estimating volcanic plume heights from depositional clast size 1 tephra deposits retain a considerable amount of information about the nature of volcanic eruptions with plume height commonly inferred from maximum clast size measurements however current methods for inferring plume height from maximum clast size lose some of the accuracy in measurements made in the field and have limited application when trying to constrain the uncertainty in these parameters here a predictive numerical model is used to determine plume height from maximum clast size found in a deposit plume height is an essential parameter for inferring eruption magnitude as it is explicitly related to mass eruption rate we investigate the effects of different atmospheric and wind profiles allowing the input conditions to be location specific for each eruption the use of a predictive model reduces the uncertainty in determining plume height as it retains the detail of measurements made in the field and incorporates the uncertainty in these measurements in the results in the form of a probability distribution we have applied this approach to fall deposits from the 1991 eruption of pinatubo one of the few plinian eruptions in which the true height of the plume is well known the predicted plume heights are in good agreement with those found from satellite measurements this new approach provides a method for determining plume height from poorly preserved fall deposits from which a limited amount of data can be sampled leading to large uncertainties in the field measurements

pyroclastic flows from the 1991 eruption of unzen volcano japan pyroclastic flows from unzen were generated by gravitational collapse of the growing lava dome as soon as the parental lobe failed at the edge of the dome spontaneous shattering of lava occurred and induced a gravity flow of blocks and finer debris the flows had a overhanging tongue like head and cone or rollershaped vortices expanding outward and upward most of the flows traveled from 1 to 3 km but some flows reached more than 4 km burning houses and killing people in the evacuated zone of kita kamikoba on the eastern foot of the volcano the velocities of the flows ranged from 15 to 25 m s on the gentle middle flank observations of the flows and their deposits suggest that they consisted of a dense basal avalanche and an overlying turbulent ash cloud the basal avalanche swept down a topographic low and formed to tongue like lobe having well defined levees it is presumed to have moved as a non newtonian fluid the measured velocities and runout distances of the flows can be matched to a bingham model for the basal avalanche by the addition of turbulent resistance the rheologic model parameters for the 29 may flow are as follows the density is 1300 kg m3 the yield strength is 850 pa the viscosity is 90 pa s and the thickness of the avalanche is 2 m the ash cloud is interpreted as a turbulent mixing layer above the basal avalanche the buoyant portions of the cloud produced ash fall deposits whereas the dense portions moved as a surge separated from the parental avalanche the ash cloud surges formed a wide devastated zone covered by very thin debris the initial velocities of the 3 june surges when they detached from avalanches are determined by the runout distance and the angle of the energy line slope a comparison between the estimated velocities of the 3 june avalanches and the surges indicates that the surges that extended steep slopes along the avalanche path detached directly from the turbulent heads of the avalanches the over running surge that reached kita kamikoba had an estimated velocity higher than that of the avalanche this farther travelled surge is presumed to have been generated by collapse of a rising ash cloud plume

the 15 september 1991 pyroclastic flows at unzen volcano japan a flow model for associated ash cloud surges large scale collapse of a dacite dome in the late afternoon of 15 september 1991 generated a series of pyroclastic flow events at unzen volcano pyroclastic flows with a volume of 1 106 m3 as dre descended the northeastern slope of the volcano changing their courses to the southeast due to topographic control after they exited a narrow gorge an ash cloud surge rushed straight ahead detaching the main body of the flow that turned and followed the topographic lows to the east the surge swept the kita kamikoba area which had been devastated by the previous pyroclastic flow events and transported a car as far as 120 m following detachment the surge lost its force after it moved several hundred meters but maintained a high temperature the deposits consist of a bottom layer of better sorted ash unit 1 a thick layer of block and ash unit 2 and a thin top layer of fall out ash unit 3 unit 2 overlies unit 1 with an erosional contact the upper part of unit 2 grades into better sorted ash at distal block and ash flow deposits the bottom part of unit 2 also consists of better sorted ash and the contact with the unit 1 deposits becomes ambiguous video footage of cascading pyroclastic flows during the 1991 1995 eruption traveling over surfaces without any topographic barriers revealed that lobes of ash cloud protruded intermittently from the moving head and sides and that these lobes surged ahead on the ground surface this fact together with the inspection by helicopter shortly after the events suggests that the protruded lobes consisted of better sorted ash and resulted in the deposits of unit 1 the highest ash cloud plume at the oshigadani valley exit and the thickest deposition of fall out ash over kita kamikoba and ohnokoba indicate that abundant ash was also produced when the flow passed through a narrow gorge in the model presented here the ash clouds from the pyroclastic flows were composed of a basal turbulent current of high concentration main body an overriding and intermediate fluidization zone and an overlying dilute cloud release of pressurized gas in lava block pores due to collisions among blocks and the resulting upward current caused a zone of fluidization just above the main body the mixture of gas and ash sorted in the fluidization zone moved ahead and to the side of the main body as a gravitational current where the ash was deposited as surge deposits the main body which had high internal friction and shear near its base then overran the surge deposits partially eroding them when the upward current of gas fluidization waned better sorted ash suspended in the fluidization zone was deposited on block and ash deposits in the distal places of block and ash deposits unit 2 probably was deposited in non turbulent fashion without any erosion of the underlying layer unit 1

