The dynamics of a lava flow with thermal layering is studied with a two-dimensional model describing a viscous fuid with Bingham rheology, flowing down a slope. We assume that lava is thermally homogeneous under the plug and that heat is transferred through the plug by conduction. Heat is lost by radiation to the atmosphere at the surface of the flow. Immediately after the emission of lava from the vent, a thin crust develops at the top of the flow. The surface temperature decreases rapidly, while the inner temperature remains high. As temperature decreases, viscosity and yield stress increase. At the same time, velocity decreases, the crust and the plug thicken and the total flow height increases. The present model, which considers the thermal effect of the plug, generates a higher inner temperature with respect to previous models, in which temperature was assumed constant in each vertical cross section of the flow. This seems to be in agreement with observations showing relatively small decreases of the inner temperature even in distal parts of the flow.
The study of volcanic premonitory phenomenon can be considered of the greatest importance for any program of mitigation of disasters, but no univocal information is yet available about forerunning phenomena of eruptive episodes in recent times (e.g. at Usu, Japan, a seismic swarm occurred 32 hours before the eruption of 1977; at Ontake, Japan, a weak seismicity was recorded 6 hours in advance to the 1979 event; the eruptions of Asama, Japan, in 1982, and of Ebeko, Kurile, in 1987, occurred apparently without any seismic precursor). Moreover, in some instances no volcanic activity followed significant seismic manifestations. The joint investigation of geochemical and seismic phenomena which can act as forerunners of some kind of volcanic activity has been carried out in areas of quiescent volcanism, at the Phlegrean Fields and Vulcano (Italy), during the last decade, and interesting information on fluctuating activity without eruption has been obtained. In the Phlegrean Fields the significant ground uplift started in 1982 was preceded by variations in chemical composition of fumaroles, and seismic activity began some months later; the composition of fumaroles changed again some months before the stop of uplift and seismicity. At Vulcano, chemical changes in fumarolic gases and/or phreatic waters have been detected in advance of three different seismic episodes of significant magnitudes. Sharp discrepancies of apparent equilibrium temperatures with respect to measured temperatures can provide the evidence of some kind of perturbation occurring within the volcanic system; a disequilibrium situation appears thus as a necessary, even if not sufficient, condition for the occurence of eruptive activity.
The results obtained during a significant time interval for the concentrations of water vapour, carbon and sulphur species in fumaroles characterized by different temperatures and chemical compositions, are here presented with the aim of distinguishing specific variations in response to the changing influence of water in volcanic activity. The strong increase in temperatures, as well as in concentrations of "magmatic" components, observed at Momotombo (Nicaragua) from 1973 to 1985, is interpreted as mainly due to the depletion of a "buffering" shallow water body, possibly induced by the exploitation of a nearby geothermal field. At the Phlegrean Fields (Italy), the data referring to the period 1980-1988 clearly indicate a feeding water reservoir at intermediate depth, and the changing water vapour concentrations are considered the consequence of changing thermal input from the low-lying magma body. Two stages of activity can be derived from the data collected at Vulcano (Italy) from 1977 to 1988. A first period, up to 1985 during which water from aquifers not connected to magma degassing may represent the main contribution- subsequently, the opening of the system appears responsible for increased temperatures and the significant chemical changes. The possibility of obtaining evidence for the different influence of water on volcanic systems is of great interest for forecasting purposes. According to the above interpretation of compositional changes observed in volcanic fumaroles, a low probability for a resumed activity can be estimated at Momotombo and, prior to 1985, at Vulcano; this probability is moderate for Phlegrean Fields and, since 1985, slightly higher at Vulcano.
Our study concerns the eastern flank of Mount Etna, particularly the Valle del Bove, the zones all around it and the NE sector of the volcano, between the NE Rift and Pernicana Fault. Volcano tectonic structures were localised and analysed. Our field research focused on dikes, particularly their dipping and dimensions, on eruptive fractures, on volcanic cones and on dislocations of blocks. From our field data we obtain a kinematic model for the surficial zone of the eastern area of Mount Etna. According to this model, the eastern part of the volcano can be divided in two sectors with a different kinematic behaviour:
1) Northern sector: its characteristic is a changement of the dislocation mechanisms during time, from pure extensional to oblique movements with left horizontal components. In this sector the extensional structures are NE-SW and ENE-WSW. We localized also a zone of structural weakness oriented N60°E-N70°E, which is used by magmas to reach the surface.
