Bull Volcanol (2003) 65:15–29 DOI 10.1007/s00445-002-0234-1
R E S E A R C H A RT I C L E
Ian C. Wright · John A. Gamble · Phil A. R. Shane
Submarine silicic volcanism of the Healy caldera, southern Kermadec arc (SW Pacific): I – volcanology and eruption mechanisms
Received: 13 September 2001 / Accepted: 30 May 2002 / Published online: 27 July 2002 © Springer-Verlag 2002
Abstract The submarine Healy volcano (southern Kermadec arc), with a 2–2.5 km wide caldera, is pervasively mantled with highly vesicular silicic pumice within a water depth of 1,150–1,800 m. Pumices comprise type 1 white–light grey pumice with ≤30 mm vesicles and weakmoderate foliation, type 2 grey pumice with millimetrescale laminae, flow banded foliation, including stretched vesicles ≤55 mm in length, and a minor finely vesicular type 3 pumice. All types are sparsely porphyritic, with undevitrified glassy groundmass (68–70% SiO2), which is microlite and lithic free. Coexisting pyroxenes yield magma temperatures of ~950 °C. Pumice density is ≤0.5 g cm–3 and vesicularity is 78–83%. Vesicle size distributions for types 1 and 2 pumice, range from ~20 µm to >20 mm, with a strong power-law relation (with d=–2.5±0.4) for vesicles <1–2 mm. Larger vesicles have variable size modes. The vesicle size distribution and packing indicates rapid magma decompression and ascent. Consideration of the pressure dependent, solubility of H2O at a magma temperature of ≤950 °C and water content of ≤6 wt%, with pumice petrography and vesicle granulometry, strongly suggests a pyroclastic eruption. Reconstructions of the submarine edifice between water depths of 1,000 and 550 m constrain the ambient hydrostatic pressure to ~6–9 MPa. Pressures >~9 MPa will limit vesicularity to less than the observed 78–83%, whereas pressure <~6 MPa require a more shallower reconstruction of the edifice and larger-volume syn-eruptive colEditorial responsibility: J. Gilbert I.C. Wright (✉) National Institute of Water and Atmospheric Research (NIWA), P.O. Box 14-901, Wellington, New Zealand e-mail:
[email protected] Tel.: +64-4-3860300, Fax: +64-4-3862153 J.A. Gamble School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand P.A.R. Shane Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand
lapse. Uniformly high vesicularity is interpreted as evidence of insulation within an eruption column comprising steam and hot pyroclasts. Most pyroclasts cool, condensing and ingesting water into steam-inflated vesicles, and then sink. Progression into pyroclastic mode would expand the eruption column, displace ambient water, reduce the hydrostatic load, and further promote vesiculation and fragmentation. Pyroclasts within the column would quench at these reduced pressures. We argue that Healy eruptions deeper than ~1,000 m cannot be pyroclastic. Volumes for the lower and upper bounds of edifice size are 2.36 and 3.58 km3, respectively, but do not account for intra-caldera pumice fill. These volumes are considered to be predominantly primary eruption output, as shown by a dearth of accessory lithics in all pumice, yielding (at an average 81% vesicularity) eruptive pumice volumes of between 10 and 15 km3. Some pyroclasts may have risen to the sea surface and be a correlative of the sea-rafted Loisels pumice; the latter occurs in some New Zealand Holocene beach sequences and has a estimated age of 590±80 calendar years. Keywords Healy caldera · Kermadec arc · Pyroclastic eruption · Silicic volcanism · Submarine eruption
Introduction Submarine pyroclastic volcanism remains an intractable phenomenon, although it is known that water depth, magnitude and rate of magma vesiculation are the predominant limiting constraints (e.g. McBirney 1963, 1971; Sparks 1978; Sheridan and Wohletz 1983; Kokelaar 1986; Wohletz 1986). Initially such pyroclastic eruptions were considered to be mostly restricted to water depths <500 m for a range of magma compositions and volatile contents (e.g. Fisher 1984; Stix 1991), although theoretically deeper submarine pyroclastic eruptions could occur (e.g. Burnham 1983). Although the palaeo-eruptive water depth can be equivocal, extensive pumiceous deposits within older exhumed marine sequences have been interpreted either as
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pyroclastic (e.g. Kokelaar and Busby 1992; Kano et al. 1996; Kano 1996; Hathway and Kelley 2000) or as pumiceous carapaces of effusive silicic dome lavas (e.g. Allen and McPhie 2000). The possible welding of submarine pyroclastic deposits has also proved contentious (e.g. Fiske and Matsuda 1964; Cas and Wright 1991; Kokelaar and Busby 1992; Legros and Druitt 2000). Within modern submarine arc–back-arc systems, where magma compositions may be silicic and have high H2O content (Plank and Langmuir 1988), recognition of pumice deposits (e.g. Reynolds et al. 1980; Gill et al. 1990; Fiske et al. 1998; Wright and Gamble 1999) and silicic caldera complexes (e.g. Yuasa et al. 1991; Wright and Gamble 1999; Fiske et al. 2001) now provides convincing evidence of pumice-bearing eruptions within water depths of 500–2,000 m. Pumice has also been recovered, although rarely, from mid-ocean ridge (MOR) segments, within water depths of at least 1,500–1,800 m (e.g. Hekinian et al. 2000; B. Murton personal communication 2000). Are such pumice deposits, within water depths ≥1,000 m, pyroclastic in origin – or are they permeable carapaces of effusive silicic lava flows? For subaerial silicic volcanism (particularly explosive eruptions), advances in observational, theoretical and exFig. 1 Location map of Healy caldera complex and other volcanoes of the modern southern Kermadec volcanic arc with bounding remnant and frontal arc ridges (shaded grey above the 2,000-m isobath), Great Barrier Island (GBI), and onshore Taupo Volcanic Zone (TVZ), New Zealand. Mean positions of the East Cape Eddy (ECE) and East Auckland Current (EAUC) are delineated by the 9.0, 8.6 and 8.2 °C isotherms at 600 m water depth (from Roemmich and Sutton 1998)
perimental studies (e.g. Fink 1987; Freundt and Rosi 1998; Gilbert and Sparks 1998) have led to a greater understanding of eruptive processes, including volatile solubility and exsolution (e.g. Zhang 1999), magma vesiculation and fragmentation (e.g. Navon and Lyakhovsky 1998), conduit dynamics (e.g. Mader 1998; Legros et al. 2000), pyroclastic/effusive flow, magma chamber decompression and syn-eruptive caldera collapse (e.g. Druitt and Sparks 1984; Martí et al. 2000). However, many studies have implicitly applied these advances solely to the subaerial environment, with the analysis of modern submarine pumice clasts largely restricted to syn- and post-eruptive dispersal mechanisms (e.g. Whitham and Sparks 1986; Cashman and Fiske 1991; Fiske et al. 1998; Wright 2001). We apply these advances in physical volcanology to the case of submarine eruption(s) at Healy silicic caldera complex in the southern Kermadec arc (Fig. 1; Wright and Gamble 1999), where the post-eruptive topography shoals to a water depth of ~1,150 m. We reconstruct, where possible, the eruptive volcanology of the Healy caldera, using pumice clast density and vesicularity, and vesicle textures and granulometry, within the constraints of known pressure solubility of volatile
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species such as H2O. Detailed mineralogy and petrogenesis of these rhyodacitic pumices (68–70 wt% SiO2) will be presented separately. Compelling evidence shows that the eruption was pyroclastic, although this requires reconstructions of the pre-eruption edifice back to a water depths between 150 to 600 m shallower than the present edifice summit.
