ALBERT

Description: SO2 fluxes emitted by Stromboli during the 27th February–2nd April 2007 effusive eruption were regularly measured both by an automatic network of scanning ultraviolet spectrometers and by traverse measurements conducted by boat and helicopter. The results from both methodologies agree reasonably well, providing a validation for the automatic flux calculations produced by the network. Approximately 22,000 t of SO2 were degassed during the course of the 35 day eruption at an average rate of 620 t per day. Such a degassing rate is much higher than that normally observed (150–200 t/d), because the cross-sectional area occupied by ascending degassed magma is much greater than normal during the effusion, as descending, degassed magma that would normally occupy a large volume of the conduit is absent. We propose that the hydrostatically controlled magma level within Stromboli's conduit is the main control on eruptive activity, and that a high effusion rate led to the depressurisation of an intermediate magma reservoir, creating a decrease in the magma level until it dropped beneath the eruptive fissure, causing the rapid end of the eruption. A significant decrease in SO2 flux was observed prior to a paroxysm on 15th March 2007, suggesting that choking of the gas flowing in the conduit may have induced a coalescence event, and consequent rapid ascent of gas and magma that produced the explosion.

Description: Published

Description: 214-220

Description: 1.2. TTC - Sorveglianza geochimica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: We performed measurements using an SO2 imaging camera of the SO2 gas mass emitted during five discrete explosive events on Stromboli volcano on 3 October 2006. The SO2 gas mass released during discrete explosions was 15–40 kg per explosion, producing 3–8% of the total daily SO2 gas emission, demonstrating that in terms of gas flux Strombolian explosions are a second-order phenomenon compared with quiescent degassing. Using the typical gas composition measured with OP-FTIR allows us to determine the total gas mass released during an explosion as 360–960 kg with a volume of 1500–4100 m3 at 1 bar. At the probable source pressure of gas slug formation of 75 MPa this gas amount would occupy a volume equivalent to a sphere with a radius of 0.8–1 m, comparable with estimates of Stromboli's conduit geometry.

Description: Published

Description: 395-400

Description: 1.2. TTC - Sorveglianza geochimica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: We describe the mineralogy, geochemistry, and mesomicrostructure of fresh subvolcanic blocks erupted during the 5 April 2003 paroxysm of Stromboli (Aeolian Islands, Italy). These blocks represent ∼50 vol.% of the total erupted ejecta and consist of fine- to medium-grained basaltic lithotypes ranging from relatively homogeneous dolerites to strongly or poorly welded magmatic breccias. The breccia components are represented by angular fragments of dolerites entrapped in a matrix of vesiculated (lava-like to scoriae) crystal-rich (CR) basalt. All of the studied blocks are cognates with the CR basalt of the normal Strombolian activity or lavas and they are often coated by a few-centimeter thick layer of crystal-poor (CP) basaltic pumice erupted during the paroxysm. We suggest that they result from the rapid increase of pressure and related subvolcanic rock failure that occurred shortly before the 5 April 2003 explosion, when the uppermost portion of the edifice inflated and suffered brecciation as the result of the sudden rise of the gas-rich CP basalt that triggered the eruption. Dolerites and magmatic matrix of the breccias show major and trace element compositions that match those of the CR basalts erupted during normal Strombolian activity and effusive events at Stromboli volcano. Dolerites consist of (a) phenocrysts normally found in the CR basalts and (b) late-stage magmatic minerals such as sanidine, An60-28 plagioclase, Fe–Mn-rich olivines (Fo68-48), phlogopite, apatite, and opaque mineral pairs (magnetite and ilmenite), most of which are never found both in lava flows and scoriae erupted during the persistent explosive activity that characterizes typical Strombolian behavior. Subvolcanic crystallization of the Stromboli CR magma, leading to slowly cooled equivalents of basalts, could result from transient drainage of the magma from the summit craters to lower levels. Fingering and engulfing of the material that collapsed from the summit crater floor into the shallow basaltic system during the late evening of 28 December 2002 coupled with the short break in the summit persistent explosions between December 2002 and March 2003 permitted the CR magma pockets to solidify as dolerites, which were confined to the uppermost portion of the system and thus not involved in the ongoing flank effusive activity. Crystal size distribution of the basaltic blocks and crystallization of the finer-grained (〈0.1 mm) mafic minerals of the dolerites over a time interval of ∼100 days closely agrees with the above interpretation. Vesicle filling (miarolitic cavities) locally found in some dolerites, with minerals deposited as vapor-phase crystallization is a result of continuous gas percolation through the rocks of the uppermost portion of the volcanic system. Poorly welded magmatic breccias formed during syn-eruptive processes of 5 April 2003, when the paroxysm strongly shattered the shallow subvolcanic system and many dolerite fragments were entrapped in the CR magma. In contrast, the high degree of welding between the dolerite clasts and the CR basaltic matrix in the strongly welded magmatic breccias provides a snapshot of subvolcanic intrusions of the CR basalt into the dolerite when, after a 2-month break in activity, CR magmas started to rise again to the summit craters. Blocks similar to these subvolcanic ejecta of 5 April 2003 were also erupted during previous paroxysms (e.g., 1930) suggesting that changes in the usual Strombolian activity (e.g., short breaks in the persistent mild explosions and/or flank effusive activity) lead to transient crystallization of dolerites in the shallow plumbing system.

