VOLUME  7 (1) - 1995

A New Geo-Volcanological Map of Filicudi Island (Aeolian Arc, Italy)

P. Manetti 1, G. Pasquarè 2, Abebe Tsegayel 3

1. Dipartimento di Scienze della Terra, Via La Pira 4, 50121 Firenze, Italy.
2. Dipartimento di Scienze della Terra, Via Mangiagalli 34, 20133 Milano, Italy.
3 . Ethiopian Institute of Geological Surveys, P. O. Box 2302, Addis Abeba, Ethiopia.

Abstract

The main geological and geo-volcanological results obtained during the field works of the island of Filicudi, are re-ported on the 10,000 scale map which accompanies this article. Based on the standards of the CNR commission for the preparation of a geological map of Italy (1:50,000 scale),various unconformity bounded stratigraphic units (UBSU) have been identified in the island of Filicudi (specificallyfour synthems, four lithosomes and various formations). It was also possible to recognise a complex succession ofvolcanological events, which started with the growing of the oldest volcanic edifice here proposed as Paleofilicudi.Successively after a long period of quiescence, the major part of the island was constructed from various volcanicedifices in the middle and upper Pleistocene. Volcanic products of the island are prevalently andesitic lava flowsintercalated with abundant pyroclastics. The final volcanic event in the island produced pyroclastic deposits whichcovered almost the whole island and locally forming very thick piles.

 

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Geology, structure and evolution of the island of Alicudi,
Aeolian Volcanic Arc, Italy

(With a 1:10,000 Scale Geological Map)

P. Manetti l, G. Pasquare´ 2, A. Tibaldi 2, Abebe Tsegayel 3

1. Dipartimento di Scienze della Terra, Via La Pira 4, 50121-Firenze, Itaiy
2. Dipartimento di Scienze della Terra, Via Mangiagalli 34, 20133-Milano, Italy
3. Ethiopian Institute of Geological Surveys, P. O. Box 2302, Addis Abeba, Ethiopia

Abstract

The island of Alicudi is surveyed at 1:5,000 scale using a stratigraphic classification scheme proposed by the National Research Council (CNR) for the new geological map of Italy. The map of Alicudi is here presented at 1:10,000 scale. A series of main discontinuities, represented by erosion and collapse surfaces, enabled us to distinguish three synthems, which represent the main phases of the geological evolution of the island. Fourteen lithostratigraphic units are recog- nised within these synthems and represent the base-units used for geological mapping. Some distinct volcanic edifices showing a well preserved morphology, which enabled us to localise the relative effusive centre, are distinguished as lithosomes. These stratigraphic data, together with morphological and structural observations, allowed us to recognise that the growth of the volcano occurred through a series of main episodes followed by phases of inactivity or volcano- tectonic collapses. In particular some of these collapses were accompanied or preceded by explosive activities and followed by lava flows and dome emplacements.

 

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40Ar/39Ar ages of the Filicudi Island volcanics:   implications for the volcanological history of the Aeolian Arc, Italy

A. P. Santo l, Y. Chen 2, A. H. Clark 2, E. Farrar 2, Abebe Tsegaye l , 3

1. Dipartimento di Scienze della Terra, Universita´di Flrenze, via La Pira 4, 50121 Firenze, Italy
2. Department of Geological Sciences, Queen´s University, Kingston, Ontario, Canada K7L 3N6
3. Ethiopian Institute of Geological Surveys, P.O. Box 2302, Addis Abeba, Ethiopia

