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Re-establishing the beast : an investigation into the spatiotemporal evolution of the Y5 phase of the Taupō 232 ± 10 CE eruption, New Zealand : a thesis submitted in partial fulfilment of a Philosophiae Doctor degree in Earth Science, Volcanic Risk Solutions, School of Agriculture and Environment, Massey University, New Zealand
(Massey University, 2023) Tapscott, Sarah Joanne
Plinian eruptions are sustained, high-energy explosive eruptions that generate buoyant plumes that reach >20 km into the atmosphere. They often produce devastating pyroclastic density currents (PDC) along with widespread tephra fall out, with significant hazards to communities around the volcanoes. Current computational modelling of Plinian eruptions considers generalized steady versus unsteady column regimes as the explanation for the formation of coeval buoyant Plinian plumes and intraplinian PDCs; however, natural eruption scenarios indicate that these regimes can oversimplify the interpretation of both PDC and plinian fall deposits. The large-Plinian Y5 phase of the Taupō 232 ± 10 CE eruption has an exceptionally widespread and well-preserved deposit that incorporates fall and coeval PDCs. Despite an extensive dataset in place for the Y5, there remain conflicting views on the interpretation of its deposit regarding eruption and sedimentation dynamics. Original studies by Walker (1980) considered the Y5 as a single eruptive unit from the perspective of the widespread fall deposit, without consideration of the intraplinian, coeval Early Flow Units (EFU) identified by Wilson & Walker (1985). Walker’s study determined that the Y5 phase involved a Plinian plume ~50 km high. Bedding characteristics in the fall deposit were considered in detail by Houghton et al (2014), who used their qualitative observations to propose the presence of 26 subunits within the Y5 fall deposited by a fluctuating plume influenced by strong changes in wind direction. Houghton et al.’s study brought the plume height down to (35 – 40 km) and denoted a vent location ~6 km SW of that proposed by Walker (1980). This Ph.D. research presents a comprehensive quantitative dataset of the deposit characteristics in the vertical stratigraphy of the upper phreatoplinian Y4 deposit, and the coeval fall and PDC deposits of the Plinian Y5 phase of the Taupō eruption. The dataset is used to reconstruct the spatiotemporal evolution of the Y5 phase and improve our understanding of Plinian eruption dynamics and sedimentation. Detailed sample collection and analysis was conducted on proximal to medial deposit exposures, whose vertical stratigraphy encompass the final stage of the Y4 (Y4-G), the Y5 fall deposit and its coeval Early Flow Units (EFU). Samples were analysed for grain size distributions, componentry, and juvenile textural characteristics. It is demonstrated that foreign lithic lithologies and their time-relative abundance in relation to other deposit characteristics play an important role in informing vent location, the evolution of the conduit and the nature of generation of erupted facies (i.e., PDC and fall). In this study, foreign lithics were subdivided by their inferred stratigraphic depth of origin below the lake floor into: F1) pre-232 ± 10 CE volcanic material (~0 – 400 m), F2) predominantly Huka Group sediments, minor Whakamaru ignimbrite and hydrothermally altered material (~400 – 3000 m), and F3) plutonic microdiorites and granitoids (>4000 m) At the boundary between the Y4-G and Y5 deposits, a decrease in obsidian abundance of c. 30 wt.%, along with an increase in F2 lithics of c. 20 wt.%, and a drop in pumice vesicularity by c. 30 % indicate a distinct change in vent location between the Y4 and Y5 phases. F2 lithologies in the Y4 differ significantly from those in the Y5, but coincide with those of the plinian Y2 deposit, suggesting similar regions of crustal excavation for Y5 and Y2 and imply a vent location comparable to that of the Y2 phase. Vertical variations in the abundance and relative proportions of different juvenile and lithic pyroclasts, in pyroclast textures and pumice densities identified in the Y5 fall deposit, following the initial clearing of the vent, define three successive stages within a relatively steady, continuous eruption. These stages are: 