Taupo Volcanic Centre Research Articles

Shallow subaqueous to emergent intra-caldera silicic volcanism of the Motuoapa Peninsula, Taupo Volcanic Zone, New Zealand – New constraints from geologic mapping, sedimentology and zircon geochronology

Motuoapa Peninsula, located in the southeast of the Taupo Volcanic Centre, New Zealand, is dominated by a silicic pyroclastic cone and overlaying lavas. The pyroclastic succession has not been recognised and studied before, and its thickness and sedimentological characteristics indicate completely different eruption mechanisms than proposed for the other pyroclastic successions within the central Taupo Volcanic Zone. Here, we present the results of field mapping and sedimentological characterisation of accessible pyroclastic deposits, and complement these data with combined U-Th and (U-Th)/He zircon geochronology providing first constraints on the succession's crystallization and eruption history.(U-Th)/He zircon eruption ages of 77.2 ± 6.3, 81.3 ± 9.2 and 34.5 ± 3.1 ka indicate that volcanic activity in the Motuoapa Peninsula occurred in two distinct eruptive episodes that were separated by ca. 45 kyrs. The earlier rhyolitic eruption at ca. 80 ka is inferred to have commenced in a shallow subaqueous environment. Its lowermost succession includes breccias and tuff breccias sourced from an extruding lava dome by autobrecciation, quench-fragmentation and localised debris flows. With gradual emergence of the growing volcanic pile, explosive hydrovolcanic activity became dominant, constructing an emergent cone by pyroclastic density currents and fall-out. The lack of exotic/accidental clasts, along with an abundance of low-vesicularity rhyolitic juvenile fragments, suggests fragmentation driven by magma-water interaction, which predominantly occurred at shallow depths within the outgassed part of the ascending magma. The frequency and thickness of ash-dominated units increases upwards, suggesting a gradual increase in explosive energy of tephra jets. The final phase of the rhyolitic activity was dominated by emplacement of viscous lava that breached the crater rim and flowed onto the SE sector of the pyroclastic cone. The remnant of the Motuoapa pyroclastic cone, along with the bedded structure of deposits that comprise fallout and surge-dominated units, appears very similar to Surtseyan tuff cones and silicic tuff/pumice cones described elsewhere. A dacitic eruption that produced a nearby lava dome at ca. 35 ka, represents a significantly younger event that occurred after substantial erosion of the earlier pyroclastic cone. The Motuoapa Peninsula deposits most likely record the evolution of a subaqueous silicic eruption, where hydrovolcanism played a fundamental role on subaerial pyroclastic cone formation in a terrestrial environment with abundant surface water availability. The similarities between the environment of the Taupo area today and the area during the Motuoapa activity at ca. 80 ka may provide an analogue model for future subaqueous eruptions in the region.

Magma residence and eruption at the Taupo Volcanic Center (Taupo Volcanic Zone, New Zealand): insights from rhyolite-MELTS geobarometry, diffusion chronometry, and crystal textures

The Taupo Volcanic Center [TVC; Taupo Volcanic Zone (TVZ), New Zealand] is home to the two most recent silicic caldera-forming eruptions in the TVZ—the giant (530 km3 DRE) Oruanui supereruption and the large (35 km3 DRE) Taupo eruption. This offers a unique opportunity to investigate similarities and differences between large and supersized volumes of rhyolitic magma at the TVC and more generally. We combine information from crystal textures (crystal size distributions), crystal compositions (Ti-in-quartz diffusion chronometry), and glass compositions (rhyolite-MELTS geobarometry) to investigate the crustal storage conditions, longevity, and eruption of these magmas. Results show differences between the storage depths of the Oruanui and Taupo magmas consistent with the differences in their glass silica contents but strikingly similar, short crustal residence times for the final crystal-poor, eruptible magma bodies (decades to centuries). Differences in groundmass textures between the two systems also likely reflect differences in ascent durations, decompression paths, and/or amounts of undercooling experienced by these systems due to the differences in their storage depths. Finally, comparing the evolution of storage depths and magma compositions at the TVC with those in a preceding central TVZ ignimbrite ‘flare-up’ supports the idea that the modern TVC may be flaring up and highlights the evolution of magma storage depths with increasing system maturity in this volcanic region.

