Magma Rheology Research Papers - Academia.edu (original) (raw)

Disordered matter still presents stringent conceptual difficulties. The often confused concepts of amorphous and glassy states (and corresponding material substances) were re-examined and previous attempts to trace a distinction are... more

Disordered matter still presents stringent conceptual difficulties. The often confused concepts of amorphous and glassy states (and corresponding material substances) were re-examined and previous attempts to trace a distinction are revised and thoroughly discussed. We examine several rheological and thermodynamical aspects of glasses in the glass transformation region as well as the dynamics of associated relaxation processes.

El objetivo de este trabajo es contribuir con la discusión de los viejos paradigmas asociados con los distintos procesos dinámicos que experimentan los magmas (desde su generación hasta su emplazamiento), en especial con el diapirismo y... more

El objetivo de este trabajo es contribuir con la discusión de los viejos paradigmas asociados con los distintos procesos dinámicos que experimentan los magmas (desde su generación hasta su emplazamiento), en especial con el diapirismo y la propagación de fracturas, así como con la difusión de los nuevos, como por ejemplo las “zonas calientes”, los pulsos magmáticos y la construcción de los plutones. Para alcanzar este objetivo, en este trabajo se mencionan y discuten los avances teóricos más recientes relacionados con la dinámica magmática, haciendo hincapié, entre otras cosas, a los parámetros físicos que controlan
el transporte de los magmas y a los modelos más aceptados que explican su generación, segregación,
ascenso y emplazamiento.

The Neogene basins of the Eastern Betic Mountains were placed in a marginal position with respect to the deep Mediterranean basin during the Tortonian to the Messinian. They contain thick gypsum–anhydrite formations resulting from the... more

The Neogene basins of the Eastern Betic Mountains were placed in a marginal position with respect to the deep Mediterranean
basin during the Tortonian to the Messinian. They contain thick gypsum–anhydrite formations resulting from the Upper Miocene
salinity crisis. The evaporitic sedimentation reached its maximum extension and stratigraphic complexity in the Fortuna basin,
which belongs to this geological setting.
Five stratigraphic units are distinguished in the Fortuna basin: (1) Lower Gypsum, (2) Chicamo Gypsum, (3) Chicamo
diatomitic–evaporitic cycles, (4) Ribera Gypsum and (5) Rambla Salada Gypsum units. Some of these units occur locally, whereas
others can be laterally correlated with those of the Archena-Mula, Guadalentín corridor and Lorca basins. Most of the gypsum units
consist of nodular-laminated secondary gypsum, however, primary selenitic, laminated gypsarenites and fine-grained
microlenticular burrowed gypsum lithofacies are also present. The evaporite units can also be characterized by the geochemical
composition of the gypsum (the sulphur and oxygen of the sulphate and the strontium isotopes), which display a wide range of
values. The extreme values indicate precipitation from Tertiary marine brines, or from meteoric waters that recycled Triassic
sulphate anion and additional non-marine contributions from country rocks and hydrothermal solutions. The intermediate values
are attributed to mixing between seawater and non-marine parent brines.
The Fortuna basin contains several outcrops of volcanic to subvolcanic igneous lamproites (Cabezos Negros, Derramadores, El
Tale). These lamproites, the so-called fortunites, crop out as individual domic volcanoes with lava flows (Cabezos Negros) or as
intrusive subvertical dikes (Derramadores, El Tale). There is good evidence of several magma–sediment interactions that produced
macro and microglobular peperitic lithofacies in the Fortuna basin. A peperite is produced when two liquids (a hot magma and a
fluidized water-rich sediment) interact and no mixing is possible due to the sharp contrast of viscosity between them. This implies
that the eruption was nearly coeval with the deposition of the host-sediment, that was water-rich. The latest evaporitic event
(Rambla Salada Gypsum Unit) in the Fortuna basin took place coevally to the emplacement of the Cabezos Negros dome and
related lavic-structures, and the intrusion of the Tale and Derramadores dikes.
This paper illustrates the presence of peperitic lithofacies in the fortunites and discusses their implications in assigning an age to
these evaporitic sediments, which in turn allows us to date the evaporitic units of the Fortuna–Lorca basins.

