Plate tectonics has become a unifying concept in the earth sciences, and within its framework rational explanations of heretofore diverse and apparently singular geologic phenomena have followed. In the Pacific, hydrogeologic provinces fit naturally within the plate-tectonics framework.

Hydrogeologic provinces in the Pacific basin are classified as: (1) continental coasts and islands at the margins of continental plates; (2) arc islands associated with the margins of ocean plates; and (3) ocean islands thought to have formed as the result of hot spots

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underlying ocean plates.

The hydrogeology of continental coasts and islands is usually an extension of continental-type hydrogeology, except that coastal hydrology often is overwhelmingly influenced by the relation between fresh water and sea water in aquifers. Arc-island hydrogeology is dominated by andesitic volcanic rocks, often acting as aquicludes, and in many places by fossil reefs, which are normally suitable as freshwater aquifers if extensive enough. The hydrogeology of ocean islands is characterized by permeable basalts, relatively impermeable sediments, and, occasionally, fossil reefs. Atolls are emerged fossil reefs overlying preexisting arcs or ocean islands.

The hydrogeology of islands is particularly crucial to successful economic development as examples of the importance of hydrogeology to economic growth show.

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