Magma Viscosity Research Articles

A melt viscosity scale for preeruptive magmas

A simplified method to estimate preeruptive melt viscosity by using only melt SiO2 content (groundmass SiO2 content) is proposed for sub-alkaline magmas. Melt viscosity is controlled by many magmatic properties (e.g., melt composition, melt water content, and temperature); however, these properties are linked by phase equilibrium in preeruptive magmas. In this study, the magmatic properties were investigated by compiling data of phase equilibria experiments performed under preeruptive conditions. Negative correlations are found between melt SiO2 contents and liquidus temperatures, and between liquidus temperatures and melt water contents. Both increasing melt SiO2 content and decreasing liquidus temperature have the effect of increasing the melt viscosity, producing a linear positive correlation between logarithmic values of melt viscosity and linear values of melt SiO2 content. For a specific melt SiO2 content, an increase in liquidus temperature causes a decrease in melt viscosity, whereas a decrease in water content causes an increase in melt viscosity. As a result of this opposing effect, the melt viscosity is strongly correlated with the melt SiO2 content (the correlation coefficient of ∼1). Based on this relationship, an empirical equation predicting logarithmic values of preeruptive melt viscosity is proposed as a linear function of melt SiO2 content, referred to as the melt viscosity scale. The equation reproduces melt viscosities for compiled experimental melts and natural melts with root-mean-square deviation of ∼0.4 and ∼0.5 log units, respectively. This method provides order-of-magnitude estimates for preeruptive melt viscosity. Its strength is in being applicable to examples for which a full dataset of preeruptive magmatic properties is lacking or has large uncertainties (e.g., slowly cooled lavas). The simplicity of this method enables us to easily and promptly estimate preeruptive melt viscosity. Combined with rheological models for multiphase magmas, the present method can be applied widely and thus greatly increase the number of case studies which include evidence-based estimates of preeruptive magma viscosity.

Viscosity measurements of crystallizing andesite from Tungurahua volcano (Ecuador).

Viscosity has been determined during isothermal crystallization of an andesite from Tungurahua volcano (Ecuador). Viscosity was continuously recorded using the concentric cylinder method and employing a Pt‐sheathed alumina spindle at 1 bar and from 1400°C to subliquidus temperatures to track rheological changes during crystallization. The disposable spindle was not extracted from the sample but rather left in the sample during quenching thus preserving an undisturbed textural configuration of the crystals. The inspection of products quenched during the crystallization process reveals evidence for heterogeneous crystal nucleation at the spindle and near the crucible wall, as well as crystal alignment in the flow field. At the end of the crystallization, defined when viscosity is constant, plagioclase is homogeneously distributed throughout the crucible (with the single exception of experiment performed at the lowest temperature). In this experiments, the crystallization kinetics appear to be strongly affected by the stirring conditions of the viscosity determinations. A TTT (Time‐Temperature‐Transformation) diagram illustrating the crystallization “nose” for this andesite under stirring conditions and at ambient pressure has been constructed. We further note that at a given crystal content and distribution, the high aspect ratio of the acicular plagioclase yields a shear‐thinning rheology at crystal contents as low as 13 vol %, and that the relative viscosity is higher than predicted from existing viscosity models. These viscosity experiments hold the potential for delivering insights into the relative influences of the cooling path, undercooling, and deformation on crystallization kinetics and resultant crystal morphologies, as well as their impact on magmatic viscosity.

