Auckland Volcanic Field Magmatism, Volcanism, and Hazard: a Review

Home , Auckland Domain, Auckland volcanic field, Lake Pupuke, Maungarei, Maungataketake, Motukorea

1 This is an Accepted Manuscript of an article published by Taylor & Francis in New Zealand 2 Journal of Geology and Geophysics on 18 March 2020, available online: 3 http://www.tandfonline.com/10.1080/00288306.2020.1736102 4 5

6 Auckland Volcanic Field magmatism, volcanism, and hazard: a review

7 8

9 Jenni L Hopkins*1, Elaine R Smid*2, Jennifer D Eccles2, Josh L Hayes3, Bruce W Hayward4, Lucy E McGee5,

10 Kasper van Wijk2, Thomas M Wilson3, Shane J Cronin2, Graham S Leonard6, Jan M Lindsay2, Karoly

11 Németh7, Ian E M Smith2 12 13 *Corresponding Author(s): [email protected] / [email protected] 14

15 1Victoria University of Wellington, PO Box 600, Wellington, New Zealand

16 2 University of Auckland, Private Bag 92019, Auckland, New Zealand

17 3 University of Canterbury, Private Bag 4800, Christchurch, New Zealand

18 4 Geomarine Research, 19 Debron Ave, Remuera, Auckland, New Zealand

19 5 University of Adelaide, Adelaide, Australia

20 6 GNS Science, PO Box 30-368, Lower Hutt, New Zealand

21 7 Massey University, Private Bag 11 222, Palmerston North, New Zealand 22 23 24 25 26 27 Manuscript prepared for submission to the IAVCEI special issue of New Zealand Journal of Geology and 28 Geophysics 29 30 31 1

33 Keywords 34 intraplate, monogenetic volcanism, chronology, tephrochronology, volcanic hazard assessment, faulting, 35 magma ascent rates, geochemistry, eruption scenarios, New Zealand

36 Abstract

37 Auckland Volcanic Field (AVF) is a basaltic intraplate volcanic field in North Island, New Zealand, 38 upon which >1.6 million people live. Seismic velocity tomography and geochemistry both suggest a 39 primary mantle source region at a depth of 70-90 km. Geochemical analysis indicates a range of magma 40 compositions, and that melts ascend with little crustal interaction. Eruptions generally began with a 41 phreatomagmatic phase forming maar and tuff rings with tephra fall, base surges, and ballistic 42 projectiles as the main hazards. Subsequent magmatic phases formed scoria cones, and sometimes 43 produced lava flows. Ages of 47 of the 53 volcanic centres reveal that the AVF first erupted ~193 ka, 44 and last erupted ~500 yrs. BP. These geochronological constraints indicate repose periods ≤0.1-13 kyr, 45 which have decreased since ~60 ka. From known geological and exposure information, and using an 46 interdisciplinary approach, eight future eruption scenarios have been developed for planning processes. 47 Outstanding questions for the AVF concern the cause of mantle melting, the structure of the underlying 48 lithosphere, magma ascent rates, controls on repose periods and eruptive volumes. Answering these 49 questions may improve our understanding of warning periods, monitoring strategies, spatiotemporal 50 risk profiles and socio-economic impacts of volcanism on New Zealand’s largest city.

51 Introduction

52 The Auckland Volcanic Field (AVF; Figure 1a) is a basaltic, intraplate volcanic field underlying 53 New Zealand’s most populated city, Auckland (population ~1.6M; Statistics New Zealand, 2018). It is 54 one of 240+ known Holocene volcanic fields around the world (Cañón-Tapia, 2016). Where volcanic 55 fields overlap with population centres, eruptions pose significant hazard (e.g. base surges, ballistics, ash 56 fall, lava flows) and associated risk (Lindsay et al., 2010, Lindsay, 2016). While eruption volume, 57 duration, and scale are generally much smaller than those at caldera or stratovolcanoes, eruptions in 58 volcanic fields typically occur at a different vent location each time (Valentine and Connor, 2015). 59 Detailed holistic and field-wide studies are required to overcome this spatial distribution challenge for 60 hazard assessments, however, these holistic studies are often infeasible due to limited resources, the 61 number of vents, and/or the areal extent of the field (Valentine and Connor, 2015). As the AVF is 62 younger (~193 ky) and contains fewer centres (53) than most volcanic fields globally, a comprehensive 63 understanding of the whole system is more easily constructed. 64 The AVF was first recognised as a volcanic field and mapped by Heaphy (1860) and Hochstetter 65 (1864). They identified 62 and 63 volcanic centres respectively, however, the number of discrete 66 eruptive centres has been an ongoing matter of discussion (Figure 1b; Searle, 1964; Hayward et al., 67 2011a). In the 1990’s, the accepted number of centres was 48 (Kermode, 1992) but subsequent research 68 identified a difference between volcanic centres and volcanic episodes (Smith and Allen, 1993; 69 Hayward et al., 2011a, 2011b, 2012). For example, Rangitoto exhibits two compositionally and 70 temporally discrete eruption events (Rangitoto 1 and 2) that are spatially proximal (Figure 1b; 71 Needham et al., 2011; McGee et al., 2013; Hopkins et al., 2017). In comparison, Purchas Hill/Te 72 Tauoma and Mt Wellington/Maungarei (Figure 1b) are a compositionally distinct, geographically 73 separate pair, but may have erupted in one episode. Within the last decade, new centres Boggust Park, 74 Cemetery Hill, and Puhinui Craters (Figure 1b) were identified from new 0.5 m resolution Light 75 Detection and Ranging (LiDAR) data. Currently, 53 centres are now recognised in the AVF (Hayward 76 et al., 2012; Hayward 2019; Hayward and Hopkins, 2019). 77 Regionally, the AVF is the most recently active in a series of volcanic fields trending northward to 78 Auckland (from south to north: Okete [2.69 - 1.8 Ma]; Ngatutura [1.83 - 1.54 Ma]; South Auckland 79 (SAVF) [1.59 - 0.51 Ma]; and Auckland [0.19 Ma - present) (Figure 1a; Briggs et al., 1989, 1994; 80 Cook et al., 2004). During the AVF’s ~193 ka lifespan (Leonard et al., 2017; Figure 1c) individual,

81 small batches of basaltic magma (<<0.1-0.7 km3 in volume; Kereszturi et al., 2013) rose from an 82 inferred depth of 70-90 km and formed maars, tuff cones, tuff rings, scoria cones, lava flows and shield 83 volcanoes (Figure 1b) within a ~30 x ~20 km elliptical area (Spörli and Eastwood, 1997; Horspool et 84 al., 2006; Kereszturi et al., 2013; Leonard et al., 2017). At present, there is no firm consensus on the 85 geodynamic processes driving mantle melting and eruptions within the AVF, or the older volcanic fields 86 in the Auckland Volcanic Province (Spörli and Eastwood, 1997; Cook et al., 2004; Hoernle et al., 2006; 4

87 Timm et al., 2010; Shane, 2017; van den Hove et al., 2017). Although most of the Holocene volcanic 88 activity in New Zealand has taken place in the Taupō Volcanic Zone (TVZ; a rifted volcanic arc in the 89 central North Island), driven by melts generated from the south-westerly subduction of the Pacific Plate 90 under the Australian Plate (Figure 1a; Wilson, 1996), there is no evidence linking this region of 91 volcanic activity to the volcanism ~250 km northwest in Auckland. 92 A future eruption in the AVF is a low probability event on human time-scales, but would have high 93 consequences - Auckland is home to ~35% of New Zealand’s total population, and generates 38% of 94 the country’s GDP (Lindsay, 2010; McDonald et al., 2017; Statistics New Zealand, 2019). For this 95 reason, future eruptions have been a long-recognised threat to the city (e.g. Searle, 1961b, 1964a; Smith 96 and Allen, 1993; Molloy et al., 2009; Hurst and Smith, 2010; Leonard et al., 2017). The dispersed nature 97 of eruptions in volcanic fields and the wide range of factors involved mean that a large amount of data 98 is needed to properly assess volcanic hazards and quantify volcanic risk. 99 Coordinated, transdisciplinary efforts, in consideration with emergency management and lifeline 100 priorities, began in the AVF in the last few decades (e.g. Smith and Allen, 1993; Deligne et al., 2015a). 101 The focus of these studies include: potential melt generation triggers (McGee et al., 2011, 2012, 2013), 102 erupted magma composition (Smith et al., 2008; McGee et al., 2012; Hopkins et al., 2017), degree of 103 crustal contamination (McGee et al., 2013; Spörli et al., 2015; Hopkins et al., 2016), tectonic setting 104 (Horspool et al., 2006; Cassidy and Locke, 2010; Le Corvec et al., 2013a, 2013b), local mantle and 105 crustal compositions and properties (Price et al., 2015; Spörli et al., 2015; Hopkins et al., 2016; Brenna 106 et al., 2018), magma ascent rates (Brenna et al., 2018), magma volumes and fluxes (McGee et al., 2013; 107 Kereszturi et al., 2013), local crustal structures and their effect on magmatic plumbing (e.g. Figure 1d; 108 Eccles et al., 2005; Cassidy and Locke, 2010; Kenny et al., 2012; Le Corvec et al., 2013a, 2013b), 109 eruption recurrence rates and spatial patterns of eruptions (Leonard et al., 2017; Hopkins et al, 2017), 110 and their styles, extent, duration, frequency, likelihood and environmental conditions (Lindsay, 2010; 111 Kereszturi et al., 2013, 2014; Agustín-Flores et al., 2015). The findings from these studies have fed into 112 applied hazard and risk focused research including: scenario building and testing (Deligne et al., 2017a; 113 Blake et al., 2017; Stewart et al., 2017; Hayes et al., 2018; Hayes et al., 2020); impact modelling 114 (Deligne et al., 2017b); clean-up and hazard response management (Dolan et al., 2003; Hayes et al., 115 2017); lava flow modelling and impact (Tsang et al., 2019; Tsang et al. in review); qualifying and 116 quantifying damage to infrastructure (Magill and Blong, 2005a, 2005b; Houghton et al., 2006; Wilson 117 et al., 2012, 2014; Deligne et al., 2015b, 2017a, 2017b; McDonald et al., 2017; Williams et al., 2017), 118 including transport networks (Blake et al., 2017) and waterways (Stewart et al., 2017); and volcanic 119 hazard communication, education and outreach (Doyle et al., 2011; Wilson et al., 2014; Dohaney et al., 120 2015; Fitzgerald et al., 2016) and evacuation assessment and planning (Lindsay et al., 2010; Tomsen et 121 al., 2014).

122 In this article, we detail the current understanding of the AVF with respect to its magmatism, 123 volcanism, and hazards, and how researchers have collated and combined this information to create 124 volcanic eruption scenarios, which can be used to forecast and plan for a range of plausible eruption 125 locations, scales, and styles (Figure 1e).

Figure 1. A) Location map for the Auckland Volcanic Field in the context of the North Island, New Zealand. Highlighted in red are all the volcanic fields of the North Island, black triangles show the other currently active volcanoes that could potentially impact Auckland. Green dashed line indicates the trace of the Dun Mountain Ophiolite Belt (DMOB), identified by the Junction Magnetic Anomaly (JMA). Black dashed lines indicate key tectonic traces discussed in the text. B) Location of the centres in the AVF, including their eruptive products and extents numbered from North to South, including their alternate names. C) Ages of the centres (c.f. Table 1 for details) coloured by 50 kyr groupings. D) Structural characteristics of the Auckland region, fault traces are from Kenny et al. (2012), trace of the DMOB from Eccles et al. (2005) and Spörli et al. (2015), diagram adapted from Kermode (1992), colouration shows differing geology described in the text; grey is Murihiku Terrane; brown is Waipapa Terrane; light green is Peridotite bearing DMOB; and dark green is Serpentinite bearing DMOB (from Eccles et al., 2005 and Spörli et al., 2015) . E) Location of the 8 modelled DEVORA scenario eruption sites from Hayes et al. (2018) (see text and Table 2 for details), dashed line indicates the “tight boundary” ellipse proposed for the extent of the field by Runge et al. (2015). Reliability Centre name / Alternate name Preferred Age (ka) Error (2sd) Method Reference Comments Grouping Albert Park 145.0 4.0 Tephrochronology Hopkins et al., 2017 2 Ash Hill 31.8 0.4 14C Hayward, 2008 2 Boggust Park > 130a Morphostratigraphy Hayward and Hopkins 2019 5 Cemetery Crater undated 5 Crater Hill 30.4 0.8 Tephrochronology Hopkins et al., 2017 2 Auckland Domain / Pukekawa 106.0 8.0 Tephrochronology Hopkins et al., 2017 See SM1 2 Grafton 106.5 Morphostratigraphy* Hopkins et al., 2017 See SM1 3 Green Mt / 19.6 6.6 Ar/Ar Leonard et al., 2017 1 Matanginui Hampton Park 57.0 32.0 Ar/Ar Leonard et al., 2017 2 Te Hopua-a-Rangi / 31.0 Tephrochronology & Hopkins et al., 2017 See SM1 3 Gloucester Park morphostratigraphy Kohuora 33.7 2.4 Tephrochronology Hopkins et al., 2017 1 Little Rangitoto / 24.6 0.6 Tephrochronology Hopkins et al., 2017 1 Maungarahiri Māngere Lagoon 59.5 Tephrochronology & Hopkins et al., 2017 3 morphostratigraphy* Māngere Mt / 59.0 20.0 Tephrochronology Hopkins et al., 2017 4 Te Pane o Mataaho Maungataketake / Elletts 88.9 4.8 Ar/Ar Leonard et al., 2017 See SM1 2 Mountain McLaughlins Mt / 48.2 6.4 Ar/Ar Leonard et al., 2017 2 Matukutūreia McLennan Hills / 41.3 2.4 Morphostratigraphy & Hopkins et al., 2017 2 Te Apunga-o-Tainui paleomagnetic Motukorea / Brown's Island 24.4 0.6 tephrochronology Hopkins et al., 2017 2 Mt Albert / Te Ahi-kā-roa-a- 119.2 5.6 Ar/Ar Leonard et al., 2017 1 Rakataura Mt Cambria / Takaroro 42.3 22.0 Ar/Ar Leonard et al., 2017 2 Mt Eden / Maungawhau 28.0 0.6 tephrochronology Hopkins et al., 2017 See SM1 1 Mt Hobson / Ōhinerangi / 34.2 1.8 Tephrochronology Hopkins et al., 2017 4 Ōhinerau Mt Richmond / Otahuhu 30.2 4.2 Tephrochronology Hopkins et al., 2017 2 Mt Roskill / Puketāpapa / 105.3 6.2 Ar/Ar Leonard et al., 2017 2 Reliability Centre name / Alternate name Preferred Age (ka) Error (2sd) Method Reference Comments Grouping Pukewīwī Mt Smart / Raratonga 20.1 0.2 Tephrochronology Hopkins et al., 2017 4 Mt St John / Te Kōpuke / 75.3 3.4 Ar/Ar Leonard et al., 2017 2 Tītīkōpuke Mt Victoria / 34.8 4.0 Tephrochronology Hopkins et al., 2017 4 Takarunga Mt Wellington / 10.0 1.0 Tephrochronology Hopkins et al., 2017 1 Maungarei North Head / 87.5 15.2 Ar/Ar Leonard et al., 2017 2 Maungauika One Tree Hill / 67.0 12.0 Tephrochronology Hopkins et al., 2017 2 Maungakiekie Onepoto / Te Kopua-o- 187.6 Tephrochronology and Hopkins et al., 2017 See SM1 2 Matakarepo morphostratigraphy Ōrākei 126.0 6.0 Tephrochronology Hopkins et al., 2017 2 Otara Hill / 56.5 Morphostratigraphy* Hopkins et al., 2017 3 Te Puke-o-Taramainuku Ōtuataua 24.2 1.8 Tephrochronology Hopkins et al., 2017 See SM1 2 Panmure Basin / 25.2 1.8 Tephrochronology Hopkins et al., 2017 2 Te Kopua Kai-a-Hiku Pigeon Mt / O Huiarangi 23.4 0.8 Tephrochronology Hopkins et al., 2017 3 Puhinui Craters undated 5 Pūkaki Lagoon / Te Pukaki >> 45º Tephrochronology Shane 2005 3 Tapu-o-Poutukeka Pukeiti / 23.7 Morphostratigraphy* This paper See SM1 4 Te Puketapapatanga a Hape Pukewairiki / > 130a Morphostratigraphy Hayward and Hopkins 2019 3 Highbrook Park Puketūtū / 29.8 4.4 Tephrochronology Hopkins et al., 2017 2 Te Motu a Hiaroa Lake Pupuke / Pupuke Moana 193.2 5.6 Ar/Ar Leonard et al., 2017 See SM1 1 Purchas Hill / Te Tauoma 10.9 0.2 14C Lindsay et al., 2011 1 Rangitoto 2 504 cal. yrs. BP 10 yrs 14C Needham et al., 2011 See SM1 1 Reliability Centre name / Alternate name Preferred Age (ka) Error (2sd) Method Reference Comments Grouping (Upper Tephra or surface lava flows) Rangitoto 1 553 cal. yrs. BP 14 yrs 14C Needham et al., 2011 1 (Lower Tephra) Mt Robertson/Sturges Park 24.3 0.8 Tephrochronology Hopkins et al., 2017 3 St Heliers / Glover Park / 161.0 36.0 Tephrochronology and Hopkins et al., 2017 3 Whakamuhu morphostratigraphy Styaks Swamp 19.1 Morphostratigraphy* Hopkins et al., 2017 3 Tank Farm / 181.0 2.0 Tephrochronology Hopkins et al., 2017 See SM1 2 Te Kopua-o-Matakamokamo Taylors Hill / Taurere 30.2 0.2 tephrochronology Hopkins et al., 2017 2 Te Pou Hawaiki > 28.0 Morphostratigraphy Affleck et al., 2001 See SM1 3 Three Kings / 31.0 1.8 Tephrochronology Hopkins et al., 2017 2 Te Tātua-a-Riukiuta Waitomokia / Moerangi / 20.3 0.2 Tephrochronology Hopkins et al., 2017 See SM1 2 Mt Gabriel Wiri Mt / 30.1 - 31.0 Ar/Ar Cassata et al., 2008 1 Te Manurewa o Tamapahore

