Egmont Volcano Research Articles

Upper-mantle seismic anisotropy in the southwestern North Island, New Zealand: Implications for regional upper-mantle and slab deformation

We employed shear-wave splitting analysis on both teleseismic SKS and S waves, and S waves from deep (150–250 km) local earthquakes collected from a dense array with 43 temporary broadband seismic stations and nine long-term seismic stations centered at Mount Taranaki to characterize the upper-mantle dynamics in the southwestern North Island of New Zealand, in areas previously unexamined for shear-wave splitting. We observed predominantly trench-parallel fast polarizations and strikingly large delay times over 3 s from teleseismic analysis. In contrast, local S analysis yielded a sharp transition of fast-polarization from trench-parallel in the northeast to trench-normal in the southwest. Trench-parallel fast-polarization from teleseismic analysis may be attributed to sub-slab trench-parallel flow or to trench-parallel fractures in the subducting slab. More importantly, we attribute large delay times to deep upper-mantle (200–400 km depth) deformation, possibly associated with the dynamic interaction between the downgoing slab and the 410-km discontinuity or with the lithosphere delamination near the Taranaki-Ruapehu line. In contrast, the trench-parallel anisotropy from the local S waves in the northeast could be caused by fluid-bearing cracks in the crust of the Taupō Volcanic Zone and/or by trench-parallel fractures in the subducting slab resulting from outer rise bending. The abrupt change to trench-normal may be related to stress variations in the downgoing slab at different depths.

Disentangling the effects of temperature and reactive minerals on soil carbon stocks across a thermal gradient in a temperate native forest ecosystem

Effects of global warming on soil organic carbon (C) can be investigated by comparing sites experiencing different temperatures. However, observations can be affected by covariance of temperature with other environmental properties. Here, we studied a thermal gradient in forest soils derived from volcanic materials on Mount Taranaki (New Zealand) to disentangle the effects of temperature and reactive minerals on soil organic C quantity and composition. We collected soils at four depths and four elevations with mean annual temperatures ranging from 7.3 to 10.5 °C. Soil C stocks were not significantly different across sites (average 162 MgC ha−1 to 85 cm depth, P > .05). Neither aluminium (Al)-complexed C, nor mineral-associated C changed significantly (P > .05) with temperature. The molecular characterisation of soil organic matter showed that plant-derived C declined with increasing temperature, while microbial-processed C increased. Accompanying these changes, soil short-range order (SRO) constituents (including allophane) generally increased with temperature. Results from structural equation modelling revealed that, although a warmer temperature tended to accelerate soil organic C decomposition as inferred from molecular fingerprints, it also exerted a positive effect on soil total C presumably by enhancing plant C input. Despite a close linkage between mineral-associated C and soil organic C, the increased abundance of reactive minerals at 30–85 cm depth with temperature did not increase soil organic C concentration at that depth. We therefore propose that fresh C inputs, rather than reactive minerals, mediate soil C responses to temperature across the thermal gradient of volcanic soils under humid-temperate climatic conditions.

‘Egmont, Who Was He?’

As part of Aotearoa New Zealand’s process of settling historical Treaty of Waitangi claims, a settlement is expected to be completed soon in relation to the maunga (mountain) known to Māori as Taranaki. In addition to recognising the maunga as a legal person, the settlement will reportedly make Taranaki Maunga the landmark’s sole official name. More than 250 years after Captain Cook imposed the name Mount Egmont on the landscape, that name will finally disappear from the map. Few people today are likely to mourn the loss of this name, but things were very different 35 years ago. In 1986, ‘Mount Taranaki or Mount Egmont’ was recognised as the official name of the maunga. The path to that compromise, in which Māori and European names sat side by side, was bitterly contested by many Pākehā (New Zealanders of European descent) who feared the removal of a name they saw as tied to their sense of identity. For Taranaki Māori, who had patiently campaigned for restoration of the Māori name, the decision was another step towards recognition of their deep connections with their sacred maunga. This article provides an account of the debate over the name of the maunga that took place in 1985-86 and looks at how identity, history, race relations and democracy were discussed in the debate. It also reflects on the reasons why there was such intense contestation over the name, and the debate’s relevance to the new Aotearoa New Zealand histories curriculum.

