Part of the NPS Geodiversity Atlas, which describes each park's significant geologic features and processes. Geodiversity is the full variety of a park's natural geologic resources — rocks, minerals, sediments, fossils, landforms and physical processes — and its soils. The atlas comes from the NPS Geologic Resources Inventory.
The monument's Geologic Resource Evaluation Report. Image from the NPS's page.
A cinder cone like no other
Sunset Crater Volcano National Monument protects the 1,000-foot (300-m) Sunset Crater cinder cone, one of the best examples of its kind on the continent, with its lava flows and volcanic features, on the southern edge of the Colorado Plateau near Flagstaff, among the ponderosa pines below the San Francisco Peaks.
It erupted in 1085 CE, the most recent eruption in the San Francisco Volcanic Field. The eruption probably lasted less than a year. Its column rose 65,000 feet (20 km) — perhaps visible across much of the Southwest — and spread ash over 270 square miles (700 km²). Lava covered only about 3 square miles (8 km²), but ash piled up far away, as at Wupatki National Monument, 12–15 miles (20–25 km) off. Ash deeper than about 6–8 inches (15–20 cm) cannot be farmed, so many fields were abandoned.
John Wesley Powell, who led the first exploration of the Colorado River through the Grand Canyon in 1869, was probably the first geologist to see the cone, and gave it its name:
A portion of the cone is of bright reddish cinders, while the adjacent rocks are of black basalt. The contrast in the colors is so great that on viewing the mountain from a distance the red cinders seem to be on fire. From this circumstance the cone has been named Sunset Peak.
The red comes from oxidation near fumaroles — vents that release volcanic gas — around the main vent in the summit crater.
Red cinders at the top of Sunset Crater. James St. John, CC BY 2.0.
It is one of 19 park units set aside mainly for volcanic resources, one of 33 with a volcano, and one of 17 with a volcano that erupted in historic or prehistoric times.
The San Francisco Volcanic Field
The Colorado Plateau is high, with thick crust and little deformation, so its rock is mostly flat-lying sedimentary layers — laid bare in places like the Grand Canyon. Along its margins lie several monogenetic volcanic fields: clusters of cinder cones, fissure volcanoes and others that each erupt only once, where magma supply is low. The Kolob, Uinkaret, Hopi Buttes and Zuni-Bandera fields are others.
The San Francisco Volcanic Field, with features labeled. USGS.
The field became active about 6 million years ago. It holds about 600 volcanoes over some 1,800 square miles, mostly cinder cones and basalt flows, with lava domes and, near the center, a large composite volcano, San Francisco Mountain. The San Francisco Peaks — including Humphreys Peak, Arizona's highest at 12,633 feet — are its eroded remains; it may once have collapsed on one flank, as Mount St. Helens did in 1980. Activity has moved from west to east, so a future eruption could come near Sunset Crater or farther east.
The San Francisco Peaks. James St. John, CC BY 2.0.
The eruption
Cinder cones are monogenetic: one period of activity. Sunset Crater rose where no volcano stood before, though older cones such as Lenox Crater were nearby. Such eruptions usually begin along a fissure, center on one vent, last months to a few years, and are mildly explosive — Strombolian, like volcanic fireworks, at 1 or 2 on the Volcanic Explosivity Index (VEI).
Sunset Crater's was the most explosive cinder-cone eruption documented: sub-Plinian, VEI 4, with an unsteady but sustained column. Unusually high carbon dioxide in the magma bubbled out fast and drove it rapidly to the surface. It was also large, about 0.12 cubic miles (0.5 km³) of dense rock.
Paricutin, in Mexico — also a cinder cone, though Sunset Crater's eruption was larger. Bodil Christensen, public domain.
It came in three phases:
- A fissure eruption along a 6-mile (10-km) northwest–southeast crack, with low fire fountains and a small lava flow; Sunset Crater lies near its northwest end.
- The highly explosive eruption that built the cone, with the Kana'a and Bonito lava flows.
- A waning stage, less explosive, with far less tephra and more of the Bonito flow.