2) Southern
sector: it is characterised by extensional dislocation
mechanisms with right horizontal components. The extensional structures
are oriented mainly NW-SE, N-S and E-W.
The movement vectors show a general trend SEward for the northern sector
and Eward and NEward for the southern sector. All the deformations observed
point out an Eward movement of the whole eastern flank of Mount Etna.
Integration of new petrological data with existing information on the geologic, structural, and petrologic features of Mount Etna allows the extent of subalkaline to transitional basal lavas and the distribution and characters of the Ancient Volcanic Centres to be better defined: they show a fairly wide variation of their compositions (basalts to basic mugearites), with chemically distinct units. They constitute a broad, quite complex base to the relatively recent Trifoglietto Unit centres, characterized by a marked TiO2 decrease with differentiation. These are found exclusively within a narrow sector open to the sea on the eastern flank of the volcano,that may be interpreted as a "sector graben". The later Mongibello sequence shows the widest chemical variation (hawaiites to trachytes) among the Ellittico centre products, whereas the Recent Mongibello products tend to evolve to definitely more mafic varieties with time (basic mugearites to hawaiites-tephntes) .
The release patterns for various forms of energy have been estimated at Etna volcano for the time interval 1983-1987. Considering onlv the eruptions which occurred during this time span, thermal and potential energy releases were estimated at about 1017 and 1015 J/year, respectively; while earthquake and volcanic tremor energy releases range between 1011 and 1013 J/year. Moreover, the cumulative and the volumetric seismic moments associated to eruptions have been estimated for the whole decade 1978-1987. They resulted 5.4.1017 Nm and 7.8.1018 Nm, respectively. Their yearly ratios range between 0.01 and 0.1.
Data concerning low-frequency seismic signals recorded on Mt. Etna on January 1990 are presented and discussed in this paper with the aim of improving the seismological researches in the context of the dynamics of magmatic fluids. Transitions of the seismic regime, associated with the recording of "wave packages" on the background signal, are taken into account and related to significant changes in the multicomponent flow patterns within the magmatic column. The approach, which at present is a qualitative one, is addressed to encourage the use of the seismological data collected on Mt. Etna in order to get a more advanced knowledge of the fluid-dynamic phenomena occurring in the volcanic apparatus, rather than speculations about the geometry of the feeding conduit system.
The
time patterns of the tremor energy, the strain-release and the space
distribution of seismicity during 1990 are analyzed in the present paper
in order to provide new data which may be useful for the interpretation of
the physical phenomena occurring in the upper crust and related to the
uprise of magma and the evolution of volcanic events. The intensity of
volcanic phenomena is evaluated by an empirical scale, based on continuous
observations of the volcanic activity.
Based on an interpretation of the gravimetric data obtained in the period 1988-1991, a model of the shallow crustal structures of the area concerned has been worked out. The surveys were carried out along two routes for a total length of about 200 km between the Campi Flegrei and Minturno (Garigliano river), the section from Castelvolturno to Capua belonging to both. The resulting model of the crustal structures has been constrained by means of the data from deep drillings carried out in the area. It shows a step pattern of the carbonate basement (density = 2.6 g/cm3) extending up to a maximum depth of 4.6 km westwards according to the tectonics that would have generated the graben. Fill sediments show a stratified pattern, their density increasing with depth (from 1.8 to 2.5 g/cm3) in agreement with data from wells. The apparently more complex structures Iying south of the Volturno Plain are chacterized by large masses, probably of volcanic origin, as can be inferred from drillings carried out in the surrounding area. Moreover, near and within the Campi Flegrei caldera, data from drillings up to the depth of about 3 km, have not shown the existence of the carbonate basement so that it is very hard to interpret the observed gravimetric anomalies in terms of a continuous basement according to the interpretation of numerous authors. The results are in agreement with models that explain the opening of the Tyrrhenian Sea and the formation of the Graben of theCampanian Plain in terms of Mantle upwelling. In fact, such a process produces curvature of the crust, distensive tectonics, migration of magmatic masses toward the surface, collapses with graben formations and volcanism.
