Experimental investigations on the explosivity of steam-driven eruptions: a case study of Solfatara volcano (Campi Flegrei) (original) (raw)

Explosive properties of water in volcanic and hydrothermal systems

Journal of Geophysical Research, 2009

This paper describes, from a thermodynamic point of view, 7 the physico-chemical conditions, under which water behaves as an explosive. 8 This phenomenon occurs frequently in hydrothermal and volcanic systems, 9 when water is brutally shifted from its initial equilibrium state. Water (ei-10 ther liquid or gas) becomes metastable or unstable, and reequilibrates by vi-11 olent demixing of a liquid-gas mixture. In a first step, a phenomenological 12 approach of metastability is given in an one-component perspective, intro-13 ducing the notion of spinodals and delimiting the extent of metastable fields. 14 The physical mechanisms (bubble nucleation, cavitation, spinodal decom-15 position), which are involved in these explosive transformations of water, are 16 detailed in what relates to the natural eruptions topic. The specific thermo-17 dynamic properties (P -v-T -H-U ) of metastable water are presented by us-18 ing the reference Wagner and Pruss equation of state. Then, the mechani-19 cal work produced by the different possible physical transformations, includ-20 ing decompression, vaporization, isobaric heating and exsolution, involved 21 in water explosions are quantified. The classic calculation of the energy bal-22

Experimental constraints on phreatic eruption processes at Whakaari (White Island volcano)

Journal of Volcanology and Geothermal Research, 2015

Vigorous hydrothermal activity interspersed by sequences of phreatic and phreatomagmatic eruptions occur at Whakaari (White Island volcano), New Zealand. Here, we investigate the influence of sample type (hydrothermally altered cemented ash tuffs and unconsolidated ash/lapilli) and fragmentation mechanism (steam flashing versus gas expansion) on fragmentation and ejection velocities as well as on particle-size and shape. Our rapid decompression experiments show that fragmentation and ejection speeds of two ash tuffs, cemented by alunite and amorphous opal, increase with increasing porosity and that both are significantly enhanced in the presence of steam flashing. Ejection speeds of unconsolidated samples are higher than ejection speeds of cemented tuffs, as less energy is consumed by fragmentation. Fragmentation dominated by steam flashing results in increased fragmentation energy and a higher proportion of fine particles. Particle shape analysis before and after fragmentation reveal that both steam flashing and pure gas expansion produce platy or bladed particles from fracturing parallel to the decompression front. Neither fragmentation mechanisms nor sample type show a significant influence on the shape. Our results emphasize that, under identical pressure and temperature conditions, eruptions accompanied by the process of liquid water flashing to steam are significantly more violent than those driven simply by gas expansion. Therefore, phase changes during decompression and cementation are both important considerations for hazard assessment and modeling of eruptions in hydrothermally active environments.

