measuring volcanic plume and ash properties from space the remote sensing of volcanic ash plumes from space can provide a warning of an aviation hazard and knowledge on eruption processes and radiative effects in this paper new algorithms are presented to provide volcanic plume properties from measurements by the michelson interferometer for passive atmospheric sounding mipas the advanced along track scanning radiometer aatsr and spinning enhanced visible and infrared imager seviri a challenge of remote sensing is to provide near real time methods to identify and so warn of the presence of volcanic ash to achieve this a singular vector decomposition method has been developed for the mipas instrument on board the environmental satellite envisat this method has been applied to observations of the ash clouds from the eruptions of nabro and the puyehue cordon caulle in 2011 and lead to a more sensitive volcanic signal flag which was capable of tracking changes in the volcanic signal spectra as the plume evolved a second challenge for remote sensing is to identify the ash plume height this is a critical parameter for the initialisation of algorithms that numerically model the evolution and transport of a volcanic plume as mipas is a limb sounder the identification of ash also provides an estimate of height provided the plume is above about 6 km this is complemented by a new algorithm stereo ash plume height retrieval algorithm saphra that identifies plume height using the parallax between images provided by along track scanning radiometer type instruments the algorithm was tested on an image taken at 14 01 gmt on 6 june 2011 of the puyehue cordon caulle eruption plume and gives a height of 11 9 1 4 km which agreed with the value derived from the location of the plume shadow 12 7 1 8 km this plume height was similar to the height observed by mipas 12 1 5 km at 02 56 gmt on 6 june the quantitative use of satellite imagery and the full exploitation of high resolution spectral measurements of ash depends upon knowing the optical properties of the observed ash laboratory measurements of ash from the 1993 eruption of mt aso japan have been used to determine the refractive indices from 1 to 20 m these preliminary measurements have spectral features similar to ash values that have been used to date albeit with slightly different positions and strengths of the absorption bands the refractive indices have been used to retrieve ash properties plume height optical depth and ash effective radius from aatsr and seviri instruments using two versions of oxford ral retrieval of aerosol and cloud orac algorithm for aatsr a new ash cloud type was used in orac for the analysis of the plume from the 2011 eyjafjallajokull eruption for the first 500 km of the plume orac gave values of plume height 2 5 6 5 km optical depth 1 2 5 and effective radius 3 7 m which are in agreement with other observations a weakness of the algorithm occurs when underlying cloud invalidates the

patterns of deposition from experimental turbidity currents with reversing buoyancy turbidity currents are turbulent sediment laden gravity currents which can be generated in relatively shallow shelf settings and travel downslope before spreading out across deep water abyssal plains because of the natural stratification of the oceans and or fresh water river inputs to the source area the interstitial fluid within which the particles are suspended will often be less dense than the deep water ambient fluid consequently a turbidity current may initially be denser than the ambient sea water and propagate as a ground hugging flow but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient sea water when this occurs all or part of the turbidity current lofts to form a buoyant sediment laden cloud from which further deposition occurs deposition from such lofting turbidity currents containing a mixture of fine and coarse sediment suspended in light interstitial fluid is explored through analogue laboratory experiments complemented by theoretical analysis using a box and cloud model particular attention is paid to the overall deposit geometry and to the distributions of fine and coarse material within the deposit a range of beds can be deposited by bimodal lofting turbidity currents lofting may encourage the formation of tabular beds with a rapid pinch out rather than the gradually tapering beds more typical of waning turbidity currents lofting may also decouple the fates of the finer and coarser sediment depending on the initial flow composition the coarse fraction can be deposited prior to or during buoyancy reversal while the fine fraction can be swept upwards and away by the lofting cloud an important feature of the results is the non uniqueness of the deposit architecture different initial current compositions can generate deposits with very similar bed profiles and grading characteristics highlighting the difficulty of reconstructing the nature of the parent flow from field data it is proposed that deposit emplacement by lofting turbidity currents is common in the geological record and may explain a range of features observed in deep water massive sands thinly bedded turbidite sequences and linked debrites depending on the parent flow and its subsequent development for example a lofting flow may lead to a well sorted largely ungraded or weakly graded bed if the fines are transported away by the cloud however a poorly sorted largely ungraded region may form if during buoyancy reversal high local concentrations and associated hindered settling effects develop at the base of the cloud

hydrometeor enhanced tephra sedimentation constraints from the 18 may 1980 eruption of mount st helens 1 uncertainty remains on the origin of distal mass deposition maxima observed in many recent tephra fall deposits in this study the link between ash aggregation and the formation of distal mass deposition maxima is investigated through reanalysis of tephra fallout from the mount st helens 18 may 1980 msh80 eruption in addition we collate all the data needed to model distal ash sedimentation from the msh80 eruption cloud four particle size subpopulations were present in distal fallout with modes at 2 2 4 2 5 9 and 8 3 settling rates of the coarsest subpopulation closely matched predicted single particle terminal fall velocities sedimentation of particles 100 m was greatly enhanced predominantly through aggregation of a particle subpopulation with modal diameter 5 9 0 2 19 3 m mammatus on the msh80 cloud provided a mechanism to transport very fine ash particles with predicted atmospheric lifetimes of days to weeks from the upper troposphere to the surface in a matter of hours in this mechanism ash particles initiate ice hydrometeor formation high in the troposphere subsequently the volcanic cloud rapidly subsides as mammatus develop from increased particle loading and cloud base sublimation rapid fallout occurs as the cloud passes through the melting level in a process analogous to snowflake aggregation aggregates sediment en masse and form the distal mass deposition maxima observed in many recent volcanic ash fall deposits this work provides a data resource that will facilitate tephra sedimentation modeling and allow model intercomparisons

mount st helens 1980 and mount pelee 1902 flow or surge the far reaching directed blast of mount st helens and the devastating nuees ardentes of mount pelee produced deposits that in many places consist of three main layers a basal gravelly or sandy layer 1 a massive or bedded ash layer 2 and a capping ash cloud layer 3 these thin and in part landscape mantling deposits are generally ascribed to pyroclastic surges and the dune bedding seen in parts of layer 2 has reinforced this interpretation there are several reasons however for preferring a pyroclastic flow origin 1 much of layer 2 is unequivocal pyroclastic flow 2 the tripartite subdivision and landscape mantling habit is similar to that displayed by ignimbrites of the low aspect ratio type 3 the deflation of a pyroclastic surge and the subsequent deposition of particulate material will tend not to conserve the fine ash and dust that are abundant in valley ponded parts of layer 2 and cannot explain the observed tendency for layer 2 to show stronger fines depletion on ridges than in valley bottoms and 4 the coarseness and the variance of grain size found within dune bedded bed sets in mount st helens layer 2 are like those of very weak pyroclastic surges and the dune bedding most likely resulted from local minor turbulence as could have been caused by surface roughness in a mountainous terrain littered with tree stumps and fallen trees in a thin depositing pyroclastic flow the directed blast and nuees ardentes at mount st helens and mount pelee are interpreted to have been violently emplaced pyroclastic flows producing deposits of low aspect ratio type the characteristics of which stem from an exceptionally high flow velocity

impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 may 1980 mount st helens fallout deposit the 18 may 1980 eruption of mount st helens started with a lateral blast that fed a pyroclastic surge which then uplifted to form a co blast plume thirty minutes later plinian activity started at the vent and fluctuated in intensity for 9 h the resulting fallout deposit documented to 600 km from vent presents some striking features 1 displacement of the overall deposit to the north of the vent 2 a secondary thickness and mass maximum at 300 km from vent 3 a total grain size distribution dominated by fine ash 62 wt of the deposit 63 m and 4 individual grain size distributions that vary dramatically in the crosswind direction from strongly bimodal in the south to skewed unimodal in the north results from a new deconvolution of the individual grain size distributions show that they are a combination of a coarse subpopulation that decreases in size with distance from vent and a constant fine subpopulation with a mean of 15 m relative proportions of each subpopulation vary asymmetrically in the crosswind directions with the fine subpopulation preponderant toward the north and the coarse one dominating the south of the deposit both reach their absolute maxima in mass on the deposit axis componentry analyses of selected samples show that blast derived material is greatly enriched toward the north of the deposit these results indicate that the co blast plume dispersed fine grained material over great distances and dominated the fine subpopulation comparison with reanalysis data of atmospheric wind fields and satellite images of the spreading ash cloud suggests contrasting ash transport and depositional processes for the early co blast plume and the later vent derived plinian plumes the co blast plume is displaced to the north it had a high overshoot height and eastward dispersion via strong winds low in the stratosphere 10 15 km the plinian plumes were lower and dispersed most of the material to the southeast as the direction of high velocity winds shifted just before the late climactic plinian eruptive phase fine ash fine subpopulation was deposited continuously throughout the deposit with an increase of sedimentation rate 300 km from the vent where there is a secondary maximum in the deposit mass and thickness fine ash probably settled by a combination of enhanced sedimentation mechanisms including not only aggregation but also gravitational convective instabilities of cloud base hydrometeor formation and destruction and entrainment of small particles by larger ones finally we show that half of the deposit by mass in the medial area was deposited by the co blast plume and that a significant proportion of the mount st helens fallout deposit is nonjuvenile which has implications for the magmatic budget of this eruption

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

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

magmatic model for the mount st helens blast of may 18 1980 analytical and numerical solutions to the hydrodynamic equations of motion constrained by physical properties of juvenile ejecta in the mount st helens blast deposit were used to investigate magmatic conditions required to produce the initial devastating blast phase of the eruption of may 18 1980 evidence that the blast was magmatic includes equivalence in volume of juvenile blast ejecta to preemption inflation of the cone substantial vesicularity of this ejecta and continued vesiculation of large juvenile clasts after eruption observed or inferred ejecta velocities of 100 to 250 m s are shown to require 0 2 to 0 7 wt water vapor preexisting in magma unloaded by a landslide 200 to 900 m thick these conditions imply total magmatic water contents of 0 7 to 1 7 wt respectively such low required water content suggests that volcanic blasts may be regarded as a normal consequence of magma intrusion into an unstable edifice

a seismically constrained mass discharge rate for the initiation of the may 18 1980 mount st helens eruption we calculate the vertical mass discharge rate from mount st helens for the first few minutes of the may 18 1980 cataclysmic eruption using a new method based on seismic constraints the observed seismic waves indicate that the seismic source is a series of single forces we model these forces as thrusts due to a combination of the momentum flux of the erupted products and the pressure of the eruptive jet the momentum discharge rate is converted to a mass discharge rate based on estimates of the velocity and jet pressure as constrained by a simple fluid dynamical model only two parameters are necessary for the calculation mach number and sound velocity the calculated mass ejected in the first 100 s is 1 6 10 11 4 6 10 11 kg since the total blast deposit is 3 2 10 11 4 1 10 11 kg one possible interpretation is that the directed blast had a significant 30 vertical component

blast ashfall deposit of may 18 1980 at mount st helens washington abstract the ashfall deposit produced by the major directed blast at mount st helens on the morning of may 18 1980 is a thin 65 mm small volume 0 03 km 3 dense rock equivalent fine grained crystal lithic ash with a large dispersal area 10 4 km 2 the blast ashfall deposit is characterized by accretionary lapilli and smaller ash aggregates within 50 km of the volcano the maximum deposit thickness finest median grain size most efficient sorting and largest and most abundant accretionary lapilli are approximately coincident and located 17 km north of the volcano adjacent to a major reentrant in the northern margin of the devastated zone it is suggested that processes of flow convergence and arrested dispersal in the blast due to topographic barriers led to higher suspended ash concentrations in that area resulting in increased aggregation and fallout of fine grained ash the most distinctive characteristic of the blast ashfall that indicates an origin from a directed blast is its combined features of chiefly dense lithic and crystal pyroclasts in a widely dispersed fine grained deposit the common presence of shredded vegetation is a further indication of a directed blast

on the formation of eruption columns following explosive mixing of magma and surface water when magma vents into the sea or a crater lake the ensuing magma water interaction can affect the style of eruption dramatically if the mass of surface water incorporated into the erupting material is small 108 kg s the height progressively decreases with surface water content this occurs when the magma and surface water begin to constitute a significant fraction of the mass at the top of the column so that an increasing fraction of the initial magmatic thermal energy is converted to the surface water rather than the entrained air the transitions in eruption style which result from changes in the mass of surface water mixing with the magma may account for observations of both buoyant plumes and wet surge during the eruptions of taal in 1965 and miyake jima in 1983 and for the changes in the eruptive activity at surtsey in 1963 1964 as the access of seawater to the vent became more restricted we also present calculations which suggest that the accretionary lapilli which are often found in wet flow deposits may result from condensation of vapor in both the cold wet collapsing fountains and in the flows themselves

the variation of large magnitude volcanic ash cloud formation with source latitude 1 very large magnitudes of explosive volcanic eruptions can produce giant ash clouds with diameters of hundreds to thousands of kilometers these ash clouds are controlled by gravity and rotational forces leading to a more radially constrained shape than clouds produced by smaller eruptions here we develop a dynamic model of the formation of large ash clouds that are produced by eruptions of constant intensity and finite duration incorporating source latitude eruption type magnitude and intensity the cloud grows as a stratified intrusion in the stratosphere to an equilibrium shape that approximates an ellipsoid of revolution rotating anticyclonically as a solid body at sufficient large distances from the equator more generally the structure of the cloud is determined by the source latitude s and the parameter ys ys nd0 1 2 wherein ys is the distance of the source from the equator is the north south gradient of the coriolis frequency n is the buoyancy frequency of the stratosphere and d0 is the maximum cloud thickness a steady solution for an equilibrium cloud exists if ys lies above a boundary that ranges from 2 at the equator to 0 at the pole these clouds move westward at an increasing rate with decreasing latitude below this boundary steady solutions appear not to exist and the nature of the breakdown of the solution at the boundary suggests that the cloud or part of it moves toward and across the equator the above parameters may be expressed in terms of latitude and cloud volume which enables the model to be applied to the ash clouds of past large magnitude eruptions the results suggest that the behavior of clouds formed from plinian phases with eruption magnitudes m 6 5 they could achieve interhemispheric transport from most latitudes for clouds from co ignimbrite sources for m 8 it is possible from most latitudes