Complex decompression and fragmentation of mingled andesite magmas driving multi-phase Plinian eruptions at Mt. Taranaki, New Zealand

Estimating the kinetics of andesite magma vesiculation and crystallization inside volcanic plumbing systems is key for unraveling andesite Plinian eruption dynamics. The conduit kinetics provide the necessary input data for estimating the magma flow rates driving magma ascent and the fragmentation mechanisms controlling shifts in eruption explosivity and style. This information is crucial for increasing knowledge on expected hazards and for developing realistic eruption scenarios. In this work, we estimate conduit magma vesiculation and crystallization kinetics during the 3300 cal BP Upper Inglewood Plinian eruptive episode of Mount Taranaki, New Zealand. This episode comprised (i) low-intensity, conduit-opening phases of dome-collapse PDCs; (ii) pre-climactic, highly explosive phases of diverse PDCs, of up to violent 18-km-runout lateral blasts; (iii) climactic phases of steady 22-km-high Plinian eruption columns; and (iv) waning phases of column-collapse PDCs. By employing synchrotron microtomography, combined with mineral/glass chemistry and electron-microscopy, we quantified 3D vesicle and crystal size and shape distributions in juvenile pyroclasts over time, and corresponding number densities ranging from 1.1 × 105 to 2.5 × 106 mm−3 for vesicles, and from 8.0 × 104 to 5.1 × 106 mm−3 for crystals. Our results indicate that tapping of chemically alike yet rheologically contrasting magmas over a multi-phase andesite eruptive episode is linked to: (a) mafic magma recharge and differentiation in multiple storage reservoirs at distinct crustal levels, (b) stepwise to rapid magma decompression while mingling, producing variable pre- and syn-eruptive degassing and crystallization, and (c) syn-eruptive changes in melt viscosity, strain rate, localized shear deformation, and conduit geometry. The earliest and least explosive eruptive phases (≈ 2 × 106 kg s−1) were produced at the slowest rates of magma decompression (0.3–0.6 MPa s−1), ascent (0.01–0.02 m s−1) and strain (< 0.002 s−1), driven by volatile diffusion and exsolution. All subsequent pre-climactic and Plinian phases (4 × 107–1 × 108 kg s−1) were produced at either rapid or intermittent rates of magma decompression (2.0–6.0 MPa s−1), ascent (0.06–0.2 m s−1) and strain (> 0.003–0.010 s−1), powered by combined magma volatile supersaturation and delayed disequilibrium degassing, decompression-induced microlite crystallization and rapid heterogeneous vesiculation kinetics, shear deformation and magma mingling. These processes enabled complex fragmentation mechanisms of the rheologically most homogeneous magmas.

Volcanic hazard scenarios for multiphase andesitic Plinian eruptions from lithostratigraphy: Insights into pyroclastic density current diversity at Mount Taranaki, New Zealand

Volcanic hazard scenarios for multiphase andesitic Plinian eruptions from lithostratigraphy: Insights into pyroclastic density current diversity at Mount Taranaki, New Zealand

High-K andesite petrogenesis and crustal evolution: Evidence from mafic and ultramafic xenoliths, Egmont Volcano (Mt. Taranaki) and comparisons with Ruapehu Volcano, North Island, New Zealand