Regional setting and Healy caldera The Healy submarine caldera is one of 13 centres (Fig. 1), which comprise both basaltic–andesitic stratovolcanoes and silicic caldera complexes, which form the southernmost 260-km segment of the modern submarine Kermadec arc (SKA) between 34°50′S and 36°50′S (Wright et al. 1996; Wright 1997; Wright and Gamble 1999). Of these volcanoes, most basaltic edifices shoal to water depths <1,000 m (and three to <500 m). Observed lithofacies record the general transition from effusive pillow lavas, massive and sheet flows, and pillow and talus breccias to fragmental and scoriaceous hyaloclastite–pyroclastic deposits within water depths of ~500–700 m (Wright 1996; Wright et al. 2002). Basalts and andesites (with 47–58 wt% SiO2) are generally porphyritic and have a vesicularity of ~20–40% (Smith and Brothers 1988; Gamble et al. 1993, 1995; Gamble and Wright 1995). Present-day hydrothermal venting (de Ronde et al. 2001) and/or associated sulfide mineralisation (Wright et al. 1998; de Ronde et al. 2002) are recorded from seven of the 13 volcanoes. The rhyodacitic Healy volcano is one of two presently known silicic submarine caldera complexes of the SKA (Wright and Gamble 1999). It is ~15 km in length (Fig. 2a) and comprises the Healy edifice shoaling to 1,150 m water depth with the main caldera on its northeastern mid-lower flank, and a southern parasitic cone (Cotton Volcano) rising to a water depth of 950 m. The caldera is circular, 2–2.5 km wide, with a smooth, flat floor at a depth of 1,660–1,690 m, some 250–400 m below the caldera rim. Seafloor photography, Hawaii Mapping Researcher MR1 side-scan imagery, and rock dredge sampling reveal the outer flanks of the complex, caldera floor and walls, and main edifice to be pervasively mantled with pumice deposits (Wright and Gamble 1999; Wright 2001) distributed over at least 50 km2 of the volcano. Massive, felsic rocks outcrop along the caldera wall. Active hydrothermal venting is recorded from the lower southern caldera wall (de Ronde et al. 2001). The main edifice consists of blocky felsic outcrops and pyroclastic breccias and lapilli. A smaller caldera (~1.3 km in diameter and 50–100-m-high caldera walls), is sited on the upper southern flank (Fig. 2).
Data and methods Data for this study were derived from 22 representative pumice sub-samples selected from 13 rock-dredge sites
Fig. 2 a Bathymetry of the Healy caldera complex based on swath MR1 survey (Wright and Gamble 1999) with 50-m contour interval. Shaded box shows area of terrain models in b and c with arrow showing view orientation. Insert box shows location of Fig. 3. b, c Terrain models of Healy caldera with, respectively, conservative and capacious reconstructed pre-eruptive topography. Vertical exaggeration is 2× with the reconstructed topography having a mesh grid of 200 m
(with a total cumulative weight of ~60 kg) covering the main Healy edifice, caldera floor and rim, and outer caldera flanks (Fig. 3). Vesicularity (V) and dry densities (ρd) of an additional 14 subsamples, previously presented by Wright and Gamble (1999), are also included in this study. Dry densities were determined from representative
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Type 1 (white) pumice
Fig. 3 Healy caldera showing distribution of camera and rock dredge sites with the latter identified by station number
13–44-cm3 pumice cubes after oven drying at 110 °C for 24 h (Houghton and Wilson 1989). Macroscopic vesicularity was determined from pumice slabs (some of which were cut both normal and parallel to flow banding), which were typically 15–30 cm2 in area, impregnated with blue epoxy resin under vacuum, and polished smooth. The polished slabs were scanned at high-resolution (≥1,000 dpi=390 pixels per mm), converted to binary images of vesicle space/phenocrysts and glass groundmass, with vesicle size distributions (VSDs) subsequently derived using OPTIMAS image analysis. Microscopic vesicularity was determined in the same manner from large-format polished thin-sections taken from the cut slabs. The macroscopic and microscopic VSDs were merged at 0.1 mm diameter, and subsequently converted from two-dimensional number per area (NA) to threedimensional vesicle distributions of number per volume (NV) using the methods of Cheng and Lemlich (1983), Mangan et al. (1993) and Cashman and Mangan (1994).
Macroscopic pumice types Pumice pyroclasts are the predominant lithology of all dredge sites, although single blocks of basalt have been recovered from both within and outside of the caldera (stations X594 and X610; Wright and Gamble 1999). Seafloor photography records pyroclasts and blocks up to ~1.2 m in size, but pyroclasts recovered with rock dredging are typically 10–20 cm in diameter, but range up to 35 cm in maximum diameter. Three distinct macroscopic pyroclast types are identified.
