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Seguam reported activity

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EVENT SPECIFIC INFORMATION

Event Name : Seguam 9100 yBP

Start: 9100 (± 1000 Years) Years BP Unknown dating method

Pyroclastic flow, surge, or nuee ardente: BibCard
Directed blast: BibCard
Eruption Type:Explosive
MaxVEI: 5 BibCard
Eruption Product: dacite BibCard
ChemYes
Otherfelsic

Description: From Jicha and Singer (2006): "At 9 ka, Wilcox volcano partially collapsed forming a 4-km-wide crater. The sector collapse was accompanied by an explosive lateral blast, which deposited a 0.45 km cubed dacitic ignimbrite (dig) to the northwest of the edifice. This sequence of events was likely similar to those during the May 1980 eruption of Mount St. Helens (Christiansen and Peterson, 1981) and the October 1902 eruption of Santa MarĂ­a volcano in Guatemala (Williams and Self, 1983). The dacitic ignimbrite retains a relatively uniform thickness of 80-100 m for 4 km until it reaches the northern shoreline. Because offshore bathymetry is not available, the distal extent of the ignimbrite is unknown, but it is clearly recognizable 10 km north of the coastline in U.S. Geological Survey (USGS) GLORIA sidescan images. A small outcrop is also preserved along the southeastern rim (Fig. 4 [in original text]). The deposit is characterized by 10-25 cm, dark-gray dacitic pumice clasts or fiamme (64.0 wt% SiO2) hosted in a lighter-gray, fine ash supported matrix of similar composition (64.9 wt% SiO2). The upper 50-70 m of the ignimbrite is nonwelded and poorly consolidated, but the base is strongly welded and contains flattened fiamme (Fig. 6 [in original text]). Four incremental-heating experiments on whole-rock samples and a phenocryst-free separate of the poorly consolidated facies of the ignimbrite gave a weighted mean 40Ar/39Ar age of 8.4+/-1.5 ka. A U-Th mineral isochron age for this sample (defined by cpx + mt + plag + glass + wr) gave an age of 10.1+/-1.3 ka (Jicha and others, 2005). Thus, we infer that emplacement of this ignimbrite and collapse of the stratovolcano occurred at 9+/-1 ka.

The Global database on large magnitude explosive volcanic eruptions (LaMEVE; 2017) reports a magnitude of 4.7, bulk eruptive volume of 0.45 cubic km and a dense rock equivalent eruptive volume of 0.188 cubic km for the eruption.

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