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The Rotoiti (~120 km3) and Earthquake Flat (~10 km3) eruptions occurred in close succession from the Okataina Volcanic Centre at ~50 ka. While accessory mineral geochronology points to long periods of crystallization prior to eruption (104–105 years) and separate thermal histories for the magmas, little was known about the rates and processes of the final melt production and eruption. Crystal zoning patterns in plagioclase and quartz reveal the thermal and compositional history of the magmatic system leading up to the eruption. The dominant modal phase, plagioclase, displays considerable within-crystal zonation: An37–74, ~40–227 ppm MgO, 45–227 ppm TiO2, 416–910 ppm Sr and 168–1164 ppm Ba. Resorption horizons in the crystals are marked by sharp increases (10–30%) in Sr, MgO and XAn that reflect changes in melt composition and are consistent with open system processes. Melt inclusions display further evidence for open system behaviour, some are depleted in Sr and Ba relative to accompanying matrix glass not consistent with crystallization of modal assemblage. MI also display a wide range in XH2O that is consistent with volatile fluxing. Quartz CL images reveal zoning that is truncated by resorption, and accompanied by abrupt increases in Ti concentration (30–80 ppm) that reflect temperature increases ~50–110°C. Diffusion across these resorption horizons is restricted to zones of

article The Okataina Volcanic Centre (OVC) contains the northeasternmost caldera complex in onshore Taupo Volcanic Zone (TVZ), New Zealand, sited largely within the Taupo Rift. Pyroclastic fall deposits and ignimbrites deposited in the Bay of Plenty coast area between 420 and 625 ka were probably erupted from OVC, and these provide a maximum age for the centre. The earliest ignimbrite for which there is good evidence for eruption from OVC is the c.550 ka Quartz-biotite Ignimbrite, exposed to the west of Lake Okataina and to SE of OVC. This ignimbrite probably correlates with one of the early ignimbrites found in the Kawerau geothermal wells, and is large enough to have been accompanied by caldera collapse. It was followed by extensive rhyolitic explosive eruptions of the Murupara Subgroup, culminating with eruption of the Matahina Ignimbrite at c.325 ka. This c.160 km 3 (magma volume) eruption was accompanied by caldera collapse to form the southern part of the present day Okataina caldera complex. A long duration sequence of rhyolite lavas and pyroclastics was then erupted on the southern and western sides of OVC, before eruption of the >100 km

Deposits of the 22.6 ka Okareka Eruption Episode from Tarawera Volcanic Complex record the sequential and simultaneous eruption of three discrete rhyolite magmas following a silicic recharge event related to basaltic intrusion. The episode started with basaltic eruption (∼ 0.01 km3 magma), and rapidly changed to a plinian eruption involving a moderate temperature (750 °C), cummingtonite-bearing rhyolite magma (T1) with a volume of ∼ 0.3 km3. Hybrid basalt/rhyolite clasts demonstrate direct basaltic intrusion that helped trigger the eruption. Crystals, shards and lapilli of two other rhyolite magmas then joined the eruption sequence. They comprise a cooler (720 °C) crystal-rich biotite–hornblende rhyolite magma (T2) (∼ 0.3 km3), and a hotter (780 °C), crystal-poor, pyroxene–hornblende rhyolite magma (T3) (∼ 4.5 km3). All mid to late-stage ash units contain various mixtures of T1, T2 and T3 components with a general increase in abundance of T3 and rapid decline of T1 with time. About 4 km3 of T3 magma was extruded as lavas at the end of the episode. Contrasts in melt composition, crystal and volatile contents, and temperatures influenced viscosity and miscibility, and thus limited pre-eruption mixing of the rhyolite magmas. The eruption sequence and the restricted direct basaltic intrusion into only one magma (T1) is consistent with the rhyolites occupying separate melt pods within a large crystal-mush zone. Melt–crystal equilibria and volatile contents in melt inclusions indicate temporary magma storage depths of

Most rhyolite eruption episodes of Tarawera volcano have emitted several physiochemically distinct magma batches (∼1–10 km3). These episodes were separated on a millennial timescale. The magma batches were relatively homogeneous in temperature and composition at pumice scale (>4 cm), but experienced isolated crystallisation histories. At the sub-cm scale, matrix glasses have trace element compositions (Sr, Ba, Rb) that vary by factors up to 2.5, indicating incomplete mixing of separate melts. Some quartz-hosted melt inclusions are depleted in compatible trace elements (Sr, Ti, Ba) compared to enclosing matrix glasses. This could reflect re-melting of felsic crystals deeper in the crystal pile. Individual quartz crystals display a variety of cathodoluminescence brightness and Ti zoning patterns including rapid changes in melt chemistry and/or temperature (∼50–100°C), and point to multi-cycle crystallisation histories. The Tarawera magma system consisted of a crystal-rich mass containing waxing and waning melt pockets that were periodically recharged by silicic melts driven by basaltic intrusion.

Tephra deposits of unknown age, and/or occurring in distal sequences that lack stratigraphic markers, are commonly identified by their glass compositions and mineralogical assemblages. Recent studies of widespread rhyolitic tephra deposits from Okataina Volcanic Centre (OVC), New Zealand, reveal a hitherto unrecognised geochemical and mineralogical diversity within individual deposits, and sometimes between and within individual lapilli in these deposits. These variations, subtle in some units, can occur both vertically and laterally through a tephra deposit, and result from the sequential or simultaneous eruption of separate batches of rhyolitic magma. This process often includes mingling of magmas in the eruption conduits, as demonstrated by disequilibrium mineral assemblages in pumice clasts. Some eruption episodes (e.g., 36 cal ka Hauparu, 17.6 cal ka Rerewhakaaitu) have tapped two or more magmas simultaneously, while others (0.7 cal ka Kaharoa; 15.4 cal ka Rotorua) have tapped multiple magmas sequentially. Some tephra deposits display the sequential and/or simultaneous emissions of different magmas from multiple vents through temporal phases of the episode (5.5 cal ka Whakatane). The resulting deposits can also show variations between different azimuths around the vent due to changes of wind direction during the episode. The internal heterogeneity in OVC tephra deposits was only recognised after comprehensive sampling through their thick proximal pyroclastic sequences and associated lavas. The internal heterogeneity highlights the inadequacy of fingerprinting tephra deposits from a small number of samples of restricted dispersal. Magma mingling at a single-clast-scale also has implications for geochronological techniques that use accessory phases (U-series) and/or require uniform melt geochemistry (fission-track on glass).

