IDOAS: A new monitoring technique to study the 2D distribution of volcanic gas emissions

Imaging Differential Optical Absorption Spectroscopy (IDOAS) is an optical remote-sensing method using scattered sunlight as light source. It combines a “pushbroom” imaging spectrometer with the DOAS technique and thus allows imaging two-dimensional trace gas distributions, e.g., in volcanic plumes. The highly sensitive and specific detection of many trace gases simultaneously (specific molecules, not just elements, e.g. SO₂, BrO, NO₂, O₃, HCHO, etc.) is possible, and the temporal and spatial variation of these gases can be measured. The IDOAS system presented here enables the taking of two-dimensional images of trace gas distributions in a volcanic plume with a spatial resolution of 100 pixels horizontally × 64 pixels vertically, each with a field of view of 0.087° in horizontal and 0.208° in vertical directions. Therefore, IDOAS provides useful information about the chemical composition and chemical variability in a volcanic plume and allows studying plume dispersal and chemical transformations. The technique was applied to map the SO₂ distribution in the plume of Mt. Etna volcano for the first time in October 2003.     

Halogen oxide measurements at Masaya Volcano,Nicaragua using active long path differential optical absorption spectroscopy

Active Long Path Differential Optical Absorption Spectroscopy (LP-DOAS) measurements of halogen oxides were conducted at Masaya Volcano, in Nicaragua from April 14 to 26, 2007. The active LP-DOAS system allowed night-time halogen measurements and reduced the ClO detection limit by an order of magnitude when compared to previous passive DOAS measurements, as wavelengths below 300 nm could be used for the DOAS retrievals. BrO was detected with an average BrO/SO₂ molecular ratio of approximately 3 × 10⁻⁵ during the day. However, BrO values were below the detection limit of the instrument for all night-time measurements, a strong indication that BrO is not directly emitted, but rather the result of photochemical formation in the plume itself according to the autocatalytic “bromine explosion” mechanism. Despite the increased sensitivity, both ClO and OClO could not be detected. The achieved upper limits for the X/SO₂ ratios were 5 × 10⁻³ and 7 × 10⁻⁶, respectively. A rough calculation suggests that ClO and OClO should be present at similar abundances in volcanic plumes. Since the DOAS technique is orders of magnitude more sensitive for OClO than for ClO, this indicates that OClO should always be detectable in plumes in which ClO is found. However, further LP-DOAS studies are needed to conclusively clarify the role of chlorine oxides in volcanic plumes.     

CO2 output and δ13C(CO2) from Mount Etna as indicators of degassing of shallow asthenosphere

 An estimated average CO₂ output from Etna's summit craters in the range of 13±3 Mt/a has recently been determined from the measured SO₂ output and measured CO₂/SO₂ molar ratios. To this amount the CO₂ output emitted diffusely from the soil (≈ 1 Mt/a) and the amount of CO₂ dissolved in Etna's aquifers (≈ 0.25 Mt/a) must be added. Data on the solubility of CO₂ in Etnean magmas at high temperature and pressure allow the volume of magma involved in the release of such an amount of this gas to be estimated. This volume of magma (≈ 0.7 km³/a) is approximately 20 times greater than the volume of magma erupted annually during the period 1971–1995. On the basis of C-isotopic data of CO₂ collected in the Etna area and of new hypotheses on the source of Mediterranean magmas, significant contributions of CO₂ from non-magmatic sources to the total output from Etna are unlikely. Such large outputs of CO₂ and also of SO₂ from Etna could be due to an anomalously shallow asthenosphere beneath the volcano that allows a continuous escape of gases toward the surface, even without migration of magma. Received: 7 August 1996 / Accepted: 9 November 1996     

Contribution des Sondes Aérologiques Motorisées à l’Etude de la Physico-Chimie des Panaches Volcaniques

