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Alagogshak reported activity



Event Name : Alagogshak 954000 yBP

Start: 954000 (± 109000 Years) Years BP K-Ar

Lava flow: BibCard BibCard
Eruption Type:Effusive
Eruption Product: andesite BibCard

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."

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