The San Francisco Volcanic Field, which covers

about 1,800 square miles, is part of northern

Arizona’s spectacular landscape. Much of the

field lies within Coconino and Kaibab National

Forests. Forest life zones in the region range from

Piñon-Juniper at lower elevations through Ponderosa Pine to Fir and Bristlecone Pine at the highest elevations. The varied forests and geologic

features of the San Francisco Volcanic Field offer

diverse recreational opportunities, including

camping, hiking, mountain biking, wildlife viewing, and winter sports.

Almost all hills and mountains between

Flagstaff and the Grand Canyon are geologically

young but extinct volcanoes of the San Francisco

Volcanic Field. Without the volcanoes, this region

would be a flat, arid plateau. Instead, the area

includes both Arizona’s highest mountain—San

Francisco Mountain, with peaks rising to 12,633

feet—and the State’s youngest volcano—Sunset

Crater, which erupted less than 1,000 years ago

and has been a National Monument since 1930.

Why does Northern Arizona have so many

geologically young volcanoes? Most volcanoes

are located near boundaries of the Earth’s

tectonic plates, but Arizona is well within the

The San Francisco Volcanic Field, Arizona

N orthern Arizona’s San Fran-

 cisco Volcanic Field, much

of which lies within Coconino

and Kaibab National Forests, is

an area of young volcanoes

along the southern margin of the

Colorado Plateau. During its 6-

million-year history, this field has

produced more than 600 volcanoes. Their activity has created a

topographically varied landscape with forests that extend

from the Piñon-Juniper up to the

Bristlecone Pine life zones. The

most prominent landmark is San

Francisco Mountain, a stratovolcano that rises to 12,633 feet and

serves as a scenic backdrop to

the city of Flagstaff.

This digital elevation model (DEM) of the San Francisco Volcanic Field shows many of the more than 600

vents which have erupted in the area during the past 6 million years. Some lava flows (flat lobate features)

are easily recognized by their proximity to vents. Larger tectonic structures such as the northeast-trending

Mesa Butte Fault and Doney Fault are also seen. Flagstaff lies at the south-central edge of the volcanic

field nestled between the base of Elden Mountain and the Observatory Mesa flow emitted from A-1 Mountain.

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 017-01

2001

The peaks of San Francisco Mountain, an eroded stratovolcano—which includes Arizona’s highest point,

Humphreys Peak at 12,633 feet—tower over the ruins of an ancient Native American pueblo in Wupatki National

Monument. The ancient inhabitants of this area must have witnessed the eruption of nearby Sunset Crater, the

State’s youngest volcano, which erupted in about A.D. 1064. San Francisco Mountain and Sunset Crater are only

two of the hundreds of volcanoes in the San Francisco Volcanic Field, which covers about 1,800 square miles of

northern Arizona. (Copyrighted photo courtesy of Michael Collier.)

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

FOREST SERVICE

DE PA RTMENT OF AGRICULT UR

U S E

SP Crater

64

89

89

180

180

40

40

17

Mesa Butte 

 Fault

Doney 

Fault

Sunset 

Crater

San Francisco

Mountain

Elden Mountain

Flagstaff

Observatory

 Mesa

Williams

0 10 MILES

0 10 KILOMETERS

SP Crater

Mesa Butte 

 Fault

Doney 

Fault

Sunset 

Crater

San Francisco

Mountain

Elden Mountain

Flagstaff

Observatory

 Mesa

Williams

10 MILES

0

0

10 KILOMETERS
interior of the North American Plate. Some

geologists suggest that there is a site of localized

melting, or “hot spot,” fixed deep within the

Earth’s mantle beneath northern Arizona. As the

North American Plate moves slowly westward

over this stationary source of molten rock

(magma), eruptions produce volcanoes that are

strung out progressively eastward.

The first volcanoes in the San Francisco

Volcanic Field began to erupt about 6 million

years ago, in an area where the town of Williams

is now. Subsequently, a several-mile-wide belt of

successively younger eruptions migrated eastward, to the area of modern Flagstaff, and even a

bit beyond, toward the valley of the Little Colorado River. Today, this belt of volcanoes extends

about 50 miles from west to east.

Although there has been no eruption for nearly

1,000 years, it is likely that eruptions will occur

again in the San Francisco Volcanic Field. With an

average interval of several thousand years between past periods of volcanic activity, it is impossible to forecast when the next eruption will

occur. U.S. Geological Survey (USGS) scientists

believe that the most probable sites of future

eruptions are in the eastern part of the field and

that the eruptions are likely to be small. These

future eruptions may provide spectacular volcanic

displays but should pose little hazard because of

their small size and the relative remoteness of the

area.

Volcanoes and Types of Magma

A volcano is an opening where magma erupts

onto the surface as lava after rising from deep

within the Earth. Not all magma is the same.

