Long Valley Caldera

Long Valley Caldera is a depression in eastern California that is adjacent to Mammoth Mountain. The valley is one of the largest calderas on earth, measuring about 32 kilometres long (east-west) and 17 kilometres wide (north-south). The elevation of the floor of the caldera is 6,500 feet (2,000 m) in the east and 8,500 feet (2600 m) in the west. The elevation of the walls of the caldera reach elevations of 9,800-11,500 feet (3000-3500 m) except in the east where the wall rises only 500 feet (150 m) to an elevation of 7,550 feet (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 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.

Hot Creek (shown in the image to the left) has cut into the floor of the caldera and passes through hot springs. The warm water of Hot Creek supports many trout, and is used at the Hot Creek Fish Hatchery. Hot Creek is part of a stream that follows part of a fault line and is well-known for its hydrothermal pools and for the contrast swimmers experience between the cold stream water and either the occasional plume of very hot water (which can burn swimmer's feet and can on rare occasions cause more serious injury) or even long and very strong upwellings of hot water when enough water is in the hydrothermal system (photo).

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.

All but one of these volcanoes, 1-2 million year old Glass Mountain (made of obsidian), was completely destroyed by the major eruption of the area 760,000 years ago, which released 600 cubic kilometres of material from vents just inside the margin of the caldera (the 1980 Mount St. Helens eruption was 1.2 km³). About half of this material was ejected in a series of pyroclastic flows of a very hot (1,500 ° Fahrenheit) mixture of noxious gas, pumice, and ash that covered the surrounding area hundreds of feet deep. One lobe of this material moved south into Owens Valley, past where Big Pine, California now lies. 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 of other matter, was blown as far as 25 miles into the air where winds distributed it as far away as eastern Nebraska and Kansas. However, much of the material ejected straight into the air fell back to earth to fill the 2- to 3- km deep caldera two-thirds to its rim.

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 overtopped 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, Map of Long Valley-Mono area

References

• Roadside Geology of Northern and Central California, Alt, Hyndman (Mountain Press Publishing Company, Missoula; 2000) ISBN 0-87842-409-1
• Geology Underfoot in Death Valley and Owens Valley, Sharp, Glazner (Mountain Press Publishing Company, Missoula; 1997) ISBN 0-87842-362-1
• USGS Volcano Hazards Program: Long Valley Observatory (http://lvo.wr.usgs.gov/index.html#glance) (public domain text)

See also: List of guidebooks about the Sierra Nevada