Conduit and Eruption Dynamics of the 1912 Vulcanian Explosions at Novarupta, Alaska.

Abstract

The Vulcanian phase of the 1912 eruption of Novarupta comprised only a very small part of the most voluminous eruption of the twentieth century, yet it served as an important transition from very powerful and sustained explosions to effusive dome growth. The Novarupta eruption is divided into five episodes: Episodes I–III comprised, cumulatively, of 60 hours of strong Plinian explosions that produced voluminous rhyolitic and dacitic ignimbrites and widespread, predominantly dacite, Plinian fall beds. Episode IV produced a dacite block apron, interpreted as the product of complete destruction of a dacite plug/dome via Vulcanian explosions, before extrusion of a rhyolite dome in Episode V. Compensatory caldera collapse during the Plinian episodes occurred 10 km from vent, preserving the ultra-proximal deposits from Episodes I–IV to within 200 m from source. This dissertation explores the pre- and syn-fragmentation conduit and eruption processes occurring throughout Episode IV through careful examination of the uniquely well-preserved Vulcanian block deposit. Block distributions suggest that Episode IV consisted of multiple small explosions that disrupted only small regions of the dome and shallow conduit. Significant textural heterogeneity, including dense, pumiceous, texturally banded, and breadcrusted blocks, and estimates of their source depths reveal that the texturally diverse magmas were juxtaposed to one another within the upper 400 m of the shallow conduit and/or dome. Blocks representing magmas in varying stages of degassing and outgassing imply that very small packages of magma arrived to, and were stored at, their fragmentation depths at/for varying times and durations. Pyroclast textures suggest that the rapid vesiculation of the pumice-forming magma after emplacement at shallow depths likely provided the energy for explosions throughout Episode IV.