Our knowledge of the physico-chemical properties of volcanic plumes remains at an embryonic state, mainly because of the prohibitive cost of measurements made from aircraft, which alone can provide samples representative of the total aerosol and gaseous emission from a volcanic source. The authors show that a small Remotely Piloted Vehicle (R.P.V.), similar in design to the one they used for some probative flights over Mt. Etna, may be of great use in this field. They can, for instance, insure frequent measurement of the SO₂/HCl ratio in volcanic emanations close to active craters at times when the eruptive vents themselves cannot be sampled directly. In June 1978, we were thus able to measure a mean ratio of SO₂/HCl (7.6) in the Mt. Etna volcanic plume. In addition, a correlation spectrometer was in operation during the airborne sampling and allowed us to obtain order of magnitude values for the Mt. Etna discharge in water vapour (170000 tons/day), sulphur dioxide (1700 t/d), hydrochloric acid (340 t/d) and hydrofluoric acid (40 t/d). Widespread use of such R.P.V’s would insure the collection of a great deal of physicochemical data from volcanic plumes, data which are presently lacking and which could extensively enhance the efficiency of chemical methods of volcano surveillance.     

Sulphur emissions to the stratosphere from explosive volcanic eruptions

 Two methods were used to quantify the flux of volcanic sulphur (as the equivalent mass of SO₂) to the stratosphere over different timescales during the Holocene. A combination of satellite-based measurements of sulphur yields from recent explosive volcanic eruptions with an appropriate rate of explosive volcanism for the past 200 years constrains the medium-term (∼10²years) flux of volcanic sulphur to the stratosphere to be ∼1 Mt a^–1, with lower and upper bounds of 0.3 and 3 Mt a^–1. The short-term (∼10- to 20-year) flux due to small magnitude (10¹⁰–10¹²kg) eruptions is of the order of 0.4 Mt a^–1. At any time the instantaneous levels of sulphur in the stratosphere are dominated by the most recent (0–3 years) volcanic events. The flux calculations do not attempt to address this very short timescale variability. Although there are significant errors associated with the raw sulphur emission data on which this analysis is based, the approach presented is general and may be readily modified as the quantity and quality of the data improve. Data from a Greenland ice core support these conclusions. Integration of the sulphate signals from presumed volcanic sources recorded in the GISP2 core provides a minimum estimate of the 10³–year volcanic SO₂ flux to the stratosphere of 0.5–1 Mt a^–1 over the past 9000 years. The short-term flux calculations do not account for the impact of rare, large events. The ice-core record does not fully account for the contribution from small, frequent events. Received: 27 September 1995 / Accepted: 13 December 1995     

First <Superscript>13</Superscript>C/<Superscript>12</Superscript>C isotopic characterisation of volcanic plume CO<Subscript>2</Subscript>

We describe analytical details and uncertainty evaluation of a simple technique for the measurement of the carbon isotopic composition of CO₂ in volcanic plumes. Data collected at Solfatara and Vulcano, where plumes are fed by fumaroles which are accessible for direct sampling, were first used to validate the technique. For both volcanoes, the plume-derived carbon isotopic compositions are in good agreement with the fumarolic compositions, thus providing confidence on the method, and allowing its application at volcanoes where the volcanic component is inaccessible to direct sampling. As a notable example, we applied the same method to Mount Etna where we derived a δ¹³C of volcanic CO₂ between −0.9 ± 0.27‰ and −1.41 ± 0.27‰ (Bocca Nuova and Voragine craters). The comparison of our measurements to data reported in previous work highlights a temporal trend of systematic increase of δ¹³C values of Etna CO₂ from ~ −4‰, in the 1970’s and the 1980’s, to ~ −1‰ at the present time (2009). This shift toward more positive δ¹³C values matches a concurrent change in magma composition and an increase in the eruption frequency and energy. We discuss such variations in terms of two possible processes: magma carbonate assimilation and carbon isotopic fractionation due to magma degassing along the Etna plumbing system. Finally, our results highlight potential of systematic measurements of the carbon isotopic composition of the CO₂ emitted by volcanic plumes for a better understanding of volcanic processes and for improved surveillance of volcanic activity.     

Major-ion bulk deposition around an active volcano (Mt. Etna, Italy)

Bulk atmospheric deposition of major cations (Na, K, Ca, Mg) and anions (Cl, F, SO₄) were measured at 15 sites around an active volcano, Mount Etna, from 2001 to 2003. Their composition indicates several natural sources, among which deposition of plume-derived volcanogenic gas compounds is prevalent for F, Cl and S. Plume-derived acidic compounds are also responsible for the prevailing acidic composition of the samples collected on the summit of the volcano (pH in the 2.45–5.57 range). Cation species have complex origin, including deposition of plume volcanogenic ash and aerosols and soil-dust wind re-suspension of either volcanic or carbonate sedimentary rocks. Variation of the deposition rates during the March 2001–March 2003 period, coupled with previous measurements from 1997 to 2000 (Appl Geochem 16:985–1000, 2001), were compared with the variation of SO₂ flux, volcanic activity and rainfall. The deposition rate was mainly controlled by rainfall. Commonly, about 0.1–0.9% of HF, HCl and SO₂ emitted by the summit crater's plume were deposited around the volcano. We estimate that ∼2 Gg of volcanogenic sulphur were deposited over the Etnean area during the 2002–2003 flank eruption, at an average rate of ∼24 Mg day⁻¹ which is two orders of magnitude higher than that typical of quiescent degassing phases.     

Petrology and sulfur and chlorine emissions of the 1963 eruption of Gunung Agung,Bali, Indonesia

 The 1963 eruption of Gunung Agung produced 0.95 km³ dense rock equivalent (DRE) of olivine±hornblende-bearing, weakly phyric, basaltic andesite tephra and lava. Evidence for magma mixing in the eruptive products includes whole-rock compatible and incompatible trace element trends, reverse and complex compositional zoning of mineral phases, disequilibrium mineral assemblages, sieve-textured plagioclase phenocrysts, and augite rims on reversely zoned orthopyroxene. Basalt magma mixed with pre-existing andesite magma shortly before eruption to yield basaltic andesite with a temperature of 1040–1100  °C at an assumed pressure of 2 kb, f O₂>NNO, and an average melt volatile content (H₂O±CO₂) of 4.3 wt.%. Magma-mixing end members may have provided some of the S and Cl emitted in the eruption. Glass inclusions in phenocrysts contain an average of 650 ppm S and 3130 ppm Cl as compared with 70 ppm and 2220 ppm, respectively, in the matrix glass. Maximum S and Cl contents of glass inclusions approach 1800 and 5000 ppm, respectively. Application of the petrologic method to products of the 1963 eruption for estimating volatile release yields of 2.5×10¹²g (Mt) of SO₂ and 3.4 Mt of Cl released from the 0.65 km³ of juvenile tephra which contributed to stratospheric injection of H₂SO₄ aerosols on 17 March and 16 May, when eruption column heights exceeded 20 km above sea level. An independent estimate of SO₂ release from atmospheric aerosol loading (11–12 Mt) suggests that approximately 7 Mt of SO₂ was injected into the stratosphere. The difference between the two estimates can be most readily accounted for by the partitioning of S, as well as some Cl, from the magma into a water-rich vapor phase which was released upon eruption. For other recent high-S-release eruptions of more evolved and oxidized magmas (El Chichón, Pinatubo), the petrologic method gives values two orders of magnitude less than independent estimates of SO₂ emissions. Results from this study of the Agung 1963 magma and its volatile emissions, and from related studies on eruptions of more mafic magmas, suggest that SO₂ emissions from eruptions of higher-S-solubility magma may be more reliably estimated by the petrologic method than may those from more-evolved magma eruptions. Received: 29 June 1994 / Accepted: 25 April 1996     

Geochemistry of gases and waters discharged by the mud volcanoes at Paternò, Mt. Etna (Italy)

 Approximately 20 km south of Mt. Etna craters, at the contact between volcanic and sedimentary formations, three mud volcanoes discharge CO₂-rich gases and Na–Cl brines. The compositions of gas and liquid phases indicate that they are fed by a hydrothermal system for which temperatures of 100–150  °C were estimated by means of both gas and solute geothermometry. The hydrothermal system may be associated with CO₂-rich groundwaters over a large area extending from the central part of Etna to the mud volcanoes. Numerous data on the He, CH₄, CO₂ composition of the gases of the three manifestations, sampled over the past 5 years, indicate clearly that variations are due to separation processes of a CO₂-rich gas phase from the liquid. The effects of these processes have to be taken into account in the interpretation of the monitoring data collected for the geochemical surveillance of Etna volcano. Received: 4 September 1995 / Accepted: 14 February 1996     

