DESCRIPTION:
Coso Volcanic Field, California

Coso Volcanic Field
Location: California
Latitude: 36.00 N
Longitude: 117.75 W
Elevation: 730 to 2,400 meters
Type: Monogenetic volcanic field
Eruptive History: Two principal periods: 4 to 2.5 million years ago (31 cubic kilometers), and 1 million to 40,000 years ago (2.5 cubic kilometers)

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.

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 development beginning in the 1980's resulted in the startup of a geothermal steam-driven 3-MW electric power plant in 1987 ...

The west side of Coso volcanic field is crossed by Highway 395 at the village of Little Lake, approximately 34 kilometers north of Inyokern, California. Most of the field, however, including all of the Pleistocene rhyolite, is a few to several kilometers to the east, within the China Lake Naval Weapons Center. ...

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