Pit crater structure and processes governing persistent activity at Masaya Volcano, Nicaragua

 Persistent activity at Masaya Volcano, Nicaragua, is characterised by cycles of intense degassing, lava lake development and pit crater formation. It provides a useful site to study the processes which govern such activity, because of its easy accessibility and relatively short cycles (years to decades). An understanding of the present activity is important because Masaya is visited by large numbers of tourists, is located close to major cities and has produced voluminous lavas, plinian eruptions and ignimbrites in the recent past. We provide structural and geophysical data that characterise the "normal" present state of activity. These indicate that the ongoing degassing phase (1993 to present) was not caused by fresh magma intrusion. It was associated with shallow density changes within the active Santiago pit crater. The activity appears to be associated predominantly with shallow changes in the pit crater structure. More hazardous activity will occur only if there are significant departures from the present gravity, deformation and seismic signatures. Received: 16 May 1997 / Accepted: 29 October 1997     

A preparation zone for volcanic explosions beneath Naka-dake crater,Aso volcano,as inferred from magnetotelluric surveys

The 1st crater of Naka-dake, Aso volcano, is one of the most active craters in Japan, and known to have a characteristic cycle of activity that consists of the formation of a crater lake, drying-up of the lake water, and finally a Strombolian-type eruption. Recent observations indicate an increase in eruptive activity including a decrease in the level of the lake water, mud eruptions, and red hot glows on the crater wall. Temporal variations in the geomagnetic field observed around the craters of Naka-dake also indicate that thermal demagnetization of the subsurface rocks has been occurring in shallow subsurface areas around the 1st crater. Volcanic explosions act to release the energy transferred from magma or volcanic fluids. Measurement of the subsurface electrical resistivity is a promising method in investigating the shallow structure of the volcanic edifices, where energy from various sources accumulates, and in investigating the behaviors of magma and volcanic fluids. We carried out audio-frequency magnetotelluric surveys around the craters of Naka-dake in 2004 and 2005 to determine the detailed electrical structure down to a depth of around 1 km. The main objective of this study is to identify the specific subsurface structure that acts to store energy as a preparation zone for volcanic eruption. Two-dimensional inversions were applied to four profiles across the craters, revealing a strongly conductive zone at several hundred meters depth beneath the 1st crater and surrounding area. In contrast, we found no such remarkable conductor at shallow depths beneath the 4th crater, which has been inactive for 70 years, finding instead a relatively resistive body. The distribution of the rotational invariant of the magnetotelluric impedance tensor is consistent with the inversion results. This unusual shallow structure probably reflects the existence of a supply path of high-temperature volcanic gases to the crater bottom. We propose that the upper part of the conductor identified beneath the 1st crater is mainly composed of hydrothermally altered zone that acts both as a cap to upwelling fluids supplied from deep-level magma and as a floor to infiltrating fluid from the crater lake. The relatively resistive body found beneath the 4th crater represents consolidated magma. These results suggest that the shallow conductor beneath the active crater is closely related to a component of the mechanism that controls volcanic activity within Naka-dake.     

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. Received: 26 September 1997 / Accepted: 11 July 1998     

Magma transfer processes at persistently active volcanoes: insights from gravity observations

Magma transfer processes at persistently active volcanoes are distinguished by the large magma flux required to sustain the prodigious quantities of heat and gas emitted at the surface. Although the resulting degassed magma has been conjectured to accumulate either deep within the volcanic edifice or in the upper levels of the sub-edifice system, no direct evidence for such active accumulation has been reported. Temporal gravity data are unique in being able to quantify mass changes and have been successfully used to model shallow magma movements on different temporal scales, but have not generally been applied to the investigation of postulated long-term accumulation of magma at greater spatial scales within volcanic systems. Here, we model the critical data acquisition parameters required to detect mass flux at volcanoes, we review existing data from a number of volcanoes that exemplify the measurement of shallow mass changes and present new data from Poas and Telica volcanoes. We show that if a substantial proportion of degassed magma lodges within the sub-edifice region, it would result in measurable annual to decadal gravity increases occurring over spatial scales of tens of kilometres and propose that existing microgravity data from Sakurajima and, possibly, Etna volcanoes could be interpreted in these terms. Furthermore, such repeat microgravity data could be used to determine whether the accumulation rate is in equilibrium with the rate of production of degassed magma as calculated from the surface gas flux and hence identify the build-up of gas-rich magma at depth that may be significant in terms of eruption potential. We also argue that large magma bodies, both molten and frozen, modelled beneath volcanoes from seismic and gravity data, could represent endogenous or cryptic intrusions of degassed magma based on order of magnitude calculations using present-day emission rates and typical volcano lifetimes.     

Probing the magma plumbing of Erebus volcano,Antarctica, by open-path FTIR spectroscopy of gas emissions

Gas emissions from Erebus volcano, Antarctica, were measured by open-path Fourier transform infrared spectroscopy to understand degassing of its magmatic system. Two degassing phonolite lava lakes were present in the summit crater during observation in December 2004. We report analyses of H₂O, CO₂, CO, SO₂, HF, HCl and OCS, (in order of molar abundance) in the plumes. Variations in the proportions of these species strongly reflect the dynamics of degassing, and sourcing of gas from different depths in the magmatic network. The highest observed ratios of CO₂ and H₂O are consistent with gas extracted from the melt at a depth of up to ∼ 2 km below the lava lakes. Magma degassing above this depth contributes to a higher H₂O/CO₂ proportion in the airborne plume. The ratio therefore reflects the balance of deeper vs. shallower contributions of volatiles and, possibly, a combination of closed- and open-system degassing. We observe a strong contrast in HF content in emissions from the two lava lakes, which we attribute to differing levels of magma ascent and/or cooling and crystallization of the magma supply. Fluxes of all gas species were determined using independent SO₂ flux determinations and measured gas ratios. In the case of CO₂ and water, ∼ 1 and ∼ 0.4 m³ s^− 1, respectively, of parental basanite magma are required to sustain the calculated output. The discrepancy between the two figures is readily explained by sequestration of part of the magma supply at depth such that it only partially degasses its complement of water.     

Gas segregation in dykes and sills

Many basaltic volcanoes emit a substantial amount of gas over long periods of time while erupting relatively little degassed lava, implying that gas segregation must have occurred in the magmatic system. The geometry and degree of connectivity of the plumbing system of a volcano control the movement of magma in that system and could therefore provide an important control on gas segregation in basaltic magmas. We investigate gas segregation by means of analogue experiments and analytical modelling in a simple geometry consisting of a vertical conduit connected to a horizontal intrusion. In the experiments, degassing is simulated by electrolysis, producing micrometric bubbles in viscous mixtures of water and golden syrup. The presence of exsolved bubbles induces a buoyancy-driven exchange flow between the conduit and the intrusion that leads to gas segregation. Bubbles segregate from the fluid by rising and accumulating as foam at the top of the intrusion, coupled with the accumulation of denser degassed fluid at the base of the intrusion. Steady-state influx of bubbly fluid from the conduit into the intrusion is balanced by outward flux of lighter foam and denser degassed fluid. The length and time scales of this gas segregation are controlled by the rise of bubbles in the horizontal intrusion. Comparison of the gas segregation time scale with that of the cooling and solidification of the intrusion suggests that gas segregation is more efficient in sills (intrusions in a horizontal plane with typical width:length aspect ratio 1:100) than in horizontally-propagating dykes (intrusions in a vertical plane with typical aspect ratio 1:1000), and that this process could be efficient in intermediate as well as basaltic magmas. Our investigation shows that non-vertical elements of the plumbing systems act as strong gas segregators. Gas segregation has also implications for the generation of gas-rich and gas-poor magmas at persistently active basaltic volcanoes. For low magma supply rates, very efficient gas segregation is expected, which induces episodic degassing activity that erupts relatively gas-poor magmas. For higher magma supply rates, gas segregation is expected to be less effective, which leads to stronger explosions that erupt gas-rich as well as gas-poor magmas. These general physical principles can be applied to Stromboli volcano and are shown to be consistent with independent field data. Gas segregation at Stromboli is thought likely to occur in a shallow reservoir of sill-like geometry at 3.5 km depth with exsolved gas bubbles 0.1–1 mm in diameter. Transition between eruptions of gas-poor, high crystallinity magmas and violent explosions that erupt gas-rich, low crystallinity magmas are calculated to occur at a critical magma supply rate of 0.1–1 m³ s^− 1.     