Types and mechanisms of strombolian explosions: characterization of a gas-dominated explosion at Stromboli

Textural and chemical analyses of bombs quenched directly from a normal explosion at Stromboli volcano (Italy) were integrated with coincident seismic, acoustic, and thermal data. The data set defines a new gas-dominated type of “strombolian” eruption, named type 0. These events are characterized by high-velocity emission (150–250 m s−1) of a few relatively small, juvenile particles, with entrained non-juvenile clasts that previously fell back into the vent to be re-erupted. For the studied event, the explosion depth was more than 250 m deep, and the particles showed little residence time in the shallow system. Slug ascent velocities over the final 20–35 m of magma-filled conduit, and the low viscosity of the resident magma, are all consistent with simple bubble burst in a “clean” conduit. This conduit type and eruption style likely fit popular slug ascent and burst models used to explain “strombolian” eruptions. In contrast, the ballistic-dominated type of explosions (type 1) are associated with larger proportions of stagnant material in the shallow system magma mix. We argue that the additional volume of this stagnant material pushes the free surface upward. Because of the larger volume of material available for entrainment into a type 1 slug burst, which has to fragment through a thick cap of degassed material, type 1 events tend to be rich in particles. In contrast, the less spectacular, gas-rich (type 0) events have little material to entrain, thus being poor in lapilli and bombs.

Effects of crystal shape‐ and size‐modality on magma rheology

AbstractErupting magma often contains crystals over a wide range of sizes and shapes, potentially affecting magma viscosity over many orders of magnitude. A robust relation between viscosity and the modality of crystal sizes and shapes remains lacking, principally because of the dimensional complexity and size of the governing parameter space. We have performed a suite of shear viscosity measurements on liquid‐particle suspensions of dynamical similarity to crystal‐bearing magma. Our experiments encompass five suspension types, each consisting of unique mixtures of two different particle sizes and shapes. The experiments span two orthogonal subspaces of particle concentration, as well as particle size and shape for each suspension type, thereby providing insight into the topology of parameter space. For each suspension type, we determined the dry maximum packing fraction and measured shear rates across a range of applied shear stresses. The results were fitted using a Herschel‐Bulkley model and augment existing predictive capabilities. We demonstrate that our results are consistent with previous work, including friction‐based constitutive laws for granular materials. We conclude that predictions for ascent rates of crystal‐rich magmas must take the shear‐rate dependence of viscosity into account. Shear‐rate dependence depends first and foremost on the volume fraction of crystals, relative to the maximum packing fraction, which in turn depends on crystal size and shape distribution.

Rheology of phonolitic magmas – the case of the Erebus lava lake

Long-lived active lava lakes are comparatively rare and are typically associated with low-viscosity basaltic magmas. Erebus volcano, Antarctica, is unique today in hosting a phonolitic lava lake. Phonolitic magmas can erupt explosively, as in the 79 CE Plinian eruption of Vesuvius volcano, Italy, and it is therefore important to understand their physical properties. The phonolite at Erebus has slightly higher silica content than that at Vesuvius yet its present activity is predominantly non-explosive. As a contribution to understanding such contrasting eruptive behaviour, we focus on the rheological differences between these comparable magmas. In particular, we evaluate the viscosity of the Erebus phonolite magma by integrating new experimental data within a theoretical and empirical framework. The resulting model enables estimation of the Erebus melt viscosity as a function of temperature, crystal and water concentrations, with an uncertainty of, at most, ±0.45log (Pa s). Using reported ranges for these parameters, we predict that the magma viscosity in the upper region of the plumbing system of Erebus ranges between 105 and 107 Pas. This is substantially higher than has been hitherto considered with significant implications for modelling the dynamics of the lava lake, conduit and magma reservoir system. Our analysis highlights the generic challenges encountered in calculation of magma viscosity and presents an approach that can be applied to other cases.

