As noted, the technology for exploiting this resource has not been demonstrated, and environmental effects are difficult to estimate. In general, the problems associated with hot-dry-rock exploitation will probably apply.
The extreme case of hot dry rock is magma, or molten lava, which may be found at temperatures higher than 650°C, in pools at the surface or in reservoirs below volcanoes. Aside from a few in national parks, the existence of such bodies and their depths are generally speculative, and practical means of extracting heat from them have yet to be demonstrated. However, there is now some evidence of magma development, and some research is being done.
Because the only known lava pools in the United States are in Hawaii Volcanoes National Park and are thus inaccessible to development, the identified, accessible resource base represented by molten magmas is zero. Some of the Alaskan volcanoes are accessible, and the probability of the existence there of molten magma bodies at drillable depths is high enough that they can be considered an undiscovered resource base. Their heat content above 300°C, to an unspecified depth, is estimated by Smith and Shaw12 to be 7900 quads. Recalculating this to a reference temperature of 80°C, and assuming that one third of the useful heat exists at depths less than 6 km, yields a resource base estimate of 3500 quads (Table 8–1). If 1 percent of this can eventually be recovered, the product would be 35 quads of useful heat (above 80°C).
Because of the lack of well-defined plans, it is impossible to discuss the potential impacts of magma exploitation. Any proposed scheme will warrant most serious environmental scrutiny before it is allowed to proceed.