Pyroclastic flow deposits of the 1991 eruption of Volc�n de Colima, Mexico

Abstract

The April 16, 1991, eruption of Volcµn de Colima represents a classical example of partial dome collapse with the generation of progressively longerrunout, Merapi-type pyroclastic flows that traveled up to 4 km along the El Cordoban gullies (East, Central and West). The flows filled the gullies with block-and-ash flow deposits up to 10 m thick, of which, after 7 years of erosion, only remnants remained in the El Cordoban West and East gullies. The El Cordoban Central gully, however, provided a well-preserved and incised longitudinal section of the 1991 deposits. The deposits were emplaced as proximal and distal facies, separated by a change in slope angle from >30 to <20. The proximal facies consists of massive, clast-supported flow units (up to 1 m thick) with andesite blocks locally supported by a matrix of coarse ash and devoid of segregation structures or grading. The distal facies consists of a massive, matrixsupported deposit up to 8 m thick, which contains dispersed andesite blocks in a fine ash matrix. In the distal facies, a train of blocks marks flow-unit upper boundaries and, although sorting is poor, some grading is present. Thin, finely stratified, or dune-bedded layers of fine ash material are locally present above or below units of both facies. Sedimentologic parameters show that the size or fraction of large pyroclasts (larger than 1 f) decreases from proximal to distal facies, as the percentage of matrix (0 to 4 f) increases, especially immediately beyond the break in slope. We propose that the propagation of the Colima pyroclastic flows is critically dependent on local slope angle, the presence of erodible slope debris, and the decrease in grain size with distance from the vent. The progressive fining is probably caused by some combination of erosion, clast breakup and deposition of larger pyroclasts, and is itself influenced by the slope angle. In the proximal region, the flows moved as granular avalanches, in which interacting grains ground each other and erosion occurred to produce an overriding dilute ash cloud. The maximum runout distance of the avalanches was controlled by the angle of repose of the material, and the volume and grain size of source and eroded material. Because the slope angle is close to the repose angle for this debris, granular avalanches were not able to propagate far beyond the change in slope. If, however, an avalanche had enough mass in finer grain size fractions, at least part of the flow continued beyond the break in slope and across the volcano apron, propagating in a turbulent state and depositing surge layers, or in an otherwise settling-modified state and depositing block-and-ash flow layers.

Paleomagnetic determination of emplacement temperatures of pyroclastic deposits: an under-utilized tool

“…Intermittent activity has been observed since 1998, with vulcanian eruptions, lava flows and growing domes that have collapsed and generated pyroclastic density currents (Saucedo et al 2002;Zobin et al 2002;Saucedo et al 2004Saucedo et al , 2005. Thirteen localities were sampled from areas where pyroclastic eruptions occurred on June 2005 (VC1-7), January 1913 (VC8-11), and June 2004 (VC12-13).…”

Caracterización granulométrica de los depósitos de abanicos aluviales en la Cuenca de Motozintla, Chiapas, México: un peligro geológico latente por eventos de inundación

“…Este tipo de depósito es comparable con la matriz de los flujos de escombro de tipo cohesivo reportados por Capra y Macías (2002) en la presa el Naranjo del Nevado de Colima y en los depósitos de flujo de escombro de Pilcaya del Volcán Nevado de Toluca (Capra y Macías, 2000). La segunda causa de la presencia de fracciones finas en los depósitos se debe a eventos de precipitación pluvial intensa y la transformación de los flujos de escombros en flujos hiperconcentrados (Mulder y Alexander, 2001 cas en la barranca de Huiloac, al NE del volcán Popocatepetl (Franco-Ramos et al, 2017), donde se presentan flujos de escombros y distancia hasta su depósito (Caballero et al, 2006;Saucedo et al, 2004). Estos depósitos se caracterizan por presentar una mejor selección, con valores de Diámetro Medio (Mdφ) = 3.65 como máximo y con Desviación Estándar de (σφ) = 0.72 como valor mínimo; estos datos indican una selección moderadamente buena para estos depósitos.…”

Basaltic pyroclastic flows of Fuji volcano, Japan: characteristics of the deposits and their origin

“…The poor sorting, massive appearance, and valley-confined distribution of this facies are characteristic of deposition from a highly concentrated flow without formation of traction-related bedforms or sorting of different grain sizes by turbulence (Lowe 1982;Postma 1986). The massive facies occurs on the gentle (<20 degrees) valley bottoms and may represent a settling-modified flow; same matrix-supported massive facies is common to block-andash flow deposits (Davis et al 1978;Boudon et al 1993;Saucedo et al 2004). The debris flow deposits show smaller standard deviation and better sorting than the massive facies.…”