Pahoehoe to `a`a transition of Hawaiian lavas: an experimental study

Basaltic lavas collected at the Muliwai a Pele lava channel, built during 1974 as part of Mauna Ulu’s eruption on Kilauea’s east rift zone, have been studied to investigate the effect of cooling and crystallization on the rheological properties of the lava. We have quantified the viscosity-strain-rate dependence of lava during cooling and crystallization, using concentric cylinder viscometry. We measured the viscosity of the crystal-free liquid between 1600 and 1230 °C, where we observed a deviation from the expected viscosity trend, marking the liquidus. We then made rheology measurements at subliquidus temperatures of 1207, 1203, 1183, 1176, and 1169 °C, varying the applied strain rates at each temperature. While the crystal-free liquid behaved as a Newtonian fluid, crystallization changed the rheological response to pseudo-plastic behavior, even at the lowest crystal volume fraction of 0.025. Pseudo-plastic behavior was observed down to a temperature of 1183 °C, with a crystal fraction of 0.15. Between 1183 and 1176 °C, the two-phase suspension transitioned from a power-law fluid to a Herschel-Bulkley fluid. At temperatures of 1176 and 1169 °C, with crystal fractions of 0.33 and 0.42, respectively, we observed lobate surface textures on the experimental samples, which remained preserved until the end of the experiments. Measurements at these temperatures indicated yield strengths of 82 ± 16 and 238 ± 18 Pa, respectively. The yield strength resulted from the development of an interconnected crystal network of diopside and enstatite by 1176 °C. By 1169 °C, diopside and plagioclase microcrystals had also appeared, and the effective viscosity was between 2000 and 5000 Pa s, depending on the strain rate. Further cooling to 1164 °C resulted in a rapid viscosity increase, to an effective viscosity in excess of 105 Pa s that exceeded the measurement range of our apparatus. The yield strength varies with crystallinity in an exponential fashion, with yield strength in Pa given by σ y = 1.25e12.93Φ c, where Φ c is the crystal volume fraction. The physical effect of crystals on the relative viscosity of magma was assessed by removing the effects of changing residual liquid viscosity, due to changing composition and temperature. To do this, we analyzed, synthesized, and measured the most evolved residual liquid from the subliquidus experiments. The effect of crystals was best captured by the Einstein-Roscoe equation for polydisperse spherical inclusions. We also measured the viscosity of the same crystal-liquid mixtures at low temperatures and strain rates using parallel-plate viscometry. The effect of crystals on magma viscosity was slightly greater at low strain rates, in agreement with theoretical models, although no single model reproduced these results well. In our experiments, the transition from pahoehoe to `a`a occurred between 1200 and 1170 °C, at viscosities between 100 and 1000 Pa s, depending on strain rate.

Structure of cavitation zones in a heavy magma under explosive character of its decompression

The paper is devoted to the investigation of a dynamics of state and structure of compressed magma flow saturated by gas and microcrystallites which is characterized by phase transitions, diffusive processes, by increase of a magma viscosity magnitude by the orders and bubbly cavitation development behind the decompression wave front formed in the result of volcanic channel depressurization. The multi-phase mathematical model, which includes well-known conservation laws for mean pressure, mass velocity, and density as well as the system of the kinetic equations describing the physical processes that occur in a compressed magma during its explosive decompression, is considered. The results of numerical analysis show that the processes of a magma saturation by cavitation nuclei as their density magnitude increases by a few orders lead to the formation of separate zone with anomalously high values of the flow parameters. As it has turned out the abnormal zone is located in the vicinity of a free surface of a cavitating magma column. The mechanism of its formation is determined by diffusion flows redistribution as the nuclei density increases as well as by the change of the distribution character of main flow parameters in the abnormal zone from a gradual to an abrupt increase of their values on the lower zone bound. Note, the mass velocity jump by the order magnitude relatively main flow allows to conclude that the flow disintegration on the lower bound of the zone is quite probable. [Supp. RAS Presidium Program, Project 2.6].

Origin of the fluorine-rich highly differentiated granites from the Qianlishan composite plutons (South China) and implications for polymetallic mineralization

