Mamaku Ignimbrite: a caldera-forming ignimbrite erupted from a compositionally zoned magma chamber in Taupo Volcanic Zone, New Zealand

Mamaku Ignimbrite was deposited during the formation of Rotorua Caldera, Taupo Volcanic Zone, New Zealand, 220–230 ka. Its outflow sheet forms a fan north, northwest and southwest of Rotorua, capping the Mamaku–Kaimai Plateau. Mamaku Ignimbrite can be divided into a partly phreatomagmatic basal sequence, and a main sequence which comprises lower, middle, and upper ignimbrite. The internal stratigraphy indicates that it was emplaced progressively from a pyroclastic density current of varying energy that became less particulate away from source. Gradational contacts between lower, middle, and upper ignimbrite are consistent with it being deposited during one eruptive event from the same source. Variations in lithic clast content and coexistence of different pumice types through the ignimbrite sequence indicate that caldera collapse occurred throughout the eruption, but particularly when middle Mamaku Ignimbrite was deposited and in the final stages of deposition of upper Mamaku Ignimbrite. Maximum lithic data and the location of lithic lag breccias in upper Mamaku Ignimbrite confirm Rotorua Caldera as the source. At least 120 m of geothermally altered intra-caldera Mamaku Ignimbrite occurs inside Rotorua Caldera. Pumice clasts in the Mamaku Ignimbrite are dacite to high-silica rhyolite and can be chemically divided into three types: high–silica rhyolite (type 1), rhyolite (type 2), and dacite (type 3). All are petrogenetically related and types 1 and 2 may be derived by up to 20% crystal fractionation from the type 3 dacite. All three types probably resided in a single, gradationally zoned magma chamber. Andesitic juvenile fragments are found only in upper Mamaku Ignimbrite and inferred to represent a discrete magma that was injected into the silicic chamber and is considered to have accumulated as a sill at the base of the magma chamber. The contrast in density between the andesitic and silicic magmas did not allow eruption of the andesitic fragments during the deposition of lower and middle Mamaku Ignimbrite. The advanced stage of caldera collapse, late in the main eruptive phase, created withdrawal dynamics that allowed andesitic magma to reach the surface as fragments within upper Mamaku Ignimbrite.     

Seismicity in the Rotorua and Kawerau geothermal systems,Taupo Volcanic Zone,New Zealand,based on improved velocity models and cross-correlation measurements

We analyze earthquakes occurring in and around the Rotorua and Kawerau geothermal systems, Taupo Volcanic Zone, New Zealand. The two data sets contain 504 and 1875 shallow (≤ 20 km deep) earthquakes, respectively, and span the 21 year period between 1984 and 2004. The arrival time data for these earthquakes are first used to calculate 1-D P- and S-wave seismic velocity models and accompanying station correction terms for both areas. In order to address the non-uniqueness of the joint hypocenter-velocity model estimation problem, we analyze suites of 1000 velocity models computed from random initial models. The final velocity models are well constrained, particularly at depths between 4 and 15 km, and consistent with the results obtained in previous seismic refraction studies of the central Taupo Volcanic Zone. Using a combination of cross-correlation-derived and catalog-based arrival times, we relocate subsets of the Rotorua and Kawerau data sets. In Rotorua, the relocated earthquakes cluster near the geothermally active parts of Rotorua City and beneath the Mount Ngongotaha rhyolite dome. Earthquake clusters and alignments reveal seismogenic structures in the mid-crust whose positions and geometries are consistent with previously published fault mechanisms and known near-surface faults. In Kawerau, the earthquakes within the geothermal field align along northeast-trending lineations, consistent with the predominant alignment of surface-mapped faults in the area.     

Palaeotemperature determinations for the 1.8-ka Taupo ignimbrite,New Zealand,and implications for the emplacement history of a high-velocity pyroclastic flow

Palaeomagnetic data from lithic clasts collected at 46 sites within layers 1 and 2 of the 1.8-ka Taupo ignimbrite, New Zealand, have been used to determine the palaeotemperatures and thermal structure of the deposit on its emplacement. Equilibrium temperatures from sites less than 30–40 km from vent are 150–300 °C, whereas at greater distances site equilibrium temperatures increase up to 400–500 °C. This variation is seen in both layer 1 and 2 deposits, with values for layer 1 being somewhat cooler, and with its increase in temperature occurring at a greater distance from vent. A temperature maximum at ~50 km from vent coincides with a zone of pink thermal-oxidation colouration of pumices previously inferred to reflect higher emplacement temperatures. Additional palaeomagnetic data collected by us and others from pumice clasts show comparable temperature variations, but these temperature estimates are shown here to be due to a chemical remanence and unreliable for accurate temperature estimates. Cooler temperatures in proximal parts of the ignimbrite are consistent with admixture of >20% cold lithic clasts at source and interaction with the pre-eruption Lake Taupo. The similar, but offset, increases in equilibrium temperatures for medial and distal layers 1 and 2 are consistent with both layers being deposited from the same flow. However, any proximal deposits left by the later, hotter material must have been subsequently eroded, or be so thin that our collection failed to sample them. Radial asymmetries in equilibrium temperatures as well as other physical parameters suggest that the deposit emplacement temperature is primarily determined at source, rather than by interaction with air during transport. These data support previous interpretations that a concentrated basal flow played a dominant role in emplacement and deposition of the Taupo ignimbrite.Editorial responsibility: T. Druitt     

Chronological and palaeomagnetic constraints on widespread welded ignimbrites of the Taupo volcanic zone,New Zealand

 Large volume (100–1000 km³), widespread rhyolitic ignimbrites are the main products of the Taupo volcanic zone (TVZ) of New Zealand, one of the most active silicic volcanic regions on Earth. Several factors have made correlation and the eruptive history of the ignimbrites difficult to resolve, including limited exposure and chronological data, broadly similar lithologies and the lack of stratigraphic successions visible in the field. We have used the isothermal plateau fission track (ITPFT) method on glass shards from the non-welded basal zones to obtain new eruption ages for the widespread units: Ongatiti (1.25±0.12 Ma), Whakamaru group (0.34±0.03 Ma), Matahina (0.34±0.02 Ma), Chimp (0.33±0.02 Ma), Kaingaroa (0.31±0.01 Ma) and Mamaku (0.23±0.01 Ma) ignimbrites. These glasses show little evidence of geochemical alteration and allow the units to be fingerprinted for correlation. The glass ages we have obtained for the late Quaternary units provide an independent check on chronological data obtained from phenocryst phases. The ITPFT method is a useful dating approach for sanidine-poor eruptives which limit the application of ⁴⁰Ar/³⁹Ar. Errors as limited as 10–30 ka can be obtained from the weighted mean of several age determinations. The thermoremanent magnetic (TRM) direction recorded in the units provides a means of correlation over a wide area of the TVZ, because each ignimbrite can be distinguished by its unique record of palaeosecular variation. These data indicate that the four separately mapped members of the Whakamaru group represent the same phase of activity, occurring within a period of 100 years. The TRM data indicate that the widespread Ahuroa ignimbrite erupted during an excursion in Earth's magnetic field, perhaps associated with the Cobb Mountain subchron (ca. 1.2 Ma). The youngest widespread welded unit, Mamaku ignimbrite (ca. 0.23 Ma), also erupted during an excursion and may represent a southern hemisphere record of the Pringle Falls geomagnetic episode found in the western United States. The palaeomagnetic and ITPFT data for the widespread late Quaternary ignimbrites suggest a major period of caldera formation at 0.34–0.30 Ma. This interval represents the eruption of multiple units from the Whakamaru caldera, followed by the formation of the Okataina and Reporoa calderas in rapid succession. Received: 20 November 1995 / Accepted: 8 May 1996     

Origin of dacites of Taupo Volcanic Zone, New Zealand

Dacites form a relatively small proportion of lavas in the Taupo Volcanic Zone, New Zealand (5km³), and occur mainly on the eastern side. In this paper their origin is considered in terms of three processes: (a) partial melting of crustal rocks; (b) fractional crystallisation of basalt and andesite; and (c) sub-surface mixing of basic and acid magma. Modelling techniques are used to calculate the most acceptable degree of fractional crystallisation and magma mixing to fit major-element data, and these values are used to compare calculated and observed trace-element values. The success or failure of the model is determined by the closeness of the two sets of values. For partial-melt models, trace-element values alone are calculated by the batch-melting equation.Results indicate that White Island dacite can best be modelled by fractional crystallisation; Manawahe by fractional crystallisation plus limited crustal contamination; Maungaongaonga by partial melting of Western Basement greywacke, and Tauhara by partial melting of this greywacke together with minor mixing with a more basic magma. Results from Parekauau and Horohoro indicate that these lavas are unlikely to have formed by any of the processes examined.     

