Journal of Volcanology and Geothermal Research
Fine ash content of explosive eruptions
Introduction
In classical sedimentology, “volcanic ash” refers to pyroclasts with diameter, D, smaller than 2 mm (if D > 2 mm, lapilli). In this paper, we define the finest particle-size classes based on fluid-dynamic behavior and focus on particles that settle in non-turbulent flow regimes distinguished by low particle Reynolds numbers, Re, where Re < 500 (Bonadonna et al., 1998). This distinction is well-suited to this paper, as the focus of our study is the longer atmospheric residence time of fine and especially very fine ash. We use the term “fine ash” to include ash particles with diameters < 1000 μm (> 0 ϕ) which fall in the intermediate flow regime (0.4 < Re < 500), and “very fine ash” to include particles with diameter < 30 μm ( > 5 ϕ; Re < 0.4) which settle according to Stokes Law in the laminar flow regime.
The terminal fall velocity of sedimenting particles, which determines residence time in the atmosphere, is sensitive to particle size and atmospheric conditions (these vary as a function of height — Fig. 1). Ash particles are not spherical, which complicates and further slows fallout (Riley et al., 2003). Ignoring the effect of shape for simplicity, we can define fine-ash particles in general terms as those with a predicted atmospheric residence time of > 30 min and very fine ash particles with residence times > 3 h. In fact, we know that mass fractions of some volcanic ash events have particle diameters < 10 μm which have predicted residence times of > 10 days. Here we use the word “predicted” because we know that calculated fallout times based on settling according to Stokes Law are not accurate or are inaccurate for very fine ash which sediments much faster. Remote sensing results (Rose et al., 2000) and distal ash sampling studies (Durant et al., 2009-this volume) strongly suggest that both fine and very fine ash mostly fall within a day of their eruption, much faster than fluid dynamics modeling suggests.
Fine ash, and more importantly, very fine ash, have not been studied as much as coarse ash and lapilli. Distal ash deposits are generally dispersed over a more extensive area and at greater distance from the volcano, and form an ephemeral, irregular covering over a large area which may be quickly reworked and further dispersed by winds and rain. Sampling is difficult unless it is done during or immediately after fallout, and the fallout areas may be very large. Only a few eruptions have well-sampled very fine ash fallout (Table 1). Consequently, assessment of ash hazards is subject to large uncertainty because we know that two important hazards of volcanic ash are strongly-skewed toward very fine ash: (1) Human and animal health effects of ash, linked to respiratory illness, is closely associated with particle size, which are especially anticipated for diameters < 10 μm (Horwell and Baxter, 2006) because aerodynamically fine particles successfully negotiate the curves of the throat and are carried to the lungs; and (2) aircraft operations (especially commercial jets) are threatened by volcanic cloud encounters, during which a variety of hazards exist such as airborne very fine ash that enters and melts in jet turbines and can cause failure (Casadevall, 1994).
This paper aims to integrate new data about the origin and distribution of fine and very fine ash from the application of laser diffraction particle-size analysis (LDPSA) to fine and very fine volcanic ash-fall samples (Table 1). This data has been used to reconstruct the “total grain-size distribution” (TGSD) (Bonadonna and Houghton, 2005) of whole tephra deposits from some recent eruptions. This integration requires a spatial analysis of deposit grain-size distributions which are weighted according to deposit characteristics (either mass or thickness) to estimate the initial grain-size distribution before atmospheric fractionation (Bonadonna and Houghton, 2005). The details of this analysis are contained in the papers cited in Table 1. TGSD can be used to assess potential hazards from explosive eruptions and will help address the following questions: (1) How is fine ash created? (2) What types of eruptions should create more fine ash? (3) What causes fine ash to fall faster than it would as simple particles? (4) What is the role of meteorology in this fallout? (5)Can this fallout be forecast?
Section snippets
Methods used
LDPSA determines the size distribution of a particle dispersion through the application of Mie theory or the Fraunhofer approximation to measured light scattering. Two instruments were used for this work which measure particle diameters from millimeter to submicron size: (1) Microtrac® SRA (Standard Range Analyzer) 9210-1-10-1 laser particle size analyzer and (2) Malvern Mastersizer 2000 laser particle size analyzer. The majority of very fine ash particles are smaller than standard sieve size
Results
Distal ash-fall particle size distributions are typically polymodal; the shapes of the distributions are neither well-sorted nor lognormal (e.g., Fig. 2). No single sample is representative of the size distribution generated in an eruption, because, in spite of the limited sorting evidenced by a single sample analysis, there is substantial sorting of lapilli and coarse ash that occurs during transport in the atmosphere. All processes, including fragmentation, and then during transport and
Production of fine and very fine ash
Pyroclasts are fragments of magma which form by a variety of processes such as rapid decompression and explosive vesiculation. During ascent in the crust, gas exsolves from magma and forms bubbles which coalesce and form an over-pressurized foam thatruptures explosively (Alidibirov and Dingwell, 1996). Hydromagmatic processes, where magma comes in contact with external water during eruption, may also produce ash in some circumstances. Large pyroclasts can be further reduced in size after
Conclusions
TGSD from several recent eruptions have been reconstructed from LDPSA of extensively sampled tephra fallout blankets. The data indicate that eruptions generate fallout with polymodal particle size distributions, which includes substantial proportions of particles < 30 μm (very fine ash) with particular significance to hazards. This material is more abundant in highly silicic explosive eruptions and those with prominent pyroclastic flows.
Since the coarse fractions of pyroclasts have short
Acknowledgements
Andrei Sarna-Wojcicki and Elmira Wan are thanked for providing access and assistance with the 18 May 1980 Mount St. Helens samples. The University of Cambridge Malvern Laboratory and staff are thanked for assistance with some of the LDPSA analyses. Komar Kawatra kindly provided access to Michigan Tech's Microtrac lab. AJD gratefully acknowledges support during the final preparation of this manuscript as a member of the GREENCYCLES Marie Curie Research Training Network. Alain Volentik helped
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