Auckland Volcanic Field Research Articles

Volcanic records of the Laschamp geomagnetic excursion from Mt Ruapehu, New Zealand

We present palaeodirectional records of the Laschamp geomagnetic excursion from lavas on Mt Ruapehu, New Zealand. Fourteen lava flows on the northwestern and southern flanks of Mt Ruapehu, with 40Ar/39Ar weighted mean plateau ages that range from 46.3±2.0 to 39.9±1.4 ka, were studied. The youngest and older flows carry a normal polarity magnetization; however, six flows, dated between 46.3±2.0 and 42.7±1.8 ka, record excursional directions. Three of these flows record southerly palaeomagnetic declinations and negative inclinations that agree well with a published Laschamp record from the Auckland Volcanic Field (AVF). Together, the AVF and Mt Ruapehu lavas currently represent the only volcanic records of the Laschamp excursion outside the Chaîne des Puys region, France. Thus, they make an important contribution to the global set of Laschamp excursion records. Virtual geomagnetic pole (VGP) groups for the New Zealand and French records early in the excursion are compatible with a dipole-dominated field that rotated to an equatorial orientation while simultaneously decaying in strength. In contrast, younger excursional flows from France and New Zealand yield separate VGP groups, which suggest either that the field had a nondipolar morphology in this later phase, or that the VGP groups were not synchronous. 40Ar/39Ar ages for the Mt Ruapehu record are on average slightly older than published northern hemisphere ages and from the relative palaeointensity minimum in the GLOPIS sedimentary stack. Although few individual ages differ significantly at the 2σ level, the spread suggests an overall excursion duration that is longer than the currently accepted 1500 years. This age spread may result from excess Ar in magmas at the time of the eruption biasing the results to slightly older ages, or from non-synchronous excursional field behaviour at near-antipodal locations, or, possibly, a precursory phase prior to the main excursion.

The spatial and temporal ‘cost’ of volcanic eruptions: assessing economic impact, business inoperability, and spatial distribution of risk in the Auckland region, New Zealand

Volcanic risk assessment has historically concentrated on quantifying the frequency, magnitude, and potential diversity of physical processes of eruptions and their consequent impacts on life and property. A realistic socio-economic assessment of volcanic impact must however take into account dynamic properties of businesses and extend beyond only measuring direct infrastructure/property loss. The inoperability input-output model, heralded as one of the 10 most important accomplishments in risk analysis over the last 30 years (Kujawaski Syst Eng. 9:281–295, 2006), has become prominent over the last decade in the economic impact assessment of business disruptions. We develop a dynamic inoperability input-output model to assess the economic impacts of a hypothetical volcanic event occurring at each of 7270 unique spatial locations throughout the Auckland Volcanic Field, New Zealand. This field of at least 53 volcanoes underlies the country’s largest urban area, the Auckland region, which is home to 1.4 million people and responsible for 35.3% (NZ$201481.2 billion) of the nation’s GDP (Statistics New Zealand 2015). We apply volcanic event characteristics for a small-medium-scale volcanic eruption scenario and assess the economic impacts of an ‘average’ eruption in the Auckland region. Economic losses are quantified both with, and without, business mitigation and intervention responses in place. We combine this information with a recent spatial hazard probability map (Bebbington and Cronin Bull Volcanol. 73(1):55–72, 2011) to produce novel spatial economic activity ‘at risk’ maps. Our approach demonstrates how business inoperability losses sit alongside potential life and property damage assessment in enhancing our understanding of volcanic risk mitigation.

Dissolution and secondary mineral precipitation in basalts due to reactions with carbonic acid