Study of Volcanic Sources at Long Valley Caldera,California, Using Gravity Data and a Genetic Algorithm Inversion Technique

We model the source inflation of the Long Valley Caldera, California, using a genetic algorithm technique and micro-gravity data. While there have been numerous attempts to model the magma injection at Long Valley Caldera from deformation data, this has proven difficult given the complicated spatial and temporal nature of the volcanic source. Recent work illustrates the effectiveness of considering micro-gravity measurements in volcanic areas. A genetic algorithm is a problem-solving technique which combines genetic and prescribed random information exchange. We perform two inversions, one for a single spherical point source and another for two-sources that might represent a more spatially distributed source. The forward model we use to interpret the results is the elastic-gravitational Earth model which takes into account the source mass and its interaction with the gravity field. The results demonstrate the need to incorporate more variations in the model, including another source geometry and the faulting mechanism. In order to provide better constraints on intrusion volumes, future work should include the joint inversion of gravity and deformation data during the same epoch.     

Genesis of lavas of the Taupo Volcanic Zone, North Island, New Zealand

The Taupo Volcanic Zone forms part of the Taupo-Hikurangi subduction system, and comprises five volcanic centres: Tongariro, Taupo, Maroa, Okataina and Rotorua. Tongariro Volcanic Centre is formed almost entirely of andesite while the other four centres contain predominantly rhyolitic volcanics and later fissure eruptions of high-Al basalt. Estimated total volume of each lava type are as follows: 2 km³ of high-Al basalt (< 0.1%); 260 km³ of andesite (< 2.5%); 5 km³ of dacite (< 0.1%); > 10,000 km³ of rhyolite and ignimbrite (> 97.4%).The location of the andesites and vent alignments suggest a source from a subduction zone underlying the area. However, the lavas differ chemically from island-arc andesites such as those of Tonga; in particular by having higher contents of the alkali elements, light REE and Sr and Pb isotopes. This suggests some crustal contamination, and it is considered that this may occur beneath the wide accretionary prism of the subduction system. Amphibolite of the subduction zone will break down between 80 and 100 km and a partial melt will rise. A multi-stage process of magma genesis is then likely to occur. High-Al basalts are thought to be derived from partial melting of a garnet-free peridotite near the top of the mantle wedge overlying the subduction zone, locations of the vents controlled largely by faults within the crust. Rhyolites and ignimbrites were probably derived from partial melting of Mesozoic greywacke and argillite under the Taupo Volcanic Zone. Initial partial melting may have been due to hydration of the base of the crust; the “water” having come from dehydration of the downgoing slab. The partial melts would rise to form granodiorite plutons and final release of the magma to form rhyolites and ignimbrites was allowed because of extension within the Taupo graben.Dacites of the Bay of Plenty probably resulted from mixing of andesitic magma with small amounts of rhyolitic magma, but those on the eastern side of the Rotorua-Taupo area were more likely formed by a higher degree of partial melting of the Mesozoic greywacke-argillite basement. This may be due to intrusion of andesite magma on this side of the Taupo volcanic zone.     

Correlation of the Hakkoda–Kokumoto Tephra, a widespread Middle Pleistocene tephra erupted from the Hakkoda Caldera, northeast Japan

Abstract A Middle Pleistocene widespread tephra referred to here as Hakkoda–Kokumoto Tephra (Hkd–Ku) has been newly recognized. Hkd–Ku, derived from the Hakkoda Caldera located in northernmost Honshu Is. of northeast Japan, covers much of Honshu Is. At the type locality in the proximal area, Hkd–Ku comprises Plinian pumice deposits and an immediately overlying ignimbrite. The fine vitric ash nature of the distal ash‐fall deposits of Hkd–Ku suggests that they are coignimbrite ash‐fall deposits. Hkd–Ku was identified using a combination of refractive indices and chemical compositions of major, trace and rare earth elements of glass shards, heavy mineral content, refractive indices of orthopyroxene and paleomagnetic polarity. On the basis of these properties, Hkd–Ku was identified in Oga and Boso Peninsulas and Osaka Plain, 830 km southwest of the source. Stratigraphic positions in Boso Peninsula and Osaka Plain within marine sediments that have a reliable chronology based on oxygen‐isotope, and litho‐, bio‐, magneto‐ and tephrostratigraphy indicate that the age of Hkd–Ku is ca 760 ka, positioned in the transition between marine oxygen‐isotope stages 19.1 and 18.4. The widespread occurrence of Hkd–Ku providing a tie line between many different Pleistocene sections over a distance of 800 km is a key marker horizon in the early part of the Middle Pleistocene. This tephra gives a time control point of ca 760 ka to marine sediments in the Oga Peninsula – where no datum plane exists between the Brunhes–Matuyama chron boundary and oxygen‐isotope stage 12 – and to the volcanostratigraphy of the Hakkoda Caldera. The distribution of Hkd–Ku showing emplacement of coignimbrite ash‐fall deposits in the area 830 km southwest of the source emphasizes the upwind transport direction, relative to the prevailing westerly winds, typical of other coignimbrite ash‐fall deposits in the Japanese islands.     