fluid dynamics of the 1997 boxing day volcanic blast on montserrat west indies 1 directed volcanic blasts are powerful explosions with a significant laterally directed component which can generate devastating high energy pyroclastic density currents pdcs such blasts are an important class of eruptive phenomena but quantified understanding of their dynamics and effects is still incomplete here we use 2 d and 3 d multiparticle thermofluid dynamic flow codes to examine a powerful volcanic blast that occurred on montserrat in december 1997 on the basis of the simulations we divide the blast into three phases an initial burst phase that lasts roughly 5 s and involves rapid expansion of the gas pyroclast mixture a gravitational collapse phase that occurs when the erupted material fails to mix with sufficient air to form a buoyant column and thus collapses asymmetrically and a pdc phase that is dominated by motion parallel to the ground surface and is influenced by topography we vary key input parameters such as total gas energy and total solid mass to understand their influence on simulations and we compare the simulations with independent field observations of damage and deposits demonstrating that the models generally capture important large scale features of the natural phenomenon we also examine the 2 d and 3 d model results to estimate the flow mach number and conclude that the range of damage sustained at villages on montserrat can be reasonably explained by the spatial and temporal distribution of the dynamic pressure associated with subsonic pdcs

are enigmatic erosional sandy wave like bedforms in jurassic bridport sands dorset due to standing waves a field of sandwave like bedforms produced by erosion rather than deposition with bedload traction is identified within the lower middle jurassic shallow marine bridport sands of the wessex basin which were previously interpreted as simple channel like erosional scours by davies in 1967 here this interpretation is refined by proposing that these sandy bedforms formed as near symmetrical scours into horizontally stratified and bioturbated very fine to fine grained sands in shallow water below standing waves possibly internal edgewaves these sandy bedforms and contemporaneous erosional surface which defines the base of the ham hill stone may represent different responses to a moorei subzone lowstand of sea level

3 d numerical simulations of eruption column collapse effects of vent size on pressure balanced jet plumes abstract buoyant columns or pyroclastic flows form during explosive volcanic eruptions in the transition state these two eruption styles can develop simultaneously we investigated the critical condition that separates the two eruption styles referred to as the column collapse condition by performing a series of three dimensional numerical simulations in the simulation results we identify two types of flow regime a turbulent jet that efficiently entrains ambient air jet type and a fountain with a high concentration of the ejected material fountain type hence there are two types of column collapse jet type and fountain type which type of collapse occurs at the column collapse condition depends on whether the critical mass discharge rate for column collapse mdr cc is larger or smaller than that for the generation of a fountain mdr jf for a given exit velocity temperature controls the relative magnitude of mdr cc relative to mdr jf and hence the type of collapse for given magma properties e g temperature and water content the column collapse condition is expressed by a critical value of the richardson number ri cc g 0 l 0 w 0 2 where g 0 is the source buoyancy l 0 is the vent radius and w 0 is the exit velocity when the jet type collapse occurs at the column collapse condition ri cc is independent of the exit velocity when the fountain type collapse occurs at the column collapse condition on the other hand ri cc decreases as the exit velocity increases as the exit velocity exceeds the sound velocity a robust flow structure with a series of standing shock waves develops in the fountain which suppresses entrainment of ambient air and enhances column collapse