Double origin of hydrothermal convective flux variations in the Fossa of Vulcano (Italy)

Soil-temperature measurements can provide information on the distribution of degassing fissures, their relationship to the internal structure of the volcano, and the temporal evolution of the system. At Vulcano Island (Italy), heat flux from a <3 km-deep magma body drives a hydrothermal system which extends across the main Fossa crater. This heat flux is also associated with variable magmatic gas flow. A high-density map of soil-temperatures was made in 1996 at a constant depth of 30 cm on the central and southern inner flanks of the Fossa crater. These measurements extended over an area covering about 0.04 km², across which the heat flux is predominantly associated with a shallow boiling aquifer. The map shows that hot zones relate to structures of higher permeability, mainly associated with a fissure system dating from the last eruptive cycle (1888–1890). From 1996 to January 2005, we studied the evolution of the heat flux for the high temperature part of the map, both by repeating our measurements as part of 14 visits, during which temperatures were measured at a constant depth, and using data from permanent stations which allowed soil-temperatures to be continuously measured for selected vertical profiles. These data allowed us to calculate the heat flux, and its variation, with good precision for values lower than about 100 W m⁻², which is generally the case in the study area. Above 100 W m⁻², although the heat flux value is underestimated, its variations are recorded with an error less than 10%. During the period 1996–2004, two increases in the thermal flux were recorded. The first one was related to the seismic crisis of November 1998 which opened existing or new fissures. The second, in November 2004, was probably due to magma migration, and was associated with minor seismic activity.     

Pit crater formation on Kilauea volcano, Hawaii.

Most of the known pit craters in Hawaii occur along the East and Southwest Rift Zones of Kilauea volcano. The pit craters typically are either astride a single rift zone fracture or between a pair of rift zone fractures. These fractures are prominent in the pit crater walls. The pit craters are elliptical in plan view, with their major diameters ranging from 8 to 1140 m. They range in depth from 6 m to 186 m. They typically develop with initially steep, locally overhanging walls, but as the walls collapse, the craters fill with talus and become shaped like inverted elliptical cones. None of the craters apparently formed as eruptive vents, although some have been subsequently filled by lava. Devil's Throat is the best-exposed pit crater along the East Rift Zone. It is sited at a `waist' between two east-striking zones of ground cracks; the spacing between the crack zones decreases towards Devil's Throat. East-striking fractures are also prominent in the pit crater walls. Pit craters along the Southwest Rift Zone typically are elongate in plan view along the direction of the rift, have large caves at their bases along the long axes of the craters, and are smaller than those of the East Rift Zone. Some closely spaced pits there have coalesced to form a trough. Based on our observations and mechanical considerations, we infer that pit craters form by stoping over an underlying large-aperture rift zone fracture, and not by piston-like collapse over broad magma bodies or voids. Flow of magma along the underlying fracture may remove stoped blocks and prevent the fracture from being choked with debris. This mechanism is consistent with pit crater location, ground crack patterns, the preferred orientation of fractures in pit crater walls, and pit crater geometry (both in map view and cross-section). The mechanism also fits with observations of stoping into a gaping rift fracture that conducted lava from Kilauea caldera during the 1920s. Additionally, the ratio of pit crater width to depth of 0.5 to 2 is consistent with pit craters forming over a nearly vertical opening mode fracture.     