The mechanism of phreatic eruptions

Journal of Geophysical Research, 1995

We investigate the mechanism for initiating phreatic eruptions following the emplacement of a shallow magmatic intrusion into water-saturated permeable rock which contains subsidiary low-permeability crack networks and disconnected cracks. Heat from the intrusion causes the local groundwater to boil and ascend through the main permeable crack network. As the ascending superheated steam heats the overlying rock, the water in the subsidiary networks and disconnected cracks will boil. The pressure exerted by the vapor in the subsidiary and disconnected cracks can lead to rapid horizontal crack propagation, resulting in an increase in crack length by more than an order of magnitude. According to the model, the eruption process starts near a free surface and migrates rapidly along thermoelastic isostresses as a result of multiple breakage of the thin surface layers above the cracks. For certain crack and rock parameters, however, the crack propagation mechanism, instead of leading to a dynamic eruption, may generate a highly cracked zone that may be removed later by fluid transport processes. The proposed mechanism gives rise to precursory phenomena observed in conjunction with many phreatic eruptions. According to the model developed here, phreatic eruptions are most likely to occur only for a rather restricted set of rock parameters. For example, the country rock should not be too strong (c• t • 10 MPa) and should be characterized by two-scale permeability structure involving a main crack network of relatively high permeability (•> 10 -•2 m 2) and a subsidiary crack network with much lower permeability (< 10 -•7 m2). Moreover, the model works better if the mean crack aspect ratio is relatively large (13 -10 -•) and the crack concentration is not too low (• > 10-2). These restrictions may explain indirectly why phreatic eruptions are not ubiquitous in volcanic regions. Introduction Hydrovolcanic phenomena vary greatly. At one end of a spectrum are hydrothermal eruptions [e.g., Lloyd, 1959; Hedenquist and Henley, 1985] that may be related to transient pressure changes in near-surface (< 300 m) regions of hydrothermal systems and that apparemly do not involve direct magmatic heating. At the other end are phreatomagmatic eruptions that involve fresh magma along with water, steam, and brecciated country rock. These eruptions apparently occur in connection with the influx of considerable quantities of water [Moore et at., 1966; Williams and McBirney, 1979; Shepherd and Sigurdsson, 1982]. Between these extremes are phreatic eruptions, which are generally thought to involve the transfer of magmatic heat to circulating groundwater and subsequent eruption of steam and country rock but, often, without the eruption of fresh magma [Oilier, 1974; Barberi et at., 1992]. This paper focuses on phreatic eruptions; however, the thermal crack propagation processes that we discuss below may be important in other types of hydrovolcanic phenomena. for the most part, the physical processes involved in initiating such eruptions are not thoroughly understood. Barberi et at. [1992] classify three broad types of phreatic eruptions: (1) fracturing phenomena following seismic events, (2) heating of shallow aquifers following magma rise, and (3) phreatic explosions at volcanoes with magma located high in the edifice. In this paper, we consider a phreatic eruption following the emplacement of a dike at shallow depths in the Earth's crest where groundwaters are free to circulate (type (2)). We estimate the rate at which superheated steam rising along the margins of the dike heats the overlying rock and then determine the conditions for the propagation of isolated, fluid-filled cracks in the heated country rock. We argue that sufficient crack propagation can lead to massive failure of the country rock near the dike. Following failure, excavation of a crater may occur in a manner analogous to the process of rock outbursts in mines [e.g., Khristianovich and Satganik, 1983], or the fractured material may be removed by fluidization processes as suggested by Holmes [ 1965]. This paper uses an order of magnitude analysis to address processes that lead up to the eruption but does not 8417 8418 GERMANOVICH AND LOWELL: PHREATIC ERUPTIONS address the details of removal of lithie material outside the eruption crater. Our analysis of the thermal boundary layer adjacent to the dike is similar to previous studies [e.g., Parmentier, 1979; Cheng and Verma, 1981]. Mechanical processes of crack propagation that result from the heating of isolated, fluid-filled cracks are also analyzed in a rather elementary way. The main contribution of this paper is the consideration of coupled thermal and mechanical processes. That is, we investigate a mechanism whereby thermal energy of magma, after being transferred to nearby groundwater, is converted into mechanical work in the shallow crust. Consequently, this paper treats three problems: (1) heating of an aquifer from below, (2) propagation of isolated cracks, and (3) failure of near-surface rocks to form a crater. The scenario we discuss would give rise to precursors such as anomalous seismicity, ground deformation, and changes in fumarolic and hot spring output that commonly occur from weeks to up to a few years prior to an eruption [Barberi et at., 1992]. The crack propagation and rock failure mechanism we propose is consistent with observed crater dimensions that range from tens to a few hundred meters in diameter and up to a few hundred meters deep [Muffler et at., 1971; Oilier, 1974; Kiente et at., 1980].

Explosive thermal interactions between molten lava and water

Experimental Thermal and Fluid Science, 1993

1] Thermohydraulic explosions were produced by Molten Fuel Coolant Interaction (MFCI) experiments using remelted shoshonitic rocks from Vulcano (Italy). The fragmentation history and energy release were recorded. The resulting products were recovered and analyzed with the scanning electron microscope. Fine particles from experiments show shape and surface features that result from melt fragmentation in brittle mode. These clasts relate to the thermohydraulic phase of the MFCI, where most of the mechanical energy is released; they are here called ''active'' particles. The total surface area of such particles is proportional to the energy of the respective explosions. Other particles from experiments show shape and surface features that result from melt fragmentation in a ductile regime. These fragments, called ''passive'' particles, form after the thermohydraulic phase, during the expansion phase of the MFCI. In order to verify thermohydraulic explosions in volcanic eruptions, we compared experimental products with samples from phreatomagmatic base-surge deposits of Vulcano. Ash particles from the experiments show features similar to those from the deposits, suggesting that the experiments reproduced the same fragmentation dynamics. To achieve discrimination between active and passive particles, we calculated shape parameters from image analysis. The mass of active particles in base-surge deposits was calculated. As the material properties for the natural samples are identical to the experimental ones, the energy measurements and calculations of the experiments can be applied. For a single phreatomagmatic eruption at Vulcano, a maximum mechanical energy release of 2.75 Â 10 13 J was calculated, representing a TNT analogue of 6.5 kt. INDEX TERMS: 8414 Volcanology: Eruption mechanisms; 8404 Volcanology: Ash deposits; 8494 Volcanology: Instruments and techniques; 8439 Volcanology: Physics and chemistry of magma bodies; 8499 Volcanology: General or miscellaneous; KEYWORDS: explosive volcanism, phreatomagmatic explosions, image processing analysis, experimental volcanology, thermohydraulic explosions Citation: Büttner, R., P. Dellino, L. La Volpe, V. Lorenz, and B. Zimanowski, Thermohydraulic explosions in phreatomagmatic eruptions as evidenced by the comparison between pyroclasts and products from Molten Fuel Coolant Interaction experiments,

