Auckland Volcanic Field Research Articles

Time scales of olivine storage and transport as revealed by diffusion chronometry at Waitomokia Volcanic Complex, Auckland Volcanic Field, New Zealand

Detailed knowledge of the pre-eruptive time scales associated with magma storage and transport is vital to improve volcanic hazard forecasting in active volcanic regions. However, quantification of the timescales of volcanic processes at mafic volcanic centres in continental intraplate settings is challenging, despite them being a source of significant hazards for human populations and infrastructure due to their limited predictability in space and time. We conducted a detailed petrological study to investigate the time scales of olivine storage and transfer throughout the eruption sequence of Waitomokia Volcanic Complex, a tuff ring and scoria cone complex in the Auckland Volcanic Field. Olivine crystal textures and compositions were determined from stratigraphically-constrained samples of the volcanic complex, from the initial phreatomagmatic phase to the final magmatic phase. Olivine crystals are typically <300 μm in length and characterised by skeletal morphologies, displaying chemical zoning in forsterite (Fo = 100*Mg/[Mg + Fe]; mol%), CaO, MnO and NiO wt% contents. We classified olivine into three major groups based on their Fo core compositions: (1) normally zoned crystals with high Fo content (Fo > 85), (2) crystals with intermediate Fo contents (84–81), and (3) reversely zoned crystals with lower Fo core content (<80). Olivine chemical zoning (diffusion) profiles were modelled in the context of a specific magmatic environment linked with changes in thermodynamic variables during storage (temperature, pressure, and oxygen fugacity). We propose that the normally zoned olivine crystals grew in one magmatic environment (ME1), which subsequently intruded into a more evolved (lower MgO) environment (ME2), where they interacted and were stored for up to 135 days before their eruption. During magma ascent to the surface, a second magma mixing event occurred between ME2 and magma within a third magmatic environment (ME3), forming reversely-zoned olivine crystals yielding notably shorter ascent times of approximately a few days. The rocks from the opening phreatomagmatic phase of the eruption show a larger range in olivine group types compared to the final magmatic phase, where those from the deeper ME1 are more abundant. The short time scales of magma transport obtained in our study, on the order of days to months, should be informative of the warning times that may be encountered between the onset of volcanic unrest and an eruption in the Auckland Volcanic Field.

Modeling SO2 dispersion from future eruptions in the Auckland Volcanic Field, New Zealand

Auckland city (pop. 1.7 M) is Aotearoa New Zealand’s largest city and an important economic hub. The city is built upon the active intraplate basaltic Auckland Volcanic Field (AVF). An AVF eruption would cause considerable impacts. An important component of volcanic risk management is assessing the likely volcanic hazards to help inform emergency planning and other preparedness activities. Previous volcanic hazard assessments for the AVF, particularly those for emergency planning scenarios, have modeled multiple volcanic hazards including lava flows, pyroclastic density currents, ballistic projectiles and tephra fall. Despite volcanic gas being an important and impactful hazard from intraplate basaltic field eruptions, there has been limited consideration of volcanic gas in AVF hazard assessment to date. This project is one of the first to quantitatively assess potential volcanic gas hazards for an explosive eruption scenario. For basaltic volcanism, sulfur dioxide (SO2) gas is typically the most consequential volcanic gas emitted. The aim of this exploratory study was to model SO2 dispersion from a high impact eruption during weather conditions conducive to high ground level pollutant concentrations. Since ground level SO2 concentrations are influenced by complex wind patterns resulting from interactions of locally driven flow circulations and topographically influenced weather, we modeled SO2 dispersion using the HYSPLIT model, a state-of-the art hybrid Eulerian and Lagrangian dispersion model widely used for volcanic gases, using high-resolution meteorological forcing fields given by the Weather Research and Forecasting (WRF) model.Modeled air parcel trajectories and ground level SO2 concentrations illustrate the effect of the converging sea breeze winds on SO2 dispersion. Under worst-case dispersion conditions, extensive areas of up to hundreds of square kilometers to the north and northwest of the eruption location would exceed New Zealand short-term (24 h) air quality standards and guidelines for SO2, indicating heightened health risks to downwind communities. Using this numerical modeling approach, this work presents a methodology for future applications to other AVF eruption scenarios, with a wider range of meteorological conditions that can help in exploring consequences for health services such as anticipated emergency department respiratory admissions.

