Thermal characterization of the Vulcano fumarole field

Ground-based thermal infrared surveys can contribute to complete heat budget inventories for fumarole fields. However, variations in atmospheric conditions, plume condensation and mixed-pixel effects can complicate vent area and temperature measurements. Analysis of vent temperature frequency distributions can be used, however, to characterise and quantify thermal regions within a field. We examine this using four thermal infrared thermometer and thermal image surveys of the Vulcano Fossa fumarole field (Italy) during June 2004 and July 2005. These surveys show that regions occupied by low temperature vents are characterised by distributions that are tightly clustered around the mean (i.e., the standard deviation is low), highly peaked (positive kurtosis) and skewed in the low temperature direction (negative skewness). This population is associated with wet fumaroles, where boiling controls maximum temperature to cause a narrow distribution with a mode at 90–100°C. In contrast, high temperature vent regions have distributions that are widely spread about the mean (i.e., the standard deviation is high), relatively flat (negative kurtosis) and skewed in the high temperature direction (positive skewness). In this dry case, fumaroles are water-free so that maximum temperatures are not fixed by boiling. As a result greater temperature variation is possible. We use these results to define two vent types at Vulcano on the basis of their thermal characteristics: (1) concentrated (localized) regions of high temperature vents, and (2) dispersed low temperature vents. These occur within a much larger region of diffuse heat emission across which surfaces are heated by steam condensation, the heat from which causes elevated surface temperatures. For Vulcano's lower fumarole zone, high and low temperature vents occupied total areas of 3 and 6 m², respectively, and occurred within a larger (430 m²) vent-free zone of diffuse heat emission. For this lower zone, we estimate that 21–43 × 10³ W of heat was lost by diffuse heat emission. A further 4.5 × 10³ W was lost by radiation from high temperature vents, and 6.5 × 10³ W from low temperature vents. Thus, radiative heat losses from high and low temperature vents within Vulcano's lower fumarole zone respectively account for 10% and 15% of the total heat lost from this zone. This shows that radiation from open vents can account for a non-trivial portion of the total fumarole field heat budget.     

Time-space variation of volcano-seismic events at La Fossa (Vulcano, Aeolian Islands, Italy): new insights into seismic sources in a hydrothermal system

We investigated the relationship between volcano-seismic events, recorded at La Fossa crater of Vulcano (Aeolian Islands, Italy) during 2004-2006, and the dynamics of the hydrothermal system. During the period of study, three episodes of increasing numbers of volcano-seismic events took place at the same time as geothermal and geochemical anomalies were observed. These geothermal and geochemical anomalies have been interpreted as resulting from an increasing deep magmatic component of the hydrothermal fluids. Three classes of seismic events (long period, high frequency and monochromatic events), characterised by different spectral content and various similarity of the waveforms, have been recognised. These events, clustered mainly below La Fossa crater area at depths of 0.5–1.1 km b.s.l., were space-distributed according to the classes. Based on their features, we can infer that such events at Vulcano are related to two different source mechanisms: (1) fracturing processes of rocks and (2) resonance of cracks (or conduits) filled with hydrothermal fluid. In the light of these source mechanisms, the increase in the number of events, at the same time as geochemical and geothermal anomalies were observed, was interpreted as the result of an increasing magmatic component of the hydrothermal fluids, implying an increase of their flux. Indeed, such variation caused an increase of both the pore pressure within the rocks of the volcanic system and the amount of ascending fluids. Increased pore pressures gave rise to fracturing processes, while the increased fluid flux favoured resonance and vibration processes in cracks and conduits. Finally, a gradual temporal variation of the waveform of the hybrid events (one of the subclasses of long period events) was observed, likely caused by heating and drying of the hydrothermal system.     

