Browsing by Issue Date, starting with "1982-07"
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- The geology, volcanic activity, and age of Bouvetøya, South AtlanticPublication . Prestvik, ToreThe volcanic cone of Bouvetøya is built up of two formations. The older formation, of predominantly pyrodastic rocks which are typically hydrothermally altered, is overlain by a formation of mainly lava flows. The rocks present on the island constitute a transitional volcanic series. K/Ar dating indicates that surface rocks were formed as early as 1.4 Ma B.P. The rocks of Nyrøysa are 0.4-0.5 Ma, showing that this platform, which formed between 1955 and 1958, represents a landslide type deposit rather than a recent eruption as has previously been suggested by several workers. Various kinds of evidence suggest that the volcanic activity, or a cycle of activity, of Bvuvet0ya is now in a declining phase.
- Coarse-Grained rocks from Ascension IslandPublication . Harris, Chris; Bell, J. D.; Atkins, F. B.The lavas and pyroclastic rocks of Ascension Island contain a suite of coarse-grained igneous blocks which range in composition from dunite to granite paralleling, but extending beyond, the compositional range of the volcanic. The mineralogy, texture and chemistry of these blocks discussed, together with their modes of occurrence. More specific features observed include partial melting seen in one suite which suggests that granitic liquids may be formed from blocks of intermediate composition; the occurrence of the rare minerals dalyite (K₂Zr Si₆O₁₅) and vlasovite (Na₂ZrSi₄O₁₁); and the crystallization ‘in situ’ of a rock type considered to reproduce several of the features displayed by the blocks. The implications of this for the origin of the coarse-grained suite as a whole are considered.
- Excursion guide for field trip V1 : 'Island of São Miguel'Publication . Martins, J. ÁvilaS. Miguel, the largest islands of the archipelago (757 km2), consists of four volcanoes with summit calderas, from W toward E: Sete Cidades, Água de Pau, Furnas and Povoação. The range formed by the latter three volcanoes rises to the height of 1105 meters (in Pico da Vara) and trends approximately E-W, but the massif of Sete Cidades, separated from the eastern area by a low narrow zone - «Região dos Picos» - formed by recent basalts, is oriented according to the regional trend, i.e., approximately WSW-ESE. That Sete Cidades trend controlled by fracture alignment supported by the linear positions of the recorded submarine eruptions, parallel to the SW coast of the island. From field relationship and some available radiometric data, it is possible to establish the following -eruptive sequence (Zbyszewski, 1974) : Nordeste basalt complex (4 m.y.) , trachytes of Povoação, Furnas and Água de Pau, with intercalations of basaltic eruptions, basalts of Sete Cidades and Picos, and the historic recorded eruptions which were trachytic (explosive) and basaltic (effusive). […].
- The historic eruptions of La Palma Island (Canaries)Publication . Hernandez-Pacheco, Alfredo; Valls, M. C.From the first contacts of the European navigators who first visited the Canary Islands, around the middle of the XIV century, 18 eruptions he.ve taken place of which seven occurred on the island of La Palma. A historic-bibliographic study as well as a detailed field work have enabled us to accurately determine the places, dates, duration and volcanological patterns as well as the petrology and geochemistry of this historic eruptive cycle. In every instance, alcaline basalitic lavas were emitted: true basalts, basanitoids and/or ankaramites. A differentiation process always took place in the magma chamber and a sequence from amphibole to olivine-bearing lavas was erupted. These variations of the chemistry and mineralogy of the lavas were related to the different stages of the emption and the height over sea level, where the corresponding eruptive vents opened. The two main structural trends of the historic subhistoric volcanism of La Palma are N 5° W and N 35° W. Both trends correspond to the predominant ones of the dike swarms of the Basal Complex of the island. The secondary trends, N 80° E and N 15° E, likewise coincide with the corresponding main directions of the dike swarms of the Basal complexes of La Gomera and Fuerteventura. The duration of these historic eruptions was between 1 to 3 months and the area covered with lava and pyroclasts was 37 km2, 5 % of the total surface of the island.
