Learn more about Volcanos in Northern Arizona
Guest post by Greg Zielinski
Driving toward the Grand Canyon along Routes 180, 64, 89 and I40 from Flagstaff and other parts of Northern Arizona or when flying into the region, one can see many mountains and conical shaped hills, as in photo above. Almost all of these are former volcanoes that erupted over the last 6 million years, although the most recent eruption was only about 950 years ago, a mere millisecond in geologic time. This article describes the type of volcanic activity in Northern Arizona including volcanic eruptions that have directly impacted the Canyon and some other noteworthy eruptions in the region.
Geology of the San Francisco Volcanic Field (SFVF)
Covering ~1800 sq mi (~4660 sq km) in Northern Arizona, the SFVF is home to about 600 cinder cones, a larger stratovolcano and several volcanic domes. These different types of volcanoes and the nature of the material from the eruption are a function of the chemical composition of the erupting magma, the molten material beneath the Earth’s surface. The type of material produced during an eruption include lava flows, ejected material or tephra (for example, ash and bigger particles, such as bombs) and gases.
Digital map of the San Francisco Volcanic Field showing many of the volcanic features found within the Field including the many cinder cones, San Francisco Peaks, Mt. Elden, Sunset Crater and Bill Williams Mountain (SW of the town of Williams) in the southwest part of the Field. Image from NASA as available at https://earthobservatory.nasa.gov/images/6585/san-francisco-peaks-volcano-field
The key element that determines the style of eruption and resulting volcanic landform and rock type is the silica content of magma. When the silica content is low, the magma is less viscous, meaning it can flow easier. As a result, many gases can escape before the magma reaches the surface so the eruption is not very explosive. The results are cinder cones and wide-spread lava flows. At the other end of the spectrum, magma high in silica is very viscous meaning the gases can not escape as easily before reaching the surface. The results are more explosive eruptions or lava that does not flow far from the erupting vent, thus it builds up into domes. Similarly, the types of minerals found in magma with the different concentrations of silica produce different rock types that are characterized by their color. The most common low-silica volcanic rock is basalt and they are black to red (from the high iron content when oxidized, like rust). Intermediate-silica content produces grayish rocks, such as andesite, whereas high-silica content magmas produce dacite and rhyolite (highest silica content) the two most common rocks with this composition. They are often tan to pink. Without getting into details, magma composition is a function of the overall geological setting and particularly, the tectonic history of the region.
Northern Arizona’s volcanism is interesting
However, the reason for this extensive area of volcanism in Northern Arizona is quite interesting from a geological perspective. Most volcanic eruptions occur at plate tectonic boundaries, such as the Ring of Fire around the Pacific Ocean. A second common area of volcanism occurs over hot spots often found in the middle of tectonic plates. These large chambers of magma beneath the Earth’s surface remain stationary, but produce lines of volcanism as the geologic plate moves over it. Yellowstone is the best example. Volcanism within the SFVF of Northern Arizona also occurs away from the boundary of a tectonic plate, but the mechanism is not the same as a hot spot. In this case, the friction caused by internal layers of the Earth has produce magma as the North American Plate moves over the area toward the west. Consequently, that magma has reached the Earth’s surface beginning ~6 million years with the oldest volcano in the field, Bill Williams Mountain, Williams AZ, mostly active ~3-4 million years. Progressively younger volcanoes occur to the east with Sunset Crater, east of Route 89, about 40 mi east of Bill Williams Mountain and northeast of Flagstaff, being the youngest in the Field (about 950 yrs old).
The question this scenario might bring to mind is: will there be another eruption in Northern Arizona. The answer is probably, maybe sometime over the next 1000 years, somewhere to the east of Sunset Crater. For now, however, such an event is likely to be well-beyond our lifetime.
Another interesting aspect of the SFVF and especially those interested in geology and volcanic eruptions is that all four of the major volcanic rocks types (and magma that produced them) are found in the area. Basalt is the most common rock type as almost all of the cinder cones in the SFVF are made of basalt as are the many lava flows around the region, such as within Flagstaff and as noted later, around the Grand Canyon. The SP Crater, about 30 mi north of Flagstaff, just south of Route 180, and its associated lava flow is a classic, text-book example of these features. Andesite makes up the San Francisco Peaks just north of Flagstaff with Humphreys Peak being the highest point in AZ at 12,633 ft. These peaks are the remnants of a much larger stratovolcano that formed from the combination of lava flows and ejected material. The former volcano may have been about 16,000 ft high which would have made it the highest mountain in the lower 48 states by far. Much of the upper part of the former mountain may have collapsed to the northeast with much of that material forming the slope on which present day Route 89 just north of Sunset Crater is located. Dacitic rocks make up Mt. Elden immediately north of Flagstaff with many lobes coming off of the main vent. On the east side of the SF Peaks, near Sunset Crater, is Sugarloaf Mountain, a dome made of rhyolite, the highest silica content within SFVF rocks.
