A review of volcanic ash aggregation
Highlights
► The state of knowledge of ash aggregation in volcanic ash clouds. ► How and why aggregation reduces the atmospheric residence time of fine ash. ► Insights from field, lab + modelling investigations on particle aggregation processes. ► Propose that integrated observations are required to fill current gaps in knowledge.
Introduction
Explosive volcanic eruptions generate large amounts (>50% of total erupted mass) of fine ash particles (here defined as particles with diameter <63 μm) which are dispersed into the atmosphere by buoyant plumes above volcanic vents and pyroclastic density currents (PDCs; Carey and Sigurdsson, 1982, Hildreth and Drake, 1992, Durant and Rose, 2009). Most volcanic ash finer than 125 μm settles out of the atmosphere as particle aggregates that have higher settling velocities than individual constituent particles (Carey and Sigurdsson, 1982, Sorem, 1982, Lane et al., 1993). While aggregation exerts a first order control on the dispersal of fine ash within eruption clouds, the physical and chemical processes involved are not completely understood despite significant progress over the past 20 years (Schumacher and Schmincke, 1995, Gilbert and Lane, 1994, James et al., 2002, James et al., 2003, Textor et al., 2006a, Textor et al., 2006b, Durant et al., 2009, Costa et al., 2010). Fine airborne particles adhere to each other as a result of electrostatic attraction, moist adhesion between particles (e.g., Sorem, 1982, Gilbert and Lane, 1994, Schumacher and Schmincke, 1991, Schumacher and Schmincke, 1995, James et al., 2002) and hydrometeor formation (e.g., Veitch and Woods, 2001, Textor et al., 2006a, Durant et al., 2009). The atmospheric residence time of fine ash determines the hazard to aviation (Casadevall, 1994), and ash fallout impacts local environments and infrastructure (Stewart et al., 2006, Spence et al., 2005, Wardman et al., this issue) and may present a health hazard over an extended period of exposure (Horwell and Baxter, 2006).
Over the past few decades, models of varying complexity have been developed for the dispersal and sedimentation of volcanic particles. These models include analytical solutions, used widely for investigations of particle sedimentation and for hazard assessments (e.g., Armienti et al., 1988, Bonadonna et al., 2005a, Bonadonna et al., 2005b, Bursik et al., 1992a, Bursik et al., 1992b, Connor et al., 2001, Connor and Connor, 2006, Glaze and Self, 1991, Hurst and Turner, 1999, Koyaguchi and Ohno, 2001, Macedonio et al., 2005, Suzuki, 1983), and numerical models for real-time forecast of plume evolution and sedimentation (e.g., Barsotti and Neri, 2008, Barsotti et al., 2008, Costa et al., 2006, Searcy et al., 1998). Both types have been validated with field data and are now used regularly for both these purposes. However, the majority of these models do not account for ash aggregation and as a consequence tend to underestimate proximal fallout and overestimate ash concentrations in the atmosphere far from source, in particular in case of ash-rich volcanic plumes.
This paper reviews the current understanding of ash aggregation, summarises observations of aggregate fallout, the structure and morphology of ash aggregates, and reviews the effects of aggregation on the dispersal of tephra. We draw together observations of eruptions, field studies of deposits, experimental studies and numerical modelling. Observations of recent eruptions indicate that the type of aggregate falling from ash clouds changes with distance from the volcano (Rosenbaum and Waitt, 1981, Hobbs et al., 1981, Sorem, 1982): proximal aggregates are larger and can contain water on deposition (liquid or frozen); distal aggregates are much smaller, fragile, and often reach the surface without direct evidence for the involvement of water in the process (we loosely use the term proximal for regions within the plume corner, i.e. <15 km depending on plume height; Bonadonna and Phillips, 2003, and distal for regions beyond the plume corner). We support improved observation and documentation of ash aggregates both during eruptions and within deposits in order to advance knowledge of this important topic.
Ash aggregates present a range of sizes, textures and shapes, from fragile sub-millimetre-size clusters of ash to centimetre-size concentric-laminated aggregates displaying variously sharp and graded laminations (e.g., Moore and Peck, 1962, Fisher and Schmincke, 1984, Reimer, 1983, Scolamacchia et al., 2005). An early line of studies on particle aggregation focused on proximal aggregates often associated with phreatomagmatic activity (e.g., Lorenz, 1974, Rosi, 1992, Gilbert and Lane, 1994). Later studies of aggregation processes brought to light the complexities of particle aggregation and the greater variety of particle aggregates from other types of eruptions, and also included more focus on laboratory study (e.g., Sorem, 1982, James et al., 2002, James et al., 2003, Durant et al., 2009). As a consequence, aggregation terminology has evolved and a number of different classification schemes have been proposed (Reimer, 1983, Schumacher and Schmincke, 1991, Thordarson, 2004, Brown et al., 2010). No classification scheme has yet been widely adopted and, over the past 20 years, the term ‘accretionary lapilli’ has been used to describe unstructured aggregates (e.g., Hayakawa, 1990, Rosi, 1992, Sisson, 1995, Watanabe et al., 1999, Trusdell et al., 2005), multiple concentric-laminated aggregates (e.g., Cole and Scarpati, 1993, Junqueira-Brod et al., 2005, Edgar et al., 2007), aggregates with a single fine-grained coating around a massive ash core (Branney, 1991) and ash-coated lithic clasts (Bednarz and Schmincke, 1990, Palladino et al., 2001). There are other examples in the literature where the term accretionary lapilli is used without an accompanying description of the aggregates. Usage of terms is inconsistent and confusion exists particularly around the term ‘accretionary lapilli’, which includes all lapilli-sized ash aggregates, but not aggregates smaller than 2 mm. Consistent use of terminology is important for clear communication of ideas. In recognition that the term ‘lapilli’ is a particle size denominator and that many aggregates commonly referred to as ‘accretionary lapilli’ are <2 mm, we propose to replace this term with ‘accretionary pellet’, which avoids particle size connotations. Accretionary pellets may be divided into three subcategories (AP1, AP2 and AP3) based only on internal structure, which avoids any implications regarding formation mechanisms. We define a second group of aggregates called particle clusters, which include ‘ash clusters’ (PC1; e.g., Sorem, 1982) and coated particles (PC2). Revised definitions for aggregate types are provided in Table 1 and illustrated in Fig. 1.
Section snippets
Visual observations
Ash aggregates have been observed falling during many historic explosive eruptions (Table 1; see examples in Fig. 1). Fallout of fine ash >100 s km downwind appears exclusively dominated by millimetre-scale loosely-bound ash clusters (PC1) that rarely survive impact, whereas aggregate fallout from the column may include ash cluster formation, but is dominated by denser, typically spherical or subspherical aggregates (e.g., AP1, AP2 and AP3). As a result of aggregation, many distal ash fall
Aggregation within ash plumes: conditions and downwind changes
Large variations in particle aggregate morphology as a function of distance from source suggests that there are multiple aggregation pathways, which implies formation processes evolve with time during transport. The availability and abundance of water in the eruption cloud exerts a dominant control on aggregation (Gilbert and Lane, 1994, Veitch and Woods, 2001, Durant et al., 2009, Costa et al., 2010, Folch et al., 2010, Textor et al., 2006a, Textor et al., 2006b). Initial fragmentation,
Experimental studies on aggregation
Experimental studies over the last couple of decades have provided fundamental insight on both accretionary pellet and ash cluster formation (Gilbert and Lane, 1994, James et al., 2002, James et al., 2003, Schumacher, 1994, Schumacher and Schmincke, 1995, Kueppers et al., 2011). As an example, even though ash clusters (PC1, Table 1) are difficult to document and analyse due to their low preservation potential, drag coefficients, aggregation coefficients and particle size distribution have been
Empirical and numerical studies on aggregation
Empirical parameterizations have been devised by some authors to understand particle aggregation. In particular, in order to simulate the Campanian Y-5 ash layer, Cornell et al. (1983) assumed aggregation of 50, 75 and 100 wt.% of particles in the size range 125–63 μm, 63–31 μm and <31 μm respectively with all aggregates (of unspecified type) having a diameter of 250 μm (φ = 2) and density of 2000 kg m−3. The distal mass accumulation maximum (the “Ritzville bulge”) of the Mount St. Helens 1980 eruption
Discussion, conclusions and suggestions for future work
Aggregation plays a fundamental role in the sedimentation of fine volcanic particles, both in proximal and distal areas, and influences atmospheric ash concentrations and tephra deposition. Fine ash particle aggregation results in greater fallout of fine ash close to source and has the effect of reducing distal (100 s–1000 s km) atmospheric ash concentrations. Even though ash-poor deposits can still be described without accounting for particle aggregation (Bonadonna and Phillips, 2003), models
Acknowledgements
AJD gratefully acknowledges support from the Natural Environment Research Council. Jennie Gilbert is kindly thanked for discussion and comments. The authors thank two anonymous reviewers for thoughtful and constructive reviews that improved the manuscript and Ulrich Kueppers, Yan Lavallée and Jacopo Taddeucci for editorial assistance.
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