Thermal, seismic and infrasonic evidences of variable degassing rates at Stromboli volcano

During June 1999, we measured the amplitude and rate (number of events per second) at which gas exited the vent at Stromboli volcano as discrete gas bursts or puffs. This allowed us to identify two styles of gas burst (puffing) activity. The first is characterized by frequent, rapidly rising puffs, the second by less frequent, slowly rising puffs. Each style persisted over 5–40-min-long durations and was associated with a high and low number of strombolian explosions per hour, respectively. Each period was also associated with characteristic delay times between the arrival of the infrasonic and thermal signals during strombolian explosions; the delays were longer during vigorous puffing periods. To explain our observations, we propose a model in which the degassing process cycles between vigorous and weak degassing phases. During vigorous degassing phases, bubble layers ascend the conduit at a frequency of 0.5–1.0 s⁻¹. This high degassing level reflects a gas-rich magma column and leads to an increased rate in the formation of shallow foams and, hence, an increase in puffing and explosive activity, as well as a higher free surface level and/or gas jet velocity. During weak phases, bubble layers ascend the conduit at a reduced frequency of 0.2–0.3 s⁻¹. During such times the magma column is poor in gas. This leads to a decreased rate of foam layer formation and hence a reduction in puffing and explosive activity, as well as a lower free surface level and/or gas jet velocity. Variations in puffing activity can thus be used to track changes in the rate at which the shallow system is supplied by fresh, gas-rich magma. Our observations indicate that the two degassing styles last from 5 to 40 min and that the switch from one to the other occurs over a matter of minutes.     

Compositionally zoned crystals and real-time degassing data reveal changes in magma transfer dynamics during the 2006 summit eruptive episodes of Mt. Etna

One of the major objectives of volcanology remains relating variations in surface monitoring signals to the magmatic processes at depth that cause these variations. We present a method that enables compositional and temporal information stored in zoning of minerals (olivine in this case) to be linked to observations of real-time degassing data. The integrated record may reveal details of the dynamics of gradual evolution of a plumbing system during eruption. We illustrate our approach using the 2006 summit eruptive episodes of Mt. Etna. We find that the history tracked by olivine crystals, and hence, most likely the magma pathways within the shallow plumbing system of Mt. Etna, differed considerably between the July and October eruptions. The compositional and temporal record preserved in the olivine zoning patterns reveal two mafic recharge events within months of each other (June and September 2006), and each of these magma supplies may have triggered the initiation of different eruptive cycles (July 14–24 and August 31–December 14). Correlation of these observations with gas monitoring data shows that the systematic rise of the CO₂/SO₂ gas values is associated with the gradual (pre- and syn-eruptive) supply of batches of gas-rich mafic magma into segments of Etna’s shallow plumbing system, where mixing with pre-existing and more evolved magma occurred.     

Compositionally diverse magmas erupted close together in space and time within a Karoo flood basalt crater complex

Geochemical data and mapping from a Karoo flood basalt crater complex reveals new information about the ascent and eruption of magma batches during the earliest phases of flood basalt volcanism. Flood basalt eruptions at Sterkspruit, South Africa began with emplacement of thin lava flows before abruptly switching to explosive phreatomagmatic and magmatic activity that formed a nest of craters, spatter and tuff rings and cones that collectively comprise a crater complex >40 km² filled by 9–18 km³ of volcaniclastic debris. Rising magma flux rates combined with reduced access of magma to external water led to effusion of thick Karoo flood basalts, burying the crater-complex beneath the >1.5 km-thick Lesotho lava pile. Geochemical data is consistent with flood basalt effusion from local dikes, and some lava flows likely shared or re-occupied vent sites active during explosive eruptions at Sterkspruit. Flood basalt magmas involved in Sterkspruit eruptions were chemically heterogenous. This study documents the rapid (perhaps simultaneous) eruption of three chemically distinct basaltic magmas which cannot be simply related to one another from one vent site within the Sterkspruit crater complex. Stratigraphic and map relationships indicate that eruption of the same three magma types took place from closely spaced vents over a short time during formation of the bulk of the crater-complex. Two magma types recognized there have not been recognized in the Karoo province before. The variable composition of flood basalts at Sterkspruit argues that magma batches in flood basalt fields may be small (0.5–1 km³) and not simply related to one another. This implies in turn that heterogeneities in the magma source region may be close to each other in time and space, and that eruptions of chemically distinct magmas may take place over short intervals of space and time without significant hybridisation in flood basalt fields.     

