Long Valley Caldera and Mono-Inyo Craters
Volcanic Field, California

Location: 37.6N, 118.8W

Long Valley caldera, located at the boundary between the Sierra Nevada and the Basin and Range Province, is one of the largest Quaternary rhyolitic volcanic centers in North America. The caldera is elliptical in shape and 10 by 20 miles (15 by 30 km) in size. The elevation of the floor of the caldera is 6,500 feet (2,000 m) in the east and 8,500 feet (2600 m) in the west. The elevation of the walls of the caldera reach elevations of 9,800-11,500 feet (3000-3500 m) except in the east where the wall rises only 500 feet (150 m) to an elevation of 7,550 feet (2,300 m).

Space Shuttle photograph of Mono Lake, the Long Valley area, and the adjacent east front of the Sierra Nevada. Look closely and you can see the contrails of two jet aircraft flying over Mono Lake and their contrails' shadows on the ground.

This photograph shows the Long Valley caldera complex and eastern Sierra Nevada from the summit of White Mountain Peak. The Bishop Tuff and Volcanic Tableland form the wooded ridge bounding the eastern part of the caldera. View is to the west. Photo by R. Forrest Hopson.

Volcanic activity began in the area about 3.6 million years ago when trachybasalt and trachyandesite lava flows covered an area of about 1,500 square miles (4,000 square km). A short time later, geologically speaking, rhyodacite was erupted as flows and domes. Geologists interpreted these more silica-rich compositions as the first eruptive products of a growing magma chamber.

The Long Valley caldera was produced by a catastrophic eruption about 730,000 years ago. The roof above the magma chamber collapsed, forcing 150 cubic miles (600 cubic km) of rhyolitic magma to the surface in the form of Plinian ash columns and associated air falls and ash flows. The volume of ash is comparable to similar caldera-forming eruptions at Yellowstone and far exceeds the volume of ash erupted from stratovolcanoes. For example, the large eruption of Tambora in 1815 produced 10 cubic miles (40 cubic km) of ash.

A three-dimensional perspective of the southern caldera boundary and Mammoth Mountain is shown on NASA's homepage Space radar images of Earth.

The Bishop tuff was erupted during the catastrophic eruption that created Long Valley caldera. This photo shows the Bishop tuff exposed in the Owens River Gorge in the eastern Sierra Nevada about 240 miles (380 km) north of Los Angeles. The White Mountains are in the background. View is to the southeast. Photo by R. Forrest Hopson.

After the catastrophic eruption, volcanism continued on the caldera floor, producing a thin layer rhyolitic tephra and lava. Pressure increased within the magma chamber and forced the overlying rocks upward, forming a resurgent dome. The resurgent dome formed within 100,000 years after the caldera.

Eruptions 500,000, 300,000, and 100,000 years ago along the periphery of the resurgent dome produced thick, steep-sided rhyolitic lava flows and domes. These volcanic products are the youngest lavas to originate from the magma chamber.

Mammoth Mountain (11,050 feet), is a composite volcano made up of about 12 rhyodacite and quartz latite domes extruded along the southwest rim of Long Valley caldera from 200,000 to 50,000 years ago. Mammoth Mountain is one of the eruptive centers that developed late in the evolutionary cycle of the Long Valley caldera complex. Photograph by R. Forrest Hopson.

Between 200,000 and 100,000 years ago, trachybasalt was erupted southwest of the caldera rim. The lava ponded behind a ridge of glacial deposits and cooled to produce spectacular columns. The basalt columns can be viewed at Devils Postpile National Monument.

The history and deposits of the Mono-Inyo Craters volcanic field overlap with Long Valley caldera in time and space. The Mono-Inyo Craters volcanic field developed along a 30-mile-long (50 km) fissure system that extends northward from Mammoth Mountain on the southwestern rim of the caldera to Mono Lake.

Mafic eruptions began 300,000-200,000 years ago within the caldera and migrated to the north. The youngest mafic lavas erupted at Black Point near Mono Lake 13,300 years ago.

This photograph of the Mono-Inyo Craters volcanic field is from the summit of Mammoth Mountain and looks to the north. Photo by R. Forrest Hopson.

Rhyolitic volcanism began on the Mono Craters chain northwest of the caldera about 35,000 years ago. Mono Craters consists of a chain of at least 30 coalesced domes, flows, and craters. The youngest feature is only 600 years old.

