Complex crater fields formed by steam-driven eruptions: Lake Okaro, New Zealand

Cristian Montanaro1,2,†, Shane Cronin2, Bettina Scheu1, Ben Kennedy3, and Bradley Scott4 1Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Theresienstrasse 41, 80333 Munich, Germany 2School of Environment, University of Auckland, Science Centre, Building 302, 23 Symonds Street, Auckland Central 1010, New Zealand 3Department of Earth Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand 4GNS Science, 114 Karetoto Road, Wairakei 3377, New Zealand

ABSTRACT tuff, as well as within deposits from phase I. waters to steam due to the sudden arrival of heat Phase II breccias display vertical variation and gas from intruding magma (or magmatic Steam-driven eruptions are caused by in lithology that reflects top-down excavation fluids). The flashed steam overpressurizes the explosive vaporization of water within the from shallow levels (10–20 m) to >70 m. Af- bottom of an aquifer, but the aquifer is likely to pores and cracks of a host rock, mainly within ter another hiatus, lake levels rose. Phase III breach near a horizontal discontinuity, such as a geothermal or volcanic terrains. Ground or hydrothermal explosions were later triggered textural break or capping/sealing layer; thus, a surface water can be heated and pressurized by a sudden lake-level drop, excavating into simultaneous top-down rarefaction wave travels rapidly from below (phreatic explosions), or deposits from previous eruptions. This case within the aquifer, and a “bottom-up” pressure already hot and pressurized fluids in hydro- shows that once a hydrothermal system is es- wave in the overlying rock results in fragmenta- thermal systems may decompress when host tablished, repeated highly hazardous hydro- tion and ejection of lithic clasts. Phreatic erup- rocks or seals fail (hydrothermal eruptions). thermal eruptions may follow that are as large tions do not require the presence of a geothermal Deposit characteristics and crater morphol- as initial phreatic events. system and thus may eject fresh, unaltered rock. ogy can be used in combination with knowl- Deposits from a phreatic eruption should show a edge of host-rock lithology to reconstruct the INTRODUCTION vertical succession of ejecta from deep sources locus, dynamics, and possible triggers of these overlain by ejecta from shallower sources. events. We investigated a complex field of >30 Volcanic eruptions triggered by explosive va- According to Mastin (1995) and Browne and craters formed over three separate episodes of porization of water are common and locally haz- Lawless (2001), hydrothermal eruptions are trig- steam-driven eruptions at Lake Okaro within ardous phenomena. They are typically termed gered within a geothermal area by a variety of the Taupo volcanic zone, New Zealand. Fresh steam-driven eruptions (Mastin, 1995; Thiéry processes that cause the sudden decompression unaltered rock excavated from initially >70 m and Mercury, 2009; Montanaro et al., 2016c). and flashing of water that is already metastable depths in the base of phase I breccia deposits Such eruptions eject large (meter-sized) clasts on and near boiling. Hydrothermal eruptions typi- showed that eruptions were deep, “bottom- ballistic trajectories, generate highly energetic cally progress from a near-surface rupture down- up” explosions formed in the absence of a pre- steam-rich density currents (surges), and expel ward (McKibbin et al., 2009). This mechanism existing hydrothermal system. These phreatic wet jets of poorly sorted rock debris (Jolly et al., is energetically very favorable, and a hydrother- explosions were likely triggered by sudden rise 2010; Breard et al., 2014; Maeno et al., 2016; mal eruption can start within a meter or so of of magmatic fluids/gas to heat groundwater Fitzgerald et al., 2017; Strehlow et al., 2017). the ground surface below a very thin cap. After within an ignimbrite 70 m below the surface. The main hazard from steam-driven eruptions is initial excavation, pressure within the deeper Excavation of a linear set of craters and associ- their unheralded sudden onset, typically having geothermal reservoir is reduced, and a flashing ated fracture development, along with contin- little seismic or other warning (Barberi et al., front and rarefaction front move progressively ued heat input, caused posteruptive establish- 1992; Hurst et al., 2014). More than 200 such downward, followed by the boiling front. Water ment of a large hydrothermal system within eruptions have occurred over the last three cen- present in joints or cracks adjacent to the devel- shallow, weakly compacted, and unconsoli- turies, causing thousands of deaths (Mastin and oping crater may also flash to steam as pressures dated deposits, including the phase I breccia. Witter, 2000). The most common steam-driven reduce suddenly, widening the vent. This pro- After enough time for extensive hydrothermal eruptions can be classified as either phreatic or cess may occur in both brittle or unconsolidated alteration, erosion, and external sediment in- hydrothermal eruptions. Both of these eruption materials (Browne and Lawless, 2001; Galland flux into the area, phase II occurred, possibly types do not directly involve or disrupt magma, et al., 2014; Montanaro et al., 2016b). Deposits triggered by an earthquake or hydrological but the power of water/gas expansion explosive- typically contain abundant hydrothermally al- disruption to a geothermal system. Phase II ly fragments wall rock and ejects fragments up- tered components and may record a progressive produced a second network of craters into ward and outward (Browne and Lawless, 2001). deepening of explosion locus through an inver- weakly compacted, altered, and pumice-rich Here, we follow Stearns and McDonalds’ sion of the pre-explosion stratigraphy. (1949) definition of phreatic explosions as being Steam-driven eruptions last from seconds to †[email protected]. caused by flashing of groundwater and surface hours (Browne and Lawless, 2001; Jolly et al.,

GSA Bulletin; September/October 2020; v. 132; no. 9/10; p. 1914–1930; https://doi.org/10.1130/B35276.1; 12 figures; 2 tables; Data Repository item 2020069. published online 22 January 2020

1914 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license.

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2014) and produce craters from a few meters up Steam-driven eruption craters are common (Nairn and Wiradiradja, 1980; Cole et al., 2006; to hundreds of meters in diameter, with depths in many volcanic terrains or areas of high heat Morgan et al., 2009; Breard et al., 2014). Evi- from few to several hundred meters (Muffler flow, such as in New Zealand (e.g., Champagne dence includes: lines or clusters of closely spaced et al., 1971; Browne and Lawless, 2001; Morgan Pool crater at Wai-o-tapu; Hedenquist and Hen- craters; a single large crater with scalloped mar- et al., 2009). Deposits produced by these explo- ley, 1985), Indonesia (e.g., Sinila and Sigludung gins and/or a complex shape; and nested craters sions are generally of low volume (<105 m3) and craters in the Dieng Plateau; Allard et al., 1989), with different depths (e.g., Scott and Cody, 1982; restricted to within hundreds of meters to a few Japan (e.g., craters on the Ontake volcano sum- Morgan et al., 2009; de Ronde et al., 2015; Mon- kilometers from crater margins. They are typi- mit area; Maeno et al., 2016), Greece (e.g., Ste- tanaro et al., 2016b). Detailed mapping of the cally very poorly sorted, matrix-supported brec- fanos crater on Nysiros; Marini et al., 1993), El ejecta blanket, as well as ballistic ejecta analyses cias (Muffler et al., 1971; Mastin, 1995; Browne Salvador (e.g., craters in the Agua Shuca thermal around the crater area, may reveal additional evi- and Lawless, 2001; Breard et al., 2014). Steam- area; Handal and Barrios, 2004), and the United dence of multiple explosion epicenters during a driven eruptions of all types occur in diverse vol- States (e.g., Mary Bay crater complex in Yel- single active episode (Nairn, 1979; Kilgour et al., canic and sedimentary rocks, showing a range lowstone National Park; Morgan et al., 2009). In 2010, 2019; Breard et al., 2014; Lube et al., 2014; in grain size, competence, alteration, fracturing, all these cases, craters were excavated within a Maeno et al., 2016; Montanaro et al., 2016b). and bedding. All these factors influence deposit wide range of host-rock lithologies having dif- In this study, we assessed the effects of varia- distribution and crater form. The geological and ferent alteration states, strengths, and permeabili- tions in the eruption-generation mechanism (bot- stratigraphic setting of an eruption site is also an ties (Maeno et al., 2016; Montanaro et al., 2016a; tom-up vs. top-down), the host-rock lithology, important factor in interpreting eruption dynam- Heap et al., 2017). These factors produced a typi- and the impact of successive eruptions on explo- ics (Browne and Lawless, 2001; Morgan et al., cally complex crater morphology. Craters from sion dynamics and crater formation processes, us- 2009; Breard et al., 2014; Valentine et al., 2015a; steam-driven eruptions may preserve evidence ing the example of Lake Okaro in New Zealand, Montanaro et al., 2016b). of multiple explosions that migrated laterally located within the Taupo volcanic zone (Fig. 1).

