Jan M. Lindsay

Age of the Auckland Volcanic Field: a review of existing data

Determining magnitude–frequency relationships, a critical first step in assessing volcanic hazard, has been hampered in the Auckland Volcanic Field (AVF) by the difficulty in dating past eruptions from the fields c. 50 centres. We assessed 186 age determinations from 27 centres for reliability and consistency. Results indicate that only three centres (Rangitoto 0.6 ka; Mt Wellington 10 ka; Three Kings 28.5 ka) are reliably and accurately dated. Eight are reasonably reliably dated within a small age range: Crater Hill, Kohuora, Mt Richmond, Puketutu, Taylors Hill and Wiri Mountain (all 32–34 ka); Ash Hill (32 ka); and Purchas Hill (11 ka). Tephrochronology of lake sediments and relative ages from stratigraphic relationships provide age constraints for a further 9 and 11 centres, respectively. Although recent Ar–Ar studies show promise, ages of AVF centres generally remain poorly understood; this has implications for any statistical treatment of the distribution of volcanism in the AVF.

The influence of probabilistic volcanic hazard map properties on hazard communication

Probabilistic volcanic hazard analysis is becoming an increasingly popular component of volcanic risk reduction strategies worldwide. While probabilistic hazard analyses offer many advantages for decision-making, displaying the statistical results of these analyses on a map presents new hazard communication challenges. Probabilistic information is complex, difficult to interpret, and associated with uncertainties. Conveying such complicated data on a static map image without careful consideration of user perspectives or context, may result in contrasting interpretations, misunderstandings, or aversion to using the map. Here, we present the results of interviews and surveys conducted with organisational stakeholders and scientists in New Zealand which explored how probabilistic volcanic hazard map properties influence map interpretation, understanding, and preference. Our results suggest that data classification, colour scheme, content, and key expression play important roles in how users engage with and interpret probabilistic volcanic hazard maps.Data classification was found to influence the participants’ perceived uncertainty and data reading accuracy, with isarithmic style maps reducing uncertainty and increasing accuracy best. Colour scheme had a strong influence on the type of hazard messages interpreted, with a red-yellow scheme conveying the message of a hazard distribution (high to low), and a red-yellow-blue scheme conveying the message of hazard state (present or absent) and/or risk. Multiple types of map content were found to be useful, and hazard curves were viewed as valuable supplements. The concept of “confidence” was more easily interpreted than upper and lower percentiles when expressing uncertainty on the hazard curves. Numerical and verbal expression in the key also had an influence on interpretation, with a combination of both a percent (e.g., 25%) and a natural frequency (e.g., 1 in 4) “probability” being the most inclusive and widely-understood expression. The importance of these map property choices was underscored by a high portion of participants preferring to receive maps in unalterable formats, such as PDF.This study illustrates how engaging with users in a bottom-up approach can complement and enhance top-down approaches to volcanic hazard mapping through a collaborative and integrative design process which may help to prevent miscommunications in a future crisis when maps are likely to be drafted and disseminated rapidly.

Primary Volcanic Landforms

Abstract Volcanic landforms on the Earth range are shapes and sizes from tiny scoria cones to enormous flood basalt or ignimbrite plateaux. Controls on the final volcanic landform include magma composition and volume, tectonic environment, nature of the crust, and posteruption erosion. Primary volcanic landforms (pre-erosion) can be divided into polygenetic and monogenetic volcanoes. The former include composite, shield, and caldera volcanoes; the latter include mafic minor centers such as scoria and spatter cones, tuff rings and cones, maars and kimberlites, as well as silicic minor centers such as lava domes and coulees. The most obvious volcanic landforms at the planetary scale are spatially, temporally, and magmatically connected volcanic provinces, such as volcanic fields, volcanic arcs, and large igneous provinces. These may contain all of the individual polygenetic and monogenetic volcanic landforms regardless of tectonic environment, providing strong evidence for universal primary controls on the character of Earths volcanic landforms.

