Rincón de la Vieja volcano, Guanacaste province, Costa Rica: geology of the southwestern flank and hazards implications

Acid rain and ongoing eruptive activity at Rincón de la Vieja volcano in northwestern Costa Rica have created a triangular, deeply eroded “dead zone” west-southwest of the Active Crater. The barren, steep-walled canyons in this area expose one of the best internal stratigraphic profiles of any active or dormant volcano in Costa Rica. Geologic mapping along the southwestern flank of the volcano reveals over 300 m of prehistoric volcanic stratigraphy, dominated by tephra deposits and two-pyroxene andesite lavas. Dense tropical forests and poor access preclude mapping elsewhere on the volcano. In the “dead zone” four stratigraphic groups are distinguished by their relative proportions of lava and tephra. In general, early volcanism was dominated by voluminous lava emissions, with explosive plinian eruptions becoming increasingly more dominant with time. Numerous phreatic eruptions have occurred in historic times, all emanating from the Active Crater. The stratigraphic sequence is capped by the Río Blanco tephra deposit, erupted at approximately 3500 yr B.P. Approximately 0.25 km³ (0.1 km³ DRE) of tephra was deposited in a highly asymmetrical dispersal pattern west-southwest of the source vent, indicating strong prevailing winds from the east and east-northeast at the time of the eruption. Grain-size studies of the deposit reveal that the eruption was subplinian, attaining an estimated column height of 16 km. A qualitative hazards assessment at Rincón de la Vieja indicates that future eruptions are likely to be explosive in style, with the zone of greatest hazard extending several kilometers north from the Active Crater.     

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     

Geochemical and isotopic evidence for seawater contamination of the hydrothermal system of Taal Volcano, Luzon, the Philippines

 The hydrologic structure of Taal Volcano has favored development of an extensive hydrothermal system whose prominent feature is the acidic Main Crater Lake (pH<3) lying in the center of an active vent complex, which is surrounded by a slightly alkaline caldera lake (Lake Taal). This peculiar situation makes Taal prone to frequent, and sometimes catastrophic, hydrovolcanic eruptions. Fumaroles, hot springs, and lake waters were sampled in 1991, 1992, and 1995 in order to develop a geochemical model for the hydrothermal system. The low-temperature fumarole compositions indicate strong interaction of magmatic vapors with the hydrothermal system under relatively oxidizing conditions. The thermal waters consist of highly, moderately, and weakly mineralized solutions, but none of them corresponds to either water–rock equilibrium or rock dissolution. The concentrated discharges have high Na contents (>3500 mg/kg) and low SO₄/Cl ratios (<0.3). The Br/Cl ratio of most samples suggests incorporation of seawater into the hydrothermal system. Water and dissolved sulfate isotopic compositions reveal that the Main Crater Lake and spring discharges are derived from a deep parent fluid (T≈300  °C), which is a mixture of seawater, volcanic water, and Lake Taal water. The volcanic end member is probably produced in the magmatic-hydrothermal environment during absorption of high-temperature gases into groundwater. Boiling and mixing of the parent water give rise to the range of chemical and isotopic characteristics observed in the thermal discharges. Incursion of seawater from the coastal region to the central part of the volcano is supported by the low water levels of the lakes and by the fact that Lake Taal was directly connected to the China sea until the sixteenth century. The depth to the seawater-meteoric water interface is calculated to be 80 and 160 m for the Main Crater Lake and Lake Taal, respectively. Additional data are required to infer the hydrologic structure of Taal. Geochemical surveillance of the Main Crater Lake using the SO₄/Cl, Na/K, or Mg/Cl ratio cannot be applied straightforwardly due to the presence of seawater in the hydrothermal system. Received: 12 February 1997 / Accepted: 26 January 1998     

Geochemical mapping of magmatic gas–water–rock interactions in the aquifer of Mount Etna volcano