Remnants of a Young Monogenetic Volcanic Field and the Fragile Balance of Anthropogenic Interaction

The rapid development of Auckland City in New Zealand from an initial rural settlement to a global urban hub produced a characteristic footprint on the Auckland Volcanic Field (AVF). This process was facilitated by increased anthropogenic activity that has resulted in the deterioration and destruction of many volcanic landforms and caused severe archaeological, cultural, geological and educational losses in an alarmingly short timescale. The AVF has 53 volcanic centres, and of these, 17% are classified as intact, 28% are partially intact, 30% are partially destroyed, and 25% are destroyed (including 13% that have no trace left). Based on surface area, approximately 40% of volcanic deposits in the AVF have been lost. The most common causes for impacts are public land use, quarrying and urban development. Regardless, there is significant potential to be found in the balance between the losses and gains of anthropogenic impacts on volcanic landforms. In the AVF and worldwide, geological studies have often been assisted by the presence of outcrops created by quarrying, mining, transport infrastructure and other modifications of volcanic landforms. Areas of significant volcanic geoheritage worldwide are often linked with these impacted volcanoes, and the information gained from these geoheritage areas assists in the management of geodiversity and geoeducation. Several volcanic centres are currently at risk of further destruction in the near future (Crater Hill, Waitomokia, Maugataketake, Kohuroa, Three Kings, St Heliers and McLaughlins Mt) and should be prioritised for any possible research before it is too late. We propose that a geological assessment should be a requirement before and, if possible, during any land development on or near a volcanic landform. Allowing access to scientists through the course of development in areas with volcanic landforms would, in turn, aid public and governing bodies in decision-making for the future of the city and its volcanoes in terms of increased knowledge of volcanic mechanisms of the AVF and awareness of the potential associated hazards.

The role of phreatomagmatism in the formation of complex monogenetic volcanic systems in a low-lying coastal plain

Monogenetic volcanoes are characterised worldwide by a single eruptive phase that is short-lived, with small eruptive volumes that produce a wide range and complexity of landforms. Investigating the evolution in eruptive styles through the active history of a volcano is key to understanding the role of phreatomagmatism in the formation of these complex monogenetic systems. The classification of the complexity of a monogenetic volcanic centre is of great significance to understanding both past volcanoes and the hazards of future volcanoes. Complexity can be defined by the number of volcanic structures created, the presence of multiple types of eruptions, and the number of conduits involved. In this paper we reconstruct the volcanic history of the Waitomokia Volcanic Complex, located in the southern lowlands of the active monogenetic Auckland Volcanic Field (AVF), New Zealand. Activity began with deposition of an initial tuff ring, followed by vent migration and establishment of a maar with a surrounding tuff ring. Four smaller tuff rings developed on the north and northeastern rim of the maar. This was followed by the noteworthy deposition of complex intercalated transitional deposits from at least two tuff rings and magmatic deposits from a scoria cone. Three scoria cones were then built within the maar, followed by minor effusion of short lava flows. Waitomokia highlights the prospect of having a complex system with ten or more landforms being created in different areas active at the same time, with associated eruptive styles and mechanisms. The possibility of a complex eruption, where several styles may occur simultaneously from distinct vents, should be considered in hazard assessment and crisis management plans.

Fire From Volcanic Activity: Quantifying the threat from an understudied hazard

Natural hazards can trigger fires that increase the area and severity of impacts beyond that caused by the initiating hazard. For some physical processes, such as earthquake, fires are a well appreciated and studied cascading hazard. However, fire from volcanic activity (FFVA) is a highly dangerous and largely understudied hazard arising from volcanic activity. To demonstrate the potential destructiveness of this hazard, we develop a FFVA ignition probability model underpinned by a widely applicable fault tree, which identifies the mechanisms that can lead to fire ignition from volcanic activity. By assigning values to each node of the fault tree, our model can be used to consider the relative probabilities associated with different fire ignition mechanisms. We couple this ignition probability model with a fire spread model to evaluate hazardous areas and associated impacts caused by FFVA. To demonstrate the applicability of our model, we use an eruption scenario for volcanic ballistic projectiles in the Auckland Volcanic Field (Aotearoa New Zealand). This case study demonstrates the potential for FFVA to compound and greatly increase the impacts caused by other volcanic hazards. We found that fire ignition and spread could increase the damaged area and monetary losses associated with the initiating hazard alone by more than six and five times, respectively. We suggest that more study is needed to better understand, evaluate, and plan for FFVA.

