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91.
Mixing of rhyolite,trachyte and basalt magma erupted from a vertically and laterally zoned reservoir,composite flow P1, Gran Canaria 总被引：1，自引：0，他引：1
The 14.1 Ma composite welded ignimbrite P1 (45 km3 DRE) on Gran Canaria is compositionally zoned from a felsic lower part to a basaltic top. It is composed of four component magmas mixed in vertically varying proportions: (1) Na-rhyolite (10 km3) zoned from crystal-poor to highly phyric; (2) a continuously zoned, evolved trachyte to sodic trachyandesite magma group (6 km3); (3) a minor fraction of Na-poor trachyandesite (<1 km3); and (4) nearly aphyric basalt (26 km3) zoned from 4.3 to 5.2 wt% MgO. We distinguish three sites and phases of mixing: (a) Mutual mineral inclusions show that mixing between trachytic and rhyolitic magmas occurred during early stages of their intratelluric crystallization, providing evidence for long-term residence in a common reservoir prior to eruption. This first phase of mixing was retarded by increasing viscosity of the rhyolite magma upon massive anorthoclase precipitation and accumulation. (b) All component magmas probably erupted through a ring-fissure from a common upper-crustal reservoir into which the basalt intruded during eruption. The second phase of mixing occurred during simultaneous withdrawal of magmas from the chamber and ascent through the conduit. The overall withdrawal and mixing pattern evolved in response to pre-eruptive chamber zonation and density and viscosity relationships among the magmas. Minor sectorial variations around the caldera reflect both varying configurations at the conduit entrance and unsteady discharge. (c) During each eruptive pulse, fragmentation and particulate transport in the vent and as pyroclastic flows caused additional mixing by reducing the length scale of heterogeneities. Based on considerations of magma density changes during crystallization, magma temperature constraints, and the pattern of withdrawal during eruption, we propose that eruption tapped the P1 magma chamber during a transient state of concentric zonation, which had resulted from destruction of a formerly layered zonation in order to maintain gravitational equilibrium. Our model of magma chamber zonation at the time of eruption envisages a basal high-density Na-poor trachyandesite layer that was overlain by a central mass of highly phyric rhyolite magma mantled by a sheath of vertically zoned trachyte-trachyandesite magma along the chamber walls. A conventional model of vertically stacked horizontal layers cannot account for the deduced density relationships nor for the withdrawal pattern. 相似文献
92.
Dr. Klaus Huber Dr. Eckhard Kleine Dr. Hans-Ulrich Lass Dr. Wolfgang Matthäus 《Ocean Dynamics》1994,46(2):103-114
Summary Major Baltic inflows (from the Kattegat) represent the only possibility for deep water renewal in the Baltic Sea that is generally characterised by stable stratification. Such an inflow occurred in January 1993 for the first time in 16 years. Observations were made by a moored station at Darß Sill and, for the first time since investigations into major Baltic inflows began, by a research vessel. In addition, the event has been simulated in an operational model.
Der Salzwassereinbruch vom Januar 1993 in die Ostsee-Messungen und Modellergebnisse
Zusammenfassung Für die im allgemeinen stabil geschichtete Ostsee bilden Salzwassereinbrüche (aus dem Kattegat) die einzige Möglichkeit zur Erneuerung des Tiefenwassers. Ein solcher Salzwassereinbruch ereignete sich nach 16 Jahren Unterbrechung erst wieder im Januar 1993. Dieser Vorgang wurde durch Messungen von einer permanenten Station auf der Darßer Schwelle und — erstmals in der Geschichte der Erforschung von Salzwassereinbrüchen —einem Forschungsschiff registriert. Außerdem liegt eine numerische Simulation mit einem operationellen Modell vor.
Insertion d'eau salée dans la mer Baltique en janvier 1993. Mesures et exploitation d'un modèle numérique
Résumé L'apport d'eau salée en provenance du Kattegat reste la seule possibilité de renouvellement des eaux profondes des couches stables de la mer Baltique. Un telle insertion d'eau salée ne s'est produite qu'au mois de janvier 1993 après une interruption de 16 ans. Ce processus a été enregistré par des mesures d'une station permanente (Darßer Schwelle) et, pour la première fois dans l'histoire de l'étude des insertions d'eau salée, par un navire scientifique. En outre une simulation numérique à l'aide d'un modèle opérationnel est présentée.相似文献
93.
