Long Valley Caldera

Long Valley Caldera
Long Valley caldera NE rim.jpg
View from northeast rim of caldera
Floor elevation6,500–8,500 feet (2,000–2,600 m)
Length20 miles (32 km) EW
Width11 miles (18 km)
Depthup to 3,000 feet (900 m)
Age760,000 yrs
LocationMono County, California
United States
Coordinates37°43′00″N 118°53′03″W / 37°43′00″N 118°53′03″W / 37.71667; -118.88417[1]
Map of the Long Valley Caldera
Early winter in Long Valley, 2017

Long Valley Caldera is a depression in eastern California that is adjacent to Mammoth Mountain. The valley is one of the Earth's largest calderas, measuring about 20 miles (32 km) long (east-west), 11 miles (18 km) wide (north-south), and up to 3,000 feet (910 m) deep.

Long Valley was formed 760,000 years ago when a supervolcanic eruption released very hot ash that later cooled to form the Bishop tuff that is common to the area. The eruption emptied the magma chamber under the area to the point of collapse. The second phase of the eruption released pyroclastic flows that burned and buried thousands of square miles. Ash from this eruption blanketed much of the western part of what is now the United States.


The caldera is a giant bowl-shaped depression, approximately 20 miles (32 km) wide, surrounded by mountains except to the southeast. The elevation of the bottom of the bowl ranges from 6,500 to 8,500 feet (2,000 to 2,600 m), being higher in the west.[2]

Near the center of the bowl, there is a resurgent dome formed by magmatic uplift. The southeastern slope from the caldera down towards Bishop, California, is filled with the Bishop Tuff, solidified ash that was ejected during the stupendous eruption that created the caldera. The Bishop tuff is thousands of feet thick and is cut by the Owens River Gorge, formed during the Pleistocene when the caldera filled with water and overtopped its rim.

The rim of the caldera is formed from pre-existing rock, rising about 3,000 feet (910 m) above the caldera floor.[2] However, the eastern rim is lower, only about 500 feet (150 m).[2]

Mammoth Mountain is a lava dome complex west of the structural rim of the caldera,[3] consisting of about 12 rhyodacite and dacite overlapping domes.[2][4] These domes formed in a long series of eruptions from 110,000 to 57,000 years ago, building a volcano that reaches 11,059 feet (3,371 m) in elevation.[5]

The Mono–Inyo Craters are a 25 mi (40 km)-long volcanic chain situated along a narrow, north–south-trending fissure system extending along the western rim of the caldera from Mammoth Mountain to the north shore of Mono Lake.[6] The Mono-Inyo Craters erupted from 40,000 to 600 years ago, from a magma source separate from the Long Valley Caldera.[7]

The caldera has an extensive hydrothermal system. Casa Diablo Hot Springs at the base of the resurgent dome hosts a geothermal power plant. Hot Creek cuts into part of the resurgent dome and passes through hot springs. The warm water of Hot Creek supports many trout, and is used at the Hot Creek Fish Hatchery.[8] The creek was closed to swimming in 2006 after geothermal activity in the area increased.[8][9] There are a number of other hot springs in the area, some of which are open to bathers.