A new scenario for the last magmatic eruption of La Soufrière of Guadeloupe (Lesser Antilles) in 1530 A.D. Evidence from stratigraphy radiocarbon dating and magmatic evolution of erupted products

La Soufrière of Guadeloupe is a dangerous volcano characterized over the last decade by moderate seismic and fumarolic unrest. In the last 15,000 years it has experienced phreatic and magmatic eruptions and unusually numerous flank collapse events sometimes associated with a magmatic eruption. We propose a new age of 1530 A.D. and a new eruptive scenario for the last magmatic eruption on the basis of a novel statistical analysis of radiocarbon age dates, and new field and geochemical data. This eruption is the only magmatic eruption likely to have occurred in Guadeloupe during the last 1400 years. The eruption mainly involved an andesitic magma which, in the first phase of the eruption, partially mixed with a slightly more differentiated magma stored in a small and shallow magma chamber. Ascent of magma to the surface generated a partial collapse of the hydrothermally altered edifice that increased the magma discharge and led to a sub-plinian phase with scoria fallout and column-collapse pyroclastic flows followed by near-vent pyroclastic scoria fountains. The eruption ended with growth of a lava dome. Our revised interpretation of the last magmatic eruption of La Soufrière constitutes the most likely key to a future magmatic eruption scenario for this volcano which displays strong evidence of unrest since 1992.     

Reconstruction and analysis of sub-plinian tephra dispersal during the 1530 A.D. Soufrière (Guadeloupe) eruption: Implications for scenario definition and hazards assessment

The last magmatic eruption of Soufrière of Guadeloupe dated at 1530 A.D. (Soufrière eruption) is characterized by an onset with a partial flank-collapse and emplacement of a debris-avalanche that was followed by a sub-plinian VEI 2–3 explosive short-lived eruption (Phase-1) with a column that reached a height between 9 and 12 km producing about 3.9 × 10⁶ m³ DRE (16.3 × 10⁶ m³ bulk) of juvenile products. The column recurrently collapsed generating scoriaceous pyroclastic flows in radiating valleys up to a distance of 5–6 km with a maximum interpolated bulk deposit volume of 11.7 × 10⁶ m³ (5 × 10⁶ m³ DRE). We have used HAZMAP, a numerical simple first-order model of tephra dispersal [Macedonio, G., Costa, A., Longo, A., 2005. A computer model for volcanic ash fallout and assessment of subsequent hazard. Comput. Geosci. 31, 837–845] to reconstruct to a first approximation the potential dispersal of tephra and associated tephra mass loadings generated by the sub-plinian Phase 1 of the 1530 A.D. eruption. We have tested our model on a deterministic average dry season wind profile that best-fits the available data as well as on a set of randomly selected wind profiles over a 5 year interval that allows the elaboration of probabilistic maps for the exceedance of specific tephra mass load thresholds. Results show that in the hypothesis of a future 1530 A.D. scenario, populated areas to a distance of 3–4 km west–southwest of the vent could be subjected to a static load pressure between 2 and 10 kPa in case of wet tephra, susceptible to cause variable degrees of roof damage. Our results provide volcanological input parameters for scenario and event-tree definition, for assessing volcanic risks and evaluating their impact in case of a future sub-plinian eruption which could affect up to 70 000 people in southern Basse-Terre island and the region. They also provide a framework to aid decision-making concerning land management and development. A sub-plinian eruption is the most likely magmatic scenario in case of a future eruption of this volcano which has shown, since 1992, increasing signs of low-energy seismic, thermal, and acid degassing unrest without significant deformation.     

Explosive activity and eruption scenarios at Somma-Vesuvius (Italy): Towards a new classification scheme

A new proposal for the classification of Somma-Vesuvius (SV) explosive activity is presented, based on a critical revision of a large set of published and unpublished stratigraphic, compositional, and physical volcanology data on the products of the past 20,000 years of activity. The new database is used to discuss the general behaviour of the volcano in terms of frequency, magnitude and intensity of the events, as well as of the length of the repose time which preceded each eruption. Several different types of eruption are recognized, each characterised by specific physical eruptive parameters: plinian, subplinian (further subdivided in subplinian I and subplinian II), violent strombolian, ash emission events. For each eruption type, a complex scenario is described, with phases of different style, duration, magnitude and intensity occurring during the course of the eruption itself. The name given to each eruption type is derived from the style of the most representative part of the eruption (in terms of duration or volume).     