Description: Published

Description: 795-813

Description: 3.5. Geologia e storia dei vulcani ed evoluzione dei magmi

Description: JCR Journal

Description: reserved

Description: The recent eruption of Stromboli in February–April 2007 offered a unique chance to test our current understanding of processes driving the transition from ordinary (persistent Strombolian) to effusive activity, and the ability of instrumental geophysical and geochemical networks to interpret and predict these events. Here, we report on the results of two years of in-situ sensing of the CO2/SO2 ratio in Stromboli's volcanic gas plume, in the attempt to put constraints on the trigger mechanisms and dynamics of the eruption. We show that large variations of the plume CO2/SO2 ratio (range, 0.9–26) preceded the onset of the eruption (since December 2007), interrupting a period of relatively-steady and low ratios (time-averaged ratio, 4.3) lasting from at least May to November 2006. By contrasting our observations with numerical simulations of volcanic degassing at Stromboli, derived by use of an equilibrium saturation model, we suggest that the pre-eruptive increase of the ratio reflected an enhanced supply of deeply-derived CO2-rich gas bubbles to the shallowplumbing system. This larger-than-normal ascent of gas bubbles was likely sourced by a 1–3 km deep gas– melt separation region (probably a magma storage zone), and caused faster convective overturning of magmas in the shallow conduit; an increase in the explosive rate and in seismic tremor, and finally the collapse of the la Sciara del Fuoco sector triggering the effusive phase. The high CO2/SO2 ratios (up to 21) observed during the effusive phase, and particularly in the days and hours before a paroxysmal explosion on March 15, 2007, indicate the persistence of the same gas source; and suggest that de-pressurization of the same 1–3 km deep magma storage zone could have been the trigger mechanism for the paroxysm itself

Description: Published

Description: 221-230

Description: 1.2. TTC - Sorveglianza geochimica delle aree vulcaniche attive

Description: 1.5. TTC - Sorveglianza dell'attività eruttiva dei vulcani

Description: JCR Journal

Description: reserved

Description: On 30 December 2002, a 25-30 × 106 m3 landslide on the NW flank of Stromboli volcano produced a tsunami that caused relevant damage to the Stromboli village and to the neighboring islands of the Aeolian archipelago. The NW flank of Stromboli has been the site of several, cubic kilometer-scale, landslides during the past 13 ka. In this paper we present sedimentological and compositional data of deep-sea cores recovered from a site located about 24 km north of the island. Our preliminary results indicate that: (i) turbidity currents were effectively generated by the large-scale failures and (ii) volcanogenic turbidity current deposits retain clues of the landslide source and slope failure dynamics. By analogy with Hawaii and the Canary islands we confirm that deep-sea sediments can be effectively used to assess the age and scale of past landslide events giving an important contribution to the tsunami hazard assessment of this region.

