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Aniakchak reported activity





Event Name : Aniakchak Black Nose Pumice

Start: 7000 Years BP Tephrochronology
Stop: 3500 Years BP Tephrochronology

Tephrafall: BibCard BibCard BibCard BibCard
Pyroclastic flow, surge, or nuee ardente: BibCard
Eruption Type:Explosive
Eruption Product: other BibCard

Description: From Bacon and others (2014): "Subsequent to Aniakchak I, Plinian eruptions ca. 7,000 14C yr B.P. from a vent northeast of the edifice summit produced the Black Nose Pumice, consisting of a lower unit of rhyodacite lava, pumice fall, and intraplinian welded ignimbrite and an upper unit of dacite pumice fall and northeast flank lava flow."

"A comparatively thin tephra bed (VanderHoek and Myron, 2004, sample ANI 99-L) beneath the principal fall deposit at Cabin Bluff (ANI 99-K) contains glass and Fe-Ti oxides compositionally similar to those of the principal deposit as well as another population of glass that is more evolved and oxides that have somewhat different compositions (appendix A, tables A1 and A2 [in original text]). We tentatively interpret the lower bed as representing one or more eruptions of Aniakchak volcano precursory to the Black Nose Pumice eruptions."

"Following Dreher’s (2002, p. 17) description of the Black Nose Pumice and prior informal usage of this term, we apply the name to dacite and rhyodacite Plinian pumice fall deposits and intercalated (intraplinian) welded rhyodacite ignimbrite north of The Gates and to correlative deposits elsewhere near the caldera rim. The unit is named for exposures 2.5 km south of The Gates at Black Nose, although a more accessible and probably more complete section is present north of The Gates. An ~2,000-m square patch of apparent fall deposit is preserved on a northeast-trending spur at ~1,800 ft (550 m) asl south of the Aniakchak River immediately east of The Gates, which indicates that a deeply incised valley headed just east of the present caldera rim at the time of the Black Nose eruptions. Similarly, a prominent 1,500 ft (460 m) asl bench ~1-2 km east-northeast of The Gates appears to be capped by Black Nose Pumice. Other remnants of Black Nose Pumice may be present on the ~400 x 700 m bench at ~2,300-ft (700 m) elevation south of The Gates. Black Nose Pumice also may be present below younger deposits on the north and west caldera rim. North of The Gates, Black Nose Pumice dips gently towards the caldera and conformably overlies 1 cm of gray ash and 1 cm of brown ash that rest upon the eroded surface of the previously described unconsolidated dacite pyroclastic deposits. The Black Nose Pumice is overlain by Aniakchak II lithic breccia. The Black Nose Pumice is subdivided into lower and upper subunits (figs. 5B and 8 [in original text])."

"The well-sorted beds of the lower Black Nose Pumice consist of highly inflated crystal-poor buff-colored pumice that is rhyodacite in composition. The pumice is chemically less evolved and lacks the hornblende phenocrysts that are present in Aniakchak II rhyodacite pumice (Dreher, 2002). Maximum pumice clast size is ~30 cm. Many clasts have pink interiors and are fractured so that they fall apart when removed from the outcrop. The unit is ~8-10 m thick north of The Gates."

"Welded ignimbrite occurs within the lower Black Nose Pumice north of The Gates ~2 m below the top of the lower Black Nose Pumice fall (figs. 5B, 8A, 8C, and 8D [in original text]). About 1-2 m thick at the locality shown in figure 8D [in original text], the ignimbrite pinches out south towards The Gates and thickens to the north to at least 10 m in topographic lows...The ignimbrite likely represents collapse of the Plinian eruption column, probably owing to vent widening and increase in eruption rate as deduced for the similar Wineglass Welded Tuff at Crater Lake, Oregon (Kamata and others, 1993)."

"The age of the lower Black Nose Pumice is constrained by tephrochronology. A 45-cm-thick bed of relatively coarse silicic pumice ~40 km southeast of the caldera near Aniakchak Bay (fig. 1 in original text) was described by VanderHoek and Myron (2004, their figures 7, 8; Cabin Bluff section, ANI 99-K)...Peat from beneath the Cabin Bluff pumice bed yielded a radio-carbon age of 7,350+/-70 yr B.P., whereas soil beneath the same bed 600 m distant gave 6,760+/-60 yr B.P. (table 1 [in original text]; VanderHoek and Myron, 2004; VanderHoek, 2009), leading us to suggest an age of ca. 7,000 yr B.P. for the lower Black Nose Pumice."

"The upper Black Nose Pumice is a Plinian fall deposit that conformably overlies the lower Black Nose Pumice. As large as 40 cm, clasts of brown pumice are silicic dacite, slightly less evolved than the underlying ignimbrite and lower Black Nose Pumice. This pumice has greater phenocryst content than the lower unit (Dreher, 2002). The well-sorted beds are variably oxidized and incipiently welded. Basal upper Black Nose Pumice fall is partly welded and forms a dark-gray band in caldera rim exposures. This nearly uniform-thickness band and the underlying variable-thickness ignimbrite, separated by ~2 m of buff-colored pumice fall, make a distinctive pair of dark-gray stripes across exposures near the rim of the northeast quadrant of the caldera (figs. 8A and 8C [in original text]). The preserved thickness of upper Black Nose Pumice north of The Gates is as much as ~15 m but varies considerably, implying modification during a significant period of erosion. The deposit is overlain by Aniakchak II lithic breccia."

"The distribution of upper Black Nose Pumice and any distal correlations have yet to be established, though the thickness, composition, and coarseness of the deposit at the caldera rim imply wide dispersal downwind. Analyzed dacite pumice from a fall deposit beneath Aniakchak II ignimbrite in lower Reindeer Creek (fig. 4 [in original text], near west edge), sampled by Dreher (2002; sample 98AC70D), has unique composition but is closest to basal upper Black Nose Pumice."

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