HR: 0800h
AN: V31B-0491    [Abstracts]
TI: Explosive Origins of Welded Block and Ash Flow Deposits
AU: * Andrews, G D
EM: gandrews@eos.ubc.ca
AF: University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada
AU: Michol, K
EM: kmichol@eos.ubc.ca
AF: University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada
AU: Russell, K
EM: krussell@eos.ubc.ca
AF: University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada
AB: The 2360 years B. P. eruption of Mount Meager, British Columbia, Canada, produced a succession of rarely- observed, welded block and ash flow deposits and non-welded equivalents, the Pebble Creek Formation. The welded block and ash flow deposits result from accumulation in a narrow, confined river valley; the accumulation was sufficiently rapid to keep the deposits above their glass transition temperature thereby allowing the succession to weld as a single cooling unit. The average integrated strain for vertical sections of the deposit is 31% implying > 50 m shortening of the thickest deposit (from 162 m to the current 112 m). Observations made in the field were supplemented by textural studies with the SEM and microscope, before being quantified by image analysis software. These data were then added to with measurements of connected and isolated porosity from the He-pycnometer at UBC. Only through the integration of these related data sets were we able to explore two significant findings: (1) Unconsolidated deposits have an average matrix porosity of ~41% and clasts have an average porosity of ~32%. Isolated porosity is pervasive (< 8%) in juvenile clasts, but is near absent in samples of matrix (< 1%). Welding and compaction cause a reduction in both connected and isolated porosity, where, equivalent amounts of strain (~38%) are recorded in matrix and clasts. Thus, both matrix and clasts are fully coupled during the welding and compaction process; in contrast, reports from pumiceous ignimbrites suggest that pumice clasts deform faster than the corresponding matrix. We present a series of strain evolution pathways that predict the theoretical welding trajectories for different pyroclastic deposits through porosity space. In this conceptual environment, coupled clast-matrix welding plots along the 1:1 diagonal. (2) The nature and distribution of isolated porosity in the juvenile clasts of rhyodacite (< 8%) suggests an explosive (rather than effusive) origin for these block and ash flow deposits, as previously thought. Therefore, they may be analogous to some Vulcanian-eruption-triggered dome collapse processes observed at Soufrière Hills Volcano, Montserrat, rather than by gravitational collapse of domes or lavas (e.g., Merapi- style). The interpretation of the presence of isolated porosity as a characteristic of explosive volcanism is not yet unanimously accepted. However, isolated porosity: (i) is an inevitable consequence of late-stage vesiculation during the ascent of magma during an explosive eruption; (ii) has been observed consistently in Plinian and Vulcanian tephras; and (iii) is not yet reported in lavas from purely effusive eruptions. We propose that the identification of isolated porosity is a strong indication of an explosive origin in tephras, and of particular importance in deposits that may otherwise be interpreted to be the products of effusive eruption based on field and textural observations. We encourage other workers to examine the deposits of observed explosive and effusive eruptions to generate a greater data set to test our hypothesis.
DE: 8404 Volcanoclastic deposits
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8486 Field relationships (1090, 3690)
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting