Shane J Cronin | The University of Auckland (original) (raw)

Papers by Shane J Cronin

Rapid changes in small areas at the apex of alluvial fans may have devastating consequences by di... more Rapid changes in small areas at the apex of alluvial fans may have devastating consequences by directing downstream flood or lahar impacts into catchments of widely varying population or infrastructure vulnerability. During a series of lahars in 1995 at Mt. Ruapehu, New Zealand, aggradation of the Whangaehu fan apex (draining the eastern edifice) caused the onset of avulsion of flows northward into the highly vulnerable Tongariro catchment. An earth training dike (or bund) was constructed to protect this catchment by retaining flows on the southern side and the normal lahar outlet path to the south. Surveys in 2001, late 2005, and following a major lahar in March 2007 now show net degradation of a channel in the Whangaehu fan apex, bordered by the bund. This indicates a net increase in the channel capacity at this site and shows that the bund remains at its effective design capacity. Past hydrological modelling used for the bund design provided a large range of discharge estimates but lacked precise constraints on the size and nature of lahars from eruption and lake-breakout events. New modelling has been carried out using Titan2D to examine the impacts of a 6 × 10 6 m 3 volume granular flow down this catchment. This simulates either an eruption or a lake breakout-induced lahar with a historically typical volumetric bulking factor of 4. These simulations predict minimum discharges between 1800 and 2100 m 3 /s at the bund site. By comparison, the largest 1995 flow at this site was estimated at around 1200 m 3 /s. Further, any single modelled flow from the normal outlet channel of Crater Lake could not be induced to overtop the bund because discharge appears to be limited by the narrow upper reaches of the Whangaehu Gorge. Theoretical discharge levels required to overtop the bund are estimated to be N 6800 m 3 /s, assuming no aggradation of the channel by the decelerating flow. Maximum potential discharge at the bund site is additionally modified by potential bifurcation of lahars above a certain size threshold at a point 2.5 km upstream of the bund. A major side channel ("the chute") effectively diverts part of the flows away to the southern side of the Whangaehu fan. This potential for splitting of the flow appears to have increased since 1995, from aggradation of lahar deposits and reworked sediment in the area immediately upstream of the divide. Despite remaining at design capacity for lahar events in the Whangaehu, erosion of new channels following the 2007 lahar renders the bund vulnerable to undercutting.

Journal of Volcanology and Geothermal Research, 2007

Small-volume pyroclastic density currents (PDCs) are generated frequently during explosive erupti... more Small-volume pyroclastic density currents (PDCs) are generated frequently during explosive eruptions with little warning. Assessing their hazard requires a physical understanding of their transport and sedimentation processes which is best achieved by the testing of experimental and numerical models of geophysical mass flows against natural flows and/or deposits. To this end we report on one of the most detailed sedimentological studies ever carried out on a series of pristine small-volume PDC deposits from the 1975 eruption of Ngauruhoe volcano, whose emplacement were also witnessed during eruption. Using high-resolution GPS surveys, a series of lateral excavations across the deposits, and bulk sedimentological analysis we constrained the geomorphology, internal structure and texture of the deposits with respect to laterally varying modes of deposition.

Journal of Volcanology and Geothermal Research, 2005

... Keywords: pyroclastic flows; block-and-ash flows; basal avalanche; emplacement mechanisms; tr... more ... Keywords: pyroclastic flows; block-and-ash flows; basal avalanche; emplacement mechanisms; transport mechanisms; facies; Merapi Volcano. Article Outline. 1. Introduction 2. Methods 2.1. Density and vesicularity analysis 2.2. Grain size analysis 2.3. ...

... WITH ASSISTANCE FROM Tauranga Locals David and Joan Kear, David Milner and Kathrin Otrel-Cass... more ... WITH ASSISTANCE FROM Tauranga Locals David and Joan Kear, David Milner and Kathrin Otrel-Cass ... Page 3. POST-CALDERA VOLCANISM IN THE YENKAHE CALDERA, TANNA ISLAND, VANUATU K. Németh 1 , SJ Cronin 1 , A. Möbis 1 , D. Charley 1,2 ...

Geological Society of America Bulletin, 2012

... was used to understand how the break in erup-tive activity and vent migration may have been r... more ... was used to understand how the break in erup-tive activity and vent migration may have been related to changes in magma composition and possible transitions between separate pulses of alkali basaltic magma, as has been postulated for other Jeju volcanoes (Brenna et al ...

