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1.
Since the March–April 1982 eruption of El Chichòn volcano, intense hydrothermal activity has characterised the 1-km-wide summit crater. This mainly consists of mud and boiling pools, fumaroles, which are mainly located in the northwestern bank of the crater lake. During the period 1998–2000, hot springs and fumaroles discharging inside the crater and from the southeastern outer flank (Agua Caliente) were collected for chemical analyses. The observed chemical fluctuations suggest that the physico-chemical boundary conditions regulating the thermodynamic equilibria of the deep rock/fluid interactions have changed with time. The chemical composition of the lake water, characterised in the period 1983–1997 by high Na+, Cl−, Ca2+ and SO42− contents, experienced a dramatic change in 1998–1999, turning from a Na+–Cl−- to a Ca2+–SO42−-rich composition. In June 2000, a relatively sharp increase in Na+ and Cl− contents was observed. At the same time, SO2/H2S ratios and H2 and CO contents in most gas discharges increased with respect to the previous two years of observations, suggesting either a new input of deep-seated fluids or local variations of the more surficial hydrothermal system. Migration of gas manifestations, enhanced number of emission spots and variations in both gas discharge flux and outlet temperatures of the main fluid manifestations were also recorded. The magmatic-hydrothermal system of El Chichòn is probably related to interaction processes between a deep magmatic source and a surficial cold aquifer; an important role may also be played by the interaction of the deep fluids with the volcanic rocks and the sedimentary (limestone and evaporites) basement. The chemical and physical changes recorded in 1998–2000 were possibly due to variations in the permeability of the conduit system feeding the fluid discharges at surface, as testified by the migration of gas and water emanations. Two different scenarios can be put forward for the volcanic evolution of El Chichòn: (1) build-up of an infra-crater dome that may imply a future eruption in terms of tens to hundreds of years; (2) minor phreatic–phreatomagmatic events whose prediction and timing is more difficult to constrain. This suggests that, unlike the diminished volcanic activity at El Chichòn after the 1982 paroxistic event, the volcano-hydrothermal fluid discharges need to be more constantly monitored with regular and more frequent geochemical sampling and, at the same time, a permanent network of seismic stations should be installed.  相似文献   

2.
Radionuclide activities (210Pb, 210Bi, 210Po) were investigated in Mount Etna plume from 1983 to 1995. At SE crater the long-term observation (12 years) of the 210Po/210Pb ratio shows that it can behave as a degassing vent not directly related to the main magma reservoir depending on the magma level inside the volcano. Since 1992, the simultaneous determination of radionuclides and sulfur in the main plume results in new constraints on the degassing model of Lambert et al. (Earth Planet. Sci. Lett., 76 (1986) 185). The 210Po/SO2 and 210Pb/SO2 ratios enable us to identify two sources of 210Po in the plume: one is magmatic, correlated with SO2, the other one is an additional component issued from the decay of 210Pb in the shallow degassing cell, and depends directly on the residence time of the gases before their emission. Estimations of the volume of degassing magma, the residence time of the gases and the proportion of undegassed magma renewing the shallow degassing cell are given for the period 1992–1995. During the 1992 eruption, the rate of degassing magma volume is estimated to have been as high as 5×106 m3/day, and the volume of the shallow degassing magma reservoir about 0.5 km3. In 1994 and 1995 the rate of non-erupted degassing magma volume was estimated to have been about 0.18 km3/year. During the entire 1983–1995 period, only 15–20% of the degassed magma was erupted.  相似文献   

