Roberto Scandone

The structure of the Campanian Plain and the activity of the Neapolitan volcanoes (Italy)

Abstract The central Campanian Plain is dominated by the structural depression of Acerra whose origin is tectonic, but may have been enlarged and further depressed after the eruption of the Campanian Ignimbrite (42-25 ka). The deposits of the Campanian Ignimbrite are possibly the results of multiple eruptions with huge pyroclastic deposits that covered all the Campanian Plain. The more recent activity of Vesuvius, Campi Flegrei and Procida occurred on the borders of Acerra depression and resulted from a reactivation of regional faults after the Campanian Ignimbrite cycle. The activity of Vesuvius produced the building of a stratovolcano mostly by effusive and plinian explosive eruptions. The Campi Flegrei area, on the contrary, was dominated by the eruption of the Neapolitan Yellow Tuff at 12 ka that produced a caldera collapse of the Gulf of Pozzuoli. The caldera formation controlled the emplacement of the recent activity of Campi Flegrei and the new volcanoes were formed only within the caldera or along its rim.

Evolution of a section of the Africa-Europe plate boundary: Paleomagnetic and volcanological evidence from Sicily

Abstract New paleomagnetic data relative to Upper Cretaceous, Neogene and Quaternary volcanic rocks from eastern Sicily definitively indicate that Sicily is a part of the African plate, which collided with the European continental plate in Middle Miocene times. These data and the tectonic evolution of Sicily as inferred from the nature, age and distribution of volcanic products, are broadly consistent with the motions of Africa relative to Europe since the Upper Trias. During the Mesozoic, eastern Sicily was affected by extensional tectonics with associated alkali basaltic volcanism, and oceanic crust was produced in the meantime between the diverging African and European plates. Near the end of Mesozoic times the two plates started to converge with consequent consumption of oceanic crust. Different times of oceanic plate consumption along the Sicily-Calabria section of the plate boundary are suggested by the occurence of andesitic volcanism of different ages. The tectonic significance of late Tertiary to present basaltic activity in eastern Sicily is also discussed.

Mount Vesuvius: 2000 years of volcanological observations

Abstract Mount Vesuvius had eruptions ranging between VEI 5+ to 0–1 during the last 2000 years. Infrequent explosive eruptions are recorded during the period 79 AD to 1631. Since the violent explosive eruption of 1631, the volcano has been in persistent activity, rebuilding the morphology that it had before that eruption. A succession of explosive and effusive eruptions occurred until 1944, with a predominance of short and violent episodes until 1872 and longer effusive eruptions since that date. Two factors mainly controlled the character of volcanic activity during this period: (1) the strength of the cone, which allowed, in the earlier period, an easy fracturing, rapid drainage, and pressure release of the magma column; (2) the interaction between magma and water, which enhanced the explosivity of several eruptions. The volcano appears to have reached a stage of quiescence because it finally attained a shape of equilibrium in which the height of the mountain is sufficient to counterbalance the buoyancy of the magma.

Chaotic collapse of calderas

Abstract A study of the formation of Krakatoan-type calderas indicates that they probably form by chaotic collapse. The energy required to form these calderas by explosive decapitation of the volcano is seldom available during caldera-forming eruptions. Gravity anomalies resulting from different mechanisms of collapse show that chaotic-collapse calderas are characterized by a negative residual Bouguer anomaly with a gentle gradient toward the center of the caldera. Calderas of the Valles type have steeper gradients near of the caldera edges. The residual anomaly derives from the low-density shattered rocks that make up the chimney above the collapsed magma-chamber. The chimney flares toward the top giving the substructure of the caldera a funnel-shaped form. During eruptions forming calderas of the chaotic-collapse type, the eruption and emplacement of coignimbrite breccia is indicative of explosions due to a pressure decrease within the magma chamber. The pressure decrease is due both to drainage of magma and to collapse of the roof of the chamber both of which cause a decrease of the lithostatic load on the magma chamber. In some cases water can gain access to the magma chamber through collapse of entire sections of the hydrothermal system of the volcano with consequent explosive vaporization within the chamber itself.

A methodology for the evaluation of long-term volcanic risk from pyroclastic flows in Campi Flegrei (Italy)

The volcanological history of Campi Flegrei suggests that the most frequent eruptions are characterized by the emplacement of pyroclastic flow and surge deposits erupted from different vents scattered over a 150-km 2 caldera. The evaluation of volcanic risk in volcanic fields is complex because of the lack of a central vent. To approach this problem,we subdivided the entire area of Campi Flegrei into a regular grid and evaluated the relative spatial probability of opening of vents based on geological,geophysical and geochemical data. We evaluated the volcanic risk caused by pyroclastic flows based on the formula proposed by UNESCO (1972), R = HUVUVa,where H is the hazard, V is the vulnerability and Va is the value of the elements at risk. The product HUV was obtained by performing simulations of type eruptions centered in each cell of the grid. The simulation is based on the energy cone scheme proposed by Sheridan and Malin [J. Volcanol. Geotherm. Res. 17 (1983) 187^202],hypothesizing a column collapse height of 100 m for eruptions of VEI = 3 and 300 m for eruptions of VEI = 4 with a slope angle of 6‡. Each simulation has been given the relative probability value associated with the corresponding cell. We made use of the GIS software ArcView 3.2 to evaluate the intersection between the energy cone and the topography. The superposition of the areas invaded by pyroclastic flows (124 simulations for VEI = 3 and 37 for VEI = 4) was used to obtain the relative hazard map of the area. The relative volcanic risk map is obtained by superimposing the urbanization maps. C 2002 Elsevier Science B.V. All rights reserved.

