Sunset Crater is one of the youngest geological features in
Arizona, only about 900 years old. It lies on the eastern edge
of the San Francisco Volcanic field. It is a classic cinder cone
whose red and orange tints are due to oxidation by steaming hot
gases which continued for some time after the other volcanic
activity subsided. The layer of cinders surrounding the crater
extends far to the east and north, covering an area of about 120
square miles. The cinders are very light weight because they are
filled with bubbles, due to the large admixture of pressurized
gas that was responsible for the ejection of this material high
into the air. The molten lava, having a lower gas content,
pushed out at the base of the cinder cone and flowed over the
surface. Little weathering or erosion of the cinder cone has
taken place due to the cool dry climate (the altitude here is
over 7000 feet above sea level). Rain and snow melt will sink
right through the cinders, not eroding the surface layers. The
western lava flow from