Fumarole monitoring with a handheld infrared camera: Volcán de Colima,Mexico, 2006–2007

This paper describes a methodology for the monitoring of fumarole temperatures at medium ranges (~ 6 km) using a handheld infrared camera (wavelength range: 8–13.5 µm). As a relationship between fumarole temperatures, gas flux and volcanic activity has been demonstrated by a number of studies, fumarole temperature data has a potential use as a monitoring tool. Volcán de Colima is an andesitic stratovolcano with a 300 m diameter summit crater formed by the destruction of the 2004 lava dome by a series of explosions in 2005. Between January 2006 and August 2007, sequences of thermal images were recorded from a viewpoint 6 km to the north during regular 24–48 hour monitoring excursions. The temperatures of fumaroles on the crater rim and the ground surface on the volcano's flanks were measured. A methodology was developed to remove data affected by clouds or volcanic water vapour based on rates of temperature change and scatter within the data. For the remaining data, it is demonstrated mathematically that at this range, typical variations in atmospheric transmissivity will affect the apparent temperatures by +/− 2 °C, while a 25% change in fumarole heat flux would change it by 5–10 °C. The mean night-time apparent temperature of the fumaroles was calculated for each excursion and showed an irregular decline over the 19 month period. Subtracting the radiant heat flux of flank rocks from those of the fumaroles removes seasonal variations and gives the clearest view of trends in the fumarole heat flux. A sharp drop in fumarole temperature during February 2007 coincided with the emergence of a lava dome in the crater. The declining fumarole temperature is interpreted to reflect decreasing gas flux from the crater in line with a change in eruptive regime from frequent, small, ash-rich explosions to slow effusion of lava.     

Thermal expansion-contraction and slope instability of a fumarole field inferred from geodetic measurements at Vulcano

Between 1987 and 1993, fumarole temperatures at the Fossa crater of Vulcano (Italy) were characterized by the highest values since the 1920’s, increasing from about 300°C in 1987 to 690°C in May 1993, before decreasing to 400°C by 1996–1997. During 1990, Vulcano’s Electronic Distance Measurement (EDM) network was expanded to provide more detailed coverage of the northern sector of the Fossa crater and, in particular, to monitor the movement of the northern flank the Fossa cone. Measurements, carried out between 1990 and 1994, showed shortening by about 6 to 7 cm along baselines measured to a small section of the northern rim. Over the following four years these baselines showed a slow extension by about 3 cm, to gradually recover part of the previous deformation. We believe that the shortening and lengthening of the EDM baselines was respectively due to the increasing and decreasing temperature of the rock body lying close to the deforming area. This caused thermal expansion, followed by contraction. The positive movement of the rim was not completely matched by a negative recovery, suggesting that a non-recoverable sliding movement was also responsible for some of the shortening of the baselines. We verified our hypothesis by calculating the expected dilatation of a rock body, as a function of the volume of rock heated and its thermal expansion coefficient, and compared the expected deformation to that observed. The geodetic investigation showed that the unstable portion affects a small length of the rim (about 100 m long) and involves a volume of about 0.8 × 10⁶ m³. However, this zone lies directly above a particularly unstable portion of the flank, as well as the main village and port on the island.     

Fluids in early stage hydrothermal alteration of high-sulfidation epithermal systems: A view from the Vulcano active hydrothermal system (Aeolian Island,Italy)

High-sulfidation (HS) epithermal systems have elements in common with passively degassing volcanoes associated with high T, acid fumarole fields or acid crater lakes. They are considered to form in two stages, the first of which involves advanced argillic alteration resulting from intense, strongly acidic fluid–rock interaction. The La Fossa hydrothermal system (Vulcano Island) represents a classic example of such an active HS system and can be considered as a modern analogue of this early stage of alteration, resulting in a core of intense silicic (90–95% pure SiO₂) alteration surrounded by alunitic alteration zones.     

New fumarolic activity on Mt. Baker: observations during April through July, 1975

On 10 March, 1975, large amounts of steam were first sighted coming from the Sherman Crater of Mt. Baker, long the site of mild fumarole activity. To document and evaluate the new activity, we conducted a series of aerial and ground-based observations and experiments including the collection of samples within the crater, installation of 35-mm automatic sequence cameras at Park and Anderson Buttes, and overflights with a mapping camera and infrared scanner. Significant changes characterizing the new activity are: development of a new, energetic fumarole in an area of earlier mild activity; growth of crevasses within and concentric to the crater walls; collapse of a central 70 m diameter plug of ice to form a warm lake; and ejection of tephra rich in analcite and sulfides from the new fumarole. Although the tephra contains minor amounts of unaltered glass, it does not differ in this or other respects from material collected from the west crater wall, where fumaroles have long been active. This suggests that the glass is older tephra derived from the fumarole vent walls.Weather and lighting conditions were such that the Park Butte sequence camera provided acceptable images of the crater area on 27% of the frames for the period 1825 Pacific Daylight Time June 5 to 1825 PDT August 2. Of these, 8% show a significant plume (>300 m above the vent). Large plumes occur during favorable weather conditions of low wind and high relative humidity, and all appear white.While there are abundant manifestations of increased heat emission from Sherman Crater, there is as yet no sign of a fundamental change in the style of activity, or that the rise of fresh magma is the cause.     

