Graham S. Leonard

Double trouble: Paired ignimbrite eruptions and collateral subsidence in the Taupo Volcanic Zone, New Zealand

Large explosive eruptions are generally rare, random events in the history of any particular volcano, volcanic area, or worldwide. In the Taupo Volcanic Zone, New Zealand, temporal clustering of eruptions occurs on a 15 smaller eruptions over a total ∼100 k.y. period. After a precursor eruption from a nearby source (and a break of years to decades), these paired eruptions in turn generated a wet ash-fall deposit and a dry pumice-fall deposit; the Mamaku ignimbrite (>145 km 3 magma); a fine-grained vitric ash-fall deposit; then the Ohakuri ignimbrite (>100 km 3 magma). Rotorua and Ohakuri, spaced ∼30 km apart, are the inferred collapse calderas associated with the Mamaku and Ohakuri ignimbrites, respectively. The early wet and dry fall deposits came from southerly sources, close to or within the subsequent Ohakuri caldera, while the fine-grained vitric ash is inferred to represent a co-ignimbrite ash from the Mamaku ignimbrite. At its southwest margin, the Mamaku ignimbrite overlies, but is also intercalated within and then overlain by, the pumice fall deposit, demonstrating that at least two widely spaced vents were active simultaneously for part of the eruption sequence. The post-Mamaku vitric ash-fall deposit underwent only trivial reworking prior to emplacement of the Ohakuri ignimbrite. This and other field evidence imply continuity, or time gaps of only days to months, in the whole paired sequence. Syneruptive volcanotectonic faulting may have permitted accumulation of >400 m of nonwelded Ohakuri ignimbrite through graben subsidence. Posteruptive faulting within years to decades of the eruption produced an ∼300 m extra-caldera offset of the Mamaku ignimbrite and collateral subsidence of a >40 km 2 area immediately south of Rotorua caldera. Temporal linkages between ignimbrite eruptions and graben subsidence, the NNE-SSW alignment of associated faulting between the Rotorua and Ohakuri calderas, and the eruption-related subsidence indicate a tectonic control on volcanism associated with Taupo Volcanic Zone rifting processes. Statistical forecasts of the frequency of large-volume explosive events based on averages may be inaccurate because of tectonic triggering effects.

High-level stratigraphic scheme for New Zealand rocks

We formally introduce 14 new high-level stratigraphic names to augment existing names and to hierarchically organise all of New Zealands onland and offshore Cambrian–Holocene rocks and unconsolidated deposits. The two highest-level units are Austral Superprovince (new) and Zealandia Megasequence (new). These encompass all stratigraphic units of the countrys Cambrian–Early Cretaceous basement rocks and Late Cretaceous–Holocene cover rocks and sediments, respectively. Most high-level constituents of the Austral Superprovince are in current and common usage: Eastern and Western Provinces consist of 12 tectonostratigraphic terranes, 10 igneous suites, 5 batholiths and Haast Schist. Ferrar, Tarpaulin and Jaquiery suites (new) have been added to existing plutonic suites to describe all known compositional variation in the Tuhua Intrusives. Zealandia Megasequence consists of five predominantly sedimentary, partly unconformity-bounded units and one igneous unit. Momotu and Haerenga supergroups (new) comprise lowermost rift to passive margin (terrestrial to marine transgressive) rock units. Waka Supergroup (new) includes rocks related to maximum marine flooding linked to passive margin culmination in the east and onset of new tectonic subsidence in the west. Māui and Pākihi supergroups (new) comprise marine to terrestrial regressive rock and sediment units deposited during Neogene plate convergence. Rūaumoko Volcanic Region (new) is introduced to include all igneous rocks of the Zealandia Megasequence and contains the geochemically differentiated Whakaari, Horomaka and Te Raupua supersuites (new). Our new scheme, Litho2014, provides a complete, high-level stratigraphic classification for the continental crust of the New Zealand region.

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.

Developing warning and disaster response capacity in the tourism sector in coastal Washington, USA.

Purpose – There has been a considerable effort over the last decade to increase awareness of the tsunami risk in coastal Washington, USA. However, contemporary research on warning systems spawned by the recent Indian Ocean tsunami tragedy highlights the need for development of an effective tsunami warning system for both residents and transient populations, including visitors and tourists. This study sets out evaluate staff training for emergencies, emergency management exercises (including drills and evacuation), and hazard signage within motels and hotels in Ocean Shores, Washington, USA.Design/methodology/approach – Data were collected from interviews with reception staff and managers at 18 hotels, motels, and other accommodation establishments.Findings – Levels of staff training and preparedness for tsunami and other hazards were found to be generally very low, although examples of “best practice” were found at a select few establishments. Larger hotels already had orientation or general training prog...

Evolution of the intra-arc Taupo-Reporoa Basin within the Taupo Volcanic Zone of New Zealand

The spatial and temporal distributions of volcaniclastic deposits in arc-related basins reflect a complex interplay between tectonic, volcanic, and magmatic processes that is typically difficult to unravel. We take advantage of comprehensive geothermal drill hole stratigraphic records within the Taupo-Reporoa Basin (TRB), and integrate them with new 40 Ar/ 39 Ar age determinations, existing age data, and new mapping to develop a four-dimensional model of basin evolution in the central Taupo Volcanic Zone (TVZ), New Zealand. Here, exceptional rhyolitic productivity and high rates of extensional tectonism have resulted in the formation of at least eight calderas and two subparallel, northeast-trending rift basins, each of which is currently subsiding at 3 to 4 mm/yr: the Taupo fault belt (TFB) to the northwest and the TRB to the southeast (the main subject of this paper). The basins are separated in the northeast by a high-standing, fault-controlled range termed the Paeroa block, which is the focus of mapping for this study, and in the southwest by an along strike alignment of smaller scale faults and an associated region of lower relief. Stratigraphic age constraints within the Paeroa block indicate that a single basin (∼120 km long by 60 km wide) existed within the central TVZ until 339 ± 5 ka (Paeroa Subgroup eruption age), and it is inferred to have drained to the west through a narrow and deep constriction, the present-day Ongaroto Gorge. Stratigraphic evidence and field relationships imply that development of the Paeroa block occurred within 58 ± 26 k.y. of Paeroa Subgroup emplacement, but in two stages. The northern Paeroa block underwent uplift and associated tilting first, followed by the southern Paeroa block. Elevations (>500 m above sea level) of lacustrine sediments within the southern Paeroa block are consistent with elevations of rhyolite lavas in the Ongaroto Gorge, the outlet to the paleolake in which these sediments were deposited, and indicate that the Paeroa block has remained relatively stable since development. East of the Paeroa block, stratigraphic relationships show that movement along the Kaingaroa Fault zone, the eastern boundary of the central TVZ, is associated with volcano-tectonic events. Stratigraphic and age data are consistent with rapid formation of the paired TRB and TFB at 339 ± 5 ka, and indicate that gradual, secular rifting is punctuated by volcano-tectonic episodes from time to time. Both processes influence basin evolution.

The nature and age of Ohakuri Formation and Ohakuri Group rocks in surface exposures and geothermal drillhole sequences in the central Taupo Volcanic Zone, New Zealand

Abstract The name “Ohakuri” has been applied both as formation and group names to surface deposits and deeply buried deposits recorded in geothermal drillholes in the central Taupo Volcanic Zone (TVZ) of New Zealand. The surface deposits are dated at 240 ± 10 ka (1 SD) and cap sequences that postdate the 320–340 ka Whakamaru group ignimbrites, whereas the buried deposits demonstrably predate the Whakamaru group ignimbrites. The Ohakuri Formation is here redefined to refer only to the surface deposits, which are also reinterpreted to be composed predominantly of primary pyroclastic deposits (mostly ignimbrite with only minor intercalated fall and secondary mass‐flow deposits). The term Waikora Formation has been used interchangeably with the term Ohakuri Group in geothermal drill core logs, but was originally defined to cover only pre‐Whakamaru sedimentary units containing abundant rounded greywacke clasts. We recommend three terms be used: Waikora Formation, which retains its original usage, Tahorakuri Formation (new) for the other volcaniclastic and sedimentary deposits between the Whakamaru group ignimbrites and the greywacke basement, and Reporoa Group (new) as an overall term for all subsurface pre‐Whakamaru lavas and deposits previously and collectively referred to by the name Ohakuri Group. The Reporoa Group thus includes the Waikora and Tahorakuri Formations, plus numerous other locally named thick ignimbrites and lava flows between the Whakamaru group ignimbrites and the greywacke basement. The term Ohakuri Group should be abandoned.

