U.S. Department of the InteriorU.S. Geological Survey

USGS Fact Sheet-108-96, version 2.1Revised May 2000

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

S

GUS S

cientists have monitoredgeologic unrest in the Long

Valley, California, area since 1980.In that year, following a swarm ofstrong earthquakes, they discov-ered that the central part of theLong Valley Caldera had begunactively rising. Unrest in the areapersists today. The U.S. GeologicalSurvey (USGS) continues to providethe public and civil authoritieswith current information on thevolcano hazards at Long Valley andis prepared to give timely warningsof any impending eruption.

as Nebraska. The Earth’s surface sank morethan 1 mile into the space once occupied bythe erupted magma, forming a large volcanicdepression that geologists call a caldera.

Today, Long Valley occupies the easternhalf of this 10-mile-wide, 20-mile-longcaldera. Magma still underlies the caldera andheats underground water. The heated waterfeeds local hot springs and natural steam ventsand drives three geothermal power plants, pro-ducing a combined 40 megawatts of electricity.

The Long Valley Caldera is only one part ofa large volcanic system in eastern Californiathat also includes the Mono-Inyo Craters vol-canic chain. This chain extends from Mam-moth Mountain at the southwest rim of thecaldera northward 25 miles to Mono Lake.Eruptions along this chain began 400,000years ago, and Mammoth Mountain itself wasformed by a series of eruptions ending 50,000years ago. The volcanic system is still active.Scientists have determined that eruptions oc-curred in both the Inyo Craters and Mono Cra-ters parts of the volcanic chain as recently as600 years ago and that small eruptions oc-curred in Mono Lake sometime between themid-1700’s and mid-1800’s.

Although no volcanic eruptions are knownto have occurred in eastern California sincethose in Mono Lake, earthquakes occur fre-quently. These earthquakes are caused bymovement along faults and by the pressure ofmagma rising beneath the Earth’s surface, twoclosely related geologic processes. In 1872,the magnitude 7.6 Owens Valley earthquakewas felt throughout most of California, and anumber of moderate (magnitude 5 to 6) earth-quakes have shaken the Long Valley area dur-ing this century.

A period of ongoing geologic unrest in theLong Valley area began in 1978, when a mag-nitude 5.4 earthquake struck 6 miles southeast

In the 1850’s, gold fever brought the firstwaves of European settlers through the LongValley area of eastern California. Today, thou-sands of visitors are attracted to the area yearround by the spectacular mountain scenery ofthe eastern Sierra Nevada. This landscapehas been sculpted over the past 4 millionyears by glaciers, earthquakes, and volcaniceruptions.

About 760,000 years ago a cataclysmic vol-canic eruption in the area blew out 150 cubicmiles of magma (molten rock) from a depthof about 4 miles beneath the Earth’s surface.Rapidly moving flows of glowing hot ash(pyroclastic flows) covered much of east-cen-tral 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 hori-zon 18 miles away) was formed about 760,000 years ago in a violent volcanic eruption that blew out 150 cubicmiles of magma (molten rock) from beneath the Earth’s surface. U.S. Geological Survey scientists are trackingcontinuing dome-like swelling centered in the low forested hills in the middle of the caldera. This swelling affects morethan 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

InyoCraters

LONGVALLEY

CALDERA

MammothLakes

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

LONGVALLEYCALDERA

Known Distribution ofWind-Blown Ash from thePrehistoric Eruption thatFormed Long Valley Caldera

500 mi

1000m

i

Much of the Long Valley area of eastern California is covered byrocks formed during volcanic eruptions in the past 2 millionyears. A cataclysmic eruption 760,000 years ago formed LongValley Caldera and ejected flows of hot glowing ash (pyroclasticflows), which cooled to form the Bishop Tuff. Wind-blown ashfrom that ancient eruption—which was more than 2,000 timeslarger than the 1980 eruption of Mt. St. Helens, Washington, thatkilled 57 people and caused several billion dollars in damage—covered much of the Western United States (inset map). Thephoto shows a volcanic cone (Panum Dome) formed by erup-tions about 600 years ago at the north end of the Mono Craters.The most recent volcanic eruptions in the area occurred inMono Lake sometime between the mid-1700’s and mid-1800’s.

of the caldera. This temblor ended two de-cades of low quake activity in eastern Califor-nia. The area has since experienced numerousswarms of earthquakes, especially in thesouthern part of the caldera and the adjacentSierra Nevada.

The most intense of these swarms began inMay 1980 and included four strong magnitude6 shocks, three of which struck on the sameday. Immediately following these shocks, sci-entists from the U.S. Geological Survey(USGS) began a reexamination of the LongValley area and detected other evidence of un-rest—a dome-like uplift in the caldera. Theirmeasurements showed that the center of thecaldera had risen almost a foot since the sum-mer of 1979, after decades of stability. Thiscontinuing swelling, which by early 2000 totalednearly 2.5 feet and affects more than 100 squaremiles, is caused by new magma rising beneaththe caldera.