AbstractOne of the leading hydrothermal alteration processes in volcanic environments is when rock‐forming minerals with high concentrations of iron, magnesium, and calcium react with CO2 and water to form carbonate minerals. This is used to the advantage of geologic sequestration of anthropogenic CO2. Here we experimentally investigate how mineral carbonation processes alter the rock microstructure due to CO2‐water‐rock interactions. In order to characterize these changes, CO2‐water‐rock alteration in Auckland Volcanic Field young basalts (less than 0.3 Ma) is studied before and after a 140 day reaction period. We investigate how whole core basalts with similar geochemistry but different porosity, permeability, pore geometry, and volcanic glass content alter due to CO2‐water‐rock reactions. Ankerite and aluminosilicate minerals precipitate as secondary phases in the pore space. However, rock dissolution mechanisms are found to dominate this secondary mineral precipitation resulting in an increase in porosity and decrease in rigidity of all samples. The basalt with the highest initial porosity and volcanic glass volume shows the most secondary mineral precipitation. At the same time, this sample exhibits the greatest increase in porosity and permeability, and a decrease in rock rigidity post reaction. For the measured samples, we observe a correlation between volcanic glass volume and rock porosity increase due to rock‐fluid reactions. We believe this study can help understand the dynamic rock‐fluid interactions when monitoring field scale CO2 sequestration projects in basalts.

Investigating the consequences of urban volcanism using a scenario approach II: Insights into transportation network damage and functionality

Transportation networks are critical infrastructure in urban environments. Before, during and following volcanic activity, these networks can incur direct and indirect impacts, which subsequently reduces the Level-of-Service available to transportation end-users. Additionally, reductions in service can arise from management strategies including evacuation zoning, causing additional complications for transportation end-users and operators. Here, we develop metrics that incorporate Level-of-Service for transportation end-users as the key measure of vulnerability for multi-hazard volcanic impact and risk assessments.A hypothetical eruption scenario recently developed for the Auckland Volcanic Field, New Zealand, is applied to describe potential impacts of a small basaltic eruption on different transportation modes, namely road, rail, and activities at airports and ports. We demonstrate how the new metrics can be applied at specific locations worldwide by considering the geophysical hazard sequence and evacuation zones in this scenario, a process that was strongly informed by consultation with transportation infrastructure providers and emergency management officials. We also discuss the potential implications of modified hazard sequences (e.g. different wind profiles during the scenario, and unrest with no resulting eruption) on transportation vulnerability and population displacement.The vent area of the eruption scenario used in our study is located north of the Māngere Bridge suburb of Auckland. The volcanic activity in the scenario progresses from seismic unrest, through phreatomagmatic explosions generating pyroclastic surges to a magmatic phase generating a scoria cone and lava flows. We find that most physical damage to transportation networks occurs from pyroclastic surges during the initial stages of the eruption. However, the most extensive service reduction across all networks occurs ~6days prior to the eruption onset, largely attributed to the implementation of evacuation zones; these disrupt crucial north-south links through the south eastern Auckland isthmus, and at times cause up to ~435,000 residents and many businesses to be displaced. Ash deposition on road and rail following tephra-producing eruptive phases causes widespread Level-of-Service reduction, and some disruption continues for >1month following the end of the eruption until clean-up and re-entry to most evacuated zones is completed. Different tephra dispersal and deposition patterns can result in substantial variations to Level-of-Service and consequences for transportation management. Additional complexities may also arise during times of unrest with no eruption, particularly as residents are potentially displaced for longer periods of time due to extended uncertainties on potential vent location. The Level-of-Service metrics developed here effectively highlight the importance of considering transportation end-users when developing volcanic impact and risk assessments. We suggest that the metrics are universally applicable in other urban environments.

Forecasting transitions in monogenetic eruptions using the geologic record

Abstract Spatial forecasting of volcanism and associated hazards in intraplate monogenetic volcanic fields is subject to large uncertainties in both data and models. We demonstrate a novel logistic regression method for mapping phreatomagmatic-magmatic eruption transition susceptibility using near-surface hydrologic, topographic, and geologic data. The method is illustrated on the Auckland volcanic field, the location of New Zealand’s largest city, Auckland. Environmental factors examined for possible influence included the thickness of water-saturated and porous sediments, substrate type and geology, vent elevation, and distance from the nearest fault. By comparing these factors with the volumes and styles of past eruption sequences, a location-specific eruption sequence forecasting model was constructed, recognizing that larger and/or longer eruptions are more likely to exhaust vent-area sources of water. Estimating volcanic hazard susceptibility in this way allows more effective planning and improved preeruption preparedness between eruptions and during future volcanic crises.

