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  • Springer Science + Business Media  (4,670)
  • 1980-1984  (4,670)
  • 1
  • 2
    Publication Date: 1980-06-01
    Description: From August to October, 1976, La Soufrière de Guadeloupe was observed, and recorded with an automated sequence camera and numerous handheld cameras. During the period of observation, the nature of volcanic activity ranged from mild steam emission to moderately energetic phreatic eruptions. Background fumarolic activity (steam emission) was characterized by the emission of generally tephra-free steam clouds 50 to 150 m above the summit. The clouds rose buoyantly above the vent and were blown downwind at prevailing wind velocities. Phreatic eruptions were well-documented on September 22, October 2, and October 4. In the latter two eruptions, small bursts of tephra-laden steam erupted at intervals of 30 to 45 min, and rose from 350 to 500 m above the summit. In the largest observed eruption, that of October 2, the steam and tephra cloud rose to a maximum height of 600 to 650 m in 20 min. A white vapor cloud and a medium gray, tephra-laden cloud were erupted simultaneously from the summit vent and both were surrounded by a vapor collar: the clouds were thoroughly mixed within 1 km downwind of the summit. The concurrent growth of clouds from separate vents (summit and flank) implies a common source. Simultaneous eruption of tephra-free and tephra-laden clouds from the same vent is puzzling and implies: (i) lateral changes in the degree of alteration of dome rocks along the elongate vent, hence erodability of the dome lavas, or (ii) differences in the gas velocities. These «mixed» clouds moved westward, downwind and downslope as a density current, along the watersheds of the R. Noire and R. des Pères with an approximate velocity of 10 to 25 m/sec. Upon reaching the sea the clouds continued to move forward, but at a decreased velocity, and spread laterally, having left behind the restrictions of valley walls. A thin gray veneer of moist tephra, ranging from several cm thick near the dome to less than 1 mm thick several km downwind, was deposited along a narrow corridor southwest of the summit. Tephra from the phreatic eruptions consisted mostly of hydrothermally altered lithic, mineral, and glass fragments derived from dome lavas; no fresh (juvenile) pyroclasts were present in the tephra. Absence of juvenile tephra at La Soufrière supports the view that activity was due to groundwater circulating in a vapor-dominated geothermal system, probably driven by a shallow heat source. At La Soufrière, most vapor-dominated systems are located in elevated areas of groundwater recharge where groundwater movement is downward and outward. The sporadic phreatic eruptions may be related to the rate of recharge of meteoric waters within the dome, the decrease in pore pressure during fortnightly tidal minimums or both. Whatever the triggering mechanism, vapor-dominated fluids eroded vent walls during phreatic eruptions and carried out fine-grained, hydrothermally altered, pre-existing dome material as tephra. ©1980 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 3
    Publication Date: 1980-03-01
    Description: O’a is the largest of the Quaternary caldera volcanoes that punctuate the axis of the Ethiopian rift valley. The known volcanic history of O’a is brief: eruptions of restricted ash-flow tuffs and «tufolavas» were followed by extensive pumice deposition with intervening paleosols, lacustrine sediments, and flows of occasional welded tuffs and rare basalts. Ensuing caldera collapse at c. 0.24 m.y. ago was accompanied by emplacement of two massive ignimbrite flow units comprising a single cooling unit: the first was much more severely welded than the second which shows lahar characteristics. Post-caldera volcanism at O’a has been sparse compared with most other Ethiopian rift centres. O’a volcano exemplifies the common rift association of a caldera set tightly between two offset segments of the Wonji fault belt. The Wonji fault belt marks the youngest tectonism of the rift floor, and in the vicinity of O’a has been active in a major way since caldera subsidence. This faulting is clearly younger than the massive rift margin faulting, which to the northeast of O’a occurred during a tectonic climax dated at c. 1.0 m.y. ago. Radiometric analysis suggests a rather regular level of initial40Ar in O’a basalt lavas sampled near to their original vents. If this level also applies to near-vent basalts dated from other parts of the Ethiopian rift, a regional rift paroxysm of crustal extension and related silicic and basaltic volcanism is evident at c. 0.30–0.20 m.y. ago. Episodic dilatation and associated volcano-tectonism separated by long periods of quiescence appears to be a general feature of continental rift valleys. ©1980 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 4
    Publication Date: 1982-09-01
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  • 5
    Publication Date: 1982-09-01
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  • 6
    Publication Date: 1982-09-01
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  • 7
    Publication Date: 1982-12-01
    Description: Some domean volcanoes built by acidic to intermediate lavas are described. They were intruded, probably in Pliocene, into the neogenic volcano-sedimentary formations of the volcanic Tabriz — Bazman Zone (Central Iran). Some of them are intrusive domes which certainly did not reach the existing topographical surface when emplaced; others are extrusive domes which exhibit some interesting features viz. : hypogenous emplacement of the Suleghan dome, typical ring shape of the Dastjerd dome, etc. A provisional classification of domean volcanism in proposed in conclusion.RésuméOn décrit quelques appareils volcaniques dôméens, constitués de laves acides à intermédiaires, mis en place probablement au Pliocène dans les formations volcano-sédimentaires de la zone volcanique de Tabriz — Bazman (Iran Central). Les uns sont des dômes intrusifs n’ayant certainement pas atteint la surface topographique contemporaine de leur mise en place; les autres sont des dômes extrusifs dont certains présentent des particularités interessantes: mise en place hypogène du dôme de Suleghan, forme typique en anneau du dôme de Dastjerd, etc. En conclusion, les auteurs proposent un essai de classification des appareils volcaniques dôméens. ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 8
    Publication Date: 1982-09-01
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  • 9
    Publication Date: 1984-12-01
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  • 10
    Publication Date: 1984-12-01
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  • 11
    Publication Date: 1984-12-01
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  • 12
    Publication Date: 1984-12-01
    Description: An aircraft survey was carried out for the study of the gaseous and particulate emissions over Mt Etna Volcano in September 1983. Samples were collected in the range from 10 to 300 km; a satisfying conservation of the elemental composition of the aerosol has been observed. Some activity fluctuation, 150 km from the emitter, was attributed to an ash fall episode. Sampling assumed that some crustal elements in the volcanic aerosol can be used to measure the plume dilution factor. New estimations of volcanic particulate fluxes are given. ©1984 Intern. Association of Volcanology and Chemistry of the Earth's Interior
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  • 13
    Publication Date: 1982-09-01
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  • 14
    Publication Date: 1982-09-01
    Description: This paper describes the main lines of investigation for the volcanic and geothermal research in Kamchatka. Methods of gas sampling in the field and gas extraction from rocks are also described. ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 15
    Publication Date: 1982-09-01
    Description: The analyses of approximately 100 high temperature gas samples from erupting lavas of Surtsey, Erta Ale, Ardoukoba, Kilauea, Mount Etna and Nyiragongo exhibit erratic compositions resulting from analytical errors, condensation effects, reactions with sampling devices, and contamination by atmospheric gases, meteoric water and organic material. Computational techniques have been devised to restore reported analyses to compositions representative of the erupted gases. The restored analyses show little evidence of short-term variations. The principal species are H_2O, CO2, SO_2, H_2, CO, H_2S, S_2, and HCl. The O_2 fugacities range from nickel-nickel oxide to a half order of magnitude below quartz-magnetite-fayalite. There is no evidence for a unique magmatic gas composition; instead, the erupted gases show regular compositional trends characterized by decreasing CO2 with progressive outgassing. The gases from more alkaline lavas (Etna, Nyiragongo) are distinctly richer in CO2, while those from less alkaline (Surtsey) or tholeiitic lavas (Erta Ale, Ardoukoba) tend to be richer in H_2O. Kilauean gases range from CO2-rich to H_2O-rich. The total sulfur contents of the erupted gases show an excellent positive correlation with lava O_2 fugacity. All restored analyses are significantly lower in H_2O and enriched in sulfur and CO2 compared to the «excess volatiles». ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 16
    Publication Date: 1982-09-01
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  • 17
    Publication Date: 1982-09-01
    Description: A field gas chromatograph, built in 1978, was used in the field to directly analyse volcanic gases before water vapor condensation. Tested in Vulcano (Italy), Kilauea (Hawaii) and Merapi (Indonesia), this field measurement technique provides the actual composition of the volcanic gas mixture. The technique avoids the depletion of sulfur gases and the dissolution of the acid gases in the condensed water during the cooling. Thus the mixture of H_2S and SO_2 in fumarolic and high temperature gases (up to 819°C) in equilibrium at the emission temperature was examined. ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 18
    Publication Date: 1982-09-01
    Description: Sulphur isotopic compositions of pyrite, anhydrite and native sulphur in volcanic ashes discharged by the 1979 eruption of Ontake volcano, Nagano, Japan were determined. The isotopic data indicate that sulphate in anhydrite and a part of native sulphur were produced by the disproportionation reaction of sulphite formed by dissolution of SO_2 in volcanic gases into water which filled a mud reservoir probably located just below the crater zone. Some part of H_2S in volcanic gases was fixed as pyrite and some was oxidised to form native sulphur. Hydrothermal alteration of country rocks to form pyrite, anhydrite and clay minerals had proceeded in the mud reservoir before eruption at temperatures ranging from 110° to 185°C which were estimated by oxygen isotopic fractionation between anhydrite and water. ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 19
    Publication Date: 1982-09-01
    Description: During the eruption of Mt Tolbachik volcano in 1975–1977 magmatic gases were collected by condensation and dissolution. The field system is described and discussed.ResumePendant l’éruption du volcan Tolbachik en 1975–1977 les auteurs ont utilisé un système de condensation par barbotage afin de récolter les condensats et les gaz magmatiques. La methode de prélèvement et décrite et discutée dans le détail. ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 20
    Publication Date: 1982-09-01
