Event Name : Augustine West Island Debris Avalanche
|Start: 350 || Years BP Tephrochronology || |
|Stop:|| 1812 ||Tephrochronology|
|Debris-avalanche, volcanic avalanche, or landslide: ||
|Eruption Type:||Not an eruption.|
From Waitt and Beget (1996): "Between about 350 yr B.P. (after tephra layer B) and historic time, three separate debris avalanches swept to the sea on the west-northwest, north-northwest, and northflanks. One of them (West Island} was large and fast, most of it having rode to sea beyond a sea cliff cut back into older deposits."
"A diamict of brecciated andesite-dacite forms West Island, separated from the northwest coast of Augustine Island by Northwest lagoon 0.5 km wide. The deposit is a mixture of angular to very angular reddish to grayish porphyritic andesite boulders as large as 4 m set in a nonsorted matrix of finer diamict. The West Island deposit comprises a central core of unmodified high conical hummocks as high as 30 m with slopes as steep as 40 degrees, surrounded on seaward sides by a wide zone of lower hummocks (all unit Bawa). The hummocks are capped by 0-30 cm of humus and five sand ashes of which three underlie the Katmai 1912 white-silt ash. The B tephra is absent from West Island and thus the hummocky diamict must predate 350 yr B.P. (table 2 [in original text]). In a gully 1.2 km back from the lagoon at altitude 85 m, a landward phase of the deposit (unit Bawi)is underlain by the (M?) tephra (but B tephra is missing."
"On the seaward side there is but a discontinuous sea cliff cut into sporadic hummocks, though the irregular cliff line is fairly straight. West Island deposit is thickly vegetated by shrub alder and a few spruce. A continuous filed of boulders ranging up to 5 m in diameter extends at least as far as 1.3 km offshore, much of it visible at the lowest spring tides and discernable as well on aerial photographs."
"Hummocks on the southwest side of West Island are nearly flat topped, apparently beveled down as much as 20 m. These southwestern hummocks are also sharply incised, and some of them capped by coarse winnowed lag of openwork boulders as large as 4 m. A few of them are overlain by poorly sorted sand, perhaps waterlaid, as high as 5 m above high-tide level. The geomorphically modified southwestmost hummocks are evidence of a great ruch of water across them. This evidence of a huge flow of water perhaps records a great sea wave (tsunami) created as West Island debris avalanche plowerd into the sea with high momentum. The modification could not have been caused by an 1883 tsunami originating from Burr Point because the low-level hummocky the north part of West Island (facing Burr Point) is unmodified and because at least one of the modified hummocks is overlain by tephra older than the 1912 Katmai ash."
"During this prehistoric period numerous domes must have been emplaced at the summit, repeatedly renewing the source for catastrophic debris avalanches. Remnants of these older domes form the east and south sides of the present summit-dome complex. Below the summit area at least three domes were emplaced on the upper flanks, one on the south (Karnishak dome), two on the northwest (domes "I" and 'H"). Another undated and nearly buried dome or lava flow diversifies the upper south flank."