Steam-driven eruptions are caused by explosive vaporization of water within the pores and cracks of a host rock, mainly within geothermal or volcanic terrains. Ground or surface water can be heated and pressurized rapidly from below (phreatic explosions), or already hot and pressurized fluids in hydrothermal systems may decompress when host rocks or seals fail (hydrothermal eruptions). Deposit characteristics and crater morphology can be used in combination with knowledge of host-rock lithology to reconstruct the locus, dynamics, and possible triggers of these events. We investigated a complex field of >30 craters formed over three separate episodes of steam-driven eruptions at Lake Okaro within the Taupo volcanic zone, New Zealand. Fresh unaltered rock excavated from initially >70 m depths in the base of phase I breccia deposits showed that eruptions were deep, “bottom-up” explosions formed in the absence of a preexisting hydrothermal system. These phreatic explosions were likely triggered by sudden rise of magmatic fluids/gas to heat groundwater within an ignimbrite 70 m below the surface. Excavation of a linear set of craters and associated fracture development, along with continued heat input, caused posteruptive establishment of a large hydrothermal system within shallow, weakly compacted, and unconsolidated deposits, including the phase I breccia. After enough time for extensive hydrothermal alteration, erosion, and external sediment influx into the area, phase II occurred, possibly triggered by an earthquake or hydrological disruption to a geothermal system. Phase II produced a second network of craters into weakly compacted, altered, and pumice-rich tuff, as well as within deposits from phase I. Phase II breccias display vertical variation in lithology that reflects top-down excavation from shallow levels (10–20 m) to >70 m. After another hiatus, lake levels rose. Phase III hydrothermal explosions were later triggered by a sudden lake-level drop, excavating into deposits from previous eruptions. This case shows that once a hydrothermal system is established, repeated highly hazardous hydrothermal eruptions may follow that are as large as initial phreatic events.Volcanic eruptions triggered by explosive vaporization of water are common and locally hazardous phenomena. They are typically termed steam-driven eruptions (Mastin, 1995; Thiéry and Mercury, 2009; Montanaro et al., 2016c). Such eruptions eject large (meter-sized) clasts on ballistic trajectories, generate highly energetic steam-rich density currents (surges), and expel wet jets of poorly sorted rock debris (Jolly et al., 2010; Breard et al., 2014; Maeno et al., 2016; Fitzgerald et al., 2017; Strehlow et al., 2017). The main hazard from steam-driven eruptions is their unheralded sudden onset, typically having little seismic or other warning (Barberi et al., 1992; Hurst et al., 2014). More than 200 such eruptions have occurred over the last three centuries, causing thousands of deaths (Mastin and Witter, 2000). The most common steam-driven eruptions can be classified as either phreatic or hydrothermal eruptions. Both of these eruption types do not directly involve or disrupt magma, but the power of water/gas expansion explosively fragments wall rock and ejects fragments upward and outward (Browne and Lawless, 2001).Here, we follow Stearns and McDonalds’ (1949) definition of phreatic explosions as being caused by flashing of groundwater and surface waters to steam due to the sudden arrival of heat and gas from intruding magma (or magmatic fluids). The flashed steam overpressurizes the bottom of an aquifer, but the aquifer is likely to breach near a horizontal discontinuity, such as a textural break or capping/sealing layer; thus, a simultaneous top-down rarefaction wave travels within the aquifer, and a “bottom-up” pressure wave in the overlying rock results in fragmentation and ejection of lithic clasts. Phreatic eruptions do not require the presence of a geothermal system and thus may eject fresh, unaltered rock. Deposits from a phreatic eruption should show a vertical succession of ejecta from deep sources overlain by ejecta from shallower sources.According to Mastin (1995) and Browne and Lawless (2001), hydrothermal eruptions are triggered within a geothermal area by a variety of processes that cause the sudden decompression and flashing of water that is already metastable and near boiling. Hydrothermal eruptions typically progress from a near-surface rupture downward (McKibbin et al., 2009). This mechanism is energetically very favorable, and a hydrothermal eruption can start within a meter or so of the ground surface below a very thin cap. After initial excavation, pressure within the deeper geothermal reservoir is reduced, and a flashing front and rarefaction front move progressively downward, followed by the boiling front. Water present in joints or cracks adjacent to the developing crater may also flash to steam as pressures reduce suddenly, widening the vent. This process may occur in both brittle or unconsolidated materials (Browne and Lawless, 2001; Galland et al., 2014; Montanaro et al., 2016b). Deposits typically contain abundant hydrothermally altered components and may record a progressive deepening of explosion locus through an inversion of the pre-explosion stratigraphy.Steam-driven eruptions last from seconds to hours (Browne and Lawless, 2001; Jolly et al., 2014) and produce craters from a few meters up to hundreds of meters in diameter, with depths from few to several hundred meters (Muffler et al., 1971; Browne and Lawless, 2001; Morgan et al., 2009). Deposits produced by these explosions are generally of low volume (<105 m3) and restricted to within hundreds of meters to a few kilometers from crater margins. They are typically very poorly sorted, matrix-supported breccias (Muffler et al., 1971; Mastin, 1995; Browne and Lawless, 2001; Breard et al., 2014). Steam-driven eruptions of all types occur in diverse volcanic and sedimentary rocks, showing a range in grain size, competence, alteration, fracturing, and bedding. All these factors influence deposit distribution and crater form. The geological and stratigraphic setting of an eruption site is also an important factor in interpreting eruption dynamics (Browne and Lawless, 2001; Morgan et al., 2009; Breard et al., 2014; Valentine et al., 2015a; Montanaro et al., 2016b).Steam-driven eruption craters are common in many volcanic terrains or areas of high heat flow, such as in New Zealand (e.g., Champagne Pool crater at Wai-o-tapu; Hedenquist and Henley, 1985), Indonesia (e.g., Sinila and Sigludung craters in the Dieng Plateau; Allard et al., 1989), Japan (e.g., craters on the Ontake volcano summit area; Maeno et al., 2016), Greece (e.g., Stefanos crater on Nysiros; Marini et al., 1993), El Salvador (e.g., craters in the Agua Shuca thermal area; Handal and Barrios, 2004), and the United States (e.g., Mary Bay crater complex in Yellowstone National Park; Morgan et al., 2009). In all these cases, craters were excavated within a wide range of host-rock lithologies having different alteration states, strengths, and permeabilities (Maeno et al., 2016; Montanaro et al., 2016a; Heap et al., 2017). These factors produced a typically complex crater morphology. Craters from steam-driven eruptions may preserve evidence of multiple explosions that migrated laterally (Nairn and Wiradiradja, 1980; Cole et al., 2006; Morgan et al., 2009; Breard et al., 2014). Evidence includes: lines or clusters of closely spaced craters; a single large crater with scalloped margins and/or a complex shape; and nested craters with different depths (e.g., Scott and Cody, 1982; Morgan et al., 2009; de Ronde et al., 2015; Montanaro et al., 2016b). Detailed mapping of the ejecta blanket, as well as ballistic ejecta analyses around the crater area, may reveal additional evidence of multiple explosion epicenters during a single active episode (Nairn, 1979; Kilgour et al., 2010, 2019; Breard et al., 2014; Lube et al., 2014; Maeno et al., 2016; Montanaro et al., 2016b).In this study, we assessed the effects of variations in the eruption-generation mechanism (bottom-up vs. top-down), the host-rock lithology, and the impact of successive eruptions on explosion dynamics and crater formation processes, using the example of Lake Okaro in New Zealand, located within the Taupo volcanic zone (Fig. 1). Lake Okaro has a subrectangular shape, and the surrounding breccia (Cross, 1963; Hedenquist and Henley, 1985) indicates that it may have resulted 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 hydrothermal eruptions were larger than the initial phreatic event. The eruptive activity that formed Lake Okaro is representative of many other complex 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 Yellowstone 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.Lake Okaro is located within the central Taupo volcanic zone (Fig. 1), north of the Wai-o-tapu geothermal field. This area is dissected by the NE-striking Ngapouri and Rotomahana fault systems, which feed fluids to numerous geothermal fields (Hedenquist and Henley, 1985; Nairn et al., 2005). Nairn et al. (2005) suggested that these fault/fluid pathways were reactivated during magma rise into the neighboring Okataina caldera, triggering the A.D. 1315 effusive and explosive, rhyolitic Kaharoa eruptions at Tarawera, as well as many of the steam-driven explosive eruptions within the Wai-o-tapu geothermal field. Contrasting with many hydrothermal craters in this area, Lake Okaro is not directly above a major fault system; it lies 0.5 km northwest of the nearest surface faults (Ngapouri and Rotomahana faults; Figs. 1 and 2). An inferred fault is mapped on the western side of the lake, and rivers running parallel to the local fault orientations on the eastern lake shore could indicate the presence of older faults and fractures across the whole Lake Okaro area (inferred faults in Figs. 1 and 2).The stratigraphy below Lake Okaro (Fig. 2) was constructed from nearby field observations and borehole logs. The deepest units recognized in drill cores (Nairn, 2003; Hardy, 2005) include an undescribed siltstone unit overlain by the Rangitaiki Ignimbrite (RI in Fig. 2). The Rangitaiki Ignimbrite is a poorly to moderately welded, dark-gray, crystal-rich tuff (Nairn, 2002). Facies include coarse ash- to pumice-flow tuffs, pumice breccias, and ash-to-lapilli-fall deposits. This ignimbrite is >100 m thick on the caldera rim at Lake Rotomahana, 3 km from Lake Okaro (Fig. 1), but it thins to only 7–39 m thick in boreholes near Lake Okaro (Nairn, 2002). The thinning likely reflects the pre-eruptive topography from displacement associated with blind fault branches of the Rotomahana fault system (inferred faults in Figs. 1 and 2). A 9-m-thick tuff-breccia, rich in accretionary lapilli, lies above the Rangitaiki Ignimbrite (Nairn, 2003). This unit is correlated with a similar breccia near Haumi Stream (Fig. 2), interbedded with a bedded lapilli-fall deposit (Rotoehu Ash; RA in Fig. 2; Nairn and Kohn, 1973; Leonard et al., 2010). The Earthquake Flat Formation overlies a sharp nonerosional break above the Rotoehu Ash (EFF in Fig. 2; Nairn and Kohn, 1973). The Earthquake Flat Formation is a weakly compacted, crystal-rich, lapilli-bearing ash, with coarsely vesicular pumice blocks; coarse (>5 mm) quartz, plagioclase, hornblende, and biotite crystals and crystal fragments are also present (Molloy et al., 2008; Leonard et al., 2010). In boreholes close to Lake Okaro, the Earthquake Flat Formation is ∼50 m thick (Figs. 1 and 2; Nairn, 2003; Hardy, 2005), and it is overlain by 0.5–2-m-thick ash-to-lapilli-fall deposits of the Waiohau Formation erupted from the Okataina volcanic complex (Nairn, 2002; Speed et al., 2002; Leonard et al., 2010; WF in Fig. 2). Above the Waiohau Formation, there is a series of younger tephras, including the 1.8 ka Taupo Pumice Formation (TPF in Fig. 3). A 1–3-cm-thick ash-fall deposit from the ca. A.D. 1315 Kaharoa eruption lies within a paleosol immediately below the Okaro breccias (Lloyd, 1959; Cross, 1963; Hedenquist and Henley, 1985; Nairn et al., 2005).We described the Okaro breccia deposits in several exposures around the lake (Figs. 1 and 3–7; Figs. DR2–DR51) and sampled the ash and lapilli-rich matrix, as well as blocks representative of the main lithologies, to be used for component and grain-size analyses. The percentage of blocks with respect to the breccia matrix was qualitatively assessed for the investigated sections. Examination of macroscopic alteration features, for example, color differences from whitish (freshest) to orange, yellow, reddish, and greenish (most altered), as well as the occurrence of alteration halos and silicified crusts and veins, indicated the relative degree of alteration within the investigated breccias. Field observations, in concert with the lake bathymetry (described later), were then used to reconstruct the eruptions scenario.Three main breccia units, Okaro Breccia I, II, and III, were distinguished based on grain-size, color, and lithology/componentry (Figs. 3–6), and they are grouped within the Okaro Breccia Formation. Cross (1963) and Hedenquist and Henley (1985) mapped the Okaro Breccia Formation out to ∼1 km from the lake shoreline, with one lobe extending ∼1.5 km to the east (Fig. 1). The whole formation is ∼13 m thick near the crater rim, but it rapidly thins to 2–3 m at ∼250 m from the eruptive center.On the western lake shores (location 1 in Fig. 1), the Okaro Breccia Formation overlies a 1-cm-thick fall unit from the A.D. 1315 Kaharoa eruption and a series of tephras from the Taupo and Okataina calderas, capping the Earthquake Flat Formation ignimbrite (Figs. 3, 4, and 7; Fig. DR1). A thin, pale brown paleosol caps the Okaro Breccia Formation, which is in turn covered by the Rotomahana Mud, a distinctive deposit from the phreatomagmatic phase of the 1886 Tarawera eruption (Fig. 7; Figs. DR3, DR4, and DR6; Nairn, 1979).The basal Okaro Breccia I is orange to yellow brown and dominated by an ash-rich matrix, supporting subrounded to angular lapilli and blocks (maximum diameter 100–150 mm; Fig. 4; Fig. DR1). The block fraction (>64 mm) consists of two main types of slightly altered to unaltered lithologies: (1) white to orange-stained pumice-rich tuff and fresh crystal-rich tuff, both with abundant lithic clasts, biotite, and quartz grains, and (2) fine-ash tuff containing abundant biotite, pumice, accretionary lapilli (0.5–1 mm sized), and lithic clasts in a pale-gray to yellow matrix. The first block lithology is predominant in the breccia (>60 modal %) and is derived from the Rangitaiki Ignimbrite, whereas the second, less abundant type (>35 modal %) is derived from the Rotoehu Ash. Rare (<5 modal %) white pumice blocks, rich in biotite and quartz, derived from the Earthquake Flat Formation, are also present.The matrix of Okaro Breccia I is reversely graded from ash-rich upward to greater lapilli content (reaching up to ∼30 wt%; Figs. 8A–8D). There is a persistent grain-size mode between 0 and –0.5 phi (1.4–1 mm; very coarse ash), as well as a long tail (from ∼30 to ∼50 wt%) extending from 1 mm to fine ash (<0.063 mm; Figs. 8A–8D). The matrix consists mainly of slightly altered to unaltered fragments of the same ignimbrite, tuff, and pumice as the blocks, together with loose biotite and quartz crystals (Figs. 8A and 8B). Ignimbrite and tuff clasts are orange-stained and subangular to angular, collectively making up ∼50 modal % of the particles (Figs. 9A and 9B). Another 30–40 modal % of the matrix particles are white to yellow pumice, which are mostly angular, highly vesicular, and rich in quartz and biotite. They are most abundant in the upper part of the breccia. Free crystals make up >10 modal % of the matrix and are mostly quartz, particularly in its middle to upper portion. The matrix particles commonly show a yellow alteration patina. Additionally, rare but distinctive red-stained, tuff-like particles occur within the matrix at the base of the unit (Figs. 9A and 9B).The Okaro Breccia I is thickest in the western (1 m) and southwestern (2.5 m) quadrants around the lake. It is not present to the north, and there are no outcrops of this member in the south and east (sections 1, 3, and 10; Fig. 7).In the southwestern sector of the lake (location 3; Fig. 1), the Okaro Breccia II is separated from the underlying Okaro Breccia I by a thinly laminated layer (1–10 cm thick) of reworked and waterlain ash and a pale brown paleosol (Figs. 5A–5B). However, at other locations (e.g., 1 in Fig. 1), no erosional