Many composite granite plutons occur in South China, accompanied by large-scale polymetallic mineralization. Each composite pluton is composed of main-phase granite and late-stage highly differentiated granite. Traditionally, the highly differentiated granite is thought to be residual melt of the former via fractionation, and ore-forming materials and fluids are from granite magma itself. We propose a different model for the origin of the granites and related mineralization, based on petrological and geochemical studies on the Qianlishan composite plutons that host the supergiant Shizhuyuan W–Sn-Bi–Mo deposit. The main-phase granite shows features of normal granites, while the highly differentiated granite is characterized by F-rich, water-deficient, low fO2, alkalinity, REE tetrad effect, and modified behavior of some trace elements, e.g., very high K/Ba and low K/Rb and Zr/Hf ratios. We suggest that the parent magma of the highly differentiated granite was derived from melting of dominantly lower crustal rocks, triggered by underplating of a new pulse of basaltic magmas; small amounts of basaltic magmas and volatiles such as fluorine were involved in the source of the granite. Addition of fluorine lowered the solidus temperature and viscosity of granite magma, and thus prolonged the process of magma evolution. This resulted in extreme fractional crystallization, and intense interaction between melt and circulating waters from country rocks, forming the unusual geochemical features of the granite. The high temperature circulating waters, along with metamorphic fluids released from deep crustal rocks, subsequently, extract ore-forming metals from country rocks through forming F-bearing complexation, forming the polymetallic deposits.

Conduit convection driving persistent degassing at basaltic volcanoes

The persistent release of gas at basaltic volcanoes where there is a low magma eruption rate can be driven by an exchange flow of magma in the conduit, in which gas-rich magma ascends, degasses and crystallises and then sinks back down the conduit. The driving force of the flow is provided by the density difference between the buoyant bubble-rich magma at depth and the dense degassed crystallised magma at shallow levels. In this study we attempt to constrain the physical and chemical processes driving an exchange flow of magma at Stromboli, Aeolian Archipelago, Italy. The model uses a simple, cylindrical geometry. We define degassing and crystallisation paths of the ascending and descending magmas, constrained by gas flux and melt inclusion data given in the literature, to produce a three-phase model of ascending and descending magmas driving persistent gas fluxes. We calculate the viscosity of the three-phase magma using end-member rheological models for bubble and crystal suspensions. Combining our modelled magma properties with analogue exchange flow experiments we can relate the regime of magma flow driving persistent degassing to pressure. At pressures ≲90MPa (≲3km) the viscosity ratio is ≲100 and the regime is predicted to be side by side flow with both ascending and descending magmas adjacent to a portion of the conduit wall. At pressures ≳90MPa (≳3km) the viscosity ratio between the ascending and descending magma is ≳100 and the flow is predicted to be core annular flow, with the ascending vesiculating magma in the inner core and the more crystalline degassed magma flowing down along the conduit wall. By analogy, we hypothesise that degassed magma would flow down along the walls in dike-like plumbing geometries that have been proposed for the deeper Strombolian system. Analogue experiments suggest that exchange flows do not overturn under conditions of maximum volume flux; we use an empirical relationship to characterise the volume flux of the exchange flow and show that the radius of a cylindrical conduit required to account for the observed persistent gas flux at Stromboli at 0.1MPa is ~1m.

Three-dimensional numerical analysis of magma transport through a pre-existing fracture in the crust

Magmas are transported through pre-existing fractures in many repeatedly erupting volcanoes. The study of this special process of magma transport is fundamentally important to understand the mechanisms and conditions of volcanic eruptions. In this paper, we numerically simulate the magma propagation process through a pre-existing vertical fracture in the crust by using the combined finite difference method (FDM), finite element method (FEM) and discontinuous deformation analysis (DDA) approach. FDM is used to analyze magma flow in the pre-existing fracture, FEM is used to calculate the opening of the fracture during magma intrusion, and DDA is used to deal with the contact of the closed fracture surfaces. Both two-dimensional (2D) and three-dimensional (3D) examples are presented. Parametric studies are carried out to investigate the influence of various physical and geometric parameters on the magma transport in the pre-existing fracture. We have considered magma chamber depth ranging from 7km to 10km under the crust surface, magma viscosity ranging from 2×10−2 to 2×10−7MPas, and the density difference between the magma and host rock ranging from 300 to 700kg/m3. The numerical results indicate that (1) the fluid pressure p varies gradually along the depth, (2) the shape of the magma body during propagation is like a torch bar and its width ranges from 2m to 4m approximately in the 3D case and 10m to 50m in the 2D case for the same physical parameters used, (3) the crust surface around the pre-existing fracture begins to increase on both sides of the fracture, forms a trough between them, then gradually uplifts during the transport of the magma, and finally takes the shape of a crater when the magma reaches the surface. We have also examined the influence of physical and geometric parameters on the minimum overpressure for magma transport in the 3D case. The numerical results show that our numerical technique presented in this paper is an effective tool for simulating magma transport process through pre-existing fractures in the crust.

