Petrographic and chemical analyses of basalt from Nihoa Island, Necker Island, French Frigate Shoals, and Midway Atoll, all in the leeward part of the Hawaiian chain, confirm that these islands are subaerial remnants of tholeiitic shield volcanoes similar to those that form the principal Hawaiian Islands. Chemistry suggests that Gardner Pinnacles may be part of the alkalic cap on a tholeiitic shield. Weighted mean potassium-argon ages of 7.0 ± 0.3 m.y. for Nihoa, 10.0 ± 0.4 m.y. for Necker, 11.7 ± 0.4 m.y. for French Frigate, and 17.9 ± 0.6 m.y. for Midway demonstrate that the ages of these volcanoes increase northwestward, continuing the trend of increasing age away from the active volcano of Kilauea shown by the main islands. The increase in age with distance along the chain, however, appears to be nonlinear. The results support the general hypothesis that the volcanoes of the Hawaiian chain have a common origin and were formed as the Pacific plate moved northwestward over a melting spot in the mantle.

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