Guido Ventura

CO2 degassing and energy release at Solfatara volcano, Campi Flegrei, Italy

In the present period of quiescence, the Solfatara volcano, 1 km far from Pozzuoli, releases 1500 t d−1 of hydrothermal CO2 through soil diffuse degassing from a relatively small area (0.5 km2). This amount of gas is comparable to that released by crater plume emissions of many active volcanoes. On the basis of the CO2/H2O ratio measured in high-temperature fumaroles inside the degassing area, we computed a total thermal energy flux of 1.19×1013 J d−1 (138 MW). Most of this energy is lost by shallow steam condensation and transferred to the atmosphere through the hot soil of the degassing area. The thermal energy released by diffuse degassing at Solfatara is by far the main way of energy release from the whole Campi Flegrei caldera. It is 1 order of magnitude higher than the conductive heat flux through the entire caldera, and, during the last 20 years, it was several times higher than the energy associated with seismic crises and ground deformation events. It is possible that changes in the energy flux from a magma body seated underneath Solfatara and/or argillification processes at relatively shallow depths determine pressurization events in the hydrothermal system and consequently ground deformation and shallow seismic swarms, as recorded during the recent episodes of volcanic unrest centered at Pozzuoli.

The Somma-Vesuvius stress field induced by regional tectonics: evidences from seismological and mesostructural data

Abstract A detailed structural and geophysical study of the Somma–Vesuvius volcanic complex was carried out by integrating mesostructural measurements, focal mechanisms and shear-wave splitting analysis. Fault-slip and focal mechanism analysis indicate that the volcano is affected by NW–SE-, NE–SW-trending oblique-slip faults and by E–W-trending normal faults. Magma chamber(s) responsible for plinian/sub-plinian eruptions (i.e. A.D. 79 and 1631) formed inside the area bounded by E–W-trending normal faults. The post-1631 fissural eruptions (i.e. 1794 and 1861) occurred along the main oblique-slip fault segments. The movements of the Vesuvius faults are mainly related to the regional stress field. A local stress field superposed to the regional one is also present but evidences of magma or gravity induced stresses are lacking. The local stress field acts inside the caldera area being related to fault reactivation processes. The present-day Vesuvius seismic activity is due to both regional and local stress fields. Shear-wave splitting analysis reveals an anisotropic volume due to stress induced cracks NW–SE aligned by faulting processes. Since the depth extent of the anisotropic volume is at least 6 km b.s.l., we deduce the NW–SE-trending oblique-slip fault system represents the main discontinuity on which lies the volcano. This discontinuity is responsible for the morphological lowering of the edifice in its southwestern side.

Relationships among crustal structure, volcanism and strike–slip tectonics in the Lipari–Vulcano Volcanic Complex (Aeolian Islands, Southern Tyrrhenian Sea, Italy)

Abstract The Lipari–Vulcano Volcanic Complex (LVVC, Aeolian Arc, Southern Tyrrhenian Sea, Italy) develops along a strike–slip fault system that bisects the Aeolian Arc. The LVVC crustal structure is investigated by applying seismic methods to two data sets: P-wavefronts generated by local and distant events constrain the shallow velocity structure; DSS data provide shape and depth of the main crustal discontinuities. The obtained velocity model shows that the LVVC shallow structure is characterized by the presence of two sharp lateral discontinuities. The northern discontinuity bounds a low velocity basin-like structure including the La Fossa Caldera and Mt. Guardia Caldera depressions. The southern discontinuity coincides with the southern boundary of a high-velocity diffractor located beneath the La Fossa Cone. Direct modeling of DSS data define three crustal discontinuities showing a complex geometry. The intra-crustal interfaces and the Moho discontinuity show an upheaval beneath the LVVC central sector. The estimated crustal velocities are lower than those generally reported for the continental crust. This feature is due to the high heat flux and fluid circulation affecting the LVVC area. The ratio between the thickness of the upper and lower crust and the presence of the Moho upheaval are consistent with that reported for continental areas affected by extensional strain. The collected geophysical data, combined with volcanological and structural information, indicate that: (a) the Mt. Guardia and La Fossa Caldera depressions represent the surface expression of a single structure whose formation is mainly due to tectonic processes; (b) the LVVC crustal structure and the evolution of the volcanism are consistent with those recognized in zones of transition from arc-related to rift volcanism. In addition, results of a preliminary mechanical analysis based on the available structural data indicate low values of the ratio between magmatic overpressure and tectonic stress. This suggests a passive mechanism of mantle upwelling. The mantle upwelling is due the post-collisional normal strike–slip tectonics affecting the Aeolian Arc.

