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Okmok reported activity





Event Name : Okmok 1817/3

Start:March 2, 1817 Observed
Stop:March 1, 1820 Observed

Tephrafall: BibCard BibCard BibCard
Pyroclastic flow, surge, or nuee ardente: BibCard BibCard BibCard
Central eruption: BibCard BibCard BibCard BibCard
Eruption Type:Explosive
MaxVEI: 3 BibCard
Duration: Possibly 3 years BibCard

Description: Grey (2003) has compiled and studied information about the 1817 Okmok eruption. From her thesis: "The first confirmed activity at Okmok was a large explosive eruption around 1817. There is a discrepancy as to the exact date and duration of the eruption. Veniaminov (1840, translated by Lydia T. Black and R.H. Geoghegan, 1984) maintains it was March 2, 1817; Postels (in Lutke, 1836) alleges it occurred on March 1, 1820. Grewingk (1850, translated 2003 by Fritz Jaensch) favors Veniaminov's assertion and 1817 is the date most often cited. However, Hantke (1951), who never visited the Aleutians, implies that Okmok was active during the entire four-year period and this is reflected in Simkin and Siebert (1994), though Hantke cites no specific references for his information. Whenever this eruption occurred (or perhaps there were two more separate eruptions?), there is general agreement about what happened during the explosive phase. During a storm with heavy SW winds, 'the range lying on the NE side of Umnak Island exploded * * * hurling great rocks for distances of up to 5 versts (~5 km)' (Veniaminov, 1840; Grewingk, 1850). The strong earthquake accompanying the eruption frightened the inhabitants of Unalaska, 120 km ENE of Okmok, who reportedly woke in the morning to find up to a foot of ash on the ground in some places (Lutke, 1836; Veniaminov, 1840; Grewingk, 1850). Iliuliuk Creek, which flows through Unalaska village, was reportedly so clogged with ash that it supported no fish for almost a year thereafter (Lutke, 1836; Grewingk, 1850). Though these reports all mention heavy ash fall in Unalaska, today no such thick ash layer can be found to substantiate this claim. It is quite possible that these eyewitness accounts overestimated the thickness of the deposit. It is also likely that any ash that was deposited in Unalaska has long since been eroded by the wind and rain action notorious in the Aleutians. Fieldworkers on Chuginadak Island in 2002 reported that the 2001 Cleveland ash fall on the island was approximately 20 cm thick up to 5 km from the vent, but still, no coherent ash layer can be found now, just some windblown pockets (J. Dehn, personal communication, 2003).

"The Aleut village of Egorkovskoe (or Adus), located on Cape Tanak (formerly called Egorkovskoi) on the northernmost end of Umnak, was destroyed by the 1817 eruption while its inhabitants were hunting in the Pribilof Islands (Grewingk, 1850). The villagers relocated to the Inanudak isthmus and in 1830 to the present site of Nikolski (Grewingk, 1850; Veniaminov, 1840). Because of the distance from the eruption source within the caldera, it is unlikely that the explosion itself was responsible for depositing the large boulders found near Cape Tanak. Wolfe and Beget (2002, and Wolfe, 2001) note a distinct absence of a thick enough tephra fall deposit at this site to bury a village. Rather, it is more likely that the boulders and gravel were carried in a syneruptive ash-laden outburst flood from Okmok caldera, either by disruption of an intracaldera lake or by melting of snow during the eruption. Radiocarbon date ranges for a 50-75 cm sand and gravel deposit found at Cape Tanak bracket the date of an outburst flood from the caldera between 1636-1951 AD from one soil sample, with a 55% probability between 1726-1813 AD, and between 1806-1931 AD for a second soil sample (Wolfe, 2001). These dates correspond well with the date of this eruption and support this hypothesis.

"It is possible that this explosive eruption originated from Cone E within the caldera because of the large, fresh pit crater in that cone that must have been formed by a forceful explosion quite recently. Alternatively, field evidence suggests that the eruption may have been through the intracaldera lake, originating at the site of Cone B, which is located close to the breach in the caldera wall (J. Beget and T. Neal, personal communication, 2003)."

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