A deeply eroded shoshonitic lava flow of the Radicofani eruptive center has been dated by whole rock 40Ar/39 Ar stepwise heating. The plateau age of 1299 ± 3 Ka agrees with the isochron age 1303 ± 5 Ka. Small amounts of excess Ar are present; therefore previously reported K/Ar ages are very likely to be affected as well. The duration of the volcanic activity thus cannot be precisely calculated, but it is thought to have been short-lived.
We present here XRD and S.E.M. investigations on wet surge samples from the island of Vulcano and the Latera volcanic complex. Although pertaining to the same cycle of activity, samples from Vulcano show quite different alteration states related to syndepositional water/rock interactions. Two main characteristics are taken into account: the evidence of corrosion effects due to HF-bearing solutions and the occurrence, as secondary minerals, of smectites of different composition and cristallinity index. Wet surge samples from the Latera volcanic complex (sampled from the pyroclastic formation named "Tufi di Poggio Pinzo") show clay minerals as secondary phases. On the basis of XRD investigations, these secondary phases are represented by smectites with a very low cristallinity index. On the basis of the available experimental data on the genesis of smectites and the observed relationships between alteration degree, corrosion effects on vitric fragments and occurrence of smectites with high C.I., the interaction between acid solutions and pyroclasts appears to be the prevailing process. Processes of vapour condensation from the eruptive cloud can result into a solution of water and magmatic acids, leading to deep alteration of the pyroclasts with production of smectites.
The present idea, arising from the available data, is that hydromagmatic products emplaced in "wet" conditions can develop high stability clay minerals as a result of acid-solutions/pyroclasts interaction. This minimizes any other post-depositional mineral production.
This work reports the results of field and laboratory studies on the
palagonitization of the Cala Pozolana di Ponente pyroclastic rocks in the
Linosa Island (Sicilian Channel, Southern Italy).
The eruptive history of this volcanic edifice see an initial hydromagmatic
activity with formation of a tuff ring followed by a magmatic one with
emission of scoriaes lava flows and intrusion of dikes. The basal pyroclastic
units (vitric tuffs) have suffered palagonitization processes. Glass shards
are completely replaced in their outer portions by authigenic minerals (smectites) that are deposited as cement between granules
too. The glass
shards vesicles are filled with calcite and zeolites (phillipsite-chabazite).
The palagonitization of the basal vitric tuffs is brought about by reactive
hydrothermal fluids heated by the magma rising through the vent. The end
of eruptive activity induced a quick temperature fall in the volcanic apparatus
that coupled with the impermeabilisation of the tuffs by deposition of
smectites caused an early cessation of the palagonitization processes.
Liquid phase Ion chromatography has been used to measure the content of HCI, HF and total Sulphur in fumarolic fluid samples.The reproducibility of the method was found to be ± 2.5% for HC1, ± 2.7% for HF and ±1.1% for total Sulphur. The analytical results for HC1 and HF of Vulcano fumarolic gaseous samples obtained with this method agree with those obtained by potentiometric analysis. The accuracy of the present method has been found to be similar to that of conventional techniques which are more complex and time consuming.
Monitoring activity carried out by GNV at the island of Vulcano is summarized. La Fossa volcano is showing, since some years, signs of potential reactivation. Unrest mostly consists of physico-chemical changes, temperature (650°C by the end of June 1991) and output increase in the crater fumarolic gases, suggestive of a progressive increase of the flux of hot deep (magmatic) gases. In the same time seismicity remained mostly at low levels and ground deformation showed only minor changes. Intense fracturing caused slope instability of La Fossa cone that culminated in a land-slide occurred in April 1988.