Shallow structure beneath the Central Volcanic Complex of Tenerife from new gravity data: Implications for its evolution and recent reactivation

We present a new local Bouguer anomaly map of the Central Volcanic Complex (CVC) of Tenerife, Spain, constructed from the amalgamation of 323 new high precision gravity measurements with existing gravity data from 361 observations. The new anomaly map images the high-density core of the CVC and the pronounced gravity low centred in the Las Cañadas caldera in greater detail than previously available. Mathematical construction of a sub-surface model from the local anomaly data, employing a 3D inversion based on “growing” the sub-surface density distribution via the aggregation of cells, enables mapping of the shallow structure beneath the complex, giving unprecedented insights into the sub-surface architecture. We find the resultant density distribution in agreement with geological and other geophysical data. The modelled sub-surface structure supports a vertical collapse origin of the caldera, and maps the headwall of the ca. 180 ka Icod landslide, which appears to lie buried beneath the Pico Viejo–Pico Teide stratovolcanic complex. The results allow us to put into context the recorded ground deformation and gravity changes at the CVC during its reactivation in spring 2004 in relation to its dominant structural building blocks. For example, the areas undergoing the most significant changes at depth in recent years are underlain by low-density material and are aligned along long-standing structural entities, which have shaped this volcanic ocean island over the past few million years.     

Volcanic risk ranking for Auckland, New Zealand. I: Methodology and hazard investigation

Volcanic eruptions typically produce a number of hazards, and many regions are at risk from more than one volcano or volcanic field. So that detailed risk assessments can be carried out, it is necessary to rank potential volcanic hazards and events in terms of risk. As it is often difficult to make accurate predictions regarding the characteristics of future eruptions, a method for ranking hazards and events has been developed that does not rely on precise values. Risk is calculated individually for each hazard from each source as the product of likelihood, extent and effect, based on the parameters order of magnitude. So that multiple events and outcomes can be considered, risk is further multiplied by the relative probability of the event occurring (probability_e) and the relative importance of the outcome (importance_o). By adding the values obtained, total risk is calculated and a ranking can be carried out.This method was used to rank volcanic hazards and events that may impact the Auckland Region, New Zealand. Auckland is at risk from the Auckland volcanic field, Okataina volcanic centre, Taupo volcano, Tuhua volcano, Tongariro volcanic centre, and Mt. Taranaki volcano. Relative probabilities were determined for each event, with the highest given to Mt. Taranaki. Hazards considered were, for local events: tephra fall, scoria fall and ballistic impacts, lava flow, base surge and associated shock waves, tsunami, volcanic gases and acid rain, earthquakes and ground deformation, mudflows and mudfills, lightning and flooding; and for distal events: tephra fall, pyroclastic flows, poisonous gases and acid rain, mudflows and mudfills, climate variations and earthquakes. Hazards from each source were assigned values for likelihood, with the largest for tephra fall from all sources, earthquakes and ground deformation, lava flows, scoria fall and base surge for an Auckland eruption on land, and earthquakes and ground deformation from an Auckland eruption in the ocean. The largest values for extent were for tephra fall and climate variation from each of the distal centres. However, these parameters do not give a true indication of risk. In a companion paper the effect of each hazard is fully investigated and the risk ranking completed.     

Geophysical evidence on the structure of the Taupo Volcanic Zone and its hydrothermal circulation

The Taupo Volcanic Zone (TVZ) of New Zealand is characterised by extensive volcanism and by high rates of magma production. Associated with this volcanism are numerous high-temperature (> 250 °C) geothermal systems through which the natural heat output of 4200 ± 500 MW is channelled. Outside the geothermal fields the heat flow is negligible. The average heat flux from the central 6000 km² of the TVZ, which contains most of the geothermal fields, is 700 mW/m³. This heat flux appears to be more concentrated along the eastern margin of the TVZ.Schlumberger resistivity measurements (AB/2 of 500 m and 1000 m) have identified 17 distinct geothermal fields with natural heat outputs greater than 20 MW. An additional six, low-heat-output geothermal fields also occur, and may represent formerly more active systems now in decline. Two extinct fields have also been identified. The average spacing between fields is 10–15 km. The distribution of geothermal fields does not appear to be directly associated with individual volcanic features except for the geothermal system that occurs within Lake Taupo and which occupies the vent of the 1800 yr.B.P. Taupo eruption. The positions of the geothermal fields do not appear to have varied for at least the last 200,000 years. These data are consistent with a model of large-scale convection occurring throughout the TVZ, in which the geothermal fields represent the upper portion of the rising, high-temperature, convective plumes. The majority of the recharge to the convection system is provided by the downward movement of cold meteoric water between the fields which suppresses the heat flow in these regions.Gravity measurements indicate that to a depth of about 2.5 km the upper layers of the TVZ consist of low-density pyroclastic infill. A seismic refraction interface with velocity change from 3.2 km/s to 5.5 km/s occurs at a similar depth. The cross-sectional area of the convection plumes (identified electrically) appears to increase at depths of 1–2 km, consistent with a decrease in permeability at the depth at which the velocity and density increase.The seismicity is dominated by swarm activity which accounts for about half of all earthquakes and is highly variable in both space and time. The small number of seismic events (and swarms) that have well determined depths show a cut off of seismicity at depths of 7–9 km. The depth of the transition from brittle to ductile behaviour of the rocks is identified with the transition from a regime where heat is transported by (hydrothermal) convection and pore pressures are near-hydrostatic to a regime where heat transport is dominantly conductive and pore pressures are lithostatic. Within the convective region, temperatures are moderated by the circulation of water so that the depth of the transition from convective to conductive heat transfer can be linked to the bottom of the seismogenic zone. Rocks must become ductile within about 1 km of the bottom of the overlying convective zone.Seismic refraction studies suggest that the crust beneath the TVZ is highly thinned with a seismic velocity of about 7.5 km/ s, typical of the upper mantle, occurring at depth of 15 km. Seismological studies indicate the upper mantle is highly attenuating beneath the TVZ. Conductive heat transfer between the bottom of the convective system, at about 8 km, and the base of the material with crustal velocities, at 15 km, is not able to provide all the heat that is discharged at the surface. Repeated intrusion from the mantle may provide the additional heat transport required.     

Pyroclastic phases of a rhyolitic dome-building eruption: Puketarata tuff ring,Taupo Volcanic Zone,New Zealand

The 14 ka Puketarata eruption of Maroa caldera in Taupo Volcanic Zone was a dome-related event in which the bulk of the 0.25 km³ of eruption products were emplaced as phreatomagmatic fall and surge deposits. A rhyolitic dike encountered shallow groundwater during emplacement along a NE-trending normal fault, leading to shallow-seated explosions characterised by low to moderate water/magma ratios. The eruption products consist of two lava domes, a proximal tuff ring, three phreatic collapse craters, and a widespread fall deposit. The pyroclastic deposits contain dominantly dense juvenile clasts and few foreign lithics, and relate to very shallow-level disruption of the growing dome and its feeder dike with relatively little involvement of country rock. The distal fall deposit, representing 88% of the eruption products is, despite its uniform appearance and apparently subplinian dispersal, a composite feature equivalent to numerous discrete proximal phreatomagmatic lapilli fall layers, each deposited from a short-lived eruption column. The Puketarata products are subdivided into four units related to successive phases of:(A) shallow lava intrusion and initial dome growth; (B) rapid growth and destruction of dome lobes; (C) slower, sustained dome growth and restriction of explosive disruption to the dome margins; and (D) post-dome withdrawal of magma and crater-collapse. Phase D was phreatic, phases A and C had moderate water: magma ratios, and phase B a low water: magma ratio. Dome extrusion was most rapid during phase B, but so was destruction, and hence dome growth was largely accomplished during phase C. The Puketarata eruption illustrates how vent geometry and the presence of groundwater may control the style of silicic volcanism. Early activity was dominated by these external influences and sustained dome growth only followed after effective exclusion of external water from newly emplaced magma.     