A 100 m-thick sequence of Carboniferous submarine, dominantly basaltic volcanic rocks at Herbornseelbach (Lahn-Dill area) is subdivided into six units. Two pillow volcanoes show a characteristic change from basal megapillows (up to 5 m across) through »normal« pillows (0.5 to 0.8 m across) to a top facies of small pillows (<0.5 m across) and/ or breccias of scoriaceous clasts and discontinuous lava stringers. Sheet flows, 0.5 to 4 m thick, can be distinguished from sills by asymmetric cooling structures and textures, vesicle concentrations near the top, basal flow wrinkles and ponding of sediments on irregular flow tops.The clastic top facies of one pillow unit dominated by vesicular lapilli is thought to have formed by submarine lava fountaining indicating eruptions in shallow water. Angular fragments in a hyaloclastite breccia, containing minor fresh sideromelane, are interpreted to have resulted from mechanical break-up of pillows or sheet flows. Deposition by mass flow in a topographic depression alternated with deposition of non-volcanic sediments and rhyolitic tuffs prior to slumping of the assemblage en masse. Thin beds of rhyolitic tuff are intercalated with sheet flows.Interpretation of secondary chemical and mineralogical alteration changes and the tectonic environment of magma formation is aided significantly by volcanological analysis. Low-temperature diagenetic processes and subsequent very low grade regional metamorphism resulted in the dominant chlorite — albite paragenesis, local epidote and pumpellyite indicating maximum temperatures around 300°C. Pronounced effects of post-depositional cold sea floor weathering and local hydrothermal alteration are still recognizable. Lavas and breccias of pillow unit II are characterized by severe loss of Ca and Na and major gain of K, chemical changes being most pronounced in clastic rocks altered at high water/rock ratios. Unit II is believed to represent a pillow volcano that stood above the sea floor exposed to oxidizing percolating seawater for a long time.The extrusive and clastic rocks range from mafic basalts, containing olivine-spinel-plagioclase phenocrysts to intermediate compositions dominated by plagioclase phenocrysts and characterized by lower compatible trace element concentrations. The volcanics most closely resemble present day E-type MORB to moderately alkalic OIB.
Zusammenfassung Eine 100 m mächtige Folge karbonischer, submariner, überwiegend basaltischer vulkanischer Gesteine in Herbornseelbach (Lahn-Dill-Gebiet) ist in 6 Einheiten untergliedert worden. Zwei Pillowvulkane zeigen einen charakteristischen Wechsel von Megapillows an der Basis (bis zu 5 m Durchmesser) über normale Pillowlaven (0.5–0.8 m Durchmesser) zu einer Topfazies, die aus kleinen Pillows (<0.5 m Durchmesser) und/oder einer Breccie aus schlakkigen Komponenten besteht. Schichtlaven, 0.5–4 m dick, können von Sills durch asymmetrische Abkühlstrukturen und Gefüge, Blasenanreicherung in der Topzone, Fließrunzeln an der Basis und Sedimenttaschen auf unregelmäßigen Stromoberflächen unterschieden werden.Die klastische Topfazies einer Pilloweinheit mit überwiegend blasigen Lapilli wird als Ablagerung submariner Lavafontänen interpretiert und weist auf Eruptionen im Flachwasser hin. Angulare Fragmente in einer hyaloklastischen Breccie mit Relikten frischer Sideromelanscherben entstanden wahrscheinlich durch das mechanische Zerbrechen von Pillow- und Schichtlaven. Die Hyaloklastite, die mit nichtvulkanischen Sedimenten assoziert sind und mit rhyolitischen Tuffen wechsellagern, sind durch mass flows in einer topographischen Mulde abgelagert worden. Zwischen den Schichtlaven liegen dünne rhyolitische Tuffe.Die Interpretation der chemischen und mineralogischen Veränderungen und des tektonischen Environments der Magmenentstehung wird durch die vulkanologische Analyse signifikant ergänzt. Diagenetische Prozesse und die anschließende sehr schwache regionale Metamorphose resultieren in der dominanten Chlorit — Albit Paragenese. Lokales Auftreten von Epidot und Pumpellyit deutet auf Temperaturen von maximal ca. 300°C. Kalte Meeresbodenverwitterung und lokale hydrothermale Veränderungen sind noch zu erkennen. Die Laven und Breccien der Pilloweinheit II sind charakterisiert durch Ca und Na Verluste und deutliche K Anreicherungen. Diese chemischen Veränderungen sind am deutlichsten in den klastischen Gesteinen, die unter hohen Wasser/Gesteinsverhältnissen verändert wurden. Einheit II repräsentiert vermutlich einen morphologisch exponierten Pillowvulkan, der lange Zeit dem oxidierenden, durchsickernden Meerwasser ausgesetzt war.Die extrusiven vulkanischen Gesteine reichen von Basalten mit Olivin-Spinell-Plagioklas Einsprenglingen bis zu intermediären Zusammensetzungen mit überwiegend Plagioklas Einsprenglingen und geringen Konzentrationen kompatibler Spurenelemente. Die Vulkanite ähneln am ehesten heutigen E-Typ MORB bis mäßig alkalischen OIB.
