IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

0P2_2K-O13

Room A1

Date/Time: July 20 17:45-18:00

Bubble morphologies, gas escape and fragmentation of crystal-rich magma
Alison C Rust, Katharine V Cashman, Julie Oppenheimer, Emma J Nicholson
University of Bristol, United Kingdom
E-mail: Alison.Rust@bristol.ac.uk

Hydrous intermediate and mafic magmas commonly erupt with abundant microlites that crystallized as volatiles
exsolved during ascent.

Both natural and experimental samples show that crystal-rich groundmasses areassociated with substantially thicker bubble walls and more complex bubble morphologies than crystal-poor melts.
The physical and chemical relationships between co-evolving bubbles and crystals are complicated. Here we
combine observations from three-phase (gas, viscous liquid, solid particle) analogue experiments and textures ofscoria from Fuego Volcano (Guatemala; October 14 and 17, 1974) to examine the effects of crystals on bubble
morphologies, and their role in gas escape and fragmentation of crystal-rich magma.

The Fuego samples are extremely crystal-rich: in a typical basaltic scoria erupted Oct.14 1974, we find amatrix of 61 vol% microlites, which together with the phenocrysts gives a total crystallinity of 70 vol% on avesicle-free basis. As is common for crystal-rich scoria, the bubbles are non-spherical and often polylobate. Similar
bubble morphologies have been ascribed to extensive bubble coalescence, with implications for interpretationsof bubble number densities and ascent conditions. However, our analogue experiments and analysis of bubble
textures in rims and cores of lapilli, indicate that bubble expansion is sufficient to generate polylobate bubbles incrystal-rich suspensions (which can be further modified by bubble coalescence).

High crystal contents and thick bubble walls resist bubble expansion, promoting bubble overpressure. Analogue
experiments indicate that if there is sufficient gas pressure, there is a transition from lobate bubbles to fracturing
as the crystal content approaches maximum packing, and if crystal distribution is inhomogeneous, the fractures
tend to propagate through regions of relatively high crystallinity. Such inter-crystal fracture networks would aid
degassing as magma ascends and could play a role in the fragmentation of magma to ash and lapilli.

Fragmentation of crystal-poor silicic magma occurs if gas is unable to escape sufficiently quickly to prevent
overpressured bubbles breaking the magma apart. For this reason, deposits from silicic Plinian eruptions containabundant fine ash composed of bubble wall remnants, and the total deposit ash size distribution is closely related
to the bubble-size distribution determined from co-erupting pumice. In contrast, the size distribution of fine ash
from the well-characterized Oct. 14 1974 subplinian/vulcanian Fuego fallout deposit correlates with the crystalrather than bubble-size distribution measured in scoria, suggesting a strong control of crystals on the production of
the fine ash. The groundmass crystals are both smaller and much more abundant (by number) than the bubbles.
There is a substantially larger median grain size (mm-scale) on a volume (mass) basis, which may be related tophenocrysts as well as bubble textures.

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