Gas-rich Magma Research Articles

Volcanic SO2 emissions and the abundance and distribution of exsolved gas in magma bodies

The mass of sulfur dioxide (SO2) released by most explosive volcanic eruptions greatly exceeds the amount originally dissolved in the erupted volume of silicate melt. Several lines of evidence suggest that this discrepancy is due to the presence of SO2-bearing gas bubbles in the magma before the time of eruption. Comparison of remote sensing data for SO2 emissions with conventional petrologic estimates suggests that the discrepancy can be resolved if andesitic and dacitic magmas contain about 1–6wt% exsolved gas, equivalent to 5–30vol%, prior to eruption. Such large mass fractions of exsolved gas in pre-eruptive magma are consistent with previously published physical models in which crystallization-induced gas exsolution gradually increases overpressuring of a magma reservoir, eventually triggering an eruption. Simple mass balance models for SO2 emissions from magma bodies in which there is an upwards increasing gradient in exsolved gas mass fraction (i.e. gas-rich at the top) yield SO2 vs. eruptive volume trends that are similar to those observed for eruptions ranging in size from 0.01 to 10km3 of magma. Despite the many uncertainties involved, these patterns are consistent with the hypothesis that subvolcanic magma reservoirs are separated from volcanic vents by cupolas of gas-rich magma that supply much of the gas released in explosive eruptions. The volume fraction of exsolved gas inferred from SO2 data for the upper regions of many pre-eruptive magma bodies (∼30vol%) is similar to the percolation threshold at which gas bubbles become sufficiently interconnected to allow permeable gas flow through a bubble network. Thus this value may reflect a physical limitation on the maximum exsolved gas volume fraction that can occur at the roof zone of a magma reservoir because any additional gas would be lost by advective flow through the permeable bubble network. Andesitic and dacitic magma in crustal reservoirs are probably gas saturated due to recharge and underplating by basaltic magma that is saturated with H2O–CO2–S gas. Comparison of repose times, eruptive volumes, and basaltic magma supply rates for a spectrum of volcanic systems suggests a relatively steady state flux of S from Earth's mantle to atmosphere through many andesitic and dacitic magma systems.

Activity of Mount Etna preceding the February 1999 fissure eruption: inferred mechanism from seismological and geochemical data

The February 1999 eruption of Mt. Etna took place through a fissure on the SSE flank of the cone of the summit SE Crater. This event was preceded by continuous activity since 1995, sometimes accompanied by violent outbursts from one or more of the three other summit craters (NE Crater, Voragine or Chasm, and Bocca Nuova), and finally by a series of 20 short-lived eruptions from the SE Crater between September 1998 and January 1999. These phenomena could be accounted for by invoking gradual invasion of a shallow small reservoir by more primitive, basic and gas-rich magma coming from depth. The shallow “chamber” is more likely to be a plexus of dikes, which had developed during the previous years (1995–1997), following variations of the local stress field owing to enhanced magma generation and accumulation at the top of the mantle. Magma injection and mixing is evidenced through geochemistry, whereas the state of stress of the volcanic pile and underlying crust is determined using earthquake distributions and focal mechanisms. The behaviour of the seismic tremor amplitude appears to be a good indicator of the state of unrest of the volcano, although not always directly linked to the relative energy of degassing phenomena.

