Shamberger, P.J., Hammer, JE, and Coombs, M

Submarine landslides are a significant process in the evolution of Hawaiian volcanoes, as well as a potential geologic hazard due to the possibility of associated tsunamis. Studies of landslide materials have had two broadly defined aims: to understand their structure and emplacement mechanisms and to examine the compositional evolution of Hawaiian volcanoes by studying the rocks exposed within them. Recent submersible dives off the western flank of Hualalai using the Japanese submersibles Kaiko and Shinkai 6500 characterized and sampled midslope pillow lava benches and an apparent landslide block. The block is ~4500m below sea level, 2x8 km in area, 300-700 m above the surrounding terrain, and similar in morphology to blocks in the Hilina and Laupahoehoe slumps (derived from Kilauea-Mauna Loa and Kohala volcanos, respectively). It is composed of at least nine distinct lithologic units, dominantly hyaloclastite and volcanic breccias. The sedimentary layers are heavily jointed, slickensided, and mineralized by zeolites, but are nearly horizontal in orientation. 15 samples were collected from this block and the glass fragments in six have been analyzed by EPMA. The glass grains are uniformly low in dissolved sulfur and are compositionally distinct from those in nearby in-place Hualalai tholeiitic pillow lavas, submarine Hualalai alkali basalts, and Mauna Loa tholeiites. The grains within indurated hyaloclastite samples are compositionally diverse, suggesting that multiple flows or eruptive events are represented in each layer; a single sample contains numerous weakly alkalic grains. We tentatively conclude that the materials in this block are shallow-erupted volcaniclastic deposits erupted from Hualalai during a period of transition between alkalic and tholeiitic stages that were subsequently transported as a package to deep water in a single mass movement event.