Vapor-dominated ("dry-steam") geothermal systems are uncommon and poorly understood compared with hot-water systems. Critical physical data on both types were obtained from U.S. Geological Survey research in Yellowstone Park. Vapor-dominated systems require relatively potent heat supplies and low initial permeability. After an early hot-water stage, a system becomes vapor dominated when net discharge starts to exceed recharge. Steam then boils from a declining water table; some steam escapes to the atmosphere, but most condenses below the surface, where its heat of vaporization can be conducted upward. The main vapor-dominated reservoir actually is a two-phase heat-transfer system. Vapor boiled from the deep (brine?) water table flows upward; most liquid condensate flows down to the water table, but some may be swept out with steam in channels of principal upflow. Liquid water favors small pores and channels because of its high surface tension relative to that of steam. Steam is largely excluded from smaller spaces'but greatly dominates the larger channels and discharge from wells. With time, permeability of water-recharge channels, initially low, becomes still lower because of deposition of carbonates and CaSO 4, which decrease in solubility with temperature. The "lid" on the system consists in part of argillized rocks and CO2-saturated condensate. Our model of vapor-dominated systems and the thermodynamic properties of steam provide the keys for understanding why the major reservoirs of The Geysers, California, and Larderello, Italy, have rather uniform reservoir temperatures near 240 ø C and pressures near 34 kg/cm • (absolute; gases other than H•.O increase the pressures).Local supply of pore liquid and great stored heat of solid phases account for the physical characteristics and the high productivity of steam wells.We suggest that vapor-dominated systems provide a good mechanism for separating volatile mercury from all other metals of lower volatility. Mercury is likely to be enriched in the vapor of these systems; the zone of condensation that surrounds the uniform reservoir is attractive for precipitating HgS.A more speculative suggestion is that porphyry copper deposits form below the deep water tables hypothesized for the vapor-dominated systems. Some enigmatic characteristics of these copper deposits are consistent with such a relationship, and warrant consideration and testing. • Publication authorized by the Director, U.S. Geologlcal Survey. 75 than a hundred meters or so •' and near centers of surface activity were found to yield slightly superheated steam (Burgassi, 1964). Some wells on the borders of the active systems * produced hot water •-The metric system is used throughout this paper. Some readers may find useful the following conversion factors: Length: 1 m = 3281 ft; 1 km= 3,281 ft ----0.6214 mi. Temperature: (øC X 9/5) + 32 = øF. Pressure: 1 kg/cm • = 0.9678 atto --0.9807 bars --14.22 psi. All pressures absolute, with 0.78 kg/cm • added to gage pressure for Yellow.stone Park, an(} 1.03 kg/...

Mean reservoir volume in relation to mean reservoir temperature for hydrothermal convection systems identified in 1978, excluding those systems for which data are not adequate to calculate other than a standard volume of 3.3 km3-Histogram of thermal energy in undiscovered hydrothermal convection systems by reservoir temperature (2ooc classes, 90o-330oc

ABSTRAGr analysis of our data indicates that four of our drill holes affected nearby springs, generally converting them temporarily into geysers.Our conclusions from Yellowstone indicate that physical relations in many commercially explored geothermal reservoirs are not as uniform as routine postdrilling measurements have indicated.' I used as the circulating fluid. Storage tanks of 1,000-gal capacity at each drill site provided a reserve water supply in the event the pump failed. Drilling mud was not

increasing frequency and intensity, the ground vibrations called volcanic tremor, localized uplift of the surface, ground cracks, and anomalous gas emissions. Of all the possible hazards from a future volcanic eruption in the Yellowstone region, by far the least likely would be another explosive caldera-forming eruption of great volumes of rhyolitic ash. Abundant evidence indicates that hot magma continues to exist beneath Yellowstone, but it is uncertain how much of it remains liquid, how well the liquid is interconnected, and thus how much remains eruptible. Any eruption of sufficient volume to form a new caldera probably would occur only from within the present Yellowstone caldera, and the history of postcaldera rhyolitic eruptions strongly suggests that the subcaldera magma chamber is now a largely crystallized mush. The probability of another major caldera-forming Yellowstone eruption, in the absence of strong premonitory indications of major magmatic intrusion and degassing beneath a large area of the caldera, can be considered to be below the threshold of useful calculation.

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