Fish Canyon Tuff Explained

Fish Canyon Tuff eruption
Date:	About 28 million years ago
Location:	Colorado, United States
Coordinates:	37.7564°N -106.9342°W
Vei:	8

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 .^[1] (see List of largest volcanic eruptions). The Fish Canyon Tuff eruption was centred 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 (SiO₂=bulk 67.5–68.5% (dacite), matrix 75–76% (rhyolite) and consistent phenocryst content (35–50%) and mineralogical 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^[2] and the Oligocene Southern Rocky Mountain ignimbrite flare-up.^[3]

The Fish Canyon Tuff eruption occurred around 28 Million years ago. Sanidine crystals from the Fish Canyon Tuff (FCTs) are used as a reference mineral in ⁴⁰Ar-³⁹Ar dating, and the current 'astronomically calibrated' age for the FCTs is 28.175 Ma.^[4]

See also

• List of large volume volcanic eruptions in the Basin and Range Province

References

1. Bachmann . O. . The Fish Canyon Magma Body, San Juan Volcanic Field, Colorado: Rejuvenation and Eruption of an Upper-Crustal Batholith . Journal of Petrology . August 2002 . 43 . 8 . 1469–1503 . 10.1093/petrology/43.8.1469 .
2. Geologic map of the central San Juan caldera cluster, southwestern Colorado . 2006 . 10.3133/i2799 .
3. Lake . Ethan T. . Farmer . G. Lang . Oligo-Miocene mafic intrusions of the San Juan Volcanic Field, southwestern Colorado, and their relationship to voluminous, caldera-forming magmas . Geochimica et Cosmochimica Acta . May 2015 . 157 . 86–108 . 10.1016/j.gca.2015.02.020 . 2015GeCoA.157...86L .
4. Phillips . D. . Matchan . E.L. . Dalton . H. . Kuiper . K.F. . Revised astronomically calibrated 40Ar/39Ar ages for the Fish Canyon Tuff sanidine – Closing the interlaboratory gap . Chemical Geology . May 2022 . 597 . 120815 . 10.1016/j.chemgeo.2022.120815 . 2022ChGeo.59720815P .