Title: The Colorado Geological Survey Asks, “Did You Know?”
Author: Vince Matthews
Publication: The Outcrop, September 2001, p. 22
Did you know that Colorado is one of the premier areas in the world for studying calderas? Calderas form when large ash flows spew out of the earth creating voids that the overlying rocks collapse into, thus creating circular or oval structural depressions. These ash flows came out hot (1100-1300 degrees F.), traveled up to 75 miles per hour, and spread as far as 90 miles from their source in Colorado.
Colorado has at least nineteen calderas including one af the world’s largest, La Garita. It is so large (22 by 47 miles) that for a long time it was hard for geologists to realize that they were mapping a giant caldera (sort of like discovering a giant oil field and not realizing it until well into development). Several younger, smaller calderas are nested within the La Garita caldera. The ash flow that caused La Garita’s collapse is also one of the world’s largest, at ~1200 cubic miles of material. This Fish Canyon ash-flow tuff has a welded zone nearly one half-mile thick.
Colorado’s calderas formed during ash flow eruptions between thirty-five and twenty-six million years ago. They are concentrated in the San Juan Mountains, but are also present in the Sawatch Range. The towns of Creede and Silverton both lie within the walls of large calderas. Rich ores are often associated with Colorado’s calderas.