Time scales of intra‐oceanic arc magmatism from combined U‐Th and (U‐Th)/He zircon geochronology of Dominica, Lesser Antilles

The island of Dominica, located in the intra-oceanic Lesser Antilles arc, has produced a series of intermediate (mostly andesitic) lava domes and ignimbrites since the early Pleistocene. (U-Th)/He eruption ages from centers across the island range from ∼3 to ∼770 ka, with at least 10 eruptions occurring in the last 80 ka. Three eruptions occurred near the southern tip of Dominica (Plat Pays Volcanic Complex) in the past 15 ka alone. Zircon U-Th ages from individual centers range from near-eruption to secular equilibrium implicating protracted storage and recycling of zircons within the crust. Overlapping zircon crystallization peaks within deposits from geographically separated vents (up to 40 km apart) indicate that magma associated with separate volcanic edifices crystallized zircon contemporaneously. Two lava domes from the southern sector of the island display exclusively young zircon rim ages (<50 ka) with narrow crystallization peaks consistent with the construction of a new magma reservoir. The younging of eruption and crystallization ages implies that the magmatic foci leading to the construction of this reservoir have migrated southward, arc-parallel over time. Overall, our data support geochemical models for the ongoing construction of a silicic intrusive complex, consisting of varying amounts of crystal mush, beneath the island. U-Pb zircon ages <1–2 Ma indicate that accumulation of this complex is entirely Quaternary in age. Together zircon U-Th and U-Pb ages for Dominica suggest that the magmatic processes and time scales operating in intra-oceanic arcs are similar to those documented for continental arcs.

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.

Geosite of a steep lava spatter cone of the 1256 AD, Al Madinah eruption, Kingdom of Saudi Arabia

UNESCO promotes geoconservation through various programs intended to establish an inventory of geologically and geomorphologically significant features worldwide that can serve as an important database to understand the Earth’s global geoheritage. An ultimate goal of such projects globally is to establish geoparks that represent an integrated network of knowledge transfer opportunities, based on a specific array of geological and geomorphological sites able to graphically demonstrate how the Earth works to the general public. In these complex geoconservation and geoeducational programs, the identification of significant geological and geomorphological features is very important. These are commonly referred to as ‘geosites’ or ‘geomorphosites’, depending on whether the feature or processes the site demonstrates is more geological or geomorphological, respectively. The Kingdom of Saudi Arabia is an extraordinary place due to its arid climate and therefore perfect exposures of rock formations. The Kingdom is also home to extensive volcanic fields, named “harrats” in Arabic, referring particularly to the black, basaltic lava fields that dominate the desert landscape. Current efforts to increase awareness of the importance of these volcanic fields in the geological landscape of Arabia culminated in the first proposal to incorporate the superbly exposed volcanic features into an integrated geoconservation and geoeducation program that will hopefully lead to the development of a geopark named, “The Harrat Al Madinah Volcanic Geopark” [1]. Here we describe one of the extraordinary features of the proposed Harrat Al Madinah Volcanic Geopark, namely a steep lava spatter cone formed during a historical eruption in 1256 AD.

Evacuation planning in the Auckland Volcanic Field, New Zealand: a spatio-temporal approach for emergency management and transportation network decisions

Auckland is the largest city in New Zealand (pop. 1.5 million) and is situated atop an active monogenetic volcanic field. When volcanic activity next occurs, the most effective means of protecting the people who reside and work in the region will be to evacuate the danger zone prior to the eruption. This study investigates the evacuation demand throughout the Auckland Volcanic Field and the capacity of the transportation network to fulfil such a demand. Diurnal movements of the population are assessed and, due to the seemingly random pattern of eruptions in the past, a non-specific approach is adopted to determine spatial vulnerabilities at a micro-scale (neighbourhoods). We achieve this through the calculation of population-, household- and car-to-exit capacity ratios. Following an analysis of transportation hub functionality and the susceptibility of motorway bridges to a new eruption, modelling using dynamic route and traffic assignment was undertaken to determine various evacuation attributes at a macro-scale and forecast total network clearance times. Evacuation demand was found to be highly correlated to diurnal population movements and neighbourhood boundary types, a trend that was also evident in the evacuation capacity ratio results. Elevated population to evacuation capacity ratios occur during the day in and around the central city, and at night in many of the outlying suburbs. Low-mobility populations generally have better than average access to public transportation. Macro-scale vulnerability was far more contingent upon the destination of evacuees, with favourable results for evacuation within the region as opposed to outside the region. Clearance times for intra-regional evacuation ranged from one to nine hours, whereas those for inter-regional evacuation were found to be so high, that the results were unrealistic. Therefore, we conclude that, from a mobility standpoint, there is considerable merit to intra-regional evacuation.