Remote and in situ plume measurements of acid gas release from La Soufrière volcano, Guadeloupe

This paper presents the first remote measurements of La Soufrière gas emissions since the fumarolic and seismic reactivation in 1992. The chemical composition of the plumes has been measured from May 2003 to September 2004 using an Open Path Fourier Transform InfraRed (OP-FTIR) spectrometer, up to 15 m downwind the South Crater. HCl is clearly detected (concentration between 2.4 and 12 ppmv) whereas SO₂ and H₂S generally remain below the detection limit of the OP-FTIR. Direct measurements of SO₂ and H₂S near the South Crater with a Lancom III analyzer show a fast decrease of their concentrations with the distance. Calculated Cl / S mass ratios are high: from 9.4 ± 1.7 at 15 m from the vent to 2.8 ± 0.6 at 140 m. The enrichment in HCl of the gas emitted at La Soufrière, observed since 1998, corresponds to the degassing of a magma enriched in Cl and depleted in S. This result agrees with isotopic measurements which suggest a magmatic origin of the gases. Readjustments inside the volcanic system may have taken place during the seismic activity beginning in 1992 and enhance the transfer of magmatic gases to the summit.     

Evidence of submarine hydrothermal discharge to the northwest of Guadeloupe Island (Lesser Antilles island arc)

During the Soviet-French survey carried out on board of the R.V. Akademik N. Strakhov, between Guadeloupe and Montserrat island in the central Lesser Antilles, evidence of submarine hydrothermal activity was discovered on the southeastern tip of the Shoe-Rock escarpment. The latter is part of a large, 130 km long, transverse structure, the Montserrat-Marie Galante fault, which crosses the Guadeloupe archipelago. Another locus of activity, probably of subdued importance nowadays, is represented by the d'Entrecasteaux dome, a small faulted seamount, located about 25 km to the SW of the former area.The evidence for hydrothermal activity is: (a) a wide range of values of conductive heat flow (q=6–11 mW/m²) and regular vertical variation at some sites with extreme values located close to the two above-mentioned features (average regional value, q = 105 mW/m², s.d. = 32); (b) occurrence of secondary mineralizations (todorokite, nontronite, etc.) replacing locally the primary matrix of basal upper Pliocene deposits; (c) geochemical anomalies in seawater near the sea bottom, showing enrichment in mantle-derived ³He (δ³He up to 10.2%), correlated with a sharp Zn anomaly (content up to 523 ppb).Further investigations along the submerged segments of the Montserrat-Marie Galante fault should probably lead to other discoveries of hydrothermal venting and/or mineralizations.     

State of the hydrothermal activity of Soufrière of Guadeloupe volcano inferred by VLF surveys

La Soufrière (1467 m) is the active island arc volcano of Guadeloupe Island in the Lesser Antilles arc. Its historical eruptions are more or less violent phreatic outbursts the last of which, in 1976–1977, led to the evacuation of nearly 70 000 persons. The subsurface structure of the volcano consists of calderas, craters, and avalanche amphitheatres nested within the composite pile of eruptive products. Since the last magmatic eruption, dated ca. 1440 AD, the four phreatic eruptions have developed radial fractures on Soufrière dome favouring the development of a huge active hydrothermal system emphasized by a tropical environment. After the eruptions, the thermal state and the stable ground water flow are completely disorganised during several years during which the slow mineralization of rocks is becoming again preponderant. Sealing of fractures and decay of rocks permeability act as a cap for upward thermal transfers. Therefore Soufrière dome operates as a valve, resealing the hydrothermal system underlying the volcano thus providing over pressurization that could lead to the next phreatic eruption. In 1992 new small seismic swarms have appeared. Several of them are recorded every year while the emission of acid gas slowly increases.In order to recognise the superficial electrical resistive and conductive zones (less than 100 m depth) as well as the cavities on Soufrière volcano, we have made Very Low Frequency (VLF) surveys in 2000. Electrical conductive zones are clearly associated with major radial faults starting from the summit in which the hydrothermal activity takes place. In the continuation of these active hydrothermal fractures hot springs are located down slope. Conversely some of the resistive zones are associated with inactive clayed and sealed or opened faults. The distribution of the conductive zones allows detailing the state of the superficial part of the hydrothermal system of La Soufrière. The distribution of vertical clayed zones associated with major faults supposes Soufrière dome constituted of more or less consolidated blocks joined side by side and lying on the hydrothermally floor of crater Amic.     