Table 1. Overview of the currently preferred ages for the Auckland Volcanic Field centres. An expanded version of this table can be found in SM Table 1. 14C calibrations were done in 2011 by CalPal.ª The age of 130 ka indicates geomorphological evidence that shows marine erosion during the last sea level high-stand related to the last interglacial at c. 130 – 120 ka. ºThe age of 45 ka is related to the eruption of the Rotoehu tephra, which at present has an ambiguous age (see discussion in SM 1). *indicates centres where morphostratigraphy suggests contemporaneous eruptions (for example no organic material between successive volcanic deposits); these are given an arbitrary difference in age of 500 years, based on a minimum time it takes to form soil horizons. Ambiguous or controversial ages highlighted in the table are discussed in SM1. Reliability grouping are assigned as per the characteristics outlined in SM Table 3.

126 Magmatism

127 Magma composition 128 Important insights about AVF mantle sources, melt generation, and ascent processes have been 129 gleaned from petrographic observations and a large number of major, trace, and isotopic analyses of the 130 eruptive products (an extensive geochemical database is available in Hopkins et al., 2017). AVF rocks 131 are characteristically porphyritic, consisting mainly of olivine with subordinate clinopyroxene 132 phenocrysts, in a groundmass of plagioclase, clinopyroxene, olivine and accessory iron-titanium oxides 133 (Searle, 1959a, 1959b, 1961a, 1961b, 1965; Heming and Barnet, 1986). The most prominent feature

134 observed in AVF major elements is a negative trend in SiO2 vs. total alkalis, i.e. rocks vary from 135 nephelinite through basanite to subalkaline basalt (TAS classification; Figure 2; McGee et al., 2013). 136 Overall, trace element patterns reveal that AVF mantle sources have a similarity to ocean island basalt 137 (OIB) trends, with high Nb and decreasing abundances of less incompatible elements (Figure 3; Huang 138 et al., 1997; McGee et al., 2013; McDonough and Sun, 1995). Trace element modelling (e.g. La/Yb and 139 Gd/Yb ratios compared to typical MORB compositions; Figure 5) places melting dominantly in the 140 garnet stability zone (i.e. > 60 km deep and likely up to 100 km deep (Figure 4; Smith et al., 2008;

141 McGee et al., 2013). This is supported by the presence of 230Th-excesses (230Th/238U > 1; Huang et al., 142 1997; McGee et al., 2011, 2013), and a broad low-velocity seismic zone at approximately 70-90 km 143 depth under the Auckland region (Horspool et al., 2006). 144 145 Influences on magma composition 146 Detailed, field-wide studies of AVF lavas show that the field is compositionally heterogeneous 147 (McGee et al., 2013, 2015). Two main processes explain the overall chemical variability found in the 148 AVF. The first, and most influential, involves the probable interaction of melts from three different 149 mantle sources (Figure 4, 5) at variable degrees of melting, and the second involves minor evolution 150 of the magma on ascent through assimilation, fractionation, and crystallisation (e.g. Smith et al., 2008; 151 Hopkins et al., 2016). The three different mantle sources proposed by McGee et al. (2013, 2015) 152 include: 1) fertile garnet peridotite which gives rise to the dominant compositions of LREE enriched, 153 alkalic OIB-like basalts, 2) trace element enriched, possibly carbonated veins within the garnet 154 peridotite that give a signature of relatively higher 206Pb/204Pb vs. 207Pb/204Pb and negative Zr-Hf 155 anomalies, and 3) a shallower, cryptically-metasomatised mantle source indicated by positive Sr 156 anomalies, higher fluid mobile/fluid immobile trace element ratios and relatively lower 206Pb/204Pb vs 157 high 207Pb/204Pb in the resultant compositions (Figure 6; McGee et al., 2013, 2015). However, 158 subsequent studies of mantle xenoliths thought to represent the lithospheric mantle composition of 159 Southern Zealandia (e.g. Scott et al., 2016; Scott et al., 2020) have suggested the geochemical data used 160 by McGee et al. (2013; 2015) for mantle modelling in the AVF need updating or at least compared to 161 these new data. Such studies are immensely useful for the petrogenesis of the AVF where no definitive 7

162 mantle xenoliths have so far been found. Although the Sr-Nd isotopic values for the AVF show very 163 little variability, and are similar to the Otago lithospheric mantle region (Scott et al., 2014a, 2014b, 164 2016, 2020), AVF Pb isotope ratios are less radiogenic than basalts from the South Island intraplate 165 volcanic fields (e.g. Scott et al., 2016). McGee et al. (2013) used the variability in Pb-isotopes combined 166 with trace element ratio modelling to suggest the interaction of three mantle sources (Figure 6d). This 167 variability is not observed in the South Island volcanic products. There are therefore some apparent 168 differences in the products of the South Island eruptives in comparison to the AVF, and, although 169 feasible to hypothesise that they would have similar mantle characteristics, this topic needs further 170 investigation before mantle models developed for the South Island can be applied to the AVF, 171 particularly considering the heterogeneity of the mantle beneath Zealandia (e.g. Dalton et al., 2017). 172 Meticulous sampling through the sequence of volcanic deposits at select, individual AVF centres 173 reveals that the compositional variability found across the field can be seen within single centres (Smith 174 et al., 2008; Needham et al., 2011; McGee et al., 2012; Linnell et al., 2016). Studies of single centres, 175 combined with the field-wide geochemical database, have been used to better understand eruptive 176 processes in the AVF. For example, at Motukorea/Browns Island, complex geochemical trends through 177 the eruption sequence are explained by exhaustion of mantle components during the melting event 178 (McGee et al., 2012). At Rangitoto, petrologic work by Needham et al. (2011) importantly revealed that 179 the far larger, subalkaline lava-producing phase of the eruption was preceded by a small volume, 180 alkaline cone-forming eruption. These results hint at the control that mantle source compositions exert 181 on the eventual eruption, and the hazards it produces. 182 183

Figure 2. Total alkali (Na2O + K2O) vs. SiO2 (after Le Maitre 2002) for all whole rock lava samples from centres in the AVF (n=755) data from Hopkins et al (2017) and references therein. Alkaline – subalkaline discrimination line from Irvine and Baragar (1971).

Figure 3. Primitive mantle-normalised multi-element patterns for whole rock lava samples from the AVF, normalization values from McDonough and Sun (1995). All data shown by grey shaded region, three of the centres in the AVF are highlighted for their volume (based on DRE from Kereszturi et al., 2013); Rangitoto2 (large centre, red symbols), Wiri Mt/Te Manurewa o Tamapahore (medium centre, blue symbols), and Purchas Hill/Te Tauoma (small centre, green symbols). Also shown for comparison is the trace element composition of the Waipapa metasediments that make up some of the crustal structure beneath the AVF (data from Price et al., 2015), and typical OIB signature (data from Sun and McDonough 1989).

Figure 4. Schematic diagram of our current understanding of the mantle processes associated with an eruption in the AVF. Depths on the left of the figure are approximate and supported by geophysical (low velocity zone under MKAZ station, see Figs. 8 and 10; aHorspool et al., 2006) and geochemical evidence ( bMcGee et al., 2013). Similarly, time frames of ascent on the right of the figure are also approximate, based on diffusion modelling in AVF xenocrystic olivines (cBrenna et al., 2018) and evidence from analogue volcanic fields (dBlake et al., 2006). Complex plumbing systems and ascent in both the deep and shallow mantle are proposed with evidence from crustal contamination studies (e.g. eHopkins et al., 2016), geochemical modelling (McGee et al. 2011; Brenna et al., 2018), or comparisons to other fields globally (e.g. Muirhead et al., 2016). Studies on crustal composition and magma ascent through the crust indicate the importance of the DMOB as a potential weakness exploited by the ascending melt. Finally, in the upper crust, magma ascent has been suggested to be influenced by pre-existing faulting or weaknesses, which can impact the final location of the eruptive centre at the surface.

Figure 5. All AVF whole rock lava samples analysed for trace elements using solution-ICP-MS. Modelled parameters of three heterogeneous mantle sources after McGee et al. (2013, 2015), normalization values from McDonough and Sun (1995). Horizontal tick marks show the % partial melting of the individual sources, and vertical tick marks show the % mixing between these partially melted sources.

Figure 6. Pb-isotope compositions for the AVF whole rock lava samples (highlighted in red in panels (A) and (C)), panels (B) and (D) show the AVF data, enlarged to allow the detail in the data to be seen. Northern Hemisphere Reference Line (NHRL) calculated after Hart (1984), analytical errors are smaller than symbol size and therefore not shown. White areas (panels (A) and (C) show key mantle domains from Stracke et al. (2005). Shaded areas show data for other intraplate basaltic volcanic fields and complexes in New Zealand, data from Scott et al. (2014a, 2014b, 2016, 2020); McGee et al. (2013) and references therein. Two areas are from the east coast of the South Island; Banks Peninsular includes volcanic fields and the Akaroa shield volcano on the east coast of the South Island, and the Otago data includes volcanic fields, the Dunedin shield volcano, Dunedin Volcanic Group, and the Westland Alpine dyke swarm. Two areas are from the west coast of the North Island; SAVF, South Auckland Volcanic Field, and Northland volcanic field (located on Fig. 1a).

184 Mantle source composition-volume links 185 McGee et al. (2013) combined geochemical characteristics with eruption volume estimates 186 (Kereszturi et al., 2013) and found a correlation between mantle sources and the size of eruptive centres 187 (Figure 7). Multi-element plots of large, lava-dominated centres versus small, tuff-dominated centres 188 show that mantle-derived signals such as negative Zr-Hf anomalies and shallower features such as 189 positive Sr and K anomalies become diluted or prominent respectively, with increasing size of eruptive 190 centre (Figure 3; McGee et al., 2013). In a follow-up study, McGee et al. (2015), identified a link 191 between degree of melting, involvement of mantle sources and melting rate. McGee et al. (2015) 192 proposed that smaller centres, generally nephelinites, were produced by the smallest degrees of melting 193 at lower melting rates from a deeper source, and larger centres resulted when these melts also 194 incorporated shallower melts with higher degrees of melting (Figure 4). This relationship was not only 195 applied to the AVF but also proposed to provide a general model for other small monogenetic fields 196 (e.g. the Wudalianchi volcanic field, China; McGee et al., 2015). The AVF is one of the few volcanic 197 fields in which we are able to explore a geochemical-eruptive volume relationship, as there is a lack of 198 extensive volume data in other fields, coupled with the dwindling accuracy of estimating volumes as 199 erosion degrades the volcanic landforms over time (McGee et al., 2015; McGee and Smith, 2016). 200 201

Figure 7. Geochemistry vs. volume plot after McGee et al. (2015) and Hopkins et al. (2017). Volume estimates from Kereszturi et al. (2013), geochemical data for the most primitive samples (i.e. those containing, e.g., Mg# ≥60) from Hopkins et al. (2017) and references therein.

