V11F-01 INVITED
Sponge Cake or Champagne? Bubbles, Magmatic Degassing and Volcanic Eruptions
Vesiculation is an unavoidable consequence of magma decompression; the extent to which bubbles travel with ascending magma or leave the system by separated or permeable flow will determine the nature of the ensuing eruption. Bubbles travel with the melt from which they exsolve if the rise time of bubbles through the melt (the ‘drift velocity') is much less than the rise rate of the magma (sponge cake). This condition is most likely to be met in viscous melts (where bubble rise velocities are low) and in melts that experience rapid decompression (high ascent velocities). Under these conditions, bubble expansion within the melt continues until sufficient bubble expansion causes coalescence and the development of a permeable network. Typical pumice vesicularities of 70-80% and permeabilities of 10-12 m2 constrain this limit under conditions appropriate for subplinian to plinian eruptions (mass fluxes > 106 kg/s). Slower rise rates (and lower mass fluxes) that characterize effusive eruptions produce silicic lavas with a wider range of vesicularities. In general, permeability decreases with decreasing sample vesicularity as bubbles deform (as evidenced by anisotropy in permeability and electrical conductivity) and pore apertures diminish. Degassing efficiency (and resulting densification of magma within the conduit) under these conditions is determined by permeability and the time allowed for gas escape. Bubbles rise through the melt if the drift velocity exceeds the velocity of magma ascent (champagne). This condition is most easily met in volatile-rich, low viscosity (mafic) melts at low to moderate fluxes. At very low magma flux, magma eruption rate is determined by the extent to which magma is entrained and ejected by rising gases (strombolian eruptions); when bubbles are too small, or are rising too slowly, they may not break the surface at all, but instead may be concentrated in a near-surface layer (surface foam). As the magma flux increases, segregation of bubble-rich from bubble-poor melt requires both longer conduits and lateral transport of degassed magma, as seen in violent strombolian eruptions. Flow transitions require coalescence, which is a dynamic process where bubble-bubble interactions are controlled by shear and gravitational (i.e. buoyancy) processes, both of which are strongly dependent on magma rheology. Also common in basaltic magmas that exhibit separated flow are by the simultaneous eruption of crystal-rich (shallow) and crystal-poor (deep) magmas. Upward increases in crystal content within the magma transport system will create rheological changes that may both slow upward bubble migration and change the size and shape of the bubble network (through deformation, coalescence, or bubble splitting). Preliminary experiments further show that abrupt rheological boundaries may concentrate bubbles at the boundary, allowing them to coalesce and move laterally prior to rising through the mush.
V11F-02
Vesicle Microtextures and Fragmentation in Basaltic Plinian Eruptions
We set out to establish how basaltic magma erupts at Plinian intensities, by matching vesicle microtextures within the products of basaltic Plinian eruptions to calculated rates for bubble growth and coalescence in basaltic magmas. We find that observed microtextures are consistent with sufficient gas overpressure for magma fragmentation during ascent-driven decompression. Vesicles in pyroclasts from basaltic Plinian eruptions at Mt. Etna, 122 B.C. (Italy) and Mt. Tarawera, 1886 A.D. (New Zealand) have complex, non-spherical shapes indicative of bubble coalescence. Because of surface tension, non-spherical bubbles relax back to spherical shape over time. Preservation of abundant vesicles with "coalescence" shapes in both sample suites implies that bubble coalescence proceeded at a faster rate than shape relaxation, immediately prior to ejection and cooling of the erupting magma. We hypothesize that fast rates of bubble coalescence are the consequence of ascent-driven, decompressive bubble growth. We test this hypothesis by calculating the required decompression rates and matrix viscosities at which bubble growth and coalescence are faster than shape relaxation. Calculated decompression rates are consistent with existing eruption rate estimates, while calculated viscosities are consistent with a melt viscosity affected by high micro-crystallinity (>50%) observed in matrix glasses from both sample suites. When considered together, we find that during eruptive conditions of faster coalescence relative to shape relaxation, gas overpressure within bubbles is predicted to be sufficient to cause brittle fragmentation of the ascending basalt magma.