Systematic analysis of major and minor elements in groundwaters from springs and wells on the slopes of Mt. Etna in 1995–1998 provides a detailed geochemical mapping of the aquifer of the volcano and of the interactions between magmatic gas, water bodies and their host rocks. Strong spatial correlations between the largest anomalies in pCO₂ (pH and alkalinity) K, Rb, Mg, Ca and Sr suggest a dominating control by magmatic gas (CO₂) and consequent basalt leaching by acidified waters of the shallow (meteoric) Etnean aquifer. Most groundwaters displaying this magmatic-type interaction discharge within active faulted zones on the S–SW and E lower flanks of the volcanic pile, but also in a newly recognised area on the northern flank, possibly tracking a main N–S volcano-tectonic structure. In the same time, the spatial distribution of T°C, TDS, Na, Li, Cl and B allows us to identify the existence of a deeper thermal brine with high salinity, high content of B, Cl and gases (CO₂, H₂S, CH₄) and low K/Na ratio, which is likely hosted in the sedimentary basement. This hot brine reaches the surface only at the periphery of the volcano near the Village of Paternò, where it gives rise to mud volcanoes called “Salinelle di Paternò”. However, the contribution of similar brines to shallower groundwaters is also detected in other sectors to the W (Bronte, Maletto), SW (Adrano) and SE (Acireale), suggesting its possible widespread occurrence beneath Etna. This thermal brine is also closely associated with hydrocarbon fields all around the volcano and its rise, generally masked by the high outflow of the shallow aquifer, may be driven by the ascent of mixed sedimentary–magmatic gases through the main faults cutting the sedimentary basement.     

Temperature observations of Crater Lake,Mt Ruapehu,New Zealand,using Temperature Telemetry Bouys

The temperature of the Crater Lake of the active volcano Ruapelm has been recorded by Temperature Telemetry Buoys, to determine if lake temperature is correlated with volcanic activity. These buoys had to be specially designed to cope with the unfavourable environment of Crater Lake. A buoy contains a thermistor to measure the lake temperature, and a radio transmitter to transmit a short signal every few minutes, the interval between signals being a function of temperature. The temperature records obtained from these buoys show that the temperature near the lake surface can vary considerably within a few hours. Some of these variations appear to be caused by disturbances in convective heat transfer occurring in the lake. The occurrence of these short term temperature variations means that there is no simple relation between Crater Lake temperatures and the volcanic activity of Ruapehu. Some rapid increases in temperature followed volcanic earthquakes, but one of the biggest increases in temperature occurred just before a group of earthquakes upder the lake.     

The magmatic- and hydrothermal-dominated fumarolic system at the Active Crater of Lascar volcano,northern Chile

Low-to-high temperature fumaroles discharging from the Active Crater of Lascar volcano (northern Chile) have been collected in November 2002, May 2005 and October 2006 for chemical and isotopic analysis to provide the first geochemical survey on the magmatic-hydrothermal system of this active volcano. Chemical and isotopic gas composition shows direct addition of high-temperature fluids from magmatic degassing, mainly testified by the very high contents of SO₂, HCl and HF (up to 87,800, 29,500 and 2,900 μmol/mol) and the high R/Ra values (up to 7.29). Contributions from a hydrothermal source, mainly in gas discharges of the Active Crater rim, has also been detected. Significant variations in fluid chemistry, mainly consisting of a general decrease of magmatic-related compounds, i.e. SO₂, have affected the fumarolic system during the period of observation, indicating an increase of the influence of the hydrothermal system surrounding the ascending deep fluids. The chemical composition of Active Crater fumaroles has been used to build up a geochemical model describing the main processes that regulate the fluid circulation system of Lascar volcano to be utilized in volcanic surveillance.     

