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Aniakchak reported activity





Event Name : Aniakchak CFE II

Start: 3430 (± 70 Years) Years BP C-14 (raw)

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Pyroclastic flow, surge, or nuee ardente: BibCard BibCard BibCard BibCard BibCard BibCard BibCard BibCard
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Eruption Type:Explosive
Eruption Product: other BibCard
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MaxVEI: 6 BibCard

Description: From Neal and others (2001): "Modern Aniakchak caldera formed about 3,500 years ago during a violent, catastrophic eruption nearly 1,000 times the size (eruptive volume) of the August 1992 eruption of Mount Spurr volcano (fig. 1 [in original text]; Miller and Smith, 1977). During the caldera-forming eruption, draining of underground magma reservoirs caused an existing stratocone to collapse, thereby creating the deep crater that persists today (fig. 4 and pl.1 [in original text]). The enormous impact of this eruption is evident in thick blankets of coarse, pumice-rich debris deposited by pyroclastic flows that flowed in all directions from Aniakchak. These deposits extend as much as 60 km, to Bristol Bay and the Pacific Ocean. Fine ash from this eruption has been identified as far away as the north shore of the Seward Peninsula, 1,100 km to the north (Riehle and others, 1987, p. 19-22; Beget and others, 1992). Moreover, careful analysis of these fall-out deposits suggests that as many as 10 distinct, smaller explosive eruptions together spanned decades to centuries leading up to the climactic caldera-forming eruption (Riehle and others, 1999)."

From Bacon and others (2014): "The spectacular 10-km-diameter Aniakchak Crater is a caldera that collapsed during the voluminous Aniakchak II eruption ca. 3,590 cal yr B.P. (ca. 3,430 14C yr B.P.). This eruption produced rhyodacite Plinian pumice fall and ignimbrite, followed first by ignimbrite with both rhyodacite and andesite juvenile clasts, then by andesite ignimbrite. The mixed and andesite ignimbrite deposits commonly are partly welded. Ignimbrite is present in valleys surrounding the edifice and extends to the coast on both northwest and southeast."

"The ignimbrite is significant for the high mobility of the pyroclastic flows that deposited it (Miller and Smith, 1977) and for dramatic compositional zonation of rhyodacite followed by andesite (Miller and Smith, 1977; Dreher, 2002; Dreher and others, 2005). Ash from this eruption has been identified over a large part of the Alaska Peninsula and on the Alaska mainland as far north as the Seward Peninsula (Riehle and others, 1987; Beget and others, 1992). The calendrical age of the eruption is suggested to be 3,590-3,588+/-11 cal yr B.P. (Coulter and others, 2012) by correlation with chemically analyzed glass shards associated with an acid spike in Greenland ice cores (Pearce and others, 2004; Denton and Pearce, 2006), which compares favorably with the calibrated +/-2 sigma age range of 3550-3870 cal yr B.P. and preferred +/-1 sigma age of 3,660+/-70 cal yr B.P. for 3,430+/-70 14C yr B.P. (table 1 [in original text])."

From VanderHoek (2009): "United States Geological Survey and NPS researchers have reported multiple lines of evidence that, when taken together, suggest a warm season eruption for the 3400 B.P. Aniakchak event. The first of these include a pattern of tephra fallout to the north-northwest (Beget and others 1992; Riehle and others 1987), indicating a wind from the south-southeast, which is more common in the region between May and September (Lea 1989b:227, Figure 5.3). Additional support for a warm season eruption is the fact that rip-up peat clasts were found entrained in the lower levels of the 3400 B.P. pyroclastic flow (Dilley 2000), suggesting flow over unfrozen ground. Third, a tsunami was generated by the pyroclastic flow striking Bristol Bay (Waythomas and Neal 1998), suggesting the flow encountered an unfrozen bay. Fourth, the tsunami deposits on the northern Bristol Bay coast also contained rip-up peat clasts (Waythomas and Neal 1998: 112), which would only have happened if the tsunami swept across the northern coast when the ground was unfrozen."

From Derkachev and others (2018): "Glass from the Br2 tephra have medium-K rhyolitic compositions and are geochemically similar to the rhyolitic population of the ~ 3.6 ka Aniakchak II glass (Kaufman et al. 2012; Davies et al. 2016; Wallace et al. 2017) (Figs. 5, 6,and 7) [in text]. Only one of the analyzed Br2 shards falls into the andesitic Aniakchak II field; however, non-analyzed brown shards described in the samples likely indicate the presence of the whole andesite Aniakchak II population. Moreover, rhyolite Br2 glass are identical in composition to rhyolite glass from both proximal and ultra-distal (Chukchi Sea) Aniakchak II tephras. A single andesitic glass shard analyzed by LA-ICP-MS has a very similar trace element composition to that of andesitic Aniakchak II shards from Chukchi Sea sediments (Fig. 4a) [in text].

Based on chemical similarity of Br2 glass to those from the 3.6 ka Aniakchak II tephra, its Holocene age (Fig. 2) [in text], and the aerial distribution of the Aniakchak II fall deposit (Davies et al. 2016; Graham et al. 2016; Pearce et al. 2017; Ponomareva et al. 2018), we suggest that the Br2 tephra correlates to the ~ 3.6 ka Aniakchak II eruption. A preliminary report on this finding by Derkachev et al. (2015) permitted Ponomareva et al. (2018) to use this site to develop a new isopach map for the Aniakchak II tephra fall deposit. The new map allows us to double the volume of this eruption from the value of 50 km3 given by Miller and Smith (1987) to~100 km3. Assuming an ash density of 0.6 g/cm3 (Kutterolf et al. 2008b) and rhyolite density of 2.6 g/cm3, the dense rock equivalent (DRE) volume of the Aniakchak II tephra fall deposit is here estimated to be 23 km3,with an erupted mass of 6.0 × 104 Mt. This corresponds to an eruption magnitude (M) of 6.8 (Pyle 1995; Mason et al. 2004).

Wide dispersal of the ~ 3.6 ka Aniakchak II tephra across the Bering Sea (Derkachev et al. 2015; Graham et al. 2016) permits significant enlargement of its known dispersal area (Davies et al. 2016) and indicates that it could also occur in northeast Asia (Fig. 1) [in text]. The Aniakchak II tephra can also serve as a major Holocene marker horizon for the Bering Sea shelf, directly linking its Holocene sedimentary archives to those from the Chukchi Sea (Pearce et al. 2017; Ponomareva et al. 2018), eastern Canada (Pyne-O'Donnell et al. 2012), and Greenland (Pearce et al. 2004; Coulter et al. 2012; Jennings et al. 2014)."

The Global database on large magnitude explosive volcanic eruptions (LaMEVE; 2016) reports a magnitude of 6.9, bulk eruptive volume of 75 cubic km and a dense rock equivalent eruptive volume of 33.5 cubic km for the Aniakchak II eruption.

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