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Explosive to Effusive Transition During the Largest Volcanic Eruption of the 20Th Century (Novarupta 1912, Alaska)

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Explosive to Effusive Transition During the Largest Volcanic Eruption of the 20Th Century (Novarupta 1912, Alaska) Geology, published online on 30 June 2014 as doi:10.1130/G35593.1 Explosive to effusive transition during the largest volcanic eruption of the 20th century (Novarupta 1912, Alaska) Chinh T. Nguyen1*, Helge M. Gonnermann1, and Bruce F. Houghton2 1Department of Earth Science, Rice University, Houston, Texas 77005, USA 2Department of Geology and Geophysics, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA ABSTRACT Episodes I–III gave way to ephemeral dome growth and Vulcanian activ- Silicic volcanic eruptions commonly show abrupt shifts between ity of Episode IV, and subsequently to stable dome growth in Episode V, powerful and dangerous (Plinian) explosive episodes and gentle effu- producing the current Novarupta dome. sion of lava. Whether the onset of magma permeability and ensuing Eruptive products consist of rhyolite and dacite, as well as andesite, gas loss controls these transitions has been a subject of debate. We which played a volumetrically negligible role (see the GSA Data Reposi- measured porosities and permeabilities in samples from the A.D. 1912 tory1). In Episode I, first rhyolite was erupted, followed by rhyolite and eruption of Novarupta volcano, Alaska, and analyzed them within the dacite erupting simultaneously, together with minor andesite. Episodes II, context of a well-constrained eruptive sequence that encompasses III, and IV were dominantly dacite, and Episode V was again rhyolite. sustained explosive and effusive activity. For the explosive samples, Both rhyolite and dacite consist of rhyolitic melt, and the contrast in bulk we find that the degree of vesicle interconnectivity, measured as the chemistry is due to the higher phenocryst content of the dacitic magma ratio of connected to total porosity, decreases with phenocryst con- (Coombs and Gardner, 2001; Hammer et al., 2002), which increases the tent and with increasing eruption intensity. Permeabilities of explo- viscosity of the dacitic magma by about one order of magnitude relative to sive samples show a weak dependence on porosity. Dome samples are the rhyolite (Mader et al., 2013). not significantly different in permeability, but are of lower porosity, which together with abundant flattened vesicles is consistent with PERMEABLE OUTGASSING bubble collapse by permeable outgassing. Quantitative analysis indi- Permeable flow of gases, once coalescing bubbles have formed an cates that outgassing alone was insufficient to affect the transition to interconnected pathway, is thought to occur above a critical porosity, fc, effusive activity. Rather, the change from explosive to effusive activity which can range from ~30% to 70% (Klug and Cashman, 1996). Above was probably a consequence of high versus low magma ascent rates. fc, permeability is thought to increase with porosity to some power, n, as bubbles continue to nucleate, grow, and coalesce. Although theoretical INTRODUCTION values of n fall near 2 (Blower, 2001), in nature they appear to be greater The degassing of magma is of critical importance for determining the (Rust and Cashman, 2011). One hypothesis is that if the rate at which style and intensity of volcanic eruptions (Jaupart and Allegre, 1991; Woods bubbles coalesce is much smaller than the rate at which the magma de- and Koyaguchi, 1994; Dingwell, 1996). The exsolution of magmatic vola- compresses, and new bubbles nucleate and grow, fc and n will increase, tiles into bubbles provides magma buoyancy and drives the ascent of the because coalescence is kinetically limited (Takeuchi et al., 2009). magma through the conduit. Exsolution of dissolved water also increases magma viscosity, which resists bubble growth. Consequently, during METHODOLOGY magma ascent the pressure inside bubbles decreases more slowly than the pressure outside, resulting in overpressure of the exsolved volatiles and Sample Analysis explosive magma fragmentation, once a critical over pressure, DPf, has We measured porosities and permeabilities in representative samples been reached (Mueller et al., 2008). from all five episodes of the Novarupta eruption (see the Data Repository). An important discovery is that magmatic gases can flow through We excluded any dense samples from Episodes IV and V that showed vesic u lar magma and escape (Eichelberger et al., 1986), once bubbles pervasive fine cracking, because these microcracks have an uncertain coa lesce to form persistent interconnected networks. This process, herein origin and would bias permeability measurements. Measured permeabili- referred to as permeable outgassing, may reduce the pressure of magmatic ties are therefore solely due to interconnected vesicles formed by bubble gases and, hence, the potential for sustained explosive fragmentation. The coa lescence. question as to what extent permeable outgassing modulates volcanic erup- tions, in particular the transition between sustained explosive and effusive Numerical Modeling activity, remains a subject of debate (Jaupart and Allegre, 