San Juan volcanic field

San Juan volcanic field
San Juan volcanic field Location in Colorado
Highest point
Coordinates	37°53′36″N 106°46′28″W / 37.89333°N 106.77444°W / 37.89333; -106.77444Coordinates: 37°53′36″N 106°46′28″W / 37.89333°N 106.77444°W / 37.89333; -106.77444
Geography
Location	Colorado, United States
Geology
Mountain type	Volcanic field

The San Juan volcanic field is part of the San Juan Mountains in southwestern Colorado. It 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 the Middle Tertiary geologic time. Within the region lie 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 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.

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.