Fish Canyon Tuff
The Fish Canyon Tuff is the large volcanic ash flow deposit resulting from one of the largest known explosive eruptions on Earth, estimated at 1,200 cu mi (5,000 km3). (see List of largest volcanic eruptions) The eruption was centered at La Garita Caldera in southwest Colorado. The tuff can be assumed to belong to one eruption due to its high chemical consistency (SiO2=bulk 67.5–68.5% (dacite), matrix 75–76% (rhyolite) and consistent phenocryst content (35–50%) and composition (plagioclase, sanidine, quartz, biotite, hornblende, sphene, apatite, zircon, Fe-Ti oxides are the primary phenocrysts). This tuff and eruption is part of the larger San Juan volcanic field and Mid-Tertiary ignimbrite flare-up.