The 17 July 1999 block-and-ash flow (BAF) at Colima Volcano: New insights on volcanic granular flows from textural analysis D. Sarocchi a, ⁎, R. Sulpizio b,c , J.L. Macías d , R. Saucedo a a Instituto de Geología/Fac. Ingeniería UASLP, Dr. M. Nava No 5, Zona Universitaria 78240, San Luis Potosí, México b CIRISIVU, c/o Dipartimento Geomineralogico, Universitá di Bari, via Orabona 4, 70125, Bari, Italy c IDPA-Milan, Italy d Departamento de Vulcanología, Instituto de Geofísica, Universidad Nacional Autónoma de México, Coyoacán 04510, D.F., México abstract article info Article history: Received 29 December 2010 Accepted 18 April 2011 Available online 28 April 2011 Keywords: block-and-ash flows granular flows grain size quantitative textural analysis particle shape Colima Volcano On July 17 1999, a strong explosion occurred at Colima Volcano (Mexico) that produced a 10 km high eruptive column. The partial column collapse originated a block-and-ash flow (BAF) that flowed to the south, along the San Antonio and Montegrande ravines, travelling 3.3 km from the volcano summit. The flow filled the ravines with a volume estimated at 7.9 × 10 5 m 3 . The erosion of these deposits occurred between 1999 and 2002 (time of sampling), providing excellent longitudinal outcrops that allowed their detailed textural study. The study was carried out by means of quantitative textural analysis: (1) Rosiwal intersections, for carrying out vertical granulometric profiles; (2) total grain-size analysis, from -11 to + 9 ϕ; and (3) Fourier and fractal analysis of the particle morphology. Grain size and morphometric parameters obtained with these methods were used to identify vertical and longitudinal variation patterns in the BAF deposit. The grain size variations allowed to infer the main particle segregation mechanisms that acted during transport and deposition of the studied BAFs. The two methods used for studying the particle shape morphologies yielded results with different accuracy and reliability. In particular, fractal analyses have been found to be the most effective in describing the particle support mechanisms that acted during transport and deposition of the studied BAFs. The results highlight the importance of the information obtained by means of these techniques, and provide new insights in transportation and deposition mechanisms of BAFs. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Gravity-driven flows in volcanic areas comprise some of the most complex and hazardous natural phenomena, and can occur either during explosive eruptions (i.e. column collapse, sector failure, and dome or lava flow failure) or during volcanic quiescence (i.e. slope instability, climatic events, and earthquakes). They include generation of pyroclastic density currents, debris avalanches, and volcaniclastic flows, which have different dynamics of transportation and emplace- ment (Reubi and Hernandez, 2000; Iverson and Vallance, 2001; Branney and Kokelaar, 2002; Sulpizio et al., 2007; Shea et al., 2008). Volcanic gravity driven flows can be described as a continuum between two end members, which are the solid particles and the fluid (water, gas or both; e.g. Branney and Kokelaar, 2002). Based on the relative ratio between the two end members the flows are classified as concentrated (high particle concentration) or diluted (prevalence of fluid fraction; e.g. Sulpizio and Dellino, 2008). The comprehension of the physics of these natural phenomena is far to be satisfactory (e.g. Iverson, 1997; Bursik et al., 2005; Sulpizio and Dellino, 2008), and this faces with the need of detailing their behaviour. Important clues for the physical constraints of flow behaviour come from the study of their deposits, which yield precious information about flow dynamics at time of deposition (Branney and Kokelaar, 2002; Sulpizio and Dellino, 2008). Among volcanic gravity-driven flows the study of those char- acterised by high-particle concentration is exceedingly important, since they encompass some of the most destructive volcanic phe- nomena. In all these phenomena the same basic forces govern motion, but differing mixture compositions, initial and boundary conditions yield varied dynamics and deposits. Examples range from dry rock avalanches (Varnes, 1978; Hutchinson, 1988), in which pore fluid may play a negligible role, to liquid-saturated debris flows (e.g. Iverson, 1997) and gas-charged pyroclastic flows, in which fluids may enhance bulk mobility (e.g. Wilson, 1984; Druitt, 1998; Salatino, 2005). These phenomena have a very hostile nature, and their direct observation is usually limited or impossible. Movies and photographs have sometimes captured volcanic gravity-flows (e.g. YouTube video, 2009, 2010), yielding important clues about their macro-scale behaviour. However, poor or little information has been obtained about the physics that governs their internal behaviour. In recent years important advancements came from laboratory and large scale Journal of Volcanology and Geothermal Research 204 (2011) 40–56 ⁎ Corresponding author. Tel.: + 52 444 8171039; fax: + 52 444 8111741. E-mail address: damiano.sarocchi@uaslp.mx (D. Sarocchi). 0377-0273/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2011.04.013 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores