This section provides descriptions of the most prominent and distinctive geologic features and processes in Sunset Crater Volcano National Monument.

Figure 3. Aerial view of Sunset Crater Volcano.

Figure 2. Aerial view of Sunset Crater Volcano.

The eruption responsible for Sunset Crater produced a 300 m (1,000 ft) high asymmetrical cinder cone that lies on the base of the east slope of the San Francisco Peaks (Figures 2 and 3) (Smiley, 1960). The crater rim encloses the main crater which is more than 120 m (400 ft) deep (Holm and Moore, 1987). The cinder cone was constructed when hot volcanic cinders and ash (1100- 1200 degrees Centigrade) were blown into the air and settled around the fissure or vent (Breed, 1976). This northwest- trending fissure was 5 km (3 mi) long at the beginning of eruptive activity, extending from Janus Crater to Yaponcha Crater, but quickly localized directly beneath present day Sunset Crater Volcano (Smiley, 1960; Hodges, 1962; Amos et al., 1981). Cinder material of the cone is loose and poorly sorted. The average particle size is 4 mm, intermixed with very fine ash and scattered ejecta of block size (more than 32 mm) (Hodges, 1962).

Formation of the cinder cone was accompanied by the eruption of dark, basaltic lava flows (present in Bonito Canyon and a small wash toward the Little Colorado River) (Figure 5). The Kana- a flow moved northeast, and the Bonito flow moved northwest (Smiley, 1960; Breed, 1976). The lava flows likely breached the edges of the original cone, rendering its asymmetrical shape (Holm, 1987). The flows were subsequently covered with more ash.

Figure 5. Geoscientist Sarah Hanson examining a fumarole deposit near the summit of Sunset Crater Volcano.

The alkali olivine basalt cone and lava flows cover ~8 square km. The extended, fallout, cinder cover blankets close to 2,000 square km (Breed, 1976; Ort et al., 2001). At the very end of the volcanic activity at Sunset Crater Volcano, hot springs and vapors escaped from the vent and deposited the brilliantly colored minerals near the top of the crater including gypsum, magnetite, hematite, sulfur, cuprite (?), opal, jarosite, voltaite, ralstonite, alunogen, ilmenite, barite, and celestine (Hanson, 2000). The yellows, oranges, and reds of these minerals give the crater the appearance of a perpetual sunset, hence the monument’s name. This coloration is enhanced by the overall oxidation of iron that lends a reddish cast to the entire cinder cone.

This eruption was small in scale relative to the Mt. Saint Helens explosion in 1980 or Krakatoa in 1883, but the eruption completely altered the existence of the peoples living in the area (now referred to as the Sinagua (Reynolds, 1982)).

Archaeological Geology
The eruption began late in 1064 A.D. and may have continued intermittently until 1067 A.D. In 2 years, a mountain was born. Whatever settlements, including pithouses, existed in the immediate area were buried (Nichols, 1944). Two pithouses were excavated in 1930 (Smiley, 1960). This created a setting similar to that at Pompeii, Italy, where an entire city was buried. The native inhabitants at Sunset Crater Volcano no doubt had a much smaller, more agriculturally based existence.

Basalt clasts contain casts of ears of corn possibly left in the path of a flow as a sacrificial offering in one masonry structure 6 km (3.7 mi) west of the monument. People witnessed this eruption and had to adapt ways of dealing with its aftermath (Ort et al., 2001). This very recent event provides a unique opportunity to study the effects of an eruption on a population.

Some single room structures still standing and those buried beneath the ash indicate that there was a small agricultural population in the area before and during the eruption. The ancient peoples no doubt fled the spewing ash, lava, and gas but resettled the area as early as A.D. 1071, once the landscape had cooled (Doolittle, 1997).

The recent volcanism continues to affect regional agriculture. Some archaeologists surmise that the dry farming/mulching practices used by the Hopi in today’s sand dune cultivation are directly related to techniques perfected after the Sunset Crater Volcanic eruption. They trace the technique back to ancient lithic covered fields (Doolittle, 1997). Volcanic ash serves as excellent lithic mulch to maintain moisture in the soil longer. Farmers (including migrating Anasazi from the Mesa Verde area) returned to the area almost immediately after the eruption and used the new ash layers to grow relatively prolific crops. They moved east, leaving multiroom dwellings behind as the ash layers were depleted and/or blew away (Reynolds, 1982; Doolittle, 1997; Ort et al., 2001).

