Event Name : Augustine Rocky Point 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."
"West of Burr Point, a diamict forms a sharp point of land (Rocky Point). The deposit has a sharply hummocky topography with a relief of 40 m and local slopes as steep as 35 degrees and comprises angular andesite boulders at least as large as 5 m. At low tide this debris forms bouldery islands (unit ob) as far as 1.5 km offshore that have been winnowed and beveled to bouldery shoals."
"Capping the coarse diamict in upward succession is weakly oxidized soil horizon 20 cm thick, an organic layer, a gray silt ash (1883 eruption?), and a distinctive white silt ash from the 1912 Katmai eruption. The irregular, curving sea-cliff line is roughly similar to that at West Island. The sharp morphology of the hummocks and that it has but one ash layer beneath the Katmai ash makes this deposit is both geomorphically and stratigraphically younger than the West Island debris-avalanche deposit."
"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."