Can Volcanic Ash Poison Water Supplies

Carol Stewart,*3 Lino Pizzolon,4 Thomas Wilson,1 Graham Leonard,I David Dewar,1 David Johnston,# and Shane Cronin33 7Private Consultant, Brooklyn, Wellington, New Zealand 8National University of Patagonia, Esquel, Argentina 6University of Canterbury, Christchurch, New Zealand IGNS Science, Avalon, Lower Hutt, New Zealand #GNS Science/Massey University, Avalon, Lower Hutt, New Zealand 77Massey University, Palmerston North, New Zealand * stewart.carol@xtra.co.nz

Community Understanding of, and Preparedness for, Earthquake and Tsunami Risk in Wellington, New Zealand

The city of Wellington, New Zealand’s capital, is exposed to a wide range of potentially devastating impacts from various natural hazards. It is situated in one of the most active seismic regions in New Zealand, creating a significant earthquake risk. Another hazard to which it is exposed is that of tsunami from local and distant sources. Given the variety of hazards that Wellington faces, consideration of how the risks from such hazards can be reduced is necessary. Preparedness activities can be undertaken to try and reduce risk, with individual household preparedness forming one such activity. Motivating citizens to prepare can be a difficult task. Educators have often long assumed that if individuals are told about the risk of hazards, then they will begin to prepare; however, this is not usually the case. This is reflected in Wellington where traditional public education (i.e. information dissemination) has predominantly been undertaken to inform people about the risk of earthquakes and tsunami. Results from evaluation surveys show that in the wake of the public education campaigns, awareness of the risk is high, but levels of comprehensive preparedness low. Consequently it is apparent that risk perception does not usually link directly with preparedness, but is amplified or attenuated through a variety of individual, social psychological and community factors. Such factors are important to the preparedness process and must be considered when developing public education programmes. Programmes should include a traditional information dissemination element to build awareness of the risk, as well as a more interactive community-based component to foster important factors such as critical awareness, self-efficacy, outcome expectancy, action coping, participation, engagement and trust.

Age and eruptive center of the Paeroa Subgroup ignimbrites (Whakamaru Group) within the Taupo Volcanic Zone of New Zealand

We here explore the temporal and spatial relationships between the contrasting sources for two eruptive episodes that collectively represent the Whakamaru Group, the largest ignimbrite-forming sequence in the ∼2 m.y. history of the Taupo Volcanic Zone in New Zealand. At 349 ± 4 ka (weighted mean at 2σ), the >1500 km 3 widespread Whakamaru Group ignimbrites and ∼700 km 3 Rangitawa Tephra fallout were erupted in association with collapse of the 40 km long by 25 km wide rectilinear Whakamaru caldera. New 40 Ar/ 39 Ar age data presented here show that the co-magmatic >110 km 3 Paeroa Subgroup ignimbrites, previously included as part of the Whakamaru Group, are slightly younger and were erupted at 339 ± 5 ka (weighted mean at 2σ). New field evidence also presented here demonstrates that the Paeroa Subgroup ignimbrites came from a source geographically separated from vents for the widespread Whakamaru Group ignimbrites. The presence of co-ignimbrite lag breccias, sizes of vent-derived lithic clasts, thicknesses of exposed and subsurface deposits, and morphologies of deposits imply that eruptions of the Paeroa Subgroup occurred from a linear source (the Paeroa linear vent zone), coinciding with the present-day northeast-striking Paeroa fault, and outside (northeast) of the earlier Whakamaru caldera collapse area. No separate caldera has been recognized, although three nearby areas may have undergone eruption-related subsidence. Residual magma from the Whakamaru or adjacent Kapenga caldera areas may have migrated toward the Paeroa linear vent zone during eruptive episodes, resulting in subsidence in either, or both, of these areas. Shallow plutons are also inferred to lie beneath near source fault blocks (Paeroa and Te Weta) on each side of the fault, and eruption-related subsidence may have been expressed as movement across the Paeroa fault and localized subsidence in the southern Paeroa fault block. Subsequent secular, rift-related displacement along the Paeroa fault has obscured the Paeroa linear vent zone.

Volcanic ashfall preparedness poster series: a collaborative process for reducing the vulnerability of critical infrastructure

Volcanic ashfall can be damaging and disruptive to critical infrastructure including electricity generation, transmission and distribution networks, drinking-water and wastewater treatment plants, roads, airports and communications networks. There is growing evidence that a range of preparedness and mitigation strategies can reduce ashfall impacts for critical infrastructure organisations. This paper describes a collaborative process used to create a suite of ten posters designed to improve the resilience of critical infrastructure organisations to volcanic ashfall hazards. Key features of this process were: 1) a partnership between critical infrastructure managers and other relevant government agencies with volcanic impact scientists, including extensive consultation and review phases; and 2) translation of volcanic impact research into practical management tools. Whilst these posters have been developed specifically for use in New Zealand, we propose that this development process has more widely applicable value for strengthening volcanic risk resilience in other settings.