surfaces or sedimentary layers are present between the two units (Fig. 4). In the northern and northeastern sectors, the Okaro Breccia II directly overlies the Kaharoa ash (Fig. 7; Figs. DR2 and DR3). The Okaro Breccia II is the most extensive deposit of all deposits mapped around the lake, covering all sectors. The breccia is at least 10 m thick in the southern and northern sectors of Lake Okaro, 6.5 m thick to the southwest, and 1.5 m thick to the east (Fig. 7).The Okaro Breccia II shows a vertical stratification with diffuse contacts between beds (Figs. 3 and 7). The basal bed is yellowish to gray, dominated by fine-grained matrix-supported subrounded to angular lapilli. The massive central bed is brown to yellow-brown and matrix- to clast-supported, and it contains angular to subrounded lapilli to coarse blocks (maximum diameter 400–600 mm; Figs. 5C–5D; Fig. DR4). The uppermost bed is brown to orange and dominated by a matrix-supported subrounded to angular lapilli (Fig. 6). The full sequence is present in the southern and western sectors around the lake, whereas to the east, northeast, and north, only the central bed is present with rare large blocks. In all the investigated sections, the central bed is characterized by distinctive altered tuff-like green clasts as blocks and/or within the matrix (Figs. 7; Figs. DR3 and DR5).The block fraction of Okaro Breccia II consists of fine-ash (>40 modal %) and accretionary lapilli–rich tuff (<30 modal %), and biotite- and quartz-rich pumice (∼30 modal %), mainly in the massive central bed (Figs. DR4–DR6). The first block lithology is derived from the Rangitaiki Ignimbrite, whereas the second type is derived from the Rotoehu Ash. The third lithology, more abundant compared to the amount found in the Okaro Breccia I, is derived from the Earthquake Flat Formation. There is a larger proportion of blocks (up to 30–40 modal % in the massive bed) cropping out in the southern, southwestern, and western sectors (Figs. 3, 5, and 7; Fig. DR5). The tuff clasts vary from pale gray to white to greenish-white and are mostly angular, whereas the white pumice particles are rounded to subangular. In the upper bed, rare silicified vein-like clasts and brecciated tuffs are found (Fig. DR4). Rare bomb sags were identified within the basal bed (Fig. DR3).The matrix grain-size distribution of Okaro Breccia II is coarsely skewed, having a large mode in the lapilli fraction and tails of ash (<15 wt%; Figs. 8E–8F). The central massive bed has the greatest ash content (Fig. 8F). The matrix contains mainly quartz- and biotite-rich pumice (>50 modal %), decreasing in content upward (Figs. 9C–9E). Tuff clasts are subordinate (<30 modal %), and their proportion increases from the center of the middle bed upward. Some matrix particles of all types show a yellow alteration patina. Up to 10 modal % of matrix particles are strongly altered, green, and rounded to angular in shape. These particles are limited to the upper half of the Okaro Breccia II (from the middle part of the massive bed upward; Figs. 6 and 8D–8E; Figs. DR3–DR5). Abundant quartz crystals (<10 modal %), either euhedral or broken, together with rare unidentified dark clasts (<1 modal %) occur in the matrix (Figs. 9C–9E).The Okaro Breccia III crops out in the southern and southwestern part of the lakeside and directly overlies the Okaro Breccia II, commonly without a weathered contact or paleosol. At the type section, a thin (up to 2 cm), fine-grained, waterlain orange ash deposit separates it from the Okaro Breccia II (Fig. 6). The Okaro Breccia III is red and mostly clast-supported, containing predominantly subrounded to angular lapilli, and it is capped by modern/recent soil. The Okaro Breccia III is limited to the western and southwestern sectors around the lake, where it is 2.5 and ∼0.5 m thick, respectively (Figs. 1 and 7).The block fraction of Okaro Breccia III mainly consists of angular fine-ash tuff (>60 modal %) and accretionary lapilli–rich tuff (>30 modal %), with a minor quantity (<15 modal %) of biotite- and quartz-rich pumice. Generally, all clast types are whitish in color and less competent than those within the other two breccias. Similar to the components of the first two breccias, the first and second block lithologies are derived from the Rangitaiki Ignimbrite and the Rotoehu Ash, respectively, whereas the pumice is derived from the Earthquake Flat Formation. Rare (<5%) altered tuff-like green blocks occur. Few large blocks (up to 30 cm) are present at the base of the breccia, whereas the upper part is lapilli-rich with rare blocks (Fig. 6). No bomb sags were identified within this member.The matrix grain-size distribution is coarsely skewed (Fig. 8H) and is lapilli dominated (>60 wt%). The matrix consists mainly of slightly or strongly altered pumice particles (>50%), subordinate (<20%) loose crystals (euhedral and broken quartz), and strongly altered fragments of tuffs (Fig. 9F). All matrix pumice particles are rounded to angular, yellowish-white, or covered by a red alteration patina. Altered tuff-like green clasts occur only in the block fraction.Lake Okaro has a roughly rectangular shape, is ∼650 m long and 400 m wide, and covers an area of 0.31 km2 (Fig. 10). High-resolution multibeam data (1 m spatial resolution) were acquired in 2014 by the Bay of Plenty Regional Council (Figs. 10 and 11). The level of Lake Okaro has varied historically (up to 1.5 m) in response to changing rainfall patterns (Cross, 1963). The lake surface had an elevation of 413 m during the 2014 survey and showed a maximum depth of 18 m in its southern sector. We calculated a water volume of 3.9 × 106 m3 using the high-resolution multibeam data.A well-developed drainage network is present west and northwest of the lake (Fig. 1), supplying sediment and water to form fans on the lake floor. On the northwestern hillsides bordering the lake, there are numerous rills, which formed immediately after the emplacement of the Rotomahana Mud during the A.D. 1886 Tarawera-Waimangu eruption (Hardy, 2005).The lake bathymetry allows reconstruction of the order of formation of the Okaro eruption craters. We determined three subareas of craters (Figs. 10 and 11; Fig. DR6), representing separate eruptive phases that produced the three separate breccia units. We defined these subareas based on the fact that younger eruption craters (1) cut the earlier-formed crater borders, (2) excavated into older breccias, and (3) typically showed fresher morphology (deeper, steeper walls and sharper margins). Older craters are smoothed and/or shallowed by infill of younger ejecta and/or lake sediment. Crater shapes were defined as circles or ovals fitting the distinct (or inferred) crater rims (Fig. 10; Fig. DR6). Average 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 definitively attributed to this phase because they are generally the most eroded, buried by sedimentation, and crosscut by later craters. They include the less obvious crater features located in the southern part of the lake (Fig. 11). Many of the craters, especially on the southwestern 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 borders are more evident, and they have relatively smooth floors. A NW-SE–elongated ridge in this area probably represents another remnant 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 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 intermediate 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 E-E′), having diameters between 33 and 80 m and depths of 1.6–3.9 m. Most of these clustered craters show low wall slopes (from 11° to 15°) and have crater depth/diameter ratios between 0.04 and 0.07. In the southwest, intermediate-sized craters (21–22 in Fig. DR6) have diameters of 77 and 92 m, intermediate wall slopes (16°–29°), and crater depth/diameter ratios of 0.03 and 0.06. All of these intermediate-sized craters cut the remnant phase I craters, as well as further excavate their crater floors (Fig. 10, profile E-E′). There may be more craters buried beneath the landslide deposits that blanket the northern part of the lake. Phase II craters cover ∼0.25 km2.Phase III craters. These craters crosscut crater walls from phases I and II, representing the most geomorphologically distinct and youngest craters occurring on the southern side of the lake (Fig. 10, profiles A-A′ and E-E′). On their eastern and southern sides, coalescing craters (25–28 in Fig. DR6) cut into a NW-SE–elongated ridge and remnants of older crater borders, both from phase I. On the northern and western sides of the phase III subareas, craters cut the rims and floors of craters from phase II (23, 24, 29 in Fig. DR6). Inferred diameters for the coalesced craters range from 53 to 110 m, and depths are between 1 m and 3 m. In general, these craters exhibit low wall slopes (from 5° to 15°) and low crater depth/diameter ratios (0.02–0.03). Craters of phase III (at least seven distinguishable) cover an area of ∼0.035 km2.Along the western and northwestern sides of Lake Okaro, several features indicate sediment inflow to the lake below creeks/valleys that drain the surrounding hills (Figs. 1, 10, and 11). A large delta occupies ∼0.05 km2 on the northwest side of the lake, and one of 0.008 km2 is present on the northeastern side. Stepped escarpments, 1–3 m high, on and above the largest delta indicate that this is the toe of a retrogressive slump feature (Fig. 10, profiles A-A′ and C-C′; Fig. 11). On the western side of the lake, small rockslide deposits have a hummocky topography and include scattered megablocks (up to 20 m wide and 1.5 m high) below many of the steeper crater wall embayments (Fig. 10, profile C-C′; Fig. 11).Potential triggers of the initial explosive activity at Lake Okaro and eruptive scenarios can be inferred from several lines of evidence, including subsurface stratigraphy and tectonic structures, as well as the alteration state and stratigraphy of the Okaro Breccia Formation.Lake Okaro lies close to the Rotomahana multithread fault system (Fig. 1; Lloyd, 1959; Hedenquist and Henley, 1985). Displacement along these faults produced a raised local topography, so that only a thin veneer of Rangitaiki Ignimbrite was emplaced, and the regional topography smoothed (Nairn, 2003). Also, a faulted volcanic sequence outcrops in the nearby Haumi Stream (Nairn et al., 2005), along with a surface fault west of Lake Okaro. Indirect evidence of further fault strands includes streams oriented parallel to the main fault trends immediately northeast of the lake (Fig. 1). Collectively, it appears that an approximately NE-SW–striking fracture zone existed at the site of Lake Okaro before the eruption (inferred fault in Figs. 1 and 2).The phase I eruption occurred soon after the explosive phase of the A.D. 1315 Kaharoa rhyolitic eruption (volcanic explosivity index [VEI] 4; Bonadonna et al., 2005; Nairn et al., 2005). Nairn et al. (2005) suggested that basalt dikes across the region near Mount Tarawera injected CO2 and heat to prime several steam-driven eruptions, e.g., the Champagne Pool at the Wai-o-tapu geothermal system. Fault displacements and/or channeling of magmatic gas up faults and fractures could have thus triggered the widespread explosive episodes, including Lake Okaro (Hedenquist and Henley, 1985; Browne and Lawless, 2001; Rowland and Simmons, 2012).Okaro Breccia I includes the deepest local lithology (Rangitaiki Ignimbrite; Nairn, 2003; Hardy, 2005) as tuff clasts at the base of the unit. There are no juvenile pyroclasts, indicating that no phreatomagmatic eruption occurred. These Rangitaiki Ignimbrite clasts show that the initial explosion began at least at a depth of ∼70 m. In addition, there are very few altered clasts in Okaro Breccia I, so no significant geothermal system existed at the eruption site.Collectively, the initial eruption of deep-seated bedrock, the absence of pervasive hydrothermal alteration of the ejecta, and the presence of faults and fractures in the area suggest that the eruption was triggered by rapid heating and overpressurization of groundwater by sudden arrival of magmatic fluids, for instance, from a dike (cf. Germanovich and Lowell, 1995; Stix and De Moor, 2018). This interpretation follows Nairn et al. (2005), who proposed that magmatic CO2 injection from a basaltic dike generated hydrothermal eruptions at Wai-o-tapu. Nearby, a shallow dike also caused both steam-driven and phreatomagmatic eruptions at Waimangu in 1886 (Nairn, 1979). Thermodynamic models (e.g., Delaney, 1987) suggest that heat is not transferred rapidly from an ascending dike to groundwater. However, large amounts of rising CO2 from a basaltic dike (>1000 t/d; Nairn et al., 2005) could have rapidly heated and pressurized shallow groundwater in fractures and aquifers above faults at the site of Lake Okaro. Germanovich and Lowell (1995) considered emplacement of magmatic fluids into a water-saturated permeable reservoir with two scales of permeability: 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 subsidiary network starts to be heated, leading to microscale pressurization. Heat and gas cause rapid propagation of cracks (seconds to hours for fractures 10−3 m to 10−1 m in size), leading to near boiling (Germanovich and Lowell, 1995). Extensional stresses and overpressures build up and eventually cause the country rock to fail, initiating decompression and an explosive eruption. This model is well suited for rocks with low tensile strength (≤10 MPa), such as the Rangitaiki Ignimbrite (4–8 MPa; Foote et al., 2011), which also has a bulk permeability of 10−16 m2 (Montanaro et al., 2017).A steam-driven event along a fracture system above a blind fault and a dike is also consistent with the linear chain of vents/craters of phase I (Fig. 11). Following phase I explosions, the elongate crater structure was filled by highly permeable ejecta. Ongoing heat transfer from the shallow intrusion may have lasted for months to years (cf. Petcovic and Dufek, 2005), heating and circulating hydrothermal fluids (cf. Rowland and Simmons, 2012). This geothermal system expanded into the shallow permeable (∼10−12 m2) deposits of the Earthquake Flat Formation surrounding the phase I craters (Fig. 12B; Tschritter and White, 2014).The new shallow hydrothermal system covered at least the area of phase II craters, extending ∼400 m north of the phase I explosion sites (Figs. 11 and 12B). The occurrence of thinly laminated lacustrine sediments between Okaro Breccia I and II (Figs. 5A and 5B) indicates a time break between these phases, with formation of a lake and related sediment accumulation. The presence of silicified sediments, silicified breccias, and abundant altered green tuff clasts, as well as large quantities of altered pumice and tuffs within Okaro Breccia II all around the lake (Figs. 9C–9E; Fig. DR4) suggests at least decades of hydrothermal activity took place before phase II explosions. In this setting, thermal and mechanical conditions at Lake Okaro were primed for producing a hydrothermal eruption, under any trigger scenario (e.g., Browne and Lawless, 2001). Possible trigger mechanisms of phase II eruption include seismic displacements and changes in surface and groundwater in the Lake Okaro area (cf. Lawless, 1988; Rowland and Simmons, 2012).The Okaro phase II eruption was likely triggered by a seismic event that fractured the hydrothermal system and reduced the confining pressure. The decompression of hydrothermal fluids probably resulted in the formation of a boiling-erosion front that penetrated and excavated down into the hydrothermal reservoir, mostly within the <70-m-depth Earthquake Flat Formation (top-down model of McKibbin et al., 2009). The eruption continued until the rate of groundwater boiling decreased and steam expansion declined to the point where rock could no longer be ejected from the crater. The collapse of crater walls, or flooding may have further contributed to stopping the eruption (Browne and Lawless, 2001).Another pause in the activity occurred between phases II and III, as indicated by a thin, waterlain mud deposit below Okaro Breccia III (Fig. 6), on the western side of the lake and positioned ∼3 m above the current lake level. This deposit indicates that a large lake formed in the depression produced during phase II. Hardy (2005) also suggested that the lake level was once much higher (>3 m), based on undercutting to the north and terracing to the east of the present Lake Okaro; our finding of lake sediments on the phase II crater rim confirms this hypothesis. The Okaro Breccia III mostly consists of tuff and minor pumice that appear more altered