That Arizona hot spot might be a volcanic field
Deborah Painter
Let’s see, when I say “Arizona hot spots”, what might come to mind for many people are the restaurants, nightclubs and sports events in Phoenix (the US state’s largest city), the Grand Canyon of the Colorado River, attracting visitors from around the world, Tombstone (the infamous “town too tough to die”, where the equally infamous 1881 gunfight at the OK Corral took place), and any portion of the desert in the daytime during August.
But how many people think of the many volcanoes in Arizona USA, part of a volcanic field that is likely not finished erupting?
Arizona, USA has seven young (Quaternary Period) volcanic fields. The three youngest fields are the San Francisco, Uinkaret and Pinacate volcanic fields. The first two of these young fields are on the Colorado Plateau of northern Arizona; the Pinacate Field is much farther south on the Arizona-Mexico border.
The San Francisco Field is the focus of this article. It is situated near Flagstaff and Williams in northern Arizona (Fig. 1). It extends approximately 5,0002km from Williams to the Little Colorado River. There are slightly over 600 cones. The field was active as recently as 932 BP (Before Present), with the eruption that formed Sunset Crater at Sunset Crater Volcano National Monument.
The spectacular San Francisco Peaks within this field are originally a single stratovolcano that experienced deep erosion (Fig. 2). Mount Elden near Flagstaff is a large volcanic dome, but the majority, including Sunset Crater, is the ‘strombolian’ type of cinder cone. Most cinder cones one sees anywhere around the world average about 304m in height. However, there are some quite small ones in the San Francisco Volcanic Field averaging 180m high and 340m wide. One cannot miss them as one travels in the area.
These cinder cones have been quiet for hundreds of thousands and, in some cases, millions of years. Their eruptions were explosive with gas and plumes of lava, but of short duration, up to but not exceeding a year for each. The smaller ones are fragile, and some have already eroded away in just a few million years. Many are privately owned and are being mined for their volcanic rocks used in industry and as landscaping stone (Fig. 3).
Are the volcanoes here likely to erupt again? Might new ones form? The Arizona Geological Survey says “yes”. It has named the area as a current moderate geologic hazard, and anticipates that new eruptions will likely take the form of a small new cinder cone. When this might happen is not known.
Why are there volcanoes here?
Arizona is not along a subduction zone or a spreading zone. Arizona’s volcanic fields are all believed to be located near and above a hot spot in the mantle, since there is a definite eastward trend in the eruptive history. A hot spot is a large plume of mantle material that is more common on the ocean floor than within a continent. Better known continental hot spots in North America are the Yellowstone hot spot and the Snake River Plain hot spot. The motion of the plate (in this case the North American Plate) is west-southwest.
The volcanoes within the San Francisco Volcanic Field are older as one travels from east to west, as is also seen in the Yellowstone and the Snake River Plain hot spots.
The San Francisco Peaks are 3,851m at their highest elevation at Humphreys Peak. They command the scenery for many kilometres and are visible along much of north US 89 and Interstate 40 in the vicinity of Flagstaff and Williams. A stratovolcano is a mountain comprised of layers of basaltic rocks formed from relatively small eruptions of weak, pulsating fountains of lava from a single crater. In the case of Humphreys Peak, this occurred repeatedly over a period of approximately one million years. The last eruption occurred approximately 400,000 years ago. Hikers are attracted to Humphreys Peak, the highest point in Arizona and part of the Humphreys Summit Trail in the Coconino National Forest.
I was fortunate to visit the area on a March day in 2002. I think spring is definitely a good time to see the San Francisco Volcanic Field since the temperatures are pleasant and there might even be some snow. A visit to the San Francisco Volcanic Field would not be complete without a stop in the city of Flagstaff, a starting point on the Arizona side of the Grand Canyon for visitors who want to go there as well.
It is also a good central location for visitors to Wupatki National Monument, Sunset Crater Volcano National Monument, Walnut Canyon National Monument, the Painted Desert, and other sites (Figs. 4 and 5). Buffalo Park is a Flagstaff city park at 2400 North Gemini Road. The telephone number to reach the park staff is (928) 213-2300.
One can picnic under the piñon and ponderosa pines, and enjoy a view of Mount Elden. There are also hiking trails. This volcano, like O’Leary Peak near Sunset Crater, never exploded. Instead, the mountain is composed of layers upon layers of magma that cooled into a dome structure. Some faults allowed the high-silica magma to reach the surface, forming lava ‘lobes’. Mount Elden formed between 6,000 and 5,000 years BP.
Proceeding along US 89 North out of Flagstaff, my friends Terril Shorb, Yvette Schnoeker-Shorb and David Hawk shared with me a pleasant half hour trip of 19km to the Sunset Crater/Wupatki Loop Road. From there, it was three kilometres to Sunset Crater Volcano National Monument, past forests of pines that grow in jagged black basalt that, due to the relatively young age of the basalt and the aridity of the region, appears fresh. In reality, the eruption took place sometime around 932 BP and Native Americans were definitely present in the region at that time to witness it. Archaeologists have found lava from the Bonito Lava Flow at Sunset Crater that had the impression of maize cobs in it. Native Americans had used the cooling lava to cook ears of maize.
