Controls on magma permeability in the volcanic conduit during the climactic phase of the Kos Plateau Tuff eruption (Aegean Arc)

International audienceX-ray computed microtomography (μCT) was applied to pumices from the largest Quaternary explosive eruption of the active South Aegean Arc (the Kos Plateau Tuff; KPT) in order to better understand magma permeability within volcanic conduits. Two different types of pumices (one with highly elongated bubbles, tube pumice; and the other with near spherical bubbles, frothy pumice) produced synchronously and with identical chemical composition were selected for μCT imaging to obtain porosity, tortuosity, bubble size and throat size distributions. Tortuosity drops on average from 2.2 in frothy pumice to 1.5 in tube pumice. Bubble size and throat size distributions provide estimates for mean bubble size (~93– 98 μm) and mean throat size (~23–29 μm). Using a modified Kozeny-Carman equation, variations in porosity, tortuosity, and throat size observed in KPT pumices explain the spread found in laboratory measurements of the Darcian permeability. Measured difference in inertial permeability between tube and frothy pumices can also be partly explained by the same variables but require an additional parameter related to the internal roughness of the porous medium (friction factor f0). Constitutive equations for both types of permeability allow the quantification of laminar and turbulent gas escape during ascent of rhyolitic magma in volcanic conduits

A new strategy for the estimation of plume height from clast dispersal in various atmospheric and eruptive conditions

Plume height is an important parameter routinely used to characterize and classify explosive eruptions. Though the strategies to estimate key eruption source parameters such as erupted volume and mass flow rate have evolved over the past few decades, the determination of plume height of past eruptions is still mostly based on empirical approaches that do not account for the new developments in plume modelling based on the interaction of plume and wind. Here we present a revised strategy for the retrieval of plume height from field data that accounts for key aspects of plume dynamics and particle sedimentation, which include: i) the effect of wind advection on the buoyant plume, ii) a new parameterization of the gravitational spreading of the umbrella cloud for distances smaller than the radius of the plume, iii) the effect of particle shape on particle sedimentation, iv) the effect of different atmospheric profiles in different climate zones, v) three-dimensional wind, temperature and pressure data, and vi) topography. In particular, as wind can affect the dynamics and height of the plume, new computed sedimentation patterns are more complex and result in non-linear relationships between downwind and crosswind deposition. Our method is tested against observations of the 2011 eruption of Shinmoedake (Japan), the 1980 eruption of Mount St Helens (USA), and the 1991 eruption of Pinatubo (Philippines). These are well-constrained examples of small, intermediate, and high intensity eruptions, respectively. Intensity scenarios are introduced to account for the non-unique relation between plume height and particle sedimentation resulting from wind advection of volcanic plumes. We further demonstrate that needle-like and disk-like particle shapes can have downwind distances 36 to 70% larger than the equivalent spheres. In addition, we find that the effect of latitude on the determination of plume height is more significant for low and intermediate intensity scenarios with a discrepancy between 7 and 20%

The mechanics of shallow magma reservoir outgassing

Magma degassing fundamentally controls the Earth's volatile cycles. The large amount of gas expelled into the atmosphere during volcanic eruptions (i.e. volcanic outgassing) is the most obvious display of magmatic volatile release. However, owing to the large intrusive:extrusive ratio, and considering the paucity of volatiles left in intrusive rocks after final solidification, volcanic outgassing likely constitutes only a small fraction of the overall mass of magmatic volatiles released to the Earth's surface. Therefore, as most magmas stall on their way to the surface, outgassing of uneruptible, crystal-rich magma storage regions will play a dominant role in closing the balance of volatile element cycling between the mantle and the surface. We use a numerical approach to study the migration of a magmatic volatile phase (MVP) in crystal-rich magma bodies (“mush zones”) at the pore-scale. Our results suggest that buoyancy driven outgassing is efficient over crystal volume fractions between 0.4 and 0.7 (for mm-sized crystals). We parameterize our pore-scale results for MVP migration in a thermo-mechanical magma reservoir model to study outgassing under dynamical conditions where cooling controls the evolution of the proportion of crystal, gas and melt phases and to investigate the role of the reservoir size and the temperature-dependent visco-elastic response of the crust on outgassing efficiency. We find that buoyancy-driven outgassing allows for a maximum of 40-50% volatiles to leave the reservoir over the 0.4-0.7 crystal volume fractions, implying that a significant amount of outgassing must occur at high crystal content (>0.7) through veining and/or capillary fracturing

Magma chamber growth during inter-caldera periods: insights from thermo-mechanical modeling with applications to Laguna del Maule, Campi Flegrei, Santorini, and Aso

Crustal magma chambers can grow to be hundreds to thousands of cubic kilometers, potentially feeding catastrophic caldera‐forming eruptions. Smaller‐volume chambers are expected to erupt frequently and freeze quickly; a major outstanding question is how magma chambers ever grow to the sizes required to sustain the largest eruptions on Earth. We use a thermo‐mechanical model to investigate the primary factors that govern the extrusive:intrusive ratio in a chamber, and how this relates to eruption frequency, eruption size, and long‐term chamber growth. The model consists of three fundamental timescales: the magma injection timescale τin, the cooling timescale τcool, and the timescale for viscous relaxation of the crust τrelax. We estimate these timescales using geologic and geophysical data from four volcanoes (Laguna del Maule, Campi Flegrei, Santorini, Aso) to compare them with the model. In each of these systems, τin is much shorter than τcool and slightly shorter than τrelax, conditions that in the model are associated with efficient chamber growth and simultaneous eruption. In addition, the model suggests that the magma chambers underlying these volcanoes are growing at rates between ~10‐4‐10‐2 km3/yr, speeding up over time as the chamber volume increases. We find scaling relationships for eruption frequency and size that suggest that as chambers grow and volatiles exsolve, eruption frequency decreases but eruption size increases. These scaling relationships provide a good match to the eruptive history from the natural systems, suggesting the relationships can be used to constrain chamber growth rates and volatile saturation state from the eruptive history alone

How do volatiles escape their shallow magmatic hearth?