V11F-03
Separated Vs. homogeneous two-phase flow in violent strombolian activity
The term ‘violent Strombolian’ was first used to describe mafic eruptions that formed ash-charged columns up to 6 km high, and dispersed material up to a few hundred km from the source (Walker, 1971). These eruptions are often discontinuous and strongly pulsatory and are typically associated with simultaneous effusive activity: they form composite deposits constituted by a cinder cone, tephra blanket, and lava flows spreading from lateral vents. This eruptive regime is typical of water-rich mafic magmas and is characterized by average mass flows (103-105 kg/s) intermediate between Hawaiian and subplinian regimes. Within this interval, there is a direct correlation between explosivity, as defined by tephra production, and magma flux. When magma flow exceeds 105 kg/s, gas segregation is no longer possible and eruptive activity takes the form of sustained columns (subplinian to plinian activity). At eruption rates below 103 kg/s passive degassing processes dominate, causing lava effusion and/or mild explosive activity (Strombolian to Hawaiian). We suggest that very shallow gas segregation processes play a fundamental role in violent strombolian dynamics, affecting both explosive and effusive activity. Simultaneous eruption of tephra from the cone and lava flows from lateral vents requires both a gas-rich mixture ascending the central conduit and gas-poor lava flowing in the lateral system. Uneven distribution of liquid and gas phases is possible only when gas and magma are characterized by different momentum, i.e. the flow is separated. At a first approximation, the phase distribution is controlled by the two-phase flow regime (bubbly, slug, churn or annular), both gas and liquid fluxes, and the ratio between conduit and dike diameters. To quantify this process, we analyze in detail the dynamics of a particularly long-lived and well-known eruption of the last century- the Paricutin eruption (1943-1952) of central Mexico. Specific two-phase flow models are then used to evaluate 1) the effect of separated flow in magma splitting between a central conduit and a lateral dike system, 2) the flow regime that can develop in the central conduit 3) the potential effect on the explosive dynamics. Our results indicate that the explosivity of the eruption was strongly increased by segregation processes, promoting shallow fragmentation of the magma, and that the pulsatory dynamics was likely related to unstable, transitional two-phase flow regimes typical of gas fluxes intermediate between slug and annular flow.
V11F-04
A Numerical Model for the Development of Magma Permeability During Eruption
Magma permeability and eruption rate are closely coupled. Permeable networks of bubbles may develop as magma rises and bubbles grow and impinge on one another. The rate of release of overpressure as gas escapes through these networks influences the rate of bubble growth, hence, the rate and style of eruption. Conversely, the eruption rate influences the changing pressure environment experienced by a rising packet of magma. This, in turn, controls the morphology and permeability of the bubble networks. I present a numerical model for the development of the permeability of a packet of magma as it rises in the volcanic conduit. An important result is that anisotropic expansion of the magma leads to anisotropic permeability: For magma expanding at low capillarity within a conduit, percolation occurs first across the conduit and permeability is enhanced across the conduit. This implies that lateral degassing predominates in such a system, facilitating rapid gas escape, reducing the likelihood of magma fragmentation and mitigating eruption explosivity.
V11F-05 INVITED
Inferring gas transport mechanisms through volcanic gas measurements
The composition of volcanic gases is controlled by a number of factors, such as the initial concentration of volatiles in the magma; the rate of volatile diffusion through the melt with respect to decompression rate; the depth and style of melt-vapour separation; the degree of equilibration between ascending gases and resident shallow magma; and the extent to which the vapour phase interacts with a hydrothermal system. All of these processes influence the proportions and fluxes of the major species in volcanic gases. Measurements of volcanic gas species from both basaltic and silicic systems allow insight into the mechanisms of gas transport, from its exsolution to its escape into the atmosphere. At Kilauea Volcano, the composition of gas (measured by FTIR) emitted during different styles of activity constrains the depth and mode of vapour-melt separation that occurs in the conduit. Distinct degassing regimes can be identified: gas accumulation at depth followed by rapid ascent is associated with the emission of CO2-rich gases; ascent and growth of large bubbles through relatively stagnant magma is accompanied by H2O-rich gases. At the andesitic Soufriere Hills Volcano, SO2 flux is measured by UV spectrometers (DOAS) during dome-building and during explosive activity. Observations suggest that an S-rich vapour phase may flow through a shear zone at the conduit walls, derived from a mafic, unerupted magma, thus bypassing shallow bubble networks. Volcanic gas measurements are thus a valuable addition to the existing pool of observational, experimental, numerical and theoretical observations elucidating gas transport through magma.
