We report particle-size distributions for andesitic ash fall from vertical eruptions and clouds above block-and-ash flows at Volcán de Colima, Mexico, during 2004–2006. We analyzed 17 samples using laser diffraction: 11 from vertical eruptions and 6 from block-and-ash flows (co-pyroclastic flow ash). Vertical eruptions produce well-sorted fall deposits, whereas co-pyroclastic fall deposits are poorly sorted, with high proportions of very fine grained ash (<30 μm). Statistical analysis shows particle-size distributions of vertical eruptions are more leptokurtic (peaked) than co-pyroclastic samples. Deconvolution of grain-size histograms shows that co-pyroclastic samples have at least one subpopulation with a mode of 8.3–8.7 ϕ (2.4–3.1 μm). Estimates of the number of particles in different size ranges show that co-pyroclastic samples contain much greater numbers of very fine particles than vertical eruption samples. Our results provide no direct evidence that milling or comminution produces hazardous fine ash particles in pyroclastic flows, but are consistent with that interpretation.

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