Type 1 pumice are grey–white to light grey, highly vesicular pyroclasts (Fig. 4a and b) that comprise ~70–80% (by volume) of the recovered dredge material. This pumice type appears to be widely distributed over the caldera floor and rim, but forms a minor component of dredged material from the Healy edifice crest. Pyroclasts are mostly 10–20 cm in length, but do range up to 30 cm, and invariably have an angular to subangular form. Two pumice subtypes (types 1a and 1b) are recognised, which have, respectively, an isotropic fabric without observable flow structure, and a highly deformed, moderately developed fibrous (aspect ratios ~1–2:1) ‘woody’ flow fabric (Kato 1987). The subtypes are not, however, mutually exclusive, being end members of a continuum with intermediary fabrics. Large coalesced, ovoid and cavernous vesicles up to ~30 mm in maximum size are commonly observed within both subtypes; however, it is possible that the true maximum vesicle size was larger because the largest vesicles usually occur at the clast margins. Coalesced vesicles commonly reveal fibrous remnants of inter-bubble walls, with type 1b pumice having deformed and stretched coalescing vesicles. Type 2 (grey) pumice Type 2 pumice are dominantly grey, vesicular, mostly flow-banded pyroclasts (Fig. 4c) that comprise ~20% (by volume) of the recovered pumices. The light grey flow laminae record differences of vesicularity and crystallinity rather than composition (Wright and Gamble 1999). This pumice appears to be mostly restricted to the main Healy edifice (and has possibly erupted from the smaller upper flank caldera), though some type 2 clasts have been recovered from the caldera floor. Pyroclasts are 10–20 cm in length (Fig. 4), but do range up to 35 cm, and mostly have a subangular form. As with type 1 pyroclasts, type 2 pumice has two subtypes distinguished on the basis of fabric. Type 2a pumice, which is relatively uncommon, has millimetre-scale complex flow banding of light grey laminae with large stretched, cavernous vesicles that range up to 55 mm in length. This pumice subtype lacks any significant planar or fibrous flow fabric. In contrast, type 2b pumice, being more common, comprises a fibrous ‘woody’ fabric with light grey, millimetre to sub-millimetre planar flowbanding laminations (Fig. 4c). Occasional laminae of intermediary light grey colour are observed and occasionally the laminae show a sinuous complexly folded fabric. Type 3 (yellow–grey finely vesicular) pumice Type 3 pumice comprises generally light grey to pale yellow, finely vesicular pyroclasts that form ~≤5% (by volume) of the dredged material, being recovered from
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Fig. 4 Representative Healy pumice fabrics; a type 1 pumice (X591-B) with b as close-up inset (scale in mm); c type 2 pumice (X611-E; scale in mm); d, e macroscopic scanned and binary images, respectively, of X607-C-1; f macroscopic binary image of X613-F-1; g microscopic binary image of X593-C-1; h microscopic binary image of X613-E-1. Black is void space (vesicularity) in binary images
two sites X593 and X607. Pyroclasts are subangular and <15 cm in length. Type 3 pumice is characterised by a finely vesicular, highly planar fabric, without alternating flow laminae, but including occasional large and highly stretched vesicles that can extend to ≤25 mm.
Petrology, mineral chemistry and geochemistry All three Healy pumice types are sparsely porphyritic with phenocrysts rarely exceeding 5% by volume. Plagioclase,
orthopyroxene, clinopyroxene and Fe–Ti oxides are the main phenocryst minerals, in that order of abundance; amphibole is rare. Accessory apatite occurs as inclusions in all phenocryst phases. Plagioclase occurs as euhedral phenocrysts up to 1 mm across and as glomeroporphyritic aggregates up to 5 mm across, which may be intergrown with pyroxene. Crystals show strong compositional zoning with some cores being up to An93 and rims consistently around An40. Microphenocrysts in the glassy groundmass define a strong preferred orientation, parallel to vesicles and are compositionally homogeneous (An40) similar to the rims of the larger phenocrysts. Orthopyroxene (100.Mg/Mg+Fe=69) phenocrysts are euhedral (up to 1 mm across) and faintly pleochroic. Clinopyroxene (100.Mg/Mg+Fe=77–79) comprises distinctive green (under plane polarised light) subhedral phenocrysts, up to 1 mm across, which may be partly embayed. Both orthopyroxene and clinopyroxene occur as rare microphenocrysts. Temperatures calculated from
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Mg–Fe exchange equilibria (Wells 1977) in the coexisting pyroxenes yield temperatures of ~950 °C. Ti-magnetite forms euhedral phenocrysts, which often contain inclusions of apatite. Amphibole (100.Mg/Mg+Fe=68) forms rare acicular crystals that are a pale red-brown and usually partially corroded. Groundmass is entirely glassy, undevitrified and microlite free. Microprobe analyses give rhyolitic compositions (SiO2 68–77%). Accessory lithics are not observed either as clasts or in the groundmass. Whole-rock analyses of the three pumice types are composition identical, straddling the standard TAS dacite–rhyolite field boundary (Wright and Gamble 1999). Healy pumices are subtly distinctive from nearby Brothers volcano dacite–rhyodacites (Wright and Gamble 1999), having lower K2O and TiO2 at given SiO2 contents.
Fig. 5 Measured bulk dry densities (and derived bulk vesicularity) for type 1 and 2 pumices; see Table 1 for basis of calculating bulk vesicularity
Pumice density and vesicle granulometry Clast density and vesicularity Clast densities (ρd) are mostly ≤0.5 g cm–3 (Fig. 5) with no significant difference in mean values for types 1 and 2 pumice (Table 1), although densities of the latter are slightly more variable, reflecting differences in vesicularity and crystallinity between flow bands (Wright and Gamble 1999). Vesicularity (derived from ρd in Table 1) is universally >75% (Fig. 5), with type 2 clasts having more variation and a marginally higher mean. Such vesicularity yields gas/liquid ratios that vary by a factor of up to 3 (Table 1) with highest ratios of 8.7 and 8.3 for type 1 and 2 pumices, respectively. However, this variable vesicularity is matched by geochemical homogeneity (Wright and Gamble 1999). Vesicle shape Petrographic textures (at both microscopic and macroscopic scales, Fig. 4d–h), shape factor (e.g. analysis (Fig. 6) and vesicle aspect ratios reveal an increasing vesicle elongation and shape complexity (as deviation from a spherical bubble) with increasing vesicle size. For all pumice types, increasing shape complexity forms a broad log–log relation with vesicle size, even for type 1 pumices that lack a significant macroscopic flow fabric.
