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On October 26, 2002, Etna volcano, Sicily (southern Italy), entered a new eruptive phase, characterised by the usual effusive activity from fissure vents located both on the southern and on the northern sector of the summit area. In addition, explosive events of unusual intensity were recorded at the beginning of the eruption and produced a substantial plume rich in fine ash particles and steam. On October 27, fine ash from the plume, blown by the wind, was reported falling on the island of Cefalonia, Greece, which is located approximately 500 km east of the volcano. In this communication, we report on the study of fine ash from the eruption as collected on Cefalonia. We elaborate on the nature of the peculiar type of explosion mechanism that led to such an unusual manifestation of Etna, which is phreatomagmatism. This finding draws a new picture of the eruptive scenario of this active volcano, and requires a careful evaluation of the hazard that similar events might pose in the future. Not just the surroundings of the volcano but also the aviation corridors of the southern Mediterranean area could be potentially affected. [Copyright &y& Elsevier]

Abstract: Large silicic explosive eruptions are the most catastrophic volcanic events. Yet, the intratelluric mechanisms underlying are not fully understood. Here we report a field and laboratory study of the Kos Plateau Tuff (KPT, 161 ka, Aegean Volcanic Arc), which provides an excellent geological example of conduit processes that control magma vesiculation and fragmentation during intermediate- to large-scale caldera-forming eruptions. A prominent feature of the KPT is the occurrence of quite unusual platy-shaped tube pumice clasts in pyroclastic fall and current deposits from the early eruption phases preceding caldera collapse. On macroscopic and SEM observations, flat clast faces are elongated parallel to tube vesicles, while transverse surfaces often occur at ~45° to vesicle elongation. This peculiar pumice texture provides evidence of high shear stresses related to strong velocity gradients normal to conduit walls, which induced vesiculation and fragmentation of the ascending magma. Either an increasing mass discharge rate without adequate enlargement of a narrow central feeder conduit or a developing fissure-like feeder system related to incipient caldera collapse provided suitable conditions for the generation of plate tube pumice within magma volumes under high shear during the pre-climactic KPT eruption phases. This mechanism implies that the closer to the conduit walls (where the stronger are the velocity gradients) the larger was the proportion of plate vs. conventional (lensoid) juvenile fragments in the ascending gas–pyroclast mixture. Consequently, plate pumice clasts were mainly entrained in the outer portions of the jet and convecting regions of a sustained, Plinian-type, eruption column, as well as in occasional lateral blast currents generated at the vent. As a whole, plate pumice clasts in the peripheral portions of the column were transported at lower altitudes and deposited by fallout or partial collapse closer to the vent relative to lensoid ones that dominated in the inner column portions. The plate tube pumice proportion decreased abruptly up to disappearance during the emplacement of the main pyroclastic currents and lithic-rich breccias related to extensive caldera collapse at the eruption climax, as a consequence of an overall widening of the magma feeder system through the opening of multiple conduits and eruptive vents, along with fissure erosion, concomitant to the disruption of the collapsing block. [Copyright &y& Elsevier]

Abstract Twenty gas samples have been collected from the natural gas manifestations of Milos Island, the majority of which is found underwater along its coast. Furthermore, three anomalous degassing fumarolic areas (Kalamos, Paleochori and Adamas) have been recognized on-land. Almost all the gases are CO 2 -dominated with CO 2 ranging from 88 to 99% vol for the samples taken underwater, while the on-land manifestations show a wider range (15–98%) due to air contamination. Methane reaches up to 1.0% vol, H 2 up to 3.2% vol and H 2 S up to 3.5% vol indicating a hydrothermal origin of the gases. The isotope composition of He points out to mantle contributions up to 45%, while the C-isotope composition of CO 2 (from −1.9 to +1.3‰ vs. V-PDB with most of the values around −0.5‰) suggests a prevailing limestone origin. Isotope composition of CH 4 , ranging from −18.4 to −5.0‰ vs. V-PDB for C and from −295 to +7‰ vs. V-SMOW for H, points to a geothermal origin with sometimes evident secondary oxidation processes. Additionally, CO 2 -flux measurements showed high values in the three fumarolic areas (up to 1100, 1500 and 8000 g/m2/d at Kalamos, Paleochori and Adamas respectively) with the highest CO 2 -flux values (up to about 23,000 g/m2/d) being measured in the sea at Kanavas with a floating chamber. The south-western part of the island was covered with a lower density prospection revealing only few anomalous CO 2 flux values (up to 650 g/m2/d). The total output of the island (30.5 t/d) is typical of quiescent closed-conduit volcanoes and comparable to the other volcanic/geothermal systems of the south Aegean active volcanic arc (Nisyros, Kos, Nea Kameni, Methana and Sousaki). [ABSTRACT FROM AUTHOR]

