4.2 Article

Pyroclast textural variation as an indicator of eruption column steadiness in andesitic Plinian eruptions at Mt. Ruapehu

期刊

BULLETIN OF VOLCANOLOGY
卷 76, 期 5, 页码 -

出版社

SPRINGER
DOI: 10.1007/s00445-014-0822-x

关键词

Computed microtomography (mu-CT); Degassing; Pyroclast; Vesicle size distribution; X-ray synchrotron

资金

  1. New Zealand Foundation for Research Science and Technology (NZ Natural Hazards Research Platform)
  2. Massey University Doctoral Research Scholarship
  3. ERC [202844]
  4. Director, Office of Science, Office of Basic Energy Sciences
  5. U. S. Department of Energy [DE-AC02-05CH11231]
  6. European Research Council (ERC) [202844] Funding Source: European Research Council (ERC)

向作者/读者索取更多资源

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 (mu-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. Meltreferenced vesicle number density ranges between 1.8x10(2) and 8.9x10(2) mm(-3), except in fibrous textures, where it spans from 0.3x10(2) to 53x10(2) mm-3. Vesicle-free magnetite number density varies within an order of magnitude from 0.4x10(2) to 4.5x 10(2) mm(-3) in samples with dacitic groundmass glass and between 0.0 and 2.3x10(2) 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.

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