San Francisco Volcanic Field

by Will Nickles and Pete Broccolo

Geologic Setting
North of Flagstaff, Arizona, lies the San Francisco Volcanic Field. This southern boundary of the Colorado Plateau covers 1800 square miles containing 600 volcanoes. This area lies within Arizona’s Coconino and Kaibab National forests. The basalts from the San Francisco Volcanic Field overlie the Moenkopi Formation.

Geologic History
The history of The San Francisco Volcanic Field began 6 million years ago. The fields were made by localized melting, called a “hot spot,” fixed deep within the Earth’s mantle. As the North American Plate moved slowly westward, more eruptions produced volcanoes in a line that stretches for 50 miles and is made of over 600 volcanoes (Preist, S.S. et. al., 2001). The eruption interval has gone from 1 per 17,000 years to 1 per 3,000 years occurred between 5 and 0.25 million years ago.” (Shoemaker, E.M. et. al., 1986). This setting has been more active as of recent and is continuing to migrate eastward. Meaning that another eruption will occur sometime in the future.

Volcano types
There are three different types of volcanoes present at The San Francisco Volcanic Field, cinder cones, stratovolcanoes, and lava domes.

The most common of these in the San Francisco Volcanic Field are cinder cones, which are relatively small and form quickly; within months to years. These volcanoes are built when gas-charged basaltic magma erupts up out of the volcano and fall back down. These lava blobs cool and fall down as dark volcanic rocks with cavities made by trapped gas bubbles. The resulting rock fragments are called “cinders” and are generally small. However, larger ciders are usually referred to as “bombs”. The fragments accumulate and build a cone-shaped hill. Lava flows in the field comprise mainly of Ah-Ah flows.

The only stratovolcano in the field is the San Francisco Mountain. Stratovolcanoes are formed by the accumulation of andesitic lava flows, cinders, ash, and volcanic mudflows.

The last type of volcano present in the San Francisco Volcanic Field are lava domes. “There are several lava domes on the field and they are characterized by very steep-sided bulbous domes made of the high silica content magmas, rhyolite, and dacite.” (Preist, S.S. et. al., 2001)

Personal Impressions
After visiting the Sunset Crater Visitor Center, we yet again got back into the vans to drive to our hiking destination. On our way we saw Ah-Ah lava flows on either side of the road along with various conifers giving you a “Pine Barrens” feel. This short drive leads to a self-guided loop trail. It’s a must to pick up a trail guide and explore an exciting volcanic landscape at the base of the Sunset Crater Volcano. On either side of the trail, basaltic lava flows are exposed and can be observed for the entire duration of the hike. Approximately halfway through the trail, the San Francisco Peak can be viewed. This point of view provides an excellent location to imagine the Stratovolcano that once stood prior to the eruption. What you see today is simply remnants, known as the San Francisco Peaks.

Reference Cited

Preist, S.S., Duffield, W.A., Malis-Clark, K., Hendley II, J.W., Stauffer, P.H., 2001. The San       Francisco Volcanic Field, Arizona. U.S. Geological Survey Fact Sheet. 017-01.

Shoemaker, E.M. Tanaka, K.L. Ulrich, G.E. Wolfe, E.W. 1986, Migration of Volcanism in the     San Francisco Volcanic Field, Arizona, Geological Society of America Bulletin, v. 97.      pg 123-141