The Earthquake Flat Pyroclastics form a c. 10 km3 rhyolite deposit erupted at c. 50 ka from the margin of Okataina Volcanic Centre, immediately following the caldera-forming eruption of the Rotoiti Pyroclastics (c. 100 km3) from vents c. 20 km to the NE. Earthquake Flat Pyroclastics deposits display textural and compositional complexity on a crystal-scale consistent with rejuvenation of a near-crystalline pluton in the upper crust. Quartz and plagioclase crystals are resorbed, whereas hornblende and biotite are euhedral. Fe–Ti oxides indicate large variations in pre-eruption temperatures (702–805 °C). Differences of up to 70 °C within pumice lapilli show that crystals were chaotically juxtaposed during magma stirring and evacuation. Chemical zoning within hornblende crystals is consistent with rimward increases of c. 50 °C. These features are consistent with a convective self-stirring process. Previous isotope studies demonstrate a long (>100 ka) crystallization history for the magma. Resorption of crystals deep in the magma may have produced a Ca-, Fe- and Mg-enriched rhyolite melt that allowed the growth of reverse-zoned hornblende. Microdiorite lithic fragments in the Earthquake Flat Pyroclastics and Rotoiti deposits and a basaltic eruption that immediately preceded the Rotoiti eruption suggest that mafic underplating beneath Okataina Volcanic Centre provided a major thermal and volatile pulse to drive the caldera eruptions.

The 17.7 ka Rerewhakaaitu eruption episode (volume ∼ 5 km3 DRE rhyolite magma) was the second of five major episodes that have built the Tarawera volcanic complex in the Okataina Volcanic Centre over the past 22 kyr. The Rerewhakaaitu episode produced a widespread tephra fall deposit, associated proximal pyroclastic flow deposits, and voluminous rhyolite lava extrusions. Two different rhyolite magmas (T1 and T2) were simultaneously erupted from the main vent area throughout much of the eruption episode. T1 magma was a crystal-poor orthopyroxene-hornblende rhyolite that is highly evolved (whole rock SiO2 = 77 wt.%), with a moderate temperature (∼ 760 °C, based on Fe–Ti oxides). T2 is a crystal-rich biotite-hornblende rhyolite that is less evolved (SiO2 = 75 wt.%), with a Fe–Ti oxide temperature of ∼ 700 °C. Ejecta from the simultaneous and sequential eruption of these two magmas include some pumice clasts with mixed (hybrid) and mingled glass compositions and crystal populations. A third rhyolite magma (T3) was extruded from another vent 3 km distant to form an apparently contemporaneous lava dome. T3 was the least evolved (SiO2 = 74 wt.%) and hottest (∼ 820 °C) of the three magmas. Saturation pressures calculated using dissolved H2O and CO2 contents of melt inclusions in quartz crystals indicate that T2 magma stagnated and crystallised at about 12 km depth, while small quartz crystals in T1 magma grew during ascent through ∼ 8 km depths. Some T1 and T2 rhyolite clasts contain vesicular brown blebs with widely variable (andesite to rhyolite) glass compositions, accompanied by olivine, clinopyroxene and calcic plagioclase crystals that are interpreted as xenocrysts derived from injected basalt. Temperatures over 1000 °C estimated from pyroxene phase equilibria in these clasts reflect intrusion of the more mafic magma, which is now identified as the priming and triggering mechanism for three of the four post-22 ka Tarawera rhyolite eruption episodes. However, the rhyolite magma bodies and conduits modelled for each episode have considerable differences in characteristics and geometry. Our preferred model for the Rerewhakaaitu episode is that eruptions occurred from three laterally and vertically isolated rhyolite magma bodies that were initially primed and triggered by basalt intrusion during a regional rifting event. The ascending hotter and less viscous T1 rhyolite magma intersected and further invigorated a stagnant pond of cooler, denser and more viscous T2 magma, and lubricated its transport to the surface.

The previously poorly documented 26–16.6 ka interval of pyroclastic volcanism from Tongariro Volcano is marked by three distal lapilli fall units (Rt1-3) exposed in ring-plain deposits. The distal Rt1-3 units are tentatively correlated to proximal scoria deposits on the upper slopes of North Crater based on their dispersal patterns, petrography and geochemistry. Lapilli in each of the Rt1-3 deposits are characterised by variable groundmass crystallinity, vesicularity and colour within individual clasts. Matrix glasses are mostly microlite-free, and compositionally diverse across the deposits (SiO2 = 62–75 wt%), with wide composition ranges occurring within single clasts. The glasses represent different melts that were mingled and mixed shortly before eruption; a finding supported by widely variable Fe–Ti oxide equilibrium temperature estimates (∼830–1,200°C). Ranges of 30–160°C (typically 70°C) occur within individual clasts. Some clinopyroxene crystals display Mg-rich (∼Mg #88) rim zones around homogeneous low-Mg (∼Mg #68) cores, with abrupt transition zones. This zoning is interpreted as resulting from the injection of a more mafic melt into a stagnating, resident magma. Crystal-melt equilibria indicate that several episodes of mafic intrusion occurred, to produce hybrid melts with zoned crystals forming isolated ponds within the resident magma. Variable mixing from the percolation of melts and the coalescence of melt ponds would explain the wide range of melt compositions and equilibrium temperatures observed in the ejecta. The magma heterogeneity was preserved by quenching on prompt eruption, with much of the short-duration chaotic mixing of melts and crystals occurring in the conduit. The Rt1-3 eruptions were from an open magmatic system consisting of one or more long-lived stagnant crystal mush zones, from which eruptions were rapidly triggered by new injections of mafic magmas from greater depths. A similar pattern of magmatic dynamics was observed in the much smaller 1995 eruptions of the neighbouring Ruapehu Volcano.

