Fish Canyon Tuff
The Fish Canyon Tuff is the large volcanic ash flow or ignimbrite deposit resulting from one of the largest known explosive eruptions on Earth, estimated at . The Fish Canyon Tuff eruption was centered at the La Garita Caldera in southwest Colorado; the caldera itself would have formed by collapse, as a result of the eruption. Studies of the tuff show that it all belongs to one eruption due to its uniform bulk-chemical composition, matrix 75–76% and consistent phenocryst content and mineralogical composition. This tuff and eruption is part of the larger San Juan volcanic field and the Oligocene Southern Rocky Mountain ignimbrite flare-up.
The Fish Canyon Tuff eruption occurred around 28 Million years ago. Sanidine crystals from the Fish Canyon Tuff are used as a reference mineral in 40Ar-39Ar dating, and the current "astronomically calibrated" age for the FCTs is 28.175 Ma.