By Doug Nichols
Recently, fireworks have been heard around town, but they were nothing compared with Colorado’s natural fireworks — volcanoes. Volcanoes in Colorado? Yes! Well, not recently, but definitely in the geological past.
The most recent volcanic eruption in Colorado took place about 4,200 years ago near the present day town of Dotsero, by the confluence of the Colorado and Eagle Rivers, in the north-central part of the state. Interstate 70 cuts across the lava flow. The Dotsero eruption created what is called a maar, a broad, low-relief crater formed by a shallow explosive eruption. The maar is 700 meters (2,300 feet) wide by 400 meters (1,300 feet) deep. In addition to a three-kilometer- (1.86 mile) long lava flow, the Dotsero eruption formed a lahar, a mudflow composed of volcanic materials. The eruption also created small cones of scoria (dark, crusty lava) along a NNE-SSW line on either side of the maar. There were lots of natural fireworks here!
For a really huge fireworks display, however, consider the La Garita Caldera located in the San Juan Mountains in southwestern Colorado, west of the town of La Garita. A caldera (from the Spanish word for caldron) is a large, basin-shaped volcanic depression formed by collapse during an eruption and having a diameter many times greater than the volcanic vent within it. Geologists believe that the eruption that created the La Garita Caldera was, perhaps, the largest known explosive eruption in all of Earth’s history. The La Garita eruption happened 26 to 28 million years ago, and the area devastated by the eruption covered a significant portion of Colorado. Ash could have fallen as far as the east coast of North America. The resulting deposit, known as the Fish Canyon Tuff, has a volume of approximately 5,000 cubic kilometers (1,200 cubic miles). The caldera is quite large, 35 by 75 kilometers (22 by 47 miles). The La Garita eruption was possibly the most energetic event on Earth since the asteroid impact at Chicxulub, Mexico, which caused the extinction of the dinosaurs.
Earlier still, a series of volcanic eruptions took place in the Thirtynine Mile Volcanic Field located in Park and Teller Counties, Colo., northwest of Cripple Creek and southeast of South Park. This area was active about 35 million years ago. Many volcanoes erupted intermittently in the Thirtynine Mile field; these were violent eruptions similar to that of the much more recent Mount St. Helens in Washington State. Falls of volcanic ash and lahars from the Guffey volcano in the Thirtynine Mile field created the conditions for fossilization at what is now Florissant Fossil Beds National Monument, west of Colorado Springs. A lahar formed a dam across an ancient drainage south of the monument area, resulting in the formation of ancient Lake Florissant. Ongoing ashfalls gradually filled the lake, trapping and exquisitely preserving fossils of leaves and flowers from plants that grew around the lake, and insects, including butterflies. The national monument also features petrified stumps of gigantic ancestral redwood trees, another consequence of Colorado’s volcanic history.
Doug Nichols was a Scientist Emeritus with the U.S. Geological Survey and a Research Associate with the Denver Museum of Nature & Science. He was a resident of Berthoud. We mourn Doug’s untimely passing in Jan. 2010.