Figure 1. Satellite image (Google Earth™, 2016) of Lake Okaro showing several geological and geomorphological features, as well as the drainage network that feeds the Lake Okaro. OB—Okaro Breccia; TVZ—Taupo volcanic zone.

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Lake Okaro has a subrectangular shape, and the surrounding breccia (Cross, 1963; Hedenquist and Henley, 1985) indicates that it may have re- sulted from multiple explosions. Hardy (2005) recognized at least three breccia units produced by an initial phreatic eruption at the southern end of the lake; this eruption was followed by a series of hydrothermal explosive events, enlarging the crater to its final shape. Some of the later hy- drothermal eruptions were larger than the initial phreatic event. The eruptive activity that formed Lake Okaro is representative of many other com- plex cases of repeated steam-driven eruptions in New Zealand (e.g., at Rotokawa, Waimangu, and Wai-o-tapu geothermal fields), as well as many other similar areas around the world (e.g., large hydrothermal eruption craters in Yellow- stone National Park, United States; Browne and Lawless, 2001; Morgan et al., 2009). Here, we examined the breccia deposits surrounding Lake Okaro and investigated the lake bathymetry to: (1) revisit the eruptive triggers, and (2) explore the relationship between eruption dynamics, including crater-forming processes, and the host-rock properties during a series of repeated steam-driven explosive events.

GEOLOGICAL SETTING AND Figure 2. Reconstructed block diagram (not to scale) illustrating the major units making STRATIGRAPHY OF LAKE OKARO up the stratigraphy at Lake Okaro, including the possible hydrological conditions before AREA the first eruption (based on data from Hardy, 2005; Hedenquist and Henley, 1985; Molloy et al., 2008; Speed et al., 2002). TPF—Taupo Pumice Formation; WF—Waiohau Formation; Lake Okaro is located within the central Taupo EFF—Earthquake Flat Formation; RA—Rotoehu Ash; RI—Rangitaiki Ignimbrite. volcanic zone (Fig. 1), north of the Wai-o-tapu geothermal field. This area is dissected by the taiki Ignimbrite is a poorly to moderately weld- to Lake Okaro, the Earthquake Flat Formation is NE-striking Ngapouri and Rotomahana fault ed, dark-gray, crystal-rich tuff (Nairn, 2002). ∼50 m thick (Figs. 1 and 2; Nairn, 2003; Hardy, systems, which feed fluids to numerous geother- Facies include coarse ash- to pumice-flow tuffs, 2005), and it is overlain by 0.5–2-m-thick ash- mal fields (Hedenquist and Henley, 1985; Nairn pumice breccias, and ash-to-lapilli-fall deposits. to-lapilli-fall deposits of the Waiohau Formation et al., 2005). Nairn et al. (2005) suggested that This ignimbrite is >100 m thick on the caldera erupted from the Okataina volcanic complex these fault/fluid pathways were reactivated dur- rim at Lake Rotomahana, 3 km from Lake Oka- (Nairn, 2002; Speed et al., 2002; Leonard et al., ing magma rise into the neighboring Okataina ro (Fig. 1), but it thins to only 7–39 m thick in 2010; WF in Fig. 2). Above the Waiohau Forma- caldera, triggering the A.D. 1315 effusive and ex- boreholes near Lake Okaro (Nairn, 2002). The tion, there is a series of younger tephras, includ- plosive, rhyolitic Kaharoa eruptions at Tarawera, thinning likely reflects the pre-eruptive topog- ing the 1.8 ka Taupo Pumice Formation (TPF as well as many of the steam-driven explosive raphy from displacement associated with blind in Fig. 3). A 1–3-cm-thick ash-fall deposit from eruptions within the Wai-o-tapu geothermal field. fault branches of the Rotomahana fault system the ca. A.D. 1315 Kaharoa eruption lies within Contrasting with many hydrothermal craters in (inferred faults in Figs. 1 and 2). A 9-m-thick a paleosol immediately below the Okaro brec- this area, Lake Okaro is not directly above a ma- tuff-breccia, rich in accretionary lapilli, lies cias (Lloyd, 1959; Cross, 1963; Hedenquist and jor fault system; it lies 0.5 km northwest of the above the Rangitaiki Ignimbrite (Nairn, 2003). Henley, 1985; Nairn et al., 2005). nearest surface faults (Ngapouri and Rotomahana This unit is correlated with a similar breccia faults; Figs. 1 and 2). An inferred fault is mapped near Haumi Stream (Fig. 2), interbedded with FIELD STUDY on the western side of the lake, and rivers run- a bedded lapilli-fall deposit (Rotoehu Ash; RA ning parallel to the local fault orientations on the in Fig. 2; Nairn and Kohn, 1973; Leonard et al., We described the Okaro breccia deposits eastern lake shore could indicate the presence of 2010). The Earthquake Flat Formation overlies in several exposures around the lake (Figs. 1 older faults and fractures across the whole Lake a sharp nonerosional break above the Rotoehu and 3–7; Figs. DR2–DR51) and sampled the Okaro area (inferred faults in Figs. 1 and 2). Ash (EFF in Fig. 2; Nairn and Kohn, 1973). The ash and lapilli-rich matrix, as well as blocks The stratigraphy below Lake Okaro (Fig. 2) Earthquake Flat Formation is a weakly compact- was constructed from nearby field observations ed, crystal-rich, lapilli-bearing ash, with coarsely 1 and borehole logs. The deepest units recognized vesicular pumice blocks; coarse (>5 mm) quartz, GSA Data Repository item 2020069, additional field information, and bathymetry maps of the in drill cores (Nairn, 2003; Hardy, 2005) include plagioclase, hornblende, and biotite crystals and investigated area, is available at http://www. an undescribed siltstone unit overlain by the crystal fragments are also present (Molloy et al., geosociety.org/datarepository/2020 or by request to Rangitaiki Ignimbrite (RI in Fig. 2). The Rangi- 2008; Leonard et al., 2010). In boreholes close [email protected].

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Figure 3. Type location of the Okaro Breccia (I—orangish; II—yellowish; III—reddish), in the southwestern part of the lake (location 1 in Fig. 1). TPF—Taupo Pumice Formation; WF—Waiohau Formation; EFF—Earthquake Flat Formation.

