Arizona is home to an extensive volcanic landscape, featuring remnants of past activity that span millions of years. While the state does not host the classic stratovolcanoes associated with the Pacific Northwest, its geology is profoundly shaped by magma that has repeatedly reached the surface. The term “volcano” in Arizona most often refers to volcanic fields, which are vast areas containing hundreds of smaller, individual eruption points such as cinder cones. This history has created some of Arizona’s most recognizable geological features and highest peaks.
Arizona’s Major Volcanic Provinces
Arizona’s volcanic features are concentrated in several distinct regions known as volcanic fields. The most prominent is the San Francisco Volcanic Field (SFVF), which spans roughly 1,800 square miles near Flagstaff on the southern edge of the Colorado Plateau. This field has been intermittently active for nearly six million years, producing over 600 individual volcanoes. Activity within the SFVF has generally migrated eastward over its history, culminating in the state’s most recent eruptions.
Further east, the Springerville Volcanic Field is the third-largest monogenetic field in the continental United States. This area in east-central Arizona covers approximately 1,200 square miles and contains over 400 distinct vents. The most recent cones here date to about 300,000 years ago.
A third area of activity is the Pinacate Peaks Volcanic Field, which straddles the Arizona-Mexico border in the Sonoran Desert. Although the largest part of this field lies in Sonora, Mexico, its geological connection makes it relevant to Arizona’s volcanic history. The Pinacate field is characterized by a large shield volcano and hundreds of smaller cones, with its most recent activity occurring about 11,000 years ago.
Types of Volcanic Features Present
Arizona’s volcanic landscape is dominated by features resulting from low-viscosity, basaltic magma, which tends to produce less explosive eruptions than the high-silica magmas found elsewhere. The most common structures found across the state are cinder cones, also known as scoria cones, such as Sunset Crater. These features are relatively small, typically less than 1,000 feet high. They are built rapidly by the accumulation of frothy, gas-charged lava fragments that cool and fall back to the ground.
The SFVF also contains a single, much larger stratovolcano, San Francisco Mountain, which hosts Humphreys Peak, the highest point in Arizona. Stratovolcanoes are built by alternating layers of lava flows and ash, representing a more complex eruption style than the surrounding cones. The field also features lava domes, like Mount Elden. These are steep-sided, bulbous protrusions formed by highly viscous lava that piles up around the vent.
Beyond these conical structures, the fields are covered by extensive basaltic lava flows that spread out across the landscape, sometimes forming vast, flat mesas. In areas where rising magma interacts violently with groundwater, a different explosive feature called a maar crater can form. These are broad, low-relief volcanic craters surrounded by a rim of ejected material, such as Crater Elegante in the Pinacate field.
Current Status and Geological Monitoring
The most accurate classification for Arizona’s youngest volcanic fields is “dormant.” This means that while no current eruption is occurring, the magma system beneath the surface is still capable of producing future activity. The most recent eruption in Arizona was the formation of Sunset Crater, which occurred around 1085 CE, making it less than 1,000 years old.
The three youngest volcanic provinces—the San Francisco, Uinkaret, and Pinacate fields—are considered geologically active and likely to erupt again. Intervals between eruptions are typically on the order of several thousand years. Future eruptions are expected to be monogenetic, meaning they create a new, single-use vent like a cinder cone. The U.S. Geological Survey (USGS) designates the San Francisco Volcanic Field’s threat potential as “Moderate.”
Geological monitoring is conducted by the USGS and the Arizona Geological Survey (AZGS) to track signs of renewed activity. Monitoring involves detecting changes in seismic activity, looking for ground deformation using GPS and satellite imagery, and analyzing gas emissions. The most probable location for a future event in the SFVF is the eastern portion of the field, where activity has recently migrated.
Scientists estimate the annual probability of a new eruption in the San Francisco Volcanic Field that could affect the Flagstaff metropolitan area is very low, approximately 1 in 100,000. Continuous monitoring is maintained to provide early warning. A future eruption would primarily pose a localized threat from lava flows and ashfall near the new vent.