Post-Miocene faults in Auckland: insights from borehole and topographic analysis

Abstract Although experiencing some seismicity, the Auckland region is generally considered to be one of New Zealands most tectonically stable areas. There is a distinct lack of mapped faults in the central Auckland area, yet regional trends indicated by several mapped faults surrounding it suggest that these faults should continue through Auckland. The overprint of Auckland Volcanic Field (AVF) deposits may mask any potential fault traces in the area. We carried out a detailed study of the topography of Auckland concealed beneath the AVF deposits, using borehole and geophysical data, and combined this with structural information gleaned from current topography and exposed geology to identify significant concealed fault offsets. The reliability of each potential fault displacement was determined using pre-defined attributes. The resulting block faulting model for Auckland supports the proposed connection between the position of the Junction Magnetic Anomaly and Auckland volcanism. The identification of several possible faults in the area covered by the AVF might indicate more structural control on volcanism than previously thought.

The 1256 AD Al Madinah historic eruption geosite as the youngest volcanic chain in the Kingdom of Saudi Arabia

Volcanic geosites, geomorphosites and geotops are the smallest ‘‘units’’ of intact geological features that are identifiable through their uniqueness, or because they are graphic examples of specific volcanic phenomena, or form a vital landscape representative of a specific volcanic processes. Here, we identify significant volcanic features that bear not only regional, but global, volcanic value in a confined area that could be organized and promoted as the first volcanic geopark in the Kingdom of Saudi Arabia: the Harrat Al Madinah Volcanic Geopark (HAMVG). Harrat Al Madinah (‘‘harrat’’ in Arabic means lava field) is among many intraplate basaltic volcanic fields (Camp and Roobol 1989; Camp et al. 1991, 1992) that are located in the western margin of the Arabian Peninsula forming a broad zone sub-parallel to the Red Sea Rift (Fig. 1), which has been active over the last 30 Ma. These harrat fields are relatively thin (typically \300 m) but cover vast areas, the largest of which is *60,000 km. The proposed HAMVG has many spectacular volcanic geosites including the last historically erupted volcanoes in the Kingdom of Saudi Arabia (Camp et al. 1987; Moufti et al. 2012). The 1256 AD eruption site is located near to the culturally significant Al Madinah city (*1.5 million population), which is one of the holiest places to Muslims. A major geotop tentatively named as ‘‘The 1256AD Al Madinah Historic Eruption Site’’ with distinct individual geosites/ geomorphosites has been selected to demonstrate the diversity of volcanic phenomena associated with intraplate volcanism of the Al Madinah Volcanic Field. Hawaiian to Strombolian type eruptions created lava spatter and scoria cones visible from major highways, allowing visitors to stop near the 1256 AD historic eruption site just 10 km SE of Al Madinah (Fig. 1). The historically documented eruption lasted for 52 days and formed a *2.25 km long chain of NW–SE-aligned scoria and lava spatter cones (Fig. 2), producing alkali-olivine basalt (*0.5 km) a0a and pahoehoe lava flows (Camp et al. 1987). At least seven cones have been identified. Most of them are nested lava spatter and scoria cones such the largest cone in the northern edge of the fissure shown on Fig. 2. A combination of energetic explosive eruptive episodes (violent Strombolian-style eruptions) interrupted by less explosive lava fountaining, lava flow outbreaks and corresponding volcanic cone rafting formed a moon-like

Geology, petrology, and petrogenesis of Little Barrier Island, Hauraki Gulf, New Zealand

Abstract Little Barrier Island is the emergent part of a large, isolated, dacite‐rhyodacite volcano in the active Hauraki Rift, 80 km northeast of Auckland. Two volcanic episodes are recognised: Waimaomao Formation was emplaced as a rhyodacite dome at 3 Ma, whereas the more extensive dacitic lavas of Haowhenua Formation were erupted between 1.2 and 1.6 Ma. All Little Barrier lavas are strongly porphyritic and contain phenocrysts of plagioclase, orthopyroxene, and hornblende. Geochemically, they are subduction related and distinct from the older lavas of the Coromandel Volcanic Group, being Zr rich but Rb and Ba poor. Their Sr and Nd isotope ratios are similar to those of the Tonga‐Kermadec arc volcanoes. Modelling of the dacite supports petrographic evidence that recharge and mixing were important in the magmatic system. Little Barrier and two dacite domes of similar age and composition near Whangarei form a northwest‐trending lineament subparallel to the Alexandra Volcanics and the Vening Meinesz Fractur...