Tephra Dispersal Research Articles

A new deep ice core in the Northern Patagonian Icefield reveals a regional cooling in a warming world

Atmospheric dispersion of volcanic ash from explosive eruptions or from subsequent fallout deposit resuspension causes a range of impacts and disruptions on human activities and ecosystems. The April–May 2015 Calbuco eruption in Chile involved eruption and resuspension activities. We overview the chronology, effects, and products resulting from these events, in order to validate an operational forecast strategy for tephra dispersal. The modelling strategy builds on coupling the meteorological Weather Research and Forecasting (WRF/ARW) model with the FALL3D dispersal model for eruptive and resuspension processes. The eruption modelling considers two distinct particle granulometries, a preliminary first guess distribution used operationally when no field data was available yet, and a refined distribution based on field measurements. Volcanological inputs were inferred from eruption reports and results from an Argentina–Chilean ash sample data network, which performed in-situ sampling during the eruption. In order to validate the modelling strategy, results were compared with satellite retrievals and ground deposit measurements. Results indicate that the WRF-FALL3D modelling system can provide reasonable forecasts in both eruption and resuspension modes, particularly when the adjusted granulometry is considered. The study also highlights the importance of having dedicated datasets of active volcanoes furnishing first-guess model inputs during the early stages of an eruption.

Sigmoidal particle density distribution in a subplinian scoria fall deposit

A general expression to describe particle density distribution in tephra fall deposits is essential to improve fallout tephra mass determination and numerical modelling of tephra dispersion. To obtain particle density distributions in tephra fall deposits, we performed high-resolution componentry and particle density analyses on samples from the 2006 subplinian eruption of Tungurahua volcano in Ecuador. Six componentry classes, including pumice and scoria, have been identified in our sample collection. We determined the class of 300 clasts in each 0.5ϕ fractions from −4.5ϕ to 3.5ϕ and carried out water pycnometry density measurements on selected size fractions. Results indicate that the mean particle density increases with ϕ up to a plateau of ~2.6 g/cm3 for clasts finer than 1.5ϕ. The density of scoria and pumice increases between −3 and 1ϕ, while dense particle density is sub-constant with grainsize. We show that the mean particle density μ of the vesicular fractions is a function of grainsize i (ϕ scale) given by a sigmoidal law: $$ \mu (i)={{{K+\beta }} \left/ {{\left( {1+\alpha {e^{-ri }}} \right)}} \right.} $$ , where K, β, α and r are constants. These sigmoidal distributions can be used to determine accurately the load of each componentry class and should be applicable to many tephra deposits and for modelling purposes.

A fast GIS-based risk assessment for tephra fallout: the example of Cotopaxi volcano, Ecuador-Part II: vulnerability and risk assessment

In order to develop efficient strategies for risk mitigation and emergency management, planners require the assessment of both the expected hazard (frequency and magnitude) and the vulnerability of exposed elements. This paper presents a GIS-based methodology to produce qualitative to semi-qualitative thematic risk assessments for tephra fallout around explosive volcanoes, designed to operate with datasets of variable precision and resolution depending on data availability. Due to the constant increase in population density around volcanoes and to the wide dispersal of tephra from volcanic plumes, a large range of threats, such as roof collapses, damage to crops, blockage of vital lifelines and health problems, concern even remote communities. To address these issues, we have assessed the vulnerability and the risk levels for five themes relevant to tephra fallout: (1) social, (2) economic, (3) environmental, (4) physical and (5) territorial. Risk and vulnerability indices for each theme are averaged to the fourth level of administrative unit (parroquia, parish). In a companion paper, Biass and Bonadonna (this volume) present a probabilistic hazard assessment for tephra fallout at Cotopaxi volcano (Ecuador) using the advection-diffusion model TEPHRA2, which is based on field investigations and a global eruption database (Global Volcanism Program, GVP). The scope of this paper is to present a new approach to risk assessment specifically designed for tephra fallout, based on a comprehensive hazard assessment of Cotopaxi volcano. Our results show that an eruption of moderate magnitude (i.e. VEI 4) would result in the possible collapse of ∼9,000 houses in the two parishes located close to the volcano. Our study also reveals a high risk on agriculture, closely linked to the economic sector, and a possible accessibility problem in case of an eruption of any size, as tephra is likely to affect the only major road running from Quito to Latacunga (Panamerican Highway). As a result, this method fits into the ongoing effort to better characterize and evaluate volcanic risk, and more specifically the risk associated with tephra fallout. Although this methodology relies on some assumptions, it can serve as a rapid and efficient starting point for further investigations of the risk level around explosive volcanoes.