than those from Okaro Breccia II, and its extent is limited to the western to southwestern area of the lake close to the source craters (Figs. 7 and 11). Further alteration of the Okaro Breccia III clasts indicates some residual hydrothermal activity within the strongly reworked/fractured ejecta of the phase I crater area (Fig. 12C). Hardy (2005) also noted that erosion and the formation of a 50 m breach in the southeastern margin of the lake resulted in catastrophic drainage. Drainage may have caused a sudden decrease in confining pressure beneath the lake bed, which we suggest triggered rapid boiling in the surficial hydrothermal system and the onset of phase III eruption (Fig. 12C). Similar events have occurred after subtle lake drainage in other shallow hydrothermal systems, excavating top-downward (cf. McKibbin et al., 2009), and producing craters of comparable size to the phase III craters, for example, in Yellowstone and Iceland (Muffler et al., 1971; Morgan et al., 2009; Montanaro et al., 2016b).Field studies of natural explosion craters (Yokoo et al., 2002; Montanaro et al., 2016a), and field-based explosion experiments using loose-to-compacted material at varying shallow depths (Murphey and Vortman, 1961; Goto et al., 2001; Ohba et al., 2002; Taddeucci et al., 2013; Graettinger et al., 2014; Valentine et al., 2014; Sonder et al., 2015; Macorps et al., 2016) have demonstrated that crater shape and size result from an interplay between explosion energy and scaled depth (physical depth divided by cube root of energy). In addition, preexisting craters appear to influence the ejecta jets and whether or not an explosion is able to vent (Taddeucci et al., 2013; Graettinger et al., 2014). Experimental results further indicate that the volume affected depends on the host-rock (substrate) strength (Galland et al., 2014; Macorps et al., 2016). In many natural locations, multiple neighboring craters may imply internal inhomogeneity in fluid storage within rock bodies (e.g., thermal circulation patterns that set up mineralization boundaries at their margins, or three-dimensional variations in the porosity and permeability characteristics of host rock; Lawless, 1988; Browne and Lawless, 2001; Rowland and Simmons, 2012; Montanaro et al., 2016a). At Lake Okaro, much of the hydrothermal water was contained within the highly permeable, weakly compacted Earthquake Flat Formation (Tschritter and White, 2014). In areas near Lake Okaro, this formation shows complex changes in deposit texture, incipient compaction, and gas-escape pipes (Leonard et al., 2010). These variations may extend at depth underneath the Lake Okaro area, thus setting up “pockets” of coexisting fluids with only secondary connections to the rest of the hydrothermal system. These pockets could explain the multiple craters, rather than the formation of one large crater area (Kilgour et al., 2019).Macorps et al. (2016) set up explosion experiments in “strong substrates” (mimicking well-consolidated sediments) and “weak substrates” (mimicking unconsolidated sediments or volcaniclastic deposits). Their results suggest the following:(1) The host rock properties affect the crater morphology (e.g., steepness of walls), the clast type, and sizes available for ejection;(2) the host rock disrupted by subsurface explosions loses its original strength and has less impact on subsequent explosions, and(3) most of the disrupted subsurface structure is filled with ejecta.(1) The host rock properties affect the crater morphology (e.g., steepness of walls), the clast type, and sizes available for ejection;(2) the host rock disrupted by subsurface explosions loses its original strength and has less impact on subsequent explosions, and(3) most of the disrupted subsurface structure is filled with ejecta.As a first approximation, strong substrates at Lake Okaro could correspond to the deep Rangitaiki Ignimbrite and Rotoehu Ash tuffs, whereas weak substrates could correspond to surficial tephra deposits (Waiohau and Taupo Pumice Formations), or the ejecta from phases I and II. The Earthquake Flat Formation, representing the thickest and main host rock involved in the first two phases of Okaro eruptions, varies from weakly compacted to possibly slightly consolidated at depth, thus acting as a substrate with intermediate strength (Macorps et al., 2016).The steepness of crater walls at Lake Okaro (Table 2; Fig. 10) appears to have been influenced by the presence of the Earthquake Flat Formation. Many of the craters formed during the first two phases were excavated within this formation and have steep sides (23°–49°). By contrast, craters excavated within unconsolidated tephra deposits or the ejecta deposits of phase I and II eruptions have low-angle slopes (6°–15°).We infer that the Okaro Breccia I eruption crater distribution was dominated by the focusing mechanism of deep hot fluids in a preexisting fracture zone. Thus, the craters were elongate and aligned along this NE-SW–striking fracture system (Figs. 1 and 2). Based on the slope values and bathymetric profiles (Table 2; Fig. 10), phase II and III craters either excavated deeper into the phase I (increasing slope angles), or deposited ejecta that filled the crater (decreasing slope angles). We suggest that the phase I eruption was hosted mostly within the Earthquake Flat Formation, and it was deeply excavated along a fracture to produce a steep-sided fissure-like crater. The Okaro phase I eruption produced a breccia that was less widespread than the phase II breccia, consistent with the greater depth of initiation of the phase I phreatic eruption.The phase II eruption produced the most widespread and thickest breccia, as well as many craters with different sizes and a broad distribution (Fig. 11). Moreover, the phase II eruption involved a wide area north of the initial eruption site, which may reflect the presence of a large hydrothermal system. The wide area may also reflect the fragmented and/or weakly compacted nature of the host rocks and the intermediate to shallow depth of the explosions within the Earthquake Flat Formation (Galland et al., 2014; Montanaro et al., 2016b).The final phase II crater shapes (Fig. 11) suggest that a series of blasts overprinted and enlarged the craters from preceding events, while maintaining a roughly circular shape (cf. Valentine et al., 2015a). The smaller nested craters may have resulted from multiple small steam explosions (Scott and Cody, 1982; Shanks et al., 2005; Morgan et al., 2009). Phase II craters in the east and southwest mostly excavated ejecta from phase I, without extending the original crater area (Figs. 10 and 11). Their coalesced shape may reflect shallow explosion loci.Craters produced during the phase III explosions are shallow compared to those produced by the previous eruptions, and the explosions recycled ejecta from the previous two eruptive phases. During phase III, almost no energy was spent in fragmenting the host substrate (cf. Montanaro et al., 2016b), and the excavation produced overlapping shallow structures, with shallow explosion loci.Breccia componentry reflects the different host rock and depths of fragmentation during the three periods of eruptions. Okaro Breccia I is dominated by weakly altered and unaltered tuffs indicating an explosion locus >70 m depth, and overburden pumice from shallow levels (Fig. 9A). The deeper, consolidated units required high energy to be fragmented and ejected, whereas less energy was consumed in “unloading” or disaggregating the Earthquake Flat Formation (Alatorre-Ibargüengoitia et al., 2010; Montanaro et al., 2016b, 2016c). The Okaro Breccia II shows a complex circumcrater variation in componentry and grain size, as well as in sorting and thickness (Fig. 7). This variability reflects the contrast between undisturbed Earthquake Flat Formation excavated in the northern sector and prefragmented ejecta from phase I in the southern sector. The radial variation of Okaro Breccia II thickness may reflect instabilities in the eruptive jets, multiple different explosion positions, as well as directed jets produced by interactions with confining crater walls (cf. Taddeucci et al., 2013; Valentine et al., 2015a, 2015b). Directed jets, in particular, produce strongly asymmetrical skirts in which the thickest deposits are