Perils in distinguishing phreatic from phreatomagmatic ash; insights into the eruption mechanisms of...

Journal of Volcanology and Geothermal Research xxx (2014) xxx–xxx

VOLGEO-05318; No of Pages 18

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Perils in distinguishing phreatic from phreatomagmatic ash; insights intothe eruption mechanisms of the 6 August 2012 Mt. Tongariro eruption,New Zealand

Natalia Pardo a, Shane J. Cronin a,⁎, Károly Németh a, Marco Brenna a, C. Ian Schipper b,c, Eric Breard a,James D.L. White d, Jonathan Procter a, Bob Stewart a, Javier Agustín-Flores a, Anja Moebis a, Anke Zernack a,Gábor Kereszturi a, Gert Lube a, Andreas Auer d, Vince Neall a, Clel Wallace a

a Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealandb School of Geography, Environment and Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealandc Japan Agency for Marine Earth Science and Technology (JAMSTEC) 2-15 Natsushima-cho Yokosuka, Kanagawa 237-0061, Japand Geology Department, University of Otago, PO Box 56, Dunedin 9054, New Zealand

⁎ Corresponding author. Tel.: +64 6 3569099x84871;E-mail address: [email protected] (S.J. Cronin).

http://dx.doi.org/10.1016/j.jvolgeores.2014.05.0010377-0273/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Pardo, N., et al., Perthe 6 August 2012 Mt. Tongariro eruption, N

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Article history:Received 20 June 2013Accepted 3 May 2014Available online xxxx

Keywords:Te MaariHydrothermalJuvenileTephraMolten-fuel coolant interactionGlass

The weak geophysical precursors of the 6 August 2012 Te Maari eruption of Mt. Tongariro and a lack of obviousjuvenile components in its proximal ballistic deposits imply that the eruption was caused by the sudden decom-pression of a sealed, hot hydrothermal system. Strong magmatic signals in pre- and post-eruption gas emissionsindicate that fresh magma had intruded to shallow levels shortly before this eruption. Here we examine the vol-canic ash produced during the August eruption with the aim of determining whether juvenile magma waserupted or not. The widely applied criteria for identifying fresh juvenile pyroclasts provided inconclusive results.The Te Maari ash sorting and trend towards a unimodal grain-size distribution increase with distance along thedispersal axis. Proximal to intermediate sites showing polymodal grain-size distributions can be related to the re-fragmentation of different pre-existing lithologies, overlapped erupted pulses and transport mechanisms, and toparticle aggregation. Between 69 and 100 vol.% of particles coarser than 3 ϕ and 45–75 vol.% of grains finer than3 ϕwere sourced from the pre-existing, commonly hydrothermally altered, vent-area lavas and pyroclasts. Freecrystals (pyroxene N plagioclase N magnetite N pyrite) make up 0–23 vol.% of particles coarser than 3 ϕ, and22–41 vol.% of grains finer than 3ϕ. Brown to black fragments of fresh glass are a small (1–15 vol.%), but notable,component. Under SEM, these blocky, glassy particles are poorly vesicular, and irregularly shaped, somewith flu-idal or bubble-wall surfaces, and others with fragmented stepped surfaces and fine adhering ash. In thin section,they contain variable amounts of microlites within an isotropic groundmass. The range in silica content of themicroprobe-analysed glass is very wide (56–77 wt.%) and cannot be correlated to any specific particle texturaltype. These chemically and texturally diverse glassy fragments are identical to mechanically broken pieces ofcountry rock lavas and pyroclasts; both their diversity, and their match with vent country rocks, argue stronglyagainst a “juvenile” origin for the glassy fragments. We conclude that rising magma provided only heat andgas into the overlying, sealed vapour-dominated hydrothermal system. A landslide from this area led to arapid decompression and ash was produced by top-down hydrothermal explosions. Careful attention must bepaid to the combination of compositions and textures of fine ash particles in such situations, aswell as to the con-text of their source vent, in order to be confident that new magma has reached the surface.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Phreatomagmatic eruptions result from the explosive interactionbetween rising magma and external water (Lorenz, 1973) through aprocess that has been experimentally and numerically explained as a

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ils in distinguishing phreaticew Zealand, J. Volcanol. Geot

Molten-fuel coolant interaction (MFCI) (Buchanan, 1974; Wohletz,1983; Zimanowski et al., 1991, 1997). This process occurs when twofluids, one above the boiling point of the other (e.g., magma andwater) establish direct contact across an interfacial surface by a shockwave. Evidence for this eruption mechanism is often difficult to extractfrom the finely fragmented ash deposits, particularly in medial to distalareas. Purely hydrothermal eruptions can also producewidely distribut-ed deposits of fine ash (cf., Rose et al., 1982; Freuillard et al., 1983;Barberi et al., 1989).

from phreatomagmatic ash; insights into the eruption mechanisms ofherm. Res. (2014), http://dx.doi.org/10.1016/j.jvolgeores.2014.05.001

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2 N. Pardo et al. / Journal of Volcanology and Geothermal Research xxx (2014) xxx–xxx

The sudden disruption of an over-pressurised hydrothermalsystem without directly interacting with magma produces a hydro-thermal explosion, sometimes termed a “phreatic” explosion(Macdonald, 1972; Barberi et al., 1989) or more specifically, a “Mag-matic-Hydrothermal” eruption (Browne and Lawless, 2001). Theseeruptions most commonly occur as isolated events, and precursorsare often absent or limited to subtly anomalous seismicity and sud-den variations in fumarole and surface water properties (Barberiet al., 1992; Martini, 1996).

In composite volcanoes, phreatomagmatic and magmatic-hydrothermal eruptions are commonly observed as opening/closingphases of larger events (e.g., Cioni et al., 1992), but are often neglecteddue to the low eruption magnitudes compared to larger, sustainederuptions. In stratovolcanoes with active hydrothermal systems or cra-ter lakes (e.g., Rowe et al., 1992) the high frequency of isolated eventsposes an imminent hazard in proximal areas. This is the case atMt. Tongariro (New Zealand), where popular walkways cross mostof the active vents, and main highways and settlements are locatedb15 km away.

The eruption through the Upper Te Maari crater near midnight on 6August 2012 was extremely short-lived (b30 s acoustic signal record;Hurst et al., in this issue; Jolly et al., in this issue). Ash was ejected at29–38m/s and the buoyant column reached at least 7.8 km and possibly~10 km above the vent within the following 25 min (Crouch et al., inthis issue; Turner et al., in this issue). Fine ash was dispersed to theeast for at least 200 km, reaching the coast of the North Island (Fig. 1).Ash particles were transported at velocities of ~80 km/h, consistentwith thewind-speed recorded at ~9–10 km a.s.l at the time of the erup-tion (cf. Crouch et al., in this issue; Turner et al., in this issue), over asmuch as 6000 km2. A second smaller eruption on21November 2012 oc-curred without precursors, but was observed and recorded by severalwalkers on the mountain at the time (https://www.youtube.com/watch?v=X2ASNu7vVGA). The ash from the November eruption dis-persed rapidly towards the southwest, and was visible on the groundonly in areas b15 km from the vent.