Bulletin of Volcanology, 2009

Volcanoes provide important contributions to atmospheric budgets of SO 2 and reactive halogens, w... more Volcanoes provide important contributions to atmospheric budgets of SO 2 and reactive halogens, which play significant roles in atmospheric oxidative capacity and radiation. However, the global source strengths of volcanic emissions remain poorly constrained. These uncertainties are highlighted here by the first measurements of gas emission rates from Ambrym volcano, Vanuatu. Our initial airborne ultraviolet spectroscopic measurements made in January 2005 indicate fluxes of 18-270 kg s -1 of SO 2 , and 62-110 gs -1 of BrO, into the atmosphere, placing Ambrym amongst the largest known contemporary point sources of both these species on Earth. We also estimate high Cl and F fluxes of~8-14 and~27-50 kg s -1 , respectively, for this period. Further observations using both airborne and spaceborne remote sensing reveal a fluctuating SO 2 output between 2004 and 2008, with a surge in the first half of 2005, and underline the substantial contribution that a single passively degassing volcano can make to the atmospheric budget of sulfur and halogens.

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.

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-...

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 ...

ABSTRACT On 16 June 2006 an overpass of IKONOS coincided with the emplacement of an active block-... more ABSTRACT On 16 June 2006 an overpass of IKONOS coincided with the emplacement of an active block-and-ash flow fed by a lava dome collapse event at Merapi Volcano (Java, Indonesia). This was the first satellite image recorded for a moving pyroclastic flow. The very high-spatial resolution data displayed the extent and impact of the pyroclastic deposits emplaced during and prior to, the day of image acquisition. This allowed a number of features associated with high-hazard block-and-ash flows emplaced in narrow, deep gorges to be mapped, interpreted and understood. The block-and-ash flow and surge deposits recognized in the Ikonos images include: (1) several channel-confined flow lobes and tongues in the box-shaped valley; (2) thin ash-cloud surge deposit and knocked-down trees in constricted areas on both slopes of the gorge; (3) fan-like over bank deposits on the Gendol-Tlogo interfluves from which flows were re-routed in the Tlogo secondary valley; (4) massive over bank lobes on the right bank from which flows devastated the village of Kaliadem 0.5 km from the main channel, a small part of this flow being re-channeled in the Opak secondary valley. The high-resolution IKONOS images also helped us to identify geomorphic obstacles that enabled flows to ramp and spill out from the sinuous channel, a process called flow avulsion. Importantly, the avulsion redirected flows to unexpected areas away from the main channel. In the case of Merapi we see that the presence of valley fill by previous deposits, bends and man-made dams influence the otherwise valley-guided course of the flows. Sadly, Sabo dams (built to ameliorate the effect of high sediment load streams) can actually cause block-and-ash flows to jump out of their containing channel and advance into sensitive areas. Very-high-spatial resolution satellite images are very useful for mapping and interpreting the distribution of freshly erupted volcanic deposits. IKONOS-type images with 1-m resolution provide opportunities to study and map the meter-scale detail of volcanic deposits. When such high-spatial-resolution satellite remote sensing data are combined with in situ field work, geomorphic analyses can be applied that allow us to more fully understand the dynamics and hazards of eruptions. In the case given here, IKONOS imagery allowed two qualitative hazard assessments for block-and-ash flow activity in drainages around Merapi. Firstly, the interpretation of IKONOS images provides insights in factors that control the propagation of secondary flows as the avulsion of the main flows is driven by longitudinal change in channel capacity due to increased sinuosity in the valley and decreased containment space. Secondly, the sinuosity and obstacles (including Sabo dams) may create over bank flows over adjacent low relief, allowing them to reach unexpectedly vulnerable areas distant from an active dome and away from the volcanically active valleys. Hazard assessment should therefore consider the geometry of secondary channels outside the principal valleys.

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.

New Zealand Journal of Geology and Geophysics, 1997

... carried out under the same analytical conditions and on the same instrument (Froggatt 1982, 1... more ... carried out under the same analytical conditions and on the same instrument (Froggatt 1982, 1983; Froggatt & Solloway 1986; Lowe 1988; Alloway 1989; Froggatt & Rogers 1990; Donoghue 1991; Pillans & Wright 1992: Stokes et al. 1992; Pillans et al. 1993; Shane & Froggatt ...

New Zealand Journal of Agricultural Research, 2000

... Fluoride: A review of its fate, bioavailability, and risks of fluorosis in grazed‐pasture sys... more ... Fluoride: A review of its fate, bioavailability, and risks of fluorosis in grazed‐pasture systems in New Zealand SJ Cronin a , V. Manoharan ab , MJ Hedley a & P. Loganathan a ... With an average 1.5% F in the SSP (Evans et al. 1971; P. Loganathan et al. unpubl. ...

Journal of Volcanology and Geothermal Research, 2008

... Eruptives from Mt Taranaki are relatively potassium-rich compared to the andesitic volcanoes ... more ... Eruptives from Mt Taranaki are relatively potassium-rich compared to the andesitic volcanoes of the Taupo Volcanic Zone, 140 km to the east ([Neall et al., 1986], [Price et al., 1992], [Stewart et al., 1996] and [Price et al., 1999]), hence the magmas are classified as high-K ...

Geophysical Journal International, 2013

ABSTRACT We demonstrate using a high-resolution Holocene volcanic event record from Mt Taranaki (... more ABSTRACT We demonstrate using a high-resolution Holocene volcanic event record from Mt Taranaki (New Zealand) how geochemical data can be used to modulate a renewal model for the estimated probability of a future eruption. The andesitic stratovolcano Mt Taranaki has an activity record punctuated by long periods of quiescence and subsequent re-awakening. Thus, the distribution of interonset times is bimodal, with the possibility of anomalously long reposes. However, we show that a bimodal renewal distribution is outperformed for eruption forecasting by a proportional hazards type model. The latter model uses the major-oxide chemistry of the ubiquitous phenocryst mineral phase titanomagnetite as a proxy for the state of the magmatic system. We find that the concentrations of TiO2 and Al2O3 (or MgO) are useful predictors of repose length. These are major substituting elements in the titanomagnetite structure, reflecting variations in magmatic pressure/temperature and oxidation-state history. Highly variable Al2O3 (or MgO) indicates mixing of different magma batches deep within the plumbing system before eruption, and correlates with longer repose times. In contrast, high variability in TiO2 results from solid-state exsolution processes in the shallow, near-surface conduit and is an indicator for shorter repose times. In the bimodal renewal model, the estimated hazard is only updated to reflect the likelihood of a long repose after the present repose-length exceeds the shorter mode. In the geochemistry modulated model, this predictive information is available at the beginning of the repose to be forecast.

Contributions to Mineralogy and Petrology, 2010

High-resolution, stratigraphically ordered samples of the Udo tuff cone and lava shield offshore ... more High-resolution, stratigraphically ordered samples of the Udo tuff cone and lava shield offshore of Jeju Island, South Korea, show complex geochemical variation in the basaltic magmas that fed the eruption sequence. The eruption began explosively, producing phreatomagmatic deposits with relatively evolved alkali magma. The magma became more primitive over the course of the eruption, but the last magma to be explosively erupted had shifted back to a relatively evolved composition. A separate sub-alkali magma batch was subsequently effusively erupted to form a lava shield. Absence of weathering and only minor reworking between the tuff and overlying lava implies that there was no significant time break between the eruptions of the two magma batches. Modelling of the alkali magma suggests that it was generated from a parent melt in garnet peridotite at c. 3 to 3.5 GPa and underwent mainly clinopyroxene ? olivine ± spinel fractionation at c. 1.5 to 2 GPa. The sub-alkali magma was, by contrast, generated from a chemically different peridotite with residual garnet at c. 2.5 GPa and evolved through olivine fractionation at a shallower level compared to its alkali contemporary. The continuous chemostratigraphic trend in the tuff cone, from relatively evolved to primitive and back to evolved, is interpreted to have resulted from a magma batch having risen through a single dyke and erupted the batch's head, core and margins, respectively. The alkali magma acted as a path-opener for the sub-alkali magma. The occurrence of the two distinct batches suggests that different magmatic systems in the Jeju Island Volcanic Field have interacted throughout its history. The polymagmatic nature of this monogenetic eruption has important implications for hazard forecasting and for our understanding of basaltic field volcanism.

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.

... WITH ASSISTANCE FROM Tauranga Locals David and Joan Kear, David Milner and Kathrin Otrel-Cass... more ... WITH ASSISTANCE FROM Tauranga Locals David and Joan Kear, David Milner and Kathrin Otrel-Cass ... Page 3. POST-CALDERA VOLCANISM IN THE YENKAHE CALDERA, TANNA ISLAND, VANUATU K. Németh 1 , SJ Cronin 1 , A. Möbis 1 , D. Charley 1,2 ...

