The United States is home to a significant number of potentially active volcanoes, a reality often overshadowed by the high-profile activity in Alaska and Hawaii. Many people mistakenly believe the continental U.S. is geologically quiescent, but the western states contain a complex and dynamic landscape shaped by recent eruptions. Determining the total number of these volcanoes is complicated because the definition of a volcano and how it is counted varies greatly depending on its size and structure. The location and tectonic setting of these features, from massive mountain cones to expansive fields of small vents, are tied to the distinct geological forces beneath the North American continent.
Defining the Count: What Qualifies as a Volcano?
The total number of potentially active volcanoes in the United States and its territories is approximately 169, but the count for the continental U.S. alone is a smaller, yet substantial, subset. The U.S. Geological Survey (USGS) considers a volcano “active” if it has erupted within the current Holocene epoch, which began about 11,650 years ago, or if it shows signs of unrest like ground deformation or seismic activity. This threshold ensures that only systems with a youthful magma supply are included in the risk assessment.
The variability in the count stems from the difference between a single volcanic edifice and a volcanic field. A classic volcano, such as Mount Shasta, is a single, large, conical mountain built by repeated eruptions from a central conduit. These stratovolcanoes are typically counted as one unit.
In contrast, a volcanic field, like the San Francisco Volcanic Field in Arizona, is a broad area containing hundreds of smaller, typically short-lived vents. Most of these are cinder cones that erupt only once. The entire field is counted as a single volcanic system for monitoring purposes because all eruption sites draw from a similar, shallow magmatic source.
Geographic Distribution: Mapping the Major Volcanic Fields
The active volcanic systems of the continental U.S. are concentrated in the western third of the country, reflecting three distinct types of tectonic settings.
Cascade Volcanic Arc
The most recognized chain is the Cascade Volcanic Arc, which stretches from northern California through Oregon and Washington. This arc is a direct result of the Cascadia Subduction Zone, a convergent boundary where the oceanic Juan de Fuca plate is sliding beneath the North American plate. As the oceanic plate descends, it releases water into the overlying mantle, generating the magma that feeds peaks like Mount St. Helens and Mount Rainier.
Yellowstone Hotspot
Further inland, the Yellowstone Volcanic Field is the surface expression of a large hotspot—a mantle plume of hot rock rising from deep within the Earth. The hotspot itself is relatively stationary, but the North American plate is slowly moving southwestward over it. This movement has created a trail of progressively older caldera systems across the Snake River Plain of Idaho, with the active Yellowstone caldera currently sitting over the plume.
Basin and Range Province
A third major area of volcanism is found in the Basin and Range Province, covering parts of Arizona, New Mexico, Utah, and Nevada. Volcanic activity here is driven by continental rifting, where the crust is being stretched and thinned in an east-west direction. This extension allows magma to rise through fractures in the crust, leading to widespread volcanic fields such as the San Francisco Volcanic Field.
Current Status: Activity Levels and Monitoring
Volcanoes in the continental U.S. are continually assessed for their potential threat to people and infrastructure, with the USGS assigning them to one of five categories: very low, low, moderate, high, or very high threat potential. This classification system combines the inherent hazards of the volcano’s eruptive style and the level of exposure of nearby communities and aviation routes. The vast majority of the systems designated as “very high threat” in the contiguous U.S. are located in the Cascade Arc, including 11 volcanoes across Washington, Oregon, and California.
To track these systems, the USGS Volcano Hazards Program operates a network of observatories that use specialized instruments to detect signs of subterranean activity. Magma movement before an eruption typically triggers changes that these tools are designed to record.
Networks of seismometers listen for swarms of small earthquakes, which signal magma fracturing rock as it moves toward the surface. Scientists also use Global Positioning System (GPS) receivers and tiltmeters to measure minute changes in ground elevation and slope, indicating the swelling or deflation of a magma chamber. Researchers monitor the output of gases like carbon dioxide and sulfur dioxide, as changes in their volume or composition can be an early indicator of rising magma. These observations allow geologists to issue timely warnings and inform public safety measures.