Abstract: Geothermal Exploration on Adak Island, Alaska--a Case History

Thomas P. Miller, Donald B. Hoover, Robert L. Smith, Carl Long

The U.S. Geological Survey, in cooperation with the U.S. Navy, has conducted a study of the geothermal potential of northern Adak Island in the central Aleutian Islands. Petrologic sampling, geochemistry, and K-Ar measurements were used to determine the volcanic history of Mount Adagdak, Mount Moffett, and Andrew Bay volcanoes. Geophysical studies including gravity, audio-magnetotelluric, telluric traverse, self-potential, and EM-16R electromagnetic techniques were used to search for possible high-level magma chambers beneath these volcanoes.

Field relations and K-Ar age dating indicate that Mount Adagdak was the youngest of the volcanoes with activity occurring from 140,000 to 342,000 years ago. The presence of central vent domes of dacitic composition (62% SiO₂) indicates the present or former existence of a high-level magma chamber. An estimate of the volume of this chamber and the age and composition of the most recent eruption were used to determine the probable solidification state of the chamber, its relation to a 300°C isotherm, and the probable present heat content of the system. The calculations indicated that a thermal anomaly probably underlies Mount Adagdak although its temperature may be relatively low, possibly much less than 300°C. This estimate is in good agreement with the geochemical thermometry of nearby hotspring waters which indicate subsurface reservoir temperatures of 180°C. A heat content of 2 to 8 × 10¹⁸ cal has been calculated for the now mostly solidified magma chamber.

The geophysical data show that a low-density, very conductive mass of rock underlies Mount Adagdak, that the low-density rock is quite near the surface, and that it is elongated in a northeast direction. Low resistivity values also imply hot or possibly molten rock.

Two shallow holes drilled by the U.S. Navy on this geophysical anomaly suggest bottom-hole temperatures, prior to stabilization, of 66°C at 550 m. The possibility of deeper drilling is now being considered to test further the geothermal possibilities on northern Adak Island.

AAPG Search and Discovery Article #90962©1978 AAPG 2nd Circum-Pacific Energy and Minerals Resource Conference, Honolulu, Hawaii