Dynamics of shallow hydrothermal eruptions: new insights from Vulcano’s Breccia di Commenda eruption

Bulletin of Volcanology, 2018

Understanding the dynamics and effects of hydrothermal eruptions is crucial to the hazard assessment in both volcanic and geothermal areas. Eruptions from hydrothermal centers may occur associated with magmatic phases, but also as isolated events without magmatic input, with the most recent examples being those of Te Maari (Tongariro, New Zealand) in 2012 and Ontake (Japan) in 2014. The most recent caldera of the Island of Vulcano (southern Italy) hosts in its center the La Fossa cone, active since 5.5 ka and now characterized by continuous fumarolic degassing. In historical times La Fossa cone has experienced several hydrothermal eruptions, with the most violent event being the Breccia di Commenda eruption, that occurred during the 13 th century AD. Based on analysis of 170 stratigraphic logs, we show that the Breccia di Commenda eruption occurred in three main phases. After an opening, low-intensity ash emission phase (Phase 1), the eruption energy climaxed during Phase 2, when a series of violent explosions produced an asymmetric shower of ballistic blocks and the contemporaneous emplacement of highly dispersed, lithic-rich, blast-like pyroclastic density currents (PDCs). The tephra units emplaced during Phase 2, ranging in volume from 0.2 to 2.7  10 5 m 3 , were covered in turn by thin ash fall deposits (Phase 3). The dynamics of the most violent and intense stage of the eruption (Phase 2) was investigated by numerical simulations. A threedimensional numerical model was applied, describing the eruptive mixture as a Eulerian-Eulerian, two-phase, non-equilibrium gas-particle fluid (plus a one-way coupled Lagrangian ballistic block fraction). At the initial simulation time, a mass of about 10 9 kg, with initial overpressure above 10 MPa, and a temperature of 250°C, was suddenly ejected from a 200-m-long, eastward inclined, NNE-SSW trending fissure. The mass release formed blast-like PDCs on both sides of the fissure and launched ballistic blocks eastwards. Field investigations and numerical simulations confirm that hydrothermal explosions at La Fossa cone includes intense ballistic fallout of blocks, emission of PDCs potentially travelling beyond the La Fossa caldera, and significant ash fallout. The hazard associated with both ballistic impact and PDC ingress, as associated with hydrothermal eruption, is significantly larger with respect to that associated with Vulcanian-type events of La Fossa.

Components and Phases: Modelling Progressive Hydrothermal Eruptions

The ANZIAM Journal, 2009

This is a review of progress made since [R. McKibbin, "An attempt at modelling hydrothermal eruptions", Proc. 11th New Zealand Geothermal Workshop 1989 (University of Auckland, 1989), 267-273] began development of a mathematical model for progressive hydrothermal eruptions (as distinct from "blasts"). Early work concentrated on modelling the underground process, while lately some attempts have been made to model the eruption jet and the flight and deposit of ejected material. Conceptually, the model is that of a boiling and expanding two-phase fluid rising through porous rock near the ground surface, with a vertical high-speed jet, dominated volumetrically by the gas phase, ejecting rock particles that are then deposited on the ground near the eruption site. Field observations of eruptions in progress and experimental results from a laboratory-sized model have confirmed the conceptual model. The quantitative models for all parts of the process are based on the fundamental conservation equations of motion and thermodynamics, using a continuum approximation for each of the components.

