The Etna eruption started on December 4, 1991 and stopped on March 31, 1993 During 473 days of continuous lava outpouring, it formed a 7 6 km² lava flow-field. This eruption is one of the most prolonged and volumetrically significant observed at Etna over the last 300 years. The regular and systematic field surveys carried out during the eruption have allowed to reconstruct the planimetric evolution of the lava field. GPS techniques used to measure lava thickness enable us to calculate a final volume of nearly 235x10⁶ m³ and a mean eruption rate about 5.8 m³/s. The eruption rate value was compared with the instantaneous flow rates, obtained by the lava channel´s size and flow velocity assessments, and with the daily effusion rate, calculated by the flow-field expansion The comparison revealed the inherent limits in measuring these parameters. In addition, various methods were used to determine the temperature and estimate the rheological properties of the lava. The temperature values measured in several parts of the lava field presented limited variability indicating an almost isothermal behaviour of tje inner molten portion of the lava flows. By contrast, the rheological parameters, such as viscosity and yeld strength, varied in function of the distance from the vent.

Back to index Volume 4

The emission of hawaiitic lavas in the summit region of Mt. Etna that started on December 1991, represents the largest eruption of the last three centuries on the volcano. Periodic collection of lavas directly sampled in the lava channels or at the moving fronts during the first year of the eruption, has revealed compositional fluctuations. Major changes were observed during the first days of the eruption, when the upper part of the volcano was supplied by a geochemically distinct-pulse of new hawaiitic magma that partly mixed with the one already standing in the feeding system. In the following, emitted material kept an almost constant composition until September 1992, when a slight increase in the evolution degree of lavas was observed with time. Weak variations in the modal abundance of phenocrysts may be responsible of the observed minor fluctuations in major element composition. On the contrary, major and trace element mass balance point to a gabbroic fractionation of about 14% by weight, to derive the latest and most evolved sample from the less evolved one of the beginning of the eruption. Crystal Size Distributions of lavas allow to detect a lower limit for the size of intratelluric phenocrysts, reveal that the rise of the magma feeding the eruption followed the same pattern during the examined time span; and that the flow of lavas onto the surface follows an isothermal regime (Tlava~1070°C). Melt inclusions in olivine phenocrysts confirm the occurrence of mixing phenomena at the onset of the eruption and allow to estimate mean SO₂ and Cl fluxes.

Back to index Volume 4

Back to index Volume 4

The space-time evolution of seismic activity forerunning the major, 1991-1993 eruptive event of Mt. Etna is analyzed. Activity resumed in late January 1991 and continued, mostly in form of swarms, until the outbreak of the eruption, culminating in a typical mainshock-aftershock sequence on December 15 (M_max=4.5) About 70% of the seismic strain was released along structures Iying in the western part of the volcano; the remainder of seismicity clustered on the main NNW-SSE trending structure along which magma uprose. Seismicity became markedly shallow only one week before the eruption outbreak, and clustered between sealevel and ca. 2 km above it during the major seismic sequences (more than 250 events) occurred on December 14-15. The hypocentral pattern and the most reliable focal mechanisms lead to hypothesize that activation of the intrusion at depth might have occurred about two months before the eruption and allow drawing a preliminary sketch of the kinematics of the volcanic system.

Back to index Volume 4

Polarization features of the tremor wavefield recorded before and during the first stages of the 1991-1993 eruption of Mt Etna, are analyzed and compared to data relating to the 1989 eruption. No significant variation with time of either direction of polarization or linearity content is found before the onset of the eruption, whereas linearity significantly increased at the summit eruption onset (December 14, 1991), then dropped in coincidence with the following outbreak of the eruptive fracture. Such behaviour, compared to that observed during the 1989 eruption, allows to critically review some Drevious considerations on the volcanic tremor source at Mt. Etna

Back to index Volume 4

Seismic data collected at temporary arrays of three-component stations, operated at Mt. Etna in 1989-1991, allow detailed insight into the shallow dynamics of the fault zone along which the dyke of the 1991-1993 eruption intruded. Three relevant swarm sequences, respectively occurred near the end of the 1989 eruption, then four months before and at the onset of the 1991-1993 eruption, indicate that the part of this structure centred on the most recent eruptive boccas was almost uninterruptely active since 1989. Focal depths (4 km b.s.l. to 2 km a.s.l.), space-time evolution of the swarms, and dynamics of the rupture during each sequence support the hypothesis that the eruptive episodes from 1989 onwards might be ascribed to a single intrusion along the fault system reflecting the regional, NNW-SSE oriented structural trend.

