HR: 1340h
AN: V53A-1541    [Abstracts]
TI: Contrasting Fragmentation Modes in the Capelas Tuff Cone, São Miguel (Azores)
AU: * Mattsson, H B
EM: hannesm@hi.is
AF: Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, Reykjavik, IS-101 Iceland
AU: Solgevik, H
EM: henrik.solgevik@geo.su.se
AF: Department of Geology and Geochemistry, Stockholm University, Stockholm, SE-106 91 Sweden
AB: The Capelas tuff cone is situated on the northern coast of São Miguel island (Azores). The cone was formed in an eruption that started in a shallow marine environment, grew above sea level and adjoined with the island of São Miguel. We sampled and photographed ash particles (using Scanning Electron Microscopy) from representative layers from a cross-sectional profile of the cone. Most investigated beds contain evidence of mixed fragmentation (i.e. both magmatic and phreatomagmatic) occurring simultaneously. Phreatomagmatic fragments are characterized by blocky and equant particle shapes, linear particle outlines, quenching cracks and few vesicles. Particles characteristic of magmatic fragmentation display irregular shapes, concave-convex particle outlines, high vesicularity and an absence of surface structures such as chemical pitting and adhering particles. Analyses show that the bulk volume of the Capelas cone consists of particles generated by phreatomagmatic fragmentation. However, the first phase of the eruption (represented by early proximal fallout deposits) also display considerable amount of magmatic fragmentation. This phase occurred in a shallow-marine setting as indicated by the abundance of broken shell fragments (molluscs) found in the deposits. Deposits formed in the later phases of the Capelas eruption display features consistent with drier Surtseyan periods (overall lower water/magma-ratios). This is deduced from the presence of grain flow deposits and pyroclastic surge deposits that occasionally display undulating layers and relatively dry structures such as strongly grain-segregated layers and brittle behavior when impacted by ballistic ejecta. We interpret the initial phase of the eruption as being generated by shallow-marine fire-fountaining activity which corresponds well to the continuous jetting reported from the Surtsey eruption (which at nighttime was incandescent). The presence of highly vesicular fragments mixed with equant shards, suggests that the first phase was associated with high eruption rate and that the magma was already vesiculated. We suggest that during the initial phase at Capelas water could only interact with the ascending magma to a limited extent at the outer edges of a sustained magma jet, thus producing clasts of contrasting fragmentation modes simultaneously. As the eruption progressed, the eruption rate probably dropped, as did the influx of water into the vent region (due to the build-up of a cone), thus producing drier features of Surtseyan-type and more efficient fragmentation.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8434 Magma migration and fragmentation
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005