Investigating the consequences of urban volcanism using a scenario approach I: Development and application of a hypothetical eruption in the Auckland Volcanic Field, New Zealand

What happens when a city has a volcanic eruption within its boundaries? To explore the consequences of this rare but potentially catastrophic combination, we develop a detailed multi-hazard scenario of an Auckland Volcanic Field (AVF) eruption; the AVF underlies New Zealand's largest city, Auckland. We start with an existing AVF unrest scenario sequence and develop it through a month-long hypothetical eruption based on geologic investigations of the AVF and historic similar eruptions from around the world. We devise a credible eruption sequence and include all volcanic hazards that could occur in an AVF eruption. In consultation with Civil Defence and Emergency Management staff, we create a series of evacuation maps for before, during, and after the hypothetical eruption sequence. Our result is a versatile scenario with many possible applications, developed further in companion papers that explore eruption consequences on transportation and water networks. However, here we illustrate one application: evaluating the consequences of an eruption on electricity service provision. In a collaborative approach between scientists and electricity service providers, we evaluate the impact of the hypothetical eruption to electricity generation, transmission, and distribution infrastructure. We then evaluate how the impacted network functions, accounting for network adaptations (e.g., diverting power away from evacuated areas), site access, and restoration factors. We present a series of regional maps showing areas with full service, rolling outages, and no power as a result of the eruption. This illustrative example demonstrates how a detailed scenario can be used to further understand the ramifications of urban volcanism on local and regional populations, and highlights the importance of looking beyond damage to explore the consequences of volcanism.

A model to assess tephra clean-up requirements in urban environments

Tephra falls can cause a range of impacts to communities by disrupting, contaminating and damaging buildings and infrastructure systems, as well as posing a potential health hazard. Coordinated clean-up operations minimise the impacts of tephra on social and economic activities. However, global experience suggests clean-up operations are one of the most challenging aspects of responding to and recovering from tephra falls in urban environments. Here, we present a method for modelling coordinated municipal-led (town/district level authorities) tephra clean-up operations to support pre-event response and recovery planning. The model estimates the volume of tephra to be removed, clean-up duration, and direct costs. The underpinning component of the model is a scalable clean-up response framework, which identifies and progressively includes more urban surfaces (e.g., roofs, and roads) requiring clean-up with increasing tephra thickness.To demonstrate model applicability, we present four clean-up scenarios for the city of Auckland, New Zealand: 1 mm and 10 mm distal tephra fall across the city, along with two local ‘wet’ eruption scenarios (low and high volume tephra deposition) from within the Auckland Volcanic Field. Depending on the modelled scenario, outputs suggest that coordinated clean-up operations in Auckland could require the removal of tens of thousands to millions of cubic metres of tephra. The cost of these operations are estimated to be NZ$0.6–1.1 million (US$0.4–0.7 million) for the 1 mm distal tephra scenario and NZ$13.4–25.6 million (US$9–17 million) for the 10 mm distal tephra scenario. Estimated clean-up costs of local eruptions range from tens of millions to hundreds of millions of dollars. All eruption scenarios indicate clean-up operations lasting weeks to months, but clean-up in some areas impacted by local eruptions could last for years. The model outputs are consistent with documented historic tephra clean-up operations. Although we use Auckland as a proof-of-concept example, the method may be adapted for any city exposed to a tephra hazard.

The Role of Cultural and Indigenous Values in Geosite Evaluations on a Quaternary Monogenetic Volcanic Landscape at Ihumātao, Auckland Volcanic Field, New Zealand

