Frequent landslides from Koolau Volcano: Results from ODP Hole 1223A

Abstract

Giant landslides on the flanks of oceanic islands are considered relatively rare but potentially devastating natural hazards. The Hawaiian Islands are known to produce some of the biggest landslides on Earth. The Nuuanu slide, largest of the Hawaiian slides, is thought to have removed a substantial part of Koolau Volcano from the island of Oahu. Ocean Drilling Program (ODP) Hole 1223A was drilled to determine the depositional history, timing, thickness and hazards associated with the Nuʻuanu landslide, the only previously known Koolau slide. Site 1223, located ∼ 260 km northeast of the island of Oahu near the crest of the 500-m-high Hawaiian Arch, was drilled to a depth of 41 m. Eight distinct sandy layers were recovered and more are likely to occur deeper in the section. Contacts of these sandy layers with pelagic clay are sharp at their base and gradational at the top. The layers contain angular fresh glass fragments with compositions that are typical of Hawaiian shield volcanoes, including the distinctive Koolau high SiO 2-type in seven layers. Most glasses (> 90%) are degassed (< 0.03 wt.% S) indicating that they were probably erupted subaerially. Pleistocene to Early Eocene Radiolaria taxa are present in the Hole 1223A cores, with mixed ages in some intervals. Seven of the sand layers are probably associated with Koolau landslides and were deposited prior to 1.77 Ma. Among the four thicker sand layers (> 1.5 m), it is unclear which, if any, are related to the Nuuanu slide. Results from Hole 1223A cores demonstrate that Hawaiian volcanoes collapse repeatedly, and the debris from these many slides can travel great distances (> 260 km) across the ocean floor and over significant bathymetric obstacles (∼ 500 m). Thus, landslides from oceanic volcanoes pose a greater risk than previously assumed.