When ignimbrite meets water: Megascale gas-escape structures formed during welding

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 ponded within an older caldera in the San Juan Mountains, Colorado. Interaction of hot pyroclastic flows with moist underlying sediments or standing water in a stream valley or shallow-lakeshore environment produced mega-scale 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.