Colorado’s Volcanoes

Our Neighborhood Peak

Feb 19 - When someone says “Colorado” and “geography,” most people will think of the Rockies but not volcanoes. While Colorado has no active volcanoes, there has been volcanic activity all along the Rockies.  Most of them are extinct, meaning they won’t erupt again.  One area has dormant volcanoes. This area could at some point become active again.

What some geologists believe to be the largest volcanic area on the planet was in what is now the San Juan Range in the southwest corner of the state.  This area was active from about 40 million years ago to 5 million years ago. The area is about 22 miles wide and 66 miles long. The remaining evidence of volcanic activity can be seen in the fifteen plus calderas located within the San Juans as well as ash and lava deposits. A caldera is a large crater formed by volcanic activity. The most well know caldera is the one in which Yellowstone National Park is located.

Moving north and east, near Colorado Springs, Florissant Fossil Bed National Monument is a park dedicated to homesteads of the area’s first European settlers and the fossilized redwood stumps they encountered. These stumps were encased in silica rich volcanic mud under hot acidic anaerobic conditions. Over time, the wood ‘absorbed’ minerals from the surrounding mud. I found this site that provides humorous Do-it-Yourself instructions for fossilizing a tree. I particularly like the part where they instruct the reader to give your tree some time to petrify and encourages the reader to, “set an alarm and check back once every million years” to view the progress.  

If you’re looking for more recent volcanic activity, there is a cluster of dormant volcanoes near Dotsero, CO.  These last erupted around 4,200 years ago, which is yesterday in geologic time. The lava flow it produced was around 2 miles long and crosses what is now I-70. We drive through there and never noticed it. I’ll know to look for it next time!

Closer to home, Hahn’s Peak, located north of Steamboat Springs, is also an extinct volcano. And, I think, the most interesting of the areas I’m writing about. Around 3 to 5 million years ago, the eruptions deposited a 400-foot think layer of rock and ash just north of the peak.  As the volcano cooled, cracks formed around the rock core so that it fell back into the volcano. This blocked the main vent effectively sealing itself off. Erosion then washed the dirt and ash from around the peak leaving the solid cone as the obvious visual clue of its existence.

View from Mount Werner

Several million years later, gold was discovered in the area that is now Denver. Prospectors traveled up stream looking for gold in the gravel along the creek banks. Flakes of gold and occasionally nuggets were found but the source of the gold that washed downstream was never found. Many of the prospectors of the time thought that Hahn’s Peak was the source of the gold. Geologic surveys over the years have identified areas around Hahn’s Peak with significant quantities of low quality silver-lead-zinc ore.  The location and quality of the ore indicates mining would not be profitable. No gold was identified by the surveys.

There are several theories about the origin of the gold.  Either the volcanic eruption pushed gold from deep in the earth to the surface, or the original vein of gold was near the surface and has completely eroded, washing the gold downstream. Remember before, I said the dirt and ash for the top of the volcano wash away. It used to be even taller!

While Hahn’s Peak erupted and was been given credit for bringing gold to Colorado’s surface, our local volcano, shown at the top of the page, doesn’t appear to have done more than push rock up then spit gas and small bits of lava. It has steep 30 to 40 degree incline typical of volcanoes and is only 470 feet from valley floor to peak. If it had erupted, instead of being very pointy, it would have a funnel shaped crater at the top. As far as we can tell, it is unnamed but at least one of the locals calls it Gibraltar. And when I call it the neighborhood volcano, it’s less than 0.5 mile from our house.

Being extinct or dormant doesn’t mean that there is no activity. Evidence of heat is just below the surface all along the Rockies. As you drive around the western slope, particularly on a cold day, you can spot geothermal areas. These are visible by the patches of melted snow and rising vapor that can completely obscure the road in fog heated by geothermal activity. There are also developed hot springs that attract locals and tourists to take a hot swim. Steamboat Springs has two developed springs, Old Town and Strawberry Park. There are additional springs in the town’s parks.

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 *Geology Underfoot on Colorado’s Western Slope by J. Shroder et. al. 2022.