Estimating the likelihood of an eruption from a volcano with missing onsets in its record

Historical eruption records are often incomplete, a problem which is exacerbated in dealing with catalogs derived from geologic records. We examine the problem of estimating the true (adjusted for missing observations) parameters and hence the hazard in a Weibull (or gamma) renewal model, which is commonly used to model time series of volcanic onsets. Robust regression, robust estimation using repeated medians, and results from the theory of inverses of thinned renewal processes failed to provide consistent estimates from simulated data. Hence we adopted a hidden Markov model framework, where the hidden state is a reflection of the number of missing onsets. This also allows for the completeness level of the record to be estimated, and offers a means of determining where in the record the missing observations are likely to be found. Tested on data from the Holocene record of Mt Taranaki, the preliminary estimates of completeness are 86–87% complete (record 7ka — present) and 78–80% complete beyond that. These figures were independently verified using a model of tephra dispersal. The estimated present hazard is approximately 20% higher than estimated without allowing for missing data.

Geochemistry of the Phlegraean Fields (Italy) proximal sources for major Mediterranean tephras: Implications for the dispersal of Plinian and co-ignimbritic components of explosive eruptions

Volcanic activity at Phlegraean Fields, Italy, produced several major marker tephras over a 50ka period. The caldera forming eruptions of the Campanian Ignimbrite (CI) and Neapolitan Yellow Tuff (NYT) are of particular importance for tephrostratigraphy in Europe. Other key eruptions from this source include the Pomici Principali (PP) and the Tufi Biancastri eruptions. We combine analyses of fresh glasses from proximal locations (i.e., juvenile clasts in proximal flow and fall deposits) with data for key tephra layers from Lago Grande di Monticchio, 120km to the east. The micron-beam major (EMPA) and trace (LA-ICP-MS) element glass dataset allows us to: (a) distinguish between tephra units produced from the Phlegraean Fields before and during the CI eruption (CI-series), and before and during the NYT and PP eruptions (NYT-series/PP); (b) discriminate between the CI and the geochemically similar Pre-CI pyroclastic deposits; (c) separate the NYT from Pre-NYT tephra units, although both major and trace elements do show significant overlap. The complex compositional overlap between Pre-NYT tephras may present a problem for tephra correlations in the 14–39ka time window and may have resulted in incorrect proximal–distal and distal–distal correlations. The diagnostic chemical criteria detailed herein permits more accurate matching of distal tephras with their proximal equivalents and hence will improve chronostratigraphy of distal settings and give insight into tephra dispersal. We show that the dispersal of PP tephra was more limited than previously thought. The surge/fall (Lower Member) and subsequent pyroclastic density current (Upper Member) phases of the NYT eruption can be recognised in distal settings. Both the NYT Lower and Upper Members are found in distal localities to the east of the Phlegraean Fields, however the Lower Member is found in the absence of the Upper Member in locations to the far north of Phlegraean Fields. Chemical compositions of the Plinian and ignimbrite phases of the CI eruption overlap extensively, but can be distinguished on a plot of Zr–Th.

