U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet-108-96, version 2.1

Revised May 2000

U.S. GEOLOGICAL SURVEY—REDUCING THE RISK FROM VOLCANO HAZARDS

S

US SG

cientists have monitored

geologic unrest in the Long

Valley, California, area since 1980.

In that year, following a swarm of

strong earthquakes, they discovered that the central part of the

Long Valley Caldera had begun

actively rising. Unrest in the area

persists today. The U.S. Geological

Survey (USGS) continues to provide

the public and civil authorities

with current information on the

volcano hazards at Long Valley and

is prepared to give timely warnings

of any impending eruption.

as Nebraska. The Earth’s surface sank more

than 1 mile into the space once occupied by

the erupted magma, forming a large volcanic

depression that geologists call a caldera.

Today, Long Valley occupies the eastern

half of this 10-mile-wide, 20-mile-long

caldera. Magma still underlies the caldera and

heats underground water. The heated water

feeds local hot springs and natural steam vents

and drives three geothermal power plants, producing a combined 40 megawatts of electricity.

The Long Valley Caldera is only one part of

a large volcanic system in eastern California

that also includes the Mono-Inyo Craters volcanic chain. This chain extends from Mammoth Mountain at the southwest rim of the

caldera northward 25 miles to Mono Lake.

Eruptions along this chain began 400,000

years ago, and Mammoth Mountain itself was

formed by a series of eruptions ending 50,000

years ago. The volcanic system is still active.

Scientists have determined that eruptions occurred in both the Inyo Craters and Mono Craters parts of the volcanic chain as recently as

600 years ago and that small eruptions occurred in Mono Lake sometime between the

mid-1700’s and mid-1800’s.

Although no volcanic eruptions are known

to have occurred in eastern California since

those in Mono Lake, earthquakes occur frequently. These earthquakes are caused by

movement along faults and by the pressure of

magma rising beneath the Earth’s surface, two

closely related geologic processes. In 1872,

the magnitude 7.6 Owens Valley earthquake

was felt throughout most of California, and a

number of moderate (magnitude 5 to 6) earthquakes have shaken the Long Valley area during this century.

A period of ongoing geologic unrest in the

Long Valley area began in 1978, when a magnitude 5.4 earthquake struck 6 miles southeast

In the 1850’s, gold fever brought the first

waves of European settlers through the Long

Valley area of eastern California. Today, thousands of visitors are attracted to the area year

round by the spectacular mountain scenery of

the eastern Sierra Nevada. This landscape

has been sculpted over the past 4 million

years by glaciers, earthquakes, and volcanic

eruptions.

About 760,000 years ago a cataclysmic volcanic eruption in the area blew out 150 cubic

miles of magma (molten rock) from a depth

of about 4 miles beneath the Earth’s surface.

Rapidly moving flows of glowing hot ash

(pyroclastic flows) covered much of east-central California, and airborne ash fell as far east

Living With a Restless Caldera—Long Valley, California

Long Valley Caldera in eastern California (here viewed from its southwest rim toward its northeast rim on the horizon 18 miles away) was formed about 760,000 years ago in a violent volcanic eruption that blew out 150 cubic

miles of magma (molten rock) from beneath the Earth’s surface. U.S. Geological Survey scientists are tracking

continuing dome-like swelling centered in the low forested hills in the middle of the caldera. This swelling affects more

than 100 square miles and is caused by magma rising beneath the Earth’s surface. (USGS photo by Roy Bailey.)