Weibull-distributed dyke thickness reflects probabilistic character of host-rock strength

Magmatic sheet intrusions (dykes) constitute the main form of magma transport in the Earth’s crust. The size distribution of dykes is a crucial parameter that controls volcanic surface deformation and eruption rates and is required to realistically model volcano deformation for eruption forecasting. Here we present statistical analyses of 3,676 dyke thickness measurements from different tectonic settings and show that dyke thickness consistently follows the Weibull distribution. Known from materials science, power law-distributed flaws in brittle materials lead to Weibull-distributed failure stress. We therefore propose a dynamic model in which dyke thickness is determined by variable magma pressure that exploits differently sized host-rock weaknesses. The observed dyke thickness distributions are thus site-specific because rock strength, rather than magma viscosity and composition, exerts the dominant control on dyke emplacement. Fundamentally, the strength of geomaterials is scale-dependent and should be approximated by a probability distribution.

Very long period conduit oscillations induced by rockfalls at Kilauea Volcano, Hawaii

Eruptive activity at the summit of Kilauea Volcano, Hawaii, beginning in 2010 and continuing to the present time is characterized by transient outgassing bursts accompanied by very long period (VLP) seismic signals triggered by rockfalls from the vent walls impacting a lava lake in a pit within the Halemaumau pit crater. We use raw data recorded with an 11‐station broadband network to model the source mechanism of signals accompanying two large rockfalls on 29 August 2012 and two smaller average rockfalls obtained by stacking over all events with similar waveforms to improve the signal‐to‐noise ratio. To determine the source centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous medium that takes topography into account. We apply a new waveform inversion method that accounts for the contributions from both translation and tilt in horizontal seismograms through the use of Green's functions representing the seismometer response to translation and tilt ground motions. This method enables a robust description of the source mechanism over the period range 1–1000 s. The VLP signals associated with the rockfalls originate in a source region ∼1 km below the eastern perimeter of the Halemaumau pit crater. The observed waveforms are well explained by a simple volumetric source with geometry composed of two intersecting cracks including an east striking crack (dike) dipping 80° to the north, intersecting a north striking crack (another dike) dipping 65° to the east. Each rockfall is marked by a similar step‐like inflation trailed by decaying oscillations of the volumetric source, attributed to the efficient coupling at the source centroid location of the pressure and momentum changes induced by the rock mass impacting the top of the lava column. Assuming a simple lumped parameter representation of the shallow magmatic system, the observed pressure and volume variations can be modeled with the following attributes: rockfall volume (200–4500 m3), length of magma column (120–210 m), diameter of pipe connecting the Halemaumau pit crater to the subjacent dike system (6 m), average thickness of the two underlying dikes (3–6 m), and effective magma viscosity (30–210 Pa s). Most rockfalls occur during episodes of sustained deflation of the Kilauea summit. The mass loss rate in the shallow magmatic system is estimated to be 1400–15,000 kg s−1 based on measurements of the temporal variation of VLP period in the two large rockfalls that occurred on 29 August 2012.

