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Okmok reported activity




Event Name : Okmok CFE II

Start: 2050 (± 50 Years) Years BP C-14 (raw)

Tephrafall: BibCard BibCard
Pyroclastic flow, surge, or nuee ardente: BibCard BibCard
Caldera/crater: BibCard BibCard
Tsunami: BibCard
Phreato-magmatic: BibCard
Geomorphologic change: BibCard
Eruption Type:Explosive
Eruption Product: other BibCard
MaxVEI: 6 BibCard
ColHeight: 14000 m BibCard

Description: From Miller and Smith (1987): "The Okmok volcanic center, Umnak Island (Fig. 1 [in original text)], is a large basaltic composite volcano with a caldera system marking the summit area. This system consists of two large partially overlapping calderas (Byers, 1959), each with an estimated diameter of about 10 km. Postglacial ash-flow tuffs exposed in sea-cliff exposures on the northwest side of the volcano are locally separated by a lava flow 6-9 m thick (Miller and Smith, 1975). A hiatus between extrusion of the lava and deposition of the upper ash-flow sheet is indicated by a well-developed erosional surface on the lava flow; in places, stream channels were cut through the lava flow and later filled by the younger ash flow. The occurrence of two major ash-flow units strongly supports the probability of two major caldera-forming eruptions in Holocene time."

"Charcoal directly beneath the upper ash-flow tuff unit on the north side of the island yielded a 14C age of 2400±+/-200 yr (sample 1, Table 1 [in original text]). This is a maximum age for the second ash-flow tuff eruption, presumably the second caldera forming eruption (Table 2 [in original text]). A minimum of about 5500 yr separates the two caldera-forming eruptions, assuming the correlations are correct."

From Beget and others (2005): "The second large caldera-forming eruption produced a pyroclastic flow deposit that buried the older pyroclastic flow and almost all other older volcanic and non-volcanic sediments on all flanks of Okmok Volcano. The deposits of this caldera-forming eruption are as much as 80 meters thick near the caldera rim, and 30 to 40 meters thick at some coastal exposures (fig. 7 [in original text]). The most distal deposits preserved on land are found 25 kilometers to the southeast across Umnak Pass on southern Unalaska Island. Sea cliff exposures on the north, east, and south flanks of the volcano show where the pyroclastic flow advanced into the sea in these directions. New radiocarbon dating on plants found buried and incinerated at the base of the pyroclastic flow indicates the second caldera-forming eruption occurred about 2,050+/-50 14C years ago (Begét and Larsen, 2001). This eruption generated a tsunami which affected the westernmost part of Unalaska Island."

From Larsen and others (2007): "...the Okmok II deposits are usually significantly thinner than Okmok I, which leads to significantly lower eruptive volume estimates. Burgisser (2005) notes the total volume of the initial rhyodacite and andesite Plinian fall deposits are ~0.5 km3, whereas the main pyroclastic flow deposits are estimated to be ~15 km 3 on-island."

The Global database on large magnitude explosive volcanic eruptions (LaMEVE; 2017) reports a magnitude of 6.7, bulk eruptive volume of 50 cubic km and a dense rock equivalent eruptive volume of 29 cubic km for the eruption.

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Page modified: March 30, 2017 14:36
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