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

4W_3E-P7

Date/Time: July 24 Poster

Towards improved characterisation of volcanic ash
Emma J Nicholson1 , Katharine V Cashman1 , Alison C Rust1 , Sigurdur R Gislason2
1

University of Bristol, Department of Earth Sciences, U.K., 2 University of Iceland, Institute of EarthSciences, IcelandE-mail: en12525@bristol.ac.uk

Explosive volcanism can generate large volumes of volcanic ash, with potentially significant economic andenvironmental implications. Recent eruptions, such as that of Eyjafjallajokull in 2010, have highlighted gaps in
our knowledge regarding both the fragmentation mechanisms that produce ash and their effect on the resulting
size distributions and morphologies of erupted pyroclasts. Mafic eruptions, in particular, exhibit a wide range oferuption styles that generate pyroclasts with highly variable sizes, shapes, vesicularities and crystallinities. We
are investigating links between eruption style and pyroclast characteristics, with the goal of using these data bothto gain insight into the processes governing fragmentation and to develop characteristic source parameters fordifferent eruption conditions.

The 2011 subglacial eruption of Grimsvotn, Iceland, involved the explosive interaction of volatile-rich basaltic
magma with subglacial meltwater. The tephra produced in this eruption contains a high proportion of fine ash, with90 percent of grains less than 100 microns in diameter, and is texturally diverse. The ash-fall samples used in thisstudy were collected 50-115 km southwest of the vent immediately following the eruption. We explore variation inquantitative shape parameters as a function of both grainsize and transport distance using multiple image analysis
techniques, including Morphologi and Scanning Electron Microscopy (SEM). Sample componentry comprises twodominant particle classes (dense/vesicular) that can be further subdivided (blocky and platy/highly, moderate, andelongate vesicular), with less than 5 percent crystalline material. Systematic trends in the relative proportionsof each particle class are evident within the fine ash fraction (10-125 microns), the most striking of which is theincrease in the percentage of dense relative to vesicular grains with decreasing grainsize.

Vesicle size distributions (VSDs) were measured on vesicular clasts and fragments where greater than 50
percent of individual bubble walls were preserved. VSD modes lie between 30 and 50 microns. By combining
morphological data on particle shapes with textural data on vesicular fragments we can reconstruct the state of themagma at the point of fragmentation. Fragmentation style appears strongly controlled by the VSD: the dominantparticle morphology, as measured by both componentry and shape parameters, changes significantly as theparticle size approaches the modal size of the vesicle population. The presence of several different morphologicalcomponents suggests fragmentation of an inhomogeneously vesicular primary magma, such as the internal
textures commonly observed within Peles tears quenched from Hawaiian fire fountains. Determining how theparticle size and shape distributions of volcanic ash are linked to the style of magma fragmentation will guide thedevelopment of characteristic input parameters for ash dispersion models.

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