Event Name : Augustine Long Beach Debris Avalanche
|Start: 1470 (± 160 Years) || Years BP C-14 (raw) || |
|Stop: 1110 (± 70 Years) || Years BP C-14 (raw) || |
|Debris-avalanche, volcanic avalanche, or landslide: ||
|Eruption Type:||Not an eruption.|
From Waitt and Beget (1996): "Between about 1400 and 1100 yr B.P. (between tephra layers H and C) one debris avalanche swept to the sea on the south, another on the southwest, and perhaps a third on the north-northwest."
"A hummocky diamict is sparsely exposed in the distal southwest quarter as small kipukas surrounded by 1976 and older pumiceous lahar deposit and as an uninterrupted hummocky belt just beyond the limit of 1976 deposits. The humocks are as much as 9 m high and 20 in diameter, one of the largest consisting mostly of one 9-m andesite boulder. Numerous large boulders protruding through swampy terrain near the southwest coast probably are of this deposit. A lithic diamict exposed along Long Beach also includes hummocks exposing boulders as large as 6 m capped by tephra layers C and M. Large boulders be in matrix of smaller angular material of identical composition. Most of the lithic clasts are porphyritic andesite, but a few are sandstone. The avalanche must have flowed down across Jurassic sandstone (farther west than now exposed) en route to the coast. Detterman (1973) mistook the south-coastal exposure containing the 6-m boulder for in-situ lava flow."
"At low tide and on aerial photographs, large-boulder lag can be seen as far as 0.5 km off the south-southwest and southwest shore; maps show highly concoluted bathymetric contours here extending to a depth of 10 m as far as 2 km offshore, some 8.5 km from the summit. Similar submarine hummocky topography extends generally 6 to 9 kilometers outboard of demonstrable debris-avalanche deposits on other asimuths; such topography off the southwest coast must record the seaward extent of debris avalanche."
"In the southwest quarter 0.6 to 1.0 km back from the south coast is semicontinuous swamp nearly at sea level devoid of large hummocks but diversified by several large boulders. This low area may have been a lagoon in board of a debris avalanche that swept mostly to sea-like Northwest lagoon now behind West Island (see below). Younger pyroclastic and laharic deposits, including from 1935, 1964, and 1976 eruptions have nearly filled the former southwest lagoon. This much-buried Long Beach debris-avalanche deposit seems to extend a similar distance from the summit cone as the younger and well-preserved West Island debris-avalanche deposit (see below)."
"The Long Beach debris-avalanche deposits (unit HCal) is perhaps only slightly older than the overlying pumiceous deposit (Southwest pyroclastic fan, unit HCpw), for in exposures along the eastern part of Long Beach where the pumiceous deposit directly overlies the lag there is no discernible soil at the top of the diamict."
"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."