Cost-benefit analysis for evacuation decision-support: challenges and possible solutions for applications in areas of distributed volcanism

During a volcanic crisis, evacuation is the most effective mitigation measure to preserve life. However, the decision to call an evacuation is typically complex and challenging, in part due to uncertainties related to the behaviour of the volcano. Cost-benefit analysis (CBA) can support decision-makers: this approach compares the cost of evacuating versus the expected loss from not evacuating, expressed as a ‘break-even’ probability of fatality. Here we combine CBA with a Bayesian Event Tree for Short-term Volcanic Hazard (BET_VHst) to create an evacuation decision-support tool to identify locations that are cost-beneficial to evacuate in the event of volcanic unrest within a distributed volcanic field. We test this approach with the monogenetic Auckland Volcanic Field (AVF), situated beneath the city of Auckland, New Zealand. We develop a BET_VHst for the AVF, extending a recently revised Bayesian Event Tree for Eruption Forecasting (BET_EF) to consider the eruptive style, phenomena produced, and the impact exceedance probability as a function of distance. The output of the BET_VHst is a probability of volcanic hazard impact at a given location. Furthermore, we propose amending the weight of the monitoring component within the BET_VHst framework to a transitional parameter, addressing limitations identified in a previous study. We examine how three possible transitional monitoring component weights affect the spatial vent likelihood and subsequent BET_VHst outputs, compared to the current default weight. For the CBA, we investigate four thresholds, based on two evacuation durations and two different estimates for the value of life that determine the cost of not evacuating. The combinations of CBA and BET_VHst are tested using a synthetic unrest dataset to define an evacuation area for each day. While suitable evacuation areas were identified, there are further considerations required before such an approach can be applied operationally to support crisis management.

Gravity and magnetic models at Rangitoto Volcano, Auckland Volcanic Field, New Zealand: Implications for basement control on magma ascent

In small-volume volcanism, pre-existing crustal structures can influence magma ascent processes. Rangitoto volcano in the Auckland Volcanic Field, New Zealand, provides a possible example of magma-structure interaction as this volcano was emplaced adjacent to an outcrop of a regional-scale basement fault, the Islington Bay Fault. In this study, we evaluated the magmatic plumbing system of Rangitoto using gravity and magnetic data acquired over the volcano and the adjacent, non-volcanic Motutapu Island. We modelled the Rangitoto internal architecture and magma plumbing using 2.5D forward and 3D inverse modelling methods. Both models are constrained by petrophysical data, while drill hole logs are only used to constrain the 2.5D model. Our model endmembers suggest that parallel magma pathways are present below the Rangitoto summit cones. In 3D magnetic models, this is evidenced by a fault-aligned pair of high-susceptibility bodies. Interpreting our models in conjunction with previously published geological and geophysical models allows us to hypothesise that the fault-parallel alignment of Rangitoto magma pathways reflects the primary influence of the Islington Bay Fault over the Rangitoto magma ascent. Magma diversion at shallow levels by other finer structures intersecting and adjacent to the fault could explain why Rangitoto erupted 3.5 km west of the Islington Bay Fault surface trace.

Spatial decision-making support for geoheritage conservation in the urban and indigenous environment of the Auckland Volcanic Field, New Zealand

Abstract It is imperative today to make geoheritage conservation an essential part of all environmental standards and operational procedures. This is because geoheritage conservation secures the preservation of in situ geoheritage elements especially in urban environments such as Auckland. Geoheritage in Auckland is strongly associated with both indigenous culture and textbook geology of monogenetic volcanism, and it can play an important role in hazard forecasting and risk mitigation. To date, there has been a lack of policy or any planning tools based explicitly on the current geopreservation inventory. Here, we present an approach to support policy making informed by a spatial multi-criteria analysis that has long been used in environmental decision-making, supported by multi-layer mapping. A systematic literature review was undertaken to define the most accepted assessment criteria used in geoheritage evaluation. We identified six criteria for the base spatial layers of our analysis, highlighting the most suitable areas for geoheritage conservation. For cultural conservation, we used available archaeological shape files, indigenous land ownership data and elevation data (the volcanic cones had multiple roles in the life of first settlers, the ancestors of the Māori). Geographic Information Systems (GIS) multi-objective land use planning is an effective procedure for achieving complex planning and preservation objectives. It allows for outcomes based on quality data and sound analysis while minimizing compromise and conflict between geoheritage, social and cultural values.

A 3D crustal shear speed model of the Auckland volcanic field, New Zealand, from multi-component ambient noise tomography

We present a 3D shear-wave speed model of the crust in the intra-plate Auckland Volcanic Field (AVF), that provides new depth information on the basement geology and is a step towards determining lithospheric controls on volcanism of the AVF, within the Zealandia continent. Beneath the AVF, our model finds a mid-crustal contact of the upturning ophiolite-rich Dun Mountain-Maitai Terrane (DMT), and the western end of the accretionary Waipapa (Caples) Terranes which are dominated by terrigenous sedimentary rocks. Westward and at depth, the DMT presents as a high-speed, thick, and horizontal part of the Gondwana-ward verging syncline seen in the DMT elsewhere in New Zealand. Along the western side of the Junction Magnetic Anomaly (which indicates the presence of the DMT), a major vertical discontinuity probably marks the contact between the horizontal DMT and the Waipapa (Caples) Terranes. The Waipapa (Caples) Terrane contain a large wedge-shaped low-speed zone tapering eastward and extending NNW-SSE alongside the DMT. In addition, the model identifies a sharp vertical discontinuity at the eastern edge of the AVF that extends from the near-surface through the low-speed wedge. We interpret this discontinuity as a relatively young subvertical fault with a significant downthrow to the west. It appears to be a reactivation of a fault zone inherited from the break-up of Gondwana. The model also imaged the largest massif of crystalline ultramafic rocks in the DMT that causes the Takapuna Gravity Anomaly and the NE striking Port Waikato-Mangatangi cross-fault system. The strike directions of the faults mirror a rhombic-rectangular fault grid inherited from the breakaway of Zealandia from Gondwana.The shear speed model was produced using frequency-time analysis of multi-component Rayleigh wave Green's functions, reversible-jump Markov chain Monte Carlo inversion of group arrival times, and the inversion of dispersion relations for shear speed as a function of depth. Plain language summaryA 3D model of shear wave speeds in the crust provides new information on the geological structure beneath the Auckland Volcanic Field (AVF). The field is positioned above the contact between ocean floor rocks of the Dun Mountain-Maitai Terrane (DMT) in the west with fast wave speeds and a prominent wedge-shaped low-speed zone in the sandstone-dominated Waipapa/Caples Terrane in the east. The model images a fold structure of the DMT similar to that seen throughout New Zealand and at depth delineates the contact as a major sub-vertical discontinuity. A second vertical discontinuity just east of the AVF is very abrupt, is also oriented NNW-SSE and is probably caused by faulting. The model has also imaged the NE-oriented Port Waikato-Mangatangi cross-fault system in the south of the region and the large massif of coarse-grained intrusive rocks in the DMT that causes the previously recognised Takapuna gravity anomaly.

The interplay between environmental and magmatic conditions in eruption style transitions within a fissure-aligned monogenetic volcanic system of Auckland, New Zealand