paleomagnetic directions and k ar dating of 0 to 1 ma lava flows from la guadeloupe island french west indies implications for time averaged field models twenty six lava flows spanning the last million years were sampled in la guadeloupe french west indies because of the lack of continuous volcano stratigraphic sections in la guadeloupe dating is necessary in order to describe the temporal evolution of the geomagnetic field in this time interval new k ar ages ranging from 50 ka to 1 ma have been obtained on andesites using the cassignol gillot technique at the universite paris sud institut de physique du globe de paris orsay laboratory additional flows with k ar ages obtained using the same dating technique were also sampled for paleomagnetic investigations more than 200 samples were analyzed using both alternating field af and thermal stepwise demagnetization techniques duplicate samplings of two flows at three different sites demonstrate that within flow dispersion is negligible for the andesitic lava flows sampled in this study direct comparison with an earlier paleomagnetic study performed on the island indicates that for the three investigated flows modern demagnetization techniques yield much better defined paleomagnetic directions the matuyama brunhes transition was recorded in a three flow section and is dated at 781 18 ka in good agreement with other recent radiometric age determinations the mean paleomagnetic pole calculated from the 23 normal polarity flows is indistinguishable from geographic north which implies that no significant persistent axial quadrupole term can be identified at this site for the last million years this result contradicts earlier results and has important implications for models of the time averaged field taf an occam algorithm was used to construct a taf model from the global volcanic database from the last 5 ma substitution of the mean direction calculated from the earlier study for the lesser antilles by our new mean value reduces the quadrupole term by more than 30 this effect which was produced by changing data from a single site demonstrates that older paleomagnetic sites may need to be reinvestigated furthermore it also highlights the limitations of taf models that can be inferred from paleomagnetic databases in their present state

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

mobility of a large volume pyroclastic flow emplacement of the campanian ignimbrite italy abstract the trachytic campanian ignimbrite originally exposed over a 30 000 km 2 area around naples italy is the product of a highly energetic gas rich eruption the deposit lies in valleys and isolated watersheds and in its medial and distal extent to the south north and east of naples the campanian pyroclastic current encountered mountains exceeding 1000 meters anisotropy of magnetic susceptibility ams measurements indicate that the campanian pyroclastic current the transport system traveled radially outward from the phlegrean fields area but the ignimbrite forming flow the deposition system that developed from the base of the transport system moved downslope from mountainsides to valleys including slopes facing the eruption source and flowed down drainage systems from intermontane basins the campanian pyroclastic current flowed 35 km over the water of the bay of naples to deposit 43 m of ignimbrite on the south shore and also overtopped a 685 1000 m high ridge of the sorrento peninsula to deposit more on the other side the distribution of the ignimbrite and the measured flow directions suggest that the campanian pyroclastic current moved across the landscape as an expanded and therefore turbulent decompressing flow rather than as a high density nonturbulent sheet like current moving over mountains by momentum acquired by eruption column collapse strong expansion is corroborated by shard morphology that indicates derivation from highly inflated pumice and suggests vesicles must have been at least 80 by volume of the original magma

dynamics and deposits generated by the kos plateau tuff eruption controls of basal particle loss on pyroclastic flow transport the explosive eruption of voluminous silicic magmas often produces widespread and massive deposits formed from pyroclastic density currents while these punctuated events dramatically alter the landscape and have potential climate altering impact our understanding of the internal structure and transport dynamics of these eruptions is hampered by a lack of direct observations we utilize the natural boundary conditions encountered by the eruption of the kos plateau tuff to probe its internal structure as well as constrain the neotectonic activity in the region and eruption duration of this moderate to large 60 km3 event at the time of the eruption 161 ka the lower sea level in the mediterranean may have resulted in flows that traversed mostly land to the north of the eruptive vent while flows to the south may have encountered an expanse of water steep topography and overwater transport can impede the transport of the dense basal portions of the flow where particles make multiple or sustained contact with the bed we use an eulerian eulerian lagrangian computational approach coupled with overwater and overland boundary conditions including topography to determine the role of bed load versus suspended load in the transport of these flows we find that a ring vent structure and eruptive fluxes greater than 2 106 m3 s are required for the spatial distribution of the kpt the maximum grain size and deposit locations of the first voluminous ignimbrite unit d can be explained by suspended flow to the south consistent with overwater transport and bed load and suspended load transport to the north consistent with overland transport however the maximum lithic size for the largest and last ignimbrite unit e requires some bed load transport in all directions we propose that the boundary conditions were significantly altered during the course of the eruption through either the in filling of a shallow sea to the south or the development of a thick pumice raft to aid saltation on the basis of the inferred eruptive flux we estimate that the duration of the eruption climax in which most of the material was erupted likely only lasted from a few hours to a day