Chemical characteristics of the crater lakes of Popocatetetl, El Chichon, and Nevado de Toluca volcanoes, Mexico

Three crater lakes from Mexican volcanoes were sampled and analyzed at various dates to determine their chemical characteristics. Strong differences were observed in the chemistry among the three lakes: Nevado de Toluca, considered as dormant, El Chichón at a post-eruptive stage, and Popocatépetl at a pre-eruptive stage. Not surprisingly, no influence of volcanic activity was found at the Nevado de Toluca volcano, while the other volcanoes showed a correlation between the changing level of activity and the evolution of chemical trends. Low pHs (<3.0) were measured in the water from the active volcanoes, while a pH of 5.6 was measured at the Nevado de Toluca Sun lake. Changes with time were observed at Popocatépetl and El Chichón. Concentrations of volcanic-gas derived species like Cl⁻, SO₄²⁻ and F⁻ decreased irregularly at El Chichón from 1983 until 1997. Major cations concentrations also diminished at El Chichón. A 100% increase in the SO₄²⁻ content was measured at Popocatépetl between 1985 and 1994. An increase in the Mg/Cl ratio between 1992 (Mg/Cl=0.085) and 1994 (Mg/Cl=0.177) was observed at Popocatépetl, before the disappearance of the crater lake in 1994. It is concluded that chemical analysis of crater lakes may provide a useful additional tool for active-volcano monitoring.     

Eruptive history of the Piton de la Fournaise volcano, Reunion Island, Indian Ocean

In order to establish a general chronology of the volcanic evolution and to determine the temporal succession of the structural units, potassium-argon measurements were made on 15 samples selected as a function of their stratigraphical position on Piton de la Fournaise volcano.The rocks of Réunion Island are essentially oceanic and basaltic lavas of two shield volcanoes: the central, now extinct Piton des Neiges and the more recent, still active, Piton de la Fournaise. Piton de la Fournaise volcano is generally thought to have been developed unconformably on the southeastern flank of the Piton des Neiges volcano. Previous studies have shown four successive phases and three calderas in the construction of Piton de la Fournaise.The subaerial basaltic shield-building lavas of Piton de la Fournaise appear to be older than previously thought: at least 530,000 y. old instead of 360,000 years. In terms of their duration and erupted volumes, the four successive phases are not equivalent. The duration of the first two phases is 240,000 years (from 530,000 to 290,000 y. B.P.) and 155,000 years (from about 220,000 to 65,000 y. B.P.). The duration of the third phase is less than 60,000 years and the fourth phase may actually be an episode of the third. The two volcanoes, Piton des Neiges and Piton de la Fournaise, were active simultaneously for at least 500,000 years. The evolution of Réunion Island appears to be consistent with activity along a developing rift. The evolution of Piton de la Fournaise is mainly linked with the structural development of the shield and to large-scale slumpings due to instability of the slope.     

Detailed gravity study of the offshore structure of Piton de la Fournaise volcano,Reunion Island

We present an interpretation of gravity data acquired in 1984 by the French R/V Jean Charcot on the submarine part of the eastern flank of Piton de la Fournaise volcano. We comment on the Bouguer anomaly map and give a quantitative interpretation of three gravity profiles. The main results are that a gravity high over Grand Brûlé, the lower subaerial part of the eastern flank, does not extend far offshore and that an anomalous topographic feature, discovered in 1982 on the submarine eastern flank, is characterized by a large negative anomaly. We propose three hypotheses to explain the origin of this anomaly, i. e., it marks the site of a new volcano, or it is a consequence of lateral volcanism from a volcano older than Piton de la Fournaise, or more probably, it represents a great landslide deposit.     

