F Barberi

Evolution of Eolian arc volcanism (Southern Tyrrhenian Sea)

Abstract The Eolian arc is located at the boundary between the converging African and European plates. Its volcanism is characterized by a marked evolution in a restricted time (less than 1 my). A progressive transition is observed from typical calc-alkaline series toward shoshonitic rocks produced by past and present activity (Vulcano and Stromboli). By comparison with circum-Pacific island arcs, the evolution of Eolian volcanism can be related to a rapid deepening of the Benioff zone. The occurrence of shoshonitic rocks and the continental nature of the crust on both sides of the plate boundary suggest that the Eolian arc is in a senile stage of evolution. Petrology and Sr isotopic data indicate a probable mantle source for Eolian volcanism.

The campanian ignimbrite: a major prehistoric eruption in the Neapolitan area (Italy)

A geological, chemical and petrographical study of the Campanian ignimbrite, a pyroclastic flow deposit erupted about 30,000 years ago on the Neapolitan area (Italy), is reported. The ignimbrite covered an area of at least 7,000 km2; it consists of a single flow unit, and the lateral variations in both pumice and lithic fragments indicate that the source was located in the Phlegraean Fields area.Textural features, areal distribution and its morphological constraints suggests that the eruption was of the type of highly expanded low-temperature pyroclastic cloud. The original composition was strongly modified by post-depositional chemical changes involving most of the major and trace elements. No primary differences in the composition of the magma have been recognized. The Campanian ignimbrite is a nearly saturated potassic trachyte, similar to many other trachytes of the Quaternary volcanic province of Campania. Its chemistry indicates an affinity with the so-called «low-K association» of the Roman volcanic province.

Volcanism of Afar: Small-scale plate tectonics implications

Structures analogous to oceanic spreading ridges, transform faults, and “leaky” fracture zones have been identified in Afar. They were formed during the past 3 to 4 m.y. and permit identification of the present plate boundaries within Afar and determination of the amount of spreading in this interval. At least two microplates are required in the zone of the junction of the African, Arabian, and Somalian plates. Variations of the spreading rate along single ridges or from one ridge to another, ridge jumping, migration of spreading with time, and counterclockwise rotation of the microplates are inferred by comparing volcanological, chronological, and geochemical data with aeromagnetic data. Deformation affects both accreting and transform plate boundaries within zones similar in width to the microplates, whose interiors are also systematically affected by regional faulting and locally by volcanism and transverse faulting. This indicates that although plate tectonics can explain most of the Afar features it fails when applied at such a small scale.

Phlegraean Fields 1982–1984: Brief chronicle of a volcano emergency in a densely populated area

The essential features of the ongoing potential pre-eruptive crisis at the Phlegraean Fields begun in August 1982 are summarized and the main problems faced by scientists responsible of volcanic hazards evaluation in such a densely populated area are discussed.

The Erta Ale volcanic range (Danakil depression, northern afar, ethiopia)

The Erta Ale range is the main volcanic unit in the Danakil rift. It is located along the axis of the northern part of the Depression, in a zone clearly related to the Red Sea rift. The detailed study of the tectonics, volcanism and petrographic succession of the whole range allows one to draw the following conclusions.1)A compl3te, tectonically controlled evolution is observed in the subaerial volcanism from simple fissure activity, to small shield volcanoes built along open fissures, and to more complex central volcanoes. The emitted products form a complete differentiation series from basalts to rhyolites, with a remarkably regular volume decrease from basic to silicic terms.2)The close relationships existing between petrological and volcanological evolution suggest that the magma fractionation has been realized at relatively shallow depth, at the emplacement level of magmatic reservoirs. The importance of a time factor in the differentiation processes in volcanic conditions is clearly stressed.3)All collected data strongly support the subcrustal origin of the whole volcanic series and the hypothesis of crustal separation with formation of new oceanic crust along the axis of the Northern Danakil Depression.

The somma-vesuvius magma chamber: a petrological and volcanological approach

The volcanic history of Somma-Vesuvius indicates that salic products compatible with an origin by fractionation within a shallow magma chamber have been repeatedly erupted («Plinian» pumice deposits). The last two of these eruptions, (79 A.D. and 3500 B.P.) were carefully studied. Interaction with calcareous country rocks had limited importance, and all data indicate that differentiated magmas were produced by crystal-liquid fractionation within the undersaturated part of petrogeny’s residua system at about 1 kb water pressure. The solid-liquid trend indicates that the derivative magmas originated by fractionation of slightly but significantly different parental liquids. Some lavas of appropriate composition were selected as parental liquids to compute the entity of the fractionation. Results suggest that in both bases a fractionation of about 70 weight % was needed to produce liquids with the composition of the pumice. The combination of all data indicates that the two Plinian eruptions were fed by a magma chamber (3–4 km deep) having a volume of approx. 2.0–2.5 km3. The temperature of the magma that initially entered the chamber was about 1100°C, whereas the temperature of the residual liquids erupted was Plinian pumice was 800° and 850°C respectively. There is no evidence that such a magma chamber existed at Vesuvius after the 79 A.D. eruption. These results have relevant practical implications for volcanic hazard and monitoring and for geothermal energy.

