Arizona is widely known for its immense canyons and vast desert landscapes, but the state holds a surprising geological past that shaped its northern plateaus. Beneath the serene appearance of its high-altitude forests and open plains lies a history of intense volcanic activity. The northern part of the state is scattered with hundreds of volcanic vents, illustrating a complex interaction between deep earth processes and the surface environment. This concentration of volcanoes has created a unique and diverse terrain that continues to be of interest to geologists.
The Scale of Volcanism: Counting Arizona’s Volcanic Fields
The number of volcanoes in Arizona is much higher than most people realize, with estimates suggesting the total across the state is well over 1,000 individual vents. The vast majority of these are concentrated in a few distinct areas known as volcanic fields. The most significant is the San Francisco Volcanic Field (SFVF), which spans roughly 1,800 square miles north of Flagstaff.
This single field alone contains more than 600 volcanoes that have erupted over the last six million years. The SFVF represents the primary concentration of volcanism in the state and includes the iconic San Francisco Peaks. Other, smaller fields contribute to the overall count, such as the Uinkaret Volcanic Field near the Grand Canyon’s North Rim.
Defining the Landscape: The Prevalence of Cinder Cones
The dominant type of volcanic feature across Arizona’s fields is the cinder cone, a relatively small and common structure formed by a specific eruption style. These steep-sided, conical hills are created during eruptions where gas-charged magma is forcefully ejected into the air as lava fountains. The fragments of frothy, basaltic material cool rapidly before falling back to earth as scoria or cinders.
The accumulation of these loose fragments around the vent builds a cone that is typically under 1,000 feet in height. Most of the 600+ volcanoes in the San Francisco Volcanic Field are these basaltic cinder cones. The magma responsible for these features is low-viscosity basalt, which often flows from the base of the cone to form thin, extensive lava flows across the surrounding landscape.
While cinder cones are numerous, the landscape also includes other, less common volcanic forms. The most prominent is the San Francisco Mountain, which contains Humphreys Peak, the highest point in Arizona. This massive structure is the remnant of a stratovolcano, built by layers of lava and ash, which contrasts sharply with the diminutive cinder cones. The field also features lava domes like Mount Elden, formed by much thicker, higher-viscosity magma that piles up near the vent instead of flowing outward.
Activity Status: Eruption History and Future Potential
The volcanic fields of Arizona are classified as potentially active, even though the majority of individual cones are considered extinct. Geologists define the San Francisco Volcanic Field as active because it has had eruptions within the last few million years and remains underlain by a deep magma system. The most recent confirmed eruption occurred at Sunset Crater, located on the eastern edge of the field near Flagstaff.
Scientific dating indicates this event took place around 1085 AD, making it Arizona’s youngest volcano. The eruption at Sunset Crater produced a significant amount of ash and cinders, which dramatically altered the landscape and impacted the local Ancestral Puebloan inhabitants. The Uinkaret Volcanic Field near the Grand Canyon also experienced activity around the same time, with eruptions occurring approximately 1,000 years ago.
Since the Sunset Crater event, the region has been quiet, though the field is continuously monitored by geological surveys. The current risk assessment for the San Francisco Volcanic Field is classified as moderate due to the recent geological history and the presence of a magma source. New eruptions, if they occur, are expected to be small-volume, basaltic events, similar to the cinder cone formation that has characterized the region.