directed blasts and blast generated pyroclastic density currents a comparison of the bezymianny 1956 mount st helens 1980 and soufriere hills montserrat 1997 eruptions and deposits we compare eruptive dynamics effects and deposits of the bezymianny 1956 bz mount st helens 1980 msh and soufriere hills volcano montserrat 1997 shv eruptions the key events of which included powerful directed blasts each blast subsequently generated a high energy stratified pyroclastic density current pdc with a high speed at onset the blasts were triggered by rapid unloading of an extruding or intruding shallow magma body lava dome and or cryptodome of andesitic or dacitic composition the unloading was caused by sector failures of the volcanic edifices with respective volumes for bz msh and shv c 0 5 2 5 and 0 05 km3 the blasts devastated approximately elliptical areas axial directions of which coincided with the directions of sector failures we separate the transient directed blast phenomenon into three main parts the burst phase the collapse phase and the pdc phase in the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into the atmosphere with much of the expansion having an initially lateral component the erupted material fails to mix with sufficient air to form a buoyant column but in the collapse phase falls beyond the source as an inclined fountain and thereafter generates a pdc moving parallel to the ground surface it is possible for the burst phase to comprise an overpressured jet which requires injection of momentum from an orifice however some exploding sources may have different geometry and a jet is not necessarily formed a major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be attributed to shock waves within an overpressured jet or alternatively to dynamic pressures and shocks within the energetic collapse and pdc phases internal shock structures related to unsteady flow and compressibility effects can occur in each phase we withhold judgment about published shock models as a primary explanation for the damage sustained at msh until modern 3d numerical modeling is accomplished but argue that much of the damage observed in directed blasts can be reasonably interpreted to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified pdc this view is reinforced by recent modeling cited for shv in distal and peripheral regions solids concentration maximum particle size current speed and dynamic pressure are diminished resulting in lesser damage and enhanced influence by local topography on the pdc despite the different scales of the blasts devastated areas were respectively 500 600 and 10 km2 for bz msh and shv and some complexity involving retrogressive slide blocks and clusters of explosions their pyroclastic deposits demonstrate strong similarity juvenile material composes 50 of the deposits implying for the blasts a dominantly magmatic mechanism although hydrothermal explosions also occurred the character of the magma fragmented by explosions highly viscous phenocryst rich variable microlite content determined the bimodal distributions of juvenile clast density and vesicularity thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater and laterally from the axial region the proximal stratigraphy of the blast deposits comprises four layers named a b c d from bottom to top layer a is represented by very poorly sorted debris with admixtures of vegetation and soil with a strongly erosive ground contact its appearance varies at different sites due to different ground conditions at the time of the blasts the layer reflects intense turbulent boundary shear between the basal part of the energetic head of the pdc and the substrate layer b exhibits relatively well sorted fines depleted debris with some charred plant fragments its deposition occurred by rapid suspension sedimentation in rapidly waning high concentration conditions layer c is mainly a poorly sorted massive layer enriched by fines with its uppermost part laminated created by rapid sedimentation under moderate concentration weakly tractive conditions with the uppermost laminated part reflecting a dilute depositional regime with grain by grain traction deposition by analogy to laboratory experiments mixing at the flow head of the pdc created a turbulent dilute wake above the body of a gravity current with layer b deposited by the flow body and layer c by the wake the uppermost layer d of fines and accretionary lapilli is an ash fallout deposit of the finest particles from the high rising buoyant thermal plume derived from the sediment depleted pyroclastic density current the strong similarity among these eruptions and their deposits suggests that these cases represent similar source transport and depositional phenomena

extensive volcaniclastic deposits at the mid atlantic ridge axis results of deep water basaltic explosive volcanic activity a recent mapping survey followed by submersible diving investigations revealed the existence of extensive volcaniclastic deposits on three adjoining progressively deeper segments of the mid atlantic ridge south west of the azores the maximum water depth at which subaqueous magmatic and hydro magmatic explosive processes can occur is controversial this discovery provides fundamental constraints on the dynamics of deep subaqueous volcanic eruptions using multibeam bathymetry sonar image interpretations submersible observations volcaniclastic sample descriptions and chemical analysis evidence of extensive deep water up to 1700 m magmatic explosive activity of basalt at the axis of a spreading mid ocean ridge is documented for the first time

delayed brittle like fragmentation of vesicular magma analogue by decompression abstract a rapid decompression experiment using syrup containing gas bubbles was conducted in order to clarify the fragmentation of vesicular magma which is a key phenomenon in volcanic eruptions we focus on brittle like fragmentation which occurs with a longer time scale than brittle fragmentation the response of the bubbly syrup to the decompression for various viscosities and porosities was tested under various initial pressures pressure differences and decompression times the response observed by high speed photography was classified using the deborah number de dt which is defined as the ratio of the viscoelastic relaxation time of the syrup to the decompression time the maximum differential stress at the bubble surface max and a measure of instantaneous brittleness at the bubble surface c at the time when the differential stress at the surface reaches the critical fracture stress c in a number of the experimental runs in which both de dt and c indicated ductile response of the material brittle like fragmentation occurred when max substantially exceeded c for all of the runs in which brittle like fragmentation was observed the onset of fragmentation was delayed substantially from the relaxation time whereas the onset occurred within the characteristic time for viscous expansion of the bubbles in many cases of brittle like fragmentation fragmentation continued after the onset triggered by a single decompression event detailed analysis indicates that brittle like fragmentation started with a sudden release of the gas in the bubbly syrup through a surface crack which might lead to an increase in local brittleness magma fragmentation may be viewed as sequential brittle like fragmentation this view may provide an explanation for the observed time delay for the onset of an explosive eruption after a triggering decompression event

dynamics of strombolian ash plumes from thermal video motion morphology and air entrainment 1 imaging volcanic plumes is essential to provide an observational basis for understanding and modeling plume dynamics during june july 2004 150 strombolian ash plumes were imaged at stromboli volcano italy with a forward looking infrared radiometer flir thermal video camera 30 hz of these 25 80 plumes were suited for different levels of quantitative analyses in this study some simple analyses are applied to constrain basic parameters for the dynamics of strombolian plumes during their initial ascent 130 m plume rise rates covered both gas thrust 15 m s 1 and buoyant regimes 15 m s 1 which in turn controlled lateral spreading rates and air entrainment rates the half angle of lateral spreading of the plume front averaged 7 3 1 6 for gas thrust regimes and 13 5 1 6 for buoyant regimes equating to mean air entrainment coefficients of 0 06 0 12 for gas thrust regimes and 0 22 0 03 for buoyant regimes these factors were also linked to plume morphologies which included jets starting plumes and thermals a rooted thermal form was observed and presumed as an intermediary between starting plumes and discrete thermals plume rise could be approximated by a power law dependence with time rooted thermals spread and entrained air at rates approaching those of a discrete thermal but rose at a rate similar to that of a starting plume phenomena including helical motion and sedimentation were visible in the flir imagery these results demonstrate that emergent plume behavior is progressive and highly transient furthermore this study offers empirical reinforcement that entrainment dynamics are intrinsically different in 1 gas thrust versus buoyantly driven regimes and 2 plume fronts versus steady plumes

was the 18 may 1980 lateral blast at mt st helens the product of two explosions the 18 may 1980 lateral blast at mt st helens has been interpreted as the product of a single explosion by some stratigraphers and as two closely spaced explosions by others the stratigraphic evidence that bears on this question is inconclusive strata change dramatically over short distances and this complexity provides wide latitude for interpretation some independent non stratigraphic evidence however suggests that the blast was the product of two explosions or clusters of explosions the independent evidence comes from eyewitness accounts and photographs from satellite sensors and from seismic records this paper reviews the pertinent evidence offers a new interpretation and concludes that the blast was indeed the product of two explosions or clusters of explosions