The physical properties of magmas play a fundamental role in controlling the eruptive dynamics of volcanoes. Magmas are multiphase mixtures of crystals and gas bubbles suspended in a silicate melt and, to date, no flow laws describe their... more

The physical properties of magmas play a fundamental role in controlling the eruptive dynamics of volcanoes. Magmas are multiphase mixtures of crystals and gas bubbles suspended in a silicate melt and, to date, no flow laws describe their rheological behaviour. In this study we present a set of equations quantifying the flow of high-viscosity (N10 5 Pa·s) silica-rich multiphase magmas, containing both crystals (24–65 vol.%) and gas bubbles (9–12 vol.%). Flow laws were obtained using deformation experiments performed at high temperature (673– 1023 K) and pressure (200–250 MPa) over a range of strain-rates (5 · 10 −6 s −1 to 4 · 10 −3 s −1), conditions that are relevant for volcanic conduit processes of silica-rich systems ranging from crystal-rich lava domes to crystal-poor obsidian flows. We propose flow laws in which stress exponent, activation energy, and pre-exponential factor depend on a parameter that includes the volume fraction of weak phases (i.e. melt and gas bubbles) present in the magma. The bubble volume fraction has opposing effects depending on the relative crystal volume fraction: at low crystallinity bubble deformation generates gas connectivity and permeability pathways, whereas at high crystallinity bubbles do not connect and act as " lubricant " objects during strain localisation within shear bands. We show that such difference in the evolution of texture is mainly controlled by the strain-rate (i.e. the local stress within shear bands) at which the experiments are performed, and affect the empirical parameters used for the flow laws. At low crystallinity (b44 vol.%) we observe an increase of viscosity with increasing strain-rate, while at high crystallinity (N44 vol.%) the viscosity decreases with increasing strain-rate. Because these behaviours are also associated with modifications of sample textures during the experiment and, thus, are not purely the result of different deformation rates, we refer to " apparent shear-thickening " and " apparent shear-thinning " for the behaviours observed at low and high crystallinity, respectively. At low crystallinity, increasing deformation rate favours the transfer of gas bubbles in regions of high strain localisation, which, in turn, leads to outgassing and the observed increase of viscosity with increasing strain-rate. At high crystallinity gas bubbles remain trapped within crystals and no outgassing occurs, leading to strain localisation in melt-rich shear bands and to a decrease of viscosity with increasing strain-rate, behaviour observed also in crystal-bearing suspensions. Increasing the volume fraction of weak phases induces limited variation of the stress exponent and pre-exponential factor in both apparent shear-thickening and apparent shear-thinning regimes; conversely, the activation energy is strongly dependent on gas bubble and melt volume fractions. A transient rheology from apparent shear-thickening to apparent shear-thinning behaviour is observed for a crystallinity of 44 vol.%. The proposed equations can be implemented in numerical models dealing with the flow of crystal-and bubble-bearing magmas. We present results of analytical simulations showing the effect of the rheology of three-phase magmas on conduit flow dynamics, and show that limited bubble volumes (b10 vol.%) lead to strain localisation at the conduit margins during the ascent of crystal-rich lava domes and crystal-poor obsidian flows.

Because of strong coupling between viscosity and temperature, the dynamics of magma flows in conduits are drastically controlled by thermal effects due to heat generation by viscous dissipation and loss to the walls by conduction. Here we... more

Because of strong coupling between viscosity and temperature, the dynamics of magma flows in conduits are drastically controlled by thermal effects due to heat generation by viscous dissipation and loss to the walls by conduction. Here we present analytical solutions and a practical procedure based on an order of magnitude analysis that permits the characterization of the regime and estimation of the main features of the flow. The ranges of validity of analytical and asymptotic solutions were bounded by using results from fully two-dimensional (2-D) numerical solutions of mass, momentum, and energy equations for magma flow inside a cylindrical conduit and the heat conduction in the surrounding host rocks. The results permitted the identification of three regimes: a conductive-heat-loss-dominated regime, an intermediate regime, and a viscous-heating-dominated regime. Some useful analytical parameterizations are proposed for estimating friction in simplified 1-D models. Temperature layering due to heat loss by conduction can lead to local crystal growth and magma solidification whereas heat generated by viscous dissipation can be responsible for crystal resorption and remelting of wall rocks.