The 6 August eruption raised many challenging questions for scien-tific advisors. One of the key questions was whether new magma hadrisen into the volcanic system. Numerous publications have focussedon criteria to distinguish magmatic from phreatomagmatic eventsbased on glass ash morphology, microtextures and microstructures,grain-size distributions, and non-juvenile lithic contents (Heiken,1972; Wohletz, 1983; Wohletz and Sheridan, 1983; Büttner et al.,1999; Dellino et al., 2004; Németh, 2010). Amongst glassy ash particlesproduced in phreatomagmatic eruptions, Büttner et al. (2002) distin-guished “active” vs. “passive” particles, with the former directly at themagma–water interface (fragmented in a brittle or ductile state), andthe latter away from the direct contact zone, fragmented by mechani-cal energy. It was not certain, however, whether glassy ash particlesas a minor component of the Te Maari eruption represented newmagma, or fragments recycled from the deposits of previous(including historic) eruptions. Glassy particles produced in eruptionphases preceding the 2004 Mt. St. Helens dome-extrusion wererecognised, via detailed micro-geochemical studies, as being recycledand the “juvenile” particles shown to be crystal-rich fragments(Cashman et al., 2008; Rowe et al., 2008). Discerning whether thereare juvenile products in small phreatomagmatic eruptions, or onlyfragments of pre-existing rock, is therefore not always straightforward,but it is crucial for hazard response and advice provision to know

Fig. 1.Distribution of the ash produced on the 6–7th of August, 2012 from the Upper Te Maari Cdistribution was reconstructed by using photographs taken from helicopters during the emerSH46, including sharp 0-thickness locations that delimit the extent of the deposit; b) extrapolastruction was based on eye-witness reports and on the Visible Infrared Imaging Radiometeeruption sampling sites and histograms illustrating the grain-size distribution of samples (See

Please cite this article as: Pardo, N., et al., Perils in distinguishing phreaticthe 6 August 2012 Mt. Tongariro eruption, New Zealand, J. Volcanol. Geot

whether magma has started to reach the surface (cf., Sparks et al.,1981; Kennedy and Russell, 2012).

2. Mt. Tongariro: a complex multi-vent composite volcano

Mt. Tongariro (Tonga-South wind, and riro-carried away; in Maorilanguage) is a ~2000 m high andesitic volcano situated within theTongariro Volcanic Centre (TgVC; Topping, 1974; Cole and Nairn,1975; Cole, 1978), at the southern end of the active Taupo VolcanicZone (TVZ) in the central North Island, New Zealand. It consists of sev-eral coalesced and overlapping cones and craters (Fig. 1), along a NE–SW trend (Mathews, 1967; Hobden et al., 1996). Gamble et al. (2003)report a maximum age of 340 ka for the volcano, and K/Ar dating byHobden et al. (1996) was used to define cone-building stages at210–200 ka, 130–70 ka and from 25 ka to the present. Historical erup-tions have occurred from the Upper Te Maari crater (1877) and Mt.Ngauruhoe (last in 1975) (Gregg, 1960).

Tongariro eruptive products are typical subduction-related andes-ites, basaltic andesites and dacites that exhibit textures and composi-tions that reflect complex mixing and mingling processes which takeplace prior to an eruption (Graham and Hackett, 1987; Gamble et al.,1999, 2003; Hobden et al., 1999; Cole et al., 2000; Price et al., 2005). Ahigh short-term geochemical variability in compositions is consistentwith complex dyke-sill systems with scattered and unconnectedmagma reservoirs throughout the crust and upper mantle (Nakagawaet al., 1999, 2002; Waight et al., 1999).

2.1. The hydrothermal system

KetetahiHot Springs, TeMaari Craters, Red Crater, andMt. Ngauruhoehost themain geothermal surface manifestations of a vapour-dominatedsystem underlying Tongariro (Wilson, 1960; Moore and Brock, 1981). Acombination of magmatic steam mixed with circulating meteoric waterat equilibrium temperatures of 230–290 °C (Wilson, 1960; Hochstein,1985) is sealed by a 200–500m thick, low-resistivity cap of highly hydro-thermally altered rocks (Walsh et al., 1998). Seismic swarms between1988 and 1994 with aligned epicentres close to Te Maari Craters havebeen interpreted by Walsh et al. (1998) as evidence of episodic magmaintrusion below the geothermal system.

2.2. Te Maari craters

These are named after the Maori chieftain, Te Maari, who died soonafter the eruption of the upper crater in 1868 (Hobden, 1997). Hector(1870; in Gregg, 1960), in 1867 only described the Lower Te MaariCrater, which contained a lake and sinter deposits. The oldest eruptionrelated to the Lower Te Maari Crater is the 13.8 ka sub-Plinian RotoairaLapilli and coeval lava flows (Topping, 1973). Topping (1974) reporteddebris flow/hyperconcentrated flowdeposits sourced from the TeMaariarea, relating to eruptions at ~2500 yrs BP. Further, lava flows eruptedfrom the Upper Te Maari crater in 1528 AD, possibly marking itsinception, with a flow towards the NW and another into the LowerCrater (Topping, 1974). The poor preservation of the common hydro-thermal to phreatomagmatic deposits sourced from Te Maari (cf.,Gregg, 1960; Cole andNairn, 1975)makes it difficult to establish a com-prehensive eruption history (Table 1). Eruptions in 1892 and 1896wereobserved and reported by Hill (1893) and Friedlaender (1898), respec-tively. Maori settlements in the area have been established since

rater: a) shows the location of sampling sites within 25 km from the source. The proximalgency response. Mass/area ratios and thickness data are reported for sites along SH1 andtion of the deposit to distal areas with a main dispersal axis towards the east. The recon-r Suite image acquired by NASA Earth Observatory at 00:55 on 7th August, 2012. Post-SM1) selected from near the dispersal axis are shown.


https://www.youtube.com/watch?v=X2ASNu7vVGA

https://www.youtube.com/watch?v=X2ASNu7vVGA


3N. Pardo et al. / Journal of Volcanology and Geothermal Research xxx (2014) xxx–xxx

Please cite this article as: Pardo, N., et al., Perils in distinguishing phreatic from phreatomagmatic ash; insights into the eruption mechanisms ofthe 6 August 2012 Mt. Tongariro eruption, New Zealand, J. Volcanol. Geotherm. Res. (2014), http://dx.doi.org/10.1016/j.jvolgeores.2014.05.001


Table 1Summary of the eye-witnessed eruptions sourced at Te Maari Craters (Gregg, 1960; Cole and Nairn, 1975).

Date Reference Reported products, eruption observations whereprovided

Interpretation

1855-Red Crater or Te Maari Hochstetter (1864) Ash Ash1869-Te Maari or Mt. Ruapehu Hill (1891), Maori oral

descriptions,R.T. Batley (1869).