Analyzing nitrogen in natural and synthetic silicate glasses by secondary ion mass spectrometry

Volatile releases through volcanic eruptions are one of the major processes contributing to the global nitrogen cycle. Past studies have often estimated the magnitude of this flux using volcanic emission measurements, which are limited to currently active systems and sensitive to atmospheric contamination. Another possible approach is the measurement of nitrogen in melt inclusions, which are parcels of magmatic melt trapped prior to eruption. This methodology requires appropriate analytical parameters for nitrogen analysis in silicate glasses by secondary ion mass spectrometry (SIMS), which have not yet been established. To this end, calibrations for nitrogen were obtained using ion implanted basaltic and rhyolitic glasses. We demonstrate that water content significantly affects the ion yields of 14N+ and 14N16O−, as well as the background intensity of 14N+ and 12C+. Application of implant-derived calibrations to natural samples provide the first reported concentrations of nitrogen in melt inclusions. These measurements were made on samples from the Bishop Tuff in California, the Huckleberry Ridge Tuff of the Yellowstone Volcanic Center, and material from the Okaia and Oruanui eruptions in the Taupo Volcanic Center. In studied material, we often find maximum nitrogen contents of less than 45ppm and that nitrogen concentration varies positively with CO2 concentration, which reflects a partial degassing trend. Using the maximum measured nitrogen contents for each eruption, we find that the Bishop released >3.4×1013, the Huckleberry Ridge released >1.4×1014, the Okaia released >1.0×1011, and the Oruanui released >4.5×1013g of nitrogen. Simple calculations suggest that with concentrations such as these, rhyolitic eruptions may ephemerally increase the nitrogen flux to the atmosphere, but are insignificant compared to the 4×1021g of nitrogen stored in the atmosphere.

A catalogue of caldera unrest at Taupo Volcanic Centre, New Zealand, using the Volcanic Unrest Index (VUI)

Caldera unrest occurs frequently at Taupo Volcanic Centre (TVC), New Zealand, occasionally resulting in deleterious socio-economic impacts. This large silicic volcano most recently erupted in 232 AD in an explosive, caldera-forming rhyolitic eruption, devastating the central North Island. Eruptions are preceded by volcanic unrest, often consisting of seismicity, deformation, degassing, and/or geothermal system changes. These phenomena may also occur due to non-magmatic processes, complicating eruption forecasting. As volcanic unrest may or may not lead to an eruption, it needs to be characterised to provide effective warnings; this is best achieved by understanding past unrest. In this research, a catalogue of caldera unrest at TVC is developed using an historical chronology methodology, spanning from 1872 to December 2011. The Volcanic Unrest Index (VUI), which is introduced by Potter et al. (2015), is estimated for the catalogue, demonstrating its use and providing a characterisation of unrest at TVC. Sixteen episodes of unrest are identified; 4 are classified as moderate unrest (VUI 3), and 12 are classified as minor unrest (VUI 2). There has been median interval of approximately 3 years between unrest episodes and a median unrest episode duration of just under 5 months. This research provides context for future caldera unrest crises at TVC and contributes to the global caldera unrest dataset.

Introducing the Volcanic Unrest Index (VUI): a tool to quantify and communicate the intensity of volcanic unrest

Accurately observing and interpreting volcanic unrest phenomena contributes towards better forecasting of volcanic eruptions, thus potentially saving lives. Volcanic unrest is recorded by volcano observatories and may include seismic, geodetic, degassing and/or geothermal phenomena. The multivariate datasets are often complex and can contain a large amount of data in a variety of formats. Low levels of unrest are frequently recorded, causing the distinction between background activity and unrest to be blurred, despite the widespread usage of these terms in unrest literature (including probabilistic eruption-forecasting models) and in Volcanic Alert Level (VAL) systems. Frequencies and intensities of unrest episodes are not easily comparable over time or between volcanoes. Complex unrest information is difficult to communicate simply to civil defence personnel and other non-scientists. The Volcanic Unrest Index (VUI) is presented here to address these issues. The purpose of the VUI is to provide a semi-quantitative rating of unrest intensity relative to each volcano’s past level of unrest and to that of analogous volcanoes. The VUI is calculated using a worksheet of observed phenomena. Ranges for each phenomenon within the worksheet can be customised for individual volcanoes, as demonstrated in the companion paper for Taupo Volcanic Centre, New Zealand (Potter et al. 2015). The VUI can be determined retrospectively for historical episodes of unrest based on qualitative observations, as well as for recent episodes with state-of-the-art monitoring. This enables a long time series of unrest occurrence and intensity to be constructed and easily communicated to end users. The VUI can also assist with VAL decision-making. We present and discuss two approaches to the concept of unrest.