Volcanic Rocks

Mono

 Lake

Mono Craters

Inyo

Craters LONG

VALLEY

CALDERA

Mammoth

Lakes

Mammoth

 Mountain

0 10 MILES

395

395

OWENS VALLEY Bishop

Bishop Tuff (arrows indicate 

directions of flow)

Mono-Inyo Craters volcanic chain

Other volcanic rocks

LONG

VALLEY

CALDERA

Known Distribution of

Wind-Blown Ash from the

Prehistoric Eruption that

Formed Long Valley Caldera

500 mi

1000 mi

Much of the Long Valley area of eastern California is covered by

rocks formed during volcanic eruptions in the past 2 million

years. A cataclysmic eruption 760,000 years ago formed Long

Valley Caldera and ejected flows of hot glowing ash (pyroclastic

flows), which cooled to form the Bishop Tuff. Wind-blown ash

from that ancient eruption—which was more than 2,000 times

larger than the 1980 eruption of Mt. St. Helens, Washington, that

killed 57 people and caused several billion dollars in damage—

covered much of the Western United States (inset map). The

photo shows a volcanic cone (Panum Dome) formed by eruptions about 600 years ago at the north end of the Mono Craters.

The most recent volcanic eruptions in the area occurred in

Mono Lake sometime between the mid-1700’s and mid-1800’s.
of the caldera. This temblor ended two decades of low quake activity in eastern California. The area has since experienced numerous

swarms of earthquakes, especially in the

southern part of the caldera and the adjacent

Sierra Nevada.

The most intense of these swarms began in

May 1980 and included four strong magnitude

6 shocks, three of which struck on the same

day. Immediately following these shocks, scientists from the U.S. Geological Survey

(USGS) began a reexamination of the Long

Valley area and detected other evidence of unrest—a dome-like uplift in the caldera. Their

measurements showed that the center of the

caldera had risen almost a foot since the summer of 1979, after decades of stability. This

continuing swelling, which by early 2000 totaled

nearly 2.5 feet and affects more than 100 square

miles, is caused by new magma rising beneath

the caldera.

In response to this escalating geologic unrest, the USGS intensified its monitoring

program in the Long Valley Caldera and

Mono-Inyo Craters volcanic system. An expanded network of seismometers installed in

1982 closely monitors earthquake activity in

the area, and other instruments track the continuing swelling in the caldera. Data from

these instruments help scientists to assess the

volcano hazards in the Long Valley area and

to recognize the early signs of possible eruptions. In cooperation with the California Office of Emergency Services and civil authorities in eastern California, the USGS has established procedures to promptly alert the

public to a possible eruption.

During the early 1990’s, trees began dying

off at several places on Mammoth Mountain

on the southwest edge of Long Valley Caldera.

Studies conducted by USGS and U.S. Forest

Service scientists show that the trees are being

killed by large amounts of carbon dioxide

For more information contact:

Earthquake Information Hotline (650) 329-4085

U.S. Geological Survey, Mail Stop 977

345 Middlefield Road, Menlo Park, CA 94025

http://quake.wr.usgs.gov/VOLCANOES/LongValley/

or

http://volcanoes.usgs.gov

See also Invisible CO2

 Gas Killing Trees at Mammoth Mountain, California (USGS Fact Sheet 172-96) and Future Eruptions

in California’s Long Valley Area—What’s Likely? (USGS Fact

Sheet 073-97)

COOPERATING ORGANIZATIONS

California Office of Emergency Services

California Division of Mines and Geology

Inyo County, California

Mono County, California

Town of Mammoth Lakes

Mammoth Mountain Ski Area

University of Nevada, Reno

U.S. Forest Service

David P. Hill, Roy A. Bailey,

 Michael L. Sorey, James W. Hendley II, and

Peter H. Stauffer

Graphic design by

 Sara Boore and Susan Mayfield

Banner design by Bobbie Myers

Earthquake activity in the Long Valley area of eastern

California increased greatly after 1978. This volcanically active area is located along the major faults

(black lines) that form the eastern edge of the Sierra

Nevada. The quake activity is caused by two closely

related geologic processes—movement along faults

and the pressure of magma (molten rock) rising beneath the Earth’s surface. Following four strong magnitude 6 shocks in May 1980, the U.S. Geological Survey

(USGS) detected dome-like swelling in the middle of

Long Valley Caldera (inset graph). In response to these

signs of renewed volcanic unrest, USGS scientists intensified their monitoring of the Long Valley area.

