Coso Volcanic Field - Geology

Geology

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 volcanic 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 present-day 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.