Showing papers on "Phreatomagmatic eruption published in 1988"

A method for prediction of volcanic eruptions

Abstract: The relationship Ω·–α Ω¨ – A = 0 describes the behaviour of materials in terminal stages of failure, where Ω is an observable quantity such as strain, and A and α are empirical constants. Drawing on analogies between failure mechanics and eruption processes at volcanoes, Ω is interpreted in terms of conventional geodetic, seismic or geochemical observations. Manipulation of Ω provides a consistent analytical basis for eruption prediction.

Fracture fillings and intrusive pyroclasts, Inyo Domes, California

Abstract: Fractures containing juvenile magmatic pyroclasts were encountered during drilling into a 600-year-old feeder dike beneath the Inyo Domes chain, California. The Inyo Domes consist of a north-south trending, 10-km-long chain of domes, rhyolitic tuff rings, and phreatic craters. Boreholes were cored through the 51-m-diameter conduit of Obsidian Dome, the largest of the Inyo Domes, and through an unvented portion of the intrusion (dike) 1 km to the south. Pyroclast-bearing fractures were intersected in both holes: (1) 7- to 40-cm-thick fractures in welded basaltic scoria and quartz monzonite country rock are adjacent to the conduit at depths of 400–411 m and 492–533 m; they contain gray, clastic deposits, which show truncated cross bedding and convolute bedding; (2) adjacent to the dike, massive fracture fillings occur at depths of 289–302 m (129 m east of the dike) and 366–384 m (95–87 m east of the dike). The fracture fillings consist of mineral clasts derived from the quartz monzonite, quartz monozonitic and basaltic lithic clasts, and juvenile glass pyroclasts. Angular mineral components are present in the same ratio as in the surrounding quartz monzonite country rock. Juvenile glassy and hyalocrystalline pyroclasts make up from less than 1% up to 22% of the deposits. They consist of blocky obsidian clasts, equant, blocky glass pyroclasts with vesicularities of 0–30%, and small pumices with vesicularities of 30–40%. Intrusive pyroclasts differ from erupted pyroclasts in their generally lower vesicularity, higher crystallinity, and the presence of solution pits and clay coatings indicative of prolonged contact with water. The presence, orientation, and texture of fracture fillings strongly resemble those of propped, man-made hydrofractures. We interpret these fractures as naturally occurring hydrofractures. The apparently horizontal fracture orientations may have been controlled by perturbations of maximum principal stress by the dikes or by preexisting sheet fractures in the quartz monzonite country rock. Assumption of elastic moduli and fracturing properties for the Sierran basement rock allows calculation of fluid overpressures 5 to 9 MPa in excess of overburden stress. These overpressures are consistent with either vapor exsolution from decompressed magma or rapid heating of groundwater. However, the textural and chemical similarity of the pyroclasts to phreatomagmatic tephra that appears late in the explosive eruption sequence suggests that heating of groundwater by the dike/conduit caused the fracturing. Such fracturing around volcanic conduits may play an important role in the development of hydrothermal circulation.

Monte Vulture volcano (Basilicata, Italy): an analysis of morphology and volcaniclastic facies

Abstract: The earliest activity of Monte Vulture, central Italy, included ignimbrites but the bulk of the volcano was built up by plinian airfall deposits. Contemporaneous remobilisation of these deposits formed an apron of lahars around the base of the main cone. The volcano was constructed on a ridge; the valley to the east and tributaries to the north and south became sediment traps for volcaniclastic materials emplaced by fluvial reworking and directly from volcanic activity. To the west the valley was swept clear by active downcutting. Instability of the west flank as a result of this erosion was probably a contributory cause of major gravitational sector collapse on the volcano's flank, terminating the main cone-building phase. The resultant scar is an amphitheatre-shaped hollow called here the Valle dei Grigi. Previous workers have attributed this feature to coalescing calderas formed by engulfment. The last volcanic phase was the production of the Monticchio calderas and associated phreatomagmatic explosions producing airfall and surge deposits. Because most of the activity at Vulture has been repeated plinian eruptions producing similar assemblages of products, detailed stratigraphy of the volcano is difficult to accomplish. To characterise Vulture in terms of its products, various facies are identified and interpreted in relation to volcanic processes, distance from vent and environmental conditions.

Early Devonian bimodal volcanic rocks of Southwestern New Brunswick: petrography, stratigraphy, and depositional setting

Abstract: to subaenastal volcanic belt of the northeastern United States and New Brunswick consists of a bimodal sequence of marine of New Brutfurian and Early Devonian volcanic and assocUted sedimentary rocks. The Mascarene lithostratigraphic terrane depositional styletf probably the northern extension of this belt. This paper describes the volcanic stratigraphy, eruptive and Passamaquoddy Bay geography and tectonic setting of a 125 km2 area of the Mascarene terrane located along the coast of rhyolitic and basaltic flo^a mapped includes a - 3.5 km thick bimodal and subalkaline volcanic sequence of ""^ded GeorgeBathoUthandoverlain'fiyroclastic rocks and terrigeneous shale and sandstone. These rocks were intruded by the St. which aremappable at ascale of iformablyby the Late DevonianPerry Formation. The area is divided into 53 hthologic units Hawaiian, StromboIian,Piiniana39 • Rhyolitic rc>cks are volumetrically most irnportanL phreatomagmatic tuff cone, and peperitic *«*»