Evolution of the Taupo Volcanic Center, New Zealand: petrological and thermal constraints from the Omega dacite

The 20 ka ~0.1 km3 Omega dacite, which erupted shortly after the 26.5 ka Oruanui super-eruption, compositionally stands out among Taupo Volcanic Zone (TVZ) magmas, which are overwhelmingly characterized by rhyolites (>90 % by volume). The previously reported presence of inherited zircons in this zircon-undersaturated magma has provided unequivocal evidence for the involvement of upper-crustal material in a 1–10 year timescale prior to the Omega eruption. However, whether this crustal involvement is characterized by wholesale, melting of preexisting crust or subordinate bulk assimilation into an already differentiated magma body remains unclear. To disentangle these processes, we describe the mineral chemistry of the major phases present in the Omega dacite and determine intensive parameters describing magma chamber conditions. Dominantly unimodal populations of plagioclase (An50–60), orthopyroxene (Mg# from 58 to 68), and clinopyroxene (Mg# from 65 to 73), along with coexisting equilibrium pairs of Fe–Ti oxides, constrain pre-eruptive temperatures to 850–950 °C, a pressure between ~3 and 7 kbars, and an oxygen fugacity of ~NNO. MELTS thermodynamic modeling suggests that this phase assemblage is in equilibrium with the bulk rock and glass compositions of the Omega dacite at these estimated P–T–fO2 pre-eruptive conditions. Combining these petrological observations with insights into conductive thermal models of magma–crust interactions, we argue that the Omega dacite more likely formed in the mid-to-lower crust via protracted processing through fractional crystallization coupled with some assimilation (AFC). Incorporation of crustal material is likely to have occurred at various stages, with the inherited zircons (and potentially parts of glomerocrysts) representing late and subordinate upper-crustal assimilants. This petrogenetic model is consistent with the presence of a differentiating crustal column, consisting of a polybaric fractional crystallization and assimilation history. On the basis of petrological, thermal, and geophysical considerations, upper-crustal reservoirs, which feed large-scale rhyolitic volcanism in the TVZ, most likely take the form of large, long-lived crystal mush zones. Following large eruptions, such as the Oruanui event, this mush is expected to crystallize significantly (up to 70–80 vol% crystals) due to syn-eruptive decompression. Hence, the Omega dacite, immediately post-dating the Oruanui event, potentially represents incoming deeper recharge of less-evolved magma that was able to penetrate the nearly solidified upper-crustal mush. Over the past 20,000 years, similar intermediate recharge magmas have incrementally reheated, reconstructed, and reactivated the upper-crustal mush zone, allowing a gradual return to rhyolitic volcanism at the Taupo Volcanic Center.

Tephra beds in deep-sea cores off northern New Zealand: implications for the history of Taupo Volcanic Zone, Mayor Island and White Island volcanoes

Twenty piston cores were collected from water depths of ∼ 650–3300 m from offshore Bay of Plenty, North Island, New Zealand. They contain tephra from 11 Okataina Volcanic Centre and 4 Taupo Volcanic Centre rhyolite eruptions during the last ∼ 50 kyr that produce a tephrostratigraphic framework across the tectonically and volcanically complex region of southern Havre Trough, Alderman Trough and the Bay of Plenty continental shelf. This allows correlation between offshore and onshore sequences up to 200 km from source volcanoes, covering much of Marine Isotope Stages 3 and 2. The framework temporally constrains poorly dated and newly recognised volcanic events. Macroscopic tephra layers from the peralkaline Mayor Island volcano are documented for the first time at pre-50, post-50, 40.5, 37.4, 22.2, and 14.2 ka, in addition to the well known 7 ka (Tuhua) event. These macroscopic layers represent some of the most explosive events from this volcano, and provide new marker horizons. They are dispersed up to 90 km north-east to east of the edifice. Minor tephra dispersal is also recorded at 17.8, 25.5, ∼ 35 and pre-50 ka. The tephra contain high SiO 2 (73–75.5 wt.%) glass and subordinate basaltic components, and each tephra represents a distinct magma batch that can be fingerprinted. The Mayor Island tephra form two temporal trends toward less evolved magma compositions, punctuated by a large caldera-forming event at 36 ka. The historically active, andesitic White Island volcano does not have widely dispersed tephra, and the oldest primary deposit found is ∼ 21 ka. Five pre-50 ka rhyolite eruptions from an unknown Taupo Volcanic Zone source provide evidence for explosive activity in a time interval poorly documented on-land. The cores demonstrate the patchy and uneven preservation of large magnitude tephra falls caused by local faulting, bioturbation and ponding in bathymetrically complex regions. Reworked tephra layers are common and often lack indicative lithological features. Such units could easily be misinterpreted as primary events without micro-beam geochemical analyses of glass shards.