  Abstract

New incremental-heating 40Ar/39Ar age data for volcanic rocks from Filicudi (Aeolian Arc, Southern Tyrrhenian Sea) are summarized. The island was built up by eruptions from many volcanic centres which show complex geometrical relationships. In addition, two other eruptive cones, La Canna and Banco di Filicudi, respectively partially and completely submerged, were recognised to the NW of the island. Only calc-alkaline rocks, lavas and pyroclastics, are present, ranging in composition from basalt, through basaltic-andesite, to high-K andesite. The geochronological data together with field observations, permitted the reconstruction of the geological history of the island. Four major volcanic activity events are recognised. These are represented by: 1) Zucco Grande formation (1.02 Ma), 2) Filo del Banco Formation (0.39 Ma); 3) Monte Palmieri formation, Sciara formation, Monte Guardia formation and Capo Graziano dome (0.25-0.19 Ma); and 4) the pyroclastic Valle La Fossa formation, and beyond the island, the emergent part of the La Canna neck (0.04 Ma). The obtained 40Ar/39Ar  age data are also discussed in the wider context of the entire Aeolian province.

 

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Gravity anomalies and structures at the island of Pantelleria

G. Berrino , P. Capuano 

Osservatorio Vesuviano, via A. Manzoni 249, 80123 Napoli, Italy

Abstract

The island of Pantelleria is an active volcanic area located in the Sicily Channel (Italy), in the median part of a continental rift system.The island represents the emerged part of a submarine volcanic field; it is entirely made up of volcanic rocks and approximately half the height of the volcanic edifice is above sea level. The structural setting of Pantelleria is defined by faults and fractures trending NW-SE and NE-SW. Two calderas of different ages, located in the central part of the island, and a resurgent block inside the younger caldera are the main volcano-tectonic features of the island. The most recent eruptive activity is represented by two submarine eruptions occurred respectively in 1831 and 1891 A.D., the presence of hot water springs and fumaroles is indicative of the persistent activity of the magmatic system. A new Bouguer anomaly map has been obtained to better define the local structural pattern due to the regional
tectonics and the volcanic activity. The anomaly field has been computed on the basis of 234 gravity stations measured in different surveys. The Bouguer anomalies have been derived with reference to the 1930 International Formula and using, in the data reduction, the value of 2500 kg/m3 for the density of the shallow rocks. Moreover the anomalies have been also computed with reference to the 1980 International Ellipsoid and an empirical formula is given to transfer the values of the anomaly from the 1930 Reference into 1980 one and vice versa. To permit the comparison of the current data and results with those of previous works, the maps and the interpretative models are based on the anomaly values computed using the 1930 International Formula. To better emphasiie the anomalies of shallow origin, the map of the vertical gravity gradient has been also drawn.   The Bouguer anomaly on the island of Pantelleria shows an average value of about 78 mGal, consistent with the general pattern of the gravity anomaly in the Sicily Channel. The distribution of the anomalies on the island outlines two different areas. The first one, in the north-western part, is characterized by the lowest values of the anomaly and
by a smooth pattern of the isoanomalies decreasing from 80 to 70 mGal; in this area the most recent volcanic activity occurred. In the remaining part of the island, where the most important volcano-tectonic structures are present, the anomaly field appears quite irregular in its remarkable and localized maxima and minima. The limit between the two areas coincides with the northern rim of the ancient caldera and is well evidenced in the gradiometric map.
The modelling of the source responsible of the Bouguer anomaly field has been done. A possible interpretative model is consistent with a basement having the top at variable depth. The basement is dipping in the north-western part of the island; it rises in the central part, and in the SW and SE of the island. Very often abrupt variations of the depth of the basement are observed in correspondence of faults and fractures evidenced or inferred by geological observations; a rise of the basement is observed in correspondence of the rim of the caldera.   The results of repeated altimetric easurements, carried out on the island since 1980, suggest that the north-western part of Pantelleria is having a small uplift while a subsidence is observed in the central-southern part, inside the older caldera. Therefore the kynematic behaviour indicates that Pantelleria appears divided in two parts like already observed in the gravity maps.

