UPDATE TO THE PROBABILISTIC VOLCANIC HAZARD ANALYSIS, YUCCA MOUNTAIN, NEVADA

A probabilistic volcanic hazard analysis (PVHA) was conducted in 1996 for the proposed repository at Yucca Mountain, Nevada. Based on data gathered by the Yucca Mountain Project over the course of about 15 years, the analysis integrated the judgments of a panel of ten volcanic experts using methods of formal expert elicitation. PVHA resulted in a probability distribution of the annual frequency of a dike intersecting the repository, which ranges from 10E-7 to 10E-10 (mean 1.6 x 10E-8). The analysis incorporates assessments of the future locations, rates, and types of volcanic dikes that could intersect the repository, which lies about 300 m below the surface. A particular focus of the analysis is the quantification of uncertainties. Since the 1996 PVHA, additional aeromagnetic data have been collected in the Yucca Mountain region, including a high-resolution low-altitude survey. A number of anomalies have been identified within alluvial areas and modeling suggests that some of these may represent buried eruptive centers (basaltic cinder cones). A program is currently underway to drill several of the anomalies to gain information on their origin and, if basalt, their age and composition. To update the PVHA in light of the new aeromagnetic and drilling data as well as other advancements in volcanic hazard modeling over the past decade, the expert panel has been reconvened and the expert elicitation process has been fully restarted. The analysis requires assessments of the spatial distribution of igneous events, temporal distributions, and geometries and characteristics of future events (both intrusive and extrusive). The assessments are for future time periods of 10,000 years and 1,000,000 years. Uncertainties are being quantified in both the conceptual models that define these elements as well as in the parameters for the models. The expert elicitation process is centered around a series of workshops that focus on the available data; alternative approaches to modeling spatial, temporal, and event definitions; preliminary assessments of model and parameter values; and feedback. Discussions and interactions among the experts are strongly encouraged and they are charged with considering the views of the larger technical community. Expert judgments are elicited in individual interviews and the technical basis for all assessments is documented. The results of the PVHA Update will be a series of probability distributions defining the annual frequency of various types of igneous events that could disrupt the repository. These distributions will be used as input to the total system performance assessment of the repository system

An ashy septingentenarian: the Kaharoa tephra turns 700 (with notes on its volcanological, archaeological, and historical importance)

Most of us are aware of the basaltic Tarawera eruption on 10th June 1886: the high toll on life (~120 people), landscape devastation, and loss of the Pink and White Terraces. But this was not the first time that Mt Tarawera produced an eruption of importance both to volcanology and human history. This edition of the GSNZ Newsletter marks the 700th anniversary of the Kaharoa eruption – its septingentenary to be precise – which occurred at Mt Tarawera in the winter of 1314 AD (± 12 years) (Hogg et al. 2003) (Fig. 1). The importance of the Kaharoa eruption is at least threefold. (1) It is the most recent rhyolite eruption in New Zealand, and the largest New Zealand eruption volumetrically of the last millennium. (2) The Kaharoa tephra is an important marker horizon in late Holocene stratigraphy and geoarchaeology (Lowe et al. 1998, 2000), and in particular helps to constrain the timing of settlement of early Polynesians in North Island (Newnham et al. 1998; Hogg et al. 2003; Lowe 2011). (3) There is a link between the soils that developed on the Kaharoa tephra, the animal ‘wasting’ disease known as ‘bush sickness’, and the birth of a government soil survey group as an independent organisation (Tonkin 2012)

Global Volcanic Hazards and Risk

Presenting the state of the art in our understanding of global volcanism, this is the first comprehensive assessment of global volcanic hazards and risk. Regional profiles and local case studies are provided online for all countries with active volcanoes, making this an invaluable reference for the disaster risk reduction community. This title is also available as Open Access

Revised calendar date for the Taupo eruption derived by ¹⁴C wiggle-matching using a New Zealand kauri ¹⁴C calibration data set