A continuous 5300-yr Holocene cryptotephrostratigraphic record from northern New Zealand and implications for tephrochronology and volcanic hazard assessment

A continuous ∼5280 calendar (cal.) yr long cryptotephrostratigraphic record of a peat core from northern New Zealand demonstrates that cryptotephra studies can enhance conventional tephra records by extending the known distribution of ash fall and enabling re-assessment of volcanic hazards. A systematic sampling strategy was used to locate peaks in glass-shard concentrations and to determine loci of individual geochemical populations, and a palynological method involving spiking samples with Lycopodium spores was adapted to facilitate accurate counting of glass-shard concentrations. Using glass shard major element compositions, and a core chronology based on eight AMS 14C ages and two visible macroscopic tephra layers, Taupo Tephra (Unit Y) (1688-1748 cal. BP) and Tuhua Tephra (6800-7230 cal. BP) (2cr-age ranges), four cryptotephras were correlated with known eruptions: Whakaipo (Unit V) (2743-2782 cal. BP), Stent (Unit Q) (4240-4510 cal. BP), and Unit K (4970-5290 cal. BP), erupted from Taupo Volcanic Centre, and Whakatane Tephra (5470-5600 cal. BP) erupted from Okataina Volcanic Centre. Mixed glass populations were found in the core, most likely an artefact of post-depositional remobilization of shards vertically (both up and down) in the peat or on its surface by wind, or a result of closely spaced eruption events, or a combination of these. A secondary glass population identified within the macroscopic Taupo Tephra was tentatively attributed to either an earlier phase within that eruption or to mixing with a slightly older Taupo-derived eruptive or (less likely) a currently unknown Okataina-derived eruptive. These results indicate that, in the absence of continuous cryptotephrostratigraphic analysis, a peak in shard concentrations may not in itself be indicative of the ‘true’ stratigraphic (ie, isochronous) level of a tephra layer. For cryptotephra studies of peat cores, we recommend (1) using a detailed sampling strategy for the analysis of distal tephra-derived glass to detect and account for any mixed populations and possible vertical spread of glass shards through the peat, and (2) analysing more shards from larger samples to help ‘capture’ sparsely represented cryptic andesitic tephra deposits.

Trends in rhyolite geochemistry, mineralogy, and magma storage during the last 50 kyr at Okataina and Taupo volcanic centres, Taupo Volcanic Zone, New Zealand

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.

Fluvial responses to volcanism: resedimentation of the 1800a Taupo ignimbrite eruption in the Rangitaiki River catchment, North Island, New Zealand

The potential for the generation of dangerous and damaging lahars and floods in response to the eruption of voluminous pyroclastic debris has become increasingly appreciated in recent years. The style and tempo of this response varies both between eruptions and between individual catchments impacted by a single eruption, so that an understanding of the factors controlling this variation is necessary for precise hazard assessment. The 1800a Taupo eruption from the Taupo Volcanic Centre in the central North Island of New Zealand devastated an area of 20,000 km 2 during eruption of a climactic ignimbrite, impacting the headwaters of all major rivers draining radially from this region. The Rangitaiki River, the subject of this paper, differs from other catchments in that the Taupo ignimbrite buried an essentially flat land surface inherited from a suite of welded ignimbrite sheets erupted between 320–340 and 230 ka. The middle reaches of the catchment are characterised by narrow, steep gorges alternating with low-gradient basins developed in tectonic half-grabens. Initially, remobilisation of pyroclastic material in the headwaters was dominated by hyperconcentrated sheet flows resulting in shallow reworking. In higher gradient areas, reintegration of drainage networks was achieved by incision of deep channels and gullies, assisted by breakouts from ephemeral lakelets developed in ignimbrite-dammed depressions. Braided, and later meandering, streams superseded this pattern as rill and gully systems stabilised and sediment yields fell leading to a decline in drainage density. Gorge reaches acted as efficient conduits for remobilised material while the basins acted as local depocentres for the temporary storage of volcaniclastic sediments, mediating the transfer of pyroclastic debris to the Bay of Plenty coast >100 km to the north. Reworking and resedimentation of pyroclastic debris began immediately after the eruption, peaking early and then rapidly declining so that most geomorphic adjustments occurred within one or two decades of the initial disturbance.