The Bonito Lava Flow extends for 3.2km around the base of the volcano and is 30.5m in thickness. The cinder cone itself is off limits to hikers (Fig. 6). It is 2,450m in elevation, taller than most cones in the area, but fragile, showing footprints easily.
In fact, the National Monument came into being because local activists feared for the integrity of the mountain. In 1928, a film production company wanted to place fire pots and set off dynamite within the crater and along its flanks to simulate a volcanic eruption for a motion picture to be named Avalanche. The local activists, led by Harold Colton, the founder of the Museum of Northern Arizona, implored President Herbert Hoover to set aside the volcano and surrounding areas as a National Monument to stop any future attempts at destroying part of the mountain.
Although hiking is not permitted on the volcano, visitors can walk an easy trail from the Visitors Center at 6082 Sunset Crater Road, Flagstaff to an area near the foot of Sunset Crater. The volcano is comprised of basalt with a summit of reddish coloured scoria (Fig. 7), mixed with gypsum, ferrohexahedrite and magnetite.
The last eruption scattered these minerals as a final flourish, hence the name of the mountain. Near the mountain are trails, such as the trail facing nearby O’Leary Peak, a lava dome (Fig. 8). To reach the National Monument by telephone, dial (928) 526-0502. It is best to call first due to occasional fires that could impact travel there.
Another 21.24km to the north on Loop Road is Wupatki National Monument, at 25137 Loop Road, Flagstaff. The telephone number for the Visitors Center is (928) 679-2365. The Visitors Center provides useful information on the town of Wupatki (Fig. 9). The tallest ‘house’ provided about 50 rooms, making homes for approximately 100 people. The red stone bricks comprising the houses are local sandstones known as Moenkopi Sandstone.
When visiting the site, I was struck by the closeness of the basalt and scoria to the structures and wondered “Did the lava flow halt, just missing homes already built or did the lava flow cool and harden and all eruptions cease prior to Native Americans settling the area?” The rangers explained that it was this latter scenario. The ancestors of the modern Zuni and Hopi people built this town over 800 years ago as a focus for their farming and trading.
The ashfall from Sunset Crater’s final eruption made the land around the volcano outside the Bonito Lava Flow very fertile. The people farmed (Fig. 10), hunted (Fig. 11), and traded with people from as far away as what is now modern day Mexico.
These people, from southern climes, introduced the concept of the ball field and a prominent round brick structure was built in a depression within the complex of houses. This was to provide all the traders diversions, recreation and perhaps even opportunities for making wagers.
Eventually, a prolonged drought brought about the end of Wupatki, since the people could no longer grow squash and other crops successfully here. They moved elsewhere with more water and left the town to the elements.
Near Wupatki was so much of that Moenkopi Sandstone in small and highly accessible ridges that I, a hopelessly addicted rock collector, was sorely tempted. I saved myself from potential trouble by telling myself. “No, Debbie, you cannot take them! This is Federal land!”
We took a look at the windblown nearby Painted Desert to the east of Wupatki National Monument as snow clouds gathered dramatically and the March afternoon was drawing to a close.
About the author
Deborah Painter is an ecologist and general environmental scientist. She lives in the United States.
References
Hanson, Sarah. 2009. Sunset Crater, a Cinder Cone Eruption that Impacted the Ancestral Puebloan Indians. Arizona Geology. Arizona Geological Survey: https://azgeology.azgs.arizona.edu/archived_issues/azgs.az.gov/arizona_geology/april09/article_sunsetcratervolcano.html
Hawaiian Volcano Observatory, January 25, 2007. United States Geological Survey. Volcano Watch: San Francisco Volcanic Field, Arizona’s Hotspot: https://www.usgs.gov/observatories/hvo/news/volcano-watch-san-francisco-volcanic-field-arizonas-hotspot
Houk, Rose. October 2, 2009. “America’s Best Idea: Sunset Crater Nearly Destroyed by Hollywood”. KNAU News Talk – Arizona Public Radio: https://www.knau.org/knau-and-arizona-news/2009-10-02/americas-best-idea-sunset-crater-nearly-destroyed-by-hollwyood
Lynch, D. J. September 1982. Volcanic Processes in Arizona. Field Notes of the Arizona Bureau of Geologic and Mineral Technology Volume 12 Number 3 pp. 1-12.
National Park Service Photo Galleries, Sunset Crater Volcano National Monument and Wupatki National Monument.
Rand McNally. 2018. DeLorme’s Arizona Atlas & Gazetteer. Garmin. 68 pages.
Schnoeker-Shorb, Yvette. Personal communication.
Shorb. Terril. Personal communication.