Rapid changes in small areas at the apex of alluvial fans may have devastating consequences by di... more Rapid changes in small areas at the apex of alluvial fans may have devastating consequences by directing downstream flood or lahar impacts into catchments of widely varying population or infrastructure vulnerability. During a series of lahars in 1995 at Mt. Ruapehu, New Zealand, aggradation of the Whangaehu fan apex (draining the eastern edifice) caused the onset of avulsion of flows northward into the highly vulnerable Tongariro catchment. An earth training dike (or bund) was constructed to protect this catchment by retaining flows on the southern side and the normal lahar outlet path to the south. Surveys in 2001, late 2005, and following a major lahar in March 2007 now show net degradation of a channel in the Whangaehu fan apex, bordered by the bund. This indicates a net increase in the channel capacity at this site and shows that the bund remains at its effective design capacity. Past hydrological modelling used for the bund design provided a large range of discharge estimates but lacked precise constraints on the size and nature of lahars from eruption and lake-breakout events. New modelling has been carried out using Titan2D to examine the impacts of a 6 × 10 6 m 3 volume granular flow down this catchment. This simulates either an eruption or a lake breakout-induced lahar with a historically typical volumetric bulking factor of 4. These simulations predict minimum discharges between 1800 and 2100 m 3 /s at the bund site. By comparison, the largest 1995 flow at this site was estimated at around 1200 m 3 /s. Further, any single modelled flow from the normal outlet channel of Crater Lake could not be induced to overtop the bund because discharge appears to be limited by the narrow upper reaches of the Whangaehu Gorge. Theoretical discharge levels required to overtop the bund are estimated to be N 6800 m 3 /s, assuming no aggradation of the channel by the decelerating flow. Maximum potential discharge at the bund site is additionally modified by potential bifurcation of lahars above a certain size threshold at a point 2.5 km upstream of the bund. A major side channel ("the chute") effectively diverts part of the flows away to the southern side of the Whangaehu fan. This potential for splitting of the flow appears to have increased since 1995, from aggradation of lahar deposits and reworked sediment in the area immediately upstream of the divide. Despite remaining at design capacity for lahar events in the Whangaehu, erosion of new channels following the 2007 lahar renders the bund vulnerable to undercutting.

Journal of Volcanology and Geothermal Research, 2007

Small-volume pyroclastic density currents (PDCs) are generated frequently during explosive erupti... more Small-volume pyroclastic density currents (PDCs) are generated frequently during explosive eruptions with little warning. Assessing their hazard requires a physical understanding of their transport and sedimentation processes which is best achieved by the testing of experimental and numerical models of geophysical mass flows against natural flows and/or deposits. To this end we report on one of the most detailed sedimentological studies ever carried out on a series of pristine small-volume PDC deposits from the 1975 eruption of Ngauruhoe volcano, whose emplacement were also witnessed during eruption. Using high-resolution GPS surveys, a series of lateral excavations across the deposits, and bulk sedimentological analysis we constrained the geomorphology, internal structure and texture of the deposits with respect to laterally varying modes of deposition.

Journal of Volcanology and Geothermal Research, 2005

... Keywords: pyroclastic flows; block-and-ash flows; basal avalanche; emplacement mechanisms; tr... more ... Keywords: pyroclastic flows; block-and-ash flows; basal avalanche; emplacement mechanisms; transport mechanisms; facies; Merapi Volcano. Article Outline. 1. Introduction 2. Methods 2.1. Density and vesicularity analysis 2.2. Grain size analysis 2.3. ...

... WITH ASSISTANCE FROM Tauranga Locals David and Joan Kear, David Milner and Kathrin Otrel-Cass... more ... WITH ASSISTANCE FROM Tauranga Locals David and Joan Kear, David Milner and Kathrin Otrel-Cass ... Page 3. POST-CALDERA VOLCANISM IN THE YENKAHE CALDERA, TANNA ISLAND, VANUATU K. Németh 1 , SJ Cronin 1 , A. Möbis 1 , D. Charley 1,2 ...

Geological Society of America Bulletin, 2012

... was used to understand how the break in erup-tive activity and vent migration may have been r... more ... was used to understand how the break in erup-tive activity and vent migration may have been related to changes in magma composition and possible transitions between separate pulses of alkali basaltic magma, as has been postulated for other Jeju volcanoes (Brenna et al ...