The 21.9 ka Okareka and 17.6 ka Rerewhakaaitu rhyolite eruption episodes began the construction of Tarawera Volcano in the Okataina Volcanic Centre, Taupo Volcanic Zone. Examination of the proximal and medial stratigraphy of these moderate‐size (c. 5 km3 magma) but poorly exposed pyroclastic deposits has increased understanding of their eruption and dispersal processes. The Okareka Tephra consists of at least nine units (A‐I), with unit A basaltic scoria at the base, overlain by the rhyolitic units B‐I. Unit C is the largest individual plinian fall deposit (c. 0.4 km3), dispersed from an eruption column that reached c. 19 km height in the presence of strong crosswinds. The other pyroclastic units record a variety of phreatomagmatic, sub‐plinian, and small ignimbrite eruptions, which were followed by extrusion of voluminous lava flows. The Rerewhakaaitu Tephra consists of 15 rhyolitic fall units A‐N. An initial short plinian phase dispersed lapillifall unit A, mostly to ENE, from columns c. 15 km ...

Post-10 ka rhyolitic eruptions from the Haroharo linear vent zone, Okataina Volcanic Centre, have occurred from several simultaneously active vents spread over 12 km. Two of the three eruption episodes have tapped multiple compositionally distinct homogeneous magma batches. Three magmas totalling ~8 km3 were erupted during the 9.5 ka Rotoma episode. The most evolved Rotoma magma (SiO2=76.5–77.9 wt%, Sr=96–112 ppm) erupted from a southeastern vent, and is characterised by a cummingtonite-dominant mineralogy, a temperature of 739±14°C, and fO2 of NNO+0.52±0.11. The least evolved (SiO2=75.0–76.4 wt%, Sr=128–138 ppm, orthopyroxene+ hornblende-dominant) Rotoma magma erupted from several vents, and was hotter (764±18°C) and more reduced (NNO+0.40±0.13). The ~11 km3 Whakatane episode occurred at 5.6 ka and also erupted three magmas, each from a separate vent. The most evolved (SiO2=73.3–76.2 wt%, Sr=88–100 ppm) Whakatane magma erupted from the southwestern (Makatiti) vent and is cummingtonite-dominant, cool (745±11°C), and reduced (NNO+0.34±0.08). The least evolved (SiO2=72.8–74.1 wt%, Sr=132–134 ppm) magma was erupted from the northeastern (Pararoa) vent and is characterised by an orthopyroxene+ hornblende-dominant mineralogy, temperature of 764±18°C, and fO2 of NNO+0.40±0.13. Compositionally intermediate magmas were erupted during the Rotoma and Whakatane episodes are likely to be hybrids. A single ~13 km3 magma erupted during the intervening 8.1 ka Mamaku episode was relatively homogeneous in composition (SiO2=76.1–76.8 wt%, Sr=104–112 ppm), temperature (736±18°C), and oxygen fugacity (NNO+0.19±0.12). Some of the vents tapped a single magma while others tapped several. Deposit stratigraphy suggests that the eruptions alternated between magmas, which were often simultaneously erupted from separate vents. Both effusive and explosive activity alternated, but was predominantly effusive (>75% erupted as lava domes and flows). The plumbing systems which fed the vents are inferred to be complex, with magma experiencing different conditions in the conduits. As the eruption of several magmas was essentially concurrent, the episodes were likely triggered by a common event such as magmatic intrusion or seismic disturbance.

The Taupo Volcanic Zone of New Zealand is the most frequently active rhyolitic zone on Earth. Since a major caldera-forming eruption episode at ∼50 ka, > 50 rhyolitic eruption episodes have occurred at Okataina Volcanic Centre (OVC) and Taupo Volcanic Centre (TVC). These two active calderas provide an opportunity to examine contemporaneous magmatic processes at high temporal and spatial resolution. Temporal trends indicate OVC intra-caldera eruption episodes tapped progressively more evolved (∼71–77 wt.% SiO 2 ), cooler (940–730 °C), and possibly shallower (∼400–150 MPa) magmas. At TVC, pre-26.5 ka activity was relatively infrequent, and eruption episodes were from shallow (∼100 MPa), cool (∼750 °C) magmas, containing hydrous mineral phases. Following the caldera-forming 26.5 ka Oruanui episode, initial eruption episodes were dacitic and were small volume hot magmas, which were probably from deep (> 400 MPa) chambers. At 12 ka, rhyolitic activity at TVC re-commenced, and eruption episodes have tapped progressively hotter (∼790–850 °C) magmas, which appear to be from deeper (∼140–300 MPa) chambers. At both OVC and TVC volcanoes, the least evolved magmas were erupted following the caldera-forming episodes. Equilibrium between glass and phenocrysts in OVC and TVC rhyolites suggests that crystallisation occurs shortly prior to eruption. Low crystal contents (∼5–15%), and the lack of pre-eruptive gradients suggests rapid convection and/or short crustal residence times. Melts are extracted from greater depths, as indicated by high whole-rock temperatures, before ponding in shallow storage chambers. The distinct pressure, temperature, and oxygen fugacity of each magma erupted, and the lack of temporal (fractionation) trends, suggests that the magmas are not derived from a single common magmatic system at their respective centres. OVC magmas are more oxidised than those from TVC, at any given temperature, suggesting the source areas are fundamentally different. Volumetrically subordinate pumice clasts in some OVC ejecta display mingled glasses and disequilibrium crystal populations resulting from the intrusion and mingling of separate rhyolite magmas prior to eruption. At OVC, some crystal-rich stagnating magmas have become reactivated by new intrusions or engulfed into larger magma bodies, and some eruption episodes were primed and triggered by mafic intrusion.