Fumarolic and diffuse soil degassing west of Mount Epomeo, Ischia, Italy

Abstract Fumarolic fluid compositions and diffuse soil emissions of hydrothermal fluids of the Donna Rachele area (0.86 km 2 , western flank of Mt. Epomeo, Ischia Island) have been studied in order to develop a conceptual geochemical model of the hydrothermal system. The degassing area was mapped and the total release of hydrothermal gas and heat associated with the diffuse emission of hydrothermal fluids was estimated. A mesostructural study was carried out in order to investigate the relations between the brittle structures and the main pathways of the uprising vapor. The fumarolic compositions are typical of hydrothermal fluids and water (>99%) represents the major component. All gas species in the H 2 O–H 2 –CO–CH 4 –CO 2 –H 2 S system are close to equilibrium concentrations at temperatures of ∼300°C and at redox conditions slightly more oxidizing than expected. The compositions of the Donna Rachele fumarolic gas approach the pure liquid equilibrium composition. This indicates a high fraction of separated vapor and suggests the presence of a highly energetic hydrothermal system at depth. The pure liquid equilibrium compositions of the Donna Rachele fumaroles, along with the historical records of shallow seismicity, the ‘explosion’ of a well in 1995, the occurrence of intense acoustic phenomena and of shallow wells discharging vapor indicate that the internal pressure of the hydrothermal system is occasionally larger than the hydrostatic pressure. To quantify the energy dissipated in the Donna Rachele area by the emission of fumarolic fluids, the hydrothermal diffuse degassing was studied by means of 336 soil CO 2 flux measurements. The highest CO 2 fluxes were measured in hydrothermally altered areas along the faults that border Mt. Epomeo. Structural data indicate that the vapor rises up along NW–SE striking normal faults related to gravity-induced stresses and affecting highly fractured lavas. The older faults, which are related to the Mt. Epomeo resurgence, act as a permeability barrier and bound the Donna Rachele diffuse degassing structure. The total hydrothermal CO 2 output was estimated to be ∼9 t d −1 . Assuming that the H 2 O/CO 2 ratio of the fluids that feed the diffuse degassing is the same as that of fumarolic effluents, the calculated heat flux is ∼40 MW. This value, which represents an important energy release, is only a part of the total thermal energy release of Ischia, where other fumarolic areas occur. The results obtained at Ischia indicate the importance of thermal energy released by diffuse degassing structures in the energy balance of quiescent volcanoes. Values of the thermal energy release from the Ischia hydrothermal system are comparable with those estimated on other quiescent volcanoes.

Tectonics, structural evolution and caldera formation on Vulcano Island (Aeolian Archipelago, southern Tyrrhenian Sea)

Abstract A detailed structural study and a review of the eruptive history of the island of Vulcano (Aeolian Arc—southern Tyrrhenian Sea) have been carried out with the purpose of understanding the evolution of both volcanic and tectonic structures. The island is composed of four main volcanoes which date from about 120 ka to historical times. Two calderas occur in the central sector of the island. The time-space evolution of the volcanism indicates a shifting of the activity from the southeastern sectors towards the northwest. Two main systems of NW—SE-trending right-lateral strike-slip faults affect the island. NE—SW- and N—S-trending normal faults are also present. Geophysical investigation indicate the NW—SE-trending strike-slip faults are likely to be the surface expression of a main regional crustal structure. This system of discontinuity is related with the stress field acting in the southern sector of the Aeolian Archipelago. Normal N—S- and NE—SW-oriented faults represent extensional fractures. The NE—SW-trending structures are related to a local stress field. The results of the kinematic analysis, as well as the geometric relationships between the faults and the constant subsidence velocities, are consistent with the opening of a pull-apart basin. Volcanological and geochronological data are also consistent with the proposed model. In the general deformative style affecting the island, the calderas can be explained as the consequence of different opening phases of a pull-apart basin.