Recent conceptual geological frameworks of continental monogenetic volcanism highlight that the small magma volume eruptions, resulting volcanic geology and edifice architecture in such settings are sensitive to variations in external or environmental conditions. These conditions, along with fluctuations in magma flux, can change rapidly over short time frames and cause dramatic changes in eruption style. Understanding the drivers of transitions in explosive to effusive behaviour within the short timescales of eruptions at individual volcanic centres is essential to accurately assess volcanic hazards in continental monogenetic settings. Wiri Mountain Volcanic Complex is one of the largest and most complex volcanic centres in the active Auckland Volcanic Field. Despite the significant removal of much of the original volcanic deposits, present-day exposures and historical images provide a unique opportunity to examine the growth and evolution of the volcanic complex. Wiri Mountain deposited an initial basal tuff ring (covering an area of approximately 0.67 km2) by predominantly pyroclastic density currents, followed by at least two smaller tuff rings erupted through the outer flanks of the first, in a transition from phreatomagmatic to Strombolian eruptive style. A 90 m high central scoria cone was then produced within the initial tuff ring, partially capped by lava spatter, clastogenic lava flows and lava flows that mostly covered all tuff rings, the scoria cone, and the surrounding area. A high-resolution stratigraphic study of the well-exposed tuff ring to capping magmatic succession was conducted to determine the changes in eruptive style and their driving forces. The deposit architecture of Wiri Mountain can be described using three volcanic stratigraphic units: a basal unit comprised of tuff, lapilli tuff and tuff breccia deposits, a middle unit comprised of juvenile-rich transitional tuff deposits of black scoria ash and lapilli, red scoria and spatter, and a capping unit comprised of agglutinate and lava flow successions. Most volcanic materials were either fragmented and ejected at near-optimal scaled depth, or the transition between near-optimal and shallower/deeper depths. Wiri Mountain provides a striking example of a fissure eruption likely controlled by a pre-existing tectonic fault that was fed by relatively stable melt sources over a sustained period. We infer that despite eruption within a water-saturated coastal plain, the initial phreatomagmatic phase was overridden by subsequent explosive and effusive magmatic phases through the formation of an increasingly established conduit, thus allowing sustained magma flux and melt supply. The transitions, both gradual and rapid, from an initial phreatomagmatic to subsequent magmatic explosive and effusive phases fit well to the general understanding of eruption style changes over time from other larger and more complex volcanoes of Auckland, and elsewhere worldwide. Wiri Mountain showcases the fine balance between the external and internal conditions that control eruption style variations and govern the formation of complex monogenetic volcanoes.

Short-Term Eruption Forecasting for Crisis Decision-Support in the Auckland Volcanic Field, New Zealand

Auckland, a city of 1.6 million people, is situated atop the active monogenetic Auckland Volcanic Field (AVF). Thus, short-term eruption forecasting is critical to support crisis management in a future event, especially to inform decisions such as calling evacuations. Here we present an updated BET_EF for the AVF incorporating new data and the results of an expert-opinion workshop, and test the performance of the resulting BETEF_AVF on eight hypothetical eruption scenarios with pre-eruptive sequences. We carry out a sensitivity analysis into the selection of prior distributions for key model parameters to explore the utility of using BET_EF outputs as a potential input for evacuation decision making in areas of distributed volcanism such as the AVF. BETEF_AVF performed well based on the synthetic unrest dataset for assessing the probability of eruption, with the vent outbreaks eventuating within the zone of high spatial likelihood. Our analysis found that the selection of different spatial prior model inputs affects the estimated vent location due to the weighting between prior models and monitoring inputs within the BET_EF, which as unrest escalates may not be appropriate for distributed volcanic fields. This issue is compounded when the outputs are combined with cost-benefit analysis to inform evacuation decisions, leading to areas well beyond those with observed precursory activity being included in evacuation zones. We find that several default settings used in past work for the application of BET_EF and CBA to inform evacuation decision-support are not suitable for distributed volcanism; in particular, the default 50-50 weighting between priors and monitoring inputs for assessing spatial vent location does not produce useful results. We conclude by suggesting future cost-benefit analysis applications in volcanic fields appropriately consider the spatial and temporal variability and uncertainty characteristic of such systems.

Using paleomagnetism to determine the heating effect of lava flows on underlying substrates

The extent to which heat from lava flows passes into underlying and adjacent materials has significant implications for volcanic hazard studies. Here we demonstrate how paleomagnetism can be used as a tool to determine the heating effects of lava flows in the pre-existing substrates over which they flow. Samples from soils taken beneath lava flows from Rangitoto and Puketutu eruptive centres (Auckland Volcanic Field, Aotearoa New Zealand) and a human-made berm beneath the June 27th Lava Flow (2014–2015; Kīlauea Volcano, Hawaii, USA) were subjected to progressive thermal demagnetization to assess the strength and stability of their remanent magnetizations. The temperature and depth to which these soils display a strong coherent magnetization represents the extent to which they were remagnetized (and therefore heated) by the overlying flow. Results suggest heating to at least 570 ℃ at depths of up to 21 cm below the substrate-flow contact. This information is valuable for constraining and validating heat transfer models, which can be used to assess the lava flows’ subterranean thermal hazard. Among many uses, this is vital for emergency management planning for buried infrastructure networks traversing regions that could be exposed to effusive volcanic activity. Further afield, in astrobiology, it might find application in determining the thickness of a substrate layer heated sufficiently by a lava flow to kill living organisms.

