The San Francisco volcanic field of northern Arizona is home to ~ 600 cinder (scoria) cones. The cones form during Strombolian eruptions when gases of rising magma burst resulting in the violent ejection of magma fragments – ranging from volcanic bombs to ash-sized particles – from the vent. The coarser material – volcanic bombs and scoria or cinders – fall back to earth quickly building a cone about the central vent. The resulting cinder cone is typically 1000 feet high and one-half mile in diameter at the base. The reddish-colored scoria – fist-sized particles – makes an excellent decorative stone, aggregate, and, in Flagstaff, it’s added to icy and snowy roads to provide traction for cars.
Online Resources:
US Geological Survey Fact Sheet, 'The San Francisco Volcanic Field, Arizona', https://pubs.usgs.gov/fs/2001/fs017-01/
US Geological Survey Fact Sheet, 'Red Mountain Volcano - a spectacular and unusual cinder cone in northern Arizona, https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5315773.pdf
US Geological Survey ~ Map showing geology, structure, and uranium deposits of the Flagstaff 1 degrees x 2 degrees quadrangle, Arizona, https://ngmdb.usgs.gov/Prodesc/proddesc_9245.htm
