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A systematic petrographic and chemical study of chronologically ordered lava samples collected during the 1983 Etna eruption, has shown minor fluctuating variations in crystallinity and chemistry, apparently correlated with variations in the rate of effusion. Comparison with the compositional variations observed in the lavas emitted during the 1971-1983 period, has revealed a repeated evolutional tendency suggesting a small size shallow storage system periodically refilled, as in 1983, by more primitive magma. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Volcanological and petrological data suggest that the Phlegraean Fields volcanic activity has been fed, at least in the last 10,500 years, by a not-refilled magma chamber where trachytic residual liquids were produced by fractionation of a trachybasaltic magma. Using estimated volumes of the erupted products and P-T data obtained through petrological studies, a conductive thermal model of the chamber was built up in order to estimate its past and present size. Results suggest a volume decrease from approximately 14 to 1.4 km of the trachybasaltic magma in 10,500 years. Trachytic liquid would also be present in the chamber in a minimum amount of 0.4 km. The model allowed some insights on the petrogenesis of the Phlegraean trachytes, suggesting that they were erupted as liquids because thermally buffered within the magma chamber. [ABSTRACT FROM AUTHOR]

Subsurface geothermal exploration has considerably added to our knowledge of the Latera volcanic complex. A syenitic body is located about 2 km below the present-day surface; K-Ar data point a 0.9 Ma age. The primary magma was a silica-saturated trachyte; undersaturated, hauyne-bearing products are found near the carbonatic wall-rocks and have been interpreted as reaction products. Subsurface data from deep drilling and geophysical surveys suggest that the Latera caldera resulted from three main successive collapse phases: ( i) formation of an old caldera, now buried, related to the eruption of ignimbrites from the syenitic magma chamber; ( ii) sinking of the eastern sector as a consequence of the formation of the nearby Bolsena caldera (∼0.3 Ma); ( iii) multistage formation of the present Latera caldera (∼0.16 Ma). [ABSTRACT FROM AUTHOR]

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 km. 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. [ABSTRACT FROM AUTHOR]

Petrological, mineralogical and chemical data of 46 ejecta deriving from the sedimentary basement beneath Somma-Vesuvius volcano are reported. The ejecta samples were collected in pumice deposits formed during two major Plinian eruptions. One of these pumice deposits was formed during the well-known 79 A.D. eruption, and the other one - the so-called Avellino pumice - during an eruption occurred about 3,500 years B.P. Most of the ejecta from both the layers are fragments of contact-metamorphosed carbonate rocks. For the ejecta of the 79 A.D. Plinian eruption, the mineralogical parageneses of the metamorphosed carbonate rocks (dolomite-Mg calcite-periclase, and dolomite-Mg calcite-brucite) allow the evaluation of the conditions under which contact metamorphism developed. Temperatures, estimated by the Mg content in the calcite coexisting with dolomite, ranged from 360° to 790°C, whereas total fluid pressure would not have exceeded 1,500-2,000 bars with a maximum depth of metamorphism (and hence of the magma chambers) of 5,000-6,000 m. The ejecta from the so-called Avellino pumice layer (characzerized prevalently by a dolomite-Mg calcite assemblage) show that contact metamorphism occurred under the same temperature range as that of the 79 A.D. ejecta, but at an higher P partial pressure and probably at an higher total fluid pressure. These differences in physico-chemical conditions of metamorphism seem to indicate that the two Plinian eruptions were fed probably by two different magma chambers. Comparison between chemical composition of the carbonate ejecta and carbonate samples of the Mesozoic sedimentary series outcropping near the volcano indicates that fragmentation of almost all the sequence were brought to the surface by the explosive Plinian eruptions. Although the data at our disposal do not provide any information on the size of the 79 A.D. eruption magma chamber, this probably had an important vertical length component of at least 2,000 m [ABSTRACT FROM AUTHOR]

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 km; 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. [ABSTRACT FROM AUTHOR]

A summary of the available data on the peralkaline rocks of S. Pietro and S. Antioco islands, together with, new chemical analyses and some preliminary K-Ar ages are reported. Peralkaline rocks occur as ignimbrites, lava flows and domes usually deeply affected by hydrothermal alteration. Pantelleritic varieties are found within the dominantly comenditic association, which display KO contents higher than NaO ones. K-Ar data indicate that these peralkaline rocks have a middle Miocene age (≅ 15 m.y.). They occur in close field association with coheval andesitic and subalkaline acid volcanics belonging to the final products of the Tertiary calc-alkaline volcanic cicle of Sardinia. [ABSTRACT FROM AUTHOR]

Three main types of recent volcanism may be distinguished in the Afar Depression: 1) oceanic volcanism of the axial ranges; 2) volcanism along the margins where an attenuated sialic crust probably occurs; 3) mainly fissural volcanism of Central-Southern Afar, with associated central volcanoes, similar as a whole to the volcanism of the Ethiopian Rift Valley. Peralkaline silicic volcanic rocks are found in all the three groups but showing some different characteristics which seem related to their geological location and which probably reflect different sources. Moreover emplacement of peralkaline granitic bodies, associated with volcanics of the same composition, marks the first stage of formation of the Afar Depression, in the Early Miocene. Axial Ranges: Erta'Ale and Boina volcanic ranges indicate that peralkaline rocks are the final liquids produced by fractionation of basalt in shallow magma chambers of central volcanoes. The parental magma is a transitional type of basalt with a mildly alkalic affinity, which fractionated under low pHO- pO conditions. Transition to peralkaline liquids is realized without passing a «true» trachytic (low silica) stage. The first peralkaline liquid is a low silica comendite and evidence exists that «plagioclase effect» was active in determining the first peralkalinity. Within the peralkaline field a fractionation mainly controlled by alkali feldspar progressively increases the peralkalinity and silica oversaturation of residual liquids (transition from comendites to pantellerites). The most peralkaline pantellerites of Boina are produced by fractionation of an alkali feldspar of constant composition (Ab Or) suggesting that these liquids lie on a «low temperature zone» of the peralkaline oversaturated system. Marginal Units: On the borders of the depression peralkaline silicics are found in volcanic massifs mainly made of metaluminous silicic products. Petrology and geochemistry suggest a complex origin. Crystal fractionation, contamination with sialic crust and chemical changes related to a volatile rich phase, all these processes probably played a role in the genesis of these peralkaline silicic rocks. Central-Southern Afar Fissural Volcanism: Mildly alkaline basalts are associated with peralkaline and metaluminous silicics; intermediate rocks are very scanty. Fractionation from deep seated magmatic bodies with selective eruptivity and partial melting at depth of associated basalts or of a common source material are possible genetic mechanisms. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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