An analogue experimental model of depth fluctuations in lava lakes

Lava lakes, consisting of molten degassing lava in summit craters of active basaltic volcanoes, sometimes exhibit complex cycles of filling and emptying on time-scales of hours to weeks such as recorded at Pu’u’O’o in Hawaii and Oldoinyo Lengai in Tanzania. Here we report on a new series of analogue laboratory experiments of two-phase flow in a reservoir-conduit-lava lake system which spontaneously generates oscillations in the depth of liquid within the lake. During the recharge phase, gas supplied from a subsurface reservoir of degassing magma drives liquid magma up the conduit, causing the lake to fill. As the magmastatic pressure in the lake increases, the upward supply of magma, driven by the gas bubbles, falls. Eventually the upflow becomes unstable, and liquid drains downwards from the lake, driven by the magmastatic pressure of the overlying lake, suppressing the ascent of any more bubbles from the chamber. At a later stage, once the lake has drained sufficiently, the descent speed of liquid through the conduit decreases below the ascent speed of the bubbles, and the recharge cycle resumes. Application of a quantitative model of the experiments to the natural system is broadly consistent with field data.     

The 1984 to 1996 cyclic activity of Lascar Volcano, northern Chile: cycles of dome growth, dome subsidence, degassing and explosive eruptions

 Lascar Volcano (5592 m; 23°22'S, 67°44'W) entered a new period of vigorous activity in 1984, culminating in a major explosive eruption in April 1993. Activity since 1984 has been characterised by cyclic behaviour with recognition of four cycles up to the end of 1993. In each cycle a lava dome is extruded in the active crater, accompanied by vigorous degassing through high-temperature, high-velocity fumaroles distributed on and around the dome. The fumaroles are the source of a sustained steam plume above the volcano. The dome then subsides back into the conduit. During the subsidence phase the velocity and gas output of the fumaroles decrease, and the cycle is completed by violent explosive activity. Subsidence of both the dome and the crater floor is accommodated by movement on concentric, cylindrical or inward-dipping conical fractures. The observations are consistent with a model in which gas loss from the dome is progressively inhibited during a cycle and gas pressure increases within and below the lava dome, triggering a large explosive eruption. Factors that can lead to a decrease in gas loss include a decrease in magma permeability by foam collapse, reduction in permeability due to precipitation of hydrothermal minerals in the pores and fractures within the dome and in country rock surrounding the conduit, and closure of open fractures during subsidence of the dome and crater floor. Dome subsidence may be a consequence of reduction in magma porosity (foam collapse) as degassing occurs and pressurisation develops as the permeability of the dome and conduit system decreases. Superimposed upon this activity are small explosive events of shallow origin. These we interpret as subsidence events on the concentric fractures leading to short-term pressure increases just below the crater floor. Received: 12 December 1996 / Accepted: 6 May 1997     

A CO2-rich gas trigger of explosive paroxysms at Stromboli basaltic volcano,Italy

In addition to rhythmic slug-driven Strombolian activity, Stromboli volcano occasionally produces discrete explosive paroxysms (2 per year on average for the most frequent ones) that constitute a major hazard and whose origin remains poorly elucidated. Partial extrusion of the volatile-rich feeding basalt as aphyric pumice during these events has led to consider their triggering by the fast ascent of primitive magma blobs from possibly great depth. Here I examine and discuss the alternative hypothesis that most of the paroxysms could be triggered and driven by the fast upraise of CO₂-rich gas pockets generated by bubble foam growth and collapse in the sub-volcano plumbing system. Data for the SO₂ and CO₂ crater plume emissions are used to show that Stromboli's feeding magma may originally contain as much as 2 wt.% of carbon dioxide and early coexists with an abundant CO₂-rich gas phase with high CO₂/SO₂ molar ratio (≥ 60 at 10 km depth below the vents, compared to ～ 7 in time-averaged crater emissions). Pressure-related modelling indicates that the time-averaged crater gas composition and output are well accounted for by closed system decompression of the basalt–gas mixture until the volcano–crust interface (～ 3 km depth), followed by open degassing and crystallization in the volcano conduits. However, both the low viscosity and high vesicularity of the basaltic magma permit bubble segregation and bubble foam growth at deep sill-like feeder discontinuities and at shallower physical boundaries (such as the volcano–crust interface) where the gas-rich aphyric basalt interacts with the unerupted crystal-rich and viscous magma drained back from the volcano conduits. Gas pressure build-up and bubble foam collapse at these boundaries will intermittently trigger the sudden upraise of CO₂-rich gas blobs that constitute the main driving force of the paroxysms. Deeper-sourced gas blobs, driving the most powerful explosions, will be the richest in CO₂ and have highest CO₂/SO₂ ratios. This mechanism is shown to account well for the dynamic, seismic and petrologic features of Stromboli's paroxysms and, hence, to provide a potential alternative interpretation for their genesis and their forecasting. Enhanced bubble foam leakage prior to a paroxysm, or foam emptying in several steps, should lead indeed to precursory upstream of CO₂-rich gas and increasing CO₂/SO₂ ratio in crater plume emissions. The recent detection of such signals prior to two explosions in December 2006 and March 2007 strongly supports this expectation and the model proposed in this study.     

