Papers by Robert B Stewart
Central European Journal of Geosciences, 2010
The Efate Pumice Formation (EPF) is a trachydacitic volcaniclastic succession widespread in the c... more The Efate Pumice Formation (EPF) is a trachydacitic volcaniclastic succession widespread in the central part of Efate Island and also present on Hat and Lelepa islands to the north. The volcanic succession has been inferred to result from a major, entirely subaqueous explosive event north of Efate Island. The accumulated pumice-rich units were previously interpreted to be subaqueous pyroclastic density current deposits on the basis of their bedding, componentry and stratigraphic characteristics. Here we suggest an alternative eruptive scenario for this widespread succession. The major part of the EPF is distributed in central Efate, where pumiceous pyroclastic rock units several hundred meters thick are found within fault scarp cliffs elevated about 800 m above sea level. The basal 200 m of the pumiceous succession is composed of massive to weakly bedded pumiceous lapilli units, each 2-3 m thick. This succession is interbedded with wavy, undulatory and dune bedded pumiceous ash and fine lapilli units with characteristics of co-ignimbrite surges and ground surges. The presence of the surge beds implies that the intervening units comprise a subaerial ignimbritedominated succession. There are no sedimentary indicators in the basal units examined that are consistent with water-supported transportation and/or deposition. The subaerial ignimbrite sequence of the EPF is overlain by a shallow marine volcaniclastic Rentanbau Tuffs. The EPF is topped by reef limestone, which presumably preserved the underlying EPF from erosion. We here propose that the EPF was formed by a combination of initial subaerial ignimbrite-forming eruptions, followed by caldera subsidence. The upper volcaniclastic successions in our model represent intra-caldera pumiceous volcaniclastic deposits accumulated in a shallow marine environment in the resultant caldera. The present day elevated position of the succession is a result of a combination of possible caldera resurgence and ongoing arc-related uplift in the region.
Central European Journal of Geosciences, 2010
The Bakony-Balaton Highland Volcanic Field (BBHVF) is located in the central part of Transdanubia... more The Bakony-Balaton Highland Volcanic Field (BBHVF) is located in the central part of Transdanubia, Pannonian Basin, with over 50 alkali basaltic volcanoes. The basanite plug of Hegyestű erupted in the first phase of volcanic activity. K/Ar and Ar/Ar ages were published for the BBHVF. K/Ar and Ar/Ar ages of the leucite-bearing basanite of HegyestÅś were conflicting. This is caused by the special Ar retention feature of leucite in this basanite. K/Ar ages measured in the usual way were 25-45% younger, but after HCl treatment of the rock, or after reducing the baking temperature of the argon extraction line from 250˚C to 150˚C, they became similar to the Ar/Ar ages. All Ar/Ar determinations were performed after HF treatment. HCl treatment dissolved olivine, nepheline, leucite, magnetite and from 1-1 sample analcime or calcite. K dissolution studies from different locations of Hegyestű have shown that K content is mostly ∼2%, but it may decrease to ∼0.3%. HCl treatment dissolved 28.0-63.5% of the K content. The calculated K concentration for the dissolved part of samples with ∼2%K was 4.02-6.42%: showing that leucite is responsible for the low temperature loss of 40 Ar(rad). Ar may release at low temperature from very fine-grained mineral, or when the Ar release mechanism changes. A 40 Ar(rad) degassing spectrum has been recorded in the 55-295˚C range of baking temperature and the data were plotted in the Arrhenius diagram. The diagram shows that a change of the structure in the 145-295˚C range caused the loss of 40 Ar(rad). On fractions of HCl treated rock 7.56±0.17 Ma isochron K/Ar age has been determined. This is regarded as minimum age of eruption and it is similar to the Ar/Ar isochron age (7.78±0.07 Ma).
Journal of Volcanology and Geothermal Research, 2014
Environmental Geochemistry and Health, 2012
Ambrym in Vanuatu is a persistently degassing island volcano whose inhabitants harvest rainwater ... more Ambrym in Vanuatu is a persistently degassing island volcano whose inhabitants harvest rainwater for their potable water needs. The findings from this study indicate that dental fluorosis is prevalent in the population due to fluoride contamination of rainwater by the volcanic plume. A dental survey was undertaken of 835 children aged 6-18 years using the Dean's Index of Fluorosis. Prevalence of dental fluorosis was found to be 96% in the target area of West Ambrym, 71% in North Ambrym, and 61% in Southeast Ambrym. This spatial distribution appears to reflect the prevailing winds and rainfall patterns on the island. Severe cases were predominantly in West Ambrym, the most arid part of the island, and the most commonly affected by the volcanic plume. Over 50 km downwind, on a portion of Malakula Island, the dental fluorosis prevalence was 85%, with 36% prevalence on Tongoa Island, an area rarely affected by volcanic emissions. Drinking water samples from West Ambrym contained fluoride levels from 0.7 to 9.5 ppm F (average 4.2 ppm F, n = 158) with 99% exceeding the recommended concentration of 1.0 ppm F. The pathway of fluoride-enriched rainwater impacting upon human health as identified in this study has not previously been recognised in the aetiology of fluorosis. This is an important consideration for populations in the vicinity of degassing volcanoes, particularly where rainwater comprises the primary potable water supply for humans or animals.