Sr isotope diversity of hot spring and volcanic lake waters from Zao volcano,Japan

The ratio of ⁸⁷Sr/⁸⁶Sr was measured from different water samples of thermal/mineral (hot spring as well as crater lake) and meteoric origins, in order to specify the location and to verify the detailed model of a volcano-hydrothermal system beneath Zao volcano. The ratio showed a trimodal distribution for the case of thermal/mineral water: 0.7052–0.7053 (Type A, Zao hot spring), 0.7039–0.7043 (Type B, Okama crater lake and Shin-funkiko hot spring), and 0.7070–0.7073 (Type C, Gaga, Aone, and Togatta hot springs), respectively. However, in comparison, the ratio was found to be higher for meteoric waters (0.7077–0.7079). The water from the central volcanic edifice (Type B) was found to be similar to that of nearby volcanic rocks in their Sr isotopic ratio. This indicates that the Sr in water was derived from shallow volcanic rocks. The ⁸⁷Sr/⁸⁶Sr ratio for water from the Zao hot spring (Type A) was intermediate between those of the pre-Tertiary granitic and the Quaternary volcanic rocks, thus suggesting that the water had reacted with both volcanic and granitic rocks. The location of the vapor–liquid separation was determined as the boundary of the pre-Tertiary granitic and the Quaternary volcanic rocks by comparing the results of this strontium isotopic study with those of Kiyosu and Kurahashi [Kiyosu, Y., Kurahashi, M., 1984. Isotopic geochemistry of acid thermal waters and volcanic gases from Zao volcano in Japan. J. Volcanol. Geotherm. Res. 21, 313–331.].     

Isotopic geochemistry of acid thermal waters and volcanic gases from Zaō volcano in Japan

The chemical composition and D/H, and ratios have been determined for the acid hot waters and volcanic gases discharging from Zaō volcano in Japan. The thermal springs in Zaō volcano issue acid sulfate-chloride type waters (Zaō) and acid sulfate type waters (Kamoshika). Gases emitted at Kamoshika fumaroles are rich in CO₂, SO₂ and N₂, exclusive of H₂O. Chloride concentrations and oxygen isotope data indicate that the Zaō thermal waters issue a fluid mixture from an acid thermal reservoir and meteoric waters from shallow aquifers. The waters in the Zaō volcanic system have slight isotopic shifts from the respective local meteoric values. The isotopic evidence indicates that most of the water in the system is meteoric in origin. Sulfates in Zaō acid sulfate-chloride waters with δ³⁴S values of around +15‰, are enriched in ³⁴S compared to Zaō H₂S, while the acid sulfate waters at Kamoshika contain supergene light sulfate (δ³⁴S = + 4‰) derived from volcanic sulfur dioxide from the volcanic exhalations. The sulfur species in Zaō acid waters are lighter in δ³⁴S than those of other volcanic areas, reflecting the difference in total pressure.     

