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E Living with a Restless Caldera— Long Valley, California

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E Living with a Restless Caldera— Long Valley, California REDUCING THE RISK FROM VOLCANO HAZARDS Living With a Restless Caldera— Long Valley, California arth scientists have Emonitored geologic un- rest in the Long Valley, Cali- fornia, area since 1980. In that year, following a swarm of strong earthquakes, sci- entists discovered that the central part of the Long Val- ley Caldera had begun ac- tively rising. Unrest in the area persists today. The USGS continues to provide the public and civil authori- ties with current information on the volcanic hazard at Long Valley and is prepared 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 to give timely warnings of cubic miles of magma (molten rock) from beneath the Earth’s surface. U.S. Geological Survey scientists are any impending eruption. 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. In the 1850’s, gold fever brought the first ered much of east-central California, and air- The Long Valley Caldera is only one part waves of European settlers through the Long borne ash fell as far east as Nebraska. The of a large volcanic system in eastern California Valley area of eastern California. Today, waves Earth’s surface sank more than 1 mile into the that also includes the Mono-Inyo Craters vol- of visitors are attracted to the area year round space once occupied by the erupted magma, canic chain. This chain extends from Mammoth by the spectacular mountain scenery of the east- forming a large volcanic depression that geolo- Mountain at the southwest rim of the caldera ern Sierra Nevada. This landscape has been gists call a caldera. northward 25 miles to Mono Lake. Eruptions sculpted over the past 4 million years by gla- Today, Long Valley occupies the eastern along this chain began 400,000 years ago, and ciers, earthquakes, and volcanic eruptions. half of this 10-mile-wide, 20-mile-long caldera. Mammoth Mountain itself was formed by a About 760,000 years ago a cataclysmic Magma still underlies the caldera and heats series of eruptions ending 50,000 years ago. The volcanic eruption in the area blew out 150 cu- underground water. The heated water feeds lo- volcanic system is still active. Scientists have bic miles of magma (molten rock) from a depth cal hot springs and natural steam vents and determined that eruptions occurred in both the of about 4 miles beneath the Earth’s surface. drives three geothermal power plants, produc- Inyo Craters and Mono Craters parts of the volca- Rapidly moving flows of glowing hot ash cov- ing a combined 40 megawatts of electricity. nic chain as recently as 600 years ago and that small eruptions occurred in Mono Lake sometime 1 0 0 0 between the mid-1700’s and mid-1800’s. 500 m m i i Although no volcanic eruptions are known LONG 0 10 MILES VALLEY to have occurred in eastern California since 395 Mono CALDERA Lake those in Mono Lake, earthquakes occur fre- Known Distribution of quently. These earthquakes are caused by move- Wind-Blown Ash from the Prehistoric Eruption that ment along faults and by the pressure of magma Mono Craters Formed Long Valley Caldera rising beneath the Earth’s surface, two closely Much of the Long Valley area of eastern California related geologic processes. In 1872, the mag- is covered by rocks formed during volcanic eruptions nitude 7.6 Owens Valley earthquake was felt in the past 2 million years. A cataclysmic eruption Inyo 760,000 years ago formed Long Valley Caldera and throughout most of California, and a number Craters LONG ejected flows of hot glowing ash, which cooled to of moderate (magnitude 5 to 6) earthquakes Mammoth VALLEY Mountain CALDERA form the Bishop Tuff. Wind-blown ash from that have shaken the Long Valley area during this ancient eruption—which was more than 2,000 times century. larger than the 1980 eruption of Mt. St. Helens, Mammoth A period of ongoing geologic unrest in the Lakes Washington, that killed 57 people and caused several billion dollars in damage—covered most of Long Valley area began in 1978, when a mag- Volcanic Rocks 395 the Western United States (inset map). The photo nitude 5.4 earthquake struck 6 miles southeast Bishop Tuff (arrows indicate shows a volcanic cone (Panum Dome) formed by of the caldera. This temblor ended two de- directions of flow) eruptions about 600 years ago at the north end of Mono-Inyo Craters volcanic chain the Mono Craters. The most recent volcanic cades of low quake activity in eastern Cali- eruptions in the area occurred in Mono Lake fornia. The area has since experienced nu- Other volcanic rocks OWENS VALLEY Bishop sometime between the mid-1700’s