- Excursion guide for field trip V2 : ‘Island of Terceira’Publication . Self, StephenThe island of Terceira is 406 km2 in area and rises to 1021 metres above sea level. It consists of four strato-volcanoes grouped along a prominent fissure zone (Fig. 1). Two volcanoes, Pico Alto and Santa Barbara, are active and the other two, Guilherme Moniz and Cinquo Picos, are believed to be extinct. The fissure zone may be the sub-aerial expression of the Terceira Rift, regarded by Krause and Watkins (1970) as a secondary spreading centre. Terceira shows a great diversity of lavas and pyroclastics for an oceanic island and is noteworthy for voluminous production of peralkaline salic magma. Of the four volcanoes forming the island ; three are composed of both basic and salic rocks and one has only salic rocks exposed. Since the emergence of the island a compositionally bimodal population of rocks has been represented. The products of over 100 eruptions in the upper Terceira Group have been recognized. These include ignimbrites, pumice fall deposits, salic lava extrusions, strombolian scoria deposits, basaltic lava flows and littoral (surtseyan) basaltic tuffs. Basaltic activity is concentrated along the fissure zone which bisects the island diagonally from NW to SE. Volumetric studies give the rate of accumulation of new crust along this small spreading centre; 5.46 km3 of new material has been erupted on the island in the past 23,000 years, of which over 4 km3 is comendite pantellerite composition. The island's economy is dominated by agriculture and dairy farming. Much of the water for the maintown of Angra do Heroísmo (approximately 20,000 population) comes from underground springs or streams in the lava tubes of a 2000-year-ald basalt in Guilherme Moniz Caldera. The island has a good system of roads. Almost the entire papulation lives around a 5 km wide coastal strip (Fig. 2).
- Project de saline a Ribeira QuentePublication . Andlauer, GerardLe project consiste à capter l'eau chaude des sources de Furnas et à la canaliser jusqu'à proximité de Ribeira Quente ou seront disposés des bassins de saumure à l'air libre. L'eau chaude servira à élever la température de l'eau de mer de manière à obtenir une evaporation intense qui accélerera le dépôt du sel dissous. Celui-ci sera produit, en quantité industrielle, et à peu de frais. Il permettra de répondre à une forte demande locale sur le plan de l'agriculture et de la pêche.
- The large Tolbachik fissure eruption in 1975-1976, KamchatkaPublication . Fedotov, S. A.; Balesta, S. T.; Gorelchik, V. I.; Flerov, G. B.; Enman, V. B.This paper discusses the results of seismological, geodetic, geophysical, geologic and petrologic investigations of the large Tolbachik fissure eruption which occurred from July 6, 1975 to Dec. 10, 1976 in Kamchatka. The eruption proceeded first in the region of North vents (July 6-Sept. 15, 1975) where the character of its activity was predominantly explosive and then after a short repose period started again in the region of South vent where its activity was predominantly effusive with outpouring of fluid lava. The total volume of erupted products is 2.3 km³. The time and locality of eruption were successfully predicted according to the character of a swarm of earthquakes recorded at depths from 0 to 20 (30) km. […].
- Interpretation of ground deformation in the AzoresPublication . Machado, FredericoGeodetic measurements in Fayal, Pico and San Jorge showed some horizontal expansion of these islands. The total expansion during the interval 1936-1979 amounts to· several metres, being considerably greater than the average sea floor spreading for that area of the Atlantic Ocean. This suggests that the spreading could be a discontinuous phenomenon, depending on short lived epochs of quick strain release. A vertical pulsation on the central part of Pico volcano was also detected; the pulsation has amplitude of about one metre and period close to one year. Its-seems due to periodic changes of pressure inside the magma chamber.
- Submarine pyroclastic rocks of the La Palma «Ophiolite» complexPublication . Staudigel, S.; Schmincke, H.-U.The basement of La Palma (Canary Islands) consists of plutonic and hypabyssal intrusive and a submarine series, 2.5 km thick. Pyroclastic rocks increase in thickness upwards in the section and dominate at the top where they are several 100 meters thick. The clastic rocks in the pillow-dominated section are well bedded to cross-bedded hyaloclastites forming layers generally < 2 m thick, and thicker beds (up to 5 m) of coarse breccias which range from incipiently fractured pillows, that came to rest nearly in situ, to coarse, well sorted breccias deposited some distance from their source. Clastic rocks in the upper section are massive to poorly bedded, dominantly lapilli- to sand-sized whit clasts, being generally highly vesicular and larger fragments being reddish oxidized displaying shapes and vesicularity intermediate between pillows and sub-aerial scoria. […].
- Observed ground deformation during the Krafla eruption of March 16, 1980Publication . Tryggvason, EysteinnThe Krafla volcano has been monitored with continuously recording tiltmeters and seismometers and frequent geodetic measurements since the beginning of the present episode of activity in 1975. The inflation-deflation sequence of the volcano showed striking regularity in 1977 and 1978 but it became more erratic in late 1979 with slow inflation interrupted by small deflations. The eruption of March 16, 1980 was preceded by rapid deflation which started at 15:15 (GMT) and intense volcanic tremor started simultaneously. The subsidence became very rapid at about 16:00, about three times more rapid than had been seen in any previous subsidence event. The eruption was first seen at about 16:20, but it may have started 10 to 20 minutes earlier. It lasted until about 22h that same night. The deflation of the volcano ceased at about 03h next morning, March 17, and a new inflation started within a few hours. Tilt observations indicate that roughly 30 x 106m3 of magma left the Krafla magma chamber, but only some 10 % of this carne to the surface as very fluid basaltic lava. New fissures extended through the Krafla volcano over a distance of a:bout 21 km and the widening od' the fissure zone was about 1.5 m.