V11F-06
From the Sound of Erta Ale Lava Lake (Ethiopia) to Eruption Dynamics Into a Magma Reservoir
The basaltic volcano of Erta Ale, located on the East African Rift, has a permanent lava lake whose behaviour presents similarity with a shallow magma reservoir. In March 2003, continuous measurements of acoustic pressure, images from video, temperature, seismicity and wind velocity were perfomed to quantify degassing of the lava lake in order to understand the eruptive behaviour of this volcano. The videos show that two types of gas bubbles break at the lava lake surface. Modelling acoustic pressure gives bubble overpressure and size. Bubbles are either large (radius 2 m) and overpressurised (4.104 Pa) or of intermediate size (radius 1 m) and weakly overpressurised (450 Pa). The large bubbles come from the conduit at the base of the lava lake whereas bubbles of intermediate size are produced by the destabilisation of a foam accumulated below the crust overlying the lava lake. Hence, their overpressure is related to capillary pressure of the rising small bubbles, suggesting that their diameter is 3.6 mm. The formation of bubbles of intermediate size is related to the local foam coalescence because of foam sluggish drainage. However, overpressure of intermediate size bubbles shows sudden peaks every eighteen hours, up to 6000 Pa. Each peak is related to a massive coalescence of a foam having reached its critical thickness. This involves a much larger number of bubbles than foam drainage, hence a much larger overpressure and energy. The rapid and massive coalescence leads to a sudden withdrawal of the foam. The disappearence of the foam suppress the buoyancy that sustained the cold and dense crust at the top of the lava lake, forcing the crust to sink. The average gas flux (6.10-3 m3s-1) is estimated over an eighteen-hour cycle from modelling the frequency of sound waves. Furthermore the diameter of the small bubbles deduced from the overpressure on synthetic waveforms can be combined with gas volume fraction observed on videos to estimate the gas flux between 3.5.10-3 m3s-1 and 7.10-3 m3s-1. The excellent agreement between these two independant methods reinforces the validity of our approach in the understanding of the physical processes at work in the lava lake.
V11F-07
A Model of Emitted Gas Ratios From Persistently-Degassing Volcanoes and Implications for Equilibrium Closed-System Degassing
Many passively degassing volcanoes exhibit remarkably constant ratios of different emitted gas species. Examples, with measured sulphur : chlorine ratios (S/Clemitted) include Stromboli, Italy (0.8 -– 0.9); Masaya, Nicaragua (1.4 –- 1.5) and Nyiragongo, Dem. Rep. Congo (0.034 –- 0.035). These ratios persist over periods of years despite variations in absolute gas fluxes of over 200%. We present a simple model for the evolution of gas ratios during volcanic degassing which shows that a constant gas ratio is exactly what one would expect if degassing processes are dominated by equilibrium, closed-system degassing. The model assumes a simplified configuration consisting of a single magma reservoir connected to the surface by a conduit in which magma convection occurs, with degassing taking place rapidly as the magma reaches the surface, followed by return of degassed magma to the deep reservoir. Using measured `primitive' and `degassed' volatile concentrations from these volcanoes, we find excellent quantitative agreement between the simple model and observed values at Stromboli and Masaya. We show that the emitted gas ratios are controlled by the initial (S0, Cl0) and degassed (Sd, Cld) volatile concentrations of the source magmas and the degassed magmas, according to the expression S/Clemitted = (S0 - Sd) / (Cl0 - Cld). The ratios are not affected by the extent of degassing, or by an influx of fresh, volatile-rich magma of the same initial volatile composition. We infer that volcanoes that exhibit strongly variable gas ratios (e.g. Etna, Italy, S/Clemitted ~ 0 –- 7) must either be controlled by open- system degassing, or tap several magmas with distinct initial volatile compositions.
V11F-08
The role of gas percolation in quiescent degassing of persistently active basaltic volcanoes
Using constraints from literature data on the petrology and texture of erupted material from Stromboli and geochemical measurements of gas emissions together with a model of gas solubility we construct a conceptual model of quiescent degassing for this volcano. We find that within a pressure range between 100MPa and 50MPa (~3.6km and ~1.8km depth respectively) vesiculating magma ascending within the conduit becomes permeable to gas flow and a transition from closed- to open-system degassing takes place. Above the transition, gas, rich in the most insoluble gases, flows up through degassing magma, and thereby becomes enriched in more soluble gases during ascent to the surface. The final gas emission is therefore a superposition of gases released from magma above the percolation transition and gas that has evolved in closed-system below the transition. Steady-state gas release from Stromboli can only be sustained via magma circulation, driven by the density variation between ascending vesiculating magma and descending degassed magma. By balancing the buoyant force of ascending vesiculating magma against the viscous resistance produced by travelling through descending, degassed magma in a simple flow model we determine that a cylindrical conduit diameter of 2.5- 2.9m produces the magma mass flow rate of 575kgs-1, required to account for the observed quiescent SO2 gas flux on Stromboli of ~2.3kgs-1 (200td-1). Ascending magma accelerates during depressurization due to increasing vesicularity and requires ~8 hours to ascend from 200MPa (~7km depth) to the surface.