Back to index Volume 4

Swarms of low-magnitude long-period earthquakes accompanied the 1991-1993 eruption of Mount Etna, Italy. We describe the physical characteristics of a suite of long-period events which occurred during 1992 and show that they have common features in the time and spectral domains. These events are closely associated with clusters of volcano-tectonic earthquakes and they coincide with episodes of landslide activity within NE crater. We interpret the long-period activity as refiecting dike or conduit resonances caused by short-term pressure transient in a high-level dike system beneath the summit of Etna. The pressure transients appear to be caused by sudden collapses of the floor of NE crater in response to sustained magrna withdrawal from the summit area.

Back to index Volume 4

The eruption which started on December 14, 1991, and continued unti 1 March 31,1993, certainly represents the most irnportant volcanic event that has occured at Mt. Etna over the last three centuries, because of its long duration and of the huge volume of the emitted lava. The eruption feeding fracture cut through the upper eastern slope of the volcano running from the SE subterminal crater (around 3000 m a.s.l.) down to 2200 m altitude on the western wall of the Valle del Bove. The development of the eruptive fissure coincided with the uppermost section of a "dry" fracture system which formed during the September-October 1989 eruption, according to a well known structural trend feeding Etnean volcanism. The associated deformation affected a large area of the volcanic structure, extending over well defined sectors. The strain pattern was carefully detected using EDM measurements carried out on the three geodetic networks which for several years have been regularly reoccupied by the Istituto Internazionale di Vulcanologia (CNR, Catania) staff, as well as by the continuously recording tilt network run by the same Institute. Data inversion mostly concerned EDM measurements, but height changes and information coming from GPS surveys carried out in the same region were also used to constrain modelling. The results obtained allowed us to suggest the effect of two concurrent sources, located at different depth. The analytical solution, based on 122 distance variations and 3 height changes, show a very close coherence between expected and observed values. The deeper source is represented by an ellipsoidal body, undergoing negative pressure variation. Its center is located at about 1.75 km below sea level immediately to the west beneath the volcano summit craters. The relative sizes of the ellipsoidal axes indicate a clear elongation striking NNW-SSE. The shallower source consists of a tensile dislocation, similarly trending in NNW-SSE direction, slightly to the East of the deeper one. This tabular opening cut through the volcanic structure between sea level and about 2000 m elevation. The northern tip of the crack surface projection is coincident with the summit crater area, while its south-eastern extension plots relatively close to the eruptive fracture trace. Modelling suggests that the deeper source could represent a magma storage zone within the volcano sedimentary basin, undergoing depressurization due to mass depletion. The shallower tensile opening is regarded as a feeder dyke path accounting for the magma uprise and eruption. The sectors of the volcanic structure which underwent the maximum contraction in relation to the recent eruption, had presented an almost steady rate areal expansion for some years. This trend has been only temporarly interrupted by the numerous eruptive events that marked the active life of the volcano in this period.

Back to index Volume 4

Vertical and horizontal ground deformation movements measured between 1991 and 1992, before and during the 1991-1993 eruption of Etna, are presented. They show a narrow trough of more than a metre of subsidence running SSE from the summit, connecting the Southeast Crater and the eruption site, flanked by two zones of inflation up to 37 cm to the east and 7 cm to the west. Elastic modelling of the data indicate a dyke at a depth of 450 metres for its top surface and 1100 metres for its base, dipping 75° to the east. The horizontal movements show that many stations to the east of this dyke have moved more than a metre eastwards. The model is compared with that derived from gravity data acquired before the eruption.

Back to index Volume 4

In this paper the GPS data collected during the first eight months of the 1991-1993 Mt. Etna eruption has beenanalysed. The results obtained, integrated with those of previous surveys, contribute to a description of the geodynamical framework in which the eruption took place. The deformations observed are coherent with a depression source, as preliminarily validated by using an appropriate Mogi model. According to this modelling, the source is located beneath the upper south-western flank of the volcano, at depth ranging from 1.5 ÷ 3.5 km below the s.l. However, far from being definitive, these results represent a starting point for future research regarding the considered eruption.

