Long Valley Caldera

Long Valley Caldera
View from northeast rim of caldera
Location	Mono County, California United States
Floor elevation	6,500–8,500 feet (2,000–2,600 m)
Long-axis direction	EW
Long-axis length	20 miles (32 km)
Width	11 miles (18 km)
Depth	up to 3,000 feet (900 m)
Geology
Type	Caldera
Age	760,000 yrs
Geography
Coordinates	37°43′00″N 118°53′03″W / 37.71667°N 118.88417°W / 37.71667; -118.88417Coordinates: 37°43′00″N 118°53′03″W / 37.71667°N 118.88417°W / 37.71667; -118.88417

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 mi (32 km) long (east-west) and 11 mi (18 km) wide (north-south), and up to 3,000 ft (910 m) deep.

Long Valley was formed 760,000 years ago when a colossal volcanic 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, but open to the southeast. The elevation of the bottom of the bowl ranges from 6,500 to 8,500 ft (2,000 to 2,600 m), being higher in the west.

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 ft (910 m) above the caldera floor. However, the eastern rim is less high, only about 500 ft (150 m).

Mammoth Mountain is a lava dome complex in the southwestern corner of the caldera, consisting of about 12 rhyodacite and dacite overlapping domes. 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.