3.
The Torfajökull central volcano in south-central Iceland contains the largest volume of exposed silicic extrusives in Iceland (225 km3). Within SW-Torfajökull, postglacial mildly alkalic to peralkalic silicic lavas and lava domes (67–74 wt.% SiO2) have erupted from a family of fissures 1–2.5 km apart within or just outside a large caldera (12×18 km). The silicic lavas show a fissure-dependent variation in composition, and form five chemically distinct units. The lavas are of low crystallinity (0–7 vol.%) and contain phenocrysts in the following order of decreasing abundance: plagioclase (An10-40), Na-rich anorthoclase (<Or23), clinopyroxene (Fs37-20), FeTi oxides (Usp32-60; Ilm93-88), hornblende (edenitic–ferroedenitic) and olivine (Fo22-37), with apatite, pyrrhotite and zircon as accessory phases. The phenocryst assemblage (0.2–4.0 mm) consistently exhibits pervasive disequilibrium with the host melt (glass). Xenoliths include sparse, disaggregated, and partially fused leucocratic fragments as well as amphibole-bearing rocks of broadly intermediate composition. The values of the silicic lavas are in the range 3.6–4.4, and these are lower than the values of comagmatic, contemporaneous basaltic extrusives within SW-Torfajökull, implying that the former can not be derived from the latter by simple fractional crystallization. FeTi-oxide geothermometry reveals temperatures as low as 750–800°C. To explain the fissure-dependent chemical variations, depletions, low FeTi-oxide temperatures and pervasive crystal-melt disequilibrium, we propose the extraction and collection of small parcels of silicic melt from originally heterogeneous basaltic crustal rock through heterogeneous melting and wall rock collapse (solidification front instability, SFI). The original compositional heterogeneity of the source rock is due to (1) silicic segregations, in the form of pods and lenses characteristically formed in the upper parts of gabbroic intrusives, and (2) extreme isostatic subsidence of the earlier, less differentiated lavas of the Torfajökull central volcano. Ridge migration into older crustal terranes, coupled with establishment of concentrated volcanism at central volcanoes like Torfajökull due to propagating regional fissure swarms, supplies the heat source for this overall process. Continued magmatism in these fissures promotes extensive prograde heating of older crust and the progressive vitality and rise of the central volcano magmatic system that leads to, respectively, SFI and subsidence melting. The ensuing silicic melts (with relict crystals) are extracted, collected and extruded before reaching complete internal equilibrium. Chemically, this appears as a two-stage process of crystal fractionation. In general, the accumulation of high-temperature basaltic magmas at shallow depths beneath the Icelandic rift zones and major central volcanoes, coupled with unique tectonic conditions, allows large-scale reprocessing and recycling of the low- , hydrothermally altered Icelandic crust. The end result is a compositionally bimodal proto-continental crust.  相似文献   

4.
 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  相似文献   

5.
Noncondensible gases from hot springs, fumaroles, and deep wells within the Valles caldera geothermal system (210–300°C) consist of roughly 98.5 mol% CO2, 0.5 mol% H2S, and 1 mol% other components. 3He/4He ratios indicate a deep magmatic source (R/Ra up to 6) whereas δ13C–CO2 values (−3 to −5‰) do not discriminate between a mantle/magmatic source and a source from subjacent, hydrothermally altered Paleozoic carbonate rocks. Regional gases from sites within a 50-km radius beyond Valles caldera are relatively enriched in CO2 and He, but depleted in H2S compared to Valles gases. Regional gases have R/Ra values ≤1.2 due to more interaction with the crust and/or less contribution from the mantle. Carbon sources for regional CO2 are varied. During 1982–1998, repeat analyses of gases from intracaldera sites at Sulphur Springs showed relatively constant CH4, H2, and H2S contents. The only exception was gas from Footbath Spring (1987–1993), which experienced increases in these three components during drilling and testing of scientific wells VC-2a and VC-2b. Present-day Valles gases contain substantially less N2 than fluid inclusion gases trapped in deep, early-stage, post-caldera vein minerals. This suggests that the long-lived Valles hydrothermal system (ca. 1 Myr) has depleted subsurface Paleozoic sedimentary rocks of nitrogen. When compared with gases from many other geothermal systems, Valles caldera gases are relatively enriched in He but depleted in CH4, N2 and Ar. In this respect, Valles gases resemble end-member hydrothermal and magmatic gases discharged at hot spots (Galapagos, Kilauea, and Yellowstone).  相似文献   