The chemical and hydrologic structure of Poa´s Volcano, Costa Rica

Comparison of the chemical characteristics of spring and river water draining the flanks of Poa´s Volcano, Costa Rica indicates that acid chloride sulfate springs of the northwestern flank of the volcano are derived by leakage and mixing of acid brines formed in the summit hydrothermal system with dilute flank groundwater. Acid chloride sulfate waters of the Rio Agrio drainage basin on the northwestern flank are the only waters on Poa´s that are affected by leakage of acid brines from the summit hydrothermal system. Acid sulfate waters found on the northwestern flank are produced by the interaction of surface and shallow groundwater with dry and wet acid deposition of SO₂ and H₂SO₄ aerosols, respectively. The acid deposition is caused by a plume of acid gases that is released by a shallow magma body located beneath the active crater of Poa´s.No evidence for a deep reservoir of neutral pH sodium chloride brine is found at Poa´s. The lack of discharge of sodium chloride waters at Poa´s is attributed to two factors: (1) the presence of a relatively volatile-rich magma body degassing at shallow depths (< 1 km) into a high level summit groundwater system; and (2) the hydrologic structure of the volcano in which high rates of recharge combine with rapid lateral flow of shallow groundwater to prevent deep-seated sodium chloride fluids from ascending to the surface. The shallow depth of the volatile-rich magma results in the degassing of large quantities of SO₂ and HCl. These gases are readily hydrolyzed and quickly mix with meteoric water to form a reservoir of acid chloride-sulfate brine in the summit hydrothermal system. High recharge rates and steep hydraulic gradients associated with elevated topographic features of the summit region promote lateral flow of acid brines generated in the summit hydrothermal system. However, the same high recharge rates and steep hydraulic gradients prevent lateral flow of deep-seated fluids, thereby masking the presence of any sodium chloride brines that may exist in deeper parts of the volcanic edifice.Structural, stratigraphic, and topographic features of Poa´s Volcano are critical in restricting flow of acid brines to the northwestern flank of the volcano. A permeable lava-lahar sequence that outcrops in the Rio Agrio drainage basin forms a hydraulic conduit between the crater lake and acid chloride sulfate springs. Spring water residence times are estimated from tritium data and indicate that flow of acid brines from the active crater to the Rio Agrio source springs is relatively rapid (3 to 17 years). Hydraulic conductivity values of the lava-lahar sequence calculated from residence time estimates range from 10⁻⁵ to 10⁻⁷ m/s. These values are consistent with hydraulic conductivity values determined by aquifer tests of fractured and porous lava/pyroclastic sequences at the base of the northwestern flank of the volcano.Fluxes of dissolved rock-forming elements in Rio Agrio indicate that approximately 4300 and 1650 m³ of rock are removed annually from the northwest flank aquifer and the active crater hydrothermal system, respectively. Over the lifetime of the hydrothermal system (100's to 1000's of years), significant increases in aquifer porosity and permeability should occur, in marked contrast to the reduction in permeability that often accompanies hydrothermal alteration in less acidic systems. Average fluxes of fluoride, chloride and sulfur calculated from discharge and compositional data collected in the Rio Agrio drainage basin over the period 1988–1990 are approximately 2, 38 and 30 metric tons/day. These fluxes should be representative of minimum volatile release rates at Poa´s in the last 10 to 20 years.     

Volcanic hazard assessment for Ruapehu composite volcano,taupo volcanic zone,New Zealand

Ruapehu is a very active andesitic composite volcano which has erupted five times in the past 10 years. Historical events have included phreatomagmatic eruptions through a hot crater lake and two dome-building episodes. Ski-field facilities, road and rail bridges, alpine huts and portions of a major hydroelectrical power scheme have been damaged or destroyed by these eruptions. Destruction of a rail bridge by a lahar in 1953 caused the loss of 151 lives. Other potential hazards, with Holocene analogues, include Strombolian and sub-Plinian explosive eruptions, lava extrusion from summit or flank vents and collapse of portions of the volcano. The greatest hazards would result from renewed phreatomagmatic activity in Crater Lake or collapse of its weak southeastern wall. Three types of hazard zones can be defined for the phreatomagmatic events: inner zones of extreme risk from ballistic blocks and surges, outer zones of disruption to services from fall deposits and zones of risk from lahars, which consist of tongues down major river valleys. Ruapehu is prone to destructive lahars because of the presence of 10⁷ m³ of hot acid water in Crater Lake and because of the surrounding summit glaciers and ice fields. The greatest risks at Ruapehu are to thousands of skiers on the ski field which crosses a northern lahar path. Three early warning schemes have been established to deal with the lahar problems. Collapse of the southeastern confining wall would release much of the lake into an eastern lahar path causing widespread damage. This is a long-term risk which could only be mitigated by drainage of the lake.     

Structural origin of Lake Singkarak in central Sumatra

Since long it has been accepted that Lake Singkarak in Central Sumatra is nothing else but a remnant of a gigantic volcano — the Singkarak volcano — which once blew off its top to form a lake. Van Bemmelen, though not referring to Singkarak lake especially, explained the numerous depressions in Sumatra as the combined result of volcanic and tectonic activities, a phenomenon he called volcano-tectonic process which caused the formation of the socalled volcano-tectonic depressions. A short visit to the lake area in the months of February and March (1970) convinced the author that Lake Singkarak is neither a volcanic ruin nor a volcano-tectonic depression in the sense of van Bemmelen. Faulting evidences, morphology and the position of the Singkarak trough plus the distribution of volcanic products north and south of the lake led to the conclusion that the Singkarak trough is a depression making part of the 1650 km graben zone which stretches from Sumatra’s northern tip untill the Semangko valley in the SE. Field evidences suggest that the lake results from a damming process by volcanic material produced by the Marapi-Singgalang-Tandikat volcanoes in the north and by the products from the Talang volcano in the south.     