The role of hydrothermal fluids in triggering the July–August 2000 seismic swarm at Campi Flegrei, Italy: evidence from seismological and mesostructural data

Abstract This study presents a detailed analysis and interpretation of the seismicity that occurred on July 2–7 and August 22, 2000, during a ground uplift episode which started on March 2000 at Solfatara crater, Campi Flegrei. Earthquakes are located using a probabilistic grid-search procedure acting on a 3-D heterogeneous earth structure. The mainshock of the July swarm depicts a spectrum characterized by a few narrow peaks spanning the 1–5-Hz frequency band. For this event, we hypothesize a direct involvement of magmatic fluids in the source process. Conversely, the spectra of the August events are typical of shear failure. For these latter events, we evaluate the source properties from P- and S-wave displacement spectra. Results for the most energetic shocks (M d around 2) yield a source radius in the order of 100 m and stress drop around 10 bars, in agreement with most of the earthquakes that occurred during the 1982–1984 bradyseismic crises. For the August swarm we identify two clusters of similar earthquakes. Application of high-resolution relative location techniques to these events allows for the recognition of two parallel alignments trending NE–SW. The relationship among source dimension and relative location evidences overlapping of sources. This may be interpreted in terms of either a heterogeneous stress field or a lubrication process acting over the fault surface. For a selected subset of the August events, we also analyze the splitting of the shear waves: results are indicative of wave propagation through a densely fractured medium characterized by a distribution of cracks oriented NE–SW. The pattern of faulting suggested by relative locations and shear-wave splitting is not consistent with the surface trace of NW–SE striking faults. However, a detailed mesostructural analysis carried out over the Solfatara area indicated the occurrence of two main crack systems striking NW–SE and NE–SW. This latter system shows a strike consistent with that derived from seismic evidence. Results from a stress analysis of the crack systems indicate that a fluid overpressure within the NW–SE-striking faults is able to form NE–SW cracks. We found that the pressure of fluids P f required to activate the NW–SE faults is less than σ Hmin , while the P f value required to open the NE–SW cracks is higher than σ Hmax . Our main conclusions are: (a) the Solfatara area is affected by two orthogonal fracture systems, and the fluid pathway during the 2000 crisis mainly occurred along the NNE–SSW/NE–SW-striking crack system; (b) the July seismicity is associated to the upward migration of a pressure front triggered by an excess of fluid pressure from a small-size magmatic intrusion; conversely, the August events are associated to the brittle readjustment of the inflated system occurring along some lubricated structures.

Silicic magma entering a basaltic magma chamber: eruptive dynamics and magma mixing — an example from Salina (Aeolian islands, Southern Tyrrhenian Sea)

The Pollara tuff-ring resulted from two explosive eruptions whose deposits are separated by a paleosol 13 Ka old. The oldest deposits (LPP, about 0.2 km3) consist of three main fall units (A, B, C) deposited from a subplinian column whose height (7–14 km) increased with time from A to C, as a consequence of the increased magma discharge rate during the eruption (1–8x106 kg/s). A highly variable juvenile population characterizes the eruption. Black, dense, highly porphyritic, mafic ejecta (SiO2=50–55%) almost exclusively form A deposits, whereas grey, mildly vesiculated, mildly porphyritic pumice (SiO2=56–67%) and white, highly vesiculated, nearly aphyric pumice (SiO2=66–71%) predominate in B and C respectively. Mafic cumulates are abundant in A, while crystalline lithic ejecta first appear in B and increase upward. The LPP result from the emptying of an unusual and unstable, compositionally zoned, shallow magma chamber in which high density mafic melts capped low density salic ones. Evidence of the existence of a short crystal fractionation series is found in the mafic rocks; the andesitic pumice results from complete blending between rhyolitic and variously fractionated mafic melts (salic component up to 60 wt%), whereas bulk dacitic compositions mainly result from the presence of mafic xenocrysts within rhyolitic glasses. Viscosity and composition-mixing diagrams show that blended liquids formed when the visosities of the two end members had close values. The following model is suggested: 1. A rhyolitic magma rising through the metamorphic basement enterrd a mafic magma chamber whose souter portions were occupied by a highly viscous, mafic crystal mush. 2. Under the pressure of the rhyolitic body the nearly rigid mush was pushed upwards and mafic melts were squeezed against the walls of the chamber, beginning roof fracturing and mingling with silicic melts. 3. When the equilibrium temperature was reached between mafic and silicic melts, blended liquids rapidly formed. 4. When fractures reached the surface, the eruption began by the ejection of the mafic melts and crystal mush (A), followed by the emission of variously mingled and blended magmas (B) and ended by the ejection of nearly unmixed rhyolitic magma (C).