Continuous soil CO<Superscript>2</Superscript> and discrete plume SO<Subscript>2</Subscript> measurements at Mt. Etna (Italy) during 1997–2000: a contribution to volcano monitoring

Continuous monitoring of soil CO₂ dynamic concentration (which is proportional to the CO₂ flux through the soil) was carried out at a peripheral site of Mt. Etna during the period November 1997–September 2000 using an automated station. The acquired data were compared with SO₂ flux from the summit craters measured two to three times a week during the same period. The high frequency of data acquisition with both methods allowed us to analyze in detail the time variations of both parameters. Anomalous high values of soil CO₂ dynamic concentration always preceded periods of increased flux of plume SO₂, and these in turn were followed by periods of summit eruptions. The variations were modeled in terms of gas efflux increase due to magma ascent to shallow depth and its consequent depressurization and degassing. This model is supported by data from other geophysical and volcanological parameters. The rates of increase both of soil CO₂ dynamic concentration and of plume SO₂ flux are interpreted to be positively correlated both to the velocity of magma ascent within the volcano and to lava effusion rate once magma is erupted at the surface. Low rates of the increase were recorded before the nine-month-long 1999 subterminal eruption. Higher rates of increase were observed before the violent summit eruption of September-November 1999, and the highest rates were observed during shorter and very frequent spike-like anomalies that preceded the sequence of short-lived but very violent summit eruptions that started in late January 2000 and continued until late June of the same year. Furthermore, the time interval between the peaks of CO₂ and SO₂ in a single sequence of gas anomalies is likely to be controlled by magma ascent velocity.Editorial responsibility: H. Shinohara     

Recycling and “re-hydration” of degassed magma inducing transient dissolution/crystallization events at Stromboli (Italy)

Intrusive degassing and recycling of degassed and dense magma at depth have been proposed for a long time at Stromboli. The brief explosive event that occurred at the summit craters on 9 January 2005 threw out bombs and lapilli that could be good candidates to illustrate recycling of shallow degassed magma at depth. We present an extensive data set on both the textures and the mineral, bulk rock and glassy matrix chemistry of the “9 Jan” products. The latter have the common shoshonitic–basaltic bulk composition of lavas and scoriae issued from typical strombolian activity. In contrast they differ by the heterogeneous chemistry of their matrix glasses and their crystal textures that testify to crystal dissolution event(s) just prior magma crystallization upon ascent and eruption. Comparison between mineral paragenesis of the natural products and experimental phase equilibria suggest water-induced magma re-equilibration. We propose that mineral dissolution is related to water enrichment of the recycled degassed magma, via differential gas bubble transfer and to some extents its physical mixing with volatile-rich magma blobs. However, all these features illustrate transient processes. Even though evidence of mineral dissolution is ubiquitous at Stromboli, its effect on the bulk magma chemistry is minor because of the subtle interplay between mineral dissolution and crystallization in magmas having comparable bulk chemistry.     

