Лонг-Вэлли: Гигантская кальдера в Калифорнии

Geologic depression near Mammoth Mountain, California, United States

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 mile (32 km) long (east west), 11 mile (18 km) wide (north south), and up to 3000 feet (914 m) deep. Long Valley 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.

The caldera is a giant bowl shaped depression, approximately 20 mile (32 km) long, surrounded by mountains except to the southeast. The elevation of the bottom of the bowl ranges from , 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, California, is filled with the Bishop Tuff, solidified ash that was ejected during the eruption that created the caldera. The Bishop tuff is 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 3000 feet (914 m) above the caldera floor. 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 11059 feet (3371 m) in elevation. The Mono–Inyo Craters are a 25 mile (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. 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.

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 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 as of 600,000 years ago. At that time, the lake surface was at an elevation near 7500 feet (2286 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 Lake Crowley, 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.

In May 1980, a strong earthquake swarm that included four Richter magnitude 6 earthquakes struck the southern margin of the Long Valley Caldera. It was associated with a dome shaped uplift of the caldera floor. These events marked the onset of the latest period of caldera unrest that is ongoing. Hot springs discharge primarily in Hot Creek Gorge, along Little Hot Creek, and in the Alkali Lakes area. The largest springs are in Hot Creek Gorge where about per second of thermal water discharge and account for about 80% of the total thermal water discharge in the caldera. At the other extreme are springs at Hot Creek Fish Hatchery which contain a small component (2–5%) of thermal water that raises water temperatures about higher than background temperatures. Use of the warm spring water in the hatchery has increased fish production because trout growth rates are faster in the warm water than in ambient stream temperatures in Long Valley.