Magma flow between summit and Pu‘u ‘Ō‘ō at K̄lauea Volcano, Hawai‘i

Volcanic eruptions are often accompanied by spatiotemporal migration of ground deformation, a consequence of pressure changes within magma reservoirs and pathways. We modeled the propagation of pressure variations through the east rift zone (ERZ) of K̄lauea Volcano, Hawai‘i, caused by magma withdrawal during the early eruptive episodes (1983–1985) of the ongoing Pu‘u ‘Ō‘ō‐Kupaianaha eruption. Eruptive activity at the Pu‘u ‘Ō‘ō vent was typically accompanied by abrupt deflation that lasted for several hours and was followed by a sudden onset of gradual inflation once the eruptive episode had ended. Similar patterns of deflation and inflation were recorded at K̄lauea's summit, approximately 15 km to the northwest, albeit with time delays of hours. These delay times can be reproduced by modeling the spatiotemporal changes in magma pressure and flow rate within an elastic‐walled dike that traverses K̄lauea's ERZ. Key parameters that affect the behavior of the magma‐dike system are the dike dimensions, the elasticity of the wall rock, the magma viscosity, and to a lesser degree the magnitude and duration of the pressure variations themselves. Combinations of these parameters define a transport efficiency and a pressure diffusivity, which vary somewhat from episode to episode, resulting in variations in delay times. The observed variations in transport efficiency are most easily explained by small, localized changes to the geometry of the magma pathway.

Petrology and geochemistry of Late Holocene felsic magmas from Rungwe volcano (Tanzania), with implications for trachytic Rungwe Pumice eruption dynamics

Rungwe in southern Tanzania is an active volcanic centre in the East African Rift System, characterised by Plinian-style explosive eruptions of metaluminous to slightly peralkaline trachytic silica-undersaturated magmas during its late Holocene history. Variations in whole-rock major and trace element compositions of erupted products have been investigated, in combination with electron microprobe data for melt inclusions and phenocrysts comprising sanidine, biotite, clinopyroxene, titanomagnetite, ilmenite, haüyne, titanite, apatite and traces of plagioclase and amphibole. Compositional variations largely reflect fractional crystallisation, with a limited influence of magma mixing. Subtle variations in whole-rock composition and mineralogical characteristics between and within deposits, suggest the existence of a chemically zoned trachytic magma chamber beneath Rungwe. For the two most important studied deposits, the Isongole and Rungwe Pumice, co-existing Fe–Ti oxides constrain pre-eruptive temperature to 915–950°C and oxygen fugacity to NNO+0.25–NNO+0.45. For the Rungwe Pumice, melt inclusions suggest that the melt was water-undersaturated (maximum inferred H2O concentration 5.5wt.%). In the range of the defined pre-eruptive temperatures, this corresponds to melt viscosities as low as 103.3Pa·s, i.e. significantly lower than magmas that typically generate highly explosive eruptions. Because no microlites formed in the conduit during ascent, which would have strongly increased the effective magma viscosity, the highly explosive nature of the eruptions may be attributable to a crucial role of exsolved CO2 and S phases, and very high ascent rates.

Viscosity of andesitic lava and its implications for possible drain-back processes in the 2011 eruption of the Shinmoedake volcano, Japan

The 850 m diameter crater of the Shinmoedake volcano was filled by andesitic lava after three subplinian eruptions on 26-27 January 2011. We analyzed blocks thrown from the lava-filled crater by subsequent Vulcanian explosions to estimate the lava’s viscosity and evaluate the possibility of drain-back processes in the crater. Petrographic work on the ejecta, including bulk and glass chemistry, phenocryst and microlite modes, and the water content of the glass enabled us to estimate the bulk viscosity of the lava to be 109.8(+15 − 12) Pa s. The conduit radius is constrained to 4.5 to 6 m by the eruption rate of preceding subplinian eruptions (450–740 m3/s dense rock equivalent). We estimate the simple drain-back rate of the lava to be 3 × 10−2 ~ 2 × 10−5 m3/s. At this rate, less than 1 percent of the total amount of the effused lava could drain back within 100 days. Synthetic aperture radar (SAR) observations did not reveal evidence of drain-back after the eruption, possibly because the chamber was sustained, at least in part by repressurization and refilling as observed by global navigation satellite system (GNSS) measurements of the volcano. This study showed that degassing and crystallization of the andesitic magma during emplacement increased magma viscosity by more than five orders of magnitude, prohibiting drain-back of the lava that filled the crater after the emplacement.