Reconstructing the tephra dispersal pattern from the Bronze Age eruption of Santorini using an advection–diffusion model

The advection–diffusion model TEPHRA2 has been used in conjunction with the downhill simplex method (DSM) and one-at-a-time (OAT) inversion methods to reconstruct the eruption conditions and seasonality consistent with the deposit patterns from the Bronze Age (‘Minoan’) eruption of Santorini. We investigated three datasets representing different depositional environments (proximal terrestrial, distal terrestrial and deep-sea core), in order to determine source conditions such as plume height, erupted mass and grain-size and recreate the tephra fall deposit from the Plinian, co-ignimbrite and combined eruptive phases. The results of the DSM and OAT method agreed adequately well with each other for erupted mass, plume height and grain-size distribution. Both approaches were able to successfully recreate the Plinian deposit but estimating conditions that created the co-ignimbrite and deep-sea core dataset were less successful. The reduced agreement is the result of the low quantity (6 to 28 deposit points) and quality (inconsistent deposit depths at localities adjacent to each other) of the datasets, and the different dynamics between co-ignimbrite and Plinian columns, with the former not well represented in the model. Different sampling methods between archaeological and volcanological disciplines and post-depositional processes which have acted on the tephra deposits since the Bronze Age can explain the discrepancy between these computed and observed deposits. The seasonality of the Minoan eruption was investigated by using seasonal wind profiles for winter, spring, summer and autumn. We find that the Bronze Age eruption of Santorini is likely to have during the spring and summer months with a main dispersal axis aligned East. Crete would have received very little ash fall, and the eruption would not have caused much disruption to the life of the inhabitants of the island.

Dynamics, stratigraphy and proximal dispersal of supraglacial tephra during the ice-confined 2004 eruption at Grímsvötn Volcano, Iceland

The basaltic, phreatomagmatic eruption of Grimsvotn volcano, Iceland, in November 2004 (G2004) lasted for 5 days, during which time two separate vents were active. Significant deposition of tephra occurred in the first 45 h only. We have subdivided the deposit into seven units (A–G) on the basis of differences in texture, grain size and componentry, and the presence of sharp contacts between the layers. The distribution of tephra lobes was used to infer the vent of origin for each unit. The G2004 deposit is poorly sorted overall and consists of non-vesicular to highly vesicular juvenile components. Units A and B comprise almost exclusively non- to poorly vesicular glass fragments, whereas units C–G contain at least 30 vol.% highly vesicular pumice. The proportion of non-juvenile fragments increases significantly in the final unit (unit F) of the main phase; non-juvenile fragments are restricted to the coarse (>0 Φ) fraction of the deposit. Main phase units C and E account for 80% of the total deposit volume, including the entire distal portion, and are interpreted to represent a mixture of (1) a widely dispersed component that fell from the upper margins of a strongly inclined (∼45°), 6–10 km high plume and (2) a locally dispersed (<3 km from source) component originating from pyroclastic density currents and minor tephra jets.

Ballistics and ash plumes discriminated by Doppler radar

[1] Small scale eruptive ash plumes at Arenal volcano (Costa Rica) were recorded using a ground-based Doppler radar (VOLDORAD). The time-velocity distribution of the mass load (i.e., Doppler radargrams) exhibits two contrasted dynamics recorded simultaneously, evidenced by distinctive velocities, life spans, and transit speeds through the radar beam. Synthetic Doppler radargrams computed with a simple ballistic model indicate that the short-lived signal is consistent with the instantaneous projection of ballistics blocks accompanying the ash plume emission. The mass of centimeter- to decimeter-sized ballistics is confidently estimated at 0.5–7 tons, whereas the ash plume mass is loosely constrained at 5.8 × 102 tons, assuming a particle diameter of 2 mm close to the vent. These quantitative estimates of the mass proportion either falling on the slopes of the volcano or ejected into the atmosphere could help in the modeling and monitoring of tephra dispersal.