In situ granulation by thermal stress during subaqueous volcanic eruptions

Geology

Some of the most complex volcanic thermodynamic processes occur when erupting magma interacts with water. In shallow water, "Surtseyan" eruptions are spectacular, and they efficiently fragment magma into fine ash particles. The aviation hazard from these eruptions depends the amount of transportable fine ash that is generated and whether it is aggregated into particle coatings or accretions. To investigate both mechanisms, we analyzed ash-encased lapilli from the Surtseyan eruptions of

Dynamics of explosive degassing of magma: Observations of fragmenting two-phase flows

Journal of Geophysical Research, 1996

Liquid explosions, generated by rapid degassing of strongly supersaturated liquids, have been investigated in the laboratory with a view to understanding the basic physical laX•sses operating during bubble nucleation and growth and the subsequent behavior of the expanding two-phase flow. Experimenkq are carried out in a shock tube and ,me monitored by high-speed photography and pressure trm•sducers. Theoretical CO2 supersaturations up to 455 times the ambient saturation concentration ,are generated by a chemical reaction; K2CO3 solution is suddenly injected into an excess of HC1 solution in such a way as to mix the two solutions rapidly. Immediately after file injection event, a bubble nucleation delay of a few milliseconds is followed by rapid nucleation ,mid explosive expmlsion of CO2 bubbles forming a highly heterogeneous foam. Enhanced diffusion due to advection in the 11ow coupled with continuous mixing of tile reactants, and hence ongoing bubble nucleation after injection, generates an increasingly accelerating flow until the reactants become depleted at peak accelerations of around 150 g and velocities of about 15 m s-•. Stretching of the accelerating two-phase mixture enhances the mixing. Liberation of CO2 vapor is spatially inhomogeneous leading to ductile fragmentation occurring throughout the flow in regions of greatest gas release as the consequence of the collision and stretching of lluid streams. Tile violence of the eruptions is controlled by using different concentrations of tile HCI and K2CO 3 solutions, which alters the CO2 supersaturation and yield and also file efficiency of the mixing process. Peak acceleration is proportional to theoretical supersaturation. Pressure tneasurements at the base of the shock tube show an initial nucleation delay and a pressure pulse related to the onset of explosive bubble fortnation. These chemically induced explosions differ t¾om liquid explosions created in other experiments. In explosions caused by sudden depressurization of C02-saturated water, the bubbles nucleate uniformly fl•roughout the liquid in a single nucleation event. Subsequent bubble growth causes the two-phase mixture to be accelerated upward at nearly constant accelerations. Explosively boiling liquids, in which heterogeneous nucleation is suppressed, experience an evaporation wave which propagates down into the liquid column at constant average velocity. Fragtnentation occurs at the shin'ply dellned leading edge of the wavefront. The chemical flows effectively simulate highly explosive volcanic eruptions as they are comparable in terms of flow densities, velocities, accelerations, and in the large range of scales present. The lm'ge accelm,'ations cause su'ong extensional strain and longitudinal deformation. Comparable delbrmation rates in volcanic systems could be sufficient to approach conditions for brittle l•,tgmentation. Tube pumice is a major component of plinian deposits and ignimbrites and preserves evidence of accelerating llow conditions. mounts of dissolved gas becomes strongly supersaturated on approaching the Earth's surface. Gas bubbles nucleate and grow explosively and the magma disintegrates into a two-phase mixture of gas and pyroclasts that accelerates to velocities of order of a few hundred meters per second along volcanic conduits [Wilson et al., 1980; Dobran, !992]. The timescales for these processes Copyright !996 by fl•e American Geophysical Union. Paper number 95JB02515. 0!48.1227/96/95JB.025 [5505.00 are very short. For example, in the plinian eruption of Mount St. Helens on May 18, 1980, estimates of chamber depth, magma discharge rates, conduit dimensions and volatile contents [ Carey et al., 1990] constrain the time that it takes for an individual parcel of magma to move from the chamber to the Earth's surface as about !0 min. Due to pressure variations most bubble growth is confined to the uppermost parts of the magma column so the timescale for explosive degassing must be substantially less. Estimates from modeling studies [e.g., Kief[kr, 198!; Dobran, 1992; Proussevitch et al., !993; Sparks et aL, 1994] suggest that timescales for prefragmentation bubble growth in p!inian eruptions are of order !0 to I00 s. Explosive volcanic flows are unlikely to be observed directly. Therefore the processes involved can only be studied by theoretical modeling, simulation in ana!ogue experiments, or 5547 5548 MADER ET AL.: FRAGMENTING 'IWO-PI IASE FI.OWS