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 completely destroyed volcano was significantly 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. Geologists call topographic depressions formed in this manner calderas.
Near the center of the caldera there is a mound called the "resurgent dome" that was formed by magmatic uplift. The area is still volcanically active and has periodic rhyolitic lava flows. There is a hydrothermal power plant near the resurgent dome. The Bishop tuff is the oldest normally magnetized tuff (that is, it was formed when the earth's magnetic north was near the north pole - as it is today). In the geologic past, water gathered in the Long Valley caldera and overtopped its rim forming the Owens River Gorge.
Mammoth Mountain (11,050 feet), is a composite volcano made up of about 12 rhyodacite and quartz latite domes extruded along the southwest rim of Long Valley Caldera from 200,000 to 50,000 years ago. Mammoth Mountain is one of the eruptive centers that developed late in the evolutionary cycle of the Long Valley Caldera complex.
The history and deposits of the Mono and Inyo Craters overlap with Long Valley Caldera in time and space. The Mono-Inyo Craters volcanic field developed along a 30-mile-long (50 km) fissure system that extends northward from Mammoth Mountain on the southwestern rim of the caldera to Mono Lake.
Over a dozen people have died in Hot Creek since the late 1960s but most of these deaths happened to individuals who ignored the numerous warning signs and attempted to use the hydrothermal pools as hot tubs (like the stream portion of the creek, these pools alternate in temperature but the eruptions in the pools are of super-heated water in already very hot water).
Detailed geology
The known volcanic history of the Long Valley area started several 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, the about 1,500 square miles (4,000 square km) were covered by lava.
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 upwarped 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 resurgent dome.
At its height 600,000 years ago, an Owens River-fed 300 foot deep lake filled the caldera and rose to an elevation of 7,800 feet above sea level. The lake was completely drained sometime in the last 100,000 years after it overtoped the southern rim of the caldera, eroded the sill and created the Owens River Gorge. A dam in the gorge has partially restored part of that lake which is now known as Lake Crowley. Since the great eruption many hot springs developed in the area and the resurgent dome has uplifted.
Notable geothermal areas in Long Valley include; Casa Diablo at the base of resurgent dome and Hot Creek which is about 5 miles from Casa Diablo and cuts into part of resurgent dome. Hydrothermal activity has altered many rocks in the caldera transforming them into travertine and clay. The Huntley clay mine is exposed on resurgent dome and appears as a brilliant white band. A white chalky clay called kaolinite is mined here.
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: these moraines are the debris left from glacial sculpting. Laurel Creek, Convict Creek, and McGee Creek all have prominent moraines.
In May of 1980, a strong earthquake swarm that included four Richter magnitude 6 earthquakes struck the southern margin of Long Valley Caldera associated with a 25-cm, dome-shaped uplift of the caldera floor. These events marked the onset of the latest period of caldera unrest that continues to this day. This ongoing unrest includes recurring earthquake swarms and continued dome-shaped uplift of the central section of the caldera (the resurgent dome) accompanied by changes in thermal springs and gas emissions. After the quake another road was created as an escape route. Its name at first was proposed as the "Mammoth Escape Route" but was changed to the Mammoth Scenic Route after Mammoth area businesses and land owners complained.
In 1982, the United States Geological Survey under the Volcano Hazards Program began an intensive effort to monitor and study geologic unrest in Long Valley Caldera. The goal of this effort is to provide residents and civil authorities in the area reliable information on the nature of the potential hazards posed by this unrest and timely warning of an impending volcanic eruption, should it develop. Most, perhaps all, volcanic eruptions are preceded and accompanied by geophysical and geochemical changes in the volcanic system. Common precursory indicators of volcanic activity include increased seismicity, ground deformation, and variations in the nature and rate of gas emissions.
See also: Mono-Inyo Craters,
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