­representative of the main lithologies, to be DR1). A thin, pale brown paleosol caps the Oka- Figs. 8A–8D). The matrix consists mainly of used for component and grain-size analyses. ro Breccia Formation, which is in turn covered slightly altered to unaltered fragments of the The percentage of blocks with respect to the by the Rotomahana Mud, a distinctive deposit same ignimbrite, tuff, and pumice as the blocks, breccia matrix was qualitatively assessed for from the phreatomagmatic phase of the 1886 together with loose biotite and quartz crystals the investigated sections. Examination of mac- Tarawera eruption (Fig. 7; Figs. DR3, DR4, and (Figs. 8A and 8B). Ignimbrite and tuff clasts are roscopic alteration features, for example, color DR6; Nairn, 1979). orange-stained and subangular to angular, col- differences from whitish (freshest) to orange, lectively making up ∼50 modal % of the particles yellow, reddish, and greenish (most altered), as Okaro Breccia I (Figs. 9A and 9B). Another 30–40 modal % of well as the occurrence of alteration halos and The basal Okaro Breccia I is orange to yellow the matrix particles are white to yellow pumice, silicified crusts and veins, indicated the relative brown and dominated by an ash-rich matrix, sup- which are mostly angular, highly vesicular, and degree of alteration within the investigated brec- porting subrounded to angular lapilli and blocks rich in quartz and biotite. They are most abun- cias. Field observations, in concert with the lake (maximum diameter 100–150 mm; Fig. 4; Fig. dant in the upper part of the breccia. Free crys- bathymetry (described later), were then used to DR1). The block fraction (>64 mm) consists of tals make up >10 modal % of the matrix and reconstruct the eruptions scenario. two main types of slightly altered to unaltered are mostly quartz, particularly in its middle to lithologies: (1) white to orange-stained pumice- upper portion. The matrix particles commonly Okaro Breccia Formation rich tuff and fresh crystal-rich tuff, both with show a yellow alteration patina. Additionally, abundant lithic clasts, biotite, and quartz grains, rare but distinctive red-stained, tuff-like particles Three main breccia units, Okaro Breccia I, II, and (2) fine-ash tuff containing abundant biotite, occur within the matrix at the base of the unit and III, were distinguished based on grain-size, pumice, accretionary lapilli (0.5–1 mm sized), (Figs. 9A and 9B). color, and lithology/componentry (Figs. 3–6), and lithic clasts in a pale-gray to yellow matrix. The Okaro Breccia I is thickest in the west- and they are grouped within the Okaro Brec- The first block lithology is predominant in the ern (1 m) and southwestern (2.5 m) quadrants cia Formation. Cross (1963) and Hedenquist breccia (>60 modal %) and is derived from the around the lake. It is not present to the north, and and Henley (1985) mapped the Okaro Breccia Rangitaiki Ignimbrite, whereas the second, less there are no outcrops of this member in the south Formation out to ∼1 km from the lake shoreline, abundant type (>35 modal %) is derived from the and east (sections 1, 3, and 10; Fig. 7). with one lobe extending ∼1.5 km to the east Rotoehu Ash. Rare (<5 modal %) white pumice (Fig. 1). The whole formation is ∼13 m thick blocks, rich in biotite and quartz, derived from Okaro Breccia II near the crater rim, but it rapidly thins to 2–3 m the Earthquake Flat Formation, are also present. In the southwestern sector of the lake (loca- at ∼250 m from the eruptive center. The matrix of Okaro Breccia I is reversely tion 3; Fig. 1), the Okaro Breccia II is separated On the western lake shores (location 1 in graded from ash-rich upward to greater lapilli from the underlying Okaro Breccia I by a thinly Fig. 1), the Okaro Breccia Formation overlies a content (reaching up to ∼30 wt%; Figs. 8A–8D). laminated layer (1–10 cm thick) of reworked and 1-cm-thick fall unit from the A.D. 1315 Kaharoa There is a persistent grain-size mode between waterlain ash and a pale brown paleosol (Figs. eruption and a series of tephras from the Taupo 0 and –0.5 phi (1.4–1 mm; very coarse ash), 5A–5B). However, at other locations (e.g., 1 in and Okataina calderas, capping the Earthquake as well as a long tail (from ∼30 to ∼50 wt%) Fig. 1), no erosional surfaces or sedimentary lay- Flat Formation ignimbrite (Figs. 3, 4, and 7; Fig. extending from 1 mm to fine ash (<0.063 mm; ers are present between the two units (Fig. 4). In

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A B

Figure 4. (A) Okaro Breccia I at the type location (location 1 in Fig. 1). The upper part grades into Okaro Breccia II. Okaro Breccia I lies on top of a dark-brown paleosol developed on the Taupo Pumice Formation (TPF), which in turns overlies the Waiohau Formation (WF). (B) Detail of Okaro Breccia I showing a thin ash layer (ca. A.D. 1315 Kaharoa Ash) that separates the breccia from the paleosol.

the northern and northeastern sectors, the Oka- lapilli–rich tuff (<30 modal %), and biotite- and trix particles of all types show a yellow alteration ro Breccia II directly overlies the Kaharoa ash quartz-rich pumice (∼30 modal %), mainly in the patina. Up to 10 modal % of matrix particles are (Fig. 7; Figs. DR2 and DR3). The Okaro Brec- massive central bed (Figs. DR4–DR6). The first strongly altered, green, and rounded to angular cia II is the most extensive deposit of all deposits block lithology is derived from the Rangitaiki in shape. These particles are limited to the upper mapped around the lake, covering all sectors. The Ignimbrite, whereas the second type is derived half of the Okaro Breccia II (from the middle part breccia is at least 10 m thick in the southern and from the Rotoehu Ash. The third lithology, more of the massive bed upward; Figs. 6 and 8D–8E; northern sectors of Lake Okaro, 6.5 m thick to abundant compared to the amount found in the Figs. DR3–DR5). Abundant quartz crystals (<10 the southwest, and 1.5 m thick to the east (Fig. 7). Okaro Breccia I, is derived from the Earthquake modal %), either euhedral or broken, together The Okaro Breccia II shows a vertical strati- Flat Formation. There is a larger proportion of with rare unidentified dark clasts <( 1 modal %) fication with diffuse contacts between beds blocks (up to 30–40 modal % in the massive bed) occur in the matrix (Figs. 9C–9E). (Figs. 3 and 7). The basal bed is yellowish to cropping out in the southern, southwestern, and gray, dominated by fine-grained matrix-support- western sectors (Figs. 3, 5, and 7; Fig. DR5). Okaro Breccia III ed subrounded to angular lapilli. The massive The tuff clasts vary from pale gray to white to The Okaro Breccia III crops out in the south- central bed is brown to yellow-brown and ma- greenish-white and are mostly angular, whereas ern and southwestern part of the lakeside and di- trix- to clast-supported, and it contains angular the white pumice particles are rounded to suban- rectly overlies the Okaro Breccia II, commonly to subrounded lapilli to coarse blocks (maximum gular. In the upper bed, rare silicified vein-like without a weathered contact or paleosol. At the diameter 400–600 mm; Figs. 5C–5D; Fig. DR4). clasts and brecciated tuffs are found (Fig. DR4). type section, a thin (up to 2 cm), fine-grained, The uppermost bed is brown to orange and dom- Rare bomb sags were identified within the basal waterlain orange ash deposit separates it from inated by a matrix-supported subrounded to an- bed (Fig. DR3). the Okaro Breccia II (Fig. 6). The Okaro Breccia gular lapilli (Fig. 6). The full sequence is present The matrix grain-size distribution of Okaro III is red and mostly clast-supported, containing in the southern and western sectors around the Breccia II is coarsely skewed, having a large predominantly subrounded to angular lapilli, and lake, whereas to the east, northeast, and north, mode in the lapilli fraction and tails of ash (<15 it is capped by modern/recent soil. The Okaro only the central bed is present with rare large wt%; Figs. 8E–8F). The central massive bed has Breccia III is limited to the western and south- blocks. In all the investigated sections, the cen- the greatest ash content (Fig. 8F). The matrix western sectors around the lake, where it is 2.5 tral bed is characterized by distinctive altered contains mainly quartz- and biotite-rich pumice and ∼0.5 m thick, respectively (Figs. 1 and 7). tuff-like green clasts as blocks and/or within the (>50 modal %), decreasing in content upward The block fraction of Okaro Breccia III matrix (Figs. 7; Figs. DR3 and DR5). (Figs. 9C–9E). Tuff clasts are subordinate (<30 mainly consists of angular fine-ash tuff >( 60 The block fraction of Okaro Breccia II con- modal %), and their proportion increases from modal %) and accretionary lapilli–rich tuff sists of fine-ash (>40 modal %) and accretionary the center of the middle bed upward. Some ma- (>30 modal %), with a minor quantity (<15