    Description: Des sublimés ont été récoltés à l’Etna en 1976, le long d’un tube en silice. La mesure de la température du dépôt et l’analyse élémentaire par activation neutronique permettent de mieux comprendre des mécanismes magmatologiques, métallogéniques et volcanologiques.On Mount Etna, in June 1976, sublimates were collected on the wall of a silicatube. Temperatures of deposition were recorded along the tube and the atomic composition has been analysed by neutronic activation. It is possible with this technique to know the temperature of condensation or sublimation for the different elements. These results should help to better understand the magma/gas relationship. ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 21
    Publication Date: 1982-09-01
    Description: The cooling of volcanic gases generates sublimates, condensates and gases. Condensates were sampled and analysed on different volcanoes: On Guadeloupe, phreatic eruptions occur when pH of condensates has maintained higher than 4 to 5 for 3 to 7 days. After the phreatic eruption, pH decreases to 1.5 to 2. At Pozzuoli and Vulcano it is possible to distinguish between gases washed by superficial waters (low mineral contents and high pH) and gases less affected by condensation and dissolution in the ground (high mineral contents and low pH).ResumeLes gaz volcaniques en se refroidissant donnent une phase solide: les sublimés, une phase liquide: les condensats et une phase gazeuse résiduelle. Les condensats ont été analysés sur place puis au laboratoire. Cette technique a été testée sur différents volcans: A la Soufriére de la Guadeloupe les éruptions phréatiques surviennent après des périodes de 3 à 7 jours où le pH des condensats est supérieur a 4 ou 5, ces éruptions sont toujours suivies d’une diminution du pH: 1,5 à 2 pendant plusieurs jours. A Pouzzoles et Vulcano les condensats des fumerolles lessivés par des acquifères superficielles ont des pH élevés et une faible mineralisation, les pH bas et les mineralisations élevées correspondant à des gaz peu affectés par les condensations dans le sol. ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 22
    Publication Date: 1982-09-01
    Description: The flow rate of SO2, HCl and HF was calculated from a transfer coefficient obtained by measuring the concentration of a tracer gas (SF_6) emitted at the plume and analysed down wind with a field gas chromatograph. The results obtained are: SO2 = 2.3 pm 0.4 t/day \ HCl = 6.4 pm 0.4 t/day \ HF = 0.11 pm 0.3 t/day ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 23
    Publication Date: 1982-09-01
    Description: A field system for analyzing gases was installed on line in a borehole into the geothermal field associated with Gorelyi volcano, which is close to Mutnovski volcano (Kamtchatka). The system consisted of a gas chromatograph and an electrode for sensing gases. Measurements were made for the duration of one week in July 1980. Variations in H_2 concentrations were less than 50%. No correlations were observed with regional seismic activity or with volcanic activity at Gorelyi volcano (10 km away). Measurements of H_2 during a longer period of time would allow a statistical treatment of the data.ResumeLe forage du gisement hydrothermal situé au pied du volcan Gorelyi au voisinage de Mutnovski (Kamtchatka) a été équipé d’un système de mesure « en ligne » de la composition des gaz. Les mesures ont été réalisées à l’aide d’un chromatographe en phase gazeuse et d’une électrode à membrane gazeuse. La mesure a durée une semaine en Juillet 1980. La variation de la concentration en H_2 n’éxcédait pas 50%. Aucune corrélation n’a pu étre faite ni avec l’activité sismique régionale enregistrée, ni avec celle du volcan voisin Gorelyi (situé à 10 km). La mesure de la concentration en H_2 pendant un temps plus long, permettrait seule un traitement statistique rigoureux. ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 24
    Publication Date: 1982-06-01
    Description: Many examples of mixed magmas in banded lavas have been studied. Another type of mixed magmas or inhomogeneity of magma reservoir found in the 1962 lava flows of Miyake-zima Volcano erupted from fissures is reported. ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 25
    Publication Date: 1982-06-01
    Description: This study discusses the focal mechanisms of 61 earthquakes and dynamic parameters of 97 earthquakes (m_i, ≥ 2.0 and focal depths 0–20 km) occurring in the region of the large Tolbachik fissure eruption in 1975–1978. Variations of these parameters during the different eruptive stages are examined. Three stages can be distinguished: 1) June 27–July 2, 1975 (from the beginning of the seismic preparation to the large earthquakes of July 2nd with M =5.0); 2) July 2nd-July 6th, 1975 (before the onset of eruption); 3) From July 6th, 1975 to the end of 1976 (during the eruption). During the second stage the 90° reorientation of the compression stress axes was observed; during the third stage the compression stress system returned to the initial strike. No variation in the orientation of the tension stress axes was noted during all these stages. Simultaneously with the reorientation of the compression stress axes in the volcanic earthquake foci during the second stage, the stress-drop values decreased strongly, whereas during the first stage these values were anomalously high. During the third stage the stress-drop values continued to decrease and approached the average regional values indicative of a calm period. All the estimates of parameters variations were made at 0.1 significance level. The above-mentioned temporal variations in the volcanic earthquake focal parameters are discussed in connection with the probable model of preparation and course of eruption. ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 26
    Publication Date: 1982-06-01
    Description: Local eruptions of acid volcanic rocks occurred in the Central Aegean region around the Miocene-Pliocene boundary. Rhyolites outcrop on the Island of Antiparos, located in the central part of the Attic-Cycladic Massif. The age of these volcanic rocks ranges from 4.0 to 5.4 m.y., and chemical and Sr isotopic data suggest they were generated by partial melting of the continental crust. At the same time, along the border of the Attic-Cycladic Massif, i.e. on the island of Patmos and Caloyeri, local eruptions of Naalkaline basalts occurred. The whole of the eruptive activity is interpreted as an expression of the marked tensional tectonic phase which has affected the Central Aegean area since Middle Miocene. The contrasting nature of the erupted volcanic rocks (crustal and sub-crustal) is attributed to the different thermal state of the lithosphere beneath the two areas, as emphasized also by the presence of a wide granitic belt, of mainly Miocene age, which developed in the median sector of the Attic-Cycladic Massif. ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 27
    Publication Date: 1982-06-01
    Description: The Vulture complex is made up of foiditic, tephritic, phonolitic-trachytic and phonolitic products. New rock analyses have been performed in order to ascertain whether the various rock types derive from a unique parental magma and, if so, to define its nature. The data presented support that the Vulture suite originated from a foiditic melt which had differentiated at low pressures. The main process determining the foidite → → tephrite → phonolitic trachyte evolution seems to be the crystal fractionation of mainly clinopyroxenes, and opaques, with the contribution of plagioclases and haüyne too in the tephrite → trachyte evolution. Additionary role must have been played by a mixing of melts at different evolution stages occurred in a shallow seated magma chamber. ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 28
    Publication Date: 1982-06-01
    Description: On February 20th, 1979, 142 inhabitants of Dieng Plateau (Indonesia) were asphyxiated by poisonous gases during a mild phreatic eruption. From later fields gas collection and analysis, the casualties are considered to be due to CO2 rich volcanic gases. ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 29
    Publication Date: 1984-12-01
    Description: A succession of heterogeneous pyroclastites with contrasted composition (hawaiite-trachyte) and homogeneous intermediate lava flows (benmoreite) is present at Puy de la Nugère (Chaîne des Puys, Massif Central, France). Mineralogical disequilibrium assemblages are present in both the explosive en effusive products. Textural, mineralogical and geochemical data on these formations are compatible with a magma mixing model. The origin of the trachyandesitic rocks in the alkaline suite of Chaîne des Puys would be controlled by fractional crystallization and magma mixing. ©1984 Intern. Association of Volcanology and Chemistry of the Earth's Interior
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  • 30
    Publication Date: 1984-12-01
    Description: A systematic petrographic and chemical study of chronologically ordered lava samples collected during the 1983 Etna eruption, has shown minor fluctuating variations in crystallinity and chemistry, apparently correlated with variations in the rate of effusion. Comparison with the compositional variations observed in the lavas emitted during the 1971–1983 period, has revealed a repeated evolutional tendency suggesting a small size shallow storage system periodically refilled, as in 1983, by more primitive magma. ©1984 Intern. Association of Volcanology and Chemistry of the Earth's Interior
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  • 31
    Publication Date: 1984-12-01
    Description: The Quaternary central volcano Longonot is situated on the floor of the Gregory Rift Valley, Kenya, at 0° 55′ S, 36° 25′ E. Although the majority of its products are lavas and pyroclastics of pantelleritic trachyte composition, small volumes of alkali basalt magma have been coerupted with pantelleritic trachyte magma to produce mixed lavas. These lavas were the first products following each of three caldera collapses and mark the start of three successive cycles of whole-rock chemical variation with time. For the first two mixed-lava eruptions identified, field, petrographic and mineralogical evidence suggests that the contrasting magmas comingled, and in places hybridized, during eruption. Whole-rock geochemistry requires the alkali basalt component to have been contaminated prior to coeruption with trachyte. Syenite is suggested as a possible contaminant of the basalt component in the last two mixed-lava eruptions. Field and whole-rock chemical evidence points to the trachyte magma chamber being underlain by a basalt magma root zone. Inputs of fresh basalt magma into the root zone may have initiated each pre-caldera pyroclastic event and subsequent caldera formation and may have also caused the trachyte magma to overturn and commence a fresh cycle of chemical evolution. Some of the hot, buoyant basalt magma was able to leak towards the surface up peripheral fractures where it was coerupted with the initial trachyte magma of each cycle. ©1984 Intern. Association of Volcanology and Chemistry of the Earth's Interior
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  • 32
    Publication Date: 1984-12-01
    Description: The Holocene volcanic activity which built up the present terminal cones of Pico de Orizaba and Popocatepetl in eastern Mexico, was characterized by repeated pyroclastic Saint-Vincent type eruptions. Radiocarbon data show that these paroxysmal events occurred at more or less regular intervals, and were followed by moderate activity producing ash and pumice falls and andesitic lava flows from the summit craters. Typical ash and scoria pyroclastic flows exhibit a heterogeneous composition given by the interaction of a dacitic component with a more basic andesitic one. Scoria bombs are characterized by banded to emulsified textures, mineralogical desequilibrium assemblages and linear chemical variations on element-element plots as exemplified by the Loma Grande flow at Pico. Periodic replenishments of the magmatic reservoir could be the major phenomenon that started mixing and consequently triggered the pyroclastic eruptions. ©1984 Intern. Association of Volcanology and Chemistry of the Earth's Interior
