Before Guffey

The volcanic story of Florissant actually begins before the Guffey Volcanic Complex with a very large eruption of a caldera 80 kilometers (~50 miles) to the west of Florissant near modern Mt. Princeton about 36.7 million years ago. The eruption caused a pyroclastic flow of superheated ash, pumice, mineral crystals, and glass to race towards Florissant at 160 kilometers per hour (~100 miles per hour)! When this glowing cloud of extremely hot volcanic material quickly settled, cooled, and solidified, it formed what we call a "welded tuff", which scientists named the Wall Mountain Tuff. Two million years after the eruption that created the Wall Mountain Tuff, time and water had eroded much of the evidence and the landscape was beginning to recover, but not for long.

Two million years later

This time, only 25 kilometers (~15 miles) away from Florissant, the Thirty-nine Mile volcanic field which included the Guffey Volcanic Complex amongst others began to change the landscape once again. Eruptions similar to the Mt. St. Helens eruption in 1980 began to mark the area with volcanic domes, lava flows, lahars (volcanic debris flows), and pyroclastic eruptions. Lahars from the Guffey Volcanic complex flowing down the valleys towards Florissant would dam the valley, creating two generations of lakes, and ash from the volcanic complex would provide silica to encourage diatom blooms and add an extra layer of preservation for fossils already preserved by diatom mats.

See Ancient Guffey Volcano and Lake Florissant Dam!

In one short trip after you visit Florissant Fossil Beds National Monument, you can see one of the lahars that possibly formed to create Lake Florissant, and then travel to see what is left of the Ancient Guffey Volcanic Complex! The online brochure: Roadside Guide to the Volcanic Beginnings of Ancient Lake Florissant in HTML and Adobe Acrobat PDF will help guide you to the two more interesting stops outside the Monument.