Magnetic measurements on La Soufriere Volcano, Guadeloupe (Lesser Antilles), 1976–1984: A re-examination of the volcanomagnetic effects observed during the volcanic crisis of 1976–1977

During the seismovolcanic crisis of 1976–1977 at La Soufrière on Guadeloupe, a magnetic network of 12 reference markers was set up. Measurements of the intensity of the earth's magnetic field, carried out up to once a day at each marker, showed volcano-magnetic variations of several nanoteslas (nT). The variations, at certain markers, were more or less concealed by transient magnetic variations due to anomalies in conductivity. As early as 1978, measurements were resumed and a telemetering network was coupled with the network of reference markers, to which 4 new markers were added. A detailed study of conductivity anomalies was carried out on the entire volcano. Contrasts in conductivity linked to the existence of a superficial conducting surface, on a SSW/NNE axis, located south of the volcano, caused a great lack of homogeneity in the field variations measured at the surface. Variations greater than about 10 nT appeared in the difference in intensity of the earth's magnetic field between two stations.No long-term magnetic variation was observed between 1978 and 1984. On the network of markers, the accuracy of measurements of volcanic effects was at best 2 nT. Measurements carried out on the telemetering network during the night refined these results, since their accuracy was 1 nT. The only significant volcanic crisis between 1978 and 1984 (5–7 January 1981) seems to be observed by telemetering stations. All the measurements carried out in periods of volcanic inactivity make it possible to re-examine the crisis of 1976–1977. Though volcanomagnetic effects over short periods cannot be accurately determined, variations with a time constant of several weeks were present over the entire volcano. These variations were as high as 7–8 nT in remote stations and they can be linked to the three major phases of eruptive activity at La Soufrière during the crisis of 1976–1977.     

Andesitic magma degassing investigated through H2O vapour–melt partitioning of halogens at Soufrière Hills Volcano, Montserrat (Lesser Antilles)

Magma degassing at Soufrière Hills Volcano (SHV) is characterised by an almost permanent SO₂ flux and a HCl production rate which mainly depends on dome growth rate. Degassing processes have been studied through textural, H₂O and halogen analyses of clasts collected between 1995 and 2006 on the dome and in pyroclastic flows and vulcanian eruption deposits. Cl, Br and I are strongly depleted in melts during H₂O degassing with no significant Cl–Br–I fractionation, whereas F is almost unaffected. All magmas erupted at SHV have followed a multi-step degassing path from the magma chamber up to a shallow depth ( 1 km, P  20 MPa). From that depth, however, effusive and explosive paths are distinct; vulcanian eruptions are the result of closed system degassing (CSD), while effusive dome growth is the result of CSD up to a very shallow depth (≤ 200 m, P  5–2 MPa) followed by open system degassing (OSD). CSD is modelled using the H₂O solubility law, the perfect gas law and partition coefficients of halogens between a rhyolitic melt and H₂O vapour (d_v − lⁱ). Gas loss characteristic of OSD is modelled using a Rayleigh law. Degassing induced crystallisation is introduced through the ratio of crystallisation and degassing rates, which ranges from 150–500. d_v − l^Cl for OSD ranges between 50–300, increasing with melt Cl content. For CSD, the lower effective d_v − l^Cl ( 20) is attributed to kinetic effects.

