Yellow, hydrothermally altered, lithic-dominated pyroclastic breccia deposits, having only a minor juvenile component, at Galeras volcano, Colombia, contain evidence that they were emplaced as hot, fluidized, true pyroclastic flows. The distribution of the flows around the caldera rim, their transport mechanisms inferred from grain-size analyses, and the presence of charcoal and coarse channel and fine overbank facies indicate that these deposits were emplaced as true pyroclastic flows. These deposits are the result of a regularly repeated eruption sequence, involving clearing of vent material as a prelude to more typical pumiceous pyroclastic flows. Recognition of the mode of origin of these deposits is important in terms of understanding Vulcanian-type eruption and flow- emplacement mechanisms. Failure to recognize the explosive eruption origin of these deposits at Galeras and other volcanoes could result in serious underestimation of potential hazards.

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