The rheology of volatile-bearing crystal mushes was constrained by deformation experiments on hydrous (2.52 wt.% H 2 O) haplogranitic magmas containing quartz crystals (solid fraction of 0.55 to 0.65) and gas-pressurized CO 2-rich bubbles... more

The rheology of volatile-bearing crystal mushes was constrained by deformation experiments on hydrous (2.52 wt.% H 2 O) haplogranitic magmas containing quartz crystals (solid fraction of 0.55 to 0.65) and gas-pressurized CO 2-rich bubbles (bubble fraction of 0.09–0.10), under simple shear using a HT–HP Paterson-type rock deformation apparatus. Variable strain rates (from 5 · 10 −6 to 4 · 10 −3 s −1) were applied at high temperature (823–1023 K) and constant confining pressure (200–250 MPa; 8–10 km depth). This study shows that the rheology of three-phase magmas is strain rate-dependent (non-Newtonian behavior). Two non-Newtonian regimes were observed: (1) shear thinning (decrease of viscosity with increasing strain rate) and (2) shear thickening (increase of viscosity with increasing strain rate). Shear thinning results from crystal size reduction and shear localization, enhanced by the presence of gas bubbles in the weak shear bands. Shear thickening becomes dominant when the solid crystal framework induces internal flow blockage due to crystal interlocking. Compared to the rheology of bubble-free, crystal-bearing systems, the presence of limited amount of gas bubbles (maximum bubble fraction of 0.10) results in a prominent decrease in viscosity; e.g., at a crystal fraction of 0.70 a decrease of about 4 orders of magnitude in relative viscosity is caused by adding a bubble fraction of 0.09. This experimental study suggests that the contemporaneous presence of crystals and bubbles induces a significant difference in the rheological behavior of magmas with respect to two-phase (bubbles or crystals + silicate melt) systems. Crystallization and efficient gas removal from magmatic bodies in the Earth's crust lead to a substantial increase of viscosity and, eventually, to their " viscous death ". On the contrary, the significant decrease of viscosity associated with the presence of limited volumes of gas could promote re-mobilization of large plutonic magma bodies and the generation of large explosive eruptions.

Using a compilation of melt compositions, meltwater contents, temperatures, and phenocryst contents, the preeruptive viscosities under magma reservoir conditions are calculated for 83 erupted magmas. The basaltic-to-rhyolitic magmas have... more

Using a compilation of melt compositions, meltwater contents, temperatures, and phenocryst contents, the preeruptive viscosities under magma reservoir conditions are calculated for 83 erupted magmas. The basaltic-to-rhyolitic magmas have preeruptive viscosities over the range 10^1 to 10^8 Pa s. Although bulk SiO2 content has often been used as a qualitative measure of preeruptive magma viscosity, the bulk SiO2 content shows a weak correlation with magma viscosity (correlation coefficient r = 0.5). Because of a wide range of phenocryst contents from 0 to ∼50 vol %, andesitic magmas have viscosities ranging from 10^2 to 10^7 Pa s, which are lower or higher than those of phenocryst-poor rhyolitic magmas with 10^5 to 10^6 Pa s. Focusing on andesitic to rhyolitic magmas, the r between bulk SiO2 contents and magma viscosities changes to −0.1. In contrast, the melt-only SiO2 content from a basaltic-to-rhyolitic melt shows a good linear correlation with melt-only viscosity (r = 0.9). Although most of the calculated viscosities of erupted magmas fall below ∼10^6 Pa s, as consistent with the previous compilation study, this paper describes 20 examples of highly viscous magmas with >10^6 Pa s, in most cases, composed of mixtures of high-silica rhyolitic melt (75–79 wt % SiO2) and abundant phenocrysts (30–55 vol %). In these highly viscous magmas, 9 examples have erupted following the precursory eruption of less viscous magma, suggesting that precursory dike propagation and conduit formation by the less viscous magma with <10^6 Pa s induced the following eruption of less eruptible, highly viscous magmas.

The transition from viscous to brittle behavior in magmas plays a decisive role in determining the style of volcanic eruptions. While this transition has been determined for one-or two-phase systems, it remains poorly constrained for... more

The transition from viscous to brittle behavior in magmas plays a decisive role in determining the style of volcanic eruptions. While this transition has been determined for one-or two-phase systems, it remains poorly constrained for natural magmas containing silicic melt, crystals, and gas bubbles. Here, we present new experimental results on shear-induced fracturing of three-phase magmas obtained at high-temperature (673–1023 K) and high-pressure (200 MPa) conditions over a wide range of strain-rates (5 6 1 3 1 ·10 − s − –4·10 − s −). During the experiments bubbles are deformed (i.e., capillary number is in excess of 1) enough to coalesce and generate a porous network that potentially leads to outgassing. A physical relationship is proposed that quantifies the critical stress required for magmas to fail as a function of both crystal (0.24–0.65) and bubble volume fractions (0.09–0.12). The presented results demonstrate efficient outgassing for low crystal fraction (<0.44), whereas high crystal fractions (>0.44) promote gas bubble entrapment and inhibit outgassing. The failure of bubble-free, crystal-bearing systems is enhanced by the presence of bubbles that lower the critical failure stress in a regime of efficient outgassing, while the failure stress is increased if bubbles remain trapped within the crystal framework. These contrasting behaviors have direct impact on the style of volcanic eruptions. During magma ascent, efficient outgassing reduces the potential for an explosive eruption and favors brittle behavior, contributing to maintain low overpressures in an active volcanic system resulting in effusion or rheological flow blockage of magma at depth. Conversely, magmas with high crystallinity experience limited loss of exsolved gas, permitting the achievement of larger overpressures prior to a potential sudden transition to brittle behavior, which could result in an explosive volcanic eruption.

Due to its unfavorable rheology, magma with crystallinity exceeding about 50 vol.% and effective viscosity > 10e6 Pa s is generally perceived to stall in the Earth's crust rather than to erupt. There is, however, irrefutable evidence for... more

Due to its unfavorable rheology, magma with crystallinity exceeding about 50 vol.% and effective viscosity > 10e6 Pa s is generally perceived to stall in the Earth's crust rather than to erupt. There is, however, irrefutable evidence for colossal eruption of batholithic magma bodies and here we analyze four examples from Spain, Mexico, USA and the Central Andes. These silicic caldera-forming eruptions generated deposits characterized by i) ignimbrites containing crystal-rich pumice, ii) co-ignimbritic lag breccias and iii) the absence of initial fall-out. The field evidence is inconsistent with most caldera-forming deposits, which are underlain by initial fall-out indicating deposition from a sustained eruption column before the actual collapse sequence. In contrast, the documented examples suggest early deep-level fragmentation at the onset of eruption and repeated column collapse generating eruption volumes on the order of hundreds of cubic kilometers almost exclusively in the form of ignimbrites. These examples challenge our understanding of magma eruptability and eruption initiation processes. In this paper, we present an analysis of eruption promoters from geologic, theoretical and experimental considerations. Assessing relevant dynamics and timescales for failure of crystal-melt mush we propose a framework to explain eruption of batholithic magma bodies that primarily involves an external trigger by near-field seismicity and crustal failure. Strain rate analysis for dynamic and static stressing, chamber roof collapse and rapid decompression indicates that large “solid-like” silicic reservoirs may undergo catastrophic failure leading to deep-level fragmentation of batholithic magma at approximately 2 orders of magnitude lower strain rates than those characteristic for failure of crystal-poor magmas or pure melt. Eruption triggers can thus include either amplified pressure transients in the liquid phase during seismic shaking of a crystal-melt mush, decompression by block subsidence or a combination of both. We find that the window of opportunity for the eruption of large silicic bodies may thus extent to crystallinities beyond 50 vol.% for strain rates on the order of > 10− 3 to 10− 4 s− 1.