The rheological transition in plagioclase‐bearing magmas

Volcanological processes, such as melt segregation, ascent, and eruption, are directly dependent on the rheological behavior of magmatic suspensions. An increase of the crystal fraction of the suspension leads to the formation of a solid‐particle network, which abruptly increases magma viscosity. The crystal fraction at which this rheological transition occurs depends on parameters such as the strain rate and the size, shape, and sorting of particles. To determine the influence of the crystal shape on the rheological transition, suspensions of plagioclase, a representative anisometric crystal, have been investigated at high temperatures and pressures. Synthetic suspensions with crystal fractions (ϕs) ranging from 0.38 to 0.75 were deformed both in compression and torsion in a Paterson apparatus at 300 MPa, 900°C and 800°C, and for strain rates between 1.0 × 10‐5 and 1.0 × 10‐3 s‐1. All suspensions exhibit a non‐Newtonian shear thinning rheological behavior. The experimental results, coupled with existing data and models at low crystal fractions (ϕs < 0.3), allow several rheological domains to be identified, from steady‐state flow to strain weakening, each characterized by a specific microstructure. In particular, a progressive evolution from a pervasive to a strain partitioning fabric is found when increasing the crystal fraction. Our results highlight the influence of both the strain rate and the shape of crystals on the rheological behavior of magmas. During crystallization, magmatic suspensions of anisometric minerals such as plagioclase would develop a solid‐particle network earlier (ϕs ~ 0.3) than suspensions of isometric minerals (ϕs ~ 0.5). Our study shows that localization of strain early in the crystallization history of mushy zones in the magma chamber, near the conduit margins, and at the base of lava flows would facilitate the mobilization, the transfer, and the final emplacement at the surface of highly viscous, feldspar‐rich magmas.

Relating vesicle shapes in pyroclasts to eruption styles

Vesicles in pyroclasts provide a direct record of conduit conditions during explosive volcanic eruptions. Although their numbers and sizes are used routinely to infer aspects of eruption dynamics, vesicle shape remains an underutilized parameter. We have quantified vesicle shapes in pyroclasts from fall deposits of seven explosive eruptions of different styles, using the dimensionless shape factor $\Omega $ , a measure of the degree of complexity of the bounding surface of an object. For each of the seven eruptions, we have also estimated the capillary number, Ca, from the magma expansion velocity through coupled diffusive bubble growth and conduit flow modeling. We find that $\Omega $ is smaller for eruptions with ${\rm{Ca}} \ll 1$ than for eruptions with Ca $\gg 1$ . Consistent with previous studies, we interpret these results as an expression of the relative importance of structural changes during magma decompression and bubble growth, such as coalescence and shape relaxation of bubbles by capillary stresses. Among the samples analyzed, Strombolian and Hawaiian fire-fountain eruptions have ${\rm{Ca}} \ll 1$ , in contrast to Vulcanian, Plinian, and ultraplinian eruptions. Interestingly, the basaltic Plinian eruptions of Tarawera volcano, New Zealand in 1886 and Mt. Etna, Italy in 122 BC, for which the cause of intense explosive activity has been controversial, are also characterized by ${\rm{Ca}} \gg 1$ and larger values of $\Omega $ than Strombolian and Hawaiian style (fire fountain) eruptions. We interpret this to be the consequence of syn-eruptive magma crystallization, resulting in high magma viscosity and reduced rates of bubble growth. Our model results indicate that during these basaltic Plinian eruptions, buildup of bubble overpressure resulted in brittle magma fragmentation.

Switching between seismic and seismo‐acoustic harmonic tremor simulated in the laboratory: Insights into the role of open degassing channels and magma viscosity

AbstractSwitching between seismic‐only harmonic tremor (SHT) and seismo‐acoustic harmonic tremor (SAHT) has been reported at few volcanoes worldwide, but its occurrence may indicate important changes in shallow conduit conditions. Switching was simulated in a laboratory experiment in which harmonic signals were produced with a flow‐driven valve and compressed air. The harmonic signals were passed through a tank of shear‐thinning viscoelastic fluid, and the resulting signals were recorded. At high fluid stiffness, a stable, open conduit was produced, and the harmonic signals generated within the experimental apparatus were efficiently transmitted into the atmosphere. At lower fluid stiffness, bubbling dominated the activity, stable pathways were not generated in the fluid, and HT was not recorded in the atmosphere. These results are compared to observations of switching at Fuego volcano, Guatemala. We conclude that at intermediate magma viscosities, the development of stable degassing pathways open to the atmosphere will allow HT generated in the conduit to be transmitted into the atmosphere. Further, subtle changes in magma properties and supply rate may control whether SHT or SAHT is recorded, providing information about the state of the shallow conduit and vent at active volcanoes.

Switching between seismic and seismo-acoustic harmonic tremor simulated in the laboratory: Insights into the role of open degassing channels and magma viscosity

Switching between seismic and seismo-acoustic harmonic tremor simulated in the laboratory: Insights into the role of open degassing channels and magma viscosity

Volatile-rich silicate melts from Oldoinyo Lengai volcano (Tanzania): Implications for carbonatite genesis and eruptive behavior

This study presents volatile, trace, and major element compositions of silicate glasses (nepheline-hosted melt inclusions and matrix glass) from the 2007–2008 explosive eruption at Oldoinyo Lengai volcano, Tanzania. The bulk compositions of the heterogeneous ash erupted in 2007–2008 are consistent with physical mixing between juvenile nephelinite magma and natrocarbonatite emplaced during the preceding ∼25 years of effusive carbonatite eruption. The melt inclusions and matrix glasses span a wide range of silica-undersaturated compositions, from ∼46wt% SiO2 and (Na+K)/Al∼3 in the least evolved melt inclusions to 38wt% SiO2 and (Na+K)/Al up to 12 in the matrix glass. The depletion in SiO2 between melt inclusions and matrix glass is accompanied by strong enrichment in all of the incompatible trace elements measured (Ba, Nb, La, Ce, Sr, Zr, Y), which is consistent with fractional crystallization of a bulk mineral assemblage with SiO2 higher than that of the melt inclusions but inconsistent with silicate melt evolution by assimilation of carbonatite. The melt inclusions are volatile-rich with 2.7wt% to 8.7wt% CO2 and 0.7wt% to 10.1wt% H2O, indicating that Oldoinyo Lengai is a hydrous system. This is contrary to the long-held assumption that Oldoinyo Lengai is relatively anhydrous, which is based on the observation that natrocarbonatite lavas are water-poor. We argue that natrocarbonatites are derived from hydrous carbonate liquid that degas H2O at low pressure. The silicate glass data show that H2O concentration is negatively correlated with incompatible element enrichment, which we attribute to crystallization of the melt in response to decompression degassing of H2O. The eruptive cycle at Oldoinyo Lengai reflects changes in bulk silicate magma viscosity due to extensive H2O-driven crystallization and explosive eruptions occur when volatiles (i.e. H2O>CO2 gas, and carbonate liquid) cannot separate from the crystal-rich nephelinite magma. Melt H2O content decreases as a function of pressure; however CO2 concentration in the melt inclusions is buffered by the presence of immiscible carbonate liquid. CO2 content increases with melt evolution parameters (e.g. increasing (Na+K)/Al) due to enhanced solubility with alkali enrichment and SiO2 depletion in the melt. The matrix glasses and evolved melt inclusions, on the other hand, experienced low pressure (<50MPa) CO2 degassing and were not buffered by a coexisting carbonate liquid. Whereas the melt inclusions are the most CO2-rich yet identified, their CO2/Nb ratios are without exception lower than that in MORB, indicating that a volatile-rich mantle source is not required for Oldoinyo Lengai.