Back to index Volume 4

Microgravity measurements made at regular intervals between June 1989 and October 1992 bracket the onset of eruptions in 1989 and 1991. Activity in 1989 culminated in the opening of a fracture network trending SSE from the summit of Etna, this fracturing provided a low stress path that channelled dyke emplacement which fed the eruption beginning in December 1991. By combining high precision gravity measurements and ground deformation data (from levelling and triangulateration surveys) we are able to detect sub-surface mass changes at several times during the 1989-92 period. We show that there was a higher level of magma (a mass increase) within the summit cones at the end of the 1989 activity than at the beginning, but that this was reduced by a small eruption in January 1990. At some time between June 1990 and June 1991, there was an intrusion into the fractured region left after the 1989 activity. These initial stages of dyke formation occurred at least 6 months beofre the channel was exploited further by magma feeding the eruption starting on 14th December. The absence of seismic or elastic strain precursors indicate that this early intrusive phase must have been largely passive, with magma filling a void or micro-fractured region. There have been no subsequent microgravity changes as large as those preceding the eruption and, by October 1992, no indication of a precursor to the end of this activity. Estimates of the dyrke mass and eruption mass suggest a shallow crustal residence time of ca. 2 months in accord with preliminary isotope data. These results illustrate the importance of microgravity monitoring on Etna and similar volcanoes as the prealrsory processes we identify here were largely undetected by seismic and ground deformation monitoring.

Back to index Volume 4

Since 1986 periodic gravity measurements have been carried out on Mt. Etna, using a network of several stations located between 700 and 3100 m a.s.l. on the slopes of the volcanic edifice. In particular, gravity data obtained from 9 survey carried out in the 1989 SSE fracture zone before and during the 1991-93 eruption are discussed. Strong gravity variations, maximum amplitude reaching 100 mGal during the eruption, were observed. The shape of the anomalies observed and a gross evaluation of the masses involved suggested the presence of shallow sources characterized by strong density contrasts. The hypothesis that these sources could be generated by water or magma movements is discussed, as the likehood of further fenomena capable of causing similar gravity changes is considered remote.

Back to index Volume 4

We propose a mechanism for self-potential (SP) anomaly generation in volcanic areas related to the rise of a magma intrusion towards the Earth´s surface. To this scope, we take profit by a series of SP measurements carried out along a profile crossing the fracture system, located in the south-eastern part of Mt. Etna (Sicily, Italy), which are presented and discussed beforehand. A new processing method of the observed SP anomalies, for a ready recognition of the source pattern, according to a tomographic representation, is adopted for outlining, as realistically as possible, the time evolution of the impending risk.

Back to index Volume 4

A geoelectromagnetic survey on Etna was performed in June 1992, to study the geophysical features of the lava tube feeding the lava flows in Valle del Bove. Measurements were carried out along profiles established on a smoothed area above the lava tube at 2000 m a.s.l., immediately upstream from the site where lava flow was artificially diverted. Ground probing radar, very low frequency (VLF) electromagnetic inductive and magnetometric techniques were employed. The purpose was to geometrically define the cross sectional area of melt in lava tube. The interpretation of electromagnetic data led to the reconstruction of a cross section of the investigated volcanic structure. The reduced geologic noise and the high electric and magnetic contrast between the molten lava in the tube and the host rock facilitated the data analysis. The model proposed was performed on the base of a "calculated" magnetization. It presents a completely demagnetized body, about 9 m² in cross-sectional area, located at 2 m depth. The body centre coincides with the VLF cross-over and is laterally marked by two radar wave diffraction points located on the buried channel upper corners, between the melt and the host rock. All the geophysical techniques used show a good sensitivity toward the studied target, although each technique can satisfactorily resolve only few physical and geometrical characteristics of the lava tube. The experimental results suggest that these techniques could also be useful for monitoring the temporal evolution of lava tubes.