Table 1 Summary physical properties and vesicle granulometry of Healy pumices
a Derived from ρ based on 5% d phenocrysts with a density of –3 2.9 g cm and 90% glass with a density of 2.3 g cm–3
For pumice slabs, with both flow-normal and flow-parallel orientations, the latter have slightly higher shape complexity (Fig. 6c) and pronounced vesicle elongation. This elongation commonly develops into fibrous ‘woody’ pumice textures, where individual vesicles form cylindrical tubes with aspect ratios as high as 15–20:1. Kato (1987) has documented comparable fibrous pumice. Increasing shape complexity (as shown by higher values of shape factor) is interpreted to be a consequence of both vesicle coalescence and strain deformation caused by flow shear. Vesicle coalescence, occurring at both macroscopic and microscopic scales, is evident with larger cavernous bubble spaces surrounded by strings of smaller vesicles. The latter can be intact, whereas others are wholly or partially coalesced with the larger adjacent pore space, leaving collapsed and remnant fibrous septae. Commonly septae of the largest vesicles (≥20 mm diameter) form ragged vesicle perimeters with ‘maze’like textures especially where a flow fabric has stretched, re-orientated and flattened bubble walls. As documented elsewhere (e.g. Orsi et al. 1992; Klug and Cashman 1994), both increasing vesicle shape complexity and strain deformation can correlate with high vesicularity, which indeed is seen within type 2 pumice. However, the relative importance of vesicle coalescence and stretching as structural mechanisms to accommodate increased ves-
Samples (n) Bulk dry density ρd (g cm–3); range Bulk dry density ρd (g cm–3); mean Bulk dry density ρd (g cm–3); SD Bulk vesicularity (%)a; range Bulk vesicularity (%); mean Bulk vesicularity (%); SD Gas/liquid ratio; range
Type 1
Type 2
Type 3
24 0.23–0.47 0.39 0.06 77.1–88.4 80.4 2.9 3.5–8.3
16 0.22–0.53 0.36 0.09 73.3–88.9 82.0 4.6 3–8.7
2 0.32–0.42 0.37 – 79.0–83.9 82.8 – 4.1–5.7
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Fig. 6 Log–log plots of vesicle shape factor (P2/4πA) where P and A are vesicle perimeter and area, respectively, versus vesicle size (given as two-dimensional vesicle area) for representative type 1 and 2 pumices (a and b), and flow parallel and normal fabric for type 2 pumice (c)
icle packing (and hence vesicularity) is unknown. Further, such structural accommodations assume one or few events of bubble nucleation. As discussed below, where vesicle nucleation is continuous or constant over an interval of time, then ‘Apollonian’ packing will also result in high vesicularity (e.g. Blower et al. 2001) without the need to deform and/or coalesce vesicles. However, the high vesicularity of Healy pumices is likely to be accommodated by both vesicle coalescence and deformation, and ‘Apollonian’ packing.
where N (r) is the number of vesicles within a size radius r, N (>r) is the number of vesicles with a radius greater than r, and d is the power law exponent. All the VSDs extend over at least two orders of magnitude of vesicle diameter. For the vesicles ≤1–2 mm in diameter, powerlaw exponents for the Healy pumices are –2.5±0.4, lying within the range of d=–2 to –3 for natural subaerial pyroclastic samples and ‘explosive’ laboratory simulations (Blower et al. 2001). These power-law size distributions extend to typically 40–80 µm for smallest sized vesicles (Fig. 7), thereafter decay exponentially. The power-law exponent is not statistically different between type 1 and 2 pumices. In contrast to the single or few bubble nucleation events implied in Poison or bimodal vesicle populations, such power-law VSDs are interpreted (where the exponent is –2 to –3) as formed by non-equilibrium degassing with continuous bubble nucleation (Proussevitch and Sahagian 1996; Blower et al. 2001). Such power-law VSDs produce a highly vesicular ‘Apollonian’ packing (Fig 8) that can exceed the 74% vesicularity of polyhedral foams (Cashman and Mangan 1994). At vesicle diameters >1–2.5 mm, all Healy pumice VSDs record a inflexion (arrowed in Fig. 7) of the size distribution to a general power-law trend (exponent of d≈–1) with a series of varying size modes. The inflexion typically occurs between 2–2.5 and 1.5–2 mm diameter for type 1 and 2 pumices, respectively. Decay of the size distribution at these larger diameters may reflect both magmatic processes such as Oswald ripening (e.g. Cashman Mangan 1994) and statistically low vesicle counts. Variable size modes (Fig. 7) are recorded between 1 mm and the largest observed vesicle diameter of ~35 mm. Such polymodal size distributions are commonly observed in subaerial pyroclastic pumices (e.g. Toramaru 1990; Orsi et al. 1992) with larger vesicles variously interpreted as pre-eruptive bubble growth and coalescence in magma chambers (Whitham and Sparks 1986) or postfragmentation expansion and coalescence within eruption columns (Klug and Cashman 1994). The inflexion at lower vesicle sizes in type 2 pumice may reflect increased coalescence in more sheared pumice. Conversely, large vesicles (e.g. >30–40 mm), associated with zones of high vesicularity in effusive silicic eruptions, may record bubble coalescence and expansion, and buoyant rise within a flow (Manley and Fink 1987). Notwithstanding, for the moment, the origin of the vesicles, a consequence of power-law VSDs is that larger vesicles mostly control the total void volume (Blower et al. 2001). For Healy, the larger vesicles do indeed dominate void space (Fig. 9).