Dome forming activity has a greater impact on atmospheric chemistry than vulcanian eruptions because OSD is much more efficient at extracting halogens. The model shows that HCl flux is a good proxy for the dome forming eruption rate. Comparison between model and measured gas compositions suggests a high HBr–BrO conversion rate (BrO/Total Br  1/3) in the SHV gas plume.

The degassing behaviour of Cl, Br and I implies similar Cl/Br ( 160) and Br/I ( 90) in initial melts, volcanic clasts and high temperature gases. The low Cl/Br at SHV compared to other island arcs ( 250–300) is attributed to a shallow, pre-eruptive Br enrichment. The almost permanent dome extrusion at SHV since 1995 has likely had a significant regional atmospheric impact because of the very efficient effusive degassing and the high conversion rate of halogens into reactive species within the gas plume.     

Numerical inversion and analysis of tephra fallout deposits from the 472 AD sub-Plinian eruption at Vesuvius (Italy) through a new best-fit procedure

A simple semi-analytical model for ash-fall deposit was applied to reconstruct the tephra deposits of the sub-Plinian 472 AD eruption of Vesuvius, Italy, which is of the scale of the reference eruptive scenario for the emergency planning, at Vesuvius. Applying a novel least-squares method, the bulk grain-size distribution, the total mass, and the eruption column height were obtained by fitting the computed ground load and granulometries with the observed ones. The analysis of the effect of three different weighting factors in the minimization procedure was also performed. Results showed that the statistical weighting factor produced the minimum bias. The best correlation between calculated and measured deposit was found, even though the quantity of the input data was not very high, as it commonly occurs for several ancient eruptions. Model results were also in agreement with estimations provided by other independent methods.     

Boundary crossing and non‐linear theory in earth‐system sciences – a proof of concept based on tsunami and post‐eruption scenarios on Java Island,Indonesia

Although volcanic eruptions are well‐known to be the trigger of some weather and climatic changes, land‐cover changes by pyroclastic‐flows and lahars do not get this recognition, neither do major hazards such as tsunami. These two earth processes are even lesser considered as being able to modify other earth processes they are not directly connected to, such as landslides or river discharge in non‐connected basins more than a hundred kilometres away. In this contribution the authors argue that these ideas are mainly driven by the process of being ‘educated’ in a single academic discipline and once put to the test interactions and retroactions between earth processes and atmospheric processes are far more reaching than commonly thought. For this study, the site of Java Island (Indonesia) was chosen to conduct (1) an analysis of a major tsunami impact – in the same area as the 2006 Java tsunami and (2) an analysis of the post‐eruption impacts of Merapi Volcano after a major eruption – excluding any ejecta in the atmosphere for the sake of the demonstration. The atmospheric feedback simulations were conducted using the regional climate model (RegCM‐4) with calibration from weather stations in Java Island. As a result, both simulations have proven that large scale deposits of pyroclasts (not introducing the ejectas sent in the atmosphere) and tsunamis can have outstanding impacts on the atmospheric situation and the bio‐geomorphologic evolution of the landscape in the following weeks to months. Interestingly enough these impacts are not limited to the area impacted by the earth process and the effect are not linear in time as they work following thresholds. These rainfalls ‘tele‐impacts’ are important enough to, in turn, modify earth‐surface processes in areas remote from the original phenomenon. This system acts in the same manner as a famous butterfly in Africa that could trigger a hurricane on the other side of the Atlantic Ocean. Copyright © 2012 John Wiley & Sons, Ltd.     