6.
Quantifying the contribution of volcanism to global mercury (Hg) emissions is important to understand the pathways and the mechanisms of Hg cycling through the Earth's geochemical reservoirs and to assess its environmental impacts. While previous studies have suggested that degassing volcanoes might contribute importantly to the atmospheric budget of mercury, little is known about the amount and behaviour of Hg in volcanic aquifers. Here we report on detailed investigations of both the content and the speciation of mercury in aquifers of active volcanoes in Italy and Guadeloupe Island (Lesser Antilles). In the studied groundwaters, total Hg (THg) concentrations range from 10 to 500 ng/l and are lower than the 1000 ng/l threshold value for human health protection fixed by the World Health Organization [WHO (1993): WHO Guidelines for Drinking Water Quality- http://www.who.int/water_sanitation_health/GDWQ/index.htlm]. Positive co-variations of (THg) with sulphate indicate that Hg-SO4-rich acid groundwaters receive a direct input of magmatic/hydrothermal gases carrying mercury as Hg0(gas). Increasing THg in a volcanic aquifer could thus be a sensitive tracer of magmatic gas input prior to an eruption. Since the complex behaviour and toxicity of mercury in waters depend on its chemical speciation, we carefully determined the different aqueous forms of this element in our samples. We find that dissolved elemental Hg0(aq) and particulate-bound Hg (HgP) widely prevail in volcanic aquifers, in proportions that highlight the efficiency of Hg adsorption onto colloidal particles. Moreover, we observe that dissolved Hg0aq and Hg(II) forms coexist in comparable amount in most of the waters, in stark contrast to the results of thermodynamic equilibrium modelling. Therefore, chemical equilibrium between dissolved mercury species in volcanic waters is either prevented by natural kinetic effects or not preserved in collected waters due to sampling/storage artefacts. Finally, we provide a first quantitative comparison of the relative intensity of aqueous transport and atmospheric emissions of mercury at Mount Etna, a very active basaltic volcano.  相似文献   

7.
On July 8, 1976, eruptive activity broke out at la Soufrière de Guadeloupe (F.W.I) after about one year of increasing seismic activity. Seismic activity continued to increase until August 1976, reaching more than 1500 events (a 200-fold increase over the preceding quiet period of a few years) and an energy output of about 1017 ergs in a day. A total of 26 major phreatic eruptions similar to the July 8 outburst took place during an eight-months period. The steam blasts that characterized the eruptions gave rise to particle- and sometimes block-charged plumes that deposited an estimated 106 m3 of solids. The H2O-rich gases emitted during the blasts presumably contained other gases (H2S, SO2, CO2...) that were partly adsorbed on solid particles. All material was erupted at temperatures of the order of 100° to 200°C.The observation of vertical migration of earthquake foci in less than a few hours and over about 6 km depth, and of abnormal variations of the geomagnetic field, indicate a deep energy source for the phreatic eruptions. A small proportion of the gases adsorbed on solid particles had a magmatic origin. However, most of the steam and the tephra seemed to originate from superficial levels of a hydrothermal system. Similar phreatic eruptions have occurred several times in recorded history. In the case of la Soufrière, the origin of the phreatic eruptions is best described by an abnormal energy input (versus steady-state) from a crustal magma chamber. The occurrence of truly magmatic eruptions is presumably inhibited by an extensive hydrothermal system. The abrupt release of more power from the magma chamber could have resulted in an explosive pyroclastic eruption.Substantial improvement of the Guadeloupe volcano observatory has followed the 1975–1977 crisis. Permanent telemetered geophysical networks and regular geochemical observations have provided a five year data base of the volcano behavior in its noneruptive state which can be compared to crisis situations.  相似文献   

8.
The Active Crater at Rincón de la Vieja volcano, Costa Rica, reaches an elevation of 1750 m and contains a warm, hyper-acidic crater lake that probably formed soon after the eruption of the Rio Blanco tephra deposit approximately 3500 years before present. The Active Crater is buttressed by volcanic ridges and older craters on all sides except the north, which dips steeply toward the Caribbean coastal plains. Acidic, above-ambient-temperature streams are found along the Active Crater's north flank at elevations between 800 and 1000 m. A geochemical survey of thermal and non-thermal waters at Rincón de la Vieja was done in 1989 to determine whether hyper-acidic fluids are leaking from the Active Crater through the north flank, affecting the composition of north-flank streams.Results of the water-chemistry survey reveal that three distinct thermal waters are found on the flanks of Rincón de la Vieja volcano: acid chloride–sulfate (ACS), acid sulfate (AS), and neutral chloride (NC) waters. The most extreme ACS water was collected from the crater lake that fills the Active Crater. Chemical analyses of the lake water reveal a hyper-acidic (pH0) chloride–sulfate brine with elevated concentrations of calcium, magnesium, aluminum, iron, manganese, copper, zinc, fluorine, and boron. The composition of the brine reflects the combined effects of magmatic degassing from a shallow magma body beneath the Active Crater, dissolution of andesitic volcanic rock, and evaporative concentration of dissolved constituents at above-ambient temperatures. Similar cation and anion enrichments are found in the above-ambient-temperature streams draining the north flank of the Active Crater. The pH of north-flank thermal waters range from 3.6 to 4.1 and chloride:sulfate ratios (1.2–1.4) that are a factor of two greater than that of the lake brine (0.60). The waters have an ACS composition that is quite different from the AS and NC thermal waters that occur along the southern flank of Rincón de la Vieja.The distribution of thermal water types at Rincón de la Vieja strongly indicates that formation of the north-flank ACS waters is not due to mixing of shallow, steam-heated AS water with deep-seated NC water. More likely, hyper-acidic brines formed in the Active Crater area are migrating through permeable zones in the volcanic strata that make up the Active Crater's north flank. Dissolution and shallow subsurface alteration of north-flank volcanoclastic material by interaction with acidic lake brine, particularly in the more permeable tephra units, could weaken the already oversteepened north flank of the Active Crater. Sector collapse of the Active Crater, with or without a volcanic eruption, represents a potential threat to human lives, property, and ecosystems at Rincón de la Vieja volcano.  相似文献   

