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Discriminating Between Hawaiian and Strombolian Eruption Styles

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Discriminating Between Hawaiian and Strombolian Eruption Styles Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G37423.1 1 Stronger or longer: Discriminating between Hawaiian and 2 Strombolian eruption styles 3 B.F. Houghton1*, J. Taddeucci2, D. Andronico3, H.M. Gonnermann4, M. Pistolesi5, 4 M.R. Patrick6, T.R. Orr6, D.A. Swanson6, M. Edmonds7, D. Gaudin2, R.J. Carey8, 5 and P. Scarlato2 6 1Geology and Geophysics, University of Hawai‘i, Honolulu, Hawaii 96822, USA 7 2Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy 8 3Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Catania, Italy 9 4Department of Earth Science, Rice University, Houston, Texas, 77005, USA 10 5Dipartimento di Scienze della Terra, Università di Firenze, Florence, Italy 11 6Hawaiian Volcano Observatory, U.S. Geological Survey, Hawaii National Park, 12 Hawaii, 96718, USA 13 7Earth Sciences Department, University of Cambridge, Cambridge CB2 3EQ, UK 14 8School of Physical Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia 15 *E-mail: [email protected] 16 ABSTRACT 17 The weakest explosive volcanic eruptions globally, Strombolian explosions and 18 Hawaiian fountaining, are also the most common. Yet, despite over a hundred years of 19 observations, no classifications have offered a convincing, quantitative way of 20 demarcating these two styles. New observations show that the two styles are distinct in 21 their eruptive timescale, with the duration of Hawaiian fountaining exceeding 22 Strombolian explosions by ~300–10,000 seconds. This reflects the underlying process of Page 1 of 15 Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G37423.1 23 whether shallow-exsolved gas remains trapped in the erupting magma or is decoupled 24 from it. We propose here a classification scheme based on the duration of events (brief 25 explosions versus prolonged fountains) with a cutoff at 300 seconds that separates 26 transient Strombolian explosions from sustained Hawaiian fountains. 27 INTRODUCTION 28 Kīlauea, Hawaii, USA, and Stromboli, Aeolian Islands, Italy, are among the most 29 intensely monitored, continually active volcanoes in the world, and their activity has 30 given rise to two of the most frequently used names for eruption styles, Hawaiian and 31 Strombolian. Both styles are also well represented in the recent eruptions at Etna, Italy. 32 Continuity of eruptive activity and of real-time geophysical and geochemical 33 observations makes these three volcanoes natural sites to delineate these eruption styles 34 rigorously. 35 Recent debate within the volcanological community clearly emphasizes that the 36 confusion in characterizing and classifying eruptions has greatly hindered our capability 37 to identify potential eruptive scenarios and assess the associated hazards at these and 38 other volcanoes (Bonadonna et al., 2014). This is particularly crucial in the cases of 39 small-scale eruptions, which are the most frequent, but the most difficult to characterize, 40 mostly due to limited dispersal of the products and/or brief durations. Thus, the 41 characterization and classification of volcanic eruptions are not only crucial to our 42 scientific understanding, but also for hazard and risk assessment, as well as 43 communication to the public. Kīlauea, Etna, and Stromboli are locations of large and 44 growing volcano-tourism operations. Their eruptions pose particular issues for 45 management agencies because the volcanoes are highly accessible. Hawaii Volcanoes Page 2 of 15 Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G37423.1 46 National Park records ~5000 visitors per day to the summit of Kīlauea, while the 47 population of Stromboli increases ten-fold to ~4000 people in the summer tourist season. 48 Etna, a UNESCO world heritage site since 2013, is one of the most visited volcanoes in 49 the world. 50 CLASSIFICATIONS 51 Both eruption names were introduced qualitatively, based on direct observations 52 of eruptions at these volcanoes (Mercalli, 1881; Macdonald, 1972). They were 53 subsequently first classified quantitatively on the basis of deposit characteristics (Walker, 54 1973), using principally the rate at which the products thin with distance from vent as 55 some measure of dispersal of the ejecta, which in turn is a proxy for mass discharge rate 56 (intensity). By these criteria, collectively all Hawaiian and Strombolian eruptions are 57 ‘weak’ with low mass eruption rate, as they have limited ranges of tephra dispersal and 58 form steep-walled pyroclastic cones or ramparts rather than aerially extensive sheet-like 59 deposits. A major issue with the use of the Walker classification for weak eruptions arises 60 because no Hawaiian deposits and no products of eruptions at Stromboli and Etna were 61 used in arriving at this classification. In fact, contrary to the Walker classification, the 62 data presented here show that normal Strombolian activity is weaker (in terms of mass 63 eruption rate, i.e., kg/s), not stronger, than Hawaiian fountains (Fig. 1). Consequently, 64 subsequent classifications avoided delineating Hawaiian and Strombolian, by either 65 excluding Hawaiian (Pyle, 1989) or grouping Strombolian and Hawaiian together 66 (Bonadonna and Costa, 2013). 