Bulletin of Volcanology, 2009

Volcanoes provide important contributions to atmospheric budgets of SO 2 and reactive halogens, w... more Volcanoes provide important contributions to atmospheric budgets of SO 2 and reactive halogens, which play significant roles in atmospheric oxidative capacity and radiation. However, the global source strengths of volcanic emissions remain poorly constrained. These uncertainties are highlighted here by the first measurements of gas emission rates from Ambrym volcano, Vanuatu. Our initial airborne ultraviolet spectroscopic measurements made in January 2005 indicate fluxes of 18-270 kg s -1 of SO 2 , and 62-110 gs -1 of BrO, into the atmosphere, placing Ambrym amongst the largest known contemporary point sources of both these species on Earth. We also estimate high Cl and F fluxes of~8-14 and~27-50 kg s -1 , respectively, for this period. Further observations using both airborne and spaceborne remote sensing reveal a fluctuating SO 2 output between 2004 and 2008, with a surge in the first half of 2005, and underline the substantial contribution that a single passively degassing volcano can make to the atmospheric budget of sulfur and halogens.

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.

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-...

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 ...

ABSTRACT On 16 June 2006 an overpass of IKONOS coincided with the emplacement of an active block-... more ABSTRACT On 16 June 2006 an overpass of IKONOS coincided with the emplacement of an active block-and-ash flow fed by a lava dome collapse event at Merapi Volcano (Java, Indonesia). This was the first satellite image recorded for a moving pyroclastic flow. The very high-spatial resolution data displayed the extent and impact of the pyroclastic deposits emplaced during and prior to, the day of image acquisition. This allowed a number of features associated with high-hazard block-and-ash flows emplaced in narrow, deep gorges to be mapped, interpreted and understood. The block-and-ash flow and surge deposits recognized in the Ikonos images include: (1) several channel-confined flow lobes and tongues in the box-shaped valley; (2) thin ash-cloud surge deposit and knocked-down trees in constricted areas on both slopes of the gorge; (3) fan-like over bank deposits on the Gendol-Tlogo interfluves from which flows were re-routed in the Tlogo secondary valley; (4) massive over bank lobes on the right bank from which flows devastated the village of Kaliadem 0.5 km from the main channel, a small part of this flow being re-channeled in the Opak secondary valley. The high-resolution IKONOS images also helped us to identify geomorphic obstacles that enabled flows to ramp and spill out from the sinuous channel, a process called flow avulsion. Importantly, the avulsion redirected flows to unexpected areas away from the main channel. In the case of Merapi we see that the presence of valley fill by previous deposits, bends and man-made dams influence the otherwise valley-guided course of the flows. Sadly, Sabo dams (built to ameliorate the effect of high sediment load streams) can actually cause block-and-ash flows to jump out of their containing channel and advance into sensitive areas. Very-high-spatial resolution satellite images are very useful for mapping and interpreting the distribution of freshly erupted volcanic deposits. IKONOS-type images with 1-m resolution provide opportunities to study and map the meter-scale detail of volcanic deposits. When such high-spatial-resolution satellite remote sensing data are combined with in situ field work, geomorphic analyses can be applied that allow us to more fully understand the dynamics and hazards of eruptions. In the case given here, IKONOS imagery allowed two qualitative hazard assessments for block-and-ash flow activity in drainages around Merapi. Firstly, the interpretation of IKONOS images provides insights in factors that control the propagation of secondary flows as the avulsion of the main flows is driven by longitudinal change in channel capacity due to increased sinuosity in the valley and decreased containment space. Secondly, the sinuosity and obstacles (including Sabo dams) may create over bank flows over adjacent low relief, allowing them to reach unexpectedly vulnerable areas distant from an active dome and away from the volcanically active valleys. Hazard assessment should therefore consider the geometry of secondary channels outside the principal valleys.

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.

New Zealand Journal of Geology and Geophysics, 1997

... carried out under the same analytical conditions and on the same instrument (Froggatt 1982, 1... more ... carried out under the same analytical conditions and on the same instrument (Froggatt 1982, 1983; Froggatt & Solloway 1986; Lowe 1988; Alloway 1989; Froggatt & Rogers 1990; Donoghue 1991; Pillans & Wright 1992: Stokes et al. 1992; Pillans et al. 1993; Shane & Froggatt ...