Different volcanic cycles have been identified in the volcanic history of the La Fossa cone. Each cycle is characterized by a similar pattern of activity, starting with surge eruption and ending with effusion of lava flow from the crater rim. Four stages have been proposed for the common eruptive model: A) Quiescent stage; B) Surge eruption stage; C) Vulcanian eruption stage; D) Lava flow stage.The absence of paleosoils and/or marked erosional surfaces between deposits that constitute a single cycle suggests a time-closed sequence of events within each cycle. On the contrary, strong erosional unconformities between the cycles suggest a discrete time interval between the end of one cycle and the start of the next.Surge deposits are the most common products in all the identified cycles, and consist of glassy fragments, with some crystals and very few lithic fragments. Dry surge deposits are present in all the identified cycles. The unimodal distribution of grains suggests a unique mechanism of emplacement. The lack of deposits near the source area, the lack of edge modification on the grains, the thin lamination, all suggest that the transport was in a very diluted cloud. Wet surge deposits have complex grain size distribution, and two or three subpopulations can be identified, suggesting more than one fragmentation process. Grain morphology and textural features of the layers suggest an initial high content of interstitial water in the deposits. The depositional unit formed at the base of the cloud through the lateral expansion of the ground of a sticky muddy medium consisting of ash and fluids. The present resting period of the volcano is interpreted as a quiescent state and, consequently, the renewal of the eruptive activity in the area should open a new volcanic cycle and could follow the general pattern that has characterized the history of the La Fossa cone. Being the major volcanic hazard linked to surge emplacement, the major risk is expected during the initial phases of the erurption in the Caldera della Fossa area The areas outside the Caldera della Fossa can be considered at low risk.
Study of the Fossa subsurface by means of deep drillings revealed the occurrence of a differential collapse that reaches its maximum in the northern area and is less pronounced in the southern one. The Lentia complex extends eastward, occupying most of the Fossa caldera at depth. The collapse occurred in several phases, the earlier one coinciding with the trachybasaltic phase of activity, originating bet veen 42 and 13 ka from eruptive structures located in the northern part of the old South Vulcano; the subsequent and main phase of collapse occurred after the emplacement of the Lentia complex, between 15,5 and 7,3 ka. In this work we propose that the collapse is probably related to regional extensional tectonics, also involving southern Lipari, that occurred along the Tindari-Letojanni wrench fault. The lowered area was later affected by intense submarine volcanic activity, resulting in thick beds of hyaloclastites and lava flows that filled the depression with several km3 of homogeneous potassic products prior to the recent Fossa and Vulcanello phases of activity. The Fossa caldera subsurface is affected by injections of several subvolcanic and intrusive bodies; an intrusive body was drilled in its southeastern part and may be related to the trachybasaltic phase of activity. Subvolcanics, evolved in composition, cut the Lentia lavic sequences in the central part of the structure. The presence at depth of thick sequences of potentially brittle lavas, with overlying propylitic paragenesis, brings new possibilities to deep geothermal exploration in this area of the island.
This paper describes the geomorphological and morpho-structural picture of the Fossa cone as seen from photo-interpretation techniques and field checks. The structural conditions of the volcano and the different active geomorphical processes indicate hazard conditions for the village of Vulcano Porto caused by possible rock falls from the unstable inner flank of the Forge crater. The authors suggest could be an abnormal evolution of the sea facing flank of the Fossa edifice triggered by deep-sited sliding surfaces.
Several oceanographic cruises, carried out within a program for the monitoring of volcanic activity on the island of Vulcano, provided new bathymetric and high-resolution seismic profiles. The obtained data show that the nearly circular base of the volcanic complex is located at about -1100/-900 m water depth and reaches a diameter of about 15 Km. The western and eastern flanks of Vulcano show different morpho-structural characteristics. The western flank displays a regular slope and a large shelf (whose break lies at -115 m with maximal width of about 1200 m) related to Late Quaternary sea-level lowering. On the eastern flank there is no shelf and the morphology is much more irregular due to the presence of NE-SW-oriented narrow ridges and incisions. Four km offshore of the eastern flank an secondary volcanic apparatus has been discovered. The apparatus (top -742 m deep) shows evidence of very recent volcanic activity. The northeastern submarine sector of Vulcano, i.e. the area of Porto Levante between Fossa and Vulcanello, is characterized by NE-SW-trending canyons, escarpments, faults, fractures, and by fumaroles. On the contrary, in the north western sector (Porto Ponente and the area in front of Lentia) the structural lineaments show an approximate NW-SE orientation. All collected data emphasize the important role played by a conjugate system of NW-SE and NE-SW tectonic trends in the evolution of the Vulcano complex. In particular, the northern, youngest sector of Vulcano appears actually controlled by NE-SW structural lineaments.