Description: Unpublished

Description: -

Description: 3.5. Geologia e storia dei vulcani ed evoluzione dei magmi

Description: 4.3. TTC - Scenari di pericolosità vulcanica

Description: reserved

Description: No abstract

Description: Published

Description: 306-308

Description: 2.3. TTC - Laboratori di chimica e fisica delle rocce

Description: 3.5. Geologia e storia dei vulcani ed evoluzione dei magmi

Description: JCR Journal

Description: reserved

Description: A multidisciplinary geological and compositional investigation allowed us to reconstruct the occurrence of flank eruptions on the lower NE flank of Stromboli volcano since 15 ka. The oldest flank eruption recognised is Roisa, which occurred at ~15 ka during the Vancori period, and has transitional compositional characteristics between the Vancori and Neostromboli phases. Roisa was followed by the San Vincenzo eruption that took place at ~12 ka during the early stage of Neostromboli period. The eruptive fissure of San Vincenzo gave rise to a large scoria cone located below the village of Stromboli, and generated a lava flow, most of which lies below sea level. Most of the flank eruptions outside the barren Sciara del Fuoco occurred in a short time, between ~9 and 7 ka during the Neostromboli period, when six eruptive events produced scoria cones, spatter ramparts and lava flows. The Neostromboli products belong to a potassic series (KS), and cluster in two differently evolved groups. After an eruptive pause of ~5,000 years, the most recent flank eruption involving the NE sector of the island occurred during the Recent Stromboli period with the formation of the large, highly K calc-alkaline lava flow field, named San Bartolo. The trend of eruptive fissures since 15 ka ranges from N30°E to N55° E, and corresponds to the magma intrusions radiating from the main feeding system of the volcano.

Description: The mapping of Stromboli was supported by a grant to S. Calvari (Project V2/01, 2005–2007, funded by the Istituto Nazionale di Geofisica e Vulcanologia and by the Italian Civil Protection). This work was partly supported by INGV through a research grant financed byMIUR-FIRB to G. Norini.We wish to thank the former Director of INGV-Sezione di Catania, A. Bonaccorso, for making additional funds available for field trip and datings.

Description: Published

Description: 101-112

Description: 3.5. Geologia e storia dei vulcani ed evoluzione dei magmi

Description: JCR Journal

Description: restricted

Description: We report on the variation of the elemental (He, Ne, and Ar) and isotopic (He and Ar) compositions of olivine and clinopyroxene-hosted fluid inclusions hosted in lavas, pyroclastics, and cumulate xenoliths from the last 60 ka of the eruptive history of Stromboli volcano, Italy. Samples belong to (i) the present-day activity as represented by shoshonitic (SHO) basalts, i.e., pumices with low porphyritic (LP) content and high porphyritic (HP) scoriae; (ii) the subalkaline versus alkaline products erupted at Stromboli during its subaerial history among the extrememagmatic series, i.e., calc-alkaline (CA) and potassic (KS); and (iii) the only known ultramafic cumulates (San Bartolo wehrlite xenoliths, SBX) outcropping in the island, carried to the surface by recent (ca. 2 ka) basaltic lava. To interpret the results better, we also investigated trace elements in the clinopyroxenes of wehrlite xenoliths and the Sr and Nd isotopes of all of the products in which the 3He/4He ratio was measured. Wehrlite xenoliths are igneous cumulates crystallized at mantle depth that mostly consist of clinopyroxene and olivine crystals with limited compositional variation. The texture, mineral chemistry, pattern of trace elements and Sr–Nd-isotope ratios (in clinopyroxene) suggest that these cumulates were produced by the early crystallization of a primitive basaltic magma with CA or HKCA–SHO affinity. The gas contents measured in themafic crystals decrease fromthewehrlite xenoliths through LP pumice, CA and KS lavas and, finally, to the HP scoria. This observation is consistent with crystallization and fluid entrapment frommantle depths to progressively shallower crustal levels. The lowgas content extracted from the HP crystals did not allow measurement of their 3He/4He ratios. Most of the investigated crystals exhibit a 3He/4He ratio in the range of 4.0–4.9 Ra, with only the KS mafic minerals exhibiting lower 3He/4He values (≤3.5 Ra). The low values of He-isotope ratios relative to those of the most-uncontaminated mantle source of the Aeolian lavas (i.e., 3He/4He ~7 Ra at Alicudi) and in common volcanic arcs suggest that the Stromboli mantle wedge is more contaminated by sediments and aqueous fluids derived by the active subduction of the Ionian slab. We also hypothesize that a process of mantle He loss that occurred during the mantle metasomatism contributed to the decrease of 3He/4He. The low 3He/4He values of the KS minerals with respect to the other Stromboli magmatic series are consistent with the higher Sr- and lower Nd-isotope ratios measured in the same samples and are attributed to source heterogeneity. Finally, data for the 3He/4He ratios from mafic minerals were compared with those of currently emitted hydrothermal fluids, which are regularly sampled for volcano surveillance. The maximum 3He/4He ratio found in the hydrothermal fluids matched the maximum ratio measured in the LP fluid inclusions (i.e., 4.6 Ra) and thus corresponds to the upper limit that should be expected for surface gases during or before high-intensity eruptive events in which a deep gas component

