Jennifer Gilbert

Pressure changes associated with the ascent and bursting of gas slugs in liquid-filled vertical and inclined conduits

At basaltic volcanoes, the sources of long-period and very-long-period seismicity and acoustic signals are frequently described in terms of fluid dynamic processes, in particular the formation and ascent of gas slugs within the magma column and their bursting at the surface. To investigate pressure changes associated with these processes, two-phase flow experiments have been carried out in vertical and inclined pipes with both single gas slugs and a continuously supplied gas phase. The ascent of individual gas slugs is accompanied by strong dynamic pressure variations resulting from the flow of liquid around the slug. These dynamic transients generate sub-static pressures below the ascending slug in viscosity-controlled systems, and produce super-static pressures when the slug reaches the surface and motion ceases in inertia-dominated systems. Conduit inclination promotes a change of regime from bubbly to slug flow and favours an increase in size and velocity of the slugs at the expense of their frequency of occurrence during continuously supplied two-phase flow. The experimental pressure data support previous theoretical analyses of oscillatory sources in ascending slugs as the slugs approach the surface and burst. Pressure oscillations are also observed during the release of gas slugs and in their wake region.

Electric potential gradient changes during explosive activity at Sakurajima volcano, Japan

We report electric potential gradient measurements carried out at Sakurajima volcano in Japan during: (1) explosions which generated ash plumes, (2) steam explosions which produced plumes of condensing gases, and (3) periods of ashfall and plume-induced acid rainfall. Sequential positive and negative deviations occurred during explosions which generated ash plumes. However, no deflections from background were found during steam explosions. During periods of ashfall negative electric potential gradients were observed, while positive potential gradients occurred during fallout of plume-induced acid rain from the same eruption. These results suggest that a dipole arrangement of charge develops within plumes such that positive charges dominate in the volcanic gas-rich top and negative charges in the following ash-rich part of the plume. The charge polarity may be reversed for other volcanoes (Hatakeyama and Uchikawa 1952). We suggest that charge is generated by fracto-emission (Donaldson et al. 1988) processes probably during magma fragmentation within the vent, rather than by frictional effects within the plume.

Melting of the glacier base during a small-volume subglacial rhyolite eruption: evidence from Bláhnúkur, Iceland

Abstract Although observations of recent volcanic eruptions beneath Vatnajokull, Iceland have improved the understanding of ice deformation and meltwater drainage, little is known about the processes that occur at the glacier base. We present observations of the products of a small-volume, effusive subglacial rhyolite eruption at Blahnukur, Torfajokull, Iceland. Lava bodies, typically 7 m long, have unusual conical morphologies and columnar joint orientations that suggest emplacement within cavities melted into the base of a glacier. Cavities appear to have been steep-walled and randomly distributed. These features can be explained by a simple model of conductive heat loss during the ascent of a lava body to the glacier base. The released heat melts a cavity in the overlying ice. The development of vapour-escape pipes in the waterlogged, permeable breccias surrounding the lava allows rapid heat transfer between lava and ice. The formed meltwater percolates into the breccias, recharging the cooling system and leaving a steam-filled cavity. The slow ascent rates of intrusive rhyolitic magma bodies provide ample time for a cavity to be melted in the ice above, even during the final 10 m of ascent to the glacier base. An equilibrium cavity size is calculated at which melting is balanced by creep closure. This is dependent upon the heat input and the difference between glaciostatic and cavity pressure. The cavity sizes inferred from Blahnukur are consistent with a pressure differential of 2–4 MPa, suggesting that the ice was at least 200 m thick. This is consistent with the volcanic stratigraphy, which indicates that the ice exceeded 350 m in thickness. Although this is the first time that a subglacial cavity system of this type has been reconstructed from an ancient volcanic sequence, it shares many characteristics with the modern firn cave system formed by fumarolic melting within the summit crater of Mount Rainier, Washington. At both localities, it appears that localised heating at the glacier base has resulted in heterogeneous melting patterns. Despite the different rheological properties of ice and firn, similar patterns of cavity roof deformation are inferred. The development of low-pressure subglacial cavities in regions of high heat flux may influence the trajectory of rising magma, with manifold implications for eruptive mechanisms and resultant subglacial volcanic landforms.