... Wallace WITH ASSISTANCE FROM Tauranga Locals David and Joan Kear, David Milner and Kathrin Ot... more ... Wallace WITH ASSISTANCE FROM Tauranga Locals David and Joan Kear, David Milner and Kathrin Otrel-Cass Symposium ... 8 Page 3. A LARGE-SCALE, NEAR-SEA LEVEL, SILICIC CALDERA-FORMING ERUPTION IN EFATE? AN ...
Journal of Volcanology and Geothermal Research, 2014
The weak geophysical precursors of the 6 August 2012 Te Maari eruption of Mt. Tongariro and a lac... more The weak geophysical precursors of the 6 August 2012 Te Maari eruption of Mt. Tongariro and a lack of obvious juvenile components in its proximal ballistic deposits imply that the eruption was caused by the sudden decompression of a sealed, hot hydrothermal system. Strong magmatic signals in pre-and post-eruption gas emissions indicate that fresh magma had intruded to shallow levels shortly before this eruption. Here we examine the volcanic ash produced during the August eruption with the aim of determining whether juvenile magma was erupted 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 the dispersal axis. Proximal to intermediate sites showing polymodal grain-size distributions can be related to the refragmentation of different pre-existing lithologies, overlapped erupted pulses and transport mechanisms, and to particle aggregation. Between 69 and 100 vol.% of particles coarser than 3 ϕ and 45-75 vol.% of grains finer than 3 ϕ were sourced from the pre-existing, commonly hydrothermally altered, vent-area lavas and pyroclasts. Free crystals (pyroxene N plagioclase N magnetite N pyrite) make up 0-23 vol.% of particles coarser than 3 ϕ, and 22-41 vol.% of grains finer than 3 ϕ. 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, some with fluidal 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 the microprobe-analysed glass is very wide (56-77 wt.%) and cannot be correlated to any specific particle textural type. These chemically and texturally diverse glassy fragments are identical to mechanically broken pieces of country rock lavas and pyroclasts; both their diversity, and their match with vent country rocks, argue strongly against a "juvenile" origin for the glassy fragments. We conclude that rising magma provided only heat and gas into the overlying, sealed vapour-dominated hydrothermal system. A landslide from this area led to a rapid decompression and ash was produced by top-down hydrothermal explosions. Careful attention must be paid to the combination of compositions and textures of fine ash particles in such situations, as well as to the context of their source vent, in order to be confident that new magma has reached the surface.
New Zealand Journal of Geology and Geophysics, 2001
The sand and silt mineralogy of four soils preserved on fluvioglacial outwash terraces at Kumara ... more The sand and silt mineralogy of four soils preserved on fluvioglacial outwash terraces at Kumara indicates that the main soil body is developed in up to 1 m of loessial coverbeds that accumulated subsequent to cessation of gravel terrace formation. Oxygen isotope abundances of the dominant mineral quartz in these soils show a higher temperature origin for the >250 |4.m
Bulletin of Volcanology, 1999
The ca. 10,500 years B.P. eruptions at Ruapehu volcano deposited 0.2–0.3 km3 of tephra on the fla... more The ca. 10,500 years B.P. eruptions at Ruapehu volcano deposited 0.2–0.3 km3 of tephra on the flanks of Ruapehu and the surrounding ring plain and generated the only known pyroclastic flows from this volcano in the late Quaternary. Evidence of the eruptions is recorded in the stratigraphy of the volcanic ring plain and cone, where pyroclastic flow deposits and several lithologically similar
New Zealand Journal of Geology and Geophysics, 1999
... Zealand is dominantly (>90%) rhyolitic, with considerably lesser andesitic activit... more ... Zealand is dominantly (>90%) rhyolitic, with considerably lesser andesitic activity and sparse (>1%) basaltic volcanism (Cole 1986; Gamble et al. ... Mount Ruapehu is a composite andesitic stratovolcano which forms part of the Tongariro Volcanic Centre (Graham & Hackett 1987 ...