Résumé A Herbornseelbach (région Lahn-Dill) existe une série carbonifère puissante de 100 m, formée de roches volcaniques sous-marines, principalement basaltiques, qui se subdivise en six unités. Deux volcans à laves en coussins montrent une évolution caractéristique qui consiste, de bas en haut, en mégacoussins (jusque 5 m de diamètre), en coussins »normaux« (0,5 à 0,8 m) et en un faciès supérieur comportant de petits coussins (<0,5 m) et/ou des brèches de clastes scoriacés melées de langues de lave discontinues. Les coulées, épaisses de 0,5 m à 4 m, se distinguent des sills par leurs structures de refroidissement asymétriques, par la concentration des vésicules à leur sommet, par des rides d'écoulement basales et par le piégeage de sédiments sur leur surface supérieure irrégulière.Dans une des unités, le faciès supérieur clastique, dominé par des lapilli vésiculaires est considéré comme le résultat de fontaines de laves sous-marines, caractérisant des éruptions en eau peu profonde. Dans une bréche hyaloclastique, des fragments anguleux, contenant du sidéromélane frais sont interprétés comme résultant de la fragmentation de coussins. Il y a eu alternance de dépôt par transport en masse dans des aires en dépression et de sédimentation de tuf rhyolitique et de matériaux non volcaniques, préalable à ces transports.L'analyse volcanologique apporte une aide précieuse à l'interprétation des modifications minéralogiques et chimiques secondaires et au cadre tectonique de ce magmatisme. La paragenèse dominante à chlorite-albite témoigne de processus diagénetiques de basse température et par conséquent d'un métamorphisme de faible degré, la présence locale de d'épidote et de pumpellyite indiquant une température maximale de l'ordre de 300°. Il est possible d'encore reconnaître les effets prononcés d'une altération post-dépôt au contact de l'eau de mer froide, ainsi que d'une altération hydrothermale. Les laves et les brèches de la 2e unité montrent une perte sévère de Ca et de Na, et un gain important de K, ces changements chimiques étant plus marqués dans les roches clastiques à rapport eau/roche élevé. L'unité n 2 doit correspondre à un volcan posé sur le fond et exposé pendant un temps prolongé à la percolation oxydante de l'eau de mer.Les roches extrusives et clastiques vont de basaltes mafiques à phénocristaux d'olivine-spinelle-plagioclase à des produits de compositions intermédiaires à phénocristaux de plagioclase dominants et à faible concentration d'éléments en trace compatibles. Ces volcanites ressemblent de très près aux types actuels E-MORB et OIB modérément alcalin.
, . ., Herbornseelbach ( Lahn-Dill), 100 6 . — 5 -, «» — 0,5 0,8 — — 0,5 — / . 0,5 4 , , . , , , . , , , , . , , . . , . , , — , , 300° C. . Ca Na, a , , . , , , ., , -- , , ; . E MORB OIB.相似文献
94.
Hans-Ulrich Schmincke 《International Journal of Earth Sciences》1967,56(1):992-1020
More than 150 flow directions measured mainly in inclined pipe vesicles and, more rarely, vesicle cylinders, spiracles, and inclined foreset beds in pillowpalagonits-complexes) of the upper 12 widespread Yakima Basalt flows on the Columbia River Plateau in south-central Washington indicate rather uniform transport directions from southeast to northwest. This strongly supports former ideas that the large Grande Ronde-Cornucopia dike swarm in northeast Oregon fed the huge lava floods. It is shown that all previous evidence indicating eruption centers for the basalts in the Cascada Range to the west is inconclusive. Cross-bedding directions, maximum pebble sizes, and distribution of heavy mineral suites in sedimentary deposits interbedded with the basalt sheets delineate in more detail two main paleoslopes whose deposits interfinger:
- a)the Cascade paleoslope to the west with east-southeast paleocurrents and a predominance of volcanic heavy minerals (hornblende, oxyhornblende, hypersthene, and clinopyroxene). 相似文献
95.