This study uses the geochemistry and petrology of xenoliths to constrain the evolutionary pathways of host magmas at two adjacent andesitic volcanoes in New Zealand’s North Island. Egmont (Mt. Taranaki) is located on the west coast of the North Island and Ruapehu lies 140km to the east at the southern end of the Taupo Volcanic Zone, the principal locus of subduction-related magmatism in New Zealand. Xenoliths are common in the eruptives of both volcanoes but the xenoliths suites are petrographically and geochemically different. Ruapehu xenoliths are predominantly pyroxene–plagioclase granulites derived from Mesozoic meta-greywacke basement and the underlying oceanic crust. The xenolith population of Egmont Volcano is more complex. It includes sedimentary, metamorphic and plutonic rocks from the underlying basement but is dominated by coarse grained, mafic and ultramafic igneous rocks. Gabbroic xenoliths (Group 1) are composed of plagioclase, clinopyroxene and amphibole whereas ultramafic xenoliths are dominated by amphibole (Group 2) or pyroxene (Group 3) or, in very rare cases, olivine (Group 4). In Group 1 xenoliths plagioclase and clinopyroxene and in some cases amphibole show cumulate textures. Amphibole also occurs as intercumulate poikilitic crystals or as blebs or laminae replacing pyroxene. Some Group 2 xenoliths have cumulate textures but near monomineralic amphibole xenoliths are coarse grained with bladed or comb textures. Pyroxene in Group 3 xenoliths has a polygonal granoblastic texture that is commonly overprinted by veining and amphibole replacement.Group 1 and most Group 2 xenoliths have major, trace element and Sr, Nd and Pb isotope compositions indicating affinity with the host volcanic rocks. Geochemical variation can be modelled by assimilation fractional crystallisation (AFC) and fractional crystallisation (FC) of basaltic parents assuming an assimilant with the composition of average crystalline basement and Group 1 xenoliths have compositions approximating the solid material extracted during these processes. Some Group 2 xenoliths have relatively unevolved Sr and Nd isotopic compositions and they are interpreted to have derived as crystal cumulates from a more primitive parental basalt or through metasomatic alteration of other xenolith types by isotopically less evolved fluids or melts. Some Group 3 xenoliths could have originally been pyroxene cumulates but the granoblastic textures of others are more consistent with an origin as restites generated during anatexis of amphibolite. Group 4 xenoliths have textures similar to those of mantle-derived peridotite xenoliths found in intraplate basalts.The geochemical variation is consistent with a system fed by mantle-derived magmas that underplated and intruded the lower crust. At this level AFC and FC and crustal anatexis generated cumulates and pyroxene restite represented by the mafic and ultramafic xenoliths. The magmas segregating at these deep levels moved higher into the crust where a complex dispersed magma storage and plumbing system formed. Here magmas evolved further through AFC and FC with the formation of cumulates and crystal mushes that are represented by some Group 1 and Group 2 xenoliths. Xenoliths were further modified by interaction with host magmas or by alteration at the side walls of magma storages and conduits.

Forecasting catastrophic stratovolcano collapse: A model based on Mount Taranaki, New Zealand