 

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Detection of ground movements on the Mt. Etna volcano using the Global Positioning System

C. Bruyninx, R. Warnant

Royal Observatory of Belgium (ROB) Avenue Circulaire, 3 B-1180 Bruxelles, Belgium 

Abstract

The ETNA92 (Sept´92) and ETNA93 (Sept´93) GPS campaigns provided the first GPS network for the detection of ground movements on Mt. Etna with 3 connections outside the geological unstable region of the volcano. Baseline repeatabilities are of the order of 10-7 for short baselines (< 50 km) and 10-8 for the long baselines (> 100 km), with a strong dependence on the height difference. The extreme tropospheric conditions and different micro-climates cause systematic errors of about 10 cm in the estimated heights if an a priori tropospheric modelling is used. Around 90% of these errors cancel out when comparing the positioning results from both campaigns with each other. This shows that GPS is a useful technique to monitor volcano deformations if it is used with care.

 

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Subsurface gases in selected sites of the Mount Etna area (Sicily)

F. Parello 1, W. D´Alessandro 2, P. Bonfanti 2, G. Dongarra´ 3 

1. Istituto di Mineralogia, Petrograf a e Geochimica, via Archirafi 36,  90I23 Palermo, Italy
2. Istituto di Geochimica dei Fluidi, CNR, via Torino 271/DS, 90133 Palermo, Italy
3. Istituto di Scienze della Terra, Salita Sperone 3I, 98I66 S. Agata di Messina, Italy 

Abstract

This paper reports the chemical analyses of the gases from Patern˜ Salinelle, Acqua Grassa Spring, S. Venerina mofettes, Fondachello Salse, located in the Etnean area (Eastern Sicily). Data of PCO2 and 222Rn in the waters of some wells and springs are also reported. The analytical data regarding soil gases on the Eastern flank of M. Etna have shown a good correlation between CO2 and 222Rn which, although in different ways, indicate the mechanism and extent of degassing. The measurements carried out in soil gases shows that CO2, when it is the prevailing gas species, acts as a carrier for 222Rn. Results obtained for the soil gases at Fondachello, where the prevailing deep gas is CH4, confirm that 222Rn transport is effected by any gaseous compound. 222Rn activity measurements in waters show similar trends for both springs and wells, the latter showing higher values. In waters with a free gas phase, 222Rn shows a greater affinity for the volatile phase. Some anomalies observed both in the PCO2 of groundwater and in the chemical composition of some gas manifestations can provide useful informations for monitoring seismic and volcanic activity in the Mount Etna area.

 

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Submarine gas-emission from Panarea Island (Aeolian Archipelago): distribution of inorganic and organic compounds and inferences about source conditions

N. Calanchi l, B. Capaccioni 2, M. Martini 3, F. Tassi 3, L. Valentini 2

1. Dipartimento di Scienze Mineralogiche, Universita´ di Bologna, Piazza di Porta San Donato 1, 40126 Bologna, Italia
2. Istituto di Vulcanologia e Geochimica Universita´ di Urbino, via Oddi 14, 61029 Urbino, Italia
3. Dipartimento di Scienze della Terra, Universita´ di Firenze, via La Pira 4, 50I21 Firenze, Italia

Abstract

On the basis of a scubadiving prospection, four main submarine fumarolic fields were recognized east of Panarea island (Aeolian Archipelago). Nine samples of fumarolic gases were collected and analyzed for inorganic and organic compounds. Gases from the area close to Panarea ("La Calcara") appear to be equilibrated at very shallow depth at temperatures lower than 100°C; a significant content of organic compounds not affected by thermal or catalytic effects is also noteworthy. Relatively higher equilibrium temperatures are shown by the submarine fumarolic fields located among scattered reefs about 2 km east of Panarea. Among then, the Basiluzzo islet, about 3 km north-east of Panarea, appear as affected by diffuse gas exhalations at very low fluxes, but with the highest recorded equilibrium temperatures (up to 200°C). Two different re-equilibration zones, the deeper and hotter producing alkenes and the shallower and cooler giving rise to re-equilibration of the inorganic carbon compounds, may be hypothesized.

