San Francisco volcano field

Most of the more than 600 volcanoes in the San Francisco Volcanic Field are monogenetic basalt cinder cones. The cinder cones are relatively small, usually less than tall, and formed within months to years. They are built when gas-charged frothy blobs of basalt magma are erupted as a lava fountain. During flight, these lava blobs cool and fall back to the ground as dark volcanic rock containing cavities created by trapped gas bubbles. If small, these fragments of rock are called “cinders” and, if larger, “bombs.” As the fragments accumulate, they build a cone-shaped hill. Once sufficient gas pressure has been released from the supply of magma, lava oozes quietly out to form a lava flow. This lava typically squeezes out from the base of the cone and tends to flow away for a substantial distance because of its low viscosity. SP Crater, north of Flagstaff, is an excellent example of a cinder cone, and its associated lava flow extends for to the north of the cone. Several zones of concentrated eruptive activity have been identified – the more silicic volcanic centers appear to have begun with basaltic activity and then evolved to more silicic compositions.