The San Juan Mountains, in southwestern Colorado, consist mainly of volcanic rocks that form the largest remnant of a major composite volcanic field that covered most of the southern Rocky Mountains in middle Tertiary time. Within the region, lies an abundance of caldera volcanoes which comprise the San Juan Volcanic Fields. There are approximately 15 calderas known in the San Juan Volcanic Fields, however it is possible that there are 2, or even 3 more, that exist in the region. The region began with many composite volcanoes that became active between 35 and 40 million years ago 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 15-18 caldera volcanoes in the region today.
La Garita Caldera, size: 35 x 75 km. The La Garita caldera was formed by one of the most explosive and energetic eruptions in earth's history. This eruption, which occurred about 27 million years ago, spewed more than 3,107 cubic miles (5,000 cubic km) of lava from its rim. To this day it is the only known eruption to reach a 9 on the VEI scale.