Maria Luisa Carapezza

The control of lava flow during the 1991–1992 eruption of Mt. Etna

Abstract All the actions carried out in 1992 to protect the village of Zafferana Etnea from being invaded by lava are described. An earthen barrier 234 m long and 21 m high was firstly built in January 1992 by accumulating with mechanical escavators 370,000 m 3 of earth, scoriae and stones. This embankment contained the lava for about one month and was overflowed by April 9, 1992. Three additional smaller earthen barriers (lenght: 90–160 m; height: 6–12 m) were built in April to gain time while the lava front was descending towards Zafferana from the overflowed first embankment. The major effort of the 1992 operation consisted of several attempts at stopping the lava front advance by diverting the flow out from the natural and extensively tunnelled channel through a skylight near the vent. The main intervention point was located in Valle del Bove at an elevation of 2000 m, at 8 km from Zafferana, in a zone almost unaccessible from land: helicopters were hence extensively used during the whole operation. Initial interventions called for attempts at plugging a tunnel by dumping into it linked concrete blocks, hedgehogs and blasted portions of the solid levee. Each intervention caused the partial obstruction of the tunnelled channel, which determined major increases of lava overflow in Valle del Bove and the consequent halt of the most advanced fronts. However, benefits were of brief duration, at the most two weeks of respite, before new lava fronts approached again and again the outskirts of Zafferana. The final successful intervention was carried out on May 27–29. An artificial channel was dug departing from the natural one. The solid separation levee was thinned to 3 m and blasted by 7000 kg of explosives. After the explosion, 2 3 of the lava flowed spontaneously in the artificial channel and then the total diversion was obtained, the tunnel being plugged by dumping into the natural flow 230 m 3 of lava boulders. As a consequence of the intervention the active natural lava front, that on May 27 was only 850 m from Zafferana, came to an halt, as did the entire flow downhill from the diversion point, bringing back the situation as it was five months earlier, a few days after the beginning of the eruption, with the new front of the diverted flow at 6–7 km from Zafferana. In June 1992, the effusion rate halved from 30 to 15 m 3 /s and with this reduced thrust the lava was no longer capable of covering long distances. Five months after the conclusive intervention, the diverted lava continues to flow over its initial natural field but remaining confined in the upper Valle del Bove, without any new threat to Zafferana.

Genesis and evolution of the fumaroles of vulcano (Aeolian Islands, Italy): a geochemical model

A geochemical model explaining the presence of fumaroles having different gas composition and temperature at the top of the crater and along the northeastern coast of Vulcano island is proposed. A pressurized biphase (liquid-vapor) reservoir at the depth of about 2 km is hypothesized. Energy and mass balance sheets controlP-T conditions in the system.P-T must vary along a boiling curve of brine as liquid is present. The CO2 content in the steam is governed by the thermodynamic properties of the fluids in the H2-NaCl-CO2 system. On the assumption that oxygen fugacity in the system is between the HM-FMQ oxygen buffers, observed SO2/H2S, CO2/CO, CO/CH4 ratios in the fumarolic gases at the Fossa crater appear in equilibrium with a temperature higher than that observed, such as may exist at depth. The more reduced gas phases present on the sea-side may result from re-equilibrium processes in shallower aquifers. The suggested model would help in monitoring changes in volcanic activity by analyzing fumarolic gases.

Helium and CO2 soil gas emission from Santorini (Greece)

Soil gas investigation is a useful tool to detect active faults. The sudden appearance of soil gas anomalies in zones of deep-reaching faults represents a promising potential precursor of earthquakes and volcanic eruptions. In volcanic areas the development of soil gas monitoring techniques is particularly important, as they can represent, together with remote sensing techniques, the only geochemical methods that can be safely applied during volcanic unrest, when it becomes impossible or too dangerous to sample crater fumaroles. A soil gas survey was carried out in June 1993 at the main island of Thera, in the Santorini volcanic complex. CO2 flux and CO2 and helium concentrations were measured at 50 cm depth for 76 points covering the entire island, with a spacing of 500 m or less. Several anomalous soil degassing sites have been detected. The main anomalies correspond to the Kolumbos line and to the Kameni line, two volcano-tectonic fault systems that controlled all the historic volcanic activity of Santorini. A third anomaly is related to a gas-leaking fault cutting the geothermal field of southern Thera. Soil gas data, together with geovolcanological and seismological evidence, indicate that the Kolumbos and Kameni lines are the most probable sites for future volcanic or seismic reactivation, and provide the basis for the establishment of a new geochemical monitoring technique at Thera.