This view of the Mono Craters is from the west. Photo by R. Forrest Hopson.

This view of the Mono Craters chain is to the south. Mono Lake is in the foreground. Panum Dome is at the center of the right margin. Northwest Coulee is near the center of the photo. North Coulee is near the center of the left margin. Photograph from the cover of Miller and others (1982).

The Inyo Craters chain is located on the northwest rim of the caldera and the west side of the caldera floor. These domes, flows, and craters are 6,000 to 500 years old. The most recent eruptions (650-550 years ago) began explosively and concluded with the production of lava domes. Dan Miller of the U.S. Geological Survey proposed that the eruptions were fed by a shallow dike.

South Glass Creek and North Glass Creek domes and Mono Craters from the summit of Deer Mountain. View is to the north. Photo by R. Forrest Hopson.

Many of the eruptions in the Inyo Craters chain were phreatic eruption. These explosive eruptions produced numerous craters.

Eruptions at the Mono-Inyo Craters volcanic field occurred at 500-year intervals over the past 2,000-3,000 years. The most recent eruption in the region was at Mono Lake between 1720 and 1850 A.D. A dome grew on the lake floor and emerged to make Paoha Island.

A sequence of earthquakes which began in 1978 generated much interest in Long Valley caldera and the Mono-Inyo Craters volcanic field and the possibility of an eruption. Earthquake activity culminated in mid-May 1980 when four magnitude 6 events were recorded in a 2-day period. Volcanologists interpreted the earthquakes, accompanying ground deformation, and an increase in activity at fumaroles as an indication of magma movement beneath the caldera. The magma did not rise to the surface. The activity prompted the U.S. Geological Survey to publish a study on the potential hazards of future volcanic eruptions (Miller and others, 1982). Volcanic unrest continues at Long Valley. In 1994, geologists investigated an area of 75 acres (30 hectare) that contained dying forest. They studied the gas in the soil and found carbon dioxide concentrations of 30-96%. Additional details are provided on a USGS homepage. The Survey continues to monitor the caldera and provides current information on the Long Valley homepage.

Sources of Information:
Bailey, R.C., 1989, Geologic map of Long Valley caldera, Mono-Inyo Craters volcanic chain, and vicinity, eastern California: U.S. Geological Survey, Miscellaneous Investigations Series Map I-1933, scale 1:62,500.

Bailey, R.C., 1987, Long Valley caldera, eastern California, in Hill, M.L., ed., Centennial Field Guide Volume 1 Cordilleran Section of the Geological Society of America, p. 163-168.

Bailey, R.C., 1983, Mammoth Lakes earthquakes and ground uplift: Precursors to possible volcanic activity?: Earthquake Information Bulletin, May-June 1983, v. 15, no. 3, p. 88-102.

Bailey, R.C., and Koeppen, R.P., 1977, Preliminary geologic map of Long Valley caldera, Mono County, California, U.S. Geological Survey Open-File Report 77-468, 2 sheets, scale 1:62,500.

Bailey, R.C., Miller, C.D., and Seih, K., 1989, Excursion 13B: Long Valley caldera and Mono-Inyo Craters volcanic chain, eastern California, in Chapin, C.E. and Zidek, J., eds., Field excursions to volcanic terranes in the western United States, Volume II: Cascades and intermountain west; Memoir 47, New Mexico Bureau of Mines & Mineral Resources, p. 227-254.

Farrar, C.D., and others, 1995, Forest-killing diffuse CO2 emission at Mammoth Mountain as a sign of magmatic unrest: Nature, v. 376, p. 675-678.

Izett, G.A., Obradovich, J.D., and Mehnert, H.H., 1982, The Bishop ash bed and some older closely related ash beds in California, Nevada, and Utah: U.S. Geological Survey Open-File Report 82-584, 60 p.

Miller, C.D., Mullineaux, D.R., Crandell, D.R., and Bailey, R.C., 1982, Potential hazards of future volcanic eruptions in the Long Valley-Mono Lake area, east-central California and southwest Nevada: U.S. Geological Survey Circular 877, 10 p.

Miller, C.D., 1985, Holocene eruption at the Inyo volcanic chain, California: implications for future eruptions in Long Valley: Geology, v. 13, p. 14-17.

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