page_title: San Juan volcanic field
section_title: Phases of Volcanism
hierarchical_section_title: San Juan volcanic field / Phases of Volcanism
caption_attribution_description: English: Cochetopa Dome, located in the Cochetopa Park Caldera in Saguache County, Colorado. The mountain is 3395 m (11,138 ft) high. It is located in the San Juan Volcanic Field.
context_page_description: 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. Within the region lie an abundance of caldera volcanoes, which comprise the San Juan Volcanic Fields. 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.
  The region began with many composite volcanoes that became active between 35 and 40 million years ago and were particularly eruptive in the time period around 35-30 million years ago. Around this time the activity changed to explosive ash-flow eruptions. Many of these volcanoes experienced caldera collapse, resulting in the fifteen to eighteen caldera volcanoes in the region today.
context_section_description: The San Juan volcanic field contains two phases of volcanism.
  The earlier volcanism is from the Oligocene age of the Paleogene Period. It consists of largely intermediate composition lavas and breccias. A few ash flow tuffs also constitute this phase of volcanism.
  The later volcanism is from the  Miocene to Pliocene in ages of the Neogene Period. It is basaltic in composition. It is usually interpreted as a partial melt of the lower crust that was erupted onto the surface.
caption: Cochetopa Dome (Cochetopa Caldera).
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