HR: 15:10h
AN: V43D-05    [Abstracts]
TI: Askja 1875, a complex Plinian eruption: Welding and its implication for eruption dynamics.
AU: * Carey, R J
EM: beccarey@hawaii.edu
AF: Geology and Geophysics, University of Hawaii, 1680 East West Rd, Honolulu, HI 96822 United States
AU: Houghton, B F
EM: bhought@soest.hawaii.edu
AF: Geology and Geophysics, University of Hawaii, 1680 East West Rd, Honolulu, HI 96822 United States
AU: Thordarson, T
EM: moinui@soest.hawaii.edu
AF: Geology and Geophysics, University of Hawaii, 1680 East West Rd, Honolulu, HI 96822 United States
AB: The Askja 1875 eruption was an eruption that included both Plinian and Phreatoplinian phases. The Plinian phase was unusual because it was accompanied by low fountaining eruptions along segments of the fissure to the north and the southwest of the main plinian vent, as well as hot pyroclastic surges from a vent in the southwest portion of the caldera. There are complex relationships amongst the proximal products of these vents. We focus here on the interplay in time and space between low fountaining and Plinian vents which have often led to welded and locally rheomorphosed deposits around the northern and southwestern caldera rim. The deposits along the northern rim of the caldera can be separated into white generally non-welded pumice fall that can be traced outwards to form the widespread medial distal fall and grey/black sometimes welded deposits which have a markedly more restricted dispersal and coarser grain size including pumice bombs with up to 9-meter diameters. The deposits in the southwest portion of the caldera are also dominated by locally dispersed products: hot pyroclastic surge and low intensity fountaining deposits, including spatter bombs up to 15-meters in diameter. The fountain deposits appear to have facilitated welding and rheomorphism of the proximal deposits. Welding appears to be due to a combination of a) the thermal contribution of large meter-sized spatter bombs which locally weld the surrounding deposit and b) heat intrinsic to the deposit, in regions remote from spatter bombs. Detailed field mapping of the type pioneered by George Walker, shows the 1875 eruption to be an example of how proximal sedimentation regimes during powerful sustained eruptions can include processes that are inherently transient and short-lived.
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
DE: 8434 Magma migration and fragmentation
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005