Size and style of the November 3, 2002 eruption of El Reventador volcano (Ecuador) assessed from tephra studies

On November 3, 2002 El Reventador volcano (Ecuador) erupted violently, causing damages near the volcano and intense ash falls in the Interandean Valley. To assess the size and style of the eruption we conducted field work to obtain tephra thickness measurements and samples in the area located ∼50–100 km from source. The thickness data (max. 40 mm) were processed to draw an isopach map that reveals a plume dispersal to the WSW of the volcano. The ash samples underwent grain size distribution (GSD) and componentry analyses, morphometric characterization, bulk and solid density determinations, and chemical analyses. The GSD results show a single mode at 3–4 ϕ in the study region, indicating a finely comminuted tephra. The componentry in the deposits reveals strong density-driven fractionation of dominant poorly to highly vesiculated juvenile ash particles, and of abundant xenolithic grains. The bulk ash density was close to 1017 kg/m 3 , whilst the solid (powder) density is ∼2640 kg/m 3 , and the juvenile ash bears the composition of the most differentiated products documented in historical times at El Reventador (silicic andesite: 58-59 wt.% SiO 2 ). The bulk volume of the tephra fall deposit is estimated at 80 × 10 6 m 3 , and the total volume with PDC deposits at 135 × 10 6 m 3 (VEI4 range), while the mass-based magnitude and intensity reach 4.2 and 10.8, respectively, in the range of subplinian eruptions. Accordingly, on November 3, 2002 El Reventador volcano released the largest explosive event in the whole North Volcanic Zone of the Andes since more than a century. Also, our study shows the importance of obtaining urgently ground-based data after any unexpected eruption for scenario elaboration, and that there is still room to technically improve the early characterization of such sudden explosive events.