Fumarolic emissions from Mount St. Augustine,Alaska: 1979–1984 degassing trends,volatile sources and their possible role in eruptive style

Gas samples were collected from high-temperature, rooted summit vents at Mount St. Augustine in 1979, 1982, and 1984. All of the gas samples exhibit various degrees of disequilibrium. Thermodynamic restoration of the analyzed gases permits partial or complete removal of these disequilibrium effects and allows inference of equilibrium gas compositions. Long-term (1979–1984) degassing trends within resampled or adjacent vents are characterized by increases (from 97.4 to 99.8 mole%) in the H₂O fraction and major decreases in the residual gases. Over this same period total gas HCl contents decreased by a factor of 3 to 10 while dry gas (H₂O-free recalculated) HCl contents increased by a factor of 1.6 to 3. Dry gas mole proportions at these sites changed from being CO₂-dominated (46% CO₂, 24% H₂ in 1979) to H₂-dominated (49% H₂, 22% CO₂ in 1984). The overall trends in gas chemistry and the stable isotope patterns in gases and condensates from the summit fumaroles can be explained by progressive magmatic outgassing coupled with increasing proportions of seawater in the fumarole emissions.Studies of the gaseous emissions following the 1976 and 1986 Mount St. Augustine eruptions confirmed the Cl- and S-rich nature of the Mount St. Augustine emanations. Seawater, possibly derived from magmatic assimilation or dehydration of near-surface seawater-bearing sediments, could supply a portion of the outgassed Cl and S. Continued seawater influx through subvolcanic fractures or permeable sediments would recharge the seawater-depleted zone and provide a near-surface Cl and S source for the next eruptive cycle,Various lines of evidence support a phreatomagmatic component in the 1976 and 1986 Mount St. Augustine eruptions. We suggest that seawater may interact with magma or volcanic gases during the early explosive phase of Mount St. Augustine eruptions and that it continues to influence high-temperature fumarole emissions as the volcanic system cools.     

Seismic experiments on Showa-Shinzan lava dome using firework shots

Seismic experiments were conducted on Showa-Shinzan, a parasitic lava dome of volcano Usu, Hokkaido, which was formed during 1943–1945 activity. Since we found that firework shots fired on the ground can effectively produce seismic waves, we placed many seismometers on and around the dome during the summer festivals in 1984 and 1985. The internal structure had been previously studied using a prospecting technique employing dynamite blasts in 1954. The measured interval velocity across the dome in 1984 ranges 1.8–2.2 km/s drastically low compared to the results (3.0–4.0 km/s) in 1954; in addition, the velocity is 0.3–0.5 km/s higher than that in the surrounding area. The variation of the observed first arrival amplitudes can be explained by geometrical spreading in the high velocity lava dome. These observations show a marked change in the internal physical state of the dome corresponding to a drop in the measured highest temperature at fumaroles on the dome from 800°C in 1947 to 310°C in 1986.     

Evolution of fumarolic gases — boundary conditions set by measured parameters: case study at Vulcano,Italy

Physical, chemical and isotopic parameters were measured in fumaroles at the Vulcano crater and in drowned fumaroles near the beach. The data were used to define boundary conditions for possible conceptual models of the system.Crater fumaroles: time variations of CO₂ and SO₂ concentrations indicate mixing of saline gas-rich water with local fresh water. Cl/Br ratios of 300– 400 favour sea-water as a major source for Cl, Brand part of the water in the fumaroles. Cl concentrations and D values revealed, independently, amixing of 0.75 sea-water with 0.25 local freshwaterin furmarole F-5 during September 1982.Patterns of parameter correlation and mass balances reveal that CO₂, S, NH₃ and B originate from sources other than sea water. The CO₂ value of ¹³C = – 2%_o favours, at least partial, origin from decomposition of sedimentary rocks rather than mantle-derived material. Radiogenic⁴He(1.3 × lO^–3 ccSTP/g water) and radiogenic⁴⁰Ar(10.6 × 10^–4 ccSTP/g water) are observed, (⁴He/⁴⁰Ar)_radiogenic = 1.2, well in the range of values observed in geothermal systems.Drowned fumaroles: strongly bubbling gas at a pond and at the beachappears to have the same origin and initial compositionas the crater fumaroles (2 km away). The fumarolic gas is modified by depletion of the reactive gases, caused by dissolution in shallow-water. Atmospheric Ne, Ar, Kr and Xe are addeden route, some radiogenic He and Ar are maintained. The Vulcano system seems to be strongly influenced by the contribution of sea-water and decomposition of sedimentary rocks. Evidence of magmatic contributions is mainly derived from heat.     

Evaluation of volcanic gas analysis from surtsey volcano, iceland 1964–1967

Methods used previously to remove compositional modifications from volcanic gas analyses for Mount Etna and Erta'Ale lava lake have bean employed to estimate the gas phase composition at Nyiragongo lava lake, based on samples obtained in 1959. H₂O data were not reported in 11 of the 13 original analyses. The restoration methods have been used to estimate the H₂O contents of the samples and to correct the analyses for atmospheric contamination, loss of sulfur and for pre- and pest-collection oxidation of H₂S, S₂, and H₂. The estimated gas compositions are relatively CO₂-rich, low in total sulfur and reduced. They contain approximately 35–50% CO₂ 45–55% H₂O, 1–2% SO₂, 1–2% H₂., 2–3% CO, 1.5–2.5% H₂S, 0.5% S₂ and 0.1% COS over,he collection temperature range 102° to 960° C. The oxygen fugacities of the gases are consistently about half an order of magnitude below quartz-magnetite-fayalite. The low total sulfur content and resulting low atomic S/C of the Nyiragongo gases appear to be related to the relatively low f_O2 of the crystallizing lava. At temperatures above 800°C and pressures of 1–1.5 k bar, the Nyiragongo gas compositions resemble those observed in primary fluid inclusions believed to have formed at similar temperatures and pressures in nephelines of intrusive alkaline rocks. Cooling to 300°C, with f_O2 buffered by the rock, results in gas compositions very rich in CH₄ (50–70%) and resembling secondary fluid inclusions formed at 200–500°C in alkaline rocks. Below 600°C the gases become supersaturated in carbon as graphite. These inferences are corroborated by several reports of hydrocarbons in plutonic alkaline rocks, and by the presence of CH₄-rich waters in Lake Kivu — a lake on the flanks of Nyiragongo volcano.     

The hydrothermal system of Volcan Puracé, Colombia

This paper presents chemical and isotopic data for thermal waters, gases and S deposits from Volcan Puracé (summit elevation 4600 m) in SW Colombia. Hot gas discharges from fumaroles in and around the summit crater, and thermal waters discharge from three areas on its flanks. The waters from all areas have D values of-75±1, indicating a single recharge area at high elevation on the volcano. Aircorrected values of³He/⁴He in thermal waters range from 3.8 to 6.7 R_A, and approach those for crater fumarole gas (6.1–7.1 R_A), indicating widespread addition of magmatic volatiles. An economic S deposit (El Vinagre) is being mined in the Rio Vinagre fault zone at 3600 m elevation. Sulfur isotopic data are consistent with a magmatic origin for S species in thermal waters and gases, and for the S ore deposit. Isotopic equilibration between S species may have occurred at 220±40°C, which overlaps possible equilibration temperatures (170±40°C) determined by a variety of other geothermometers for neutral thermal waters. Apparent CH₄–CO₂ equilibration temperatures for gases from thermal springs (400±50°C) and crater fumaroles (520±60°C) reflect higher temperatures deeper in the system. Hot magmatic gas ascending through the Rio Vinagre fault zone is though to have precipitated S and generated thermal waters by interaction with descending meteoric waters.     

Fumarole distribution,morphology, and encrustation mineralogy associated with the 1986 eruptive deposits of mount St. Augustine,Alaska

Numerous rootless fumaroles were developed on pyroclastic flows and a lava flow generated during the March 1986 eruptive cycle of Mount St. Augustine. Gases issued from fumarole vents with four different shapes: fissure, phreatic explosion crater, single/multiple ovoid opening, and diffuse, multiple opening. Fumarole distribution and morphology were controlled by preeruption drainage and topography, as well as by the thickness, compaction, and settling of the flow deposits. Fumarole temperatures measured in June and July 1986 ranged from 75°–394°C. Varying amounts of colorful and often roughly zoned encrustations are associated with all fumarole vent shapes. Only six types of crystalline phases were detected by X-ray diffraction, with gypsum the most abundant mineral, followed by anhydrite, sulfur, tridymite, halite, and soda alum. Scanning electron microscopy and energy dispersive X-ray analysis revealed a number of amorphous phases, mainly halogen-rich, as well as other minor crystalline phases. The mineral assemblages in the encrustations suggest formation conditions for these deposits within a general range of 25°–250°C in an oxidizing environment. Many of the amorphous phases are metastable and upon cooling of the fumarole lose nonstructural water and crystallize to more stable forms. The high halogen contents of the fumarole condensates and the mineralogy, chemistry, and morphology of the encrustations support leaching of the andesitic ash and lava flow by condensed acid vapors as the primary source for the chemical components contained in the encrustations. Comparison of traceelement (Sr, Ba, V, Co, Ni, and Cr) contents in unaltered and altered ash suggests that trace-element distribution follows a pattern of isomorphic substitution in the encrustation phases.     

Geochemistry of the thermal springs and fumaroles of Basse-Terre Island,Guadeloupe, Lesser Antilles

 The purpose of this work was to study jointly the volcanic-hydrothermal system of the high-risk volcano La Soufrière, in the southern part of Basse-Terre, and the geothermal area of Bouillante, on its western coast, to derive an all-embracing and coherent conceptual geochemical model that provides the necessary basis for adequate volcanic surveillance and further geothermal exploration. The active andesitic dome of La Soufrière has erupted eight times since 1660, most recently in 1976–1977. All these historic eruptions have been phreatic. High-salinity, Na–Cl geothermal liquids circulate in the Bouillante geothermal reservoir, at temperatures close to 250  °C. These Na–Cl solutions rise toward the surface, undergo boiling and mixing with groundwater and/or seawater, and feed most Na–Cl thermal springs in the central Bouillante area. The Na–Cl thermal springs are surrounded by Na–HCO₃ thermal springs and by the Na–Cl thermal spring of Anse à la Barque (a groundwater slightly mixed with seawater), which are all heated through conductive transfer. The two main fumarolic fields of La Soufrière area discharge vapors formed through boiling of hydrothermal aqueous solutions at temperatures of 190–215  °C below the "Ty" fault area and close to 260  °C below the dome summit. The boiling liquid producing the vapors of the Ty fault area has δD and δ¹⁸O values relatively similar to those of the Na–Cl liquids of the Bouillante geothermal reservoir, whereas the liquid originating the vapors of the summit fumaroles is strongly enriched in ¹⁸O, due to input of magmatic fluids from below. This process is also responsible for the paucity of CH₄ in the fumaroles. The thermal features around La Soufrière dome include: (a) Ca–SO₄ springs, produced through absorption of hydrothermal vapors in shallow groundwaters; (b) conductively heated, Ca–Na–HCO₃ springs; and (c) two Ca–Na–Cl springs produced through mixing of shallow Ca–SO₄ waters and deep Na–Cl hydrothermal liquids. The geographical distribution of the different thermal features of La Soufrière area indicates the presence of: (a) a central zone dominated by the ascent of steam, which either discharges at the surface in the fumarolic fields or is absorbed in shallow groundwaters; and (b) an outer zone, where the shallow groundwaters are heated through conduction or addition of Na–Cl liquids coming from hydrothermal aquifer(s). Received: 9 November 1998 / Accepted: 15 July 1999     

Fumaroles in ice caves on the summit of Mount Rainier—preliminary stable isotope, gas, and geochemical studies

