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Koniuji reported activity



Event Name : Koniuji 4600 yBP

Start: 4600 (± 2000 Years) Years BP Ar/Ar

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

Description: From Jicha (2009): "Two experiments from a plagioclase-phyric, mafic andesite (58.1 wt.% SiO2) that flowed over the 5.8 ka unit and formed a headland on the northwest corner of the island gave a 40Ar/39Ar age of 4.6+/-2.0 ka (Figs. 2-4 [in original text]). Large boulder-sized xenoliths of the 5.8 ka lava were observed within the 4.6 ka lava (Fig. 3 [in original text])."

"The composition of the eruptive products evolved slightly as a 4.6 ka mafic andesite (58.1 wt.% SiO2) flowed over the 5.8 ka basaltic andesitic unit and formed a headland on the northwest corner of the island. Several meters of tephra blanket the 4.6 ka and the older lava flows, but these deposits were not thoroughly investigated and may or may not be from Koniuji."

"A broad crescent-shaped scarp, concave to the west, makes up the western half of the Koniuji Island, but this feature is narrower and much smaller in area than the deep seated failures observed at large Aleutian stratovolcanoes such as Kiska, Tanaga, Kanaga, and Great Sitkin (Coombs and others, 2007). I suggest that the steep-sided western edge of the island may be a collapse scar. Because the 5.8 ka basaltic andesitic lava is located within this feature and the northern edge of the scarp is formed by the 4.6 ka andesite, the age of the collapse must be b 4.6 ka. The sector collapse at Koniuji is similar to the thin-skinned failures at Gareloi (178.8 degrees W), which did not excavate deeply into the edifice (Coombs and others, 2007)."

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