Discriminating the long distance dispersal of fine ash from sustained columns or near ground ash clouds: The example of the Pomici di Avellino eruption (Somma-Vesuvius,Italy)

Ash samples from tephra layers correlated with the Pomici di Avellino (Avellino Pumice) eruption of Somma-Vesuvius were collected in distal archives and their composition and particle morphology investigated in order to infer their behaviour of transportation and deposition. Differences in composition and particle morphologies were recognised for ash particles belonging to the magmatic Plinian and final phreatomagmatic phases of the eruption. The ash particles were dispersed in opposite directions during the two different phases of the eruption, and these directions are also different from that of coarse-grained fallout deposits. In particular, ash generated during magmatic phase and injected in the atmosphere to form a sustained column shows a prevailing SE dispersion, while ash particles generated during the final phreatomagmatic phase and carried by pyroclastic density currents show a general NW dispersion. These opposite dispersions indicate an ash dispersal influenced by both high and low atmosphere dynamics. In particular, the magmatic ash dispersal was first driven by stratospheric wind towards NE and then the falling particles encountered a variable wind field during their settling, which produced the observed preferential SE dispersal. The wind field encountered by the rising ash clouds that accompanied the pyroclastic density currents of the final phreatomagmatic phase was different with respect to that encountered by the magmatic ash, and produced a NW dispersal. These data demonstrate how ash transportation and deposition are greatly influenced by both high and low atmosphere dynamics. In particular, fine-grained particles transported in ash clouds of small-scale pyroclastic density currents may be dispersed over distances and cover areas comparable with those injected into the stratosphere by Plinian, sustained columns. This is a point not completely addressed by present day mitigation plans in case of renewal of activity at Somma-Vesuvius, and can yield important information also for other volcanoes potentially characterised by explosive activity.     

Overview of the 1990–1995 eruption at Unzen Volcano

Following 198 years of dormancy, a small phreatic eruption started at the summit of Unzen Volcano (Mt. Fugen) in November 1990. A swarm of volcano-tectonic (VT) earthquakes had begun below the western flank of the volcano a year before this eruption, and isolated tremor occurred below the summit shortly before it. The focus of VT events had migrated eastward to the summit and became shallower. Following a period of phreatic activity, phreatomagmatic eruptions began in February 1991, became larger with time, and developed into a dacite dome eruption in May 1991 that lasted approximately 4 years. The emergence of the dome followed inflation, demagnetization and a swarm of high-frequency (HF) earthquakes in the crater area. After the dome appeared, activity of the VT earthquakes and the summit HF events was replaced largely by low-frequency (LF) earthquakes. Magma was discharged nearly continuously through the period of dome growth, and the rate decreased roughly with time. The lava dome grew in an unstable form on the shoulder of Mt. Fugen, with repeating partial collapses. The growth was exogenous when the lava effusion rate was high, and endogenous when low. A total of 13 lobes grew as a result of exogenous growth. Vigorous swarms of LF earthquakes occurred just prior to each lobe extrusion. Endogenous growth was accompanied by strong deformation of the crater floor and HF and LF earthquakes. By repeated exogenous and endogenous growth, a large dome was formed over the crater. Pyroclastic flows frequently descended to the northeast, east, and southeast, and their deposits extensively covered the eastern slope and flank of Mt. Fugen. Major pyroclastic flows took place when the lava effusion rate was high. Small vulcanian explosions were limited in the initial stage of dome growth. One of them occurred following collapse of the dome. The total volume of magma erupted was 2.1×10⁸ m³ (dense-rock-equivalent); about a half of this volume remained as a lava dome at the summit (1.2 km long, 0.8 km wide and 230–540 m high). The eruption finished with extrusion of a spine at the endogenous dome top. Several monitoring results convinced us that the eruption had come to an end: the minimal levels of both seismicity and rockfalls, no discharge of magma, the minimal SO₂ flux, and cessation of subsidence of the western flank of the volcano. The dome started slow deformation and cooling after the halt of magma effusion in February 1995.     

