite eruptive plumes can be distinguished on the images analysis of the spreading umbrella plume during the climactic phase suggests that over the first 4 5 hours the plume spread laterally as a gravitational intrusion before becoming advected in the ambient winds we compare the rate of plume growth to a model of a radially symmetric continuously fed intrusion to describe the motion the rate of plume growth shows good agreement with this simple model for the first 4 5 hours from column top heights we determine eruption rates and obtain an independent estimate of the total volume of magma erupted from june 12 16 of 5 5 km3 dense rock equivalent dre in good agreement with an independent estimate of the total eruptive volume made from deposits on land and submarine ash fall of up to 5 3 km3 dre we also show that secondary explosions in the pinatubo ignimbrite deposits produced ash plumes reaching altitudes as high as 19 km in september 1991 this study indicates that realistic interpretation and monitoring of major eruption plumes can be accomplished by analysis of high temporal resolution satellite data

reconstruction of eruption column dynamics on the basis of grain size of tephra fall deposits 2 application to the pinatubo 1991 eruption the granulometric methods to reconstruct eruption column dynamics developed in paper 1 are applied to the tephra fall deposits of the climactic plinian phase of the 1991 eruption at pinatubo the tephra fall deposits are composed of two units layer c1 which corresponds to the first half of the climactic phase and layer c2 the second half the granulometric estimates of the expansion rate of the umbrella cloud for layers c1 and c2 are 7 1010 and 3 1010 m3 s respectively which agree with the observations of satellite images these estimates indicate that the magma discharge rate decreased from 9 108 to 3 108 kg s during the eruption the grain size distribution at the top of the eruption column is characterized by depletion of coarse clasts suggesting a distinct deceleration in the gas thrust region down to several tens of meters per second particularly during the second half of the climactic phase the total amount of layers c1 and c2 is estimated to be 3 1012 kg and the very fine particles which did not deposit in the accessible on land area may occupy up to 60 of the total ejecta the effective duration of the eruption estimated by the present methods a few hours or less is substantially shorter than a previous estimate from the real time observation based on infrasonic data 10 hours this discrepancy may be attributed to the overestimation of the magma discharge rate due to the entrainment of the ambient air at the cloud top and or to the underestimation of the total amount of the ejecta due to the effect of the contemporaneous plinian activity and generation of pyroclastic flows it is also suggested that the intensity of eruption was fluctuating with time so that the instantaneous observations such as cloud height in satellite images do not always show a quantitative agreement with the time averaged features predicted by the present methods

a three dimensional numerical simulation of spreading umbrella clouds 1 during explosive volcanic eruptions an eruption column buoyantly rises as a turbulent plume and an umbrella cloud spreads laterally as a gravity current at the neutral buoyancy level the source conditions of explosive eruptions such as mass discharge rates of magma at vents have been estimated from the field observations e g satellite images on the height of the eruption columns and the spreading rate of umbrella clouds on the basis of the one dimensional 1 d plume model and the gravity current model however these simplified models contain empirical constants entrainment coefficient of turbulent plume k and froude number of gravity current that should be justified we developed a time dependent three dimensional 3 d numerical model of eruption clouds to independently determine the values of these constants the 3 d model is designed to calculate quantitative features of turbulent mixing between an eruption cloud and the ambient air without any a priori empirical constants by applying a sufficiently fine grid size with a third order accuracy scheme it has reproduced the fundamental features of eruption clouds including eruption columns pyroclastic flows coignimbrite ash clouds and umbrella clouds the altitudes of the spreading umbrella clouds in the 3 d simulations are consistent with those of the neutral buoyancy level of eruption columns estimated by the 1 d plume model with the entrainment coefficient k 0 1 the relationship between the volumetric flow rate and the spreading rate of the umbrella cloud in the 3 d simulations is explained by the axisymmetrical gravity current model with 0 2 for eruption clouds in the tropical atmosphere and 0 1 for those in the midlatitude atmosphere on the other hand the total column height in the 3 d simulations strongly oscillates even for a constant mass discharge rate and its time average tends to be substantially greater than the column height estimated by the 1 d plume model with k 0 1 particularly for large scale eruption clouds in the midlatitude atmosphere we recommend a method to estimate the mass discharge rate from the observation of the column height or the spreading rate of umbrella cloud the method and the accuracy of the 3 d simulations are tested on the basis of the field data e g the total height of the eruption column the altitude and the spreading rate of the umbrella cloud and the mass discharge rate of the pinatubo 1991 eruption

