Okmok 8250 yBP
Start: 8250 yBP [1]
Event Type: Explosive
- Phreato-magmatic [2]
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)."
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)."
References Cited
[1] Late-Quaternary sea level changes, Umnak Island, Aleutians: their effects on ancient Aleuts and their causes, 1974
Black, R. F., 1974, Late-Quaternary sea level changes, Umnak Island, Aleutians: their effects on ancient Aleuts and their causes: Quaternary Research, v. 4, n. 3, p. 264-281.[2] Preliminary volcano-hazard assessment for Okmok Volcano, Umnak Island, Alaska, 2005
Beget, J.E., Larsen, J.F., Neal, C.A., Nye, C.J., and Schaefer, J.R., 2005, Preliminary volcano-hazard assessment for Okmok Volcano, Umnak Island, Alaska: Alaska Division of Geological & Geophysical Surveys Report of Investigation 2004-3, 32 p., 1 sheet, scale 1:150,000.
full text of report 3.4 MB
map sheet 53.2 MB [3] Physical volcanology of a sub-plinian and phreatomagmatic eruption at Okmok Volcano, Alaska: implications for explosive mafic volcanism, 2004
Wong, L.J., 2004, Physical volcanology of a sub-plinian and phreatomagmatic eruption at Okmok Volcano, Alaska: implications for explosive mafic volcanism: University of Alaska Fairbanks unpublished M.S. thesis, 117 p.Complete Eruption References
Preliminary volcano-hazard assessment for Okmok Volcano, Umnak Island, Alaska, 2005
Beget, J.E., Larsen, J.F., Neal, C.A., Nye, C.J., and Schaefer, J.R., 2005, Preliminary volcano-hazard assessment for Okmok Volcano, Umnak Island, Alaska: Alaska Division of Geological & Geophysical Surveys Report of Investigation 2004-3, 32 p., 1 sheet, scale 1:150,000.
full text of report 3.4 MB
map sheet 53.2 MB 
Physical volcanology of a sub-plinian and phreatomagmatic eruption at Okmok Volcano, Alaska: implications for explosive mafic volcanism, 2004
Wong, L.J., 2004, Physical volcanology of a sub-plinian and phreatomagmatic eruption at Okmok Volcano, Alaska: implications for explosive mafic volcanism: University of Alaska Fairbanks unpublished M.S. thesis, 117 p.
Geology of Umnak Island, eastern Aleutian Islands, as related to the Aleuts, 1976
Black, R. F., 1976, Geology of Umnak Island, eastern Aleutian Islands, as related to the Aleuts: Arctic and Alpine Research, v. 8, n. 1, p. 7-35.
Late-Quaternary sea level changes, Umnak Island, Aleutians: their effects on ancient Aleuts and their causes, 1974
Black, R. F., 1974, Late-Quaternary sea level changes, Umnak Island, Aleutians: their effects on ancient Aleuts and their causes: Quaternary Research, v. 4, n. 3, p. 264-281.