Research Article| November 01, 2012 Forecasting catastrophic stratovolcano collapse: A model based on Mount Taranaki, New Zealand Anke V. Zernack; Anke V. Zernack 1INR, Massey University, Private Bag 11 222, Palmerston North, New Zealand Search for other works by this author on: GSW Google Scholar Shane J. Cronin; Shane J. Cronin 1INR, Massey University, Private Bag 11 222, Palmerston North, New Zealand Search for other works by this author on: GSW Google Scholar Mark S. Bebbington; Mark S. Bebbington 1INR, Massey University, Private Bag 11 222, Palmerston North, New Zealand Search for other works by this author on: GSW Google Scholar Richard C. Price; Richard C. Price 2Faculty of Sciences, University of Waikato, Private Bag 3105, Hamilton, New Zealand Search for other works by this author on: GSW Google Scholar Ian E.M. Smith; Ian E.M. Smith 3School of Environment, University of Auckland, Private Bag 92 019, Auckland, New Zealand Search for other works by this author on: GSW Google Scholar Robert B. Stewart; Robert B. Stewart 1INR, Massey University, Private Bag 11 222, Palmerston North, New Zealand Search for other works by this author on: GSW Google Scholar Jonathan N. Procter Jonathan N. Procter 1INR, Massey University, Private Bag 11 222, Palmerston North, New Zealand Search for other works by this author on: GSW Google Scholar Author and Article Information Anke V. Zernack 1INR, Massey University, Private Bag 11 222, Palmerston North, New Zealand Shane J. Cronin 1INR, Massey University, Private Bag 11 222, Palmerston North, New Zealand Mark S. Bebbington 1INR, Massey University, Private Bag 11 222, Palmerston North, New Zealand Richard C. Price 2Faculty of Sciences, University of Waikato, Private Bag 3105, Hamilton, New Zealand Ian E.M. Smith 3School of Environment, University of Auckland, Private Bag 92 019, Auckland, New Zealand Robert B. Stewart 1INR, Massey University, Private Bag 11 222, Palmerston North, New Zealand Jonathan N. Procter 1INR, Massey University, Private Bag 11 222, Palmerston North, New Zealand Publisher: Geological Society of America Received: 05 Feb 2012 Revision Received: 26 Apr 2012 Accepted: 27 Apr 2012 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2012 Geological Society of America Geology (2012) 40 (11): 983–986. https://doi.org/10.1130/G33277.1 Article history Received: 05 Feb 2012 Revision Received: 26 Apr 2012 Accepted: 27 Apr 2012 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Anke V. Zernack, Shane J. Cronin, Mark S. Bebbington, Richard C. Price, Ian E.M. Smith, Robert B. Stewart, Jonathan N. Procter; Forecasting catastrophic stratovolcano collapse: A model based on Mount Taranaki, New Zealand. Geology 2012;; 40 (11): 983–986. doi: https://doi.org/10.1130/G33277.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Regular large-scale edifice collapse and regrowth is a common pattern during the long lifespans of andesitic stratovolcanoes worldwide. The >130 k.y. history of Mount Taranaki, New Zealand, is punctuated by at least 14 catastrophic collapses, producing debris avalanche deposits of 1 to >7.5 km3. The largest of these sudden events removed as much as one-third of the present-day equivalent cone. The resulting deposits show similar sedimentary and geomorphic features, suggesting similar proto-edifice characteristics, failure trigger mechanisms, and runout path conditions. Each collapse was followed by sustained renewed volcanism and cone regrowth, although there are no matching stepwise geochemical changes in the magma erupted; instead a stable, slowly evolving magmatic system has prevailed. Last Glacial climatic variations are also uncorrelated with the timing or magnitudes of edifice collapse. We demonstrate here that, if the magmatic composition erupted from stratovolcanoes is constant and basement geology conditions are stable, large-scale edifice collapse and the generation of catastrophic debris avalanches will be governed by the magma supply rate. Using a mass balance approach, a volume-frequency model can be applied to forecasting both the probable timing and volume of future edifice failure of such stratovolcanoes. In the Mount Taranaki case, the maximum potential size of a present collapse is estimated to be 7.9 km3, while the maximum interval before the next collapse is <16.2 k.y. The current annual collapse probability is ∼0.00018, with the most likely collapse being a small one (<2 km3). You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Subedifice collapse of an andesitic stratovolcano: The Maitahi Formation, Taranaki Peninsula, New Zealand

The Maitahi Formation is a large-volume (>7.5 km 3 ) debris-avalanche deposit that resulted from a mid-Pleistocene (ca. 0.24–0.21 Ma) collapse of Pouakai Volcano, Taranaki Peninsula, New Zealand. It is best exposed at intermediate distances (12–14 km) from its source and consists of heterolithologic megaclast-rich and megaclast-poor facies. There are four megaclast and clast types: (1) volcanogenic sedimentary deposits derived from the cone and ring plain; (2) extrusive and intrusive igneous rocks derived primarily from the cone; (3) Pliocene mudstone-rich, subedifice bedrock; and (4) pyroclastic deposits derived from the cone and ring plain. Transport of intact, weakly to moderately lithified, volcanogenic sedimentary and bedrock megaclasts and megaclast domains, tens of meters in diameter, indicates that the W-NW–directed Maitahi debris avalanche was dominantly viscous and nonturbulent. Normally faulted, stratified megaclasts reflect flow dilation, whereas jigsaw-puzzle fractured megaclasts and clasts are evidence for early-stage compression and limited particle-particle and particle-substrate collisions. Folded and elongated megaclasts and clasts and injected interclast matrix reflect the influences of syn- and postavalanche compression, loading, penetrative shear, and pore-fluid overpressures. A significant subedifice bedrock and ring-plain megaclast and clast content distinguishes the Maitahi Formation from all other Taranaki Peninsula debris-avalanche deposits. Coupled with its large volume, the subedifice bedrock content also suggests that the Maitahi Formation originated from a deep-seated edifice collapse, unlike subsequent, and apparently shallower, edifice collapses at the nearby and genetically similar Egmont Volcano.