Vesicle size distributions and volumes
Eruptive mechanism Vesicle size distributions (VSDs) of Healy pumices (Fig. 7) generally represent size distributions of powerlaw form (Sarda and Graham 1990; Simakin et al. 1999; Blower et al. 2001), described by: (1)
Eruption model 1 (pyroclastic eruption) The physical constraints of the Healy eruption can be broadly reconstructed by considering the volatile content within the pre-eruptive melt, degree of vesicularity and
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Fig. 7 Representative vesicle size (closed circles and primary axis) and cumulative void (open circles and secondary axis) distributions as log–log plots of Healy type 1 (a–c) and 2 (d–f) pumices, with the size distribution given as natural logarithms of number of vesicles (NV) per unit volume (cm3). Arrows mark the inflexion points in the power-law size distribution, with a least-squares fit exponent (d) ±1 standard error given for the vesicle diameters <2 mm
the ambient pressure of eruption. Although CO2, S and H2O are the main degassing volatile species of arc magma (e.g. Gerlach et al. 1994), including the Taupo Volcanic Zone and southern Kermadec volcanoes (Giggenbach et al. 1993; de Ronde et al. 2001), for simplicity we consider H2O as the sole saturated volatile component. H2O is both the predominant volatile species within subduction-arc systems (e.g. Plank and Langmuir 1998; Hochstaedter et al. 1990), and the prime determinant of explosive volcanism (e.g. Anderson et al. 1989; Dunbar et al. 1989). Where ambient pressure is ≤200 MPa, H2O solubility is strongly pressure dependent (Burnham 1975; Stopler 1982; Holtz et al. 1995) as given by Henry’s law: (2) where CS is the saturated concentration, k is the solubility constant and P is the ambient pressure. Rhyodacitic melts typically have k values of 4–4.5×10–6 (Dingwell 1998). A conservative reconstruction of the Healy precaldera topography (Fig. 2b) to a water depth of 1,000 m (with a seawater density correction of 1.027 g cm–3) establishes an ambient hydrostatic pressure (PH) of 10.3 MPa (at the time of eruption), at which magma at the summit vent would have equilibrated. Equation (2)
indicates that the erupted pumice would remain partially H2O saturated at this depth with a post-eruptive CS of 1.28 wt%. A larger volume edifice reconstruction to a summit crest of 550 m (Fig. 2c), with a concomitant ambient pressure of 5.6 MPa, would yield a post-eruptive CS of 0.95 wt%. Such solubility is broadly consistent with estimated volatile content from whole-rock analysis (LOI values) and electron microprobe analyses of glass. Such topographic reconstructions require whatever the initial CS, of the melt, some ~1–1.3 wt% H2O has remained soluble in the quenched melt, and hence is unavailable for magma vesiculation and fragmentation. For a conservative reconstruction, and assuming that water vapour behaves as an ideal gas (and accounting for 1.3 wt% of H2O remaining saturated), a theoretical vesicularity can be calculated using the gas law: (3) where P is pressure (equivalent to PH of 10.3 MPa), V is the gas volume (vesicularity), n is the number of moles of gas, R is the gas constant, T is temperature (°K), and we assume a melt density of 2,300 kg m–3. Typical rhyolitic magma eruptive temperatures are 800–900 °C (Dingwell 1998), but we have used 950 °C as determined directly from coexisting pyroxenes within Healy pumices. Solving Eq. (3) for V, over a range of initial theoretical H2O contents between 3 and 9 wt% produce, at the ambient hydrostatic pressure of 10.3 MPa, eruption vesicularities of 56.6–85.4% (Fig. 10a). Where determined directly, pre-eruptive H2O contents for arc–back-arc magmas are typically 3–6 wt% (e.g. Hervig et al. 1989; Barclay et al. 1998). At the conservative reconstruction ambient pressure of 10.3 MPa, such H2O contents would translate to 57–78% vesicularity, which is close, but still
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Fig. 8 Theoretical two-dimensional Apollonian packing for vesicles with a power-law size distribution (after Blower et al. 2001)
less than the observed vesicularity (Table 1). Credible increases of eruption temperature and melt density (Fig. 10a) will increase vesicularity marginally. Conversely, other volatiles (e.g. CO2) dissolved in the melt will decrease the solubility of H2O (Mysen 1976; Anderson et al. 1989), but may increase vesicularity as a separate phase. Reconstruction of the pre-eruption edifice topography to shallower depths (with a concomitant reduction in hydrostatic pressure) will obviously increase vesicularity closer to that observed. Swath surveys of adjacent SKA Fig. 9 Representative volume histogram distributions of Healy type 1 (a–c) and 2 (d–f) pumices
