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Home , Cinder cone, Conical hill, Volcanic field

T SER RES VIC FO E

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P U S R A U R LT TMENT OF AGRICU

U.S. GEOLOGICAL SURVEY and the U.S. FOREST SERVICE—OUR VOLCANIC PUBLIC LANDS Red Mountain —a Spectacular and Unusual in Northern

ed Mountain, located in R the of northern Arizona, 25 miles northwest of Flagstaff, is a volcanic cinder cone that rises 1,000 feet above the surrounding landscape. It is unusual in having the shape of a “U,” open to the west, and in lacking the symmetrical shape of most cinder cones. In addi- tion, a large natural amphithe- ater cuts into the cone’s north- east fl ank. Erosional pillars called “hoodoos” decorate the amphitheater, and many dark mineral crystals erode out of its walls. Studies by U.S. This aerial view of Red Mountain cinder cone in northern Arizona shows the deeply sculpted north- Geological Survey (USGS) and east fl ank of the volcano. The deep depression in the center of the image is a natural amphitheater, Northern Arizona University whose origin has been something of a geologic mystery. The back wall of the amphitheater is a scientists suggest that Red nearly vertical 800-foot cliff, which tapers off to the right and left. The dipping layers of volcanic Mountain formed in eruptions cinders that make up the cone can be seen in the walls of the amphitheater. The center of the vol- about 740,000 years ago. cano, the site of the vent where molten rock () was erupted, is on the far side of the ridge. (Copyrighted photo courtesy of Michael Collier.)

Red Mountain is one of several hundred not had enough time to expose their internal A visitor standing in the center of the cinder cones within a swath of volcanic features. Although human quarrying creates amphitheater, nearly surrounded by tower- landscape that extends 50 miles eastward frequently changing glimpses into a few ing cliffs of cinders, might think that this from Williams, Arizona, through Flagstaff of the cones in the volcanic field, quarries is the center of the volcano, the location of to the canyon of the Little River. generally are unsafe for tourists and public the vent where molten rock (magma) was call this belt of volcanoes the San access commonly is denied. erupted. However, the actual center of erup- Francisco , named for San Near Red Mountain a large Forest tion was over the back wall of the amphi- Francisco Mountain, whose tallest peak is Service sign along Highway 180 invites the theater, out of sight. The amphitheater is a 12,633 feet above sea level, the highest ele- motorist to exit and visit this cinder cone. A geologic feature that formed after the erup- vation in Arizona. Red Mountain rises about parking lot is located about a quarter mile tion ended and continues to be enlarged by 1,000 feet above the surrounding landscape, off the highway. A gentle, uphill, 30-minute today. and its crest is at 7,965 feet elevation. The walk from the parking lot brings the visitor San Francisco Volcanic Field has been active into a natural amphitheater carved into the for about 6 million years, and Red Mountain northeast flank of Red Mountain. The back An “Ideal” Cinder Cone is roughly 740,000 years old. wall of the amphitheater is a nearly verti- An “ideal” cinder cone forms when Red Mountain is unusual in that its cal 800-foot cliff, which tapers off to the eruption occurs on flat ground. From deep internal structure is exposed. This is not the right and left. Truncated layers of volcanic within the , magma charged with gas case at most cinder cones in the San Fran- cinders form ledges and color bands across (like a carbonated drink) rises through a cisco Volcanic Field, because erosion has the amphitheater walls. vertical pipe-shaped conduit and erupts as

