Under the surface: Textural analysis of complex, multi-component Vulcanian bombs produced during the hybrid effusive-explosive phase of the 2011-2012 Cordón Caulle eruption, Chile

The ascent, eruption, and deposition of volcanic pyroclasts is complex, but the resultant rocks have distinctive textural markers that indicate the unseen processes that were operating during a given eruption. These textures can be used to build a picture of the sequence of events and the eruptive environment. Vulcanian eruptions, short-lived, intermittent blasts interpreted as the clearing of a conduit plug, produce ballistic pyroclasts with textures that are directly correlated with the makeup of the plug material. A late phase of the recent eruption of Puyehue-Cordón Caulle (2011-2012, Southern Chile) produced a striking array of, colourful, and texturally diverse Vulcanian bombs. The eruption began on June 4th 2011 with Plinian to Sub-Plinian activity, transitioning to a phase of obsidian lava effusion on June 15th, and then to a hybrid effusive-explosive phase (vulcanian bomb ejection coeval with an effusive obsidian lava flow) in January 2012. Transitions from explosive to effusive activity are often described as singular, definitive, one-way events, at odds with the hybrid effusive-explosive activity seen at Puyehue-Cordón Caulle. Textures in these bombs indicate that the constituent melts have experienced several (possibly countless) episodes of fragmentation, sintering, densification, shearing, and vesiculation within a conduit-scale breccia pack, conceptually equivalent to a conduit-scale tuffisite vein. In all examined bombs, centimetre to micron scale clasts of basaltic-andesite (~SiO2 54-55 wt%) are found, with textures that indicate a magmatic origin. Although volumetrically minor, co-mingling of a hotter, mafic magmatic component has implications for the anomalously hot rhyolite, as well as the onset and longevity of the hybrid eruption phase. Textural and geochemical characteristics of bombs elucidate complex processes in the shallow conduit and vent, advancing the understanding of tuffisite veins and Vulcanian eruption dynamics, which are far from straightforward.