Geothermal energy is normally dissipated at the surface. Hot water and steam extracted from drilled wells so far have constituted the main economic sources of geothermal energy. Geothermal areas have been developed or explored in several countries around the Pacific margin.

The geothermal areas are commonly in regions of Quaternary volcanism, though hydrogen and oxygen isotopic studies of the waters fail to reveal the presence of magmatic water. The waters are of meteoric origin. Geothermal systems are of two main types--hot-water systems and vapor systems. Hot-water systems are numerically dominant and are typically located in discharge areas at low levels. Evidence suggests that their locations are affected by, or related to, regional groundwater flow in which there may be a strong horizontal component. Temperature controls include rainfall, rock permeabilities and local heat flux. Vapor systems are usually in elevated recharge areas where groundwater movement normally would be downward and outward; temperatures and pressures are controlled by the ther odynamic properties.

The fluids in hot-water systems are mineralized to varied extents. The energy available from hot-water systems is limited by the amount of steam that can be extracted, and disposal of the remaining effluent can constitute a pollution problem. A vapor system yields relatively more energy and no pollution problem.

In the Pacific region a close relation exists among the distribution of thermal springs, known geothermal fields, and Quaternary volcanism. No specific relation exists between hydrothermal areas and active volcanoes. Rather, all tend to be located above subduction zones or on spreading ridges. This suggests the presence of a large common heat source.

The known geothermal potential in the region is limited, but the ultimate potential is unknown. Future expansion may be in two main directions. The first involves testing the ultimate production capacity of selected known fields, seeking additional zones where no superficial indications exist, and investigating regional hydrology in selected geothermal areas. Geologic, geophysical, and geochemical explorations are required in combination with exploratory drilling. The second promising direction is toward the use of low-boiling-point fluids in equipment and methods for increasing permeability of the rocks.

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