1) the continuous excavation of the conduit at relatively low mass eruption rate shown through higher lithic:pumice ratios, finer overall grain size and higher pumice densities compared to later stages of the Y5; 2) increasing mass eruption rate towards a climax with relatively steady conduit erosion coinciding with deepening fragmentation, exhibited in increasingly larger grain sizes and relatively lower total lithic abundances, yet higher relative proportions of F2 and F3 lithics; and 3) a moderate decrease in mass eruption rate and the acceleration of conduit erosion (shown through a rapid increase in F1 abundance and decreasing grain size), promoting the potential early onset of caldera collapse that led to the Y6 ignimbrite producing blast event. Vertical bedding features in the Y5 fall deposit are shown to be laterally discontinuous and pinch out over length scales of 101-103 m. This precludes the possibility that coarse-fine fluctuations were caused by mass partitioning of material during partial column collapse, or by variations in wind direction. Instead, I suggest that the bedform features identified in the Y5 deposit result from gravitational instabilities in the umbrella cloud, sedimenting as tephra swathes. Additionally, the intraplinian EFUs were differentiated by their characteristics into two main types: Type 1 centimetre to metre thick, massive, pink-orange to cream coloured, coarser grained deposits that are topographically confined; and Type 2 decimetre to centimetre thick, massive to moderately stratified, white-grey, finer grained deposits that have mounted topography. The anomalously high proportion of ash (<10 µm at 4 – 27 wt.%) in the EFU deposits, in conjunction with a lack of evidence for enrichment of dense clasts (i.e., lithics and crystals), indicates that there was minimal to no mass partitioning that would be expected in the case of partial column collapse. In addition, the inferred high particle concentration of the Type 1 flows and their high temperature emplacement indicates that the materials that propagated to form the EFU PDCs is likely to have originated from lower heights around the jet where entrained air had limited effect to cool the mixture. A lack of variation in the proportion of lithic types and juveniles between Type 1 and Type 2 with relative height compared to the Y5 fall suggests that the EFU are a product of one generation mechanism and that the deposit types 1 and 2 represent contrasts in relative volume, runout distance, and/or topographical constraints on runout of individual flows. The EFU are entirely contained within fall activity and become more abundant, voluminous and/or increase in flow mobility with increasing mass eruption rate during the Y5 phase. The generation mechanism for the EFU PDCs strongly aligns with the modelling and field observations for gargle dynamics, where a dense sheath formed by recycled pre-existing material in a basin-like vent structure develops on an eruptive jet. This dense sheath produces PDCs simultaneous with a sustained Plinian column that occurs seemingly without interruption. Similarities can be drawn with deposits from other, historical large-Plinian eruptions such as the Bishop Tuff, 0.76 Ma and Novarupta, 1912, which also involved phases of coeval fall and PDC deposition analogous to the Y5 and EFUs, and were likely produced through gargle dynamics. This study has shown that through the detailed, quantitative characterisation of deposit features in plinian eruption deposits involving coeval fall and PDCs, the temporal changes in eruptive behaviour, conditions at source and the nature of sedimentation can be identified. Interpretations indicate that the Y5 phase of the Taupō 232 ± 10 CE eruption was a large, steady, and extremely powerful eruption beyond the general depiction of a ‘standard’ Plinian event. Using quantitative analysis such as this may help build upon our knowledge base of the eruption and sedimentation dynamics of large Plinian eruptions by providing a field-based foundation for the reconstruction of the spatiotemporal evolution of such events. This is intended to provide a pathway for the amalgamation of field data and computational eruption models, ultimately improving our ability to forecast and mitigate explosive eruption hazards at similar volcanoes globally.