Violent earthquake. An incandescent “flame”wasseen through an ash cloud that would burst andfall from the top of the cloud. Intense steam.

Formation time of the Upper Te Maari crater (?).Ballistics and ash plume.

06/1886-Ruapehu (?) Gregg (1960) Ash Ash11/1892-Te Maari (Red Craterand Ngauruhoe unrest withlocal eye-witness evidencethat the latter erupted for2–3 h, generating an ashplume the day after the TeMaari major event)

Hill (1893) A “flow” stripped the vegetation in thedeep valleyrunning between Ketetahi springs and TeMaari toa width of 10–30 m depending on the valleyshape. This terminated within 170 m of thecraters at a steep cliff, also stripped of coveringsoil. Sand, mud and “stones” partly filled thevalley. A fringe of around 20 m to either side ofthe stripped area showed dead vegetation“blackened or reddened”, appearing “scalded”(but not charred). Residual pools of water in thevalley were strongly caustic and clear. Above theLower Te Maari crater a new fissure had opened,extending upslope. Strong degassing preventedaccess to the rift and sulphur was depositedaround it. Sand and small “stones” surrounded thevent area. In addition, grey “dull” dense pumiceash to fine lapilli. Ash and fine pumice lapilliscattered thinly over the ridges extending abovethe Te Maari crater, thinning towards Blue Lakeand absent down-slope (north) of Te Maari.

Hydrothermal or/and phreatomagmaticexplosions, generating wet surges. Some of thesesurges were concentrated in the valley extendingbelow the crater and possibly transformed laterinto lahars. The surges were warm and associatedwith acidic steam orwater aerosols that “scalded”and damaged vegetation in a singe-like zonelaterally extending from the central concentratedpart of the flow in the valley base.The dense pumice lapilli lobe distributed to thesouth of the Te Maari suggests that at least oneeruption event emitted new magma. This phase,involving fresh vesiculated magma appeared tohave followed after the initial vent-clearingexplosions. Scattered thin fall indicates thisphase was brief and the total eruption could haveoccurred within 1–2 h.

13/11/1896-Te Maari Thames Star (1896) andAuckland Star (1896).

Eruptions commenced 12.40 on 13 Nov 1896, thefirst one lasting 40 min. A second, starting at1500 h continued until dusk. Fall of coarse ash tofine lapilli. Eye-witnesses at the scene reportedearthquakes and incandescence.

Opening hydrothermal or/and phreatomagmaticphases with ejection of ballistics.

24/11/1896-Te Maari Evening Post (Vol. LII-158, 25/11/1896)

Quiet for a few days after previous activity. At0930, a large black ash plume suddenly rose fromTe Maari, with a simultaneous steam eruptionfrom Ruapehu and a steam plume from Ketetahi.

Hydrothermal or/and phreatomagmatic phase(ash fallout).

1/12/1896-Te Maari Evening Post (Vol. LII-163, 1/12/1896)

An ash plume of great height was erupted,obscuring the view to Ngauruhoe and Ruapehu(from Taupo).

Hydrothermal or/and phreatomagmatic phase(ash fallout).

3/12/1896 Hawera and Norman byStar (Vol. XXXIII-3414,4/12/1896)

Themain ash/steam plumes described arose froma fresh outlet above Te Maari (upper Te Maaricrater). Steam rose from Ruapehu and Ketetahi.

Degassing

12/1896 Friedlaender (1898) Vigorous steam plumes, with occasionally abrown ash component, began in early December.Roaring and thundering were heard for “some”minutes from the crater during Friedlander's visit(?). A blue-coloured gas was noted in contrast tothe white steam of Ketetahi.

Minor hydrothermal or/and phreatomagmaticpulses; fumarolic activity, H2S gas release.

14-15/1896 Hill (1897)Friedlaender (1898)(Bay Of Plenty Times,Vol. XXIV, 13-01-1897)Evening Post, Vol. LIII-4,6-01-1897

Thundering and ejection of a vertical ash/steamcloud of “considerable” height, following adetached cloud from an earlier explosion. Thegrowing ash column had a dark-red glow at itsbase (caused by reflection from below).Incandescent blocks/bombs were ejected onparabolic paths, and lightening occurred in manyparts of the column. The eruption wasaccompanied by a continuous thundering roar andbright blue “flames” rose 30 m above the crater.The entire witnessed eruption lasted for 15 min.The large bombs scarcely seemed hot in the dayand had a viscous, angular, form. The uppervolcanic conewas covered by bluish-grey fine ash.Incandescence was only seen for shortperiods and not seen during daytime eruptions.Intervening activity was constant energeticsteaming. The Upper Te Maari crater excavateddeeper into the up-slope area, with a block of4.5 tonnes hurled 800 m from the crater. Furtherobservations from an ascent on 1 Jan 1897 (Bay OfPlenty Times, 1897) revealed a 33 m wide riftextending upslope. The Lower crater was filledwith ejecta. Mud, sand and rock covered much ofthe area, reaching Ketetahi and Red Crater. Newsteamvents also appeared 400 mdownslope fromthe Te Maari Craters.Fall deposits at Waihohonu were coarse ash, in

Most violent phase, with a probable pyroclasticflow, ejection of ballistics and ash fallout; similarto a vulcanian style. The magmatic component isinferred from the incandescent bombs.Generation of new rifts and steam vents.The proximal mud-sand- and rocks reachingKetetahi may correspond to surge deposits.Lahars.




Table 1 (continued)

Date Reference Reported products, eruption observations whereprovided

Interpretation

contrast to other eruptions. Also none of theprevious eruptions generated as much lighteningaccording to settlers' descriptions, withprevious eruptions producing very fine ash withthe consistency of flour. Fine grey ash was thickenough to allow observers to fill several smallsample bottles with ash from the roofs of houses.Local reports were that the ash deposit extendedfrom between Waihohonu stream and LakeRotoaira, with some ash drifting as far as Tokaanu.The deposit was up to 5 cm thick along thenorthern Desert Road, with areas of vegetation“destroyed”. The ashfall reached Napier (114 kmSW),where dust on drying clotheswas reported inthe morning of the 15th. Lahars/mudflowsentered Lake Rotoaira (Evening Post, 1897).

14-15/1896


1250–1300 AD and the local oral traditions also contain several refer-ences to volcanic activity sourced at Mt. Tongariro (Lowe et al., 2002;Lowe, 2008; Cashman and Cronin, 2008).

The observed 1892 and 1896 eruptions seem to have been similar incharacter to the 2012 eruptions, with explosions producing ballisticsand pyroclastic density currents, including brief paroxysmal phasesejecting incandescent ballistics, dense pumice lapilli and fine lithic ash(Hill, 1893; Friedlaender, 1898). The climactic phases produced thelargest ash plumes, with ash up to 5 cm thick along State HighwaySH1 (23 km to the east; Fig. 1) and reaching Napier, 115 km to thesoutheast (Hill, 1897). Proximal pyroclastic density currents may haveoccurred, scalding vegetation with acid-like damage, along with laharsthat reached Lake Rotoaira, 10 km NNE from the source (Hill, 1893).