Polygenic eruptions on Alba Patera,Mars

A combination of photogeologic mapping, analysis of Viking Orbiter thermal inertia data, and numerical modelling of eruption conditions has permitted us to construct a new model for the evolution of the martian volcano Alba Patera. Numerous digitate channel networks on the flanks of the volcano are interpreted to be carved by sapping due to the release of non-juvenile water from unconsolidated flank deposits. Using the thermal inertia measurements, we estimate the particle size of these deposits to be 3–10 µm, which, together with theoretical modelling of the disperison of explosively derived volcanic materials, leads us to conclude that the flank deposits on Alba Patera are low-relief pyroclastic flows. The recognition of numerous late-stage summit and sub-terminal lava flows thus makes Alba Patera a unique martian volcano that is transitional between the older pyroclastic-dominated highland paterae and the more recent effusive central-vent volcanoes such as the Tharsis Montes.     

Geology of the Side Crater of the Erebus volcano,Antarctica

The summit cone of the Erebus volcano contains two craters. The Main crater is roughly circular (∼ 500 m diameter) and contains an active persistent phonolite lava lake ∼ 200 m below the summit rim. The Side Crater is adjacent to the southwestern rim of the Main Crater. It is a smaller spoon-shaped Crater (250–350 m diameter, 50–100 m deep) and is inactive. The floor of the Side Crater is covered by snow/ice, volcanic colluvium or weakly developed volcanic soil in geothermal areas (a.k.a. warm ground). But in several places the walls of the Side Crater provide extensive vertical exposure of rock which offers an insight into the recent eruptive history of Erebus. The deposits consist of lava flows with subordinate volcanoclastic lithologies. Four lithostratigraphic units are described: SC 1 is a compound lava with complex internal flow fabrics; SC 2 consists of interbedded vitric lavas, autoclastic and pyroclastic breccias; SC 3 is a thick sequence of thin lavas with minor autoclastic breccias; SC 4 is a pyroclastic fall deposit containing large scoriaceous lava bombs in a matrix composed primarily of juvenile lapilli-sized pyroclasts. Ash-sized pyroclasts from SC 4 consist of two morphologic types, spongy and blocky, indicating a mixed strombolian-phreatomagmatic origin. All of the deposits are phonolitic and contain anorthoclase feldspar.     

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.     

Evolution structurale du volcan actif du Piton de la Fournaise,Ile de la Réunion — Océan indien occidental

This structural study shows that the Piton de la Fournaise volcano was built over four periods separated by 3 calderas. Each stage, dated by K/Ar and CI4 data, and characterized by its own stratigraphy, intrusive system and collapses, is analysed in detail. The stratigraphical study shows lithological and petrological units within some of these stages. The lavas of Piton de la Fournaise are alkaline basalts ranging in composition from picrite to hawaiite. The feeder dikes systems are radial and converging to the volcanic paleocenters of each period. However, the majority of intrusions and surface cones are concentrated along rifts named « Reunion type » because of there wideness. The uplift of magma in these rift zones causes displacement and sumpling of the unsupported seaward flank of the volcano. Collapse structures with variable diameter, formed at different phases of the volcano history. Some are compared to calderas in relation to an intermediate magma chamber, others seem to be due to the bulge and strecht of the massif. The 3 calderas of great size (8–15 km) separating each stage are related to a lower and larger magmatic chamber. This geological study of Fournaise leads us to purpose an evolutive pattern of the volcano based on paleogeographical and paleostructural reconstitutions. The first Fournaise was built over a rift trending N 120 of the old neighbouring volcano of Piton des Neiges. The activity of this rift progressively decreased all through time with the development of a curved intrusive system where most eruptions took place. As in the Hawaiian rifts, the influence of gravitational stresses is invoked to explain the migration of the intrusive zones.     

