HR: 13:55h
AN: V23E-02    [Abstracts]
TI: Textural signatures in pyroclasts of the 1875 eruption of Askja volcano: understanding conduit and vent processes in wet phreatoplinian and dry subplinian/Plinian phases.
AU: * Carey, R J
EM: beccarey@hawaii.edu
AF: University of Hawaii at Manoa, 1680 East West Rd, Honolulu, hi 96825, United States
AU: Houghton, B F
EM: bhought@soest.hawaii.edu
AF: University of Hawaii at Manoa, 1680 East West Rd, Honolulu, hi 96825, United States
AU: Thordarson, T T
EM: tthordar@staffmail.ed.ac.uk
AF: University of Edinburgh, Edinburgh EH9 3JW Scotland, United Kingdom, Edinburgh, 96825, United Kingdom
AB: The eruption of Askja on March 28-29, 1875 is one of four "type" phreatoplinian examples described by Self and Sparks, 1978 and the only example with documented historical records. The phreatoplinian phase was preceded and followed by purely "dry"or magmatic fragmentation during subplinian and Plinian phases, defining a sequence of fluctuating eruption intensity and contrasting style. In this study we will assess the roles of magmatic volatiles and/or external water driving phreatoplinian eruptions and the factors which control abrupt shifts between "dry" (subplinian, Plinian) and "wet" (phreatoplinian) eruption processes. Macroscopically similar populations of highly vesicular pumice characterize all three phases. Density data also shows that pumices from each fall phase have a very similar narrow range of density. The similarity between the bulk density data for all units is thus suggestive of similar vesiculation histories. We present quantitative evidence from image anlayis of selected clasts from each layer to contrast the patterns of nucleation, growth and coalescence of vesicles in melt of the phreatoplinian and plinian/subplinian pumices. Preliminary data supports two hypotheses: (1) Vesiculation was the prime driving mechanism for even the phreatoplinian phase of the 1875 eruption and (2) shifts between "dry" and "wet" phases of the 1875 eruption were not driven by conduit processes (e.g. changing ascent rate, variable amounts of vesiculation and degassing) but instead were an artifact of changing vent position and the availability of surface water.
DE: 8428 Explosive volcanism
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting