Red Mountain is one of several hundred 

cinder cones within a swath of volcanic 

landscape that extends 50 miles eastward 

from Williams, Arizona, through Flagstaff 

to the canyon of the Little Colorado River. 

Geologists call this belt of volcanoes the San 

Francisco Volcanic Field, named for San 

Francisco Mountain, whose tallest peak is 

12,633 feet above sea level, the highest elevation in Arizona. Red Mountain rises about 

1,000 feet above the surrounding landscape, 

and its crest is at 7,965 feet elevation. The 

San Francisco Volcanic Field has been active 

for about 6 million years, and Red Mountain 

is roughly 740,000 years old.

 Red Mountain is unusual in that its 

internal structure is exposed. This is not the 

case at most cinder cones in the San Francisco Volcanic Field, because erosion has 

not had enough time to expose their internal 

features. Although human quarrying creates 

frequently changing glimpses into a few 

of the cones in the volcanic field, quarries 

generally are unsafe for tourists and public 

access commonly is denied. 

Near Red Mountain a large Forest 

Service sign along Highway 180 invites the 

motorist to exit and visit this cinder cone. A 

parking lot is located about a quarter mile 

off the highway. A gentle, uphill, 30-minute 

walk from the parking lot brings the visitor 

into a natural amphitheater carved into the 

northeast flank of Red Mountain. The back 

wall of the amphitheater is a nearly vertical 800-foot cliff, which tapers off to the 

right and left. Truncated layers of volcanic 

cinders form ledges and color bands across 

the amphitheater walls.

A visitor standing in the center of the 

amphitheater, nearly surrounded by towering cliffs of cinders, might think that this 

is the center of the volcano, the location of 

the vent where molten rock (magma) was 

erupted. However, the actual center of eruption was over the back wall of the amphitheater, out of sight. The amphitheater is a 

geologic feature that formed after the eruption ended and continues to be enlarged by 

erosion today.

An “Ideal” Cinder Cone

An “ideal” cinder cone forms when 

eruption occurs on flat ground. From deep 

within the Earth, magma charged with gas 

(like a carbonated drink) rises through a 

vertical pipe-shaped conduit and erupts as 

FOREST SERVICE

DE PA RTMENT OF AGRICULT UR

U S E

This aerial view of Red Mountain cinder cone in northern Arizona shows the deeply sculpted northeast fl ank of the volcano. The deep depression in the center of the image is a natural amphitheater, 

whose origin has been something of a geologic mystery. The back wall of the amphitheater is a 

nearly vertical 800-foot cliff, which tapers off to the right and left. The dipping layers of volcanic 

cinders that make up the cone can be seen in the walls of the amphitheater. The center of the volcano, the site of the vent where molten rock (magma) was erupted, is on the far side of the ridge. 

(Copyrighted photo courtesy of Michael Collier.)

 ed Mountain, located in 

 the Coconino National 

Forest 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 addition, a large natural amphitheater cuts into the cone’s northeast fl ank. Erosional pillars 

called “hoodoos” decorate 

the amphitheater, and many 

dark mineral crystals erode 

out of its walls. Studies by U.S. 

Geological Survey (USGS) and 

Northern Arizona University 

scientists suggest that Red 

Mountain formed in eruptions 

about 740,000 years ago.

U.S. GEOLOGICAL SURVEY and the U.S. FOREST SERVICE—OUR VOLCANIC PUBLICLANDS

Red Mountain Volcano—a Spectacular and Unusual 

Cinder Cone in Northern Arizona

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 024–02

2002

R
The amphitheater of Red Mountain has 

many “hoodoos” (left, center foreground), rather bizarre landforms that are 

especially common in the western part of 

the amphitheater. These 10- to 20-foottall, upward-tapering earth pinnacles of 

cinders are capped by 1- to 3-foot-wide 

boulders of dense lava. 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.

This alluring view shows the 

foot trail leading to the amphitheater of Red Mountain. 

Roughly the last half of the 

foot trail follows a normally 

dry stream bed. The stream 

sand includes countless 

black shiny grains, a few as 

large as walnuts (see inset). 