Volcanic plume monitoring using atmospheric electric potential gradients

By monitoring perturbations of the natural atmospheric electric potential gradient, it is possible to detect and track particle laden volcanic plumes. The perturbations are produced by electrical charge that resides on the solid particles and liquid droplets of the plume, and the ions within it. A network of potential gradient monitoring stations around a volcano can provide data on eruption time, magnitude, plume dispersion direction and areas of ashfall. Our laboratory experiments have produced electrically charged ash by fragmenting pumice. These results support the theory that charging is due to the magma fragmentation process and should therefore be ubiquitous in all particulate volcanic plumes.

The aerodynamic behaviour of volcanic aggregates

A large proportion of solid material transported within the atmosphere during volcanic eruptions consists of particles less than 500 μm in diameter. The majority of these particles become incorporated into a wide range of aggregate types, the aerodynamic behaviour of which has not been determined by either direct observation or in the laboratory. In the absence of such data, theoretical models of fallout from volcanic plumes make necessarily crude assumptions about aggregate densities and fall velocities. Larger volcanic ejecta often consists of pumice of lower than bulk density. Experimental data are presented for the fall velocities of porous aggregates and single particles, determined in systems analogous to that of ejecta falling from a volcanic plume. It is demonstrated that the fall of aggregates may be modelled in identical fashion to single particles by using a reduced aggregate density dependent on the porosity, and a size corresponding to an enclosing sphere. Particles incorporated into aggregates attain a substantially higher fall velocity than single particles. This is due to the larger physical dimensions of the aggregate, which overcomes the effect of lower aggregate density. Additionally, the internal porosity of the aggregate allows some flow of fluid through the aggregate and this results in a small increase in fall velocity. The increase in fall velocity of particles incorporated into aggregates, rather than falling individually, results in the enhanced removal of fine material from volcanic plumes.

The significance of garnet in the Permo-Carboniferous volcanic rocks of the Pyrenees

Garnet occurs within rhyolitic to dacitic lavas, ignimbrites and volcaniclastic sediments of the Pyrenean calc-alkaline Permo-Carboniferous volcanic rocks. On the basis of texture and major and trace element composition, the garnet is interpreted as having a phenocrystal as opposed to a xenocrystal or restite origin. The Pyrenean silicic volcanics are strongly peraluminous, consistent with derivation as a partial melt from a sedimentary source. They were erupted contemporaneously with the Hercynian thermal anomaly, which produced regional granulite facies metamorphism and anatectic granites. The ‘Late Hercynian granodiorites’ were also intruded at about this time. Geochemical data suggest that the petrogenesis of the volcanic rocks differed from that of the granodiorites. REE garnet/matrix partition coefficients for garnets from a welded rhyolitic ignimbrite are similar to those reported for other phenocrystal garnets in rhyolites. REE abundances imply that the garnets crystallized from a HREE depleted melt and this is considered to reflect the effects of garnet in the restite. Experimental constraints suggest that the garnet crystallized at pressures >4–5kb, implying that the rhyolites originated from > 14–17km depth and were erupted rapidly from these depths in order for the garnet to survive. These figures place constraints on the thermal regime prevailing in the late stages of the Hercynian orogeny.