volcanoes (e.g. Rumble III, IV and V) show that such edifices can shoal to water depths of ≤500 m (Wright 1996; Wright et al. 2002). From these latter observations, and based on the morphology of the post-eruptive edifice and caldera, a more voluminous reconstruction (Fig. 2c) is made with a summit shoaling to 550 m water depth. This reconstruction is ~600 m shallower than the present edifice, but with ≤10–25° slopes akin to other Kermadec edifices including the parasitic Cotton Volcano (Fig. 2a). For this reconstruction, with the same magma parameters and assumptions for Eq. (3), a hydrostatic pressure of 5.6 MPa would sustain theoretical vesicularities of 73.9 to 91.8% for initial H2O contents between 3 and 9 wt% (Fig. 10b). For the credible range of 3–6 wt% H2O in the melt, such theoretical vesicularity of 74–87% is mostly coincident with that observed. For Healy, these conservative and more voluminous reconstructions form the broad bounds of a pyroclastic eruption model where the initial discharging vent occurred between 550 and 1,000m water depth (Fig. 11). Eruption model 2 (effusive lava dome) An alternative to a pyroclastic eruption is a model of effusive lava dome formation where the eruptive vent is close to or deeper than the 1,150-m water depth of the present edifice summit, with a high hydrostatic pressure that prevents sustained pyroclastic eruption (Fig. 11), as given by Eqs. (2) and (3). Within this effusive model, associated with silicic dome growth, the pumice form an external carapace, and possibly underlying units of degassed, vesicular pumice interbedded with dense obsidian (e.g. Eichelberger et al. 1986; Fink and Manley 1987). The pumice is interpreted to be a highly inflated perme-
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Fig. 10 Theoretical vesicularity curves from Eq. (3), for the confining ambient hydrostatic pressures of a 10.3 and b 5.6 MPa (equivalent to water depths of 1,000 and 550 m, respectively), of gas void space (vesicularity) as a function of varying H2O content within a rhyodacitic melt of different eruption temperatures and magma densities. Residual soluble H2O within the melt, for each confining pressure, is determined from Eq. (2)
able foam formed by ‘open-system’ degassing where volatiles exsolve from the melt before or during ascent, with the uppermost pumice layer formed by late-stage vesiculation (Eichelberger et al. 1986; Fink et al. 1992). Pumice units within the flow are formed by volatile expulsion because of extensive microlite growth, buoyant volatile migration along brittle fractures to the upper surface and extensive bubble coalescence (Fink 1983; Manley and Fink 1987). Such pumice units can have vesicularity as high as 70–80% (Manley and Fink 1987). These eruptions are characterised by prolonged residence within a crustal magma chamber, relatively slow magma ascent and decompression within the conduit and concomitant ‘opensystem’ gas loss, an increase of magma viscosity associated with crystallisation of ubiquitous microlites within the dome, and subsequent low magma discharge rates and emplacement as effusive flows with late-stage vesiculation as the flow surface equilibrates with the ambient pressure (Melnik and Sparks 1999). The upper pumice units may be microlite-poor though not necessarily microlite-free. Pumiceous carapaces are recorded with submarine silicic lava domes (e.g. Kano et al. 1991; Allen and McPhie 2000). For subaerial silicic flows, late-stage vesiculation may include further migration of volatiles and dome over-pressuring to generate endogenic explosive eruptions (Newhall and Melson 1983; Fink and Manley 1987; Robertson et al. 1998).
Fig. 11 Theoretical vesicularity curves from Eqs. (2) and (3), as a function of water depth (hydrostatic pressure), for a rhyodacitic melt (with magma density of 2,300 kg m–3), at credible initial water contents and magma temperature of 850 and 950 °C. Interpreted pressure (water depth) range of the initial Healy vent and possible quenching pressures of pyroclasts within the eruption column are shown. Grey fill is a postulated field for submarine pyroclastic eruption assuming 70% vesicularity threshold for fragmentation and using credible magma parameters
Eruption mechanism: pyroclastic eruption versus effusive lava dome For Healy, the distinction between pyroclastic or effusive eruption mechanisms is based upon a number of observations. Firstly, Healy pumice groundmass comprises undevitrified glass that is universally microlite-free. Such textures are consistent with eruptions with high discharge and gas content, and rapid decompression, ascent and crystallisation. In contrast, microlite crystallisation, even within the uppermost microlite-poor pumices, is more characteristic of degassing lava domes where ascent and decompression is slow, and associated with prolonged crystallisation in the vent conduit (Sparks 1997; Melnik and Sparks 1999). Secondly, the vesicle granulometry, consistent with the groundmass petrography, indicate rapid decompression and ascent associated with pyroclastic eruptions. Power-law VSDs of Healy pumice (Fig. 7), with exponents of d ≈–2 to –3, are consistent with non-equilibrium degassing, and continuous bubble nucleation during rapid decompression (Proussevitch and Sahagian 1996; Blower et al. 2001), as observed from both natural pyroclastic samples and ‘explosive’ laboratory experiments. Power-law VSDs occur where magma systems cannot degas efficiently, but rather involve viscous magma, rapid magma ascent rates, and ultimately explosive pyroclastic eruption (Simakin et al. 1999; Blower et al. 2001). Thirdly, we have yet to recover vitrophyric clasts, or identify retrograde obsidian textures, although it would be a fair expectation to recover such samples if Healy comprised effusive lavas or dome growth. Naturally, our rock dredge sampling is
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possibly limited to the outer surficial units, but sampling of the inner, post-collapse caldera wall, as evinced by recovery of basalt (Wright and Gamble 1999), indicates that at least some inner sections of the edifice have been sampled. Lastly, the effusion of silicic lava domes is rarely, if ever, contemporaneous with caldera formation, being typically a post-caldera eruptive phase. From these observations, we conclude there is compelling evidence that the Healy eruption was pyroclastic.