Distal record of multi-sourced tephra in Onepoto Basin, Auckland, New Zealand: implications for volcanic chronology, frequency and hazards

We have documented 80 tephra beds dating from ca. 9.5 to >50 ka, contained within continuously deposited palaeolake sediments from Onepoto Basin, a volcanic explosion crater in Auckland, New Zealand. The known sources for distal (>190 km from vent) tephra include the rhyolitic Taupo Volcanic Centre (4) and Okataina Volcanic Centre (14), and the andesitic Taranaki volcano (40) and Tongariro Volcanic Centre (3). The record provides evidence for four new events between ca. 50 and 28 ka (Mangaone Subgroup) suggesting Okataina was more active than previously known. The tephra record also greatly extends the known northern dispersal of other Mangaone Subgroup tephra. Ten rhyolitic tephra pre-date the Rotoehu eruption (>ca. 50 ka), and some are chemically dissimilar to post-50 ka rhyolites. Some of these older tephra were produced by large-magnitude events; however, their source remains uncertain. Eight tephra from the local basaltic Auckland Volcanic Field (AVF) are also identified. Interpolation of sedimentation rates allow us to estimate the timing of 12 major explosive eruptions from Taranaki volcano in the 27.5–9.5-ka period. In addition, 28 older events are recognised. The tephra are trachytic to rhyolitic in composition. All have high K2O contents (>3 wt%), and there are no temporal trends. This contrasts with the proximal lava record that shows a trend of increasing K2O with time. By combining the Onepoto tephra record with that of the previously documented Pukaki crater, 15 AVF basaltic fall events are constrained at: 34.6, 30.9, 29.6, 29.6, 25.7, 25.2, 24.2, 23.8, 19.4, 19.4, 15.8 and 14.5 ka, and three pre-50 ka events. This provides some of the best age constraints for the AVF, and the only reliable data for hazard recurrence calculations. The minimum event frequency of both distal and local fall events can be estimated, and demonstrates the Auckland City region is frequently impacted by ash fall from many volcanoes.

Vertical deformation at Lake Taupo, New Zealand, from lake levelling surveys, 1979–99

Lake Taupo, occupying much of the Taupo Volcanic Centre and spanning the Taupo Fault Belt, has routinely been monitored for vertical deformation since 1979 using a portable lake levelling survey technique. Regular monitoring continues to the present time. This technique is used at a number of fixed points around the lake to efficiently determine their height changes. Long‐term trends and relatively short‐term departures from the trends have been observed, although only one short‐term event, in 1983, was confidently correlated with seismicity. That event provided clear examples of pre‐, co‐, and post‐seismic deformation, resulting in vertical displacements exceeding 60 mm. Distinct styles of deformation have been identified, both in time and space. Four episodes of deformation have occurred during the study period, each one terminated by an abrupt transition to the next style. A clear difference in style can also be seen between the Taupo Fault Belt and the adjoining Taupo Volcanic Centre; sustained subsidence rates recorded within the Taupo Fault Belt initially exceeded 11 mm/yr following the 1983 deformation and seismic event but decreased steadily until virtually ceasing by early 1999. The subsidence zone had a half‐width of 12 km and represented the southern end of an 80 km long trough extending northeastward along the axis of the fault belt. Within the Taupo Volcanic Centre, the long‐term trends were far less distinct until mid 1996, when uplift of the central portion commenced abruptly, averaging c. 11 mm/yr before ceasing 3 years later.

Environmental change during the last glacial maximum (c. 25 000‐c. 16 500 years BP) at Mt Richmond, Auckland Isthmus, New Zealand

A 2 m section at the base of Mt Richmond contains a palynological record of the last glacial maximum (LGM) (c. 25 000‐c. 16 500 14C years BP) vegetation of the Auckland Isthmus. Three silicic tephra layers derived from the Taupo Volcanic Centre (Okaia Tephra c. 23 500 14C years BP, Kawakawa Tephra c. 22 500 14C years BP) and the Okataina Volcanic Centre (Okareka Tephra c. 18 000 14C years BP), both centres lying within the Taupo Volcanic Zone, provide the basis of the chronology supported by radiocarbon dates. The pollen diagram is divided into two pollen zones separated by the deposition of a locally derived basaltic ash. From c. 25 000–23 000 14C years BP the site was initially a eutrophic lake fringed by Leptospermum and Typha, which altered to a Cyperaceae/Leptospermum‐dominated swamp. The regional vegetation at the time was beech‐dominated forest; canopy conifers were present but formed a minor part of the local forest. From c. 23 000–16 500 14C years BP regional forest was further restricted to local patches in extensive shrubland/grassland. Temperatures may have been depressed by more than 4–5°C. The eruption of a local volcano dammed the swamp outlet resulting in a return to lacustrine conditions. Local volcanism may have accelerated vegetation change already under way as a result of climate change to cooler conditions. This record provides a view of LGM vegetation of the Auckland Isthmus and a template for earlier cold stages of the Quaternary.

