Grain size distribution and sedimentology in volcanic mass-wasting flows: implications for propagation and mobility
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ORIGINAL PAPER
Grain size distribution and sedimentology in volcanic mass‑wasting flows: implications for propagation and mobility Symeon Makris1 · Irene Manzella1 · Paul Cole1 · Matteo Roverato2 Received: 22 November 2019 / Accepted: 8 July 2020 © Geologische Vereinigung e.V. (GV) 2020
Abstract The sedimentological characteristics of mass-wasting flow deposits are important for assessing the differences between phenomena and their propagation and emplacement mechanisms. In the present study, nine volcanic debris avalanche deposits and eight lahar deposits are considered, from the literature. Their sedimentology is expressed in the descriptive statistics: median grain size, sand, gravel and finer particle fractions, skewness and sorting. Analysis of the data confirms that lahars and debris avalanches diverge in their grain size distribution and in their evolution during propagation. Water saturation in lahars is the main factor enabling debulking, a mechanism that is not recorded in the data derived from debris avalanches deposits. On the contrary, evidence of comminution of particles due to particle-particle interactions is observed in debris avalanches, and not in lahars. These findings support previous studies suggesting that although water content in debris avalanches plays a role in propagation, the effects of inertial collision of solid fragments are more important than fluid effects, confirming that particle-particle interactions are the main factor influencing the mobility of non-saturated mass wasting flows. Keywords Debris avalanche · Runout · Volcanic · Lahar · Grain size distribution
Introduction The long runout of large mass-wasting flows was first reported by Heim (1882) and have subsequently been further studied in diverse settings, even extraterrestrial, by several authors (including but not limited to: Hsü 1975; Davies 1982; Siebert 1984; Glicken 1991; Corominas 1996; Legros 2002; Hungr and Evans 2004; Davies and McSaveney 2012; Manzella and Labiouse 2013; van Wyk de Vries and Delcamp 2015). The mobility of both volcanic debris avalanches (VDA) and non-volcanic debris avalanches (DA) is far greater than what would be predicted by simple frictional models (Legros 2002). This is commonly expressed by small apparent coefficients of friction, i.e. the H/L ratio, initially introduced by Heim (1932), between elevation loss (H) and runout in the direction of flow (L) during propagation (Scheidegger 1973; Hsü 1975). This coefficient of friction is * Symeon Makris [email protected] 1
School of Geography, Earth and Environmental Science, University of Plymouth, Plymouth, UK
Department of Earth Sciences, University of Geneva, Geneva, Switzerland
2
used in literature as a measure of mobility of VDAs and DAs (e.g. Shreve 1968; Erismann 1979). Simple frictional models would predict values of ̴0.5–0.6 for DAs and VDAs, however, they typically exhibit H/L values of 0.1–0.2 (see Fig. 1 and Table 2) (Scheidegger 1973; Hsü 1975; Davies 1982; Ui 1983; Legros 2002; Dufresne 2009). Although ma
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