Seismic events under vulcano,Aeolian Islands,Italy

This paper describes the seismic activity of Vulcano - an active volcano of the Aeolian Islands group - from 1977 to 1980. Two main swarms of low energy seismic events are recognizable on the basis of the entire set of data. The events, localized very near the crater of Vulcano where a fumarole field is present, have been classified into two classes:i) very shallow events (as far as 1 km in depth); andii) deeper events (as far as 3 km in depth). Double events and small earthquakes localized on the rim of the old caldera have also been recorded in a little percentage. The seismic events of Vulcano can be attributed to an intensive fumarolic activity.     

Signs of potential renewal of eruptive activity at La Fossa (Vulcano,Aeolian Islands)

Since the end of the last magmatic eruption (1890), activity of La Fossa (southern Tyrrhenian Sea, Italy) has consisted of fumarolic emissions of fluctuating intensity. Fluids are discharged principally at two fumarolic fields located in the northern rim of the active crater and at the beach sited at its northern foot. Increased thermal, seismic and geochemical activity has been recorded since 1978, when an earthquake of M=5.5 occurred in the region. This paper combines available geophysical and geochemical information in order to develop a tentative interpretation of two episodes of apparent unrest which occurred in 1985 and 1987–1988, enhancing the risk of renewal of the eruptive activity. The 1985 unrest consisted essentially of a sharp build up of the internal pressure in the shallow hydrothermal system, which was induced by the injection of hot gases of magmatic origin. The crater fumaroles displayed significant increases in CO₂ and other acid species, but their outlet temperature did not change. Conversely, the 1987–1988 episode was characterized by appreciable modifications at the crater fumaroles, with only secondary effects at the fumarole system of the beach. The sliding of part of the eastern flank of the La Fossa cone into the sea occurred on 20 April 1988, when the region was affected by crustal dilatation producing a seismic sequence of relatively high intensity. Both episodes of unrest were accompanied by increases of local microseismic activity, which affected the nothern sector of the island in 1985, and the southern one in 1988. Finally, a phase of appreciable areal contraction was detected in 1990, probably due to the effect of the cooling and crystallization of magma at relatively shallow depths, accompanying the increased thermal activity at the crater fumaroles. Regional tectonic stress seems to play an important role in the transition of the volcanic system from a phase of relative stability to a phase of apparent unrest, inducing the heating and the expansion of shallow hydrothermal fluids. Available information is insufficient to indicate whether or not the volcano is building towards the renewal of a magmatic eruption, and there is no evidence to hypothesize episodes of significant magma migration. The frequency of measurements of many parameters needs to be increased in order to learn more about the temporal relationships between geochemical and geophysical variations preceding and accompanying periods of increased thermal activity. This will probably be a valid tool for recognizing short-term precursors of a future eruption, reducing the risk of false alarms.     

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.     

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.     

Surface mercury geochemistry as a guide to volcanic vent structure and zones of high heat flow in the valley of ten thousand smokes, Katmai National Park, Alaska

