Seismic evidence has long indicated that the south flank of Kilauea Volcano is mobile. Examination of triangulation, trilateration, and leveling data obtained throughout the 20th century shows that the south flank of Kilauea has been displaced upward and away from the rift zones by as much as several metres. The amount of horizontal displacement approximates the probable amount of dilation that accompanies the intrusion of magma as dikes in the rift zones and is greatest for p~riods of most intense intrusive activity, as evidenced by the frequency of eruptions. Displacement and seismic events on the south flank take place soon after intrusive activity, indicating that the displacement is the result of forceful intrusion in the rift zones, not the cause of relatively passive intrusion. The dikes are thought to be nearly vertical or to dip steeply southward, on the basis of both interpretation of seismic and displacement data for Kilauea itself and comparison with dikes exposed in older, eroded Hawaiian shield volcanoes. In contrast to the south flank, seismic and geodetic data indicate that the north flank is virtually immobile. This contrast is believed to reflect the fact that Kilauea was built on the south slope of Mauna Loa and was consequently influenced by the gravitational stress system of Mauna Loa, which favors displacement away from the volcanic edifice. The north flank of Kilauea is effectively buttressed by Mauna Loa, whereas thr south flank is unbuttressed and free to move away from the edifice when prompted by forceful intrusion of magma. The active part of the east rift zone of Kilauea has apparently migrated several kilometres southward with time. This is shown by the location of recent vents and by the location of the axis of a positive gravity anomaly along the north edge of the active part of the rift zone. The southward migration helps explain several features of the geometry of the east rift zone, particularly its prominent bend near Kilauea Caldera. The southwest rift zone and the caldera also show some evidence of southward migration. The Hilina fault system is considered to be a gravity-controlled system not directly related to the rift zones. Gravitational instability resulting from uplift and seaward displacement is eventually relieved by normal faulting along the seaward part of the south flank. The Hilina faults are thought to bottom at shallow depth without interseeting magma reservoirs, except possibly along part of the lower east rift zone, where the fault system impinges upon the rift zone. Strains have been accumulating within the Hilina system throughout this century, and a high level of instability may have been reached. We anticipate a subsidence event in the not too distant future, possibly similar to the damaging events of 1823 and 1868. (While this paper was in press, such an event occurred on November 29, 1975.) I ACKNOWLEDG ENTS Most staff members of the Haw iian Volcano Observatory took part in the 1970, 197 , and 1973 surveys; we thank them all, especially R. T. Okamura, ...

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The Coso Range lies at the west edge of the Great Basin, adjacent to the southern part of the Sierra Nevada. A basement complex of pre-Cenozoic plutonic and metamorphic rocks is partly buried by --35 km 3 of late Cenozoic volcanic rocks that were erupted during two periods, as defined by K-At dating: (1) 4.0-2.5 m.y., ~31 km 3 of basalt, rhyodacite, dacite, andesite, and rhyolite, in descending order of abundance, and (2) _<1.1 m.y., nearly equal amounts of basalt and rhyolite, most of the rhyolite being _<0.3 m.y. old. Vents for the volcanic rocks of the younger period are localized on and near a horst of basement rocks within a concavity defined by the distribution of vents of the older period. The alignment of many vents and the presence of a considerable number of roughly north-trending normal faults of late Cenozoic age reflect basin and range tectonics dominated by roughly east-west lithospheric extension. Fumaroles, intermittently active thermal springs, and associated altered rocks occur within and immediately east of the central part of the field of Quaternary rhyolite, in an area characterized by various geophysical anomalies that are evidently related to an active hot-water geothermal system. This system apparently is heated by a reservoir of silicic magma at _>8-kin depth, itself produced and sustained through partial melting of crustal rocks by thermal energy contained in mantle-derived basaltic magma that intrudes the crust in response to lithospheric extension.The youthfulness of some of the volcanic rocks and the presence of fumaroles, intermittently active hot springs, and associated hydrothermally altered rock prompted the selection of this area for study by the U.S. Geological Survey in its Geothermal Research Program. In conjunction with ongoing and planned investigations by workers at universities and at the China Lake Naval Weapons Center, within whose bounds most of the volcanic field lies, a comprehensive program of geologic, geophysical, geochemical, geodetic, and hydrologic studies was formulated in 1974; several progress reports have been published and are cited below. This report summarizes our present knowledge of the geology of the area, based principally on information gained from field mapping, rock chemistry, K-Ar dating, and study of thin sections; it also synthesizes our understanding of the geothermal system on the basis of all types of data now available and is intended to provide background for the related papers in this issue.Earlier work in the Coso Range included several topical studies. Ross and Yates [1943] and Dupuy [1948] described mercury occurrences that were once worked commercially from altered rocks in a few fumarolic areas. Power [1958, 1959] and geologists of Lucius Pitkin, Inc. [1976], examined the general geology and uranium mineralization in the northwestern part of the range. Chesterman [1956] described late Cenozoic pyroclastic deposits, some of which have been mined for pumice intermittently for many years. Schultz [1937] studied a volcaniclastic-rich s...

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