G. Wörrier J. -M. Beusen N. Duchateau R. Gijbels H. -U. Schmincke 《Contributions to Mineralogy and Petrology》1983,84(2-3):152-173
Twenty six whole rocks, seven matrix and fifty three mineral separates from the compositionally zoned late Quaternary Laacher See tephra sequence (East Eifel, W Germany) were analyzed by instrumental neutron activation. These data document the chemical variation within the Laacher See magma chamber prior to eruption with a highly fractionated phonolite at the top and a more mafic phonolite at its base as derived from other data. Incompatible elements such as Zn, Zr, Nb, Hf, U, light and heavy rare earths are extremely enriched towards the top whereas compatible elements (e.g. Sr, Sc, Co, Eu) are strongly depleted. Semicompatible elements (Ta and some middle REE) are depleted at intermediate levels. This chemical variation is shown by whole rock and matrix data indicating the phonolite liquid was compositionally zoned regardless of phenocryst content. Hybrid rocks (phonolite-basanite) show the largest concentrations for compatible elements. All elements (except Rb) display continuous compositional variations with regard to the stratigraphic position of pumice samples. From these data we are able to distinguish three main units: An early erupted highly fractionated magma, the main volume of evolved phonolite and a mafic phonolite as the final products. The extreme variation of trace element distribution coef ficients (K) for 9 mineral phases with respect to stratigraphic position (resp. matrix composition) cannot be explained by conventional mechanisms. We postulate a significant modification of the trace element content of the phonolite melt by liquid-liquid controlled differentiation processes subsequent to and/or contemporaneous with (fractional) crystallization which caused disequilibrium between phenocrysts and host matrix. Therefore, our “distribution coefficients” deviate from equilibrium partition coefficients equivalent to the amount of this post crystallization modification of the matrix composition. The relationship between varying K and matrix composition is demonstrated by a ΔK-ΔM-diagram (variation of K versus variation of matrix, M). Different parts of this diagram relate to different parameters (T, P, polymerization, complex-building, equilibrium crystallization in a zoned magma column and post crystallization disequilibrium effects) which are responsible for the variation of distribution coefficients in general. The ΔK-ΔM-diagram may allow to distinguish between different processes affecting the distribution coefficients measured in natural volcanic rocks from a differentiating magma system. 相似文献
96.
Stephen R. Tait Gerhard Wrner Paul Van Den Bogaard Hans-Ulrich Schmincke 《Journal of Volcanology and Geothermal Research》1989,37(1)
The physical mechanism by which chemical zonation develops in magma chambers has been controversial partly because unambiguous geological constraints have been lacking. The 11,000 years B.P. eruption of Laacher See Volcano produced a zoned tephra deposit and also ejected crystal-rich nodules which provide a snapshot of the materials crystallising at the magma chamber margins. New data on petrography and chemical compositions of nodules, their cumulate minerals and interstitial glasses are used to deduce the chemical evolution of the phonolite melt due to fractional crystallisation of the mineral assemblages. These data, together with those on the vertical zonation of the melt in the bulk of the chamber, are shown to be consistent with a model of stratification of the chamber by convective fractionation, in which a thin boundary layer of residual melt from fractional crystallisation ascends at the chamber side and accumulates at the roof. Crystallisation could have provided buoyancy to drive convection by enriching incompatible volatile components (mainly water) in the residual melt. Available fluid dynamic studies of single- and double-diffusive boundary layers are used to assess convection in the Laacher See chamber. The boundary layer is likely to have been: (1) laminar, which implies that the density gradient in the chamber steepened upwards; (2) in the counterflow regime, in which compositional and thermal layers flow in opposite directions; and (3) thin ( 10 cm), estimated from theory for a flat wall, suggesting that wall morphology could be important in determining boundary layer characteristics. Estimates of mass transfer rates due to this mechanism suggest that the chamber could have become stratified in a time of the order of 103 years. 相似文献
97.