Gravity and aeromagnetic constraints on the structure of the Woods Mountains volcanic center, southeastern California

 The Woods Mountain volcanic center is a well-exposed, mildly alkaline volcanic center that formed during the Miocene in southeastern California. Detailed geologic mapping and geochemical studies have distinguished three major volcanic phases: precaldera, caldera forming, and postcaldera. Geologic mapping indicates that caldera formation occurred incrementally during eruptions of three large ignimbrites and continued into a period of voluminous intracaldera lava-flow eruptions. Rhyolitic ignimbrites and lava flows within the caldera are associated with large amplitude, circular gravity, and magnetic minima that are among the most prominent gravity and magnetic anomalies in southeastern California. Analysis of a Bouguer gravity anomaly map, reduced-to-the-pole magnetic intensity map, and three-dimensional gravity and magnetic models indicates that there is a single, funnel- to bowl-shaped caldera approximately 4 km thick and approximately 10 km wide at the surface. This model is consistent with other siliceous, pyroclastic-filled calderas on continental crust, except that most siliceous volcanic centers associated with more than one eruption are characterized by more than one caldera. Received: 20 December 1997 / Accepted: 15 October 1998     

Crustal heat transfer in the Taupo Volcanic Zone (New Zealand): comparison with other volcanic arcs and explanatory heat source models

The Taupo Volcanic Zone (TVZ) is a 200-km-long volcanic arc segment which developed ≤2 Ma ago within the continental crust of the North Island of New Zealand and lies at the southern end of the much larger Tonga-Kermadec arc system. The total crustal heat transfer of the TVZ is at present c. 2600 MW/100 km, most of the heat being transferred by convective geothermal systems. The rate of transfer is anomalously high in comparison to that of other active arcs, and arguably the highest world wide for such a setting. Heat transfer of other active arcs appear to vary almost linearly with subduction speed (about 150 MW/100 km for 10 mm/yr). The mass rate of common type arc extrusions (basalts, andesites, dacites) also increases almost linearly with subduction speed. This allows separation of the TVZ heat transfer into a “normal” component, associated with extrusions and intrusions of andesites and dacites (about 600 MW/100 km), and an “anomalous” component of about 2000 MW/100 km, related to extrusions and intrusions of rhyolitic melts whose generation is not directly controlled by subduction processes.Rhyolitic melts in the TVZ are partial melts of dominantly crustal origin. Comparison with other arcs indicates that the long-term extrusion rate of TVZ rhyolites (about 400 kg/s per 100 km) is also the highest world wide for this setting. The occurrence of voluminous Quaternary rhyolitic pyroclastics is a rare phenomenon and appears to be associated with a few arc segments (TVZ, Sumatra, Kyushu) that undergo significant crustal deformation.Various models have been proposed to explain the phenomenon of the anomalously high heat transfer within the TVZ. Models which require only heat transfer from plumes and subcrustal melts, either ponded at the crust/mantle boundary or intruding a spreading crust, are not suitable because the associated heat transfer at the contact is too low by a factor 2 to explain the required transfer rate of about 0.8 W/m² representing the “anomalous” crustal heat component of the TVZ. Heat generation by focussed plastic deformation within the ductile lithosphere is an alternative mechanism to explain “endogenous crustal heating” which yields heating rates that are also too low by a factor of two, although important parameters (average yield strength of lithosphere and opening rate of the TVZ) are not well known. A further search for a suitable combination of heat source models is required.     

The 26.5 ka Oruanui eruption, New Zealand: an introduction and overview

The 26.5 ka Oruanui eruption, from Taupo volcano in the central North Island of New Zealand, is the largest known ‘wet’ eruption, generating 430 km³ of fall deposits, 320 km³ of pyroclastic density–current (PDC) deposits (mostly ignimbrite) and 420 km³ of primary intracaldera material, equivalent to 530 km³ of magma. Erupted magma is >99% rhyolite and <1% relatively mafic compositions (52.3–63.3% SiO₂). The latter vary in abundance at different stratigraphic levels from 0.1 to 4 wt%, defining three ‘spikes’ that are used to correlate fall and coeval PDC activity. The eruption is divided into 10 phases on the basis of nine mappable fall units and a tenth, poorly preserved but volumetrically dominant fall unit. Fall units 1–9 individually range from 0.8 to 85 km³ and unit 10, by subtraction, is 265 km³; all fall deposits are of wide (plinian) to extremely wide dispersal. Fall deposits show a wide range of depositional states, from dry to water saturated, reflecting varied pyroclast:water ratios. Multiple bedding and normal grading in the fall deposits show the first third of the eruption was very spasmodic; short-lived but intense bursts of activity were separated by time breaks from zero up to several weeks to months. PDC activity occurred throughout the eruption. Both dilute and concentrated currents are inferred to have been present from deposit characteristics, with the latter being volumetrically dominant (>90%). PDC deposits range from mm- to cm-thick ultra-thin veneers enclosed within fall material to >200 m-thick ignimbrite in proximal areas. The farthest travelled (90 km), most energetic PDCs (velocities >100 m s⁻¹) occurred during phase 8, but the most voluminous PDC deposits were emplaced during phase 10. Grain size variations in the PDC deposits are complex, with changes seen vertically in thick, proximal accumulations being greater than those seen laterally from near-source to most-distal deposits. Modern Lake Taupo partly infills the caldera generated during this eruption; a 140 km² structural collapse area is concealed beneath the lake, while the lake outline reflects coeval peripheral and volcano–tectonic collapse. Early eruption phases saw shifting vent positions; development of the caldera to its maximum extent (indicated by lithic lag breccias) occurred during phase 10. The Oruanui eruption shows many unusual features; its episodic nature, wide range of depositional conditions in fall deposits of very wide dispersal, and complex interplay of fall and PDC activity.     

Volcanic risk ranking for Auckland, New Zealand. II: Hazard consequences and risk calculation

In a companion paper, a methodology for ranking volcanic hazards and events in terms of risk was presented, and the likelihood and extent of potential hazards in the Auckland Region, New Zealand investigated. In this paper, the effects of each hazard are considered and the risk ranking completed. Values for effect are proportions of total loss and, as with likelihood and extent, are based on order of magnitude.Two outcomes were considered – building damage and loss of human life. In terms of building damage, tephra produces the highest risk by an order of magnitude, followed by lava flows and base surge. For loss of human life, risk from base surge is highest. The risks from pyroclastic flows and tsunami are an order of magnitude smaller. When combined, tephra fall followed by base surge produces the highest risk. The risks from lava flows and pyroclastic flows are an order of magnitude smaller. For building damage, the risk from Mt. Taranaki volcano, 280 km from the Auckland CBD, is largest, followed by Okataina volcanic centre, an Auckland volcanic field eruption centred on land, then Tongariro volcanic centre. In terms of human loss, the greatest risk is from an Auckland eruption centred on land. The risks from an Auckland eruption centred in the ocean, Okataina volcanic centre, and Taupo volcano are more than an order of magnitude smaller. When combined, the risk from Mt. Taranaki remains highest, followed by an Auckland eruption centred on land. The next largest risks are from the Okataina and Tongariro volcanic centres, followed by Taupo volcano.Three alternative situations were investigated. As multiple eruptions may occur from the Auckland volcanic field, it was assumed that a local event would involve two eruptions. This increased risk of a local eruption occurring on land so that it was equal to that of an eruption from Mt. Taranaki. It is possible that a future eruption may be of a similar, or larger size, to the previous Rangitoto eruption. Risk was re-calculated for local eruptions based on the extent of hazards from Rangitoto. This increased the risk of lava flow to greater than that of base surge, and the risk from an Auckland land eruption became greatest. The relative probabilities used for Mt. Taranaki volcano and the Auckland volcanic field may only be minimum values. When the probability of these occurring was increased by 50%, there was no change in either ranking.Editorial responsibility: J. S. Gilbert     

Variations in the chemical and isotopic composition of fluids discharged from the Taupo Volcanic Zone, New Zealand

Contents of H₂O, CO₂ and Cl in well discharges from six explored geothermal systems of the Taupo Volcanic Zone, New Zealand, point to the existence of two distinct source fluids. The fluid present in discharges from systems along the eastern boundary is characterised by high CO₂ contents, 1.6 ± 0.5 , at mole ratios of 3.9 ± 1.5. High (0.06) and (12) weight ratios in these waters suggest that all four constituents are derived from associated andesitic rock. Geothermal discharges in the western parts of the TVZ, dominated by rhyolitic magmatism, are characterised by low CO₂ contents, 0.12 ± 0.05 , and low (0.14 ± 0.1) ratios. Again, relative Cl, B, Li and Cs contents agree with those of this potential source rock. High and ratios in the east are typical of fluids affected by the addition of volatiles released from subducted marine sediments. For the western systems, these ratios resemble more closely those expected for mantle-derived volatiles. The isotopic compositions of all deep waters point to the presence of variable amounts of a magmatic component, some 14 ± 5% in the eastern and 6 ± 2% in the western systems. The observed variations are explained in terms of interaction of volatiles released from the subducted sediments with material of the mantle wedge to form a volatile-charged, high-alumina basalt. Its convective rise, in a direction opposite to that of the down-going slab, leads to high enrichment in volatiles of the magmas generated beneath the eastern parts of the TVZ and increases their ability to intrude the continental crust. Further fractional crystallisation and assimilation leads to the formation of volatile-rich andesitic melts, partly extruded to form the volcanoes of the andesitic arc, partly intruded to act as source rocks for the high-gas geothermal systems. Batches of high-alumina basalt, depleted in subducted volatiles, travel farther west to pond beneath a zone of crustal extension. Following extensive fractionation, highly siliceous melts, carrying predominantly mantle-type volatiles, rise beneath the western part of the TVZ to supply both heat and volatiles to the geothermal systems there.     