New Zealand Journal of Agricultural Research, 2000

... Fluoride: A review of its fate, bioavailability, and risks of fluorosis in grazed‐pasture sys... more ... Fluoride: A review of its fate, bioavailability, and risks of fluorosis in grazed‐pasture systems in New Zealand SJ Cronin a , V. Manoharan ab , MJ Hedley a & P. Loganathan a ... With an average 1.5% F in the SSP (Evans et al. 1971; P. Loganathan et al. unpubl. ...

Journal of Volcanology and Geothermal Research, 2008

... Eruptives from Mt Taranaki are relatively potassium-rich compared to the andesitic volcanoes ... more ... Eruptives from Mt Taranaki are relatively potassium-rich compared to the andesitic volcanoes of the Taupo Volcanic Zone, 140 km to the east ([Neall et al., 1986], [Price et al., 1992], [Stewart et al., 1996] and [Price et al., 1999]), hence the magmas are classified as high-K ...

Geophysical Journal International, 2013

ABSTRACT We demonstrate using a high-resolution Holocene volcanic event record from Mt Taranaki (... more ABSTRACT We demonstrate using a high-resolution Holocene volcanic event record from Mt Taranaki (New Zealand) how geochemical data can be used to modulate a renewal model for the estimated probability of a future eruption. The andesitic stratovolcano Mt Taranaki has an activity record punctuated by long periods of quiescence and subsequent re-awakening. Thus, the distribution of interonset times is bimodal, with the possibility of anomalously long reposes. However, we show that a bimodal renewal distribution is outperformed for eruption forecasting by a proportional hazards type model. The latter model uses the major-oxide chemistry of the ubiquitous phenocryst mineral phase titanomagnetite as a proxy for the state of the magmatic system. We find that the concentrations of TiO2 and Al2O3 (or MgO) are useful predictors of repose length. These are major substituting elements in the titanomagnetite structure, reflecting variations in magmatic pressure/temperature and oxidation-state history. Highly variable Al2O3 (or MgO) indicates mixing of different magma batches deep within the plumbing system before eruption, and correlates with longer repose times. In contrast, high variability in TiO2 results from solid-state exsolution processes in the shallow, near-surface conduit and is an indicator for shorter repose times. In the bimodal renewal model, the estimated hazard is only updated to reflect the likelihood of a long repose after the present repose-length exceeds the shorter mode. In the geochemistry modulated model, this predictive information is available at the beginning of the repose to be forecast.

Contributions to Mineralogy and Petrology, 2010

High-resolution, stratigraphically ordered samples of the Udo tuff cone and lava shield offshore ... more High-resolution, stratigraphically ordered samples of the Udo tuff cone and lava shield offshore of Jeju Island, South Korea, show complex geochemical variation in the basaltic magmas that fed the eruption sequence. The eruption began explosively, producing phreatomagmatic deposits with relatively evolved alkali magma. The magma became more primitive over the course of the eruption, but the last magma to be explosively erupted had shifted back to a relatively evolved composition. A separate sub-alkali magma batch was subsequently effusively erupted to form a lava shield. Absence of weathering and only minor reworking between the tuff and overlying lava implies that there was no significant time break between the eruptions of the two magma batches. Modelling of the alkali magma suggests that it was generated from a parent melt in garnet peridotite at c. 3 to 3.5 GPa and underwent mainly clinopyroxene ? olivine ± spinel fractionation at c. 1.5 to 2 GPa. The sub-alkali magma was, by contrast, generated from a chemically different peridotite with residual garnet at c. 2.5 GPa and evolved through olivine fractionation at a shallower level compared to its alkali contemporary. The continuous chemostratigraphic trend in the tuff cone, from relatively evolved to primitive and back to evolved, is interpreted to have resulted from a magma batch having risen through a single dyke and erupted the batch's head, core and margins, respectively. The alkali magma acted as a path-opener for the sub-alkali magma. The occurrence of the two distinct batches suggests that different magmatic systems in the Jeju Island Volcanic Field have interacted throughout its history. The polymagmatic nature of this monogenetic eruption has important implications for hazard forecasting and for our understanding of basaltic field volcanism.

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.

... WITH ASSISTANCE FROM Tauranga Locals David and Joan Kear, David Milner and Kathrin Otrel-Cass... more ... WITH ASSISTANCE FROM Tauranga Locals David and Joan Kear, David Milner and Kathrin Otrel-Cass ... Page 3. POST-CALDERA VOLCANISM IN THE YENKAHE CALDERA, TANNA ISLAND, VANUATU K. Németh 1 , SJ Cronin 1 , A. Möbis 1 , D. Charley 1,2 ...