Gas and particle emissions from Soufrière Hills Volcano, Montserrat, West Indies: characterization and health hazard assessment

 The Soufrière Hills Volcano, Montserrat, erupting since 18 July 1995, intensified its degassing in early 1996 with the continuing growth of the lava dome inside the summit crater. During this period of increased activity, between 11 and 18 March 1996, we measured gases and particles within the visible plume to determine whether at that time it posed a health risk to the population of Plymouth, the capital town, which is 5 km southwest (downwind) and was then still occupied. Gravimetric measurements were made of total suspended particles (TSP) and particles having an aerodynamic diameter of less than 10 μm (PM₁₀). Measurements were made of sulphur dioxide (SO₂), hydrochloric acid (HCl), hydrofluoric acid (HF), nitric acid (HNO₃), acetic acid (CH₃COOH), formic acid (HCOOH), and particulate sulphate (SO₄ ^2–), chloride (Cl^–), nitrate (NO₃ ^–), fluoride (F^–), methanesulphonate (CH₃SO₃ ^–), acetate (CH₃COO^–), formate (HCOO^–), ammonium (NH₄ ⁺), sodium (Na⁺) and acidity (H⁺). Trace metals having human health implications [chromium (Cr), nickel (Ni), cobalt (Co), copper (Cu), zinc (Zn), arsenic (As), selenium (Se), cadmium (Cd), tin (Sn), mercury (Hg) and lead (Pb)] were also determined. Mean concentrations of HCl, SO₂ and HF obtained in the town of Plymouth were 14.0, 5.9 and 0.8 ppbv, respectively. Corresponding concentrations in the mixed plume on the crater edge were 533, 168 and 22 ppbv. There were no direct emissions of HNO₃, although nitrate was detected in coarse particles at the source. Higher concentrations of CH₃COOH and HCOOH were measured close to the crater. Mean TSP and PM₁₀ were 64 and 15 μg m^–3 in Plymouth, and 455 and 47 μg m^–3 on the upper volcano slope. Aerosols were highly acidic at the source but rapidly neutralised during transport. Trace metals were enriched in the aerosol relative to crater surface material. The concentrations of the acid gases, sulphur dioxide in particular, and particles were found to be too small to pose a health hazard at the time of these measurements, when relatively modest volcanic activity was occurring. Received: 9 September 1998 / Accepted: 29 August 1999     

Surge in sulphur and halogen degassing from Ambrym volcano,Vanuatu

Volcanoes provide important contributions to atmospheric budgets of SO₂ and reactive halogens, which play significant roles in atmospheric oxidative capacity and radiation. However, the global source strengths of volcanic emissions remain poorly constrained. These uncertainties are highlighted here by the first measurements of gas emission rates from Ambrym volcano, Vanuatu. Our initial airborne ultraviolet spectroscopic measurements made in January 2005 indicate fluxes of 18–270 kg s^-1 of SO₂, and 62–110 g s^-1 of BrO, into the atmosphere, placing Ambrym amongst the largest known contemporary point sources of both these species on Earth. We also estimate high Cl and F fluxes of ~8–14 and ~27–50 kg s^-1, respectively, for this period. Further observations using both airborne and spaceborne remote sensing reveal a fluctuating SO₂ output between 2004 and 2008, with a surge in the first half of 2005, and underline the substantial contribution that a single passively degassing volcano can make to the atmospheric budget of sulfur and halogens.     