The edifice of Mount Rainier, an active stratovolcano, has episodically collapsed leading to major debris flows. The largest debris flows are related to argillically altered rock which leave areas of the edifice prone to failure. The argillic alteration results from the neutralization of acidic magmatic gases that condense in a meteoric water hydrothermal system fed by the melting of a thick mantle of glacial ice. Two craters atop a 2000-year-old cone on the summit of the volcano contain the world's largest volcanic ice-cave system. In the spring of 1997 two active fumaroles (T=62°C) in the caves were sampled for stable isotopic, gas, and geochemical studies.Stable isotope data on fumarole condensates show significant excess deuterium with calculated δD and δ¹⁸O values (−234 and −33.2‰, respectively) for the vapor that are consistent with an origin as secondary steam from a shallow water table which has been heated by underlying magmatic–hydrothermal steam. Between 1982 and 1997, δD of the fumarole vapor may have decreased by 30‰.The compositions of fumarole gases vary in time and space but typically consist of air components slightly modified by their solubilities in water and additions of CO₂ and CH₄. The elevated CO₂ contents (δ¹³C_CO2=−11.8±0.7‰), with spikes of over 10,000 ppm, require the episodic addition of magmatic components into the underlying hydrothermal system. Although only traces of H₂S were detected in the fumaroles, most notably in a sample which had an air δ¹³C_CO2 signature (−8.8‰), incrustations around a dormant vent containing small amounts of acid sulfate minerals (natroalunite, minamiite, and woodhouseite) indicate higher H₂S (or possibly SO₂) concentrations in past fumarolic gases.Condensate samples from fumaroles are very dilute, slightly acidic, and enriched in elements observed in the much higher temperature fumaroles at Mount St. Helens (K and Na up to the ppm level; metals such as Al, Pb, Zn Fe and Mn up to the ppb level and volatiles such as Cl, S, and F up to the ppb level).The data indicate that the hydrothermal system in the edifice at Mount Rainier consists of meteoric water reservoirs, which receive gas and steam from an underlying magmatic system. At present the magmatic system is largely flooded by the meteoric water system. However, magmatic components have episodically vented at the surface as witnessed by the mineralogy of incrustations around inactive vents and gas compositions in the active fumaroles. The composition of fumarole gases during magmatic degassing is distinct and, if sustained, could be lethal. The extent to which hydrothermal alteration is currently occurring at depth, and its possible influence on future edifice collapse, may be determined with the aid of on site analyses of fumarole gases and seismic monitoring in the ice caves.     

The crater lake and hydrothermal system of Mount Pinatubo,Philippines: evolution in the decade after eruption

The June 1991 eruption of Mount Pinatubo, Philippines breached a significant, pre-eruptive magmatic-hydrothermal system consisting of a hot (>300 °C) core at two-phase conditions and surrounding, cooler (<260 °C) liquid outflows to the N and S. The eruption created a large, closed crater that accumulated hydrothermal upwellings, near-surface aquifer and meteoric inflows. A shallow lake formed by early September 1991, and showed a long-term increase in level of ~1 m/month until an artificial drainage was created in September 2001. Comparison of the temporal trends in lake chemistry to pre- and post-eruptive springs distinguishes processes important in lake evolution. The lake was initially near-neutral pH and dominated by meteoric influx and Cl–SO₄ and Cl–HCO₃ hydrothermal waters, with peaks in SO₄ and Ca concentrations resulting from leaching of anhydrite and aerosol-laden tephra. Magmatic discharge, acidity (pH~2) and rock dissolution peaked in late 1992, during and immediately after eruption of a lava dome on the crater floor. Since cessation of dome growth, trends in lake pH (increase from 3 to 5.5), temperature (decline from 40 to 26 °C), and chemical and isotopic composition indicate that magmatic degassing and rock dissolution have declined significantly relative to the input of meteoric water and immature hydrothermal brine. Higher concentrations of Cl, Na, K, Li and B, and lower concentrations of Mg, Ca, Fe, SO₄ and F up to 1999 highlight the importance of a dilute hydrothermal contribution, as do stable-isotope and tritium compositions of the various fluids. However, samples taken since that time indicate further dilution and steeper trends of increasing pH and declining temperature. Present gas and brine compositions from crater fumaroles and hot springs indicate boiling of an immature Cl–SO₄ geothermal fluid of near-neutral pH at approximately 200 °C, rather than direct discharge from magma. It appears that remnants of the pre-eruptive hydrothermal system invaded the magma conduit shortly after the end of dome emplacement, blocking the direct degassing path. This, along with the large catchment area (~5 km²) and the high precipitation rate of the area, led to a rapid transition from a small and hot acid lake to a large lake with near-ambient temperature and pH. This behavior contrasts with that of peak-activity lakes that have more sustained volcanic gas influx (e.g., Kawah Ijen, Indonesia; Poas and Rincón de la Vieja, Costa Rica).Editorial responsibility: H. Shinohara     

Factors influencing the height of Hawaiian lava fountains: implications for the use of fountain height as an indicator of magma gas content