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

improved prediction and tracking of volcanic ash clouds abstract during the past 30 years more than 100 airplanes have inadvertently flown through clouds of volcanic ash from erupting volcanoes such encounters have caused millions of dollars in damage to the aircraft and have endangered the lives of tens of thousands of passengers in a few severe cases total engine failure resulted when ash was ingested into turbines and coating turbine blades these incidents have prompted the establishment of cooperative efforts by the international civil aviation organization and the volcanological community to provide rapid notification of eruptive activity and to monitor and forecast the trajectories of ash clouds so that they can be avoided by air traffic ash cloud properties such as plume height ash concentration and three dimensional ash distribution have been monitored through non conventional remote sensing techniques that are under active development forecasting the trajectories of ash clouds has required the development of volcanic ash transport and dispersion models that can calculate the path of an ash cloud over the scale of a continent or a hemisphere volcanological inputs to these models such as plume height mass eruption rate eruption duration ash distribution with altitude and grain size distribution must be assigned in real time during an event often with limited observations databases and protocols are currently being developed that allow for rapid assignment of such source parameters in this paper we summarize how an interdisciplinary working group on eruption source parameters has been instigating research to improve upon the current understanding of volcanic ash cloud characterization and predictions improved predictions of ash cloud movement and air fall will aid in making better hazard assessments for aviation and for public health and air quality

magma fragmentation speed an experimental determination abstract the propagation speed of a fragmentation front combined with the ascent velocity of magma is in all likelihood a controlling factor in the dynamics of explosive volcanic eruptions direct measurement of the fragmentation speed in natural systems appears to be impossible at present fortunately laboratory experiments can provide information on the propagation speed of the fragmentation front here we present the results of fragmentation speed determinations using a so called fragmentation bomb these are to the best of our knowledge the first in situ fragmentation speed determinations performed on magma natural magma samples merapi basaltic andesite mount st helens dacite and unzen dacite have been investigated in the temperature range of 20 950 c and at pressures up to 25 mpa two techniques have been employed firstly in experiments at 20 c dynamic pressure transducers were placed above and below the magma samples and the fragmentation speed of the magma sample was derived from an analysis of the decompression curves secondly at elevated temperatures an alternative technique was introduced and successfully employed this involved the severing via fragmentation of conducting wires placed within the samples at various heights fragmentation speeds are very low falling in the range of 2 70 m s and increasing with an increase in the magnitude of the decompression step responsible for the fragmentation the first high temperature determination seems consistent with low temperature results implications for explosive volcanism are discussed briefly

a model for the mechanism of the may 18 1980 mount st helens blast abstract a model is suggested for the mechanism of the blast that accompanied the may 18 1980 mount st helens eruption the model explains the large duration of the blast process and accounts for several explosive episodes recognized during the blast the mechanism is based on the idea of destruction of the near solidus porous cryptodome by a fragmentation wave termed the f wave originating from cryptodome decompression induced by a landslide the f wave propagates from a free surface to the interior of the cryptodome at velocity n the products of cryptodome fragmentation a gas pyroclast mixture are ejected into the atmosphere at velocity u calculations show that for pressure of gas in the cryptodome pores of 20 mpa and porosity 0 4 0 7 the f wave travels at 5 m s and velocity of pyroclast ejection is 100 250 m s for a cryptodome with diameter of 700 m the duration of blast fragmentation is estimated at 140 s energy released during the blast is estimated as 4 8 7 2 1015 j blast power is 3 4 5 1 1013 wt gas pyroclastic mixture mass flux is 5 0 7 5 103kg m2 s the maximum discharge is 1 9 2 9 sx 109 kg s and the weight fraction of water vapor in the cryptodome is 0 006 0 04 several explosive episodes during the blast are accounted for by fragmentation wave stoppage due to blockage of the exploding cryptodome by landslide material

the 1982 eruptions of el chichon volcano mexico 2 observations and numerical modelling of tephra fall distribution tephra fallout from the a 1 march 29 0532 ut b april 4 0135 ut and c april 4 1122 ut 1982 explosive eruptions of el chichon produced three tephra fall deposits over southeastern mexico bidirectional spreading of eruption plumes as documented by satellite images was due to a combination of tropospheric and stratospheric transport with heaviest deposition of tephra from the ene tropospheric lobes maximum column heights for the eruptions of 27 32 and 29 km respectively have been determined by comparing maximum lithic clast dispersal in the deposits with predicted lithic isopleths based on a theoretical model of pyroclast fallout from eruption columns these column heights suggest peak mass eruption rates of 1 1 108 1 9 108 and 1 3 108 kg s maximum column heights and mass eruption rates occured early in each event based on the normal size grading of the fall deposits sequential satellite images of plume transport and the production of a large stratospheric aerosol plume indicate that the eruption columns were sustained at stratospheric altitudes for a significant portion of their duration new estimates of tephra fall volume based on integration of isopach area and thickness yield a total volume of 2 19 km3 1 09 km3 dre dense rock equivalent or roughly twice the amount of the deposit mapped on the ground up to one half of the erupted mass was therefore deposited elsewhere as highly dispersed tephra

the 16 september 1986 eruption of lascar volcano north chile satellite investigations landsat thematic mapper tm images acquired in 1984 and 1985 revealed a pronounced thermal anomaly on lascar volcano north chile subsequent images showed that the anomaly was persistent but variable and that after a significant eruption on 16 september 1986 it was weaker and divided into several sources tm studies and other observations of lascar indicate that the persistent thermal anomaly may be due to high temperature fumaroles within the summit crater goes weather satellite images and field investigations confirm that the 16 september event was a short lived vulcanian type eruption which produced an ash column that reached 15 km altitude the ash cloud can be tracked on geos images to about 400 km downwind and covered an area 112 000 km2 transport occurred in the upper troposphere at speeds up to 180 km h ash fall from the plume was well sorted and moderately fine grained md 200 m formation and fallout of ash as aggregates was unimportant in the deposition of the ash layer although small the lascar 16 september eruption is significant because few historic eruptions have been recorded in the central andes little would have been known of the eruption in the absence of remote sensed data

settling behaviour of concentrated dispersions and some volcanological applications abstract experiments have been carried out on the settling of non aggregating poorly sorted aqueous dispersions laced with lowdensity particles in order to gain insight into the sedimentation behaviour of volcaniclastic flows which contain pumice two poorly sorted dispersions of silicon carbide particles with continuous size distributions were studied plastic beads were added as a light component experiments were carried out at solids concentrations between 10 and 60 vol dispersions of low concentration generated normally graded deposits in which particles of different densities were in approximate hydraulic equivalence above a critical concentration 45 to 60 depending on the grain size distribution particle segregation was suppressed and the dispersion settled to form a virtually ungraded bed hindered settling at intermediate concentrations generated normally graded deposits with elutriation pipes floating rafts of plastic and capping layers of fines elutriated by escaping pore fluid buoyancy forces resulted in strong hydraulic inequivalence between light and dense components in the deposits from intermediate concentrations two mechanisms of elutriation pipe growth were observed at intermediate concentrations pipes grew progressively upwards in the deposit as the latter aggraded whereas at high concentrations they nucleated and grew uniformly throughout the whole dispersion elutriation pipes form readily during sedimentation of some poorly sorted aqueous dispersions the presence of elutriation pipes cannot be used alone to distinguish between deposits of epiclastic and pyroclastic flows reverse grading of pumice pumice rafts elutriation pipes and layers of elutriated fines should characterise deposits from pumice rich submarine mass flows despite differences in the fluid phase the experiments also have implications for fluidization mechanisms in pyroclastic flows hindered settling of a concentrated poorly sorted dispersion is a type of self fluidization in which the sedimentation of large particles causes a sufficiently strong vertical fluid flux to fluidize or elutriate smaller ones hindered settling is intrinsic to all pyroclastic flows as they spread out and deflate across the landscape and may reinforce other gas sources as an effective fluidization mechanism during emplacement of type 2 and type 3 ignimbrites and of coignimbrite lag breccias

volcanic eruption plume top topography and heights as determined from photoclinometric analysis of satellite data photoclinometry a shape from shading technique is applied to satellite data to determine the three dimensional height and morphology of the upper optical surface of a plume from the eruption of redoubt volcano alaska the technique can be applied to visible images of volcanic plumes that have uniform scattering properties and is most effective on images with moderate incident sun angles and plume transport perpendicular to the azimuth of the sun the two most significant sources of error are the finite resolution of the image data and the choice of image data number dn value for a flat plume element provided the plume spans at least 10 image pixels useful results can be obtained for the central portion of the plume the closest approximation to a flat plume element is found to be the visible dn value corresponding to the coldest pixel along a transect errors in the element altitude due to uncertainty in the dn value for a flat pixel amount to 10 20 m per pixel and are cumulative along the plume transect for redoubt this results in an uncertainty of approximately 300 m at the highest points the method indicates that the redoubt plume rose to an altitude of approximately 3 km while traveling a distance of 150 km from the vent and that the surface topography of the plume exhibits influences of the ambient wind through simple wave structures this technique will have a tremendous impact on studies of plume spreading dynamics and the time integrated evolution of plumes

pyroclastic density current from the 1888 phreatic eruption of bandai volcano ne japan abstract the great eruption of july 15 1888 at bandai volcano produced large phreatic explosions and a major debris avalanche toward the northern foot this eruption is characterized as follows 1 there was no significant precursor 2 the eruption and collapse of the volcanic edifice were triggered by an earthquake with a magnitude of about 5 3 the initial explosions were accompanied with a pyroclastic density current 4 the main explosive eruption quickly ceased a few minutes after the eruption began 5 a subsequent late stage plume rose buoyantly to a height of 5000 m falling with a heavy warm rain which generated a lahar and 6 ejected materials were made up wholly of pre existing rock fragments with no juvenile glass shards the main branch of the pyroclastic density current cascaded from the summit for a distance of 6 km along the biwazawa valley on the southeast flank the heim coefficient of the reconstructed energy line slope for the density current is 0 223 to 0 296 the estimated velocity using these values is less than 100 m s the density current deposit consists of normally graded parallel bedded or scour fill cross bedded thin layers of coarse ash and lapilli except for coarse massive breccia near the vent the sedimentary structures and grain size data suggest that the pyroclastic materials were transported in traction saltation and true suspension the estimated velocity is competent for transportation of the deposit the time sequence suggests that the seismic shocks and subsequent collapse of the edifice broke the confined hydrothermal system and caused the phreatic explosions the pressurized hydrothermal fluids had been presumably stored due to self sealing of the system because hydrothermal activities had been vigorous at the summit region until the july 15 seismic shocks collapse of the dry and dense initial eruption column generated the density current which spilled toward the opposite side of the crater along the topographic lows the change from the collapse to the late stage ascending plume results from a decrease in the rock fragment contents which made the plume light and wet this is because that a decrease in the fraction of rock fragments causes an increase in the fraction of steam produced within the plume the occurrence of the water flushed ash fall and lahar during the late stage eruption is consistent with this model

pyroclastic flows and surges over water an example from the 1883 krakatau eruption pyroclastic deposits from the 1883 eruption of krakatau are described from areas northeast of the volcano on the islands of sebesi sebuku and lagoendi and the southeast coast of sumatra massive and poorly stratified units formed predominantly from pyroclastic flows and surges that traveled over the sea for distances up to 80 km granulometric and lithologic characteristics of the deposits indicate that they represent the complement of proximal subaerial and submarine pyroclastic flow deposits laid down on and close to the krakatau islands the distal deposits exhibit a decrease in sorting coefficient median grain size and thickness with increasing distance from krakatau crystal fractionation is consistent with the distal facies being derived from the upper part of gravitationally segregated pyroclastic flows in which the relative amount of crystal enrichment and abundance of dense lithic clasts diminished upwards the deposits are correlated to a major pyroclastic flow phase that occurred on the morning of 27 august at approximately 10 a m energetic flows spread out away from the volcano at speeds in excess of 100 km h and traveled up to 80 km from source the flows retained temperatures high enough to burn victims on the sw coast of sumatra historical accounts from ships in the sunda straits constrain the area affected by the flows to a minimum of 4x103 km2 at the distal edge of this area the flows were relatively dilute and turbulent yet carried enough material to deposit several tens of centimeters of tephra the great mobility of the krakatau flows from the 10 a m activity may be the result of enhanced runout over the sea it is proposed that the generation of steam at the flow seawater interface may have led to a reduction in the sedimentation of particles and consequently a delay in the time before the flows ceased lateral motion and became buoyantly convective the buoyant distal edge of these ash and steam laden clouds lifted off into the atmosphere leading to cooling condensation and mud rain

simulation of the 1980 eruption of mount st helens using the ash tracking model puff abstract the dispersal of volcanic ash from the may 18 1980 eruption of mount st helens msh has been simulated using the lagrangian ash tracking model puff previous applications of the model were limited to smaller short lived eruptions with ash dispersal occurring mainly within the troposphere two high resolution atmospheric reanalysis datasets era 40 and ncep ncar 40 allowed msh ash cloud dispersal to be simulated up to 30 km elevation the 1980 eruption was divided into two distinct eruptive phases 1 an initial relatively short lived blast surge phase that injected ash up to 30 km and 2 a subsequent nine hour plinian phase that maintained an average eruption column height of 16 km using puff the two phases of the msh eruption were modeled separately based on a range of individual input parameters and then combined to produce an integrated simulation of the entire eruption the trajectory and areal extent of the modeled atmospheric ash cloud best match the actual distribution of msh ash when input parameters are set to values inferred from satellite and radar data collected on may 18 1980 the prevailing wind field exerts the strongest control on the advection and ultimate position of the modeled ash cloud making the maximum column height and the vertical distribution of ash the most sensitive of the puff input parameters for this event the results indicate that the puff model works well at simulating the dispersal of ash injected well into the lower stratosphere from a moderate relatively long lived eruption such as msh however attempts to use puff to recreate some granulometric aspects of the msh fallout deposit such as the maximum particle size as a function of distance from source were not successful puff consistently predicts much greater fallout distances for small ash particles

sediment laden gravity currents with reversing buoyancy there are many natural occurrences of sediment laden gravity currents in which the density of the interstitial fluid is less than that of the ambient fluid although the bulk density of the current is greater such currents are driven by the excess density of suspended particles however after sufficient particles have sedimented the current will become buoyant cease its lateral motion and ascend to form a plume examples of such currents include brackish underflows in deltas turbidity currents and pyroclastic flows experimental studies are described which show that due to sedimentation sediment laden gravity currents decelerate more rapidly than saline currents of the same density there is little difference in the experiments between a sediment laden current with neutrally buoyant interstitial fluid and one with buoyant interstial fluid until sufficient sediment has been lost to cause the latter kind of current to lift off a marked deceleration is then observed and a plume is generated with lift off occurring along the length of the current the resulting buoyant plume then generates a gravity current below the upper surface of the fluid in the tank the deposit from a current with buoyant fluid shows a fairly abrupt decrease in thickness beyond the lift off distance and has a flatter profile than that from a simple sediment current a theoretical model is presented which is based on the two layer shallow water equations and incorporates a model of the sedimentation in which particles are assumed to be uniformly suspended by the turbulence of the current the model shows good agreement with the observed lengths of the experimental currents as a function of time and predicts the lift off distance reasonably well these processes have implications for the behaviour of turbidity currents the interpretation of turbidites mixing processes in the oceans and the lift off of pyroclastic flows

a model for wet aggregation of ash particles in volcanic plumes and clouds 2 model application 1 the occurrence of particle aggregation has a dramatic effect on the transport and sedimentation of volcanic ash the aggregation process is complex and can occur under different conditions and in multiple regions of the plume and in the ash cloud in the companion paper costa et al develop an aggregation model based on a fractal relationship to describe the rate particles are incorporated into ash aggregates the model includes the effects of both magmatic and atmospheric water present in the volcanic cloud and demonstrates that the rate of aggregation depends on the characteristics of the initial particle size distribution the aggregation model includes two parameters the fractal exponent df which describes the efficiency of the aggregation process and the aggregate settling velocity correction factor e which influences the distance at which distal mass deposition maxima form both parameters are adjusted using features of the observed deposits here this aggregation model is implemented in the fall3d volcanic ash transport model and applied to the 18 may 1980 mount st helens and the 17 18 september 1992 crater peak eruptions for both eruptions the optimized values for df 2 96 3 00 and e 0 27 0 33 indicate that the ash aggregates had a bulk density of 700 800 kg m 3 the model provides a higher degree of agreement than previous fully empirical aggregation models and successfully reproduces the depositional characteristics of the deposits investigated over a large range of scales including the position and thickness of the secondary maxima

