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.
Computed microtomography (μ-CT) Degassing Pyroclast Vesicle size distribution X-ray synchrotron
Supplementary Material ESM- 1Dataset for groundmass glass, mineral rims, and whole-rock geochemistry. For EMPAspread-sheets, coloured lines separate individual clastswhereas each line marked as ID represents a separate analytical point within each lapillus.Unfortunately, the interception of microlites and nanoliths within the beam could not be avoided in fibrous clasts, and precise glass geochemistry for this texture could not be obtained with the available facilities. (XLS 5274 kb)
Supplementary Material ESM- 2Density and Porosity data obtained with envelope-and-gas pycnometry for: (a) Sw: Shawcroft (steady column); (b)Mgt: Managtoetoenui (Oscillatory column); (c) Oru: Oruamatua (collapsing, phreatomagmatic); (d) Okp: Okupata (collapsing, magmatic, and sourced at the Southern Crater). Solid density values obtained in powders of each texture are shown in (e); modal proportion of each texture within each unit is shown in (f). (XLSX 114 kb)
Supplementary Material ESM- 3Details on textural analysis based on X-ray synchrotron and computed microtomography. ESM-3.1: Methodology details. ESM-3.2: Groundmass crystallinity, corrected vesicularity and mafic crystallinity; ESM-3.3: Decompression rates calculated following the corrected method of Toramaru (2006). (DOCX 95.2 kb)
Supplementary Material ESM- 4
Video showing (1) Renders of some of the textures, followed by (2) scrolls across: (a)sheared pumice type from the oscillatory column case (Lower-Mangatoetoenui). Vesicle shapes vary from subspherical to ellipsoidal and walls are smooth and thin compared to other pumice types. The field of view equals 800 × 300 pixels;(b) Microsheared texture from the steady phase of the Upper-Mangatoetoenui unit, with highly elongated and oriented vesicles. Field of view equals 800 × 370 pixels;(c) Microvesicular pumice clastfrom steady columns (Shawcroft). Large vesicles are distorted and have thick vesicle walls compared to other pumice types. Field of view equals 600 × 350 pixels;(d) Microvesicular pumice clast from phreatomagmatic, collapsing columns (Oruamatua). Vesicles are distorted and have thick walls. The field view is 600 × 400 pixels. (e) Fibrous pumice clasts from the magmatic, collapsing columns (Okupata-Poruahu). Vesicles are distorted, locally oriented and have the thinnest walls of all pumice types. (MOV 54235 kb)