Long Valley Caldera

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). The elevation of the floor of the caldera is 6,500 ft (2,000 m) in the east and 8,500 ft (2,600 m) in the west. The elevation of the caldera walls reach 9,800–11,500 ft (3,000–3,500 m), except in the east, where the wall rises only 500 ft (150 m) to an elevation of 7,550 ft (2,300 m).

Long Valley was formed 760,000 years ago when a huge volcanic eruption released very hot ash that later cooled to form the Bishop tuff that is common to the area. The eruption was so colossal that the magma chamber under the now destroyed volcano was emptied to the point of collapse. The collapse itself caused an even larger secondary eruption of pyroclastic ash 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.