2.3. Holocene volcanism

Holocene volcanic eruptions at Tongariro have ranged from effusive,lava-flow-producing events, to highly explosive Plinian eruptions(Topping, 1974; Hackett and Houghton, 1989; Donoghue et al., 1995).Fall deposits are mainly distributed to the east and southeast. At leasteight known, and several other suspected, large-scale sub-Plinian toPlinian eruptions occurred from Tongariro between the 13.8 ka BP(0.2 km3) Rotoaira Lapilli, and the 9.7 ka BP (0.9 km3) Poutu Lapilli.Due to chemical similarities between the tephra and vent-rim aggluti-nate and its distribution, the 13.8 ka BP Rotoaira Lapilli is most likelysourced from the Lower Te Maari crater (Hobden, 1997). Lower-magnitude eruptions subsequent to the 9.7 ka BP event depositedthin, fine-medium grained ash beds, alongwith proximal bombs, blocksand agglutinates around the vent areas (Topping, 1973, 1974; Toppingand Kohn, 1973; Donoghue et al., 1995; Moebis, 2010).

3. Methodology

Fresh ash from the 6 August 2012 eruption was sampled along statehighways (SH) SH1 and SH46 (Fig. 1) as soon as the roads were acces-sible (7–12 h after the eruption) (See Supplementary material SM 1).Both bulk samples and specific mass/area samples were taken fromclean, flat, hard surfaces of measured area (Fig. 2a). Intermittent andpatchy rain showers, during and shortly after the eruption, may have lo-cally modified the original deposits with rain-dropmarks in some loca-tions (Fig. 2a). Further bulk samples were taken during the followingtwo days. These were from the ash dispersal axis at Waipakihi hut(25 km from the source; collected by H. Keys from the Department ofConservation), along SH5 and in Wairoa (150 km from the source; col-lected by Ashleigh Phillips, Ravensdown Fertiliser Ltd). Samples were


dried at b40 °C, weighed, sieved at half-ϕ intervals, and size fractionsN1 ϕ (b500 μm) were analysed with a Partica LA-950V2 (Horiba Scien-tific) Laser Scattering Particle Size Distribution Analyzer (LPDA). A totalof 16 samples were analysed, with five to ten runs per sample carriedout until reproducible curves were obtained. Analyses were run usingthe Mie scattering method (de Boer et al., 1987; Horiba Inc., 2012),with a refractive index of 1.56 (andesitic tephras). Grain-size distribu-tions (SM 1) were combined from the datasets using Gradistat V.8.0(Blott and Pye, 2001).

Bulk and sieved samples were cleaned with several (~10) bathcycles in distilled water and a final one in acetone (Fig. 2c). Althoughwe are aware that Lautze et al. (2012) pointed out the effects of thismethod on the surface chemical etching, adhering and colour varia-tion, the previous examination of Te Maari Grains revealed abundantthin coatings (micron-scale) on the grain surfaces. The removal ofcolloidal surface dust and film layers that prevented seeing any tex-ture characteristic to distinguish lithics from juvenile particles wastherefore needed.

Sieved and cleaned samples from sites at 11.5, 25, and 132 km alongthe dispersal axis (NP-A6-1; HK-1; and VEN-K4 in Fig. 1), and anotheroff-axis (AZ-A6-1; in Fig. 1a) were subjected to a detailed componentanalysis. Grains were counted (300 per sample) under a Leica WildMZ8 binocularmicroscope, in each of the 0.0, 1.0, 2.0, and3.0ϕ size frac-tions. Additionally, density-separated fractions previously cleaned withHCl (10%) in an ultrasonic bath were also examined and glass particlesfrom the 3 ϕ fraction were hand-picked, mounted on a stub and gold-coated for examination under a FEI Quanta Scanning Electron Micro-scope (SEM) at the Microscopy Imagery Centre of Massey University.Images were taken with a 20 kV accelerating voltage and a 55 μAbeam current, at a 9.1–9.8 mm working distance. Particles were takendirectly from two bulk samples (HK-1 at 25 km and GIS-1 at 150 kmfrom the source), and the 3 and 4 ϕ size fractions from two other sam-ples (NP-K2 at 9 km and HK-1 at 25 km from the source), and mountedin epoxy. Thin sections were produced (Fig. 2d) for a comparative com-ponent analysis based on 500 counts from each sub-sample. Isotropicgrains were identified in thin sections and imaged with a Zeiss SigmaVP FEG SEM at the University of Otago, in order to examine 2D-textures and produce semi-quantitative element distribution maps(95 maps in total). Backscattered secondary electron (BSE) imageswere taken with the Zeiss SEM (218 in total, covering samples NP-K2,HK-1, and GIS-1), using an accelerating voltage between 8 and 12 kVand an aperture size of 60 μm, at 8.1mmworking distance. Groundmassglass compositions of ash grains from the bulk samples, and from thesame grains analysed at the University of Otago, were measured witha JEOL JXA-8230 SuperProbe Microanalyser (EPMA) at Victoria



Fig. 2.TeMaari ash deposit: a)Grey ash coatedmost of the vegetation and clean depositswere found onflat,metal road fences over the Poutu Canal. Ash deposits from all locations showedsome degree of moisture and excavated pits indicating impact from rain drops; b) ash aggregates (red arrows) between 6mmand a few (~1)mmwere identified. Ash samples after siev-ing and cleaning in distilled water and acetone (c) were also mounted in thin sections (d). L1: Fresh dark grey andesites; L2: fresh pale grey to brown andesites; L3: white, hydrothermallyaltered lithics with disseminated pyrite and mottled textures; L4: multicoloured hydrothermally altered lithics; Pl: plagioclase; Px: pyroxene; G: glassy particles were found but grade intexture to L1 type. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


University of Wellington, using a 15 kV accelerating voltage and 8 nAcurrent. All major elements except Na were analysed with peak/back-ground counting times of 30/15 s. To guard against loss, Na was mea-sured first with counting times of 10/5 s. When possible, weattempted to maintain a 20 μm spot size, but often had to reduce thison a spot-by-spot basis, to avoid microlites. For comparison, furtherglass and crystal grains were also analysed at the University of Otagowith a JEOL 8600 electron microprobe. Standard operating conditionswere an accelerating voltage of 15 kV, a beam current of 1 nA, 100 slive count time, with a 10 μm beam diameter.

4. Results

4.1. Ash dispersion, volume and grain-size distribution

The available data were insufficient to construct reliable isopachs,but the greatest ash thicknesses, and grain sizes, define an axis to theeast (Fig. 1), with a minor secondary lobe to the north, towards siteAZ-A6 (Fig. 1). The deposit boundaries along SH46 and SH1 weresharp. Between 100 and 150 km from the source, measurements wereaugmented by inhabitants' reports of ash presence. Despite this, the dis-tal boundaries remain uncertain (Fig. 1a). Ash coveragewas often not ina complete layer, butmade up of irregular aggregates of a fewmm, up toa maximum of 6 mm (Fig. 2b). Based on the available fall data andapplying the Legros (2000)method, as well as an ArcMap volume com-parison in digital elevation models from pre- and post-eruption topog-raphy (Procter et al., in this issue), a minimum volume of ~231,000–280,400 m3 for the total ejecta is estimated (considering that 1 mm ofash was deposited within the first 9.6 km; 1.5 mm within the first5.5 km, and 2.5 mm within the first 4.5 km from the source).