Inside pyroclastic surges - a characterisation of the flow behaviour, hazard impact mechanisms and sedimentation processes through large-scale experiments : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand
(Massey University, 2020) Brosch, Ermanno
Dilute pyroclastic density currents (or pyroclastic surges) are frequently occurring and highly dangerous volcanic flows generated during explosive volcanic eruptions. These fast and ground-hugging flows of hot volcanic particles and gas swiftly sweep across landscapes to cause significant risk to life and infrastructure at many volcanoes globally. Understanding the flow dynamics of pyroclastic surges, and developing a quantitative understanding of the mechanisms behind their hazard impacts, are thus important requisites for the development and testing of robust hazard mitigation strategies. Despite strong progress through field, theoretical, laboratory and numerical approaches during the past 50 years, the understanding of the flow and hazard behaviour of pyroclastic surges is still highly fragmentary. An important reason behind current gaps in knowledge stems from the hostile nature of these currents, which, to date, has prevented any direct observations and internal measurements. This leaves many theories behind flow and hazard models untested and un-validated. Over the past ten years, large-scale experiments have provided a novel approach to generate the missing ‘view’ inside pyroclastic density currents. While recent experiments have improved the understanding of the internal flow behaviour of dense pyroclastic density currents, comparable large-scale experimental studies for pyroclastic surges remain outstanding. This thesis describes the first systematic series of large-scale experiments that aimed to obtaining detailed measurements of the dangerous interior conditions of pyroclastic surges. Conducted at the international eruption simulator facility PELE (the Pyroclastic flow Eruption Large-scale Experiment), this study aimed to provide answers to the following three research questions: What are the flow-internal processes that cause the extreme destruction potential of pyroclastic surges? What is the detailed internal structure of pyroclastic surges? What are the particle-transport and sedimentation processes occurring in the basal region of dilute PDCs? Through the development of new measurement techniques and the refinement of existing set-up approaches, a systematic series of experiments were completed, which provided comprehensive datasets of dilute PDC analogues. The main results and implications of this research are as follows. Direct measurements of the internal velocity and density structure inside experimental flows show that turbulence is an important driver of the destructiveness of pyroclastic density currents. The effects of turbulence manifest themselves through three cumulative mechanisms. First, most of the flow energy becomes focussed into large eddies whose turbulent excursions generate destruction-causing dynamic pressures that exceed traditionally estimated mean values manifold. Second, self-developed pulsing inside flows, associated with the propagation of large coherent turbulence structures, leads to high dynamic pressures, propagating and perpetuating downstream. Third, the characteristic frequencies associated with large eddy motion are able to excite resonance in large buildings. The characterisation of gas-particle interactions inside the dynamically and kinematically scaled experimental pyroclastic surges revealed that commonly assumed near-homogeneous coupling between particles and gas is the exception, while strong to intermediate feedback loops between gas and solid phases are omnipresent throughout most of the evolving flow. This leads to interesting mesoscale turbulence effects, which are here shown to modify and control the evolving vertical flow stratification, the spatiotemporally varying deposition mechanisms, and the generation of turbulence in addition to the typically assumed shear and buoyancy processes. The spatially variable velocity of the leading front of the experimental surges is here demonstrated to behave differently to aqueous particle-laden gravity currents. The propagation of flow-internal pulses inside the highly turbulent gravity currents is a key mechanism in determining the flow runout and consequent hazard characteristics. The deposits of the experimental pyroclastic surges are here shown to have strong similarities to real-world deposits. Simultaneous measurements of the evolving structure of the lower flow boundary and the accreting deposit add complexity to our current qualitative view of the sediment transport and deposition mechanisms inside pyroclastic surges. The occurrence of mesoscale turbulence modifies particle supply into the lower flow boundary and, through the rapid passage of large-eddy passages in this region, gives rise to dynamic changes between a range of bedload transport processes and deposition rates. A correlation of the contribution of the different regions of the experimental surge to the spatially and temporally evolving deposit provides new insights in to how real-world deposits originate and how best to sample them to characterise the behaviour of past PDC-forming eruptions. Parts of the experimental datasets obtained during this research form the first international benchmark case to test, validate and compare the current range of numerical dilute PDC flow and hazard models.
Paleoseismology, seismic hazard and volcano-tectonic interactions in the Tongariro Volcanic Centre, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, (Palmerston North, Manawatu), New Zealand
(Massey University, 2017) Gómez Vasconcelos, Martha Gabriela
At the southern part of the Taupo Rift, crustal extension is accommodated by a combination of normal faults and dike intrusions, and the Tongariro Volcanic Centre coexists with faults from the Ruapehu and Tongariro grabens. This close coexistence and volcanic vent alignment parallel to the regional faults has always raised the question of their possible interaction. Further, many periods of high fault slip-rate seem to coincide with explosive volcanic eruptions. For some periods these coincidences are shown to be unrelated; however, it remains important to evaluate the potential link between them. In the Tongariro Graben, the geological extension was quantified and compared to the total geodetic extension, showing that 78 to 95% of the extension was accommodated by tectonic faults and only 5 to 22% by dike intrusions. Within the latter, 4 to 5% was accommodated by volcanic eruptions and 18 to 19% by arrested dike intrusions, with an unknown percentage of hybrid extension. Short-term variations in fault slip-rates and volcanic activity for the last 100 ka in the Tongariro Volcanic Centre may have been influenced by static stress transfer between adjacent faults (within <20 km from the source) and dike intrusions (within <10 km), or by fluctuations in magma input through time. The amount of magma involved in the rifting process will condition the predominant extension mechanism and thus influence the predominant type of volcano-tectonic interaction. A record of volcanic and seismic activity for the last 250 ka was assembled, from new and published studies. This was used to analyse the spatiotemporal associations between volcanic and seismic activity in the southern Taupo Rift. Data on the faulting history, slip-rate variation and seismic hazard of the Upper Waikato Stream, Wahianoa, Waihi and Poutu faults formed the core of the analysis. These faults are capable of producing a MW 7.2 earthquake with a single-event displacement of 2.9 m, posing an important hazard to the region. Data gathered in this study provides an update to the National Seismic Hazard Model for New Zealand.
