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John Wesley Powell named �Sunset Mountain� in 1892 during his famous expedition (Cotton, 1945). President Herbert Hoover established Sunset Crater Volcano as a National Monument on May 26, 1930, to protect both the delicate volcanic features and any archaeological resources in the area. The land was transferred from the U.S. Forest Service to the National Park Service August 10, 1933, and was renamed on November 16, 1990. The primary natural resources of the park are the volcanic flows, ashfalls, craters, and ridges.
This park is part of the Geologic Resource Evaluation program because of the unique geologic resources and human impacts to these resources. Information gathered at this park may also be used to represent other parks with similar resources or patterns of use, especially when the findings are evaluated for Servicewide implications.
At an average elevation of 305 m (1,000 ft) above the valley floor, the rim of Sunset Crater Volcano dominates the landscape of the 3,040 acre national monument. It is part of a N 60 W trending 10 km long chain that includes Rows of Cones, Gyp Crater, and Vent512 (Blaylock, 1996). The base of the volcano is at 2,134 m (7,000 ft) above sea level (Hodges, 1962). The monument, located in north- central Arizona, is along the very southern edge of a feature called the Colorado Plateau Province (Figure 1). Covering parts of Colorado, Utah, Arizona, and New Mexico, the Colorado Plateau is a region of high plateaus and broad, rounded uplands separated by vast rangelands. The rangelands are underlain by large elliptical stratigraphic basins.
The structural fabric of gently warped, rounded folds contrasts with the intense deformation and faulting of the terranes bordering the Colorado Plateau. Northeast and east of the Colorado Plateau are the jagged peaks of the Rocky Mountains. The Mesozoic- age overthrust belt marks the west- northwest edge of the Colorado Plateau (Figure 1). The extensional, normal- faulted Basin and Range Province borders the Colorado Plateau to the west and south. The Rio Grande Rift, tearing a ragged scar in the landscape, forms the southeastern border.
The Colorado Plateau is also known for its laterally extensive monoclines that formed during the Late Cretaceous � Tertiary (Figure 1). The basins adjacent to the steep limbs of the monoclines have been filled with sediment eroded from these folds.
Many of the features present on the Colorado Plateau today were molded by the processes of erosion. The destructive forces of wind and rain, running water, and freezing temperatures attacked the uplifts as soon as all the tectonic havoc started in the Late Cretaceous. The Colorado Plateau has been uplifted about 3,660 m (12,000 ft) since the end of the Cretaceous about 66 million years ago (Fillmore, 2000). Some of this uplift occurred geologically rapidly.
The rate of erosion increased as the rate of uplift increased. The Colorado River carved its present course within the last 6 million years. With uplift, streams throughout the Colorado Plateau began to dissect the topography with unprecedented vigor, carving the rocks and carrying away the dismantled strata into the landscape we see today. The Grand Canyon located northwest of Sunset Crater Volcano is a relatively recent development on the plateau. However, few geologic features are more recent than the cinder cone at Sunset Crater Volcano National Monument, the youngest volcano in Arizona.
Last erupting around A.D. 1064, Sunset Crater Volcano is part of northern Arizona�s San Francisco Volcanic Field, much of which lays within Coconino and Kaibab national forests. This is an area of young volcanoes. Volcanism dates from late Miocene or Pliocene to recent (Hodges, 1962). Nearly all the hills and mountains between Flagstaff and the Grand Canyon are the young (geologically) but extinct volcanoes of the San Francisco Volcanic Field. The field covers about 4,662 square km (1,800 square miles) of northern Arizona�s pi�on- juniper and ponderosa pine to fir and bristlecone pine forested, semi- arid landscape.
This field has produced more than 550 basaltic vents during its 6 million years of volcanic activity (Holm and Moore, 1987). Extreme volcanic activity has created a topographically varied landscape. The most prominent regional landmark is San Francisco Mountain. This stratovolcano rises to 3,851 m (12,633 ft). As Arizona�s highest peak, it dominates the high, semi- arid horizon.
Most volcanoes are located near boundaries of the Earth�s lithospheric tectonic plates, including the Cascades in Washington and Oregon and the New Zealand volcanoes. However, Arizona is well within the interior of the North American Plate. Similar to Yellowstone National Park, a site of localized melting, or �hot spot,� is fixed deep within the Earth beneath northern Arizona.
As the North American Plate moves slowly westward over this stationary source of hot, molten rock in Arizona, eruptions produce volcanoes that extend progressively eastward in a roughly linear fashion. This is similar to the string of volcanics crossing southern Idaho including Craters of the Moon National Monument.
The first volcanoes began to erupt about 6 million years ago in the vicinity of the current Williams, Arizona. Following these early eruptions, a belt (several km wide) of successively younger eruptions migrated eastward for about 80 km (50 mi). At present the belt of volcanoes stretches just beyond the area of modern Flagstaff. Although there has been no significant volcanic activity in the San Francisco Volcanic Field for nearly 1,000 years, it is likely that eruptions will continue in the future. The average interval between large eruptions is several thousand years. Predicting future events is difficult.
Future activity may produce a cinder cone like the one at Sunset Crater Volcano. Cinder cones are relatively small structures. They are usually less than 1,000 feet tall and form quickly, within months to years. They are built as a result of gas- charged frothy blobs of basalt magma rising quickly to the surface. These blobs are erupted as an upward spray or lava fountain. As projectiles during flight, the lava blobs cool and fall back to the ground. As the fragments accumulate, they build a cone- shaped hill.
The eruptive activity at Sunset Crater was no doubt witnessed by Native Americans inhabiting the Wupatki area. The rapid geomorphological change must have affected their way of life. The aftermath of a volcanic eruption is pristinely preserved at Sunset Crater Volcano National Monument.
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