Applying the Cas and Wright (1987) classification, ash samples arewell to poorly sorted (poorly to very poorly sorted in epiclastic-equivalent; SM1). Along SH1, grain-size distributions range fromunimodal to polymodal in localities b12 km from the source, and areunimodal beyond 25 km from the source. Samples taken north of the


Poutu Channel (Fig. 1) are symmetric to finely skewed and dominantlyplatykurtic, whereas south of the channel samples are symmetric anddominantly mesokurtic. Sorting and a tendency towards unimodaldistributions increase with distance along the dispersal axis (Fig. 1b,SM1). Along this axis, samples have three modes at b12 km from thesource, with mode1 = 4.8 ϕ (38 μm), mode2 = 2.2 ϕ (213 μm), andmode3 = 8.2 ϕ (4 μm). Beyond 25 km from the source, distributionschange from symmetrical to finely-skewed, and the main mode rangesfrom 4.8 ϕ (38 μm) to 5.7 ϕ (19 μm; 132 km from the source). The totalfine ash content (b4 ϕ; 63 μm) ranges from 39–74% within the first12 km from the source, to 65–99% between 25 and 150 km from thesource and along the dispersion axis. Grain-size distributions alongSH46 are symmetrical to coarsely skewed, and range from unimodalto polymodal, with a main mode1 = 5.7 ϕ (19 μm). The total fine ashcontent within the northern secondary lobe ranges from 60 to 79%.

4.2. Component analysis

Counted grains in thin sections were classified according to theirlithology into free crystals, lithics, and glassy particles (Fig. 2c, d). Re-sults indicate that 69–100 vol.% of particles coarser than 3 ϕ, and45–75 vol.% offiner particles, are lithic clastswith variable degrees of al-teration (L1–L4 in Fig. 2c; Table 2; Fig. 3). Unaltered, porphyritic, dense,lava fragments grade from black (L1) to pale grey (L2), and contain pla-gioclase (Pl), pyroxene (Px), and magnetite (Mt) phenocrysts. Hydro-thermally altered clasts range from whitish-grey (L3) with mottledtextures (induced by disseminated oxides and sulphides) to altered,yellow and red, aphanitic clasts (L4). In all samples, lithics of type L3are most common. Unaltered lava fragments (L1 and L2) resemble typi-cal reported Tongariro lithologies (cf., Donoghue, 1991; Donoghue et al.,1991; Hobden, 1997). Angular, free crystals occur with a relative abun-dance of pyroxene N plagioclase N magnetite N pyrite, comprising0–23 vol.% of particles coarser than 3 ϕ and 22–41 vol.% of finer grains.Finally, brown to black, dense to vesicular, fresh to slightly altered glassyfragments (“G” in Fig. 2c),make up 1–15% of the 3 to 0ϕ size fractions. It



Table 2Ash componentry photographed with a Leica Wild MZ8 binocular microscope. Pl; Plagioclase; Px: Pyroxene; Mt: Magnetite; Py: Pyrite. Raw and normalised data are presented.

Sample Distance from thesource

Crystals Fresh fragments Altered fragments Totalcounts

ϕ-Size (km) Pl Px Mt py Brown, glassy Dark grey to black, porphyritic Pale grey, porphyritic Brown porphyritic Whitish grey Red, yellow, orange

Features Angular, subhedral-euhedral

Irregular, from vesicular,aphanitic to porphyritic,dense

Subangular to rounded,dense, porphyritic

Subangular to subrounded,dense, porphyritic

Dense with variable degreesof alteration. Some withsilicified matrix

Subangular to rounded,common mottled textureand disseminated pyrite

Highly altered,ranging from denseto vesicular

Raw dataNP-A6-1 0 11 East 2 5 5 1 13NP-A6-2 1 3 2 52 81 12 95 55 300NP-A6-3 2 44 23 1 26 18 58 7 98 25 300NP-A6-4 3 88 32 3 11 9 38 3 71 45 300HK-1 0 25 East 3 1 17 20 66 151 42 300HK-1 1 6 4 27 24 65 119 55 300HK-1 2 15 12 16 46 52 110 49 300HK-1 3 59 26 7 13 22 131 42 300VEN-K4 3 132 East 55 11 1 9 13 48 125 38 300VEN-K4 4 120 28 10 2 4 32 29 50 25 300NP-A3-0.0f 0 12 Northeast 9 2 16 31 4 62NP-A3-0.5f 1 28 21 14 69 191 131 22 476NP-A3-2.0f 2 40 30 7 89 64 110 74 414NP-A3-3.0f 3 67 36 3 45 60 62 27 300AZ-A6-1 0 6 North 18 11 13 75 21 105 57 300AZ-A6-1 1 23 2 23 33 37 14 101 67 300AZ-A6-1 2 35 33 1 12 14 44 3 107 51 300AZ-A6-1 3 65 39 1 5 13 13 22 2 100 40 300

Normalised dataNP-A6-1 0 11 East 0 0 0 0 0 15 38 0 38 8 100NP-A6-2 1 1 0 0 0 1 17 27 4 32 18 100NP-A6-3 2 15 8 0 0 9 6 19 2 33 8 100NP-A6-4 3 29 11 0 1 4 3 13 1 24 15 100HK-1-0.0f 0 25 East 1 0 0 6 7 22 50 14 100HK-1-1.0f 1 2 1 0 9 8 22 40 18 100HK-1-2.0f 2 5 4 0 5 15 17 37 16 100HK-1-3.0f 3 20 9 0 2 4 7 44 14 100VEN-K4-3.0f 3 132 East 18 4 0 3 4 16 42 13 100VEN-K4-4.0f 4 40 9 3 1 1 11 10 17 8 100NP-A3-0.0f 0 12 Northeast 0 0 0 15 3 26 50 6 100NP-A3-0.5f 1 6 4 0 3 14 40 28 5 100NP-A3-2.0f 2 10 7 0 2 21 15 27 18 100NP-A3-3.0f 3 22 12 1 15 0 20 21 9 100AZ-A6-0.0f 0 6 North 6 0 0 0 4 4 25 7 35 19 100AZ-A6-1.0f 1 8 1 0 0 8 11 12 5 34 22 100AZ-A6-2.0f 2 12 11 0 0 4 5 15 1 36 17 100AZ-A6-3.0f 3 22 13 0 2 4 4 7 1 33 13 100

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Fig. 3.Relative proportions of the components identified in the 0–3ϕ size fractions of ash samples taken at (a) 11, (b) 25, and (c) 132 km from the source along the dispersal axis. In the lastone, only the 3 and 4 ϕ size fractions could be analysed. Two more samples off-axis were analysed at (d) 12 km northeast, and (e) 6 km north from the source. Corresponding data areshown in Table 2.





is also apparent that intermediate textures show a complete gradationfrom black, porphyritic fresh L1 lithic to brown G glassy grains. Lithicabundance decreases with higher free-crystal content in finer grain-size fractions, but no trend is observed with distance (Table 2; Fig. 3).The brown, glassy component does not show any clear trend withgrain-size and decreases in abundance with distance (Fig. 3).