New unspiked K–Ar ages of volcanic rocks of the central and western sector of the Aeolian Islands: reconstruction of the volcanic stages

Abstract A geochronological study of the Filicudi, Salina, Lipari and Vulcano Islands (Aeolian Archipelago) using the unspiked potassium–argon technique provides new age data which, combined with stratigraphic correlation, better constrain the temporal evolution of volcanism. The unspiked K–Ar age of the oldest exposed lavas on Filicudi, 219±5 ka, is significantly younger than the previous estimation of 1.02 Ma. In the general context of Aeolian volcanism, this new date suggests that the volcanism of the western sector of the Aeolian Archipelago is younger than previously thought. Geochronological data point out on the rapid transition from calc–alkaline to potassic volcanism. The distribution of the K–Ar ages within the Salina–Lipari–Vulcano group shows that the volcanism started on Lipari and propagated over time northward on Salina and southward on Vulcano. Geochronological and geophysical data suggest that the onset of volcanism in the central sector of the Aeolian Arc may be due to a mantle upwelling structure located below Lipari. A change in the style of the eruptions occurred in the Salina–Lipari–Vulcano system at about 100 ka from the present. Low-energy magmatic eruptions occurred between 188 and about 100 ka. From about 100 ka to the present, higher-energy eruptions and low-energy events due to magma–water interaction also occurred. This change in the style of activity, together with the appearance of evolved products (i.e. rhyolites) during the last 50 ka, is consistent with the formation of magmatic reservoirs located at shallower depth with respect to those of the 188–100-ka period. The new geochronological data and available petrological models reveal that a change in the deep source of the primary magmas occurred in a relatively short time interval.

Seismic study of the ‘41st Parallel’ Fault System offshore the Campanian–Latial continental margin, Italy

Abstract A set of seismic reflection lines, collected in the Southern Tyrrhenian Sea offshore the Campanian–Latial continental margin has been selected and interpreted. The aim is to characterize: (1) the structural features of the 41st Parallel Line (41PL), an E–W elongated magnetic anomaly zone separating the Northern Tyrrhenian domain from the Southern Tyrrhenian domain; and (2) to study the relationships between the 41PL and the Ortona–Roccamonfina Line (ORL), a tectonic structure transversal to the Apennines along which the Northern Apennine Arc merges the Southern Arc. The interpretation of the seismic lines is discussed in light of the available geological (stratigraphic and structural) and geophysical (gravimetric and magnetic) information. Results show that the Campanian–Latial continental margin is characterized by a series of structural lows and highs that match with the main structures on the mainland (Mt. Massico horst, the Volturno and Garigliano depressions). The study area is characterized by ESE–WNW to E–W and NE–SW striking faults. The activity of these faults developed during Pliocene–Early Pleistocene times. ESE–WNW to E–W faults display structures consistent with strike–slip movements. These faults, which are located on the maximum gradient of the E–W elongated magnetic alignment of 41PL, are responsible for the SSE translation of the offshore sector of the Mt. Massico horst. The horizontal dislocation of Mt. Massico suggests left-lateral movements for the ESE–WNW to E–W faults. The NE–SW faults that affect the Mt. Massico horst, which represents the southern tip of ORL, show seismic features consistent with normal movements, as also revealed by inland data. Since: (1) background seismicity is virtually absent along the study area; and (2) the uppermost seismic reflectors seem unaffected by faults, it is very likely that both the ORL and 41PL fault zones are now inactive, at least in the Campanian–Latial area. Seismic data indicate that ORL is older than 41PL and support the interpretation that the 41PL represents a deep-seated transfer fault system formed in response to the different rates of opening of the Tyrrhenian Sea. The strike–slip movements along the 41PL faults and the normal movements along the ORL faults are consistent with a NW–SE extension, which is responsible for the longitudinal extension in the Southern Apennines belt.

The role of flank failure in modifying the shallow plumbing system of volcanoes: An example from Somma‐Vesuvius, Italy

The joined interpretation of structural, geophysical, historical and DTM-extracted data collected on the Somma-Vesuvius volcano reveals the presence of a spoon-shaped paleo-depression, which is masked by the well known Vesuvius subsidence caldera. Such depression was caused by failure of the Somma southwestern flank. The flank failure was triggered by dike intrusion along a pre-existing structural discontinuity. This event occurred between 20–25 and 18ka and marked a change in the composition of erupted products and style of activity, which shifted from an effusive and strombolian activity to a plinian-type activity. The flank failure induced a reorientation of the gravity-induced stress trajectory, which, below the depression, resulted parallel to the surface. This prevented the ascent of magma to the surface allowing the formation of a shallow reservoir in which more felsic magma evolved. When the magma pressure exceeded a critical value, plinian activity began.