Unusual cloud formation on Mount Egmont

Unusual cloud formation on Mount Egmont

Relating magma composition to eruption variability at andesitic volcanoes: A case study from Mount Taranaki, New Zealand

Acquiring accurate eruption records and understanding the volcanic processes behind eruption periodicity are important in the development of realistic hazard assessments and volcanic emergency planning. Here, we use a detailed study of the Holocene ( 0.5 km 3 ) eruptions appear to be statistically predictable because they tend to occur just prior to a period of repose, and they erupt the most strongly evolved magmas. The fundamental properties of magma-volcano systems identified here offer a paradigm for constraining the time scales and nature of magmatic processes, in addition to providing a foundation for more robust probabilistic time-varying hazard forecasts.

The New Zealand National Seismograph Network

### Tectonic Settings & Seismicity in New Zealand New Zealand's active tectonics are dominated by the oblique convergence of the Pacific plate and the Australian plate, which produces earthquakes, volcanoes, active geological deformation, and steep terrain. The tectonic setting (Figure 1) makes New Zealand one of the most seismically active countries in the world with more than 15,000 earthquakes ( ML ≥ 2) located per year (http://www.geonet.org.nz/earthquake/). Shallow earthquakes (depth <40 km) cluster in a band that extends along the east coast of the North Island above the Hikurangi subduction zone, through the western South Island in proximity to the Alpine fault, to the southernmost tip of the South Island above the Puysegur subduction zone (Figure 2A). Another band of shallow seismicity maps the Taupo volcanic zone (TVZ), and the cluster beneath the western part of the central North Island is associated with the Cape Egmont fault zone and Egmont Volcano (Mount Taranaki). Figure 2B shows the deep seismicity (≥40 km) of New Zealand. The deep earthquakes beneath the North Island and the upper part of the South Island define the dipping Benioff zone within the subducting Pacific plate. In the Fiordland region the deep seismicity is associated with the subduction of the Australian plate beneath the Pacific plate. Detailed overviews of the seismicity of New Zealand can be found in Robinson (1986), Reyners (1989), Anderson and Webb (1994), and Eberhart-Phillips (1995). ### The GeoNet Project The New Zealand GeoNet is a strongly integrated data collection and analysis system consisting of national and regional-scale sensor networks. In 2001, GNS Science, a New Zealand government-owned research institute, received funding from the New Zealand Earthquake Commission (EQC), a New Zealand government mandated natural disasters insurer, to develop GeoNet as a non-profit “public good” initiative to enhance New Zealand's ability to respond to and prepare for natural hazards such as earthquakes, volcanic eruptions, tsunamis, …

