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Location Location: 35.4N, 111.5WElevation: 8,026 feet (2,447 m) Sunset Crater is in the eastern part of the San Francisco volcanic field. 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. 2
Facts Sunset Crater, one of the youngest cinder 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. 3
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. 4
In the 1920s, H.S. Colton saved the cone from severe damage by averting the attempt of a Hollywood movie company to blow it up in order to simulate an eruption. This led to the establishment of the Sunset Crater National Monument. 5
Ancient Indians undoubtedly witnessed the eruption of Sunset Crater Volcano in A.D. 1065 which blanketed the region with black cinder. Today the volcano's rim of red cinders and the lava flows near the cone seem to have cooled and hardened to a jagged surface only yesterday. To protect this fragile resource, Sunset Crater Volcano is closed to climbing and hiking. 6
Sunset Crater gets its name from the topmost cap of red, oxidized material which makes the volcano appear to be lit by the sunset. Sunset Crater appeared 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. 7
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 lava flows: the Kana-A flow in 1064 and the Bonito flow in 1180. They destroyed all living things in their paths. In a final burst of activity, around 1250, lava containing iron and sulfur 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 still to glow from intense volcanic heat. 8
Magma - molten rock and gases • Central vent • Cloud of ash, cinders, and bombs • Cone formed from larger lava fragments • Lava flow from base of volcano 9
Biblography • www.volcanoworld.com • Volcanoes of the world • Volcanoes online • Michigan Technological University Volcanoes Page 10
