Description: From Bacon and others (2014): "The oldest recognized postglacial explosive eruption, andesitic Aniakchak I, left nonwelded ignimbrite in valleys below the edifice and fines-poor welded ignimbrite or agglutinated fall deposits high on its flanks between ca. 9,500 and 7,000 years ago. A small caldera may have collapsed at the source of the ignimbrite, likely just west of the present Vent Mountain. Aniakchak I andesite is geochemically distinct among the postglacial Aniakchak suite for its high K2O, Zr, and other incompatible-element contents."
"Aniakchak I and andesitic Aniakchak II exposures can be indistinguishable in the field. Where stratigraphic evidence for a pre-3,400-yr B.P. age is lacking, however, these units can be readily identified by their chemical compositions. Even though their silica concentrations and crystal contents are similar, Aniakchak I andesite has uniquely high K2O, Zr, and many other incompatible-element concentrations (see "Geochemistry" section [in original text]). Systematic search for Aniakchak I using geochemical fingerprinting has not been done, so the true maximum extent of pyroclastic flows from that eruption remains to be determined."
"Deposits of unconsolidated and partly welded andesite bombs and ash found low in north-northwest flank gully exposures, in Birthday Creek drainage and Birthday Pass, and below the caldera rim north of The Gates have distinctive Aniakchak I chemistry. These are considered to have been emplaced by pyroclastic flows during one or a few closely spaced eruptions that constitute the Aniakchak I event. The deposits in valleys clearly postdate the last major glacial advance. Large patches of Aniakchak I partly welded ignimbrite, consisting of flattened bombs as large as ~1 m and lithic fragments in an indurated matrix, are present on north and south sides of the floor of the Birthday Pass beheaded glacial valley at elevations between ~2,100 ft (640 m) and ~2,450 ft (750 m) asl (fig. 6B; see also fig. 10A [in original text]). Vertical cooling joints cut the entire ~30-m-thick exposure on the north side of the valley, where differences in induration suggest three or more flow units within a single cooling unit. Possibly, cliff exposures of these remnants of partly welded Aniakchak I ignimbrite were quarried by an early Holocene glacier that originated higher on ancestral Aniakchak, before formation of the caldera that we associate with Aniakchak II. The implication is that the Aniakchak I eruption was not accompanied by caldera collapse or, if a caldera formed, it either was too small to engulf the high-edifice source of the Birthday Creek glacier or it rapidly filled with ice that then spilled out Birthday Pass. Alternatively, presence of partly welded Aniakchak I ignimbrite on either side of Birthday Pass but not in the center of the valley could reflect deposition of ignimbrite along the margins of a glacier that occupied the upper valley at the time of the eruption. In that scenario, nonwelded ignimbrite lower in Birthday Creek valley was deposited in the valley floor below the terminus of the glacier, now marked by a low north-north-west-trending ridge that may be the glacier’s terminal moraine 1.8 km west of the pass at ~1,850 ft (560 m) elevation (see fig. 7 [in original text]). In that case, the glacier could have been beheaded by caldera collapse associated with the Aniakchak I eruption."
The Global database on large magnitude explosive volcanic eruptions (LaMEVE; 2016) reports a magnitude of 6.0, bulk eruptive volume of 30 cubic km and a dense rock equivalent eruptive volume of 12 cubic km for the Aniakchak I caldera-forming eruption.