The Ihumātao Peninsula is located within the wider Mangere region in South Auckland, New Zealand. Significant sites of geological, cultural and ecological value are a recorded feature of this area. One of the most significant sites in the region is the Otuataua Stonefields Historic Reserve (OSHR). Geologically, this area is a Quaternary lava flow field with tuff rings, scoria and spatter cones. It sits within the Auckland Volcanic Field (AVF), and the landscape of the OSHR and the wider Ihumātao Peninsula may be seen as the physical expression of a unique convergence of ecological, geological and cultural values. Geosite evaluation methods applied to the Ihumātao Peninsula, following two distinct methods, shed light on potential high geoheritage values the region holds. These values may be looked at as a good base to develop effective geoeducational, geoconservational and geotouristic programs. The study also showed that implementing management strategies to add and conserve geosite values in the region could provide positive outcomes; however, reduction of its main geosite values would be inevitable and irreversible should proposed urban development take place on a block of land immediately bordering the OSHR. The Ihumātao Peninsula is one of several areas of South Auckland where urbanization has left significant areas relatively untouched until the present, whereby they are now threatened by intense housing and industrial development. The geological features of these areas unquestionably hold geological heritage values, allowing understanding of the interplay between low coastal land and rising basaltic magma. This interplay has resulted in a landscape potentially featuring the greatest volcanic geodiversity of the entire AVF. In addition, this relatively undeveloped land provides an unbroken physical and cultural record dating to arrival of the first humans in New Zealand. Of particular note is the physical artefacts and archaeological sites telling a story of settlers attracted to, utilizing and shaping this discrete region of the AVF. Geosite evaluations demonstrate that high geoheritage values of regions like the Ihumātao Peninsula are influenced by the strong cultural link between the community (in particular the indigenous population) and the volcanic landscape. These cultural factors could be given more weight in currently used geosite evaluation methods, enabling such geoheritage values to be demonstrated in a more explicit and meaningful way and providing a basis for further community education and protection of specific sites within the geographical context of the Ihumātao Peninsula.

Os isotopic constraints on crustal contamination in Auckland Volcanic Field basalts, New Zealand

The Auckland Volcanic Field (AVF) represents the youngest and northernmost of three subjacent Quaternary intraplate basaltic volcanic fields in the North Island, New Zealand. Previous studies on AVF eruptive products suggested that their major- and trace-element, and Sr-, Nd- and Pb-isotopic signatures primarily reflect their derivation from the underlying asthenospheric and lithospheric mantle. All AVF lavas however ascend through a ca. 20–30km thick continental crust, and some do carry crustal xenoliths, posing the question whether or not crustal contamination plays a role in their formation. Here we present new Os and Pb isotopic data, and Os and Re concentrations for 15 rock samples from 7 AVF volcanic centres to investigate mantle and crustal petrogenetic processes. The samples include the most primitive lavas from the field (Mg# 59–69) and span a range of eruption sizes, ages, locations, and geochemical signatures. The data show a large range in Os concentrations (6–579ppt) and 187Os/188Os isotope ratios from mantle-like (0.123) to highly radiogenic (0.547). Highly radiogenic Os signatures together with relatively low Os contents in most samples suggest that ascending melts experienced contamination primarily from metasedimentary crustal rocks with high 187Os/188Os ratios (e.g., greywacke). We further demonstrate that <1% metasedimentary crustal input into the ascending melt can produce the radiogenic Os isotope signatures observed in the AVF data. This low level of crustal contamination has no measurable effect on the corresponding trace element ratios and Sr-Nd-Pb isotopic compositions. In addition, high Os contents (195–578ppt) at slightly elevated but mantle-like Os isotopic compositions (187Os/188Os=0.1374–0.1377) in some samples suggest accumulation of xenocrystic olivine-hosted mantle sulphides from the Permian-Triassic ultramafic Dun Mountain Ophiolite Belt, which traverses the crust beneath the Auckland Volcanic Field. We therefore infer that the AVF Os isotopic compositions and Os contents reflect contamination from varying proportions of heterogeneous crustal components, composed of Waipapa and Murihiku terrane metasediments, and ultramafic rocks of the Dun Mountain Ophiolite Belt. This demonstrates, contrary to previous models that primitive lavas from the Auckland Volcanic Field do show evidence for variable interaction with the crust.