Future research directions on the physics of explosive volcanic eruptions

Scientific research can take unexpected, even counter-intuitive, directions because of technical innovation, the occasional brilliant idea that overturns conventional wisdom and new observations that provide previously unexpected insights into the way in which nature works. For these reasons no one is certain what the future holds in terms of breakthroughs. This chapter highlights some of the most recent developments in research on the physics of explosive volcanism. It pin-points cardinal areas of study poised for new research and anticipates major future developments. Advances in remote sensing and computational power are two examples of technical developments which are currently having dramatic impacts on understanding the physics of explosive volcanism. Such technical innovations, together with many good ideas and observations, are changing perceptions of the mechanisms of explosive volcanism. With an increasingly populated and ecologically stressed world, the potential effects of explosive volcanism are being exacerbated. Several megacities, e.g. Tokyo, Naples and Mexico City, now exist close to active volcanoes, and in many parts of the world economic development and population expansion have combined such that the risk of major volcanic disasters increases year by year. Volcanic activity has both local and global environmental effects. For example, fallout of volcanic ash from eruption plumes can disrupt air, sea, road and rail traffic, inhibit electrical communications, cause respiratory problems for people and animals, pollute water, damage crops, cause failure of building roofs and generally bring havoc to local communities. On a larger scale, volcanic aerosols from some events, such as the 1991 Pinatubo

Ice‐melt rates by steam condensation during explosive subglacial eruptions

Subglacial volcanism melts cavities in the overlying ice. These cavities may be flooded with meltwater or they may be fully or partially drained. We quantify, for the first time, heat transfer rates by condensation of steam on the walls and roof of a fully or partially drained subglacial eruption cavity. Our calculations indicate that heat fluxes of up to 1 MW m−2 may be obtained when the bulk vapor in the cavity is in free convection. This is considerably smaller than heat fluxes inferred from ice penetration rates in recent subglacial eruptions. Forcing of the convection by momentum transfer from an eruption jet may allow heat fluxes of up to 2 MW m−2, consistent with values inferred for the Gjalp 1996 subglacial eruption. Vapor-dominated cavities in which vapor-liquid equilibrium is maintained have thermal dynamic responses that are an order of magnitude faster than the equivalent flooded cavities.

The stratigraphy of a proximal late Hercynian pyroclastic sequence: the Vilancós region of the Pyrenees

Volcanic activity, the result of crustal differentiation during the Hercynian orogeny, generated eight explosive eruptions in the Vilancos region of the Spanish Pyrenees. The volcanic products comprise the Erill Castell Volcanic Formation of Stephanian age, which crops out as a 20 km long, WNW-trending strip < 2 km wide dipping steeply to the south. The Vilancos region represents a small fragment of an originally extensive regional terrain of silicic centres. The explosive eruptions mainly generated strongly peraluminous and phenocrystal garnet-bearing subaerial ignimbrite facies. Proximal intra-formational breccias represent a substantial volume of the preserved erupted product and one phreatoplinian deposit is exposed. Mass-flow deposits are common, and small-volume basalt, andesite and rhyolite lava flows, minor tuffs and palaesols also occur. Electron microprobe data show that each garnet-bearing member of the Vilancos region has a distinct garnet composition. This is used as geochemical fingerprinting tool to aid mapping and correlation between proximal intra-formational breccias and ignimbrite of the same eruption. Within one debris-flow deposit (the Vilancos Breccia Member) at least three garnet populations occur. Two of these are derived from pyroclastic members within the mapped region, the other comes from an unexposed rhyolite lava source.

Electrostatic phenomena in volcanic eruptions

Electrostatic phenomena have long been associated with the explosive eruption of volcanoes. Lightning generated in volcanic plumes is a spectacular atmospheric electrical event that requires development of large potential gradients over distances of up to kilometres. This process begins as hydrated liquid rock (magma) ascends towards Earths surface. Pressure reduction causes water supersaturation in the magma and the development of bubbles of supercritical water, where deeper than c. 1000 m, and water vapour at shallower depths that drives flow expansion. The generation of high strain rates in the expanding bubbly magma can cause it to fracture in a brittle manner, as deformation relaxation timescales are exceeded. The brittle fracture provides the initial charge separation mechanism, known as fractoemission [1]. The resulting mixture of charged silicate particles and ions evolves over time, generating macro-scale potential gradients in the atmosphere and driving processes such as particle aggregation. For the silicate particles, aggregation driven by electrostatic effects is most significant for particles smaller than c. 100 μm. Aggregation acts to change the effective aerodynamic behaviour of silicate particles [2], thus altering the sedimentation rates of particles from volcanic plumes from the atmosphere. The presence of liquid phases also promotes aggregation processes [3] and lightning.