Monte Vulture Volcano (Basilicata,Italy): an analysis of morphology and volcaniclastic facies

The earliest activity of Monte Vulture, central Italy, included ignimbrites but the bulk of the volcano was built up by plinian airfall deposits. Contemporaneous remobilisation of these deposits formed an apron of lahars around the base of the main cone. The volcano was constructed on a ridge; the valley to the east and tributaries to the north and south became sediment traps for volcaniclastic materials emplaced by fluvial reworking and directly from volcanic activity. To the west the valley was swept clear by active downcutting. Instability of the west flank as a result of this erosion was probably a contributory cause of major gravitational sector collapse on the volcano's flank, terminating the main cone-building phase. The resultant scar is an amphitheatre-shaped hollow called here the Valle dei Grigi. Previous workers have attributed this feature to coalescing calderas formed by engulfment. The last volcanic phase was the production of the Monticchio calderas and associated phreatomagmatic explosions producing airfall and surge deposits. Because most of the activity at Vulture has been repeated plinian eruptions producing similar assemblages of products, detailed stratigraphy of the volcano is difficult to accomplish. To characterise Vulture in terms of its products, various facies are identified and interpreted in relation to volcanic processes, distance from vent and environmental conditions.     

Array analyses of volcanic earthquakes and tremor recorded at Las Cañadas caldera (Tenerife Island, Spain) during the 2004 seismic activation of Teide volcano

We analyze data from three seismic antennas deployed in Las Cañadas caldera (Tenerife) during May–July 2004. The period selected for the analysis (May 12–31, 2004) constitutes one of the most active seismic episodes reported in the area, except for the precursory seismicity accompanying historical eruptions. Most seismic signals recorded by the antennas were volcano-tectonic (VT) earthquakes. They usually exhibited low magnitudes, although some of them were large enough to be felt at nearby villages. A few long-period (LP) events, generally associated with the presence of volcanic fluids in the medium, were also detected. Furthermore, we detected the appearance of a continuous tremor that started on May 18 and lasted for several weeks, at least until the end of the recording period. It is the first time that volcanic tremor has been reported at Teide volcano. This tremor was a small-amplitude, narrow-band signal with central frequency in the range 1–6 Hz. It was detected at the three antennas located in Las Cañadas caldera. We applied the zero-lag cross-correlation (ZLCC) method to estimate the propagation parameters (back-azimuth and apparent slowness) of the recorded signals. For VT earthquakes, we also determined the S–P times and source locations. Our results indicate that at the beginning of the analyzed period most earthquakes clustered in a deep volume below the northwest flank of Teide volcano. The similarity of the propagation parameters obtained for LP events and these early VT earthquakes suggests that LP events might also originate within the source volume of the VT cluster. During the last two weeks of May, VT earthquakes were generally shallower, and spread all over Las Cañadas caldera. Finally, the analysis of the tremor wavefield points to the presence of multiple, low-energy sources acting simultaneously. We propose a model to explain the pattern of seismicity observed at Teide volcano. The process started in early April with a deep magma injection under the northwest flank of Teide volcano, related to a basaltic magma chamber inferred by geological and geophysical studies. The stress changes associated with the injection produced the deep VT cluster. In turn, the occurrence of earthquakes permitted an enhanced supply of fresh magmatic gases toward the surface. This gas flow induced the generation of LP events. The gases permeated the volcanic edifice, producing lubrication of pre-existing fractures and thus favoring the occurrence of VT earthquakes. On May 18, the flow front reached the shallow aquifer located under Las Cañadas caldera. The induced instability constituted the driving mechanism of the observed tremor.