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pyroclastic flow (ash-flow) General term for a hot, high-concentration flow of pumice or lithic clasts, entrained and transported in a fluidized ash matrix. Pyroclastic flows include a wide range of phenomena, from ignimbrites (large volume, pumiceous) to block-and-ash flows (small volume, lithic). The flow originates by the gravitational collapse of a dense, turbulent, eruption column at the source vent and moves down-slope as a coherent flow. Fluidization of the ash matrix, which contributes to the high mobility of such avalanches, is achieved by (a) the diffusion and release of gas during breakage and the attribution of ash and pumice particles entrained within the flow and (b) air ingested and compressed at the front of the advancing flow margin. Where the ash matrix is the dominant component, the term ‘ashflow’ is applied by American authors, although British authors prefer to use the term ‘pyroclastic flow’.