Similar components were identified in thin section (Fig. 2c), al-though it was difficult to correlate these exactly with stereoscopic ob-servations. Lithic clasts were subdivided into: unaltered black (L1) togrey (L2), porphyritic lithics that cannot be distinguished from eachother in thin section (1–24%); white, hydrothermally altered clasts(L3) that are transparent with mottled textures (20–72%); and red-yellowish brown, hydrothermally altered clasts (L4: 1–6%). Similarly,free crystals include Pl, Px, Mt, and Pyrite (9–28%). Isotropic glass was

Fig. 4. Fresh glass shards hand-picked from the 3–4 ϕ size fractions (a) and further examinedwsurfaces. The blue arrow in “i” points to the stepped surfaces identified in bubble-wall shards. (the web version of this article.)


found in proportions b6% of the 2–3 ϕ fraction, increasing to 25% inthe 4 ϕ fraction, and reaching up to 46% in a distal location off axis(sample GIS-1 at 150 km from the source).

4.3. Glassy ash morphology, microtextures and microstructures

Hand-picked glassy particles (Fig. 4a) viewed under SEM showa range of shapes including: angular blocky particles (Fig. 4b, c);fluidal grains with molten and bulbous surfaces (Fig. 4d, e); irreg-ular shapes (Fig. 4f); and cuspate, vesicle-wall shards (Fig. 4g–j).Most of them are poorly vesicular (Fig. 4b–e), with rare moderate-ly vesicular variants (Fig. 4f). At higher magnifications, steppedsurfaces are observed (Fig. 4k–l). After strong acid cleaning(10% HCl, followed by an ultrasonic bath in distilled water for

ith the SEM (b-l). The red arrow in “d” points to an example of the rare, smooth, “molten”For interpretation of the references to colour in this figure legend, the reader is referred to




15 min), many particles still retain adhered ash on their surfaces(Fig. 4j, k).

4.4. Glassy ash petrography and element mapping

The 3 and 4 ϕ fractions were cleaned in distilled water and acetone,before mounting in epoxy under vacuum and thin-sectioning. In thinsections, completely isotropic glass particles were very rare. Threeglassy populations were identified (Fig. 5):

• Type-1 (T1): angular, equant to irregular, brown glass, varying frommicrolite-rich, dense (T1a) to less common, moderately vesiculargrains (T1b). Microlite-poor clasts are typically vesicle-wall shards(T1c), rarely with plagioclase microlites (T1d) and disseminated

Fig. 5. Glass particles identified and classified in thin section according to their co


oxides. The latter type (T1d) is only distinguishable from T1b withback-scattered electron (BSE) imagery. BSE images (SM 2) and ele-ment mapping of T1 shards (Fig. 6) show subhedral plagioclase,orthopyroxene and clinopyroxene (commonly zoned), andmagnetite(rare olivine).

• Type-2 (T2): angular, transparent, microlite-bearing glass varyingfrom equant and dense (T2a) to less common and poorly vesicular(T2b). T2 particles (Fig. 7a–c) have subhedral to euhedral, commonlyzoned plagioclase and pyroxene microlites, with minor subhedralmagnetite, rare olivine, and an acicular, silica phase (tridymite identi-fied under SEM). Apatite microlites andmicrophenocrysts were iden-tified during element mapping.

• Type-3 (T3): yellowish-brown, rounded, commonly devitrified, andcontaining rounded opaque inclusions. Although this type can look

lour in transmitted light, microlite content, vesicularity, and overall texture.



Fig. 6. Example of type-1 glass fragments (a) and shards (b); (c) shows the element maps produced for the grain shown in “b”.


isotropic with optical microscopy, BSE images reveal their alterationto silica precipitates (SM 2). Compared to element-mapped type 1-and-2 clasts, T3 clasts show significant alkali loss in the groundmass,or they are altered to an amorphous silica phase with a concentricstructure and contain pyrite (rounded inclusions) (Fig. 7d, e). Theseparticles were thus added to the lithic population in the componentanalysis.

4.5. Comparison to proximal observations

Once the proximal area was accessible (September, 2012), coarser,pyroclastic density current (PDC) deposits were sampled for comparison.A thin-sectioned vesicular, ballistic clast sampled from the proximal zone(sample TS-42) had groundmass glass thatwas brown andmicrolite-rich,similar to the T1 fresh brown glass (Fig. 8a–d). Such vesicular ballisticswere extremely rare in the vent area, but some had moss growing ontheir surface. Grains from the 1 and 2 ϕ size fractions of PDC depositswere manually broken and examined under the microscope. Non-juvenile, but unweathered, brown, black and grey, porphyritic lava-fragments in the 2 ϕ size fractions were crushed, yielding T1-like brownmicrolite-rich glass grains (Figs. 5, 8e). Similarly,whitish-grey, hydrother-mally altered clasts yieldedmottled T2-like clear glass grains (Figs. 5, 8f).

4.6. X-ray diffraction

Powder-XRD analysis of crushed samples carried out during the re-sponse period for rapid mineral identification demonstrated the


presence of quartz, feldspar, gypsum, pyrite, and cristobalite in samplesfrom the August eruption. For comparison, samples from the Novemberevent lack gypsum.

4.7. Glassy particle geochemistry

Compositions from brown T1 and transparent T2 ash from the Au-gust and November (T1 only) eruptions were analysed by EMPA,alongwith the particles frommanually crushed lithic clasts and ground-mass of a glassy vesicular ballistic clast (TS-42) (SM 3) (Figs. 9–10). Thedata ranged from 57 to 75wt.% SiO2, but concentrated in two clusters athigh (N75 wt.%) and mid-silica values (62–67 wt.%). The other majorelements have values that scatter both higher and lower than the fieldsdefined by all published compositions from Te Maari and nearby erup-tive centres (Figs. 9 and 10).

5. Discussion

If the dominant glass T1 in Te Maari samples was juvenile, the highmicrolite content would indicate shallow degassing and stalling ofmagma rise. This is consistent with the clusters of earthquakes at~500 m below the surface and the decreased seismicity in the week be-fore the eruption, relative to the activity reported in July (Hurst et al., inthis issue). The glassy particles represent 2% to 7% of the total volume(which would indicate only 5000–17,500 m3 of ejected magma). Evenif the erupted magma represented only 10% of the intruded volume,



Fig. 7. Example of type-2 glass fragments (a) and shards (b); (c) shows the element maps produced for the grain shown in “b”. Note the identification of dark-grey mineral phases in “a”(trydimite and crystobalite), and the presence of apatite phenocrysts in “b”; (d)–(e) show an example of devitrified glass particles with pyrite inclusions.


the small injection would have occurred without major geodetic orother surface changes (cf., Jolly et al., in this issue).

