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]). The most recently active feeding conduit of Half Cone is marked by the center of the radially symmetrical Cobweb dacite flow (fig. 13B [in original text])."
"The final product of the Half Cone vent was crystal-rich dacite (~65-66 weight percent SiO2) that spread radially to form the Cobweb lava flow (figs. 13A and B [in original text]), now heavily mantled with tephra of the 1931 eruption. The flow has arcuate pressure ridges that are approximately concentric about a low central cratered cone above the vent, which is located somewhat west of center of the 1.5 km (north-south) by 2 km (east-west) lava field (fig. 10A [in original text]). At least six rifts traverse the flow surface from the center to the edges like spokes of a wheel. The longest of these appears to have channeled late-erupted lava northeast all the way to the margin of the flow field. Late-moving lava also emerges as short toes from the north and south margins of the field. The lava terminates on the northwest near the base of the Half Cone cliff. We adopt the name Cobweb for this lava flow after B.R. Hubbard (1932), who called it the "Avernian Cobweb." "The poisonous vapors of Avernus, Italy, killed many birds, and Hubbard witnessed the same at Aniakchak, which seems why he adopted this name" (W. Hildreth, written commun., 2014)."