University of Birmingham The evolution of volcanic systems following sector collapse Watt, Sebastian DOI: 10.1016/j.jvolgeores.2019.05.012 License: Creative Commons: Attribution-NonCommercial-NoDerivs (CC BY-NC-ND) Document Version Peer reviewed version Citation for published version (Harvard): Watt, S 2019, 'The evolution of volcanic systems following sector collapse', Journal of Volcanology and Geothermal Research, vol. 384, pp. 280-303. https://doi.org/10.1016/j.jvolgeores.2019.05.012 Link to publication on Research at Birmingham portal Publisher Rights Statement: Checked for eligibility: 25/06/2019 General rights Unless a licence is specified above, all rights (including copyright and moral rights) in this document are retained by the authors and/or the copyright holders. 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Take down policy While the University of Birmingham exercises care and attention in making items available there are rare occasions when an item has been uploaded in error or has been deemed to be commercially or otherwise sensitive. If you believe that this is the case for this document, please contact [email protected] providing details and we will remove access to the work immediately and investigate. Download date: 06. Oct. 2021 1 2 3 4 1 The evolution of volcanic systems following sector collapse 5 2 6 7 3 Sebastian F.L. Watt 8 4 School of Geography, Earth and Environmental Sciences, University of Birmingham, 9 10 5 Edgbaston, Birmingham B15 2TT, U.K. 11 12 6 e-mail: [email protected] 13 7 tel: 0121 414 6131 14 15 8 16 9 Abstract 17 18 10 Sector collapses affect volcanic edifices across all tectonic settings and involve a rapid 19 20 11 redistribution of mass, comparable in scale to the largest magmatic eruptions. The eruptive 21 12 behaviour of a volcano following sector collapse provides a test of theoretical relationships 22 23 13 between surface loading and magma storage, which imply that collapse-driven unloading 24 14 may lead to changes in eruption rate and erupted magma compositions. Large sector collapses 25 26 15 are infrequent events globally, with all historical examples being relatively small in 27 28 16 comparison to many of the events documented in the geological record. As a result, 29 17 exploration of the impacts of sector collapse on eruptive behaviour requires detailed 30 31 18 investigation of prehistoric collapses, but this is often hindered by poorly-resolved 32 19 stratigraphic relationships and dating uncertainties. Nevertheless, observations from a number 33 34 20 of volcanoes indicate sharp changes in activity following sector collapse. Here, a global 35 21 synthesis of studies from individual volcanoes, in both arc and intraplate settings, is used to 36 37 22 demonstrate a number of common processes in post-collapse volcanism. Multiple examples 38 3 39 23 from large (>5 km ) sector collapses in arc settings show that collapse may be followed by 40 24 compositionally anomalous, large-volume and often effusive eruptions, interpreted to 41 42 25 originate via disruption of a previously stable, upper-crustal reservoir. These anomalous 43 26 eruptions highlight that magma compositions erupted during periods of typical (i.e. 44 45 27 unperturbed by sector collapse) volcanism may not be representative of the range of 46 47 28 compositions stored within a vertically extensive crustal reservoir. If eruptible magma is not 48 29 present, upper-crustal reservoirs may rapidly solidify following collapse, without further 49 50 30 eruption, allowing more mafic compositions to ascend to the surface with only limited upper- 51 31 crustal modification, resulting in edifice regrowth at temporarily elevated eruption rates. 52 53 32 Subsequent re-establishment of an upper-crustal reservoir further supports a relationship 54 55 33 between surface loading and crustal storage, but long-term chemical and mineralogical 56 34 differences between pre- and post-collapse evolved magmas imply that a newly-developed 57 58 1 59 60 61 62 63 35 reservoir can overprint the influence of a preceding reservoir, forming a spatially and 64 36 compositionally distinct plumbing system. These broad patterns are replicated in intraplate 65 66 37 settings, despite differences in scale and melting processes; current evidence suggests that 67 38 post-collapse evolution of intraplate volcanoes can be explained by unloading-induced 68 69 39 destabilisation of the magma plumbing system, rather than increased melt production. What 70 71 40 emerges from an apparently diverse set of observations is a systematic behaviour that 72 41 strongly supports a coupling between edifice growth and magma ascent, storage and 73 74 42 pressurisation. Eruption rates, erupted compositions, and the style of volcanism at any 75 43 particular system may thus be modulated from the surface, and long-term shifts in surface 76 77 44 behaviour may thus occur without any changes in the deep parts of magmatic systems. 78 79 45 Observations of sharp post-collapse changes in erupted compositions, including the ascent of 80 46 primitive mafic magmas, also require a crystal-dominated mid- to upper-crustal reservoir, 81 82 47 consistent with recent models of crustal magmatic systems. 83 48 84 85 49 Keywords: sector collapse; magma storage; eruptive behaviour; edifice growth and 86 87 50 destruction; debris avalanche 88 51 89 90 52 Highlights 91 53 • Global synthesis of volcano-magmatic evolution following sector collapse 92 93 54 • Decompression-driven reservoir disruption leads to anomalous post-collapse 94 95 55 eruptions 96 56 • Rapid regrowth via mafic volcanism; subsequent re-establishment of shallow storage 97 98 57 • Implies direct modulation of shallow plumbing system development by surface load 99 100 58 • Common patterns in intraplate and arc settings 101 59 102 103 60 1. Introduction 104 61 Volcanic edifices across all tectonic settings are prone to structural instability and the 105 106 62 generation of large landslides (Ui, 1983; Siebert, 1984), resulting in a redistribution of 107 108 63 volcanic rock across the surrounding land surface. Landslides formed by edifice collapse 109 64 span a wide range of dimensions, and their scars have been identified on volcanoes ranging 110 111 65 from submerged seamounts to large ocean islands, and across subaerial composite volcanoes 112 66 in both arc and intraplate settings (Siebert et al., 1987, 2006; Moore et al., 1989; Deplus et al., 113 114 67 2001; Coombs et al., 2007; Staudigel and Clague, 2010). The triggers for such landslides are 115 116 117 2 118 119 120 121 122 68 varied. Although some, such as the sector collapse of Mount St Helen’s in 1980, are directly 123 69 associated with large magmatic eruptions (Glicken, 1996), structural failure is not always 124 125 70 linked with magma ascent (Siebert et al., 1987; McGuire, 1996). 126 71 127 128 72 Historical observations and deposit characteristics suggest that structural failure of volcanic 129 130 73 edifices generally occurs in a sudden, catastrophic event (although this may follow a long 131 74 period of more gradual flank spreading (e.g., Moore et al., 1989; Neri et al., 2004; Wooller et 132 133 75 al., 2004; Karstens et al., 2019); and failure itself may occur over several, shortly-spaced 134 76 stages (Glicken, 1996; Hunt et al., 2013)). The base of the failure plane in large edifice 135 136 77 collapses may lie deep within the volcano structure (Crandell, 1989; Glicken, 1996; Watt et 137 138 78 al., 2014) and even intersect basement rock (e.g., Wadge et al., 1995; Shea et al., 2008), and 139 79 in many cases cuts the central conduit. The mobilised mass may be remarkably large, in some 140 141 80 cases accounting for >10% of the edifice volume. Sector collapses thus profoundly alter both 142 81 the morphology of a volcano and the distribution of mass above an active magma plumbing 143 144 82 system, potentially reducing the thickness of overlying rock by a kilometre or more. 145 146 83 147 84 Theoretical analyses suggest that mass redistribution following edifice collapse can influence 148 149 85 pressurisation and failure conditions in stored magma bodies (Pinel and Jaupart, 2005; Pinel 150 86 and Albino, 2013). It is therefore plausible that major collapses may be followed by changes 151 152 87 in eruption rate or style, or in the composition of erupted magma. Anecdotal evidence from 153 88 several individual volcanoes supports this idea (e.g., Tibaldi, 2004; Hora et al., 2009; 154 155 89 Manconi et al., 2009), but it is not clear that there is a common pattern to post-collapse 156 157 90 activity. In addition, some volcanoes show no apparent change in behaviour following large- 158 91 scale edifice failure (e.g., Ponomareva et al., 2006; Zernack et al., 2012). Given the diversity 159 160 92 of volcanic systems affected by edifice collapse, this is perhaps unsurprising, but the limited 161 93 current understanding of the impacts of sector collapse on volcano-magmatic processes 162 163 94 provides the motivation for this work.