Description: From Bacon and others (2014): "Half Cone is the crescentic remnant of an andesite-dacite composite edifice that abuts the northwest caldera wall, its unsupported southeast half having been destroyed during explosive eruptions late in this volcano's life (fig. 13A [in original text])."
"In the north wall of Half Cone, coarse poorly sorted pumice fall layers form wedges tapering away from both sides of the truncated, crater-filling dome (fig. 13C [in original text]). Beds of the lowest approximately one-third of the western wedge, which are rich in coarse, relatively dark dense clasts, are crosscut by lighter colored pumiceous beds of the middle third. A small remnant of a lava dome rests on the latter beds and is overlain by the uppermost third of the wedge beds, which are intermediate in color. These deposits are overlain by the ca. 400 yr B.P. Pink Pumice fall sequence (see next section [in original text]). Thick exposures of lava in the west wall are compositionally identical to the crater-filling dome and may be equivalent. Xenoliths of distinctive white felsite are common in these dacite lavas. The northeasternmost lava in the Half Cone wall, thick dacite that has prominent vitrophyre columns above its base, is likely correlative with the crater-filling dome judging from its proximity and similar appearance. The western near-vent fall-deposit wedge of Half Cone lies on silicic andesite (63 weight percent SiO2) lava of unique composition known only from that locality. Subhorizontal vitrophyre columns ~6-10 cm across, perpendicular to a caldera-facing vertical joint surface that strikes east-northeast, suggest that this lava may have effused from a buried vent near the northwest caldera wall and encountered ice or deep snow in the caldera before the eruption of Half Cone."
"A series of Plinian eruptions ca. 400 yr B.P. produced widespread pumice falls and destroyed much of the original Half Cone edifice (Neal and others, 2001; Browne, 2006). Pyroclastic flows and surges affected much of the caldera floor. Canyons to the east of Half Cone expose more than 40 m of pyroclastic-flow deposits related to this event. Outside the caldera, the pumice fall is 10 cm thick at least 50 km to the north. Fine ash may extend as far as 330 km (Riehle and others, 1999). Total erupted volume is estimated at 0.75 to 1.0 km3 (Neal and others, 2001). The areally extensive units are two volumetrically subequal, successive Plinian fall deposits known by the informal unit names of the Pink Pumice (fig. 13F [in original text]) and the overlying Brown Pumice; the contact between Pink and Brown Pumice falls is gradational through a modest thickness. The dacitic Pink Pumice (63-67 weight percent SiO2) is highly vesicular and carries few phenocrysts. Two horizons within the Pink Pumice deposit have relatively coarse pumice and lithic clasts. The normally graded Brown Pumice layer consists of relatively crystal-rich andesite pumice (58-62 weight percent SiO2). White felsite (77 weight percent SiO2), granitic, and crystal tuff xenoliths are common in the fall deposits. Five radiocarbon dates are for materials associated with the Pink Pumice (table 2 [in original text]). We consider the weighted mean of 380+/-50 yr B.P. (table 1 [in original text]) for the two youngest dates, which are for wood and for which stratigraphic context is clear, to provide the most accurate constraint on the Pink Pumice eruption."
From Neal and others (2001): "The...eruption occurred at Half Cone, at the base of the northwest caldera wall (fig. 5 [in original text]), and was one of the most violent events in recent history at Aniakchak. An estimated 0.75 to 1.0 km 3 of material (about the size of the May 18, 1980, eruption of Mount St. Helens, excluding the debris avalanche) destroyed a preexisting edifice at Half Cone and inundated most of the caldera floor with pyroclastic flows, surges, and fallout many meters thick. Near the vent, these deposits are more than 40 m thick. A 10-cm accumulation of pumiceous lapilli and coarse ash from this eruption can be found at least 50 km to the north, at the surface or just below the modern root mat, and fine ash may extend as far as 330 km (Riehle and others, 1999). During the final phase of this eruption, a lava flow filled the basin formed during the collapse of Half Cone."
The Global database on large magnitude explosive volcanic eruptions (LaMEVE; 2016) reports a magnitude of 4.9, bulk eruptive volume of 0.875 cubic km and a dense rock equivalent eruptive volume of 0.150 cubic km for the eruption.