U.S. Department of the Interior USGS Fact Sheet 024–02 U.S. Geological Survey 2002 The amphitheater of Red Mountain has many “hoodoos” (left, center fore- ground), rather bizarre that are especially common in the western part of the amphitheater. These 10- to 20-foot- tall, upward-tapering earth pinnacles of cinders are capped by 1- to 3-foot-wide boulders of dense . The boulder “sombrero” capping each pinnacle (see above) protects the underlying cinders from erosion. These hoodoos and other odd-looking spires, ridges, and ribs found in the amphitheater were almost certainly sculpted by water and wind erosion. a fountain of frothy lava that may spray as back to Earth. Many gas bubbles remain Periodically, the flanks of the growing high as 2,000 feet into the air. trapped in the fragments. If small, these may become so steep that lobes and As an individual blob of this frothy fragments of rock are called “cinders,” and sheets of cinders slide downward. When molten rock flies through the air, it cools if larger, “bombs.” As eruption continues, lava fountaining ends, a symmetrical quickly enough to solidify before falling cinders accumulate to form a . cone-shaped hill, commonly indented by a summit crater, has been added to the land- scape. Internally, the cone is a pile of loose This alluring view shows the foot trail leading to the am- cinders in layers that dip away from the phitheater of Red Mountain. volcano’s vent in all directions. Roughly the last half of the During the waning stage of an ideal foot trail follows a normally cinder-cone eruption, the magma has lost dry stream bed. The stream most of its gas content. This gas-depleted sand includes countless magma does not fountain but oozes quietly black shiny grains, a few as into the crater or beneath the base of the large as walnuts (see inset). cone as lava. Because it contains so few gas These grains are sometimes bubbles, the molten lava is denser than the mistaken for “Apache tears,” bubble-rich cinders. Thus, it burrows out which are composed of along the bottom of the cinder cone, lifting , the volcanic glass highly valued by ancient the less-dense cinders like a cork on water, cultures for crafting ar- and advances outward, creating a lava flow rowheads, knives, scrapers around the cone’s base. When the eruption and other tools. However, ends, a symmetrical cone of cinders sits at the abundant black grains at the center of a surrounding pad of lava. Red Mountain are crystals of minerals (pyroxene and amphibole) eroded from the Red Mountain Cinder Cone volcano. A close look at the Field studies conducted by U.S. walls of the amphitheater Geological Survey (USGS) and Northern will reveal crystals of these Arizona University scientists suggest minerals still embedded in cinders, waiting to be that Red Mountain grew on a nearly flat plucked out by water and surface that may have sloped gently to the wind erosion. north. However, little else about this vol- cano mimics the features of an ideal cinder 0 eastward enough to give the same ef- fect as a westerly wind, but this seems 0 unlikely because the driving force for rising magma is the buoyancy of very hot volcanic gases. Like a cork released in water, such gases tend to rise vertically rather than follow an inclined path. • The most likely possibility is that the waning-stage lava flow of the Red Mountain eruption rafted the west- ern section of the cinder cone away, Amphitheater like wood on flowing water. When RED gas-poor molten lava burrows its way MOUNTAIN outward beneath a cinder cone, it may either leave the cone undisturbed or carry pieces piggyback, literally float- ing pieces on the surface of the denser lava. Many examples of both situations are known worldwide; a spectacular example of rafting is found at National Monument, northeast of Flagstaff. At Red Mountain, geolo- gists have discovered several outcrops Approximate of layered cinder deposits, some of which are hundreds of feet wide and tens of feet thick, at the top of the lava flow. Typically, these “floaters” form on the surface of an otherwise fairly flat flow. Apparently, molten lava This digital elevation model (DEM) of the Red Mountain area shows the features of the cinder oozed out beneath the west base of cone. During the eruption that formed it, Red Mountain grew on a nearly fl at surface, which was Red Mountain and rafted away much eventually covered by a lava fl ow that extruded from the base of the cone during the eruption’s of that side of the cinder cone, creating waning phase. This lava fl ow rafted away some of the volcanic cinders that formed the western its U shape. side of the cone. When viewed from above, Red Mountain today is a U-shaped open to the west, rather than a symmetrical “ideal” cinder cone, with a large natural amphitheater carved into its northeast fl ank. U.S. Geological Survey and Northern Arizona University scientists think Origin of the Amphitheater that the amphitheater may have been blasted out by one or more steam explosions shortly after The origin of the Red Mountain amphi- eruption ceased at the volcano’s vent 740,000 years ago. theater is something of a geologic mystery. The truncated cinder layers exposed in cone. When viewed from above, Red The shape of Red Mountain and its the amphitheater walls are clear evidence Mountain is a U-shaped landform open to overall pattern of cinder layers raises the that material has been removed. However, the west, rather than a symmetrical cone. question of why a symmetrical cone was how this removal was accomplished is not The base of the U is a curving ridge that not created around the vent at the center of entirely clear. forms the highest part of the mountain. the U. Three possible explanations are: It seems unlikely that the entire am- The nearly half-mile-long arms of the U • If the lava-fountaining phase of eruption phitheater was created by water erosion, slope down to the west and merge with the occurred during a time of sustained because there is so little surface area to gently rolling surface of the Red Mountain wind blowing from west to east, most catch rainwater and snowmelt and funnel lava flow. cinders could have been blown east- it down channels to erode the side of the By carefully measuring the orienta- ward creating the asymmetrical shape volcano. An intriguing, though speculative, tion of cinder layers over all parts of Red of Red Mountain. However, eruptions possibility is that one or more steam explo- Mountain, geologists have mapped a radial of the type that built Red Mountain sions created an amphitheater-shaped hole pattern of layers, dipping away from the usually last several years to a decade in the side of Red Mountain shortly after middle of the U in all directions. At the or longer, and it seems unlikely that a eruption ceased. Newly erupted cinders amphitheater, all the exposed layers dip westerly wind could have persisted for probably cooled to about 600°F as they uniformly to the northeast. This pattern such a period of time. fell back to earth, but then remained well indicates that the vent is somewhere in the • Perhaps the conduit through which above the boiling temperature of water middle of the U and not at the amphitheater. magma rose to the surface was inclined for some time. Rainwater seeping into the GETTING TO RED MOUNTAIN