Large-scale silicic alkalic magmatism associated with the Buckhorn Caldera, Trans-Pecos Texas, USA: comparison with Pantelleria, Italy

Three major rhyolite systems in the northeastern Davis and adjacent Barrilla Mountains include lava units that bracketed a large pantelleritic ignimbrite (Gomez Tuff) in rapid eruptions spanning 300,000 years. Extensive silicic lavas formed the shields of the Star Mountain Formation (37.2 Ma-K/Ar; 36.84 Ma ³⁹Ar/⁴⁰Ar), and the Adobe Canyon Formation (37.1 Ma-K/Ar; 36.51-³⁹Ar/⁴⁰Ar). The Gomez Tuff (36.6 Ma-K/Ar; 36.74-³⁹Ar/⁴⁰Ar) blanketed a large region around the 18×24 km diameter Buckhorn caldera, within which it ponded, forming sections up to 500 m thick. Gomez eruption was preceded by pantelleritic rhyolite domes (36.87, 36.91 Ma-³⁹Ar/⁴⁰Ar), some of which blocked movement of Star Mountain lava flows. Following collapse, the Buckhorn caldera was filled by trachyte lava. Adobe Canyon rhyolite lavas then covered much of the region. Star Mountain Formation (~220 km³) is composed of multiple flows ranging from quartz trachyte to mildly peralkalic rhyolite; three major types form a total of at least six major flows in the northeastern Davis Mountains. Adobe Canyon Formation (~125 km³) contains fewer flows, some up to 180 m thick, of chemically homogenous, mildly peralkalic comendite, extending up to 40 km. Gomez Tuff (~220 km³) may represent the largest known pantellerite. It is typically less than 100 m thick in extra-caldera sections, where it shows a pyroclastic base and top, although interiors are commonly rheomorphic, containing flow banding and ramp structures. Most sections contain one cooling unit; two sections contain a smaller, upper cooling unit. Chemically, the tuff is fairly homogeneous, but is more evolved than early pantelleritic domes. Overall, although Davis Mountains silicic units were generated through open system processes, the pantellerites appear to have evolved by processes dominated by extensive fractional crystallization from parental trachytes similar to that erupted in pre- and post-caldera lavas. Comparison with the Pantelleria volcano suggests that the most likely parental magma for the Buckhorn series is transitional basalt, similar to that erupted in minor, younger Basin and Range volcanism after about 24 Ma. Roughly contemporaneous mafic lavas associated with the Buckhorn caldera appear to have assimilated or mixed with crustal melts, and, generally, may not be regarded as mafic precursors of the Buckhorn silicic rocks, They thus form a false Daly Gap as opposed to the true basalt/trachyte Daly gap of Pantelleria. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.     

A magnetic model for deep plutonic bodies beneath the central Taupo Volcanic Zone, North Island, New Zealand

The central Taupo Volcanic Zone (TVZ) is a region of intense Quaternary rhyolitic volcanism and geothermal activity in the North Island of New Zealand from which about 14,000 km³ of pyroclastics and lavas have been erupted during the last 1.6 Ma. Analysis of aeromagnetic surveys over the TVZ showed the presence of long-wavelength (10 to 25 km) magnetic anomalies which roughly follow the trend of the currently active eastern TVZ, from the north of Lake Taupo to the east of Lake Rotorua. An interpretation of the long-wavelength magnetic anomalies using 3-D magnetic modelling suggests that these anomalies are caused by the magnetic effects of < 3 km thick sequence of volcanic rocks and deeper magnetised bodies within the non-magnetic upper crust (4–7 km depth) beneath the young (age < 0.7 Ma), currently active eastern TVZ. The deep magnetised bodies are interpreted as solidified rhyolitic sub-volcanic plutons that have cooled down to below their Curie temperature.Although the existence of plutonic bodies beneath the TVZ has been postulated prior to this study, this magnetic interpretation result appears to be the first geophysical model of such bodies.