Sunset Crater appeared in the winter of 1064-65 when molten rock sprayed out of a crack in the ground high into the air, solidified, then fell to earth as large bombs or smaller cinders. As periodic eruptions continued over the next 200 years, the heavier debris accumulated around the vent creating a 1,000 foot cone. The lightest, smallest particles blew the farthest, dusting 800 square miles of northern Arizona with ash. Perhaps as spectacular as the original pyrotechnics were two subsequent lava flows: the Kana-A flow in 1064 and the Bonito flow  in 1180. They destroyed all living things in their paths.

The processes that created Sunset Crater also created a sculpture garden of extraordinary forms at its base. As new gas vents opened suddenly, spatter cones sprouted from the ground like minatures of the cone itself. Moving lava developed a crust on the surface where it cooled; caves were formed as the hotter material beneath drained away. Partially cooled lava, pushing through cracks like toothpaste from a tube, solidified into wedge-shaped squeeze-ups, grooved from scraping against the harder rock.

In a final burst of activity, around 1250, lava containing iron and sulphur shot out of the vent. The red and yellow oxidized particles fell back onto the rim as a permanent "sunset" so bright that the cone appears to glow from intense volcanic heat.

The Sunset Crater volcanic chain, comprised of four vents forming a volcanic chain 10 km long oriented N60W and three associated flows, is in eastern part of the San Francisco volcanic field, Arizona.

At the base of Sunset Crater, a 1-mile foot trail loops through the dramatic volcanic landscape. The brittle aa [pronounced ah-ah], similar to the lava from Hawaiian volcanoes, and the deep, loose cinders at Sunset Crater do not seem a hospitable environment, yet many species thrive here: ponderosa pines, Abert's squirrels, Steller's jays, pinyon pines, one-seed juniper, jackrabbits and pronghorn antelope, along with lizards, snakes, coyotes and bobcats.

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 amount 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 evaporation. 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.

Sunset Crater is in the eastern part of the San Francisco volcanic field. Sunset Crater, one of the youngest scoria cones in the contiguous United States, began erupting between the growing seasons of 1064 and 1065 A.D. Eruptions continued in the area for many decades. The cone was named by John Wesley Powell, first director of the U.S. Geological Survey, for the topmost cap of oxidized, red spatter which makes it appear bathed in the light of the sunset. The red, pink, and yellow colors at the top of the cone are silica, gypsum, and iron oxide that formed from fumaroles.

Sunset Crater is a nearly symmetrical cone made of dark-gray scoria and scattered bombs. The cone is about 1,000 feet (300 m) high and 1 mile (1.6 km) in base diameter.

The eruption at Sunset Crater covered and area of 800 square miles (2,100 square kilometers) with lapilli and ash. The eruption was Strombolian in style and large, with eruption columns as high as several hundred feet (meters). The tephra covers parts on the Bonito and Kana-a lava flows which were erupted from Sunset Crater. The Bonito lava flow erupted from the west and northwest base of the cone and covered an area of 1.8 square miles (4.6 square kilometers). The Kana-a lava flow was erupted from the base of the east side of the cone and traveled 6 miles (9.6 km) down a wash.

The Sunset eruption is unusual because the volume of volcanic products (about 0.7 cubic miles, 3 cubic km) is large for a strombolian event, the air fall dispersal was large, and the discharge rate for magma was high.

The Sunset event had a severe effect on the Sinagua Indians that lived in the area, forcing them to temporarily leave.

Glossary

Aa: Hawaiian word used to describe a lava flow whose surface is broken into rough angular fragments.

Ash: Fine particles of pulverized rock blown from an explosion vent. Measuring less than 1/10 inch in diameter, ash may be
either solid or molten when first erupted. By far the most common variety is vitric ash, glassy particles formed by gas bubbles
bursting through liquid magma.

Bomb: Fragment of molten or semi-molten rock, 2 1/2 inches to many feet in diameter, which is blown out during an eruption.
Because of their plastic condition, bombs are often modified in shape during their flight or upon impact.

Cinder cone: A volcanic cone built entirely of loose fragmented material (pyroclastics.)

Lapilli: Literally, "little stones"; round to angular rock fragments measuring 1/10 inch to 2 1/2 inches in diameter, which may be
ejected in either a solid or molten state.

Magma: Molten rock beneath the surface of the earth.

Pyroclastic: Pertaining to fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent.

San Francisco Mountain: San Francisco Mountain is the most prominent volcano in the San Francisco volcanic field. The volcanic field covers 1,935 square miles (5,000 square km) and includes more than 600 vents. Most of the vents erupted basalt lava flows. Basaltic scoria cones are scattered throughout the field. Intermediate to silica-rich eruptions formed a few localized volcanic centers.

Scoria: A bomb-size (> 64 mm) pyroclast that is irregular in form and generally very vesicular. It is usually heavier, darker, and more crystalline than pumice.

Spatter cone: A low, steep-sided cone of spatter built up on a fissure or vent; it is usually of basaltic material.

Strombolian eruption: A type of volcanic eruption characterized by jetting of clots or fountains of fluid basaltic lava from a
central crater.

Tephra: Materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air.

Vent: The opening at the earth's surface through which volcanic materials issue forth.