Anthony Finizola

Fluid circulation at Stromboli volcano (Aeolian Islands, Italy) from self-potential and CO2 surveys

This work addresses the study of fluid circulation of the Stromboli island using a dense coverage of self-potential (SP) and soil CO2 data. A marked difference exists between the northern flank and the other flanks of the island. The northern flank exhibits (1) a typical negative SP/altitude gradient not observed on the other flanks, and (2) higher levels of CO2. The general SP pattern suggests that the northern flank is composed of porous layers through which vadose water flows down to a basal water table, in contrast to the other flanks where impermeable layers impede the vertical flow of vadose water. In the Sciara del Fuoco and Rina Grande–Le Schicciole landslide complexes, breccias of shallow gliding planes may constitute such impermeable layers whereas elsewhere, poorly permeable, fine-grained pyroclastites or altered lava flows may be present. This general model of the flanks also explains the main CO2 patterns: concentration of CO2 at the surface is high on the porous north flank and lower on the other flanks where impermeable layers can block the upward CO2 flux. The active upper part of the island is underlain by a well-defined hydrothermal system bounded by short-wavelength negative SP anomalies and high peaks of CO2. These boundaries coincide with faults limiting ancient collapses of calderas, craters and flank landslides. The hydrothermal system is not homogeneous but composed of three main subsystems and of a fourth minor one and is not centered on the active craters. The latter are located near its border. This divergence between the location of the active craters and the extent of the hydrothermal system suggests that the internal heat sources may not be limited to sources below the active craters. If the heat source strictly corresponds to intrusions at depth around the active conduits, the geometry of the hydrothermal subsystems must be strongly controlled by heterogeneities within the edifice such as craters, caldera walls or gliding planes of flank collapse, as suggested by the correspondence between SP–CO2 anomalies and structural limits. The inner zone of the hydrothermal subsystems is characterized by positive SP anomalies, indicating upward movements of fluids, and by very low values of CO2 emanation. This pattern suggests that the hydrothermal zone becomes self-sealed at depth, thus creating a barrier to the CO2 flux. In this hypothesis, the observed hydrothermal system is a shallow one and it involves mostly convection of infiltrated meteoric water above the sealed zone. Finally, on the base of CO2 degassing measurements, we present evidence for the presence of two regional faults, oriented N41° and N64°, and decoupled from the volcanic structures.

Inner structure of La Fossa di Vulcano (Vulcano Island, southern Tyrrhenian Sea, Italy) revealed by high-resolution electric resistivity tomography coupled with self-potential, temperature, and CO2 diffuse degassing measurements

La Fossa cone is an active stratovolcano located on Vulcano Island in the Aeolian Archipelago (southern Italy). Its activity is characterized by explosive phreatic and phreatomagmatic eruptions producing wet and dry pyroclastic surges, pumice fall deposits, and highly viscous lava flows. Nine 2-D electrical resistivity tomograms (ERTs; electrode spacing 20 m, with a depth of investigation >200 m) were obtained to image the edifice. In addition, we also measured the self-potential, the CO2 flux from the soil, and the temperature along these profiles at the same locations. These data provide complementary information to interpret the ERT profiles. The ERT profiles allow us to identify the main structural boundaries (and their associated fluid circulations) defining the shallow architecture of the Fossa cone. The hydrothermal system is identified by very low values of the electrical resistivity ( 400 Ω m). Inside the crater it is possible to follow the plumbing system of the main fumarolic areas. On the flank of the edifice a thick layer of tuff is also marked by very low resistivity values (in the range 1–20 Ω m) because of its composition in clays and zeolites. The ashes and pyroclastic materials ejected during the nineteenth-century eruptions and partially covering the flank of the volcano correspond to relatively resistive materials (several hundreds to several thousands Ω m). We carried out laboratory measurements of the electrical resistivity and the streaming potential coupling coefficient of the main materials forming the volcanic edifice. A 2-D simulation of the groundwater flow is performed over the edifice using a commercial finite element code. Input parameters are the topography, the ERT cross section, and the value of the measured streaming current coupling coefficient. From this simulation we computed the self-potential field, and we found good agreement with the measured self-potential data by adjusting the boundary conditions for the flux of water. Inverse modeling shows that self-potential data can be used to determine the pattern of groundwater flow and potentially to assess water budget at the scale of the volcanic edifice.