Dating it. An eruption date of 1064 CE, widely reported, came from tree rings in wooden beams at Wupatki that narrow abruptly. But other things narrow tree rings too. Dendrochemistry — studying Lassen's Cinder Cone (1666 CE) and Paricutin (1943) — showed that an eruption can raise phosphorus in wood and lower the strontium isotope ratio. Wupatki wood from 1085–1090 CE carries both signatures.
The cone and its lava
Most cinder cones stand 100–150 m high and rarely exceed 200–300 m. Sunset Crater is about 300 m (1,000 ft), reaching 8,042 feet, about 1 mile across, with a crater about 350 feet deep. It is a pile of cinders (scoria) and ash around a central vent, with slopes averaging about 30 degrees, near the angle of repose. Loose cinders erode easily under feet, so the cone has been closed to hiking since 1973.
Cinders on the flank of Sunset Crater. James St. John, CC BY 2.0.
Two lava flows came from the cone's base: the Kana'a from its east side, running northeast, and the Bonito from its west, running northwest. Both are ropy pāhoehoe near the vent and rough 'a'ā farther out, with mostly collapsed lava tubes. The Lava Flow Trail crosses pāhoehoe with squeeze-ups, a lava tube, hornitos, and pieces of the cone rafted away by the lava; the 'A'ā Trail and Lava's Edge Trail show the rougher lava and the flow front. The whole monument counts as pseudokarst — cave-like features formed without dissolving rock.
The Bonito lava flow. Image from the NPS's page.
People, plants and astronauts
The eruption endangered the Sinagua (ancestral Puebloan) people. Those nearest had to leave fields buried in ash, but farther away a thin ash layer acted as mulch, holding moisture and raising yields. Impressions of corn cobs and stalks in basalt from a site near the volcano suggest people placed corn around an active hornito, perhaps in ritual, and carried the "cornrocks" home to a pithouse.
The field is a natural laboratory for erosion of young volcanoes. Sunset Crater's steep, nearly textbook shape shows its youth; older cones nearby are progressively rounder and lower. Inverted topography forms because basalt resists erosion: flows that filled valleys end up as ridges once the land around them wears away.
Two rare plants grow only on young cinders, both U.S. Fish and Wildlife Service species of concern: Sunset Crater penstemon (Penstemon clutei), found only in the field's northeast, and cinders phacelia (Phacelia serrata), found only here and at El Malpais National Monument.
Cinders phacelia. C. Griffing, NPS.
South of the monument, the Cinder Lake crater field — dug in the eruption's ash and blasted with explosives to mimic the Moon — was used for Apollo astronaut training in the 1960s and 1970s.
Hazards
Sunset Crater itself should not erupt again, but another vent in the field could, after a long interval. In 2022, scientists from 40 organizations, including the USGS, spent a month role-playing a future eruption there — how to track magma rising and protect Flagstaff. The USGS's 2018 national volcanic threat assessment rates the field moderate, the middle of five categories, with a threat score of 50.6: its hazard is relatively low, but exposure of people and aviation is relatively high.
The area lies in the Northern Arizona Seismic Belt and is a moderate seismic hazard: the USGS's 2014 map gives a 2% chance in 50 years of shaking above 14–16% of gravity, roughly Modified Mercalli VI. North-central Arizona can expect 10 to 20 damaging earthquakes in 10,000 years.
Geologic map of the monument, from the Geologic Resources Inventory. NPS.
The monument's Geologic Resources Inventory products are in the NPS DataStore, and a Soil Resources Inventory project has been completed. More: the USGS page on the San Francisco Volcanic Field and the 2018 threat assessment.
Sources
Based on "NPS Geodiversity Atlas—Sunset Crater Volcano National Monument, Arizona," National Park Service; a work of the United States government in the public domain. Photographs by James St. John are used under their Creative Commons Attribution 2.0 licence, as published on Flickr. A map adapted from a 2016 publication, and photographs courtesy of individuals, are not reproduced. The source spells the western flow "Bonito" and once "Bonita", and the nearby cone "Lenox" and once "Lennox"; this page uses the first of each.
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
1
0
0
0
Comments