modeling dense pyroclastic basal flows from collapsing columns 1 a two layer model for the formation of dense pyroclastic basal flows from dilute collapsing volcanic eruption columns is presented the collapsing dilute current is described by depth averaged isothermal continuum conservation equations the dense basal flow is modelled as a granular avalanche of constant density simulations demonstrate that pyroclastic flow formation and behavior is dependent upon slope conditions when the dilute part of the current has lost most of its mass the dilute current runout increases with decreasing particle size and increasing initial column height if the dilute current has transferred its mass to the dense basal flow on volcanic slopes with inclination angle greater than the friction angle of the basal flow then the basal flow will continue to propagate until frictional forces bring it to rest if the dilute current terminates on lower angled slopes frictional forces dominate the basal flow preventing further front propagation

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

pressure evolution during explosive caldera forming eruptions abstract caldera forming eruptions of silicic magmas result from a complex coupling of the mechanics and fluid dynamics of the associated magma chamber field studies of caldera forming eruption products suggest that great pressure variations occur inside the magma chamber and associated conduits during these eruptions pressure evolution during explosive caldera forming eruptions is investigated through a simple model that describes the first order quantitative behaviour of the chamber we consider a piston like model that assumes a coherent block subsiding along circular sub vertical ring faults into the magma chamber this subsidence occurs after significant decompression of the chamber by an initial central vent eruption we assume that the initial pressure distribution in the chamber is magmastatic once collapse has begun the chamber roof is supported by the magma so that magma pressure at the chamber roof increases to lithostatic we suggest that pressure variations during caldera forming eruptions are mainly controlled by variations in magma volatile content regardless of what induces the formation of ring faults the model suggests that the occurrence of explosive caldera forming events depends on the strength of the chamber walls and the depth water content and aspect ratio of the magma chamber no significant differences exist between model results for a cylindrical or a more realistic elliptical magma chamber geometry of comparable aspect ratio assuming a constant strength of the host rock the mass fraction of magma that must be erupted during the central vent phase in order to trigger caldera collapse ranges for deep gas poor chambers from a few percent up to 40 for shallow gas rich chambers the model suggests that zoned chambers tend to collapse earlier than homogeneous chambers dike shaped chambers will erupt less magma than sill like chambers before caldera collapse initiates although dike like geometries are not associated with stress fields appropriate to create ring faults the model suggests that once initiated caldera collapse will tend to force out most or all of the volatile rich magma from the chamber for volatile rich magma chambers the total volume of erupted magma during caldera forming event is of the same order as the chamber volume the model also explains the variation in the erupted mass during the different phases of explosive caldera forming eruptions and is in good agreement with natural examples

interaction of pyroclastic density currents with human settlements evidence from ancient pompeii integrating field observations and rock magnetic measurements we report how a turbulent pyroclastic density current interacted with and moved through an urban area the data are from the most energetic turbulent pyroclastic density current of the a d 79 eruption of vesuvius italy which partially destroyed the roman city of pompeii our results show that the urban fabric was able to divide the lower portion of the current into several streams that followed the city walls and the intracity roads vortices revealed by upstream particle orientations and decreases in deposit temperature formed downflow of obstacles or inside cavities although these perturbations affected only the lower part of the current and were localized they could represent in certain cases cooler zones within which chances of human survival are increased our integrated field data for pyroclastic density current temperature and flow direction collected for the first time across an urban environment enable verification of coupled thermodynamic numerical models and their hazard simulation abilities