on the crater side opposite to the jet direction (Graettinger et al., 2015), which is consistent with the Okaro Breccia II.Considering (1) the location of the craters excavated by phase I, (2) the confinement of the lower bed of Okaro Breccia II to the southern sector, as well as (3) the greatest deposit thickness (up to 10 m) in the southern, southwestern, and western sectors, we suggest that the phase II hydrothermal eruption began in the south. The deepest excavation of phase II is indicated by lapilli-to-large blocks derived from the Rangitaiki Ignimbrite and Rotoehu Ash, concentrated in the middle stratigraphic levels of the Okaro Breccia II in the western and southern sectors (Figs. 5 and 7; Figs. DR4 and DR5). As the phase II eruption progressed, it expanded into substrate to the north. The northern to eastern sectors of Okaro Breccia II (up to 9 m thick) contain abundant altered pumice and silicified clasts, but few large blocks (Fig. 7). This thick deposit lobe that was possibly produced by explosions directed from a crater in the northern area (Fig. 12B).Craters from phase III excavated the ejecta from phase II (Fig. 12C). The third set of explosions excavated weak and permeable substrate, enabling efficient crater formation (cf. Montanaro et al., 2016b). Many of the recognized craters lack obvious raised rims (Figs. 10 and 11), which probably indicates breccia dispersal into lake water (Morgan et al., 2009), consistent with the sediment-covered depression and smoothed morphologies of craters from the second eruptive phase (Fig. 10). Secondary hydrothermal dissolution and collapses may also have modified the original crater shapes (Scott and Cody, 1982; Morgan et al., 2009). The thickest Okaro Breccia III occurs in the western and southwestern sectors (Fig. 7) outside the lake, and the dispersal direction was possibly affected by the presence of previous craters in the area.Slumps and rockslide deposits at Lake Okaro provide evidence that crater enlargement continued after the eruptions. However, these deposits are confined to the discharge area of drainage networks, suggesting that mass movements occurred long after the eruptive phases. This inference is also supported by the lack of slump and landslide deposits at the steeper and deeper craters in the eastern, southeastern, and southern sectors of the lake (Figs. 10 and 11). The northern part of the lake received a sudden influx of fine sediment during the erosion of the 1886 Rotomahana Mud (a fine phreatomagmatic deposit; Hardy, 2005). On the western lake side, the sediment load from the surrounding drainage network may have destabilized semiconsolidated to consolidated Okaro breccias and older formations (e.g., Earthquake Flat Formation) on the rim of steep crater walls.Based on breccia stratigraphy, subsurface geological structure, and new crater morphology data at Lake Okaro, we can distinguish deep “bottom-up” from subsequent “top-down” steam-driven eruptions. The history of eruptions from this area demonstrates that hazardous steam-driven explosive events may persist where a geothermal system forms in a newly disturbed site. At this locality, we see evidence for three separate episodes of explosive eruptions.Deep-seated bedrock clasts and dominantly unaltered clasts in the first (phase I) explosion breccia show that no geothermal system was previously present at this site, but groundwater was suddenly heated and pressurized to produce a phreatic explosion. A fracture and fault network likely focused gas or magmatic fluids and produced a linear chain of craters. After this event, a new geothermal system was established within porous, weak pumice and nonwelded tuffs. After some time, during which hydrothermal alteration affected most of the Okaro area, phase II eruption was initiated from the flashing of pressured water and steam within the upper hydrothermal system. Phase II breccia contains highly altered pumice and tuff clasts, along with silicified blocks. The lower part of Okaro Breccia II has a shallow source, and higher parts have a deeper source, indicating that the locus of explosions deepened. The phase II eruption reached depths of the phase I event, as well as spreading laterally across a wide area of weak pumice deposits. Triggering of the phase II event could have involved a variety of processes, such as a seismic event or changes in the water levels of the hydrothermal system. Variably sized cluster of craters were formed during phase II. During the next pause, a widespread lake formed across the area, and hydrothermal activity continued. A third phase of explosions occurred mainly within the shallow breccia deposits of earlier eruptions. Phase III appears to have been generated by a sudden >3 m drop in the lake level.Formation of the large crater field at Okaro was produced by steam-driven eruptions, but these eruptions had different character (bottom-up, followed by top-down) and fluids in different conditions (initially ambient and subsequently hot). This combination has been rarely reported in New Zealand or elsewhere in the world (e.g., Mary Bay crater complex in Yellowstone; Shanks et al., 2005; Morgan et al., 2009). The geological and morphological evidence from this case suggests that observable crater sizes, shapes, and distributions are partially controlled by the different excavated host rocks, as well as by the presence of pre-eruption craters and by preexisting fracture zones. This study also shows that deposit componentry can help to distinguish phreatic from hydrothermal eruptions, at least where the local stratigraphy is well known and variable enough over the depth of explosion excavation.In a broader sense, our findings suggest that in hydrothermally active environments, the assessment of potential eruptive scenarios depends on a detailed understanding of shallow geology and the extent of the hydrothermal system. These key factors control, for instance, eruption directivity, longevity, and number of explosions, and their associated hazard. Also, once an area has been disturbed by a phreatic eruption, the subsequent formation of a new geothermal system should be very closely monitored, because later hydrothermal eruptions can be equally as large and hazardous as the initial phreatic event.Montanaro and Cronin acknowledge funding from the New Zealand Ministry of Business, Innovation and Employment Smart Ideas grant “Stable power generation and tourism with reduced geothermal explosion hazard.” Kennedy acknowledges the New Zealand Marsden grant “Shaking magma to trigger eruptions.” We acknowledge the Bay of Plenty Regional Council, particularly Andy Bruere, and David Hamilton from Waikato University for allowing the access to the multibeam data used in this study. We also acknowledge the New Zealand Department of Conservation (Te Papa Atawhai), and Stephanie Kelly from the Rotorua Lakes Council, who supported and allowed this research to take place. We further acknowledge Larry Mastin, two anonymous reviewers, and the Associate Editor Jocelyn McPhie, whose comments significantly improved the manuscript.

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由蒸汽驱动的喷发形成的复杂的火山口场：新西兰奥卡罗湖

