Long Valley Caldera

"For the town in New Jersey, see Long Valley, New Jersey":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 convert|32|km|mi long (east-west) and convert|17|km|mi wide (north-south). The elevation of the floor of the caldera is convert|6500|ft|m in the east and convert|8500|ft|m in the west. The elevation of the caldera walls reach 9,800-11,500 feet (3,000-3,500 m) except in the east where the wall rises only 500 feet (150 m) to an elevation of convert|7550|ft|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.

Geography of the caldera

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.

Notable geothermal areas in Long Valley include Casa Diablo at the base of resurgent dome; and Hot Creek which is about 5 miles (8 km) 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.

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 swimmers' 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.

Detailed geology

Origins of the caldera

The tectonic causes of the volcanism that have produced the Long Valley Caldera are still largely unexplained and are therefore a matter of much ongoing research. Long Valley is not above a hotspot as is Yellowstone or Hawaii, nor is it the result of subduction such as that which produces the volcanism of the Cascades.

The known volcanic history of the Long Valley Caldera 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, about 1,500 square miles (4,000 square kilometers) 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 degree Fahrenheit or 800 degree Celsius) 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 (40 km) 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.

ince the eruption

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 (91 m) deep lake filled the caldera and rose to an elevation of 7,800 feet (2,400 m) 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.

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.

Recent geology

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 10 inch (25-cm), dome-shaped uplift of the caldera floor.cite web|url=http://lvo.wr.usgs.gov/index.html#glance|title=Long Valley Caldera at a Glance|work=USGS Volcano Hazards Program, Long Valley Observatory (public domain source) ] 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.

Hydrothermal system

The Long Valley Caldera hosts an active hydrothermal system that includes hot springs, fumaroles (steam vents), and mineral deposits. Hot springs exist primarily in the eastern half of the caldera where land-surface elevations are relatively low; fumaroles exist primarily in the western half where elevations are higher. Mineral deposits from thermal activity are found on an uplifted area called the resurgent dome, at Little Hot Creek springs, Hot Creek Gorge, and other locations in the south and east moats of the caldera.cite web|url=http://lvo.wr.usgs.gov/HydroStudies.html|title=Hydrologic Studies in Long Valley Caldera|work=Volcano Hazards Program: Long Valley Observatory|publisher=USGS (public domain source)]

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 250 liters 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 5°C 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.

In hydrothermal systems the circulation of groundwater is driven by a combination of topography and heat sources. In Long Valley Caldera, the system is recharged primarily from snow-melt in the highlands around the western and southern rims of the caldera. The meteoric water infiltrates to depths of a few kilometers where it is heated to at least 220°C by hot rock near geologically young intrusions. Upflow occurs in the west moat where the heated water with lower density rises along steeply inclined fractures to depths of 1-2 km. This hydrothermal fluid flows laterally, down the hydraulic gradient, from the west to the southeast around the resurgent dome and then eastward to discharge points along Hot Creek and around Crowley Lake. Reservoir temperatures in the volcanic fill decline from 220°C near the Inyo Craters to 50°C near Crowley Lake due to a combination of heat loss and mixing with cold water.

Hot Creek has been a popular local swimming hole for decades. 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). Recent geothermal instability has led to its temporary closure for swimming. Officials are unsure when (if ever) Hot Creek will officially reopen for swimming.

Tourism and hiking

The largest tourist attraction in the caldera is the Mammoth Mountain Ski Area: the area offers skiing and snowboarding in the winter, and mountain biking in the summer.

Hot Creek is also a tourist attraction, allowing tourists to soak in warm water. However, as of 2006, Hot Creek has been closed by the United States Forest Service, due to an increase in geothermal activity.

Hiking and off-road vehicle driving is available throughout the caldera, and in the glacial valleys of the Sherwin Range, immediately to the south of the caldera. Hikers can hike to several lakes in these glacial valleys, including Valentine Lake, Convict Lake, Lake Dorothy, and Laurel Lakes.

Crowley Lake, at the south end of the caldera, is noted for its fishing.

The nearest hotel accommodations to the caldera are in Mammoth Lakes, California. There are also campgrounds scattered throughout the caldera, and in the mountains near the edge of the caldera.

History and ecology

The history and ecology of the caldera is most closely associated with that of Owens Valley, please see that article for more information.

ee also

*Mono-Inyo Craters
*List of books about the Sierra Nevada for further reading
*

References

*cite book|title=Roadside Geology of Northern and Central California|last=Alt|first=David|coauthors=Donald Hyndman|publisher=Mountain Press Publishing Company|location=Missoula, Montana|year=2000|id=ISBN 0-87842-409-1
*cite book|last=Harris|first=Stephen L.|title=Fire Mountains of the West|year=2005|edition=3rd edition|publisher=Mountain Press Publishing Company|location=Missoula, Montana|id=ISBN 0-87842-511-X
*cite book|title=Geology Underfoot in Death Valley and Owens Valley|last=Sharp|first=Robert P.|coauthors=Allen F. Glazner|publisher=Mountain Press Publishing Company|location=Missoula, Montana|year=1997|id=ISBN 0-87842-362-1

External links

*VNUM|1=120314-A
* [http://pubs.usgs.gov/of/2000/of00-221/of00-221.pdf Bibliography of Literature Pertaining to Long Valley Caldera and Associated Volcanic Fields] , (PDF) Retrieved on August 12 2007