Abstract
Although volcanic eruptions represent short periods in the whole history of a volcano, the large amount of loose pyroclastic material produced, combined with aeolian processes, can lead to continuous, long-lasting reworking of volcanic products. Driven by wind, these processes significantly influence the geomorphology and prolong the impacts of eruptions on exposed communities and ecosystems. Since such phenomena are of interest to scientists from a range of disciplines (e.g. volcanology, atmospheric and soil sciences), a well-defined, common nomenclature is necessary to optimize the multidisciplinary characterisation of both processes and deposits. We, therefore, first describe ash wind-remobilisation processes and provide definitions for appropriate terms consistent with the World Meteorological Organization’s classification of lithometeors. Secondly, we investigate the aeolian transport and deposition of ash from the 2011 Cordόn Caulle (Chile) tephra-fallout through field observations and on a physical characterisation of systematically collected ground and airborne material. In the arid regions of the Argentinian Patagonia steppe, two main types of secondary deposits have been identified associated with: i) non-erodible surface roughness elements (e.g. vegetation and rocks), and ii) pre-existing mounds or similar erodible bedforms. Grainsize analysis shows that wind-remobilised particles have a specific size range, from <0.4 to 500 m, with a 95% of the material between 1 and 255 m, median values of 25-135 m and modes of 30-95 m. We find that 15-40% of the remobilised material ranges from 63-125 m, coinciding with the size range which minimises the wind threshold friction velocity. Interestingly, particle shape analysis shows that for this size fraction, remobilised particles display the largest differences in shape descriptors (convexity, solidity and circularity) with respect to the primary ash, indicating abrasion and rounding due to saltation. Although particle (size and shape) and deposit features (morphology and structures) alone are insufficient to interpret transport mechanisms, their combination suggests that whilst saltation is the most common particle transport mechanism, suspension and creep also play an important role. In fact, our results suggest that saltation induces short-term suspension of fine particles (20-63 m).
Highlights
The main objectives of this paper are to: (i) correlate volcanological terms used to describe wind-remobilisation of volcanic ash with atmospheric definitions of aeolian phenomena, mostly developed by the World Meteorological Organisation (WMO); (ii) describe depositional features of ash remobilised by these phenomena, using the 2011-Cordón Caulle (CC) eruption, Chile, primary tephra fallout as a case study and (iii) characterise aeolian transport and deposition mechanisms of CC ash according to the size and shape of particles combined with field observations
Field observations, from a 2016 campaign, were made along an NNW-SEE transect from the Paso Samoré (ChileArgentina border), through Villa La Angostura (VLA) in the Andes; San Carlos de Bariloche (SCB); Pilcaniyeu (PC) to Ingeniero Jacobacci (IJ) in the Patagonian steppe (Figures 2B, 3A), and were integrated with previous descriptions of the primary tephra-fallout deposit made by Bonadonna et al (2015) and Pistolesi et al (2015)
We find that in some proximal areas, the thickness of Unit III has locally increased by a factor of two since the study of Pistolesi et al (2015) (Figures 3C,D Paso Samoré, VLA), whilst in distal areas these variations become more pronounced, varying from 1 to 40 cm (Figures 3C,D, CO, IJ)
Summary
Volcanic eruptions are often short lived and represent discrete time windows within the whole history of a volcano, the life cycle of volcanic ash is a continuous process due to the action of erosive processes (e.g., remobilisation by wind and/or water) on a large amount of loose volcanic material produced by explosive eruptions (Cas and Wright, 1987).Studies of recent eruptions [e.g., Katmai 1902, Alaska; Eyjafjallajökull 2010, Iceland; Hudson 1991, Cordón Caulle (CC) 2011 and Calbuco 2015, Chile] have demonstrated that syn- and post-eruptive aeolian remobilisation of volcanic ash exacerbates the impact of primary tephra fallout, and extends it to larger areas over prolonged periods of time (Bitschene, 1995; Hadley et al, 2004; Wilson et al, 2011; Thorsteinsson et al, 2012; Elissondo et al, 2016; Reckziegel et al, 2016; Forte et al, 2018). Volcanological studies of aeolian ash remobilisation primarily focus on associated impacts (Bitschene, 1995; Hincks et al, 2006; Wilson et al, 2011; Carlsen et al, 2015; Forte et al, 2018) or the characterisation of both the associated deposits (Hobbs et al, 1983; Liu et al, 2014; Miwa et al, 2018) and the physical processes through laboratory experiments (Douillet et al, 2014; Del Bello et al, 2018) and numerical modelling (Barsotti et al, 2010; Leadbetter et al, 2012; Folch et al., 2014; Reckziegel et al, 2016; Mingari et al, 2017) Despite this significant effort, no studies have yet correlated wind transport with deposition processes of loose volcanic particles. A first attempt to converge understanding of both mineral dust erosion and wind remobilisation of volcanic ash was undertaken by Langmann (2013), who compared the emission, atmospheric load and deposition of mineral dust versus volcanic ash
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