Estimation of SO<Subscript>2</Subscript> abundance in the eruption plume of Mt. Etna using two MIVIS thermal infrared channels: a case study from the Sicily-1997 Campaign

In this paper, an algorithm is developed based on the split-window technique, to estimate the SO₂ abundance in the plume of Mt. Etna volcano using the multispectral infrared and visible imaging spectrometer (MIVIS). The MIVIS data were remotely sensed in the thermal infrared (TIR) during the Sicily-1997 Campaign. In this study, the MODTRAN 3.5 code has been used to simulate the radiance at the sensor; the radiative transfer model was input along with the data of radio-sounding performed simultaneously with the MIVIS flight using a mobile radio-theodolite. From the SO₂ map, derived from the MIVIS image, the SO₂ flux along the axis of the plume was computed knowing the wind speed at the plume altitude. The SO₂ flux is variable along the plume axis. The average SO₂ flux (about 45 kg s^-1 on 12 June and about 30 kg s^-1 on 16 June) emitted from the vents is compared with the correlation spectrometer (COSPEC) measurements carried out by other teams (from the ground and from a light aircraft flying under the plume) during the MIVIS flight. Finally, by means of this algorithm it should be easier, with respect to the previously described procedure to monitor the SO₂ flux of a specific volcano such as Mt. Etna.     

Impact of volcanic fluoride and SO2 emissions from moderated activity volcanoes on the surrounding vegetation

Studies in the regions of the volcanoes Etna (Italy) and Masaya (Nicaragua) show that the continuous emissions of gaseous pollutants (HF and SO₂) from moderated activity volcanoes causes a chronic pollution in the surrounding vegetation with certain economical and ecological consequences.Reciprocally the measure of the pollutants in the plants growing in volcanic regions may be a simple and fast method to investigate some characteristics of the volcanic plume: for example, intensity of the emissions of gas, direction and extent of the plume.     

SO2 emissions at Mt. Etna with particular reference to the period 1993–1995

The variations in sulfur dioxide (SO₂) emission from the Summit Craters of Mt. Etna were determined, with particular reference to the period 1993–1995. Vehicle-based weekly measurements of SO₂ flux, using a correlation spectrometer (COSPEC), suggest new input of magma into the main feeder system of the volcano between 1993 and 1995. Minimal flux values (<1000 t/day) preceded the two eruptive events in the period 1987–1995. Only approximately 9.5% of the magma that contributed the SO₂ emission was erupted during the same period. Received: 3 November 1997 / Accepted: 21 September 1998     

Monitoring Etna volcanic plumes using a scanning LiDAR

In this paper, we use data obtained from LiDAR measurements during an ash emission event on 15 November 2010 at Mt. Etna, in Italy, in order to evaluate the spatial distribution of volcanic ash in the atmosphere. A scanning LiDAR system, located at 7?km distance from the summit craters, was directed toward the volcanic vents and moved in azimuth and elevation to analyse different volcanic plume sections. During the measurements, ash emission from the North East Crater and high degassing from the Bocca Nuova Crater were clearly visible. From our analysis we were able to: (1) evaluate the region affected by the volcanic plume presence; (2) distinguish volcanic plumes containing spherical aerosols from those having non-spherical ones; and (3) estimate the frequency of volcanic ash emissions. Moreover, the spatial distribution of ash mass concentration was evaluated with an uncertainty of about 50?%. We found that, even during ash emission episodes characterised by low intensity like the 15 November 2010 event, the region in proximity of the summit craters should be avoided by air traffic operations, the ash concentration being greater than 4?×?10^?3?g/m³. The use of a scanning permanent LiDAR station may usefully monitor the volcanic activity and help to drastically reduce the risks to aviation operations during the frequent Etna eruptions.     

Holocene tephrochronology record of large explosive eruptions in the southernmost Patagonian Andes