Swelling of a lava plug associated with a Vulcanian eruption at Sakurajima Volcano,Japan, as revealed by infrasound record: case study of the eruption on January 2, 2007

In order to clarify the time relation of the expansion of a gas pocket and failure of its overlying plug of lava during Vulcanian eruptions, infrasound records and video images of the Vulcanian eruption that occurred at Sakurajima volcano on January 2, 2007 were analyzed with respect to their origin times. Weak (≤3 Pa) and slowly increasing air pressure preceded the impulsive compression phase by 0.25–0.32 s, and a longer-period rarefaction phase of infrasound waves was recognized at all microphone stations. The velocity of the compression phase was assumed to be supersonic (ca. 400 m/s) up to 850 m above the crater bottom from other recent explosions. On the other hand, the propagation velocity of the preceding weak signal was regarded to be similar to the air sound velocity because the lack of impulsiveness is unlikely to be related to the main compression phase. Therefore, the estimated origin time of the main compression phase was delayed by 0.5–0.7 s from the preceding phase. The origin time of the preceding phase coincided with the onset of the isotropic expansion process of the pressurized gas pocket, which was obtained by the waveform inversion of the explosion earthquake. In contrast, the origin time of the main impulsive phase coincided with the time when the expansion rate reached its peak. This observation suggests that the volumetric increase of the gas pocket caused swelling of the surface of the crater bottom and its subsequent failure. When the expansion velocity exceeded a threshold level, the main impulsive compression phase radiated with a high velocity by the sudden releases of the pressurized gases. The volumetric change at the source was estimated to be 280–560 m³ from the preceding phase of the infrasound. This volume change indicates that the vertical displacement of the swelling ground was on the order of 1.0 m, assuming the radius of the lava plug was ca. 10 m.     

Airborne measurements of particle and gas emissions from the December 1994–January 1995 eruption of Popocatépetl volcano (Mexico)

Airborne and ground-based (correlation spectrometer, cascade impactor, and photoelectric counter together with intake filter probes) measurements are described for the volcanic emissions from Popocatépetl volcano (Mexico) from December 23, 1994 to January 28, 1995. Measurements of SO₂ restarted 48 h after the eruption onset of December 21, 1994. Maximum sulfur dioxide (4560 t d⁻¹) plus 3.8×10⁴ t d⁻¹ of particulate matter were ejected on December 24, 1994. The maximum rate of ejection occurred coincidentally with the maximum amplitude of harmonic tremor and the maximum number of seismic type B events. Sulfur dioxide emission rates ranged from 1790 to 2070 t d⁻¹ (December 23–24, 1994). Afterwards, sulfur dioxide emission rates clearly indicated a consistent decline. However, frequent gas and ash emission puffs exhibited SO₂ fluxes reaching values as high as 3060 t d⁻¹. The emission SO₂ baseline for the period of study (February 1994–January 1995) was about 1000 t d⁻¹. Ejection velocity of particulate matter was approximately 270 m s⁻¹ reaching a height of about 2.5 km over the summit. The immediate aerosol dispersion area was estimated at 6.0×10⁴ km² maximum. The microscopic structure of particles (aerosol and tephra) showed a fragile material, probably coming from weathered crustal layers. X-ray fluorescence and neutron-activation analysis from the impactor samples found the following elements: Si, Al, Ca, S, P, Cl, K, Ni, Fe, Ti, Sc, Cu, Zn, Mn, Sr, Cr, Co, Y, Br, Se, Ga, Rb, Hg and Pb. Morphological analysis shows that ash samples might be from pulverized basaltic rock indicating that the Popocatépetl eruption of December 21, 1994 was at low temperature. The microscopic structure of puff material showed substance aggregates consisted of fragile rock, water and adsorbed SO₂. These aggregates were observed within water droplets of approximately 1 mm and even larger. Sulfur transformations in the droplets occurred intensively. Volcanic ash contained 5–6% of sulfur during the first expulsion hours. Elemental relative concentrations with respect to Al show that both Si and S have relative concentrations >1, i.e., 13.73 and 2.17, respectively in agreement with the photoelectric counter and COSPEC measurements.     