Environmental response to a large, explosive rhyolite eruption: sedimentology of post-1.8 ka pumice-rich Taupo volcaniclastics in the Hawke's Bay region, New Zealand

The 1.8 ka eruption of the Taupo ignimbrite from the Taupo Volcanic Centre in the central North Island of New Zealand produced a ∼30 km 3 ignimbrite covering an area ca. 160 km in diameter, centred on Lake Taupo. Remobilisation of its pyroclastic detritus triggered major alluviation in the years to decades after its emplacement. In the Hawke's Bay region in the eastern North Island, three large rivers rise in the ignimbrite-impacted mountainous headwaters, in areas that were buried by up to 40 m of loose, unconsolidated pumiceous material. Analysis of preserved volcaniclastic sediments along valleys shows that resedimentation occurred in two main stages characterised by contrasting sediment:water ratios. The first comprised erosion and redeposition of ignimbrite debris by lahars, e.g. debris flows and hyperconcentrated flows. The second was marked by decreasing sediment:water ratios and the re-establishment of braided fluvial systems in the former river valleys, as recorded by such deposits as trough, planar, low-angle and antidune cross-bedded, planar-bedded, and ripple cross-laminated sands and gravels. However, there is no systematic chronological trend towards lower pumice contents in these fluvial facies. Four characteristic sets of co-occurring lithofacies were produced during the sedimentary response: (1) Laharic remobilisation association; (2) Upper-flow regime-dominated stream-flow association; (3) Lower-flow regime-dominated stream-flow association; and (4) Lacustrine association. Individual lithofacies were strongly controlled by the unusual hydrodynamic properties of pumice, particularly its low and variable density. Automated settling tube analysis of water-saturated pumiceous sediment confirms that sieve analysis is misleading with respect to the hydrodynamic behaviour of such sediments. Recasting granulometric data in terms of effective hydraulic grain size shows better sorting of mixed component sediments, but statistical sorting and mean grain-size parameters do not discriminate between the deposits of tractional and overcapacity parent flows. Therefore, particle behaviour is addressed by using both lithofacies and petrofacies in the description and classification of the remobilisation deposits.

Measurement and implication of "effective" viscosity for rhyolite flow emplacement

Hazardous explosive activity may sporadically accompany the extrusion of silicic lava domes. Model- ling of the emplacement of silicic domes is therefore an important task for volcanic hazard assessment. Such modelling has been hampered by a lack of a sufficiently accurate rheological database for silicic lavas with crys- tals and vesicles. In the present study, the parallel-plate viscometry method was applied to determine the shear viscosity of five natural rhyolitic samples from a vertical section through the Ben Lomond lava dome, Taupo Volcanic Centre, New Zealand. Rheological measure- ments were performed at volcanologically relevant tem- peratures (780-950°C) and strain rates (10 -5 -10 -7 s -1 ). Although these samples are in the metastable state, vis- cosity determinations, melt composition, as well as water and crystal contents of samples were demonstrably sta- ble during experiments. For samples containing up to 5 vol.% microlites, the composition of the melt, rather than the physical effect of suspended crystals, had great- er influence on the effective viscosity of the silicic mag- ma. Samples with 10 vol.% microlites and containing a flow banding defined by microlites show no significant orientational effects on apparent viscosity. The rheologi- cal measurements were used together with a simple cool- ing model to construct thermal and viscosity profiles re- vealing conditions during the emplacement of the Ben Lomond lava dome.

Stratigraphy and dispersal of the Mangaone Subgroup pyroclastic deposits, Okataina Volcanic Centre, New Zealand