New mafic magma refilling a quiescent volcano: Evidence from He‐Ne‐Ar isotopes during the 2011–2012 unrest at Santorini, Greece

In 2011–2012, Santorini was characterized by seismic-geodetic-geochemical unrest, which was unprecedented since the most-recent eruption occurred in 1950 and led to fear an eruption was imminent. This unrest offered a chance for investigating the processes leading to volcanic reactivation and the compositional characteristics of involved magma. We have thus analyzed the He-Ne-Ar-isotope composition of fluid inclusions in olivines and clinopyroxenes from cumulate mafic enclaves hosted in cogenetic dacitic lavas of the 1570–1573 and 1925–1928 eruptions of Nea Kameni. These unique data on Aegean volcanism were compared with those of gases collected in quiescent periods and during the unrest. The 3He/4He ratios (3.1–4.0 Ra) are significantly lower than the typical arc-volcano values (R/Ra ∼ 7–8), suggesting the occurrence of magma contamination in Santorini plumbing system, which would further modify the 3He/4He ratio of parental magmas generated in the local metasomatized mantle. The 3He/4He values of enclaves (3.1–3.6 Ra) are comparable to those measured in gases during quiescent periods, confirming that enclaves reflect the He-isotope signature of magma residing at shallow depths and feeding passive degassing. A significant increase in soil CO2 flux from Nea Kameni and anomalous compositional variations in the fumaroles were identified during the unrest, accordingly with previous studies. Simultaneously, 3He/4He ratios up to 4.0 Ra were also measured, demonstrating that the unrest was due to the intrusion into the shallow plumbing system of a more-primitive 3He-rich magma, which is even volatile richer and less contaminated than mafic magma erupted as enclaves. This new intrusion did not however trigger an eruption.

L’attività recente del cratere del Lago Albano di Castelgandolfo

Lake Albano is a complex maar that fed the last phases of Colli Albani volcanic activity. The study of several new stratigraphic sections opened by archeological excavations and civil works has revealed the existence of two previously unknown, primary explosive volcanic deposits, and of several lahar deposits, distributed mainly in the Ciampino plain. Morphological analysis, radiometric dating, the distribution of the early human settlements in the area and the revision of the ancient history and myths of Roma, are coherent in indicating that the activity of lake Albano is much younger than previously believed and extends into Holocene. Until the 4th century B.C. catastrophic exondations have occurred from the lowest rim of the lake, with lahar emplacement on the northern slope. The repetition of these phenomena was prevented by a drain-tunnel dug by the Romans. The overflows were possibly triggered by sudden injections, in the lake bottom, of hot and CO2-rich fluids that are certainly present underneath the volcano. The presence of several gas emission sites, the high CO2 flux in zones corresponding to structural highs of the carbonate basement, the existence of pressurised aquifers also at shallow depth and the reported sudden increase of water temperature and gas release in relation to earthquakes, indicate that a similar hazard persists nowadays.RiassuntoIl Lago Albano è una depressione craterica complessa, che ha alimentato le ultime fasi di attività eruttiva del vulcano dei Colli Albani. Lo studio di numerose nuove sezioni stratigrafiche aperte da scavi archeologici e per lavori stradali ha messo in evidenza l’esistenza di due depositi vulcanici primari, entrambi esplosivi, mai prima riconosciuti e di vari depositi da colate di fango, distribuiti prevalentemente nella Piana di Ciampino. Le analisi morfologiche, le datazioni radiometriche, la distribuzione dei primi insediamenti umani nell’area e la revisione della storia antica e dei miti di Roma concordano nell’indicare che l’attività del cratere del Lago Albano è molto più recente di quanto ritenuto, estendendosi all’Olocene. Fino a circa il IV secolo a.C. si sono verificati fenomeni catastrofici di esondazione del lago, con fuoriuscita delle acque dalla soglia più bassa del bordo craterico prospiciente la piana di Ciampino, poi prevenuti dai Romani con l’escavazione di un tunnel di drenaggio. Questi fenomeni potrebbero essere stati provocati dall’improvvisa iniezione sul fondo del lago di fluidi caldi ricchi di CO2, sicuramente presenti nel sottosuolo del vulcano. Le numerose manifestazioni di gas della zona, l’alto flusso di CO2 in corrispondenza di alti strutturali del basamento carbonatico, l’evidenza di acquiferi pressurizzati anche a debole profondità, l’improvviso aumento del rilascio del gas e della temperatura delle acque, in occasione di eventi sismici, indicano che un simile pericolo esiste anche ai giorni nostri.

Geochemical surveillance of the Solfatara of Pozzuoli (Phlegraean Fields) during 1983

Geochemical surveillance of the Phlegraean Fields area has been intensified since 1983, in response to the increased uplift rate (brady-seismic activity).Fumarolic gases from Solfatara (Pozzuoli) were sampled and analyzed monthly. A Reducing Capacity (RC) monitoring unit was installed at Soffione, the most active fumarole in the Solfatara system.The preliminary analysis of the RC temporal variations suggest they are consistent with the rate of the seismic energy release.The composition of fumarolic gases indicates that the equilibrium temperature and pressure are higher than those of sampling.The observed variations in CH4 content are explained as an increase of pressure (from 1982 to the end of 1983) of about 70% at an extimated depth of 3–3.5 km.Finally, during 1983, there were no geochemical indications of both rising magma and significant accumulation of energy at shallow depth.