A reconnaissance survey of Hg° was designed to model the 1912 Novarupta vent structure and delineate zones of near-surface high heat flow. Statistical analysis of 294 regolith samples collected at 127 sites from fossil/active fumaroles and relatively unaltered ash-flow tuff and air-fall ejecta indicates two Hg° sample populations; one associated with fumarole deposits and the other corresponding to the relatively unaltered regolith. Measured Hg° concentrations range from 12 to 6913 ppb. Sample threshold (upper limit of background Hg° concentrations; fumarole deposits: 351 ppb, unaltered regolith: 110 ppb) and contrast (sample value/threshold) determinations enable differentiation between relict and actively accumulating Hg° contents.All significant Hg° anomalies (contrast values > 1.5) occur within Novarupta Basin. High-magnitude, active Hg² enrichment was found on the Turtle, a dome-like feature northeast of the Novarupta extrusive dome. The Hg° data, coupled with available geologic and geophysical evidence, suggest a shallow intrusion beneath the Turtle. Results from two Novarupta Basin sample traverses do not support the presence of a near-surface magma feeder dike connecting the proposed intrusion beneath the Turtle with the Novarupta dome. Based on the Hg° data, the preferred vent model is one generated by collapse of supporting walls after the major eruptive phase into a cored-out explosive vent. Collapse of vent walls is initiated along a series of deep-extending faults which subsequently serve as migration pathways for Hg° to the surface. The overall vent morphology is funnel-like with subsidence concentrated in the narrow funnel center. Results of this study show that Hg° surveys can further constrain the morphology of the 1912 vent and aid in delineating zones of near-surface high heat flow in this region.     

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.     

Thermal features at Volcanoes in the cascade range,as observed by aerial infrared surveys

There have been no substantial changes in the thermal patterns at the summit of Mount Rainier in the period September 1964–September 1966, within the detection limits of the infrared instrumentation. Some differences in radiance are attributed to differences in snow cover. The highest apparent temperature is at a snow-free area on the west flank of the summit cone, several hundred feet below the west crater rim. An anomaly at this site was recorded on both infrared surveys, but no prior reports of thermal activity here have been made by ground parties. Other anomalous thermal zones at the summit are on the northern quadrants of both crater rims. A very small, low-temperature fumarole reported on Mount Adams was not detected, nor were any other thermal manifestations recorded. One anomaly consisting of a close-spaced cluster of thermal spots was detected at The Boot on Mount St. Helens and corresponds to a known fumarole area. The only thermal feature seen on Mount Shasta is near the summit at a thermal spring that has been observed by many climbers. Two anomalies were found on the north flank of Lassen Peak. Thermal activity had not been previously reported at either site, though one is in a known solfatarized area. No ground investigation has been made at the other location. Much of the other thermal activity in the Lassen Peak area is in the northeast quadrant of Brokeoff Caldera. Most of these features are well documented in the literature; others not previously described are in fairly accessible areas and doubtless result from springs and fumaroles related to Brokeoff Caldera.     

Genesis of chlorine and sulphur in fumarolic emissions at Vulcano Island (Italy): assessment of pH and redox conditions in the hydrothermal system

Chlorine- and sulphur-bearing compounds in fumarole discharges of the La Fossa crater at Vulcano Island (Italy) can be modelled by a mixing process between magmatic gases and vapour from a boiling hydrothermal system. This allows estimating the compounds in both endmembers. Magma degassing cannot explain the time variation of sulphur and HCl concentrations in the deep endmember, which are more probably linked to reactions of solid phases at depth, before mixing with the hydrothermal vapours. Based on the P–T conditions and speciation of the boiling hydrothermal system below La Fossa, the HCl and S_tot contents in the hydrothermal vapours were used to compute the redox conditions and pH of the aqueous solution. The results suggest that the haematite–magnetite buffer controls the hydrothermal fO₂ values, while the pH has increased since the end of the 1970s. The main processes affecting pH values may be linked to Na–Ca exchanges between evolved seawater, feeding the boiling hydrothermal system, and local rocks. While Na is removed from water, calcium enters the solution, undergoes hydrolysis and produces HCl, lowering the pH of the water. The increasing water–rock ratio within the hydrothermal system lowers the Ca availability, so the aqueous solution becomes less acidic. Seawater flowing towards the boiling hydrothermal brine dissolves a large quantity of pyrite along its path. In the boiling hydrothermal system, dissolved sulphur precipitates as pyrite and anhydrite, and becomes partitioned in vapour phase as H₂S and SO₂. These results are in agreement with the paragenesis of hydrothermal alteration minerals recovered in drilled wells at Vulcano and are also in agreement with the isotopic composition of sulphur emitted by the crater fumaroles.     