0 10 MILES

0 10 KILOMETERS 89

180 SP Crater Red 64 Mountain

Sunsett Craratterr 180 San FranciscoFrancisco Mountain

Williams 89 40

Flagstaff 40 17

To reach Red Mountain from Flagstaff, head northwest on Highway 180. This highway snakes through the central part of the San Francisco Volcanic Field on its way toward the Grand Canyon. About 25 miles from Flagstaff (at milepost 247 along the shoulder of the highway), turn left at a large Forest Service sign that announces the Red Mountain Geologic Area. Drive about a quarter mile on the dirt road to a parking space at the trailhead. The walk from there to the base of Red Mountain takes about 30 minutes. Carry plenty of drinking water on a hot day (at least 1 liter of water per person). Trees in the natural amphitheater provide some shade.

cone and circulating through the still hot boulders of dense lava. The boulder “som- COOPERATING ORGANIZATIONS cinders may have quickly deposited a brero” capping each pinnacle protects the strong mineral cement that bound cinders underlying cinders from erosion. Geolo- U.S. Department of Agriculture, Forest Service together, creating the equivalent of a sealed gists call this rather bizarre type of land- Northern Arizona University “pressure cooker.” Eventually, the pressure form an earth pinnacle or “hoodoo.” Much of the trapped superheated water may of the amphitheater is decorated with hoo- have exceeded the strength of that cooker, doos and other odd-looking spires, ridges, For more information contact: resulting in one or more steam explosions. and ribs, all of which were almost certainly Two characteristics of the cinder layers sculpted by water and wind erosion. U.S. Geological Survey seen in the amphitheater support this sce- The work of USGS and Northern Ari- 2255 N. Gemini Dr. Flagstaff, AZ 86001 nario. Although most cinder-cone volca- zona University scientists, in cooperation (928) 556–7148 noes are piles of loose cinders, the cinders with the U.S. Forest Service, has led to a http://volcanoes.usgs.gov/ exposed in the walls of the amphitheater better understanding of the geologic histo- or are partly cemented into hard outcrops by ry of Red Mountain and the San Francisco a mineral cement. Also, the cinders range Volcanic Field. This work is only part of U.S. Forest Service in color from nearly black to reddish and the USGS Volcano Hazards Program’s Coconino National Forest, Peaks Ranger District brownish tints, indicating contact and ongoing efforts to protect people’s lives 5075 N. Highway 89 chemical interactions with hot water and and property in all of the volcanic regions Flagstaff, AZ 86004 steam. of the United States, including Wyoming, (928) 526–0866 Even if a steam-blast scenario is cor- eastern , the Pacific Northwest, http://www.fs.fed.us/r3/coconino/ rect, the initial amphitheater opening has , and . See also The San Francisco Volcanic Field, Arizona (USGS been enlarged by normal surface-water Fact Sheet 017–01), What are Volcano Hazards? (USGS and wind erosion during the approxi- Fact Sheet 002–97), Volcanoes (USGS General Interest Susan S. Priest, Wendell A. Duffi eld, Nancy R. Riggs, Publication 94–0195), and Volcanoes of Northern Arizona mately 740,000 years since it formed. For Brian Poturalski, and Karen Malis-Clark by Wendell Duffi eld (Grand Canyon Association, 1997). example, here and there, especially in the Edited by James W. Hendley II and Peter H. Stauffer western part of the amphitheater, 10- to This Fact Sheet and any updates to it are available on line at 20-foot-tall, upward-tapering pinnacles of Graphic design by Steven L. Scott http://geopubs.wr.usgs.gov/fact-sheet/fs024-02/ cinders are capped by 1- to 3-foot-wide Banner design by Bobbie Myers

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