Dingwell, Donald Bruce

The degassing kinetics of ascending magma strongly affect eruption dynamics. The kinetics are in turn influenced by magma properties. The investigation of the relationship between magma properties and eruption dynamics is a key element in revealing the processes characterizing magmatic flows within the shallow conduit. To explore the effects of physical properties on degassing in basaltic eruptive systems, we have designed and carried out experiments on the slow decompressive response of analogue magmas, composed of silicone-oil-based suspensions, using a shock-tube apparatus. Four series of experiments were performed: 1) particle-free silicone oils with viscosity ranging from 1 to 1000 Pa s were used to constrain the liquid response; 2) silicone oils with variable proportion of suspended micrometric spherical particles were employed to assess the effect of different crystal fractions; 3) suspensions of elongated particles in silicone oils were used to investigate the role of crystal shape; 4) the effects of saturation time and pressure were examined. The rheology of both spherical- and elongated-particle-bearing suspensions were characterized by concentric cylinder rotational rheometry. The flow dynamics of the bubbly fluid, from the process of bubble nucleation up to the development of a permeable bubble network, were constrained using image analysis. Different fluid regimes were distinguished: (i) nucleation, (ii) foam build-up and (iii) foam oscillation. By comparing results obtained from the different series of experiments, we were able to assess the primary role played by the presence of particles on the evolution of the gas volume fraction within the samples. Particle fraction has a dominant role at high concentration, affecting the motion of the fluid. Finally, particle shape influences the long-term degassing efficiency of the fluid. Using scaling considerations, such observations are applied to mafic to intermediate systems. The results of our experimental investigation contribute to constraining vesiculation processes in magmas of various crystallinities at shallow depths.

Solfatara crater is locatedwithin the Campi Flegrei caldera to the west of Naples (Italy). It is one of the largest fumarolic manifestations known, and the rocks hosting the hydrothermal system are affected by intense hydrothermal alteration. Alteration can result in changes of degassing behavior, and in the formation of a cap rock thereby increasing the probability of phreatic eruptions. Here, we investigate the effects of alunitic (solfataric) alteration on the mineralogy, the physical properties (porosity, density, permeability) and the mechanical properties (strength) of the rocks involved, aswell as its influence on fragmentation and ejection behavior. Our results showthat the pristine mineralogy of deposits fromthe vicinity of the Solfatara cryptodome and from Pisciarelli is almost completely replaced by amorphous silica and alunite. The differences in the degree of alteration among the samples series are reflected in the investigated properties and behavior aswell as in the analysis of the experimentally generated particles. Alunitic alteration increases porosity and permeability,whereas it reduces density, elastic wave velocity and strength leading to higher fragmentation and ejection speeds for the sample series examined in this study. Our results also show that alteration results in the generation of a high fraction of fines (particle sizes b10 μm) during fragmentation, mainly composed of alunite crystals. Due to their potential for inducing chronic disease, dispersion of such material should represent a serious health hazard on a local scale and the evaluation of precautions should be considered.

The 5th April 2003 paroxysmal event was the strongest explosion that has occurred at Stromboli in the last 50 years. This event lasted only few minutes and was characterised by two violent explosions, followed by gas and pyroclast emission. In order to constrain models of the dynamics of the paroxystic event the viscosity of anhydrous and hydrous Stromboli high potassium (HK)-basaltic melts have been measured. Viscosity has been investigated in the low viscosity range with the falling sphere method at superliquidus temperatures (1423 to 1673 K) and 0.5 GPa and in the high viscosity range with micropenetration near the glass transition temperature (723 to 1035 K) at atmospheric pressure. Falling sphere experiments were performed in a piston cylinder apparatus with melts whose water content varies from nominally anhydrous (0.02 wt.% H2O) to 4.16 wt.% H2O. The combination of high- and low-viscosity data permits a general description of the viscosity as a function of temperature and water contentusing a modified Tamman–Vogel–Fulcher equation. Using these new viscosity data, an estimation of the flow regime and magma velocity is performed. Our data suggest that the ascent of magma from the 7–8 km deep reservoir to a shallower reservoir located at about 3 km of depth, may occur within minutes. Moreover, we infer a turbulent flow regime. Finally, our estimates of the ascent velocity agree qualitatively with results from petrological studies (e.g. [Bertagnini, A., Métrich, N., Landi, P., Rosi, M., 2003. Stromboli volcano (Aeolian Archipelago, Italy): an openwindowon the deep-feeding system of a steady state basaltic volcano. Journal of Geophysical Research 108, 2336–2350.]), which indicate a turbulent flow regime and rapid ascent velocities such to inhibit volatile-loss-induced crystallization.We conclude that hazard evaluation at Stromboli Island should incorporate the likelihood of very rapid ascent of less-evolved melts from depth.

