Explosive to effusive transition during the largest volcanic eruption of the 20th century (Novarupta 1912, Alaska)

Abstract

Silicic volcanic eruptions commonly show abrupt shifts between powerful and dangerous (Plinian) explosive episodes and gentle effusion of lava. Whether the onset of magma permeability and ensuing gas loss controls these transitions has been a subject of debate. We measured porosities and permeabilities in samples from the A.D. 1912 eruption of Novarupta volcano, Alaska, and analyzed them within the context of a well-constrained eruptive sequence that encompasses sustained explosive and effusive activity. For the explosive samples, we find that the degree of vesicle interconnectivity, measured as the ratio of connected to total porosity, decreases with phenocryst content and with increasing eruption intensity. Permeabilities of explosive samples show a weak dependence on porosity. Dome samples are not significantly different in permeability, but are of lower porosity, which together with abundant flattened vesicles is consistent with bubble collapse by permeable outgassing. Quantitative analysis indicates that outgassing alone was insufficient to affect the transition to effusive activity. Rather, the change from explosive to effusive activity was probably a consequence of high versus low magma ascent rates.

Magma Fragmentation

“…Consequently, fragmentation of permeable magma requires an increase in fragmentation overpressure of approximately 10 5 √ k/φ relative to impermeable magma, where k is the Darcian permeability (Figure 8) (Mueller et al 2008). To what extent this modulates explosive activity remains a subject of debate (Nguyen et al 2014). By the same token, permeable magma does not need to fragment to ash-sized particles in order to release the compressed vapor from within bubbles (Klug & Cashman 1996) and transition to a low-viscosity gas-pyroclast mixture.…”

Permeability During Magma Expansion and Compaction

“…The following “natural” high‐silica samples were included for comparison to the Glass Mountain samples: Plinian fall from Little Glass Mountain represents an explosive phase similar age and close proximity to the Glass Mountain samples (Rust & Cashman, ). Plinian fall from the 7.7 ka climactic eruption of Mount Mazama formed Crater Lake caldera, Oregon (CLCP samples of Klug & Cashman, ) and (Klug & Cashman, , excluding the Welded Tuff samples). Plinian fall from Episodes I–III of the 1912 eruption of Novarupta, Alaska (Adams, Houghton, Fagents, & Hildreth, ; Adams, Houghton, Hildreth, ; Fierstein & Hildreth, ; Hildreth & Fierstein, , ), where mass eruption rates of approximately 5, 1.6, and 1.1 × 10 8 kg s −1 , respectively, provide a powerful Plinian reference (Nguyen et al, ). “White microvesicular,” crystal‐poor, rhyolitic Plinian fall from the “Taupo Plinian” (Table ), which is Unit 5 of the 181 CE Taupo eruption, New Zealand (Houghton et al, , ), and provides another high‐intensity Plinian reference with discharge rates of about 10 8 kg s −1 and plume heights between 25 and 37 km. Fall from the bottom part of Unit I of the Upper El Cajete member of Valles Caldera, New Mexico (e.g., Wolff et al, ; Self et al, ) represents an eruption column that may have been in a transitional regime between fall‐ and flow‐producing conditions (Wolff et al, ) and therefore constitutes a distinct end‐member regime of activity within the suite of Plinian samples (Table ). Pumices from the 0.76 Ma Bishop Tuff ignimbrite, Long Valley Caldera, California, were collected in the unconsolidated, extremely poorly sorted, whitish pumice‐rich Sherwin subunit of Ig1Eb (Hildreth & Wilson, ; Wilson & Hildreth, ). Although no estimate of mass discharge rate exists for the caldera‐forming phase of the eruption itself, the mass discharge rate of the precaldera Plinian phase peaked at about 7.5 × 10 8 kg s −1 (Gardner et al, ).…”

“…These samples were deposited from pyroclastic density currents and therefore provide a caldera‐forming, high‐intensity reference (Table ). Rhyolitic (Swanson et al, ) dome samples from the 600 year old Obsidian Dome, California, were the basis for the original outgassing hypothesis by Eichelberger et al (). Samples from Little Glass Mountain and Glass Mountain lava flows represent a later phase of the same eruptions that produced the Glass Mountain Plinian samples (Donnelly‐Nolan et al, ). Some of the samples were analyzed by Rust and Cashman () and additional samples were analyzed by us (Table ). The 1912 Novarupta, Alaska, Episode V formed a lava dome and provides another effusive reference (Nguyen et al, ).…”

From magma ascent to ash generation: investigating volcanic conduit processes by integrating experiments, numerical modeling, and observations

“…Highly nonlinear relationships between magma shearing, degassing and crystallization, and magma permeability and pressurization of the shallow conduit system, or interaction of magma with ground water, control temporal transitions in eruptive style and unsteadiness in eruptive processes. For example, transition from periodic explosions to effusive activity may occur when sufficient permeable outgassing develops, reducing pressurization within the conduit , Kozono and Koyaguchi 2009, Nguyen et al 2014, Spina et al 2016a. Viscous heating near conduit margins [Costa et al 2007a], and frictional heating along faults [Kendrick et al 2014a, b], can locally change effective viscosity , crystallization, volatile exsolution , kinetics and gas loss [ Kendrick et al 2013, which control magma flow cyclicity in lava dome eruptions [Lavallée et al 2012], thus possibly resulting in transient changes in the flow regime.…”