蒸汽驱动的喷发是由主体岩石的孔隙和裂缝内（主要在地热或火山地形内）水的爆炸性汽化引起的。地下水或地表水可以从下面快速加热并加压（潜水爆炸），或者当主岩石或密封失效时（热液喷发），热液系统中已经热且加压的流体可能会减压。沉积特征和火山口形态可与基质岩石岩性知识结合使用，以重建这些事件的发生地，动力学和可能的触发因素。我们调查了新西兰陶波火山区内奥卡罗湖由三个不同的蒸汽驱动喷发形成的30个火山口的复杂区域。从最初开始挖掘的未改变的新鲜岩石> 一期角砾岩沉积物底部的70 m深度表明，在没有预先存在的热液系统的情况下，喷发是深的，“自下而上”的爆炸。这些潜水爆炸可能是由岩浆流体/气体的突然上升引发的，以加热地表以下70 m的火成岩中的地下水。线性坑的开挖和相关的裂缝发展以及持续的热量输入，导致浅水，弱压实和未固结沉积物（包括I角砾岩）内大型热液系统的后继建立。经过足够的时间进行热液蚀变，侵蚀和外部沉积物大量涌入该区域后，发生了第二阶段，可能是由于地震或地热系统的水文破坏引起的。第二阶段产生了第二个火山口网络，形成了压实度低，蚀变和富含浮石的凝灰岩，以及第一阶段的沉积物。第二阶段角砾岩在岩性方面显示出垂直变化，反映了浅层自上而下的开挖（10-20 m）至> 70 m。再次中断后，湖泊水位上升。第三阶段热液爆炸后来是由突然的湖水位下降引发的，从先前的喷发中挖掘出沉积物。该案例表明，一旦建立了热液系统，可能会发生与最初的潜水活动一样大的重复性高危险性热液喷发。水爆炸性汽化引发的火山喷发是普遍的局部危险现象。它们通常被称为蒸汽驱动爆发（Mastin，1995；Thiéry和Mercury，2009； Montanaro等，2016c）。此类喷发会在弹道上喷射出大块（米级）碎屑，产生高能量的富含蒸汽的密度流（浪涌），并排出分类不佳的岩屑的湿喷流（乔利等人，2010年；布雷德等人，2014年； Maeno等人，2016; Fitzgerald等人，2017; Strehlow等人，2017）。蒸汽驱动喷发的主要危害是它们未曾预料到的突然发作，通常几乎没有地震或其他预警（Barberi等，1992； Hurst等，2014）。在过去的三个世纪中，已经发生了200多次此类喷发，造成了数千人死亡（Mastin和Witter，2000）。最常见的蒸汽驱动喷发可分为潜水喷发或热液喷发。这两种喷发类型都不直接涉及或破坏岩浆，但是水/天然气膨胀的力量会爆炸性地炸破围岩，并向上和向外喷射碎片（Browne和Lawless，2001年）。在这里，我们遵循Stearns和McDonalds（1949）的地下爆炸的定义，这种爆炸是由地下水和地表的闪蒸引起的由于侵入岩浆（或岩浆流体）产生的热量和气体突然到达，水会变成蒸汽。闪蒸的蒸汽使含水层的底部过压，但含水层可能会在水平不连续处附近破裂，例如质地破裂或封盖/密封层。因此，同时发生的自上而下的稀疏波在含水层中传播，而上覆岩石中的“自下而上”的压力波导致碎屑岩碎裂和喷出。潜水爆发不需要地热系统的存在，因此可以喷出未变的新鲜岩石。潜水喷发的沉积物应显示出深层喷发的垂直演替，而浅层喷发则覆盖了喷发。根据Mastin（1995）和Browne and Lawless（2001）的研究，地热区域内的热液喷发是通过多种过程触发的导致已经亚稳且接近沸腾的水突然减压和闪蒸。热液喷发通常从近地表破裂向下发展（McKibbin等，2009）。这种机制在能量上非常有利，并且水热喷发可以在非常薄的帽盖下方一米左右的地面开始。初始开挖后，深层地热储层内的压力降低，闪蒸锋面和稀疏锋面逐渐向下移动，随后为沸腾前沿。由于压力突然降低，从而扩大了通风口，因此在靠近正在发展的火山口的接缝或裂缝中存在的水也可能闪蒸为蒸汽。这个过程可能会发生在脆性或未固结的材料中（Browne和Lawless，2001； Galland等，2014； Montanaro等，2016b）。矿床通常含有丰富的热液蚀变成分，并可能通过爆炸前地层的倒置来记录爆炸轨迹的逐渐加深。蒸汽驱动的喷发持续数秒至数小时（Browne and Lawless，2001; Jolly et al。，2014）和生产的火山口直径从几米到几百米不等，深度从几米到几百米（Muffler等，1971； Browne and Lawless，2001； Morgan等，2009）。这些爆炸产生的沉积物通常数量很少（< 105立方米），距离火山口边缘几百米到几公里。它们通常是分类很差，由基质支撑的角砾岩（Muffler等，1971； Mastin，1995； Browne and Lawless，2001； Breard等，2014）。各种类型的火山喷发发生在不同的火山岩和沉积岩中，显示出一定的粒度，能力，蚀变，压裂和层理。所有这些因素都会影响沉积物的分布和火山口的形成。喷发地点的地质和地层环境也是解释喷发动态的重要因素（Browne和Lawless，2001; Morgan等，2009; Breard等，2014; Valentine等，2015a; Montanaro等。 ，2016b）。蒸汽驱动的火山口在许多火山地形或高热流地区很常见，例如在新西兰（例如，Wai-o-tapu的香槟池火山口；Hedenquist和Henley，1985年），印度尼西亚（例如，迪恩高原的Sinila和Sigludung火山口； Allard等人，1989年），日本（例如，Ontake火山顶峰地区的火山口； Maeno等人，2016年），希腊（例如Nysiros的Stefanos火山口； Marini等，1993），萨尔瓦多（例如，阿瓜舒卡热区的火山口； Handal和Barrios，2004）和美国（例如，黄石国家公园的Mary Bay火山口综合体） ； Morgan等，2009）。在所有这些情况下，火山口都在具有不同蚀变状态，强度和渗透率的多种基质岩石岩性中被挖掘出来（Maeno等人，2016; Montanaro等人，2016a; Heap等人，2017）。这些因素产生了通常复杂的陨石坑形态。蒸汽驱动火山喷发的火山口可能保留了多次横向爆炸的证据（Nairn和Wiradiradja，1980； Cole等，2006； Morgan等，2009； Breard等，2014）。证据包括：密集的火山口的线或簇；具有扇形边缘和/或复杂形状的单个大火山口；以及深浅不一的嵌套陨石坑（例如Scott和Cody，1982； Morgan等，2009； de Ronde等，2015； Montanaro等，2016b）。弹射层的详细映射以及弹坑区域周围的弹道弹射分析，可能会揭示出在单个活跃事件中多次爆炸震中的其他证据（Nairn，1979； Kilgour等，2010，2019； Breard等， 2014年； Lube等人，2014年； Maeno等人，2016年； Montanaro等人，2016b）。我们使用奥卡罗湖（Lake Okaro）为例，评估了火山喷发生成机制（自下而上与自上而下），宿主岩石的岩性以及连续喷发对爆炸动力学和火山口形成过程的影响。新西兰，位于陶波火山带内（图1）。奥卡罗湖具有矩形下的形状，周围的角砾岩（克罗斯，1963年；海顿奎斯特和亨利，1985年）表明，它可能是由多次爆炸造成的。Hardy（2005）认识到湖南端最初的潜水喷发产生了至少三个角砾岩单元。火山喷发之后发生了一系列热液爆炸事件，将火山口扩大到其最终形状。一些后来的热液喷发比最初的潜水事件要大。形成冈冈湖的火山爆发活动代表了新西兰其他许多反复的蒸汽驱动爆发的复杂案例（例如，在Rotokawa，Waimangu和Wai-o-tapu地热田），以及该地区周围的许多其他类似地区世界（例如，美国黄石国家公园的大型热液喷发坑）； Browne and Lawless，2001； Morgan等，2009）。在这里，我们检查了奥卡罗湖附近的角砾岩沉积物，并调查了湖测深法，以：（1）重新探查喷发触发，以及（2）探索火山爆发动力学（包括火山口形成过程）与岩心期间岩体性质之间的关系。奥卡罗湖位于陶波中央火山区（图1），位于怀奥塔普地热田以北。东北向的Ngapouri和Rotomahana断层系统将这一区域划分开来，将流体输送到众多地热田中（Hedenquist和Henley，1985； Nairn等，2005）。Nairn等。（2005年）表明这些断层/流体通道在岩浆上升到邻近的Okataina火山口时被重新激活，引发了Tarawera的AD 1315爆发性和爆炸性流变型Kaharoa喷发，以及Wai-奥塔普地热田。与该地区许多热液陨石坑相反，奥卡罗湖不在主要断层系统的正上方。它位于最近的地面断层（Ngapouri和Rotomahana断层；图1和2）西北0.5公里处。推断出的断层被绘制在湖的西侧，以及与东部湖岸局部断层方向平行的河流可能表明在整个Okaro湖地区存在较早的断层和裂缝（图1和2中推断出的断层）。Okaro湖下方的地层（图2）。它是根据附近的野外观测和井眼测井数据构建的。钻芯中公认的最深单元（Nairn，2003； Hardy，2005）包括由Rangitaiki Ignimbrite覆盖的未描述的粉砂岩单元（图2中的RI）。Rangitaiki Ignimbrite是一种焊接不良至中度焊接，深灰色，富含晶体的凝灰岩（Nairn，2002）。岩相包括从灰烬到浮石的凝灰岩，浮石角砾岩和从灰烬到小珠落的沉积物。这种火山岩在Rotomahana湖的破火山口边缘厚于100 m，距Okaro湖3公里（图1），但是在Okaro湖附近的钻孔中，它的厚度只有7–39 m（Nairn，2002）。变薄可能反映了与罗托马哈纳断层系统的盲断分支（图1和图2中推断的断层）相关的位移所引起的喷发前的地形。Rangitaiki Ignimbrite上方有一个9米厚的凝灰岩角砾层，富含增生性lapilli（Nairn，2003年）。该单元与Haumi河附近的类似角砾岩相关（图2），中间夹有层状落石沉积物（Rotoehu Ash；图2中的RA； Nairn和Kohn，1973； Leonard等人，2010）。地震平地层覆盖了Rotoehu灰上方的一个陡峭的非侵蚀性断裂（图2中的EFF； Nairn和Kohn，1973）。地震平地层是一种压实度低，富含晶体，含青金石的灰烬，带有粗大的水疱浮石块。粗（> 5 mm）石英，斜长石，角闪石以及黑云母晶体和晶体碎片也存在（Molloy等，2008； Leonard等，2010）。在靠近Okaro湖的井眼中，地震平整层厚约50 m（图1和2； Nairn，2003； Hardy，2005），并覆盖有0.5–2 m厚的灰烬至洛皮利岩。 Okataina火山复合体喷出了Waiohau组的秋季沉积物（Nairn，2002； Speed等，2002； Leonard等，2010； WF在图2中）。在怀奥豪组之上，有一系列年轻的特弗拉斯，包括1.8 ka陶波浮石组（图3中的TPF）。大约1–3厘米厚的灰分沉积。公元1315年，卡哈罗阿火山喷发位于Okaro角砾岩正下方的古土壤中（Lloyd，1959； Cross，1963； Hedenquist和Henley，1985； Nairn等，2005）。我们描述了湖周围几次暴露的Okaro角砾岩沉积物（图1和3-7；图DR2-DR51），并采样了灰分和青金石基质以及代表主要岩性的块体用于成分和粒度分析。定性评估了相对于角砾岩基质块的百分比。检查宏观变化特征，例如，从白色（最新鲜）到橙色，黄色，红色和绿色（变化最大）的颜色差异，以及发生变化的光晕和硅化的结壳和静脉，表明变化的相对程度在调查的角砾岩中。然后，结合湖泊测深法（稍后描述）进行野外观测，以重建喷发情景。三个角砾岩单元，Okaro Breccia I，II和III根据晶粒大小，颜色和岩性/成分（图3–6）进行了区分，并归入Okaro Breccia组。Cross（1963）以及Hedenquist和Henley（1985）将Okaro Breccia组绘制到距湖岸线约1 km处，其中一个波瓣向东延伸约1.5 km（图1）。整个岩层在火山口边缘附近约13 m厚，但在距喷发中心约250 m处迅速变薄至2-3 m。在西部湖岸（图1中的位置1），Okaro Breccia组上覆一个来自AD 1315 Kaharoa火山喷发的厚1厘米的坠落单元以及来自Taupo和Okataina火山口的一系列提弗拉，封盖了地震平地层的火成岩（图3、4和7；图DR1）。薄薄的浅棕色古土壤覆盖在Okaro角砾岩层上，Okaro Breccia I基底为橙色至黄色，而又被Rotomahana泥浆覆盖，Rotomahana泥浆是1886年Tarawera喷发的岩浆相的独特沉积物（图7;图DR3，DR4和DR6; Nairn，1979）。棕褐色，以富含灰分的基质为主，支撑成亚圆角成角状的lapilli和块状（最大直径100–150 mm；图4；图DR1）。块级分数（> 64 mm）由两种主要类型组成，这些类型略有改变，变为未改变的岩性：（1）白色至橙色染色的浮石凝灰岩和新鲜的晶体凝灰岩，均具有丰富的岩屑，黑云母和石英晶粒（2）粉煤灰凝灰岩，含丰富的黑云母，浮石，增生性lapilli（0.5-1毫米大小）和石屑，呈浅灰色至黄色基质。第一类岩性主要是角砾岩（> 60莫代尔％）是由Rangitaiki Ignimbrite衍生而来的，而第二种较不丰富的类型（> 35莫代尔％）是由Rotoehu Ash衍生的。还存在来自地震平坦地层的稀有（<5模态％）的富含白云母和石英的白色浮石块.Okaro Breccia I的基质从富含灰分的向上反向分级至更大的青金石含量（达到约30 wt％；图8A-8D）。在0到–0.5 phi（1.4–1毫米；非常粗糙的灰）之间存在持续的粒度模式，以及从1毫米延伸到细灰（<<重量百分比）的长尾巴（从约30到约50 wt％）。 0.063毫米；图8A-8D）。基质主要由与块体相同的着火物，凝灰岩和浮石的轻微改变成未改变的碎片，以及松散的黑云母和石英晶体组成（图8A和8B）。着火的岩屑和凝灰岩屑是橙色的，角以下到角，共同构成了约50莫代尔％的颗粒（图9A和9B）。另有30–40模态％的基质颗粒为白色至黄色浮石，大部分呈棱角形，高度囊泡状，并富含石英和黑云母。它们在角砾岩的上部最丰富。自由晶体占基体的> 10模态％，并且主要是石英，尤其是在其中上部。基质颗粒通常显示出黄色的变色铜绿。此外，在单元底部的基质内还出现了罕见但独特的红色染色，凝灰岩状颗粒（图9A和9B）.Okaro Breccia I在西部（1 m）和西南（2.5 m）最厚湖周围的象限。它不存在于北方，在南部和东部没有该成员的露头（第1、3和10节；图7）。在湖西南部（位置3；图1），Okaro Breccia II与底层的Okaro Breccia I由一层薄薄的层压层（1-10厘米厚）的重做过的粉煤灰和浅褐色的古土壤（图5A-5B）组成。但是，在其他位置（例如，图1中的1），在两个单元之间（图4）没有侵蚀表面或沉积层。在北部和东北部，Okaro Breccia II直接覆盖在Kaharoa火山灰上（图7；图DR2和DR3）。Okaro Breccia II是湖泊周围所有矿床中分布最广泛的矿床，涵盖所有领域。在Okaro湖的南部和北部，角砾岩的厚度至少为10 m，而西南角的厚度至少为6.5 m，以及1。东面5 m厚（图7）。Okaro Breccia II显示出垂直分层，床间有分散接触（图3和7）。基床为淡黄色至灰色，主要由细颗粒基质支持的亚圆形至角状lapilli所主导。巨大的中央床为棕色至黄棕色，基质为碎屑状支撑，其中包含成角度的至次圆形的小圆石至粗块（最大直径400-600 mm；图5C-5D；图DR4）。最上层的床是棕色到橙色的，并由基质支持的亚圆角成角状青紫质为主（图6）。全部序列出现在湖周围的南部和西部，而在东部，东北和北部，只有中央河床存在稀有的大块体。在所有调查的部分中，中央床的特征是块体和/或基质内部有明显改变的凝灰岩状绿色碎屑（图7；图DR3和DR5）。Okaro Breccia II的块状部分由细灰组成（> 40模态％ ）和富集性lapilli的凝灰岩（<30莫代尔％），以及富含黑云母和石英的浮石（〜30莫代尔％），主要在巨大的中央床中（图DR4-DR6）。第一种岩性来自Rangitaiki Ignimbrite，而第二种岩性来自Rotoehu Ash。第三种岩性，比Okaro Breccia I中发现的岩性更丰富，来自地震平坦地层。在南部，西南和西部地区，出现了较大比例的块体（块状床中高达30–40模态％）（图3、5和7；图DR5）。凝灰岩碎屑从浅灰色到白色到绿白色不等，大多呈棱角形，而白色浮石颗粒呈圆形至近角形。在上层床上，发现了罕见的硅化脉状碎屑和角砾状凝灰岩（图DR4）。在基床内发现了罕见的炸弹下陷（图DR3）.Okaro Breccia II的基体晶粒尺寸分布是粗斜的，在lapilli分数和灰分的尾部具有较大的模态（<15 wt％;图8E –8F）。中央块状床具有最大的灰分含量（图8F）。基质主要包含富含石英和黑云母的浮石（> 50莫代尔％），其含量向上降低（图9C-9E）。凝灰岩碎屑是次要的（<30模态％），并且它们的比例从中间床的中心向上增加。所有类型的某些基质颗粒均显示出黄色的变色铜绿。多达10莫代尔％的基质颗粒发生了强烈变化，呈绿色并变圆了成角度的形状。这些颗粒仅限于Okaro Breccia II的上半部分（从块状床的中部向上；图6和8D-8E；图DR3-DR5）。基质中出现大量的石英晶体（<10模态％），无论是浅色的还是破碎的，以及稀有的未识别的深色碎屑（<1模态％）。Okaro Breccia III种植在南部和西南部湖边的一部分，直接覆盖Okaro Breccia II，通常没有风化接触或古土壤。在型材部分，薄的（最大2厘米）细颗粒，水渍橙色烟灰沉积物将其与Okaro Breccia II分开（图6）。Okaro布雷西亚三世（Okaro Breccia III）是红色的，大部分由碎屑支撑，主要包含亚角化成角状的lapilli 并且被现代/最近的土壤所覆盖。Okaro Breccia III仅限于湖周围的西部和西南部，厚度分别为2.5和〜0.5 m（图1和7）。Okaro Breccia III的块状部分主要由角状细灰凝灰岩组成。 （> 60莫代尔％）和增生的青金石凝灰岩（> 30莫代尔％），以及少量（<15莫代尔％）的富含黑云母和石英的浮石。通常，所有类型的克拉斯特颜色都发白，胜于其他两个角砾岩中的克拉斯特。与前两个角砾岩的成分相似，第一和第二块岩性分别来自Rangitaiki Ignimbrite和Rotoehu Ash，而浮石则来自地震平地层。发生稀有（<5％）的变凝灰状绿色块。角砾岩的底部几乎没有大块（长达30厘米），而上部是富含小块的羊角石（图6）。在该构件内未发现炸弹下陷。基体粒度分布粗略偏斜（图8H），并以青金石为主（> 60 wt％）。基质主要由略微或强烈变化的浮石颗粒（> 50％），次要（<20％）疏松晶体（石英和碎石英）以及凝灰岩的强烈变化碎片组成（图9F）。所有基质浮石颗粒均被修圆成角形，黄白色或被红色蚀变铜绿覆盖。凝灰岩状的绿色碎屑仅在块状部分中出现。冈仓湖大体呈矩形，长约650 m，宽约400 m，面积0.31 km2（图10）。高清晰度多光束数据（1 m空间分辨率）是由丰盛湾区域委员会于2014年获得的（图10和11）。由于降雨模式的变化，奥卡罗湖的水位在历史上有所变化（最大1.5 m）（Cross，1963）。在2014年的调查中，该湖面的高程为413 m，南部区域的最大深度为18 m。我们利用高分辨率的多波束数据计算出了3.9×106 m3的水量。湖西和西北存在发达的排水网络（图1），为湖底提供了沉积物和水以形成扇形。在与湖泊接壤的西北部山坡上，有许多小溪，这些小溪是在公元1886年塔拉威拉-怀曼古火山喷发（罗威玛娜泥浆）侵袭后立即形成的（哈迪，2005年）。湖泊测深法可以重建奥卡罗火山爆发坑的形成顺序。我们确定了火山口的三个子区域（图10和11；图DR6），代表了产生三个独立角砾岩单元的独立喷发相。我们根据较年轻的火山口（1）切割较早形成的火山口边界，（2）挖掘成较旧的角砾岩和（3）通常显示较新的形态（更深，更陡峭的壁和更陡峭的边缘）这一事实来定义这些分区。较年轻的火山口通过注入较年轻的喷射水和/或湖泊沉积物而变得光滑和/或变浅。陨石坑的形状被定义为适合不同（或推断）陨石坑边缘的圆形或椭圆形（图10；图DR6）。测量了平均陨石坑直径和倾斜角度以及深度（从陨石坑边缘到陨石坑最深处），表1和表2列出了这些值。很少有陨石坑最终归因于这一阶段，因为它们通常最易受侵蚀，被沉积物掩埋，后来被后来的陨石坑划破。它们包括位于湖南部的不太明显的火山口特征（图11）。许多陨石坑，特别是在西南和南部的陨石坑，仅以原始陨石坑壁的残留物为代表（图10，剖面A-A'，BB'和EE'）。在湖的东南部，火山口的边界更加明显，并且它们的地板相对较光滑。该区域的NW-SE伸长脊可能代表了在该序列早期形成的另一个残留火山口（图10，剖面EE'）。少数具有可推断形状的环形山（图DR6中的1-3）的直径在62至120 m之间，轮缘到坑底的深度分别为1.8和5 m。第一阶段的火山口壁非常陡峭（从22°到49°；表2），并且火山口深度/直径比很低（0.03-0.04）。从阶段I推断出的火山口形成了一条可能沿着断裂系统定向的线性链（图1和2中推断出的Rotomahana断层）。在推测出的链的北面可能有陨石坑，但是如果陨石坑存在的话，它们随后会被II期喷发所摧毁。根据推断的分区，第一阶段喷发约0.1 km2，占当前湖泊面积的三分之一。至少有20个II期陨石坑穿过了I期陨石坑区域。这些火山口散布在整个湖底（图10；表1）。在北部湖泊中，会出现大的火山口（图DR6中的4、6、7），直径在110至190 m之间，深度在9至18 m之间（图10，剖面AA'至DD'）。这些弹坑显示出中等的壁坡度（从18°到25°），并且弹坑深度/直径比从0.05到0.12。在大的陨石坑中（或被切开）存在着被沉积物覆盖的小凹陷和圆形坑（例如，图DR6中的5和8-13）（图10，剖面DD'）。这些特征的直径为47-70 m，深度为1.4-2.8 m，深度/直径比为0.02-0.06。在湖的东部和东南部（图10，EE'剖面）分布着小而聚拢的火山口簇（图DR6中的14-20），直径在33至80 m之间，深度在1.6-3.9 m之间。