A recent model for the submarine growth of Hawaiian volcanoes indicates that these volcanoes are composed mainly of fragmental lava debris formed as lavas enter the ocean. This model has major implications for locating earthquake hypocenters and for the landslide hazard potential of these and other ocean island volcanoes. Observations from submersible dives and analyses of volcanic glasses collected from the western submarine flank of Mauna Loa indicate that subaerially erupted pillow lavas are abundant at depths of 950 to 1900 m below sea level. Fragmental lava is an important component of ocean island volcanoes, as witnessed during the most recent eruption of Kilauea volcano, but probably is the dominant lithology only in the upper 1 km of the submarine section. A submarine dike complex was discovered 17 km west of the assumed axis of Mauna Loa's southwest rift, which indicates that its intrusive complex is much broader than previously suspected (∼20 km vs. ∼8 km). The great width of this dike complex may be a consequence of crustal unloading following the South Kona landslide or a normal feature of Hawaiian rift zones that was previously unrecognized.