 

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Explosion quakes analysis at Stromboli: Experimental data and synthetic seismograms

D. Ereditato, G. Luongo

Dipartimento di Geofisica e Vulcanologia, Largo S. Marcellino 10, 80138 Napoli

Abstract

This paper reports the results of an analysis of explosion quakes at Stromboli. The data utilized were obtained from the Osservatorio Vesuviano´s Semaforo San Vincenzo permanent three-component analogical station. Stromboli is a volcano characterized by persistent seismic activity consisting in explosion quakes, i.e. transient seismic events occurring non-stop one after the other. The tipical Strombolian activity mostly consist in explosive phenomena affecting its four active summit craters (Lo Bascio et al., 1973; Ntepe & Dorel, 1990). The explosions, usually accompanied by a loud roar which can sometimes be heard from the sea, cause pyroclastic material to be emitted together with jets of gas from one or more craters. Explosion quakes are seismic events associated with such explosive volcanic phenomena, although
single seismic events are not always accompanied by volcanic activity that can be observed at craters (Capaldi et al. 1978). A distinctive feature of explosion quakes is the presence on seismograms of two, often clearly distinct, seismic phases. The polarization of the two main seismic phases of explosion quakes was analysed: the first phase has a low frequency (<2 Hertz) and is followed, a few seconds later, by a second one with a higher frequency (>3-4 Hertz). The
first phase shows a strong linear polarization that can be associated with a compressionale wave with the incidence angle near the horizontal surface; the second one shows a more chaotic motion. We performed synthetic seismograms using the Crosson & Bame (1985) low frequency generation model with a free-surface correction. Calculations were performed by using discrete wave number by Bouchon & Aki (1977). There is a good agreement between theoretical results and data as regards the first low-frequency seismic phase, a~though the model does not seem physically fully reasonable.

 

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Magma chamber processes preceding the Pitigliano Formation eruption
(Latera volcanic complex, central Italy): 
evidence from cognate plutonic clasts

A. Renzulli 1, B. G. J. Upton 2, G. Nappi 1

1. Istituto di Vulcanologia e Geochimica, Universita´ di Urbino, via Oddi 14, 61029 Urbino, Italy
2 . Department of Geology and Geophysics, University of Edinburgh, U. K

Abstract

The Pitigliano Formation, comprising pumice flows, welded tuffs and lava flows was produced during the culminating eruption of the Latera caldera, just before the effusive post-caldera phases. It is compositionally zoned from phonolitic at the base to tephriphonolitic at the top. The mineral assemblage in the volcanic sequence is regarded as mixtures of free-growing phenocrysts at the magma chamber interior, cognate crystals stripped from the solidification zones of cumulates ("crystal mush layers") plating the side-walls and roof chamber and xenocrysts from thermally metamorphosed country rocks. The Formation also contain abundant coarse grained clasts. These are mainly syenitic; most are leucocratic and mesocratic foiditic varieties containing hauyne and/or subordinate leucite although there is a gradation through silica-saturated syenites to scarcer quartz syenites. Rare clasts of feldspathoidal monzosyenites and gabbros occur, as do foidolites composed of cumulus plagioclase enclosed by intercumulus oikocrysts of leucite and/or hauyne.  Since the feldspathoidal syenites possess drusy and uncompacted textures, in contrast to the silica-saturated and oversaturated syenites, they are inferred to have had less complex sub-solidus evolution and to be younger than the latter. Whereas all the plutonic clasts are regarded as products of the same magmatic system that erupted to form the Pitigliano Formation, the foiditic syenites are considered to be directly cognate to the observed phonolitic-tephriphonolitic volcanics. It is suggested that the leucocratic foiditic syenites grew as upper side-wall and roofing cumulates around the phonolitic upper part of the magma chamber prior to the eruption that produced the Pitigliano Formation. They are regarded as near-ideal orthocumulates in which a variety of incompatible trace elements was progressively concentrated in the intercumulus, from which titanite, apatite and melanite garnets crystallised. A proportion of the volatile-rich magmatic residues was, however, lost in the production of drusy void spaces. The bulk composition of the leucocratic orthocumulates (foiditic syenites), however, is inferred to be closely similar to that of the phonolite melts from which they grew.  Modal layering within the side-wall cumulates is suggested by the rare occurrence of plutonic clasts exhibiting contrasting leucocratic and mesocratic feldspathoidal syenite layers up to several cm thick.  Mixing processes should have occurred in the deeper levels of the magma chamber as inferred from different populations of feldspars arld clinopyroxenes, together with some compositional zonings of crystals within the Pitigliano formation.