Deposits of the 36 ka rhyodacitic Hauparu plinian eruption from Okataina Volcanic Centre contain pumices with a wide range of compositions. Type 1 pumices are crystal-poor ( Type 1 heterogeneous clasts were produced by short-lived mingling of magmas in the conduit during eruption. Phase equilibrium and trace element data are best explained by the homogeneous pumices being derived from a series of semi-isolated pockets of magma at depths > 16 km. Such a depth is close to the seismic velocity transition interpreted to reflect silicic crust overlying mafic cumulates and intrusions, where crustal melting is likely to generate rhyolite magmas. The diversity in the Hauparu Type 1 glasses may reflect heterogeneities in the source region. Crystal-rich Type 2 magma stagnated and crystallised at a shallower depth than Type 1 magma. It was apparently reactivated and/or entrained during ascent of the Type 1 magma. Hauparu is one of a series of rhyodacites that erupted following the caldera-forming Rotoiti ignimbrite eruption at ∼50 ka. The evacuation of shallow magma storage areas during the Rotoiti eruption, and the crustal disruption caused by the caldera collapse, appears to have allowed hot melts (900–990 °C) to rapidly rise from their source areas without being hindered by crystallising magma bodies at shallower depths, or being incorporated into such reservoirs.

The c. 5600 cal. yrBP Whakatane eruption episode consisted of a sequence of intracaldera rhyolite eruptions from at least five vents spread over 11 km of the Haroharo linear vent zone within Okataina Volcanic Centre. Initial vent‐opening eruptions from the Haroharo vent produced coarse lithic clast “blast beds” and pyroclastic density currents (surges). These were immediately followed by eruption of very mobile pumiceous pyroclastic surges from the Makatiti vent 6 km to the southwest. Major plinian eruptions from the Makatiti vent then dispersed Whakatane Tephra pumice fall deposits (bulk volume c. 6 km3) across the northeastern North Island while smaller explosive eruptions produced pyroclastic flows and falls from the Haroharo‐Rotokohu vents and at the Pararoa vent on the caldera rim 11 km northeast from Makatiti. The pyroclastic eruptions at all vents were followed by the extrusion of lava flows and domes; extruded lava volumes ranged from 0.03 km3 for the Pararoa dome to 7.5 km3 for the Makat...

Examination of glass and crystal chemistry in the Rotoiti Pyroclastics (>100 km3 of magma) demonstrates that compositional diversity was produced by mingling of the main rhyolite magma body with small volumes of other magmas that had been crystallizing in separate stagnant magma chambers. Most (>90%) of the Rotoiti deposits were derived from a low-K2O, cummingtonite-bearing, rhyolitic magma (T1) discharged throughout the eruption sequence. T1 magma is homogeneous in composition (melt SiO2=77.80±0.28 wt.%), temperature (766±13 °C) and oxygen fugacity (NNO+0.92±0.09). Most T1 phenocrysts formed in a shallow (∼200 MPa), near water-saturated (awater=0.8) storage chamber shortly before eruption. Basaltic scoria erupted immediately before the rhyolites, and glass-bearing microdiorite inclusions within the rhyolite deposits, suggest that basalt emplaced on the floor of the chamber drove vigorous convection to produce the well-mixed T1 magma. Lithic lag breccias contain melt-bearing biotite granitoid inclusions that are compositionally distinct from T1 magma. The breccias which overlie the voluminous T1 pyroclastic flow deposits resulted from collapse of the syn-Rotoiti caldera. Post-collapse Rotoiti pumices contain T1 magma mingled with another magma (T2) that is characterized by high-K glass and biotite, and was cooler and less oxidised (712±16 °C; NNO−0.16±0.16). The mingled clasts contain bimodal disequilibrium populations of all crystal phases. The granitoid inclusions and the T2 magma are interpreted as derived from high-K magma bodies of varying ages and states of crystallization, which were adjacent to but not part of the large T1 magma body. We demonstrate that these high-K magmas contaminated the erupting T1 magma on a single pumice clast scale. This contamination could explain the reported wide range of zircon U–Th ages in Rotoiti pumices, rather than slow crystallization of a single large magma body.

