Ash flows follow explosive volcanic eruptions and occur when dense ash sinks and flows down the flanks of an erupting volcano. These scorching hot flows can travel hundreds of miles per hour, knocking down trees and obliterating anything else in their path. Ash flows can form volcanic igneous rocks such as welded tuffs, where the incredible temperature of the flow causes ash to fuse together. Ash flows may also contain various sizes of other pyroclastic material.
Following an epic period in the region’s geologic history, Colorado now has ash-flow tuffs that cover thousands of square miles in the southwestern part of the state. Between 25 and 37 million years ago when ash flows were erupting with gusto, it is estimated that two-thirds of the state was covered with ash flow deposits. The Fish Canyon Tuff surrounding the present-day site of Creede, Colorado is one of the largest ash flows in the world. It contains approximately 1,200 cubic miles of material that was deposited red hot and has a welded zone more than a half-mile thick.
Outcrops of the 1,200-cubic-mile deposit of Fish Canyon Tuff are well displayed along Highway 160 southwest of South Fork.