Viscous heating can play an important role in the dynamics of fluids with a strongly temperature-dependent viscosity because of the coupling between the energy and momentum equations. The heat generated by viscous friction produces a... more

Viscous heating can play an important role in the dynamics of fluids with a strongly temperature-dependent viscosity because of the coupling between the energy and momentum equations. The heat generated by viscous friction produces a local increase in temperature near the tube walls with a consequent decrease of the viscosity and a strong stratification in the viscosity profile which can trigger instabilities and a transition to secondary flows.
In this paper we present two separate theoretical models: a linear stability analysis and a direct numerical simulation (DNS) of a plane channel flow. In particular DNS shows that, in certain regimes, viscous heating can trigger and sustain a particular class of secondary rotational flows which appear organized into coherent structures similar to roller vortices. This phenomenon can play a very important role in the dynamics of magma flows and, to our knowledge, it is the first time that it has been investigated by a direct numerical simulation.

This contribution presents a semiempirical model describing the effective relative viscosity of crystal- bearing magmas as function of crystal fraction and strain rate. The model was applied to an extensive data set of magmatic... more

This contribution presents a semiempirical model describing the effective relative viscosity of crystal- bearing magmas as function of crystal fraction and strain rate. The model was applied to an extensive data set of magmatic suspensions and partially molten rocks providing a range of values for the fitting parameters that control the behavior of the relative viscosity curves as a function of the crystal fraction in an intermediate range of crystallinity (30–80 vol % crystals). The analysis of the results and of the materials used in the experiments allows for evaluating the physical meaning of the parameters of the proposed model. We show that the model, by varying the parameters within the ranges obtained during the fitting procedure, is able to describe satisfactory the effective relative viscosity as a function of crystal fraction and strain rate for suspensions having different geometrical characteristics of the suspended solid fraction.

Abstract The physical properties of magmas play a fundamental role in controlling the eruptive dynamics of volcanoes. Magmas are multiphase mixtures of crystals and gas bubbles suspended in a silicate melt and, to date, no flow laws... more

Abstract The physical properties of magmas play a fundamental role in controlling the eruptive dynamics of volcanoes. Magmas are multiphase mixtures of crystals and gas bubbles suspended in a silicate melt and, to date, no flow laws describe their rheological behaviour. In this study we present a set of equations quantifying the flow of high-viscosity (> 10 5 Pa·s) silica-rich multiphase magmas, containing both crystals (24–65 vol.%) and gas bubbles (9–12 vol.%). Flow laws were obtained using deformation experiments performed at high temperature (673–1023 K) and pressure (200–250 MPa) over a range of strain-rates (5 · 10 − 6 s − 1 to 4 · 10 − 3 s − 1 ), conditions that are relevant for volcanic conduit processes of silica-rich systems ranging from crystal-rich lava domes to crystal-poor obsidian flows. We propose flow laws in which stress exponent, activation energy, and pre-exponential factor depend on a parameter that includes the volume fraction of weak phases (i.e. melt and gas bubbles) present in the magma. The bubble volume fraction has opposing effects depending on the relative crystal volume fraction: at low crystallinity bubble deformation generates gas connectivity and permeability pathways, whereas at high crystallinity bubbles do not connect and act as “lubricant” objects during strain localisation within shear bands. We show that such difference in the evolution of texture is mainly controlled by the strain-rate (i.e. the local stress within shear bands) at which the experiments are performed, and affect the empirical parameters used for the flow laws. At low crystallinity ( 44 vol.%) the viscosity decreases with increasing strain-rate. Because these behaviours are also associated with modifications of sample textures during the experiment and, thus, are not purely the result of different deformation rates, we refer to “apparent shear-thickening” and “apparent shear-thinning” for the behaviours observed at low and high crystallinity, respectively. At low crystallinity, increasing deformation rate favours the transfer of gas bubbles in regions of high strain localisation, which, in turn, leads to outgassing and the observed increase of viscosity with increasing strain-rate. At high crystallinity gas bubbles remain trapped within crystals and no outgassing occurs, leading to strain localisation in melt-rich shear bands and to a decrease of viscosity with increasing strain-rate, behaviour observed also in crystal-bearing suspensions. Increasing the volume fraction of weak phases induces limited variation of the stress exponent and pre-exponential factor in both apparent shear-thickening and apparent shear-thinning regimes; conversely, the activation energy is strongly dependent on gas bubble and melt volume fractions. A transient rheology from apparent shear-thickening to apparent shear-thinning behaviour is observed for a crystallinity of 44 vol.%. The proposed equations can be implemented in numerical models dealing with the flow of crystal- and bubble-bearing magmas. We present results of analytical simulations showing the effect of the rheology of three-phase magmas on conduit flow dynamics, and show that limited bubble volumes (

That rising bubbles may significantly affect magma mixing paths has already been demon strated by analogue experiments. Here, for the first time, bubble-advection experiments are performed employing volcanic melts at magmatic... more

That rising bubbles may significantly affect magma mixing paths has already been demon strated by analogue experiments. Here, for the first time, bubble-advection experiments are performed employing volcanic melts at magmatic temperatures. Cylinders of basaltic glass were placed below cylinders of rhyolite glass. Upon melting, interstitial air formed bubbles that rose into the rhyolite melt, thereby entraining tails of basaltic liquid. The formation of plume-like filaments of advected basalt within the rhyolite was characterized by microCT and subsequent high-resolution EMP analyses. <br><br> Melt entrainment by bubble ascent appears to be an efficient mechanism for mingling volcanic melts of highly contrasting compositions and properties. MicroCT imaging reveals bubbles trailing each other and multiple filaments coalescing into bigger ones. Rheological modelling of the filaments yields viscosities of up to 2 orders of magnitude lower than for the surrounding rhyolitic l...

Colli Albani (Roma) is a composite quaternary volcano that became active at approximately 600 ka and that is now considered to be in a quiescent state. Both explosive and effusive products are remarkably undersaturated and belong to the... more

Colli Albani (Roma) is a composite quaternary volcano that became active at approximately 600 ka and that is now considered to be in a quiescent state. Both explosive and effusive products are remarkably undersaturated and belong to the HK-Series. In spite of its very silica poor compositions, the volcano displays features identical to those of felsic volcanoes, like low aspect ratio ignimbrites, tens of cubic kilometres large, and a related central (8x8 km) collapse caldera. Despite the widespread literature about the structure and history of the volcano, the mechanisms governing the explosive activity of these very undersaturared magmas are still unknown. In order to improve our understanding on the origin of these high-energy eruption, we carried out a detailed investigation of one of the largest explosive events in the history of the volcano (407 ka, Vulcano Laziale phase), the Pozzolane nere formation (PNR).. The PNR formation is characterized by a basal scoria fall deposit sho...

Lava dome eruptions are sometimes characterised by large periodic fluctuations in extrusion rate over periods of hours that may be accompanied by Vulcanian explosions and pyroclastic flows. We consider a simple system of nonlinear... more

Lava dome eruptions are sometimes characterised by large periodic fluctuations in extrusion rate over periods of hours that may be accompanied by Vulcanian explosions and pyroclastic flows. We consider a simple system of nonlinear equations describing a 1D flow of lava extrusion through a deep elastic dyke feeding a shallower cylindrical conduit in order to simulate this short-period cyclicity. Stick-slip conditions depending on a critical shear stress are assumed at the wall boundary of the cylindrical conduit. By analogy with the behaviour of industrial polymers in a plastic extruder, the elastic dyke acts like a barrel and the shallower cylindrical portion of the conduit as a die for the flow of magma acting as a polymer. When we applied the model to the Soufrie􏰃re Hills Volcano, Montserrat, for which the key parameters have been evaluated from previous studies, cyclic extrusions with periods from 3 to 30 h were readily simulated, matching observations. The model also reproduces the reduced period of cycles observed when a major unloading event occurs due to lava dome collapse.

