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Most of the major known geothermal systems of the world are associated in some manner with recent volcanism and thermal springs. Both phenomena occur in sufficient numbers in Alaska as to indicate potentially large geothermal resources. The volcanic areas of principal geothermal interest are (1) the Aleutian arc (Fig. 1), a seismically active volcanic arc-trench system extending some 2,500 km across the North Pacific and Alaska mainland; and (2) the Wrangell Mountains volcanic pile in east-central Alaska, which underlies an area of some 10,000 km2 and ranges in age from Miocene to Holocene. Volcanism in these two areas is both tholeiitic and calc-alkaline in character and is related to the convergence of the North American and Pacific Plates. Both regions contain evidence of silicic and explosive volcanism, indicating high-level near-surface magma chambers with attendant large heat reservoirs. The distribution of volcanic rocks is shown on Plate 12 (Plafker and others, this volume).

Although other volcanic provinces occur in the state, they appear to be less important from a geothermal standpoint. The western Alaska volcanic province, consisting of Pliocene to Holocene olivine tholeiite and alkali basalt, underlies scattered areas totaling about 25,000 km2 chiefly on the Seward Peninsula, the Norton Sound area (including St. Lawrence Island), the Yukon River delta, and the Pribilof Islands. Similar, widely scattered Quaternary volcanic rocks occur in the central and eastern interior of Alaska. These basaltic volcanic provinces appear to be related to an extensional tectonic regime because of their composition and mode of occurrence. Near-surface

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