202 Magma ascent 203 While mantle source compositions have been shown to have a large influence on the final chemistry 204 of erupted AVF products, additional studies have indicated that the modification of parental magmas 205 by fractionation, crystallisation, and crustal assimilation play a much lesser, secondary role (Smith et 206 al., 2008; McGee et al., 2013; Hopkins et al., 2016). Geochemical studies of these processes also 207 provide important insights regarding magma ascent dynamics, crustal lithologies, and ascent pathways. 208 For example, Smith et al. (2008) found that compositional variations within the Crater Hill eruption 209 sequence match those expected by deep ‘near source’ clinopyroxene fractionation, shallow olivine and 210 augite fractionation, and concluded that the magma erupted via a dyke rather than stalling in a magma 211 chamber. The geochemical evidence shows that individual magma batches in the AVF are generated 212 and ascend through the mantle via complex ascent processes in isolation, even from successive

213 eruptions within the same volcanic centre (Figure 4). For example, 230Th and 226Ra excesses were 214 observed in both Rangitoto’s earlier-erupted alkaline and later-erupted subalkaline magmas, although

215 negatively correlated (McGee et al., 2011). In order to generate these large 230Th excesses but small 226Ra 216 excesses in the earlier alkali basalt (notably, one of the highest found in the literature for continental 217 intraplate and OIB), it was suggested that the melts ascended quickly from depths of ≥80 km and that 218 they suffer minimal overprinting by crustal processing via flow in high porosity channels (Huang et al., 219 2000; McGee et al., 2013). In contrast, the later, more voluminous subalkaline basalt was proposed to 220 ascend from a shallower melting region via slower, diffuse melt movement which would allow greater

221 ingrowth of 226Ra (Figure 4). 222 Complex ascent histories for Auckland magmas were also suggested by Brenna et al. (2018), who 223 examined water and major element diffusion in mantle-derived xenocrystic olivines in Lake 224 Pupuke/Pupuke Moana lavas. Major element diffusion profiles indicate that the xenocrysts were 225 entrained in the magma during deep crustal or mantle storage on timescales varying from one month up 226 to several years at depths 27-80 km, thereafter rising to the surface over ~30-40 days, giving a relatively 227 slow average ascent rate of 0.01-0.03 m/s. In contrast, water diffusion profiles in the dehydrated rims 228 of the same xenocrystic olivines indicate that Lake Pupuke/Pupuke Moana magmas rose more rapidly 229 through the crust from 1-2 km depth over <12 hours, at speeds of 1-10 m/s (Figure 4). This 230 demonstrates that a single magma batch may ascend at variable rates as it rises through the mantle and 231 crust. 232 There is no clear evidence for crustal magma storage in the AVF, past or present, suggesting that 233 shallow magma ascent rates are too quick to support stalling for significant periods of time (Lindsay, 234 et al., 2010; Mazot et al., 2013; Hopkins et al., 2016). Several physical observations also support the 235 idea of rapid crustal magma ascent for example; Houghton et al. (1999) inferred fast magma ascent at 236 Crater Hill, as indicated by deposits from Hawaiian-style fire fountaining, an eruption style primarily 237 controlled by magma ascent rate; and Kereszturi et al. (2014) noted that using an ascent rate of 0.1 m/s

238 at the vent opening in lava flow models resulted in lava flow lengths and thicknesses similar to those 239 observed in the AVF. 240 The presence of rare crustal xenocrysts and crustal xenoliths in Auckland lavas, combined with 241 analogue models of xenolith settling rates and dike propagation, have also been proposed as evidence 242 for fast ascent (0.01–6.0 m/s from 100 km depth, taking ~10-100 days; Cassidy et al., 1986; Blake et 243 al., 2006; Spörli et al., 2015; Brenna et al., 2018). Xenoliths are uncommon in the AVF, however, where 244 found they are mostly distinct populations of lithic cargo within phreatomagmatic tuff ring deposits 245 (e.g. Searle, 1959b; Searle 1962; Brothers and Rogers 1969; Jones, 2007; Spörli and Black, 2013; Spörli 246 et al., 2015; Brenna et al., 2018). For example, at Lake Pupuke/Pupuke Moana (Figure 1b), xenocrysts 247 are exclusively olivine-rich ultramafic inclusions, proposed to have been assimilated in the upper 248 mantle (e.g. Brothers and Rodgers, 1969; Brenna et al., 2018). In comparison, Mesozoic and Cenozoic 249 sedimentary clasts linked to shallow-level crustal Waitemata, Te Kuiti and Waipapa Group sediments 250 are found in several tuff ring deposits throughout the field (Spörli et al., 2015), and mafic schistose and 251 non-schistose, amphibolite-grade metabasite xenoliths have been found at St Heliers/Glover 252 Park/Whakamuhu and Taylors Hill/Taurere indicative of interaction of ascending magma with melange 253 formation along the eastern boundary of the DMOB in the crust (Spörli et al., 2015). The xenoliths at 254 some centres show evidence of reaction rims indicating high temperature interactions (e.g. at Mt 255 Wellington/Maungarei; Figure 1b), but this evidence is absent in others (e.g. at St Heliers/Glover 256 Park/Whakamuhu; Figure 1b) (Spörli et al., 2015). 257 Rapid shallow-crustal ascent is also supported by whole rock major and trace element modelling, 258 which has confirmed that crustal assimilation has very little influence on the geochemical variations 259 seen in the AVF (McGee et al., 2013). More specifically, Hopkins et al. (2016) analysed Sr-Nd-Pb-Os- 260 Re isotopes in primitive lavas and found that, while Sr-Nd-Pb systems show very little variability, the 261 more sensitive Re-Os isotopes suggest interaction with two main types of crust: metasediments of the 262 Waipapa and Murihiku terrains and dunite-dominated ultramafic rocks of the Dun Mountain – Maitai 263 terranes, commonly referred to as the Dun Mountain Ophiolite Belt (DMOB) (Figure 1d; Figure 4, 264 Hopkins et al., 2016). The input from these sources was modelled to be ≤1%, which precludes detection 265 in the traditional major, trace or isotope (Sr, Nd, and Pb) systems (Hopkins et al., 2016). While the 266 geochemical composition of both the lavas and their cargo (e.g. minerals and xenoliths) can tell us much 267 about the source compositions of melts and subsurface processes acting on the magma, details about 268 the tectonic setting as well as the geophysical structure of the mantle and crust are needed to complete 269 a conceptual eruptive model for the AVF. 270 271

272 Structure

273 Tectonic setting 274 Several events in Auckland’s tectonic history influence magma chemistry and perhaps the location 275 of the AVF. Subduction occurred directly beneath the Auckland area during the Miocene, and is the 276 presumed cause for the presence of slab fluid-metasomatized lithospheric mantle, which comprises one 277 of the three hypothesised mantle sources (McGee et al., 2013). Auckland magmas erupt through late 278 Palaeozoic to Mesozoic metasedimentary basement terranes which accreted on the Gondwana margin 279 (Bradshaw, 1989). These are overlain by dominantly deep-water Miocene Waitemata Basin sandstones 280 and mudstones, in addition to some Pliocene to Quaternary shallow marine to terrestrial (fluvio- 281 lacustrine) sediments. The total crustal thickness in Auckland is ~25-30 km, as indicated by shear 282 velocity gradients, see Figure 8 (Horspool et al., 2006). 283 Currently, the AVF occurs distal (>400 km) to the Hikurangi subduction zone (Figure 1a), where 284 the Pacific Plate (to the southeast) is subducting beneath the Australian Plate (to the northwest) and, 285 more proximally, to the west of a zone of geodetically resolved oblique extension associated with the 286 Hauraki Rift axis (Figure 1a; Pickle, 2019). The axis of extension of the Hauraki Rift parallels the 287 accreted basement fabric delineated by the Junction Magnetic Anomaly (JMA; Figure 1a) (Kenny et 288 al., 2012), a significant geophysical marker throughout New Zealand (Figure 9). The NNW-SSE 289 trending JMA is interpreted to comprise the intra-crustal trace of the ultramafic DMOB succession 290 (Hatherton, 1967; Hatherton and Sibson, 1970, Eccles et al., 2005), which contrasts with those of the 291 accreted forearc Murihiku Terrane to the west (Briggs et al., 2004) and silicic succession of the Waipapa 292 Terrane to the east (e.g. Figure 1d; Spörli et al., 1989). As a result of this, the DMOB is clearly observed 293 in ambient noise-derived velocity models (Ensing et al., 2017) and total magnetic intensity anomaly 294 maps (Figure 9; Cassidy and Locke, 2010). These models show that under the AVF, the DMOB takes 295 a complex, sinuous form of eastward dipping shear zones and likely transects the crust (Figure 1d), 296 with its influence seen in the topology of the Moho and lithospheric mantle structure. Therefore, it is 297 hypothesised to represent a zone of weakness connecting the mantle to the surface; additionally, the 298 bounding faults associated with the DMOB have been proposed to exert control over the E-W surface 299 expression and AVF boundary (Figure 4; e.g. Hatherton and Sibson, 1970; Spörli and Eastwood, 1997; 300 Eccles et al., 2005; Williams et al., 2006; Cassidy and Locke, 2010; Kenny et al., 2012; Spörli et al., 301 2015; Hopkins et al., 2016). More regionally, intraplate volcanism in northern New Zealand shows 302 some spatial correlation to the location of the DMOB; the AVF and SAVF are located directly on it, 303 however, the other fields (Okete and Ngatutura) are offset to the west (Figure 1a). 304 305 Causes for melting under Auckland 306 Researchers commonly rely on local geophysical measurements of seismicity, as well as the 307 gravitational and magnetic field, to make inferences about the subsurface structure, properties, and 12

308 origin of melting in a volcanic field. However, in the Auckland region, urbanisation, infrastructure 309 density, and a lack of high-resolution local sources for seismic tomography have reduced the ability to 310 utilise traditional geophysical methods. For example, Figure 10 displays the epicentres of the 118 311 earthquakes recorded by Geonet (https://www.geonet.org.nz/data) in the Auckland region over the past 312 30 years. Of those earthquakes, only four had a magnitude greater than M3 and very few even sit within 313 the proposed boundary of the AVF. As a result of this, high-resolution geophysical images of the 314 subsurface under the AVF generally terminate in the shallow crust (Cassidy and Locke, 2010; Davy, 315 2008), and geophysical investigations of the deeper structure under the AVF lack clear resolution 316 (Eccles et al., 2005; Mooney, 1970; Horspool et al., 2006; Eberhart-Phillips et al., 2010). 317 Our current understanding of the crust and mantle under the AVF is captured in a 2D model that 318 traverses the upper North Island of New Zealand (Horspool et al., 2006). Surface waves and receiver 319 functions from teleseismic earthquakes were converted to 1D profiles under an array of seismic stations 320 to reveal a zone of low seismic shear-wave velocity at approximately 70-90 km depth beneath Auckland 321 (Figure 8). This was interpreted by Horspool et al. (2006) as a potential zone of partial melt, supported 322 by geochemical observations (McGee et al., 2013), suggesting that erupted materials stem from magma 323 sources at a similar depth (~80 km; Figure 4). However, this image is based on data from a single 324 station (MKAZ) ~20 km the south-east of the AVF (Figure 10). Consequently, we have not yet been 325 able to identify the dominant controls on regional upwelling flow beneath the AVF and can only infer 326 potential causes by comparing the AVF to other intraplate fields in NZ (e.g. Hoernle et al., 2006; Timm 327 et al., 2010; Scott et al., 2016) and around the world (e.g. Conrad et al., 2010; Conrad et al., 2011; 328 Putirka et al., 2012; Davies and Rawlinson, 2014; Kaislaniemi and van Hunen, 2014; Brenna et al., 329 2015; van den Hove et al., 2017). The interplay between magma fluxes and regional tectonic stresses 330 in particular seem to have a large influence on the predictability of eruptions (either temporally or 331 volumetrically) in some volcanic fields (e.g. Valentine and Perry, 2007; Demidjuk et al., 2007; Conrad 332 et al., 2010; Aranda-Gomez et al., 2013; Brenna et al., 2015). Explanations for melting under Auckland 333 most closely match those in time-predictable fields, specifically those that involve disturbances in 334 mantle flow patterns and/or delamination, which in turn induce melting and upwelling (Hodder, 1984; 335 Spörli et al., 2015). However, eruptions in the AVF are neither strictly time- nor volume-predictable 336 and the cause of melting remains enigmatic (Hopkins et al., 2017; Mortimer and Scott, 2020). 337 In order to localise melting and volcanism, some degree of entrainment of enriched material may 338 be required (Demidjuk et al., 2007), or 3D regional lithospheric structure, plate motion and background 339 mantle flow must combine to localise upwelling flow (McGee et al., 2011, 2013; Davies and Rawlinson, 340 2014). The outstanding questions for the AVF are whether edge-driven flow (i.e. flow excited by crustal 341 delamination or lateral variations in lithospheric thickness), and/or shear-driven flow (i.e., movement 342 in response to regional plate motion), play an important role in the melt origins (Hodder, 1984; Spörli 343 et al., 2015). Although no chemical evidence exists to support this idea, it is possible that factors

344 influencing regional mantle flow patterns (e.g. subduction east of the North Island) are inducing 345 upwelling under the AVF, similar to the mechanisms proposed by Brenna et al. (2015) at the Jeju 346 volcanic field. 347 Where local seismicity is lacking (as in Auckland; Figure 10), researchers are employing newly 348 developed crustal imaging techniques, such as the inversion of ambient seismic noise (e.g. ocean waves; 349 Ensing et al., 2017), a method that has proven very effective for shallow targets (e.g. Shapiro et al., 350 2005; Brenguier et al., 2007; Li et al., 2009; Lin et al., 2007). Preliminary results from the correlation 351 of ambient ocean noise confirm the AVF crust is not only strongly heterogeneous, but they also provide 352 a first insight in a subsurface projection of the contact between oceanic and continental lithosphere that 353 underlies the AVF (Ensing et al., 2017). These deep crustal heterogeneities and structures (such as the 354 ultramafic DMOB, Figure 9) seem to provide a conduit for magma to the surface, while shallow faults 355 offer some control on the final vent locations of individual centres (Spörli and Eastwood, 1997; Magill 356 et al., 2005; Le Corvec et al., 2013b). 357 358 Shallow tectonic influences on volcanism 359 The cause and nature of shallow structural control on the AVF eruptions has been difficult to 360 determine (Spörli and Eastwood, 1997; Le Corvec et al., 2013a). The elliptical pattern of volcanic vents 361 within the AVF form on a N-S oriented long axis (Figure 1e; Spörli and Eastwood, 1997), it is 362 interpreted to describe the mantle source geometry at depth, and the influence of the DMOB’s bounding 363 faults to the east and west (Spörli and Eastwood, 1997; Kenny et al., 2012; Le Corvec et al., 2013b). 364 However, studies have shown that individual vent locations within the field are predominantly 365 influenced by the location of shallow fractures or faults (Le Corvec et al., 2013b) or variations in the 366 stress fields toward the surface. For example, Magill (2005) speculated that there are groups and 367 alignments of vents in largely NW-SE and SW-NE orientations, reflecting dominant fractures and 368 faulting within the basement structure. However, fault identification within the Auckland region is 369 challenging due to the sedimentary, volcanic and urban cover and the low rates of tectonic deformation 370 and associated seismicity far-field to the plate boundary, as discussed above (e.g. Figure 10; Stirling et 371 al., 2012). In order to resolve this challenge, Kenny et al. (2012) utilised exposed geology, topography 372 and the Auckland regional borehole database (Howe et al., 2011; now expanded into the New Zealand 373 Geotechnical Database [https://www.nzgd.org.nz/]) to identify offsets in the Waitemata Group surface. 374 They inferred 64 obscured fractures or faults that cannot be geologically mapped at the surface. Many 375 of these faults have NNW-SSE, WSW-ENE or N-S orientations as seen in Figure 1d. Kenny et al. 376 (2012) reveal that most AVF volcanic centres are located within 500 m of these newly inferred fractures 377 or faults, supporting the theory that ascending magma exploits shallow level weaknesses in the crust. 378 At present, however, any direct geophysical expression of the magmatic plumbing, and how for 379 example dyke propagation may utilise the pre-existing structural pathways, has yet to be resolved.

Figure 8. Shear wave velocity profile for northern New Zealand, originally published as the top panel of Fig. 11 in Horspool, N. A., M. K. Savage, and S. Bannister (2006), Implications for intraplate volcanism and back-arc deformation in northwestern New Zealand, from joint inversion of receiver functions and surface waves, Geophys. J. Int., 166(3), 1466–1483. The profile is created from interpolation between sparse 1D profiles at broadband stations indicated by triangles. Locations of intraplate basalt fields (black bars) are projected onto the profile at top: Kaikohe-Bay of Islands Volcanic Field (KBIVF), Whangarei Volcanic Field (WVF) and Auckland Volcanic Field (AVF). Note the region of low seismic velocity below MKAZ at ~70-90 km depths, interpreted to be a region of partially molten mantle is located ~ 20 km to the south east of the AVF. The interpreted Moho is indicated by the blacked dotted line showing the crustal thickness varies between 25 -30 km, and the cross section represented in Figure 4 is outlined by the black box. The seismic stations are show by grey triangles with their name abbreviation station codes defined, for example MKAZ - Moumoukai.

