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.
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