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Edgecumbe reported activity




Event Name : Mt Churchill White River East Lobe

Start: 1147 Years BP C-14 (cal)

Tephrafall: BibCard BibCard BibCard BibCard BibCard BibCard BibCard
Eruption Type:Explosive
Eruption Product: rhyolite BibCard
MaxVEI: 6 BibCard
ColHeight: 22600 m BibCard

Description: From Preece and others (2014): "The White River Ash (WRA) is an important Holocene chronostratigraphic marker through-out eastern Alaska (USA) and in Yukon and western Northwest Territories (Canada) (Lerbekmo and Campbell, 1969; Pewe, 1975), and provides age control for peat studies (Harris and Schmidt, 1994; Robinson and Moore, 1999, 2000), glacial fluctuations (Denton and Karlen, 1977), paleoecological studies (Slater, 1985), and archaeological and anthropological studies (Workman, 1979; Moodie and others, 1992) (Fig. 1 [in original text]). WRA has been defined by its geographic distribution and its stratigraphic position, combined with radiocarbon dating, with less emphasis on its petrographic and geochemical features. The northern lobe erupted between A.D. 150 and 500 (Jensen and Froese, 2006) or ca. 1900 yr B.P. (Lerbekmo and others, 1975), while the eastern lobe erupted A.D. 803 or 1147 cal. yr B.P. (Clague and others, 1995). Cryptotephra studies in lake and peat cores have extended the distribution of the eastern lobe to southeastern Alaska (sites A-C, Fig. 1 [in original text]), northern British Columbia (site D, Fig. 1 [in original text]), Newfoundland, northern Europe, and Greenland (Lakeman and others, 2008; Payne and others, 2008; Addison and others, 2010; Pyne-O’Donnell and others, 2012; Jensen and others, 2012)."

"The exact vent area of the WRA has been disputed, and two possible locations have been suggested. Lerbekmo and Campbell (1969) postulated that the WRA vent was located on the floor of a deep valley beneath the Klutlan Glacier near what they identified as a large pumice mound (4 in Fig. 2 [of original text]). McGimsey and others (1992) and Richter and others (1995) questioned the likelihood of a vent located at the floor of a valley lacking an edifice or nearby volcano, and proposed that Mount Churchill was the vent. Lerbekmo (2008) questioned whether Mount Churchill was a volcano, and reiterated that the vent for WRA was underneath the Klutlan Glacier."

From Richter and others (1995): "The White River Ash is a bilobate Plinian fallout deposit covering more than 340,000 km square and containing an estimated 25-50 km cubed of tephra (bulk volume) (Bostock 1952; Berger 1960) (Fig. 1 [in original text])."

From Lerbekmo and others (1975): "A northern lobe and an eastern lobe have axis lengths in excess of 500 and 1000 km respectively."

"Unfortunately, glass compositions are similar in the northern and eastern lobes of the WRA tephra, and published analyses cover a significant range, making it difficult to assign a particular tephra sample to a specific lobe (Addison and others, 2010; Payne and others, 2008; Lakeman and others, 2008; Froese and Jensen, 2005; Richter and others, 1995; Beget and others, 1992; Downes, 1985)."

"Tephra samples of the WRA contain phenocrysts of plagioclase, amphibole, magnetite, ilmenite, and trace amounts of orthopyroxene and apatite within highly vesicular, frothy color-less glass (Table S8 in the Supplemental File [see footnote 1] [in original text]). Biotite occurs in the eastern lobe in trace amounts (<~1%) in some pumice clasts from locations 4 and 6 (Fig. 2 [in original text])."

"Previous studies have demonstrated that the eastern and northern lobes of the WRA have different ilmenite compositional ranges (Lerbekmo and others, 1975; Downes, 1985; Richter and others, 1995) and this distinction has been used to define geochemical groups WRA-E and WRA-N."

"WRA deposits can be assigned to WRA-E or WRA-N, and are best identified using ilmenite compositions...Within WRA-E, WRA-Ea is clearly older than WRA-Eb. The 1147 cal yr B.P. (Clague and others, 1995) WRA-Ea deposits either do not show or only weakly display systematic changes in glass or Fe-Ti oxide composition with stratigraphic position. On geochemical plots, WRA-Eb samples are on linear extensions toward higher silica content and lower temperature estimates compared to WRA-Ea samples, strongly suggesting a genetic link. WRA-Eb samples represent a younger eruption or eruptions from the evolving Mount Churchill magmatic system."

From Payne and others (2008): "The LNA 100 tephra shows geochemical similarity to WRA tephra. Dating evidence does not show a consistent sequence of radiocarbon dates but samples from peat containing the ash layer suggest that the tephra was deposited between approximately 1260 and 1375 cal yr BP. The most likely origin of this tephra is therefore one of the WRA eruptions, most probably the younger, eastern lobe event. Clague et al. (1995) presented ten radiocarbon assays on this tephra spanning 791 to 1416 cal yr BP and opted for a weighted mean of four of these dates to assign the eruption an age estimate of ca. 1147 cal yr BP. The dates in this study would suggest an older date for this tephra, although this conclusion is complicated by the dates being out of sequence (Table 6 [in text])."

The Global database on large magnitude explosive volcanic eruptions (LaMEVE; 2017) reports a magnitude of 6.1, bulk eruptive volume of 50 cubic km and a dense rock equivalent eruptive volume of 23 cubic km for the eruption.

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