Figure 9. Total magnetic intensity (TMI) anomaly map of the Auckland region at 430 m above sea level. The orange to yellow, sub-linear, long-wavelength magnetic high running the length of the figure represents the Junction Magnetic Anomaly (JMA), caused by the DMOB (see text for discussion). Short wavelength anomalies (positive, dark red peaks and adjacent negative, dark blue-purple troughs) represent the often bipolar anomalies associated with volcanic centres. Figure modified from Cassidy and Locke (2010).

Figure 10. Map of the Auckland region with the epicentres of all 118 catalogued earthquakes (from Geonet catalogue (https://www.geonet.org.nz/data) in the last 30 years (in cyan circles). Magnitude of earthquakes corresponds with circle size shown in the key. Black triangles indicate seismic stations with their station codes detailed, for example MKAZ – Moumoukai (Fig. 8). The light shaded area shows the extent of the urban Auckland area. The AVF boundary (see also Fig. 1e) is marked by the dashed ellipse.

380 Magnitude and Frequency of AVF eruptions

381 Temporal eruptive patterns 382 Over the last decade, researchers have made significant advances in determining the numerical ages 383 of the centres in the AVF (Supplementary Material (SM) Table 1). For a full review of the dating 384 methods, see Lindsay and Leonard (2009) and Lindsay et al. (2011), and for critical advances in some 385 of the dating techniques since these publications, see SM 1. In Table 1 we present the currently 386 recognised ages of each of the 53 volcanic centres in the AVF. We highlight our preferred age based 387 on reconciliation of all data published to date, and include a reliability grouping, following the approach 388 originally proposed by Lindsay et al. (2011), but modified slightly to include recent advances in dating 389 techniques (refer to discussions in SM 1 and SM Table 1 and SM Table 2 for full details). Combining 390 the results from all past studies brings the total number of dated centres in the AVF up from 12 (Lindsay 391 et al., 2011) to 47 (Figure 11; Hopkins et al., 2017; Leonard et al., 2017) out of 53, with only six centres 392 with no direct ages (Hopkins et al., 2017; Hayward and Hopkins, 2019). With eruption ages for most 393 volcanic centres, we have constructed one of the most complete chronologies for a volcanic field in the 394 world, and from this, the spatial and temporal evolution of the field can be assessed.

395 The oldest recognised centre in the field is Pupuke/Pupuke Moana at 193.2 ± 2.8 ka (40Ar/39Ar

396 analysis; Leonard et al., 2017), and the youngest is Rangitoto (2) at 504 ± 5 yrs. BP (14C analysis; 397 Needham et al., 2011). Hopkins et al. (2017) and Leonard et al. (2017) highlighted a number of key 398 findings by combining their results with previous research (Figure 11a), including: 1) a flare-up in 399 activity ca. 28-35 ka; 2) a ≤10 kyr hiatus between the youngest and penultimate eruptions; 3) evidence 400 for grouping or coupling of eruptions; 4) multiple eruptions occurring during paleomagnetic excursions; 401 5) preference for eruptions to occur on pre-existing faults (Figure 1d); 6) repose periods that vary 402 between ≤0.1 to 13 kyrs, with 23 of the 47 centres (~49%) showing repose periods of <1000 yrs.; and 403 7) an overall increase in the rate of volcanism since ca. 60 ka. 404 405 Eruption styles and magnitudes 406 Eruptive styles in the AVF reflect both magmatic (composition, volume, and flux of the ascending 407 melt) and environmental variables (the presence/quantity or absence of water and the type of country 408 rock and geologic structures the magma encounters along the ascent pathway) (Németh et al., 2012; 409 Kereszturi et al., 2014). In volcanic fields, where the magma volume can vary and vent location is 410 generally not static, the relative importance of these factors can be highly changeable between eruptions. 411 Therefore, constraining the ranges of the variables involved in past eruptions, through detailed field 412 studies, allows us to more robustly forecast future eruption characteristics (e.g. Kereszturi et al., 2014; 413 Kereszturi et al., 2017; Hayes et al., 2017; Ang, 2019). See SM Table 3 for a detailed overview of 414 likely AVF eruption hazards, summary of characteristics, and considerations and requirements for 415 hazard and risk assessments. 15

416 Past studies have shown environmental influences on the style of AVF eruptions are possibly more 417 significant than magmatic influences (e.g. Allen and Smith, 1994; Kereszturi et al., 2014). In particular, 418 Kereszturi et al. (2014) highlight the prominence of phreatomagmatic phases in the AVF, and suggest 419 this is due to the presence of underlying water-saturated sediments and a shallow groundwater table in 420 the Auckland region, especially in the south of the field. They note that while unpredictable internal 421 forces (e.g. magma supply) influence eruption style and transitions, the geographical mapping of 422 external influences (e.g. groundwater distribution) can inform susceptibility maps in advance of 423 eruptions, aiding hazard assessments (Kereszturi et al., 2017). 424 In the AVF, approximately 83% of eruptions are known to have begun with a “wet” 425 phreatomagmatic vent-opening phase (e.g. Allen and Smith, 1994; Kereszturi et al., 2014), where the 426 ascending magma interacted explosively with ground- and/or surface-water. This is then sometimes 427 followed by a “dry” magmatic explosive or effusive phase once magma-water interaction has stopped 428 either through exhaustion or exclusion of available water source(s). The phreatomagmatic phases of an 429 AVF eruption generate low eruption columns (Kereszturi et al., 2014) and are associated with base 430 surges (e.g. Brand et al., 2014), and the creation of maar craters and tuff rings (e.g. Németh et al., 2012), 431 and relatively widespread tephra dispersal (e.g. Hopkins et al., 2017) (Figure 1b, SM Table 3). The 432 magmatic phases range from Hawaiian (lava fountaining) to Strombolian (explosive) styles, and 433 generally form scoria cones, scoriaceous tephra fall and lava flows (e.g. Németh et al., 2012) (Figure 434 1b, SM Table 3). There is minimal evidence in the AVF for “sensu stricto” Surtseyan style eruptions 435 (e.g. Agustín-Flores et al., 2015; Cronin et al., 2018), where the initial stages of the eruption occurred 436 in shallow standing water, suggesting that external water was abundant at least during the onset of the 437 eruptions (Kokelaar, 1983). Nevertheless, with ~35% of the field area in Auckland currently covered 438 by ≤30 m of water, combined with future sea-level rise, the Surtseyan style of volcanism could 439 potentially become increasingly prevalent in Auckland (Agustín-Flores et al., 2015). 440 Estimates for the volume of individual eruptions reported by Allen and Smith (1994) and Kermode 441 (1992) were updated by Kereszturi et al. (2013) using volcano geology-based geometrical 442 considerations, pre-existing size data and a LiDAR survey-based Digital Surface Model, taking into 443 account all types of erupted material (excluding distal tephra deposits). A minimum volume of

444 magmatic material for the field of 1.7 km3 is reported in Dense Rock Equivalent (DRE), composed of 445 ~78% lava flows, ~6% of scoria cones, ~6% of crater lava infill, ~5% in tephra/tuff rings, and ~4% 446 phreatomagmatic crater lava infills (Kereszturi et al., 2013). The smallest centre has a calculated volume 447 of 0.000076 km3 (Ash Hill); the largest centre (Rangitoto) is nearly four orders of magnitude larger at 448 0.70 km3, and comprises ~41% of the field’s total volume. When the volume estimates are plotted 449 against the new temporal reconstruction, a stepped sequence is seen (Figure 11b), highlighting that 450 most eruptions are small-volume, punctuated by a few large-volume eruptions. Although our 451 understanding of the chronology of the AVF eruptions is remarkably detailed, there are no clear patterns

452 in the spatio-temporal eruptive behaviour (Figure 1c). As a result, we remain unable to forecast the 453 timing, volume, or location of a future eruption. We do, however, understand many fundamental details 454 about the characteristics of the eruptions, and what potentially influences the sizes and styles of the 455 eruption. This information informs scenario modelling (discussed below) and provides critical 456 constraints to complex modelling parameters. 457 458

Figure 11. A) Relationship between age (including 2sd error bars), grey shading delineates different temporal eruption periods (discussed in the text). All data is reported in Table 1, proposed coupled eruptions are coloured in yellow, and flare up is highlighted by red bars, see text for discussion. B) Relationship between age (excluding error for clarity) and cumulative volume (minimum volumes, in dense rock equivalence (DRE) from Kereszturi et al., 2013). As with panel A, grey shading shows periods of variable volume vs. time relationships, black arrows show large volume increases, showing a stepped sequence with smaller-volume eruptions punctuated by a few large-volume eruptions. This cumulative age pattern can be split into four phases: (1) the oldest phase between 200-70 ka where a slow progression of small-volume eruptions built up a cumulative volume of 220 x 106 m3 over 130 ka (1.7 x 106 m3/kyr), (2) an older middle phase between 70-~30 ka, including the large-volume eruption of One Tree Hill/Maungakiekie, giving a total cumulative eruptive volume of 363 x 106 m3 over 40 ka (9 x 106 m3/kyr), (3) a younger middle phase from ~30-20 ka, which began with a large-volume eruption at Three Kings/Te Tātua-a-Riukiuta, and includes multiple small volume eruptions within a short time frame (the “flare-up”) during which 275 x 106 m3 of material erupted over 10 ka (27.5 x 106 m3/kyr), and 4) the youngest phase during which the volume increases to 795 x 106 m3 over 20 ka (40 x 106 m3/kyr), and includes the eruption of Rangitoto. Figures adapted from Leonard et al., 2017.

459 Volcanic hazards in the AVF

460 History of scenario-based planning in the AVF 461 Due to the city of Auckland’s large population, economic value, and its identity as a key node for 462 critical infrastructure, robust assessment and management of volcanic risk has been a major focus for 463 New Zealand scientists and government (Department of Prime Minister and Cabinet, 2011; Lloyd’s 464 City Risk Index, 2018). A challenge has been translating the complex and evolving volcanic science 465 into useful and useable formats for disaster risk management planning. One important tool has been 466 the development and application of “scenarios” (research-informed, postulated sequences of events 467 during a future eruption), which can be used to summarise multidisciplinary volcanology science in a 468 usable format to inform risk management. Eruption scenarios have been used extensively to manage 469 and plan for a future AVF eruption. In 1997, five eruption scenarios were produced for the Auckland 470 Regional Council (ARC; now Auckland Council [AC]) to assist with contingency planning (Johnston 471 et al., 1997). These scenarios featured both phreatomagmatic and magmatic eruption styles and were 472 used to demonstrate the variety of impacts that could occur to the built environment. They were 473 primarily qualitative, with extensive scenario narratives and accompanying maps of hazardous 474 phenomena, but gave limited consideration to varying unrest sequences that could occur (all scenarios 475 included a generic 25-day precursory seismic sequence). However, subsequent research has shown 476 there is considerable uncertainty associated with seismic unrest sequences in the AVF (Blake et al., 477 2006; Sherburn et al., 2007; Brenna et al., 2018). 478 In 2008, an ‘all-of-nation’ emergency management exercise was held using a hypothetical AVF 479 eruption scenario termed “Exercise Rūaumoko” (MCDEM 2008; Lindsay et al., 2010; Deligne et al., 480 2017a). It simulated an eruption lead-up, and tested the protocols and response of the Auckland Volcano 481 Science Advisory Group (AVSAG; MCDEM 2008; Doyle et al., 2011). The results from this scenario 482 testing coupled with further studies showed: (1) the need for increased scope for disaster response and 483 recovery planning (Brunsdon and Park, 2009); (2) a demonstration of how Bayesian Event Tree 484 modelling could be utilised for real-time eruption forecasting during a future AVF eruption (Lindsay et 485 al., 2010), and (3) an economic model, which found that the direct economic costs of business 486 inoperability would cost between NZ$1-$10B, with indirect effects potentially much greater 487 (McDonald et al. 2017). Exercise Rūaumoko scenario has subsequently been further developed into an 488 educational and outreach tool used for training and eruption scenario simulation (Dohaney et al., 2015; 489 Fitzgerald et al., 2016). Knowledge developed through Exercise Rūaumoko, the long-term 490 transdisciplinary Determining Volcanic Risk in Auckland (DEVORA) research programme, and other 491 studies on the AVF have allowed for more complex eruption scenarios and decision-making tools to be 492 developed that are better aligned with contemporary emergency management needs. 493 494 18

495 The DEVORA Scenarios 496 As part of the DEVORA research programme a new suite of eight scenarios (the “DEVORA 497 Scenarios”; Figure 1e) were developed (Hayes et al., 2018; Hayes et al., 2020) (Table 2). They include 498 complex spatio-temporal eruption sequences using scenario narratives and quantitative geospatial 499 hazard layers (Figure 1e). To ensure the scenarios were both credible and useful, a wide variety of 500 stakeholders (physical volcanologists, geophysicists, geochemists, disaster risk researchers, policy 501 advisors, geotechnical engineers, infrastructure managers, and emergency management officials) were 502 engaged and embedded throughout the design and development of the scenario suite (Hayes et al., 503 2020). The scenarios can be used for diverse disaster risk reduction activities such as table-top exercises, 504 educational/outreach activities, and economic loss modelling (Hayes et al., 2018). 505 In each of the eight scenarios, eruption volume, duration, lead time, volcanic centre location, and 506 volcanic hazards were varied within a credible range for the AVF, excluding, in its current iteration, 507 another ‘Rangitoto shield-building’ scenario as this is complex and highly atypical in the field’s history 508 (Table 2). The locations of the scenarios were chosen based on four key criteria, these included; being 509 located in the “tight” elliptical field boundary limits (Runge et al., 2015; Figure 1e); being 510 geographically spread across Auckland; allowing the exploration of different eruption styles and 511 hazards (see SM Table 3); and allowing the exploration of impacts to different exposed assets (Hayes 512 et a., 2018; Figure 1e). Details of the scenario locations and names can be found on Figure 1e, and an 513 overview of the different components attributed to each scenario is detailed in Table 2. Each of the 514 scenarios were allowed to play out to produce hazard footprints of the associated hazards (details in SM 515 Table 3), their duration, and volume (Hayes et al., 2018). 516 An example of the use of the scenarios can be seen in the Māngere Bridge scenario (Scenario C; 517 Figure 1e), which models an eruption in a shallow estuarine setting, and is placed so the eruption 518 impacts a wide variety of urban assets including critical transport links (road, rail, air), water systems, 519 electricity supplies, industrial, and residential dwellings (Deligne et al., 2017a, Blake et al., 2017; 520 Stewart et al., 2017). This scenario is discussed in detail in Deligne et al. (2017a), who use the hazard 521 map produced to model evacuation zone designation and evaluate the consequences of an eruption on 522 the electricity service provisions for Auckland city. Companion publications were also produced to 523 discuss the impacts to transportation (Blake et al., 2017) and water sectors (Stewart et al., 2017) for this 524 scenario. The eruption spans 10 weeks (two weeks of non-activity, four weeks of unrest, and four weeks 525 of eruptive activity), and is comparable to that of Maungataketake (Deligne et al., 2017a). The key 526 outcomes indicate that infrastructure will be severely damaged, but will likely still be able to provide a 527 partial service. Outage duration for a number of key critical infrastructure services are also estimated, 528 with outage duration for electricity >1 year (Deligne et al., 2017a); telecommunications <2 weeks 529 (Deligne et al., 2017a); roads and rail outages >7 weeks (Blake et al., 2017); aviation impacted for ~3

530 months (Blake et al., 2017); and water supply restrictions for >1 year coupled with raw sewage 531 discharge for >2 years (Stewart et al., 2017). 532 533 534

Lead time Duration Bulk Volume Scenario Purpose Eruption Style (Days) (days) (km3)

A - Auckland Airport A short-lived eruption directly affecting Auckland Airport Phreatomagmatic 8 4 1.8 x 10-2 An eruption affecting an important critical infrastructure node that passes through the Phreatomagmatic - B - Ōtāhuhu 13 32 3.4 x 10-2 Auckland isthmus. magmatic An eruption that includes a wide variety of eruption phenomena that would affect many Phreatomagmatic - C - Mangere Bridge 28 28 1 x 10-1 urban assets. magmatic A large volume eruption that directly impacts a predominantly residential area with a long- D - Mt. Eden Suburb Magmatic 45 240 1.3 x 10-1 lived eruption sequence. An eruption of both phreatomagmatic and Strombolian styles that directly impacts the Phreatomagmatic - E - Waitematā Port 3 27 1.2 x 10-2 Waitematā Port. magmatic Phreatomagmatic - F - Birkenhead An eruption that directly affects the Auckland Harbour Bridge. 15 160 1.9 x 10-2 magmatic An eruption that considers implications of a Surtseyan style eruption that affects the main G - Rangitoto Channel Surtseyan 10 8 1.4 x 10-2 shipping channel. An eruption that occurs near the location of the most recent eruption within the AVF. Also Phreatomagmatic - H - Rangitoto Island 660 109 1.8 x 10-1 considers a protracted unrest sequence. magmatic

Table 2. The DEVORA Scenarios, their key characteristics, and the purpose for their development.