The heights of lava fountains formed in Hawaiian-style eruptions are controlled by magma gas content, volume flux and the amounts of lava re-entrainment and gas bubble coalescence. Theoretical models of lava fountaining are used to analyse data on lava fountain height variations collected during the 1983–1986 Pu'u 'O'o vent of Kilauea volcano, Hawaii. The results show that the variable fountain heights can be largely explained by the impact of variations in volume flux and amount of lava re-entrainment on erupting magmas with a constant gas content of 0.32 wt.% H₂O. However, the gas content of the magma apparently declined by 0.05 wt.% during the last 10 episodes of the eruption series and this decline is attributed to more extensive pre-eruption degassing due to a shallowing of the sub-vent feeder dike. It is concluded that variations in lava fountain height cannot be simply interpreted as variations in gas content, as has previously been suggested, but that fountain height can still be a useful guide to minimum gas contents. Where sufficient data are available on eruptive volume fluxes and extent of lava entrainment, greatly improved estimates can be made of magma gas content from lava fountain height.     

Chemistry and mineralogy of high-temperature gas discharges from Colima volcano, Mexico. Implications for magmatic gas–atmosphere interaction

Gases, condensates and silica tube precipitates were collected from 400°C (Z2) and 800°C (Z3) fumaroles at Colima volcano, Mexico, in 1996–1998. Volcanic gases at Colima were very oxidized and contain up to 98% air due to mixing with air inside the dome interior, close to the hot magmatic body. An alkaline trap method was used to collect gas samples, therefore only acidic species were analysed. Colima volcanic gases are water-rich (95–98 mol%) and have typical S/C/Cl/F ratios for a subduction type volcano. δD-values for the high-temperature Z3 fumarolic vapour vary from −26 to −57‰. A negative δD–Cl correlation for the Z3 high-temperature fumarole may result from magma degassing: enrichment in D and decrease in the Cl concentration in condensates are likely a consequence of input of “fresh” batches of magma and an increasing of volcanic activity, respectively.The trace element composition of Colima condensates generally does not differ from that of other volcanoes (e.g. Merapi, Kudryavy) except for some enrichment in V, Cu and Zn. Variations in chemical composition of precipitates along the silica tube from the high-temperature fumarole (Colima 1, fumarole Z3), in contrast to other volcanoes, are characterized by high concentrations of Ca and V, low concentration of Mo and a lack of Cd. Mineralogy of precipitates differs significantly from that described for silica tube experiments at other volcanoes with reduced volcanic gas. Thermochemical modelling was used to explain why very oxidized gas at Colima does not precipitate halite, sylvite, and Mo- and Cd-minerals, but does precipitate V-minerals and native gold, which have not been observed before in mineral precipitates from reduced volcanic gases.     

Gas geochemistry of the Valles caldera region, New Mexico and comparisons with gases at Yellowstone, Long Valley and other geothermal systems

Noncondensible gases from hot springs, fumaroles, and deep wells within the Valles caldera geothermal system (210–300°C) consist of roughly 98.5 mol% CO₂, 0.5 mol% H₂S, and 1 mol% other components. ³He/⁴He ratios indicate a deep magmatic source (R/R_a up to 6) whereas δ¹³C–CO₂ values (−3 to −5‰) do not discriminate between a mantle/magmatic source and a source from subjacent, hydrothermally altered Paleozoic carbonate rocks. Regional gases from sites within a 50-km radius beyond Valles caldera are relatively enriched in CO₂ and He, but depleted in H₂S compared to Valles gases. Regional gases have R/R_a values ≤1.2 due to more interaction with the crust and/or less contribution from the mantle. Carbon sources for regional CO₂ are varied. During 1982–1998, repeat analyses of gases from intracaldera sites at Sulphur Springs showed relatively constant CH₄, H₂, and H₂S contents. The only exception was gas from Footbath Spring (1987–1993), which experienced increases in these three components during drilling and testing of scientific wells VC-2a and VC-2b. Present-day Valles gases contain substantially less N₂ than fluid inclusion gases trapped in deep, early-stage, post-caldera vein minerals. This suggests that the long-lived Valles hydrothermal system (ca. 1 Myr) has depleted subsurface Paleozoic sedimentary rocks of nitrogen. When compared with gases from many other geothermal systems, Valles caldera gases are relatively enriched in He but depleted in CH₄, N₂ and Ar. In this respect, Valles gases resemble end-member hydrothermal and magmatic gases discharged at hot spots (Galapagos, Kilauea, and Yellowstone).     