That rising bubbles may significantly affect magma mixing paths has already been demon strated by analogue experiments. Here, for the first time, bubble-advection experiments are performed employing volcanic melts at magmatic... more

That rising bubbles may significantly affect magma mixing paths has already been demon strated by analogue experiments. Here, for the first time, bubble-advection experiments are performed employing volcanic melts at magmatic temperatures. Cylinders of basaltic glass were placed below cylinders of rhyolite glass. Upon melting, interstitial air formed bubbles that rose into the rhyolite melt, thereby entraining tails of basaltic liquid. The formation of plume-like filaments of advected basalt within the rhyolite was characterized by microCT and subsequent high-resolution EMP analyses. <br><br> Melt entrainment by bubble ascent appears to be an efficient mechanism for mingling volcanic melts of highly contrasting compositions and properties. MicroCT imaging reveals bubbles trailing each other and multiple filaments coalescing into bigger ones. Rheological modelling of the filaments yields viscosities of up to 2 orders of magnitude lower than for the surrounding rhyolitic l...

Key Points: • We observe permeability and outgassing in sheared synthetic three-phase magma • Fractures develop at strains over 2 with crystal fraction of 0.24 • A large control on sample viscosity is the type of deformation fabric... more

Key Points: • We observe permeability and outgassing in sheared synthetic three-phase magma • Fractures develop at strains over 2 with crystal fraction of 0.24 • A large control on sample viscosity is the type of deformation fabric Abstract A major factor determining the explosivity of silicic eruptions is the removal of volatiles from magma through permeability-controlled outgassing. We studied the microstructural development of permeability during deformation of highly viscous magma by performing simple shear experiments on bubble (0.12–0.36 volume fraction) and crystal-bearing (0–0.42 volume fraction) silicate melts. Experiments were performed under torsion, at high temperature and pressure (723–873 K and 150–200 MPa) in a Paterson deformation apparatus at bulk shear strains between 0 and 10. The experimental setup allows for gas escape if bubble connectivity is reached on the sample periphery. Three-dimensional imaging and analysis of deformed bubbles was performed using X-ray tomography. The development of localized deformation in all samples, enhanced by crystal content, leads to brittle fracture at bulk strains > 2 and sample-wide fracturing in samples deformed to strains > 5. A decrease in both bubble fraction and dissolved volatile content with increasing strain, along with strain-hardening rheological behavior, suggests significant shear-induced outgassing through the fracture networks, applicable to shallow conduit degassing in magmas containing crystal fractions of 0–0.42. This study contributes to our understanding of highly viscous magma outgassing and processes governing the effusive-explosive transition.

Observations of volcanoes extruding andesitic lava to produce lava domes often reveal cyclic behavior. At Soufrière Hills Volcano, Montserrat, cycles with subdaily and multiweek periods have been recognized on many occasions.... more

Observations of volcanoes extruding andesitic lava to produce lava domes often reveal cyclic behavior. At Soufrière Hills Volcano, Montserrat, cycles with subdaily and multiweek periods have been recognized on many occasions. Observations clearly show that the period of subdaily cycles is modulated by the multiweek cycle. The subdaily and multiweek cycles have been modeled separately as stick-slip magma flow at the junction between a dyke and an overlying cylindrical conduit and as the filling and discharge of magma through the elastic-walled dyke, respectively. Here, we couple these two models to describe the behavior over a period of well-observed multiweek cycles, with accompanying subdaily cycles, from 13 May to 21 September 1997. The coupled model captures well the asymmetrical first-order behavior: the first 40% of the multiweek cycle consists of high rates of lava extrusion during short period/high amplitude subdaily cycles as the dyke reservoir discharges itself. The remainder of the cycle involves increasing pressurization as more magma is stored, and extrusion rate falls, followed by a gradual increase in the period of the subdaily cycles.

