%0 Journal Article %T Pyroclast textural variation as an indicator of eruption column steadiness in andesitic Plinian eruptions at Mt. Ruapehu %+ Volcanic Risk Solutions %+ United States Geological Survey [Reston] (USGS) %+ Institut des Sciences de la Terre d'Orléans - UMR7327 (ISTO) %+ School of Geography, Environment and Earth Sciences [Wellington] %+ School of Environment %A Pardo, Natalia %A Cronin, Shane J. %A Wright, Heather M.N. %A Schipper, C. Ian %A Smith, Ian %A Stewart, Bob %< avec comité de lecture %@ 0258-8900 %J Bulletin of Volcanology %I Springer Verlag %V 76 %P 17 p. %8 2014 %D 2014 %R 10.1007/s00445-014-0822-x %K Computed microtomography (μ-CT) Degassing Pyroclast Vesicle size distribution X-ray synchrotron %Z Sciences of the Universe [physics]/Earth Sciences/Volcanology %Z Environmental Sciences/Global ChangesJournal articles %X Between 27 and 11 cal. ka BP, a transition is observed in Plinian eruptions at Mt. Ruapehu, indicating evolution from non-collapsing (steady and oscillatory) eruption columns to partially collapsing columns (both wet and dry). To determine the causes of these variations over this eruptive interval, we examined lapilli fall deposits from four eruptions representing the climactic phases of each column type. All eruptions involve andesite to basaltic andesite magmas containing plagioclase, clinopyroxene, orthopyroxene and magnetite phenocrysts. Differences occur in the dominant pumice texture, the degree of bulk chemistry and textural variability, the average microcrystallinity and the composition of groundmass glass. In order to investigate the role of ascent and degassing processes on column stability, vesicle textures were quantified by gas volume pycnometry (porosity), X-ray synchrotron and computed microtomography (μ-CT) imagery from representative clasts from each eruption. These data were linked to groundmass crystallinity and glass geochemistry. Pumice textures were classified into six types (foamy, sheared, fibrous, microvesicular, microsheared and dense) according to the vesicle content, size and shape and microlite content. Bulk porosities vary from 19 to 95 % among all textural types. Melt-referenced vesicle number density ranges between 1.8 × 102 and 8.9 × 102 mm−3, except in fibrous textures, where it spans from 0.3 × 102 to 53 × 102 mm−3. Vesicle-free magnetite number density varies within an order of magnitude from 0.4 × 102 to 4.5 × 102 mm−3 in samples with dacitic groundmass glass and between 0.0 and 2.3 × 102 mm−3 in samples with rhyolitic groundmass. The data indicate that columns that collapsed to produce pyroclastic flows contained pumice with the greatest variation in bulk composition (which overlaps with but extends to slightly more silicic compositions than other eruptive products); textures indicating heterogeneous bubble nucleation, progressively more complex growth history and shear-localization; and the highest degrees of microlite crystallization, most evolved melt compositions and lowest relative temperatures. These findings suggest that collapsing columns in Ruapehu have been produced when strain localization is prominent, early bubble nucleation occurs and variation in decompression rate across the conduit is greatest. This study shows that examination of pumice from steady phases that precede column collapse may be used to predict subsequent column behaviour. %G English %L insu-00985851 %U https://insu.hal.science/insu-00985851 %~ OBSPM %~ SDE %~ INSU %~ BRGM %~ CNRS %~ UNIV-ORLEANS %~ ISTO %~ OPENAIRE %~ OSUC %~ GIP-BE %~ PSL %~ OBSPM-PSL %~ TEST3-HALCNRS