1991;Woods and To assess the possible conditions of magma ascent, fragmentation, Koyaguchi, 1994; Dingwell, 1996; Melnik and Sparks, 1999; Castro and and permeable outgassing, we performed numerical modeling of inte- Gardner, 2008). grated magma ascent and bubble growth for Episodes I–III, as well as for hypothetical scenarios at lower discharge rates (see the Data Repository). THE 1912 ERUPTION OF NOVARUPTA, ALASKA The models constitute isothermal, one-dimensional conduit flow of the To address this question, we characterized the porosities and per- ascending magma, coupled with diffusive bubble growth for both H2O and meabilities of samples from a single eruption, the A.D. 1912 eruption of CO2 (Gonnermann and Houghton, 2012). Among other parameters, the Novarupta in Alaska (Fierstein and Hildreth, 1992; Hildreth and Fierstein, model calculates gas pressure within bubbles, which together with f and 2000, 2012; Adams et al., 2006a, 2006b). From 6 to 8 June 1912, the k1 (where k1 is Darcian permeability; see details in the Data Repository) explosive stage of the Novarupta eruption started with three Plinian epi- allows for the prediction of magma fragmentation. sodes, which lasted ~60 h, albeit with two pauses in eruptive activity. Mass eruption rates for the Plinian Episodes I–III have been estimated at ~5, 1.6, and 1.1 × 108 kg s–1, respectively. This sustained explosive activity of 1GSA Data Repository item 2014261, sample information, methodology for poros ity and permeability measurements, and details on the numerical modeling, is available online at www .geosociety .org /pubs /ft2014 .htm, or on request from editing@ geosociety .org or Documents Secretary, GSA, P.O. Box 9140, Boulder, *E-mail: [email protected]. CO 80301, USA. GEOLOGY, August 2014; v. 42; no. 8; p. 1–4; Data Repository item 2014261 | doi:10.1130/G35593.1 | Published online XX Month 2014 GEOLOGY© 2014 Geological | August Society 2014 of | America.www.gsapubs.org For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 1 Geology, published online on 30 June 2014 as doi:10.1130/G35593.1 100 Episode I Episode II Episode III Episode IV Episode V ) 18 90 3 14 80 Figure 1. Connected porosity as 10 70 function of total porosity, with cumulative volume of erupted Cumulative magma eruption volume magma (inset, as dense rock 6 60 equivalent; Hildreth and Fier- stein, 2012). Red and blue lines 2 are drawn to guide the eye. Epi- 50 sode I samples have distinctly Cumulative volume of erupted magma (km 6 June 7 June 1912 8 June 1912 lower ratios of connected to total porosity than samples of 40 other episodes, which may be a kinetically limited coalescence consequence of kinetically lim- permeable outgassing 30 ited coalescence (i.e., bubble Connected Porosity (%) Plinian I nucleation and growth outpace Plinian II rate at which bubbles coalesce). 20 Plinian III Pumiceous IV 10 Dome V Dome V w/ cracks 0 0 10 20 30 40 50 60 70 80 90 100 Total Porosity, φ (%) RESULTS erupting magma to have reached DPf, the characteristic viscous time, tvis is 2 ~h/DPf, and the characteristic permeable time, tk is ~L m/k1DPf, both had to Porosity exceed the characteristic decompression time, tdec ~DP/Ṗ. Here h is melt Sample porosities range approximately between 30% and 60% for viscosity, L ~100 m is the characteristic path length of permeable gas flow, Episode V and 60%–85% for Episodes I–IV (Fig. 1). All samples have m ~10–5 Pa∙s is the viscosity of the vapor phase (Rust and Cashman, 2011), connected porosity that is lower than total porosity by ~10%–20% (i.e., and Ṗ is the decompression rate. The condition tvis tdec implies that there not all vesicles are interconnected). Episode I samples have a distinctly is insufficient time for bubble growth during decompression, resulting in lower ratio of connected to total porosity than all other samples. Given the the build-up of overpressure. At the same time, tk tdec indicates that there high eruption rates of Episode I, this may be due to less time for bubble is insufficient time for permeable outgassing, providing an upper bound, coalescence relative to the other episodes, although Episode I samples are k L2m/h, at which permeability does not prevent the build-up of overpres- also of substantially lower phenocryst content (1%–3%) than those from sure. Because the fall deposits from Episodes I–III lack textural evidence Episodes II–IV (30%–50%), which may also affect bubble coalescence for pervasive shear fragmentation, the shortest pathway for permeable out- (Takeuchi et al., 2009; Rust and Cashman, 2011). The trend in connected gassing is radially toward the conduit margins, where flow fracturing may versus total porosity for the dome samples of Episode V is similar to that provide enhanced permeability (Stasiuk et al., 1993; Gonner mann and for Episodes II–IV, but at distinctly lower total porosities (Fig. 1). This, Manga, 2003; Tuffen et al., 2003). together with the flattened vesicle shapes of Episode V samples, is consis- At magmatic temperatures (850 °C; see the Data Repository) and tent with bubble collapse and loss of porosity during permeable outgas- pressures similar to DPf, the water content of the Novarupta rhyolitic sing (Eichelberger et al., 1986; Westrich and Eichelberger, 1994).
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