Volcanism in the United States is often associated with the highly active regions of Alaska and Hawaii, yet the contiguous United States (the Lower 48) contains a substantial number of significant volcanic features. These geological structures represent a long history of magmatic activity and continue to shape the landscape across several western states. Understanding the scope of this activity requires defining what constitutes a volcano and assessing its potential for future activity. This exploration focuses exclusively on the volcanic landscape and monitoring status within the contiguous US.
The Official Tally of Volcanoes
A precise numerical tally of volcanoes depends heavily on the geological criteria used to define them. The United States Geological Survey monitors a large number of volcanoes across the nation, with a significant fraction located in the continental US. These features include large stratovolcanoes, cinder cones, and extensive volcanic fields.
The official count of geologically young volcanoes in the entire US is approximately 169, with over 50 located within the Lower 48 states. This total includes any vent or group of vents identified as having erupted in the last 10,000 years, or those showing signs of geothermal or seismic activity. The count is dynamic, as ongoing geological research occasionally identifies new features or reclassifies existing ones based on new data.
Categorizing Volcanic Risk and Status
Simply tallying the number of volcanoes does not convey the level of hazard each one poses to surrounding communities. Scientists differentiate volcanoes based on their activity and potential for eruption, using a framework that moves beyond simple labels like “active” or “extinct.”
The United States utilizes a formal threat assessment ranking system to categorize the risk level of monitored volcanoes. This system places volcanoes into categories such as High Threat, Moderate Threat, and Low Threat, based on the potential for an eruption and the proximity to populated areas. Factors like the history of past eruptions, seismic activity, ground deformation, and gas emissions contribute to a volcano’s placement within this framework. This classification helps allocate resources for monitoring, ensuring the most hazardous systems receive the greatest attention.
Key Volcanic Provinces of the Lower 48
The vast majority of the continental US volcanoes are concentrated in distinct geological settings, primarily along the western edge of the continent. The most recognized of these is the Cascade Range, a chain of peaks stretching from northern California through Oregon and into Washington. This arc is a direct result of the subduction of the Juan de Fuca tectonic plate beneath the North American plate, driving magma generation.
The Cascade volcanoes include large stratovolcanoes like Mount Shasta, Mount Hood, and Mount Rainier, which are the most prominent and frequently monitored systems in the Lower 48. Further inland, the Yellowstone Plateau Volcanic Field, centered in Wyoming but extending into Montana and Idaho, represents a different type of magmatic activity. This area is powered by a mantle plume, or hot spot, leading to massive caldera-forming eruptions in the distant past. These two provinces represent the major concentrations of volcanic hazard and scientific interest in the contiguous United States.