Insights on tephra settling velocity from morphological observations

In this study we present a systematic and detailed morphological characterization of tephra particles from different eruptions (Fontana Lapilli, Masaya, Nicaragua; Keanakāko'i Formation, Kilauea, USA; recent dome explosions of Soufriere Hills volcano, Montserrat) and the calculation of their Terminal Fall Velocity (TFV) as obtained based on different drag prediction models (i.e., Wilson and Huang, 1979; Haider and Levenspiel, 1989; Ganser, 1993; Dellino et al., 2005). In particular, particle sphericity, and, therefore, particle surface area, is essential for the calculation of TFV of irregular-shape particles but is of complex determination. Various attempts have been proposed. According to our results, 2D morphological characterization of volcanic particles is a fast and simple application for a wide range of particle size and provides consistent sphericity and settling-velocity values. 3D scanning also provides a promising tool for lapilli-sized tephra (> 2 cm). In contrast, gas-adsorption-derived surface area is not suitable for the calculation of TFV of volcanic particles mostly because it mainly describes the surface contribution of nanometric pores that are not expected to affect significantly TFV and because bulk-sample analysis is representative of neither individual particles nor of the whole particle population. Settling velocities calculated using values of surface area derived from gas adsorption analyses are up to two orders of magnitude lower than the values obtained through 2D analysis. In addition, our results also show how the influence of particle shape on TFV increases with particle size. In particular, calculated TFV converges at small particle sizes (≥ 3 ϕ) regardless of the model applied, suggesting that the spherical assumption is appropriate for this size fraction (discrepancies with the spherical model are within 10%). Discrepancies with the spherical model increase with particle size up to about 50% and depend on the choice of both the TFV model and the morphological parameterization used. In particular, the drag prediction model of Ganser (1993) is sensitive to the effect of particle morphology on TFV and is well suited for all sizes and Reynolds numbers of typical tephra particles. Finally, our results show how individual size categories (whole- and half-ϕ) are not associated with individual TFV values but with a range of values, which increases with class size. Nonetheless, the half-ϕ system is associated with a smaller standard deviation than the whole-ϕ system, and is therefore more appropriate for the modeling of tephra dispersal. In any case, for dispersal modeling purposes, it is more appropriate to indicate a range of settling velocities for each size class rather than giving an average value.

Three dimensional transport speed of wind-drifted ash plumes using ground-based radar

[1] The main utilization of mobile ground-based Doppler radars is to quantify the dynamics of eruptive activity by aiming directly at the emission source. We show that they can also provide information on the initial lateral transport speed of weak ash plumes bent over by crosswind. The method is illustrated by measurements made with a transportable volcano Doppler radar (VOLDORAD) at Arenal volcano, Costa Rica. The near-source displacements of the plume are tracked through echo onsets induced by ash entering successive probed volumes in the radar beam. A constant transport velocity is commonly reached within a few seconds of the initial ash emission, as wind advection and buoyancy take over momentum. The plume azimuth and upraise angles are constrained by comparing the amplitude decrease of the radar echoes as a function of distance from the source with results from a simple geometric plume model. The three dimensional vector of the ash cloud transport speed is then reconstructed with an accuracy of a few percent. This method may have applications for volcano monitoring, for determining pyroclast fluxes, for the modeling of tephra dispersal, and for remote measurements of volcanic gas fluxes.

Late Quaternary tephra layers around Raoul and Macauley Islands, Kermadec Arc: implications for volcanic sources, explosive volcanism and tephrochronology