Xenoliths from the sub-volcanic lithosphere of Mt Taranaki, New Zealand

Mount Taranaki is located 140 km west of the Taupo Volcanic Zone and represents the most westerly expression of subduction-related volcanism on the North Island of New Zealand. Taranaki is a predominantly high-K arc volcano but compositions range from basaltic andesite to andesite with minor dacite and basalt. The sub-volcanic basement under Taranaki is thought to comprise calc–alkaline plutonic and metamorphic rocks of the Median Batholith, overlain by a sequence of Cretaceous and Tertiary sediments. Taranaki lavas contain abundant xenoliths that represent samples of the upper to lower crust beneath the volcano. The xenolith suite has been initially organised into six groups based on petrography, geochemistry and inferred genetic relationships: supracrustal sedimentary rocks (1), mafic hornfels (2), garnet gneiss (3), granite and granodiorite (4), finely banded amphibolitic gneiss (5) and gabbros and ultramafic rocks (6). Groups 1, 3 and 4 are derived from the Median Batholith basement and Cretaceous–Tertiary sediments of the Taranaki Basin while Groups 2, 5 and some fine grained gabbros from Group 6 could either be derived from the Median Batholith or be cognate xenoliths. Group 6 gabbros and ultramafic rocks are dominated by clinopyroxene, amphibole and plagioclase and are predominantly cumulate in origin. The Egmont xenoliths can also be classified into the Type I and Type II xenoliths defined by Frey and Prinz (1978). Type I dunite and wehrlite xenoliths are only present in basaltic andesite host rocks and are sourced from depleted upper mantle whereas Type II xenoliths predominate in the more siliceous andesites and are sourced from the lower crust. The separate source depths for the two rock types can be explained by the “hot zone” model where the andesites have much greater interaction with the lower crust than the basaltic andesites. Some xenoliths contain glass of rhyolitic to trachyitic compositions with up to 6% K 2O that represent partial melts of the sub-volcanic lower crust and may give rise to the andesite magma compositions by mixing with lower crustal residual crystals. The widespread occurrence of amphibole in the Egmont xenolith suite reflects the fluid-rich environment of arc magma systems.

Sedimentary signatures of cyclic growth and destruction of stratovolcanoes: A case study from Mt. Taranaki, New Zealand

The long-term behaviour of andesite stratovolcanoes is characterised by a repetition of edifice growth phases followed by collapse. This cyclic pattern represents a natural frequency at varying timescales in the growth dynamics of stratovolcanoes worldwide. Around the > 130 ka Mt. Taranaki (Egmont volcano), New Zealand, coastal–cliff successions at 20–40 km distance comprise repeating packages of lithologically and sedimentologically distinctive mass-flow deposits. Varying depositional mechanisms and source properties of these units record growth and collapse cycles of the central edifice. These are used to construct a model for cyclic volcaniclastic sedimentation in the surrounds of stratovolcanoes. During edifice-construction phases, thick packages of tabular, predominantly monolithologic, hyperconcentrated-flow and debris-flow deposits accumulate with intercalated tephra beds. The mass-flow units commonly contain large proportions of fresh pumice or juvenile-lithic andesite. Intervals of quiescence separating eruptive periods are characterised by landscape re-adjustment, accompanied by deposition of fluvial and aeolian sediments, along with steady accretion of medial ash. In contrast, brief episodes of destruction are marked by wide-spread, distinctively clay-rich, polylithologic debris-avalanche deposits and related marginal debris flow units. The growth stages can be terminated by an eruption-triggered sector collapse, or by external forces once the edifice exceeds a critical stable height or profile (dependent on eruptive style and local geo-tectonic conditions). Once the edifice becomes metastable, regional tectonic earthquakes or shallow-level intrusion events are likely triggers for collapse. Although the resulting debris avalanches represent the greatest individual hazard from such andesite stratovolcanoes, their frequency is relatively low compared with other types of mass-flows generated during edifice-growth phases. Accurate forecasts of future hazard from mass-flow events are therefore dependent on recognition of both the frequency of a stratovolcano's growth cycle and its current position in that cycle.

Using titanomagnetite textures to elucidate volcanic eruption histories

Mineral assemblages in volcanic rocks record both pre-eruptive conditions and changes experienced by magma as it rises. Titanomagnetite in andesitic magmas is especially sensitive to changes in temperature and oxygen fugacity immediately prior to and during eruptions. Two end-member eruption states can be distinguished by examining titanomagnetite textures in erupted rocks. Slow-ascent eruptions—characterized by near-stagnant magma bodies and slow effusion of lava domes—show solid-state exsolution of titanohematite/ilmenite lamellae within titanomagnetite hosts. By contrast, fast-ascent eruptions—characterized by rapid chilling of magma in sub-Plinian eruptions—contain titanomagnetites without such exsolution features. This mineralogical distinction is particularly useful in examining very fine-grained distal tephra layers where other characteristic properties of the two eruptions types are not present. Such tephra records in lake deposits typically provide the most precise long-term eruption records from andesitic volcanoes. Using an example from Mount Taranaki, New Zealand, we show that by classifying eruption styles within such sequences, the underlying magmatic system processes at a volcano can be elucidated and separated from other environmental factors such as vent/crater configuration.