Detailed record of SO2 emissions from Pu'u `O`o between episodes 33 and 34 of the 1983–86 ERZ eruption,Kilauea, Hawaii

A tripod-mounted correlation spectrometer was used to measure SO₂ emissions from Pu`u `O`o vent, mid-ERZ, Kilauea, Hawaii between Episodes 33 and 34 (June 13 to July 6, 1985). In 24 repose days, 906 measurements were collected, averaging 38 determinations/day. Measurements reflect 13% of the total 576 hours of the repose and 42% of the bright daylight hours. The average SO<sub>2</sub> emission for the 24-day repose interval is 167±83 t/d, a total of 4000 tonnes emitted for the entire repose. The large standard deviation reflects the puffing character of the plume. The overall rate of SO<sub>2</sub> degassing gently decreased with a zero-intercept of 44–58 days and was interrupted by two positive peaks. The data are consistent with the gas emanating from a cylindrical conduit of 50 meter diameter and a length of 1700 meters which degasses about 50% of its SO<sub>2</sub> during 24 days. This is in support of the Pu'u `O`o model of Greenland et al. (1987). 36 hours before the onset of Episode 34 (July 5–6, 1985), elevated SO₂ emissions were detected while the magma column was extremely active ultimately spilling over during dome fountaining. A mid-repose anomaly of SO₂ emission (June 21–22, 1985) occurs two days before a sudden increase in the rate of summit inflation (on June 24, 1985), suggesting magma was simultaneously being injected in both the ERZ and summit reservoir until July 24 when it was channelled only to the summit reservoir. This implies degassing magma is sensitive to perturbations within the rift zone conduit system and may at times reflect these disturbances. Periods of 7–45 min are detected in the daily SO₂ emissions, which possibly reflect timing of convective overturn in the cylindrical magma body. If the 33–34 repose interval is considered representative of other repose periods, the ERZ reposes of Jan 1983–Jan 1986 ERZ activity, contributed 1.6 × 10⁵ tonnes of SO₂ to the atmosphere. Including summit fuming from non-eruptive fumaroles (2.7 × 10⁵ tonnes SO₂); 28% of the total SO₂ budget from Kilauea between Jan 1983 to Jan 1986 was contributed by quiescent degassing, and the remainder was released during explosive fountaining episodes.     

Equilibrium calculations in volcanic gaseous systems

Equilibria calculations of high-temperature volcanic gases from lava lakes are carried out on the basis of best volcanic gas samples. The equilibrium gas composition at temperatures from 800° to 1400°K and pressures up to 25 kilobars (in ideal gas system) was calculated using the free energy minimization model as well as the Newton-Raphson methods. It is shown that the juvenile «magmatic gas » of basaltic magma consists of three components: H₂O, SO₂, CO₂; the water vapor being about 60%. The increase of temperature under constant pressure results in the increase of the SO₂ concentration and in the simultaneous decrease of H₂S. Under the same conditions the ratios CO/CO₂ and H₂/H₂O are found to increase. Methane cannot be a component of «magmatic gas» corresponding to the elemental composition of basaltic lava gases. The calculated values of \(P_{O_2 } \) are in good agreement with the experimental data obtained from direct measurements of \(P_{O_2 } \) in lava lakes and experiments with basaltic melts.     

Modelling the lava dome extruded at Soufrière Hills Volcano, Montserrat, August 2005–May 2006: Part II: Rockfall activity and talus deformation