Sedimentology, eruptive mechanism and facies architecture of basaltic scoria cones from the Auckland Volcanic Field (New Zealand)

Scoria cones are a common type of basaltic to andesitic small-volume volcanoes (e.g. 10−1–10−5km3) that results from gas-bubble driven explosive eruptive styles. Although they are small in volume, they can produce complex eruptions, involving multiple eruptive styles. Eight scoria cones from the Quaternary Auckland Volcanic Field in New Zealand were selected to define the eruptive style variability from their volcanic facies architecture. The reconstruction of their eruptive and pyroclastic transport mechanisms was established on the basis of study of their volcanic sedimentology, stratigraphy, and measurement of their pyroclast density, porosity, Scanning Electron Microscopy, 2D particle morphology analysis and Visible and Near Visible Infrared Spectroscopy. Collection of these data allowed defining three end-member types of scoria cones inferred to be constructed from lava-fountaining, transitional fountaining and Strombolian type, and explosive Strombolian type. Using the physical and field-based characteristics of scoriaceous samples a simple generalised facies model of basaltic scoria cones for the AVF is developed that can be extended to other scoria cones elsewhere. The typical AVF scoria cone has an initial phreatomagmatic phases that might reduce the volume of magma available for subsequent scoria cone forming eruptions. This inferred to have the main reason to have decreased cone volumes recognised from Auckland in comparison to other volcanic fields evolved dominantly in dry eruptive condition (e.g. no external water influence). It suggests that such subtle eruptive style variations through a scoria cone evolution need to be integrated into the hazard assessment of a potentially active volcanic field such as that in Auckland.

Optimal likelihood-based matching of volcanic sources and deposits in the Auckland Volcanic Field

In monogenetic volcanic fields, where each eruption forms a new volcano, focusing and migration of activity over time is a very real possibility. In order for hazard estimates to reflect future, rather than past, behavior, it is vital to assemble as much reliable age data as possible on past eruptions. Multiple swamp/lake records have been extracted from the Auckland Volcanic Field, underlying the 1.4 million-population city of Auckland. We examine here the problem of matching these dated deposits to the volcanoes that produced them. The simplest issue is separation in time, which is handled by simulating prior volcano age sequences from direct dates where known, thinned via ordering constraints between the volcanoes. The subproblem of varying deposition thicknesses (which may be zero) at five locations of known distance and azimuth is quantified using a statistical attenuation model for the volcanic ash thickness. These elements are combined with other constraints, from widespread fingerprinted ash layers that separate eruptions and time-censoring of the records, into a likelihood that was optimized via linear programming. A second linear program was used to optimize over the Monte-Carlo simulated set of prior age profiles to determine the best overall match and consequent volcano age assignments. Considering all 20 matches, and the multiple factors of age, direction, and size/distance simultaneously, results in some non-intuitive assignments which would not be produced by single factor analyses. Compared with earlier work, the results provide better age control on a number of smaller centers such as Little Rangitoto, Otuataua, Taylors Hill, Wiri Mountain, Green Hill, Otara Hill, Hampton Park and Mt Cambria. Spatio-temporal hazard estimates are updated on the basis of the new ordering, which suggest that the scale of the ‘flare-up’ around 30 ka, while still highly significant, was less than previously thought.

Error in topographic attributes for volcanic hazard assessment of the Auckland Volcanic Field (New Zealand)

ABSTRACTRemotely sensed topographic datasets are a major source of information in modelling environmental and geomorphic processes. In this investigation, four of the most popular remotely sensed topographic datasets available for the Auckland Volcanic Field (AVF, New Zealand) were compared using high-accuracy control points, such as real-time kinematic global positioning system (RTK GPS) profiles as well as a terrestrial laser scanning (TLS) surface. The LiDAR (light detection and ranging) data were found to be the most accurate with a root-mean-square error (RMSE) of ±0.9 m, while other datasets such as contour-derived digital elevation model (DEM), Shuttle Radar Topography Mission (SRTM) digital terrain model (DTM) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) global DEM (GDEM) were found to be accurate at 5–10 m levels. As part of the error assessment, an extensive comparison was carried out between a range of popular terrain attributes (e.g. elevation, volume and slope angle) to determine their variability as a function of input data properties (e.g. surveying technique and data structure). This study shows that the eruptive volumes of monogenetic volcanoes are sensitive to the input data type and its spatial resolution. The moderately vegetated lava flow fields of Rangitoto have an uncertainty in eruptive volume estimate by ±15% due to the overall surface roughness. For hazard assessment purposes in the AVF, resampled LiDAR datasets are reliable; however, other datasets such as contour-derived DEM and SRTM DTM can be used to estimate eruptive volumes of monogenetic volcanoes.