and mid-1800’s. merous swarms of earthquakes, especially in U.S. Department of the Interior U.S. Geological Survey the southern part of the caldera and the adja- Earthquake activity in the Long Valley area of Mono eastern California increased greatly after 1978. This cent Sierra Nevada. Lake 2 volcanically active area is located along the major The most intense of these swarms began faults (black lines) that form the eastern edge of the in May 1980 and included four strong magni- UPLIFT IN MIDDLE Sierra Nevada. The quake activity is caused by two tude 6 shocks, three of which struck on the same 1 OF CALDERA closely related geologic processes—movement day. Immediately following these shocks, sci- FEET along faults and the pressure of magma (molten rock) rising beneath the Earth’s surface. Following entists from the U.S. Geological Survey 0 four strong magnitude 6 shocks in May 1980, the (USGS) began a reexamination of the Long 1900 1920 1940 1960 1980 2000 U.S. Geological Survey (USGS) detected dome-like YEAR Valley area and detected other evidence of un- swelling in the middle of Long Valley Caldera (inset graph). In response to these signs of renewed rest—a dome-like uplift in the caldera. Their LONG measurements showed that the center of the VALLEY volcanic unrest, USGS scientists intensified their monitoring of the Long Valley area. caldera had risen almost a foot since the sum- CALDERA mer of 1979 after decades of stability. This con- Such emissions of volcanic gas, as well as earth- tinuing swelling, which now totals nearly 2 feet quake swarms and ground swelling, commonly and affects more than 100 square miles, is precede volcanic eruptions. When these events caused by new magma rising beneath the MAY 1980 precede an eruption of a “central vent” volcano, caldera. such as Mount St. Helens, Washington, they In response to this escalating geologic normally last only a few weeks or months. EARTHQUAKES unrest, the USGS intensified its monitoring pro- 1978 to 1995 However, such symptoms of volcanic unrest gram in Long Valley Caldera and Mono-Inyo magnitude may persist for decades or even centuries at Craters volcanic system. An expanded network 6 to 6.2 large calderas, such as Long Valley Caldera. of seismometers installed in 1982 closely moni- 5 to 6 3 to 5 Recent studies indicate that only about one in tors earthquake activity in the area, and other six such episodes of unrest at large calderas instruments track the continuing swelling in the worldwide actually culminates in an eruption. caldera. Data from these instruments help sci- During the early 1990’s, trees began dy- Over the past 4,000 years, small to mod- entists to assess the volcanic hazard in the Long ing off at several places on Mammoth Moun- erate eruptions have occurred somewhere along Valley area and to recognize the early signs of tain on the southwest edge of Long Valley the Mono-Inyo volcanic chain every few hun- possible eruptions. In cooperation with the Cali- Caldera. Studies conducted by USGS and U.S. dred years, and the possibility remains that geo- fornia Office of Emergency Services and civil Forest Service scientists show that the trees are logic unrest in the Long Valley area could take authorities in eastern California, the USGS has being killed by large amounts of carbon diox- only weeks to escalate to an eruption. Nonethe- established procedures to promptly alert the ide (CO2) gas seeping up through the soil from less, geologists think that the chances of an erup- public to a possible eruption. magma deep beneath Mammoth Mountain. tion in the area in any given year are quite small. To provide reliable and timely warning PLANNED USGS RESPONSE TO UNREST IN THE LONG VALLEY AREA prior to an eruption, scientists of the USGS Volcano Hazards Program continue to closely Geologic Behavior Condition USGS Response monitor geologic unrest in the Long Valley area TYPICAL BEHAVIOR: Since 1980, ROUTINE MONITORING. When of eastern California and in other volcanic re- typical background geologic activity in the appropriate, information calls placed to gions of the United States, including Hawaii, Long Valley area has included as many as USGS personnel, Town, County, and State 20 earthquakes of magnitude 2 or smaller GREEN (OES, California Division of Mines and the Pacific Northwest, Wyoming, and Alaska. a day, occasional swarms of magnitude 3 Geology) authorities, and locally operating This ongoing work helps to better protect the and larger earthquakes (felt locally), and (NO Federal agencies (U.S. Forest Service, lives and property of American citizens from uplift of the center of Long Valley Caldera Bureau of Land Management) regarding IMMEDIATE volcanic hazards.
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