Discussion Healy submarine pyroclastic eruption For Healy Volcano, the interpretation of a pyroclastic eruption provides new evidence of and insight into explosive submarine volcanism. For credible pre-eruptive magma parameters (Fig. 11), and accounting for posteruptive H2O solubility, 75–80% vesicularity can be generated at a hydrostatic pressure of ~10 MPa (=1,000 m water depth). Such vesicularity can sustain pyroclastic eruptions (Sparks 1978; Mader 1998), but is less than that observed at Healy. Healy vesicularity of 78–83% can be generated at hydrostatic pressures of ≤9 MPa (Fig. 11), but this is interpreted to be, at least for Healy, near the lower limit of pyroclastic eruption. High vesicularity (>80%) can be generated by reducing the hydrostatic pressure and/or increasing the H2O content, but does require a concomitant increase of the pre-eruptive edifice from a conservative (1,000 m water depth) towards a shallower (550 m water depth) reconstruction (Fig. 2). Pyroclastic eruption implies insulation of the eruption column from the enclosing ambient water. Steamsheathed pyroclastic column (Kokelaar 1983), have been interpreted for a number of silicic caldera complexes (e.g. Kokelaar and Busby 1992; Fiske et al. 1998, 2001; Allen and McPhie 2000), and a similar mechanism is proposed for Healy. Uniformly high vesicularity of Healy pumice is consistent with mostly magmatic explosivity (Sheridan and Wohletz 1983; Houghton and Wilson 1989), and hence insulation within a submarine column, rather than a mostly phreatomagmatic eruption with variable vesicularity. Nevertheless, phreatomagmatic processes must have occurred at the margins of the eruption column, but we consider these to have affected a negligible proportion of the total pyroclastic products. Most pyroclasts erupted within the column would cool, condense water from previously steam-inflated vesicle spaces, ingest further water and then sink (Cashman and Fiske 1991). Although we have yet to provide convincing evidence of fines-depleted bedded pumice sequences on the Healy edifice and adjacent flanks, consistent with pyroclast density fall-out from an eruption column (e.g. Cashman and Fiske 1991; Allen and McPhie 2000; Fiske et al. 2001), we contend that this reflects the limitation of our rock dredging and seafloor photography techniques rather than their absence. Further, the larger and
more buoyant pyroclasts will be the last to sink (e.g. Cashman and Fiske 1991; Fiske et al. 2001) and hence may the form outer mantle of pyroclastic products. Development of a submarine eruption column also has implications for pressure evolution during the eruption. For an eruption commencing at the edifice crest within a water depth of 900–1,000 m, the magma pressure required to initiate the eruption needs to exceed the equivalent hydrostatic pressure of 9–10 MPa. In reality, the magma pressure required to initiate the eruption would exceed both the confining lithostatic and initial overpressure (Martí et al. 2000), and initial hydrostatic pressure. As the eruption progressed into full pyroclastic mode, expansion of the submarine column, comprising steam and steam-inflated pyroclasts, would progressively displace ambient water and reduce the overlying hydrostatic pressure. The density of the eruption column, dependent on the ratio of constituent steam and steam-inflated pyroclasts, is estimated to be ~0.3–0.5 of the displaced water with associated reduction of the hydrostatic pressure. (Fig. 11). Reduction of the effective hydrostatic pressure at the corresponding water depth will both further promote vesiculation and pyroclastic eruption, and increase the water depth limit at which such eruptions can occur (Fig. 11). Development of a steam-sheathed also requires that the main phase of the Healy eruption be sustained at a high-mass discharge. This allows both the near-complete vesiculation of the magma (Houghton and Wilson 1989), and conversion of magma into steam and steam-inflated pyroclasts at a sufficiently high rate to sustain the eruption column, and prevent collapse because of the hydrostatic load. It is probable that the eruption column is metastable at these reduced sub-hydrostatic pressures. The height of the eruption column is unknown, but could have extended through to the sea surface. Fiske et al. (2001) have inferred an eruption column breaching the sea surface for the Myojin eruption. Dispersal of sea-rafted pumices may also indicate that this occurred at Healy. Pyroclasts within the Healy steam-sheathed column, will have quenched at pressures less than the equivalent ambient hydrostatic pressure, and may well have variable trajectories (and hence quenching pressures) within the column (Fig. 11). Within these constraints of uncertain edifice reconstruction (Fig. 2) and pre-eruptive magma water content for Healy (Fig. 10), we can broadly establish the ambient hydrostatic pressure of the eruption as ~6–9 MPa. Hydrostatic pressure >~9 MPa will suppress significant H2O exsolution and vesiculation to values less than the 78–83% observed (Fig. 11), whereas pressure <~6 MPa requires a progressively less credible reconstruction and larger-volume syn-eruptive collapse and destruction of the edifice (Fig. 2c). The equivalent water depths are dependent on the height of the associated eruption column. We do not envisage Healy eruptions deeper than ~1,000 m being pyroclastic (Fig. 11). The interpretation of high mass discharge is also consistent with the recovery of basalt from the inner caldera wall, possibly being late-stage mafic volcanism associated with caldera col-
26
lapse, discharge of all silicic melt, and destruction of the magma chamber (Druitt and Francaviglia 1992). Similarly, the eruption volume can be broadly established, within the bounds of the conservative and more voluminous edifice reconstructions. Volumes of the two edifice reconstructions (Fig. 2) are 2.36 and 3.58 km3, respectively, but obviously do not account for any thickness of intra-caldera pumice deposits. This volume range is interpreted as being predominantly the volume of primary eruption output, as suggested by the dearth of accessory lithics at both macroscopic and microscopic scales in all pumice types. Country rock of the original edifice, fragmented by the eruption, is interpreted to have collapsed inwards, forming units beneath the caldera floor and, hence, is unsampled. Conversion of the edifice volumes into eruptive pumice volumes, using an average 81% vesicularity, yields ~10–15 km3 of pumice from, respectively, the conservative and more voluminous edifices. Such volumes are higher than the 5 km3 originally proposed by Wright and Gamble (1999), but only require an average of <0.4-m-thick pumice deposit over the immediate 50 km2 of the Healy Volcano. Such syn-eruptive destruction and collapse of 2.4–3.6 km2 of the Healy edifice is not, though, dissimilar in magnitude to the eruption of Myojin Knoll (Fiske et al. 2001). Although not addressed here in detail, caldera collapse is interpreted to be syn-eruptive and catastrophic, and possibly tsunamiogenic. Further, caldera collapse (relative to subaerial eruptions) may be more dynamic because of the overlying hydrostatic load if the eruption column does not breach the sea surface. Morphologically the caldera is similar to either the ‘plate’- or ‘funnel’-type caldera of Lipman (1997), although the caldera diameter of <5 km would suggest the latter is more likely. Pumice dispersal and age With the ejection of ~10–15 km3 of pumice, most pyroclasts would cool within the water column and sink; however, a proportion may have had sufficient buoyancy to rise fully through the water column and cool, in part, at the sea surface. Submarine eruption and hydrothermal plumes can buoyantly rise to the sea surface, producing sea-rafted pumice that is subsequently dispersed by ambient currents and winds (Gass et al. 1963; Kokelaar and Durant 1983; Fiske et al. 1998; Cashman and Fiske 1991). Syn-eruptive plumes have been observed during two modern eruptions from shallow (<250 m water depth) Kermadec volcanoes (Latter et al. 1992). Did Healy pumice reach the sea surface and if so, was it widely distributed? There is tantalising evidence from the New Zealand region to suggest that this indeed occurred. A sea-rafted pumice deposit (Loisels Pumice; Wellman 1962; McFadgen 1985) comprising multiple-sourced clasts (Shane et al. 1998) is relatively common within Holocene beach-sequences of North Island, New Zealand, and even recorded from the Chatham Islands (McFadgen 1994) some
