The pivotal role of Vulcanian activity in ending the explosive phase of rhyolitic eruptions: the case of the Big Obsidian Flow eruption (Newberry Volcano, USA)

Explosive volcanism produces myriad hazards from pyroclastic flows to widespread ejecta dispersal. Effusive eruptions can emit noxious, damaging gases and extensive lava flows. A single eruption of silicic magma can produce both types of activity, sometimes concurrently. Understanding what drives such eruptions, their transitions in styles, and associated hazards requires detailed observations of eruptions and analysis of resulting products. The physical and geochemical characteristics of pyroclasts serve as snapshots of final conduit processes before their egress. Here, we examine the explosive phase of the 640 CE rhyolitic eruption of Newberry Volcano (OR, USA), which began with sub-Plinian pyroclastic fallout and terminated with the effusion of the Big Obsidian Flow. To better understand shallow conduit dynamics, we combine previous work on the geochemistry and textures of obsidian pyroclasts with our detailed physical and textural analyses of > 2000 pumices from 37 layers pulled from 4 pits along the dispersal axis. We analyze the density, mass, and volume distribution of ash and lapilli layers and also examine external the texture of the latter. As with the rhyolitic eruptions of Volcán Chaitén in 2008 and Cordón Caulle in 2011–2012 (both in Chile), we find transitory Vulcanian pulses to be the link between the initial Subplinian phase and subsequent effusive activity. We also concur with recent studies that the syn-eruptive formation of obsidian by ash agglomeration and sintering along the conduit edges above the fragmentation level modulates eruptive style, ultimately leading to the subaerial effusion of rhyolitic lava.