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AC

B D

Figure 5. (A) Contact between Okaro Breccia I and II showing a laminated layer separating the two breccias and an erosional channel. (B) Detail showing the laminae. The laminated layer overlies a discordant contact on a pale-brown layer, possibly corresponding to the rem- nants of a paleosol. (C–D) Detailed view of the massive central bed of Okaro Breccia II. Most of the blocks are Rangitaiki Ignimbrite (RI) and Rotoehu Ash (RA).

modal %) of ­biotite- and quartz-rich pumice. rounded to angular, yellowish-white, or covered ing sediment and water to form fans on the lake Generally, all clast types are whitish in color by a red alteration patina. Altered tuff-like green floor. On the northwestern hillsides bordering and less competent than those within the other clasts occur only in the block fraction. the lake, there are numerous rills, which formed two breccias. Similar to the components of the immediately after the emplacement of the Ro- first two breccias, the first and second block Lake-Floor Morphology tomahana Mud during the A.D. 1886 Tarawera- lithologies are derived from the Rangitaiki Waimangu eruption (Hardy, 2005). Ignimbrite and the Rotoehu Ash, respectively, Lake Okaro has a roughly rectangular shape, whereas the pumice is derived from the Earth- is ∼650 m long and 400 m wide, and covers an Crater Areas quake Flat Formation. Rare (<5%) altered tuff- area of 0.31 km2 (Fig. 10). High-resolution multi- The lake bathymetry allows reconstruction like green blocks occur. Few large blocks (up beam data (1 m spatial resolution) were acquired of the order of formation of the Okaro eruption to 30 cm) are present at the base of the breccia, in 2014 by the Bay of Plenty Regional Council craters. We determined three subareas of cra- whereas the upper part is lapilli-rich with rare (Figs. 10 and 11). The level of Lake Okaro has ters (Figs. 10 and 11; Fig. DR6), representing blocks (Fig. 6). No bomb sags were identified varied historically (up to 1.5 m) in response to separate eruptive phases that produced the three within this member. changing rainfall patterns (Cross, 1963). The separate breccia units. We defined these subar- The matrix grain-size distribution is coarsely lake surface had an elevation of 413 m during eas based on the fact that younger eruption cra- skewed (Fig. 8H) and is lapilli dominated (>60 the 2014 survey and showed a maximum depth ters (1) cut the earlier-formed crater borders, (2) wt%). The matrix consists mainly of slightly or of 18 m in its southern sector. We calculated a excavated into older breccias, and (3) typically strongly altered pumice particles (>50%), sub- water volume of 3.9 × 106 m3 using the high- showed fresher morphology (deeper, steeper ordinate (<20%) loose crystals (euhedral and resolution multibeam data. walls and sharper margins). Older craters are broken quartz), and strongly altered fragments A well-developed drainage network is present smoothed and/or shallowed by infill of younger of tuffs (Fig. 9F). All matrix pumice particles are west and northwest of the lake (Fig. 1), supply- ejecta and/or lake ­sediment. Crater shapes were

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defined as circles or ovals fitting the distinct (or A inferred) crater rims (Fig. 10; Fig. DR6). Av- erage crater diameters and slope angles were measured, together with the depth (from the crater rim to the deepest point in the crater), and these value are listed in Tables 1 and 2. Phase I craters. Few craters were defini- tively attributed to this phase because they are generally the most eroded, buried by sedi- mentation, and crosscut by later craters. They include the less obvious crater features locat- ed in the southern part of the lake (Fig. 11). Many of the craters, especially on the south- western and southern sides, are represented only by remnants of their original crater walls (Fig. 10, profiles A-A′, B-B′, and E-E′). In the southeastern part of the lake, the crater bor- ders are more evident, and they have relatively smooth floors. A NW-SE–elongated ridge in this area probably represents another rem- nant crater that formed early in the sequence (Fig. 10, profile E-E′). The few craters with shapes that can be extrapolated (1–3 in Fig. DR6) have diameters between 62 and 120 m, and rim-to-crater-floor depths of 1.8 and B 5 m. Craters from phase I exhibit very steep walls (from 22° to 49°; Table 2) and have a low crater depth/diameter ratio (0.03–0.04). Inferred craters from phase I form a linear chain likely oriented along a fracture system (inferred Rotomahana fault in Figs. 1 and 2). There may have been craters northward of this inferred chain, but if they existed, they were later destroyed by the phase II eruption. Based on the inferred subareas, the phase I eruption involved ∼0.1 km2, or one third of the current lake area. Phase II craters. At least 20 phase II craters crosscut phase I crater areas. These craters are scattered across the lake floor (Fig. 10; Table 1). In the northern lake, large craters (4, 6, 7 in Fig. DR6) occur, having diameters between 110 and 190 m and depths between 9 and 18 m (Fig. 10, profile A-A′ to D-D′). These craters show inter- mediate wall slopes (from 18° to 25°) and have crater depth/diameter ratios from 0.05 to 0.12. Small sediment-covered depressions and circular craters (e.g., 5 and 8–13 in Fig. DR6) are present among, or cut, the large craters (Fig. 10, profile D-D′). These features have diameters of 47–70 m, depths between 1.4 and 2.8 m, and depth/diameter ratios of 0.02–0.06. Clusters of small, coalesced craters (14–20 in Fig. DR6) occur in the eastern and southeastern parts of the lake (Fig. 10, profile Figure 6. (A) Contact (red dotted line) between Okaro Breccia II and III; the gray- E-E′), having diameters between 33 and 80 m and brownish Okaro Breccia II shows typical highly altered green clasts and contrasts with depths of 1.6–3.9 m. Most of these clustered cra- the reddish matrix of Okaro Breccia III, in which (rare) large blocks made of Rangitaiki ters show low wall slopes (from 11° to 15°) and Ignimbrite (RI) can be also found. (B) Detail of the fine-grained orange layer separating have crater depth/diameter ratios between 0.04 the two breccias, indicating a potential hiatus between the events that generated Okaro and 0.07. In the southwest, intermediate-sized Breccia II and III. craters (21–22 in Fig. DR6) have diameters of 77

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Figure 7. Stratigraphic sections of the outcropping units surrounding Lake Okaro. For each section, locations are given in Figure 1. Con- tinuous lines connect discontinuities (e.g., paleosol, erosional surfaces) between units or formations, whereas dashed lines indicate gradual or unclear contacts and inferred stratigraphic correlations.