Fig. 12 Microprobe glass analyses of Healy pumice and widespread tephra from onshore New Zealand (Taupo Pumice) and Kermadec Islands (Sandy Bay Tephra) as comparative data, including the possible correlative with part of the Loisels sea-rafted pumice
650 km east of New Zealand. The most reliable 14C dates for the Loisels Pumice yield an age of 590±80 calendar years (McFadgen 1994), and is younger than the underlying and well-dated Taupo Pumice at A.D. 232±15 calendar years (Sparks et al. 1995). Glass from Loisels Pumice defines two compositional fields (Fig. 12), with Healy pumice glasses coinciding with medium-low K Loisels pumice samples (Fig. 12) recovered from Bay of Plenty, Great Barrier Island and Chatham Islands. Further, Loisels pumices, including those from the Bay of Plenty, are macroscopically similar to Healy pumice with flow foliations and light and dark flow laminae (Shane et al. 1998). Dispersal from Healy Volcano south to New Zealand, and further east, is consistent with the known oceanographic current patterns (Fig. 1) where surface oceanic transport (the East Auckland current) flows north-west–south-east (Stanton et al. 1997) and includes re-circulation of a significant part of the flow within the warm-cored, anticyclonic East Cape Eddy back to New Zealand (Roemmich and Sutton 1998). The latter, flows over southern Kermadec volcanoes, including the Healy caldera (Wright 2001). Laboratory experiments show pumice can float for extended periods (in the order of weeks to months), before becoming water-logged and sinking (Whitham and Sparks 1986; Manville et al. 1998). Such timescales would be sufficient to re-circulate within the East Cape Eddy at recorded surface speeds of 30–50 cm s–1 back to the North Island coastline. Transport of sea-rafted pumices over significant distances is known from modern eruptions (e.g. Coombs and Landis 1966; Fiske et al. 2001). If it is accepted that Healy pumices are indeed correlatives of parts of the Loisels Pumice, then it provides an age control for the Healy pumice eruptions, with a best estimate of 590±80 calendar years. Such an age of <700 years is consistent with the fresh and unaltered appearance of the Healy pumice clasts and pristine topography of the caldera.
27
Summary The Healy caldera is pervasively mantled with highly vesicular rhyodacitic pumice within water depths of 1,150–~1,800 m. Three pumice types are recognised. Type 1 is a white–light grey pumice with large cavernous vesicles ≤30 mm long and predominantly without a foliation fabric, but including elongate ‘woody’ pumice, whereas type 2 is predominantly grey, millimetre-scale laminae with a complex and sinuous flow banded foliation, including large and stretched cavernous vesicles ≤55 mm long. Type 3 is relatively rare, finely vesicular, pumice. Density and vesicularity for all pumice are ≤0.5 g cm–3 and >75%, respectively. Pumice fabrics and vesicle granulometry record a magmatic history of early bubble coalescence and subsequent continuous vesiculation. For vesicles <1–2 mm, VSDs for type 1 and 2 pumices have a strong power-law relation (where d=–2.5±0.4), which indicates ‘Apollonian’ vesicle packing consistent with continuous vesiculation associated with rapid magma decompression, gas loss and ascent. For vesicles >1–2 mm, variable size modes form a broad power-law relation. Pressure dependence of H2O solubility, and generated void space at credible magma parameters of ≤6 wt% H2O and a temperature of ≤950 °C, and observations of Healy pumice petrography and VSDs provide compelling evidence of a pyroclastic eruption. Conservative and larger volume edifice reconstructions to 1,000 and 550 m water depths, respectively, provide bounds to the preeruptive topography. Within these constraints we establish the hydrostatic pressure of the eruption as 6–9 MPa. Hydrostatic pressure >~9 MPa will suppress exsolution, and vesicularity to less than that observed, whereas pressure <~6 MPa requires a bigger volume reconstruction and larger syn-eruptive destruction of the edifice. We interpret the eruption was mostly insulated by a metastable steam-sheathed eruption column comprising steam and hot pyroclasts. Most pyroclasts would cool, condensing water from previously steam-inflated vesicle spaces, ingesting further water, and then sink to produce density fractionated bedded pumice sequences on the seafloor. As the eruption column expanded, a reduction in hydrostatic pressure would further promote vesiculation and pyroclastic fragmentation. Pyroclasts within the column would be quenched at reduced pressures. Some pyroclasts may have buoyantly risen through the water column and cooled in part at the sea surface. Major-element geochemistry and present-day sea-surface currents suggest such pumice clasts may be a correlative of the sea-rafted Loisels pumice that forms a widespread unit within northern North Island, New Zealand Holocene beach sequences. The Loisels pumice has a best age estimate of 590±80 calendar years. The high vesicularity, and interpreted development of a column, suggests a high-mass discharge rate of eruption. Similarly, although we presently lack direct evidence, we suggest caldera collapse was syn-eruptive and catastrophic. Volumes of the conservative and larger vol-
ume edifices (from the observed post-eruption topography) are 2.36 and 3.58 km3, respectively, but do not account for intra-caldera pumice. Conversion of the edifice volumes into eruptive volumes yields 10–15 km3 of pumice. Acknowledgements Staff of the Challenger Division and School of Ocean and Earth Sciences, Southampton Oceanography Centre (UK) are warmly thanked for their hospitality and discussion, where much of the analysis and writing were completed during a sabbatical visit by I.C.W. Thanks are due also to Steve Sparks and Jon Blower (University of Bristol) for insightful discussion. R. Garlick completed the terrain modelling and volume calculations. S. Bush (VUW) completed the difficult task of producing the pumice slabs and thin-sections. We thank Richard Fiske and Steve Sparks for making incisive comments on an early draft manuscript and reviews by Steve Carey and Hugh Tuffen that improved the final paper. This work was jointly funded by Victoria University of Wellington, Foundation of Research Science and Technology (New Zealand) contract C01X0038, and a NSOF-funded NIWA Sabbatical Award to I.C.W.
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