Taupo volcano in central North Island, New Zealand, is the most frequently active and productive rhyolite volcano on Earth. Its latest explosive activity about 1800 years ago generated the spectacular Taupo eruption, the most violent eruption known in the world in the last 5000 years. We present here a new accurate and precise eruption date of AD 232 ± 5 (1718 ± 5 cal. BP) for the Taupo event. This date was derived by wiggle-matching 25 high-precision ¹⁴C dates from decadal samples of Phyllocladus trichomanoides from the Pureora buried forest near Lake Taupo against the high-precision, first-millennium AD subfossil Agathis australis (kauri) calibration data set constructed by the Waikato Radiocarbon Laboratory. It shows that postulated dates for the eruption estimated previously from Greenland ice-core records (AD 181 ± 2) and putative historical records of unusual atmospheric phenomena in ancient Rome and China (c. AD 186) are both untenable. However, although their conclusion of a zero north–south ¹⁴C offset is erroneous, and their data exhibit a laboratory bias of about 38 years (too young), Sparks et al. (Sparks RJ, Melhuish WH, McKee JWA, Ogden J, Palmer JG and Molloy BPJ (1995) ¹⁴C calibration in the Southern Hemisphere and the date of the last Taupo eruption: Evidence from tree-ring sequences. Radiocarbon 37: 155–163) correctly utilized the Northern Hemisphere calibration curve of Stuiver and Becker (Stuiver M and Becker B (1993) High-precision decadal calibration of the radiocarbon timescale, AD 1950–6000 BC. Radiocarbon 35: 35–65) to obtain an accurate wiggle-match date for the eruption identical to ours but less precise (AD 232 ± 15). Our results demonstrate that high-agreement levels, indicated by either agreement indices or χ² data, obtained from a ¹⁴C wiggle-match do not necessarily mean that age models are accurate. We also show that laboratory bias, if suspected, can be mitigated by applying the reservoir offset function with an appropriate error value (e.g. 0 ± 40 years). Ages for eruptives such as Taupo tephra that are based upon individual ¹⁴C dates should be considered as approximate only, and confined ideally to short-lived material (e.g. seeds, leaves, small branches or the outer rings of larger trees)

Tephrochronology: principles, functioning, application

Tephrochronology is a unique method for linking and dating geological, palaeoecological, palaeoclimatic or archaeological sequences or events. The method relies firstly and fundamentally on stratigraphy and the law of superposition, which apply in any study that connects or correlates deposits from one place to another. Secondly, it relies on characterising and hence identifying or „fingerprinting‟ tephra layers using either physical properties evident in the field or those obtained from laboratory analysis, including mineralogical examination by optical microscopy or geochemical analysis of glass shards or crystals (e.g., Fe-Ti oxides, ferromagnesian minerals) using the electron microprobe and other tools. Thirdly, the method is enhanced when a numerical age is obtained for a tephra layer by (1) radiometric methods such as radiocarbon, fission-track, U-series, or Ar/Ar dating, (2) incremental dating methods including dendrochronology or varved sediments or layering in ice cores, or (3) age-equivalent methods such as palaeomagnetism or correlation with marine oxygen isotope stages or palynostratigraphy. Once known, that age can be transferred from one site to the next using stratigraphic methods and by matching compositional characteristics, i.e., comparing ‘fingerprints’ from each layer. Used this way, tephrochronology is an age-equivalent dating method

Beyond eruptive scenarios: assessing tephra fallout hazard from Neapolitan volcanoes