Description: Published

Description: 39-53

Description: 5V. Sorveglianza vulcanica ed emergenze

Description: JCR Journal

Description: restricted

Description: At Stromboli volcano, Italy, continuous seismic monitoring and periodic, visual observations of volcanic activity for surveillance purposes began in the mid-1980s. Since 1985, two eruptions have occurred, one lasting from December, 1985 until April, 1986, and one in May, 1993. There have also been two small overflows, in 1990 and 1994. Since these episodes of lava effusion, the persistent Strombolian activity of the volcano has had several fluctuations during the past 15 years. Some episodes climaxed in powerful explosions. According to seismic records, these paroxysms consisted of a variable number of explosion quakes in rapid succession (i.e. from tens of seconds to a few minutes), associated with a notable increment in the amplitude of volcanic tremor. Throughout these episodes - which are called explosive sequences - lapilli, fragments of old rock, and bombs of varying dimensions were ejected, affecting an area greater than the crater terrace where the active craters are located. In this article, we describe the explosive sequences recorded at Stromboli between 1985 and 1999. We provide a characterization in terms of reduced displacement and duration for nine episodes occurring in 1998 and 1999. Their reduced displacements range from 15 to 124 cm2; their durations are between 6 and 18 min. We find no change in the frequency content of the seismic signal several minutes before and during the sequences. Considering medium- to long-term behavior, the spectral amplitude of the seismic signal decreases or has low values over several months preceding the occurrence of the paroxysms. This feature is common to 20 of the 22 explosive sequences, and is indicative of internal conditions that periodically characterize the feeder. We surmise that the paroxysms are the result of the partial obstruction of the volcanic conduit when the magma column is low or dropping. The onset of the explosive sequence, causing the sudden removal of the material which forms the obstruction, would trigger a sudden depressurization of the conduit and the rapid rise of magma from depth.

Description: Published

Description: 137-150

Description: JCR Journal

Description: reserved

Description: Crustal tectonic seismicity in the Southern Tyrrhenian Sea is characterized by the high occurrence rates of earthquakes to the west of the alignment of Salina, Lipari and Vulcano islands in the Aeolian archipelago. Only a few earthquakes affect the crustal region east of these islands, whereas intermediate and deep seismicity plays a relevant role. Based on this evidence, two aspects of the seismic swarm recorded at the Aeolian Island Seismic Network between June 6 and 17, 1999 looked anomalous. The first aspect concerned the number of earthquakes (78) that affected the Stromboli submarine edifice in a short time interval. Secondly, despite the low maximum magnitude Md 3.2 reached, the cumulative strain release was conspicuous in comparison with previous swarms in this region. We localized the swarm about 6 km northeast of Stromboli, at a depth between 8 and 12km. The source region was identified using standard methods of hypocentral location, as well as azimuth analysis. It is worth noting that the volcanic activity at Stromboli did not change significantly during the swarm nor throughout the following months. Therefore, the seismic swarm had no link with volcanic activity observed at the surface. Most of the earthquakes shared similar waveform and frequency content, and can be divided into families. We identified some earthquakes - with magnitude up to Md 3 - having relatively low frequency content at different seismic stations. This anomalous feature leads us to hypothesize the presence of fluid circulation and/or propagation of seismic waves in a ductile medium. Our hypothesis is in agreement with studies on marine geology, which highlight various forms of submarine volcanism in the southern basin of the Tyrrhenian Sea.

Description: This work was financially supported by the EC project MULTIMO (Contract No. EVG1-CT-2000-00021).

Description: Published

Description: 121-136

Description: JCR Journal

Description: reserved