Geologic Formations

O'Leary Peak, seen from the colorful rim of Sunset Crater Volcano

The cinder cone we know as Sunset Crater Volcano serves as the centerpiece for an amazing display of volcanic features in and around the national monument. It shares the horizon with hundreds of other cinder cones, and examples of two other volcano types. Nearby O’Leary Peak is a lava dome, a rounded mountain formed from repeated, piled-up lava flows. The San Francisco Peaks, the often-snow-capped mountain just north of Flagstaff, is the only stratovolcano in the San Francisco Volcanic Field. Stratovolcanoes form from accumulated layers of lava, cinders, ash, and mudflows; they are the steep-sided, pointed-top volcanoes that most people think of when they hear the word.

The processes that created Sunset Crater Volcano also created a sculpture garden of extraordinary forms around its base. As new gas vents opened suddenly, spatter cones sprouted from the ground like miniatures of the cone itself. Moving lava developed a crust on the surface as it cooled; caves and lava tubes formed beneath the crust as the lava drained away. Partially cooled lava, pushing through cracks like toothpaste from a tube, solidified into wedge-shaped squeeze-ups, grooved from scraping against the harder rock. Hawaiian terms are used to describe the different types of lava flows: Aa lava has a rough, jagged surface, while pahoehoe lava looks smooth or ropy.

All of these features, and more, can be seen along the Lava Flow Trail. For a quick sample, take a Virtual Field Trip.