Geological constraints on the 2003–2005 ash emissions from the Nakadake crater lake,Aso Volcano,Japan

The active crater of Nakadake at Aso Volcano, southwestern Japan, has been occupied by a lake during its dormant periods. Multiple ash emissions were observed from the crater lake between July 2003 and August 2005. The largest events occurred on 10 July 2003 and 14 January 2004. On 10 July 2003, ash (41 tons) was dispersed throughout an area extending 14 km east-northeast of the vent, and on 14 January 2004 ash (32 tons) extended 8 km to the east-southeast. Thereafter, small ash emissions were frequent at the crater lake, in which the water level fell considerably from April to August 2005. During this period major ash emitting events, producing mostly white aggregated ash, occurred on June 10–12, June 21 (4.2 tons) and July 25 (1.2 tons). Ash emissions at the Nakadake crater in 2003–2005 were classified into three types: gentle release of white aggregated ash from fumaroles inside the crater lake (e.g. daytime of 14 April, 10–12 June and 25 July 2005); emission of black ash from an almost dried-up vent (21 June 2005); and short-time (ca. 20 s) small-explosion-triggered gas-and-ash emission through the crater lake (10 July 2003, 14 January 2004 and 14 April 20h41m 2005). All products from these ash emissions consisted of fine-grained (< 1 mm) glass shards, crystals and lithic fragments, and contained neither lapilli nor blocks. Although the glass shards show varying degrees of crystallization and alteration, clear glass shards, which appear fresh, are probably juvenile materials. These observations suggest that the 2003–2005 ash emissions from the crater lake of Nakadake Volcano are related to newly ascending magma.     

Ground-based thermal imaging of lava lakes at Erebus volcano,Antarctica

Mount Erebus, a large intraplate stratovolcano dominating Ross Island, Antarctica, hosts the world's only active phonolite lava lakes. The main manifestation of activity at Erebus volcano in December 2004 was as the presence of two convecting lava lakes within an inner crater. The long-lived Ray Lake, ~ 1400 m² in area, was the site of up to 10 small Strombolian eruptions per day. A new but short-lived, ~ 1000–1200 m² lake formed at Werner vent in December 2004 sourced by lava flowing from a crater formed in 1993 by a phreatic eruption. We measured the radiative heat flux from the two lakes in December 2004 using a compact infrared (IR) imaging camera. Daily thermal IR surveys from the Main Crater rim provide images of the lava lake surface temperatures and identify sites of upwelling and downwelling. The radiative heat outputs calculated for the Ray and Werner Lakes are 30–35 MW and 20 MW, respectively. We estimate that the magma flux needed to sustain the combined heat loss is ~ 250–710 kg s^− 1, that the minimum volume of the magma reservoir is 2 km³, and that the radius of the conduit feeding the Ray lake is ~ 2 m.     

Volcanic hazards at Fossa of Vulcano: Data from the last 6,000 years

Stratigraphic reconstruction of the complete sequence of deposits that formed the Fossa cone of Vulcano has distinguished four principal eruptive cycles: Punte Nere, Palizzi, Commenda, and Pietre Cotte. At least three additional eruptive cycles, one of which ends with the Campo Sportivo lava, occur between deposits of the Punte Nere and Palizzi cycles. However, exposure is inadequate for their characterization. The assignment of the modern deposits that follow the Pietre Cotte lava is uncertain.Deposits of each cycle follow a similar stochastic pattern that is controlled by a decrease in the effect of water/melt interaction. The normal sequence of pyroclastic products for each cycle starts with wet-surge beds, followed by dry-surge horizons, fall deposits, and finally lava flows. Absolute age determinations have been made on each cycle-ending lava flow.Wet-surge deposits normally occur near the crater rim, whereas dry-surge deposits are more widespread, reaching the surrounding caldera wall in many places. Thick fall deposits are confined to a zone extending about 800 m from the crater rim. Lava flows normally reach the base of the cone. The greatest hazard at Fossa is related to surge eruptions. The thickness of dry-surge deposits on the flanks of the cone increases away from the crater, but they pinch out toward the source near the crater rim. SEM analysis of the surface textures of juvenile glass clasts from dry-surge deposits confirms that the dominant control on the eruptive mechanism is water/melt interaction. Only slight modifications are induced on grain surfaces during transport. Particles from the Palizzi dry-surge beds lack surface textures characteristic of fall pyroclasts which suggests that ballistic fragments were not incorporated into the dense portion of the turbulent surge cloud. A quantitative analysis of the dispersal of products from the Palizzi cycle allowed creation of a computer-generated map for this eruption.Paper presented at the IUGG Inter-disciplinary Symposium on Volcanic Hazard, Hamburg, August 1983.     

