Sunset Crater National Monument in northern Arizona is defined by dramatic geology. The park’s centerpiece is the Sunset Crater cinder cone, a steep-sided hill of loose volcanic fragments. This prominent landform is recognizable by the reddish and yellowish coloration near its rim, which gives the feature its name.
Defining Volcanic Status
Determining the status of a volcano requires specific terminology used by volcanologists. A volcano is considered active if it has erupted within the Holocene epoch (the last 11,700 years), or if it shows signs of unrest, such as seismic activity or gas emissions. This definition means a volcano can be active even if it is not currently erupting.
The term dormant is often used as a shorthand for an active volcano that is currently quiet but has the potential to erupt again. The most difficult classification is extinct, which implies a volcano will never erupt again because its magma supply has been permanently cut off. This is a high-bar classification because a volcano can be quiet for tens of thousands of years before reawakening.
For scientific cataloging, the Global Volcanism Program often classifies a volcano as extinct if it has not erupted in the past 10,000 years. This time-based rule is not absolute, as some volcanic systems can remain “restless” with a hot magma chamber after long periods of inactivity. The distinction between a permanently sealed vent and a temporarily quiet system complicates the question of extinction.
The Eruption History of Sunset Crater
The formation of the Sunset Crater cone resulted from the San Francisco Volcanic Field’s most recent eruption. Scientific consensus places the start of the eruption around 1085 CE. Refined analysis, including chemical anomalies found in tree rings, helped confirm the 1085 CE date as the event’s precise timing.
The eruption began along a lengthy fissure, nearly ten kilometers long, which initially produced a “curtain of fire” of molten rock. Activity along this fissure eventually concentrated at the vent that became the Sunset Crater cinder cone. This was a highly explosive basaltic eruption, classified as sub-Plinian with a Volcanic Explosivity Index (VEI) of 4.
The eruption column soared an estimated 20 to 30 kilometers into the atmosphere, depositing an ash blanket that covered an area exceeding 2,100 square kilometers. The resulting cinder cone rose approximately 340 meters high, created by the accumulation of cinders and volcanic bombs.
Lava flows, known as the Bonito and Kana-a flows, also emerged from the base of the cone. These flows extended several kilometers across the surrounding landscape.
This explosive event had a profound impact on the local Sinagua people, forcing the temporary abandonment of settlements in the area. Archaeological evidence shows that villages and agricultural fields were buried by the thick layer of cinder and ash.
Sunset Crater within the San Francisco Volcanic Field
Sunset Crater is a single, relatively small vent within a much larger geological province known as the San Francisco Volcanic Field (SFVF). This extensive field covers approximately 4,700 square kilometers of northern Arizona and contains over 600 individual volcanoes. The field has been active for about six million years, with the center of activity gradually migrating eastward over time.
Most of the volcanoes in the SFVF, including Sunset Crater, are monogenetic cinder cones, meaning they are built by a single, relatively short-lived eruption. In this type of field, a new eruption rarely re-activates an old cone because the plumbing system that fed the first eruption becomes sealed off. When magma rises again, it typically finds a new path to the surface, creating a new vent.
This distinction is fundamental to understanding Sunset Crater’s status; the individual cone is considered sealed and unlikely to erupt again. However, the SFVF itself is a large, active volcanic region with a history of recurrent activity. The field includes a variety of volcanic landforms, from small cinder cones to the large stratovolcano of San Francisco Mountain, the highest point in Arizona.
The field’s ongoing activity is a function of underlying geological processes that allow magma to rise from the mantle. The San Francisco Volcanic Field is a classic example of distributed volcanism, where hundreds of small, single-use volcanoes form across a broad area.
The Likelihood of Future Activity
When assessing the probability of future eruptions, volcanologists differentiate between the individual cinder cone and the entire volcanic field. Sunset Crater, as a monogenetic cone, is considered permanently sealed and is not expected to erupt again.
The San Francisco Volcanic Field as a whole is classified by the United States Geological Survey (USGS) as a “moderate threat” system. Although the last eruption was nearly 1,000 years ago, the geological record indicates that eruptions occur within the field at an average interval of several thousand years.
Based on the pattern of eastward migration, the most probable location for the next eruption is in the eastern part of the field, closer to Flagstaff. Hazard assessments indicate that the annual probability of a new vent forming within the Flagstaff area is statistically low. The USGS monitors the field for signs of unrest, such as increased seismicity or ground deformation, which would signal rising magma.
Any future eruption is expected to be a basaltic event, producing a new cinder cone and associated lava flows. While these events are not explosive enough to cause widespread devastation, they would still pose a hazard to local infrastructure and communities.