Geology of the San Juan Mountains
Around 30 million years ago, this was an area of huge volcanos and calderas. Some of the largest eruptions in the known history of the Earth covered the region in thick lava and ash flows. The lava, ash, and intrusive igneous rock in between were subsequently uplifted, almost completely covered in icefields during the Ice Age, then carved by glaciers into the mountains that we see today. This volcanic history gives the San Juans their unique and rugged characteristics that distinguish them from most other mountain ranges in the Colorado Rockies.
The broader San Juan Mountains consist of numerous sub-ranges including the remarkable Sneffels Range, Grenadier Range, Needle Mountains, San Miguel Range, and more. Large portions of these mountains are protected within the Weminuche Wilderness and Uncompaghre Wilderness areas, among others.
Although I've posted about 200 photos from the San Juans in this gallery, I have nearly 1000 photos from the San Juans in the search archives if you want to see more! You can also read lots of trip reports from the San Juans here.