The poorly-to-well sorted, polymodal to unimodal grain-size distri-butions, and the fine-grained nature of the vitric components of TeMaari ash samples could be consistent with a highly efficient


fragmentation mechanism, such as MCFI within phreatomagmaticeruptions (e.g., Wohletz, 1983; Büttner et al., 2002). However, the ma-terial lithology excludes MCFI in this instance because it is not afragmented fresh magma. Hence these grainsize patterns more likelyrepresent a sampling of the range of fragmental and hydrothermally



Fig. 8. Proximal fragments chosen for manual fragmentation. Resulting fragments resemble the brown, microlitic glass particles found in the ash fallout when observed in transmittedlight: (a)–(d) Fresh vesicular ballistic TS-42 in (a) transmitted and (b) BSE-light; (c) and (d) show the fragments resulting after breaking the clast manually; (e) shows brown microliticglass fragments produced by breaking pale grey andesitic lithic clasts; (f) shows transparent, microlitic glass fragments of variable vesicularity produced by breaking white, partially al-tered clasts.


altered materials in the eruption vent areas. Thus a more likely mecha-nism is gas-driven ejection ofmaterial fragmented in the previous erup-tions (e.g., Carey and Sigurdsson, 1982), or of crushed rock, such asgouge, that formed along the margins of viscous intrusions (e.g.,Kennedy and Russell, 2012). The presence of multiple modes in thegrain-size distribution of samples b12 km from the source possibly indi-cates a strong country rock lithology control, which becomes less effi-cient with transport. Some of the local ash deposits may also beformed from elutriated ash from the surges generated by this eruption(cf., Lube et al., in this issue), which were generated from explosionson the flank immediately south of the Upper Te Maari crater. Here thedeposits disrupted by the eruption are demonstrably diamictons of var-ious types including proximal vent breccias of past Upper Te Maarieruptions, clay-rich matrix tuffs and highly altered clay horizons (cf.,Procter et al., in this issue). Within the bimodal to polymodal samples,the coarsest mode corresponds to andesite lava fragmentswith variabledegrees of hydrothermal alteration and to the largest phenocrysts freedfrom such rocks. The intermediate mode corresponds dominantly tocrystals and glassy particles. The finest-grained mode (6.7–8.2 ϕ alongdispersion axis) appears to result from fine to very fine ash, derivedfrom the clay-rich altered materials observed in the hydrothermally al-tered deposits seen in thewalls of the eruptive chasm formed by debrisavalanche and the explosive blast at the eruption onset (see Procter


et al., in this issue). The fine ash formed weak aggregates, seen insome locations in the field, but these were destroyed during sampling,drying and grain-size analysis. The more complex polymodal grain-size distributions along SH46 (AZ-A5 and AZ-A6) could be further influ-enced by the overlapping accumulation of fall from the main plumewith additional elutriated ash drifting from the proximal PDCs.

A high lithic fraction within the ash is consistent with deposits ofmany phreatomagmatic eruptions (e.g. Barberi et al., 1989; Houghtonand Nairn, 1991; Houghton and Smith, 1993; White, 1996). Thetraditional method to distinguish eruption mechanisms is the studyof hand-picked glassy particles with the SEM (Heiken, 1972).Zimanowski et al. (1991) identified “interactive” particles, which arenormally b130 μm (N3 ϕ), angular to subround, with smooth and/orplanar surfaces, and are thought to indicate undercooled meltthat was fragmented by shock waves. Passive fragments (usually90–500 μm in experiments) were described as rounded, drop-likeforms, generated from molten magma. Later experimental and naturalsample comparisons (Büttner and Zimanowski, 1998; Büttner et al.,1999, 2002; Dellino and Kyriakopoulos, 2003) concluded that only afew microfeatures are definitively linked to MFCI processes. Many fea-tures, such as blocky shapes and conchoidal fractures, are not uniqueto a single process and only demonstrate that the material (melt orglass) has broken in a brittle fashion. Stepped surfaces reflect the brittle



Fig. 9. Total alkali-silica diagram. Classification fields after Le Maitre et al. (1989). Reference compositions are shown for the TVZ, including b10 ka rhyolites and products from Ruapehu,and for different eruptive centres on Tongariro (coloured fields). Coloured symbols are EPMA analyses made since the August 2012 eruption. Each data point represents the average ofmultiple analyses from a single particle, normalised to 100%. “BulkAsh” refers tomaterial thatwas analysed immediately after the eruption, butwhichwasnot examined and characterisedin thin section. It contains a range of types T1–T2. Brown glasses (T1) from August to November eruptions and Transparent (T2) glasses from August were identified in thin section beforeanalysis. Non-juvenile lithic lapilli from the surge deposit are those used in the crushing experiments. Bomb no. TS-42 was collected after, but not necessarily produced by the 2012 erup-tions. 1(Froggatt, 1981, 1982, 1983; Lowe, 1986, 1988; Alloway et al., 1994; Froggatt and Rogers, 1990; Donoghue, 1991; Stokes et al., 1992; Shane and Froggatt, 1994; Carter et al., 1995;Eden and Froggatt, 1996; Lowe andNewnham, 1999; Shane et al., 2002; Smith et al., 2005, 2006; Lowe et al., 2008). 2(Lowe, 1988; Donoghue, 1991; Lowe et al., 2008;Moebis et al., 2011).3(Donoghue, 1991). 4(Moebis et al., 2011). 5(Hobden, 1997, Hobden et al., 1999; Griffin, 2007; Shane et al., 2008). 6(Moebis, 2010).


response of any glass to the passage of a shock wave. Adhered ash andovergrown films can also be produced by vapours released by a hydro-thermal system.

Under SEM, the TeMaari glass-shardmorphologies show no distinc-tive features that unequivocally identify whether they are juvenile orrecycled older particles. There is a lack of distinctive juvenile particlemicrostructures, along with a high degree of similarity to particlesmanufactured by crushing non-juvenile lithic clasts. This implies thatthe Te Maari eruptions had no direct magma involvement. The widespread of the EPMA data from these particles, and the overlapping com-position of the crushed lithic clasts and older ballistic fragments(Figs. 8–10), also indicate that they are unlikely to be derived from a ju-venile magma.Whilst the glassy ash compositions of the 2012 TeMaarieruptions overlapwith those from the previous TgVC eruptions general-ly (Figs. 9, 10), they mainly plot within fields for the pre-2012 tephrafrom Te Maari (Moebis, 2010). This overlap with the previous TeMaari compositions is not direct evidence against new magmaappearing. However, the lack of any tight compositional range, linkedto any specific juvenile grain-type, leads to the conclusion that there isno identifiable juvenile-magmatic component. During this investiga-tion, no single distinctive parameter, or set of them, could be found todiscriminate between juvenile magmatic and recycled fresh glassy ashparticles from the same volcano. The conclusion that no fresh magmawas erupted is consistentwith the absence of high temperature impactsin the vent area (Lube et al., in this issue) and the extremely short dura-tion of the eruption.