climatic and tectonic controls on weathering in south china and indochina peninsula clay mineralogical and geochemical investigations from the pearl red and mekong drainage basins 1 results of clay mineralogy major element geochemistry and sr and nd isotopes in 93 argillaceous samples collected from drainage basins of the pearl red and mekong rivers reveal different degrees of chemical weathering in southeast asia despite similar climate conditions across these regions the kaolinite illite ratio illite chemistry index and illite crystallinity can be used as indicators of chemical weathering intensity these mineralogical proxies combined with the k2o na2o cao molar ratio chemical index of alteration cia and weathering trends observed from major element results indicate intensive silicate weathering in the pearl river basin moderate to intensive in the mekong river basin and moderate in the red river basin although a significant modification of epsilon nd 0 values in our riverine sediments during chemical weathering and transport is unlikely sr 87 sr 86 ratios are controlled by various states of chemical weathering of high sr minerals such as plagioclase rich in na and ca with a linear decrease trend from the pearl mekong to red river basins our results suggest that it is not the warm climate with heavy monsoon precipitation but tectonics playing the most significant role in controlling weathering and erosion processes in south china and indochina peninsula strong physical erosion caused by tectonic activities and river incision along the eastern margin of the tibetan plateau and along the red river fault system is responsible for high contents of primary minerals in the lowlands of red and mekong river basins

a new scenario for the last magmatic eruption of la soufriere of guadeloupe lesser antilles in 1530 a d evidence from stratigraphy radiocarbon dating and magmatic evolution of erupted products la soufriere of guadeloupe is a dangerous volcano characterized over the last decade by moderate seismic and fumarolic unrest in the last 15 000 years it has experienced phreatic and magmatic eruptions and unusually numerous flank collapse events sometimes associated with a magmatic eruption we propose a new age of 1530 a d and a new eruptive scenario for the last magmatic eruption on the basis of a novel statistical analysis of radiocarbon age dates and new field and geochemical data this eruption is the only magmatic eruption likely to have occurred in guadeloupe during the last 1400 years the eruption mainly involved an andesitic magma which in the first phase of the eruption partially mixed with a slightly more differentiated magma stored in a small and shallow magma chamber ascent of magma to the surface generated a partial collapse of the hydrothermally altered edifice that increased the magma discharge and led to a sub plinian phase with scoria fallout and column collapse pyroclastic flows followed by near vent pyroclastic scoria fountains the eruption ended with growth of a lava dome our revised interpretation of the last magmatic eruption of la soufriere constitutes the most likely key to a future magmatic eruption scenario for this volcano which displays strong evidence of unrest since 1992

modelling pyroclastic density currents from a subplinian eruption at la soufriere de guadeloupe west indies france we have used a three dimensional non equilibrium multiphase flow numerical model to simulate subplinian eruption scenarios at la soufriere de guadeloupe lesser antilles france initial and boundary conditions for computer simulations were set on the basis of independent estimates of eruption source parameters i e mass eruption rate volatile content temperature grain size distribution from a field reconstruction of the 1530 ce subplinian eruption this event is here taken as a reference scenario for hazard assessment at la soufriere de guadeloupe a parametric study on eruption source parameters allowed us to quantify their influence on the simulated dynamics and in particular the increase of the percentage of column collapse and pyroclastic density current pdc intensity at constant mass eruption rate with variable vent diameter numerical results enabled us to quantify the effects of the proximal morphology on distributing the collapsing mass around the volcano and into deep and long valleys and to estimate the areas invaded by pdcs their associated temperature and dynamic pressure significant impact temperature 300 c and dynamic pressure 1 kpa in the inhabited region around the volcano is expected for fully collapsing conditions and mass eruption rates 2 107 kg s we thus combine this spatial distribution of temperature and dynamic pressure with an objective consideration of model related uncertainty to produce preliminary pdc hazard maps for the reference scenario in such a representation we identify three areas of varying degree of susceptibility to invasion by pdcs very likely to be invaded and highly impacted susceptible to invasion and moderately impacted and unlikely to be invaded or marginally impacted the study also raises some key questions about the use of deterministic scenario simulations for hazard assessment where probability distributions and uncertainties are difficult to estimate use of high performance computing techniques will in part allow us to overcome such difficulties but the problem remains open in a scientific context where validation of numerical models is still necessarily an incomplete and ongoing process nevertheless our findings provide an important contribution to the quantitative assessment of volcanic hazard and risk at la soufriere de guadeloupe particularly in the context of the current unrest of the volcano and the need to prepare for a possible future reawakening of the volcano that could culminate in a magmatic explosive eruption supplementary information the online version of this article 10 1007 s00445 020 01411 6 contains supplementary material which is available to authorized users