New Phytologist, 2005
The induced accumulation of mercury (Hg) by plants was investigated for the species Phaseolus vul... more The induced accumulation of mercury (Hg) by plants was investigated for the species Phaseolus vulgaris (Bush bean), Brassica juncea (Indian mustard), and Vicia villosa (Hairy vetch). • All plants were grown in modified Hg-contaminated mine tailings and were treated with sulphur-containing ligands to induce Hg accumulation. The effects of varied substrate Hg concentration and humic acid (HA) level on the induced plant-Hg accumulation for B. juncea were examined. • Thiosulphate salts (ammonium and sodium) mobilised Hg in the substrates and caused an increase in the Hg concentration of roots and shoots of all tested plant species. Root Hg accumulation was positively correlated to extractable Hg for (NH 4 ) 2 S 2 O 3 -treated B. juncea plants grown in HA-amended substrates. However, shoot Hg translocation for this species was inhibited at 1.25 g HA kg − 1 of substrate. • Mercury-thiosulphate complexes could be translocated and accumulated in the upper parts of the plants up to 25 times the Hg concentration in the substrate. We conclude that shoot Hg accumulation in the presence of thiosulphate salts is dependent upon plant species characteristics (e.g. root surface area) and humic acid content.
Journal of Volcanology and Geothermal Research, 2007
The extrusion of viscous andesite lava forming domes can terminate in explosive activity. To unde... more The extrusion of viscous andesite lava forming domes can terminate in explosive activity. To understand the driving forces behind this behaviour, a study of an AD1655 eruption episode at Mt. Taranaki was carried out. We propose that simple changes in magmatic conditions of a single hydrous melt during ascent caused sudden changes in explosivity and gave rise to pumice with highly variable vesicularities and colour. Fractionation of hornblende + plagioclase + clinopyroxene + Fe-Ti oxide at the onset of magma ascent, and step-wise crystallisation of plagioclase ± clinopyroxene in different parts of a single melt within the conduit was controlled by rates of initial rise, capping by an impermeable lava dome, and differential rates of vesiculation and volatile exsolution. This resulted in a vertical stratification in the conduit, comprising a viscous, hypocrystalline lava cap, that overlay alternating zones of grey, brown and grey magma-foams. Horizontal gradients in geochemistry in the conduit are also indicated by different clast textures. The eruption consisted of an initial extrusive phase followed by three pulses of sub-plinian activity. Each phase or pulse, corresponded to individual layers within the conduit. Ejecta included block-and-ash flow deposits, three pyroclastic pumice-flow deposits of alternating grey, brown and grey pumice, as well as fallout deposits dominated by grey pumice. The brown magma foam contained more microlites, had a more-evolved matrix glass, and a higher temperature than the grey magma foams above and below. Its eruption destabilised the sub-plinian eruption column because it was more degassed. It fragmented less efficiently than the grey magma foams due to its lower viscosity, preventing pressure build-up in bubbles. Incomplete mixing at interfaces between brown and grey magma phases gave rise to banded pumices.
Gold Bulletin, 1999
... and Robyn Simcock ... Conor C. McSweeney and Mark P. O'Connell are PhD postgraduate rese... more ... and Robyn Simcock ... Conor C. McSweeney and Mark P. O'Connell are PhD postgraduate researchers, while Andrew Walker is a graduate of Queens University Belfast and contributed in organic synthesis through a postdoctoral fellowship at UCc. REFERENCES ...
Geology, 2008
Mineral assemblages in volcanic rocks record both pre-eruptive conditions and changes experienced... more Mineral assemblages in volcanic rocks record both pre-eruptive conditions and changes experienced by magma as it rises. Titanomagnetite in andesitic magmas is especially sensitive to changes in temperature and oxygen fugacity immediately prior to and during eruptions. Two end-...
Environmental Practice, 2004
Environmental Pollution, 2005
Experiments were carried out in plant growth chambers and in the field to investigate plant-mercu... more Experiments were carried out in plant growth chambers and in the field to investigate plant-mercury accumulation and volatilisation in the presence of thiosulphate (S 2 O 3 )-containing solutions. Brassica juncea (Indian mustard) plants grown in Hg-contaminated Tui mine tailings (New Zealand) were enclosed in gastight volatilisation chambers to investigate the effect of ammonium thiosulphate ([NH 4 ] 2 S 2 O 3 ) on the plant-Hg volatilisation process. Application of (NH 4 ) 2 S 2 O 3 to substrates increased up to 6 times the Hg concentration in shoots and roots of B. juncea relative to controls. Volatilisation rates were significantly higher in plants irrigated only with water (control) when compared to plants treated with (NH 4 ) 2 S 2 O 3 . Volatilisation from barren pots (without plants) indicated that Hg in tailings is subject to biological and photochemical reactions. Addition of sodium thiosulphate (Na 2 S 2 O 3 ) at 5 g/kg of substrate to B. juncea plants grown at the Tui mine site confirmed the plant growth chambers studies showing the effectiveness of thio-solutions at enhancing shoot Hg concentrations. Mercury extraction from the field plots yielded a maximum value of 25 g/ha. Mass balance studies revealed that volatilisation is a dominant pathway for Hg removal from the Tui mine site. A preliminary assessment of the risks of volatilisation indicated that enhanced Hg emissions by plants would not harm the local population and the regional environment.
