Modeling of the Terminal Velocities of the Dust Ejected Material by the Impact

We compute the distribution of velocities of the particles ejected by the impact of the projectile released from NASA Deep Impact spacecraft on the nucleus of comet 9P/Tempel 1 on the successive 20 hours following the collision. This is performed by the development and use of an ill-conditioned inverse problem approach, whose main ingredients are a set of observations taken by the Narrow Angle Camera (NAC) of OSIRIS onboard the Rosetta spacecraft, and a set of simple models of the expansion of the dust ejecta plume for different velocities. Terminal velocities are derived using a maximum likelihood estimator. We compare our results with published estimates of the expansion velocity of the dust cloud. Our approach and models reproduce well the velocity distribution of the ejected particles. We consider these successful comparisons of the velocities as an evidence for the appropriateness of the approach. This analysis provides a more thorough understanding of the properties of the Deep Impact dust cloud.Comment: Comments: 6 pages, 2 Postscript figures, To appear in the proceedings of "Deep Impact as a World Observatory Event - Synergies in Space, Time", ed. Hans Ulrich Kaeufl and Chris Sterken, Springer-Verla

"Explosive Energy" during volcanic eruptions from fractal analysis of pyroclasts

Despite recent advances by means of experiments and high-resolution surveys and the growing understanding of the physical processes before and during volcanic eruptions, duration and type of eruptive activity still remain highly unpredictable. This uncertainty hinders appropriate hazard and associated risk assessment tremendously. In an effort to counter this problem, experimentally generated pyroclasts have been studied by fractal statistics with the aim of evaluating possible relationships between eruption energy and fragmentation efficiency. Rapid decompression experiments have been performed on three differently porous sample sets of the 1990–1995 eruption of Unzen volcano (Japan) at 850 °C and at initial pressure values above the respective fragmentation threshold [U. Kueppers, B. Scheu, O. Spieler, D. B. Dingwell, Fragmentation efficiency of explosive volcanic eruptions: a study of experimentally generated pyroclasts. J. Volcanol. Geotherm. Res. 153 (2006) 125–135.,O. Spieler, B. Kennedy, U. Kueppers, D.B. Dingwell, B. Scheu, J. Taddeucci, The fragmentation threshold of pyroclastic rocks. EPSL 226 (2004) 139–148.]. The size distribution of generated pyroclasts has been studied by fractal fragmentation theory and the fractal dimension of fragmentation (Df), a value quantifying the intensity of fragmentation, has been measured for each sample. Results showthat size distribution of pyroclastic fragments follows a fractal law(i.e. power-law) in the investigated range of fragment sizes, indicating that fragmentation of experimental samples reflects a scale-invariant mechanism. In addition, Df is correlated positively with the potential energy for fragmentation (PEF) while showing a strong influence of the open porosity of the samples. Results obtained in this work indicate that fractal fragmentation theory may allow for quantifying fragmentation processes during explosive volcanic eruptions by calculating the fractal dimension of the size distribution of pyroclasts. It emerges fromthis study that fractal dimension may be utilised as a proxy for estimating the explosivity of volcanic eruptions by analysing their natural pyroclastic deposits

Nature and efficiency of pyroclast generation from porous magma: Insights from field investigations and laboratory experiments

