Volcanic Features

We have visited several of the volcanic features described on this webpage. The largest volcanic region visited on our previous trips is the San Francisco Volcanic Field, which covers an area of almost 5,000 km² around Flagstaff in northern Arizona. Over 800 volcanoes occur in this area, and about 600 of these volcanoes are cinder cones. Cinder cones are formed when the gas-charged lava erupts in a spray that falls back to the land to create the characteristic cone-shaped hill of red cinders around the vent (photograph below). Eventually, basalt, which has a lower gas content, flows from the base of the cinder cone. Sunset Crater erupted as recently as the year 1250. It rises 300 m above the surrounding land and has a diameter at its base of 1.6 km. Capulin Volcano in northeastern New Mexico also is a large cinder cone.

Cinder Cone, Sunset Crater National Monument, Arizona
Photograph by Mark Eberle, March 1999

A less common type of volcano in the San Francisco Volcanic Field is the stratovolcano or composite volcano. A composite volcano has the classical volcano shape that steepens as it gets higher and forms a sharp-peaked mountain. Composite volcanoes are formed by alternating layers of cinders and flows of basalt. Composite volcanoes sometimes erupt violently, destroying much of themselves in the process. The ring of summits and saddleback ridges comprising the San Francisco Peaks (below) are what remains of the once larger volcano after its eruption and subsequent erosion. The photograph was taken from the eastern exposure, which is lower than the others. Humphrey Peak, the highest of the resulting summits, is 3,854 m (12,643 feet) in elevation, the highest point in Arizona. Remnants of another good example of a composite volcano can be seen at Valle Grande in the Jemez Mountains near Albuquerque and Santa Fe, New Mexico.

Remnants of Composite Volcano, San Francisco Peaks, Arizona
Photograph by Mark Eberle, March 1999

Another volcanic feature we visit on our fieldtrips is the Valley of Fires lava flow in the Tularosa Basin of southern New Mexico. The molten lava emerged from vents in the valley floor and flowed 71 km (44 miles) south through the basin toward White Sands (White Sands and the Tularosa Basin are described elsewhere on this website). At its thickest point, the basalt is 50 m (165 feet) deep. Accumulations of wind-blown material and weathered basalt allow plants to grow among the cracks and depressions of the lava bed (photograph below). Most of the volcanic activity in New Mexico occurred along the Rio Grande Rift Valley and points farther west; however, volcanoes are generally limited to northern New Mexico (e.g., Valle Grande and Capulin Volcano).

Valley of Fires Lava Flow, Carrizozo, New Mexico
Photographs by Matt Withroder, March 2003

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