For regionally widespread Holocene tephra layers in southernmost Patagonia, correlations based on both chemical and chronological data indicate their derivation from five large-volume (>1 km³) explosive eruptions of four different volcanoes in the southernmost Andes. Bulk-tephra and tephra-glass major and trace-element chemistry and Sr isotopic ratios unambiguously distinguish different source volcanoes, and imply that two of the regionally widespread tephra (MB₁ and MB₂) were derived from Mt. Burney (52°S), one (R₁) from Reclus (51°S), one (A₁) from Aguilera (50°S) and one (H₁) from Hudson volcano (46°S). The H₁ tephra derived from the Hudson volcano, which is located at the southern end of the Andean Southern Volcanic Zone (SVZ; 33–46°S), contains distinctive greenish andesitic glass with FeO > 4.5 wt.% and TiO₂ > 1.2 wt.%. In contrast, rhyolitic glass in tephra derived from the eruptions of Mt. Burney, Reclus and Aguilera volcanoes, which are located in the Andean Austral Volcanic Zone (AVZ; 49–55°S), is clear and transparent and has significantly lower FeO and TiO₂. Tephra derived from these three AVZ volcanoes all contain plagioclase, orthopyroxene, minor clinopyroxene and amphibole. Biotite occurs only in the Aguilera A₁ tephra, which also has the highest bulk-tephra and tephra-glass K₂O and Rb contents. Averages of new and published ¹⁴C ages determined on organic material in soil and sediment samples above and below these tephra constrain the uncalibrated ¹⁴C age of the R₁ eruption of Reclus volcano to 12,685 ± 260 years BP, the MB₁ and MB₂ eruptions of Mt. Burney to 8,425 ± 500 and 3,830 ± 390 years BP, the Hudson H₁ eruption to 6,850 ± 160 years BP, and the A₁ eruption of Aguilera volcano to 3,000 ± 100 years BP. The volume of the largest of these eruptions, H₁ of the Hudson volcano, is estimated as >18 km³. The volume of the Reclus R₁ eruption is estimated at >10 km³, the Aguilera A₁ eruption at between 4 and 9 km³, and the younger Mt. Burney MB₂ eruption at ≥2.8 km³. The volume of the older MB₁ Mt. Burney eruption is the least well constrained, but must have been larger than the younger MB₂ eruption. The data indicate that the frequency of explosive activity of volcanic centers in the AVZ is lower than in the southern SVZ.     

Dynamics of carbon dioxide emissions,crystallization, and magma ascent: hypotheses,theory, and applications to volcano monitoring at Mount St. Helens

 Measurements of CO₂ fluxes from open-vent volcanos are rare, yet may offer special capabilities for monitoring volcanos and forecasting activity. The measured fluxes of CO₂ and SO₂ from Mount St. Helens decreased from July through November 1980, but the record includes variations of CO₂/SO₂ in the emitted gas and episodes of greatly increased fluxes of CO₂. We propose that the CO₂ flux variations reflect two gas components: (a) a component whose flux decreased in proportion to 1/ √t with a CO₂/SO₂ mass ratio of 1.7, and (b) a residual flux of CO₂ consisting of short-lived, large peaks with a CO₂/SO₂ mass ratio of 15. We propose two hypotheses: (a) the 1/ √t dependence was generated by crystallization in a deep magma body at rates governed by diffusion-limited heat transfer, and (b) the gas component with the higher CO₂/SO₂ was released from ascending magma, which replenished the same magma body. The separation of the total CO₂ flux into contributions from known processes permits quantitative inferences about the replenishment and crystallization rates of open-system magma bodies beneath volcanos. The flux separations obtained by using two gas sources with distinct CO₂/SO₂ ratios and a peak minus background approach to obtain the CO₂ contributions from an intermittent source and a continuously emitting source are similar. The flux separation results support the hypothesis that the second component was generated by episodic magma ascent and replenishment of the magma body. The diffusion-limited crystallization hypothesis is supported by the decay of minimum CO₂ and SO₂ fluxes with 1/ √t after 1 July 1980. We infer that the magma body at Mount St. Helens was replenished at an average rate (2.8×10⁶m³d^–1) which varied by less than 5% during July, August, and September 1980. The magma body volume (2.4–3.0 km³) in early 1982 was estimated by integrating a crystallization rate function inferred from CO₂ fluxes to maximum times (20±4 years) estimated from the increase of sample crystallinity with time. These new volcanic gas flux separation methods and the existence of relations among the CO₂ flux, crystallization rates, and magma body replenishment rates yield new information about the dynamics of an open-vent, replenished magma body. Received: 15 February 1995 / Accepted: 30 March 1996     

Eddy covariance imaging of diffuse volcanic CO<Subscript>2</Subscript> emissions at Mammoth Mountain,CA, USA

Use of eddy covariance (EC) techniques to map the spatial distribution of diffuse volcanic CO₂ fluxes and quantify CO₂ emission rate was tested at the Horseshoe Lake tree-kill area on Mammoth Mountain, California, USA. EC measurements of CO₂ flux were made during September–October 2010 and ranged from 85 to 1,766 g m⁻² day⁻¹. Comparative maps of soil CO₂ flux were simulated and CO₂ emission rates estimated from three accumulation chamber (AC) CO₂ flux surveys. Least-squares inversion of measured eddy covariance CO₂ fluxes and corresponding modeled source weight functions recovered 58–77% of the CO₂ emission rates estimated based on simulated AC soil CO₂ fluxes. Spatial distributions of modeled surface CO₂ fluxes based on EC and AC observations showed moderate to good correspondence (R ² = 0.36 to 0.70). Results provide a framework for automated monitoring of volcanic CO₂ emissions over relatively large areas.     