AbstractA suite of deep‐sea cores were collected along transects up to 100 km across the fore‐arc and back‐arc regions of the predominantly submarine Kermadec arc near Raoul and Macauley islands, southwest Pacific. The cores reveal a macroscopic tephra record extending back &gt;50 ka. This is a significant addition to the dated record of volcanism, previously restricted to fragmented late Holocene records exposed on the two islands. The 27 macroscopic tephra layers display a wide compositional diversity in glass (∼50–78 wt% SiO2). Many tephra layers comprise silicic shards with a subordinate mafic shard population. This could arise from magma mingling and may reflect mafic triggering of the silicic eruptions. Broadly, the glass compositions can be distinguished on diverging high‐K and low‐K trends, most likely arising from different source volcanoes. This distinction is also reflected in the tephra records exposed on Raoul (low‐K) and Macauley (high‐K) islands, the likely source areas. Heterogeneous tephra comprising shards of both high‐ and low‐K affinity, silicic and mafic compositions, and more homogeneous tephra with subordinate outlier shard compositions, are best explained by post‐depositional mixing of separate eruption deposits or contemporaneous eruptions. Evidently, the slow sedimentation rates of the calcareous oozes (∼101–102 mm ka−1) were insufficient to adequately separate and preserve closely spaced eruption deposits. This exemplifies the difficulty in assessing eruption frequencies and magmatic trends, and erecting a tephrostratigraphy, using geochemical fingerprinting in such environments. Despite these difficulties, the ca. 5.7 ka Sandy Bay Tephra erupted from Macauley Island can be correlated over a distance of &gt;100 km, extending east and west of the island, showing that the mostly submerged volcanoes are capable of wide tephra dispersal. Hence there is potential for developing chronostratigraphies for the southwest Pacific beyond the region covered by the extensive rhyolite marker beds from the Taupo Volcanic Zone. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Three‐dimensional volcanic aerosol dispersal: A comparison between Multiangle Imaging Spectroradiometer (MISR) data and numerical simulations

The three‐dimensional reconstruction of volcanic plumes is a central goal to enhance our understanding on dispersal processes. In this paper we use data from the Multiangle Imaging Spectroradiometer (MISR) on board NASA's Terra spacecraft combined with a stereo matching retrieval procedure. We show the potential of MISR in capturing important features of volcanic plumes like column height, optical depth, type, and shape of the finest particles of two highly explosive eruptions occurring on Mount Etna in 2001 and 2002. This work tests how tephra dispersal models reconstruct the 3‐D shape of volcanic clouds. We compare MISR data with FALL3D, an Eulerian model for the transport and deposition of volcanic ash and aerosols coupled with the Weather Research and Forecasting mesoscale meteorological model. Agreement between simulations and MISR data is good regarding both events, although it could be improved by increasing the accuracy of the meteorological data, a better constraint on volcanological input parameters like the height of the eruptive column and improving our understanding of processes such as aggregation phenomena and volcanic cloud microphysics.

A simple semi-empirical approach to model thickness of ash-deposits for different eruption scenarios

Abstract. The impact of ash-fall on people, buildings, crops, water resources, and infrastructure depends on several factors such as the thickness of the deposits, grain size distribution and others. Preparedness against tephra falls over large regions around an active volcano requires an understanding of all processes controlling those factors, and a working model capable of predicting at least some of them. However, the complexity of tephra dispersion and sedimentation makes the search of an integral solution an almost unapproachable problem in the absence of highly efficient computing facilities due to the large number of equations and unknown parameters that control the process. An alternative attempt is made here to address the problem of modeling the thickness of ash deposits as a primary impact factor that can be easily communicated to the public and decision-makers. We develop a semi-empirical inversion model to estimate the thickness of non-compacted deposits produced by an explosive eruption around a volcano in the distance range 4–150 km from the eruptive source. The model was elaborated from the analysis of the geometric distribution of deposit thickness of 14 world-wide well-documented eruptions. The model was initially developed to depict deposits of potential eruptions of Popocatépetl and Colima volcanoes in México, but it can be applied to any volcano. It has been designed to provide planners and Civil Protection authorities of an accurate perception of the ash-fall deposit thickness that may be expected for different eruption scenarios. The model needs to be fed with a few easy-to-obtain parameters, namely, height of the eruptive column, duration of the explosive phase, and wind speed and direction, and its simplicity allows it to run in any platform, including a personal computers and even a notebook. The results may be represented as tables, two dimensional thickness-distance plots, or isopach maps using any available graphic interface. The model has been tested, with available data from some recent eruptions in México, and permits to generate ash-fall deposit scenarios from new situations, or to recreate past situations, or to superimpose scenarios from eruptions of other volcanoes. The results may be displayed as thickness vs. distance plots, or as deposit-thickness scenarios superimposed on a regional map by means of a visual computer simulator based on a user-friendly built-in computer graphic interface.

