Magma degassing during the 1912 eruption of Novarupta, Alaska: Textural analyses of pyroclasts representing changes in eruptive intensity and style
University of Hawaii, Honolulu
Three episodes of explosive activity occurred during the 1912 eruption of Novarupta, Alaska before a switch to effusive dome growth. Episode I was characterized by two chemically distinct magmas erupting in steadily changing proportions with contemporaneous generation of ignimbrites and Plinian fall deposits. Episodes II and III followed and were classical plinian events characterized by a stable plume generating a succession dominated by fall deposits of chemically homogeneous dacitic pumice. Episode IV is defined by the extrusion of a texturally heterogeneous ephemeral dacite dome/plug destroyed through Vulcanian explosions and hence recorded only in an apron of blocks. A rhyolitic melt then emerged and created the stable Episode V dome which occupies the vent today. This dissertation explores the influence of conduit process, e.g., vesiculation and degassing, on these events. Pumice from the waxing and stable Plinian phases (episodes I-III) of the eruption show similar vesiculation histories. Strong, disequilibrium degassing and delayed nucleation and growth of vesicles produced high vesicularities, thin glass walls, high bubble number densities (10 8 to 10 9 cm-3 ), and a predominance of bubbles of diameter 50 to 125 Å’Âºm. Early bubble growth was controlled by diffusion and decompression, but at a bubble size â€šÃ Âº30 Å’Âºm in diameter, coalescence became the dominant manner of growth. However, the earliest erupted rhyolitic pumice from Episode I and the last erupted dacite clasts in Episode III were exceptions with different vesiculation and presumably ascent histories. Early rhyolitic pyroclasts show stretched bubbles which are consistent with shearing during the opening of the conduit. During Episode III, successive samples shift towards a decreased mean vesicularity, and textures are increasingly controlled by bubble collapse which foreshadowed the shift to decoupled degassing and episodes IV and V dome extrusion. Based on the 1912 fall deposits, pauses in eruptive activity were caused by internal changes to melt rheology in the conduit as well as external forces, e.g., changes in vent geometry. On a longer time scale, slowed magma ascent and degassing, probably beginning in portions of the melt along the margins of the conduit, led to a permanent shift from explosive to effusive eruption style.
Minerals Data and Information Rescue in Alaska (MDIRA)