Some magma contains as much as 75% silica

(SiO2

), whereas other magma contains as little as

about 50%. The more silica in a magma, the

higher its viscosity, or resistance to flow. Viscosity controls the type of volcano that forms.

Eruptions of high-viscosity magma build very

steep-sided lava domes. Low-viscosity magma

produces cinder cones and thin sheet-like lava

flows, and intermediate-viscosity magma creates

moderately steep mountains called stratovolcanoes.

Most of the more than 600 volcanoes in the San

Francisco Volcanic Field are basalt cinder cones.

Basalt has the lowest viscosity of all common magmas. Cinder cones are relatively small, usually less

than 1,000 feet tall, and form within months to years.

They are built when gas-charged frothy blobs of

basalt magma are erupted as an upward spray, or lava

fountain. During flight, these lava blobs cool and fall

back to the ground as dark volcanic rock containing

cavities created by trapped gas bubbles. If small,

these fragments of rock are called “cinders” and, if

larger, “bombs.” As the fragments accumulate, they

build a cone-shaped hill. Once sufficient gas pressure

has been released from the supply of magma, lava

oozes quietly out to form a lava flow. This lava typically squeezes out from the base of the cone and

tends to flow away for a substantial distance because

of its low viscosity. SP Crater, 25 miles north of

Flagstaff, is an excellent example of a cinder cone

and its associated lava flow.

Stratovolcanoes

 Stratovolcanoes have moderately steep

slopes and form by the accumulation of layer

upon layer of intermediate-viscosity (andesite)

lava flows, cinders, and ash, interspersed with

deposits from volcanic mudflows (lahars) at

lower elevations. These tall, cone-shaped volcanoes, such as Mount Rainier, Washington, and

Mount Fuji, Japan, normally rise to a central

peak and are built up by countless eruptions

over hundreds of thousands of years.

San Francisco Mountain is the only stratovolcano in the San Francisco Volcanic Field

and was built by eruptions between about 1 and

0.4 million years ago. Since then, much of the

mountain has been removed to create the

“Inner Basin.” The missing material may have

been removed quickly and explosively by an

eruption similar to the 1980 eruption of Mount

St. Helens, Washington, or it may have been

removed slowly and incrementally by a combination of large landslides, water erosion, and

glacial scouring.

Lava Domes

The San Francisco Volcanic Field also

includes several lava domes. Lava domes are

formed by dacite and rhyolite magmas, which

have high silica contents. Dacite and rhyolite are

so viscous that they tend to pile up and form very

steep-sided bulbous masses (domes) at the site of

eruption. Domes can be active for decades or

sometimes centuries. If a lava dome grows

entirely by internal inflation, similar to a balloon,

it is called an endogenous dome. If, however,

magma breaks out through a dome’s flank during

inflation and adds new lava layers to the outer

surface, the final dome is called exogenous.

Elden Mountain, at the eastern outskirts of

Flagstaff, is an excellent example of an exogenous

dacite dome and consists of several overlapping

lobes of lava. Sugarloaf Mountain, at the entrance

The work of USGS scientists, in cooperation

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

understanding of the history of volcanism in

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 the Pacific

Northwest, eastern California, Wyoming,

Alaska, and Hawaii.

Susan S. Priest, Wendell A. Duffield, Karen Malis-Clark,

James W. Hendley II, and Peter H. Stauffer

Graphic design by

Stephen L. Scott

Banner design by Bobbie Myers

COOPERATING ORGANIZATIONS

Bureau of Land Management

National Park Service

U.S. Department of Agriculture, Forest Service

For more information contact:

U.S. Geological Survey

2255 N Gemini Dr.

Flagstaff, AZ 86001

(520) 556-7148

http://volcanoes.usgs.gov/

or

U.S. Forest Service

Coconino National Forest,

Peaks Ranger District

5075 N. Highway 89

Flagstaff, AZ 86004

(520) 526-0866

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

See also What are Volcano Hazards? (USGS Fact Sheet

002-97), Volcanoes (USGS General Interest Publication

94-0195), andVolcanoes of Northern Arizona by Wendell

Duffield (Grand Canyon Association, 1997)

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

http://geopubs.wr.usgs.gov/fact-sheet/fs017-01

Printed on recycled paper

to San Francisco Mountain’s Inner Basin, is a

rhyolite lava dome. This dome is thought to be

endogenous, but its forest cover hides direct

evidence of its internal structure.

Elden Mountain is a steep-sided lava dome in the

San Francisco Volcanic Field. Lava domes are

formed by dacite and rhyolite magmas, which have

high silica contents. Dacite and rhyolite are so

viscous that they tend to pile up and form very

steep-sided bulbous masses (domes) at the site of

eruption.

SP Crater, in the San Francisco Volcanic Field,

is an excellent example of a cinder cone and

associated lava flow. This flow extends 4 miles

from the cone and is only about 100 feet thick.