Hawaiian volcanoes are enormous edifices that frequently experience slope failure, generating landslides, debris flows, and turbidity currents. The collapses of Hawaiian volcanoes have generated some of the largest landslides on Earth, and large tsunamis have likely accompanied them (Moore, 1964; Moore and Moore, 1988; Moore et al., 1989, 1994). Deposits from dozens of major landslides, some with lengths of 200 km and volumes >1000 m3, have been recognized along the Hawaiian Ridge (Fig. F1) (Moore et al., 1994). Large landslides have also been recognized on the flanks of other ocean volcanoes such as those at Reunion Island (Lenat et al., 1989) and the Canary Islands (Carracedo, 1999; Masson et al., 2002).
The island of Oahu is the source for the giant Nuuanu Landslide, which broke away from the northeast flank of Koolau Volcano. This giant landslide is described as a debris avalanche by Moore et al. (1994) and undoubtedly generated turbidity currents that extend many hundreds of kilometers from the islands (Rees et al., 1993) as observed for other Hawaiian landslides (e.g., Moore et al., 1989; Garcia and Hull, 1994). Massive blocks from this slide are found as far as 230 km away from Oahu (e.g., Moore et al., 1989; Naka et al., 2000). The largest block in the debris field is the Tuscaloosa Seamount, which is ~30 km long, 17 km wide, and at least 2 km tall. The debris avalanche deposit is spread over a 23,000 km2 area (Normark et al., 1993; Naka et al., 2000) and has distal portions that extend up the Hawaiian Arch. In order to reach the upper portion of the arch (the target site for drilling), the landslide would have had to traverse the deep moat on the northeast side of Oahu and travel uphill for over 100 km.
Sampling the debris avalanche deposit by gravity and piston cores has proven difficult because a carapace of younger debris (turbidites and associated deposits) overlies the deposit. The thickness and depositional history of the debris avalanche, therefore, are poorly known. Estimates of the thickness of the distal portion of the debris avalanche deposit vary from 1 to 100 m, but for planning purposes we had assumed that the thickness was <10 m (Rees et al., 1993). The age of the landslide was poorly constrained; it apparently occurred near the end or after the formation of the Koolau Volcano, which has surface flows that are 1.8-2.6 Ma based on K-Ar dating by Doell and Dalrymple (1973).
There are many questions about the Nuuanu debris avalanche that may be addressed by drilling at Site 1223 (Fig. F1). First, is the method of emplacement of the Nuuanu debris avalanche a single catastrophic event, or does it consist of multiple collapses? As mentioned above, the timing of the giant Nuuanu debris avalanche is unknown; however, paleomagnetic and biostratigraphic data may help to constrain its age. Second, determining the thickness of the landslide deposit at the distal end and providing ground truth for the seismic data will aid in estimating the volume of the debris avalanche deposit. Finally, studying the debris avalanche deposits from Nuuanu will help to gain an insight into potential hazards related to giant landslides on the flanks of ocean island volcanoes.