These grains are sometimes 

mistaken for “Apache tears,” 

which are composed of 

obsidian, the volcanic glass 

highly valued by ancient 

cultures for crafting arrowheads, knives, scrapers 

and other tools. However, 

the abundant black grains at 

Red Mountain are crystals 

of minerals (pyroxene and 

amphibole) eroded from the 

volcano. A close look at the 

walls of the amphitheater 

will reveal crystals of these 

minerals still embedded 

in cinders, waiting to be 

plucked out by water and 

wind erosion.

a fountain of frothy lava that may spray as 

high as 2,000 feet into the air.

As an individual blob of this frothy 

molten rock flies through the air, it cools 

quickly enough to solidify before falling 

back to Earth. Many gas bubbles remain 

trapped in the fragments. If small, these 

fragments of rock are called “cinders,” and 

if larger, “bombs.” As eruption continues, 

cinders accumulate to form a conical hill. 

Periodically, the flanks of the growing 

hill may become so steep that lobes and 

sheets of cinders slide downward. When 

lava fountaining ends, a symmetrical 

cone-shaped hill, commonly indented by a 

summit crater, has been added to the landscape. Internally, the cone is a pile of loose 

cinders in layers that dip away from the 

volcano’s vent in all directions.

During the waning stage of an ideal 

cinder-cone eruption, the magma has lost 

most of its gas content. This gas-depleted 

magma does not fountain but oozes quietly 

into the crater or beneath the base of the 

cone as lava. Because it contains so few gas 

bubbles, the molten lava is denser than the 

bubble-rich cinders. Thus, it burrows out 

along the bottom of the cinder cone, lifting 

the less-dense cinders like a cork on water, 

and advances outward, creating a lava flow 

around the cone’s base. When the eruption 

ends, a symmetrical cone of cinders sits at 

the center of a surrounding pad of lava.

Red Mountain Cinder Cone

Field studies conducted by U.S. 

Geological Survey (USGS) and Northern 

Arizona University scientists suggest 

that Red Mountain grew on a nearly flat 

surface that may have sloped gently to the 

north. However, little else about this volcano mimics the features of an ideal cinder 
cone. When viewed from above, Red 

Mountain is a U-shaped landform open to 

the west, rather than a symmetrical cone. 

The base of the U is a curving ridge that 

forms the highest part of the mountain. 

The nearly half-mile-long arms of the U 

slope down to the west and merge with the 

gently rolling surface of the Red Mountain 

lava flow.

By carefully measuring the orientation of cinder layers over all parts of Red 

Mountain, geologists have mapped a radial 

pattern of layers, dipping away from the 

middle of the U in all directions. At the 

amphitheater, all the exposed layers dip 

uniformly to the northeast. This pattern 

indicates that the vent is somewhere in the 

middle of the U and not at the amphitheater.

The shape of Red Mountain and its 

overall pattern of cinder layers raises the 

question of why a symmetrical cone was 

not created around the vent at the center of 

the U. Three possible explanations are:

• If the lava-fountaining phase of eruption 

occurred during a time of sustained 

wind blowing from west to east, most 

cinders could have been blown eastward creating the asymmetrical shape 

of Red Mountain. However, eruptions 

of the type that built Red Mountain 

usually last several years to a decade 

or longer, and it seems unlikely that a 

westerly wind could have persisted for 

such a period of time.

• Perhaps the conduit through which 

magma rose to the surface was inclined 

eastward enough to give the same effect as a westerly wind, but this seems 

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 western section of the cinder cone away, 

like wood on flowing water. When 

gas-poor molten lava burrows its way 

outward beneath a cinder cone, it may 

either leave the cone undisturbed or 

carry pieces piggyback, literally floating pieces on the surface of the denser 

lava. Many examples of both situations 

are known worldwide; a spectacular 

example of rafting is found at Sunset 

Crater National Monument, northeast 

of Flagstaff. At Red Mountain, geologists have discovered several outcrops 

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 

hills on the surface of an otherwise 

fairly flat flow. Apparently, molten lava 

oozed out beneath the west base of 

Red Mountain and rafted away much 

of that side of the cinder cone, creating 

its U shape. 

Origin of the Amphitheater

The origin of the Red Mountain amphitheater is something of a geologic mystery. 

The truncated cinder layers exposed in 

the amphitheater walls are clear evidence 

that material has been removed. However, 

how this removal was accomplished is not 

entirely clear.