The contribution of fluid geochemistry to the volcano monitoring of Stromboli

The persistent open-vent mild explosive activity of Stromboli volcano is episodically interrupted by more violent and dangerous explosive events (major explosions and paroxysms). According to the nature of erupted products, paroxysms can be related either to the explosion of overpressured gas pockets located in the proximity of the magma column or to the sudden uprise of hot, gas-rich magma from a deep part of the plumbing system. In both cases, these more energetic explosive events should be preceded by an escape or a preferential uprise of the highly mobile volatiles which, in turn, should produce gas leakage anomalies at the surface in sites of high vertical permeability, such as deep-reaching faults. In order to identify such gas leaking sites, a systematic CO 2 soil flux survey has been carried out on the island using an accumulation chamber. Four hundred sixty-one points have been measured with a high density in the summit crater area, where high flux values have been found (10 −3–10 −2 cm/s). Anomalous points are concentrated along the main NE–SW axial feeding system of the volcano. CO 2 soil flux decreases from the crater zone to the base of the volcanic cone, where, however, an interesting gas leakage anomaly occurs (Pizzillo mofette). In the Pizzillo area thermal water wells also occur, whose chemistry indicates an origin by sea water heated by hot gas. Soil gases have been sampled in the sites with the highest CO 2 flux in the crater area and at the base of the cone. Chemical and isotopic analyses indicate the presence of a deep gas component especially in the crater zone samples (high CO 2, appreciable contents of He and H 2, 3He/ 4He values up to 3.55, δ 13C of CO 2=−2 ‰). The SC5 low-flux fumarole on the crater rim has shown, during six years, a remarkable persistence of temperature (93–95°C), with only minor fluctuations of chemistry. Appearance of anomalous peaks of H 2 in correspondence with strombolian explosions was observed during a 2-h 30-min experiment of continuous recording of hydrogen content of the fumarole. Together with previous data on H 2O, CO 2 and He, this experiment confirms that strombolian blasts produce rapid fluctuations in the fumarolic gas composition. In the light of this study, SC5 fumarole and Pizzillo mofette look promising sites for the testing of a continuous geochemical monitoring system of Stromboli volcano.

Modelling the dynamics and thermodynamics of volcanic degassing

The rates of passive degassing from volcanoes are investigated by modelling the convective overturn of dense degassed and less dense gas-rich magmas in a vertical conduit linking a shallow degassing zone with a deep magma chamber. Laboratory experiments are used to constrain our theoretical model of the overturn rate and to elaborate on the model of this process presented by Kazahaya et al. (1994). We also introduce the effects of a CO2–saturated deep chamber and adiabatic cooling of ascending magma. We find that overturn occurs by concentric flow of the magmas along the conduit, although the details of the flow depend on the magmas' viscosity ratio. Where convective overturn limits the supply of gas-rich magma, then the gas emission rate is proportional to the flow rate of the overturning magmas (proportional to the density difference driving convection, the conduit radius to the fourth power, and inversely proportional to the degassed magma viscosity) and the mass fraction of water that is degassed. Efficient degassing enhances the density difference but increases the magma viscosity, and this dampens convection. Two degassing volcanoes were modelled. At Stromboli, assuming a 2 km deep, 30% crystalline basaltic chamber, containing 0.5 wt.% dissolved water, the ∼700 kg s–1 magmatic water flux can be modelled with a 4–10 m radius conduit, degassing 20–100% of the available water and all of the 1 to 4 vol.% CO2 chamber gas. At Mount St. Helens in June 1980, assuming a 7 km deep, 39% crystalline dacitic chamber, containing 4.6 wt.% dissolved water, the ∼500 kg s–1 magmatic water flux can be modelled with a 22–60 m radius conduit, degassing ∼2–90% of the available water and all of the 0.1 to 3 vol.% CO2 chamber gas. The range of these results is consistent with previous models and observations. Convection driven by degassing provides a plausible mechanism for transferring volatiles from deep magma chambers to the atmosphere, and it can explain the gas fluxes measured at many persistently active volcanoes.

Reappraisal of the 1982 eruptions of El Chichón Volcano, Chiapas, Mexico: new data from proximal deposits