Structural evolution of Campi Flegrei caldera in light of volcanological and geophysical data

Abstract The structural evolution of Campi Flegrei (CF) has been investigated by combining on land and offshore gravimetric and aeromagnetic data, with surface geological and volcanological evidences and subsurface drilling information. A relatively small (4–5 km across) collapsed circular structure, produced by eruption along a ring fracture, has been identified for the first time in the Agnano-Astroni-Solfatara (AAS) sector. Data permitted the definition of the geometry of large CF caldera, which is 13 km across and includes an inner more collapsed zone, 11–12 km across with a total sinking of about 1,600 m. Its volume (80 km 3 ) is consistent with that estimated for the Campanian Ignimbrite erupted 35,000 a B.P.—A further collapse occurred after the eruption of the Neapolitan Yellow Tuffs (10 km 3 ; about 12,000 a B.P.) and finally the AAS structure was formed in the last phase of activity, about 4,000 a B.P.—The CF volcanic and structural evolution is therefore characterized by the repetition of a similar eruption mechanism, which includes the formation of a ring fracture, magma uprise and eruption from several points along the fracture and collapse of the central sector into the emptied magma chamber. The decrease with time of the dimensions of the resulting structure (diameter of the collapsed area; total sinking) reflects the progressively decreasing volume of the magma available in the chamber.

A review on phreatic eruptions and their precursors

Abstract An analysis of the reported information on 132 historical phreatic events indicates that most of these (115) were not followed by a magmatic or phreatomagmatic eruption. Only in 18 cases precursory phenomena are reportedly lacking, whereas they are recognized for the majority (72) of the considered cases. These precursors have been analyzed as a function of the kind of phreatic event and of its possible triggering mechanism. Unfortunately, the quality of the available information is poor. However, data suggest that anomalous seismicity, ground deformation, changes in the output, chemistry and temperature of fumaroles and thermal springs are potential mid-term precursors, whereas seismic tremor is a common short-term precursor.

Magmatic and phreatomagmatic phases in explosive eruptions of Vesuvius as deduced by grain-size and component analysis of the pyroclastic deposits

Abstract Grain-size and component analyses have been carried out on pyroclastic deposits of three well-known explosive eruptions of Vesuvius: those of 79 A.D., 1631 and 1906. These eruptions cover a wide energy spectrum, from Plinian to Strombolian, and each includes a transition from a magmatic to a well-documented and clearly distinguishable phreatomagmatic phase of activity. The chronicles of the eruptions are revisited in light of the granulometric and component data obtained from the study of the deposits and interpretation of the dynamics of the main eruptive episodes is provided for each eruption. The phreatomagmatic products (originated from explosive interaction of magma with groundwater) have characteristic grain-size and component distribution patterns distinct from associated purely magmatic pyroclastic products: in any eruption phreatomagmatic products exhibit: (a) a marked increase in both the lithic/juvenile and the crystal/total juvenile ratios, and (b) a preferential fragmentation of the juvenile fraction. Moreover, eruption energy, degree of evolution of erupted magma, lithic content and depth of provenance are all clearly correlated. The abundance of lithics and crystals in phreatomagmatic deposits can be ascribed to (a) preferential fragmentation of the aquifer-hosting rocks due to explosive vaporization of ground water; (b) vent flaring and craterization possibly related to overpressure conditions following the entrance of large quantities of steam in the conduit; (c) indirect enrichment in both crystal and lithic fractions by removal of juvenile fines from the eruptive cloud. In each eruption some of the deposits with “phreatomagmatic” component distribution patterns also exhibit evidence of steam condensation (accretionary lapilli, vesiculated tuff, mud coating, soft deformations) and some do not: these are therefore to be considered sufficient but not necessary diagnostic features for a hydromagmatic origin of the deposit (“wet” in the sense of Sheridan and Wohletz, 1983). “Dry” phreatomagmatic products correspond to higher energy conditions allowing production and maintenance of overheated steam. The magma/water ratio in terms of heat exchange surface (i.e. the degree of primary fragmentation of the magma) is certainly the main factor controlling the efficiency and the energy balance of the interaction. A series of observations strongly suggest that a primarily fragmented magma is a basic requisite for the explosive magma/water interaction: (a) occurrence of short phreatomagmatic episodes interrupting lava fountains activity (1906 eruption) in connection with drawdown of the magma column in the conduit; (b) parallel increase in depth of magma fragmentation level and magma/water interaction level (as deduced by the nature of lithics), and (c) preferential occurrence of phreatomagmatic activity at the end of explosive eruptions (when juvenile gas pressure declines and water from the confining aquifer more easily enters the conduit).

Plio-quaternary volcanism in Ecuador

Extensive sampling, major element chemistry on over 300 samples and K-Ar radiometric dating have been carried out on the Ecuadorian Upper Tertiary–Quaternary volcanoes. The results show important space–time variations of the volcanic activity, between Late Miocene time and the present. In Late Miocene time a continuous volcanic belt, located approximately along the present volcanic front (VF), affected the whole country from the Cuenca basin to the south, up to Colombia to the north. Major changes occurred at about 5 Ma: volcanic activity stopped south of the Guayaquil fault belt and never resumed; to the north the active volcanic axis shifted eastward to the Cordillera Real (CR) area with a simultaneous relative decrease in intensity. Since Early Quaternary time the volcanic belt widened westward to the Western Cordillera where the volcanism resumed at about 1.5–1.0 Ma, giving rise to the very wide active volcanic zone of Ecuador. The Plio-Quaternary products show significant longitudinal and latitudinal chemical and mineralogical changes. Volcanics of the VF and Interandean Depression contain amphibole and define a calc-alkaline trend with a K 2 O content lower than that of the CR products, which are characterized by a mostly anhydrous phenocryst assemblage. In both areas andesites dominate, but extreme compositions (basaltic andesites and rhyolites) are more diffuse in the CR than the VF. No significant transverse zoning has been detected in the northern region (north of the Chota-Mira transverse tectonic line). The observed temporal and spatial variations are interpreted as a result of the subduction of the Carnegie Ridge anomalous oceanic crust, underthrusting of which began approximately 6 Ma ago.