emplacement of the taupo ignimbrite by a dilute turbulent flow an ignimbrite is a pumice rich deposit that records the passage of a ground hugging ash flow a pyroclastic flow generated by the collapse of a volcanic eruption column1 geologists study such deposits to reconstruct the parent eruptions and to predict the consequences of future eruptions the 1 800 yr old taupo ignimbrite of new zealand has been interpreted to represent en masse emplacement by an avalanche like flow with a volumetric solids concentration in excess of 30 ref 2 the evidence for this is equivocal however and here we propose the alternative view that the deposit was emplaced by a relatively dilute and turbulent density current3 we present an isothermal hydraulic model the results of which taken together with existing observations suggest that the total flow rate of solids and gas was about 40km3s 1 for around 15 minutes this intense flux resulted in a flow which had a near vent solids concentration of 0 3 by volume was about 1 km thick and travelled outward from the vent with a typical speed of 200 ms 1 in view of the good agreement between predicted and observed radial trends in the taupo deposit we suggest that the origin of other ignimbrites with similar characteristics should be reconsidered

pyroclastic flow erosion and bulking processes comparing field based vs modeling results at tungurahua volcano ecuador pyroclastic density currents pdcs are high temperature and high velocity mixtures that threaten populations in the vicinity of many active volcanoes deciphering the cause of their remarkable mobility is essential for volcanic hazard analysis but remains difficult because of the complex processes occurring within the flows here we investigate the effect of bulking on dense pdc mobility by means of a double approach first we estimate the amount of material incorporated into scoria flows emplaced during the august 2006 eruption of tungurahua volcano ecuador for this we carry out a detailed analysis of 3d corrected digital images of well exposed scoria flow deposits componentry analysis indicates that pdc bulking occurs principally on the steep 25 upper slope of the volcano and the deposits typically comprise 40 50 wt of non juvenile i e accessory and accidental material secondly we develop a simple stress related grain by grain equation of erosion combined with two simple depth averaged geophysical mass flow models that compare the bulking mechanism to a non fluidized and a fluidized flow two behaviors based on coulomb and plastic rheologies are used to reproduce on a first order basis the 2006 tungurahua pdcs cross check comparisons between these modeled cases and the erosion pattern inferred from field based data allow us to evaluate the accuracy of our modeling assumptions regardless of the rheological regime the pdc induced erosion pattern of the 2006 tungurahua eruption can only be reproduced by fluctuations of the flow s basal shear stress during emplacement such variations are controlled by flow thinning thickening processes notably through the formation of a thick non erosive flow body that pushes a thin frictional erosive front during pdc emplacement the input volume of juvenile material as well as the thickness of the erodible layer available prior to the eruption are additional key parameters our work highlights complexities in pdc erosion and bulking processes that deserve further study in terms of hazard assessment our findings reveal that incorporation and bulking translate into increased flow mobility i e the augmented flow mass enhances both flow velocity and runout distance up to 20 these outcomes should be considered closely for hazard analysis at many other andesitic volcanoes worldwide where similar pdc events are common

sensitivity of the dynamics of volcanic eruption columns to their shape this paper presents a one dimensional steady state model to investigate the sensitivity of the dynamics of sustained eruption columns to radius variations with height due to thermal expansion of the entrained air and decreases in atmospheric pressure with height in contrast to a number of previous models using an equation known as the entrainment assumption the new model is based on similarity arguments to derive an equation set equivalent to the model proposed by woods bull volcanol 50 169 193 1988 this approach allows investigation of the effect of gas compressibility on the entrainment rate of ambient air which has been little examined for a system in which a decrease in pressure significantly affects the density stratification of a compressible fluid the new model provides results that include two end members one in which the volume change within the eruption columns affects only the radial expansion without changing the vertical motion and the other is the converse the woods bull volcanol 50 169 193 1988 model can be regarded as being between those two end members the range of uncertainty arises because the extremely high temperature of discharged materials from a volcanic vent and the exceptional terminal height of the eruption columns allow significant expansion of the gas component in the eruption columns making them behave differently from common turbulent plumes this study indicates that the maximum height of the eruption columns is affected considerably by this uncertainty particularly when the eruption columns extend above a height of 10 km at which the pressure is about one fourth the pressure at the ground surface column collapse may also be suppressed in wider parameter ranges than previously estimated however the uncertainty can be reduced by measuring column radii through a vertical profile during actual volcanic eruptions accordingly this paper suggests that appropriate observation of eruption column shapes is essential for improving our understanding of the dynamics of eruption columns

