Coso Volcanic Field
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| Coso Volcanic Field | |
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 A basaltic lava flow that is typical of the process that created the stepped terraces of Coso as it flowed across the landscape, producing a more or less flat surface eroding to a sheer front.
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| Elevation | 7,874 feet (2,400 m) |
| Location | California, USA |
| Range | Coso Range |
| Coordinates | |
| Topo map | USGS Cactus Pk (CA) |
| Type | Monogenetic volcanic field |
| Last eruption | 39,000 yrs |
The Coso Volcanic Field is located at the west edge of the Basin and Range province. Initiation of volcanism at Coso preceded the onset of Basin and Range crustal extension there, as expressed by normal faulting. The earlier of the two principal periods of volcanism began with the emplacement of basalt flows over a surface of little relief. Then, during the ensuing period of approximately 1.5 million years, eruptive activity included chemically more evolved rocks erupted upon a faulted terrain of substantial relief. Following a 1.5-million-year hiatus with few eruptions, a bimodal field of basalt lava flows and rhyolite lava domes and flows developed on Basin and Range terrain of essentially the same form as today's landscape. Many of the young basalt flows are intercanyon, occupying parts of the presentday drainage system.
The Coso Volcanic Field is best known for its Pleistocene rhyolite. Thirty-eight rhyolite domes and flows form an elongate array atop a north-trending 8 x 20-kilometer horst of Mesozoic bedrock. Nearby uneroded constructional forms are exhibited by most domes. Many are nested within tuff ring craters, and a few filled and overrode their craters to feed flows a kilometer or two long. The two oldest domes contain several percent phenocrysts; the rest are essentially aphyric. Obsidian is exposed locally on most extrusions, and analyses of fresh glass indicate that all of the rhyolite is of the so-called high-silica variety; SiO2 content is essentially constant at 77 percent. Other major-element constituents are nearly invariant. However, trace-element contents vary and help define 7 age groups, each of unique chemical composition.
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[edit] Geothermal
The Coso Volcanic Field is also well known as a geothermal area. Fumaroles are present along faults bounding the rhyolite-capped horst and locally within the rhyolite field. A multi-disciplinary program of geothermal assessment carried out in the 1970s defined a potential resource of 650 megawatts electric with a nominal life span of 30 years. Judged by the youthfulness of the rhyolite lavas and by a zone of low seismic velocity crust roughly beneath the rhyolite, a magma body may be the source of thermal energy for the geothermal system.
Commercial power development began in the 1980's. Located within the China Lake U.S. Naval Air Weapons Station near Ridgecrest, CA, power plants at the Coso Geothermal Field are currently operated by Caithness Energy, LLC (Reno, NV). It currently produces 270 MW from four geothermal power plants. More than 100 wells have been drilled throughout the field, with production depths from 2,000 to 12,000 feet, and temperatures from 200° to 350°C. Coso began generating electricity in 1987. Since then, improvements have resulted in more efficient use of the resource. (Source: Geothermal Resources Council Bulletin)
[edit] Seismicity
The Coso Volcanic Field is one of the most seismically active regions in the United States, producing dozens of tremors in the M1 and M2 range each week. Tremors in the M3 range occur at a rate of 2-6 per month and M4 action happens couple-three times each year. The last M4 temblor occurred March 28, 2000 and measured M4.3. The most recent activity in the M5 range happened in 1996 and 1998 when tremors of M5.3 and M5.1, and M5.2 and M5.0, respectively, occurred with a day of each other. These tremors were actually recorded along the eastern side of the Coso Volcanic Field, some 15 miles from today's action.
Earthquake swarms are common in the Coso area, often producing hundreds of tremors over short periods of time, like a few days. This kind of brisk and robust seismically is common in volcanic areas, such as Long Valley Caldera located near Mammoth Lakes, and Yellowstone Caldera at Yellowstone. The Coso Volcanic Field shows stunning examples of volcanic activity, probably last active 30-40,000 years ago, but ash emission and small cone building episodes may be Holocene (>10,000 years) in age.
