The Coso Range of southeastern California is underlain principally by Mesozoic granitic rocks that are partly veneered by upper Cenozoic volcanic rocks. The volcanic units (in apparent decreasing age) include (1) widespread basaltic flows, (2) dacitic flows and tuff, and (3) rhyolitic domes and flows and basaltic cones and flows. These volcanic rocks are encompassed by an oval-shaped zone of late Cenozoic ring faulting that measures about 40 km east to west and 45 km north to south and that defines a structural basin. Most of the Coso Range and a slice of the adjacent Sierra Nevada lie within this ring structure. The youngest volcanic rocks are Pleistocene and, with associated active fumaroles, occupy a north-trending structural and topographic ridge about 18 km by 10 km near the center of the basin. The ring structure and associated volcanic rocks suggest a large underlying magma chamber that has periodically erupted lava to the surface during the past few million years.

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