Mixing of rhyolite,trachyte and basalt magma erupted from a vertically and laterally zoned reservoir,composite flow P1, Gran Canaria

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.     

Contributions to the Deformation Analysis in Germany Based on Precise and Continuous GPS Measurements

Modern space geodetic techniques enable deformation monitoring of continental plate interiors with high spatial and temporal coverage. Resolving data and results are currently evaluated for their application for the integrated assessment of seismic hazard and risk in Germany. This goes especially for regions where earthquakes are generally rare but high magnitudes are still not unrealistic while vulnerability of today’s society is steadily growing. The present contribution deals with the continuous monitoring of tectonic fracture systems in Germany using the GPS. The estimation of the station velocities with GPS and the resulting geodetic strain is supposed to provide additional input to the earthquake hazard assessment. Unfortunately, the low expected and currently seen velocities (<1–2 mm/year) make it extremely difficult to distinguish between noise and a tectonic signal. Because of the short observation interval the velocity uncertainties are about 2 mm/year in the horizontal components. The essential goal of this program is to provide and model highly precise deformation data and to discuss its needs for a better assessment of geological hazard, especially for the most active tectonic regions in Germany, the Rhine-Graben, the Swabian Alb, the Alpine foreland, and the Vogtland. Here we present preliminary results from 2 years of measurements at currently 150 GPS stations throughout Germany. The time span of this program has proven to be too short and the density of the station network to be not dense enough yet for reliable significant horizontal station velocities and supporting the earthquake hazard assessment.     

Flow directions in Columbia River Basalt flows and paleocurrents of interbedded sedimentary rocks,South-Central Washington

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:

1. a)
   the Cascade paleoslope to the west with east-southeast paleocurrents and a predominance of volcanic heavy minerals (hornblende, oxyhornblende, hypersthene, and clinopyroxene).     

The major Baltic inflow in January 1993 — Measurements and modelling results

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.

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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.

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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.

     

Iblean diatremes 3: volcanic processes on a Miocene carbonate platform (Iblean Mountains,SE-Sicily): a comparison of deep vs. shallow marine eruptive processes

Evolution and magma fragmentation processes of two contrasting, well-exposed diatreme complexes interbedded with Late Miocene calcareous marine sediments in distinct sedimentary environments of a carbonate platform (Iblean Plateau, Sicily) are compared with each other. The nephelinitic Cozzo Molino diatreme (CMD) to the east developed in shallow water (0–80 m water depth); the alkali basaltic Valle Guffari seamount (VGS) to the west grew on a deeper water carbonate ramp (150–200 m water depth). We focus on the dominant boundary conditions inferred to have governed depth of magma fragmentation and subaqueous emplacement mechanisms: water depth, physical nature of host rocks, magma composition, and inferred differences in initial volatile concentrations. There are gross similarities in the composition of the two moderately evolved magmas. The low-viscosity magmas in both diatremes were laden with xenoliths originating from mantle to lower crustal sites. Although similar, the eastern shallow water CMD was likely more volatile-rich, with magma fragmented prior to reaching the surface and the surrounding tephra cone was partly emergent. The eruptions of the entirely submarine VGS diatreme complex in the deeper water environment were dominated by interaction of soft sediment and alkali basaltic magma or a pre-fragmented volatile-particle mixture. Eruption columns were, thus, strongly damped and the submarine complex never pierced the water surface.     

Deposition of rheomorphic ignimbrite D (Mogán Formation), Gran Canaria, Canary Islands, Spain

