Event Name : Emmons Lake Volcanic Center CFE II-C2/Dawson Tephra
|Start: 26000 || Years BP Ar/Ar || |
|Pyroclastic flow, surge, or nuee ardente: ||
|Eruption Product: || rhyolite ||
|MaxVEI: ||6 ||
|ColHeight: ||35000 m ||
From Waythomas and others (2006): "Major Plinian/ulta-Plinian eruption. Extensive ignimbrite sheets."
"Major explosive events with high column of ash and pyroclastic debris. Extensive ash dispersal and formation of mobile and far traveling pyroclastic flows."
"...the 26,000-year-old event, produced non-welded, highly mobile, pumice-rich pyroclastic flows of rhyolitic composition. The flows crop out over an area of about 3000 km2 surrounding the Emmons Lake volcanic center and up to 60 km from the caldera. A rhyolitic ash fall deposit found 1700 km away in the Klondike region of northwestern Canada has been correlated with the 26,000-year-old event at the Emmons Lake volcanic center (Mangan and others, 2003)."
From Mangan and others (2003): "The C2 eruption produced a 10 km wide, 12 km long, and >215 m deep caldera in the eastern half of the ELVC (Fig. 3 [in original text]). Rhyolitic pumice flows generated by column collapse extend in all directions from the caldera, covering a minimum area of 2500 km2. The ignimbrite sheet flowed away from the caldera and terminated beyond the present coastlines of the Pacific Ocean and the Bering Sea more than 60 km away. The C2 deposits exposed in sea cliffs and stream banks on the north side of the caldera overlie fluvial gravel deposits or fill paleovalleys, indicating that the ignimbrite was directed along preexisting drainages (Fig. 5 [in original text]). Most C2 deposits on the north side of the caldera are overlain by glacial deposits associated with the last glacial maximum (LGM) (Waythomas and others 2001). On the northeast side of the caldera, the ignimbrite was channeled through preexisting valleys where it locally ponded and formed thick, massive accumulations of pumice flow that now exhibit near-vertical gas escape structures. On the south side of the caldera, the ignimbrite flowed out over glacier ice and was reworked into lateral moraines composed almost entirely of pumiceous debris (Fig. 5D [in original text])."
"The most revealing outcrop of a C2 pumice flow is a 25 m thick exposure near the Cathedral River valley on the north east side of the caldera (Fig. 6 [in original text]). At this location, the flow has at least eight distinct subunits. Each subunit is <1-10 m thick, clast supported, and massive to faintly bedded. The subunits are defined by concentrations of angular, pebble-size lithics associated with concave-up to subhorizontal bounding surfaces. The median grain size of the subunits ranges from small pebble to very coarse sand-size material (-2.75phi to 0 phi). Fines in the matrix are characterized by cuspate glass shards and pumice lapilli, with minor lithics and crystals (plagioclase and orthopyroxene with lesser clinopyroxene, and Fe-Ti oxides). The coarse fraction consists of rounded to subrounded pumice clasts a few centimetres to a few decimetres in diameter and smaller, pebble-size, angular to subangular lithics. The pumice is a frothy rhyolite glass containing a few volume percent plagioclase and orthopyroxene with lesser clinopyroxene, Fe-Ti oxide, small apatite needles, and rare zircon. The lowest subunit in the sequence contains the largest percentage of lithics, 70 wt.%; the uppermost subunit has only 15 wt.%."
"Most other outcrops of C2 are massive accumulations of clast-supported pumice, 5-30 m thick. The deposits are frequently stained pink or orange from high temperature alteration and occasionally contain near-vertical fossil fumaroles. The lithic contents are typically less than 10wt.%,and pumice clasts are subrounded to subangular with diameters of 5-70 cm."
"The most prominent tephra bed in the Klondike district, however, is the Dawson tephra, which was first described by Naeser and others (1982) and later by Preece and others (2000), Westgate and others (2000), and Froese and others (2002) who suggest that the source volcano for Dawson tephra is somewhere in the eastern Aleutian arc. In this paper, we present physical, geochemical, and geochronologic data that identify the Emmons Lake Volcanic Center (ELVC) on the Alaska Peninsula (Fig. 1 [in original text]) as the source of Dawson tephra. The ELVC is located approximately 1700 km southwest of the Klondike localities of Dawson tephra and is the site of one of several caldera-forming eruptions on the Alaska Peninsula that produced extensive pyroclastic fall deposits (Miller and Smith 1987)."
"Deposits of Dawson tephra in Yukon Territory are described in Naeser and others (1982), Westgate and others (2000), Preece and others (2000), and Froese and others (2002). The tephra is a fine, light-colored rhyolitic ash (medium silt size, 5.30φ) consisting of 93 wt.% glass and 7 wt.% crystals (plagioclase, orthopyroxene, magnetite, and ilmenite, with trace amounts of clinopyroxene, apatite, and zircon). The tephra consists principally of thin, bubble-wall glass shards. Typical exposures of the tephra show beds of reworked or primary, normally graded tephra 15-30 cm in thickness."
"Deposits of Dawson tephra have been found in at least 20 localities in the placer mining region of the Klondike-Sixtymile area where they are enclosed in loess and muck exposed in mine cuts (Fig. 1 [in original text])."
"We have yet to identify conclusively the specific source vent of the C2 eruption within the ELVC."
From Davies and others (2016): "...while Dawson tephra is found commonly in the Yukon, it has rarely been found in the interior of Alaska. Explanations for this have included invoking a more complicated plume trajectory based on satellite data of modern plume movement (e.g. Beget and others, 2004, 2005), or a paucity of late MIS-2 records around the Fairbanks area (e.g. Muhs and others, 2003; Froese and others, 2006). However, recent work has shown that MIS 2 sediments are present, and that Dawson tephra can be found in locations around Fairbanks, but generally as a thin, poorly preserved layer (Jensen and others, 2016). It is also found as a thick deposit in the Gulf of Alaska (Beget and others, 2004; Jensen and others, 2013), supporting Beget’s hypothesis that complicated plume dynamics are a more likely cause for its limited distribution in the central interior of Alaska."
From Mangan and others (2003): "For the C2 event, we estimate an eruptive volume of >50 km3 and a column height of 30-40 km (above present sea level). It is thus not surprising that thick, distal ash deposits accumulated in the Yukon Territory about 1700 km beyond the source volcano; however, it is surprising that Dawson tephra has not yet been recognized elsewhere in Alaska. If new deposits of Dawson tephra are discovered, these should assist in correlation of late Quaternary stratigraphic records that delimit the beginning of the last glaciation."
From Froese and others (2006): "Dawson tephra erupted in the late winter or spring as evidenced by the interbedding of tephra with thin surface icing horizons, while the bulk of the tephra was mobilized later into the valley bottoms, likely in the same season. The extraordinary thickness of Dawson tephra at sites in the Klondike area thus includes significant reworking of the tephra, with a probable primary thickness of less than 15 cm... Winter deposition of the tephra may have, in part, minimized the terrestrial ecological impacts of the eruption on zonal ‘‘Steppe-tundra’’ vegetation through the retransportation of tephra from hillslopes to the riparian areas, where the tephra became incorporated into local fluvial systems."
The Global database on large magnitude explosive volcanic eruptions (LaMEVE; 2017) reports a magnitude of 6.7, bulk eruptive volume of 50 cubic km and a dense rock equivalent eruptive volume of 21.28 cubic km for the eruption.