Effusive history of the Grande Découverte Volcanic Complex, southern Basse-Terre (Guadeloupe, French West Indies) from new K–Ar Cassignol–Gillot ages

The Grande Découverte Volcanic Complex (GDVC), active since at least 0.2 Ma, is the most recent volcanic complex of the Basse-Terre Island (Guadeloupe, Lesser Antilles Arc). A detailed geochronological study using the K–Ar Cassignol–Gillot technique has been undertaken in order to reconstruct the history of effusive activity of this long-lived volcanic system. Twenty new ages permit to suggest that the GDVC experienced at least six main effusive stages, from 200 ka to present time. To the north of the GDVC, the GDS (Grande Découverte–Soufrière volcano) has been active since at least 200 ka, and to the south, the TRMF (Trois-Rivières–Madeleine Field), started to be emplaced 100 ka. Morphological investigations suggest that the whole TRMF volcanism was emitted from vents distinct from the GDS, most probably a large E–W fissure network linked to the Marie-Galante rift. The mean age of 62 ± 5 ka, obtained for the E–W Madeleine–Le Palmiste alignment suggests that a fissure-opening event occurred at that time. However, whole-rock major and trace element signatures are similar for both systems, suggesting that a common complex magma-plumbing system has fed the overall GDVC. We report very young ages for lava flows from the TRMF, which implies that < 10 ka volcanic activity is now identified for both massifs. Although hazards associated with such effusive volcanism are much lower than those associated with potential flank-collapse of the Soufrière lava dome or a magmatic dome eruption with explosive phases within the GDS, the emplacement of relatively large Holocene age lava flows (3–1 × 10⁸ m³) suggests that a revised integrated volcanic hazard assessment for Southern Basse-Terre should now consider the potential for renewed future activity from two Holocene volcanic centers including the TRMF.     

Textures, water content and degassing of silicic andesites from recent plinian and dome-forming eruptions at Mount Pelée volcano (Martinique, Lesser Antilles arc)

Previous petrological and phase-equilibrium experimental studies on recent silicic andesites from Mount Pelée volcano have evidenced comparable pre-eruptive conditions for plinian and dome-forming (pelean herein) eruptions, implying that differences in eruptive style must be primarily controlled by differences in degassing behaviour of the Mount Pelée magmas during eruption. To further investigate the degassing conditions of plinian and pelean magmas of Mount Pelée, we study here the most recent Mount Pelée's products (P1 at 650 years B.P., 1902, and 1929 eruptions, which cover a range of plinian and pelean lithologies) for bulk-rock vesicularities, glass water contents (glass inclusions in phenocrysts and matrix glasses) and microtextures. Water contents of glass inclusions are scattered in the plinian pumices but on average compare with the experimentally-deduced pre-eruptive melt water content (i.e., 5.3–6.3 wt.%), whereas they are much lower in the dominant pelean lithologies (crystalline, poorly vesicular lithics and dome samples). This indicates that the glass inclusions of the pelean products have undergone strong leakage and do not represent pre-eruptive water contents. The water content of the pyroclast matrix glasses are thought to closely represent the residual water content in the melt at the time of fragmentation. Determination of the water contents of both the pre-eruptive melt and matrix glasses allows the estimation of the amount of water exsolved upon syn-eruptive degassing. We find the amount of water exsolved during the eruptive process to be higher in the pelean products than in the plinian ones, typically 90–100 and 65–70% of the initial water content, respectively. The vesicularities calculated from the amount of exsolved water compare with the measured vesicularities for the plinian pumices, consistent with a closed-system, near-equilibrium degassing up to fragmentation. By contrast, the low residual water contents, low groundmass vesicularities and extensive groundmass crystallization of the pelean products are direct evidence of open-system degassing. Microtextural features, including silica-bearing and silica-free voids in the pelean lithologies may represent a two-stage vesiculation.     

Evolution of a magma-driven earthquake swarm and triggering of the nearby Oldoinyo Lengai eruption, as resolved by InSAR, ground observations and elastic modeling, East African Rift, 2007

