Long Valley Caldera

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

Layers of the Bishop tuff, in a rock quarry in Chalfant Valley, about 25 km (16 mi) southwest of the Long Valley Caldera, laid down in phases of a major eruption 760,000 years ago.

The known volcanic history of the 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 sq mi (3,900 km²) were covered by lava.

All but one of these volcanoes, 1–2 million year old Glass Mountain (made of obsidian),[10]^:264 were destroyed by the major (volcanic explosivity index (VEI) 7) eruption of the area 760,000 years ago, which released 600 cubic kilometres (144 cu mi) of material from vents just inside the margin of the caldera.[11] (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 noxious gas, 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, California. 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 mi (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.

Cross-section through Long Valley

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.[2] 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.[12]

The Long Valley volcano is unusual in that it has produced eruptions of both basaltic and silicic lava in the same geological place.[13]

Water from the Owens River filled the caldera to a depth of 300 m (984 ft) as of 600,000 years ago. At that time, the lake surface was at an elevation near 7,500 ft (2,286 m).[14] 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 10 in (250 mm) dome-shaped uplift of the caldera floor.[15][16] These events marked the onset of the latest period of caldera unrest that is ongoing.[15] This ongoing unrest includes recurring earthquake swarms and continued dome-shaped uplift of the central section of the caldera accompanied by changes in thermal springs and gas emissions.[15] 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 Loop 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.[15] The goal is to provide residents and civil authorities with reliable information on the nature of the potential hazards posed by this unrest and timely warning of an impending volcanic eruption, should it develop.[15] Most, perhaps all, volcanic eruptions are preceded and accompanied by geophysical and geochemical changes in the volcanic system.[15] Common precursory indicators of volcanic activity include increased seismicity, ground deformation, and variations in the nature and rate of gas emissions.[15]

Hot Creek Fish Hatchery at base of Resurgent Dome

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.[17]

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 L (66 US gal) 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 (9.0 °F) 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.[17]

Hot Creek in the summer

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 water from snow-melt and rainfall infiltrates to depths of a few kilometers (miles) where it is heated to at least 220 °C (428 °F) 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 (0.62–1.24 mi). 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 (428 °F) near the Inyo Craters to 50 °C (122 °F) near Crowley Lake due to a combination of heat loss and mixing with cold water.[17]

Hot Creek has been a popular swimming hole for decades. Over a dozen people have died in Hot Creek since the late 1960s, but most of these deaths happened to people 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 of when (if ever) Hot Creek will officially reopen for swimming.

Hydrothermal activity has altered many rocks in the caldera, transforming them into travertine and clay. At the Huntley clay mine white chalky clay called kaolinite is mined; the kaolinite is exposed on the resurgent dome and appears as a brilliant white band.