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redoubt reported activity





Event Name : Redoubt Early Explosive Phase

Start: 888000 (± 63000 Years) Years BP K-Ar

Lava flow: BibCard
Tephrafall: BibCard
Lahar, debris-flow, or mudflow: BibCard
Pyroclastic flow, surge, or nuee ardente: BibCard
Lava dome: BibCard
Eruption Type:Explosive
Eruption Product: other BibCard

Description: From Till and others (1994): "The lowest deposits exposed record one or more explosive eruptions that produced a small dome or shallow intrusive complex, pyroclastic deposits, and lahar deposits. Magma erupted or intruded during this stage is silicic andesite to dacite in composition; products of the eruption(s) are exposed low on all flanks of the volcano. Where the base of these deposits is exposed, it occurs on granite. A juvenile block from a pyroclastic deposit in the upper part of the deposits that were erupted during this stage yielded a K-Ar age of 0.888 Ma (Table 3 [in original text]). This age is, therefore, a minimum age for the onset of volcanic activity at Redoubt Volcano,"

"The lowest exposures on the northwest side of the volcano are composed of interlayered till, fluvial deposits, block-and-ash flow deposits, and pumiceous pyroclastic flow deposits that dip shallowly to the north. The base is obscured. Clasts in the block-and-ash and pumice flows are silicic andesite in composition (62.0-62.7% SiO2)."

"Two pyroclastic units have been differentiated. The lower unit (Qxp) is composed predominantly of till and fluvial sediments. However, pumiceous pyroclastic flow deposits composed of hornblende-bearing white pumice in a light-gray or light-brown ash matrix are interlayered with the sediments and are exposed on several ridges. Correlations between ridges are difficult, but least two emplacement units occur on most ridges, and two separate exposures of substantially thick units exist. The thicker flow is 24-30 m thick, indurated, and consists of 50-70% white subangular to subrounded horn-blende-beating pumice clasts, less than 2% light-gray hornblende-phyric juvenile lithic fragments, and a light-gray fine ash matrix. Locally, the deposit is clast supported. The largest pumice clasts, 45-60 cm in diameter, occur at the base of the flow. Most pumice clasts are less than 10 cm across and are normally graded. Grading of the juvenile lithic clasts was not noted. A 2-4- cm-thick layer of fine ash is concentrated at the base of the flow (Fig. 5 [in original text]). Several centimeters of laminated lapilli and ash occur at the top of the flow. Above the laminated ash, locally there are thin beds of strongly graded lapilli to ash-sized material, some of which may be primary airfall tephra. A second pyroclastic flow deposit, 15 m thick where exposed, is quite similar. It also has fine ash concentrated at its base and top, and the coarsest pumice clasts (10 cm in diameter) concentrated at the bottom. Pumice clasts in the main body of the deposit are pebble-sized and appear to be unsorted. This deposit contains 40% subangular to subrounded pumice clasts, 15% subangular to subrounded juvenile lithic clasts, and 45% matrix of light-brown ash-sized particles. Thinner (3 cm-2 m) pumiceous pyroclastic flow deposits are clast or matrix supported, contain rare lithic clasts, and commonly are overlain by laminated beds of fine ash."

"Above the sediments and interlayered pumice flow deposits, at least two block-and-ash flow deposits occur. Both are matrix supported and contain clasts of juvenile hornblende-phyric silicic andesite, some of which are radially fractured or are bombs. The matrix is sand-sized and crystal-rich. In general, the relative amount of volcanic material increases up section."

"The upper unit (Qxb) is dominated by block- and-ash flow deposits and contains subordinate fluvial deposits. Numerous poorly consolidated to unconsolidated block-and-ash flow deposits 0.5-60 m thick are distinguished in section by variations in matrix color or intervening concentrations of fine ash. Most emplacement units are internally unstructured and do not show sorting. Some emplacement units contain graded beds, cross bedding, and concentrations of granitic clasts that suggest the unit was reworked by water. Clasts are dominantly subangular to subrounded reddish-gray to light-gray hornblende-phyric silicic andesite (juvenile), but also include minor granitic clasts and scoriaceous basalt (dark-gray clinopyroxene-phyric flow rock). Pumice occurs rarely in some units. The largest juvenile lithic clasts observed were 2-3 m in diameter and were concentrated at the base of an emplacement unit. The matrix of most units is sand-sized and crystal-rich; rare pumiceous sand-sized grains are present. A large, glassy juvenile hornblende-phyric silicic andesite clast from one of these block-and-ash flow deposits yielded a K-Ar age of 0.888 Ma (Table 3; locality 1, Fig. 3 [in original text])."

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