9.
The character and location of seismic activity accompanying the onset of the 1991–1993 eruption at Mt. Etna are compatible with the surface evidence of the volcanic pile rupture. Both the epicentral distribution and the focal mechanisms of a swarm that occurred on December 14, 1991, agree with magma ascent occurring along the main NNW-SSE-trending structure of the volcano and the consequent opening of a system of effusive fissures with the same trend. A typical mainshock-aftershock sequence, recorded the day after and indicating transcurrent displacement occurring along the second-principal structure of Etna (NE-SW), depicts the tectonic response of the volcanic pile and the underlying basement to major stresses applied by the magma push.  相似文献   

10.
The eruptive history of Etna during the past 450 years provides data on effusion rates, volumes of magma involved, and the nature of the eruptive conduits. These data are interpreted in terms of a two-part intravolcanic magma reservoir which feeds the flank eruptions through dike-like conduits. The structural framework of the volcano which controls the spatial distribution of eruptive sites is partly inherited from the basement and partly controlled by the central magma column and the surrounding caldera boundary faults. Hydraulic fracturing theory predicts that the central magma column will fail at depths below 1 km if the tensile strength of the conduit rocks is about 100 bars and that a peak fracturing capability will be reached between 1 and 2 km depth. This inference agrees well with the peak of flank eruptive activity at 1.4 km below the summit observed in the data on the loci of eruptions. The average flank-eruption feeding dike is defined and shown to be capable of the observed maximum effusion rates (20–100 m3 s−1) from magmatic pressure differences of 30–150 bars  相似文献   

11.
Methods used previously to remove compositional modifications from volcanic gas analyses for Mount Etna and Erta'Ale lava lake have bean employed to estimate the gas phase composition at Nyiragongo lava lake, based on samples obtained in 1959. H2O data were not reported in 11 of the 13 original analyses. The restoration methods have been used to estimate the H2O contents of the samples and to correct the analyses for atmospheric contamination, loss of sulfur and for pre- and pest-collection oxidation of H2S, S2, and H2. The estimated gas compositions are relatively CO2-rich, low in total sulfur and reduced. They contain approximately 35–50% CO2 45–55% H2O, 1–2% SO2, 1–2% H2., 2–3% CO, 1.5–2.5% H2S, 0.5% S2 and 0.1% COS over,he collection temperature range 102° to 960° C. The oxygen fugacities of the gases are consistently about half an order of magnitude below quartz-magnetite-fayalite. The low total sulfur content and resulting low atomic S/C of the Nyiragongo gases appear to be related to the relatively low fO2 of the crystallizing lava. At temperatures above 800°C and pressures of 1–1.5 k bar, the Nyiragongo gas compositions resemble those observed in primary fluid inclusions believed to have formed at similar temperatures and pressures in nephelines of intrusive alkaline rocks. Cooling to 300°C, with fO2 buffered by the rock, results in gas compositions very rich in CH4 (50–70%) and resembling secondary fluid inclusions formed at 200–500°C in alkaline rocks. Below 600°C the gases become supersaturated in carbon as graphite. These inferences are corroborated by several reports of hydrocarbons in plutonic alkaline rocks, and by the presence of CH4-rich waters in Lake Kivu — a lake on the flanks of Nyiragongo volcano.  相似文献   

