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

California’s Restless Giant—the Long Valley Caldera

Scientists 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 volcanic hazard 

at Long Valley and is prepared to give timely 

warnings of any impending eruption.

In the 1850s, 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, a landscape 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—600 cubic kilometers (km3

)—

of magma (molten rock) from a depth of 

about 4 miles (6 km) beneath the Earth’s 

surface. Rapid flows of glowing hot ash 

(pyroclastic flows) covered much of eastcentral California, and airborne ash fell as far 

east as Nebraska. The Earth’s surface sank 

more than 1 mile (1.6 km) into the space 

vacated by the erupted magma, forming a 

large volcanic depression that geologists call 

a caldera.

Long Valley Caldera in eastern California (here viewed from its southwest rim toward its northeast rim on the 

horizon 18 miles, or 30 kilometers, away) was formed 760,000 years ago in a violent volcanic eruption that blew 

out 150 cubic miles (600 cubic kilometers) 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 (250 square kilometers) and is caused 

by magma rising beneath the Earth’s surface. (USGS photo by Roy Bailey.)

Today, broad and flat Long Valley, 

through which the Owens River lazily meanders on its way south, occupies the eastern half 

of this caldera, which is 10 miles (16 km) by 

20 miles (32 km) in extent. Magma still underlies the caldera and heats underground water, 

feeding local hot springs and steam vents. The 

underground heat now drives three geothermal 

power plants, producing a combined 40 megawatts of electricity.

Volcanic rocks

Mono

 Lake

Mono Craters

Inyo

Craters LONG

VALLEY

CALDERA

Mammoth

Lakes

Mammoth

 Mountain

0 10 MILES

5 KILOMETERS

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—more than 2,000 times larger than the 1980 eruption of 

Mount St. Helens, Washington, which 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-1700s and mid-1800s.

Long Valley Caldera is part of a large 

volcanic system in eastern California that also 

includes the Mono-Inyo Craters chain. This 

chain extends from Mammoth Mountain at 

the southwest rim of the caldera northward 

25 miles (40 km) to Mono Lake. Eruptions 

along this chain began 400,000 years ago, and 

Mammoth Mountain was formed by a series of 

eruptions ending 58,000 years ago. The volcanic system is still active—eruptions occurred in 

both the Inyo Craters and Mono Craters parts 

of the volcanic chain as recently as 600 years 

ago, and small eruptions occurred in Mono 

Lake sometime between the mid-1700s and 

mid-1800s.

Although no volcanic eruptions are 

known to have occurred in eastern California 

since those in Mono Lake, earthquakes occur 

frequently. These are caused by movement 

along faults and by the pressure of magma 

rising beneath the surface, two closely related 

geologic processes. In 1872, a magnitude 7.4 

earthquake centered 80 miles (125 km) south 

of Long Valley was felt throughout most of 

California, and moderate (magnitude 5 to 6) 

earthquakes have shaken the Long Valley area 

since 1978.

In 1978 a magnitude 5.4 earthquake 

struck 6 miles southeast of the caldera, heralding a period of geologic unrest in the Long 

Valley area that is still ongoing. That temblor 

ended two decades of low quake activity in 

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2014–3056

August 2014
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 on the same day. 

Following these shocks, scientists from 

the U.S. Geological Survey (USGS) began 

a reexamination of the Long Valley area, 

and they soon detected other evidence of 

unrest—a dome-like uplift within the caldera. Measurements showed that the center 

of the caldera had risen almost a foot (30 

centimeters) since the summer of 1979—

after decades of stability. This swelling, 

which by 2014 totaled more than 2.5 feet 

(75 centimeters) and affected more than 100 

square miles (250 km2

), is caused by new 

magma rising beneath the caldera.

In response to this increased unrest, 

USGS intensified its monitoring in the 

Long Valley region. Today, a state-of-theart network of seismometers and geodetic 

equipment closely monitors earthquake 

activity and the swelling in the caldera. Data 

from these instruments help scientists to 

assess the volcanic hazard in the Long Valley 

area and to recognize 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 1990s, trees began 

dying at several places on Mammoth Mountain on the southwest edge of Long Valley 

Caldera. Studies conducted by USGS and 

U.S. Forest Service scientists showed that 

the trees are being killed by large amounts of 

carbon dioxide (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 they precede an eruption of a 

“central vent” volcano, such as Mount St. 