Monitoring volcanic hazard using eddy covariance at Solfatara volcano, Naples, Italy

An eddy covariance (EC) station was deployed at Solfatara crater, Italy, June 8–25, 2001 to assess if EC could reliably monitor CO₂ fluxes continuously at this site. Deployment at six different locations within the crater allowed areas of focused gas venting to be variably included in the measured flux. Turbulent (EC) fluxes calculated in 30-min averages varied between 950 and 4460 g CO₂ m⁻² d⁻¹; the highest measurements were made downwind of degassing pools. Comparing turbulent fluxes with chamber measurements of surface fluxes using footprint models in diffuse degassing regions yielded an average difference of 0% (±4%), indicating that EC measurements are representative of surface fluxes at this volcanic site. Similar comparisons made downwind of degassing pools yielded emission rates from 12 to 27 t CO₂ d⁻¹ for these features. Reliable EC measurements (i.e. measurements with sufficient and stationary turbulence) were obtained primarily during daytime hours (08:00 and 20:00 local time) when the wind speed exceeded 2 m s⁻¹. Daily average EC fluxes varied by ±50% and variations were likely correlated to changes in atmospheric pressure. Variations in CO₂ emissions due to volcanic processes at depth would have to be on the same order of magnitude as the measured diurnal variability in order to be useful in predicting volcanic hazard. First-order models of magma emplacement suggest that emissions could exceed this rate for reasonable assumptions of magma movement. EC therefore provides a useful method of monitoring volcanic hazard at Solfatara. Further, EC can monitor significantly larger areas than can be monitored by previous methods.     

Mid-ocean ridge popping rocks: implications for degassing at ridge crests

The vesicle size distribution (VSD) and rare gas abundances in popping rocks from 14°N on the Mid-Atlantic Ridge provide constraints on the behavior of volatiles during ridge crest volcanism. These popping rocks, which contain 16–18 volume percent vesicles, are rare mid-ocean ridge basalt (MORB) magmas which appear to have retained much of their volatile inventory. The logarithm of vesicle population density displays the same linear correlation with decreasing size in two of the samples studied. This implies that continuous and simultaneous nucleation and bubble growth have occurred during magma ascent, with no significant perturbations due to accumulation, coalescence or loss of bubbles. In contrast, most MORB magmas display low vesicularities and we suggest that they have suffered some degree of pre-eruptive vesicle loss. We tentatively propose that large vesicles are produced by coalescence when MORB melt is at rest in chambers and conduits, and may be lost during early gas-rich episodes. Most MORB would represent residual liquids which erupt after vesicle loss has occurred, whereas popping rocks would represent a rare case where physical sorting of vesicles from melt did not occur, because storage in a magma chamber did not occur.The rare gas concentrations in the studied popping rocks are the highest yet measured in glassy ridge basalts ([He] > 50 μccSTP/g). The rare gas abundance pattern of these popping rocks probably resembles the pattern for non-vesiculated MORB magma and potentially reflects that of the depleted mantle source. This pattern is similar to the “mean MORB” pattern (computed from MORB glasses with⁴⁰Ar/³⁶Ar > 10,000) although a higher enrichment in He (and possibly Ne) compared to the heavier rare gases is observed in MORB. The overall similarity in abundance patterns for MORB and popping rocks indicates that vesiculation and vesicle loss do not fractionate the ArKrXe relative abundances from those in non-vesiculated magma, and that the modern flux ratios of these gases at ridges are similar to their elemental ratios in the depleted mantle. The degassing flux of He at ridge crests estimated from the MORB He deficit relative to popping rocks is comparable to the flux derived from the³He budget for the abyssal ocean. This suggests that degassing at ridges may be strongly influenced by the dynamics and style of submarine volcanism.     