这些成簇的火山口大多数显示出较低的壁坡度（从11°到15°），并且火山口深度/直径比在0.04到0.07之间。在西南部，中型火山口（图DR6中的21-22）直径为77和92 m，中间壁坡度（16°–29°），以及坑深/直径比为0.03和0.06。所有这些中等大小的陨石坑都切割了剩余的I相陨石坑，并进一步挖掘了其陨石坑底部（图10，剖面EE'）。在覆盖湖北部的滑坡沉积物下方可能埋藏着更多的火山口。第二阶段的陨石坑覆盖约0.25 km2。第三阶段的陨石坑。这些陨石坑横穿了第一和第二阶段的陨石坑壁，代表了在湖南侧出现的地貌上最独特，最年轻的陨石坑（图10，剖面AA'和EE'）。在东部和南部，聚结的火山口（图DR6中的25-28）切成NW-SE延长的山脊和较旧的火山口边界的残余物，均来自第一阶段。在第三阶段分区的北侧和西侧，环形山从第二阶段开始切割环形山的边缘和底部（图DR6中的23、24、29）。合并的环形山的推断直径范围为53至110 m，深度在1 m至3 m之间。通常，这些陨石坑的壁坡度低（从5°到15°），陨石坑的深度/直径比也很低（0.02-0.03）。第三阶段的火山口（至少可以区分七个）占地约0.035平方公里，沿着Okaro湖的西侧和西北侧，有几个特征表明沉积物流入了小溪/山谷下方的湖泊，从而使周围的山丘排水（图1， 10和11）。大的三角洲在湖的西北侧约占0.05 km2，东北侧约占0.008 km2。在最大三角洲之上和之上的高1–3 m的阶梯状悬崖表明这是倒塌坍塌特征的趾部（图10，剖面AA'和CC'；图11）。在湖的西侧，小的岩质滑坡沉积物具有高高的地形，并在许多较陡峭的火山口壁下方有分散的巨型区块（宽达20 m，高1.5 m）（图10，CC'剖面；图11）。可以从几条证据线推断出奥卡罗湖初始爆炸活动和爆发场景的潜在诱因，包括地下地层和构造构造，以及奥卡罗布雷西亚cia层的蚀变状态和地层。 Rotomahana多线程故障系统（图1； Lloyd，1959； Hedenquist和Henley，1985）。沿这些断层的位移产生了局部凸起的地形，因此仅覆盖了一个薄薄的Rangitaiki Ignimbrite薄板，区域地形得以平滑（Nairn，2003）。也，断裂的火山层序在附近的豪米河中露头（Nairn等，2005），以及奥卡罗湖以西的地表断层。进一步断层带的间接证据包括与湖东北侧主要断层趋势平行的水流（图1）。总的来说，在喷发之前（在图1和图2中推断出的断层），在Okaro湖的地盘似乎存在一个大约NE-SW撞击的断裂带。I喷发发生在AD 1315 Kaharoa爆炸阶段之后不久。流纹岩喷发（火山爆发指数[VEI] 4； Bonadonna等，2005； Nairn等，2005）。Nairn等。（2005年）提出，塔拉威拉山附近地区的玄武岩堤防注入了二氧化碳和热量，引发了几次由蒸汽驱动的喷发，例如怀奥塔普地热系统的香槟池。断层的位移和/或岩浆气向上断裂和裂缝的窜动可能引发了广泛的爆炸性事件，包括奥卡罗湖（Hedenquist和Henley，1985; Browne和Lawless，2001; Rowland和Simmons，2012）.Okaro Breccia I包括最深的局部岩性（Rangitaiki Ignimbrite； Nairn，2003； Hardy，2005），凝灰岩碎屑陷于单元底部。没有幼年的火山碎屑，表明没有发生岩浆喷发。这些Rangitaiki Ignimbrite碎屑表明，初始爆炸至少在约70 m的深度开始。此外，冈仓布雷西亚一世的碎屑蚀变很少，因此喷发处不存在明显的地热系统。集体而言，深层基岩的初始喷发，喷射体无普遍的热液蚀变，而且该地区的断层和裂缝的存在表明喷发是由岩浆流体突然到达引起的，例如从堤防中突然涌入而导致的地下水的快速加热和超压（cf. Germanovich and Lowell，1995; Stix and De Moor， 2018）。这种解释遵循Nairn等。（2005年），他提出从玄武岩堤坝注入岩浆CO2会在Wai-o-tapu产生热液喷发。附近的浅堤在1886年在Waimangu也引起了蒸汽驱动的喷发和岩浆喷发（Nairn，1979）。热力学模型（例如，Delaney，1987）表明热量没有从堤坝迅速转移到地下水。然而，玄武岩堤坝大量增加了二氧化碳（> 1000 t / d； Nairn等，2005）可能会在Okaro湖现场的断层上方的裂缝和含水层中迅速加热并加压浅层地下水。Germanovich and Lowell（1995）考虑将岩浆流体注入具有两个渗透率规模的水饱和渗透性储层中：大块岩石中的渗透率低（<10-17 m2），而裂缝网络周围的渗透率高（> 10-12 m2） ）。在这种情况下，当辅助网络的流体开始加热时，达到了爆炸性的条件，从而导致了小规模的加压。热量和气体导致裂缝快速蔓延（裂缝10−3 m至10−1 m的裂缝从几秒到几小时不等），导致沸腾快（Germanovich and Lowell，1995）。拉伸应力和超压累积，最终导致该乡村岩石破裂，引发减压和爆炸性喷发。该模型非常适用于抗张强度较低（≤10MPa）的岩石，例如Rangitaiki Ignimbrite（4– 8 MPa; Foote等，2011），其体渗透率也为10-16 m2（Montanaro等）等（2017）。沿着盲断层和堤坝上方的裂缝系统的蒸汽驱动事件也与第一阶段的喷口/火山口的线性链相一致（图11）。在第一阶段爆炸之后，细长的火山口结构被高渗透性的射流填充。来自浅层侵入体的持续传热可能持续了数月至数年（参见Petcovic和Dufek，2005），加热和循环热液（参见Rowland和Simmons，2012）。这个地热系统扩展到了围绕第一阶段火山口的地震平地层的浅层可渗透（约10-12 m2）沉积物（图12B； Tschritter和White，2014）。新的浅层热液系统至少覆盖了II期火山口的区域，在I期爆炸场以北约400 m处延伸（图11和12B）。Okaro Breccia I和II之间（图5A和5B）出现了薄层状湖相沉积物，这表明这些阶段之间存在时间间隔，形成了湖泊和相关的沉积物堆积。整个湖中的Okaro Breccia II内都存在硅化的沉积物，硅化的角砾岩和大量变化的绿色凝灰岩碎屑，以及大量变化的浮石和凝灰岩（图9C-9E；图DR4）表明至少存在在第二阶段爆炸之前发生了热液活动。在这种情况下，在任何触发条件下，奥卡罗湖的热力和机械条件都为产生热液喷发做好了准备（例如Browne和Lawless，2001年）。II期火山爆发的可能触发机制包括奥卡罗湖地区的地震位移以及地表水和地下水的变化（cf. Lawless，1988; Rowland and Simmons，2012）。Okaro II期火山爆发很可能是由地震事件触发的。热液系统降低了围压。热液的减压可能导致沸腾侵蚀锋面的形成，该锋面渗透并向下挖掘进入热液储层，主要在<70 m深度的地震平坦地层内（McKibbin等，2009年的自上而下的模型） ）。喷发一直持续到地下水沸腾速率下降和蒸汽膨胀下降到不再能从火山口喷出岩石为止。火山口墙的坍塌，（Browne and Lawless，2001）。活动的另一个停顿发生在II和III期之间，如Okaro Breccia III下方稀薄的水积泥沉积（图6）所示。湖的西侧，位于当前湖水平面以上〜3 m处。该沉积物表明在第二阶段产生的凹陷中形成了一个大湖。Hardy（2005）还指出，基于现在的Okaro湖北侧的底切和东侧的梯田，该湖的水位曾经更高（> 3 m）。我们在II期火山口边缘发现的湖泊沉积物证实了这一假设。Okaro Breccia III主要由凝灰岩和次要浮石组成，看起来比Okaro Breccia II的浮雕和化石的变化更大，其范围仅限于靠近源火山口的湖的西部到西南地区（图7和11）。Okaro Breccia III碎屑的进一步变化表明，在I期火山口区域的强烈返工/破裂射流中残留了一些水热活动（图12C）。Hardy（2005）还指出，侵蚀和湖东南缘50 m裂缝的形成导致了灾难性的排水。排水可能导致了湖床下方的围压突然下降，我们建议在表层热液系统中引发快速沸腾，并引发第三期喷发（图12C）。在其他浅层热液系统中进行细微的湖泊排水之后，也发生了类似的事件（从上向下挖掘）（参见McKibbin等，2009），例如在黄石和冰岛（Muffler等人，1971; Morgan等人，2009; Montanaro等人，2016b）生产与第三阶段陨石坑相当的陨石坑。等人，2002年；蒙塔纳罗等人，2016a），以及在不同的浅深度使用松散压实材料进行的基于现场的爆炸实验（Murphey和Vortman，1961年； Goto等人，2001年； Ohba等人， 2002年； Taddeucci等人，2013年； Graettinger等人，2014年； Valentine等人，2014年； Sonder等人，2015年； Macorps等人，2016年）证明了陨石坑的形状和大小是爆炸之间相互作用的结果能量和标定深度（物理深度除以能量的立方根）。另外，先前存在的火山口似乎会影响喷射器的喷射，以及爆炸是否能够排出（Taddeucci等人，2013年；Graettinger等，2014）。实验结果进一步表明，受影响的体积取决于基质岩石（基质）的强度（Galland等，2014； Macorps等，2016）。在许多自然位置，多个相邻的火山口可能暗示着岩体内流体存储的内部不均匀性（例如，热循环模式在其边缘处设置了矿化边界，或者说宿主岩的孔隙度和渗透率特征发生了三维变化； 1988; Browne和Lawless，2001; Rowland和Simmons，2012; Montanaro等人，2016a）。在奥卡罗湖，大部分的热水都包含在高渗透性，压实度较低的地震平坦地层中（Tschritter和White，2014年）。在Okaro湖附近的地区，该地层显示出沉积物质地，初期压实的复杂变化，和排气管（Leonard等，2010）。这些变化可能会在Okaro湖区的深处延伸，从而建立了共存流体的“口袋”，仅与热液系统的其余部分形成了二次连接。这些口袋可以解释多个陨石坑，而不是一个大陨石坑的形成（Kilgour等，2019）。（2016）在“强基质”（模拟固结良好的沉积物）和“弱基质”（模拟未固结沉积物或火山碎屑沉积物）中进行了爆炸实验。他们的结果表明以下几点：（1）基质岩石的特性影响火山口的形态（例如壁的陡度），碎屑类型和可弹出的尺寸;（2）受地下爆炸破坏的基质岩石失去其原始强度，并且对后续爆炸的影响较小，（3）大部分破裂的地下结构都充满了射出物。（1）主体岩石的性质会影响弹坑的形态（例如，壁的陡度），碎屑类型和可用于弹出的大小；（2）主体岩石（3）大部分破裂的地下结构充满了射弹。作为第一近似，奥卡罗湖的坚固基底可能对应于深Rangitaiki Ignimbrite和Rotoehu灰凝灰岩，而较弱的基质可能对应于表层的特弗拉沉积物（Waiohau和Taupo浮石形成），或第一和第二阶段的喷出物。地震平地层代表了Okaro喷发的前两个阶段所涉及的最厚的主要宿主岩，从弱压实到可能在深度上固结不等，因此可以作为具有中等强度的基质（Macorps等，2016）。奥卡罗湖火山口壁的陡度（表2;图10）似乎受到了影响地震平坦地层的存在。前两个阶段形成的许多坑在该地层中被开挖，并具有陡峭的侧面（23°–49°）。相比之下，在未固结的特非拉沉积物或第一和第二阶段喷发的喷出沉积物中开挖的火山口具有低角度斜率（6°–15°）。先前存在裂缝区域的高温流体。因此，陨石坑是细长的，沿着这个NE-SW撞击断裂系统排列（图1和2）。根据斜率值和测深曲线（表2；图10），II和III期陨石坑或者更深地挖掘到I期（倾斜角增加），或者沉积了充满火山口的喷出物（减小了倾斜角）。我们建议第一期喷发主要发生在地震平坦地层内，并且沿着裂缝进行了深挖，产生了一个陡峭的裂隙状火山口。Okaro一期火山喷发角砾岩不及二期角砾岩蔓延，这与一期潜伏性火山爆发的起始深度更大相符。二期喷发产生了最广泛，最厚的角砾岩，以及许多火山口不同的大小和广泛的分布（图11）。此外，第二阶段的喷发涉及初始喷发地点以北的广阔区域，这可能反映出大型热液系统的存在。宽阔的区域也可能反映了主体岩石的破碎和/或压实性质以及地震平地层内爆炸的中深度到浅深度（Galland等，2014; Montanaro等，2016b）。 II期陨石坑的形状（图11）表明，一系列爆炸使先前事件形成的陨石坑被套印并扩大，同时保持大致圆形（参见Valentine等，2015a）。较小的嵌套火山口可能是由多次小型蒸汽爆炸引起的（Scott和Cody，1982； Shanks等，2005； Morgan等，2009）。东部和西南部的第二阶段陨石坑大部分是从第一阶段挖出的喷出物，而没有扩展原始的陨石坑区域（图10和11）。它们的合并形状可能反映了浅层的爆炸轨迹。与之前的喷发相比，第三阶段爆炸产生的火山口较浅，爆炸从前两个喷发阶段回收弹射。在第三阶段中，几乎没有能量花费在破碎基质上（cf.Montanaro et al。，2016b），开挖产生了重叠的浅层结构，具有浅层爆炸轨迹。角砾岩成分反映了不同的宿主岩石和破碎深度爆发的三个时期。Okaro Breccia I的凝灰岩变化不大且未发生变化，这表明爆炸地点> 70 m深度，浅层覆盖了浮石（图9A）。较深的合并单位需要将高能破碎和排出，而在“卸载”或分解地震平地层时消耗的能量更少（Alatorre-Ibargüengoitia等，2010； Montanaro等，2016b，2016c）。Okaro Breccia II在成分和晶粒尺寸以及分选和厚度方面显示出复杂的外接刀变化（图7）。这种变化反映了北部地区开挖的原状地震平地层与南部地区第一阶段的预碎顶射之间的对比。Okaro Breccia II厚度的径向变化可能反映了喷发射流的不稳定性，多个不同的爆炸位置以及与封闭火山口壁相互作用产生的定向射流的不稳定性（参见Taddeucci等，2013； Valentine等，2015a， 2015b）。定向喷气机，尤其是 产生强烈不对称的裙缘，其中最厚的沉积物位于与射流方向相反的火山口一侧（Graettinger等人，2015），这与Okaro Breccia II一致。考虑（1）第一阶段挖掘的火山口的位置，（2）Okaro Breccia II的下层床仅限于南部，以及（3）南部，西南和西部最大的沉积层厚度（最大10 m），我们认为第二热液喷发始于南部。II期最深的开挖是由Rangitaiki Ignimbrite和Rotoehu Ash产生的大块块状岩块指示的，它们集中在西部和南部地区的Okaro Breccia II的中层地层中（图5和7；图5和7）。 DR4和DR5）。随着第二阶段喷发的进行，它扩展到北部的基底。Okaro Breccia II的北部至东部部分（厚度最大为9 m）包含大量的浮石和硅化碎屑，但很少有大块体（图7）。这种厚厚的沉积物可能是由北部地区一个火山口的爆炸所产生的（图12B）。第三阶段的火山口挖掘了第二阶段的喷出物（图12C）。第三组爆炸挖掘了弱而透水的基质，从而有效地形成了火山口（参见Montanaro等人，2016b）。许多公认的火山口缺乏明显的凸起边缘（图10和11），这可能表明角砾岩向湖水中扩散（Morgan等，2009），这与第二次爆发时沉积物覆盖的洼地和火山口形态平滑相关相（图10）。二次热液的溶解和坍塌也可能改变了原始的火山口形状（Scott和Cody，1982； Morgan等，2009）。Okaro Breccia III最厚发生在湖外的西部和西南部（图7），该地区以前的火山口的存在可能影响了扩散方向.Okaro湖的坍落度和岩石滑坡沉积物提供了火山口扩大的证据爆发后继续。但是，这些沉积物仅限于排水网络的排放区域，这表明在喷发阶段很久之后就发生了物质运动。该推论还得到了该湖东部，东南部和南部部分较陡峭和较深的火山口缺乏坍落度和滑坡沉积物的支持（图10和11）。在1886年Rotomahana泥浆侵蚀（一种精细的岩浆沉积物； Hardy，2005年）的侵蚀过程中，湖的北部突然流入大量细沙。在西湖侧，来自周围排水网络的泥沙负荷可能已经不稳定，半固结成固结的Okaro角砾岩和陡峭的火山口壁边缘上的较旧的地层（例如，地震平地层）。在Okaro湖的新火山口形态数据中，我们可以将深层“自下而上”与随后的“自上而下”的蒸汽驱动喷发区分开。该地区的爆发历史表明，在新近受干扰的地热系统形成的地方，危险的蒸汽驱动爆炸事件可能会持续存在。在这个地方，我们看到了三个独立爆发性爆发事件的证据。第一次（第一阶段）爆炸角砾岩中的深层基岩碎屑和显着不变的碎屑表明，该位置以前没有地热系统存在，但是地下水突然被加热并加压以产生潜水爆炸。断裂和断层网络可能会聚集气体或岩浆流体，并产生线性的陨石坑链。此事件之后，在多孔，弱浮石和非焊接凝灰岩中建立了新的地热系统。在一段时间之后，热液蚀变影响了奥卡罗大部分地区，第二阶段喷发是由上部热液系统中压力水和蒸汽的闪蒸引起的。II期角砾岩包含高度改变的浮石和凝灰岩碎屑，以及硅化块。Okaro Breccia II的下部具有较浅的来源，较高的部分具有较深的来源，表明爆炸的地点加深了。II期喷发达到了I期事件的深度，并在较薄的浮石沉积区域中横向扩散。II期事件的触发可能涉及多种过程，例如地震事件或热液系统水位的变化。在第二阶段形成了大小各异的火山口簇。在下一个暂停期间，整个区域形成了一个宽阔的湖泊，热液活动继续进行。爆炸的第三阶段主要发生在较早爆发的浅角砾岩沉积物中。第三阶段似乎是由于湖泊水位突然下降> 3 m而产生的。奥卡罗大火山口场的形成是由蒸汽驱动的喷发形成的，但这些喷发具有不同的特征（自下而上，然后是自顶向下）和在不同条件下（最初是环境温度，然后是高温）的流体。这种组合在新西兰或世界其他地方很少见（例如，黄石公园的玛丽湾火山口综合体；尚克斯等人，2005年；摩根等人，2009年）。该案例的地质和形态学证据表明，可观察到的火山口的大小，形状和分布部分受不同的开挖基质岩，喷发前火山口和裂缝区域的存在的控制。这项研究还表明，至少在局部地层是众所周知的且在爆炸开挖深度范围内变化足够大的情况下，沉积成分可以帮助区分潜水与热液喷发。从广义上讲，我们的发现表明，在热液活跃的环境中，对潜在喷发情景的评估取决于对浅层地质和热液系统范围的详细了解。这些关键因素控制着例如喷发的方向性，寿命，爆炸次数及其相关危险。另外，一旦某个地区被潜水爆发扰乱，就应该密切监视新地热系统的后续形成，因为后来的热液喷发可能与最初的潜水事件一样大且危险。蒙塔纳罗和克罗宁承认资金来自新西兰商业，创新和就业部的“聪明创意”授予“稳定的发电和旅游业，减少了地热爆炸的危险。” 肯尼迪（Kennedy）承认新西兰马斯登（Marsden）拨款“动摇岩浆引发火山爆发。我们感谢丰盛湾地区委员会，特别是怀卡托大学的安迪·布鲁尔（Andy Bruere）和戴维·汉密尔顿（David Hamilton）允许访问本研究中使用的多光束数据。我们也感谢新西兰自然保护部（Te Papa Atawhai）和罗托鲁瓦湖理事会的斯蒂芬妮·凯利，他们支持并允许进行这项研究。我们还要感谢两位匿名审稿人Larry Mastin和副编辑Jocelyn McPhie，他们的评论大大改善了手稿。他支持并允许进行这项研究。我们还要感谢两位匿名审稿人Larry Mastin和副编辑Jocelyn McPhie，他们的评论大大改善了手稿。他支持并允许进行这项研究。我们还要感谢两位匿名审稿人Larry Mastin和副编辑Jocelyn McPhie，他们的评论大大改善了手稿。