Economic risk assessment of Mount Egmont : the potential economic implications of a volcanic eruption in Taranaki : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Economics, Massey University, Palmerston North, New Zealand
(Massey University, 2006) Aldridge, Coral Louise
New Zealand is home to a large number of volcanoes, many of which threaten the North Island, with damaging ground hugging hazards or disruptive ash deposits. As little as 2mm of ash will put grazing animals off their feed, completely disrupting the agricultural environment, transport is affected and equipment is vulnerable. The most likely damaging event from an eruption is ash, the potentially unknown area of which is determined by wind direction and strength. The 1995/1996 Ruapehu eruption was geologically considered minor with no more than 10mm of ash deposited, yet the economic consequences and disruption were significant, estimates put the minimum cost of the eruption at $130million made up almost singularly of tourism revenue losses and damage to the hydro-electric turbines. There has been little work completed in assessing economic impact of a natural disaster in an economy prior to the event. While the expected scale of any disaster is frequently assessed on historical evidence for planning purposes, social or economic studies tend to consider vulnerable sectors during evacuation and recovery as opposed to a monetary figure or the economic impact. The most recognised volcanic event (and standard example) in recent history was the Mt St Helens eruption in 1980; this eruption killed 57 people and caused damage in excess of US$1billion. Mt Egmont is the visible headstone of Taranaki's volcanic history but is only the youngest location in a series of destructive volcanoes in the area. There have been no known eruptions within the region since 1755, with eight recorded eruptions in the 300 years prior. It is generally accepted that any future events from Mt Egmont will follow the same path as historic eruptions, explosive ash emissions with gentle lava extrusions. Three eruption scenarios, all skewed towards a more likely smaller eruption, are considered in the overall analysis of the region; future studies may concentrate on rare catastrophic eruptions or the evacuation of New Plymouth. The first scenario is limited largely to the national park with ash fall only within the region, the third scenario pushes ash over much of the North Island and has damaging hazards throughout Taranaki A final consideration is made to investigate how an economy responds to increased volcanic threat without an eruption. If precursors to volcanic activity extend for a long period of time the threat of economic stagnation, reduced investment, emotional stress and permanent relocation from the region will increase. Early warning systems and increased disaster planning has greatly reduced the number of deaths caused by volcanic eruptions, in many ways it has also increased economic vulnerability as danger zones become populated. Taranaki has a low population density with rich natural resources and an economy largely geared towards dairy farming and the extraction of oil and gas. The five largest sectors in Taranaki create $8,910.18million in total output or 57.83% of regional output; these are oil and gas extraction, dairy manufacturing, dairy farming, meat processing and wholesale trade. Oil and gas exploration adds an additional $331.72million to economic output. There is a lot of high level energy infrastructure in Taranaki from gas pipelines connecting fields, production stations and delivery systems to the multitude of high voltage power lines connecting two power generation stations with the national grid. All oil and gas production and much of the gas transmission system is based within Taranaki, this industry alone is estimated to contribute more than $1billion a year to the national economy. One factor of Taranaki's gas monopoly is the significant downstream impact any regional disruption in supply could have on the national economy and social well being. Oil and gas is vital to many aspects of New Zealand business not just within Taranaki but day to day business operations, manufacturing processes and power generation capacity. Iconic industries are those businesses that may have an impact on the local community above that of direct economic loss, that are socially as well as economically significant. These firms are predominantly the largest employers and contributors to the local and national economy, and are the most likely to consider permanent relocation outside the region in the case of a large ongoing event. Research was completed on significant industries to gain a more detailed impression of the largest contributors to the local and national economies and potential disruption. These enterprises include electricity generators and gas production, Fonterra, Ballance, Yarrows and Westgate Port. The National Park, tourism and the airline industry were also considered separately due to their individual importance and likelihood to be affected by an eruption. The results of the input-output scenario analyses show an immediate value added decline in the regional economy ranging between $519.09million and $2,505.21million due to volcanic eruption. Input-output captures the overall regional impact of an eruption, the immediate reduction in output as a result of evacuation and physical influences. However an eruption of any magnitude will also have a national impact on the economy which should not be forgotten. Iconic industries were considered separately to take into account some of the largest regional contributors to the national economy. Risk assessment of the iconic industries enabled the assessment of more long term, wide reaching and national effects of an eruption which are not captured in input-output assessments. The gas industry will have the most detrimental economic effect, literally closing the entire gas dependent manufacturing sector throughout the North Island for a number of weeks. Although the Whareroa dairy factory contributes considerable value to national exports with 100% of production being exported milk volumes normally processed could, with the exception of approximately two weeks during the peak season, be absorbed by other factories in the North Island limiting national impact. It is impossible to determine the degree of flow on effects from all of the businesses affected; many interdependencies wouldn't openly be recognised until they occurred. New Zealand has been lucky in that recent volcanic activity has been minor and sporadic in nature; consequently the public perception of risk has been skewed towards events which in geologic records would not even register. An eruption would overwhelm local civil defence resources almost immediately, the surrounding communities would be flooded with evacuees and the economic ripples would be widely felt. This is particularly the case with Taranaki and the critical high level infrastructure. Mitigating economic risk can only be done by locationally spreading risk, with adequate protection measures (financial or physical) and by increasing public awareness.
Physical and social impacts of past and future volcanic eruptions in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science, Massey University, Palmerston North, New Zealand
(Massey University, 1997) Johnston, David Moore
The North Island of New Zealand contains a number of active and potentially active volcanoes. Although the probability of an eruption affecting a significant portion of the North Island is relatively low in any one year, the probability of one occurring in the future is high. The potential impacts of a large eruption are significant and the risk cannot be ignored. The timing of the next eruption cannot yet be determined but its probable effects can reasonably be assessed. The 1945 eruption of Mount Ruapehu dispersed ash over a wide area of the North Island over a period of several months. Individual ash falls were only a few millimetres thick in communities closest to the volcano (<50 km) and trace amounts in communities farther away. Ash falls were mostly of nuisance value in affected communities, causing minor eye and throat irritations, soiling interiors of houses and damaging paintwork. More significant impacts included crop damage, low wool quality on farms close to the mountain, disruption to skiing, the removal of army vehicles from Waiouru and numerous disruptions to water and electricity supplies. The 1995-1996 eruptions caused similar physical effects to the 1945 eruption but had considerably greater social and economic impacts. Over the past 50 years the risk has increased significantly due to an increased population, higher visitor usage and a more technologically advanced infrastructure. With increasing development and population growth the risk from similar or larger eruptions will continue to increase. A community's infrastructure provides the services and linkages which allow society to function. These 'lifelines', involving electricity, water, sewerage and roading. are vulnerable to damage and/or disruption from a range of volcanic hazards. The most threatening hazards include pyroclastic falls, pyroclastic flows and surges, lava extrusions (flows and domes), lahars, debris avalanches and volcanic gases. Unfortunately there are very few quantitative measurements of the impacts of volcanic eruptions on community 'lifelines'. With direct observations of eruption impacts, combined with theoretical considerations, it is possible to form a conceptual model of the likely impacts of a given event. These can then be used to predict likely effects, which may then be utilised in risk analysis (and scenarios). Two eruption scenarios are considered: 1) a 0.1 km3 andesitic eruption of Ruapehu composite volcano during a northwesterly wind, affecting Hastings District; 2) a 4 km3 rhyolitic eruption from the Okataina caldera during a westerly wind, affecting Whakatane District. The choice of scenarios is designed to illustrate the contrast between a disruptive moderate-sized eruption from a cone volcano (Ruapehu) and the destructive impacts of a large caldera eruption (Okataina). The Ruapehu scenario will have disruptive short term impacts on Hastings District, with the recovery process spontaneous, immediate and rapid. The infrastructure of Whakatane will be severely damaged by the Okataina