535 Future hazard and risk research directions 536 The applications of the scenarios for hazard and risk planning have provided insights into the 537 potential complexity associated with managing and recovering from a future AVF eruption. Loss and 538 impact results can be heavily influenced by assumptions relating to a selected scenario’s characteristics. 539 Thus, characterisation of the uncertainty associated with the impacts of AVF eruptions through 540 probabilistic methodologies is the necessary next step. This has begun with research to derive 541 probabilities for the DEVORA eruption scenarios at every location in the AVF, which are dependent 542 on local environmental conditions. The inclusion of hazard footprints for end users, investigations into 543 evacuation planning strategies in the context of variable locations, the detection of eruption warning 544 signals, development of tools to support rapid crisis decision-making, and examinations into the effects 545 of an eruption on the many aspects of New Zealand life and environment (e.g. social, financial, political, 546 cultural), amongst numerous other considerations, are also essential and comprise future plans to inform 547 effective AVF risk management. An AVF unrest and eruption will be a major event requiring 548 considerable resources to manage, so it is necessary to consider strategic planning issues in advance, 549 including risk governance, to avoid doing this under duress during the actual crisis. 550 Finally, we stress the overriding importance of maintaining and extending a) effective science 551 coordination to ensure fit-for-purpose knowledge, capacity and capability is available and continues to 552 advance; and b) strong science-practitioner-policy relationships and structures (both formal and 553 informal) that ensure science effectively interfaces with disaster risk management structures to 554 continuously improve the ability to inform effective AVF risk management (Fearnley and Beaven, 555 2018; Daly and Johnston, 2015). 556

557 Conclusions

558 In the 60 years since in-depth geological research on the AVF began, the field has become one of 559 the most well-studied in the world. High-resolution mapping and reliable, comprehensive age and 560 geochemical data has allowed us to examine spatio-temporal, geochemical, and volume patterns over 561 the field’s lifetime. Detailed field studies have revealed a strong environmental control (e.g. presence 562 of groundwater) on the hazards produced. Geophysical investigations thus far, though hampered by the 563 lack of seismicity in the Auckland region, have shown a low-velocity melt zone at ~70-90 km depth 564 under Auckland, potentially indicating the depth of initial melting. 565 Improving our understanding of the source to surface processes that govern the formation and 566 evolution of the AVF has been critical, however, the way in which these results are fed into hazard, risk 567 and social studies is also a vitally important aspect of on-going research. This holistic approach is highly 568 unique and has resulted in, among many other research products, eruption scenarios detailing the ranges 569 of hazard styles and scales in various locations around the city. These valuable tools have provided a 570 strategy to estimate damage and impacts from future eruptions. 21

571 Overall, we have a unique and detailed knowledge of the AVF, however, many unknowns still exist 572 and remain to be investigated, including the processes causing melting under Auckland, the relationship 573 of the older volcanic fields within the North Island to the AVF, the drivers for increases in eruption 574 volumes and eruption rates over the field’s lifetime, and why the last eruption was volumetrically much 575 larger than other eruptions in the field’s history. Additional questions remain about magma ascent 576 through the crust and its impact on warning times, as well as the composition and structure of the mantle 577 and crust and how it controls ascending magma. Finally, we highlight the need to improve our 578 knowledge of the potential volcanic hazards (particularly tephra fall, volcanic earthquakes, land 579 deformation, shockwaves, volcanic tsunami, ballistic projectiles and fire) associated with a future 580 Auckland eruption to improve eruption scenarios, risk assessments, and disaster risk reduction efforts. 581 With continued and sustained efforts to address these gaps in our knowledge, a quantitative risk 582 assessment for Auckland is likely to become a reality in the coming years. 583 584 585 586 587 588 589 590

591 Acknowledgements

592 Much of the recent research detailed within this publication derives from the Determining Volcanic 593 Risk in Auckland (DEVORA) programme. This is a collaborative research programme funded by the 594 Auckland Council and the New Zealand Earthquake Commission (including JLH’s Postdoctoral 595 Research grant #17/U745). Some of the research referenced in this publication are sourced from 596 presentations from DEVORA Forum meetings (yet to be published). Links to these presentations can 597 be found at the DEVORA web page: http://bit.do/DEVORAPresentations. The authors would like to 598 thank Marco Brenna and an anonymous reviewer, and editor James Scott for their constructive and 599 positive feedback on this manuscript.

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Supplementary Material (SM Tables 1 – 3) Reliability Centre name / Alternate name Recognised ages (ka, 1sd error) Method Reference Preferred Age (ka) Error (2sd) Method Reference Comments Grouping Albert Park 145.0 +/- 2.0 Tephrochronology Hopkins et al., 2017 145.0 4.0 Tephrochronology Hopkins et al., 2017 2 146.9 +/- 2.8 Ar/Ar Leonard et al., 2017 Ash Hill 31.8 +/- 0.2 14C Hayward 2008 31.8 0.4 14C Hayward 2008a 2 Equal to or older than Wiri Mt/Te Manurewa Morphostratigraphy Hayward et al., 2011a o Tamapahore Boggust Park > 130ª Morphostratigraphy Hayward et al., 2011a > 130 Morphostratigraphy 5 Cemetery Crater no data undated 5 Crater Hill 117 +/- 5.0 K-Ar McDougall et al., 1969 30.4 0.8 Tephrochronology Hopkins et al., 2017 2 T. Itaya unpublished data in 60 +/- 30 K-Ar Allen and Smith 1994 32.1 +/- 5.4 Ar/Ar Cassata et al., 2008 Mono lake excursion Paleomagnetic signature Cassata et al., 2008 30.4 +/- 0.4 Tephrochronology Hopkins et al., 2017 Auckland Domain/ 148-152 K-Ar McDougall et al., 1969 106.0 8.0 Tephrochronology Hopkins et al., 2017 See SM1 2 Pukekawa ≥ 60 (14C minima) 14C Grenfell and Keny 1995 > 45 º Tephrochronology P. Shane pers. comms. 106.0 +/- 4.0 Tephrochronology Hopkins et al., 2017 younger than Grafton Park Morphostratigraphy Hayward et al., 2011a Grafton older than the Domain/Pukekawa Morphostratigraphy Hayward et al., 2011a 106.5 Morphostratigraphy* See SM1 3 Green Mt/ 19.8 +/- 8.9 14C Sameshima 1990 19.6 6.6 Ar/Ar Leonard et al., 2017 1 Matanginui 19.6 +/- 3.3 Ar/Ar Leonard et al., 2017 Hampton Park 55.0 +/- 5.0 K-Ar Mochizuki et al., 2004 57.0 32.0 Ar/Ar Leonard et al., 2017 2 26.6 +/- 8.1 Ar/Ar Cassata et al., 2008 57.0 +/- 16.0 Ar/Ar Leonard et al., 2017 unusual p.mag varience the same as Otara Paleomagnetic signature Cassata et al., 2008 younger than Otara Morphostratigraphy Hayward et al., 2011a Te Hopua-a-Rangi/ ≥ 28.9 Tephrochronology (Poihipi) Molloy et al., 2009 31.0 Tephrochronology & Hopkins et al., 2017 See SM1 3 Gloucester Park 51.6 +/- 3.2 Ar/Ar Leonard et al., 2017 morphostratigraphy younger than One Tree Hill/Maungakiekie Morphostratigraphy Hayward et al., 2011a Kohuora 33.3 +/- 0.6 14C Searle 1965, and Grant-Taylor 33.7 2.4 Tephrochronology Hopkins et al., 2017 1 34.4 +/- 2.1 14C and Rafter 1971 > 25.4 Tephrochronology (KOT) Newnham et al., 2007 33.7 +/- 1.2 Tephrochronology Hopkins et al., 2017 older than Crater Hill Morphostratigraphy Hayward et al., 2011a Little Rangitoto/ 20.7 +/- 2.2 Ar/Ar Leonard et al., 2017 24.6 0.6 Tephrochronology Hopkins et al., 2017 1 Maungarahiri 24.6 +/- 0.3 Tephrochronology Hopkins et al., 2017 younger than Ōrākei Basin Morphostratigraphy Kermode, 1992 Māngere Lagoon 59.5 Tephrochronology & Hopkins et al., 2017 59.5 Tephrochronology & Hopkins et al., 2017 3 morphostratigraphy morphostratigraphy* older than Māngere Mountain Morphostratigraphy Hayward et al., 2016 Māngere Mt/ 21.9 +/- 0.3 14C Searle 1959a; 1965 and Grant- 59.0 20.0 Tephrochronology Hopkins et al., 2017 4 Te Pane o Mataaho 32.3 +/- 1.4 14C Taylor and Rafter 1971 34.9 +/- 0.4 14C 2007 -unpublished 70.3 +/- 3.6 Ar/Ar Leonard et al., 2017 59.0 +/- 10.0 Tephrochronology Hopkins et al., 2017 younger than One Tree Hill/Maungakiekie, Morphostratigraphy Hayward et al., 2011a older than Mt Smart/Raratonga Fergusson & Rafter 1959; 88.9 4.8 Ar/Ar Leonard et al., 2017 See SM1 2 Grant-Taylor and Rafter Maungataketake/ 33.4 to > 55.0 14C 1963; Polach et al., 1969; McDougall et al., 1969 Elletts Mountain 74 - 138 K-Ar McDougall et al., 1969 140 - 177 OSL Marra et al., 2006 Reliability Centre name / Alternate name Recognised ages (ka, 1sd error) Method Reference Preferred Age (ka) Error (2sd) Method Reference Comments Grouping 38.1 +/- 1.9 Thermoluminescence Wood 1991 88.9 +/- 2.4 Ar/Ar Leonard et al., 2017 80-140 Morphostratigraphy Augustin-Flores et al., 2014 McLaughlins Mt/ 110 +/- 40 K-Ar T. Itaya unpublished data in 48.2 6.4 Ar/Ar Leonard et al., 2017 2 Allen and Smith 1994 Matukutūreia 48.2 +/- 3.2 Ar/Ar Leonard et al., 2017 McLennan Hills/ 31.7 +/- 0.2 14C Polach et al., 1969 41.3 2.4 Morphostratigraphy & Hopkins et al., 2017 2 Te Apunga-o-Tainui 40 - 124 K-Ar McDougall et al., 1969 paleomagnetic 33-64 K-Ar Mochizuki et al., 2007 48.8 +/- 2.5 Thermoluminescence Wood 1991 Laschamp p.mag excursion Paleomagnetic signature Shibuya et al., 1992 42.6 +/- 3.8 Ar/Ar Cassata et al., 2008 34.7 +/- 2.4 Ar/Ar Leonard et al., 2017 Older than Mt Richmond/Otahuhu Morphostratigraphy Hayward et al., 2011a Motukorea/ 14.3 +/- 6.0 Ar/Ar Leonard et al., 2017 24.4 0.6 tephrochronology Hopkins et al., 2017 2 Brown's Island 24.4 +/- 0.3 Tephrochronology Hopkins et al., 2017 ≥ 7-9 Morphostratigraphy Bryner 1991 Mt Albert/ > 30 14C Fergusson & Rafter 1959; 119.2 5.6 Ar/Ar Leonard et al., 2017 1 Grant-Taylor and Rafter 1963

