Long Valley Caldera is a volcanic caldera 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. The caldera was formed 760,000 years ago when a very large eruption released 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.
Geography
The caldera is a giant bowl-shaped depression, approximately 20 miles (32 km) long, 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. Near the center of the bowl, magmatic uplift has formed a resurgent dome. The southeastern slope from the caldera down towards Bishop is filled with the Bishop Tuff, solidified ash that was ejected during the eruption that created the caldera. The Bishop tuff is 1,500 meters (4,900 ft) thick in the caldera floor, 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. However, the eastern rim is lower, only about 500 feet (150 m). Mammoth Mountain is a lava dome complex west of the structural rim 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. The Mono–Inyo Craters are a 25-mile-long (40 km) 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. The Mono-Inyo Craters erupted from 40,000 to 600 years ago, from a magma source separate from the Long Valley Caldera. 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. The creek was closed to swimming in 2006 after geothermal activity in the area increased. The area has a number of other hot springs, some of which are open to bathers.
Geology The source of volcanism at Long Valley is still an active subject of research and debate. Most studies link volcanic activity to regional extension from the Basin and Range Province. Intrusions of mantle-derived basalt rise into the deep crust which supplies heat and volatiles that generate and repeatedly recharge a shallow silicic reservoir by partial melting of continental crust. Long Valley is not above a hotspot, nor is it the result of subduction as in the Cascades.
The known volcanic history of Long Valley Caldera area started a few million years ago when magma began to collect several miles below the surface. Volcanic activity became concentrated in the vicinity of the present site of Long Valley Caldera 3.1 to 2.5 million years ago with eruptions of rhyodacite followed by high-silica rhyolite from 2.1 to 0.8 million years ago. After some time, a cluster of mostly rhyolitic volcanoes formed in the area. All told, about 1,500 square miles (3,900 km2) were covered by lava. All but one of these volcanoes, 1–2-million-year-old Glass Mountain (made of obsidian), were destroyed by the major (VEI-7) eruption of the area 760,000 years ago, which released 600 cubic kilometers (144 cu mi) of material from vents just inside the margin of the caldera. (The 1980 Mount St. Helens eruption was a VEI-5 eruption releasing 1.2 km3 (0.29 cu mi).) About half of this material was ejected in a series of pyroclastic flows of a very hot (1,500 °F (820 °C)) mixture of gases, pumice, and volcanic ash that covered the surrounding area hundreds of feet deep. One lobe of this material moved south into Owens Valley, past present-day Big Pine. Another lobe moved west over the crest of the Sierra Nevada and into the drainage of the San Joaquin River. The rest of the pyroclastic material, along with 300 km3 (72 cu mi) of other matter, was blown as far as 25 miles (40 km) into the air where winds distributed it as far away as eastern Nebraska and Kansas. The eruption initially produced a caldera 2–3 km (1.2–1.9 mi) deep. However, much of the ejecta went straight up, fell down, and filled the initial caldera about two-thirds full.
Eruptions
Subsequent eruptions from the Long Valley magma chamber were confined within the caldera with extrusions of relatively hot (crystal-free) rhyolite 700,000 to 600,000 years ago as the caldera floor was uplifted to form the resurgent dome followed by extrusions of cooler, crystal-rich moat rhyolite at 200,000-year intervals (500,000, 300,000, and 100,000 years ago) in clockwise succession around the dome. The declining volcanic activity and increasingly crystalline lava extruded over the last 650,000 years, as well as other trends, suggest that the magma reservoir under the caldera has now largely crystallized and is unlikely to produce large-scale eruptions in the future. The Long Valley volcano is unusual in that it has produced eruptions of both basaltic and silicic lava in the same geological place. Water from the Owens River filled the caldera to a depth of 300 meters (984 ft) as of 600,000 years ago. At that time, the lake surface was at an elevation near 7,500 feet (2,300 m). The lake drained sometime in the last 100,000 years after it overtopped the southern rim of the caldera, eroded the sill, and created the Owens River Gorge. A human-made dam in the gorge has created Crowley Lake, a partial restoration of the original lake. Since the great eruption, many hot springs developed in the area, and the resurgent dome has uplifted. During the last ice age, glaciers filled the canyons leading to Long Valley, but the valley floor was clear of ice. Excellent examples of terminal moraines can be seen at Long Valley. Laurel Creek, Convict Creek, and McGee Creek each have prominent moraines.
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