 

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The magmatic evolution of the "modern" activity of the Nevado de Colima volcano in relation to the Colima Volcanic Complex activity (Mexico)

N. Calanchi 1, F. Lucchini 1, C. Navarro-Ochoa 2, P. L. Rossi 1, J. Sanchez-Perez 2

1. Dipartimento di Scienze Mineralogiche, piazza di Porta S. Donato 1, 40126 Bologna Italy
2. Comision Federal de Electricidad, Calle ONCE #1226, Fraccionamiento La Misiòn, Ensenada, Baja California, Mexico

Abstract

The geologic history of the "modern" Nevado de Colima magmatic activity reported in literature is divided in two periods (Nevado II and III), separated by a caldera-forming event, ranging from 0.2 Ma up to some thousand years b.p. The rocks of Nevado 1I show medium-K calc-alkaline character and range in composition from high-SiO2 andesite to dacite. Their magmatic evolution is compatible with crystal fractionation processes involving Pl, Cpx, Opx and opaques as fractionating phases; however, a limited mixing between andesitic liquids and more basic calc-alkaline magmas, supported by reverse zoning of both Pl and Cpx and by trace elements geochemistry, cannot be ruled out. Nevado III andesites differ from the Nevado II ones mainly for their higher Mgv and for the characteristic presence of Mg-hastingsite. The marked enrichment in incompatible elements jointly with disequilibrium features in the mineral assemblage suggest that these rocks can be derived by mixing processes between the Nevado II andesites and alkaline basaltic magmas like that of the coeval Volcan Erita. Mixing process is supported also by amphibole compositions. In the framework of the geologic evolution of the Colima Volcanic Complex, the Nevado III volcanism represents the last activity of Nevado de Colima volcano and is the oldest documented example of interaction between calc-alkaline and slightly alkaline magmas occurred in the area.

 

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Geological map of the San Venanzo volcano (Central Italy):
explanatory notes

F. Stoppa, S. Sforna

Dipartimento di Scienze della Terra, Università di Perugia, piazza Università, 06100 Perugia, Italy

Abstract

The volcanics of the San Venanzo area (Terni) were erupted from three local vents that built up small asymmetric pyroclastic structures associated with lava flows: San Venanzo-maar to the north; Pian di Celle tuff-ring, less than one kilometre south of San Venanzo; and Celli lapilli-cone, about 500 metres east of Pian di Celle. The volcanic morphologies, and the maar-diatreme crater structures in particular, are fairly well preserved. Explosive activity produced about 6X106 m3 of pyroclastic material, mostly lapilli-sized, whereas more than 1x106 m3 of lava was outpoured. The overall sequence shows that the initial crater-forming explosions were followed by strombolian activity. The San Venanzo eruptions caused progressive widening and deepening of the craters, which remained set in the sedimentary substratum. The initial products of San Venanzo volcano consist of breccias containing large accessory-blocks coming from the sedimentary substratum. Late products consist of mainly lapilli-sized juvenile deposits. Pyroclastic surge and minor pyroclastic flow are the main depositional mechanisms. The presence of nucleated, concentric-shelled glassy "lapilli" formed in subvolcanic conditions is typical and is associated with the mobilisation and eruption of diatremic breccia
(tuffisite). The juvenile fraction of the pyroclastic rocks contains large amounts of calcite up to 50% in vol., whose composition and texture are suggestive of a magmatic origin. Numerous levels of fine-grained primary calcite, with volcanic-sedimentary structures, are considered as products of the fall of carbonatite ash.

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