Enhanced knowledge of pre- and syn-eruptive processes is vital to deal with the increasing threat imposed to population and infrastructure by volcanoes that have been active historically and may potentially erupt in future. For many years, most of this knowledge was received from experiments on analogue materials and/or numerical models. In order to increase their significance and applicability for the “real” case, the natural complexity may not be oversimplified and the input parameters must be reliable and realistic. In the light of this, a close connection of field and laboratory work is essential. Volcanic eruptions may be phreatic, phreatomagmatic or magmatic, the latter scenario of which was addressed in this study. Rising magma is subject to decreasing lithostatic pressure. As a direct consequence, volatiles become increasingly oversaturated and bubbles will nucleate and grow depending on initial volatile content and melt viscosity. Both factors directly influence the diffusivity that limits the rate of bubble growth. Increasing amounts of bubbles increase the buoyancy difference to the surrounding rocks and lead to an acceleration of the rising melt-bubble mixture. Beside these limiting factors, the overpressure in the gas bubbles greatly depends on the magma’s ascent speed as it controls the residence time to conditions favourable to degassing (a combination of lithostatic pressure and magma temperature) and the time of overpressure reduction due to degassing. Volcanic eruptions occur when the bubbly melt can no longer withstand the exerted stress that derives from the overlying weight (lithostatic pressure), the expanding gas bubbles (internal gas overpressure) and different ascent velocities in the conduit (velocity profile). The melt will be fragmented and the gas-pyroclast mixture will be erupted. This study has combined close investigation of the deposits of the 1990-1995 eruption of Unzen volcano, Japan and detailed laboratory investigations on samples of this eruption and other volcanoes. The field work intended to reveal the density distribution of samples from within the eruptive products. Although all samples already underwent one eruption, their physical state (e.g. crystallinity, porosity) mostly remained close to sub-surface pre-eruption conditions due to their high viscosity and accordingly allowed their usage for the analysis of the fragmentation behaviour. In that purpose, rapid decompression experiments that simulate volcanic eruptions triggered by internal gas overpressure have been performed at 850 °C to evaluate fragmentation threshold and fragmentation efficiency. Laboratory investigations of that kind are one approach to bridge the gap between observational field volcanology and risk assessment as they reveal information on what can not be investigated closely but what is essential to know for realistic eruption models and the adjacent hazard mitigation. Changing the experimental conditions and close investigation of the artificial products reveals the influence of physical properties on the fragmentation behaviour. The density distribution inside a dome and the upper part of the conduit is crucial to the eruptive style of an explosive volcano. This information cannot be collected during an ongoing eruption but is important for future hazard assessment via modelling conduit flow and dome collapse/explosion behaviour. Therefore, the percentage of the mass fractions of all rock types in the primary and secondary volcanic deposits must be evaluated. For this purpose and at the lowest logistic effort, field-based density measurements have been performed on Unzen volcano, Japan. The resultant density distribution was found to be generally bimodal at constant peak values but changing peak ratios. The most abundant rock types at Unzen exhibited an open porosity of 8 and 20 vol.%, respectively. The porosity was found to be arranged in layers of cm- to dm-scale that were oriented subparallel to flow direction, i.e. subvertical within the conduit and flank-parallel within the dome lobes. The achieved results allowed for an internal picture of the dome during the last eruption of Unzen volcano. The evaluated picture of the density distribution within the uppermost parts of the conduit and the dome itself allowed for insights into and a better understanding of magma ascent and degassing conditions at Unzen volcano during its last eruption. Knowledge of the density distribution is additionally required to draw conclusions from the results of laboratory investigations on the fragmentation behaviour to the monitored behaviour of Unzen volcano during its last eruption. In the laboratory, the fragmentation behaviour upon rapid decompression has been investigated in a modified fragmentation bomb (Spieler et al., 2004). At 850 °C, initial overpressure conditions as high as 50 MPa have been applied to sample cylinders (25 mm diameter, 60 mm length) drilled from natural samples. In a first step, the minimum overpressure required to cause complete sample fragmentation (= fragmentation threshold, ΔPfr) has been evaluated. Results from samples of several volcanoes (Unzen, Montserrat, Stromboli, and Mt. St. Helens) showed that ΔPfr mainly depended on open porosity and permeability of the specific sample as these parameters were controlling pressure build-up and loss. The experiments further revealed that sample fragmentation was not the result of a single process but the result of a combination of simultaneously occurring processes as indicated by Alidibirov et al. (2000). The degree of influence of a single process to the fragmentation behaviour was found to be porosity-dependent. Further experiments at initial pressure conditions above ΔPfr and close investigation of the artificially generated pyroclasts allowed evaluating the fragmentation efficiency upon changing physical properties of the used samples. The efficiency of sample size reduction was investigated by grain-size analysis (dry sieving for particles bigger than 0.25 mm and wet laser refraction for particles smaller than 0.25 mm) and surface area measurements (by Argon adsorption). Results of experiments with samples of different porosities at different initial pressure values showed that the efficiency of fragmentation increased with increasing energy. The energy available for fragmentation was estimated from the open porosity and the applied pressure. A series of abrasion experiments was performed to shed light on the grain size reduction due to particle-particle interaction during mass movements. The degree of abrasion was found to be primarily depending on porosity and experimental duration. The results showed that abrasion may change the density distribution of block-and-ash flows (BAF) by preferentially abrading porous clasts. However, during the short drying interval prior to the analysis of the experimental pyroclasts, abrasion-induced grain-size reduction only played a minor role and was assumed to be negligible