Evaluating long-range volcanic ash hazard using supercomputing facilities: application to Somma-Vesuvius (Italy), and consequences for civil aviation over the Central Mediterranean Area

Volcanic ash causes multiple hazards. One hazard of increasing importance is the threat posed to civil aviation, which occurs over proximal to long-range distances. Ash fallout disrupts airport operations, while the presence of airborne ash at low altitudes near airports affects visibility and the safety of landing and take-off operations. Low concentrations of ash at airplane cruise levels are sufficient to force re-routing of in-flight aircrafts. Volcanic fallout deposits spanning large distances have been recognized from the Somma-Vesuvius volcano for several Holocene explosive eruptions. Here we develop hazard and isochron maps for distal ash fallout from the Somma-Vesuvius, as well as hazard maps for critical ash concentrations at relevant flight levels. Maps are computed by coupling a meteorological model with a fully numeric tephra dispersal model that can account for ash aggregation processes, which are relevant to the dispersion dynamics of fine ash. The simulations were carried out using supercomputing facilities, spanning on entire meteorological year that is statistically representative of the local meteorology during the last few decades. Seasonal influences are also analyzed. The eruptive scenario is based on a Subplinian I-type eruption, which is within the range of the maximum expected event for this volcano. Results allow us to quantify the impact that an event of this magnitude and intensity would have on the main airports and aerial corridors of the Central Mediterranean Area.

Widespread dispersal of Icelandic tephra: how does the Eyjafjöll eruption of 2010 compare to past Icelandic events?

AbstractThe Eyjafjöll AD 2010 eruption is an extraordinary event in that it led to widespread and unprecedented disruption to air travel over Europe – a region generally considered to be free from the hazards associated with volcanic eruptions. Following the onset of the eruption, satellite imagery demonstrated the rapid transportation of ash by westerly winds over mainland Europe, eventually expanding to large swathes of the North Atlantic Ocean and the eastern seaboard of Canada. This small‐to‐intermediate size eruption and the dispersal pattern observed are not particularly unusual for Icelandic eruptions within a longer‐term perspective. Indeed, the Eyjafjöll eruption is a relatively modest eruption in comparison to some of the 20 most voluminous eruptions that have deposited cryptotephra in sedimentary archives in mainland Europe, such as the mid Younger Dryas Vedde Ash and the mid Holocene Hekla 4 tephra. The 2010 eruption, however, highlights the critical role that weather patterns play in the distribution of a relatively small amount of ash and also highlights the spatially complex dispersal trajectories of tephra in the atmosphere. Whether or not the preservation of the Eyjafjöll 2010 tephra in European proxy archives will correspond to the extensive distributions mapped in the atmosphere remains to be seen. The Eyjafjöll 2010 event highlights our increased vulnerability to natural hazards rather than the unparalleled explosivity of the event. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Modeling tephra dispersal in absence of wind: Insights from the climactic phase of the 2450 BP Plinian eruption of Pululagua volcano (Ecuador)

