Earth Notes: The Geologic Mystery of Red Mountain

Oct 23, 2019

About 30 miles north of Flagstaff sits one of the region’s most magnificent-yet-perplexing geological features. Red Mountain is a cinder cone that formed nearly 750,000 years ago in the San Francisco Volcanic Field. 

Red Mountain's distinctive amphitheater juts out of the pinyon, juniper and ponderosa-dotted landscape of the Coconino National Forest.
Credit Ryan Heinsius / KNAU

Most of its interior, however, is exposed forming an unusual rust-colored amphitheater with cliffs that tower nearly a thousand feet above the pinyon, juniper and ponderosa pine of the surrounding Coconino National Forest.

The area is accessed by a fairly flat 30-minute hike. At its base visitors can explore the intricate maze of what’s left of the formerly active cinder cone and see for themselves what all the debate is about.

The walls of Red Mountain rise nearly 1,000 feet above the forest floor.
Credit Ryan Heinsius / KNAU

It appears to have been scooped out of the landscape by a giant prehistoric backhoe. This volcanic oddity could easily be mistaken for an alien world, and doesn’t resemble anything else in the area. It’s lined with hundreds of 10-to-20-foot towers made of compacted cinders called hoodoos as well as other enigmatic spires and sculpted formations.

A maze of hoodoo formations made of compacted cinders lines the base of Red Mountain.
Credit Ryan Heinsius / KNAU

Geologists don’t entirely agree on how Red Mountain formed. Some say erosion ate away at the cinder cone eventually revealing its inner volcanic plumbing. But others believe a steam explosion blasted out the amphitheater, much like a pressure cooker that’s gotten way too hot. According to each camp, there’s evidence in the cinder layers to support their theories, and the truth could ultimately be a combination of the two. But many questions remain and scientists are far from closing the book on Red Mountain’s genesis.

The mystery of Red Mountain is likely to persist, deepening the allure of one of northern Arizona’s most intriguing and seemingly out-of-place natural features.