Field-based density measurements as tool to identify preeruption dome structure: set-up and first results from Unzen volcano, Japan

For an improvement in the quality of conduit flow and dome-related explosive eruption models, knowledge of the preeruption or precollapse density of the rocks involved is necessary. As close investigation is impossible during eruption, the best substitute comes from quantitative investigation of the eruption deposits. The porosity of volcanic rocks is of primary importance for the eruptive behaviour and, accordingly, a key-parameter for realistic models of dome stability and conduit flow. Fortunately, this physical property may be accurately determined via density measurements. We developed a robust, battery-powered device for rapid and reliable density measurements of dry rock samples in the field. The density of the samples (sealed in plastic bags at 250 mbar) is determined using the Archimedean principle. We have tested the device on the deposits of the 1990–1995 eruption of Unzen volcano, Japan. Short setup and operation times allow up to 60 measurements per day under fieldwork conditions. The rapid accumulation of correspondingly large data sets has allowed us to acquire the first statistically significant data set of clast density distribution in block-and-ash flow deposits. More than 1100 samples with a total weight of 2.2 tons were measured. The data set demonstrates that the deposits of the last eruptive episode at Unzen display a bimodal density distribution, with peaks at 2.0F0.1 and 2.3F0.1 g/cm3, corresponding to open porosity values of 20 and 8 vol.%, respectively. We use this data set to link the results of laboratory-based fragmentation experiments to field studies at recently active lava domes

Fragmentation efficiency of explosive volcanic eruptions: A study of experimentally generated pyroclasts

Products of magma fragmentation can pose a severe threat to health, infrastructure, environment, and aviation. Systematic evaluation of the mechanisms and the consequences of volcanic fragmentation is very difficult as the adjacent processes cannot be observed directly and their deposits undergo transport-related sorting. However, enhanced knowledge is required for hazard assessment and risk mitigation. Laboratory experiments on natural samples allow the precise characterization of the generated pyroclasts and open the possibility for substantial advances in the quantification of fragmentation processes. They hold the promise of precise characterization and quantification of fragmentation efficiency and its dependence on changing material properties and the physical conditions at fragmentation. We performed a series of rapid decompression experiments on three sets of natural samples from Unzen volcano, Japan. The analysis comprised grain-size analysis and surface area measurements. The grain-size analysis is performed by dry sieving for particles larger than 250 Am and wet laser refraction for smaller particles. For all three sets of samples, the grain-size of the most abundant fraction decreases and the weight fraction of newly generated ash particles (up to 40 wt.%) increases with experimental pressure/potential energy for fragmentation. This energy can be estimated from the volume of the gas fraction and the applied pressure. The surface area was determined through Argon adsorption. The fragmentation efficiency is described by the degree of fineparticle generation. Results show that the fragmentation efficiency and the generated surface correlate positively with the applied energy

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

High-speed imaging of Strombolian explosions: The ejection velocity of pyroclasts

Explosive volcanic eruptions are defined as the violent ejection of gas and hot fragments from a vent in the Earth's crust. Knowledge of ejection velocity is crucial for understanding and modeling relevant physical processes of an eruption, and yet direct measurements are still a difficult task with largely variable results. Here we apply pioneering high-speed imaging to measure the ejection velocity of pyroclasts from Strombolian explosive eruptions with an unparalleled temporal resolution. Measured supersonic velocities, up to 405 m/s, are twice higher than previously reported for such eruptions. Individual Strombolian explosions include multiple, sub-second-lasting ejection pulses characterized by an exponential decay of velocity. When fitted with an empirical model from shock-tube experiments literature, this decay allows constraining the length of the pressurized gas pockets responsible for the ejection pulses. These results directly impact eruption modeling and related hazard assessment, as well as the interpretation of geophysical signals from monitoring networks

Quantification of magma ascent rate through rockfall monitoring at the growing/collapsing lava dome of Volcán de Colima, Mexico