Three field
surveys, carried out in the years 1986-1988 in the southern
Aeolian Islands, provide a complete set of seismic informations on this small, active volcanic center. A first evidence of a sharp P-velocity excess
in the crater area is obtained by constant-offset recording of shots, fired
at sea from
different azimuths around the islands of Lipari and Vulcano. The velocity
anomaly corresponds to a very dense monzogabbroic intrusion, formerly penetrated
by a drill-hole. The lateral extent of such a shallow body is mapped by
inversion of more than 1400 P-traveltimes by local earthquakes. Observation
of teleseismic advances at the center of the volcanic area suggests that
the positive velocity anomaly could extend at depth; this statement is
confirmed by two reversed DSS profiles, which constrain a local upheaval
of both the Moho and the intra-crustal layers.
TestoSince 1975 the Lipari-Vulcano area
(Aeolian Islands) has been the
subject of a systematic study by the Ground Deformation Group of the International
Institute of Volcanology (National Research Council, C.N.R. Catania). The
approach adopted consists essentially in two different methodologies, one
with a discontinuous character (geodimeter trilateration measurements)
the other continous (tilt).
In 1975 work was begun on the research field regarding the ground deformation
horizontal component: a geodimeter trilateration network was set up covering
the whole island of Vulcano and the central-southern part of the island
of Lipari. Recently a new network has been set up (March 1987) in the northern
part of the island of Vulcano with the task of monitoring thelocal deformation
pattern linked to processes occurring in the shallow portion of the "Fossa"
volcanic edifice. As regards the continous type methodology, from 1977
bore-hole tilt stations have been set up on the island of Vulcano which
are equipped with high precision biaxial sensors. The present paper contains
a synthetic discussion of data from the Lipari-Vulcano network, in terms
of cumulative areal dilatation
particular evidence is given to the comparison of the pattern deduced
from the complete network measurements (Lipari-Vulcano network or "Major"
Network) and that from the network sector on the island of Vulcano alone
("Minor" Network). Compressive phases recorded on the "Minor" network can
be associated with dilatation ones from the "Major" network, thus indicating
an expansion of the "Fossa" edifice within the hypothetical framework of
a different dilatation rate between the two investigated areas. Significant
variations of other parameters (seismicity, temperature and reducing capacity
of the fumarole gases) are correlable with the deformation phases described.
Finally, particular attention is given to the ground deformation pattern
characterizing the year 1988. In fact, during the course of this year a
Special Surveillance Plan on Vulcano (Summer 1988), was prompted by important
modifications of the geochemicall parameters which manifested since the
end of 1987.In this context, the observations on the horizontal component
of ground deformation evidenced by the geodimeter trilateration network
set up on the "Fossa" edifice assume a certain importance. In the lines
forming the network, contractional phases are observed to have prevailed;
on account of the geometry of the network this means that the volcanic
structure is on the whole undergoing dilatation. As regards
the continous observations from the tilt station located at the southwest
base of the "Fossa" cone (GPL), during 1988 a deformation event took place
beginning in July and becoming markedly accentuated in the last decade
of September. This event indicates a generalized inflation towards the
northern quadrants (or a deflation towards the southern ones) and appears
to be well correlated with the occurrence of the seismic sequence of the
9th-lSth August in the central-southern part of the island of Vulcano (the
area of "The Piano").
A considerable rise in temperature in the fumarolic
gases, observed
at Vulcano throughout 1988, was not accompanied by correlated geophysical
markers indicating clear renewal of the volcanic activity
Microseismicity remained at very low energy levels and was monitored
during two months by use of a temporary array of three-component digital
stations, in addition to the permanent seismic network of the Aeolian Islands.
Comparison of the seismic activity recorded dunng thls experiment with
the seismicity detected during a previous survey carried out in 1987, suggests
that no substantial changes occurred in both the hypocentral volumes and
energy release. Relocation of the events within a three-dimensionally heterogeneous
medium, observation of waveforms similarities and kinematic analysis for
reconnalssance of the seismic phases allow to draw a preliminary quantitative
picture of the seismic behaviour of the volcanic field during the survelllance
period.