The determination of eruptive parameters is crucial in volcanology, not only to document past eruptions, but also for tephra fallout hazard assessments. In most tephra fallout studies, eruptive parameters have been determined either by empirical techniques or analytical models, but the uncertainty of such parameters is usually not well described. We have applied both empirical and analytical models to characterize the climactic phase of the 2450 BP Plinian eruption of Pululagua (BF2 layer) and explore the variations in the total erupted mass, column height and total grain size distribution. Both approaches yield comparable results in the total mass of tephra erupted (4.5 ± 1.5 × 10 11 kg), while they show some discrepancies for the determination of the column height (36–20 km from empirical techniques and 30–20 km from analytical techniques). The total grain size distribution of the BF2 layers varies with the different techniques used for the calculation and significantly affects the outputs of analytical models. Furthermore, the determination of the total grain size distribution depends strongly on the number and spatial distribution of the sample location. Inverting tephra fallout deposits on the total accumulation (or thickness) gives a good constraint on the total mass erupted but not on the column height. However, inverting on individual grain size classes better constrains the possible range of column heights (but cannot resolve particle release height). Results from the inversion on individual grain size classes show that large diffusion coefficients are necessary to model the BF2 layer and might be required to model proximal tephra deposits in order not to overestimate the total erupted mass. Additionally, we used a statistical method (smoothed bootstrap approach) to quantify the uncertainty in eruptive parameters such as column height and total erupted mass. Our uncertainty analysis yields a mean total erupted mass of 4.5 ± 0.3 × 10 11 kg and a mean column height of 30 ± 3 km. Results from the uncertainty analysis compare well with results from other approaches. Finally, although the climactic phase of the 2450 BP Plinian eruption of Pululagua occurred in relatively calm atmospheric conditions, our results show that the dispersion of the BF2 layer was influenced by slight northeasterly wind conditions.

Simulations of tephra dispersal from the 1991 explosive eruptions of Hudson volcano, Chile

The 1991 explosive eruptions of Hudson volcano in southern Chile produced 2.7 km 3 (dense rock equivalent) of basalt and trachyandesite tephra during the period August 8–15. The initial basaltic phase (phase I, August 8–9) produced a maximum column height of 12 km above sea level (ASL) and tephra fallout was directed to the north and northeast by the prevailing winds. The paroxysmal trachyandesitic phase (phase II, August 12–15) involved at least three separate events with a maximum ∼ 18-km-high (ASL) eruption column inferred from satellite temperature data. During the initial 24 h of this phase the plume was advected almost directly south, before swinging towards the east as the wind changed direction. The plume was ultimately directed to the southeast and stayed relatively fixed at this bearing for the remainder of the eruption. These temporal variations in the main dispersal direction during the earlier stages of the phase II eruption produced a much wider overall deposit than would be expected from a plume with a relatively fixed transport direction (e.g., latter stages of phase II). The Lagrangian ash tracking model PUFF was utilized to simulate the 1991 explosive eruptions and was able to successfully reproduce the aerial distribution and temporal evolution of the plumes. The optimal agreement between the observed and simulated plumes occurs when the highest concentration of ash particles coincides with the tropopause, a height that is typically lower than the maximum observed column heights for the 1991 eruptions. Gravitational settling of the laterally spreading umbrella region (e.g., Pinatubo 1991) may result in the concentration of ash at this level. This may account for differences in column height estimates between ground- or satellite-based and lithic-based models. The plume associated with the paroxysmal phase (August 12–15, 1991) produced a multilayered deposit composed of alternating layers of fine ash and pumice lapilli. The highly stratified nature of the fall deposit is likely the result of multiple eruptive events coupled with a time varying wind field. A strongly changing wind direction that occurred during the earlier stages of the paroxysmal eruption could have produced variations in the dominant grain size being deposited between fine ash and pumice lapilli during individual eruptive sequences.

