Rob Blair

A two-volume set, published in 1996 (The Western San Juan Mountains) and 2011 (The Eastern San Juan Mountains). Each volume is a collection of essays on the geology, ecology, and history of topic area; the eastern volume covers the area from Alamosa to Durango east-west and Gunnison to Chama, New Mexico north-south; the western volume covers Lake City to Cortez east-west and Ridgway to Durango north-south; obviously some overlap. Since I’m doing a geology trip to the area latter this year (2018), I was interested in the geology chapters. The San Juan Mountains of Colorado have one of the largest collections of caldera volcanoes in the world (somewhere around 16 of them; the exact count isn’t clear since some of the later eruptions destroyed earlier evidence); these volcanoes were responsible for the Oligocene Ignimbrite Outbreak, which dumped volcanic ash all over western North America. The La Garita Caldera was responsible for the Fish Lake tuff, about 5000 km³ of volcanic ash (to put that in perspective, the Mt. St. Helens eruption of 1980 produced about 0.25 km³); this is the largest volcanic eruption known; in fact, the United States Geological Survey had to add a new level to the Volcano Explosivity Index (level 9) just for La Garita.

The geology chapters are very good, they use the current consensus explanation for the San Juans (shallow angle subduction of the Farallon Plate, starting in the Triassic) while noting that there are alternative explanations. The nongeology chapters, while usually interesting, are uneven and sometimes seem to be added as afterthoughts. Each book closes with a geological/ecological/historical road log for the area.… (más)