Diverse welding, crystallization, and structural features develop when a hot ignimbrite encounters external water, depending largely on volatile-rock ratios. Such processes are spectacularly documented by a regional ignimbrite where it ponded within an older caldera in the San Juan Mountains, Colorado (USA). Interaction of hot pyroclastic flows with moist underlying sediments or standing water in a stream valley or shallow-lakeshore environment produced megascale gas-escape structures, quenched adjacent tuff, inhibited welding, and generated nonplanar crystallization zones. This site provides a context for reviewing examples of ignimbrite-water interaction elsewhere.

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