experiments on gas ash separation processes in volcanic umbrella plumes abstract we present a series of analogue laboratory experiments which simulate the separation of ash and gas and the formation of secondary intrusions from finite volcanic umbrella plumes we examined the lateral spreading of mixtures of freshwater and particles released into a laboratory tank containing a uniformly stratified aqueous solution for times smaller than the sedimentation time of particles through the intrusion the current remains coherent and intrudes laterally as some of the particles settle into the underlying ambient fluid a layer of particle depleted fluid develops below the upper surface of the current and the density of the residual fluid is reduced over longer times the intrusion ceases to be coherent with small fingers of relatively buoyant particle depleted fluid rising from the upper part of the intrusion into the overlying fluid meanwhile the lateral motion of the injected solution induces a return flow in the ambient fluid which sweeps some of the particles sedimenting from the lower surface of the intrusion inwards as a result relatively dense particle laden fluid collects below the intrusion and then sinks into the underlying fluid eventually this fluid reaches a new neutral buoyancy height where it intrudes to form a second laterally spreading current below the original intrusion the process then repeats to form further weaker intrusions below these results of the separation of the ash and volcanic gas in an umbrella plume are consistent with field observations at sakurajima volcano where positively charged plumes thought to consist of volcanic gas have been observed above negatively charged plumes of ash this work also suggests that volcanic aerosols may form up to a kilometer above the original injection height of the ash in a strong wind shear this could result in very different trajectories of the ash and gas and so be important for evaluating the impact of ash plumes on both aviation safety and volcanic aerosol formation the separation of the ash from the umbrella plume after a time of order 10 3 10 4 s corroborates recent models of the may 18 1980 eruption at mount st helens that after a few hours the motion of the ash was simply controlled by wind dispersal and gravitational settling the complex motion of the sedimenting ash may also explain some of the observations of multiple intrusions in short lived umbrella plumes such as observed at redoubt alaska in april 1990

interpretation of umbrella cloud growth and morphology implications for flow regimes of short lived and long lived eruptions new numerical and analytical modeling shows that the growth of a volcanic umbrella cloud expressed as the increase of radius with time proceeds through regimes dominated by different force balances four regimes are identified regime ia is the long time behavior of continuously supplied intrusions in the buoyancy inertial regime regime iia is the long time behavior of continuously supplied turbulent drag dominated intrusions regime ib is the long time behavior of buoyancy inertial intrusions of constant volume and regime iib that of turbulent drag dominated intrusions of constant volume power law exponents for spreading time in each regime are 3 4 ia 5 9 iia 1 3 ib and 2 9 iib both numerical modeling and observations indicate that transition periods between the regimes can be long lasting and during these transitions the spreading rate does not follow a simple power law predictions of the new model are consistent with satellite data from seven eruptions and together with observations of umbrella cloud structure and morphological evolution support the existence of multiple spreading regimes

volcano monitoring using short wavelength infrared data from satellites data from remote sensing satellites operating in the short wavelength infrared can be used to measure temperatures of about 150 c and above the gain settings of the landsat thematic mapper tm sensors are such that to record pixel integrated temperatures from 150 c to over 1000 c requires the use of several spectral bands if the radiant source occupies less than a whole pixel as is the case for fumaroles or a crusted lava surface with incandescent cracks then the pixel integrated temperature is less than the actual surface temperature of the hot areas and appears to be different at each wavelength by determining pixel integrated temperature in two spectral bands we can estimate both the temperature and size of hot areas which occupy less than a complete pixel in some cases our observations provide the only available data on the state of activity of a particular volcano in others they support and refine field observations our landsat tm data reveal otherwise unobserved precursor and subsequent activity related to the september 1986 eruption of lascar volcano chile and supplement other data on the march 1986 eruption of augustine volcano alaska they also indicate the continued presence of lava lakes at erta ale ethiopia in the absence of any ground based observations and document minor eruptive activity at mount erebus antarctica landsat multispectral scanner data have been used to measure the variation in temperature along a lava flow on sierra negra galapagos islands which occurred in 1979

chronology and pyroclastic stratigraphy of the may 18 1980 eruption of mount st helens washington many timed observations make it possible to subdivide the 9 hour plinian eruption of mount st helens on may 18 1980 into six phases defined by eruption style the phases are correlated with stratigraphic subunits of ashfall tephra and pyroclastic flow deposits the suite of pyroclastic deposits indicates that the eruption became more pumice rich and compositionally diverse with time perhaps owing to concurrent eruption of less evolved gas poor parts of the magma body with the more evolved gas rich parts the paroxysmal phase i 0832 0900 consisted of landslides lithic pyroclastic flows of a lateral blast and other explosions and a weak pre plinian column phase i pyroclastic deposits include lithic ash flow deposits intercalated with and overlying the voluminous debris avalanche deposit and basal pumice lapilli tephra that underlies a pisolitic ash layer the early plinian phase ii 0900 1215 consisted of vertical ejection of tephra with an early pulse of small pyroclastic flows on the upper flanks 1010 1035 a brief period of lithic ash ejection 1035 1100 and a pumice rich pulse that accompanied growth in height of the eruption column 1100 1215 deposits include minor pyroclastic flows on the crater rim and a reversely graded sequence of proximal tephra that include the lower pumice lapilli layer the lower lithic ash layer and the middle pumice lapilli layer all of which consist of evolved white dacitic pumice 63 64 sio2 during the early ash flow phase iii 1215 1500 the height of the eruption column decreased vertical ejection of tephra ceased and pyroclastic flows were fed from intermittent fountains phase iii deposits consist of a poorly exposed sequence 12 m of ash flow tuff that consist of many thin flow units 2 m each containing pumiceous white dacite 63 64 sio2 and denser gray silicic andesite 61 62 sio2 and fine grained ash cloud deposits interbedded with a nongraded middle pumice ash layer the climactic phase iv 1500 1715 developed in two stages fountain fed pyroclastic flows followed by a short pulse 1625 1715 of vigorous vertical ejection of tephra these stages were accompanied by the peak seismic energy release and peak eruption column height respectively climactic deposits consist of a thick 35 m sequence of thick lapilli rich ash flow sheets 4 12 m each with white and gray pumice and streaky scoria bands 60 sio2 in pumice breccia clasts and the reversely graded upper pumice lapilli layer that is interbedded with fine grained ash cloud deposits during the late ash flow phase v 1715 1815 eruption intensity waned but included a brief episode of small pyroclastic flows 1745 1815 phase v deposits consist of small distributary lobes of ash flow tuff containing white and gray pumice and minor fine ash deposits phase vi activity 1815 to may 19 1980 consisted of a low energy ash plume with transient increases in intensity while seismicity continued at depth sustained vertical discharge of phase ii prodeced evolved dacitewith high s cl ratios ash flow activity of phase iii is attributed to decreases in gas content indicated by reduced s cl ratios and increased clast density of the less evolved gray pumice climactic events are attributed to vent clearing and exhaustion of the evolved dacite