During many lava dome-forming eruptions, persistent rockfalls and the concurrent development of a substantial talus apron around the foot of the dome are important aspects of the observed activity. An improved understanding of internal dome structure, including the shape and internal boundaries of the talus apron, is critical for determining when a lava dome is poised for a major collapse and how this collapse might ensue. We consider a period of lava dome growth at the Soufrière Hills Volcano, Montserrat, from August 2005 to May 2006, during which a  100 × 10⁶ m³ lava dome developed that culminated in a major dome-collapse event on 20 May 2006. We use an axi-symmetrical Finite Element Method model to simulate the growth and evolution of the lava dome, including the development of the talus apron. We first test the generic behaviour of this continuum model, which has core lava and carapace/talus components. Our model describes the generation rate of talus, including its spatial and temporal variation, as well as its post-generation deformation, which is important for an improved understanding of the internal configuration and structure of the dome. We then use our model to simulate the 2005 to 2006 Soufrière Hills dome growth using measured dome volumes and extrusion rates to drive the model and generate the evolving configuration of the dome core and carapace/talus domains. The evolution of the model is compared with the observed rockfall seismicity using event counts and seismic energy parameters, which are used here as a measure of rockfall intensity and hence a first-order proxy for volumes. The range of model-derived volume increments of talus aggraded to the talus slope per recorded rockfall event, approximately 3 × 10³–13 × 10³ m³ per rockfall, is high with respect to estimates based on observed events. From this, it is inferred that some of the volumetric growth of the talus apron (perhaps up to 60–70%) might have occurred in the form of aseismic deformation of the talus, forced by an internal, laterally spreading core. Talus apron growth by this mechanism has not previously been identified, and this suggests that the core, hosting hot gas-rich lava, could have a greater lateral extent than previously considered.     

Petrological characteristics and volatile content of magma from the 2000 eruption of Miyakejima Volcano, Japan

Among the series of eruptions at Miyakejima volcano in 2000, the largest summit explosion occurred on 18 August 2000. During this explosion, vesiculated bombs and lapilli having cauliflower-like shapes were ejected as essential products. Petrological observation and chemical analyses of the essential ejecta and melt inclusions were carried out in order to investigate magma ascent and eruption processes. SEM images indicate that the essential bombs and lapilli have similar textures, which have many tiny bubbles, crystal-rich and glass-poor groundmass and microphenocrysts of plagioclase, augite and olivine. Black ash particles, which compose 40% of the air-fall ash from the explosion, also have similar textures to the essential bombs. Whole-rock analyses show that the chemical composition of all essential ejecta is basaltic (SiO₂=51–52 wt%). Chemical analyses of melt inclusions in plagioclase and olivine phenocrysts indicate that melt in the magma had 0.9–1.9 wt% H₂O, <0.011 wt% CO₂, 0.04–0.17 wt% S and 0.06–0.1 wt% Cl. The variation in volatile content suggests degassing of the magma during ascent up to a depth of about 1 km. The ratio of H₂O and S content of melt inclusions is similar to that of volcanic gas, which has been intensely and continuously emitted from the summit since the end of August 2000, indicating that the 18 August magma is the source of the gas emission. Based on the volatile content of the melt inclusions and the volcanic gas composition, the initial bulk volatile content of the magma was estimated to be 1.6–1.9 wt% H₂O, 0.08–0.1 wt% CO₂, 0.11–0.17 wt% S and 0.06–0.07 wt% Cl. The basaltic magma ascended from a deeper chamber (10 km) due to decrease in magma density caused by volatile exsolution with pressure decrease. The highly vesiculated magma, which had at least 30 vol% bubbles, may have come into contact with ground water at sea level causing the large explosion of 18 August 2000.Editorial responsibility: S. Nakada, T. DuittAn erratum to this article can be found at     

Chemistry and metal contents of magmatic gases: the new Tolbachik volcanoes case (Kamchatka)

The gaseous products of new Tolbachik volcanoes were studied during 1975 to 1977 throughout all eruptive stages and during the post eruptive activity. In investigations the northern break-out gases emitted during the eruption from the moving and consolidated lava flows there have been detected H₂O (the main component), H₂, HF, HCl, SO₂ and H₂S, CO₂, CO, NH₃, CH₄ and other hydrocarbons, NH₄Cl predominated in compositions of condensates and subtimates on lava flows and the most characteristic microcomponents were Zn, Cu, Pb, Sn, Ag and others. Sampling of gases and condensates at the southern break-out was conducted immediately from the flowing melt. In gases there have been detected H₂O (98 mol. %). HCl and H₂ (0.9 mol. % each) as well as HF, SO₂, H₂S, CO₂ and in small quantities O₂ and N₂, Gases reached the equilibrium state atT andP sampling and were characteristic of gas composition of the southern break-out magma. HCl, HF and H₂SO₄ were predominant during condensate and sublimate mineralization. The major raicrocomponents were represented by Pt, Sb, As, Zn, Cu, Pb, Ni, Co and others. Comparison of compositions of gases and of products of their reactions at the northern and at the southern break-outs allows us to assume the presence of the deeper magma source at the northern break-out and of shallow magma source at the southern break-out.     