Te Ahi-kā-roa-a- 129-140 K-Ar McDougall et al., 1969 Rakataura 117.6 +/- 5.2 K-Ar Cassata et al., 2008 119.2 +/- 2.8 Ar/Ar Leonard et al., 2017 Mt Cambria/Takaroro 42.3 +/- 11.1 Ar/Ar Leonard et al., 2017 42.3 22.0 Ar/Ar Leonard et al., 2017 2 Mt Eden/ 28.4 +/- 0.3 14C East and George 2003 28.0 0.6 tephrochronology Hopkins et al., 2017 See SM1 1 Maungawhau 14-16.2 Thermoluminescence Adam 1986; Phillips 1989; Wood 1991 154 +/- 4.0 K-Ar McDougall et al., 1969 21.2 +/- 3.3 Ar/Ar Leonard et al., 2017 28.0 +/- 0.3 Tephrochronology Hopkins et al., 2017 Morphostratigraphy Hayward et al., 2011a Younger than Te Pou Hawaiki, Three Kings/Te Tātua-a-Riukiuta, One Tree Hill/Maungakiekie and the Domain/Pukekawa Mt Hobson/ 56.9 +/- 5.8 Ar/Ar Leonard et al., 2017 34.2 1.8 Tephrochronology Hopkins et al., 2017 4 Ōhinerangi/Ōhinerau 34.2 +/- 0.9 Tephrochronology Hopkins et al., 2017 Older than Three Kings/Te Tātua-a-Riukiuta Morphostratigraphy Hayward et al., 2011a Mt Richmond/ 29.9 +/- 0.6 14C Sandiford et al., 2002 30.2 4.2 Tephrochronology Hopkins et al., 2017 2 Otahuhu 24.3 +/- 4.8 Ar/Ar Leonard et al., 2017 30.2 +/- 2.1 Tephrochronology Hopkins et al., 2017 Mt Roskill/ 105.3 +/-3.1 Ar/Ar Leonard et al., 2017 105.3 6.2 Ar/Ar Leonard et al., 2017 2 Puketāpapa/ Blake p.mag excursion? Paleomagnetic signature Shibuya et al., 1992 Pukewīwī Older than Three Kings/Te Tātua-a-Riukiuta Hayward et al., 2011a Mt Smart/ 16.4 +/- 1.8 Ar/Ar Leonard et al., 2017 20.1 0.2 Tephrochronology Hopkins et al., 2017 4 Raratonga 20.1 +/- 0.1 Tephrochronology Hopkins et al., 2017 Younger than One Tree Hill/Maungakiekie Morphostratigraphy Hayward et al., 2011a and Māngere Mt/Te Pane o Mataaho, older than Mt Wellington/Maungarei Mt St John/ 19.5 +/- 0.3 14C Horrocks et al., 2005 75.3 3.4 Ar/Ar Leonard et al., 2017 2 Te Kōpuke/ 75.3 +/- 1.7 Ar/Ar Leonard et al., 2017 Tītīkōpuke Older than Mt Eden/Maungawhau and Three Morphostratigraphy Eade 2009 Kings/Te Tātua-a-Riukiuta Mt Victoria/ 57.6 +/- 7.4 Ar/Ar Leonard et al., 2017 34.8 4.0 Tephrochronology Hopkins et al., 2017 4 Takarunga 34.8 +/- 2.0 Tephrochronology Hopkins et al., 2017 Reliability Centre name / Alternate name Recognised ages (ka, 1sd error) Method Reference Preferred Age (ka) Error (2sd) Method Reference Comments Grouping Mt Wellington/ 10.0 - 10.6 14C Searle 1965; Grant-Taylor and 10.0 1.0 Tephrochronology Hopkins et al., 2017 1 Rafter 1971; Fergusson and Rafter 1959. Maungarei 57-82 K-Ar McDougall et al., 1969 11.7 +/- 1.4 Thermoluminescence Wood 1991 10.1 +/- 0.2 Tephrochronology (Opepe Lowe et al., 2008; Molloy et mingled shards, Rotoma above al 2009; Hopkins et al., 2017 in sediment cores) Younger than Purchas Hill/Te Tauoma Morphostratigraphy Hayward et al., 2011a North Head/ 87.5 +/- 7.6 Ar/Ar Leonard et al., 2017 87.5 15.2 Ar/Ar Leonard et al., 2017 2 Maungauika 80 - 140 Morphostratigraphy Agustin-Flores et al., 2015 One Tree Hill/ 16.5 - 20 Thermoluminescence Adam 1986; Phillips 1989; 67.0 12.0 Tephrochronology Hopkins et al., 2017 2 Wood 1991 Maungakiekie 83.0 +/- 5.0 K-Ar McDougall et al., 1969 52.8 +/- 3.8 Ar/Ar Leonard et al., 2017 67.0 +/- 6.0 Tephrochronology Hopkins et al., 2017 Older than Te Hopua-a-Rangi/Gloucester Morphostratigraphy Hayward et al., 2011a Park, Three Kings/Te Tātua-a-Riukiuta, Mt Eden/Maungawhau, Mt Hobson/Ōhinerangi/Ōhinerau, Mt Smart/Raratonga Onepoto/ ≥ 28.5 14C Fergusson & Rafter 1959; 187.6 Tephrochronology and Hopkins et al., 2017 See SM1 2 Grant-Taylor and Rafter 1963, morphostratigraphy 1971 Te Kopua-o- 248 +/- 28 Ar/Ar Shane and Sandiford 2003 Matakarepo > 45 º Tephrochronology Shane and Sandiford 2003 > 150 - 220 Tephrochronology Lindsay and Leonard 2009 187.6 Tephrochronology Hopkins et al., 2017 Equal in age or older than Tank Farm/Te Tephrochronology & Hopkins et al., 2017 Kopua-o-Matakamokamo morphostratigraphy Equal in age, or younger than Tank Farm/Te Morphostratigraphy Lindsay et al., 2011 Kopua-o-Matakamokamo Ōrākei > 83 Tephrochronology Molloy et al., 2009 126.0 6.0 Tephrochronology Hopkins et al., 2017 2 126.0 +/- 3.0 Tephrochronology Hopkins et al., 2017 Older than Little Rangitoto/Maungarahiri Morphostratigraphy Kermode 1992 Otara Hill/ 56.5 Morphostratigraphy Hopkins et al., 2017 56.5 Morphostratigraphy* Hopkins et al., 2017 3 Te Puke-o-Taramainuku unusual p.mag varience the same as Hampton Paleomagnetic signature Cassata et al., 2008 Park Equal to or younger than Hampton Park Morphostratigraphy Sibson 1968 Ōtuataua 29-36 K-Ar McDougall et al., 1969 24.2 1.8 Tephrochronology Hopkins et al., 2017 See SM1 2 24.2 +/- 0.9 Tephrochronology Hopkins et al., 2017 Panmure Basin/ 31.7 - 32.7 14C Fergusson & Rafter 1959; 25.2 1.8 Tephrochronology Hopkins et al., 2017 2 Grant-Taylor and Rafter 1963; Polach et al., 1969 Te Kopua Kai-a-Hiku 189.0 +/- 0.6 K-Ar McDougall et al., 1969 > 17.6 Tephrochronology (Rerewhakaaitu) 25.2 +/- 0.9 Tephrochronology Hopkins et al., 2017 Older than Mt Wellington/Maungarei Morphostratigraphy Pigeon Mt/O Huiarangi 23.4 +/- 0.4 Tephrochronology Hopkins et al., 2017 23.4 0.8 Tephrochronology Hopkins et al., 2017 3 Puhinui Craters ≥ McLaughlins Mt/Matukutūreia (statellite craters?)Morphostratigraphy undated 5 Pūkaki Lagoon/Te Pukaki >> 45º Tephrochronology Shane 2005 >> 45 Tephrochronology Shane 2005 3 Tapu-o-Poutukeka Pukeiti/ 32.0 +/- 6.0 K-Ar McDougall et al., 1969 23.7 Morphostratigraphy* Hayward and Hopkins See SM1 4 Reliability Centre name / Alternate name Recognised ages (ka, 1sd error) Method Reference Preferred Age (ka) Error (2sd) Method Reference Comments Grouping Te Puketapapatanga a 11.4 +/- 3.6 Ar/Ar Leonard et al., 2017 2019 Hape 15.3 +/- 0.7 (discredited) Tephrochronology Hopkins et al., 2017 Equal in age, or older than Otuataua Morphostratigraphy Younger than Waitomokia/Moerangi/Mt GabrielMorphostratigraphy Searle 1959a Pukewairiki/ > 130ª Morphostratigraphy Hayward et al., 2011a > 130 Morphostratigraphy Hayward et al., 2011 3 Highbrook Park Older than Styaks Swamp Morphostratigraphy Allen and Smith 1994 Puketūtū/ 77.0 +/- 9.0 K-Ar McDougall et al., 1969 29.8 4.4 Tephrochronology Hopkins et al., 2017 2 Te Motu a Hiaroa 16-38 K-Ar Mochizuki et al., 2007 33.6 +/- 3.7 Ar/Ar Cassata et al., 2008 22.8 +/- 3.3 Thermoluminescence Wood 1991 29.8 +/- 2.2 Tephrochronology Hopkins et al., 2017 Mono lake excursion Paleomagnetic signature Shibuya et al., 1992 Lake Pupuke/ > 42 14C Fergusson & Rafter 1959; 193.2 5.6 Ar/Ar Leonard et al., 2017 See SM1 1 Grant-Taylor and Rafter 1963 Pupuke Moana 178-263 K-Ar McDougall et al., 1969 141 +/- 10 Thermoluminescence Wood 1991 207 - 260 Ar/Ar Hall and York 1984 200.1 +/- 7.4 Ar/Ar Cassata et al., 2008 193.2 +/- 2.8 Ar/Ar Leonard et al., 2017 Similar age to Tank Farm/Te Kopua-o-MatakamokamoMorphostratigraphy and Onepoto/Te Kopua-o-MatakarepoHayward et al., 2011 Purchas Hill/Te Tauoma 10.9 +/- 0.1 14C Lindsay et al., 2011 10.9 0.2 14C Lindsay et al., 2011 1 Equal to or older than Mt Wellington/MaungareiMorphostratigraphy Hayward 2006 Rangitoto 2 502 - 504 cal. yrs. BP 14C Needham et al., 2011 504 cal. yrs. BP 10 yrs 14C Needham et al., 2011 See SM1 1 (Upper Tephra or surface lava 146-465 ka K-Ar McDougall et al., 1969 flows) 560 -659 yrs Thermoluminescence Adam 1986; Phillips 1989; Wood 1991 140 -950 yrs Magnetic Declination Robertson, 1983, 1986

Rangitoto 1 532 - 615 cal. yrs. BP 14C Needham et al., 2011 553 cal. yrs. BP 14 yrs 14C Needham et al., 2011 (Lower Tephra) 565 ca; yrs. BP Obsidian hydration Stevenson et al., 1996; Lowe 1 et al., 2000 <1314 AD Tephrochronology (Kaharoa) Newnham et al., 2010

Rangitoto 214 - 1114 yrs 14C Elliot unpublished; Brothers and Golson 1959; Fergusson and Rafter 1959; Polach et al 1969; McDougall et al 1969; Davidson 1972; Grant Taylor and (Historic) Rafter 1963; Law 1975; Robertson1986; Mt Robertson/Sturges Park 24.3+/- 0.4 Tephrochronology Hopkins et al., 2017 24.3 0.8 Tephrochronology Hopkins et al., 2017 3 St Heliers/ Glover Park/ > 45 º Tephrochronology Lindsay et al., 2011 161.0 36.0 Tephrochronology and Hopkins et al., 2017 3 Whakamuhu 161 +/- 18.0 Tephrochronology Hopkins et al., 2017 morphostratigraphy Styaks Swamp Equal to or younger than Green Morphostratigraphy Hayward et al., 2011a 19.1 Morphostratigraphy* Hopkins et al., 2017 3 Mt/Matanginui 19.1 Morphostratigraphy Hopkins et al., 2017 Tank Farm/ Tuff Crater/ > 45 º Tephrochronology Leonard et al., 2017 181.0 2.0 Tephrochronology Hopkins et al., 2017 See SM1 2 Te Kopua-o-Matakamokamo 181+/- 1.0 Tephrochronology Hopkins et al., 2017 Equal in age or older than Onepoto/Te Kopua-o-MatakarepoMorphostratigraphy Lindsay et al., 2011 Taylors Hill/Taurere 27.4 +/- 1.6 Ar/Ar Leonard et al., 2017 30.2 0.2 tephrochronology Hopkins et al., 2017 2 30.2 +/- 0.1 Tephrochronology Hopkins et al., 2017 Mono lake excursion Paleomagnetic signature Cassata et al., 2008 Te Pou Hawaiki Older than Mt Eden/Maungawhau Morphostratigraphy Affleck et al., 2001 > 28.0 Morphostratigraphy Affleck et al., 2001 See SM1 3 Three Kings/ 28.6 +/- 0.2 14C Eade 2009 31.0 1.8 Tephrochronology Hopkins et al., 2017 2 Te Tātua-a-Riukiuta 32.7 +/- 0.8 14C Fergusson & Rafter 1959; Grant-Taylor and Rafter 1963 31.0 +/- 0.9 Tephrochronology Hopkins et al., 2017 Younger than One Tree Hill/Maungakiekie, Morphostratigraphy Hayward et al., 2011a Reliability Centre name / Alternate name Recognised ages (ka, 1sd error) Method Reference Preferred Age (ka) Error (2sd) Method Reference Comments Grouping Mt Hobson/Ōhinerangi/Ōhinerau, Mt St Waitomokia/Moerangi/ >15.6 Tephrochronology (Rotorua) P. Shane unpublished 20.3 0.2 Tephrochronology Hopkins et al., 2017 See SM1 2 Mt Gabriel 20.3 +/- 0.1 Tephrochronolohy Hopkins et al., 2017 Older than Pukeiti/Te Puketapapatanga a HapeMorphostratigraphy Searle 1959a Wiri Mt/ 30.3-32.9 14C Searle 1965; Polach et al., 30.1 - 31.0 Ar/Ar Cassata et al., 2008 1 1969; McDougall et al., 1969; Grant-Taylor and Rafter 1971 Te Manurewa o Tamapahore 33- 64 K-Ar McDougall et al., 1969 27.0 +/- 5.0 K-Ar Mochizuki et al., 2004 30.1 - 31.0 Ar/Ar Cassata et al., 2008 Mono lake excursion Paleomagnetic signature Cassata et al., 2008 Equal to or younger than Ash Hill Morphostratigraphy Hayward et al., 2011a

Supplementary Table 1 (SM Table 1). Full reference data set of all recognised ages for the AVF centres including reference and methods for analysis. 4C calibrations were done in 2011 by CalPal.ª The age of 130 ka indicates geomorphological evidence that shows marine erosion during the last sea level high-stand related to the last interglacial at c. 130 – 120 ka (Hayward and Hopkins 2019). ºThe age of 45 ka is related to the eruption of the Rotoehu tephra, which at present has an ambiguous age (see discussion in SM 1).*indicates centres where morphostratigraphy suggests contemporaneous eruptions (for example no organic material between successive volcanic deposits); these are given an arbitrary difference in age of 500 years, based on a minimum time it takes to form soil horizons. Number of centres Reliability Number of centres (pre-DEVORA; as Definition Group (current) per Lindsay et al., 2011) 12 (including 2 1 Reliable reproducible age determined eruptions from 3 Rangitoto) Reliable single age determination or several reliable ages spanning a relatively small range, or strong palaeomagnetic correlation with centres 2 23 8 of known age, or high confidence correlation of tephra sedimentation rates calculated Minimum age based on tephrochronology and sedimentation rate in core, 3 11 9 morphostratigraphy, or low confidence correlation of tephra 4 Varied, conflicting or near-detection-limit age determinations 5 12

5 No direct or minimum age date determined 3 18

Supplementary Table 2 (SM Table 2). Definitions and details for the reliability groupings shown on Table 1 expanded from Lindsay et al., 2011. Eruption Hazard Summary Considerations for Hazard and Risk Assessment Requirements for hazard assessment

 Location + basal area (hazard A new edifice (maar, tuff ring, tuff cone, and/or scoria cone) has formed during each eruption within the footprint) The formation of edifices (e.g. maars, tuff rings, tuff cones, and scoria cones) will be devastating within their AVF (Allen and Smith 1994; Kereszturi et al., 2013). The databases of Allen and Smith (1994) and hazard footprint, either through explosions (associated high dynamic forces) and/or thick deposit accumulation Edifice formation Kereszturi et al. (2013) indicate that tuff ring radii (including the central maar) ranges between 220 and (lava and/or tephra). However, the type of edifice that forms will be dependent on the style of eruption that 1370 m in the AVF, while scoria cone radii ranges between 60 and 526 m (Kereszturi et al., 2013). Scoria occurs. cones build up to heights less than 150 - 200 m (Smith and Allen, 1993).  Eruption Style (type of edifice)

At least 34 eruption sequences in the AVF have produced lava flows, generally occurring later in the eruption sequence (Smith and Allen 1993; Allen and Smith 1994; Kereszturi et al., 2014b). Previous  Hazard footprint eruptions have resulted in either pahoehoe or a’a’ lava, however, Rangitoto produced both and represents Kereszturi et al. (2012) used LiDAR to quantify lava flow susceptibility within the AVF, concluding that hilly the largest volume of lava produced in the field (Nowak 1995; Needham et al., 2011; Kereszturi et al., areas typical in northern portions of the field are more likely to have channelized lava flows in comparison to the relatively flat southern areas. Importantly, the urban development (built environment and alteration of the Lava flows 2014b; Rhode 2016). Morphology of lava flows in the AVF are highly variable, but for eruptions that have natural environment) of Auckland may also influence where lava flows travel, a factor seldom considered included an effusive phase, the median area affected by lava flows is 0.6 km2, with a minimum of 0.1 km2 within lava flow hazard modelling approaches to date (e.g. Kereszturi et al., 2014b) and this is an area of (North Head; Fig. 1b), and a maximum of 25 km2 (Rangitoto) (Kereszturi et al., 2014b). The longest lava active research within the DEVORA research programme (Tsang et al., 2019).  Type of lava flow flow in the AVF thus far has been measured at ~11 km from Mt St John (Eade, 2009). Thicknesses of lava flows have reached up to 50 m in Auckland (e.g. Mt Eden/Maungawhau; Allen and Smith, 1994).