Fluid geochemistry of the San Vicente geothermal field (El Salvador)

The volcano Chichontepeque (San Vicente) is one of the nine recent volcanoes making up the El Salvador sector of the WNW-ESE-trending active Central American volcanic belt. Thermal activity is at present reduced to a few thermal springs and fumaroles. The most important manifestations (Agua Agria and Los Infernillos Ciegos) are boiling springs and fumaroles located on the northern slope of the volcano (850 m a.s.l.) along two radial faults. The chloride acid waters of the Los Infernillos area are partly fed by a deep hydrothermal aquifer (crossed at 1100–1300 m by a geothermal exploration well), which finds a preferential path to the surface through the radial fault system. C02 is the most important gas (>90%) of the Los Infernillos Ciegos and Agua Agria fumaroles. Part of the Los Infernillos gases may also come from a deeper, hotter source, given their high HCl/Stot. ratio and their more reducing conditions. The application of geothermometric and geobarometric methods to the gases and thermal waters suggests that both thermal areas are linked to the identified 1100–1300 m reservoir, whose temperature (250°C), lateral extension and chemical composition, as resulting from this study, are of interest for industrial development.

CO2 and H2S concentrations in the atmosphere at the Solfatara of Pozzuoli

The CO2 and H2S concentration in the Solfatara atmosphere has been measured. The concentrations of both gases are higher neraby the more active areas and decrease away from them. A sharp horizontal and vertical gradient of the CO2 content has been recognized.Such gradient is assumed to result from a diffusion of gas from the ground to the atmosphere.The total output of CO2 has been computed based on a turbulent diffusion model. The obtained value is in good agreement with previously abserved values (Italianoet al., 1984).The feasibility of monitoring the atmosphere of Solfatara for either gas hazard and surveillance of volcanic activity has also been evaluated.

The Control of Lava Flows at Mt. Etna

Because of intense urbanization, many of the historic lava flows of Mt. Etna, fed by flank eruptions, have caused significant damage to cities, villages and lifelines. The fear of legal consequences for centuries has prevented any intervention on lava flows to reduce damage until 1983, when a lava flow was dangerously approaching a village and authorization was given. In 1983, for the first time in the world, an attempt was made to stop the flow front by diverting the lava out of its natural channel, through a breach opened by blasting the levee. The technique adopted proved very complex and thermally perturbed the lava causing overflows that prevented completion of the initial plan. Only a short partial diversion was obtained, but the desired result was however achieved as most of the lava outflowed, because of the obstruction of a nearby lava tube. The technique was improved in 1992 and a total diversion of the flow was obtained. The construction of oblique earthen barriers to divert the flows towards less damaging paths was successfully employed in 1983 and 2001. In 1992 a large earthen barrier, built orthogonally to the flow direction delayed the flow advance for nearly one month. A positive experience has been therefore acquired at Mt. Etna to protect settlements from lava flows, but some legal questions have not been yet fully solved. From the Civil Protection viewpoint, it is essential that volcanologists improve their capability of reliably assessing the distance that a lava flow may travel and whether the eruption will continue long enough for lava to reach sites of relevant socioeconomic interest.

Eruptive activity at Turrialba volcano (Costa Rica): Inferences from 3He/4He in fumarole gases and chemistry of the products ejected during 2014 and 2015

A new period of eruptive activity started at Turrialba volcano, Costa Rica, in 2010 after almost 150 years of quiescence. This activity has been characterized by sporadic explosions whose frequency clearly increased since October 2014. This study aimed to identify the mechanisms that triggered the resumption of this eruptive activity and characterize the evolution of the phenomena over the past 2 years. We integrate 3He/4He data available on fumarole gases collected in the summit area of Turrialba between 1999 and 2011 with new measurements made on samples collected between September 2014 and February 2016. The results of a petrological investigation of the products that erupted between October 2014 and May 2015 are also presented. We infer that the resumption of eruptive activity in 2010 was triggered by a replenishment of the plumbing system of Turrialba by a new batch of magma. This is supported by the increase in 3He/4He values observed since 2005 at the crater fumaroles and by comparable high values in September 2014, just before the onset of the new eruptive phase. The presence of a number of fresh and juvenile glassy shards in the erupted products increased between October 2014 and May 2015, suggesting the involvement of new magma with a composition similar to that erupted in 1864–1866. We conclude that the increase in 3He/4He at the summit fumaroles since October 2015 represents strong evidence of a new phase of magma replenishment, which implies that the level of activity remains high at the volcano. This article is protected by copyright. All rights reserved.