Long-lived shield volcanism within a monogenetic basaltic field: The conundrum of Rangitoto volcano, New Zealand

Drilling through the edifice of Rangitoto, the youngest and largest volcano in the “monogenetic” Auckland volcanic field, reveals the multistage eruptive and magmatic history of a small basalt shield volcano. Activity commenced calendar year (cal.) 6000 cal. yr B.P., involving minor effusive and pyroclastic volcanism until 650 cal. yr B.P. This period either represents an early, less productive phase of a single polygenetic volcano, or, alternatively, Rangitoto is better described as a volcanic complex that includes one or more buried edifices concealed by the main structure. A voluminous shield-building phase occurred 650–550 cal. yr B.P., erupting isotopically uniform subalkalic basalts (Mg# 60–64). Four batches of magma distinguished by trace-element chemistry were erupted sequentially, but they lack genetic connection via fractional crystallization or assimilation. Two of the magma batches display linear trends of decreasing incompatible trace-element abundance and increasing ratios of moderately incompatible to highly incompatible elements with decreasing age. This is consistent with cycles of progressive partial melting at the source. The final phase of activity (ca. 550–500 cal. yr B.P.) was explosive and less voluminous, producing scoria cones at the summit. This phase involved more diversity in magma compositions, including more mafic subalkalic basalt, and alkali basalt, pointing to sourcing of magmas simultaneously from different depths in the mantle. Rangitoto volcano contributes to a growing body of evidence showing that major periods of volcanism in “monogenetic” basalt fields occur at centers that have experienced multiple eruption episodes. Changes in magma composition accompany changes in eruption style, but a lack of an obvious shared pattern in magmatic evolution at various volcanoes points to the localized mantle heterogeneity and conduit systems. Hazard scenarios for regions traditionally classified as “monogenetic” need to encompass the possibility of prolonged episodes of activity and reawakening of volcanoes, a significant implication where infrastructure is built on such regions.

Sensitivity to volcanic field boundary

Volcanic hazard analyses are desirable where there is potential for future volcanic activity to affect a proximal population. This is frequently the case for volcanic fields (regions of distributed volcanism) where low eruption rates, fertile soil, and attractive landscapes draw populations to live close by. Forecasting future activity in volcanic fields almost invariably uses spatial or spatio-temporal point processes with model selection and development based on exploratory analyses of previous eruption data. For identifiability reasons, spatio-temporal processes, and practically also spatial processes, the definition of a spatial region is required to which volcanism is confined. However, due to the complex and predominantly unknown sub-surface processes driving volcanic eruptions, definition of a region based solely on geological information is currently impossible. Thus, the current approach is to fit a shape to the known previous eruption sites. The class of boundary shape is an unavoidable subjective decision taken by the forecaster that is often overlooked during subsequent analysis of results. This study shows the substantial effect that this choice may have on even the simplest exploratory methods for hazard forecasting, illustrated using four commonly used exploratory statistical methods and two very different regions: the Auckland Volcanic Field, New Zealand, and Harrat Rahat, Kingdom of Saudi Arabia. For Harrat Rahat, sensitivity of results to boundary definition is substantial. For the Auckland Volcanic Field, the range of options resulted in similar shapes, nevertheless, some of the statistical tests still showed substantial variation in results. This work highlights the fact that when carrying out any hazard analysis on volcanic fields, it is vital to specify how the volcanic field boundary has been defined, assess the sensitivity of boundary choice, and to carry these assumptions and related uncertainties through to estimates of future activity and hazard analyses.