The 1669 eruption of Mt Etna was one of the most voluminous and devastating of its flank eruptions in historical times. Despite a large body of relevant research, knowledge of the timing and duration of magma transfer and magma recharge through the internal plumbing system preceding and during the eruption is still limited. To address that lack of knowledge, we apply a three-way integrated method, linking systems analysis of crystals, a time-integrated study of zoned olivine populations, and a forward-modelling approach using thermodynamic calculations. Analysis of 202 olivine crystals erupted during the initial (pre-March 20, i.e. SET1) and the final (post-March 20; i.e. SET2 and MtRs) stages of the eruption reveals the existence of three magmatic environments (MEs) in which the majority of the olivine cores [M1 (= Fo75–78)] and rims [i.e. M5 (= Fo51–59) and M3 (= Fo65–69)] formed. Application of the rhyolite-MELTS software allowed us to constrain the key intensive variables associated with these MEs. We find that temperature, water content and oxidation state vary between these MEs. Application of diffusion modelling to the zoned olivine crystals allowed us to reconstruct the timing and chronology of melt and crystal transfer prior to and during the 1669 flank eruption. We find that, following the formation of the olivine cores [M1 (= Fo75–78)], the reservoir M1 was intruded by batches of more evolved, degassed and possibly aphyric M5-type magma, commencing 1·5 years prior to eruptive activity. This is the origin of the SET1 olivine rims (i.e. Fo51–59). In the months prior to eruption, timescale data show that recharge activity along the newly established pathway M1–M5 increased notably. Starting in November 1668, only a few weeks after the first intrusive episode into the M1 reservoir, a second pulse of magma injections (M3-type magma) occurred and a new pathway M1–M3 opened; this is how the SET2 olivine rims (i.e. Fo65–69) formed. For several weeks a bifurcated transport system with two dominant magma pathways developed along M1–M5 and M1–M3 dyke injections. Accompanied by vigorous seismicity, in the days immediately before eruption the local magma transfer dynamics changed and the M1–M5 recharge activity slowed down, as shown by a relative lack of crystals recording shorter timescales. M1–M3 recharge, however, remained high and persisted following the eruption onset on March 11, during which the SET1 lavas were drained. We propose that the change of the local magma transfer dynamics might be linked to changes in the local stress field brought on during eruption. This may potentially have been due to repeated dyke injections into Etna’s shallow plumbing system disrupting the early M1–M5 pathway and at the same time stabilizing the M1–M3 route as a dominant feeder. This transfer of system feeding would reproduce the observed syn-eruptive recharge and mixing in the weeks following eruption onset, culminating in the eruption of the later SET2 lavas.

The sub-volcanic basement at Mt Etna (Italy) comprises thick sedimentary sequences. An understanding of the physical, mechanical, and microstructural properties of these sequences, and an appreciation of their variability, is important for an accurate assessment of the structural stability of Mt Etna. Here, we present a combined field and laboratory study in which we explore the extent of variability of the materials comprising the sedimentary basement of Mt Etna. To this end, we sampled twelve different lithological units that span the sediments of the Apenninic-Maghrebian Chain (from both the Sicilide and Ionides sequences) and the Hyblean Plateau. X-ray diffraction analysis of the blocks collected show that calcite and quartz are the predominant mineral phases. Textural analysis highlights the wide variability in rock microstructures,with features such as the presence/absence of fractures or veins, pore size and shape, and grain size and shape varying tremendously between the samples. One consequence of this microstructural, textural, and mineralogical variability is that the rock units are characterised by very different values of porosity, P-wave velocity, uniaxial compressive strength, and static Young’s modulus. For example, strength and Young’s modulus vary by a factor of twenty and an order of magnitude, respectively. Our study affirms the vast heterogeneity of the sub-volcanic sedimentary basement of Mt Etna and, on this basis, weurge cautionwhen selecting potentially oversimplified input parameters formodels of flank stability.

The inherent instability of volcanic edifices, and their resultant propensity for catastrophic collapse, is a constant source of volcanic risk. Structural instability of volcanic edifices may be amplified by the presence of carbonate rocks in the sub-volcanic strata, due to the debilitating response of carbonates to thermally-induced alteration. Nonetheless, decarbonation reactions (the primary weakening mechanism), may stall when the system becomes buffered by rising levels of a reaction product, carbon dioxide. Such thermodynamic stalling might be inferred to serve to circumvent the weakness of volcanic structures. However, the present study shows that, even when decarbonation is halted, rock physical properties continue to degrade due to thermal microcracking. Furthermore, as a result, the pathways for the escape of carbon dioxide are numerous within a volcanic edifice. Therefore, in the case of an edifice with a subvolcanic sedimentary basement, the generation of carbon dioxide via decarbonation is unlikely to hinder its impact on instability, and thus potentially devastating flank collapse.

Experimental volcanology is a powerful tool to reconstruct the dynamics of magmatic fluids within the conduit. More specifically analogue models, allow constraining the conduit dynamics by independently examine physical variables and their reciprocal relationships. Accurate scaling of the experiments to the natural systems is necessary to derive quantitative information on the studied processes. Here we present a suite of experiments investigating the decompressive response of magma analogues with different properties (i.e. fluid viscosity, suspended particle shape and/or content) and their scaling to the natural basaltic systems. In the experiments Ar-saturated silicone oils with different viscosities are used as proxies for volatile-bearing mafic magmas. Varying percentages of micrometric particles are added to the fluid to investigate the role of crystals content as well as crystal shape on the dynamics of the expanding flow. Through decompression, the degassing mixture is characterized by a regime of periodical oscillations of the bubbly front determined by phases of foam collapse and renewal. We find that time-scale of these oscillations has important implications for understanding the cyclical eruptive behaviour observed at basaltic volcanoes. Applicability of the experimental results to natural mafic systems has been verified in the scaling by using a set of a-dimensional numbers. The experimental dataset has been finally used to validate a numerical code implemented in the Openfoam framework. The original compressible multiphase solver twoPhaseEulerFoam was implemented to take into account the multicomponent nature of the fluid mixtures (liquid and gas) and their phase transition, as also reproduced in the experiments. Decompression experiments and their scaling to volcanic system provided fundamental information on the dynamics of volatiles within the shallow conduit. Furthermore, they are an invaluable tool to validate complex numerical codes for multiphase multicomponent mixtures.

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.

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.

Preface