The Thirtynine Mile volcanic area, Central Colorado volcanic field, is an extinct volcanic area located in Park and Teller counties, Colorado, northwest of Cripple Creek and southeast of South Park.[1] The area was the site of significant volcanism in the Tertiary Period about 35 million years ago. Ashfall and lahars (mudflows) from the volcanoes created the conditions for fossilization at what is now Florissant Fossil Beds National Monument.
The field is named for Thirtynine Mile Mountain, a local peak that is composed of volcanic rocks from the field. The field also contains the Guffey volcanic center.[2]