12.
Gases, condensates and silica tube precipitates were collected from 400°C (Z2) and 800°C (Z3) fumaroles at Colima volcano, Mexico, in 1996–1998. Volcanic gases at Colima were very oxidized and contain up to 98% air due to mixing with air inside the dome interior, close to the hot magmatic body. An alkaline trap method was used to collect gas samples, therefore only acidic species were analysed. Colima volcanic gases are water-rich (95–98 mol%) and have typical S/C/Cl/F ratios for a subduction type volcano. δD-values for the high-temperature Z3 fumarolic vapour vary from −26 to −57‰. A negative δD–Cl correlation for the Z3 high-temperature fumarole may result from magma degassing: enrichment in D and decrease in the Cl concentration in condensates are likely a consequence of input of “fresh” batches of magma and an increasing of volcanic activity, respectively.The trace element composition of Colima condensates generally does not differ from that of other volcanoes (e.g. Merapi, Kudryavy) except for some enrichment in V, Cu and Zn. Variations in chemical composition of precipitates along the silica tube from the high-temperature fumarole (Colima 1, fumarole Z3), in contrast to other volcanoes, are characterized by high concentrations of Ca and V, low concentration of Mo and a lack of Cd. Mineralogy of precipitates differs significantly from that described for silica tube experiments at other volcanoes with reduced volcanic gas. Thermochemical modelling was used to explain why very oxidized gas at Colima does not precipitate halite, sylvite, and Mo- and Cd-minerals, but does precipitate V-minerals and native gold, which have not been observed before in mineral precipitates from reduced volcanic gases.  相似文献   

13.
This study presents baseline data for future geochemical monitoring of the active Tacaná volcano–hydrothermal system (Mexico–Guatemala). Seven groups of thermal springs, related to a NW/SE-oriented fault scarp cutting the summit area (4,100m a.s.l.), discharge at the northwest foot of the volcano (1,500–2,000m a.s.l.); another one on the southern ends of Tacaná (La Calera). The near-neutral (pH from 5.8 to 6.9) thermal (T from 25.7°C to 63.0°C) HCO3–SO4 waters are thought to have formed by the absorption of a H2S/SO2–CO2-enriched steam into a Cl-rich geothermal aquifer, afterwards mixed by Na/HCO3-enriched meteoric waters originating from the higher elevations of the volcano as stated by the isotopic composition (δD and δ18O) of meteoric and spring waters. Boiling temperature fumaroles (89°C at ~3,600m a.s.l. NW of the summit), formed after the May 1986 phreatic explosion, emit isotopically light vapour (δD and δ18O as low as −128 and −19.9‰, respectively) resulting from steam separation from the summit aquifer. Fumarolic as well as bubbling gases at five springs are CO2-dominated. The δ13CCO2 for all gases show typical magmatic values of −3.6 ± 1.3‰ vs V-PDB. The large range in 3He/4He ratios for bubbling, dissolved and fumarolic gases [from 1.3 to 6.9 atmospheric 3He/4He ratio (R A)] is ascribed to a different degree of near-surface boiling processes inside a heterogeneous aquifer at the contact between the volcanic edifice and the crystalline basement (4He source). Tacaná volcano offers a unique opportunity to give insight into shallow hydrothermal and deep magmatic processes affecting the CO2/3He ratio of gases: bubbling springs with lower gas/water ratios show higher 3He/4He ratios and consequently lower CO2/3He ratios (e.g. Zarco spring). Typical Central American CO2/3He and 3He/4He ratios are found for the fumarolic Agua Caliente and Zarco gases (3.1 ± 1.6 × 1010 and 6.0 ± 0.9 R A, respectively). The L/S (5.9 ± 0.5) and (L + S)/M ratios (9.2 ± 0.7) for the same gases are almost identical to the ones calculated for gases in El Salvador, suggesting an enhanced slab contribution as far as the northern extreme of the Central American Volcanic Arc, Tacaná.  相似文献   

14.
Heat and mass transfer rates were studied at the Niragongo lava lake during two expeditions directed by H. Tazieff in 1959 and 1972. The results of this study are as follows:Heat is transferred to the surface of the lake by the movement of lava; gas discharge is a result and not the cause of convection. The chemical composition of the gases and magma has changed very little between 1959 and 1972, whereas the mass and energy outputs differ by an order of magnitude. In 1977 a catastrophic explosion seems to have been caused by tectonic factors, stopping the slow convection of magma under the volcano and hence reducing surface manifestations in the form of the lava lake and escaping fumarolic and magmatic gases. The gas discharge was, in tons day−1, 5000 for H2O, 11,000 for CO2, 1000 for SO2 in 1959, and in 1972 7700 for H2O, 180,000 for CO2 and 23,000 for SO2. These values correspond to an energy transfer of 0.9 × 109 W in 1959 and 16 × 109 W in 1972.  相似文献   