In response to this escalating geologic un-rest, the USGS intensified its monitoringprogram in the Long Valley Caldera andMono-Inyo Craters volcanic system. An ex-panded network of seismometers installed in1982 closely monitors earthquake activity inthe area, and other instruments track the con-tinuing swelling in the caldera. Data fromthese instruments help scientists to assess thevolcano hazards in the Long Valley area andto recognize the early signs of possible erup-tions. In cooperation with the California Of-fice of Emergency Services and civil authori-

ties in eastern California, the USGS has es-tablished procedures to promptly alert thepublic to a possible eruption.

During the early 1990’s, trees began dyingoff at several places on Mammoth Mountainon the southwest edge of Long Valley Caldera.Studies conducted by USGS and U.S. ForestService scientists show that the trees are beingkilled by large amounts of carbon dioxide

For more information contact:Earthquake Information Hotline (650) 329-4085

U.S. Geological Survey, Mail Stop 977345 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 Moun-tain, California (USGS Fact Sheet 172-96) and Future Eruptionsin California’s Long Valley Area—What’s Likely? (USGS Fact

Sheet 073-97)

COOPERATING ORGANIZATIONSCalifornia Office of Emergency Services

California Division of Mines and GeologyInyo County, California

Mono County, CaliforniaTown of Mammoth Lakes

Mammoth Mountain Ski AreaUniversity of Nevada, Reno

U.S. Forest Service

David P. Hill, Roy A. Bailey, Michael L. Sorey, James W. Hendley II, and

Peter H. StaufferGraphic design by

Sara Boore and Susan MayfieldBanner design by Bobbie Myers

Earthquake activity in the Long Valley area of easternCalifornia increased greatly after 1978. This volcani-cally active area is located along the major faults(black lines) that form the eastern edge of the SierraNevada. The quake activity is caused by two closelyrelated geologic processes—movement along faultsand the pressure of magma (molten rock) rising be-neath the Earth’s surface. Following four strong magni-tude 6 shocks in May 1980, the U.S. Geological Survey(USGS) detected dome-like swelling in the middle ofLong Valley Caldera (inset graph). In response to thesesigns of renewed volcanic unrest, USGS scientists in-tensified 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 inthe 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, typicalbackground geologic activity in the Long Val-ley area has included as many as 20 earth-quakes of magnitude 2 or smaller a day, occa-sional swarms of magnitude 3 and largerearthquakes (felt locally), and uplift of thecenter of Long Valley Caldera at a rate ofabout 1 inch a year. Swarms including magni-tude 4 earthquakes may occur about once ayear.

ROUTINE MONITORING. When appropri-ate, information calls placed to USGS per-sonnel, Town, County, and State (OES andCalifornia Division of Mines and Geology)authorities, and locally operating Federalagencies (U.S. Forest Service and Bureau ofLand Management) regarding felt earth-quakes and notable changes in other types ofgeologic activity, such as ground deforma-tion, volcanic gas emissions, and fumaroleactivity.

INTENSE UNREST (may occur about once a de-cade): For example, a quake swarm with at leastone magnitude 5 earthquake and (or) evidence ofincreased magma movement or pressure at depth,as indicated by a pronounced increase in the rate ofground deformation.

INTENSIFIED MONITORING. Set upemergency field headquarters in theLong Valley area. WATCH message sentby USGS to California OES, whichpromptly notifies local authorities.

ERUPTION LIKELY within hours or days(may occur every few hundred years): Strongevidence of magma movement at shallowdepth.

GEOLOGIC HAZARD WARNING issuedby USGS to Governors of California andNevada and others, who inform the public.Continue intensive on-site monitoring.

ERUPTION UNDERWAY (may occur ev-ery few hundred years).

SUSTAINED MONITORING AND COM-MUNICATION. Maintain intensive monitor-ing and continuously keep civil authorities in-formed on progress of eruption and likelyfuture developments.

(NOIMMEDIATE

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 asearthquake swarms and ground swelling, com-monly precede volcanic eruptions. When theseevents precede an eruption of a “central vent”volcano, such as Mount St. Helens, Washing-ton, they normally last only a few weeks ormonths. However, such symptoms of volcanicunrest may persist for decades or even centu-ries at large calderas, such as Long ValleyCaldera. Recent studies indicate that onlyabout one in six such episodes of unrest atlarge calderas worldwide actually culminatesin an eruption.

Over the past 4,000 years, small to moderateeruptions have occurred somewhere along theMono-Inyo volcanic chain every few hundredyears, and the possibility remains that geologicunrest in the Long Valley area could take onlyweeks to escalate to an eruption. Nonetheless,geologists think that the chances of an eruptionin the area in any given year are quite small.

To provide reliable and timely warning prior toan eruption, scientists of the USGS Volcano Haz-ards Program continue to closely monitor geo-logic unrest in the Long Valley area of easternCalifornia and in other volcanic regions of theUnited States, including Hawaii, the PacificNorthwest, Wyoming, and Alaska. This ongo-ing work helps to better protect peoples livesand property from volcano hazards.

Printed on recycled paper

2

1

0

1900 1920 1940 1960 1980 2000YEAR

Mono Lake

UPLIFTIN MIDDLE

OF CALDERA

FE

ET

LONGVALLEY

CALDERA

MAY 1980

EARTHQUAKES1978 through 1999

magnitude

6 to 6.25 to 63 to 5