The 15.8 ka Rotorua rhyolite eruptive episode from the Okataina Volcanic Centre comprises a plinian pumice fall deposit followed by the extrusion of two rhyolitic lava domes or flows and their associated block-and-ash flows, with a total volume >1 km 3 (dense rock equivalent). Variations in mineralogy, whole-rock, glass and mineral chemistry, and calculated magmatic properties suggest that two distinct rhyolitic magmas were sequentially tapped during the eruption. The first magma erupted (T1) is characterized by low SiO 2 ( c . 76.5 wt% in glass), calcic feldspars (An 44 ), magnesian hornblendes (MgO c . 14.45 wt%), clinopyroxene, and high temperatures ( c . 835 °C) and f O 2 (1.8–2.1 ΔFMQ (where FMQ is the fayalite–magnetite–quartz buffer)). The second magma (T2) was more evolved and is characterized by higher SiO 2 ( c . 77.4 wt% in glass), Na-rich feldspars (An 24 ), less magnesian hornblendes (MgO c . 11.8 wt%), biotite, and low temperatures ( c . 750 °C) and f O 2 (0.65–1.1 ΔFMQ). Both magmas are homogeneous, but evidence for some magma mingling indicates that they were in contact during eruption. However, there was only a minor degree of hybridization, perhaps reflecting the contrasting temperatures and viscosities of the two magmas. The crystal-rich, poorly vesicular nature of the T2 ejecta indicates that it originated from a cooling, high-level magma chamber that was reactivated by intrusion of hotter, volatile-rich T1 magma. The ponding of rhyolite magmas at shallow depth and their subsequent reactivation by later rhyolitic intrusion may be an important process in the compositional evolution and eruption dynamics of many Okataina Volcanic Centre rhyolite magma bodies.

A series of large hydrothermal eruptions occurred across the Waiotapu geothermal field at about the same (prehistoric) time as the ~AD1315 “Kaharoa” rhyolite magmatic eruptions from Tarawera volcano vents, 10–20 km distant. Triggering of the Waiotapu hydrothermal eruptions was previously attributed to displacement of the adjacent Ngapouri Fault. The Kaharoa rhyolite eruptions are now recognised as primed and triggered by multiple basalt intrusions beneath the Tarawera volcano. A ~1000 t/day pulse of CO2 gas is recorded by alteration mineralogy and fluid inclusions in drill core samples from Waiotapu geothermal wells. This CO2 pulse is most readily sourced from basalt intruded at depth, and although not precisely dated, it appears to be associated with the Waiotapu hydrothermal eruptions. We infer that the hydrothermal eruptions at Waiotapu were primed by intrusion of the same arrested basalt dike system that drove the rhyolite eruptions at Tarawera. This dike system was likely similar at depth to the dike that generated basalt eruptions from a 17 km-long fissure that formed across the Tarawera region in AD1886. Fault ruptures that occurred in the Waiotapu area in association with both the AD1886 and ~AD1315 eruptions are considered to be a result, rather than a cause, of the dike intrusion processes.

Products of the ∼5 km3 (DRE), ∼5-yr duration Kaharoa eruption episode display two main high-silica rhyolite compositions; T1 erupted early (as plinian pyroclastics), and T2 erupted late (mostly as lavas). The T1 and T2 eruptive types are defined by crystal contents and compositional variations in whole rock, glass, plagioclase and biotite. Stratigraphically intermediate pyroclastic deposits have an intermediate composition (T1+2). A small volume of rhyodacite pyroclastics, mingled with injected basalt, was also erupted. The Kaharoa rhyolites were erupted from multiple sources spread along an 8-km linear vent zone, but the changes in eruptive compositions were largely controlled by position in the eruption sequence and magma discharge rates, rather than vent locations. Data from the Kaharoa eruptive types, vent locations, eruption sequence and discharge rates can be combined with concepts of magma chamber evacuation processes to produce a preliminary dimensional model of the pre-eruption rhyolite magma body. Our model magma body is sill-like, ∼8 km long, 1 km wide, 1.4 km thick, and located at ∼6–7 km depth in the upper crust. T1 magma overlay T2 magma in the upper levels of the chamber, with each magma layer internally mixed to a homogeneous composition along an axial extent defined by the vent locations. An underlying third rhyolite magma (T3) is recognised as the silicic end-member that was modified by basalt to form the rhyodacite eruptives. The rhyolite magma stratification survived multiple injections of basalt magma, which primed and finally triggered the Kaharoa eruptions. The T1+2 eruptives resulted from syn-eruption mingling in the conduit of the two main rhyolite magma types. Thickness of the T1 layer in the model can be estimated at 0.25 km; the T2 layer was somewhat thicker. Thicknesses of the underlying T3 and basalt layers are uncertain. Post-eruption geothermal heat flow indicates a residual magma volume of ≥6 km3, suggesting that the pre-eruption magma volume was ≥11 km3.

The ∼AD 1305 Kaharoa eruption episode of Tarawera Volcanic Complex was the largest (∼4 km3 magma) eruption in New Zealand in the last 1000 years. High-silica (∼76% SiO2) rhyolite was erupted from seven vents along an 8-km linear zone to form pumiceous pyroclastic deposits and lava domes. Initial vent-clearing explosions were followed by a series of plinian pumice eruptions that spread tephra downwind, and pyroclastic density current deposits over the volcano slopes. The early pyroclastic eruptions were followed by extrusion of a dome in the summit vent and the migration of further plinian activity to adjacent vents. The episode finished with the extrusion of three summit lava domes, accompanied by block-and-ash flows. Two main types of rhyolite were erupted: (1) a low-Zr, low-Sr, high-Rb type comprising the early plinian pyroclastics and (2) a high-Zr, high-Sr, low-Rb type comprising the late eruptives including the summit domes. Basaltic material (basalt and basaltic andesite) is commonly present as free clasts within the Kaharoa pyroclastic deposits, along with rare clasts of granodiorite, diorite, gabbro and olivine clinopyroxenite. The basaltic clasts are often veneered by Kaharoa rhyolite. Basaltic material is also common as inclusions within rhyolite pumices, along with olivine and augite xenocrysts derived from the basalt. The basaltic clasts and inclusions contain complexly zoned plagioclase and corroded olivine (commonly with a reaction rim) consistent with disequilibrium caused by magma mixing. Some basaltic clasts and inclusions are hornblende-free; others contain groundmass hornblende. ‘Plumes’ of brown glass often extend from hornblende-bearing basaltic inclusions into the surrounding rhyolite. These plumes demonstrate magma mixing, as does the presence of rhyolitic glass and quartz xenocrysts in some basaltic inclusions. Most basaltic inclusions have crenulate boundaries, suggesting they were fluidal when coming into contact with the rhyolite magma. Diorite, gabbro and olivine clinopyroxenite clasts have mineralogy similar to the basalts, commonly contain patches of fine-grained basaltic material, and are considered co-magmatic with the basalts. Intrusion of basalt magma into the base of the rhyolite magma chamber appears to have occurred for some time before the Kaharoa eruptions began, allowing considerable mixing with the rhyolite magma, transfer of water to some basalt inclusions and formation of hornblende in their groundmass. Conversely, the hornblende-free basalt inclusions did not have time to react with the rhyolite magma and must have been intruded shortly before the start of the eruption. Intrusion of this hornblende-free basalt appears to have triggered the Kaharoa eruption, after the magma chamber had been primed by the earlier intrusions.