Micro-XRF-inferred depositional history of the Orakei maar lake sediment sequence, Auckland, New Zealand

Lake sediment records offer insights into past climate and environmental changes. There are, however, few continuous lake sediment records from the Southern Hemisphere mid-latitudes that span the last glacial interval (LGI) and have the requisite chronological control and sampling resolution. Orakei maar paleolake in the Auckland Volcanic Field, New Zealand, is an exception, as it contains a high-resolution record of continuous lacustrine sedimentation from its formative phreatomagmatic eruption ~ 130 ka, until post-glacial sea-level rise breached the crater tuff rim and connected the lake to the sea ~ 9 ka. We used micro-XRF core scanning, dry bulk density, loss-on-ignition and visual facies descriptions to investigate the depositional history of Orakei maar lake as a response to regional and global drivers of climate change, and to erosional events in the lake catchment. The climate history of the lake was divided into six depositional phases: (I) Early warming with frequent in-wash events, followed by climate fluctuations coeval with marine isotope stages (MIS) 5e to mid-5c, (II) Warm, quiescent depositional conditions during mid-MIS 5c to 5a, (III) A colder, windier interval during MIS 4, (IV) Warmer conditions with dominantly autochthonous sedimentation during MIS 3, (V) Cold conditions followed by a slow temperature increase and the onset of sea-level rise during late MIS 2, (VI) Warm conditions that culminated in formation of a peat unit at the top of the Orakei lacustrine sediment sequence, which was terminated by an influx of massive marine muds into the basin at 9.75 ka. Comparison of the inferred climate evolution at Orakei with climate inferences from the global marine benthic MIS record during LGI shows general agreement, though inferred climate changes consistently occur earlier at Orakei than in the MIS. There is also general agreement in temperature changes inferred from the Orakei record and from other regional lake sediment records, but more detailed comparison requires additional proxy climate data such as pollen, organic geochemistry and biomarkers, to better understand discrepancies between some records. This study demonstrated the great potential of the Orakei record for paleoclimate inference and the applicability of micro-XRF core scanning data for addressing questions about paleoclimate and paleoenvironment. It also highlighted past intervals that require further study.

Visitation Rate Analysis of Geoheritage Features from Earth Science Education Perspective Using Automated Landform Classification and Crowdsourcing: A Geoeducation Capacity Map of the Auckland Volcanic Field, New Zealand

The increase in geoheritage studies has secured recognition globally regarding the importance of abiotic natural features. Prominent in geoheritage screening practices follows a multicriteria assessment framework; however, the complexity of interest in values often causes decision making to overlook geoeducation, one of the primary facets of geosystem services. Auckland volcanic field in New Zealand stretches through the whole area of metropolitan Auckland, which helps preserve volcanic cones and their cultural heritage around its central business district (CBD). They are important sites for developing tourist activities. Geoeducation is becoming a significant factor for tourists and others visiting geomorphological features, but it cannot be achieved without sound planning. This paper investigates the use of big data (FlickR), Geopreservation Inventory, and Geographic Information System for identifying geoeducation capacity of tourist attractions. Through landform classification using the Topographic Position Index and integrated with geological and the inventory data, the underpromoted important geoeducation sites can be mapped and added to the spatial database Auckland Council uses for urban planning. The use of the Geoeducation Capacity Map can help resolve conflicts between the multiple objectives that a bicultural, metropolitan city council need to tackle in the planning of upgrading open spaces while battling of growing demand for land.

A Holocene palaeomagnetic secular variation record from Lake Pupuke, New Zealand

ABSTRACT Palaeomagnetic records from three cores of lake sediment have been merged, producing a new Holocene geomagnetic secular variation record for northern New Zealand. The cores are from Lake Pupuke, a maar lake in Auckland Volcanic Field (AVF). They contain tephra from Rangitoto Volcano (530 ± 10 yr BP), Taupō (1718 ± 10 yr BP), Tūhua/Mayor Island (7637 ± 100 yr BP) and Rotomā tephra from Ōkataina Volcanic Centre (9423 ± 120 yr BP). These tephra dates are supplemented by selected radiocarbon age estimates. The cores were correlated using tephra and fine-scale variations in magnetic susceptibility. The natural remanent magnetisation is strong, stable and carried by fine-grained titanomagnetite from AVF’s basaltic volcanoes. The resulting palaeosecular variation (PSV) record spans from 10,000 to 1500 yr BP. It shows well-defined, millennial-scale swings in direction between 10,000 and 8000 yr BP and between 4000 and 1500 yr BP, while between 8000 and 4000 yr BP variations are of higher frequency and lower amplitude and centre on the geocentric axial dipole field direction. This is in excellent agreement with published PSV records from Mavora Lakes, 1000 km further south, indicating the regional nature of PSV – resulting from broad-scale changes in the circulation of conducting iron-rich fluid of Earth’s outer core.