Diffuse emission of CO2 from the Fossa crater,Vulcano Island (Italy)

 Numerous measurements of CO₂ degassing from the soil, carried out with the accumulation chamber method, indicate that in the period April–July 1995 the upper part of the Fossa cone released a total output of 200 t d^–1 of CO₂, which corresponds to approximately 1000 t d^–1 of steam. These large amounts of fluids are of the same order of magnitude as those released by the high temperature fumarolic field located inside the crater. The spatial distribution of soil gas fluxes shows that the main structures releasing CO₂ are the inner slopes of the crater and a NW–SE line, located NE of the crater rim, which correspond to the main direction of Vulcano Island active faults. The comparison of the φCO₂ maps with the soil temperature distribution, derived from both direct measurements and airborne infrared images, indicates the occurrence of extensive condensation of fumarolic steam within the upper part of the Fossa cone, whose total amount is comparable to the rainfall budget. Part of the condensate which originates from this process contributes to the recharge of the phreatic aquifer of Porto Plain, modifying the chemical and isotopic composition of the groundwater. Received: 1 September 1995 / Accepted: 8 January 1996     

Seismological studies of the soufriere of St. Vincent, 1953-79: Implications for volcanic surveillance in the lesser antilles

The Soufriere of St. Vincent has been monitored for more than 25 years as part of a regional programme in the Lesser Antilles. In that time the volcano has erupted twice but our studies have shown no discernible change in regional seismicity before either event. However, very small seismic events were observed in the crater during the 1971–1972 eruption and were detected before the start of the 1979 explosive eruption; we believe that they were generated by thermally induced hydraulic fracturing within the lava mass inside the crater lake. We conclude that seismographic monitoring of Lesser Antillean volcanoes can give ambiguous results but that at least one instrument must be placed within 1 km of the vent if the earliest signs of activity are to be detected.     

Combined electromagnetic, geochemical and thermal surveys of Taal volcano (Philippines) during the period 2005–2006

Since 1572, 33 phreatic to phreatomagmatic eruptions have occurred on Taal volcano (Philippines), some of them causing several hundred casualties. Considering the time delay between two consecutive eruptions, there is an 88% probability that Taal volcano should have already erupted. Since 1992, several phases of seismic activity have been recorded accompanied by ground deformation, opening of fissures, and surface activity. The volcanic activity of Taal appears to be controlled by dike injections and magma supply, buffered by a hydrothermal system that releases fluids and heat through boiling and subsequent steaming. In early 2005, a multidisciplinary project was launched for studying the hydrothermal activity. To map the hydrothermal system, combined surveys were carried out to investigate self-potential, total magnetic field, ground temperature and carbon dioxide soil degassing, along with satellite thermal imaging of the Main Crater Lake. The elevated temperatures and high concentrations of carbon dioxide, as well as electromagnetic anomalies, indicate large-scale hydrothermal degassing. This process is enhanced along the tectonic features (e.g., crater rim and faults) of the volcano, while active fissures opened along the E–W northern flank during the 1992–1994 seismic activity. Heat and fluids from the hydrothermal system are essentially released in the northern part of the crater, which is bounded to the South by a suspected NW–SE fault along which seismicity seems to take place, and dikes are thought to be intruded. During the January 2005 surveys, a new seismic crisis started, and the felt earthquakes prompted spontaneous evacuation of hundreds of inhabitants living on the volcano. Repeated surveys show changes of self-potential, total magnetic field, and ground temperature with time, without any noticeable spatial enlargement. These observations suggest that the northern flank located between the crater rim and the 1992–1994 fissures is connected with a deep thermal source in Main crater and is reactivated during seismic crises. This sector could be subjected to flank failure.