Event Name : Augustine Long Beach Pyroclastic Flow
|Start: 1470 (± 160 Years) || Years BP C-14 (raw) || |
|Stop: 1110 (± 70 Years) || Years BP C-14 (raw) || |
|Lahar, debris-flow, or mudflow: ||
|Pyroclastic flow, surge, or nuee ardente: ||
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. Pumiceous pyroclastic fans were shed to the southeast and southwest and lithic pyroclastic flows and lahars(?) to the south and southeast."
"A pumiceous sandy diamict as thick as 16 m overlies the Long Beach debris-avalanche deposit and an associated lahar deposit (units HCal and HClw) and directly underlying C tephra in coastal cliffs of the southwest flank. The pycroclastic deposit is 75-80 percent sand matrix: among the coarser casts three-quarters are pumice, one quarter lithics as large as 5 cm (rarely as large as 30 cm). The deposit is graded by density pumice clasts are concentrated in upper 3 m of deposit, lithic clasts in lower 10 m, though rare lithics lie throughout the deposit. The upper 3 m of the deposit is pale red, oxidized from high temperature. It has all the characteristics of pumiceous pyroclastic-flow 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."