Only a small fraction (approx. 1–20%) of magmas generated in the mantle erupt at the surface. While volcanic eruptions are typically considered as the main exhaust pipes for volatile elements to escape into the atmosphere, the contribution of magma reservoirs crystallizing in the crust is likely to dominate the volatile transfer from depth to the surface. Here, we use multiscale physical modelling to identify and quantify the main mechanisms of gas escape from crystallizing magma bodies. We show that most of the outgassing occurs at intermediate to high crystal fraction, when the system has reached a mature mush state. It is particularly true for shallow volatile-rich systems that tend to exsolve volatiles through second boiling, leading to efficient construction of gas channels as soon as the crystallinity reaches approximately 40–50 vol.%. We, therefore, argue that estimates of volatile budgets based on volcanic activity may be misleading because they tend to significantly underestimate the magmatic volatile flux and can provide biased volatile compositions. Recognition of the compositional signature and volumetric dominance of intrusive outgassing is, therefore, necessary to build robust models of volatile recycling between the mantle and the surface. This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics’

Transition of eruptive style: Pumice raft to dome-forming eruption at the Havre submarine volcano, southwest Pacific Ocean

Transitions in eruptive style are common at volcanoes. Understanding how and why these transitions occur remain open questions. The 2012 eruption of the submarine Havre volcano in the Kermadec arc (southwest Pacific Ocean) produced a raft of floating pumice followed by a pair of domes from the same vent. Here, we used measurements on erupted magmas and constraints on the eruption rate, combined with a model for magma ascent, to identify the dominant controls on the transition in eruption style. During the raft-forming stage, magma ascent was fast enough that little gas was lost. Magma reached the seafloor with great enough vesicularity to be buoyant and produce clasts that could float. As the eruption waned, the eruption rate decreased, and the conduit narrowed. Sufficient gas was then lost to the surrounding country rocks during ascent such that the erupted magma was no longer buoyant relative to seawater. Most of the original dissolved water in the magma was lost to the crust surrounding the conduit during the dome-forming stage

Long-term magmatic evolution reveals the beginning of a new caldera cycle at Campi Flegrei

Understanding the mechanisms that control the accumulation of large silicic magma bodies in the upper crust is key to determine the potential of volcanoes to form caldera-forming eruptions. Campi Flegrei is an active and restless volcano, located in one of the most populated regions on Earth, which has produced two cataclysmic caldera-forming eruptions and numerous smaller eruptive events over the last 60,000 years. Here we combine the results of an extensive petrological survey with a thermo-mechanical model to investigate how the magmatic system shifts from frequent, small eruptions to large caldera-forming events. Our data reveal that the most recent eruption of Monte Nuovo is characterized by highly differentiated magmas akin to those that fed the pre-caldera activity and the initial phases of the caldera-forming eruptions. We suggest that this eruption is an expression of a state shift in magma storage conditions, whereby significant amounts of volatiles start to exsolve in the shallow reservoir. The presence of an exsolved gas phase has fundamental consequences for the physical properties of the reservoir and may indicate that a large magma body is currently accumulating underneath Campi Flegrei

A connection between magma chamber processes and eruptive styles revealed at Nisyros-Yali volcano (Greece)

Arc volcanoes generally emit water-rich, high-viscosity silicic magmas, which are prone to erupt explosively. However, effusive behavior is a common occurrence despite the high-H2O, high viscosity conditions. The contrasting shift from effusive to explosive behavior (and vice-versa) at any individual volcano raises the question on what controls eruptive style. Permeability development in conduits allows magma to outgas and is clearly a key factor. However, an important question is whether magma reservoir processes can also have an influence on eruptive styles. The answer could have direct impact on predicting eruptive behavior. Hence, we explore this potential connection by analyzing nine alternating effusive and explosive silicic deposits that were emplaced during distinct eruptions at the active Nisyros-Yali volcanic center. The lavas and pyroclastic deposits are compositionally similar. This indicates a negligible influence of the bulk rock composition on different eruptive styles. The crystal contents vary between units, without any clear correlation with eruptive style (from nearly aphyric to ~45 vol% crystals). Mineral textures and chemistry do show variations between effusive and explosive eruptions, with a larger proportion of resorbed plagioclase and, in most cases, more evolved amphiboles present in the lava flows. Mineral thermo-barometry and hygrometry show that the storage zones of magmas generating effusive eruptions evolved towards colder and more water-rich conditions (710–790 °C; 5.6–6.5 wt% H2O) than their explosive counterparts (815–850 °C; 4.2–4.6 wt% H2O). At storage pressures of 1.5–2 kbar, relevant for Nisyros-Yali, the volatile saturation level is reached at >5 wt% H2O. Therefore, it is likely that the magmas reached water-saturation before generating effusive eruptions, and were undersaturated before explosive events. We hypothesize that the presence of exsolved volatiles in the subvolcanic reservoir can enhance the outgassing potential of the magma during conduit ascent. Hence, the rhyolitic effusive-explosive transitions can be influenced by the pre-eruptive exsolved versus dissolved state of the volatiles in the magma chamber. This can lead to the less explosive eruptions for the most water-rich reservoir conditions