The magma source of small-scale intraplate monogenetic volcanic systems in northern New Zealand

The Auckland intraplate volcanic province consists of four small fields of basaltic monogenetic volcanoes. Each of these was active for a period of ~1 Myr and during the last 3 Myr the locus of magmatic activity migrated northward in discrete ~50 km steps. The most recent of these volcano fields is the Auckland Volcanic Field (AVF) in which individual eruptions have produced discrete magma batches within the composition range nephelinite to sub-alkaline basalt. This range of compositions is explained by partial melting involving variable melt/solid ratios together with modal heterogeneity in their mantle source. Eruptions in the field have been irregularly spaced during its 200 kyr life with repose periods varying between ≤0.1 and 13 kyr. Although most of Auckland's volcanoes represent a single period of eruption, paired eruption sequences in which smaller more alkaline magma batches have been followed by more voluminous less alkalic magmas have also occurred. A significant flare-up in activity at about 30 kyr produced compositionally discrete eruptions from at least five spatially separate locations all within a temporal interval of as little as 100 years. An interpretation of these data is that each event (volcano) in the AVF represents a discrete batch of magma extracted from an source with the capacity of producing a compositional variety of magma. Such a source forms when upwelling mantle undergoes adiabatic rise and exists as a metastable entity at the asthenosphere-lithosphere boundary for prolonged periods (105–106 years) during which time it is tapped to create individual magma batches. The shorter term magma extraction events that produced individual volcanoes in the AVF are suggested to be the result of crustal scale tectonic events. The behaviour of these four spatially separate monogenetic volcanic systems at temporal intervals of <106 years is likely linked to the long term tectonic evolution of the Pacific-Australian Plate boundary beneath the New Zealand region.

Dilemma of Geoconservation of Monogenetic Volcanic Sites under Fast Urbanization and Infrastructure Developments with Special Relevance to the Auckland Volcanic Field, New Zealand

Geoheritage is an important aspect in developing workable strategies for natural hazard resilience. This is reflected in the UNESCO IGCP Project (# 692. Geoheritage for Geohazard Resilience) that continues to successfully develop global awareness of the multifaced aspects of geoheritage research. Geohazards form a great variety of natural phenomena that should be properly identified, and their importance communicated to all levels of society. This is especially the case in urban areas such as Auckland. The largest socio-economic urban center in New Zealand, Auckland faces potential volcanic hazards as it sits on an active Quaternary monogenetic volcanic field. Individual volcanic geosites of young eruptive products are considered to form the foundation of community outreach demonstrating causes and consequences of volcanism associated volcanism. However, in recent decades, rapid urban development has increased demand for raw materials and encroached on natural sites which would be ideal for such outreach. The dramatic loss of volcanic geoheritage of Auckland is alarming. Here we demonstrate that abandoned quarry sites (e.g., Wiri Mountain) could be used as key locations to serve these goals. We contrast the reality that Auckland sites are underutilized and fast diminishing, with positive examples known from similar but older volcanic regions, such as the Mio/Pliocene Bakony–Balaton UNESCO Global Geopark in Hungary.

Modelling spatial population exposure and evacuation clearance time for the Auckland Volcanic Field, New Zealand

