DE82 905361 COVER View southeastward at geothermal development in the Geysers steam field. Large steam plumes rise from condensing towers of numerous electrical powerplants: smaller plumes rise from geothermal wellheads. Northwest-flowing Big Sulphur Creek has excavated the canyon that transects the photograph from upper right to lower left. Hydrothermal leakage from the steam reservoir occurs dong faults that cut rocks of the Franciscan assemblage along the northeast side of Big Sulphur Creek canyon. The altered rocks are prone to extensive landslides, many of which are visible here. Prominent flat-topped peak on the central horizon is Cobb Mountain, a 1.1-million year-old silicic volcanic center of the Clear Lake Volcanics. . RESEARCH IN THE GEOTHERMAL=, NORTHERN CALIFORNIA GEYSERS-C-LAKE Landsat image of a part of northwestern California, showing Clear Lake in the center, Lake Berryessa to the southeast, Suisun Bay still farther south (lower right corner), and the Pacific Ocean to the west. Agricultural development in the western Sacramento Valley is visible northeast of Clear Lake. Two smaller closed valleys, visible west of Clear Lake, that are dined along a northwestward trend are Little Lake Valley to the northwest and Ukiah Valley with Lake Mendocino on the southeast.. The present area of geothermal development in the Geysers steam field is encircled. Northwest-trending hogback ridges, composed of late Mesozoic sedimentary rocks of the Sacramento Valley that mark the west side of the Great Valley physiographic province,contrast with the more heterogeneous intricately dissected terrane of the Coast Range province to the west. Sutter Buttes, a center of late Cenozoic volcanism in the center of Sacramento Valley, is visible in the upper right corner. Along the southwest coast can be seen the rift of the San Andreas fault, trending southeast from Point Arena and passing through Bodega Head and Tomales Bay in the lower part of the photograph.