The tectonic setting of Mount Vesuvius and the correlation between its eruptions and the earthquakes of the Southern Apennines

Abstract Mount Vesuvius is emplaced on a regional NE-SW-trending fault that accommodates the stretching of the lithosphere caused by a backward retreat of the Calabrian arc. The dynamics of the Calabrian arc controls the temporal occurrence of earthquakes in the Southern Apennines and in Sicily. By means of a detailed statistical approach, we identified a significant correlation between seismic events occurring in different subsets of this geodynamic domain: seismicity changes in the Southern Apennines follow those in the Calabrian arc after 18–21 years, while seismicity changes in Sicily follow those in the Calabrian arc after 8–10 years. The seismicity changes in these three areas appear also to have affected the eruptive activity of Vesuvius in the period 1631–1944. The major effusive-explosive eruptions of this period followed the seismicity changes in the Southern Apennines after 6–13 years and those in the Calabrian arc after 36–39 years. From a tectonic point of view, this indicates a direct link between the eruptive activity of Vesuvius and the dynamics of the Calabrian arc. The backward retreat of the arc produces strain pulses propagating to adjacent areas. From a volcanological point of view, we speculate that the arrival of an extension strain pulse in the area of Vesuvius may trigger the fast movement of magma-filled cracks that stay in unstable equilibrium in the roots of the volcano.

The feeding system of Vesuvius between 1754 and 1944

The occurrence of short-term variations of Sr isotopic ratios in the Vesuvian historical lavas, and the occurrence of different types of nodules, which formed in different P-T conditions, permit the identification of two different deep magmatic reservoirs. The cumulative effusion rate of Vesuvius between 1694 and 1944 allows a calculation of the magma production rate in the same period. Based on these data, a model of the Vesuvian feeding system is presented. Between 1754 and 1944 mixing of magmas probably occurred in two deep-seated magma reservoirs. We calculated the volume of such reservoirs (∼ 0.1 km3). The increased effusion rate of Vesuvius after 1858 is nicely explained by the simultaneous activity of both reservoirs. Geometrical constraints on the Vesuvian conduit are also discussed.

The evaluation of volcanic risk in the Vesuvian area

Abstract Volcanic Risk has been defined as the product: R = Value × Vulnerability × Hazard, where value is the total amount of lives or properties at risk for a volcani eruption, the vulnerability is the percentage of value at risk for a given volcanic event, and the hazard is the probability that a given area may be affected by a certain volcanic phenomenon. We used this definition to evaluate the Risk of loss of human lives for volcanic eruptions of Vesuvius. Value has been determined based on the total number of inhabitants living in areas that could be affected by an eruption. Vulnerability is based on the relative probability of deaths as a result of different volcanic phenomena (tephra fall, pyroclastic flows, etc.). Hazard is evaluated based on the absolute probability of a given phenomenon in a certain area. This last parameter is the most difficult to evaluate. We subdivided the activity of Vesuvius, that produces risk of loss of human lives, into three classes of eruptions, based on the Volcanic Explosivity Index. We assume that the events of each class are distributed according to a poissonian distribution (this is demonstrated for VEI = 3, and inferred for the other classes), so that we can evaluate the absolute probability of an eruption for each class within a given time span. We use a time window of 10 years and evaluate the probabilities of occurrence of at least one eruption for VEI = 3, 4, 5; the probability is respectively: P 3 = 0.09896, P 4 = 0.01748, P 5 = 0.00298 We have made a hazard evaluation for the entire Vesuvian area as well as an evaluation of Volcanic Risk. The obtained map shows that the areas with higher risk are on the southern side of Vesuvius, in the coastal region where each town is characterized by an average Risk of ∼ 1000 inhabitants/10 years. The risk regularly decreases with increasing distance from the volcano. The risk is mostly due to the events with VEI = 3 and 4, as the most destructive effects of VEI = 5 are counterbalanced by the lower probability of such events. The map of volcanic risk is not intended to predict the loss of human lives of an eruption, but to give a methodology that permits to identify those areas where it is necessary to operate in order to reduce the risk of an eruption before it occurs.

Modelling of Surface Ground Deformations in the Phlegraean Fields Volcanic Area, Italy

A preliminary finite elements model of the ground deformations observed at Phlegraean Fields is proposed. The model assumes an oblate-spheroid magma chamber at the depth of 5.4 km with major semiaxis of 1.5 km and minor semiaxis of 0.75 km. The dimensions of the magma chamber have been evaluated by using a thermal model based on the assumptions that a progressively cooling huge magmatic body is responsible for the volcanic activity at Phlegraean Fields in the last 35,000 years. Surface deformations caused by an over-pressure of 30 MPa in the magma chamber have been calculated. Constant, and temperature-dependent elastic parameters of the surrounding medium have been considered. Vertical displacements of the order of those presently observed at Phlegraean Fields can be obtained only with temperature-dependent elastic properties of the medium.

Effusion rate and energy balance of Paricutin eruption (1943–1952), Michoacan, Mexico

Abstract The annual effusion rate of the Paricutin eruption shows an exponential decrease with time. This trend was observed during eruptions in other volcanoes and is explained by a simple model of the temporal changes of effusion rate proposed by Machado (1974). The regular trend of the effusion rate permits, in this case, a forecast of the length of the eruption. The energy balance of the eruption shows that part of the available energy was absorbed in a process of differentiation of the magma and this fact probably shortened the eruption.