We have performed leaching experiments on the fine (< 2 mm) particulate sampled in seven active and quiescent volcanic systems in the Mediterranean area. We reacted the particulate both in pure water and in a synthetic gastric solution. The amount of As, Mn, Pb, Ba, U and Ni leached by pure water exceeded the MAC limits for drinking water in all the materials under investigation. We defined a tolerable ash intake index (TAI) to evaluate the impact of ash ingestion, and we find that 0.2 g and 12 g of ingested fine ash from Vesuvius and Vulcano are enough to exceed the safety limits for Pb and As. Six grams of fine ashes from Stromboli are sufficient to overstep the safety limits for As. Based on our mineralogical characterisation of the particulate, we expect that the submillimetric ash fraction, with a higher surface/volume ratio, releases a greater relative amount of trace metals, which are concentrated in the thin surface layer produced by the reaction of the pristine volcanic particles with coexisting volcanic gases. This means that our measurements represent lower bounds to the actual amount of metal released in aqueous solutions by the volcanic ashes from the locations under investigation. Our results place the first constraints on the mobilisation of toxic elements from volcanic ash, which are necessary to assess the associated potential health risk of volcanic areas. [ABSTRACT FROM AUTHOR]

Arc volcanoes generally emit water-rich, high-viscosity silicic magmas, which are prone to erupt explosively. However, effusive behavior is a common occurrence despite the high-H 2 O, high viscosity conditions. The contrasting shift from effusive to explosive behavior (and vice-versa) at any individual volcano raises the question on what controls eruptive style. Permeability development in conduits allows magma to outgas and is clearly a key factor. However, an important question is whether magma reservoir processes can also have an influence on eruptive styles. The answer could have direct impact on predicting eruptive behavior. Hence, we explore this potential connection by analyzing nine alternating effusive and explosive silicic deposits that were emplaced during distinct eruptions at the active Nisyros-Yali volcanic center. The lavas and pyroclastic deposits are compositionally similar. This indicates a negligible influence of the bulk rock composition on different eruptive styles. The crystal contents vary between units, without any clear correlation with eruptive style (from nearly aphyric to ~45 vol% crystals). Mineral textures and chemistry do show variations between effusive and explosive eruptions, with a larger proportion of resorbed plagioclase and, in most cases, more evolved amphiboles present in the lava flows. Mineral thermo-barometry and hygrometry show that the storage zones of magmas generating effusive eruptions evolved towards colder and more water-rich conditions (710–790 °C; 5.6–6.5 wt% H 2 O) than their explosive counterparts (815–850 °C; 4.2–4.6 wt% H 2 O). At storage pressures of 1.5–2 kbar, relevant for Nisyros-Yali, the volatile saturation level is reached at >5 wt% H 2 O. Therefore, it is likely that the magmas reached water-saturation before generating effusive eruptions, and were undersaturated before explosive events. We hypothesize that the presence of exsolved volatiles in the subvolcanic reservoir can enhance the outgassing potential of the magma during conduit ascent. Hence, the rhyolitic effusive-explosive transitions can be influenced by the pre-eruptive exsolved versus dissolved state of the volatiles in the magma chamber. This can lead to the less explosive eruptions for the most water-rich reservoir conditions. • In water-rich silicic systems effusions can be generated by magmas with higher water contents than explosions. • The paper presents the comparative example of effusive and explosive deposits erupted at Nisyros-Yali. • Explosive eruptions were generated by water-rich magmas that were water-undersaturated during pre-eruptive storage. • Effusive eruptions were generated by water-richer magmas that were water-supersaturated during pre-eruptive storage. • The role of pre-eruptive exsolved volatiles in controlling eruptive styles is explored. [ABSTRACT FROM AUTHOR]