Magma viscosity is strongly temperature-dependent. When hot magma flows in a conduit, heat is lost through the walls and the temperature decreases along the flow causing a viscosity increase. For particular values of the controlling... more

Magma viscosity is strongly temperature-dependent. When hot magma flows in a conduit, heat is lost through the walls and the temperature decreases along the flow causing a viscosity increase. For particular values of the controlling parameters the steady-flow regime in a conduit shows two stable solutions belonging either to the slow or to the fast branch. As a consequence, this system may show an hysteresis effect, and the transition between the two branches can occur quickly when the critical points are reached. In this paper we describe a model to study the relation between the pressure at the inlet and the volumetric magma flow rate in a conduit. We apply this model to explain an hysteric jump observed during the dome growth at Soufrie`re Hills volcano (Montserrat), and described by Melnik and Sparks [1999] using a different model.

"We present a study of high-temperature, uniaxial deformation experiments of natural, partially-crystallized magma from the Monte Nuovo (1538 AD) trachytic eruption. The experiments were performed using a high-temperature uniaxial Geocomp... more

"We present a study of high-temperature, uniaxial deformation experiments of natural, partially-crystallized magma from the Monte Nuovo (1538 AD) trachytic eruption. The experiments were performed using a high-temperature uniaxial Geocomp LoadTrac II press at dry atmospheric conditions and under controlled deformation rates. Each experiment involved deforming cores of natural (i.e., crystal- and vesicle-bearing) scoriaceous samples isothermally (600 to 800 °C) at constant displacement rates (CDR) corresponding to strain rates between 10−7 and 10−4 s−1. Measured viscosities vary between 1010 and 1013 Pa s. The flow be- havior of these complex natural materials are fully described by a simplified Herschel–Bulkely equation in terms of consistency K and flow index n.We estimate the combined effects of crystals and pores on the rhe- ology of these multiphase suspensions. Our results demonstrate that the presence of pores has a major im- pact on the rheological response of magmas and may produce a marked decrease of their viscosity. At the same time, the presence of pores leads to a strong decrease in the strength of the magma inducing local and temporal variation in the deformation regimes (ductile vs. brittle). Brittle failure was in fact observed at T=600 °C and strain rates of 10−5 s−1 and at T=800 °C for the highest applied strain rate (10−4 s−1), respectively. This study constitutes an important step toward the estimation of multiphase rheological evo- lution of Monte Nuovo magmas and toward the general understanding of the full complexities governing the dynamics of magma transport in natural systems.
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Laboratory measurements of viscosity were done for basalt erupted in 1707 AD from Fuji volcano, Japan, using a concentric cylinder rotational viscometer at temperatures of 1297–1157 °C, 1 atm pressure, and fO2 near the Ni–NiO buffer. On... more

Laboratory measurements of viscosity were done for basalt erupted in 1707 AD from Fuji volcano, Japan, using a concentric cylinder rotational viscometer at temperatures of 1297–1157 °C, 1 atm pressure, and fO2 near the Ni–NiO buffer. On cooling, elongated plagioclase crystals with a mean length/width ratio of ca. 8.5 appeared at 1237 °C, followed by olivine at 1157 °C. At progressively lower temperatures, the total crystal volume fraction increased monotonously to ca. 0.25; viscosity increased from 38.9 to 765 Pa s at a shear strain rate of 1 s−1. This basalt magma behaves as a Newtonian fluid at temperatures greater than 1217 °C, but shear-thinning behavior occurs at temperatures less than 1197 °C because of the suspended plagioclase crystals. This behavior is well approximated as a power law fluid. At the onset of shear thinning, the crystal volume fraction was between 0.06 and 0.13, which is attributed to the pronounced lath-shape of plagioclase crystals. The relative viscosity increases monotonously with increase of crystal volume fraction at a constant shear strain rate, and with decrease of shear strain rate at a constant crystal volume fraction. A modified form
of the Krieger–Dougherty equation is introduced herein. It enables us to describe the dependencies of relative viscosity on both the crystal volume fraction and shear strain rate, and consequently the onset of shear-thinning behavior.