H. -U. Schmincke 《Bulletin of Volcanology》1974,38(2):594-636
Peralkaline silicic welded ash-flow tuffs differ characteristically in a number of properties from most calc-alkaline welded tuffs, due to their generally lower viscosity and higher temperatures. For example, individual cooling units are relatively small (less than 30 m thick, less than 5 km3 in volume); rocks can be thoroughly welded and crystallized to feldspar, quartz, and mafic minerals; several primary deformational structures (e.g. lineations, stretching of pumice, folds, ramp structures) indicate late stage laminar creep, resulting from the low yield strength of the nearly homogeneous glass of very low viscosity. Theoretical considerations also suggest that peralkaline melts are of low viscosity and high temperature, as inferred from,e.g., their chemical composition (high iron- and alkali-, and low alumina-concentrations). The low viscosity may also be due to trapping of volatiles. Absence or paucity of OH-bearing phenocryst phases, paucity of pyroclastic rocks, other than ash flow tuffs, formed from highly explosive eruptions, and apparently high crystallization temperatures, indicate that peralkaline silicic magmas are comparatively dry. The common occurrence of peralkaline ash-flow tuffs may be due to an increased water content of the magmas, resulting also in amphibole phenocrysts in some welded tuffs, or to specific volcanotectonic conditions. Ash flows of peralkaline composition move as particularly dense particulate flows. This type of flowage and the very rapid welding of the low viscosity glass lead to a high degree of homogenization of the fine glass shards. This in turn inhibits complete degassing of the collapsing ash flow. Semiclosed systems result where gas overpressures can develop and where volatiles play an important role by fluxing crystallization and transporting dissolved matter. Several types of vesicles can form under these conditions: (a) Spherical vesicles within collapsed ash and pumice particles formed after deposition of the ash flow. (b) Round or irregular vesicles transsecting pyroclastic particles, vesicle sheets, and large cavities, several m in diameter, may form in a largely homogenized ash-flow tuff beneath tightly welded layers. (c) Lensoid cavities formed during granophyric crystallization of large pumice particles. Small ash particles of peralkaline composition may assume spherical shapes due to their low viscosity and in some cases, expansion of bubbles. They form during transport and are preserved under low load pressure in the top part of cooling units. Globule lavas and most froth flows are interpreted as welded ash-flow tuffs, some of their unusual features being due to their peralkaline composition. 相似文献
98.
A. A. Gurenko Thor H. Hansteen Hans-Ulrich Schmincke 《Contributions to Mineralogy and Petrology》1996,124(3-4):422-435
Picritic units of the Miocene shield volcanics on Gran Canaria, Canary Islands, contain olivine and clinopyroxene phenocrysts
with abundant primary melt, crystal and fluid inclusions. Composition and crystallization conditions of primary magmas in
equilibrium with olivine Fo90-92 were inferred from high-temperature microthermometric quench experiments, low-temperature microthermometry of fluid inclusions
and simulation of the reverse path of olivine fractional crystallization based on major element composition of melt inclusions.
Primary magmas parental for the Miocene shield basalts range from transitional to alkaline picrites (14.7–19.3 wt% MgO, 43.2–45.7
wt% SiO2). Crystallization of these primary magmas is believed to have occurred over the temperature range 1490–1150° C at pressures
≈5 kbar producing olivine of Fo80.6-90.2, high-Ti chrome spinel [Mg/ (Mg+Fe2+)=0.32–0.56, Cr/(Cr+Al)=0.50–0.78, 2.52–8.58 wt% TiO2], and clinopyroxene [Mg/(Mg+Fe)=0.79–0.88, Wo44.1-45.3, En43.9-48.0, Fs6.8-11.0] which appeared on the liquidus together with olivine≈Fo86. Redox conditions evolved from intermediate between the QFM and WM buffers to late-stage conditions of NNO+1 to NNO+2. The
primary magmas crystallized in the presence of an essentially pure CO2 fluid. The primary magmas originated at pressures >30 kbar and temperatures of 1500–1600° C, assuming equilibrium with mantle
peridotite. This implies melting of the mantle source at a depth of ≈100 km within the garnet stability field followed by
migration of melts into magma reservoirs located at the boundary between the upper mantle and lower crust. The temperatures
and pressures of primary magma generation suggest that the Canarian plume originated in the lower mantle at depth ≈900 km
that supports the plume concept of origin of the Canary Islands.
