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Krystal TremblayUpward Bound Geology Student |
| Formation of a Cinder Cone |Sunset Crater |Bonito Lava Flows | | |
Sunset Crater is just one of the many cinder cones that lie within the San Francisco volcanic field that covers about 2,000 square miles of the southwestern Colorado plateau.
Cinder Cones, such as Sunset Crater, are formed by explosive volcanic eruptions. This happens when magma, or a mixture of molten rock and dissolved gases, rises upward from its underground source. As the magma rises closer to the earth's surface, the pressure drops and gases are released from the mountain. It is the high percentage of oxygen in the magma that makes it thick and therefore, creates a violent explosion. When the magma reaches the surface it is then called lava. As the gas-charged lava is blown into the air it quickly solidifies and falls as cinders around the vent. The cinders accumulate over time and create a circular or oval shaped cone that usually rises to approximately 1,000 feet.
Sunset Crater is one of the youngest cinder cones in The United States, erupting first in the winter of 1064 A.D. and continued erupting for the following 200 years. The eruption began along a 6 to 9 mile long fissure that tapped into a basalt source in the upper mantle. This style of eruption is called curtain of fire. Not long after the initial fissure phase the eruption centered itself at the northwest end of the fracture where the Sunset cinder cone grew. Almost 800 square miles of the Northern Arizona area was covered with ash and other small particles during the explosion.
Around the time of its final activity, Sunset Crater acquired its colorful appearance from yellow and red oxidized cinders that shot out of the vent. The ring of colorful cinders reminded the people of a bright sunset, giving the cone the name Sunset Crater.
There were two types of eruptions from the Sunset Crater, the Kana A and the Bonito lava flows. Together these two flows make up the other 25% of the magma that came from Sunset Crater. Approximately 75% of the magma at Sunset Crater erupted explosively as scoria. which, formed the cone.
The Kana A flow flowed to the northeast for seven miles in 1064-1065 A.D. The Bonito lava flow came in three stages in 1180 A.D., filling in a basin. The flows destroyed all living things in their paths. These two lava flows both consisted of AA lava (pronounced ah-ah). AA is a Hawaiian term for lava flows that has a surface consisting of rough, broken, lava blocks. As basaltic type lava flows further from its source, it cools, loses its gas, and becomes more viscous. A thick, brittle crust develops at its surface and continues to move forward slowly. The cooler outer region break up to produce a rough, jagged surface that can be sharp enough to cut a man's shoes.
This site was created in response to Upward Bound's field trip to the southwest for college credit. Eight students and five adults visited Nevada, Utah, as well as Arizona in order to do certain field experiments. While in Arizona where Sunset Crater lies, we looked at the different types of rocks, the lava flows, as well as the seismic activity around the area. We had to discover what minerals were present in the surrounding rock and answer questions concerning the cone's composition and characteristics. While in the tourist center we were able to observe an actual seismograph (although it was out of order at that time) and analyze past earthquakes in the area. The experience was great but I wish that it wasn't so windy out there.
(Photo Courtesy of Wendell Duffield, U.S. Geological Survey)
This page was last updated on Monday, May 20, 2002
This page was created by Krystal Tremblay
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