In a critical period, when all energy resources must be evaluated, geothermal energy is an attractive alternative to fossil fuels. Geothermal resources can be categorized by three main mechanisms of heat transfer: (1) hydrothermal convection systems, (2) hot igneous systems where magma generated in the mantle or deep crust moves upward, and (3) conduction-dominated regimes.

Most geothermal exploration has been directed toward the hydrothermal convection systems in areas of young volcanic or seismic activity. Temperatures above 200° are most actively sought.

Compared with other sources, geothermal generation of electricity is small. However, the growth rate--a doubling time of 10 years--is significant. Geothermal energy is especially important to certain countries, including El Salvador, Mexico, Japan, New Zealand, and in California. It is rapidly increasing in importance in the Philippines, and probably will in Chile and Nicaragua. Although the emphasis at present is on the easily exploited vapor-dominated systems, within the next 5 years the more abundant hot-water systems will become more important.

With changes in price and advancement of technology, low-temperature hot-water convection systems and geopressured pore fluids may eventually become more extensively utilized.