Auckland, New Zealand's largest city (population of ~1.6 million), is situated atop the monogenetic Auckland Volcanic Field (AVF). As in many places faced with volcanic activity, evacuation is seen as the best risk mitigation strategy for preserving lives in the event of volcanic unrest and/or an eruption. However, planning for an evacuation can be challenging. In particular, the uncertainty in vent location resulting from the monogenetic nature of the field makes identifying neighbourhoods to be evacuated impractical until well into the pre-eruption unrest period. This study uses spatial analysis methods to assess exposure for both population and private transport ownership as well as to identify those areas requiring public transport support for an evacuation. These data were overlaid on a range of possible vent locations across the AVF using a 500 × 500 m grid. At each possible vent location, a 5 km evacuation zone is modelled, following the official contingency plan for evacuation in a future AVF event. In order to simulate vent location uncertainty leading up to a future eruption, a range of buffer distances were applied around the modelled vent locations.The exposure data derived were then used to model evacuation clearance time, which considered four phases: 1) the time taken to decide to call an evacuation; 2) the public notification time; 3) the evacuee's time to prepare; and 4) evacuee's travel time to beyond the evacuation zone. The length of time involved in phases 1 to 3 are all independent of the vent location; our analysis found these phases could be completed within 36 h, with over 80% confidence. Travel times to beyond the evacuation zone were modelled using the exposure analysis for population and private transport ownership combined with road network data and vehicle carrying capacity. This revealed travel times for this phase ranging from less than 1 up to 11 h, depending on traffic congestion, when considering no vent uncertainty. By combining the times modelled for all four phases, we found that when there is high certainty in the vent location, the median total evacuation clearance time with no congestion is approximately 37 h. However, include a 10 km vent uncertainty buffer into the model, the evacuation clearance time can increase to between 38 and 55 h, dependent on traffic congestion. A vent in the densely populated inner Auckland and CBD area would result in the greatest population required to evacuate, and also the greatest need for public transport support given the low vehicle ownership in this area. Our results can be used to inform emergency management decision making, and the model can be adapted for other regions as well as for other hazards.

Composite development and stratigraphy of the Onepoto maar lake sediment sequence (Auckland Volcanic Field, New Zealand)

Abstract. The accurate and precise reconstruction of Quaternary climate as well as the events that punctuate it is an important driver of the study of lake sediment archives. However, until recently lake sediment-based palaeoclimate reconstructions have largely concentrated on Northern Hemisphere lake sequences due to a scarcity of continuous and high-resolution lake sediment sequences from the Southern Hemisphere, especially from the southern mid-latitudes. In this context, the deep maar lakes of the Auckland Volcanic Field of northern New Zealand are significant as several contain continuous and well-laminated sediment sequences. Onepoto Basin potentially contains the longest temporal lake sediment record from the Auckland Volcanic Field (AVF), spanning from Marine Isotope Stage 6e (MIS 6e) to the early Holocene when lacustrine sedimentation was terminated by marine breach of the south-western crater tuff ring associated with post-glacial sea-level rise. The Onepoto record consists of two new, overlapping cores spanning ca. 73 m combined with archive material in a complete composite stratigraphy. Tephrochronology and 14C dating provide the fundamental chronological framework for the core, with magnetic relative palaeo-intensity variability downcore, and meteoric 10Be influx into the palaeolake to refine the chronology. The µ-XRF (micro X-ray fluorescence) downcore variability for the entirety of the lake sediment sequence has been established with measurement of a range of proxies for climate currently underway. This work will produce the first continuous record of the last 200 kyr of palaeoclimate from northern New Zealand to date.

Seismic monitoring of the Auckland Volcanic Field during New Zealand's COVID-19 lockdown

Abstract. The city of Auckland, New Zealand (Tāmaki Makaurau, Aotearoa), sits on top of an active volcanic field. Seismic stations in and around the city monitor activity of the Auckland Volcanic Field (AVF) and provide data to image its subsurface. The seismic sensors – some positioned at the surface and others in boreholes – are generally noisier during the day than during nighttime. For most stations, weekdays are noisier than weekends, proving human activity contributes to recordings of seismic noise, even on seismographs as deep as 384 m below the surface and as far as 15 km from Auckland's Central Business District. Lockdown measures in New Zealand to battle the spread of COVID-19 allow us to separate sources of seismic energy and evaluate both the quality of the monitoring network and the level of local seismicity. A matched-filtering scheme based on template matching with known earthquakes improved the existing catalogue of five known local earthquakes to 35 for the period between 1 November 2019 and 15 June 2020. However, the Level-4 lockdown from 25 March to 27 April – with its drop in anthropogenic seismic noise above 1 Hz – did not mark an enhanced detection level. Nevertheless, it may be that wind and ocean swell mask the presence of weak local seismicity, particularly near surface-mounted seismographs in the Hauraki Gulf that show much higher levels of noise than the rest of the local network.