Ngauruhoe cone, in southern Taupo Volcanic Zone, New Zealand, has grown rapidly over the last 2,500 years in an alternation of effusive, strombolian, vulcanian, and sub-plinian eruptions of andesitic magma. At times growth has been 'staccato' in fashion as evidenced in the historical record. Each historical eruption typically lasted days to months, alternating with repose periods of years to decades. Major historic eruptions occurred in 1870 1949 1954–1955 and 1973–1975, encompassing wide variations in eruptive style over short timescales. The early period of cone building appears to have been dominated by a more continuous form of activity characterised by a series of numerous frequent explosive eruptions, with associated lava flows. The 2.2-km3 cone has grown in a piecemeal sectorial manner reflecting constant modification to the morphology of the summit, which has funnelled eruption products to specific sectors of the cone. Eruption rates can be calculated on several different timescales. Discharge rates averaged over individual eruptive pulses vary by two orders of magnitude (2.7–280 m3 s–1), reflecting variations in high level magma ascent rates and processes such as degassing, which are, in turn, reflected in contrasting eruptive styles. Lower rates (e.g. 0.65 m3 s–1) are obtained by averaging the discharge over an entire eruption lasting several months and may correspond to the ascent rate of magma batch(es) feeding the eruption. The long-term growth rate of Ngauruhoe is 0.9 km3 ky–1. This is an average rate reflecting the long-term deep supply rate of magma to crustal reservoirs. By looking at eruption rates on these different timescales we are better able to constrain processes occurring at various depths within the plumbing system. There are few detailed studies of the growth patterns of young volcanic cones, but such data are essential in understanding the dynamics of andesitic systems. More than 60 lavas and pyroclastic units mapped on different sectors of Ngauruhoe cone have been correlated by flow chronology and their distinctive compositions into five groups. Although the cone has grown rapidly, Ngauruhoe shows little evidence for the existence of large crustal magma reservoirs and long-lived magma batches. Instead, abrupt and non-systematic changes in magma chemistry and isotopic composition between and within the five groups indicate that the volcano has an open-system, multi-process, multi-directional character and erupts small (

The c. AD 1314 Kaharoa eruption from Tarawera volcano, North Island, New Zealand, was a complex, long-duration, multi-episode rhyolitic event. Up to 13 discrete sub-Plinian to Plinian-style phases occurred in two stages from at least two different vents along an 8-km-long fissure. The early explosive stage produced seven pumice lapilli fall units dispersed solely to the southeast of the volcano, while a later stage produced six fall units dispersed almost entirely to the north and northwest. The duration of fallout for these 13 lapilli units is estimated to have totaled about 1.5 days. The presence of massive-textured ash layers between many of the lapilli fall units suggests that longer-duration, pyroclastic density-current producing phases and more quiescent fallout intervals occurred between the intense explosive phases of the eruption (total duration of ash fallout ~ 7.5 days). The ash deposition extended the minimum duration of the explosive episodes to at least 9 days, based on settling rates of the ash. Possibly the explosive phase of eruptive activity lasted for ~ 2 weeks, including some brief hiatuses and pyroclastic density-current deposition, and was followed by ~ 4 years of mainly lava-dome-forming activity. At least 15 km3 (> 7.3 km3 dense rock equivalent, DRE) of tephra was erupted during the main explosive episodes, and at least 9.1 km3 DRE during the whole eruption, more than doubling the previous estimate. This may be an underestimate by up to 50 % of the fallout volume. Eruption column heights for the two most widely dispersed fall units are estimated to have ranged from 25–28 km above the vents and durations of individual lapilli fall unit-forming phases ranged from 1.5 to 4 h, but the ash-producing phases lasted much longer (individually up to 30 h) and represent a longer-lived shut-down to each eruptive episode. Time-averaged mass discharge rates are estimated to have varied by more than one order of magnitude during the main Plinian-style phases, from 107 to 108 kg/s. However, the explosive phases were unsteady, as indicated by fluctuating grain-size with stratigraphic height in the lapilli fall units and common capping ash beds. The north-northwesterly dispersal of the later stage fall units, and the possible duration of ~ 4 days for these episodes, indicate an unusual period of southerly to southeasterly winds. Therefore, the main explosive part of the Kaharoa eruption is most likely to have occurred during the austral winter to spring months. The Kaharoa eruption style is also shared by the three previous rhyolite-dominated eruptions that have built Tarawera volcano over the past 22,000 years. Knowledge of the widespread pumice and ash fallout from the Kaharoa eruption is important in quantifying the hazard from this type of extended-duration explosive activity, and has implications for future similar rhyolitic activity in New Zealand.