Seismic activity occurring in the area of Vulcano
(SouthernTyrrenian Sea)and surroundings during the period 1985-1988 ia analyzed. Both regional
tectonic earthquakes and local volcanic shocks mainly affecting the "Gran
Cratere" eruptive center are investigated, with particular reference to
hypocenter distribution, earthquake rate temporal pattern and frequency-magnitude
relationship The whole seismic evolution of the area during the last decade
has also been taken into account and all available informa-
tion have been used for a preliminary and qualitative evaluation of
the present (January 1989) eruption probability.
During the summer of 1988 the volcanic surveillance of the Fossa Grande
volcano was intensified as a consequence of certain variations in its activity.
These variations included a more than 100°C increase in the maximum
recorded temperature of the crater fumaroles, changes in the chemical composition
of the discharged fluids, opening of new fractures and reactivation of
some old ones and increased CO2 output
from the soil in the area of Vulcano harbour. Maximum measured temperatures
were 416°C on June 29th and 474°C on September 4th. Fifty two fluid
samples were collected from six crater fumaroles and analyzed in a chemical
laboratory installed on the island; the survey was performed by personnel
of CNR and of the Universities of Bari, Palermo, Perugia and Pisa.
The results were as follows:
a) significant (synchronous) variations in chemical composition have
been recorded for all fumaroles, whereas not all the fumaroles exhibited
significant temperature changes.
b) variations in major constituents observed both in space (between
different fumaroles sampled in the same day) and in time (in the same fumarole
sampled weekly or every two weeks) have to be ascribed to a unique process:
mixing of deep fluids (linked to the presence of a magmatic body) with
shallower ones produced through total evaporation of lateral aquifers.
c) the distribution of H2 and CO is
influenced by the decreasing P and T conditions encountered by the ascending
fluids towards the surface.
d) peculiar changes in HCI, HF and sulphur species have been observed
only in the hottest vents.
By using CO2 , CH4 , H2 and CO contents of gases discharged by Baia di Levante fumaroles, the temporal variations of temperature and pressure (PCO2 and Ptot) of the gas feeding aquifer, have been investigated for the period 1982-1990. The estimates of equilibrium temperatures and total pressures ranges from 180 to 240°C and from 10 to 25 bar respectively. Maximum values refer to June 1985 and to August 1988, when drastic change in the chemical composition of high temperature crater fumaroles and an increase of microseismic activity were observed. In 1985 and in 1988, the sudden inlet of high enthalpy fluids from depth, directly detectable in the crater fumaroles, has affected the Baia di Levante hydrothermal field, producing an increment in T and P of the hot aquifers feeding these surface manifestations. Results show that monitoring of CO, H2 and CH4 contents in gas discharged by Baia di Levante fumaroles represent a contribution to assess the risk of phreatic explosion in this area.
This report concludes two year of study on the chemical composition of waters from wells in the Vulcano Porto area (island of Vulcano, Southern Italy), sampled at constant intervals of about one month. The conclusions reached after the first year of study were confirmed by the data collected during the second year. The hydrogeochemistry of the wells in question is subject to seasonal variations, not due to normal dilution by rain water, but rather to more intense exploitation of the wells during the summer, with conseguent pumping from deeper levels. One indication of the possible evolution of the volcanic system came from the Pisciò well the waters of which show a continual increase in sulfate ion concentration, comparable with that observed in gases emitted from the fumarole field of the crater.
The behaviour of Po-210 in 5 fumaroles on the crater rim at Vulcano Island was studied during the summer of 1988. For emission temperatures between 300 and 400°C great variations are observed, probably due to interactions of gases with their surrounding environment before their exit. Above 400°C the temperature is high enough to ensure that Po-210 stays in gases, and the measured variations of Po-210 content could be ascribed to a contribution of magmatic gases. The activity ratio Po-210/Rn-222 at the hottest fumarole allows an estimation of the volume of the vapour reservoir (1,000,000 tons) and its residence time (10 years) to be made.
