Event Name : Augustine South Point 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."
"South Point, which just 0.6 km seaward of the general line of the south coast, consists of a diamict of angular clasts of porphyritic andesite as large as 6 m in intermediate diameter. Its hummocky surface has local relief of 10 m. Clearly a debris-avalanche deposit, it has no discernable levees upslope, where it has been deeply buried by younger eruption deposits, especially in 1963-64."
"Like the geomorphic relation at Northeast Point, the South Point debris avalanche overrode and partly buried a sea cliff that now lies 200 m back from South Point. The sharply hummocky topography is similar both seaward and behind the arrested sea cliff. The present sea cliff (seaward of buried one) is 30 m high, the minimum thickness of South Point debris-avalanche deposit there. That South Point is far broader and projects much farther seaward than Northeast Point suggests the latter’s greater age, consistent with tephra stratigraphy."
"The tephra-and-humus sequence that overlies the debris-avalanche deposit ranges from 0 to 8 m thick, typically 1-2 m thick, having the C tephra at its base. Though this is the youngest of the large debris avalanches forming Augustine’s south and east coast, it too is eroded back into a high cliff. It is thus geomorphically much older than any of the uncliffed debris-avalanche deposits on the northwest and north coasts."
"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."