3-D density structure and geological evolution of Stromboli volcano (Aeolian Islands, Italy) inferred from land-based and sea-surface gravity data

We present the first density model of Stromboli volcano (Aeolian Islands, Italy) obtained by simultaneously inverting land-based (543) and sea-surface (327) relative gravity data. Modern positioning technology, a 1 × 1 m digital elevation model, and a 15 × 15 m bathymetric model made it possible to obtain a detailed 3-D density model through an iteratively reweighted smoothness-constrained least-squares inversion that explained the land-based gravity data to 0.09 mGal and the sea-surface data to 5 mGal. Our inverse formulation avoids introducing any assumptions about density magnitudes. At 125 m depth from the land surface, the inferred mean density of the island is 2380 kg m^−3, with corresponding 2.5 and 97.5 percentiles of 2200 and 2530 kg m^−3. This density range covers the rock densities of new and previously published samples of Paleostromboli I, Vancori, Neostromboli and San Bartolo lava flows. High-density anomalies in the central and southern part of the island can be related to two main degassing faults crossing the island (N41 and N64) that are interpreted as preferential regions of dyke intrusions. In addition, two low-density anomalies are found in the northeastern part and in the summit area of the island. These anomalies seem to be geographically related with past paroxysmal explosive phreato-magmatic events that have played important roles in the evolution of Stromboli Island by forming the Scari caldera and the Neostromboli crater, respectively.

Hydrothermal fluid flow disruptions evidenced by subsurface changes in heat transfer modality: The La Fossa cone of Vulcano (Italy) case study

Detecting volcanic unrest is of primary importance for eruption forecasting, especially on volcanoes characterized by highly dangerous , and often seemingly unpredictable, phreatic or phreatomagmatic eruptions. We present a simple and innovative analysis of shallow vertical temperature profiles to depths of 70 cm. These data were recorded at La Fossa cone of Vulcano (Aeolian Islands, Italy), during an episode of increased hydrothermal and seismic activities that occurred between September and December 2009. This work involves the use of the coefficient of determination (R 2) on vertical temperature profiles in order to identify changes in conductive versus convec-tive heat transfer modality. The increase in convective heat transfer can be related to the disruption of the hydrothermal system due to its pressurization and/or variation of ground permeability between the hydrothermal system and the surface. While raw temperature data do not evidence any significant variation during the period investigated and the classic temperature gradient is highly influenced by seasonal variations, the fluctuation of R 2 displayed striking spikes that coincided with the seismic swarm inside the volcanic edifice. Such a low-cost device associated with easy real-time data processing could constitute a very promising, yet deceptively simple, technique to monitor hydrothermal systems, in order to assess the hazard posed by high-energy eruptions for populations living close to active volcanoes.

RAMAN STUDY OF SECONDARY MINERALS IN A RECENT LAVA TUBE

In this paper, we present first the technical adaptations made for a field use of the laboratory in situ spectrometer, its capabilities and limits as well. Secondly, we present identification of secondary mineral phases growing in the lava tube with a particular emphasis on the study of white speleothems and dendrites by mean of XRD, SEM-EDX and Raman spectroscopy.

Fluid Circulation and Permeability Changes in the Summit Area of Stromboli Volcano

The main problem posed by the 2002―2003 eruptive crisis at Stromboli was in assessing the flank stability of the volcanic edifice. Indeed, this eruptive crisis has been characterized by important effusive and collapse events inside the Sciara del Fuoco sector collapse area: in particular, two flank sector collapses in the aerial and submarine part of the edifice and the opening of a fracture set close to gliding plane in the summit part of the Sciara del Fuoco sector collapse. Detailed fluid flow mapping and reiterative profiles carried out with self-potential, temperature, and soil gas measurements since 1994 in the Fossetta―Sciara del Fuoco―Fossa area, show the importance of old structural boundaries, such as the Large Fossa crater, in the opening of part of the 2002―2003 set of fractures. Within this structural framework, other morphological changes occurred during the eruptive crisis, such as the opening of new sets of fracture and collapse events. These were probably located along other crater boundaries formed since the largest paroxysmal event of September 1930, which dramatically modified the morphology inside the Large Fossa crater area. The multidisciplinary approach to fluid flow measurements during an eruptive crisis demonstrates the powerful potential of these techniques as a complement to deformation measurements in assessing the weakness planes of a volcanic edifice and the risk of sector collapses.