International audienceThe most recent eruptive phase of Volc'an de Colima, Mexico, started in 1998 and was characterized by dome growth with a variable effusion rate, interrupted intermittently by explosive eruptions. Between November 2009 and June 2011, activity at the dome was mostly limited to a lobe on the western side where it had previously started overflowing the crater rim, leading to the generation of rockfall events. As a consequence of this, no significant increase in dome volume was perceivable and the rate of magma ascent, a crucial parameter for volcano monitoring and hazard assessment could no longer be quantified via measurements of the dome's dimensions. Here, we present alternative approaches to quantify the magma ascent rate. We estimate the volume of individual rockfalls through the detailed analysis of sets of photographs (before and after individual rockfall events). The relationship between volume and infrared images of the freshly exposed dome surface and the seismic signals related to the rockfall events were then investigated. Larger rockfall events exhibited a correlation between its previously estimated volume and the surface temperature of the freshly exposed dome surface, as well as the mean temperature of rockfall mass distributed over the slope. We showed that for larger events, the volume of the rockfall correlates with the maximum temperature of the newly exposed lava dome as well as a proxy for seismic energy. It was therefore possible to calibrate the seismic signals using the volumes estimated from photographs and the count of rockfalls over a certain period was used to estimate the magma extrusion flux for the period investigated. Over the course of the measurement period, significant changes were observed in number of rockfalls, rockfall volume and hence averaged extrusion rate. The extrusion rate was not constant: it increased from 0.008±0.003 to 0.02±0.007m3 s−1 during 2010 and dropped down to 0.008±0.003m3 s−1 again in March 2011. In June 2011, magma extrusion had come to a halt. The methodology presented represents a reliable tool to constrain the growth rate of domes that are repeatedly affected by partial collapses. There is a good correlation between thermal and seismic energies and rockfall volume. Thus it is possible to calibrate the seismic records associated with the rockfalls (a continuous monitoring tool) to improve volcano monitoring at volcanoes with active dome growth

Floating stones off El Hierro, Canary Islands: xenoliths of pre-island sedimentary origin in the early products of the October 2011 eruption

The eruption that started off the south coast of El Hierro, Canary Islands, in October 2011 has emitted intriguing eruption products found floating in the sea. These specimens appeared as floating volcanic "bombs" that have in the meantime been termed "restingolites" (after the close-by village of La Restinga) and exhibit cores of white and porous pumice-like material. Currently the nature and origin of these "floating stones" is vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The "restingolites" have been proposed to be either (i) juvenile high-silica magma (e.g. rhyolite), (ii) remelted magmatic material (trachyte), (iii) altered volcanic rock, or (iv) reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have collected and analysed the structure and composition of samples and compared the results to previous work on similar rocks found in the archipelago. Based on their high silica content, the lack of igneous trace element signatures, and the presence of remnant quartz crystals, jasper fragments and carbonate relicts, we conclude that "restingolites" are in fact xenoliths from pre-island sedimentary rocks that were picked up and heated by the ascending magma causing them to partially melt and vesiculate. They hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies in the Canary Islands as well as in similar Atlantic islands that rest on sediment/covered ocean crust (e.g. Cape Verdes, Azores). The occurrence of these "restingolites" does therefore not indicate the presence of an explosive high-silica magma that is involved in the ongoing eruption

Time-series analysis of fissure-fed multi-vent activity: a snapshot from the July 2014 eruption of Etna volcano (Italy)

The April to May 2010 eruption of Eyjafjallajökull (Iceland) volcano was characterized by a large compositional variability of erupted products. To contribute to the understanding of the plumbing system dynamics of this volcano, we present new EMPA and LA-ICP-MS data on groundmass glasses of ash particles and minerals erupted between April 15 and 22. The occurrence of disequilibrium textures in minerals, such as resorption and inverse zoning, indicate that open system processes were involved in determining the observed compositional variability. The variation of major and trace element data of glasses corroborates this hypothesis indicating that mixing between magma batches with different compositions interacted throughout the whole duration of the eruption. In particular, the arrival of new basaltic magma into the plumbing system of the volcano destabilized and remobilized magma batches of trachyandesite and rhyolite compositions that, according to geophysical data, might have intruded as sills over the past 20 years beneath the Eyjafjallajökull edifice. Two mixing processes are envisaged to explain the time variation of the compositions recorded by the erupted tephra. The first occurred between basaltic and trachyandesitic end-members. The second occurred between trachyandesite and rhyolites. Least-squares modeling of major elements supports this hypothesis. Furthermore, investi- gation of compositional histograms of trace elements allows us to estimate the initial proportions of melts that interacted to generate the compositional variability triggered by mixing of trachyandesites and rhyolites.Published515V. Dinamica dei processi eruttivi e post-eruttiviJCR Journa