eruption scenario and suffer effects for many years. The social and economic impacts of both scenarios will be determined not only by direct physical consequences but also by the interaction of social and economic factors. Residents of both Whakatane and Hastings were surveyed in February 1995 to measure their understanding of volcanic hazards. This was repeated following the Ruapehu eruptions in November 1995. Few residents have copies of specific volcanic hazard information and few have undertaken any form of information searching prior to the 1995 Ruapehu eruption. The 1995 eruption resulted in a small increase in the numbers searching for information on volcanic hazards in both communities. Although some agencies are perceived as more credible than others as the source of volcanic hazard information, no one agency has a monopoly on perceived credibility (i.e. different people recognise different agencies as the best source of information on volcanic hazard information and warnings). During the 1995 Ruapehu eruption the media (TV, radio and newspaper) were the principal sources of information about what was happening. Different people rely on different channels for information and this should also be acknowledged when issuing warnings and releasing public information. Whakatane and Hastings supply interesting contrasts. Both were subjected to intense media coverage during the 1995 Ruapehu eruption, but Whakatane was spared any direct effects, whereas Hastings experienced the hazard directly, in the form of ash falls in September and October 1995. Only Hastings' respondents showed a significant change in the perceived volcanic threat. However, even though there was no significant change in the perception of volcanic threat in Whakatane, residents still continued to perceive the volcanic threat as being higher than Hastings residents. Experiencing the direct and indirect impacts of the 1995 Ruapehu eruption may make subsequent warnings and information releases more salient, thereby enhancing the likelihood of engaging in protective actions or other forms of response. This is likely to be the case for those individuals and organisations that experienced the greatest impacts. However, the relatively benign impacts may make many prone to a "normalisation bias", whereby individuals or organisations believe that the volcanic eruptions did not affect them negatively, therefore the negative impacts of future volcanic events will also avoid them. This may be prevalent in communities close to Ruapehu which escaped the direct ash falls as a consequence of favourable wind directions. This conclusion suggests that the 1995-1996 Ruapehu eruptions may have both improved and reduced individual, organisational and community preparedness for future volcanic events.
Forecasting the consequences of the failure of the eastern rim of Crater Lake, Mount Ruapehu : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Earth Science at Massey University, Palmerston North, New Zealand
(Massey University, 2011) Phillips, Emma; Phillips, Emma
A numerical code for simulating dry flows of granular material, Titan2D, was used to model a range of possible collapse scenarios and resulting debris avalanches from a possible failure of the eastern crater rim of Mount Ruapehu, New Zealand. The eastern rim of Crater Lake, Mount Ruapehu consists of a stratigraphic sequence of intercalating volcaniclastic diamictons, pyroclastics and lavas, some of which are highly hydrothermally altered. This rim is under outward pressure from Crater Lake and constitutes one of the steepest parts of the active volcano. Its sudden failure could involve up to 50 million m³ of rock material, almost certainly generating a debris avalanche and/or a break-out lahar up to 9 times the size of the March 2007 event. A failure of hydrothermally altered flank materials on this side of the volcano has already occurred (c. 4600 yrs Mangaio Fm. (Donoghue & Neall, 2001)). A quantitative hazard and risk analysis of this scenario has never been undertaken, despite ongoing hydrothermal alteration and considerable sapping of both the inside and outside of the rim from explosive eruptions and base surges during the 1995/1996 and 2007 eruptions. New stratigraphic data were integrated with existing high-resolution topographic information and aerial photography to produce a detailed map of the eastern rim to highlight the distribution of contrasting stratigraphic sequences and the distribution of those units with the largest degree of alteration. This information was used as the first step towards defining the likelihood of different failure volumes and geometries to be tested in numerical hazard simulations. A quantitative scenariobased hazard forecast for partial or full collapse of the crater rim and subsequent events was determined. Simulated data of flow run out, inundation, diversion, velocity and mass transport were analysed to identify the resulting hazards for the Whangaehu and Tongariro River catchments. The results of this research suggest that the Mangatoetoenui, Upper Waikato, Tongariro and Whangaehu River catchments could be greatly affected by a sudden collapse of the eastern rim and any subsequent lahar events.
