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Okmok reported activity

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EVENT SPECIFIC INFORMATION

Event Name : Okmok 8250 yBP

Start: 8250 Years BP Tephrochronology

Tephrafall: BibCard BibCard
Pyroclastic flow, surge, or nuee ardente: BibCard BibCard
Phreato-magmatic: BibCard
Eruption Type:Explosive
Other""

Description: From Miller and Smith (1987): "Black (1975) reported 14C dating of a "20-to 30-cm thick" tephra deposited directly on a cultural horizon near the west end of Umnak Island 70 km from Okmok (Fig. 1 [in original text]). The tephra thickens towards Okmok; Black concluded that it represents a catastrophic eruption of the volcano at about 8250 B.P."

From Beget and others (2005): "At least four significant eruptions of mafic scoria (~54 wt. % SiO2; Wong, 2004) occurred during the time period between the older and younger caldera-forming eruptions and these are mainly found to the northeast and south of Okmok caldera. These units are poorly understood, yet contain evidence for explosive interactions between external water and magma, producing phreatomagmatic deposits. The most extensive of these is described in Wong (2004), and consists of thick deposits of scoria and rock fragments deposited to the south and east of the present caldera. In addition, a thin surge bed containing incinerated plant remains occurs between deposits of the first and second caldera-forming eruptions on southern Unalaska Island. This surge, which is associated with a thick tephra-fall deposit and is also locally exposed on the east end of Umnak Island, demonstrates that highly mobile pyroclastic flows capable of traveling across water to distances of 20 kilometers and more from Okmok Volcano can be generated independently of caldera-forming eruptions. Radiocarbon dates from charcoal in the surge suggest it occurred about 8,000 years ago, similar in age to an ash deposit near Nikolski, which was attributed to a large eruption of Okmok (Black, 1974)."

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