and 92 m, intermediate wall slopes (16°–29°), and ern and southern sides, coalescing craters (25– Mass Movement Morphologies crater depth/diameter ratios of 0.03 and 0.06. All 28 in Fig. DR6) cut into a NW-SE–elongated Along the western and northwestern sides of these intermediate-sized craters cut the remnant ridge and remnants of older crater borders, both of Lake Okaro, several features indicate sedi- phase I craters, as well as further excavate their from phase I. On the northern and western sides ment inflow to the lake below creeks/valleys crater floors (Fig. 10, profile E-E′). There may be of the phase III subareas, craters cut the rims that drain the surrounding hills (Figs. 1, 10, and more craters buried beneath the landslide deposits and floors of craters from phase II (23, 24, 29 in 11). A large delta occupies ∼0.05 km2 on the that blanket the northern part of the lake. Phase II Fig. DR6). Inferred diameters for the coalesced northwest side of the lake, and one of 0.008 km2 craters cover ∼0.25 km2. craters range from 53 to 110 m, and depths are is present on the northeastern side. Stepped es- Phase III craters. These craters crosscut cra- between 1 m and 3 m. In general, these craters carpments, 1–3 m high, on and above the largest ter walls from phases I and II, representing the exhibit low wall slopes (from 5° to 15°) and low delta indicate that this is the toe of a retrogressive most geomorphologically distinct and youngest crater depth/diameter ratios (0.02–0.03). Craters slump feature (Fig. 10, profiles A-A′ and C-C′; craters occurring on the southern side of the lake of phase III (at least seven distinguishable) cov- Fig. 11). On the western side of the lake, small (Fig. 10, profiles A-A′ and E-E′). On their east- er an area of ∼0.035 km2. rockslide deposits have a hummocky topography

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A E

B F

C G

D H

Figure 8. Grain-size distributions of samples collected at the type location (TS; location 1 in Fig. 1). Weight percent (e.g., wt%) and cu-

mulative frequency distribution (e.g., Σwt%) are represented by bins and curves, respectively, as a function of φ = –log2 d, where d is the grain size in millimeters measured by sieving. (A–D) Matrix grain-size distribution in the lower half of Okaro Breccia I (OBI). (E–G) Size distribution of matrix in Okaro Breccia II (OBII) base, middle, and upper parts, respectively. (H) Size distribution of matrix in the middle part of Okaro Breccia III (OBIII).

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A B

CD

EF

Figure 9. Microscope images of the 0.5 mm particles sieved from the breccia matrices showing the main components: variably altered frag- ments of Rangitaiki Ignimbrite and Rotoehu Ash tuffs (orange, yellow, and red), pumice clasts, single crystals, and (not identified) altered greenish and red-stained clasts. (A–B) Componentry of the base (0–10 cm) and middle (20–30 cm) parts of Okaro Breccia I (OBI). (C–E) Componentry of Okaro Breccia II (OBII) base, middle, and upper parts, respectively. (F) Componentry of the middle part of Okaro Brec- cia III (OBIII).

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C

D

A

B E

Figure 10. Topography around Lake Okaro and high-resolution (1 m) bathymetry. Segments (named CP) for crater slope estimates are indicated. Profiles A-A′ to E-E′ show the main morphologies observable in the lake, including interpreted explosion craters, slumps, and rockslides. Crater size refers to maximum width measured along the profile and may differ from the average values for single crater listed in Table 1. In the inlet, the contour interval is 2.5 m for the topography, and 5 m for the lake bathymetry. The vertical exaggeration (V.E.) of the profiles is 5.5:1. Bathymetric data are the property of the Bay of Plenty Regional Council.

and include scattered megablocks (up to 20 m to the main fault trends immediately northeast Okaro Breccia I, so no significant geothermal wide and 1.5 m high) below many of the steeper of the lake (Fig. 1). Collectively, it appears that system existed at the eruption site. crater wall embayments (Fig. 10, profile C-C′; an approximately NE-SW–striking fracture zone Collectively, the initial eruption of deep-seated Fig. 11). existed at the site of Lake Okaro before the erup- bedrock, the absence of pervasive hydrothermal tion (inferred fault in Figs. 1 and 2). alteration of the ejecta, and the presence of faults DISCUSSION The phase I eruption occurred soon after the and fractures in the area suggest that the eruption explosive phase of the A.D. 1315 Kaharoa rhyo- was triggered by rapid heating and overpressur- Eruption Triggers and Mechanisms litic eruption (volcanic explosivity index [VEI] ization of groundwater by sudden arrival of mag- 4; Bonadonna et al., 2005; Nairn et al., 2005). matic fluids, for instance, from a dike (cf. Ger- Phase I: Phreatic Eruption Nairn et al. (2005) suggested that basalt dikes manovich and Lowell, 1995; Stix and De Moor, Potential triggers of the initial explosive activ- across the region near Mount Tarawera injected 2018). This interpretation follows Nairn et al.

ity at Lake Okaro and eruptive scenarios can be CO2 and heat to prime several steam-driven (2005), who proposed that magmatic CO2 injec- inferred from several lines of evidence, including eruptions, e.g., the Champagne Pool at the Wai- tion from a basaltic dike generated hydrothermal subsurface stratigraphy and tectonic structures, o-tapu geothermal system. Fault displacements eruptions at Wai-o-tapu. Nearby, a shallow dike as well as the alteration state and stratigraphy of and/or channeling of magmatic gas up faults and also caused both steam-driven and phreatomag- the Okaro Breccia Formation. fractures could have thus triggered the wide- matic eruptions at Waimangu in 1886 (Nairn, Lake Okaro lies close to the Rotomahana mul- spread explosive episodes, including Lake Oka- 1979). Thermodynamic models (e.g., Delaney, tithread fault system (Fig. 1; Lloyd, 1959; Heden- ro (Hedenquist and Henley, 1985; Browne and 1987) suggest that heat is not transferred rap- quist and Henley, 1985). Displacement along Lawless, 2001; Rowland and Simmons, 2012). idly from an ascending dike to groundwater.

these faults produced a raised local topography, Okaro Breccia I includes the deepest local However, large amounts of rising CO2 from so that only a thin veneer of Rangitaiki Ignim- lithology (Rangitaiki Ignimbrite; Nairn, 2003; a basaltic dike (>1000 t/d; Nairn et al., 2005) brite was emplaced, and the regional topography Hardy, 2005) as tuff clasts at the base of the unit. could have rapidly heated and pressurized shal- smoothed (Nairn, 2003). Also, a faulted volcanic There are no juvenile pyroclasts, indicating that low groundwater in fractures and aquifers above sequence outcrops in the nearby Haumi Stream no phreatomagmatic eruption occurred. These faults at the site of Lake Okaro. Germanovich (Nairn et al., 2005), along with a surface fault Rangitaiki Ignimbrite clasts show that the ini- and Lowell (1995) considered emplacement of west of Lake Okaro. Indirect evidence of further tial explosion began at least at a depth of ∼70 m. magmatic fluids into a water-saturated perme- fault strands includes streams oriented parallel In addition, there are very few altered clasts in able reservoir with two scales of permeability:

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TABLE 2. CRATER SLOPE VALUES (AVERAGED) FOR CRATERS PRODUCED BY THE THREE ERUPTIVE PHASES, LAKE OKARO, NEW ZEALAND Slope Slope Slope phase I phase II phase III Cp1 26 29 Cp2 23 23 Cp3 23 16 Cp4 26 6 Cp5 49 11 Cp6 42 10 Cp7 23 15 Cp8 34 11 Cp9 47 15 Cp10 25 Cp11 25 Cp12 24 Cp13 28 Cp14 25 Cp15 22 Cp16 23 Cp17 14 Cp18 15 Cp19 10 Cp20 5 Cp21 9 Cp22 22 10 Cp23 18 Note: See map in Figure 10 for crater profile (Cp) locations.