The Mangaone Subgroup from the Okataina Volcanic Centre consists of silicic plinian pyroclastic units that lie between the Oruanui deposit from the Taupo Volcanic Centre (corrected 14C calendar age=26.5ka) and the Rotoiti (flow)/Rotoehu (fall) deposits from Okataina Volcanic Centre (K–Ar age=64ka). In this study we present a new stratigraphy for the subgroup that is significantly revised from the earlier work and was developed after extensive field mapping and laboratory study. We now recognize and describe a total of 12 rhyolitic units (as opposed to the 8 that had been recognized previously), with a total bulk eruptive volume of approximately 81km3. The vents for all the Mangaone Subgroup units were within the Haroharo caldera and possibly migrated through time, first within a roughly north–south trending zone in the central part of the caldera and then toward the eastern part of the caldera. We present new radiometric ages suggesting that the whole sequence was erupted in a relatively short time span (possibly 10–15ka). Our findings of more eruptions in a shorter time span than in previous work underscores the utility of detailed field studies, particularly when volcanic hazards are being assessed. All the fall units have plinian dispersals but show various degrees of magma-water interaction. The majority of the eruptions were large and intense enough to generate pyroclastic flows, yet flows were observed in only 2 units and is volumetrically small.

Late Quaternary evolution of a hyperactive rhyolite magmatic system: Taupo volcanic centre, New Zealand

Taupo volcanic centre has been active for c. 300 000 years. Since the c. 26.5 ka caldera‐forming Oruanui eruption 28 eruptions have occurred from Taupo, varying between 0.1 and >45 km 3 in minimum bulk volume, and with repose periods ranging from c. 20 to 6000 years. All magma erupted post‐26.5 ka is compositionally and mineralogically distinct from pre‐Oruanui and Oruanui eruptives, and is inferred to have formed at or after 26.5 ka. Four post‐Oruanui magma types are identified on the basis of whole rock and mineral compositions: one dacitic, forming three eruptions between 20.5 ka and 17 ka, and three subtly distinct rhyolite compositions erupted in discrete periods from 11.8 to 9.95, 7.05 to 2.75 and 2.15 to 1.74 ka. Stepwise compositional variations between, and limited variation within, rhyolite groups suggest emplacement of three petrogenetically separate batches of magma within only 10 000 years. The 15–35 km 3 of magma erupted at 1.77 ka evidently appeared in <10 3 years; this short residence time may have contributed to the lack of zonation within this magma chamber. Taupo is unusual amongst large rhyolite volcanoes in terms of the high frequency of activity since 26.5 ka, rapid stepwise changes in rhyolite compositions, and insignificant differentiation within individual subgroups. These traits are attributed to the combined effects of the extensional arc setting, thermal energy from mafic magma, and the shallow slope of the plagioclase‐saturated rhyolite liquidus.

The ∼10 ka multiple vent pyroclastic eruption sequence at Tongariro Volcanic Centre, Taupo Volcanic Zone, New Zealand:: Part 1. Eruptive processes during regional extension

A sequence of six major (each ∼1 km 3) and several minor pyroclastic eruption episodes occurred in the Tongariro Volcanic Centre (TgVC) during a period of uniquely intense activity at ∼10 ka ( 14 C years B.P.). This andesitic/dacitic sequence, named Pahoka–Mangamate (PM), had an apparent duration of ∼200 to 400 years. Rhyolite tephras erupted from Taupo are interbedded within the PM deposits. Our mapping of PM ejecta distribution has shown that multiple vents between Tongariro and Ruapehu volcanoes were active during each of the ∼10 ka episodes. The PM vents are located on a 20-km long NNE-trending linear vent zone, lying within a graben structure defined by regional fault zones. Some of these faults appear to have ruptured during the ∼10 ka eruptive sequence. The pattern of fault and vent zones suggests that an episode of accelerated extension (rifting) occurred across the TgVC at 10 ka, marked by normal displacements of the graben margin faults, and by dike intrusion beneath the vent zone on the graben axis. Juvenile ejecta from each of the PM eruptions is chemically and petrographically diverse, indicating that a number of separate magma bodies were tapped during the ∼10 ka eruption sequence (see companion paper). Despite this compositional diversity, each of the PM pyroclastic deposits is characterised by poorly vesicular juvenile lapilli with fracture-bounded surfaces, indicating interaction at depth between rapidly rising magmas and abundant groundwater. Widespread (plinian) dispersal of the PM tephras demonstrates that relatively high eruption columns were produced, despite this water-cooling of the erupting magmas. The PM eruptions and associated faulting were triggered by rifting processes. The coincident eruption of voluminous rhyolite tephras from a linear multiple vent zone in the Taupo Volcanic Centre, 50 km to NE of Tongariro, demonstrates that the 10 ka extensional episode was of regional extent.