The c. AD 1305 Kaharoa eruptive episode consisted of a complex sequence of basalt‐triggered high‐silica rhyolite eruptions from at least seven vents along an 8 km linear zone across Tarawera Volcano. Initial plinian eruptions from a summit vent spread Kaharoa Tephra southeast across the North Island, accompanied by phreatomagmatic explosions and pyroclastic density currents from vents opened on the northern flanks of the volcano. The early plinian phase was ended by extrusion of Crater Dome in the summit vent, with explosive activity migrating to two adjacent vents (Tarawera and Ruawahia) to the southwest and northeast. Renewed plinian eruptions, apparently from the northern vents, produced tephra falls dispersed northeast‐northwest from the volcano. Extrusion of the three summit lava domes was accompanied by voluminous block‐and‐ash flows generated by collapse of the growing Ruawahia and Wahanga Domes, forming large fans to north, southeast, and northwest of the volcano. Calculation of Kaharoa l...

The six eruption episodes of the ∼10 ka Pahoka–Mangamate (PM) sequence (see companion paper) occurred over a ?200–400-year period from a 15-km-long zone of multiple vents within the Tongariro Volcanic Centre (TgVC), located at the southern end of the Taupo Volcanic Zone (TVZ). Most TgVC eruptives are plagioclase-dominant pyroxene andesites and dacites, with strongly porphyritic textures indicating their derivation from magmas that ascended slowly and stagnated at shallow depths. In contrast, the PM pyroclastic eruptives show petrographic features (presence of phenocrystic and groundmass hornblende, and the coexistence of olivine and augite without plagioclase during crystallisation of phenocrysts and microphenocrysts) which suggest that their crystallisation occurred at depth. Depths exceeding ∼8 km are indicated for the dacitic magmas, and >∼20 km for the andesitic and basaltic andesitic magmas. Other petrographic features (aphyric nature, lack of reaction rims around hornblende, and the common occurrence of skeletal microphenocrystic to groundmass olivine in the andesites and basaltic andesites) suggest the PM magmas ascended rapidly immediately prior to their eruption, without any significant stagnation at shallow depths in the crust. The PM eruptives show three distinct linear trends in many oxide–oxide diagrams, suggesting geochemical division of the six episodes into three chronologically-sequential groups, early, middle and late. Disequilibrium features on a variety of scales (banded pumice, heterogeneous glassy matrix and presence of reversely zoned phenocrysts) suggest that each group contains the mixing products of two end-member magmas. Both of these end-member magmas are clearly different in each of the three groups, showing that the PM magma system was completely renewed at least three times during the eruption sequence. Minor compositional diversity within the eruptives of each group also allows the PM magmas to be distinguished in terms of their source vents. Because petrography suggests that the PM magmas did not stagnate at shallow levels during their ascent, the minor diversity in magmas from different vents indicates that magmas ascended from depth through separate conduits/dikes to erupt at different vents either simultaneously or sequentially. These unique modes of magma transport and eruption support the inferred simultaneous or sequential tapping of small separate magma bodies by regional rifting in the southern Taupo Volcanic Zone during the PM eruption sequence (see companion paper).

The Kaharoa eruption, the most recent rhyolitic volcanic event in New Zealand, resulted in the deposition of the compositionally distinctive Kaharoa Tephra over at least 30 000 km2 of northern and eastern North Island. The tephra forms an isochronous marker bed for various late-Holocene stratigraphic and palaeoen vironmental studies but is particularly important for evaluating the chronology of New Zealand's notably short prehistory because it was erupted within the last millennium, close to the time of first Polynesian colonization. We derive a definitive radiocarbon age, previously ambiguous, for the Kaharoa Tephra of 665615 BP using cluster analysis of 22 radiocarbon ages relevant to the Kaharoa eruptive episode. The error-weighted mean age obtained on unscreened ages, minus outliers, is supported by statistically identical ages obtained from three sets of screened ages selected to minimize the effects of inbuilt age or contamination. Based on the intercepts method and 1 sigma range of Northern Hemisphere calibration curves, and after subtraction of 27 years for the interhemispheric offset, the radiocarbon age 665615 BP corresponds to calibrated dates ranging from c. 650–560 cal. BP (i.e. cal. ad 1300–1390). The approximate midpoint of this range provides a colloquial calen dar date for the Kaharoa Tephra of c. 600 cal. BP ( c. cal. ad 1350).

New K‐Ar age determinations indicate that the exposed portion of the Tongariro volcanic complex has grown steadily since at least 275 ka, with intervals of vigorous cone growth at 210–200, 130–70, and 25 ka to the present day.

White Island is an active andesitic-dacitic composite volcano surrounded by sea, yet isolated from sea water by chemically sealed zones that confine a long-lived acidic hydrothermal system, within a thick sequence of fine-grained volcaniclastic sediment and ash. The rise of at least 106 m3 of basic andesite magma to shallow levels and its interaction with the hydrothermal system resulted in the longest historical eruption sequence at White Island in 1976–1982. About 107 m3 of mixed lithic and juvenile ejecta was erupted, accompanied by collapse to form two coalescing maar-like craters. Vent position within the craters changed 5 times during the eruption, but the vents were repeatedly re-established along a line linking pre-1976 vents. The eruption sequence consisted of seven alternating phases of phreatomagmatic and Strombolian volcanism. Strombolian eruptions were preceded and followed by mildly explosive degassing and production of incandescent, blocky juvenile ash from the margins of the magma body. Phreatomagmatic phases contained two styles of activity: (a) near-continuous emission of gas and ash and (b) discrete explosions followed by prolonged quiescence. The near-continuous activity reculted from streaming of magmatic volatiles and phreatic steam through open conduits, frittering juvennile shards from the margins of the magma and eroding loose lithic particles from the unconsolidated wall rock. The larger discrete explosions produced ballistic block aprons, downwind lobes of fall tephra, and cohesive ‘wet’ surge deposits confined to the main crater. The key features of the larger explosions were their shallow focus, random occurrence and lack of precursors, and the thermal heterogeneity of the ejecta. This White Island eruption was unusual because of the low discharge rate of magma over an extended time period and because of the influence of a unique physical and hydrological setting. The low rate of magma rise led to very effective separation of magmatic volatiles and high fluxes of magmatic gas even during phreatic phases of the eruption. While true Strombolian phases did occur, more frequently the decoupled magmatic gas rose to interact with the conduit walls and hydrothermal system, producing phreatomagmatic eruptions. The form of these ‘wet’ explosions was governed by a delicate balance between erosion and collapse of the weak conduit walls. If the walls were relatively stable, fine ash was slowly eroded and erupted in weak, near-continous phreatomagmatic events. When the walls were unstable, wall collapse triggered larger discrete phreatomagmatic explosions.