The summit hydrothermal system of Stromboli. New insights from self-potential,temperature, CO<Subscript>2</Subscript> and fumarolic fluid measurements,with structural and monitoring implications

Accurate and precisely located self-potential (SP), temperature (T) and CO₂ measurements were carried out in the summit area of Stromboli along 72 straight profiles. SP data were acquired every metre and T data every 2.5 m. CO₂ concentrations were acquired with the same density as T, but only along seven profiles. The high density of data and the diversity of the measured parameters allows us to study structures and phenomena at a scale rarely investigated. The shallow summit hydrothermal activity (Pizzo–Fossa area) is indicated by large positive SP, T and CO₂ anomalies. These anomalies are focused on crater faults, suggesting that the fracture zones are more permeable than surrounding rocks at Stromboli. The analysis of the distribution of these linear anomalies, coupled with the examination of the geologic, photographic and topographic data, has led us to propose a new structural interpretation of the summit of Stromboli. This newly defined structural framework comprises (1) a large Pizzo circular crater, about 350 m in diameter; (2) a complex of two concealed craters nested within the Pizzo crater (the Large and the Small Fossa craters), thought to have formed during the eruption of the Pizzo pyroclastites unit; the Small Fossa crater is filled with highly impermeable material that totally impedes the upward flow of hydrothermal fluids; and (3) The present complex of active craters. On the floor of the Fossa, short wavelength SP lows are organized in drainage-like networks diverging from the main thermal anomalies and converging toward the topographic low in the Fossa area, inside the Small Fossa crater. They are interpreted as the subsurface downhill flow of water condensed above the thermal anomalies. We suspect that water accumulates below the Small Fossa crater as a perched water body, representing a high threat of strong phreatic and phreatomagmatic paroxysms. T and CO₂ anomalies are highly correlated. The two types of anomalies have very similar shapes, but the sensitivity of CO₂ measurements seems higher for lowest hydrothermal flux. Above T anomalies, a pronounced high frequency SP signal is observed. Isotopic analyses of the fluids show similar compositions between the gases rising through the faults of the Pizzo and Large Fossa craters. This suggests a common origin for gases emerging along different structural paths within the summit of Stromboli. A site was found along the Large Fossa crater fault where high gas flux and low air contamination made gas monitoring possible near the active vents using the alkaline bottle sampling technique.     

Chemical evolution and volcanic activity of the active crater lake of Poás volcano, Costa Rica, 1993–1997

Concentrations of chloride and sulfate and pH in the hot crater lake (Laguna Caliente) at Poás volcano and in acid rain varied over the period 1993–1997. These parameters are related to changes in lake volume and temperature, and changes in summit seismicity and fumarole activity beneath the active crater. During this period, lake level increased from near zero to its highest level since 1953, lake temperature declined from a maximum value of 70°C to a minimum value of 25°C, and pH of the lake water increased from near zero to 1.8. In May 1993 when the lake was nearly dry, chloride and sulfate concentrations in the lake water reached 85,400 and 91,000 mg l⁻¹, respectively. Minimum concentrations of chloride and sulfate after the lake refilled to its maximum volume were 2630 and 4060 mg l⁻¹, respectively. Between January 1993 and May 1995, most fumarolic activity was focused through the bottom of the lake. After May 1995, fumarolic discharge through the bottom of the lake declined and reappeared outside the lake within the main crater area. The appearance of new fumaroles on the composite pyroclastic cone coincided with a dramatic decrease in type B seismicity after January 1996. Between May 1995 and December 1997, enhanced periods of type A seismicity and episodes of harmonic tremor were associated with an increase in the number of fumaroles and the intensity of degassing on the composite pyroclastic cone adjacent to the crater lake. Increases in summit seismic activity (type A, B and harmonic tremor) and in the height of eruption plumes through the lake bottom are associated with a period of enhanced volcanic activity during April–September 1994. At this time, visual observations and remote fumarole temperature measurements suggest an increase in the flux of heat and gases discharged through the bottom of the crater lake, possibly related to renewed magma ascent beneath the active crater. A similar period of enhanced seismic activity that occurred between August 1995 and January 1996, apparently caused fracturing of sealed fumarole conduits beneath the composite pyroclastic cone allowing the focus of fumarolic degassing to migrate from beneath the lake back to the 1953–1955 cone. Changes in the chemistry of summit acid rain are correlated changes in volcanic activity regardless of whether fumaroles are discharging into the lake or are discharging directly into the atmosphere.