Assessment of volcanic hazards is necessary for risk mitigation. Typically, hazard assessment is based on one or a few, subjectively chosen representative eruptive scenarios, which use a specific combination of eruptive sizes and intensities to represent a particular size class of eruption. While such eruptive scenarios use a range of representative members to capture a range of eruptive sizes and intensities in order to reflect a wider size class, a scenario approach neglects to account for the intrinsic variability of volcanic eruptions, and implicitly assumes that inter-class size variability (i.e. size difference between different eruptive size classes) dominates over intra-class size variability (i.e. size difference within an eruptive size class), the latter of which is treated as negligible. So far, no quantitative study has been undertaken to verify such an assumption. Here, we adopt a novel Probabilistic Volcanic Hazard Analysis (PVHA) strategy, which accounts for intrinsic eruptive variabilities, to quantify the tephra fallout hazard in the Campania area. We compare the results of the new probabilistic approach with the classical scenario approach. The results allow for determining whether a simplified scenario approach can be considered valid, and for quantifying the bias which arises when full variability is not accounted for

Connecting with tephras: principles, functioning, and applications of tephrochronology in Quaternary science

Tephrochronology is a unique method for linking and dating geological, palaeoecological, palaeoclimatic, or archaeological sequences or events. The method relies firstly on stratigraphy and the law of superposition, which apply in any study that connects or correlates deposits from one place to another. Secondly, it relies on characterising and hence identifying or ‘fingerprinting’ tephra layers using either physical properties evident in the field or those obtained from laboratory analysis, including mineralogical examination by optical microscopy or geochemical analysis of glass shards or crystals (e.g., Fe-Ti oxides, ferromagnesian minerals) using the electron microprobe and other tools. Thirdly, the method is enhanced when a numerical age is obtained for a tephra layer by (1) radiometric methods such as radiocarbon, fission-track, U-series, or Ar/Ar dating, (2) incremental dating methods including dendrochronology or varved sediments or layering in ice cores, or (3) age-equivalent methods such as palaeomagnetism or correlation with marine oxygen isotope stages or palynostratigraphy. Once known, that age can be transferred from one site to the next using stratigraphic methods and by matching compositional characteristics, i.e., comparing ‘fingerprints’ from each layer. Used this way, tephrochronology is an age-equivalent dating method

Connecting and dating with tephras: principles, functioning, and application of tephrochronology in Quaternary research

Tephrochronology, the characterisation and use of volcanic-ash layers as a unique chronostratigraphic linking, synchronizing, and dating tool, has become a globally-practised discipline of immense practical value in a wide range of subjects including Quaternary stratigraphy, palaeoclimatology, palaeoecology, palaeolimnology, physical geography, geomorphology, volcanology, geochronology, archaeology, human evolution, anthropology, and human disease and medicine. The advent of systematic studies of cryptotephras – the identification, correlation, and dating of sparse, fine-grained glass-shard concentrations ‘hidden’ within sediments or soils – over the past ~20 years has been revolutionary. New cryptotephra techniques developed in northwestern Europe and Scandinavia in particular and in North America most recently adapted or improved to help solve problems as they arose, have now been applied to sedimentary sequences (including ice) on all the continents. The result has been the extension of tephra isochrons over wide areas hundreds to several thousands of kilometres from source volcanoes. Taphonomic and other issues, such as quantifying uncertainties in correlation, provide scope for future work. Developments in dating and analytical methods have led to important advances in the application of tephrochronology in recent times. In particular: (i) the ITPFT (glass fission-track) method has enabled landscapes and sequences to be dated where previously no dates were obtainable or where dating was problematic; (ii) new EMPA protocols enabling narrow-beam analyses (<5 um) of glass shards, or small melt inclusions, have been developed, meaning that small (typically distal) glass shards or melt inclusions <~10 um in diameter can now be analysed more efficaciously than previously (and with reduced risk of accidentally including microlites in the analysis as could occur with wide-beam analyses); (iii) LA-ICPMS method for trace element analysis of individual shards <~10 um in diameter is generating more detailed ‘fingerprints’ for enhancing tephra-correlation efficacy (Pearce et al., 2011, 2014; Pearce, 2014); and (iv) the revolutionary rise of Bayesian probability age modelling has helped to improve age frameworks for tephras of the late-glacial to Holocene period especially