Alagogshak 954000 yBP
Start: 954000 yBP ± 109000 Years [1]
Event Type: Effusive
Description: From Hildreth and others (1999): "...four craggy outliers of ridge-capping andesite 6-10 krn west of the Alagogshak vent (fig. 2 [in original text]) may also have erupted there."
"The northernmost outlier, at peak 3603 on the divide between tributaries of Angle Creek, consists of three, gently northwest dipping, andesite lava flows (57-61 percent SiO2), each as thick as 100 m. Thick flow-breccia and glass columnar zones suggest ice-contact emplacement. The southernmost (and smallest) of the outliers, capping peak 4647 on the divide between Kejulik River and Takayofo Creek, is a single lava flow of silicic andesite (62 percent SiO2) as thick as 200 m. Between them, on the Takayofo-Angle Creek divide, a third outlier consists of coarse flow-breccia and four lava flows, three of which are atypically mafic (52-53 percent SiO2) for Alagogshak and unusually rich in big clinopyroxene and plagioclase phenocrysts. Overlying these on the northwestern spur of the outlier (peak 4281), the fourth lava consists of a 100-m-thick flow of ordinary Alagogshak-type andesite (58 percent SiO2). Finally, the fourth and westernmost outlier, which caps the ridge dividing two tributaries of Takayofo Creek (fig. 2 [in original text]), consists of a single phenocryst-rich andesite lava flow (61-62 percent SiO2), as thick as 200 m. Shew and Lanphere (1992) reported a K-Ar age of 954+/-109 ka for plagioclase separated from this flow."
"The northernmost outlier, at peak 3603 on the divide between tributaries of Angle Creek, consists of three, gently northwest dipping, andesite lava flows (57-61 percent SiO2), each as thick as 100 m. Thick flow-breccia and glass columnar zones suggest ice-contact emplacement. The southernmost (and smallest) of the outliers, capping peak 4647 on the divide between Kejulik River and Takayofo Creek, is a single lava flow of silicic andesite (62 percent SiO2) as thick as 200 m. Between them, on the Takayofo-Angle Creek divide, a third outlier consists of coarse flow-breccia and four lava flows, three of which are atypically mafic (52-53 percent SiO2) for Alagogshak and unusually rich in big clinopyroxene and plagioclase phenocrysts. Overlying these on the northwestern spur of the outlier (peak 4281), the fourth lava consists of a 100-m-thick flow of ordinary Alagogshak-type andesite (58 percent SiO2). Finally, the fourth and westernmost outlier, which caps the ridge dividing two tributaries of Takayofo Creek (fig. 2 [in original text]), consists of a single phenocryst-rich andesite lava flow (61-62 percent SiO2), as thick as 200 m. Shew and Lanphere (1992) reported a K-Ar age of 954+/-109 ka for plagioclase separated from this flow."
References Cited
[1] Map showing potassium-argon ages from the Mount Katmai and adjacent parts of the Naknek and Afognak quadrangles, Alaska Peninsula, Alaska, 1992
Shew, Nora, and Lanphere, M. A., 1992, Map showing potassium-argon ages from the Mount Katmai and adjacent parts of the Naknek and Afognak quadrangles, Alaska Peninsula, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF 2021-E, unpaged, 1sheet, scale 1:250,000.[2] Alagogshak volcano: A Pleistocene andesite-dacite stratovolcano in Katmai National Park, 1999
Hildreth, W., Fierstein, J., Lanphere, M. A., and Siems, D. F., 1999, Alagogshak volcano: A Pleistocene andesite-dacite stratovolcano in Katmai National Park: in Kelley, K.D. (ed.), Geologic studies in Alaska by the U.S. Geological Survey, 1997, U.S. Geological Survey Professional Paper PP 1614, p. 105-113.
Complete Eruption References
Alagogshak volcano: A Pleistocene andesite-dacite stratovolcano in Katmai National Park, 1999
Hildreth, W., Fierstein, J., Lanphere, M. A., and Siems, D. F., 1999, Alagogshak volcano: A Pleistocene andesite-dacite stratovolcano in Katmai National Park: in Kelley, K.D. (ed.), Geologic studies in Alaska by the U.S. Geological Survey, 1997, U.S. Geological Survey Professional Paper PP 1614, p. 105-113.


Map showing potassium-argon ages from the Mount Katmai and adjacent parts of the Naknek and Afognak quadrangles, Alaska Peninsula, Alaska, 1992
Shew, Nora, and Lanphere, M. A., 1992, Map showing potassium-argon ages from the Mount Katmai and adjacent parts of the Naknek and Afognak quadrangles, Alaska Peninsula, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF 2021-E, unpaged, 1sheet, scale 1:250,000.