low permeability (<10−17 m2) in the bulk rock, but high permeability around crack networks (>10−12 m2). In this case, explosive-eruptive conditions are reached when the fluid of the Figure 11. Shaded relief map of Lake Okaro. The bathymetry scale (color bar on the side) subsidiary network starts to be heated, leading is indicated in meters above sea level. Three sets of craters were identified, and each set is to microscale pressurization. Heat and gas cause interpreted to be associated with an eruptive phase. Craters of phase II are the most numer- rapid propagation of cracks (seconds to hours for −3 −1 ous in the lake-floor morphology. Slumped deltas and rockslides are present in the northern fractures 10 m to 10 m in size), leading to and western sides of the lake, whereas depressions located within the phase II craters are near boiling (Germanovich and Lowell, 1995). possibly craters filled by deposits from phase III eruption. Phase III craters are the most Extensional stresses and overpressures build up evident craters observable in the bathymetry. Arrows indicate the distribution of the differ- and eventually cause the country rock to fail, ini- ent breccia deposits (in brackets) around the lake. Multibeam data are the property of the tiating decompression and an explosive eruption. Bay of Plenty Regional Council. OB I, II, III—Okaro Breccia I, II, and III. This model is well suited for rocks with low ten- sile strength (≤10 MPa), such as the Rangitaiki Ignimbrite (4–8 MPa; Foote et al., 2011), which TABLE 1. MORPHOLOGICAL FEATURES OF CRATERS PRODUCED BY THE also has a bulk permeability of 10−16 m2 (Mon- THREE ERUPTIVE PHASES, LAKE OKARO, NEW ZEALAND tanaro et al., 2017). Crater diameter Crater depth Depth/ Eruptive (m) (m) diameter phase A steam-driven event along a fracture system Crater 1 62 1. 8 0.03 I above a blind fault and a dike is also consistent Crater 2 120 4.3 0.04 I with the linear chain of vents/craters of phase Crater 3 120 5 0.04 I Crater 4 165 18 0.11 II I (Fig. 11). Following phase I explosions, the Crater 5 110 1. 3 0.01 II Crater 6 190 8.8 0.05 II elongate crater structure was filled by highly Crater 7 125 14.8 0.12 II permeable ejecta. Ongoing heat transfer from Crater 8 50 2.8 0.06 II Crater 9 50 1. 5 0.03 II the shallow intrusion may have lasted for months Crater 10 60 2.8 0.05 II to years (cf. Petcovic and Dufek, 2005), heating Crater 11 47 2.2 0.05 II Crater 12 70 2 0.03 II and circulating hydrothermal fluids (cf. Rowland Crater 13 59 1. 4 0.02 II and Simmons, 2012). This geothermal system Crater 14 65 3.7 0.06 II Crater 15 33 1. 6 0.05 II expanded into the shallow permeable (∼10−12 Crater 16 42 2 0.05 II 2 Crater 17 38 2.4 0.06 II m ) deposits of the Earthquake Flat Forma- Crater 18 57.5 3.9 0.07 II tion surrounding the phase I craters (Fig. 12B; Crater 19 65 2.7 0.04 II Crater 20 80 2.8 0.04 II Tschritter and White, 2014). Crater 21 92 5.6 0.06 II Crater 22 77 2 0.03 II Crater 23 96 2.8 0.03 III Phase II: Deeply Excavating Hydrothermal Crater 24 93 2.2 0.02 III Eruption Crater 25 99 3.3 0.03 III Crater 26 87 2.4 0.03 III The new shallow hydrothermal system cov- Crater 27 108 2.8 0.03 III Crater 28 110 2.3 0.02 III ered at least the area of phase II craters, extend- Crater 29 53 1. 2 0.02 III ing ∼400 m north of the phase I explosion sites Note: See map in Supplemental Figure DR6 for crater locations (see text footnote 1). (Figs. 11 and 12B). The occurrence of thinly

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A

B

C

Figure 12. Sketch (not to scale) of the Okaro phreatic-hydrothermal eruption evolution. (A) An opening phase of the eruptive cycle at Lake Okaro involving a heat (and possibly fluid) pulse provided by an arrested shallow dike intrusion (1). The model shows heated fluid of the subsidiary network flashing to produce microcrack pressurization (2–4). The country rock can fail according to a bottom-up mechanism (5), starting a phreatic eruption. While the flashing front progresses through almost- boiling water (6–7), the expansion ejects the fragmented rocks and unloads the poorly consolidated layers. (B) Seismic activity associated with fault displacements likely triggered the second hydrothermal eruption (1). Following the decrease in confining pressure (2), rapid boiling in the surficial part of the hydrothermal system is triggered (3). The boiling-front pen- etrates downwards into the hydrothermal reservoir, followed by the explosion front according to a top-down (4) mechanism (McKibbin et al., 2009), where the steam expands, fragmenting and dispersing the surrounding broken rocks. Eruption con- tinues until the rate of boiling decreases (5–6) and steam expansion ceases to provide sufficient energy to eject rocks from the crater. (C) Okaro Breccia III lithic clasts, as well as the crater distribution, suggest that a final eruption occurred in an area with residual, but still intense hydrothermal activity located within the inferred Okaro Breccia I crater area. The (sudden) drainage of the lake (1) triggers decompression of the underlying hydrothermal system (2). A similar top-down mechanism (3–5) is envisaged for these explosions. OB I, II, III—Okaro Breccia I, II, and III.

