New report describes Yellowstone's volcanic and hydrothermal hazards

Porkchop Geyser, in the Norris Geyser Basin, formed by a hydrothermal explosion in September 1989 (Fournier and others, 1991). Blocks of silica sinter thrown from the geyser's throat form a berm around the new crater. The silica will likely continue to dry out and disaggregate over time, subduing evidence of the explosive activity. Photo by Jim Peaco, 2003 (NPS stock photo).