The U.S. Geological Survey (USGS), in cooperation with the California Office of Emergency Services (OES)

and local authorities, has established these procedures to respond to different levels of geologic unrest in

the Long Valley area of eastern California.

PLANNED USGS RESPONSE TO UNREST IN THE LONG VALLEY AREA

Geologic Behavior Condition USGS Response

TYPICAL BEHAVIOR: Since 1980, typical

background geologic activity in the Long Valley area has included as many as 20 earthquakes of magnitude 2 or smaller a day, occasional swarms of magnitude 3 and larger

earthquakes (felt locally), and uplift of the

center of Long Valley Caldera at a rate of

about 1 inch a year. Swarms including magnitude 4 earthquakes may occur about once a

year.

ROUTINE MONITORING. When appropriate, information calls placed to USGS personnel, Town, County, and State (OES and

California Division of Mines and Geology)

authorities, and locally operating Federal

agencies (U.S. Forest Service and Bureau of

Land Management) regarding felt earthquakes and notable changes in other types of

geologic activity, such as ground deformation, volcanic gas emissions, and fumarole

activity.

INTENSE UNREST (may occur about once a decade): For example, a quake swarm with at least

one magnitude 5 earthquake and (or) evidence of

increased magma movement or pressure at depth,

as indicated by a pronounced increase in the rate of

ground deformation.

INTENSIFIED MONITORING. Set up

emergency field headquarters in the

Long Valley area. WATCH message sent

by USGS to California OES, which

promptly notifies local authorities.

ERUPTION LIKELY within hours or days

(may occur every few hundred years): Strong

evidence of magma movement at shallow

depth.

GEOLOGIC HAZARD WARNING issued

by USGS to Governors of California and

Nevada and others, who inform the public.

Continue intensive on-site monitoring.

ERUPTION UNDERWAY (may occur every few hundred years).

SUSTAINED MONITORING AND COMMUNICATION. Maintain intensive monitoring and continuously keep civil authorities informed on progress of eruption and likely

future developments.

(NO

IMMEDIATE

RISK)

(WATCH)

(WARNING)

(ALERT)

RED

GREEN

YELLOW

ORANGE

(CO2) gas seeping up through the soil from

magma deep beneath Mammoth Mountain.

Such emissions of volcanic gas, as well as

earthquake swarms and ground swelling, commonly precede volcanic eruptions. When these

events precede an eruption of a “central vent”

volcano, such as Mount St. Helens, Washington, they normally last only a few weeks or

months. However, such symptoms of volcanic

unrest may persist for decades or even centuries at large calderas, such as Long Valley

Caldera. Recent studies indicate that only

about one in six such episodes of unrest at

large calderas worldwide actually culminates

in an eruption.

Over the past 4,000 years, small to moderate

eruptions have occurred somewhere along the

Mono-Inyo volcanic chain every few hundred

years, and the possibility remains that geologic

unrest in the Long Valley area could take only

weeks to escalate to an eruption. Nonetheless,

geologists think that the chances of an eruption

in the area in any given year are quite small.

To provide reliable and timely warning prior to

an eruption, scientists of the USGS Volcano Hazards Program continue to closely monitor geologic unrest in the Long Valley area of eastern

California and in other volcanic regions of the

United States, including Hawaii, the Pacific

Northwest, Wyoming, and Alaska. This ongoing work helps to better protect peoples lives

and property from volcano hazards.

Printed on recycled paper

2

1

0

1900 1920 1940 1960 1980 2000

YEAR

Mono 

Lake

UPLIFT

IN MIDDLE

OF CALDERA

FEET

LONG

VALLEY

CALDERA

MAY 1980

EARTHQUAKES

1978 through 1999

magnitude

6 to 6.2

5 to 6

3 to 5