Ecosystem Rebound
The eruption responsible for the cinder cone volcano at Sunset Crater Volcano National Monument changed the area’s landscape forever. A blanket of smothering ash and pyroclastic flow covered the area, burning and/or burying everything for kilometers around. The introduction of so much volcanic material had a profound effect on the ecosystem.

Entire species populations may have been eradicated from the area. Sunset Crater Volcano offers the unique opportunity to study the ecosystem changes inherent with volcanic eruptions (Breternitz, 1967). Because the event was so recent, plant and animal species populations are still in the process of adapting to the new landscape.

This volcano and its relatively undeveloped landscape provide an unparalleled opportunity to study plant succession and ecological change in a dry volcanic landscape.

Unique Volcanic Features
In addition to the cinder cone and fresh lava flows, other phenomena associated with volcanism including lava bombs, lava tubes, and xenoliths are abundant at Sunset Crater Volcano National Monument. Xenoliths of the host or country rock including limestones and sandstones are embedded in the flows. They are easily recognized by their contrast in color with the lava.

Lava bombs are large blobs of molten rock ejected during violent eruptions. During their fall to earth, they revolve,spin, and become elongate. They often form football or other rounded shapes and range in size from a few centimeters to several meters in diameter. These bombs can be ejected quite far from the volcano’s center. They are found littering the slopes and lava flows of Sunset Crater Volcano.

A crust builds over the top of the lava as it flows downhill (Breed, 1976). Beneath the crust, the molten lava is still flowing. When the source is depleted or the flow diverts elsewhere, the material remaining under the crust drains and the remnant tube or lava conduit remains hollow. The tube at Sunset Crater is commonly referred to as an “ice cave” because the air inside is usually below freezing. Any water present in the tube freezes and typically remains frozen due to the insulating properties of the lava (Breed, 1976).

References:

Amos, R.C., Self, S., Crowe, B., 1981, Pyroclastic activity of Sunset Crater; evidence for a large magnitude, high dispersal strombolian eruption. Eos, Transactions, American Geophysical Union, vol.62, no.45, p.1085.

Breed, W.J., Collier, M., 1976, Molten rock and trembling Earth; the story of a landscape evolving. Plateau, vol.49, no.2, p.2- 13.

Breternitz, D.A., 1967, The eruption(s) of Sunset Crater; dating and effects. Plateau, vol.40, no.2, p.72- 76.

Doolittle, W.E., 1998, Innovation and diffusion of sand and gravel-mulch agriculture in the American Southwest; a product of the eruption of Sunset Crater.
In: Volcanoes and man. Raynal, J.P., ed., Quaternaire (Paris), vol.9, no.1, p.61- 69.

Hanson, S.L., Simmons, W.B., Falster, A.U., Morrison, S.J., 2000, Eruption history of Sunset Crater Volcano and associated fumarole deposits, northern Arizona. Abstracts with Programs - Geological Society of America, vol.32, no.7, p.501.

Hodges, C.A., 1962, Comparative study of S.P. and Sunset Craters and associated lava flows. Plateau, vol.35, no.1, p.15- 35.

Holm, R.F., 1987, Significance of agglutinate mounds on lava flows associated with monogenetic cones; an example at Sunset Crater, northern Arizona.
Geological Society of America Bulletin, vol.99, no.3, p.319- 324.

Holm, R.F., Moore, R.B., 1987, Holocene scoria cone and lava flows at Sunset Crater, northern Arizona. In: Rocky Mountain section of the Geological Society of America. Beus, S.S., ed., Geol. Soc. Am., Boulder, CO, United States (USA) p.393- 397.

Nichols, T., 1944, Sunset Crater [National Monument, Arizona]. Natural History, vol.53, no.10, p.448- 451.

Ort, M.H., Elson, M.D., Hooten, J.A., Duffield, W.A., Champion, Duane E., 2001, Interaction between humans and the Sunset Crater eruption, northern
Arizona. Abstracts with Programs - Geological Society of America, vol.33, no.6, p.294.

Reynolds, S.J., 1982, Geologic features of northeastern Arizona. Fieldnotes from the State of Arizona, Bureau of Geology and Mineral Technology, vol.12, no.1, p.1- 8.

Smiley, T.L., 1960, Geology and Dating of Sunset Crater, Flagstaff, Arizona. In: The geology and dating of Sunset Crater, Flagstaff, Arizona. Smiley, T.L. ed., N. Mex. Geol. Soc., Guidebook, 9th Field Conf.