15.
Fifty-three major explosive eruptions on Iceland and Jan Mayen island were identified in 0–6-Ma-old sediments of the North Atlantic and Arctic oceans by the age and the chemical composition of silicic tephra. The depositional age of the tephra was estimated using the continuous record in sediment of paleomagnetic reversals for the last 6 Ma and paleoclimatic proxies (δ18O, ice-rafted debris) for the last 1 Ma. Major element and normative compositions of glasses were used to assign the sources of the tephra to the rift and off-rift volcanic zones in Iceland, and to the Jan Mayen volcanic system. The tholeiitic central volcanoes along the Iceland rift zones were steadily active with the longest interruption in activity recorded between 4 and 4.9 Ma. They were the source of at least 26 eruptions of dominant rhyolitic magma composition, including the late Pleistocene explosive eruption of Krafla volcano of the Eastern Rift Zone at about 201 ka. The central volcanoes along the off-rift volcanic zones in Iceland were the source of at least 19 eruptions of dominant alkali rhyolitic composition, with three distinct episodes recorded at 4.6–5.3, 3.5–3.6, and 0–1.8 Ma. The longest and last episode recorded 11 Pleistocene major events including the two explosive eruptions of Tindfjallajökull volcano (Thórsmörk, ca. 54.5 ka) and Katla volcano (Sólheimar, ca. 11.9 ka) of the Southeastern Transgressive Zone. Eight major explosive eruptions from the Jan Mayen volcanic system are recorded in terms of the distinctive grain-size, mineralogy and chemistry of the tephra. The tephra contain K-rich glasses (K2O/SiO2>0.06) ranging from trachytic to alkali rhyolitic composition. Their normative trends (Ab–Q–Or) and their depleted concentrations of Ba, Eu and heavy-REE reflect fractional crystallisation of K-feldspar, biotite and hornblende. In contrast, their enrichment in highly incompatible and water-mobile trace elements such as Rb, Th, Nb and Ta most likely reflect crustal contamination. One late Pleistocene tephra from Jan Mayen was recorded in the marine sequence. Its age, estimated between 617 and 620 ka, and its composition support a common source with the Borga pumice formation at Sør Jan in the south of the island.  相似文献   

16.
Popocatépetl volcano in central Mexico has been erupting explosively and effusively for almost 4 years. SO2 emission rates from this volcano have been the largest ever measured using a COSPEC. Pre-eruptive average SO2 emission rates (2–3 kt/d) were similar to the emission rates measured during the first part of the eruption (up to August 1995) in contrast with the effusive–explosive periods (March 1996–January 1998) during which SO2 emission rates were higher by a factor of four (9–13 kt/d). Based on a chronology of the eruption and the average SO2 emission rates per period, the total SO2 emissions (up to 1 January 1998) are estimated to be about 9 Mt, roughly half as much as the SO2 emissions from Mount Pinatubo in a shorter period. Popocatépetl volcano is thus considered as a high-emission rate, passively degassing eruptive volcano. SO2 emission rates and SO2 emissions are used here to make a mass balance of the erupted magma and related gases. Identified excess SO2 is explained in terms of continuous degassing of unerupted magma and magma mixing. Fluctuations in SO2 emission rate may be a result of convection and crystallization in the chamber or the conduits, cleaning and sealing of the plumbing system, and/or SO2 scrubbing by the hydrothermal system.  相似文献   

17.
Geological surveys, tephrostratigraphic study, and 40Ar/39Ar age determinations have allowed us to chronologically constrain the geological evolution of the lower NW flank of Etna volcano and to reconstruct the eruptive style of the Mt Barca flank eruption. This peripheral sector of the Mt Etna edifice, corresponding to the upper Simeto valley, was invaded by the Ellittico volcano lava flows between 41 and 29 ka ago when the Mt Barca eruption occurred. The vent of this flank eruption is located at about 15 km away from the summit craters, close to the town of Bronte. The Mt Barca eruption was characterized by a vigorous explosive activity that produced pyroclastic deposits dispersed eastward and minor effusive activity with the emission of a 1.1-km-long lava flow. Explosive activity was characterized by a phreatomagmatic phase followed by a magmatic one. The geological setting of this peripheral sector of the volcano favors the interaction between the rising magma and the shallow groundwater hosted in the volcanic pile resting on the impermeable sedimentary basement. This process produced phreatomagmatic activity in the first phase of the eruption, forming a pyroclastic fall deposit made of high-density, poorly vesicular scoria lapilli and lithic clasts. Conversely, during the second phase, a typical strombolian fall deposit formed. In terms of hazard assessment, the possible occurrence of this type of highly explosive flank eruption, at lower elevation in the densely inhabited areas, increases the volcanic risk in the Etnean region and widens the already known hazard scenario.  相似文献   

