Event Name : Augustine Prehistoric Lava Flow
|Start: 350 || Years BP Tephrochronology || |
|Stop:|| 1812 ||Tephrochronology|
|Lava flow: ||
From Waitt and Beget (1996): "Between about 350 yr B.P. (after tephra layer B) and historic time... Augustine's only conspicuous lava flow was emplaced on the north flank."
"A conspicuous lava flow on the middle north flank consists of massive porphyritic andesite ranging from light gray to oxidized light reddish brown. It contains about 10 percent plagioclase phenocrysts as large as 4 mm. The apex of the conspicuous landform at about altitude 650 m apparently marks the site of a flank vent, about 600 m below the present summit, indeed below the base of the entire summit-dome complex. The flow terminated 450 m below its apex at a distance of 1.75 km."
"This lava flow is usually considered as the last part of the 1883 eruption (Kienle and Forbes, 1976; Siebert and others, 1987, 1989, 1995; Swanson and Kienle, 1988). Kienle and Swanson (1980) had called it "prehistoric" but gave no evidence. Overlying the upper east side of the lava flow is a sharp-crested ridge at least 640 m long and as thick as 15 m consisting of diamict including many angular fragments as large as 5 m of dome-rock porphyritic andesite. In any one small area of this deposit the andesite boulders are diversely black, reddish, and gray. This clearly mixed deposit is a debris-avalanche levee. The debris can be traced downslope to altitude 450 m, below which it is discontinuous but aligned with a sharp levee well defined below altitude 200m, the west edge of the Burr Point debris-avalanche deposite (see below [in original text]). The Burr Point debris avalanche occurred at or near the beginning of the 1886 eruption: the lava flow is older. The lava flow appears to be overlain by just one coarse diamict, and thus it must be younger than the Rocky Point debris avalanche, which would have swept down the lava flow had it then existed...The stratigraphy level beneath the Burr Point diamict and its position on the north flank suggest that the lava flow occurred late during an eruption that began with the Rocky Point debris avalanche."
"A hypothetical alternative for its age is that North Slope lava flow began the 1883 eruption and was later overridden by the Burr Point debris avalanche. Problems with such a model include: (a) there is no evident baking of the base of the overlying debris avalanche as might have occurred had the lava been fresh and hot, and (b) a vent at mid-altitude on the flank should have provided the outlet for magma and relieved vent pressure, precluding injection of magma higher that would cause a debris avalanche later in the eruption."