 Vent location

Potential PDC hazard is a key factor for evacuation planning as it is expected to be the greatest risk to life in a Based on geological investigations, the type of PDC expected is the more dilute ‘base surge’, which occurs future AVF eruption (Lindsay et al., 2010). Modelling based on the Maungataketake eruption (Fig. 1b), Base surges during phreatomagmatic eruptions in the AVF (Cassidy et al., 1986; Smith and Allen 1993; Allen et al., suggests under a worst-case scenario, surges may contain enough dynamic pressure to completely destroy (Pyroclastic 1996). Over 80% of previous AVF eruptions have included phreatomagmatic eruption phases (Kereszturi everything within 2.5 km of the vent (Brand et al., 2014). Heavy structural damage would occur to most  PDC hazard footprint density currents et al., 2014a). Depending on eruption dynamics and topography, base surges are expected to travel from 1 structures out to 4 km, and damage would decay 4-6 km from the vent (Brand et al., 2014). However, this [PDC]) km to up to 5-6 km from the vent (Németh et al., 2012; Brand et al., 2014; Sandri et al., 2014; Agustín- assumes PDC inundation occurs radially from the vent and no influence of eruption dynamics, topography, nor Flores et al., 2015a,b). the built environment on PDC energy, directivity and run-out. Further research is ongoing to obtain a better understanding of how surges will interact with the built environment in Auckland.  Hazard intensity (dynamic pressure, deposit thickness, temperature) attenuation

 Vent location

Due to the prevalence of westerly winds in Auckland, areas exposed to greatest tephra fall hazard are areas Tephra fall deposits have been observed from all known eruptions within the AVF (Kereszturi et al., northeast and east (Smith and Allen, 1993; Molloy et al., 2009; Hopkins et al., 2015). However, complex lobe 2013). Limited systematic collection of volume, mass, and thickness data has been conducted for AVF patterns identified by correlating tephra from lake cores to potential eruption sources indicates the importance tephra fall deposits, particularly for medial-distal areas – with deposit preservation and access both Tephra fall of varying wind conditions during an eruption (Hopkins et al., 2017). Proximal areas are likely to be buried  Tephra hazard footprint limiting factors (Hayes et al., 2018). Thus, there is a reliance on information from other basaltic eruptions beneath deep tephra deposits (see edifice formation) (Houghton et al., 2006). Tephra deposition in medial to (e.g. Kīlauea Iki 1959; Kīlauea 1983; Etna 2001; Etna 2002–2003) to characterise future AVF tephra distal locations (e.g. >5 km from vent) will likely only accumulate to a few millimetres to centimetres, but this hazard (Houghton et al., 2006). can still be highly disruptive to urban functionality (Wilson et al., 2012).

 Hazard intensity (static loading, deposit thickness) attenuation Eruption Hazard Summary Considerations for Hazard and Risk Assessment Requirements for hazard assessment

Volcanic earthquakes are likely to occur in a future eruption, due to the ascent of magma and potential The entire field is likely to be exposed to volcanic earthquakes during a future eruption (Sherburn et al., 2007). deformation and stress-field changes (Sherburn et al., 2007). As there is no observed paleoseismic  Location, depth and magnitude The severity of ground-shaking will be dependent on a number of factors, but it is likely some structural Volcanic evidence during AVF's lifespan, it is necessary to use seismic records from eruptions at analogue damage to the built environment will occur. Earthquakes may also have a considerable social impact, earthquakes volcanoes to infer potential earthquake phenomena during a future AVF eruption. Based on historically influencing evacuation and other behaviours. In general, ground shaking severity will decay with distance from active volcanic fields such as Michoacán-Guanajuato volcanic field, earthquake magnitudes of up to 5.5 the earthquake epicentre, but geological conditions (e.g. soil type) can influence ground shaking locally. ML associated with the eruptions are considered credible (Blake et al., 2006; Sherburn et al., 2007).  Number of earthquakes  Ground shaking intensity and duration

Land deformation (inflation, subsidence, ground cracking) is expected to be associated with future AVF Land deformation will likely be used for volcano monitoring during a future AVF eruption to identify potential  Hazard footprint Volcanic land volcanism due to magma ascent and withdrawal (Smith and Allen 1993). We are unaware of any studies surficial outbreaks (Samsonov et al., 2010). Land deformation can be highly damaging to roads and buried deformation which observed geological evidence of deformation in previous AVF eruptions, and so experiences from services within several kilometres of intruding magma (Blong 1984). We are unaware of any studies which analogue volcanoes form the basis for assumptions. attempt to assess potential impacts of ground deformation for a future AVF eruption.

 Duration of deformation  Rate of deformation

 Hazard footprint Coastal areas are susceptible to tsunami produced by a future AVF eruption. Explosion generated tsunami Eruption induced Volcanic eruptions within or near coastal areas of the AVF could produce local source tsunami from either formed in proximal offshore areas of the AVF could reach shorelines within minutes and persist for hours tsunami collapse of edifice or due to underwater explosions (Smith and Allen 1993; de Lange and Healy 2001 (Pratsetya 1998).  Hazard intensity (speed, dynamic pressure) attenuation

 Spatial density of ballistics Volcanic ballistic projectiles are clasts of rock and incandescent material over 10 cm in diameter and can Volcanic ballistic projectiles can be ejected to distances several kilometres from the vent, but the spatial be generated from any explosive eruptions (both magmatic and phreatomagmatic) in the AVF. They could density of impacts and impact energies will be complex (Fitzgerald et al. 2014; Tsunematsu et al. 2016). Volcanic ballistic affect areas several kilometres from the vent (Smith and Allen 1993). Houghton et al. (1999) observed Impact energy (Joules) of volcanic ballistic impacts can damage building materials (Williams et al. 2017). Hot projectiles tephra beds with large numbers of bombs leaving marked impact sags and soft sediment deformation incandescent ballistics produced during dry eruptions can contain enough heat to ignite vegetation and associated with the Crater Hill eruption. common household contents (e.g. sofas), leading to fires (Blong 1984).  Impact energy

Volcanic gas can be a hazard to human and animal health, the built environment and agriculture, particularly  Hazard footprint Harmful gases such as CO2, SO2, H2S, HCl and HF, among others, can escape from a rising melt body for a high dose (a function of concentration and duration of exposure). Gas dispersal will depend on vent immediately prior to and during a future AVF eruption (Smith and Allen 1993; Stewart et al. in review). Volcanic gas location, meteorological conditions, dispersion height and timing (Stewart et al. in review). Gas is likely to The resulting plume can then be dispersed downwind, potentially affecting large areas. A baseline soil emissions pool in topographical depressions, but may also affect all of the Auckland region. Ongoing work is aiming to CO flux has been established in Auckland to compare during unrest scenarios and confirmed that, at 2 quantify the potential gas emission hazard from future AVF eruptions, and to model gas dispersion from present, sources of CO in the AVF are biogenic with no volcanic component (Mazot et al., 2013). 2 selected eruption scenarios.  Hazard intensity (ambient concentrations) attenuation Eruption Hazard Summary Considerations for Hazard and Risk Assessment Requirements for hazard assessment

 Hazard footprint Shock waves are atmospheric disturbances that propagate away from a vent during explosive eruptions There has been limited detailed investigation into the expected physical characteristics of shock waves from (Nairn 1976). Shock waves are anticipated to occur during AVF eruptions, particularly associated with Shock waves AVF or other analogue eruptions, but dynamic pressures produced by shock waves elsewhere have damaged PDCs from phreatomagmatic eruptions (Smith and Allen 1993; Agustín-Flores et al., 2014; Agustín-Flores structures several kilometres from the vent (Magill et al., 2013). et al., 2015a).  Hazard intensity (dynamic pressure) attenuation

Supplementary Table 3 (SM Table 3). Detailed information on the eruption hazards, their consideration and requirements for hazard and risk assessment. These details provide evidence for the bridge between the physical volcanology and the hazard and risk management and policy applications. 1 Supplementary Material 1 (SM 1)

2 Dating techniques employed in the AVF 3 There are a number of dating techniques that have been used to determine the ages of AVF centres. The 4 key methods include: 5 (1) Radiocarbon dating (14C) of organic material, most commonly plant material (wood, peat) preserved 6 immediately above or below eruptive products but sometimes found within the deposits themselves 7 (e.g. Searle, 1961a; Hayward, 2008; Lindsay and Leonard, 2009); 8 (2) Isotopic analysis of samples of erupted lava flows, initially using K-Ar (e.g. McDougall et al., 1969) 9 and more recently 40Ar/39Ar techniques (e.g. Cassata et al., 2008; Leonard et al., 2017); 10 (3) Tephrochronology, through either the inference of a minimum age based on the presence of a 11 known-age tephra within or overlying volcanic deposits (e.g. Sandiford et al., 2001; Molloy et al., 2009; 12 Zawalna-Geer et al., 2016), through sedimentation rate calculations (e.g. Hopkins et al., 2015), or 13 through geochemical correlation of tephra with the eruptive products at source (e.g. Hopkins et al., 14 2015, 2017); 15 (4) Age equivalence dating, most commonly through paleomagnetic analysis (e.g. Shibuya et al., 1992; 16 Cassidy, 2006; Mochizuki et al., 2006; Cassata et al., 2008; Cassidy and Hill, 2009; Cassidy and Locke, 17 2010); 18 (5) Stratigraphy, producing relative ages based on stratigraphic relationships between erupted products 19 of different centres (e.g. Searle, 1961a; Kermode 1992; Lindsay et al., 2011; Hayward et al., 2016) and 20 relative sea levels (e.g. Hayward et al., 2011a; Agustín-Flores et al., 2015); and 21 (6) Statistical modelling and predictions (e.g. Bebbington and Cronin 2011; Green et al., 2014; 22 Kawabata et al., 2016). 23 For a full review of the methods and how they have been used in the AVF specifically, see Lindsay and 24 Leonard (2009) and Lindsay et al. (2011). However, since these publications were produced, a number 25 of advances have been made in some of the dating techniques. These advances are reviewed below. 26

27 Recent Advances in dating techniques 28 Historically, isotopic analysis has been used as a dating tool in the AVF. However, original studies by 29 McDougall et al. (1969), using the K-Ar decay system, have been shown to produce erroneously old 30 ages due to excess 40Ar included in the phenocrysts of the whole rock analyses (SM Table 2). This 31 problem was overcome by Mochizuki et al. (2007; K-Ar system) and Cassata et al. (2008; 40Ar/39Ar 32 system) who showed that by removing the phenocrysts and using just groundmass, their analyses 33 produced much more viable results. However, young basaltic rocks, such as those that make up the 34 AVF, are notoriously difficult to date through these methods due to their low radiogenic Ar content 35 coupled with their young age (Leonard et al., 2017). Improvements in the 40Ar/39Ar analytical 36 techniques (e.g. Fleck et al., 2014), coupled with increasing sensitivity in gas mass-spectrometry and 37 incremental heating techniques (e.g. Lanphere, 2000), have made analysis of young basaltic material 38 much more effective. In a recent field-wide study, Leonard et al. (2017) published ages for twenty-three 39 centres using these improved methods of 40Ar/39Ar dating. Their results produced a total of nine entirely 40 new ages (for centres which previous had no ages constraints), and fourteen ages for centres with pre- 41 existing (but often poorly constrained) age determinations. 42 In addition, Hopkins et al. (2017) improved the chronology of the AVF eruptions through a 43 multidisciplinary approach, combining tephrochronology, geochemistry, existing ages and spatial 44 analysis of tephra deposits. Prior to this study, the basaltic tephra horizons in the AVF maar sediment 45 cores had been identified and analysed for their major elements, allowing some deposits to be correlated 46 between the deposition centres (e.g. Sandiford et al., 2001; Shane and Hoverd, 2002; Molloy et al., 47 2009; Shane et al., 2013; Hopkins et al., 2015; Zawalna-Geer et al., 2016). However, the sources of 48 these basaltic tephra horizons remained unknown, and therefore the usefulness of the basaltic tephra for 49 understanding the evolution of the field was limited to eruption frequencies. To further this work, 50 Hopkins et al. (2017) developed a method using trace elements in volcanic glass to correlate tephra 51 deposits within the maar sediment cores, both between the cores themselves and then back to their 52 source centre. Linking the tephra deposits back to their source allowed the relative and, in many cases, 53 numerical ages to be determined from the core stratigraphies, and resulted in the construction of a field- 54 wide chronology. 55 56 Currently Ambiguous ages in the AVF 57 1. Rangitoto 58 Currently ambiguous ages could not be discussed without starting at the eruption(s) of the AVF’s 59 youngest and most unique volcanic centre, Rangitoto, with studies claiming one (e.g. Nichol, 1992), 60 two (e.g. Horrocks et al., 2005; Needham et al., 2011; McGee et al., 2011), three or more (e.g. Linnell 61 et al., 2016; ) eruption phases for this centre (e.g. Shane et al., 2013; Zawalna-Geer et al., 2016; Lowe 62 et al., 2017). The seminal study of Needham et al. (2011) identified two tephra horizons (correlated by 63 glass-shard major element composition), separated by peat. These tephra horizons were dated through 64 14C dating, producing mean ages from 10 samples of 553 ± 7 cal. yrs. BP and 504 ± 5 cal. yrs. BP. The 65 evidence for multiple pulses of ashfall, split by a small break in time, is also confirmed by 66 archaeological evidence where footprints from humans and dogs have been found preserved between 67 the tephra layers on adjacent Motutapu Island (Nichol, 1982) (though this proves that Auckland was 68 occupied by humans during the AVF’s youngest eruptions, there are no written records or clear oral 69 histories from this time known to researchers [Smith and Allen, 1993]). Geochemical analysis of the

70 products of the eruption(s) revealed bimodal geochemistries; an alkalic (SiO2 ≤ 47 wt.%) and a sub-

71 alkalic (SiO2≥ 48 wt.%) signature, further supporting the hypothesis of two key eruptions. However, 72 multiple basaltic cryptotephra horizons were identified in two Lake Pupuke/ Pupuke Moana cores which 73 were interpreted as intermittent activity from ca. 1500 to 500 cal. yrs. BP (Shane et al., 2013). More 74 recently (2014), coring of Rangitoto lava flows and the original sea floor underneath revealed a 75 potentially 6 kyr eruption history (Linnell et al., 2016). However, the fidelity of both the cryptotephra 76 research and the interpretation of the drill core stratigraphy was questioned by Hayward and Grenfell 77 (2015) and Hayward (2017) respectively, and the older ages are almost universally not accepted within 78 the AVF research community. Continued research into this conundrum has brought up subsequent 79 evidence refuting the older ages (Cronin et al., 2018). Tephra deposits found on Motutapu Island are 80 proposed to have resulted from surge flow, transported over water, and thus capturing the presumed 81 initial, emergent growth phase of the volcano. The geochemistry of these surge deposits correlate to the 82 geochemistry found at the base of the drill core (Linnell et al., 2016) and with the tephra found on 83 Motutapu Island (Needham et al., 2011) and North Cone, relating to the earliest phase of eruptions, pre- 84 dating the shield-forming phase which has the more evolved geochemistry. This evidence logically 85 suggests that the early phase of eruptions from Rangitoto were phreatomagmatic, and, once seawater 86 was excluded from the active vent site, the secondary phase of the eruption was magmatic linked to the 87 shield building phase. Cronin et al, (2018) proposed that the entire Rangitoto sequence was erupted 88 between ca. 650 – 550 cal. yrs. BP, and as hypothesised here, was actually remarkably similar in its 89 physical evolution to the other large volcanoes in the field, for example, Mt Eden/ Maungawhau, Three 90 Kings/ Te Tātua-a-Riukiuta, and One Tree Hill/ Maungakiekie. A short-lived eruption sequence is 91 supported by the similar, young, radiometric dates of the lower-most lava flows in the Rangitoto drill 92 core, previously thought to have erupted 6 ky apart (L. McGee, unpublished data). Paleomagnetic dating 93 of the lava flows in the Rangitoto drill core are ongoing and may be able to further confirm this 94 hypothesis. Our ability to date the different phases of the eruption highlights the complexity of the older 95 eruptions that we cannot resolve through the uncertainties of dating older material. 96 97 2. Otuataua, Pukeiti/ Te Puketapapatanga a Hape, Maungataketake/ Ellets Mt and Waitomokia/ 98 Moerangi/ Mt Gabriel 99 Otuataua, Pukeiti/ Te Puketapapatanga a Hape/ Te Puketapapatanga a Hape, Maungataketake/ Ellets 100 Mt and Waitomokia/ Moerangi/ Mt Gabriel are four volcanic centres that sit in the south-west of the 101 field. Waitomokia/ Moerangi/ Mt Gabriel is located ~1.8 km to the north of Otuataua, Otuataua and 102 Pukeiti/ Te Puketapapatanga a Hape overlap in their spatial footprint (with no observed paleosols 103 between the deposits, suggesting minimal time lapse between their eruptions) and Maungataketake/ 104 Ellets Mt is centred ~1.2 km to the south of Otuataua (Fig. 1b). Searle (1959a) identified a lava flow 105 from Pukeiti/ Te Puketapapatanga a Hape passing below those of Otuataua, and noted that the Pukeiti/ 106 Te Puketapapatanga a Hape lava flows are ash-covered, whereas the Otuataua flows are not. This led 107 Searle (1959a) to suggest that an eruption must have occurred after Pukeiti/ Te Puketapapatanga a Hape- 108 -but before Otuataua--from another nearby volcano, most likely from either Waitomokia/ Moerangi/ 109 Mt Gabriel or Maungataketake/ Ellets Mt. However, this is very unlikely; the currently preferred ages 110 for Waitomokia/ Moerangi/ Mt Gabriel and Maungataketake/ Ellets Mt are 20.3 ± 0.1 ka (Hopkins et 111 al., 2017) and 88.9 ± 2.4 ka (Leonard et al., 2017), respectively. In comparison, the currently preferred 112 age for Otuataua is 24.2 ± 0.9 ka (Hopkins et al., 2017), which provides a minimum age for the Pukeiti/ 113 Te Puketapapatanga a Hape eruption based on morphostratigraphy. This suggests that neither 114 Waitomokia/ Moerangi/ Mt Gabriel nor Maungataketake/ Ellets Mt eruption could have occurred 115 between the two eruptions of Otuataua or Pukeiti/ Te Puketapapatanga a Hape. 116 Geochemical and archeological evidence supports the idea of a minimal time break between the 117 Otuataua and Pukeiti/ Te Puketapapatanga a Hape eruptions. Hayward et al (2011a) suggested that 118 Pukeiti/ Te Puketapapatanga a Hape either occurred shortly before or coevally with Otuataua. 119 Geochemical analysis of the products from the two eruptions shows that the Pukeiti/ Te 120 Puketapapatanga a Hape lavas have a more primitive geochemical signature to the Otuataua lavas 121 (Hopkins unpublished data), suggesting the Pukeiti/ Te Puketapapatanga a Hape source evolved into 122 the Otuataua source, as seen within single centres and between other centres in the field. In addition, 123 archaeological excavations of the older flows from Otuataua showed evidence for ash burying the 124 surface before being buried by the younger flows from Otuataua. We therefore think it is mostly likely 125 that Pukeiti/ Te Puketapapatanga a Hape was the source of initial activity, or was produced early on in 126 the eruption history of the pair, followed by ash-producing, phreatomagmatic eruptions from Otuataua 127 that covered the existing lava flows (from potentially both Otuataua and Pukeiti/ Te Puketapapatanga a 128 Hape), followed and buried by further lava flows from Otuataua. It is therefore highly likely that the 129 age of the Pukeiti/ Te Puketapapatanga a Hape centre immediately predates the Otuataua centre. 130 131 3. Te Hopua-a-rangi/ Gloucester Park 132 At present, Te Hopua-a-rangi/ Gloucester Park has two contrasting ages associated with it: 1) 51.6 ± 133 3.2 (1sd; 40Ar/39Ar age from Leonard et al., 2017), and 2) 31.0 ka (sedimentation rate calculation from 134 Te Hopua-a-rangi/ Gloucester Park drill core linked to Poihipi tephra towards the base of the core). The 135 morphostratigraphic constraint is that it erupted through One Tree Hill/Maungakiekie lavas (Hayward 136 et al., 2011a; Hopkins et al., 2017), and therefore must be younger, and has a residual magnetic anomaly 137 of c.0 (Cassidy and Locke 2010). The 40Ar/39Ar age from Leonard et al. (2017) also sits within error of 138 the 40Ar/39Ar age obtained for One Tree Hill/Maungakiekie, we therefore hypothesise two options. 139 Either: 1) the Te Hopua-a-rangi/ Gloucester Parkeruption occurred very shortly after the emplacement 140 of the One Tree Hill/Maungakiekie lava flows or 2) the sample taken for 40Ar/39Ar analysis was sourced 141 from One Tree Hill/ Maungakiekie, rather than Te Hopua-a-rangi/ Gloucester Park. At present, the latter 142 is the more highly favoured option, giving a younger numerical age to Te Hopua-a-rangi/ Gloucester 143 Park. However, no basaltic material was recovered at the base of the core drilled and used for 144 sedimentation rate estimations of age, therefore the sedimentation rate estimates give a minimum age 145 for the eruption of Te Hopua-a-rangi/ Gloucester Parkof 31 ka. To help resolve this uncertainty, 146 resampling of the Te Hopua-a-rangi/ Gloucester Parkcrater for 40Ar/39Ar dating could be undertaken, or 147 if appropriate material could be found, 14C dating would likely allow the age to be resolved. 148 149 4. Mt Eden/ Maungawhau and Te Pou Hawaiki 150 Mt Eden/ Maungawhau, as one of the larger centres in the AVF with good exposure of its erupted scoria 151 and lava flow products, has been well studied by numerous dating techniques, leading to a confident 152 assignment of an age of 28 ka for its eruption. Conversely, Te Pou Hawaiki has never been numerically 153 dated, and its relative eruption age in relation to Mt Eden/ Maungawhau has been debated (e.g. Affleck 154 et al., 2001; Hayward et al., 2011a; Kenny, 2014; Hopkins et al., 2017). It is identified as an apparent 155 small welded scoria or spatter cone (Hayward, 2019), and is surrounded by lava flows and lava caves 156 that have orientations suggestive of a source from the Te Pou Hawaiki centre (Kenny, 2014). Using 157 gravity data and thorough paleo-topographic mapping, Affleck et al. (2001) suggested that Te Pou 158 Hawaiki may have been a surface manifestation of a much larger centre, infilling a depression in the 159 Waitemata paleosurface. Both these theories are supported by geochemical data (McGee et al., 2013; 160 Hopkins unpublished data) which suggests that the geochemical signature of the erupted products can 161 be linked to a very large amount of partial melting, and thus a large eruption volume (e.g. McGee et al., 162 2015). It is therefore possible that rather than the suggested satellite cone, or paired eruption with Mt 163 Eden/ Maungawhau, it stands alone as an individual large eruption. However, it remains numerically 164 undated with morphological evidence suggesting that it predates Mt Eden/ Maungawhau at ~28 ka (e.g. 165 Bartrum 1928; Affleck et al., 2001), but postdates Three Kings/ Te Tātua-a-Riukiuta at ~31 ka (Allen 166 and Smith, 1994). 167 168 5. Onepoto/ Te Kopu-o-Matakarepo, Tank Farm/ Tuff Crater/ Te Kopua-o-Matakamokamo and Lake 169 Pupuke/ Pupuke Moana 170 The age of three centres in the north-east of the field, Onepoto/ Te Kopu-o-Matakarepo, Tank Farm/ 171 Tuff Crater/ Te Kopua-o-Matakamokamo, and Lake Pupuke/ Pupuke Moana (Fig. 1b), are contentious 172 in their relative eruption order. Recent dating through 40Ar/39Ar techniques has resolved part of this by 173 giving an age of 193.2 ± 2.8 ka (Leonard et al., 2017) to the Lake Pupuke/ Pupuke Moana centre, 174 currently believed to be the oldest centre in the field. Morphostratigraphic evidence suggests that Tank 175 Farm/ Tuff Crater/ Te Kopua-o-Matakamokamo and Onepoto/ Te Kopu-o-Matakarepo erupted close in 176 time, or that Tank Farm/ Tuff Crater/ Te Kopua-o-Matakamokamo is slightly older than Onepoto/ Te 177 Kopu-o-Matakarepo; a small terrace identified in the Tank Farm/ Tuff Crater/ Te Kopua-o- 178 Matakamokamo crater shows evidence of draped Onepoto/ Te Kopu-o-Matakarepo tuff (Lindsay et al., 179 2011). However, material from a sediment core drilled in the crater of Onepoto/ Te Kopu-o-Matakarepo 180 in 2000-2001 contradicts this notion. The base of the 2000-2001 Onepoto core ended in scoria and 181 lapilli, and was presumed to be the Onepoto/ Te Kopu-o-Matakarepo crater base (Shane and Hoverd, 182 2002). The basal material was analysed by 40Ar/39Ar techniques, however, potentially erroneous results 183 meant that an age range of >150-220 ky was the most reliable estimate given for this material (Lindsay 184 et al., 2011). This basal deposit was also found within a repeat sediment core, “On2” (drilled in 2011). 185 At ~4 m above the basal material in the On2 core, researchers found another basaltic horizon (called 186 AVFc; Hopkins et al., 2015), followed by ~10 m of tephra-barren lacustrine sediment (Hopkins et al., 187 2015). If Onepoto/ Te Kopu-o-Matakarepo were the oldest of these three eruptions, one would 188 anticipate that two more tephra horizons (sourced from Lake Pupuke/ Pupuke Moana and Tank Farm/ 189 Tuff Crater/ Te Kopua-o-Matakamokamo) would be found within these lower sections of the core. 190 However, only one horizon is found, suggesting that Onepoto/ Te Kopu-o-Matakarepo is the middle 191 eruption (temporally) of these three. Determining which of Lake Pupuke/ Pupuke Moana or Tank Farm/ 192 Tuff Crater/ Te Kopua-o-Matakamokamo is represented above Onepoto/ Te Kopu-o-Matakarepo’s 193 basal material, and is therefore, younger, is challenging. Tank Farm/ Tuff Crater/ Te Kopua-o- 194 Matakamokamo was a moderately small eruption (DRE = 5.87x106 m3; Kereszturi et al., 2013) and 195 Lake Pupuke/ Pupuke Moana a much larger eruption (DRE = 46.7x106 m3 Kereszturi et al., 2013), 196 however, Tank Farm/ Tuff Crater/ Te Kopua-o-Matakamokamo is located just 600 m NNE of Onepoto/ 197 Te Kopu-o-Matakarepo, and Lake Pupuke/ Pupuke Moana is ~ 3 km NNE. Hopkins et al. (2017) 198 suggested that this relatively thin tephra horizon was more likely to be linked to the eruption of Tank 199 Farm/ Tuff Crater/ Te Kopua-o-Matakamokamo rather than Lake Pupuke/ Pupuke Moana due the small 200 scale of the Tank Farm/ Tuff Crater/ Te Kopua-o-Matakamokamo eruption. However, the further 201 distance and/or potential of an unfavourable wind direction could be used to argue that this thin tephra 202 horizon (AVFc) belongs to Lake Pupuke/ Pupuke Moana. The geochemical correlation for these 203 deposits was inconclusive due the lack of material available from Tank Farm/ Tuff Crater/ Te Kopua- 204 o-Matakamokamo (Hopkins et al., 2017). Therefore, the relative age of these eruptions remains 205 proposed in the literature, but in reality is unresolved. 206 207 6. Auckland Domain/Pukekawa and Grafton 208 Always considered to be some of the older volcanoes in the AVF (Searle, 1961a; Kermode, 1992), 209 Auckland Domain/Pukekawa and Grafton have been challenging to date due to their lack of obvious, 210 datable erupted products. Their proximity to one another has allowed their relative age to be identified 211 due to their physical relationship, showing that Auckland Domain/Pukekawa erupted through the 212 Grafton tuff ring (Hayward et al., 2011b). Numerical ages have been more difficult to obtain, with 14C 213 dating (Grenfell and Kenny, 1995) and tephrochronology (Rotoehu in peat cores from Auckland 214 Domain/Pukekawa crater; Lindsay et al., 2011) giving a minimum age of ~45 ka (Hayward ad Hopkins 215 2019). More recent advances in tephrochronology with geochemical correlations allowed Hopkins et 216 al. (2017) to propose an age for Auckland Domain/Pukekawa eruption of 106 ± 4.0 ka based on the 217 correlation of basaltic tephra found in Ōrākei Basin, Onepoto/ Te Kopua-o-Matakarepo, and Glover 218 Park/Whakamuhu cores. The similarity of the geochemical products of these two eruptions has also led 219 some authors to hypothesise that these eruptions were coupled, occurring very closely in space and time 220 (e.g. Hayward, 2019; Hayward and Hopkins 2019). 221 222 223 7. Influence of the age of Rotoehu Tephra 224 Rotoehu Tephra is the oldest identifiable rhyolitic tephra in the Auckland maar cores. Its age 225 therefore influenced the calibrated sedimentation rate ages assigned to cored tephra older than the next 226 youngest rhyolitic tephra, Tahuna Tephra (39.3 ± 1.2 ka; Hopkins et al., 2017). Hopkins et al. (2017) 227 used a conservative age of 52 ± 7 ka for Rotoehu Tephra so as to accommodate the wide range of ages 228 that had been determined by many different methods over the past 50 years. A more rigorous assessment 229 of the various dating techniques used (Hayward and Hopkins, 2019) suggests that an age of 45.5±2 ka 230 is more realistic (e.g. Shane et al., 2006; Allan et al., 2008; Danisik et al, 2012; Flude and Storey 2016). 231 If this younger age for Rotoehu Tephra is used, the ages determined by Hopkins et al. (2017) become 232 6-9 kyr younger (Table 2; see Hayward and Hopkins, 2019, Table 3). 233

234 Supplementary Material 2 (SM 2)

235 Reference 236 Within this manuscript there are multiple references to the presentations given at the annual DEVORA 237 Research Forums. Links to these presentations can be found at the following webpage: 238 http://bit.do/DEVORAPresentations