|
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.
Back to index Volume
7.1
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.
Back to index Volume
7.1
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.
Back to index Volume
7.1
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.
Back to index Volume
7.1
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.
Back to index Volume
7.1
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.
Back to index Volume
7.1
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.
Back to index Volume
7.1
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.
Back to index Volume
7.1
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.
Back to index Volume
7.1
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.
Back to index Volume
7.1
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.
Back to index Volume
7.1
|