Mantle heterogeneity controls on small-volume basaltic volcanism: REPLY

Eruptions of basaltic material in small-scale volcanic fields located in intraplate settings display a very diverse range in physical and chemical characteristics. Despite its relevance to the understanding of volcanic hazards, the relationship between physical properties of eruptions (explosivity, volume, location) and chemical composition of erupted products has, to date, not been investigated. Here we present a relationship between mantle heterogeneity and extents of partial melting, and both erupted volumes and eruptive style from the Auckland Volcanic Field (New Zealand), and we suggest that this provides a general model for small-scale “monogenetic” magmatic systems globally. Small volcanic centers consistently take the form of nephelinitic tuff rings and scoria cones, whereas larger centers are produced from effusive eruptions of less alkalic magmas. Nephelinitic melts are generated by melting of a deep, carbonated source, whereas less alkalic melts are the products of melting of a shallower, noncarbonated source. U-Th-Ra isotope data from eruptions closely paired in space and time show that mixing between magmas is extremely limited as a consequence of different ascent mechanisms due to differential segregation of melts from varying sources (early, carbonated melts ascending by higher porosity channels, and later, uncarbonated melts by a more diffusive regime). This suggests that extraction of melt is nearly instantaneous in these environments. Our results stress the importance of melting and magma dynamics in determining the size and style of eruptions in small volcanic fields, and suggest that mantle controls should be an important consideration in volcanic hazard assessment.

Shallow-seated explosions in the construction of the Motukorea tuff ring (Auckland, New Zealand): Evidence from lithic and sedimentary characteristics

At least 52 eruption centres are scattered within the ~360km2 Auckland Volcanic Field (AVF). Motukorea, now an island in the Waitemata Harbour, is one of 39 AVF volcanoes that experienced a phreatomagmatic explosive phase, before a magmatic phase. The volcano erupted through a ~200–300m-thick, consolidated, mudstone/sandstone sequence of the Miocene Waitemata Group, which overlies the Waipapa Terrane greywacke basement. Detailed field descriptions of the sedimentary characteristics of the early phreatomagmatic deposits were carried out, along with examination of lithics. The ejecta ring deposit comprises 55 to 60vol.% lithics, of which Waitemata Group fragments constitute approximately 90vol.%, whereas <10vol.% are Waipapa fragments, suggesting a dominance of shallow fragmentation. The sedimentary characteristics of the stratigraphic sequence at Motukorea suggest a dominance of wet surges at the beginning of the eruption with progression into drier sequences upwards. This is reflected in increasing inter-bedded juvenile-pyroclast-dominated fall deposits up-sequence. These characteristics are attributed to the changing hydrogeological conditions within the diatreme and the host rocks. These findings shed light on the eruption dynamics of phreatomagmatic eruptions through consolidated rocks in the AVF and enable the depiction of a scenario of future eruptions within the field in similar substrates.

Tools and techniques for developing tephra stratigraphies in lake cores: A case study from the basaltic Auckland Volcanic Field, New Zealand

Probabilistic hazard forecasting for a volcanic region relies on understanding and reconstructing the eruptive record (derived potentially from proximal as well as distal volcanoes). Tephrostratigraphy is commonly used as a reconstructive tool by cross-correlating tephra deposits to create a stratigraphic framework that can be used to assess magnitude–frequency relationships for eruptive histories. When applied to widespread rhyolitic deposits, tephra identifications and correlations have been successful; however, the identification and correlation of basaltic tephras are more problematic. Here, using tephras in drill cores from six maars in the Auckland Volcanic Field (AVF), New Zealand, we show how X-ray density scanning coupled with magnetic susceptibility analysis can be used to accurately and reliably identify basaltic glass shard-bearing horizons in lacustrine sediments and which, when combined with the major and trace element signatures of the tephras, can be used to distinguish primary from reworked layers. After reliably identifying primary vs. reworked basaltic horizons within the cores, we detail an improved method for cross-core correlation based on stratigraphy and geochemical fingerprinting. We present major and trace element data for individual glass shards from 57 separate basaltic horizons identified within the cores. Our results suggest that in cases where major element compositions (SiO2, CaO, Al2O3, FeO, MgO) do not provide unambiguous correlations, trace elements (e.g. La, Gd, Yb, Zr, Nb, Nd) and trace element ratios (e.g. [La/Yb]N, [Gd/Yb]N, [Zr/Yb]N) are successful in improving the compositional distinction between the AVF basaltic tephra horizons, thereby allowing an improved eruptive history of the AVF to be reconstructed.

