Vince Neall

Stratigraphy, age, and correlation of voluminous debris avalanche events from an ancestral Egmont Volcano: implications for coastal plain construction and regional hazard assessment

Abstract Two previously unrecognised debris‐avalanche deposits have been identified on the eastern flanks of Egmont Volcano beneath a thick mantle of tephric and andic soil material that has mostly subdued their topographic expression. The Ngaere Formation is a c. 23 14C ka large volume (>5.85 km3) debris‐avalanche deposit that is widely distributed over 320–500 km2 of the north‐east, south‐east, and south portions of the Egmont ring plain. The second deposit, Okawa Formation, is a c. 105 ka large volume (>3.62 km3) debris‐avalanche deposit that has been mapped over a minimum area of 255 km2 in northern and north‐eastern Taranaki. Both debris‐avalanche formations contain axial facies with hummocks composed mainly of block‐supported brecciated andesitic debris. A less conspicuous marginal facies, texturally resembling a mudflow, is more extensive. A third debris‐avalanche deposit (Motunui Formation) is extensively preserved along the north Taranaki coast where it is truncated by a c. 127 ka wave cut surface (NT2) and closely overlies a c. 210 ka wave cut surface (NT3). The source of this debris‐avalanche deposit is unknown. Side‐scan sonar and shallow seismic profiling have been useful in accurately delineating the distribution of combined Okawa and Motunui debris‐avalanche deposits in the offshore environment but cannot distinguish between the two deposits or enable onshore spatial and volumetric estimates for each unit to be revised. However, the widespread occurrence of debris‐avalanche rock material offshore does emphasise the importance of this lag material altering the orientation of the coast influencing both wave climate and rates of coastal erosion. Similarly, the extensive onshore occurrence of debris‐avalanche rock material appears to be a significant factor in widening of the north Taranaki coastal plain and preservation of the NT2 and NT3 uplifted marine terrace surfaces. Initiation of collapse by magmatically‐induced seismicity is apparently common at many stratovolcanoes. Emplacement of Ngaere Formation was immediately preceded by a magmatic fall unit and is directly overlain by a closely spaced sequence of 13 fall units. In contrast, there is no evidence to indicate that an eruptive event triggered or immediately followed the Okawa debris‐avalanche event, but seismically induced gravitational sliding cannot be discounted. Egmont Volcano has repetitively collapsed over its c. 127 ka history and has generated at least five voluminous landscape‐forming debris‐avalanche deposits. Probabilistically‐based return times are calculated at c. 1967 14C yr for volumes ≥0.15 km3 and c. 21 000 14C yr for volumes ≥7.5 km3. Despite lower return times in comparison to tephra emission, Egmont Volcano is an inherently unstable cone because it comprises interbedded lavas and unconsolidated volcaniclastic deposits with a high slope angle overlying a faulted basement of Tertiary sediments. Should eruptive activity recommence and coincide with significant upper cone dilation, then the likelihood of a gravitational cone collapse is expected to increase although critical thresholds remain to be modelled. Fortunately, the Taranaki Regional Volcanic Contingency Plan is based on pre‐emptive evacuation which is intended to minimise loss of life in advance of an eruptive and/or cone collapse event occurring.

Introducing the Volcanic Unrest Index (VUI): a tool to quantify and communicate the intensity of volcanic unrest

Accurately observing and interpreting volcanic unrest phenomena contributes towards better forecasting of volcanic eruptions, thus potentially saving lives. Volcanic unrest is recorded by volcano observatories and may include seismic, geodetic, degassing and/or geothermal phenomena. The multivariate datasets are often complex and can contain a large amount of data in a variety of formats. Low levels of unrest are frequently recorded, causing the distinction between background activity and unrest to be blurred, despite the widespread usage of these terms in unrest literature (including probabilistic eruption-forecasting models) and in Volcanic Alert Level (VAL) systems. Frequencies and intensities of unrest episodes are not easily comparable over time or between volcanoes. Complex unrest information is difficult to communicate simply to civil defence personnel and other non-scientists. The Volcanic Unrest Index (VUI) is presented here to address these issues. The purpose of the VUI is to provide a semi-quantitative rating of unrest intensity relative to each volcano’s past level of unrest and to that of analogous volcanoes. The VUI is calculated using a worksheet of observed phenomena. Ranges for each phenomenon within the worksheet can be customised for individual volcanoes, as demonstrated in the companion paper for Taupo Volcanic Centre, New Zealand (Potter et al. 2015). The VUI can be determined retrospectively for historical episodes of unrest based on qualitative observations, as well as for recent episodes with state-of-the-art monitoring. This enables a long time series of unrest occurrence and intensity to be constructed and easily communicated to end users. The VUI can also assist with VAL decision-making. We present and discuss two approaches to the concept of unrest.

Volcanism and historical ecology on the Willaumez Peninsula, Papua New Guinea

Abstract: The role of natural disasters has been largely overlooked in studies of South Pacific historical ecology. To highlight the importance of rapid-onset natural hazards, we focus on the contributions of volcanism in shaping landscape histories. Results of long-term research in the Willaumez Peninsula on New Britain in Papua New Guinea illustrate the wide range and complexity of potential relationships between volcanic activity and human responses. Despite frequent severe volcanic impacts, human groups have responded creatively to these challenges and over time may have developed particular strategies that coped with the demands of repeated refuging and recolonization.

Recognition and paleoclimatic implications of late-Holocene glaciation on Mt Taranaki, North Island, New Zealand

Evidence for the timings of inter-hemispheric climate fluctuations during the Holocene is important, with mountain glacier moraine systems routinely used as a proxy for climate. In New Zealand such evidence for glacier expansion during the late Holocene is fragmentary and is limited to glaciers in a narrow zone within the Southern Alps. Here, we present the first evidence for late-Holocene glacier expansion on the North Island of New Zealand in the form of two unconsolidated debris ridges on the south side of the stratovolcano, Mt Taranaki/Mt Egmont, at ~1920 m a.s.l. The two ridges are aligned north–south along the western and eastern sides of a small basin (Rangitoto Flat), which is formed between the main Taranaki cone (to the north), and the parasitic cone of Fanthams Peak (to the south). The approximate age of the ridges is constrained by dated eruptive events and the relationship between ridge locations and the spatial positioning of adjacent volcanic landforms. We propose the ridges formed as two lateral moraines on the margins of a cirque glacier during the final construction phase of Fanthams Peak between 3.3 and 0.5 ka BP, during late-Holocene time. This time interval accords with published cosmogenic 10Be dating of moraine-building episodes in the Southern Alps, indicating the Mt Taranaki moraines are a response to the same regional climatic forcings.

Conceptual Development of a National Volcanic Hazard Model for New Zealand

We provide a synthesis of a workshop held in February 2016 to define the goals, challenges and next steps for developing a national probabilistic volcanic hazard model for New Zealand. The workshop involved volcanologists, statisticians, and hazards scientists from GNS Science, Massey University, University of Otago, Victoria University of Wellington, University of Auckland, and University of Canterbury. We also outline key activities that will develop the model components, define procedures for periodic update of the model, and effectively articulate the model to end-users and stakeholders. The development of a National Volcanic Hazard Model is a formidable task that will require long-term stability in terms of team effort, collaboration and resources. Development of the model in stages or editions that are modular will make the process a manageable one that progressively incorporates additional volcanic hazards over time, and additional functionalities (e.g. short-term forecasting). The first edition is likely to be limited to updating and incorporating existing ashfall hazard models, with the other hazards associated with lahar, pyroclastic density currents, lava flow, ballistics, debris avalanche, and gases/aerosols being considered in subsequent updates.