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laminated lacustrine sediments between Okaro catastrophic drainage. Drainage may have caused (mimicking unconsolidated sediments or vol- Breccia I and II (Figs. 5A and 5B) indicates a a sudden decrease in confining pressure beneath caniclastic deposits). Their results suggest the time break between these phases, with formation the lake bed, which we suggest triggered rapid following: of a lake and related sediment accumulation. The boiling in the surficial hydrothermal system and (1) The host rock properties affect the crater presence of silicified sediments, silicified brec- the onset of phase III eruption (Fig. 12C). Similar morphology (e.g., steepness of walls), the clast cias, and abundant altered green tuff clasts, as events have occurred after subtle lake drainage type, and sizes available for ejection; well as large quantities of altered pumice and in other shallow hydrothermal systems, excavat- (2) the host rock disrupted by subsurface ex- tuffs within Okaro Breccia II all around the ing top-downward (cf. McKibbin et al., 2009), plosions loses its original strength and has less lake (Figs. 9C–9E; Fig. DR4) suggests at least and producing craters of comparable size to the impact on subsequent explosions, and decades of hydrothermal activity took place be- phase III craters, for example, in Yellowstone and (3) most of the disrupted subsurface structure fore phase II explosions. In this setting, thermal Iceland (Muffler et al., 1971; Morgan et al., 2009; is filled with ejecta. and mechanical conditions at Lake Okaro were Montanaro et al., 2016b). As a first approximation, strong substrates at primed for producing a hydrothermal eruption, Lake Okaro could correspond to the deep Rangi- under any trigger scenario (e.g., Browne and Relationships Among Crater Morphology, taiki Ignimbrite and Rotoehu Ash tuffs, whereas Lawless, 2001). Possible trigger mechanisms of Breccia Distribution, Host Lithology, and weak substrates could correspond to surficial phase II eruption include seismic displacements Eruption Style tephra deposits (Waiohau and Taupo Pumice and changes in surface and groundwater in the Formations), or the ejecta from phases I and II. Lake Okaro area (cf. Lawless, 1988; Rowland Field studies of natural explosion craters (Yo- The Earthquake Flat Formation, representing and Simmons, 2012). koo et al., 2002; Montanaro et al., 2016a), and the thickest and main host rock involved in the The Okaro phase II eruption was likely trig- field-based explosion experiments using loose- first two phases of Okaro eruptions, varies from gered by a seismic event that fractured the hy- to-compacted material at varying shallow depths weakly compacted to possibly slightly consoli- drothermal system and reduced the confining (Murphey and Vortman, 1961; Goto et al., 2001; dated at depth, thus acting as a substrate with pressure. The decompression of hydrothermal Ohba et al., 2002; Taddeucci et al., 2013; Graet- intermediate strength (Macorps et al., 2016). fluids probably resulted in the formation of a tinger et al., 2014; Valentine et al., 2014; Sonder The steepness of crater walls at Lake Okaro boiling-erosion front that penetrated and ex- et al., 2015; Macorps et al., 2016) have demon- (Table 2; Fig. 10) appears to have been influ- cavated down into the hydrothermal reservoir, strated that crater shape and size result from an enced by the presence of the Earthquake Flat mostly within the <70-m-depth Earthquake Flat interplay between explosion energy and scaled Formation. Many of the craters formed during Formation (top-down model of McKibbin et al., depth (physical depth divided by cube root of the first two phases were excavated within this 2009). The eruption continued until the rate of energy). In addition, preexisting craters appear formation and have steep sides (23°–49°). By groundwater boiling decreased and steam expan- to influence the ejecta jets and whether or not an contrast, craters excavated within unconsoli- sion declined to the point where rock could no explosion is able to vent (Taddeucci et al., 2013; dated tephra deposits or the ejecta deposits of longer be ejected from the crater. The collapse of Graettinger et al., 2014). Experimental results phase I and II eruptions have low-angle slopes crater walls, or flooding may have further con- further indicate that the volume affected de- (6°–15°). tributed to stopping the eruption (Browne and pends on the host-rock (substrate) strength (Gal- We infer that the Okaro Breccia I eruption cra- Lawless, 2001). land et al., 2014; Macorps et al., 2016). In many ter distribution was dominated by the focusing natural locations, multiple neighboring craters mechanism of deep hot fluids in a preexisting Phase III: Shallow Hydrothermal Explosions may imply internal inhomogeneity in fluid stor- fracture zone. Thus, the craters were elongate Another pause in the activity occurred be- age within rock bodies (e.g., thermal circulation and aligned along this NE-SW–striking fracture tween phases II and III, as indicated by a thin, patterns that set up mineralization boundaries at system (Figs. 1 and 2). Based on the slope val- waterlain mud deposit below Okaro Breccia III their margins, or three-dimensional variations in ues and bathymetric profiles (Table 2; Fig. 10), (Fig. 6), on the western side of the lake and po- the porosity and permeability characteristics of phase II and III craters either excavated deeper sitioned ∼3 m above the current lake level. This host rock; Lawless, 1988; Browne and Lawless, into the phase I (increasing slope angles), or de- deposit indicates that a large lake formed in the 2001; Rowland and Simmons, 2012; Montanaro posited ejecta that filled the crater (decreasing depression produced during phase II. Hardy et al., 2016a). At Lake Okaro, much of the hydro- slope angles). We suggest that the phase I erup- (2005) also suggested that the lake level was thermal water was contained within the highly tion was hosted mostly within the Earthquake once much higher (>3 m), based on undercutting permeable, weakly compacted Earthquake Flat Flat Formation, and it was deeply excavated to the north and terracing to the east of the pres- Formation (Tschritter and White, 2014). In areas along a fracture to produce a steep-sided fissure- ent Lake Okaro; our finding of lake sediments on near Lake Okaro, this formation shows complex like crater. The Okaro phase I eruption produced the phase II crater rim confirms this hypothesis. changes in deposit texture, incipient compac- a breccia that was less widespread than the phase The Okaro Breccia III mostly consists of tuff and tion, and gas-escape pipes (Leonard et al., 2010). II breccia, consistent with the greater depth of minor pumice that appear more altered than those These variations may extend at depth underneath initiation of the phase I phreatic eruption. from Okaro Breccia II, and its extent is limited to the Lake Okaro area, thus setting up “pockets” The phase II eruption produced the most the western to southwestern area of the lake close of coexisting fluids with only secondary con- widespread and thickest breccia, as well as many to the source craters (Figs. 7 and 11). Further al- nections to the rest of the hydrothermal system. craters with different sizes and a broad distribu- teration of the Okaro Breccia III clasts indicates These pockets could explain the multiple craters, tion (Fig. 11). Moreover, the phase II eruption some residual hydrothermal activity within the rather than the formation of one large crater area involved a wide area north of the initial eruption strongly reworked/fractured ejecta of the phase I (Kilgour et al., 2019). site, which may reflect the presence of a large crater area (Fig. 12C). Hardy (2005) also noted Macorps et al. (2016) set up explosion experi- hydrothermal system. The wide area may also that erosion and the formation of a 50 m breach ments in “strong substrates” (mimicking well- reflect the fragmented and/or weakly compact- in the southeastern margin of the lake resulted in consolidated sediments) and “weak substrates” ed nature of the host rocks and the intermediate