Helens, Washington, they normally last only 

a few weeks or months. However, symptoms 

of volcanic unrest may persist for decades or 

centuries at large calderas, such as Long Valley Caldera. 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 the 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 

before an eruption, scientists of the USGS 

California Volcano Observatory continue to 

closely monitor geologic unrest in the Long 

Valley area of eastern California. The USGS 

Volcano Hazards Program monitors other volcanic regions of the United States, including 

Hawaii, the Pacific Northwest, Wyoming, and 

Alaska, to better protect people’s lives and 

property from volcano hazards.

David P. Hill, Roy A. Bailey,

James W. Hendley II, Peter H. Stauffer, 

and Mae Marcaida

Graphic design by

 Sara Boore, Susan Mayfield, 

and Vivian Nguyen

Banner design by Bobbie Myers

COOPERATING ORGANIZATIONS

California Office of Emergency Services

California Geological Survey

Inyo County, California

Mono County, California

Town of Mammoth Lakes

Mammoth Mountain Ski Area

University of Nevada, Reno

U.S. Forest Service 

For more information visit:

http://volcanoes.usgs.gov/observatories/calvo/

Mono

Lake

EARTHQUAKES

1978 through 2013

magnitude

3 to 5

5 to 6

6 to 6.2

5 MILES

5 KILOMETERS

0

1975 1985 1995 2005 2015

Year

0

1

2

3

Relative uplift, in feet

UPLIFT 

IN MIDDLE 

OF CALDERA

LONG 

VALLEY 

CALDERA

LONG 

VALLEY 

MAY 1980 MAY 1980

Earthquake activity in the Long Valley area of 

eastern California increased greatly after 1978. 

This area is located along 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 monitoring of the area.

Volcano Alert Levels Used by USGS Volcano Observatories

Alert Levels are intended to inform people on the ground about a volcano’s status and are issued in conjunction with the Aviation Color Code.

Term Description

NORMAL Volcano is in typical background, noneruptive state 

or, after a change from a higher level, volcanic activity has ceased and volcano has returned to noneruptive background state.

ADVISORY

Volcano is exhibiting signs of elevated unrest above known background level 

or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible

WATCH

Volcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain,

OR

eruption is underway but poses limited hazards.

WARNING Hazardous eruption is imminent, underway, or suspected.

renewed increase.

Notifications are issued for both increasing and decreasing volcanic activity and are accompanied by text with details (as known) about the

nature of the unrest or eruption and about potential or current hazards and likely outcomes.

 The USGS Alert-Notification System for Volcanic Activity

Volcano is in typical background, noneruptive state 

or, after a change from a higher level, volcanic activity has ceased and volcano has returned to noneruptive background state.

YELLOW

ORANGE

RED

Eruption is imminent with significant emission of volcanic ash into the atmosphere likely

OReruption is underway or suspected with significant emission of volcanic ash into the atmosphere 

GREEN

Color Description

Volcano is exhibiting signs of elevated unrest above known background level

or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible

Volcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain,

OR

eruption is underway with no or minor volcanic-ash emissions [ash-plume height specified, if possible].

Aviation Color Code Used by USGS Volcano Observatories

Color codes, which are in accordance with recommended International Civil Aviation Organization (ICAO) procedures, are intended to inform the aviation sector about a volcano’s status and are issued in conjunction with an Alert Level. Notifications are issued for both increasing and 

decreasing volcanic activity and are accompanied by text with details (as known) about the nature of the unrest or eruption, especially in 

regard to ash-plume information and likely outcomes.

[ash-plume height specified, if possible].

renewed increase.

The four Volcano Alert Levels are intended to inform people on the ground about a volcano’s status. 

Alerts are issued in conjunction with the Aviation Color Code, which is used to provide succinct 

information about volcanic-ash hazards to flying aircraft. For more information on Volcano Alert 

Levels and the Aviation Color Code, see http://volcanoes.usgs.gov/activity/alertsystem/index.php. ISSN 2327–6916 (print)

ISSN 2327–6932 (online)

http://dx.doi.org/10.3133/fs20143056