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University of Missouri Department of Geological Sciences

Rock type: Basaltic andesite

Location:        SP crater, AZ

gold line

SP crater is a small volcano called a cinder cone in northern Arizona, between Flagstaff and the

Grand Canyon.  It is made of a pile of lava fragments that were thrown out of a single vent, such as

the one you see here. Expanding gases caused the lava to fragment and be hurled out of the vent in

pieces, and many gas bubbles can been seen in this piece. As the eruption calmed down, lava

formed a flow that traveled some distance from the vent. A similar volcano called Paricutin formed

in Mexico, starting in 1943, growing to 1100 feet high in only one year, and ending in 1952. The

exact age of SP crater is not known, but nearby Sunset Crater last erupted about 900 years ago.

Basaltic andesite Basaltic andesite Basaltic andesite

Photo by Alan Whittington, MU Geology

USGS fact sheet on the San Francisco Volcanic Field, Arizona
MU Geology Department