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  • 33
    Publication Date: 1984-12-01
    Description: The water steam convection existing below a fumarolic area creates hydraulic, thermal and electric anomalies, the electric anomaly being probably the most correlated to a change of the deep thermal activity. The anomaly range depends on the position of the site connected with the configuration of the magmatic activity, but also on the local hydrogeologic conditions. The spatial and temporal surveillance of the eruptions based on these convective data should take into consideration these conditions.ResumeLa convection de la vapeur d'eau formée en profondeur dans une zone fumerollienne crée en surface une anomalie hydrique, thermique et électrique, cette dernière pouvant être la plus sensible à une variation de l'activité thermique profonde. L'amplitude des anomalies dépend de la position du site par rapport à la configuration de l'activité magmatique, mais aussi des conditions hydrogéologiques locales. La surveillance spatiale et temporelle des éruptions à partir du phénomène convectif hydrique doit tenir compte de ces conditions. ©1984 Intern. Association of Volcanology and Chemistry of the Earth's Interior
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  • 34
    Publication Date: 1984-12-01
    Description: Une campagne de mesures de contraintes a eu lieu en 1983 au voisinage de l'Etna. Ces mesures, effectuées par une technique de surcarottage au fond de forages peu profonds (5 à 6 mètres), montrent un changement radical du régime des contraintes entre la région au Sud de l'Etna et le pied du versant Nord de l'edifice volcanique.An in-situ rock stress survey was carried out in the Etna region in 1983. Measurements made with the overcoring technique at the bottom of shallow (5–6 m deep) boroholes have shown a strong variation in stress field between the region on the South of Etna and the zone at the northern foot of the volcano. ©1984 Intern. Association of Volcanology and Chemistry of the Earth's Interior
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  • 35
    Publication Date: 1984-12-01
    Description: A long range transport model and its application to an Etna plume are described. The model is a classical segmented plume lagrangian trajectory model. Firstly, air mass trajectories are computed by using the actual winds observed at the E.C.M.W.F. (Reading U.K.). The model then determines 5-day trajectories starting every 3 hours and made of 3-hour segments. Vertical movements of air parcels are taken into account by using the vertical component of the synoptic wind. Secondly, diffusion, SO_2 to SO_4 transformation and dry deposition are calculated for each trajectory segment. Sensitivity tests are presented and the results of the simulation for an experimental field period (21–25 september 1983) are examined. ©1984 Intern. Association of Volcanology and Chemistry of the Earth's Interior
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  • 36
    Publication Date: 1984-12-01
    Description: During the entire life of Cantal volcano, the main lava types were emitted during rather separated stages of activity. Basalts are scarce in the trachyandesitic complexes, whereas latites are very uncommon in the basaltic units. In both cases, incomplete mechanical mixing would have been active in their genesis as it is suggested by field and microscopic observations. Sanissage composite dome is formed of poorly mixed acid and basic lavas, with still clearly observable reaction relationship between components. Combe de Saure plug shows a dispersion of tiny (smaller than 1 cm) trachyandesitic relics within a prevalent basaltic groundmass, the result of which being a nearly homogeneous melt, so called « leucobasalt », with D.I. similar to that of a hawaiitic rock. From these observations it is suggested that a large part of intermediate Cantal lavas (D.I similar to those from hawaiites-mugearites-benmoreites) may derive from homogeneized mixing, when new basaltic magma entered the chamber filled with differentiated melt. ©1984 Intern. Association of Volcanology and Chemistry of the Earth's Interior
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  • 37
    Publication Date: 1984-12-01
    Description: Depuis septembre 1980, des mesures de la concentration de Radon dans le sol ont été entreprises sur l'Etna en vue d'étudier le comportement de ce gaz radioactif naturel vis-à-vis de l'activité du volcan et de son utilisation en tant qu'un des paramètres possibles de la prévision des éruptions volcaniques. La technique de mesure employée est celle des détecteurs solides de traces nucléaires plastiques (LR-115 type II, Kodak Pathé). Le réseau implanté, principalement en collaboration avec les mesures de température du sol ( Archambault et al. ) et de polarisation spontanée ( M. Aubert ) comprend une vingtaine de stations sitées sur le versant Sud et Sud-Est du volcan. Les densités de traces de particules alpha observées varient de 50 à 2000 traces/cm2/semaine selon la localisation des stations, malgré un bruit de fond à peu près constant. Sur l'ensemble des résultats aucune règle de variation saisonnière de l'émanation n'apparaît; certaines zones présentent les plus fortes densitésen hiver (Torré del Filosofo, Vulcarollo), d'autres au printemps (Etna Sud, Citelli). Le problème majeur de cette technique est la nécessité, pour obtenir une analyse fine de la variation de l'émanation du radon-222, de changer tous les 15 jours ou tous les mois les échantillons. Nous espérons remédier à ce problème par l'emploi de sondes de mesures automatiques, dont la mise au point est en cours, et pouvant être reliées au système Argos déjà existant.Since September 1980, measurements of radon concentration in the ground have been carried out on Etna using Radon as a forerunner geophysical parameter of volcanic eruptions. The technique used is the Solid State Nuclear Tracks detectors' method (LR 115 Type II, Kodak). The stations network, in connection with soil temperature ( Archambault et al. , 1980) and spontaneous polarisation measurements ( M. Aubert , pers. commun.) is composed of 20 stations located on the southern and south-eastern flanks of the volcano. The observed alpha track densities vary between 50 and 2000 tracks/cm2/week according to the location for a background almost constant. Seasonal variations seem to affect measurements, but their influence seems to be governed by no rule. The main problem in the used technique is the necessity of changing the detectors every two weeks or every month in order to obtain a good analysis of the variations in Rn-222 concentration. In order to remedy such deficiency, an automatic electronic device which can be connected with the yet existing Argos system is being arranged. ©1984 Intern. Association of Volcanology and Chemistry of the Earth's Interior
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  • 38
    Publication Date: 1984-12-01
    Description: L'éruption du 28-3-1983 a été précédée, dans la nuit entre le 27 et 28 Mars, par l'ouverture de fractures sur le flanc sud de l'Etna entre les côtes 2900 et 2300. Ces ruptures correspondent à des failles à rejet surtout vertical et à des fentes. La disposition générale dessine un graben d'orientation N-S à NE-SW, dont la largeur se réduit progressivement de 80 m à environ 25 m au point d'émission des laves, alors que le rejet vertical des failles augmente vers le sud (maximum 1 m 50). En quelques points on a pu noter une légère composante en décrochement. L'extension horizontale provoquée par l'ouverture de ce graben a été estimée à environ 3 m. Ces donnés sont confrontées à celles obtenues par d'autres méthodes et leur interprétation est discutée en rapport avec le mécanisme de l'éruption et les relations entre magmatisme et tectonique.The Etna eruption of March 28th, 1983 was preceded, during the night between March 27th and 28th, by the opening of fractures on the southern flank of Mt. Etna between 2300 and 2900 meters a.s.l. The fractures are essentially dip-slip faults and fentes . The resulting general pattern is that of a graben oriented from N-S to NE-SW that becomes progressively narrow passing from about 80 m to 25 m in width at the lava flow emission site, while the vertical fault throw increases towards the south reaching there the maximum value of 1.50 m. In a few places it was possible to detect a slight horizontal component of movement. The horizontal opening due to the graben formation has been estimated to be about 3 m in width. These data have then been compared with others obtained using different methods and their interpretation has been discussed in relation to the eruption mechanism and the relationships between magmatism and tectonics. ©1984 Intern. Association of Volcanology and Chemistry of the Earth's Interior
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  • 39
    Publication Date: 1984-12-01
    Description: L'Etna a connu, de 1977 à 1983, une période d'activité exceptionnelle, tant par la fréquence et l'extension des fracturations latérales de l'édifice que par la violence et la répétition des paroxysmes aux cratères centraux. L'éruption multifissurale de 1971 et l'éruption excentrique de 1974 sur le flanc Ouest apparaissent comme les prodromes de l'activité actuelle. Celle-ci a débuté en juillet 1977 par l'exacerbation de l'activité sommitale qui, de modérée persistante, est devenue paroxysmique et intermittente. Jusqu'en mars 1978, le Cratère NE a eu ainsi une vingtaine d'éruptions tout à fait inusitées, ne durant que quelques jours ou même quelques heures, avec puissantes fontaines de lave et coulées rapides atteignant jusqu'à 5 km de long. D'autres paroxysmes uniquement explosifs ont eu lieu en avril 1980 au Cratère Central, puis au Cratère SE. Un début d'activité persistante (mai-août 1980) dans cette dernière bouche a été interrompu par la reprise de violentes manifestations explosivo-effusives au Cratère NE en septembre 1980 et février 1981. Les éruptions latérales surtout ont été remarquables: en avril-juin 1978 (fracturation du versant SE, de 3000 à 2600 m d'altitude), août 1978 (SE, ENE, 3000–2300 m), novembre 1978 (SE, 3000–1650 m), août 1979 (SE, E, NE, 3000–1600 m), mars 1981 (NNW, 2600–1120 m), mars-août 1983 (S, 2700–2260 m). Les 4 premières de ces éruptions ont été précédées et accompagnées de violentes fontaines de lave au Cratère SE. Toutes ont été accompagnées ou suivies d'explosions phréatiques, principalement à la bouche Ouest du Cratère Central. La coulée du 4 août 1979 sur le flanc oriental est descendue en une demi-journée près du village de Fornazzo. Celle du 17 mars 1981 — la plus désastreuse — a, dans les 14 h qui ont suivi son apparition, coupé toutes les communications terrestres au pied Nord de l'Etna, sur un front par endroits large de 800 m: en moins de 5 jours, 18 × 106 m3 de lave ont été émis, recouvrant 6 km2 de terres en grande partie cultivables. Du 28 mars au 8 août 1983, les laves s'épanchant de façon tranquille, mais à un rythme soutenu, ont lentement enseveli plusieurs propriétés et implantations touristiques du versant Sud, entre 2300 et 1100 m d'altitude. Chacun de ces paroxysmes éruptifs se signale par des augmentations considérables de l'amplitude du tremor séismique, dont l'intensité n'est cependant pas directement liée à l'importance des phénomènes observés. Il est probable que la turbulence due à la montée du magma dans les conduits principaux influe notablement sur l'énergie du tremor: de fait, celui-ci est très réduit après la phase paroxysmique initiale (par exemple 1983), bien que l'effusion de lave persiste. Avant les éruptions, l'énergie du tremor atteint des niveaux relativement élévés pendant des intervalles de temps qui durent de 1 à 5 mois. Ce phénomène peut résulter d'un mécanisme de recharge du système de fractures et de dykes conduisant, à