Environmental and Experimental Botany, 2008
Phytofiltration may be a cost-effective approach for treating Hg-contaminated wastewater. We inve... more Phytofiltration may be a cost-effective approach for treating Hg-contaminated wastewater. We investigated the removal of Hg from solutions by Indian mustard [Brassica juncea (L.) Czern.] grown in hydroponic conditions with solutions containing Hg concentrations from 0 to 10 mg/L. Plants were enclosed in gastight volatilisation chambers to assess the effect of Hg concentrations on plant transpiration, accumulation and volatilisation. We also determined the speciation and site of origin of volatilised Hg. Solution Hg concentrations of 5 and 10 mg/L detrimentally affected transpiration. Roots concentrated Hg 100-270 times (on a dry weight basis) above initial solution concentrations. The plants translocated little Hg to the shoots, which accounted for just 0.7-2% of the total Hg in the plants. Volatilisation from planted vessels increased linearly as a function of Hg concentrations in solutions. Most Hg volatilisation occurred from the roots. Volatilised Hg was predominantly in the Hg(0) vapour form. Volatilisation was dependant on root uptake and absorption of Hg from the ambient solution. Production of Hg(0) vapour in the solutions may result from the activity of root-associated algae and Hg-resistant bacteria. Phytofiltration effectively removed up to 95% of Hg from the contaminated solutions by both volatilisation and plant accumulation. However, Hg(0) vapours released from living roots may have unforeseen environmental effects.
Bulletin of Volcanology, 2009
Page 1. RESEARCH ARTICLE Merging eruption datasets: building an integrated Holocene eruptive reco... more Page 1. RESEARCH ARTICLE Merging eruption datasets: building an integrated Holocene eruptive record for Mt Taranaki, New Zealand Michael B. Turner & Mark S. Bebbington & Shane J. Cronin & Robert B. Stewart Received ...
Nature, 1998
Page 1. Nature © Macmillan Publishers Ltd 1998 8 The possibility of turning base metals into gold... more Page 1. Nature © Macmillan Publishers Ltd 1998 8 The possibility of turning base metals into gold has intrigued many scientists since the early alchemists, and the discovery of significant gold uptake by plants has long been a 'philosopher's stone'. ...
The 6 August 2012 Te Maari eruption produced violent and widespread "cold" Pyroclastic Density Cu... more The 6 August 2012 Te Maari eruption produced violent and widespread "cold" Pyroclastic Density Currents (PDCs) following unroofing of the pressurized hydrothermal system. Despite an erupted volume of only~5 ×10 5 m 3 , and lacking any juvenile component, the 340,000 m 3 of PDCs spread over an area of 6.1 km 2 and had mobilities that were on the order of volcanic blasts or blast-like PDCs. This great mobility was due to strong lateral focussing of explosion energy, producing jets with initial velocities N 100 m/s. We present a type-stratigraphy for these hydrothermal-derived low-temperature PDCs that show a tripartite deposit sequence. Each of the deposit units dominates respectively three outward-gradational sedimentary facies, reflecting transitions in the propagating PDC transport and depositional mechanisms. The largest PDCs, directed west and east of the Upper Te Maari area were generated from outer-cone breccias and tuffs that were mostly highly hydrothermally altered. Landsliding and the geometry of the hydrothermal area led to the directed jetting. Initial particle-laden jets laid sheets, grading into lobes of proximal massive sand to gravel-rich facies dominated by unit A and extending up to 1 km from the vents. As the jets were collapsing, a vertically and longitudinally stratified PDC developed within the first few hundred metres from source. Exponential thinning and coarse-tail grading-dominated fining with radial distance of massive unit A resulted from fast deposition and progressive depletion of the most concentrated flow region behind the PDC head. Markedly slower tractional sedimentation from the passing PDC body and tail deposited the highly stratified and ripple-bedded fine-coarse ash of unit B. This formed distinctive dune fields of the medial dunebedded ash-rich facies. Upwards in depositional sequences the waning of the current can be seen, by replacement of higher-energy bedforms to progressively lower ones. Downstream progressive waning and further depletion are characterised by the development of the distal wavy to planar-bedded ash-rich facies. This is increasingly dominated by the uppermost deposition unit C of laminated fine-med ash deposited by gently turbulent, dilute phoenix clouds. These high energy PDCs, sourced from flank hydrothermal systems should be regarded as a serious threat in any multihazard assessment of a stratovolcano, even though they may not be one of the major magmatic vent sites. In addition, further detailed studies of these hydrothermal jetting events and their deposits should be pursued in order to better understand large-volume volcanic blasts, which appear to be a larger scale sibling phenomenon.
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Papers by Robert B Stewart