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 Sunset Crater, the Bonita flow, is still fresh and dark. Closer inspection, however, will show that lichen is already spotting the surface of the lava. Some trees have taken root in a few cinder patches and grass and shrubs are moving in from the edges of the flow. The most common type of lava in this flow is aa, which is extremely rough and hard to walk on.
There are about 400 cinder cones in the San Francisco Volcanic field, of which Sunset Crater is one of the youngest. These are remnants of the last volcanic activity in this region, long after San Francisco Peak itself had taken its current form. The activity from these cinder cones first drove the local population away from the area. When they later returned, they found that the cinders acted to conserve the moisture in the soil by curbing eveporation. This led to a short lived population explosion on the edges of the cinder fields. Wupatki National Monument is adjacent to Sunset Crater. Several of the small communities that were built nearby can be seen on the loop drive.
Species Lists for Sunset Crater National Monument are available from the National Park Service. The official webpage for the park is also available from the NPS.
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