Application of a newly developed theory to volcanological observations of explosive eruptions shows that previous estimates of pre-explosion gas pressures may be overestimated by an order of magnitude.

Basalt intruded into Tarawera Rift during the 1886 eruption occupies an en echelon pattern of dike-filled eruptive fissures. The leftstepping pattern of dilation fractures is consistent with dextral/normal movement on an underlying, older, master fault which appears to have influenced location of the earlier rhyolite eruptive vents of Tarawera Volcanic Complex, and controlled the orientation of deep basaltic intrusions. Although the Tarawera Rift was previously regarded as a pure normal fault, the 1886 dike pattern provides further geological evidence for the occurrence of dextral shear in the Taupo Volcanic Zone.

The analysis of the chronopotentiometric response of a region composed of a mixture of electronic conducting solid solution electrode and solid electrolyte, under the influence of a short galvanostatic pulse, is described. Such a technique may be used to characterise and compare the merit of a particular configuration of such a region. The application of a statistical model of a mixed-phase electrode is outlined, and the results obtained are discussed.

Recent eruptions of White Island volcano have shown an interesting cyclicity in the composition of ejecta during alternating strombolian and phreatomagmatic events. While strombolian eruptions elsewhere have been preceded by phreatomagmatic explosions, this form of cyclicity has not been noticed.

A 10 mm thick, c. 15 700 calendar yr BP (c. 13 100 14 C yr BP) rhyolitic tephra bed in the well-studied montane Kaipo Bog sequence of eastern North Island was previously correlated with Maroa-derived Puketarata Tephra. We revise this correlation to Okataina-derived Rotorua Tephra based on new compositional data from biotite phenocrysts and glass. The new correlation limits the known dispersal of Puketarata Tephra (sensu stricto, c. 16 800 cal yr BP) and eliminates requirements to either reassess its age or to invoke dual Puketarata eruptive events. Our data show that Rotorua Tephra comprises two glass-shard types: an early-erupted low-K2O type that was dispersed mostly to the northwest, and a high-K2O type dispersed mostly to the south and southeast, contemporary with late-stage lava extrusion. Late-stage Rotorua eruptives contain biotite that is enriched in FeO compared with biotite from Puketarata pyroclastics. The occurrence of Rotorua Tephra in Kaipo Bog (100 km from the source) substantially extends its known distribution to the southeast. Our analyses demonstrate that unrecognised syn-eruption compositional and dispersal changes can cause errors in fingerprinting tephra deposits. However, the compositional complexity, once recognised, provides additional fingerprinting criteria, and also documents magmatic and dispersal processes.

The Taupo volcanic zone (TVZ) has been active since 2 Ma and has erupted >104 km3 of dominantly rhyolitic magma during the last 1 m.y. Most of the volcanism is concentrated in a 125×60 km area forming the central TVZ and is expressed largely as six major caldera volcanoes, Rotorua, Okataina, Kapenga, Mangakino, Maroa, and Taupo, marked by localized collapse of the underlying basement and clustering of known or inferred vent sites. These centers have activity spans from 150 to 600 ka and have each erupted at least 300 to 1000 km3 of magma. All centers except Rotorua are known or inferred to have had complex histories of multiple caldera collapse, which have occurred alongside general basement collapse within the TVZ accompanying regional extension. Deep-seated NE trending basement lineations and/or faults have influenced vent sites at Okataina, Maroa, and Taupo. Welded ignimbrites are prominent in the pre-140 ka record; their absence since then is attributed to the effects of surface water on eruption styles rather than to a change in eruptive behavior. Volcanism from the centers has been overwhelmingly rhyolitic (>97% SiO2 69–77 wt%) with minor high-A1 basalt and dacite and traces of andesite, mostly as lithic fragments in ignimbrites from Okataina and Mangakino. Although insignificant in volume, the basalt is important as a low-Si end-member in mixing relationships with the rhyolite (at one extreme generating the dacites) and occasionally as a trigger for the rhyolitic eruptions. The current average rhyolite magma eruption rate from the central TVZ is ∼0.27 m3 s−1, equally divided between Okataina and Taupo, a figure close to the long-term average for the last 1.1 Ma. However, geothermal heat flow data imply that a further 1.4–1.8 m3 s−1 of magma may be intruded within the crust. The ratio of inferred intruded material to erupted material is higher at centers where lava extrusions are volumetrically significant (Okataina, Maroa), and this is correlated with lower phenocryst equilibration temperatures in the eruptives. Evidence for resurgent doming and long-term (>105 years) magma cycles documented at similarly sized rhyolitic calderas in the western United States is absent from the TVZ centers; this is attributed to the young faulted crust of the region, preventing the formation of sufficiently large high-level magma chambers. In overall terms, the central TVZ is comparable in size and longevity to the Yellowstone system, but its individual eruptions have very much shorter recurrence intervals and smaller volumes.