Slab or Crust—K 2O enrichment at Egmont volcano, New Zealand

Slab or Crust—K 2O enrichment at Egmont volcano, New Zealand

Probabilistic tephra fall simulation for the Auckland Region, New Zealand

The Auckland Region, New Zealand is at significant risk from tephra falls originating both from the local Auckland Volcanic Field (AVF) and several distant, large-volume centres. We use geological data and observations of historical eruptions to develop a catalogue of simulated tephra dispersal patterns for the Auckland Region, using the ASHFALL model. Simulated patterns relate to individual eruptive phases and can be combined to simulate events that vary in volume and duration. AVF eruption parameters were chosen to replicate fine tephra fall dispersed from small-volume phreatomagmatic eruptions. Modelling of eruptions from distant volcanoes was carried out in two stages. First, the sensitivity of modelled tephra thickness to various model parameters was tested. By far the largest thickness variations were the result of varying wind conditions, followed by eruption volume. The second stage involved modelling volumes that best replicated thicknesses of preserved events within the Auckland Region, allowing the catalogue to be created. Only about 1% of simulations from distant volcanoes reached the Auckland Region, suggesting that preserved tephra layers are either the result of reasonably long-lived eruptions or a number of eruptive phases. For eruptions originating within the AVF, mean modelled tephra thickness on land is small—approximately 1.6 mm near the coast and thinning rapidly inland. Mean andesitic thicknesses are dominated by eruptions from Egmont volcano, which typically deposit tephra over a narrow section of the Auckland Region with a maximum thickness of approximately 1.7 mm. Rhyolitic eruptions are more widespread, with mean thickness varying from 30 mm in the south of the Region to 9 mm in the north. When all centres are combined and weighted by their conditional likelihood of occurrence, mean thickness ranges from 7 mm in the south to 1.5 mm in the north. However, maximum thicknesses of 150 mm for the AVF, 12 mm for andesitic centres and 830 mm for rhyolitic centres all have the potential to cause much greater damage within parts of the Auckland Region.

VolcaNZ—A volcanic loss model for Auckland, New Zealand

VolcaNZ is a probabilistic volcanic loss model developed for the Auckland Region in New Zealand that currently considers tephra fall hazards from the Auckland Volcanic Field (AVF), Tuhua volcano, Okataina volcanic centre, Taupo volcano, Tongariro volcanic centre and Egmont volcano. In this first version of the model, structural and non-structural damage to residential building envelopes and associated cleanup costs are calculated using Monte Carlo simulation. VolcaNZ assigns a Minimum and Maximum Damage Value to groups of buildings for every simulation, dependent on tephra thickness. A Central Damage Value, representing loss as a percentage of total replacement cost, is then randomly selected between these limits. Even with small-thickness falls, non-structural damage is expected to roof and wall coatings, air-conditioning units, aerials and satellite dishes due to the corrosive and abrasive properties of tephra. An average loss of $583, attributed to non-structural damage, was assigned to all residential buildings impacted by any thickness of tephra greater than 0.1 mm. The costs of tephra removal from buildings, cleaning of building exteriors and tephra transport and disposal are also calculated within the model, assuming much of the cleanup process will be carried out by homeowners. Losses from all simulations are plotted against calculated Average Recurrence Intervals (ARIs) to produce loss curves. Structural damage does not become apparent until ARIs of approximately 8000 years. $1 billion losses, due to structural damage, occur at about 35,000 years and this increases to about $26 billion at 1 million years. Loss due to non-structural damage is constant at approximately $160 million for ARIs above about 600 years. Between 600 and 3000 years, cleanup loss is approximately $50 million, increasing to over $450 million at a return period of 1 million years. At ARIs between 600 and 3000 years, total loss is approximately $210 million, increasing to $10 billion at 100,000 years and over $26 billion at 1 million years. Because we only consider residential building damage and associated cleanup, these values greatly underestimate total loss from the next volcanic event to impact Auckland. Loss calculations will be improved by adding additional hazard and loss modules to VolcaNZ, resulting in a complete catastrophe loss model.