Volcanic Activity and the Grand Canyon
Grand Canyon is famous for the exposed layers of sedimentary rocks and the long record of Earth’s history preserved in those layers, although there have been times in the more recent past when volcanism had a big influence on what was happening within the Canyon. More recent studies on the age of these lava flows (Crow et al. 2015 and references therein) indicate that from about 850K to 100K years ago, 17 basaltic lava flows entered the Canyon, as evidenced by remnants along the Canyon walls, and in the process dammed the Colorado River producing a series of lakes. Most of the lava originated from the Unikaret Volcanic Field, between the Toroweap and Hurricane Faults, on the North Rim in the western part of the Canyon. Most of the flows entered the Canyon between River Miles 179 (Lava Falls Rapids and Vulcan’s Throne) and 195 with some around Miles 176-178 and others extending up to Miles 246-253. Dating of the volcanic rocks associated with these flows indicate five overall episodes of lava entered the Canyon with the three oldest episodes entering around the Lava Falls area, a fourth episode in the Whitmore Wash area and the youngest period again around Lava Falls. The resulting dams were about 35 meteres high (115 ft) to at least 330 meters (1082 ft) and possibly up to 640 meters (3000 ft) high. Evidence of the sediments left in these lakes is isolated, but it is believed some of the lakes may have extended into Utah beyond present-day Lake Powell. Geological evidence suggests that some of the dams failed catastrophically producing large outburst floods. Some of the dams may have only lasted 10s-100s of years, while others could have lasted for several millennia. There also is evidence of volcanic activity flowing into the ancestral Colorado River from the Parashant’s Volcanic Field north of the Canyon around 5.5 MY to 10 MY ago, that is prior to the formation of the Canyon as it is today.
View to the west from Toroweap toward Whitmore Canyon showing two basaltic lava flows that cascaded into the Grand Canyon around Lava Falls Rapids (River Mile 179). Figure modified from the National Park Service as available at www.nps.gov/para/learn/nature/parashant-s-volcanic-fields-and-lava-dams-in-the-grand-canyon.htm. Picture from J. Axel.
“Biggie” Eruptions in the Southwestern US
The eruptions of the SFVF have not been very big from a geological perspective, although if one was there when it erupted, it would certainly seem to be a big eruption. This raises the question of where, if any, large volcanic eruptions occurred in Arizona and the Southwest in general. These are the types of eruptions that produce large calderas, often circular features that form from the collapse of the volcano into the void left by the erupted magma below. Many of these calderas now have lakes inside of them. The answer is yes, not only in both Arizona and the Southwest, but in the West as a whole. The largest in most recent geological time and most explosive eruption in AZ occurred about 26.9 MY ago in the Chiricahua Mountains. The Turkey Creek caldera explosively erupted about 500-1000 cu km of rhyolitic magma with the deposits now making up Chiricahua National Monument in SE AZ. A similarly large eruption occurred 1.2 MY ago a little farther east. The Valles Cadera in New Mexico’s Jemez Mountains erupted 400 cu km of rhyolite with a large deposit covering much of Bandalier National Monument. Other large eruptions outside of the Southwest include Mt. Mazama, OR, which is now Crater Lake, the Long Valley Caldera in Southern California and the real-biggie, Yellowstone. They erupted anywhere between about 150 and 2500 cu km of rhyolitic material. To put all of these large eruptions into perspective, only about 0.5 cu km of material erupted from Sunset Crater, a mere “burp” compared to these large caldera forming eruptions in the region.
Summary
Volcanic activity in the recent geologic past is responsible for much of the landscape of Northern Arizona as one travels to the Grand Canyon and particularly in the San Francisco Volcanic Field (SFVF). In fact, the most recent eruption, Sunset Crater, occurred only about 950 years ago and no doubt, there will be another eruption in the eastern part of the area in the future. However, when and where is unknown, but probably not in our lifetime. Interestingly, the type of volcanic activity found in the SFVF include the four major types of volcanic rocks, as well as both lava flows and more explosive types of volcanic material. Furthermore, volcanic activity in the region had a major impact on the Colorado River and the Canyon. As many as 17 basaltic lava flows entered the Canyon, most from the Unikaret Volcanic Field on the North Rim, and in the process dammed the river around River Miles 179-195 forming a series of extensive lakes that may have lasted from 10s to 1000s of years. Although the volcanism in northern Arizona is not large from a volcanology point-of-view, very large eruptions have occurred in the past in AZ (Turkey Creek Caldera, Chiricahua National Monument) and the Southwest as a whole (Valles Caldera, NM).
References and Other Resources
Cook, R.S., Karlstrom K.E., McIntosh, W. Peters, L, Crossey, L., and Eyster, A., 2015. A new model for Quaternary lava dams in Grand Canyon based on 40Ar/39Ar dating, basalt geochemistry, and field mapping. Geosphere, v. 11, no. 5, p. 1305-1342, doi:10.1130/GES01128.1.
USGS Fact Sheet 017-01, 2001, The San Francisco Volcanic Field, Arizona.
www.nps.gov/para/learn/nature/parashant-s-volcanic-fields-and-lava-dams-in-the-grand-canyon.htm
www.usgs.gov/volcanoes/san-francisco-volcanic-field/science/geology-and-history-san-francisco-volcanic-field (Source of First Photo)
www.usgs.gov/volcanoes/uinkaret-volcanic-field
Guest Blogger: Greg Zielinski is a retired professor in the fields of climatology, meteorology and geology. Internationally known for his work on the impact of volcanic eruptions on climate through ice core research, he has been a featured scientist in many documentaries including NOVA. He has published several books on weather and climate and was the Maine State Climatologist.