The contemporaneous emission of low-K and high-K trachybasalts and the role of the NE Rift during the 2002 eruptive event,Mt. Etna,Italy

Mount Etna volcano erupted almost simultaneously on its northeastern and southern flanks between October 27 and November 3, 2002. The eruption on the northeastern flank lasted for 8 days, while on the southern flank it continued for 3 months. The northeastern flank eruption was characterized by the opening of a long eruptive fracture system between 2,900 and 1,900 m.a.s.l. A detailed survey indicates that the fractures’ direction shifted during the opening from N10W (at the NE Crater, 2,900 m) to N45E (at its lowest portion, 1,900 m) and that distinct magma groups were erupted at distinct fracture segments. Based on their petrological features, three distinct groups of rocks have been identified. The first group, high-potassium porphyritic (HKP), is made up of porphyritic lavas with a Porphyritic Index (P.I.) of 20–32 and K₂O content higher than 2 wt%. The second group is represented by lavas and tephra with low modal phenocryst abundance (P.I. < 20) named here oligo-phyric (low-phyric), and K₂O content higher than 2 wt% (HKO, high-potassium oligophyric). The third group, low-potassium oligophyric (LKO), consists of tephra with oligophyric texture (P.I. < 20) but K₂O content < 2 wt%. K-rich magmas (HKP and HKO) are similar to the magma erupted on the southern flank, and geochemical variations within these groups can be accounted for by a variable degree of fractionation from a single parent magma. The K-poor magma (LKO), erupted only in the upper segment of the fracture, cannot be placed on the same liquid line of descent of the HK groups, and it is similar to the magmas that fed the activity of Etna volcano prior to the eruption of 1971. This is the first time since then that a magma of this composition has been documented at Mt. Etna, thus providing a strong indication for the existence of distinct batches of magma whose rise and differentiation are independent from the main conduit system. The evolution of this eruption provides evidence that the NE Rift plays a very active role in the activity of Mt. Etna volcano, and that its extensional tectonics allows the intrusion and residence of magma bodies at various depths, which can therefore differentiate independently from the main open conduit system.     

Temporal variations in gravity at Mt. Etna (Italy) associated with the 1989 and 1991 eruptions

 Results are presented from 11 microgravity surveys on Mt. Etna between 1987 and 1993, a period including the major 1989 and 1991–1993 flank eruptions and subordinate 1990 activity. Measurements were made with LaCoste and Romberg D-62 and D-157 gravity meters along a network around the volcano between 1000 and 1900 m a.s.l. and, since 1992, a N–S summit profile. Gravity changes of as much as 200 μGal were observed at scales from the size of the summit region to that of the volcano. None was associated with significant changes in ground elevation. The data show an increase in gravity for 2 years before the 1989 eruption. The increase is attributed to the accumulation of magma (0.25–1.7×10⁹m³) in an elongate zone, oriented NNW–SSE, between 2.5 and 6 km below sea level. Part of this magma was injected into the volcanic pile to supply the 1989 and 1990 eruptions. It also probably fed the start of the 1991–1993 eruption, since this event was not preceded by significant gravity changes. A large gravity increase (up to 140 μGal) detected across the volcano between June and September 1992 is consistent with the arrival in the accumulation zone of 0.32–2.2×10⁹m³ of new magma, thus favoring continued flank effusion until 1993. A large gravity decrease (200 μGal) in the summit region marked the closing stages of the 1991–1993 event and is associated with magma drainage from the upper levels of Etna's central feeding system. Received: 15 July 1995 / Accepted: 27 October 1997