origin of variations in organic matter abundance and composition in a lithologically homogeneous maar type oil shale deposit gerce pliocene hungary despite having an homogeneous lithology the largest hungarian maar type deposit gerce oil shale pliocene has previously been shown to exhibit substantial variations in organic matter quantity and quality with depth this het erogeneity is also reflected in the present study by large variations in bitumen abundance and composition for 23 samples from gerce well 6 core based on the above bitumen data four samples were selected that were representative of the whole set which exhibit contrasting features scanning and transmission electron microscopy showed the occurrence of extensively altered botryococcus colonies in this deposit gc ms and gc c ir ms of the saturated hydrocarbon fractions of the bitumen of these samples reveal a predominant algal contribution along with a variable bacterial input the relative abundance of these two contributions in the four selected samples is also reflected by dif ferences in ftir and solid state 13 c nmr spectra of the isolated kerogens curie point pyrolysis gc ms of these kerogens revealed a relatively high terrestrial contribution in one sample and confirmed the variable input of algae and bacteria the above diaerences in relative contributions account for the variations in organic matter quantity and quality observed along the core 2000 elsevier science ltd all rights reserved

reversing buoyancy in turbidity currents developing a hypothesis for flow transformation and for deposit facies and architecture a turbidity current that contains fresher or otherwise less dense water than its surroundings may initially be denser than the ambient and propagate as a bottom hugging flow but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient seawater it is proposed that this reversal in buoyancy may be a significant mechanism controlling the structure and facies of turbiditic deposits buoyancy reversal followed by lofting may directly affect the relative distribution of fine and coarse material in the deposit while buoyancy reversal itself may mediate the transformation between dilute and highly concentrated suspension flows particularly in distal regions and thus lead to the formation of complex turbiditic beds in particular the generation of distal co genetic debrites may be expected similar transformations occur within dilute pyroclastic density currents where a mobile basal concentrated flow termed a surge derived pyroclastic flow develops through rapid sedimentation from the suspended load of the overlying surge the physical mechanisms involved in these processes are discussed leading to the proposal of some associated facies models these are compared with field data from the northern apennines with some striking similarities being noted as well as some differences on the basis of this discussion some directions are suggested for future experimental and modelling work on the topic

atmospheric and oceanic conditions and the extremely low bothnian bay sea ice extent in 2014 2015 the winter of 2014 2015 was the first for which we can be certain that the northernmost embayment of the baltic sea the bothnian bay remained partially ice free we assess atmospheric and oceanic drivers responsible for this extreme event in terms of atmospheric drivers mild bothnian bay winters are characterized by positive north atlantic oscillation nao associated with the advection of air from the north atlantic in winter 2014 2015 nao was at a record high level and resulted in a northerly storm track associated with vigorous southwesterlies that drove ice toward the northeastern end of the bay furthermore we found that the 1985 2015 baltic sea ice extent distribution differs from any other preceding 30 winter period since 1720 with a high confidence based on climate projections it is very likely that in the next 50 years partially ice free bothnian bay will go from being an extremely rare event to a new normal

remote sensing of active volcanoes the synoptic coverage offered by satellites provides unparalleled opportunities for monitoring active volcanoes and opens new avenues of scientific inquiry thermal infrared radiation can be used to monitor levels of activity which is useful for automated eruption detection and for studying the emplacement of lava flows satellite radars can observe volcanoes through clouds or at night and provide high resolution topographic data in favorable conditions radar inteferometery can be used to measure ground deformation associated with eruptive activity on a centimetric scale clouds from explosive eruptions present a pressing hazard to aviation therefore techniques are being developed to assess eruption cloud height and to discriminate between ash and meterological clouds the multitude of sensors to be launched on future generations of space platforms promises to greatly enhance volcanological studies but a satellite dedicated to volcanology is needed to meet requirements of aviation safety and volca

estimation of eruption source parameters from umbrella cloud or downwind plume growth rate abstract we introduce a new method to estimate mass eruption rate mer and mass loading from the growth of a volcanic umbrella cloud or downwind plume using satellite images or photographs where ground based observations are available this new method is compared with pre existing models and documented mass eruption rate given in the research literature we applied the method to five well studied eruptions mount st helens 1980 redoubt 1990 pinatubo 1991 hekla 2000 and eyjafjallajokull 2010 and to five less well documented eruptions kliuchevsko i 1994 okmok 2008 kasatochi 2008 sarychev peak 2009 and bezymianny 2012 the mass eruption rate is obtained by estimation of the radius of the umbrella cloud with time or by estimation of the width of the downwind plume with distance from the volcano the results given by the new method show a more fully characterized mer as a function of time than do the results given by pre existing methods and allow a faster remote assessment of the mass eruption rate even for volcanoes that are difficult to study the method thus may provide an additional important path to the estimation of source parameters and the forecasting of ash cloud propagation in addition in cases where numerous methods are available use of the method yields new independent measures of mass eruption rate hence an ability to estimate uncertainty in mass eruption rate which could be used in probabilistic estimations of ash cloud propagation

the fragmentation threshold of pyroclastic rocks in response to rapid decompression porous magma may fragment explosively this occurs when the melt can no longer withstand forces exerted upon it due to the overpressure in included bubbles this occurs at a critical pressure difference between the bubbles and the surrounding magma in this study we have investigated this pressure threshold necessary for the fragmentation of magma here we present the first comprehensive high temperature experimental quantification of the fragmentation threshold of volcanic rocks varying widely in porosity permeability crystallinity and chemical composition we exposed samples to increasing pressure differentials in a high temperature shock tube apparatus until fragmentation was initiated experimentally we define the fragmentation threshold as the minimum pressure differential that leads to complete fragmentation of the pressurized porous rock sample our results show that the fragmentation threshold is strongly dependent on porosity high porosity samples fragment at lower pressure differentials than low porosity samples the fragmentation threshold is inversely proportional to the porosity of the other factors permeability likely affects the fragmentation threshold at high porosity values whereas chemical composition crystallinity and bubble size distribution appear to have minor effects the relationship for fragmentation threshold presented here can be used to predict the minimum pressure differential necessary for the initiation or cessation of the explosive fragmentation of porous magma

mechanisms for ground surface fracturing and incipient slope failure associated with the 2001 eruption of mt etna italy analysis of ephemeral field data abstract during the july august 2001 eruption of mt etna development of extensional fractures faults and grabens accompanied magma intrusion and subsequent volcanic activity during the first days of the eruption we performed an analysis of attitude displacement and propagation of fractures and faults exposed on the ground surface in two sites torre del filosofo and valle del leone located along the same fracture system in the region surrounding the valle del bove depression on the eastern flank of mt etna fractures and faults formed as the consequence of a shallow intruding dyke system that fed the several volcanic centres developed along the fracture system the investigated sites differ in slope attitude and in geometrical relationships between fractures and slopes in particular the fracture system propagated parallel to the gentle slope

dimensions and dynamics of co ignimbrite eruption columns very powerful volcanic eruptions cannot always form classical plinian eruption columns1 4 instead collapsing fountains may develop above the vent and shed pyroclastic flows which spread laterally along the ground5 the upper part of these hot dense pyroclastic flows may become buoyant through the entrainment heating and expansion of ambient air coupled with the sedimentation of larger clasts suspended in the flow the buoyant material may rise in a co ignimbrite eruption column carrying massive quantities of fine dust and volatiles into the stratosphere6 7 here we present a model of this process and show that the co ignimbrite columns associated with the eruptions of toba8 9 75 000 years ago and tambora10 in 1815 may have ascended only about 32 and 23 km the latter is comparable with the less powerful 1982 plinian eruption column of el chichon11 this corroborates arguments that the mass of sulphuric acid aerosols injected into the stratosphere and not the eruptive power determines the climatic impact of an eruption12 13