plus ou moins brève échéance, à l'événement éruptif. Plusieurs périodes de recharge peuvent se succéder avant une même éruption (3 avant celle de 1983). Dans les quelques jours qui précèdent certains paroxysmes (août 1979, sept. 1980, févr. et mars 1981), on constate une rapide diminution de l'amplitude du tremor, qui peut être associée à des variations du champ de contrainte déterminant, localement, la genèse de fractures nouvelles ou la fermeture/ouverture de fractures pré-existantes.A period of exceptional eruptive activity for both size and frequency of lateral eruptions, and violence of paroxysms at the summit craters characterized Etna from 1977 to 1983. The complex fissure eruption of 1971 and the eccentric eruption of 1974 on the W flank appear to be precursors of the present activity, which began in July 1977 with the change in style of summit eruptions from moderate persistent activity to intermittent paroxysms. The NE Crater produced a score of such unusual eruptions up to March 1978, lasting a few days or even a few hours with powerful lava fountaining and flows up to 7 km long with high effusion rates. Other solely explosive paroxysms occurred in April 1980 in the Chasm (Central Crater) and later at the SE Crater. A return to a period of gentle persistent activity (May–August 1980) at the latter vent was interrupted by the recommencement of violent explosive and effusive events at the NE Crater in September 1980 and February 1981. Particularly remarkable lateral (flank) eruptions occurred in April–June 1978 (fracturing of the SE slope from 3000 to 2600 m altitude), August 1978 (SE, ENE, 3000–2300 m), November 1978 (SE, 3000–1650 m), August 1979 (SE, E, NE, 3000–1600 m), March 1981 (NNW, 2600–1120 m) and March–August 1983 (S, 2700–2260 m). The first four of these eruptions were preceded and accompanied by violent lava fountaining at the SE Crater. All were accompanied and/or followed by phreatic explosions, principally from the Bocca Nuova (or W Central Crater). The flow emitted on 4th August 1979 from the eastern flank reached next to the village of Fornazzo in half a day. That of 17 March 1981 — the most destructive — cut all communications (roads and railways) round the northern foot of the mountain in the 14 hours following its appearance. It was in places 800 m wide, and in less than 5 days, 18 × 106 m3 of lava had been emitted, covering 6 km2 of largely agricultural land. From 28th March to 6th August 1983, the slow but sustained emission of lava gradually covered and destroyed several tourist installations between 2300 and 1100 m altitude. Each of these eruptive paroxysms was marked by a substantial increase in the trace amplitude of harmonic seismic tremor, the intensity of which being not directly proportional to the observed activity. It is likely that turbulence caused by the rising of magma in the main conduits had an important effect in the energy of the tremor: this is considerably reduced after the initial paroxysmal phase ( e.g. in 1983), yet the lava effusion may continue. Tremor energy attains relatively high levels from 1 to 5 months before eruptions. This may be caused by the recharging of the system of dykes and fissures, leading up to an eruptive event after a variable period. Several distinct periods of recharging may precede one eruption (there were three before the 1983 eruption). In the few days preceding the start of some of the eruptions (Aug. 1979, Sep. 1980, Feb. and Mar. 1981), a rapid diminution in the amplitude of tremor occurred. This may be associated with changes in the stress field caused by the opening of new fractures or the closing/opening of pre-existing ones. ©1984 Intern. Association of Volcanology and Chemistry of the Earth's Interior
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  • 40
    Publication Date: 1984-12-01
    Description: En juin et juillet 1983, pendant l'éruption, on a procédé à une campagne de mesures magnéto-telluriques sur l'Etna. Elle a consisté en l'exécution de six sondages lents et d'une cinquantaine de mesures audio-magnéto-telluriques (AMT). On discute l'interprétation tant géophysique que structurale des données recueillies en tenant compte des conditions particulières dans lesquelles l'opération s'est effectuée. La méthode AMT identifie très bien la fracture majeure liée à l'éruption qui se manifeste par un baisse très sensible de la résistivité apparente. Les sondages lents, pour leur part, mettent en évidence des formations conductrices qu'il est difficile d'accorder avec le schéma proposé par R. Schick et al . L'observation d'une évolution dans le temps de certaines mesures en AMT est discutée ainsi que les conséquences qu'on pourrait en tirer quant à la surveillance directe ou indirecte de la situation du magma en profondeur.Magneto-telluric measurements were carried out during the Etna eruption in June–July 1983. Six slow soundings and about 50 audiomagneto-telluric (AMT) measurements formed the survey programme. Geophysical and structural interpretations of data are given with taking into account the particular operational conditions. The major fracture associated with the eruption has been identified by the AMT method quite well in a very clear low of the apparent resistivity. On the other hand, slow soundings have shown conductive rock formations which it is difficult to fit the Schick et al. 's model. The time evolution of certain AMT measurements is discussed together with the possible implication for a direct or indirect surveillance of the magma situation in depth. ©1984 Intern. Association of Volcanology and Chemistry of the Earth's Interior
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  • 41
    Publication Date: 1984-12-01
    Description: An extensive set of samples representative of the evolution of Etna since the beginning of its activity in the Quaternary has been analyzed for trace element contents by neutron activation on whole rock samples. Major element data were taken from Tanguy (1980). Several petrogenetic processes have played a role in the observed chemical variations: binary trace element plots form linear arrays extrapolating to the origin for most second row and higher transition elements and the rare alkali metals thus indicating their hygromagmaphile (HYG) character; 3d transition elements show two types of behavior when plotted against an HYG element. Concentrations remain relatively constant for the less evolved rocks in the range from tholeiite through transitional basalts to alkali basalts. On the contrary, concentrations drop rapidly in the series evolving from alkali basalts to trachytes; in addition to the latter two general trends, geochemical regularities specific to Etnean lavas are born out. Rare alkali metals, as well as potassium, for instance, show a trend of abnormal enrichment in the most recent historical eruptions. The recurrence in time of rather primary magmas indicates that the petrogenetic evolution of the lavas cannot be explained by the differentiation of a single batch of mantle derived magma. However, the chemical composition of the mantle source has remained relatively homogeneous with a nature very near the chondritic one. Mineralogically, the source must differ from that for oceanic basalts to account for Th/Ta differences. Early tholeiitic primary magma evolving into later alkali basalts is adequately modeled by variations of the degree of partial melting of a closed system source. Highly evolved products have episodically been emitted as a result of extensive crystal fractionation, during which no basalts are erupted. These episodes are abruptly interrupted by the eruption of basalts presumably indicating recharge of the magma plumbing system by pristine mantle derived material. Selective contamination in alkali elements from the sedimentary basement is also indicated by the data. The presence of partially digested sedimentary xenoliths is an additional argument for this process. ©1984 Intern. Association of Volcanology and Chemistry of the Earth's Interior
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  • 42
    Publication Date: 1980-06-01
    Description: Tertiary volcanic rocks of Carriacou occupy two-thirds of the island. The volcanics include volcaniclastics, lava flows and dome lavas and range in composition from basalts to andesites. Carriacou basalts fall into two petrographic types (a) clinopyroxene-plagioclase-phyric basalts and (b) olivine microphyric basalts; the latter having higher MgO and lower Al_2O_3 than the clinopyroxene basalts. Both types are unusually rich in mafic minerals compared with Lesser Antilles basalts in general, although similar types have been reported from the nearby island of Grenada. The potash to silica ratios are relatively high and confirm the similarity between Carriacou and Grenada basalts and the differences between these basalts and basalts from other islands of the Lesser Antilles. The basaltic andesites and andesites from Carriacou correspond closely in mineralogical and chemical composition with typical andesites found elsewhere in the Lesser Antilles. The geochemistry of the volcanics shows that the olivine microphyric basalts display tholeiitic affinities whereas the clinopyroxeneplagioclase-phyric basalt, basaltic andesites and andesites are calcalkaline. The compositional gradation in both the geochemistry and mineralogy of these volcanics suggests that fractional crystallization played an important role in the derivation of the various magma. ©1980 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 43
    Publication Date: 1980-03-01
    Description: In the frame of a geochemical surveillance survey of Vulcano, the phreatic waters, mainly from wells located at the Northern basis of the active volcanic centre, were studied over a time span of sixteen months. On the basis of fundamental chemical composition, contamination by a shallow aquifer of brackish water and hydrothermal alteration of rocks seem the main processes to which the observed chemical picture can be attributed. An R-mode factor analysis procedure allowed to distinguish a principal factor associated to sodium, magnesium, chloride, possibly representing the contribution of the brackish aquifer, and three minor but distinct factors. These are respectively associated to calcium and sulfate, hydrogencarbonate, boric acid, and are considered as reflecting the influence of gaseous compounds from volcanic emanations. The variation in time of this influence appears in correlation with the variation of the observed temperature at the hottest fumaroies of the crater. ©1980 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 44
    Publication Date: 1980-06-01
    Description: Stratigraphic studies on the active and potentially active volcanoes of the Lesser Antilles have revealed two main types of andesitic pyroclastic deposit. One with dense clasts in a poorly vesicular ash represents nuée ardente eruptions of Pelean type and the other group of vesicular pumice and ash represent both Plinian airfall and ash-pumice flow eruptions. The pumiceous deposits can be divided into airfall lapilli, airfall ash, crystal-pumice surge, ashpumice flow and ash hurricane types. No pumice eruptions have been witnessed in the Lesser Antilles during the period of written history although the stratigraphy of archaeological sites shows they occurred in pre-Columbian times. Detailed stratigraphic studies of Mt. Pelée, Martinique, and the Quill, St. Eustatius, show that, throughout their history, pumice eruptions have alternated with nuée ardente eruptions with approximately equal frequency. The widespread occurrence of pumiceous deposits on many of the West Indian volcanoes and the frequent alternations in the stratigraphic sections suggest the high probability that they will be witnessed in the future. On Martinique, some on the late prehistoric pumiceous pyroclastic flow deposits (the ash hurricanes) have been traced 20 km from the central vent to the out-skirts of Fort de France, indicating that they are the major hazard in the Lesser Antilles. Measured stratigraphic sections show that the Pelean type nuée ardente deposits are separated from the pumiceous pyroclastic deposits by others of intermediate vesicularity and appearance. The presence of such deposits of intermediate vesicularity could provide a future warning of impending change in pyroclastic style. As no such deposits formed on Mt. Pelée this century the present «safer» episode of nuée ardente (Pelean type) activity is expected to continue. ©1980 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 45