 Rheomorphic ignimbrite D (13.4 Ma, Upper Mogán Formation on Gran Canaria), a multiple flow–single cooling unit, is divided into four major structural zones that differ in fabric and finite strain of deformed pyroclasts. Their structural characteristics indicate contrasting deformation mechanisms during rheomorphic flow. The zones are: (a) a basal zone (vitrophyre) with pure uniaxial flattening perpendicular to the foliation; (b) an overlying shear zone characterized by asymmetric fabrics and a significantly higher finite strain, with an ellipsoid geometry similar to stretched oblate bodies; (c) a central zone with a finite strain geometry similar to that of the underlying shear zone but without evidence of a rotational strain component; and (d) a slightly deformed to non-deformed top zone where the almost random orientation of subspherical pyroclasts suggests preservation of original, syn-depositional clast shapes. Rheomorphic flow in D is the result of syn- to post-depositional remobilization of a hot pyroclastic flow as shown by kinematic modeling based on: (a) the overall vertical structural zonation suggested by finite strain and fabric analysis; (b) the relation of shear sense to topography; (c) the interrelationship of the calculated vertical cooling progression at the base of the flow (formation of vitrophyre) and the related vertical changes in strain geometry; (d) the complex lithification history; and (e) the consequent mechanisms of deformational flow. Rheomorphic flow was caused by load pressure due to an increase in the vertical accumulation of pyroclastic material on a slope of generally 6–8°. We suggest that every level of newly deposited pyroclastic flow material of D first passed through a welding process that was dominated by compaction (pure flattening) before rheomorphic deformation started. Received: 25 June 1997 / Accepted: 28 October 1998     

Peralkaline felsic magmatism at the Nemrut volcano,Turkey: impact of volcanism on the evolution of Lake Van (Anatolia) IV

Nemrut volcano, adjacent to Lake Van (Turkey), is one of the most important peralkaline silicic centres in the world, where magmatism for ~570,000 years has been dominated by peralkaline trachytes and rhyolites. Using onshore and Lake Van drill site tephra samples, we document the phenocryst and glass matrix compositions, confirming a complete spectrum from very rare mafic to dominantly silicic magmas. Magma mixing has been common and, along with the multi-lineage nature of the magmas, indicates that Nemrut has been a very open system where, nevertheless, compositionally zoned caps developed during periods of relative eruptive quiescence. Geothermometry suggests that the intermediate-silicic magmas evolved in an upper crustal magma reservoir at temperatures between 1100 and 750 °C, at fO₂ close to the FMQ buffer. The silicic magmas either were halogen poor or exsolved a halogen-rich phase prior to or during eruption. An unusual Pb-rich phase, with up to 98.78 wt% PbO, is interpreted as having exsolved from the intermediate-rhyolitic magmas.     

Models for the origin of accretionary lapilli

Binding between initially cohesionless ash particles to form concentric accretionary lapilli is provided primarily by the capillary forces of liquid bridges from condensed moisture and by electrostatic attraction. Capillary forces are strong bonds if the particles are in close contact, but they decrease rapidly with increasing particle spacing. Electrostatic attraction between charged ash particles is much weaker but effective over larger distances, increasing the frequency of collision between them.Experimental results of liquid film binding of volcanic ash showed that agglomeration was most successful between 15 and 25 wt.%, defining the agglomeration window for the formation of accretionary lapilli. Below 5–10 wt.% and above about 25–30 wt.% of water, concentric agglomeration was inhibited. Particles <350 m could be selected from a wider particle population in the experiments using only small amounts of water, which can explain the growth of accretionary lapilli in pyroclastic surges around agglomeration nuclei. Experiments testing the behavior of volcanic ash in electric fields showed that ash clusters formed instantaneously when the ash entered the field between a corona discharge gun and a grounded metal plate. The maximum grain size incorporated into the artificial clusters was about 180 m but >90 wt.% of ash was <45 m.Accretionary lapilli form in turbulent ash clouds when particles carrying liquid films of condensed moisture collide with each other and when the binding forces exceed the grain dispersive forces. Larger particles >500 m act as agglomeration nuclei in surges, accreting ash <350 m around them. In pyroclastic flows the aggregates are thought to originate from already size-sorted ash at the interface between the lower avalanche part of the flow and its overriding elutriation cloud. The fine-grained rims around accretionary lapilli found close to source are interpreted to be accreted dominantly by electrostatic attraction of very fine ash similar to clustering in elutriation clouds.