Alagogshak 954000 yBP

Start: 954000 yBP ± 109000 Years [1]

Event Type: Effusive

Event Characteristics:

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."

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.
full-text PDF 504 KB

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.
full-text PDF 504 KB

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.