    Publication Date: 1980-03-01
    Description: The paper describes the course of the Large Tolbachik fissure eruption taking place in Kamchatka from July 6, 1975 to December 10, 1976. The eruption zone extended for 30 km. The formation of monogenic scoria cones nearly 300 m high, lava tubes and basalt sheets up to 80 m thick and more than 40 km2 in area and subsidence of the Plosky Tolbachik summit caldera to a depth of more than 400 m were observed during the eruption. The volume of eruption products amounted to more than 2 km3. It was the largest basalt eruption which has taken place in the Kurile-Kamchatka volcanic belt in historic time. ©1980 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 46
    Publication Date: 1980-03-01
    Description: The paper discusses the results of geodetic investigations performed in the region of the large 1975–1976 Tolbachik fissure eruption in Kamchatka. Using data from repeated triangulation and trigonometric levelings, horizontal and vertical displacements have been detected in an area of 3,500 km2. Two zones have been recognized: the tension and uplift zone that is probably due to magma intrusion from depths to the surface along the line of new cones and the extensive compensative subsidence zone located at a distance of 20–50 km from the nearest newly-formed cones. Measurements made with small distance measuring device showed the dynamics of feeding basalt dykes intrusion and made it possible to determine their width (a little greater than 1 m) and magma and gas overpressure (50–250 bar). Data have been obtained on dimensions and growth of cones and on vertical ground deformation in the area of new cones during and after the eruption. ©1980 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 47
    Publication Date: 1980-03-01
    Description: The temperature of the Crater Lake of the active volcano Ruapelm has been recorded by Temperature Telemetry Buoys, to determine if lake temperature is correlated with volcanic activity. These buoys had to be specially designed to cope with the unfavourable environment of Crater Lake. A buoy contains a thermistor to measure the lake temperature, and a radio transmitter to transmit a short signal every few minutes, the interval between signals being a function of temperature. The temperature records obtained from these buoys show that the temperature near the lake surface can vary considerably within a few hours. Some of these variations appear to be caused by disturbances in convective heat transfer occurring in the lake. The occurrence of these short term temperature variations means that there is no simple relation between Crater Lake temperatures and the volcanic activity of Ruapehu. Some rapid increases in temperature followed volcanic earthquakes, but one of the biggest increases in temperature occurred just before a group of earthquakes upder the lake. ©1980 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 48
    Publication Date: 1980-03-01
    Description: The gas composition of recent thermal waters is in perfect conformity with their geologictectonic positions. The composition of gas-liquid inclusions in the basic-hyperbasic xenoliths of basaltoid lavas depends on the depth from which xenoliths are supplied. (The depth is determined by using the methods of geological thermobarometers). By comparing available data we came to the conchision that the gas compositions of thermal waters reflect the depth of generation of deep fluid which heats them and depends on the penetration depth of the fluid-conducting fissure zones. ©1980 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 49
    Publication Date: 1980-03-01
    Description: Temperature and chemical composition of condensates of higher temperature fumaroles, located on the rim of the crater of Vulcano, were studied over a period of one year. Maximum temperature changed from 216°C (June 1977) to 291°C (May 1978). HCl is the main constituent of condensates which, in comparison with data from fumaroles of other volcanoes, contain also noteworthy quantities of boron and bromine. Since these chemical characteristics suggested the possible inflow of water of marine origin into the volcanic conduit, experimental data on seawater-rock reactions at high temperatures and pressures were taken into account. A model is drafted for the volcanic system, in which the occurrence of an aquifer interposed between the magma chamber and the surface is considered. The vaporization of this aquifer would feed the fumaroles giving rise, through different mixing processes with surface waters, to the observed differences in temperature and chemical composition. However, data available at present are not sufficient to support the proposed model, which must be mainly considered as a working hypothesis. ©1980 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 50
    Publication Date: 1980-03-01
    Description: Three composite cones have grown on the southern edge of the previously existing Atitlán Cauldron, along the active volcanic axis of Guatemala. Lavas exposed on the flanks of these cones are generally calc-alkaline andesites, but their chemical compositions vary widely. Atitlán, the largest and most southerly of the three cones, has recently erupted mainly pyroclastic basaltic andesites, while the flanks of San Pedro and Tolimán are mantled by more silicic lava flows. On Tolimán, 74 different lava units have been mapped, forming the basis for sequential sampling. Rocks of all three cones are consistently higher in K_2O, Rb, Ba and REE than other Guatemalan andesites. Atitlán’s rocks and late lavas from Tolimán have high Al_2O_3 content, compared to similar andesites from other nearby cones. All major and trace element data on the rocks are shown to be consistent with crystal fractionation involving phases observed in the rocks. If such models are correct, significant differences in the relative proportions of fractionation phases are necessary to explain the varied compositions, in particular higher Al_2O_3 rocks have fractionated less plagioclase. We speculate that inhibition of plagioclase fractionation could occur in chambers where PH_2O is greater and when repose intervals are shorter. The distribution of volcanic vents throughout Guatemala which show this postulated «inhibition of plagioclase fractionation» is systematic with such vents lying just to the south of the main axis. The andesites of the three cones cannot be simply related to the late-Pleistocene rhyolites which are apparently associated with cauldron formation, because unlike the andesites, the rhyolites have markedly depleted heavy REE abundances. Recent dacitic lavas from vents south of San Pedro volcano and silicic pyroclastic rocks which mantle the slopes the San Pedro may reflect residual post-cauldron rhyolitic volcanism. ©1980 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 51
    Publication Date: 1980-03-01
    Description: The gaseous products of new Tolbachik volcanoes were studied during 1975 to 1977 throughout all eruptive stages and during the post eruptive activity. In investigations the northern break-out gases emitted during the eruption from the moving and consolidated lava flows there have been detected H_2O (the main component), H_2, HF, HCl, SO_2 and H_2S, CO2, CO, NH_3, CH_4 and other hydrocarbons, NH_4Cl predominated in compositions of condensates and subtimates on lava flows and the most characteristic microcomponents were Zn, Cu, Pb, Sn, Ag and others. Sampling of gases and condensates at the southern break-out was conducted immediately from the flowing melt. In gases there have been detected H_2O (98 mol. %). HCl and H_2 (0.9 mol. % each) as well as HF, SO_2, H_2S, CO2 and in small quantities O_2 and N_2, Gases reached the equilibrium state at T and P sampling and were characteristic of gas composition of the southern break-out magma. HCl, HF and H_2SO_4 were predominant during condensate and sublimate mineralization. The major raicrocomponents were represented by Pt, Sb, As, Zn, Cu, Pb, Ni, Co and others. Comparison of compositions of gases and of products of their reactions at the northern and at the southern break-outs allows us to assume the presence of the deeper magma source at the northern break-out and of shallow magma source at the southern break-out. ©1980 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 52
    Publication Date: 1983-12-01
    Description: A geochronological and structural study of dykes is used in order to define the stress field in Sardinia. The Plio-quaternary volcanism of Sardinia (between 5 and 〈0.2 My old) is mostly fissural and contemporaneous with an intraplate regime which reactivated normal faults of various orientations. Dykes in the Logudoro, Montiferro and Monte Arci areas (west-central and northern Sardinia) are 3.4 to 1.8 My old. Their orientations vary around a north-south mean direction which is the most frequent in the Tyrrhenian Basin. The reactivation of normal faults of various orientations may be linked to a stress regime characterised by a maximum vertical principal stress σ _1 and horizontal principal stresses σ _2 and σ _3 having close values and which can easily interchange. In these conditions, the magmatie liquid can intrude into preexisting fractures of different orientations, especially into the fractures that have an orientation close to the plane σ _1 σ _2 (with N-S orientation in Sardinia). This plane is concordant with the convergent motion of Europe and Africa plates. The influence of the pre-existing fractures on dyke orientations seems more important in this area than in southern France where there is a different tectonic regime ( Féralt ) and Campredon , 1983).RésuméL’analyse structurale et géochronologique des dykes, les alignements et les zones de densité maximale des centres volcaniques plioquaternaires sont utilisés pour essayer de déterminer l’évolution du champ des contraintes en Sardaigne. Depuis 5 Ma l’activité tectonique en Sardaigne est caractérisée par un domaine continental en distension faisant transition aux domaines océaniques du bassin tyrrhénien. La mise en place des produits volcaniques plioquaternaires (5÷〈0.2 Ma), de nature essentiellement fissurale, est rapportée à une tectonique intraplaque en extension induisant la réactivation de failles normales. Les dykes (3.4÷1.8 Ma) montrent une oscillation autour de la direction méridienne, direction structurale la mieux exprimée dans le bassin tyrrhénien. La formation et le rejeu de failles normales sans orientation préférentielle sont normalement considerés comme liés à une direction de contrainte principale maximale σ _1 verticale et impliquent également des contraintes σ _2 et σ _3 horizontales de valeur semblable, qui peuvent permuter facilement. La direction moyenne N-S en Sardaigne et en Tyrrhénienne indiquerait done une direction préférentielle méridienne de σ _2. L’existence d’ailleurs de fractures d’orientation sensiblement variables dans les différents secteurs de l’île (N-S; M.te Arci: N-S÷NE-SW Montiferro: NNW-SSE Logudoro) indiquerait une remarquable influence du système de fracturation préexistant dans le socle. Cela est compatible avec un champ de contraintes σ _1 vertical et σ _2 proche de σ _3 caractérisé par la mise en place des liquides magmatiques selon d’anciennes fractures de préférence par rapport à leur proximité au plan σ _1 σ _2. La direction préférentiellement méridienne du plan σ _1 σ _2 en Sardaigne est compatible avec le rapprochement récent des plaques européennes et africaines. ©1983 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 53