a laboratory study of ash flows hot ash flows propagate from volcanic vents during some explosive volcanic eruptions as a flow entrains and heats ambient air and simultaneously deposits pyroclasts part of the flow may become less dense than the air and ascend from the flow if entrainment of air is more important than sedimentation in controlling the flowbehaviour then a large buoyant ash cloud may form above the flow conversely if sedimentation of particles dominates then a much smaller fraction of the particles are able to ascend in the buoyant cloud we have carried out a series of analog laboratory experiments to simulate the motion and buoyancy generation in dilute ash flows propagating along the ground the first series of experiments investigated buoyancy generation through entrainment using mixtures of methanol ethylene glycol and water the second series examined the role of sedimentation upon the generation of buoyancy using particle laden fresh water currents propagating in a saline ambient fluid the experiments showed that the rate of entrainment increases while the sedimentation rate decreases as the downward slope of the terrain increases suggesting that massive ash laden clouds are more likely to rise from flows traveling on steeper slopes changes in slope angle also control the motion of ash flows the experiments suggest that flow separation and hydraulic jumps resulting from sudden changes in slope angle can enhance entrainment or sedimentation and increase buoyancy observations of the the interactions of ash flows with topography at redoubt volcano in april 1990 and at mount st helens on may 18 1980 are consistent with our laboratory observations

ground coupled acoustic airwaves from mount st helens provide constraints on the may 18 1980 eruption abstract the may 18 1980 mount st helens eruption perturbed the atmosphere and generated atmosphere to ground coupled airwaves which were recorded on at least 35 seismometers operated by the pacific northwest seismograph network pnsn from 102 distinct travel time picks we identify coherent airwaves crossing washington state primarily to the north and east of the volcano the travel time curves provide evidence for both stratospheric refractions at 200 to 300 km from the volcano as well as probable thermospheric refractions at 100 to 350 km the very few first hand reports of audible volcano sounds within about 80 km of the volcano coincide with a general absence of ground coupled acoustic arrivals registered within about 100 km and are attributed to upward refraction of sound waves from the coherent refracted airwave arrivals we identify at least four distinct sources which we infer to originate 10 s 114 s 180 s and 319 s after the onset of an 8 32 11 pdt landslide the first of these sources is attributed to resultant depressurization and explosion of the cryptodome most of the subsequent arrivals also appear to be coincident with a source located at or near the presumed volcanic conduit but at least one of the later arrivals suggests an epicenter displaced about 9 km to the northwest of the vent this dislocation is compatible with the direction of the sector collapse and lateral blast we speculate that this concussion corresponds to a northern explosion event associated with hot cryptodome entering the toutle river valley

the role of laboratory experiments in volcanology experiment is central to scienti c methodology within science in general experiments are used for four primary purposes i e as a tool to explore novel phenomena and to provide systematic observations of processes to determine the values of key parameters to test hypotheses and theoretical models and to validate computational models which themselves can be considered as numerical experiments the study of volcanic eruptions and magmatic processes is becoming increasingly systematic quantitative and rigorous the drive is towards providing detailed quantitative descriptions and models of the ow phenomena in terms of fundamental physico chemical processes this is motivated in no small way by an increasing need for reliable and quantitative predictions of volcanic eruptions on which to base rational hazard and risk management decisions during volcanic crises a natural consequence of this development of the scienti c study of volcanic ow phenomena is that rigorous laboratory experimentation is becoming an increasingly important feature within volcanic research the contribution of experimental research to our understanding of volcanic processes is di cult to overstate the determination of the values of key parameters has traditionally taken place in the laboratory however more recently experiments have become much more widely applied to the problem of understanding complex dynamical processes and their underlying mechanisms without models experimental theoretical numerical we are limited to qualitative interpretations of eld observations and remote sensing field data are of course essential and provide the measure against which ultimately the applicability of observations from other sources must be assessed however there are several reasons why the scienti c study of volcanic phenomena cannot rely solely on eld data direct observations of eruption processes in the eld are limited to those parts of an eruption that are accessible this restricts detailed observations to a limited range of above surface processes moreover logistical constraints mean that the systematic collection of data will usually only be possible for volcanic eruptions near an established observatory and for activity for which there has been some forewarning as a result of these constraints direct eld based observations of natural volcanic eruption processes are generally incomplete and uncertain another source of natural data derives from observations made on volcanic deposits in principle processes can be inferred from the features

experimental studies of particle laden plumes experiments are described in which buoyant plumes of fresh water and solid particles are generated in a tank of salt water dependence of the behavior of the plumes on particle concentration and grain size was studied at low concentrations behavior was similar to single component plumes and the principal effect of the particles was to reduce the buoyancy flux measurements of particle concentration across the plumes demonstrated that there were substantial amounts of particle reentrainment from an outer sedimenting veil reentrainment caused the plume margins to become denser than the ambient fluid total particle flux in the plumes increased with height and can be several times greater than the flux at the plume source plumes with high particle concentration exhibit several kinds of convective instability which generated gravity currents and rapid sedimentation gravity currents were generated at the margins of concentrated plumes these flows are attributed to reentrainment causing the flow margins to have excess density when the plume density was close to but less than the density of the ambient fluid a fountain forms high in the plumes which collapsed asymmetrically to generate a gravity current on one side of the plume when the density of the particle laden flow was greater than the ambient fluid a symmetrical fountain formed and gravity currents moved across the tank floor depositing sediment at a well defined distance from the source buoyant fluid ascended from the flow fronts to form a ring of plumes around the central fountain these plumes coalesced to form a broad buoyant cloud the plumes are generated by the sedimentation from the gravity currents which resulted in the fresh water and some entrained particles being released all the experimental plumes ascended to the top of the tank and spread out as gravity currents beneath the upper surface particles often fall from this layer in discrete parcels of particle rich fluid with shapes similar to thermals the fall velocities were much greater than the terminal velocities of individual particles and the sedimenting region is inhomogeneous these observations indicate that sedimentation is enhanced by convective instabilities when particle rich fluid is emplaced above particle free fluid the experiments indicate that there may be several different mechanisms of generating pyroclastic flows and surges from the collapse of volcanic eruptions columns the experiments also demonstrate that the sedimentation of particles from a plume has an important influence on dynamical behavior

analysis of active volcanoes from the earth observing system abstract a study of volcanic activity and its effects on the atmosphere is one of 28 interdisciplinary investigations for the earth observing system eos due to be launched in 1997 and 1999 the volcanology investigation will include long and short term monitoring of selected volcanoes the detection of precursory activity associated with unanticipated eruptions and the detailed study of on going eruptions the data collected will allow us to address two aspects of volcanism volcanic padforms and the atmospheric effects of eruptions a variety of instruments on the two nasa eos platforms together with supplemental data from the japanese and european platforms will enable the study of local to regional scale thermal and deformational features of volcanoes and the chemical and structural features of volcanic eruption plumes and aerosols this investigation fits well within the overall goal of the eos project which is to study the regional and global interrelationships between components of the earth system because it specifically investigates the links between volcanism atmospheric chemistry and short term 1 3 year climate change

magma fragmentation dynamics experiments with analogue porous low strength material experiments were conducted on the fragmentation of analogue low strength porous material plastiprin by rapid decompression in a shock tube type apparatus the porous samples length 365 mm cross section dimensions 40 40 mm pressurized by air to pressures up to 0 9 mpa were rapidly decompressed to 0 1 mpa rapid decompression of samples caused fragmentation and ejection of the fragmentation products into a large volume tank the process of analogue material fragmentation was documented using high speed cinematography and dynamic pressure measurements the duration of the fragmentation event is significantly shorter than that of the ejection event the fragmentation of material precedes the acceleration of fragments as a result of fragmentation sub parallel fractures are generated the characteristic fragment size decreases as the initial pressure differential increases the ejected fragments obtain velocities of 60 m s the mechanisms of material fragmentation during unloading and fragmentation wave propagation are discussed the experimental results provide insight into the fragmentation dynamics of highly viscous magmas in which brittle failure at high strain rate is possible