Additional data from proximal areas enable a reconstruction of the stratigraphy and the eruptive chronology of phases III and IV of the 1982 eruption of El Chichon Volcano. Phase III began on 4 April at 0135 GMT with a powerful hydromagmatic explosion that generated radially fast-moving (∼100 ms–1) pyroclastic clouds that produced a surge deposit (S1). Due to the sudden reduction in the confining pressure the process continued by tapping of magma from a deeper source, causing a new explosion. The ejected juvenile material mixed with large amounts of fragmented dome and wall rock, which were dispersed laterally in several pulses as lithic-rich block-and-ash flow (F1). Partial evacuation of juvenile material from the magmatic system prompted the entrance of external water to generate a series of hydromagmatic explosions that dispersed moisture-rich surge clouds and small-volume block-and-ash flows (IU) up to distances of 3 km from the crater. The eruption continued by further decompression of the magmatic system, with the ensuing emission of smaller amounts of gas-rich magma which, with the strong erosion of the volcanic conduit, formed a lithic-rich Plinian column that deposited fallout layer B. Associated with the widening of the vent, an increase in the effective density of the uprising column took place, causing its collapse. Block-and-ash flows arising from the column collapse traveled along valleys as a dense laminar flow (F2). In some places, flow regime changes due to topographic obstacles promoted transformation into a turbulent surge (S2) which attained minimum velocities of approximately 77 ms–1 near the volcano. The process continued with the formation of a new column on 4 April at 1135 GMT (phase IV) that emplaced fall deposit C and was followed by hydromagmatic explosions which produced pyroclastic surges (S3).

Mantle helium in ground waters of eastern North America: Time and space constraints on sources

Mantle helium in continental environments is generally considered to be the result of active volcanism and/or active extension. The latest episodes of volcanism in northeastern North America are the track of the New England hotspot (95–190 Ma) and the closure of the Iapetus sea (before 300 Ma). Thus, the identification of mantle helium in young ground waters of central New England is counter to the conventional wisdom. On the basis of evaluation of helium evolution in emplaced magmas, we postulate an “aged” mantle source for the excess helium component in ground waters of central New England that is either (1) a local, near-surface–emplaced, gas-rich magma that has retained significant volatiles (e.g., in fluid inclusions) or (2) a deeply emplaced gas-rich magma with high initial 2 He/ 4 He (10 −5 ) and helium transport (with dispersion) through the crust over time. This gas-rich initial condition may support the concept of a volatile-enriched mantle wedge and thus explain the increased buoyancy flux of the New England hotspot as it traversed eastern North America, as has been suggested by others.

Pahoehoe and aa in Hawaii: volumetric flow rate controls the lava structure

The historical records of Kilauea and Mauna Loa volcanoes reveal that the rough-surfaced variety of basalt lava called aa forms when lava flows at a high volumetric rate (>5–10 m3/s), and the smooth-surfaced variety called pahoehoe forms at a low volumetric rate (<5–10 m3/s). This relationship is well illustrated by the 1983–1990 and 1969–1974 eruptions of Kilauea and the recent eruptions of Mauna Loa. It is also illustrated by the eruptions that produced the remarkable paired flows of Mauna Loa, in which aa formed during an initial short period of high discharge rate (associated with high fountaining) and was followed by the eruption of pahoehoe over a sustained period at a low discharge rate (with little or no fountaining). The finest examples of paired lava flows are those of 1859 and 1880–1881. We attribute aa formation to rapid and concentrated flow in open channels. There, rapid heat loss causes an increase in viscosity to a threshold value (that varies depending on the actual flow velocity) at which, when surface crust is torn by differential flow, the underlying lava is unable to move sufficiently fast to heal the tear. We attribute pahoehoe formation to the flowage of lava at a low volumetric rate, commonly in tubes that minimize heat loss. Flow units of pahoehoe are small (usually <1 m thick), move slowly, develop a chilled skin, and become virtually static before the viscosity has risen, to the threshold value. We infer that the high-discharge-rate eruptions that generate aa flows result from the rapid emptying of major or subsidiary magma chambers. Rapid near-surface vesiculation of gas-rich magma leads to eruptions with high discharge rates, high lava fountains, and fast-moving channelized flows. We also infer that long periods of sustained flow at a low discharge rate, which favor pahoehoe, result from the development of a free and unimpeded pathway from the deep plumbing system of the volcano and the separation of gases from the magma before eruption. Achievement of this condition requires one or more episodes of rapid magma excursion through the rift zone to establish a stable magma pathway.