Abstract
The abundant fine ash produced in the 2011 subglacial eruption of Grímsvötn, Iceland, highlights the fragmentation efficiency of mafic hydromagmatic eruptions, which is considerably higher than for comparable "dry" eruptions. Ash from the 2011 eruption can be divided into three morphological components-vesicular particles, shards, and dense fragments-distinguished by the size and abundance of constituent vesicles. We use the vesicle characteristics to define a new shape factor, the concavity index, which provides an unbiased way to classify individual ash particles as either bubbly (vesicular particles and shards) or dense. The relative proportion of bubbly and dense particles varies systematically with grain size, with the proportion of bubbly grains decreasing as the particle size approaches the modal bubble diameter. Measured bubble volume distributions are similar to those of rapidly quenched pyroclasts from Hawaiian fountains and suggest a comparable degassing history during magma ascent. Yet concordance between the size distributions of ash and of bubbles in the Grímsvötn samples stands in contrast to the size distributions in Hawaiian fountains, where pyroclasts are orders of magnitude larger than individual bubbles. We propose that the Grímsvötn ash formed by brittle disintegration of vesicular pyroclasts and that fragmentation efficiency was amplified by residual thermal stresses in glass quenched by glacial water. The strong control of resulting particle sizes and morphologies by the size and spatial distribution of bubbles demonstrates that the bubble population cannot be ignored when modeling hydromagmatic fragmentation.
Original language | English |
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Pages (from-to) | 239-242 |
Number of pages | 4 |
Journal | Geology |
Volume | 43 |
Issue number | 3 |
DOIs | |
Publication status | Published - 1 Jan 2015 |
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2011 Grímsvötn eruption: table of ash shape parameters
Rust, A. (Creator), Liu, E. (Creator) & Cashman, K. (Creator), University of Bristol, 8 Jan 2015
DOI: 10.5523/bris.765l15d6gbsj1vty17u2a8ky2, http://data.bris.ac.uk/data/dataset/765l15d6gbsj1vty17u2a8ky2
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Profiles
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Professor Alison C Rust
- School of Earth Sciences - Head of School, Professor of Physical Volcanology
- Cabot Institute for the Environment
- Geophysics
- Petrology (formerly BEEST)
- Volcanology
Person: Academic , Member, Professional and Administrative