Eruptive and shallow conduit dynamics during Vulcanian explosions: insights from the Episode IV block field of the 1912 eruption of Novarupta, Alaska
Résumé
The study of ~1300 juvenile and lithic blocks from a Vulcanian phase of the 1912 eruption of Novarupta provides new insight into the state of the magma as an eruption passes from sustained Plinian to dome growth. Blocks that were predominantly ballistically ejected were measured and sampled within an ~2–3-km radius from vent and supply a picture of a dynamic and complex shallow conduit prior to magma fragmentation in repeated small explosions. Extreme conduit heterogeneity is expressed in the diverse range of dacitic block types, including pumiceous, dense, banded, and variably welded breccia clasts, all with varied degrees of surface breadcrusting. We present new maps of block lithology and size, making Episode IV the most thoroughly mapped Vulcanian deposit to date. Sectorial regions rich in specific lithologies together with the block size data suggest multiple, small explosions. Modeling of block trajectories to reproduce the field data indicates that ejection velocities range from 50 to 124 m/s with a median of ~70 m/s. We propose that individual explosions originated from a heterogeneous shallow conduit characterized both by the juxtaposition of magma domains of contrasting texture and vesiculation state and by the intimate local mingling of different textures on short vertical and horizontal length scales at the contacts between these domains. In our model, each explosion disrupted the conduit to only shallow depths and tapped diverse, localized pockets within the conduit. This contrasts with existing models for repetitive Vulcanian explosions, and suggests that the dynamics of Episode IV were more complex than a simple progressive top-down evacuation of a horizontally stratified conduit.