crystal scale anatomy of a dying supervolcano an isotope and geochronology study of individual phenocrysts from voluminous rhyolites of the yellowstone caldera a voluminous 600 km3 and long lived 520 75 ka phase of rhyolitic eruptions followed collapse of the yellowstone caldera 640 ka whether these eruptions represent a dying cycle or the growth of a new magma chamber remains an important question we use new u th zircon ages and 18o values determined by ion microprobe and sanidine pb isotope ratios determined by laser ablation to investigate the genesis of voluminous post caldera rhyolites the oldest post caldera rhyolites erupted between 520 and 470 ka exhibit extreme age and oxygen isotopic heterogeneity requiring derivation from individual parcels of low 18o melts we find a progressive increase in zircon homogeneity for rhyolite eruptions from 260 to 75 ka with homogeneous low 18o zircon values of 2 7 2 8 that are in equilibrium with low 18o host melts for the majority of the youngest eruptions new sanidine pb isotope data define separate arrays for post caldera rhyolites and the caldera forming tuffs that preceded them indicating that they were not sourced from a mushy lava creek tuff batholith that remained after caldera collapse rather our new age and isotopic data indicate that the post caldera rhyolites were generated by remelting of a variety of intracaldera source rocks consisting of pre lava creek tuff volcanic and plutonic rocks and earlier erupted post lava creek tuff rhyolites batch assembly of low 18o melts starting at 260 ka resulted in progressive homogenization followed by differentiation and cooling up until the last rhyolite eruption 75 ka a trend that we interpret to be characteristic of a dying magma reservoir beneath the yellowstone caldera

the 26 5 ka oruanui eruption new zealand an introduction and overview abstract the 26 5 ka oruanui eruption from taupo volcano in the central north island of new zealand is the largest known wet eruption generating 430 km3 of fall deposits 320 km3 of pyroclastic density current pdc deposits mostly ignimbrite and 420 km3 of primary intracaldera material equivalent to 530 km3 of magma erupted magma is 99 rhyolite and 90 pdc deposits range from mm to cm thick ultra thin veneers enclosed within fall material to 200 m thick ignimbrite in proximal areas the farthest travelled 90 km most energetic pdcs velocities 100 m s 1 occurred during phase 8 but the most voluminous pdc deposits were emplaced during phase 10 grain size variations in the pdc deposits are complex with changes seen vertically in thick proximal accumulations being greater than those seen laterally from near source to most distal deposits modern lake taupo partly infills the caldera generated during this eruption a 140 km2 structural collapse area is concealed beneath the lake while the lake outline reflects coeval peripheral and volcano tectonic collapse early eruption phases saw shifting vent positions development of the caldera to its maximum extent indicated by lithic lag breccias occurred during phase 10 the oruanui eruption shows many unusual features its episodic nature wide range of depositional conditions in fall deposits of very wide dispersal and complex interplay of fall and pdc activity

the respiratory health hazards of volcanic ash a review for volcanic risk mitigation studies of the respiratory health effects of different types of volcanic ash have been undertaken only in the last 40 years and mostly since the eruption of mt st helens in 1980 this review of all published clinical epidemiological and toxicological studies and other work known to the authors up to and including 2005 highlights the sparseness of studies on acute health effects after eruptions and the complexity of evaluating the long term health risk silicosis non specific pneumoconiosis and chronic obstructive pulmonary disease in populations from prolonged exposure to ash due to persistent eruptive activity the acute and chronic health effects of volcanic ash depend upon particle size particularly the proportion of respirable sized material mineralogical composition including the crystalline silica content and the physico chemical properties of the surfaces of the ash particles all of which vary between volcanoes and even eruptions of the same volcano but adequate information on these key characteristics is not reported for most eruptions the incidence of acute respiratory symptoms e g asthma bronchitis varies greatly after ashfalls from very few if any reported cases to population outbreaks of asthma the studies are inadequate for excluding increases in acute respiratory mortality after eruptions individuals with pre existing lung disease including asthma can be at increased risk of their symptoms being exacerbated after falls of fine ash a comprehensive risk assessment including toxicological studies to determine the long term risk of silicosis from chronic exposure to volcanic ash has been undertaken only in the eruptions of mt st helens 1980 usa and soufriere hills montserrat 1995 onwards in the soufriere hills eruption a long term silicosis hazard has been identified and sufficient exposure and toxicological information obtained to make a probabilistic risk assessment for the development of silicosis in outdoor workers and the general population a more systematic approach to multi disciplinary studies in future eruptions is recommended including establishing an archive of ash samples and a website containing health advice for the public together with scientific and medical study guidelines for volcanologists and health care workers