L. Lirer

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.

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.

The Ottaviano eruption of Somma-Vesuvio (8000 y B.P.): a magmatic alternating fall and flow-forming eruption

The Ottaviano eruption occurred in the late neolithic (8000 y B.P.). 2.40 km3 of phonolitic pyroclastic material (0.61 km3 DRE) were emplaced as pyroclastic flow, surge and fall deposits. The eruption began with a fall phase, with a model column height of 14 km, producing a pumice fall deposit (LA). This phase ended with short-lived weak explosive activity, giving rise to a fine-grained deposit (L1), passing to pumice fall deposits as the result of an increasing column height and mass discharge rate. The subsequent two fall phases (producing LB and LC deposits), had model column heights of 20 and 22 km with eruption rates of 2.5 × 107 and 2.81 × 107 kg/s, respectively. These phases ended with the deposition of ash layers (L2 and L3), related to a decreasing, pulsing explosive activity. The values of dynamic parameters calculated for the eruption classify it as a sub-plinian event. Each fall phase was characterized by variations in the eruptive intensity, and several pyroclastic flows were emplaced (F1 to F3). Alternating pumice and ash fall beds record the waning of the eruption. Finally, owing to the collapse of a eruptive column of low gas content, the last pyroclastic flow (F4) was emplaced.

Stromboli and its 1975 eruption

On November 4, 1975 in the evening, an eruption took place at Mt. Stromboli. On the following day lava flowed on the Sciara del Fuoco downward to the sea, accompanied by an intense explosive activity at the crater plane. Direct observations on the volcanic activity were carried out since November 6 while a seismic survey was made from Nov. 7 to 12. The total volume of the lava outpoured during this period of activity that lasted 21 days, was estimated to be about 104 m3. This paper reports the results of direct observations, and of the petrological, radioactive disequilibria and seismic activity studies performed for this eruption.The eruption was preceded by an insignificant change of seismic activity, which was monitored by a seismic station located about 2 km East of the crater. A shallow seismicity was strietly related to crater explosions accompanying the eruptive phenomenon. Radioactive disequilibria showed a lack of disequilibrium between228Ra and232Th explainable in terms of a fast rising of magma in the conduit.Chemical analyses of lava samples and deep seismic sounding data indicate a correspondence between the depth (10–15 km) at which crystallization pressure of phenocrysts occurs and a low velocity laver.

K-ar ages of the yemen plateau

Abstract The temporal evolution of the Yemen Trap series is examined in the light of K-Ar radiometric data. Rifting and volcanic activity characterize the Yemen plateau between 30 and 20 m.y. confirming a fairly common history of the Afro-Arabian plate on both sides of the Red Sea.

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.

Sr isotope geochemistry of megacrysts from continental rift and converging plate margin alkaline volcanism in South Italy

Abstract Mineral phases of two-clinopyroxene alkaline lavas from continental rift and plate margin volcanism in South Italy have been analyzed for their Sr isotopic composition and concentration. Sr isotope disequilibria are observed between megacrysts and groundmass in all seven analysed Campanian potassic lavas, but not in a lava from Stromboli, a volcano in the Eolian arc. Variations in 87 Sr/ 86 Sr ratios for different phases in the lavas are likely to reflect primary Sr isotope variations in the primitive lavas (rather than crustal contamination effects). It is suggested that the observed mineral disequilibria point to the intimate association of a range of primary magmas and small-scale source heterogeneities for the Campanian volcanism. The lack of mineral disequilibria for Stromboli suggests that here source heterogeneities are absent or else exist on a very much larger scale. It is therefore unlikely that there is any genetic connection between these two types of alkaline volcanism in South Italy.

Tephrostratigraphy of the A.D. 79 pyroclastic deposits in perivolcanic areas of Mt. Vesuvio (Italy)

Abstract Correlations between pyroclastic deposits in perivolcanic areas are often complicated by lateral and vertical textural variations linked to very localized depositional effects. In this regard, a detailed sampling of A.D. 79 eruption products has been performed in the main archaeological sites of the perivolcanic area, with the aim of carrying out a grain-size, compositional and geochemical investigation so as to identify the marker layers from different stratigraphic successions and thus reconstruct the eruptive sequence. In order to process the large number of data available, a statistical approach was considered the most suitable. Statistical processing highlighted 14 marker layers among the fall, stratified surge and pyroclastic flow deposits. Furthermore statistical analysis made it possible to correlate pyroclastic flow and surge deposits interbedded with fall, interpreted as a lateral facies variation. Finally, the passage from magmatic to hydromagmatic activity is marked by the deposition of pyroclastic flow, surge and accretionary lapilli-bearing deposits. No transitional phase from magmatic to hydromagmatic activity has been recognized.

Mineralogy and geochemistry of the “low-potassium series” of the Campania volcanics (south Italy)

Abstract The Roccamonfina volcano, the Phlegraean Fields and Ischia Island belong to the K -Quaternary volcanism of the Roman Volcanic Province. The “low-K series” of these volcanic areas are characterized by common features such as: mineralogical assemblage, crystallization patterns and the increase of p O 2 and p H 2 O conditions during the process of fractionation. However, even though the three series show the same mineralogical assemblages, the various phases seem to have slightly different roles. In fact, the different Ba distribution in the Mg-biotite—K-feldspar pairs seems to depend on the order of appearance of the Mg-biotite with respect to the K-feldspar. In addition, in the three trends there are similarities in the pattern distributions of the K/Rb ratio vs. the Rb/Sr ratio, indicating that the differentiation processes controlling the evolution are almost the same for these suites. On the other hand, the different absolute values of these ratios in the supposed parental liquids suggest a heterogeneous source or different depth or condition of magma generation. The location in the petrogenys residual system of the supposed original liquids and their derivatives is close to the Ab—Or join. Thus the parental magma was saturated and this degree of saturation was almost constant during the fractionation, with derivative liquids remaining on the high-temperature ridge of the two-feldspar surface.

Mineralogy and geochemistry of the « Low potassium » series of the roccamonfina volcanic suite (Campania, South Italy)

In the Campania area volcanic rocks belonging both to the trachytic (low K) and tephritic (high K) series occur, the high K series being well subordinate to the low K one. Volcanology of the trachytic series shows that: 1) the most widespread products are trachytes and alkali-trachytes, 2) in the stratigraphic sequences the most evolved (salic) volcanics are the oldest.The Roccamonfina trachytic series includes several Hy and Q normative lavas, whereas the majority of the Phlegraean Fields rock samples are Ne normative. In the A F M diagram, the lack of the Fe-rich phases gives a linear trend similar to the calc-alkaline series.An important feature of the Roccamonfina trachytic series is the coexistence of two clinopyroxenes of diopsidic and salitic composition observed in all the studied samples. The phenocrystic assemblage (clinopyroxenes, Mg-olivine, Mg-biotite) indicates that the crystallization of the melt probably occurred under relatively highpH2O conditions. The crystallization of the diopsidic pyroxene was controlled mainly by a relatively high silica activity of the magma, whereas the salitic pyroxene, which crystallized at a lithostatic pressure lower than the diopside, might be related to the increase of the alumina activity in the final trachytic liquid. The occurrence of two clinoproxenes suggests two main episodes of crystallization of the parental latitic magma, one at a depth of about 15 km producing diopside phenocrysts and the second at a shallower depth where salite was in equilibrium with the residual melt.The volcanic rocks of the Roccamonfina trachytic series (latites, trachytes and alkalitrachytes) have likely been generated by fractionation of a latitic melt, whereas the strongly porphyritic trachybasalts formed by crystal settling of pyroxenes and olivine from a more evolved latitic melt.