A Silicic Pyroclastic-flow Eruption and Pyroclastic Surges from the Kikai Caldera Volcano in the Last Interglacial Stage

Abstract: The Kikai caldera volcano located under water in East China Sea is one of the most gigantic calderas in southern Kyushu. At the caldera, a violent eruption occurred from the submarine vent, at ca. 70-80 ka. The eruption is interpreted to have been phreatomagmatic throughout. Each eruptive phase of the eruption sequence generated its own characteristic deposits. The sequence of the events can be summarized as fallows ; (1) a small phreatomagmatic eruption, which generated the fine grained ash including accretionary lapilli, (2) the catastrophic pyroclastic-flow eruption, which formed a large-scale pyroclastic flow (the Nagase pyroclastic flow), two pyroclastic surges (Nishinoomote-1 member : Ns-1, Nishinoomote-3 member : Ns-3), and a wide-spread co-ignimbrite ash fall (Nishinoomote-2 member : Ns-2).The Nagase pyroclastic flow came down from the rim of the caldera, and entered the sea. Then, the flow body, which included a large amount of large pumice blocks and heavy lithic fragments, was disintegrated as gas-particle flow by violent phreatomagmatic explosions, or continued subaqueously as water-supported mass flow. Dilute and fine-particle-rich pyroclastic surges, probably with a density much less than that of water, 1.0 g/cm3, generated off the top or head of subaerial Nagase pyroclastic flow. They could cross on the smooth surface of the sea, becoming water-cooled, vaporish and depleted in large clasts which dropped into the sea. Eventually, the cool and wet pyroclastic surges attacked the islands around the caldera, and deposited as Ns-1 and Ns-3.Ns-2 co-ignimbrite ash fall, composing of glass shards were generated from the upper convective part of the eruption column of the Nagase pyroclastic flow. Included accretionary lapilli indicate that the eruption column was very moisture because of much sea water flash-out subaerially for very violent explosions from the submarine vent. Ns-2 is probably correlated with the Kikai-Tozurahara ash which was found in central Japan more than 500 km off the source.

Minera1 chemistry and geochemistry

Abstract: The Nevado del RU~', located 120 km west of Bogota. is one of the currently active andesitic volcanoes that lies north of the Central Cordillera of Colombia at the intersection of two dominant fault systems originating in the Palaeozoic basement. The pre-volcanic basement is formed by Palaeozoic gneisses intruded by pre-Cretaceous and Tertiarygranitic batholiths. They are covered by lavas and volcaniclastic rocks from an eroded volcanic chain dissected during the late Pliocene. The geologic history of the Nevado del Ruiz records two periods of building of the compound volcano. The stratigraphie relations and the K-Ar dating indicate that effusive and explosive volcanism began approximately 1 Ma ago with eruption of differentiated andesitic lava andpyroclastic flows and andesitic domes along a regional structural trend. Cataclysmic eruptions opened the second phase of activity. The Upper sequences consist of block-lavas and lava domes ranging from two pyroxene-andesites to rhyodacites. Holocene to recent volcanic eruptions, controled by the intense tectonic activity at the intersection of the Palestina fawlt with the regional fault system, are similar in eruptive style and magma composition to eruptions of the earlier stages of building of the volcano. The youngest volcanic activity is marked by lateral phreatomagmatic eruptions, voluminous debris avalanches. ash flow tuffs and pumice falls related to catastrophic collapse during the historic eruptions including the disastrous eruption of 1985. Pleistocene to historic eruptive products span the compositional range from basaltic andesite to rhyodacite. Disequilibrium textures such as bimodal plagioclase, pyroxene and glass populations are evidenced especialy in the dacites. Abundant low-iron amphiboles are pargasitic and seem to have crystallized in equilibrium with rare phlogopites. Crystallization temperatures from the assemblage cpx-opx and Fe-Ti oxides range between 1 175-930 OC. Basaltic andesites are only found in the Basa1 Unit of Ruiz. The later lavas and pyroclastic flows become progressively more evolved with time. Acid andesites and dacites 160-65 % SiO2) are the most common rock-type and show a large variability of their KzO contents. The bimodality of more evolved lavas is evidenced by their Th/Ta ratios (19-25 against 7-15) from a common source with a partial fusion giving a Th/Ta ratio near5. Similar REEpatterns for all the lavas and their major minera1 phases are also consistent with a single magmatic source. However, only small differences between the lavas of the two successive phases of the building of the volcano are apparent on the multi-element spidergrams. lt appears that the hiatus between the Basa1 Unit and the final stage corresponds to a small but significant episode : certain elements change through time, in pameular LIL-and HFS elements such as Rb. Ba, Th. Hf and Zr and the heavy REE increase with decreasing age. These differences probably result