During the Summer 1988, twelve soil CO2 surveies have been carried out in the Vulcano Porto area. These surveies have shown that, from the first ten days of August, an increase of the CO2 flux from soil was going on. This trend was also coincident with variations of other parameters such as temperature, chemical composition, and output of fumarolic gases in the crater area.Moreover, computed convective release of energy in the Vulcano Porto area (1÷4-1019 ergs/day) is of the same order of magnitude as that released in the crater area.
Nitrogen, noble gases (He, Ne, Ar) and some of their isotopes (4He, 20Ne, 22Ne, 36Ar, 38Ar, 40Ar) were measured in the fumaroles at the crater of Vulcano (Italy), during a special survey from June to September 1988. Since 1979 the survey has been carried out on two fumaroles (F1 and F5) but it was extended to other medium-temperature fumaroles (F5AT, FO, FA) during the summer of 1988. In 1987 a decrease was recorded in the 40Ar/36Ar, N2/Ar, He/Ne and He/Ar ratios; these are still decreasing towards values close to atmospheric in all the sampled fumaroles. The excess of the radiogenic component (4He, 40Ar), that was clearly detected in the period 1982-1986, is thus masked by the increase of the atmospheric species (in particular Ar). The Ne/Ar and N2/Ar ratios suggest that an inflow of a non-degassed air saturated water in the volcanic system is the likely explanation for the observed variation.
Chemical data of fumarolic gases collected at Vulcano during summer 1988 are presented. Simultaneous important variations in the chemical composition of fluids were recorded in the six fumaroles under study; increases in emergence temperature were, on the other hand, not comparable in all the fumaroles. These variations can be related to an input of a deep component of probable magmatic origin. The correlation between changes in the chemical composition of crater fluids and in seismic activity localized at the crater suggests both phenomena have a common origin.
Measurements of Hg levels in soil, plants and atmosphere were performed on samples collected in June 1988 at Vulcano island. The metal content both in soil and plants is of the same order of magnitude as that found in unpolluted areas. Anomalous values have been found only in the thermal muds and in the crater. The concentration of atmospheric gaseous Hg was higher than the background levels and had the maximum values near the crater. These results and their possible relationship with the volcanic activitv are discussed.
A pool of trace elements, selected among those of interest in the geochemistry of thermal fluids, were determined in ground waters and fumarolic fluids from the Vulcano Island. Analytical data herein reported are from five surveys, three of them carried out in 1988 and two in 1989. Data from 1988 surveys, that is subsequently to the volcanic emergency on the island, are of great concern for the results of an almost identical research performed in 1983, prior to the volcanic crisis, allow the necessary direct comparison to establish the relationships between behavior of the elements and volcanic activity troughout this area. Analytical results point out that the content of those elements which undergo selective mobilization under hydrothermal conditions (e. g., As, Sb, Hg, rare alkali metals, B, etc.) is significantly higher than the corresponding values reported for unpolluted fresh waters. Apart from Mo and V, both yielding contents higher than 10 mg/L, the remaining trace metals do not exhibit any enrichment. He and Rn analyses were run on the fluids issued by around-crater fumaroles and along-beach active vents. This paper is aimed at establishing in detail the distribution pattern of the analysed pathfinder elements and at assessing their significance as premonitory markers for the volcanic hazard.
The underwater gaseous manifestations located around the coasts of
Vulcano and Lipari were sampled frequently in 1988 whilst in the summer
of that year their equilibrium temperatures showed significant variations,
accompanying the increase of the volcanic activity of Vulcano and suggesting
that these manifestations are probably connected to the main feeding system
of the fumaroles of the crater of Vulcanos.
The relative fO2 values of the
gases collected in the various sampling sites fall along trends subparallel
to the hematite-magnetite buffer, and they seem to be influenced by the
degree of alteration of the country rocks.
In order to monitor both the variations in the S/Cl ratio and the outputs of the acid gases such as HCl, HF and the sulphur gases (H2S+SO2) released from the fumaroles, two sets of experiments were carried out at the crater of Vulcano (Aeolian Islands) using the alkaline traps method. This method was used because the considerable advantage gained by employing an integral monitoring of these parameters during the sampling interval. The method, first proposed by Noguchi and Kamiya (1963), is described and discussed and the results are interpreted and critically evaluated.