Phreatomagmatic volcanic hazards where rift systems meet the sea, a study from Ambae Island, Vanuatu
(Elsevier, 2009) Nemeth K; Cronin SJ
Ambae Island is a mafic stratovolcano located in the northern Vanuatu volcanic arc and has a NE-SW rift-controlled elongated shape. Several hundred scoria cones and fissure-fed lava fields occur along its long axis. After many decades of quiescence, Ambae Island erupted on the 28th of November 2005, disrupting the lives of its 10,000 inhabitants. Its activity remained focused at the central (crater-lake filled) vent and this is where hazard-assessments were focused. These assessments initially neglected that maars, tephra cones and rings occur at each tip of the island where the eruptive activity occurred < 500 and < 300 yr B.P. The products of this explosive phreatomagmatic activity are located where the rift axis meets the sea. At the NE edge of the island five tephra rings occur, each comparable in size to those on the summit of Ambae. Along the NE coastline, a near-continuous cliff section exposes an up to 25 m thick succession of near-vent phreatomagmatic tephra units derived from closely spaced vents. This can be subdivided into two major lithofacies associations. The first association represents when the locus of explosions was below sea level and comprises matrix-supported, massive to weakly stratified beds of coarse ash and lapilli. These are dominant in the lowermost part of the sequence and commonly contain coral fragments, indicating that the loci of explosion were located within a reef or coral sediment near the syn-eruptive shoreline. The second type indicate more stable vent conditions and rapidly repeating explosions of high intensity, producing fine-grained tephra with undulatory bedding and cross-lamination as well as megaripple bedforms.
Understanding the Holocene explosive eruption record of the Tongariro Volcanic Centre, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand
(Massey University, 2010) Moebis, Anja
The Tongariro Volcanic Centre has experienced many VEI 1-4 eruptions over the last 12 000 cal. yrs. B.P., dominantly from Ruapehu, Ngauruhoe and Red Crater. The historic record of 150 years alone is insufficient to provide a robust understanding of future volcanic hazard, necessitating a quantification of eruption parameters from the geological record. The major obstacle to this is untangling a complex sequence of interdigitating, fine-grained and poorly distinguishable tephras from the three source volcanoes. With detailed mapping and using volcanic glass chemistry, tephras from the three sources were discriminated. This has led to a revision of the age of Ngauruhoe to be at least 6500 cal. yrs. B.P., around 4000 years earlier than previously thought. It also provides the most detailed explosive eruption frequency and magnitude record from the area since 12 000 cal. yrs. B.P. Ruapehu and Ngauruhoe tephras were characterised by initial phreatomagmatic explosions that transformed into dry magmatic (strombolian) phases. Magma-water interaction is shown by basal layers of pale-brownish-grey fine ash, containing blocky glass shards with small isolated spherical vesicles, and exhibiting surface conchoidal and step-like fractures. The magmatic phase ash is microlite-rich, with dark glass containing elongate vesicles with thin bubble walls and irregular surfaces. The largest eruption recognised from Ngauruhoe, produced a distinct dark purple tephra, with a well-constrained volume of 26.6 x106 m3, and a probable eruption column height of about 15 km. The total tephra volume from Ngauruhoe is estimated to be 952 x 106 m3, around 50% of the known lava volume. A climactic eruption period of Ngauruhoe occurred between ~ 2900 and 2700 cal. yrs. B.P., during which 64% of its known explosive eruptions occurred, including its largest known events. This phase, representing 3% of the volcano’s lifespan, produced 57% of its pyroclastic output. Over the last 12 000 cal. yrs. B.P., the frequency of Ruapehu eruptions appears to have increased about 2000 yrs B.P., but this may reflect better preservation and exposure of the more recent tephras. Bursts in Ruapehu explosive activity have occurred out of phase with those from Ngauruhoe. The minor pyroclastic cone of Red Crater represents an eruption site that was active for at least ~ 4000 cal. yrs. B.P. and has mainly been characterised by effusive events. Since around 900 cal. yrs. B.P. minor explosive events have occurred from this location, increasing in magnitude from 400 cal. yrs. B.P.

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