An earthquake swarm struck the North Tanzania Divergence, East African Rift over a 2 month period between July and September 2007. It produced approximately 70 M > 4 earthquakes (peak magnitude Mw 5.9), and extensive surface deformation, concurrent with eruptions at the nearby Oldoinyo Lengai volcano. The spatial and temporal evolution of the entire deformation event was resolved by Interferometric Synthetic Aperture Radar (InSAR) observations, owing to a particularly favorable acquisition programming of the Envisat and ALOS satellites, and was verified by detailed ground observations. Elastic modeling based on the InSAR measurements clearly distinguishes between normal faulting, which dominated during the first week of the event, and intermittent episodes of dike propagation, oblique dike opening and dike-induced faulting during the following month. A gradual decline in the intensity of deformation occurred over the final weeks. Our observations and modeling suggest that the sequence of events was initiated by pressurization of a deep-seated magma chamber below Oldoinyo Lengai which opened the way to lateral dike injection, and dike-induced faulting and seismicity. As dike intrusion terminated, silicate magma ascended the volcano conduit, reacted with the carbonatitic magma, and set off a major episode of explosive ash eruptions producing mixed silicate-carbonatitic ejecta. The rise of the silicate magma within the volcano conduit is attributed to bubble growth and buoyancy increase in the magma chamber either due to a temporary pressure drop after the termination of the diking event, or due to the dynamic effects of seismic wave passage from the earthquake swarm. Similar temporal associations between earthquake swarms and major explosive ash eruptions were observed at Oldoinyo Lengai over the past half century.     

Crystal-rich mass flow deposits related to the eruption of a sublacustrine silicic cryptodome (Early Permian Collio Basin, Italian Alps)

Sedimentological, granulometric and petrographic data are presented from a detailed study on a crystal-rich mass flow deposit, which is presumably related to the eruption of a sublacustrine cryptodome. The deposit forms a prominent intercalation in the Lower Permian Collio Formation in the Italian Alps north of Brescia. Outcrops of the 10–20-m-thick volcaniclastic deposit (Dasdana I Beds, DB) can be traced over 12 km from east to west. The DB consists of a thick, crystal-rich, sandy–gravelly lower subunit representing a sequence of amalgamated Bouma-a(b) divisions overlain by a thin, well-bedded, sandy–muddy subunit that is rich in outsize porphyritic silicic fragments. Modal and computer-aided image analyses reveal that the crystal-rich lower subunit contains up to 80% of volcanogenic crystals. Some samples contain up to 60% of porphyritic fragments, which have a phenocryst content of about 20%. The wide textural range from cryptocrystalline, poikilomosaic, to rarer medium-grained granophyric groundmass, the irregular to lensoid shapes of the porphyritic fragments, and the presence of basement and sedimentary clasts suggest that the DB originated from a sublacustrine eruption of a partially extrusive cryptodome (ca. 1.6 km³). Two other porphyritic felsic cryptodomes (Dosso dei Lupi, Dosso del Bue), described briefly here, emplaced into the Collio Formation sometime after the DB event, and expose flat bases and tilted sediments at their sides. Textures observed in these domes are comparable to those found in the DB porphyritic fragments.     

Upwards migration of seismic focii: A forerunner of the 1989 eruption of Mt Etna (Italy)

Data from a portable array of three-component digital stations, run at Mt Etna from 1988 to early 1990, highlight the seismic behaviour of the volcano before the 1989 eruption, one of the most significant in terms of energy of the last two decades. After a two-year period of weak and discotinuous seismicity, the depth of the seismically active volumes was observed to become shallower a few months before the volcanic event. The overall migration of the events, inferred by hypocentral locations and decreases of S-P time differences at two stations, agrees with other geophysical forerunners and allows further insights into the changes in the stress field leading to the eruption.     

Probabilistic hazard analysis of Citlaltépetl (Pico de Orizaba) Volcano, eastern Mexican Volcanic Belt

Citlaltépetl or Pico de Orizaba is the highest active volcano in the North American continent. Although Citlaltépetl is at present in repose, its eruptive history reveals repetitive explosive eruptions in the past. Its relatively low eruption rate has favored significant population growth in areas that may be affected by a potential eruptive activity. The need of some criteria for hazards assessment and land-use planning has motivated the use of statistical methods to estimate the time and space distribution of volcanic hazards around this volcano. The analysis of past activity, from late Pleistocene to historic times, and the extent of some well-identified deposits are used to calculate the recurrence probabilities of eruptions of various size during time periods useful for land-use planning.