18.
Shallow shear-type seismic activity occurring beneath the Etna volcano during 1990–1995 has been analysed for hypocenter locations, focal mechanisms and stress tensor inversion. The results have been examined jointly with Electronic Distance Measurements and tiltmeter data collected in the same period and reported in the literature. Significant seismicity located in the upper 10 km was found to be confined to the time intervals in which ground deformation data indicated inflation of the volcano edifice (e.g., the periods preceding the December 1991–March 1993 and August 1995–March 1996 eruptive phases). The shocks mostly occurred in a sector approximately centered on the crater area and elongated in the East–West direction. The causative seismogenic stress shows a low-dip East–West orientation of σ1. In agreement with existing knowledge on relationships between local fault systems and magma uprise processes, the shallow seismicity in question is tentatively explained as being due to lateral compression by magma inside a nearly North–South system. The volcano deflation phase revealed by Electronic Distance Measurements and tilt data during the 1991–1993 major eruption was not accompanied by any significant shear-type shallow event. Below the depth of 10 km, the North–South prevailing orientation of σ1 reflects the dominant role of the regional stress.  相似文献   

19.
Data for major, minor and trace elements in groundwaters from Mt. Etna volcano collected in 1994, 1995 and 1997 were analyzed using Cluster Analysis (CA). Two groups of sampling sites were identified (named clusters A and B), mainly on the basis of their different salinity and content of dissolved CO2. The highest levels of both of these parameters were observed in the sites of cluster A, located in the lower south-western and central eastern flanks of the volcano. For both of the statistical groups CA was repeated, taking into account the mean values of each parameter in time, and the results allowed us to recognize four distinct groups of parameters for each group of sites on the basis of their temporal patterns. Four different types of temporal patterns were recognized: concave, convex, increasing, decreasing. The observed changes were basically interpreted as a result of the different response of dissolved chemical elements to changes in the aqueous environment and/or in their solubility/mobility in water due to different rates of input of magmatic gases to Etna’s aquifers. The main changes occurred in 1995, when Etna’s volcanic activity resumed after a two-year period of rest. The temporal changes of the majority of the studied parameters (water temperature, water conductivity, Eh, pH, Al, Mg, B, Ca, Cl−, Hg, Mn, Mo, Na, Ni, Se, Si, Sr, Cr Zn and pCO2) were not cluster-dependent, therefore they were not apparently affected by differences in water salinity between the two groups of sampling sites. A limited number of parameters (Ti, K, Li, HCO3−, As, Fe, SO42−, Cu and V), however, manifested different behaviors, depending on the cluster of sites to which they belonged, thus suggesting their apparent dependency on water salinity.  相似文献   

20.
Wide variations were measured in the diffuse CO2 flux through the soils in three selected areas of Mt Etna between August 1989 and March 1993. Degassing of CO2 from the area of Zafferana Etnea-S. Venerina, on the eastern slope of the volcano, has been determined to be more strongly influenced by meteorological parameters than the other areas. The seasonal component found in the data from this area has been excluded using a filtering algorithm based on the best fitting equation calculated from the correlation between CO2 flux values and those of air temperature. The filtered data appear to have variations temporally coincident with those from the other areas, thus suggesting a common and probably deep source of gas. The highest fluxes measured in the two most peripheral areas may correlate well with other geophysical and volcanological anomalous signals that preceded the strong eruption of 1991–1993 and that were interpreted as deep pressure increases. Anomalous decreases in CO2 fluxes accompanied the onset and the evolution of that eruption and have been interpreted as a sign of upward migration of the gas source. The variations of CO2 flux at the 1989 SE fracture have also given interesting information on the timing of the magmatic intrusion that has then fed the 1991–1993 eruption.  相似文献   

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