    Publication Date: 1983-09-01
    Description: Seismic data collected at four volcanoes in Central America during 1973 and 1974 indicate three sources of seismicity: regional earthquakes with hypocentral distances greater than 80 km, earthquakes within 40 km of each volcano, and seismic activity originating at the volcanoes due to eruptive processes. Regional earthquakes generated by the underthrusting and subduction of the Cocos Plate beneath the Caribbean Plate are the most prominent seismic feature in Central America. Earthquakes in the vicinity of the volcanoes occur on faults that appear to be related to volcano formation. Faulting near Fuego and Pacaya volcanoes in Guatemala is more complex due to motion on a major E-W striking transform plate boundary 40 km north of the volcanoes. Volcanic activity produces different kinds of seismic signatures. Shallow tectonic or A-type events originate on nearby faults and occur both singly and in swarms. There are typically from 0 to 6 A-type events per day with b value of about 1.3. At very shallow depths beneath Pacaya, Izalco, and San Cristobal large numbers of low-frequency or B-type events are recorded with predominant frequencies between 2.5 and 4.5 Hz and with b values of 1.7 to 2.9. The relative number of B-type events appears to be related to the eruptive states of the volcanoes; the more active volcanoes have higher levels of seismicity. At Fuego Volcano, however, low-frequency events have unusually long codas and appear to be similar to tremor. High-amplitude volcanic tremor is recorded at Fuego, Pacaya, and San Cristobal during eruptive periods. Large explosion earthquakes at Fuego are well recorded at five stations and yield information on near-surface seismic wave velocities (α=3.0±0.2 km/sec.). ©1983 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 54
    Publication Date: 1983-06-01
    Description: A detailed gravity survey was carried out on the island of Vulcano, Aeolian Islands, Italy. Gravity was measured on 107 stations and the Bouguer anomalies were computed by assuming geological densities. Aim of this survey was to complete the island structural pattern relatively to the shallower structures. Separation of the gravity anomaly field was carried out by means of data filtering, and two main components were discerned. The λ〉2.2 km wavelength component, filtered out of the longer wavelength components, was interpreted quantitatively along a NW profile. The best fitting model consists of an upper layer of recent pyroclastic products (p=2.1 g/cm3) lying upon a highly compacted pyroclastic series or lavas (p=2.4 g/cm3). The shorter wavelength residual gravity field (λ〈2.2 km) is characterized by two anomalies, located on Vulcanello and the «Fossa di Vulcano» crater. Vulcanello anomaly could be interpreted, given the geothermal state of the area, as due to an increase of the rock density consequent to propylization processes by high temperature fluids (T〉200°C). «Fossa di Vulcano» anomaly is instead attributable to the local volcanic chimney. A schematic comprehensive model of Vulcano is also presented, which accounts for the available main geological and geophysical data. ©1983 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 55
    Publication Date: 1983-09-01
    Description: Au cours des 40.000 dernières années, l’activité de la Montagne Pelée a consisté en l’émission de coulées de ponces, nuées ardentes peléennes et de type Mérapi mettant en place des produits de composition andésitique à dacitique, ainsi que de quelques nuées ardentes de type Saint-Vincent responsables de l’émission de produits plus basiques (andésites basaltiques, basaltes) accompagnés par des phénomènes de mélanges magmatiques. Deux éruptions de ce type, SV1 et SV2 datées à 25700±1200 et 22300±1200 ans B.P., ont été étudiées au point de vue du déroulement de l’éruption, de la composition chimique des produits, et de la minéralogie. Elles suivent toutes les deux un même scénario éruptif, évoluant depuis un pôle andésitique vers un pôle plus basique, avec un stade intermédiaire de mélange magmatique. Les volumes émis sont très importants, de l’ordre de 1 km3 ou plus. Les différences chimiques peu marquées (3 à 6% SiO_2), les variations modérées et progressives de la minéralogie ainsi que des conditions physiques de cristallisation ( T : 920°–930° à 950°–970°C) indiquent le cogénétisme des différents magmas. Ces résultats, ainsi que l’étude de l’activité éruptive récente de la Montagne Pelée, suggèrent qu’au cours de la période intermédiaire (20.000 à plus de 40.000 ans B.P.), deux chambres magmatiques existaient sous le volcan, à des profondeurs peu différentes. Le déclenchement de ces éruptions de type Saint-Vincent peut s’expliquer par l’injection de magma moins différencié originaire de la chambre inférieure, dans la chambre superficielle, accompagnée par l’émission des magmas des deux réservoirs. Au cours de la période récente (13.500 ans B.P. à maintenant), l’activité cyclique de la Montagne Pelée serait sous l’influence prédominante d’une chambre magmatique assez superficielle et permanente. Le déclenchement des éruptions associerait des processus de surpression en volatil et de réalimentation périodique par injection de magma profond. On peut supposer que ce changement de caractère dans l’activité de la période récente est une conséquence des deux éruptions SV1 et SV2, susceptibles d’avoir perturbé la structure profonde du volcan en raison de l’importance des volumes émis.During the last 40,000 years B.P. the eruptive activity of Mont Pelée (Martinique) has been exclusively pyroclastic, including mainly pumice flow deposits, Pelean-type and Merapi-type nuée ardente deposits, characterized by an andesitic to dacitic magma composition. In addition, a few Saint-Vincent-type nuée ardente deposits are present. Their products are compositionally more basic (basalt, basaltic andesite) and show some characteristic magma mixing features. Two well defined Saint-Vincent type eruptions, named SV1 and SV2 have been studied here. They have been dated by the C14 method respectively at 25,700±1,200 and 22,300±1,200 years B.P. Both follow a similar eruptive pattern, evolving from an andesitic to a more basic magma composition, through an intermediate stage of magma mixing. The volume of ejected products is extensive (1 km3 or more), compared with other deposits such as the Pelean-type nuée ardente. The moderate and progressive variations of magma composition (3 to 6 % SiO_2), mineralogy and crystallization pressure-temperature conditions ( T : 920°–930° to 950°970°C, using Fe-Ti oxides geothermometer) demonstrate the cogenetic nature of these various magmas. These results, as well as the study of the recent activity of Mont Pelée suggest that during a former period (about 40,000 to 20,000 years B.P.), two magmatic chambers existed rather close to one another. The triggering of these Saint-Vincent type nuée ardente eruptions might involve injections of less-differentiated magma from a lower to a shallower reservoir, followed by the emptying of both reservoirs. During the recent period (less than 13,500 years B.P.), the cyclic eruptive activity of Mont Pelée Volcano has been controlled mainly by a relatively shallow and permanent magmatic chamber. The triggering of eruptions has depended on two processes: volatile overpressure and periodic replenishment of this superficial reservoir by deeper and less-differentiated magma injections. This change in eruptive character results perhaps from succession of SV1 and SV2 Saint-Vincent type eruptions; the volcano deep-structure might have changed, as a consequence of the extensive volume of ejected products. ©1983 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 56
    Publication Date: 1983-03-01
    Description: Five years of geochemical surveillance at Vulcano have shown interesting changes in the physico-chemical characters of the fumaroles at the crater and of the phreatic waters in the surrounding area. In particular, 1) The maximum temperature of the fumaroles, which had been of about 200°C for at least 30 years, rose suddenly to above 300°C: after a period of rather constant values, a decreasing trend has recently been observed; 2) An increase of CO2, S compounds and HF with respect to HCl, B, Br occurred in the gas composition contemporaneous to the rise in temperature. This increase is now gradually declining; 3) Higher concentrations of sulphate ion and silica in phreatic waters accompanied the thermal changes observed at the fumaroles. According to a model previously proposed ( Martini et al. , 1980), the increase in the above mentioned factors would reflect an increasing magmatic contribution to the volcanic system, which might be due to an increased permeability to fluids at depth. Because a sensible seismic shock occurred in the area a few months after the first chemical and thermal variations had been observed, such variations may be regarded as forerunners of the seismic event, and may be used to forecast some months in advance changes occurring at depth. ©1983 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 57
    Publication Date: 1983-06-01
    Description: The Dras volcanic rocks form a part of the ophiolite belt along the Indus Suture Zone in the Kashmir Himalaya. These volcanic rocks have suffered alteration as in any other ophiolite zone. Three types of alterations spilitisation, submarine weathering and ridongitisation were suggested. The spilitic mineralogy appears to be secondary and must have developed due to the reaction of these rocks with hot sea water. Depletion of MgO and CaO and enrichment of K_2O of these rocks relate to the submarine weathering at lower temperatures. Rodingitisation effect is reflected in the chemistry of some rocks with enriched CaO and depleted SiO_2. The trace elements — Co, Cr, Ni, Cu, Pb and Rb do not show any considerable changes during alteration. ©1983 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 58
    Publication Date: 1983-06-01
    Description: Great volcanic eruptions that eject much dust into the stratosphere are known to change climatic factors, e.g. temperature and solar radiation. It is known, on the other hand, that superimposed on the normal variation in the size of tree-rings is a variation caused by inequalities of climate. Therefore, the notion that tree-rings may preserve the date of several great volcanic eruptions seems probable. In order to support this statement, numerical investigations have been performed. Treering graphs of two Hungarian trees were compared with the chronological order of great volcanic eruptions. The mathematical correlation between the variation of the dust veil index (DVI) is examined, based on two-variate numerical computation. It is concluded that during periods subsequent to very great eruptions, exceptionally narrow rings were formed. A remarkable correlation has been found between the variation of mean values of the ring width and that of the dust veil index. Consequently, growth rings of trees in some cases record dates of great volcanic eruptions by responsing to temporary climatic changes caused by volcanic activity. ©1983 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 59
    Publication Date: 1983-03-01