paroxysmal dome explosion during the merapi 2010 eruption processes and facies relationships of associated high energy pyroclastic density currents an 11 minute sequence of laterally directed explosions and retrogressive collapses on 5 november 2010 at merapi indonesia destroyed a rapidly growing dome and generated high energy pyroclastic density currents pdcs spreading over 22 km2 with a runout of 8 4 km while contemporaneous co genetic valley confined pdcs reached 15 5 km this event formed stage 4 of the multi stage 2010 eruption the most intense eruptive episode at merapi since 1872 the deposits and the widespread devastating impact of associated high energy pdcs on trees and buildings show striking similarities with those from historical volcanic blasts montagne pelee martinique bezymianny russia mount st helens usa soufriere hills montserrat we provide data from stratigraphic and sedimentologic analyses of 62 sections of the first unequivocal blast like deposits in merapi s recent history we used high resolution satellite imagery to map eruptive units and flow direction from the pattern of extensive tree blowdown the stratigraphy of stage 4 consists of three depositional units u0 u1 u2 that we correlate to the second third and fourth explosions of the seismic record both u1 and u2 show a bi partite layer stratigraphy consisting each of a lower l1 layer and an upper l2 layer the lower l1 layer is typically very coarse grained fines poor poorly sorted and massive and was deposited by the erosive waxing flow head the overlying l2 layer is much finer grained fines rich moderately to well sorted with laminar to wavy stratification l2 was deposited from the waning upper part and wake of the pdc field observations indicate that pdc height reached 330 m with an internal velocity of 100 m s 1 within 3 km from the source the summit s geometry and the terrain morphology formed by a major transversal ridge and a funneling deep canyon strongly focused pdc mass towards a major constriction thereby limiting the loss of kinetic energy this favored elevated pdc velocities and high particle concentration promoted overspilling of pdcs across high ridges into other river valleys and generated significant dynamic pressures to distances of 6 km that caused total destruction of buildings and the forest the merapi 2010 eruption highlights that explosive and gravitational disintegration of a rapidly growing dome can generate devastating high energy high velocity pdcs this constitutes a credible high impact scenario for future multi stage eruptions at merapi and at other volcanoes that pose particular monitoring crisis response and risk reduction challenges

temporal change of characteristics of shallow volcano tectonic earthquakes associated with increase in volcanic activity at kuchinoerabujima volcano japan abstract shallow volcano tectonic vt earthquakes recorded at the kuchinoerabujima island volcano in southwest japan are analyzed in order to clarify the role of hydrothermal activity in the development of volcanic seismicity from analysis of shallow vt earthquakes in 2006 two specific episodes of elevated seismicity are observed in april and november 2006 the vt earthquakes have hypocenters at depths of 0 0 4 km beneath the summit crater and normal fault focal mechanisms with wnw ese extension consistent with the tensional stress field indicated by the alignment of craters and fissures although the hypocenters and focal mechanisms are found to be largely invariant during these episodes the corner frequencies of the vt earthquakes underwent a pronounced increase and decrease accompanying the changes in seismicity rates the corner frequencies increased to 20 25 hz approximately one month prior to the onset of elevated seismicity and then decreased to 10 15 hz in the period of peak seismicity the rupture length also decreased at the onset of seismicity thereafter increasing as the seismicity continued the peak seismicity in terms of the daily number of vt events was accompanied by inflation around the crater suggestive of a pressure increase in the volcanic system it is inferred that the increase in shallow vt seismicity and rupture length is related to the development of a fractured zone the pressure increase in the volcanic system is attributed to the intrusion of hydrothermal fluids which is supported by an observed increase in fumarolic temperature and activity the preceding monochromatic events are thus considered to be generated by the effect of fluid filled cracks the shortening of rupture length is then inferred to be related to the closing of non fluid filled cracks in the fracture zone under the increasing pressure field leading to a transition from monochromatic events to low frequency and shallow vt seismicity