Monitoring Colima Volcano,Mexico, using satellite data

The Colima Volcanic Complex at the western end of the Mexican Volcanic Belt is the most active andesitic volcano in Mexico. Short-wavelength infrared data from the Landsat Thematic Mapper satellite were used to determine the temperature and fractional area of radiant picture elements for two January data acquisitions in 1985 and 1986. The 1986 data showed four 28.5 m by 28.5 m pixels (picture elements) whose hot subpixel components had temperatures ranging from 511–774° C and areas of 1.8–13 m². The 1985 data had no radiating areas above background temperatures. Ground observations and measurements in November 1985 and February 1986 reported the presence of hot fumaroles at the summit with temperatures of 135–895° C. This study demonstrates the utility of satellite data for monitoring volcanic activity.     

The determination of deep temperatures by means of the CO-CO2-H2-H2O geothermometer: an example using fumaroles in the Campi Flegrei,Italy

Chromatographic analyses of fumarolic gases, collected in sampling bottles containing an alkaline solution, have been carried out using a thermal conductivity detector and a flame ionization detector, after catalytic conversion of CO and CH₄. The latter method enables the concentration of carbon monoxide to be measured with sufficient accuracy for use in a CO-CO₂-H₂-H₂O geothermometer. Application of this geothermometer to fumaroles in the crater of Solfatara in the Campi Flegrei, Italy, indicates that they are fed from a steam reservoir at 250±15 °C and at 10^–36±2atm of oxygen. On the other hand, the CH₄-CO₂-H₂-H₂O geothermobarometer seems to re-equilibrate at superficial temperatures and cannot be used for infering thermodynamic conditions at depth. Regular sampling of these fumaroles together with a geothermometric interpretation of the gas analyses provides a means of monitoring, with comparative accuracy, the chemical and thermal evolution of the hydrothermal reservoir below the Solfatara crater. Such monitoring would probably detect an increase in temperature at depth and the injection of magmatic gas into the reservoir.     

Magmatic processes revealed by textural and compositional trends in Merapi dome lavas

Syn-eruptive degassing of volcanoes may lead to syn-eruptive crystallization of groundmass phases. We have investigated this process using textural and compositional analysis of dome material from Merapi volcano, Central Java, Indonesia. Samples included dome lavas from the 1986–88, 1992–93, 1994 and 1995 effusive periods as well as pyroclastic material deposited by the November 1994 dome collapse. With total crystallinities commonly in excess of 70% (phenocrysts+microlites), the liquids present in Merapi andesites are highly evolved (rhyolitic) at the time of eruption. Feldspar microlites in dome rocks consist of plagioclase cores (Ab₆₃An₂₉Or₈) surrounded by alkali feldspar rims (Ab₅₃An₅Or₄₂), compositional pairs which are not in equilibrium. A change in the phase relations of the ternary feldspar system caused by degassing best explains the observed transition in feldspar composition. A small proportion of highly vesicular airfall tephra grains from the 1994 collapse have less evolved glass compositions than typical dome material and contain rimless plagioclase microlites, suggesting that the 1994 collapse event incorporated less-degassed, partially liquid magma in addition to fully solidified dome rock.As decompression drives volatile exsolution, rates of degassing and resultant microlite crystallization may be governed by magma ascent rate. Microlite crystallinity is nearly identical among the 1995 dome samples, an indication that similar microlite growth conditions (P_H2O and temperature) were achieved throughout this extrusive period. However, microlite number density varied by more than a factor of four in these samples, and generally increased with distance from the vent. Low vent-ward microlite number densities and greater microlite concentrations down-flow probably reflect progressively decreasing rates of undercooling at the time of crystal nucleation during extrusion of the 1995 dome. Comparison between dome extrusion episodes indicates a correlation between lava effusion rate and microlite number density, suggesting that extrusion slowed during 1995. Crystal textures and compositions in the 1992–93 and 1994 domes share the range exhibited by the 1995 dome, suggesting that transitions in crystallization conditions (i.e., rates of undercooling determined by effusion rate) are cyclic.