    Description: Published major-element analyses of Ethiopian volcanic rocks have been subjected to a systematic discriminant analysis. The plateau regions can be subdivided according to the proportions of alkaline and tholeiitic basalts. In northern Ethiopia, these subprovinces show increasing basalt alkalinity with time. The most voluminous basalts have lowest magnesium values, independent of the degree of alkalinity. Rift and Afar basalt chemistry falls within the spectrum observed for the plateau basalts, with no perceptible difference resulting from lithospheric attenuation beneath Afar. However, silicic volcanics of the Rift-Afar floor differ in bulk terms from those of the plateau margins in showing a stronger bias towards peralkalinity, and having higher Na/K values. Two particularly voluminous volcanic episodes have occurred in Ethiopia, dated at ?30–19 m.y. and 4.5–0 m.y. and which link well with one model for seafloor spreading in the Red Sea and Gulf of Aden. Evidence for a mantle hotspot under Ethiopia remains ambiguous. ©1983 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 60
    Publication Date: 1983-03-01
    Description: The origin of dike-like bodies along the Hilina fault scarp on the south flank of Kilauea Volcano. Hawaii has been the subject of recent controversy. Some geologists favour an origin by intrusion of magma from below, others favour « intrusion » of lava derived from above — lava derived from fluid surface flows which poured down open cracks. In order to distinguish between deep versus surface sources for the bodies, a suite of dike and other samples were analyzed for S, H_2O, and Cl. All surface flows are degassed, whereas known dikes are volatile-rich. Samples of the Hilina dikes, and dikes from the Ninole Formation, Mauna Loa are degassed, indicating that these dikes were surface-fed — formed by magma which had been de-volatized by surface transport. A model is presented whereby the Hilina dikes form in talus and lava cones that drape the Hilina fault scarp. Seismic activity during eruption may have played an important role in the formation of the Hilina dikes. Similar dikes in the Ninole Formation probably formed in a similar environment. ©1983 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 61
    Publication Date: 1983-03-01
    Description: The island of Aegma includes some of the oldest volcanic rocks in the south Hellenic Arc previous radiometric dates range from 3.87 to 4.4 Ma. The volcanic sequence is divided into nine units on the basis of field relations, petrography and geochemistry, and the characteristic paleomagnetic polarity of each unit has been determined. Two new radiometric dates (2.1 and 3.9 Ma) show that the volcanism spans the Pliocene. A detailed chronology is proposed using the paleomagnetic data which places the most voluminous volcanism in the mid Pliocene. ©1983 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 62
    Publication Date: 1983-03-01
    Description: Schmincke and Swanson (1967) explained laminar flowage structures as indicators for flow direction of pyroclastic flows that show a radial flow pattern away from the source. Several other authors have reported similar examples, but the influence of pre-flow topographic relief has not been analyzed. Flow lineations were measured for the Ata pyroclastic flow deposit, southwestern Japan. This deposit has covered an undulating basement topography. Preferred orientation of crystals and lithic fragments were measured on thin sections cut parallel to sedimentary layering. The following three factors which control the flow lineation have been recognized. 1) Flow lineations oriented radially away from the source, as described by previous authors, were obtained only for samples collected from the surface of the pyroclastic flow plateau where the basement valleys were nearly filled by earlier flow units. 2) Lineations near the floor of narrow valleys were parallel to the strike of the valley. 3) Flow lineations near the wall of valleys tend to be parallel to the dip of the valley walls. These data suggest that the initial radial movement of pyroclastic flows from the source gradually changes direction to parallel the strike of deep valleys due to confining effect of valley wall. Flows which are trapped within a valley, tends to move towards the bottom of the valley just prior to the final settlement. After the basement topographic relief has been filled up with earlier flow units, the later flows maintain their original radial movement until final settlement. ©1983 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 63
    Publication Date: 1982-12-01
    Description: The presence of self potentials on the upper part of Stromboli Volcano is associated with the existence of convection cells of gases and condensed liquids whose rising parts are situated above the hot zones. A model of a convection cell fed by a hot fluids through its lower middle part was calculated and applied to the various structures of the upper part of the volcano. This model which takes account of a process which is probably fairly common in volcanic system, gives a clear explanation of the various potential zones. Although other interpretations could be made, the fact that the positive and negative potentials are of the same importance, the limited size of the convection zone and the presence of gas under pressure beneath permeable grounds lead us to conclude that the phenomenological cause of these potentials is probably the mechanism of electro-filtration. The results of self-potential measurements showed very close correlation with those we obtained previously by magnetotelluric profiling. These two geophysical methods were very efficient in the precise delimitation of hot zones which give rise to resistivity and potential anomalies. The depth of those zones was estimated from the aspects of the potential anomalies and from the geological environment. The two methods were furthermore powerful tools in the investigation of volcanological processes and in particular the «Strombolian» rythmic eruption. The geometrical disposition of the feeding channels involves the obstruction of the path of the gases by the lava. Generally, the force of pressure exerted by the gas increases quickly and in consequence the plug of lava is ejected and shattered. When the gas flow lessens, the forces of viscosity increase quicker than the forces of pressure, the discharging outlets are then obstructed and thus there is great hazard that the accumulated energy will be liberated in a very violent fashion. In this case there will be a paroxysmal-type eruption. This mechanism is the consequence of a recent accident in the volcano evolution: the landslip of a segment of the cone, probably associated with the subsidence of the central caldera; the lava spread over the collapsed sector and the rhythmic eruptions described above constructed the present craters. ©1982 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 64
    Publication Date: 1981-09-01
    Description: The volcanic history of Somma-Vesuvius indicates that salic products compatible with an origin by fractionation within a shallow magma chamber have been repeatedly erupted («Plinian» pumice deposits). The last two of these eruptions, (79 A.D. and 3500 B.P.) were carefully studied. Interaction with calcareous country rocks had limited importance, and all data indicate that differentiated magmas were produced by crystal-liquid fractionation within the undersaturated part of petrogeny’s residua system at about 1 kb water pressure. The solid-liquid trend indicates that the derivative magmas originated by fractionation of slightly but significantly different parental liquids. Some lavas of appropriate composition were selected as parental liquids to compute the entity of the fractionation. Results suggest that in both bases a fractionation of about 70 weight % was needed to produce liquids with the composition of the pumice. The combination of all data indicates that the two Plinian eruptions were fed by a magma chamber (3–4 km deep) having a volume of approx. 2.0–2.5 km3. The temperature of the magma that initially entered the chamber was about 1100°C, whereas the temperature of the residual liquids erupted was Plinian pumice was 800° and 850°C respectively. There is no evidence that such a magma chamber existed at Vesuvius after the 79 A.D. eruption. These results have relevant practical implications for volcanic hazard and monitoring and for geothermal energy. ©1981 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 65
    Publication Date: 1981-09-01
    Description: K/Ar-determinations, major and trace element chemical analyses and magnetic data are reported for rocks from Santa Maria (Azores), Madeira and Porto Santo, and Gran Canaria. Based on these data, the age of the basement of Santa Maria is believed to have formed between about 5.2. and 4.6 m.y.; the unconformably overlying pillow complex interbedded with fossiliferous calcarenites about 3.8 to 3.3 m.y. with the capping subaerial basalt being part of the same magmatic phase. The major erosional phase levelling the basement is thus approximately synchronous with the major Pliocene regression (R2) on Gran Canaria ( Lietz and Schmincke , 1975), possibly indicating a widespread eustatic event. The upper part of the submarine, partly fossiliferous series of Porto Santo was dated as ca. 12 to 13 m.y., and the Quaternary age for the major basalt formation in Eastern Madeira ( Watkins and Abdel Monem , 1971) is confirmed. The ages of several formations on Gran Canaria were slightly revised. Chemical differences between basement (shield) and later posterosional series on Gran Canaria are re-emphasized by the new data, while such differences are much less pronounced between the basement and younger series on Santa Maria. ©1981 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 66
    Publication Date: 1981-09-01
    Description: The composition of carbonate, sulphate, halide and other phases trapped as fluid inclusions in apatite crystals in ijolite and urtite from sub-volcanic complexes in East Africa have been determined using the electron micro-probe. The bulk composition of these inclusions is alkali-rich and closely comparable to the natrocarbonatite lava from the active volcano of Oldoinyo Lengai in N. Tanzania. The inclusions are interpreted as having been originally immiscible alkali carbonate-rich droplets within a nephelinitic melt. The evidence suggests that magma of the composition of natrocarbonatite lava is that most likely for the parent magma during carbonatite petrogenesis. ©1981 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 67
    Publication Date: 1981-09-01
    Description: A new method of tide gauge data analysis is described for detecting ground deformation in active volcanic areas. This method was tested in the Phlegraean Fields, an active volcanic area near Naples where, in 1970 after a large ground uplift, a dense tide-gauge network was installed. The results of the tide-gauge analysis were checked with levelling data, and a good agreement between the data sets has been found. The observational continuity of the tide-gauge network gave the time evolution of the ground deformation and showed short time oscillations. The good results obtained at the Phlegraean Fields using the tide-gauge network and the data analysis procedure are encouraging. ©1981 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 68
    Publication Date: 1981-09-01
    Description: An analysis of the structural lineaments, as observed on the middle and upper slopes of Etna volcano, was made with the aim of checking stress distribution within the volcanic structure. The observed features suggest that the deformation pattern of the volcanic edifice is compatible with a deviating stress field dominated by an E-W sinistral shear, with the maximum stress axes being oriented NE (σ1) and NW (σ3) respectively. Such a framework appears to be consistent with the active regional stress field, as deduced from structural data and focal mechanism analyses. The role of the active stress field in the penetration, uprising and eruption of magma in the Etnean area is therefore discussed and some more general suggestions about the geodynamic evolution of eastern Sicily are made. ©1981 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 69
    Publication Date: 1981-09-01