It seems unlikely that the entire amphitheater was created by water erosion, 

because there is so little surface area to 

catch rainwater and snowmelt and funnel 

it down channels to erode the side of the 

volcano. An intriguing, though speculative, 

possibility is that one or more steam explosions created an amphitheater-shaped hole 

in the side of Red Mountain shortly after 

eruption ceased. Newly erupted cinders 

probably cooled to about 600°F as they 

fell back to earth, but then remained well 

above the boiling temperature of water 

for some time. Rainwater seeping into the 

This digital elevation model (DEM) of the Red Mountain area shows the features of the cinder 

cone. During the eruption that formed it, Red Mountain grew on a nearly fl at surface, which was 

eventually covered by a lava fl ow that extruded from the base of the cone during the eruption’s 

waning phase. This lava fl ow rafted away some of the volcanic cinders that formed the western 

side of the cone. When viewed from above, Red Mountain today is a U-shaped landform 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 

that the amphitheater may have been blasted out by one or more steam explosions shortly after 

eruption ceased at the volcano’s vent 740,000 years ago.

Approximate

Amphitheater

RED

MOUNTAIN

0

0
cone and circulating through the still hot 

cinders may have quickly deposited a 

strong mineral cement that bound cinders 

together, creating the equivalent of a sealed 

“pressure cooker.” Eventually, the pressure 

of the trapped superheated water may 

have exceeded the strength of that cooker, 

resulting in one or more steam explosions.

Two characteristics of the cinder layers 

seen in the amphitheater support this scenario. Although most cinder-cone volcanoes are piles of loose cinders, the cinders 

exposed in the walls of the amphitheater 

are partly cemented into hard outcrops by 

a mineral cement. Also, the cinders range 

in color from nearly black to reddish and 

brownish tints, indicating contact and 

chemical interactions with hot water and 

steam.

Even if a steam-blast scenario is correct, the initial amphitheater opening has 

been enlarged by normal surface-water 

and wind erosion during the approximately 740,000 years since it formed. For 

example, here and there, especially in the 

western part of the amphitheater, 10- to 

20-foot-tall, upward-tapering pinnacles of 

cinders are capped by 1- to 3-foot-wide 

COOPERATING ORGANIZATIONS

U.S. Department of Agriculture, Forest Service

Northern Arizona University

For more information contact:

U.S. Geological Survey

2255 N. Gemini Dr.

Flagstaff, AZ 86001

(928) 556–7148

http://volcanoes.usgs.gov/

or

U.S. Forest Service

Coconino National Forest,

Peaks Ranger District

5075 N. Highway 89

Flagstaff, AZ 86004

(928) 526–0866

http://www.fs.fed.us/r3/coconino/

See also The San Francisco Volcanic Field, Arizona (USGS 

Fact Sheet 017–01), What are Volcano Hazards? (USGS 

Fact Sheet 002–97), Volcanoes (USGS General Interest 

Publication 94–0195), and Volcanoes of Northern Arizona

 by Wendell Duffi eld (Grand Canyon Association, 1997).

This Fact Sheet and any updates to it are available on line at 

http://geopubs.wr.usgs.gov/fact-sheet/fs024-02/

boulders of dense lava. The boulder “sombrero” capping each pinnacle protects the 

underlying cinders from erosion. Geologists call this rather bizarre type of landform an earth pinnacle or “hoodoo.” Much 

of the amphitheater is decorated with hoodoos and other odd-looking spires, ridges, 

and ribs, all of which were almost certainly 

sculpted by water and wind erosion.

The work of USGS and Northern Arizona University scientists, in cooperation 

with the U.S. Forest Service, has led to a 

better understanding of the geologic history of Red Mountain and the San Francisco 

Volcanic Field. This work is only part of 

the USGS Volcano Hazards Program’s 

ongoing efforts to protect people’s lives 

and property in all of the volcanic regions 

of the United States, including Wyoming, 

eastern California, the Pacific Northwest, 

Hawaii, and Alaska.

Susan S. Priest, Wendell A. Duffi eld, Nancy R. Riggs, 

Brian Poturalski, and Karen Malis-Clark

Edited by James W. Hendley II and Peter H. Stauffer

Graphic design by Steven L. Scott

Banner design by Bobbie Myers

GETTING TO RED MOUNTAIN

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.

Printed on recycled paper

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