GEOLOGY AND PETROLOGY OF MAHUKONA VOLCANO, HAWAII

The submarine Mahukona Volcano, west of the island of Hawaii, is located on the Loa loci line between Kahoolawe and Hualalai Volcanoes. The west rift zone ridge of the volcano extends across a drowned coral reef at about - 1150 m and a major slope break at about - 1340 m, both of which represent former shorelines. The summit of the volcano apparently reached to about 250 m above sea level (now at - 1100 m depth) and was surmounted by a roughly circular caldera. A second rift zone probably extended toward the east or southeast, but is completely covered by younger lavas from the adjacent subaerial volcanoes. Samples were recovered from nine dredges and four submersible dives. Using subsidence rates and the compositions of flows which drape the dated shoreline terraces, we infer that the voluminous phase of tholeiitic shield growth ended about 470 ka, but tholeiitic eruptions continued until at least 435 ka. Basalt, transitional between tholeiitic and alkalic basalt, erupted at the end of tholeiitic volcanism, but no postshield-alkalic stage volcanism occurred. The summit of the volcano apparently subsided below sea level between 435 and 365 ka. The tholeiitic lavas recovered are compositionally diverse.