Northern Arizona is famous for the Grand Canyon, but the region holds another powerful story beneath its surface. The San Francisco Volcanic Field, located around Flagstaff, is one of the most active volcanic landscapes in the Southwest. It produced more than six hundred volcanic cones over the past six million years and its most recent eruption, at Sunset Crater, occurred only about one thousand years ago. A new seismic study now shows that the ground beneath this quiet forested region contains far more activity than previously understood.
Researchers used dense arrays of seismic instruments to create a detailed map of the crust under the San Francisco Peaks. These peaks form the highest mountains in Arizona and are the eroded remains of a much older and much larger volcano. What the scientists discovered under this landscape offers important clues about what still lies underground.
The study identified two bodies of molten material beneath the region. One sits in the middle of the crust at depths of fifteen to eighteen kilometers. The other lies deeper, near the base of the crust. These are not simply warm zones left over from ancient eruptions. The seismic signatures match the presence of real melt, the kind found under active volcanic systems. This result is unusual for a volcanic field dominated by small eruptions because such fields do not normally maintain long lived magma reservoirs.
The shallow magma body is large enough to hold the ingredients for a new silicic eruption. Silicic eruptions tend to be explosive and produce large ash plumes. The location of this melt zone is also significant. It sits at the depth where the crust transitions from brittle to ductile. This is the level where rising magma often slows and gathers before it moves upward again. In volcanic systems around the world, this depth is a common staging point for eruptions.
One detail from the study adds an important piece of context. In 2009 an unusual swarm of small earthquakes struck the area near the San Francisco Peaks. At the time, the cause was uncertain. The new seismic images show that the swarm lined up almost exactly above the shallow magma reservoir. This suggests that the earthquakes may have been caused by pressure changes or movement within the melt zone itself. The crust responded to activity below, which is a key sign that this system is still interacting with its surroundings.
The deeper magma body raises a different kind of question. It appears to be sitting along a sharp step in the crust that was not mapped before. This step marks a sudden change in crustal thickness. The study links this feature to a period of lower crust removal in the geological past, a process that thinned the lithosphere and allowed hotter material from deeper in the mantle to rise. The result is a natural ramp that helps guide magma upward. It channels melt toward the same region that produced the giant volcano responsible for the San Francisco Peaks.
The presence of this crustal step is important because it acts as a focusing point for melt that rises from below. Many monogenetic volcanic fields (those built from many small eruptions instead of a single large volcano) do not have such structures. Without a natural focusing mechanism, magma tends to erupt in many locations as small cones. In Arizona, the crustal step created a zone where melt accumulated, allowing a stratovolcano to grow in a field that otherwise produces small eruptions.
The combination of two active melt zones, a known crustal ramp, and a recent history of seismic activity changes the way scientists view the volcanic field. The study does not suggest that an eruption is imminent. It does, however, show that enough molten material remains to produce a future explosive event if new magma rises into the system. This is important because basaltic magma, when injected into a shallow silicic reservoir, can rapidly destabilize it.
Flagstaff and several surrounding communities lie close to this volcanic field. Millions of people visit the region each year. The new findings give scientists a more detailed picture of what sits beneath this part of Arizona. They also help refine hazard assessments by showing where magma is likely to move and how the crust above it is likely to respond.
The San Francisco Volcanic Field has long been considered active, but the new study brings sharper resolution to that picture. It shows a landscape shaped by ancient eruptions that still carries the deep structure needed to support future activity. It also confirms that this region is not dormant. Melt is still present. Pathways still exist. The crust has already shown signs of movement.
Modern volcanic science cannot predict specific eruption dates, but it can show where the conditions for future activity are most likely to develop. Beneath the forests of northern Arizona, those conditions are still in place.
Source:
This article is based on the 2026 Geological Society of America study “Tectonic controls on magma storage beneath a distributed volcanic field,” available at:
https://doi.org/10.1130/G54058.1