67 A quantitative demarcation between the two styles, however, would be 68 particularly useful, because eruptive activity at basaltic volcanoes shifts frequently Page 3 of 15 Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G37423.1 69 between both eruptive styles (Spampinato et al., 2012). Three volcanoes, Stromboli, 70 Kīlauea, and Etna, are of exceptional value to address quantitative classification of 71 basaltic explosive eruptions as both duration and erupted mass are known for numerous 72 events. Elsewhere, durations of Strombolian and Hawaiian events are generally well 73 constrained, but there is a paucity of data for erupted mass and hence mass discharge rate, 74 due both to their local dispersal and the high risk in the near-field. For this reason we 75 explore possible classification criteria using initially well-constrained eruptions at 76 Kīlauea, Stromboli and Etna. We then use a larger data set of events of known duration as 77 validation for our new approach. 78 EXPLOSIONS AT STROMBOLI 79 Stromboli, the ‘type locality’ for Strombolian explosions, has shown an 80 extraordinary level and diversity of activity for at least 1300 years (Rosi et al., 2013; 81 Taddeucci et al., 2015). Eruptions have been described qualitatively (Table 1) as normal, 82 major, or paroxysmal explosions (Rosi et al., 2013). Normal activity (Fig. 2) typically 83 involves <20-second-long explosions which eject centimeter- to meter-sized pyroclasts to 84 heights of 50–400 m (Rosi et al., 2013), on time scales of fewer than 5 to more than 25 85 events per hour. Data in Rosi et al. (2013) suggest that the durations of normal explosions 86 range between 1.3 and 30 seconds (mean 7 seconds). In the most detailed analysis of 87 individual events, Patrick et al. (2007) list 136 explosions recorded in June-July 2004 88 with durations between 6 and 41 seconds (average 15 +/ 6 seconds). The erupted mass 89 of normal explosions has been estimated at between 1 and 104 kg (Ripepe et al., 1993; 90 Harris et al., 2013; Gaudin et al., 2014; Bombrun et al., 2015). The high variability of 91 mass ejected during each event also led to classification issues among the normal Page 4 of 15 Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G37423.1 92 Strombolian events (Leduc et al., 2015). Recent use of high speed imagery (Gaudin et al., 93 2014; Taddeucci et al., 2015) shows that each normal explosion consists of multiple sub- 94 second pulses, each releasing a meter-diameter pocket of gas. A similar range of erupted 95 mass and duration was also recorded during normal Strombolian explosions at Yasur 96 volcano, Vanuatu (Fig. 1), during 10–12 July 2011 (Gaudin et al., 2014). 97 Larger events known as “major explosions” are recorded several times each year, 98 while paroxysms occur “every few decades” (Rosi et al., 2013; Gurioli et al., 2013). Both 99 are related to the rapid rise of gas-rich magma and are characterized by durations of tens 100 of seconds to a few minutes and eruptive masses of 105 and 107-108 kg respectively. 101 Although mass discharge rates for paroxysms overlap with those of Hawaiian fountains 102 (Fig. 1), all three types of activity at Stromboli are of short duration, relative to Hawaiian 103 activity. Background activity to all types of explosive eruptions at Stromboli consists of 104 two forms of shallow-derived outgassing: passive gas-streaming and small gas bursts 105 (‘puffing’) (Burton et al., 2007; Harris and Ripepe, 2007). 106 FOUNTAINS AT KĪLAUEA 107 Kīlauea, the reference volcano for Hawaiian fountaining, has been in near- 108 continuous eruption since 1983. Forty-seven Hawaiian fountaining episodes were 109 recorded at Puʻu ʻŌʻō between January 1983 and July 1986, each sustained at fountain 110 heights of 30–470 m for at least 5 h and up to 12 days, erupting 4 × 109 to 7 × 1010 kg of 111 magma (Wolfe et al., 1988). Single fountaining episodes during two other prolonged 112 eruptions, in 1959 and 1969, had fountain heights of 30–579 m, were sustained between 2 113 h and 7 days, and erupted masses of 3 × 109 to 1 × 1011 kg (Richter et al., 1970; Swanson 114 et al., 1979). These fountains are clearly distinguished from any Strombolian explosions Page 5 of 15 Publisher: GSA Journal: GEOL: Geology DOI:10.1130/G37423.1 115 by their longer durations (Fig. 1) despite almost total overlap in erupted mass and mass 116 eruption rates with Strombolian paroxysms. Hawaiian fountains are sustained in the sense 117 that continuous mass discharge is maintained for hours to days, but are also unsteady in 118 nature, i.e., fluctuate in height and mass eruption rate at frequencies of up to 1 Hz (Fig. 119 3).
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