Gravity, magnetic and seismic surveys of the caldera complex, Lake Taupo, North Island, New Zealand

The Taupo volcanic centre lies at the southern end of the Taupo Volcanic Zone in the central North Island of New Zealand. Major rhyolitic eruptions of 400 and 35 km 3 (dense rock equivalent) occurred from this centre 26.5 and 1.8 thousand years ago, respectively. As a result of these eruptions much of the volcanic centre has collapsed and now lies beneath the waters of Lake Taupo. Gravity data measured on Lake Taupo show that the most intense (negative) Bouguer anomaly to occur anywhere within the Taupo Volcanic Zone lies over the northern part of the lake. This anomaly is interpreted to outline the area of caldera collapse caused by the 26.5 ka eruption which has been subsequently modified by the 1.8 ka eruption and filled by approximately 3 km of low-density, uncompacted, volcaniclastic material. A thick laminated sequence of seismic reflectors forming a distinct unit in the seismic sections can be recognized over most of the lake area and lies beneath material interpreted to have been erupted in the 1.8 ka eruption. This sequence of reflectors is interpreted to be the result of the lacustrine deposition of unconsolidated material immediately following the 26.5 ka eruption. A prominent magnetic anomaly within the western part of the collapse structure is interpreted to be caused by a body of more highly magnetised material lying at or near the base of the low density caldera fill. The laminated sequence of reflectors tilts and thickens towards this magnetic anomaly suggesting that the magnetised material occurs in the deepest, part of the 26.5 ka collapse structure. The southern margin of the intense low-gravity anomaly is marked by a zone of faulting observed in the seismic reflection data. Magnetic and seismic data in the southern part of the lake are interpreted as a welded ignimbrite sheet overlain in places by rhyolite lavas similar to the pre-26.5 ka surface exposed west of the lake. This surface appears to have been down-faulted by approximately 500 m and is covered by more recent low-density volcaniclastic material. The thickness of the volcaniclastic material increases northward as the pre-26.5 ka surface is tilted downwards towards the area of major collapse. The northward tilt of this surface suggests that subsidence in the southern part of the lake may also be a response to the 26.5 ka eruption.

Lithic types in ignimbrites as a guide to the evolution of a caldera complex, Taupo volcanic centre, New Zealand

Taupo volcanic centre is one of two active rhyolite centres in the Taupo Volcanic Zone (TVZ), and has been sporadically active over the past ca. 300 ka. At least four large-scale ignimbrites have erupted from the centre, including the well documented 26.5 ka Oruanui ignimbrite and 1.8 ka Taupo ignimbrite. Because stratigraphy of earlier ignimbrites and their sources are masked by later volcanism, disrupted by regional tectonics and obscured by poor exposure, indirect methods must be applied in order to determine their source regions. In this paper detailed componentry, density and petrology of lithic fragments from three ignimbrites (Rangatira Point, Oruanui, Taupo) are used to reveal aspects of the sub-Taupo caldera geology, including the evolution of the Taupo volcanic centre, to assist in ignimbrite correlation and to evaluate structures within the Taupo caldera complex. Lithic fragments identify a complex subsurface geology. The Rangatira Point ignimbrite sampled dominantly rhyolite lavas, plus a variety of welded ignimbrites, rare high-silica dacites and a single dolerite. Most lithic fragments in the Oruanui ignimbrite are andesite with minor rhyolite, welded ignimbrite, dacite and rounded greywacke, while in the Taupo ignimbrite, rhyolite is again the dominant lithic component with subordinate welded ignimbrites, andesite, and greywacke. The densities of lithic fragments indicate similar ranges of values for all lava types, and thus density is a poor indicator of lithology. Care must, therefore, be taken before interpreting subcrustal stratigraphy using density as the sole criterion. The petrography and geochemistry of lithic types are more specific, and the variation can be used to identify sources for the ignimbrites. Both pumice chemistry and rhyolite lithic fragments from the Rangatira Point ignimbrite are comparable to domes exposed at the southern end of the Western Dome Complex and, combined with limited outcrop information, suggest the most likely source for this unit is in the northern part of the Taupo caldera complex. The dominance of andesite lithic fragments in the Oruanui ignimbrite suggests a major andesite cone existed beneath the source area, and the different lithic suites between Oruanui and Taupo ignimbrites suggest these ignimbrites came, at least in part, from mutually exclusive collapse structures. We believe that the Oruanui caldera is sited principally in the northwestern part of present-day Lake Taupo and the Taupo caldera in the northeastern part. Identification of abundant ignimbrite lithics in the Taupo ignimbrite, which are considered to represent an intracaldera facies of an earlier ignimbrite, that is not exposed at the surface, suggest there was a further (pre-Oruanui) ignimbrite caldera in the Taupo ignimbrite eruptive vent region.