Our results indicate that the presence of active or passive glass shardsdoes not always provide conclusive evidence for phreatomagmatism.


This is particularly true if eruptions excavate and recycle the depositsof earlier vulcanian andphreatomagmatic deposits. Failure of overheatedand pressurised, vapour-saturated hydrothermal systems, with a violentexpansion of water vapour may also produce shock waves and re-fragmentation of existing tephras, dikes, and vesicular agglutinatesaround the vent. Recycled fragments have been recognised before instrombolian and phreatomagmatic eruptions (Houghton and Smith,1993). However, the definition of “recycled” typically applies to coarseparticles (−4 to−3 ϕ particles) falling back into a vent during eruption.Here we observe fragments “recycled” not during the course of an erup-tion, but by re-ejection of pyroclastic material from the previous erup-tions. These grains extend down to ash grade, and include glassyfragments that can be distinctively angular or fluidal, but not exclusivelyso. That they are chemically andmorphologically indistinguishable fromjuvenile clasts here has major implications for the unequivocal identifi-cation of a phreatomagmatic origin from the geological record, especiallywhen the unambiguous lithic content is ≥90%.

Finally, the presence of multiple modes (usually 2 to 3) in the sam-ples collected within the first 12 km from the source reveals some as-pects of the fragmentation and dispersion of ejecta. Bimodal topolymodal grain-size distributions, atypical for fall deposits, most likelyrepresent the transport mechanism rather than fragmentation, sincemost of the particles were essentially excavated from fragmental par-ent deposits. The poor sorting could reflect a relatively highly concen-trated ash cloud compared to the typical ash-clouds of magmaticeruptions, or a combination of ash sources from both fall out of themain column as well as elutriated ash from the surges. A higher particleconcentration could promote interaction, abrasion, and mixing of



Fig. 10. Harker diagrams illustrating the major element glass geochemistry. Symbols and references are the same as in Fig. 9.


particles without opportunity for sorting. In addition, and consideringthe multiple eruption pulses identified by radar observations (Couchet al., in this issue), bimodal to polymodal distributions likely also re-flect the cumulative deposition from multiple injection pulses of ashinto the atmosphere. The influence of these injections may becomeweaker with distance from the source, and preferential sorting ofcoarse particles could generate the unimodal distributions that pre-dominate beyond 25 km.

The 2012 Te Maari eruption also shows that even a hydrothermaleruption can produce and disperse fine ash that may reach the tropo-pause (9.6 km above TgVC) and pose a risk to aviation.


6. Conclusions

In understanding the hazards associated with this eruption, earlyevaluation of the ash products revealed a small proportion of fresh-appearing volcanic glass. Its provenance was attributed to either(1) the recycling and steam-driven fragmentation and ejection ofolder glassy pyroclasts, vesicular lavas, or dikes in the vent region; or(2) excavation and fragmentation of a newly intruded dike thatcrystallisedmicrolites in theweek(s) before the eruption. This led to se-rious doubts about our ability to unambiguously identify “juvenile”pyroclasts in fine-ash deposits on a volcano that has a long history of




past phreatomagmatic, strombolian, and vulcanian eruptions. Lavas andpyroclastic deposits produced by earlier eruptions from the same volcanoare easily recycled with very well preserved morphology under visiblelight microscopy and the SEM. These may show abundant microstruc-tures, such as stepped surfaces, that are relict or freshly formed by suddendecompression, as well as relict molten surfaces, fluidal forms and vesi-cles. In addition, isotropic glass in thin sections, with clear textures onBSE images and good EMPA analytical totals, may also originate fromfragmented country-rock lavas and pre-existing volcaniclastic deposits.

Definitive proof for excluding freshmagma inputwithin an eruptionis also not straightforward from ash-grade deposits. Here our evidenceis the wide range of glass compositions, microlite textures and mineralcontent in glassy particles. The particle properties overlap those ofvent-area country rock, which we tested by comparing erupted ashwith fines produced by manual crushing of country rock. We find thatthe most diagnostic evidence that new magma is not represented inthese ash deposits is the lack of any population of clasts with distinctglass compositions characterising ash particles with specific or uniformshapes or textures. The disconnection between the physical and chem-ical properties of the particles highlights their mixed provenance.

Based on these findings, we can demonstrate that the role of magmaduring the Upper TeMaari eruptions in 2012was solely to contribute togas overpressures and elevated temperatures in the pre-existing,vapour-dominated hydrothermal system. Any explosive magma–water interaction during this eruption seems highly unlikely. Increasedseismic activity and the changes in gas compositionsmeasured in fuma-roles (Christenson et al., 2013) demonstrated that fresh magma wasinjected below the crater area in the month before the eruption. Thatan eruption resulted from this intrusion must be attributed to thesealing of the hydrothermal system, with build-up of gas pressuresand temperatures leading up to an ultimate explosive failure. A land-slide (Procter et al., in this issue) in the early seconds of eruption(Jolly et al., in this issue) destabilised the hydrothermal system andled to its sudden decompression. The resulting shock wave may havefragmented (and excavated) existing vent-area deposits and top-down explosions eviscerated the top of the hydrothermal system.

Acknowledgements

Thisworkwas supported by the “Livingwith Volcanic Risk” programof the New Zealand Natural Hazards Research Platform, funded by theMinistry of Business, Innovation and Employment (C05X0907). Wewish to thank: TomWilson (University of Canterbury), ChristinaMcGilland Emma Phillips (Macquarie University) who assisted during re-sponse planning and sampling strategy design before the eruption.We acknowledge the local community, in particular the Police Depart-ment at Turangi, the Tuwharetoa Iwi, Professor John Ham (HirirangiSchool, Turangi), and Harry Keys (Department of Conservation) for fa-cilitating the communication and logistics during the emergency re-sponse. David Prior and Kat Lilly helped by providing access to andtraining on the ZEISS-SEMat theUniversity of Otago, andDougHopcroftassistedwith the FEI-SEM atMassey University.We acknowledge LanceCurrie, GlenysWallace,Matt Irwin (all IAE,Massey University), AshleighPhillips (Ravensdown Fertiliser Ltd), and the Kotemaori and MohakaSchools for their assistance during sampling and laboratory handling.We also thank Gill Jolly, Colin Wilson, Bruce Christenson, GrahamLeonard, Brad Scott, Mike Rosenberg, Geoff Kilgour, Ben Kennedy, andAdrian Pittari for their valuable input during the syn-eruption discus-sions of ash origin. Dr. Kate Arentsen contributed to the logistics coordi-nation during response and participated in the final edition of thismanuscript.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.jvolgeores.2014.05.001.


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