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

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.

Towards a comprehensive distal andesitic tephrostratigraphic framework for New Zealand based on eruptions from Egmont volcano

AbstractThe chronology and glass composition of 43 andesitic tephra layers in palaeolake sediments in northern New Zealand provide the basis for a fine‐resolution tephrostratigraphy of the interval 10–70 cal. ka. Their ages are constrained by 14 interbedded, (mostly) well‐dated rhyolitic tephra layers. The andesitic tephra have the potential to subdivide time intervals (1–5 kyr) bracketed by well known rhyolitic layers, including periods of rapid climate change such as the last glacial–interglacial transition and the Younger Dryas. The source of the distal andesitic tephra is identified as Egmont volcano (some 270 km S‐SW) on the basis of glass shard composition. The tephra contain high‐K2O (3–6 wt%) andesitic‐dacitic (SiO2 = 60–73 wt%) glass, with commonly heterogeneous shard populations (2–10 wt% SiO2). Within stratigraphic intervals of &lt; 10 kyr, individual tephra layers can be distinguished on the basis of their SiO2 and K2O contents, and variability in these contents can also be a distinguishing characteristic. The tephra record greatly extends the dated pyroclastic and geochemical record of Egmont volcano, and demonstrates that the volcano has frequently produced widely dispersed tephra over the last 70 kyr at a generally constant rate. Copyright © 2005 John Wiley &amp; Sons, Ltd.

An integrated model for the temporal evolution of andesites and rhyolites and crustal development in New Zealand's North Island

The Taupo Volcanic Zone (TVZ) is the southern expression of the Tonga–Kermadec subduction system within the continental crust of New Zealand's North Island. It is a region of rapid extension and high heat flow containing one of the most productive, frequently active rhyolitic magmatic systems on Earth. Rhyolites make up >80% of the total erupted material with rhyolitic magmatism being prevalent from the earliest stages of development of the central TVZ. At the present day, the principal focus of andesitic magmatism is at the southern extremity in the Tongariro Volcanic Centre and, on the basis of the patterns of volcanism preserved in the geological record elsewhere in the TVZ, we predict that these volcanoes will eventually founder and be buried by later rhyolite as crustal extension propagates southwards. In the TVZ, andesitic magmatism precedes the development of large-scale rhyolitic systems. Glass inclusions and groundmasses in andesites and xenoliths from Ruapehu, which is the largest active volcano of the Tongariro Volcanic Centre, and Taranaki (Egmont Volcano), which lies to the west outside the TVZ, have major and trace element compositions that show similarities to those of TVZ rhyolites and dacites. Pyroxene and plagioclase crystals contained within xenoliths show morphological and compositional similarities to phenocrysts in the host andesites. TVZ rhyolites and andesites show overlapping Sr and Nd isotopic compositions. We argue that andesites are generated through the interaction of mantle-derived magmas with lower crustal melts and restites and differences between Ruapehu and Taranaki andesites reflect contrasting physical conditions during melting as well as subtle variations in the mantle and crustal source compositions. Magmas produced by these processes are complex mixtures of crustal melt, strongly fractionated mantle melt, restitic crystals, lithic lower crustal fragments (represented by xenoliths), and phenocrysts derived from mantle melts. In many cases, the crustal component appears to dominate and to have been further increased during crystal fractionation and assimilation of crustal melt at higher levels as magmas rose towards the surface. Rhyolitic magmatic systems in the TVZ evolve as crustal extension and thinning cause rising geothermal gradients so that crustal melting becomes progressively more significant. Earlier andesitic intrusives are remelted and recycled as the system reaches thermal maturity. Andesites and rhyolites in the TVZ are directly related and represent different aspects of a continuous, evolutionary spectrum of petrological processes by which new crust is formed, grows, and is recycled.