The San Juan volcanic field is part of the San Juan Mountains in southwestern Colorado. It consists mainly of volcanic rocks that form the largest remnant of a major composite volcanic field that covered most of the southern Rocky Mountains in the Middle Tertiary geologic time.[1] There are approximately fifteen calderas known in the San Juan Volcanic Fields; however, it is possible that there are two or even three more in the region.[1]
The region began with many composite volcanoes that became active between 35 and 40 million years ago, with peak activity in the time period around 35-30 million years ago.[1] Around this time the activity began to include explosive ash-flow eruptions.[2] Many of these volcanoes experienced caldera collapse, resulting in the fifteen to eighteen caldera volcanoes in the region today.[1]
^ abcdSteven, Thomas A.; Lipman, Peter W. (1976). "Calderas of the San Juan Volcanic Field, Southwestern Colorado". U.S. Geological Survey Professional Papers. Washington, DC: U.S. Government Printing Office. 958: 1–35. Retrieved 2012-05-16.
^Lipman, Peter W.; Steven, Thomas A.; Mehnert, Harald H. (1970-08-01). "Volcanic History of the San Juan Mountains, Colorado, as Indicated by Potassium–Argon Dating". Geological Society of America Bulletin. 81 (8): 2329–2352. Bibcode:1970GSAB...81.2329L. doi:10.1130/0016-7606(1970)81[2329:VHOTSJ]2.0.CO;2. ISSN 0016-7606.
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