The size distribution of particles in pyroclastic fall deposits has been interpreted as a strong indication of efficiency of the fragmentation process during the parent explosions. Fall deposits, however, contain two distinct populations of pyroclasts: juvenile, i.e., magmatic clasts fragmented from the magma, and wall-rock particles derived from walls of the vent and volcanic conduit. While sizes and shapes of the former are always sensitive indicators of eruptive processes, properties of the wall-rock population may reflect various external influences. Here we describe how the wall-rock component of a powerful historical eruption largely inherited its size distribution from properties of the host rocks, and how this, in turn, strongly biases the entire deposit grain size distribution of the near-vent deposits. The use of wall-rock particles to infer style of fragmentation and eruption must always be coupled with other independent lines of evidence.

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