FIG. 1: Digital elevation map of the San Francisco
Volcanic Field (Photo courtesy of the USGS)
sunset crater volcano
a cinder cone eruption that impacted the ancestral puebloan indians

The San Francisco Volcanic Field lies on the southern margin of the Colorado Plateau near its boundary with the Basin and Range Province (Figure 1). This field, covering an area of ~5000 km2, extends eastward in an irregular belt from near Williams, Arizona, to the Little Colorado River just east of Flagstaff. Volcanism began approximately 6 million years ago in the western part of the field and has since migrated eastward producing over 600 volcanic vents. Activity culminated with the eruption of Sunset Crater Volcano (Figure 2) just over 900 years ago. This 300 m (1000 ft) tall cinder cone is now preserved by the National Park Service, one of three monuments administered by the Flagstaff Area National Monuments. The other two parks, Wupatki and Walnut Canyon National Monuments preserve archaeological features.

A thousand years ago, the landscape in and around Sunset Crater Volcano National Monument was different than it is today. The prominent cinder cone had not yet formed and the stark lava flows had yet to pour onto the surface. Instead a forested valley was occupied by settlements of hunters and gatherers that lived in pit houses and grew corn to supplement their diet. Archaeologists refer to these people as the Sinagua, which is a contraction of the Spanish words “sin” and “agua” meaning without water. The Sinagua were the ancestors of the Pueblo Indians of the Four Corners area, including the Hopi Indians, who refer to them as Hisatsinom (those who lived long ago). The lives of these people were profoundly changed in the late 11th century when a volcano was born.

FIG. 2: Sunset Crater Volcano, ~900 year old cinder cone
The exact date for the onset of eruptive activity at Sunset Crater remains elusive. Based on the presence of diminished tree ring widths from roof beams found in pueblo ruins at Wupatki National Monument, Smiley (1958) suggested the initial eruption occurred during the winter of AD 1064/1065. Correlating these dates to the eruption of Sunset Crater is suspect, however, as the provenance of these trees cannot be determined and drought, fire, and insect infestation could have produced diminished rings (Ort et al., 2002). Early paleomagnetic surveys were not able to confirm the initial eruption date but suggested that it continued for just over 100 years, concluding with the Bonito lava flow in AD 1180 (Champion, 1980; Shoemaker and Champion, 1977). More recent paleomagnetic studies show the onset of the eruption occurred between AD 1040 and AD 1100 (Ort et al., 2002). 

The ~115 year duration has also been reevaluated because cinder cones typically erupt only once, and for much shorter durations. Extensive field examinations reveal no weathering and/or erosion horizons within the layered blanket of fallout cinders suggesting that the pulses of activity all occurred within a short period of time. The lack of geochemical variation between the cinder eruptions and lava flows (Hanson et al, 2008), as well as the recent paleomagnetic and dendrochemical studies (Ort et al., 2002), are also consistent with a much shorter eruption duration. A later eruption of shorter duration also correlates with population increases at pueblo villages such as Wupatki (~AD 1100), which may be related to enhanced agricultural conditions subsequent to the eruption of Sunset Crater. Thus, the eruption that formed Sunset Crater Volcano may have begun several years later than originally estimated and continued for a much shorter time, perhaps as little as a month or two, or as long as a few years (Ort et al., 2002).

FIG. 3: The Bonita Lava Flow
Regardless of the date, eruptive activity began along a 10 km (6 mi) long NE-SW trending fissure that produced a series of coeval vents including Sunset Crater, Rows of Cones, Gyp Crater, and Vent 512. This fissure eruption ended shortly after it began as activity became focused on the northern end of the fissure. Strombolian eruption built Sunset Crater Volcano and blanketed an area of at least 2000 km2 with cinders. Two lava flows, the Kana’a and the Bonito flows, effused from the base of the cone. The Kana’a flow extruded from the eastern base and continued in a northeasterly direction along Kana’a wash for about 5 miles (8 km) (Holm and Moore, 1987). The Bonito flow (Figure 3) effused from the northwestern base of the volcano and, at least in a broad sense, resulted from three pulses of activity (Holm, 1987) with little, if any, time elapsing between these events. Just west of the vent, lava ponded in a low area accumulating lava to a depth of perhaps over 30 m in the center to only a few meters along the margins (Moore, 1974 and Holm, 1987). This flow carried (rafted) mounds of red oxidized agglutinate (pieces of the early cone) several hundreds of meters to the west and northwest (Holm, 1987). Continued lava fountaining filled in the “holes” created by rafting, and ultimately produced a nearly symmetric cone (Hanson et. al, 2008a).

FIG. 4: Corn Rock (Photo Credit:  Helga Teiwes, courtesy of Desert Archaeology, Inc.)
During the waning stages of the eruption, cinders on the rim were oxidized to a red scoria and cemented by silica, gypsum and iron oxide. Five distinct fumarole incrustations are present near the central vent of the volcano. These incrustations are composed predominantly of gypsum which is locally overgrown with opal or a thin coating of sulfur. Numerous accessory minerals, including voltaite, jarosite, magnetite and hematite, occur throughout (Hanson et al., 2000 and Hanson et al., 2008b). One mineral, ferrohexahedrite, is new to the state of Arizona.