Received: 23 October 1995/Accepted: 21 February 1996 相似文献
99.
Thor H. Hansteen Andreas Klügel Hans-Ulrich Schmincke 《Contributions to Mineralogy and Petrology》1998,132(1):48-64
Gabbroic and ultramafic xenoliths and olivine and clinopyroxene phenocrysts in basaltic rocks from Gran Canaria, La Palma,
El Hierro, Lanzarote and La Gomera (Canary Islands) contain abundant CO2-dominated fluid inclusions. Inclusion densities are strikingly similar on a regional scale. Histogram maxima correspond to
one or more of the following pressures: (1) minimum 0.55 to 1.0 GPa (within the upper mantle); (2) between 0.2 and 0.4 GPa
(the Moho or the lower crust); (3) at about 0.1 GPa (upper crust). Fluid inclusions in several rocks show a bimodal density
distribution, the lower-density maximum comprising both texturally early and late inclusions. This is taken as evidence for
an incomplete resetting of inclusion densities, and simultaneous formation of young inclusions, at well-defined magma stagnation
levels. For Gran Canaria, pressure estimates for early inclusions in harzburgite and dunite xenoliths and olivine phenocrysts
in the host basanites overlap at 0.9 to 1.0 GPa, indicating that such magma reservoir depths coincide with levels of xenolith
entrainment into the magmas. Magma chamber pressures within the mantle, inferred to represent levels of mantle xenolith entrainment,
are 0.65–0.95 GPa for El Hierro, 0.60–0.68 GPa for La Palma, and 0.55–0.75 GPa for Lanzarote. The highest-density fluid inclusions
in many Canary Island mantle xenoliths have probably survived in-situ near-isobaric heating at the depth of xenolith entrainment.
Inclusion data from all islands indicate ponding of basaltic magmas at Moho or lower crustal depths, and possibly at an additional
higher level, strongly suggestive of two main crustal accumulation levels beneath each island. We emphasize that repeated
magmatic underplating of primitive magmas, and therefore intrusive accretion, are important growth mechanisms for the Canary
Islands, and by analogy, for other ocean islands. Comparable fluid inclusion data from primitive rocks in other tectonic settings,
including Iceland, Etna and continental rift systems (Hungary, South Norway), indicate that magma accumulation close to Moho
depths shortly before eruption is not, however, restricted to oceanic intraplate volcanoes. Lower crustal ponding and crystallization
prior to eruption may be the rule rather than the exception, independent of the tectonic setting.
Received: 30 May 1997 / Accepted: 6 February 1998 相似文献
100.
Andrey A. Gurenko Hans-Ulrich Schmincke 《Contributions to Mineralogy and Petrology》1998,131(1):95-110
Major elements, S, F, Cl concentrations and relative proportions of S6+ to total S were analyzed with electron microprobe in sideromelane glass shards from Pleistocene volcaniclastic sediments
drilled during ODP Leg 157. Glasses are moderately to strongly evolved and represent a spectrum from alkali basalt, basanite
and nephelinite through hawaiite, mugearite and tephrite to phonolitic tephrite. Measured S6+/ΣS (0.03–0.98) and calculated Fe2+/Fe3+ (2.5–5.8) ratios in the melt yield preeruptive redox conditions ranging from NNO−1.4 to NNO+2.1. The morphology of the glass
shards, variations of S and Cl concentrations (0.010–0.127 wt% S, 0.018–0.129 wt% Cl), calculated preeruptive temperatures
(1030–1200 °C) and oxygen fugacities suggest that glasses deposited even within the same ash layers have diverse origin and
may have resulted from both submarine and subaerial eruptions. Most vesicle-free glasses are characterized by high concentrations
of S and represent undegassed or slightly degassed submarine lavas, whereas vesiculated glasses with low concentrations of
S and Cl are strongly degassed and can be ascribed to the eruptions in shallow water or on land. Sideromelane glass shards
at Sites 953 are thought to have resulted from submarine eruptions northeast of Gran Canaria, glasses at Site 954 represent
mostly volcaniclastic material of shallow water submarine and subaerial eruptions on Gran Canaria and Tenerife, and glasses
deposited at Site 956 resulted from submarine or explosive eruptions on Tenerife.
Received: 8 April 1997 / Accepted: 27 October 1997 相似文献