Tephra dispersal and eruption dynamics of wet and dry phases of the 1875 eruption of Askja Volcano, Iceland

The 1875 rhyolitic eruption of Askja volcano in Iceland was a complex but well-documented silicic explosive eruption. Eyewitness chronologies, coupled with examination of very proximal exposures and historical records of distal deposit thickness, provide an unusual opportunity for study of Plinian and phreatoplinian eruption and plume dynamics. The ∼ 17 hour-long main eruption was characterized by abrupt and reversible shifts in eruption style, e.g., from ‘wet’ to ‘dry’ eruption conditions, and transitions from fall to flow activity. The main eruption began with a ‘dry’ subplinian phase (B), followed by a shift to a very powerful phreatoplinian ‘wet’ eruptive phase (C1). A shift from sustained ‘wet’ activity to the formation of ‘wet’ pyroclastic density currents followed with the C2 pyroclastic density currents, which became dryer with time. Severe ground shaking accompanied a migration in vent position and the onset of the intense ‘dry’ Plinian phase (D). Each of the fall units can be modeled using the segmented exponential thinning method (Bonadonna et al. 1998), and three to five segments have been recognized on a semilog plot of thickness vs. area1/2. The availability of very proximal and far-distal thickness data in addition to detailed observations taken during this eruption has enabled calculations of eruption parameters such as volumes, intensities and eruption column heights. This comprehensive dataset has been used here to assess the bias of volume calculations when proximal and distal data are missing, and to evaluate power-law and segmented exponential thinning methods using limited datasets.

Monitoring and forecasting Etna volcanic plumes

Abstract. In this paper we describe the results of a project ongoing at the Istituto Nazionale di Geofisica e Vulcanologia (INGV). The objective is to develop and implement a system for monitoring and forecasting volcanic plumes of Etna. Monitoring is based at present by multispectral infrared measurements from the Spin Enhanced Visible and Infrared Imager on board the Meteosat Second Generation geosynchronous satellite, visual and thermal cameras, and three radar disdrometers able to detect ash dispersal and fallout. Forecasting is performed by using automatic procedures for: i) downloading weather forecast data from meteorological mesoscale models; ii) running models of tephra dispersal, iii) plotting hazard maps of volcanic ash dispersal and deposition for certain scenarios and, iv) publishing the results on a web-site dedicated to the Italian Civil Protection. Simulations are based on eruptive scenarios obtained by analysing field data collected after the end of recent Etna eruptions. Forecasting is, hence, supported by plume observations carried out by the monitoring system. The system was tested on some explosive events occurred during 2006 and 2007 successfully. The potentiality use of monitoring and forecasting Etna volcanic plumes, in a way to prevent threats to aviation from volcanic ash, is finally discussed.

Evidence for Recent Large Magnitude Explosive Eruptions at Damavand Volcano, Iran with Implications for Volcanic Hazards

Damavand is a large dormant stratovolcano in the Alborz Mountains of northern Iran located in one of the most populous provinces, which could be adversely affected by tephra fall from Damavand. The youngest known eruption is a lava flow on the western flanks with an age of 7.3 ka. The volcanic products are predominantly porphyritic trachyandesite. Three major young pumice deposits, named here as Mallar, Karam Poshteh and Reyneh pumices, are identified, with provisional ages in the interval >7.3 ka and < 25 ka. The deposits cover much of the southern and western flanks of the volcano. The Mallar and Reyneh deposits consist of extensive basal pumice fall deposits with dispersal axes to the east, overlying pumiceous pyroclastic flow deposits extending up to 20 km from the summit and late-stage lahar deposits. The middle unit (Karam Poshteh) consists of a coarse-grained pumice fall deposit with proximal welded facies dispersed to the west, but lacks pyroclastic flow deposits. Based on reconnaissance field data they were formed by explosive eruptions of VEI4. Some of the villages on the flanks of the volcano are built on pyroclastic flow and lahar fans, and thus would be at high risk in the event of future explosive eruptions. We present an analysis of wind data and the applications of a computer tephra dispersal model to assess tephra fall hazards. Explosive eruptions of Damavand in the future would adversely affect large cities in the neighbouring provinces to the east, reflecting the dominant regional stratospheric wind directions.