This eruption was most certainly witnessed by the ancestors of today’s Pueblo Indians. The lava fountain would have been visible from quite a distance, perhaps 30 to 50 kilometers, and the ash plume for hundreds of kilometers (Elson and Ort, 2003).  Even today Sunset Crater is sacred to many of the areas indigenous people. The volcano is home to one of the Hopi spirit beings, the Kana’a Katsina and stories of the eruption are embedded in Hopi oral history. There is physical evidence as well. Archaeologists have excavated basalt cobbles with corn impressions from nearby pit houses. These “corn rocks” (Figure 4) were likely deliberately formed by placing corn on the rim of a hornito and allowing the spatter to cover it (Elson, et al., 2002; Elson and Ort, 2003). 

FIG. 5: Pueblo ruins at Wupatki National Monument.
(Photo courtesy of Victoria Allen)

After the eruption was over, in ~ AD 1100, a thriving community began to flourish 30 km (20 mi) to the north at Wupatki (Figure 5). Archaeologists have suggested this rapid growth may be related to the volcanic eruption. Agricultural areas near the volcano were covered with a thick layer of cinder and ash that made it unsuitable for growing crops. Further from the vent, farming may have been enhanced by a thin ash layer that acted as a water retaining mulch (Hooten, et al., 2001; Ort et al., 2002). Thus, the eruption may have caused a shift in the population to places such as Wupatki where farming in this arid climate may have been temporarily enhanced.

Future volcanic activity in this area is certainly possible. While there is no way to predict just when or where the next eruption will occur, it will probably happen somewhere in the eastern San Francisco Volcanic Field. How would an eruption impact today’s inhabitants of northern Arizona?


Champion, D.E., 1980. Holocene geomagnetic secular variation in the western United States:
Implications for the global geomagnetic field:  US Geological Survey Open-File Report, p. 80-824.

Elson, M.D., and Ort, M.H. 2003. Collaborative Research at Sunset Crater Volcano, in Elson,
M.D. ed.  Archaeology Southwest, v. 17, p. 4-6.

Elson, M.D., Ort, M.H., Hesse, S.J., and Duffield, W.A., 2002. Lava corn and ritual in the northern southwest:
American Antiquity, v. 67, p. 119-135.

Hanson. S.L., Falster, A.U., and Simmons, W.B., 2000.  Mineralogy of Fumarole Deposits from Sunset Crater Volcano, Northern Arizona: Rochester Mineralogical Symposium Abstracts, Rochester, New York, USA, p. 9.

Hanson, S.L, Duffield, W., and Plescia, J.  (2008a) Quaternary volcanism in the San Francisco Volcanic Field:
Recent basaltic eruptions that profoundly impacted the northern Arizona landscape and disrupted the lives of nearby residents.  In Field Guide to plutons, volcanoes, faults, reefs, dinosaurs, and possible glaciation in selected areas of Arizona, California, and Nevada (E.M. Duebendorfer and E.I. Smith, eds.). Geological Society of America Field Guide 11, p. 173-186.

Hanson, S.L., Falster, A.U., and Simmons, W.B. (2008b) Mineralogy of Fumarole Deposits at Sunset Crater Volcano National Monument, Northern Arizona:  Rocks and Minerals, 83 (6) p.534-544.

Holm, R.F., 1987. Significance of agglutinate mounds on lava flows associated with monogenetic cones:
An example from Sunset Crater, Northern Arizona: Geological Society of America Bulletin v. 99 p. 319-324.

Holm, R.F., and Moore, R.B., 1987. Holocene scoria cone and lava flows at Sunset Crater, northern Arizona:
Geological Society of America Centennial Field Guide – Rocky Mountain Section393-397.

Hooten, J.A., Ort, M.H., and Elson, M.D., 2001. Origin of cinders in Wupatki National Monument:
Technical Report No. 2001-12, Desert Archaeology Inc. 20p.

Moore, R.B., 1974. Geology, petrology and geochemistry of the eastern San Francisco Volcanic Field, Arizona: 
Ph.D. Thesis.  Univ. New Mexico, Albuquerque, New Mexico, USA.

Ort, M.H., Elson, M.D., and Champion, D.E., 2002. A paleomagnetic dating study of Sunset Crater Volcano:
Technical Report No. 2002-16, Desert Archaeology, Inc. 16p.

Shoemaker, E.M., and Champion, D.E., 1977. Eruption history of Sunset Crater, Arizona: 
Investigator’s Annual Report.  Manuscript on file, Flagstaff Area National Monuments Headquarters, Wupatki, Sunset Crater Volcano, and Walnut Canyon National Monuments, Flagstaff, Arizona.

Smiley, T.L., 1958. The geology and dating of Sunset Crater, Flagstaff, Arizona, in Anderson, R.Y and Harshbarger J.W.,eds., Guidebook of the Black Mesa Basin, Northeastern Arizona. New Mexico Geological Society, Socorro, NM, p. 186-190.



Sarah Hanson
Associate Professor
Earth Science Dept.

Adrian College
Adrian, MI

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