    Description: A geochemical model explaining the presence of fumaroles having different gas composition and temperature at the top of the crater and along the northeastern coast of Vulcano island is proposed. A pressurized biphase (liquid-vapor) reservoir at the depth of about 2 km is hypothesized. Energy and mass balance sheets control P-T conditions in the system. P-T must vary along a boiling curve of brine as liquid is present. The CO2 content in the steam is governed by the thermodynamic properties of the fluids in the H_2-NaCl-CO2 system. On the assumption that oxygen fugacity in the system is between the HM-FMQ oxygen buffers, observed SO_2/H_2S, CO2/CO, CO/CH_4 ratios in the fumarolic gases at the Fossa crater appear in equilibrium with a temperature higher than that observed, such as may exist at depth. The more reduced gas phases present on the sea-side may result from re-equilibrium processes in shallower aquifers. The suggested model would help in monitoring changes in volcanic activity by analyzing fumarolic gases. ©1981 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 70
    Publication Date: 1981-09-01
    Description: Plinian eruptions are amongst the most powerful of explosive volcanic events, and the extensive pumice deposits which they produce have an exceptionally wide dispersal because of the great eruptive plume height. Historical data on 12 plinian eruptions, and available quantitative data on the deposits of these and 37 other plinian eruptions are collated in this review to characterise further the plinian eruptive style and its products and to establish the known limits of their variation. The deposit volumes have been recomputed according to a standard procedure to provide a better basis for comparison, and they vary over 4 orders of magnitude to reach a maximum of about 100 km3. Almost all volcanic magma compositions apart from the most mafic are represented among the juvenile products; rhyolitic and dacitic deposits account for 80% of the total volume and basaltic ones less than 1%. Compositional zoning is very common. Plinian eruptions are of open vent type and produce deposits which tend to be homogeneous in grain size and constitution through their thickness. Considerable departures from homogeneity often however exist. Finer grained beds which often interrupt the continuity can be produced by a number of different mechanisms, the features of which are summarised. In a significant proportion of plinian deposits the finer beds are the deposits of intraplinian pyroclastic flows, or are related to such flows; pyroclastic flows such as may be attributable to column collapse thus do not form exclusively at the end of the plinian phase. The most recent work indicates that major phreatoplinian eruptions dominated by the copious inflow of water into the vent can produce deposits quite as widely dispersed and as voluminous as the biggest plinian eruptions, and it appears that the characteristics of the grain size populations of the two types tend to converge in the most powerful eruptions. ©1981 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 71
    Publication Date: 1981-09-01
    Description: Known tsunamis of volcanic origin are reviewed and classified according to their causes. Earthquakes accompanying eruptions (excluding tectonic events which apparently triggered eruptions), pyroclastic flows, and submarine explosions have each accounted for about 20% of cases. Ten causes of volcanic tsunamis are discussed. From the risk point of view, those due to landslides are particularly dangerous. Eruptions at calderas are more likely to generate tsunamis than eruptions elsewhere. Of those killed directly by volcanic eruptions, nearly a quarter have died as a result of tsunamis. By transfer of energy to sea waves, a violent eruption, which would be comparatively harmless on land, extends greatly the radius over which destruction occurs. Krakatoa, 1883, is the only eruption sequence for which sufficient data exist for a detailed study of tsunamis. The times at which air and water waves generated by this sequence were recorded have been reread, and new origin times have been calculated and compared with observations made at the time. Origin times of successive pairs of air and water waves agree closely, except in some cases in which the tsunami arrived up to 15 minutes early, thus giving an apparent origin time 15 minutes before that of the corresponding air wave. This is explained by postulating that these tsunamis did not originate at the focus of the explosions, but at distances along the path towards the tide gauge, equivalent to those which would be covered by a tsunami in the time interval observed. The calculated point at which the largest recorded tsunami originated coincides with the outer edge of a bank of volcanic debris laid down during the eruption. This is interpreted as part of an unwelded ignimbrite deposit, the violent emplacement of which, within a minute or so of the explosion, generated the tsunami. A satisfactory correlation is established between explosions and deposits laid down by the eruptions, as described from a geological section close to the source vent. An outline is given of a proposed numerical index to define tsunamigenic potential at a given volcano. Such an index could be used to calculate the expected amplitudes of tsunamis at particular places in the vicinity, and hence could serve as a basis for tsunami risk contingency planning. ©1981 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 72
    Publication Date: 1981-09-01
    Description: Ophiolites with different magmatic characteristics are closely associated in space with one another in northern Pindos. Some have affinities with ocean-floor magmas (Group I), and others represent melts which are frequently strongly depleted in «incompatible» elements (Group II). Group I is composed of cumulates, dolerites and lavas, whereas Group II occurs mainly as pillows and dykes, and postdates Group I. The two groups have different geochemical, mineralogical and petrographic features. They exhibit different Ti, Cr, Ni, Y, Zr, P, Si and Mg contents, and clinopyroxenes and spinels of Group I have higher Ti/Al and Ti/Mn ratios, and lower Cr/(Cr + Al) values respectively than those of Group II. Many rocks of Group II are chemically similar to boninites and associated rocks as well as to low-Ti basalts from other areas and ophiolitic complexes. It is concluded that geochemical and mineralogical data alone do not allow a definitive answer about the original tectonic setting of the investigated rocks, although a genesis above a subduction zone seems to be plausible hypothesis. ©1981 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 73
    Publication Date: 1981-09-01
    Description: Petrological characteristics of basaltic rocks from the Tyrrhenian deep-sea are discussed and related to the geotectonic situation. For the first time, distinctly alkaline basalts (hawaiites) have been found in the Tyrrhenian deep-sea. These are typical within-plate basalts related to the tensional fracturing of the Tyrrhenian area. A suggested age of 100,000 years is among the youngest indications for the Tyrrhenian Sea volcanism. Since the Miocene, magmatic activity in the inner Tyrrhenian sea basin evolved from ocean-floor basalts to ocean island tholeiites and transitional basalts, with alkaline basalts as the most recent products. Seamounts in the southern part of the Tyrrhenian deep-sea (Palinuro, Marsili) add shoshonitic and calcalkaline lavas (some with high Mgvalue) to the complexity of the Tyrrhenian magmatic evolution. ©1981 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 74
    Publication Date: 1981-09-01
    Description: A statistical analysis of the eruptive events occured on Mount Etna from 1323 to 1980 has been made. The statistical methods used are: i) the goodness of fitting tests, and ii) the calculation of the probability value «p» for confidence intervals of 95%. The result was a non-equiprobable distribution of eruptive events with a greater concentration in November, March and May. This concentration could be correlated to cyclic factors of astronomical nature connected with variations in the Earth velocity rate, whereas it does not seem to be affected by variations in the amount of rainfall and snowfall. ©1981 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 75
    Publication Date: 1981-09-01
    Description: Fluorine and chlorine have been determined in samples of lavas and pyroclastic products from the islands of Lipari and Vulcano, which form the southernmost portion of the Aeolian arc, because their present distribution may provide additional information for a better understanding of the differentiation and eruptive mechanisms affecting these volcanoes. On the basis of previous data which did not reveal a significant presence of Cl- and F-bearing minerals, these two elements are expected to concentrate in the residual melt during differentiation, following a distribution pattern similar to that of lithium that is a typical residual constituent. The analytical results are in good agreement with the hypothesis which attributes a positive role to potassium in the solubility of volatiles in the melt phase, while silica would favour their escaping in a gaseous form. For rocks representing the last stages of differentiation a sharp increase in litium concentration is observed, and the relative loss of fluorine and chlorine is attributed to their releasing as constituents of fumarolic fumes. If the system is to a certain extent sealed, a « normal » passive enrichment for all the considered elements may occur. ©1981 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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  • 76
    Publication Date: 1981-09-01
    Description: Refilling of undifferentiated magma at high temperature in the shallow chamber of Stromboli volcano induces both partial gas release of the crystallizing shoshonic liquid and a beginning of melting of salite-augite megacrysts in equilibrium with the liquid. In the new magma, crystallization of pyroxenes resumes with a chrome-diopside composition. The study of silicate melts inclusions in pyroxenes reveals the importance of refilling processes in the evolution products stored in the magma reservoir.RésuméDans le liquide magmatique de nature shoshonitique contenu dans le réservoir du volcan Stromboli (Iles Eoliennes) se séparent au cours de la différenciation des phénocristaux d’augite. La température du liquide estimée par le géothermomètre de Nielsen et Drake (1979) varie entre 1132° et 1123° C ± 20°C, les températures minimales de cristallization mesurées par thermométrie optique sont de l’ordre de 1110 ± 5° C ( Clocchiatti , 1975). L’augite renferme des lamelles orientées de magnétite et des microcristaux de plagioclase (An 66) soulignant les plans de croissance cristalline. L’injection d’un liquide moins différencié entraîne dans le réservoir une augmentation très nette de la température et le mélange des deux liquides. L’élévation de température a pour conséquence la déstabilisation de l’augite qui montre un front interne de corrosion et le dégazage partiel du liquide qui entraîne une baisse de la pression des fluides. Dans les nouvelles conditions le minéral stable est une diopside chromifère, la température du liquide varie entre 1214° et 1193° ± 20° C ( Nielsen et Drake , 1979) la température minimale de crystallisation se situe au voisinage de 1130° ± 5° C. après la résorption partielle de l’augite se séparent des plagioclases (An 77) et de l’olivine (Fo 74). Le mélange des deux liquides aboutit à un liquide unique dont la composition est très proche de celle de la mésostase. La comparaison entre la composition chimique des liquides inclus et celles des roches totales ( Rosi , 1981) montre que ces dernières sont fortement cumulatives en pyroxène. ©1981 Intern. Association of Volcanology and Chemistry of the Earth’s Interior
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