Mantle heterogeneity controls on small-volume basaltic volcanism

Eruptions of basaltic material in small-scale volcanic fields located in intraplate settings display a very diverse range in physical and chemical characteristics. Despite its relevance to the understanding of volcanic hazards, the relationship between physical properties of eruptions (explosivity, volume, location) and chemical composition of erupted products has, to date, not been investigated. Here we present a relationship between mantle heterogeneity and extents of partial melting, and both erupted volumes and eruptive style from the Auckland Volcanic Field (New Zealand), and we suggest that this provides a general model for small-scale “monogenetic” magmatic systems globally. Small volcanic centers consistently take the form of nephelinitic tuff rings and scoria cones, whereas larger centers are produced from effusive eruptions of less alkalic magmas. Nephelinitic melts are generated by melting of a deep, carbonated source, whereas less alkalic melts are the products of melting of a shallower, noncarbonated source. U-Th-Ra isotope data from eruptions closely paired in space and time show that mixing between magmas is extremely limited as a consequence of different ascent mechanisms due to differential segregation of melts from varying sources (early, carbonated melts ascending by higher porosity channels, and later, uncarbonated melts by a more diffusive regime). This suggests that extraction of melt is nearly instantaneous in these environments. Our results stress the importance of melting and magma dynamics in determining the size and style of eruptions in small volcanic fields, and suggest that mantle controls should be an important consideration in volcanic hazard assessment.

Excavation of buried Dun Mountain–Maitai terrane ophiolite by volcanoes of the Auckland Volcanic field, New Zealand

We present a description of the crustal rocks that underlie the Auckland Volcanic Field (AVF) based on a diverse suite of country rock-derived lithic clasts in the phreatomagmatic tuff of Glover Park and Taylors Hill volcanoes. The clasts are dominated by mafic schistose and non-schistose, amphibolite grade, meta-igneous (metabasite) rocks. Structural and mineralogical studies of these rocks reveal a complex structural and metamorphic history, including metasomatism and retrogressive metamorphism, which can be linked to their deformation history. These metabasites have no genetic relationship with associated AVF basalts. Location, composition, deformation, metamorphism and metasomatism indicate that the metabasite clasts come from a melange along the eastern boundary of the ophiolite that causes the regional Junction Magnetic Anomaly (JMA), which passes beneath the Auckland region and connects up with exposures of the Dun Mountain–Maitai terrane of the South Island. This conclusion is supported by one specimen from a suite of lower-grade metamorphic, greywacke-type clastic ejecta that contains shell fragments which we interpret as equivalents of the Atomodesma fragments in the Dun Mountain–Maitai terrane. Other lithic ejecta are sandstones from the underlying Miocene Waitemata Group, Mesozoic greywacke with prehnite veins, and chert. Pervasive cataclasite networks in the lithic clasts indicate that the Glover Park and Taylors Hill volcanoes mined a crustal fault zone within the Mesozoic basement several hundred metres deep, at an unconformity between basement rocks and overlying Cenozoic sediments.

Spatio-volumetric hazard estimation in the Auckland volcanic field

The idea of a volcanic field ‘boundary’ is prevalent in the literature, but ill-defined at best. We use the elliptically constrained vents in the Auckland Volcanic Field to examine how spatial intensity models can be tested to assess whether they are consistent with such features. A means of modifying the anisotropic Gaussian kernel density estimate to reflect the existence of a ‘hard’ boundary is then suggested, and the result shown to reproduce the observed elliptical distribution. A new idea, that of a spatio-volumetric model, is introduced as being more relevant to hazard in a monogenetic volcanic field than the spatiotemporal hazard model due to the low temporal rates in volcanic fields. Significant dependencies between the locations and erupted volumes of the observed centres are deduced, and expressed in the form of a spatially-varying probability density. In the future, larger volumes are to be expected in the ‘gaps’ between existing centres, with the location of the greatest forecast volume lying in the shipping channel between Rangitoto and Castor Bay. The results argue for tectonic control over location and magmatic control over erupted volume. The spatio-volumetric model is consistent with the hypothesis of a flat elliptical area in the mantle where tensional stresses, related to the local tectonics and geology, allow decompressional melting.