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Dispersed volcanic fields are found through the western U.S.

The U.S. Geological Survey (USGS), Volcano Hazards Program, is responsible for monitoring all potentially active volcanoes in the country. Several volcanic centers distributed throughout the southwestern U.S., and the USGS arm of YVO is responsible for monitoring volcanoes in the Four Corners states (Utah, Colorado, New Mexico, and Arizona).

Sunset Crater, a basaltic cinder cone in the San Francisco Volcanic Field
Sunset Crater, a basaltic cinder cone in the San Francisco Volcanic Field, Arizona, erupted in 1085 A.D.

The volcanoes of the American Southwest include over forty volcanic fields containing about 1400 single volcanic vents that have erupted over the course of the past 2.5 million years. Most of these volcanoes are monogenetic, meaning that they only produced one eruptive episode that may have lasted for days to years. These mid-continent volcanoes occur where the crust is thin due to the tectonic processes of rifting and extension.

Magma rises through thin crust.

Rifting and extension cause the crust to fracture and thin, providing space for the mantle to rise upward. As mantle material moves to shallower depths, the confining pressure drops. Magma continues to rise because it is more buoyant than surrounding crustal rock. Upon reaching the surface, magma can erupt as lava flows and cinder cones—for example, Sunset Crater, Arizona; Dotsero, Colorado; and the Carrizozo lava flow, New Mexico. Magma may also stall beneath the surface, melt surrounding rock, and accumulate in large reservoirs that can erupt very violently, as was the case over one million years ago at Valles caldera, New Mexico.

Future eruptions will occur, but not necessarily in the same spot as previous ones. 

The locations of volcanic fields and individual volcanoes are dictated by zones of weak crust and areas of pre-existing magma storage. Future eruptions will not necessarily occur from the location of a previous vent. This is the nature of monogenetic volcanic fields—eruption sites open for a single episode of activity, and subsequent eruptions can occur anywhere within the field. This is the inherent volcanic hazard in the southwestern U.S.—new eruptions can originate at unknown locations within broad areas of widely varying land use patterns.

Even though volcanic eruptions are comparatively rare in the American Southwest, the Four Corners states were host to numerous volcanic eruptions in the past 10,000 years and will experience volcanic activity in the future. This is why YVO is investing in monitoring for potential signs of volcanic unrest in these states.