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to shallow depth of the explosions within the deepest excavation of phase II is indicated by la- steam-driven eruptions. The history of erup- Earthquake Flat Formation (Galland et al., 2014; pilli-to-large blocks derived from the Rangitaiki tions from this area demonstrates that hazardous Montanaro et al., 2016b). Ignimbrite and Rotoehu Ash, concentrated in the steam-driven explosive events may persist where The final phase II crater shapes Fig. 11( ) sug- middle stratigraphic levels of the Okaro Breccia a geothermal system forms in a newly disturbed gest that a series of blasts overprinted and en- II in the western and southern sectors (Figs. 5 site. At this locality, we see evidence for three larged the craters from preceding events, while and 7; Figs. DR4 and DR5). As the phase II erup- separate episodes of explosive eruptions. maintaining a roughly circular shape (cf. Val- tion progressed, it expanded into substrate to the Deep-seated bedrock clasts and dominantly entine et al., 2015a). The smaller nested craters north. The northern to eastern sectors of Okaro unaltered clasts in the first (phase I) explosion may have resulted from multiple small steam Breccia II (up to 9 m thick) contain abundant al- breccia show that no geothermal system was explosions (Scott and Cody, 1982; Shanks et al., tered pumice and silicified clasts, but few large previously present at this site, but groundwa- 2005; Morgan et al., 2009). Phase II craters in blocks (Fig. 7). This thick deposit lobe that was ter was suddenly heated and pressurized to the east and southwest mostly excavated ejecta possibly produced by explosions directed from a produce a phreatic explosion. A fracture and from phase I, without extending the original cra- crater in the northern area (Fig. 12B). fault network likely focused gas or magmatic ter area (Figs. 10 and 11). Their coalesced shape Craters from phase III excavated the ejecta fluids and produced a linear chain of craters. may reflect shallow explosion loci. from phase II (Fig. 12C). The third set of explo- After this event, a new geothermal system Craters produced during the phase III explo- sions excavated weak and permeable substrate, was established within porous, weak pumice sions are shallow compared to those produced enabling efficient crater formation (cf. Montan- and nonwelded tuffs. After some time, during by the previous eruptions, and the explosions aro et al., 2016b). Many of the recognized cra- which hydrothermal alteration affected most of recycled ejecta from the previous two erup- ters lack obvious raised rims (Figs. 10 and 11), the Okaro area, phase II eruption was initiated tive phases. During phase III, almost no energy which probably indicates breccia dispersal into from the flashing of pressured water and steam was spent in fragmenting the host substrate (cf. lake water (Morgan et al., 2009), consistent with within the upper hydrothermal system. Phase II Montanaro et al., 2016b), and the excavation the sediment-covered depression and smoothed breccia contains highly altered pumice and tuff produced overlapping shallow structures, with morphologies of craters from the second erup- clasts, along with silicified blocks. The lower shallow explosion loci. tive phase (Fig. 10). Secondary hydrothermal part of Okaro Breccia II has a shallow source, Breccia componentry reflects the different dissolution and collapses may also have modi- and higher parts have a deeper source, indicat- host rock and depths of fragmentation during fied the original crater shapes (Scott and Cody, ing that the locus of explosions deepened. The the three periods of eruptions. Okaro Breccia 1982; Morgan et al., 2009). The thickest Okaro phase II eruption reached depths of the phase I is dominated by weakly altered and unaltered Breccia III occurs in the western and southwest- I event, as well as spreading laterally across a tuffs indicating an explosion locus >70 m depth, ern sectors (Fig. 7) outside the lake, and the wide area of weak pumice deposits. Triggering and overburden pumice from shallow levels dispersal direction was possibly affected by the of the phase II event could have involved a va- (Fig. 9A). The deeper, consolidated units re- presence of previous craters in the area. riety of processes, such as a seismic event or quired high energy to be fragmented and ejected, changes in the water levels of the hydrothermal whereas less energy was consumed in “unload- Posteruption Evolution system. Variably sized cluster of craters were ing” or disaggregating the Earthquake Flat For- formed during phase II. During the next pause, mation (Alatorre-Ibargüengoitia et al., 2010; Slumps and rockslide deposits at Lake Okaro a widespread lake formed across the area, and Montanaro et al., 2016b, 2016c). The Okaro provide evidence that crater enlargement contin- hydrothermal activity continued. A third phase Breccia II shows a complex circumcrater varia- ued after the eruptions. However, these deposits of explosions occurred mainly within the shal- tion in componentry and grain size, as well as in are confined to the discharge area of drainage low breccia deposits of earlier eruptions. Phase sorting and thickness (Fig. 7). This variability networks, suggesting that mass movements oc- III appears to have been generated by a sudden reflects the contrast between undisturbed Earth- curred long after the eruptive phases. This in- >3 m drop in the lake level. quake Flat Formation excavated in the northern ference is also supported by the lack of slump Formation of the large crater field at Okaro sector and prefragmented ejecta from phase I and landslide deposits at the steeper and deeper was produced by steam-driven eruptions, but in the southern sector. The radial variation of craters in the eastern, southeastern, and south- these eruptions had different character (bottom- Okaro Breccia II thickness may reflect instabili- ern sectors of the lake (Figs. 10 and 11). The up, followed by top-down) and fluids in different ties in the eruptive jets, multiple different explo- northern part of the lake received a sudden influx conditions (initially ambient and subsequently sion positions, as well as directed jets produced of fine sediment during the erosion of the 1886 hot). This combination has been rarely report- by interactions with confining crater walls (cf. Rotomahana Mud (a fine phreatomagmatic de- ed in New Zealand or elsewhere in the world Taddeucci et al., 2013; Valentine et al., 2015a, posit; Hardy, 2005). On the western lake side, (e.g., Mary Bay crater complex in Yellowstone; 2015b). Directed jets, in particular, produce the sediment load from the surrounding drain- Shanks et al., 2005; Morgan et al., 2009). The strongly asymmetrical skirts in which the thick- age network may have destabilized semiconsoli- geological and morphological evidence from est deposits are on the crater side opposite to the dated to consolidated Okaro breccias and older this case suggests that observable crater sizes, jet direction (Graettinger et al., 2015), which is formations (e.g., Earthquake Flat Formation) on shapes, and distributions are partially controlled consistent with the Okaro Breccia II. the rim of steep crater walls. by the different excavated host rocks, as well as Considering (1) the location of the craters ex- by the presence of pre-eruption craters and by cavated by phase I, (2) the confinement of the CONCLUSIONS preexisting fracture zones. This study also shows lower bed of Okaro Breccia II to the southern that deposit componentry can help to distinguish sector, as well as (3) the greatest deposit thick- Based on breccia stratigraphy, subsurface phreatic from hydrothermal eruptions, at least ness (up to 10 m) in the southern, southwestern, geological structure, and new crater morphol- where the local stratigraphy is well known and and western sectors, we suggest that the phase II ogy data at Lake Okaro, we can distinguish variable enough over the depth of explosion hydrothermal eruption began in the south. The deep “bottom-up” from subsequent “top-down” excavation.

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Fry, B., and Miller, C., 2014, Seismo-acoustic evidence Foote, L., Scheu, B., Kennedy, B., Gravley, D., and Dingwell, for an avalanche driven phreatic eruption through a be- ACKNOWLEDGMENTS D., 2011, Experemental calibration of hydrothermal ex- headed hydrothermal system: An example from the plosions: A case study on Lake Okaro, New Zealand: 2012 Tongariro eruption: Journal of Volcanology and Montanaro and Cronin acknowledge funding from Geoscience Society of New Zealand, (November), Geothermal Research, v. 286, p. 317–330, https://doi​ the New Zealand Ministry of Business, Innovation p. 38–39. .org/10.1016/​j.jvolgeores.2014.04.008. and Employment Smart Ideas grant “Stable power Fitzgerald, R.H., Kennedy, B.M., Wilson, T.M., Leonard, Kilgour, G., Manville, V., Della Pasqua, F., Graettinger, generation and tourism with reduced geothermal ex- G.S., Tsunematsu, K., and Keys, H., 2017, The com- A., Hodgson, K.A., and Jolly, G.E., 2010, The 25 munication and risk management of volcanic ballis- September 2007 eruption of Mount Ruapehu, New plosion hazard.” Kennedy acknowledges the New tic hazards, in Fearnley C.J., Bird, D.K., Haynes, K., Zealand: Directed ballistics, Surtseyan jets, and ice- Zealand Marsden grant “Shaking magma to trigger McGuire, W.J., and Jolly, G., eds., Observing the Vol- slurry lahars: Journal of Volcanology and Geothermal eruptions.” We acknowledge the Bay of Plenty Re- cano World: Cham, Switzerland, Springer, Advances in Research, v. 191, p. 1–14, https://doi​.org/10.1016/​ gional Council, particularly Andy Bruere, and David Volcanology (An Official Book Series of the Interna- j.jvolgeores.2009.10.015. Hamilton from Waikato University for allowing the tional Association of Volcanology and Chemistry of the Kilgour, G., Gates, S., Kennedy, B., Farquhar, A., Mcspor- access to the multibeam data used in this study. We Earth’s Interior [IAVCEI], Barcelona, Spain), p. 1–27, ran, A., and Asher, C., 2019, Phreatic eruption dynam- also acknowledge the New Zealand Department of https://doi​.org/10.1007/11157_2016_35. ics derived from deposit analysis: A case study from Conservation (Te Papa Atawhai), and Stephanie Kelly Galland, O., Gisler, G.R., and Haug, O.T., 2014, Mor- a small, phreatic eruption from Whakāri/White Island, phology and dynamics of explosive vents through New Zealand: Earth, Planets, and Space, v. 71, p. 36, from the Rotorua Lakes Council, who supported and cohesive rock formations: Journal of Geophysical Re- https://doi​.org/10.1186/s40623-019-1008-8. allowed this research to take place. We further ac- search–Solid Earth, v. 119, p. 4708–4728, https://doi​ Lawless, J.V., 1988, Punctuated equilibrium and paleohy- knowledge Larry Mastin, two anonymous reviewers, .org/10.1002/2014JB011050. drology, in Proceedings of the 10th New Zealand Geo- and the Associate Editor Jocelyn McPhie, whose com- Germanovich, L.N., and Lowell, R.P., 1995, The mechanism thermal Workshop, p. 165–169. ments significantly improved the manuscript. of phreatic eruptions: Journal of Geophysical Re- Leonard, G.S., Begg, J.G., and Wilson, C.J.J., 2010, Geology search–Solid Earth, v. 100, p. 8417–8434, https://doi​ of the Rotorua Area: Lower Hutt, New Zealand, Insti- .org/10.1029/94JB03096. tute of Geological & Nuclear Sciences Limited, 99 p. 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