Back to index Volume 4

CO₂ soil gas fluxes have been systematically investigated since 1989 in two flank zones of the Mt. Etna (Zafferana-S.Venerina and Paternò) selected as preferential degassing sites. CO₂ flux and CO₂and He soil concentrations, as well as Rn activity, have been also measured, with varying frequency, at 1600 m elevation, near the fracture opened in 1989 on the SE flank of the volcano. In the latter zone, CO₂ flux and Rn activity have been also continuously recorded for some months in 1992. Data indicate that CO₂ emissions from the two peripheral zones have probably a common deep source. In march 1991, several months before the 1991 eruption onset, an increase in CO₂ flux is observed, which is temporally related to a pressure build-up at depth recorded by the tilt stations. About three months before the eruption, an anomalous CO₂ degassing is observed at the 1989 fracture and at Zafferana-S. Venerina, possibly corresponding to an early magma intrusion into the fracture as suggested by the motion recorded at the nearby tilt station. The eruption onset (14 December 1991) is preceded by a CO₂ flux decrease, suggesting that as magma rises up, degassing preferentially occurs through the main conduits of the summit area. After the eruption onset, some rapid fluctuations of gas emission (CO₂, He and Rn) from the 1989 fracture appear temporally correlated to some transient geophysical phenomena. Our results indicate that soil gas monitoring along Etna flanks can be useful for detecting early phases of volcanic unrest, while monitoring of degassing from summit or subsummit fractures such as the 1989 one, can provide early evidences of near surface magma intrusion, which would be precious for civil defence purposes.

Back to index Volume 4

SO₂ flux variations from Mt. Etna volcano could be monitored for the first time during a long-lasting eruption, started in December 1991. The SO₂ flux variations were then compared with the trends of explosive activity and degassing at the Summit Craters, as well as with tilt variations and seismicity measured during the same period. This comparison leads us to make the hypothesis, for the period taken into account, of a lateral emptying of the shallow central magma feeding system of Etna (from December 1991 to May 1992), followed by two consecutive magma uprises, at the end of May and at the beginning of September 1992, respectively. These observations provide a new contribution to a better understanding of how the plumbing system of Mt. Etna volcano works.

Back to index Volume 4

Two hydrogeochemical surveys were carried out both in the Eastern and Southern areas of M. Etna before and after the paroxysmal phase of the December, 1991 eruption. The analytical results show that the CO₂ input is the most important leaching factor in the water-rock interaction; this evidence is confirmed by the study of the mineral equilibria. The water salinity, the slight thermal anomaly and the high content of some minor and trace elements (Rn, He, SiO₂ , Sr, Li, B, Fe, Mn, NH₃ , As, etc.) are due to the deep origin gases uprising, most of all in the Eastern Etnean region. The Galleria Val Calanna site, sampled shortly before it was buried by the eruption, exhibited geochemical anomalies stressing the role played by gaseous input in the vicinity of the fractures the magma issues from. As regards the ongoing volcanic activity, we can hypothesize that the water rock interaction processes related to the deep origin input was more intense before than after the eruption paroxysmal phase; based on the changes of the chemical composition between the two hydrogeochemical surveys we identified a few sites particularly suitable for geochemical monitoring in the surveillance of Etna.

Back to index Volume 4

In 1991-1993 Mt. Etna produced its highest eruption over three centuries. The lava threatened the village of Zafferana Etnea and several interventions were successfully carried out to protect the village. They include the building of lava retaining dams and the final total diversion of the flow into an artificial channel. The paper describes the state of the volcano before the eruption onset, the opening of the feeding fracture and the development of the eruption together with the civil protection problems that have been faced and solved during the emergency.

Back to index Volume 4

Four earthen barriers were built in 1992 in the upper and lower Val Calanna to delay the advance of the lava flow. All four barriers especially the biggest one proved very effective and stopped the lava front for about one month. The paper provides some technical details of the work carried out and describes some explosion tests performed to destroy the flank of newly formed lava tunnels.

Back to index Volume 4

The use of the explosive for divering the Mt. Etna lava flow in May 1992 is described. Explosive was placed against a thinned wall separating the natural lava channel from an artificial specially escavated channel. The solid septum to be blasted was thermally insulated bv a rock wool cover. The required quantity of explosive was estimated to 7.000 kg (6.000 kg of PE4 and 1.000 kg of Tritonal) taking into account the lack of complete adhesion between the charge and the septum and the dispersion of the explosive energy caused by the fluid lava. Six shaped charges (each of 11 kg of Compound B) were also used to destroy a massive lava layer present in the upper part of the septum. The explosive destroyed totally the separation septum causing 2/3 of the lava to divert into the artificial channel. Total diversion was then achieved by dumping solid lava blocks into the natural channel up to block a tunnel located just downstream from the intervention point.
 

Back to index Volume 4