Recent studies have shown that the collapse of large volcanoes due to gravitational instability plays an important role in shaping volcanic environments. Detailed offshore bathymetric surveys of the Hawaiian Ridge (Moore and Normark, 1994), Reunion Island (Lenat et al., 1989), and the Canary Islands (Masson et al., 2002) reveal extremely large landslides. In the case of Hawaii, one of the Nuuanu Landslides caused by the collapse of Koolau Volcano on Oahu extends more than 200 km from the island. In the Canary Islands, the debris extends to 30 km. Landslides on both island chains might have generated huge tsunamis (Moore, 1964; Moore and Moore, 1988; Moore et al., 1989). Herrero-Bervera et al. (2002) estimated the age of Nuuanu Landslides at 2.1–1.8 Ma. However, the size, the age, and the number of Nuuanu Landslides are still in question. Site 1223 is ~300 km from Oahu and ~100 km to the northeast of the presently defined Nuuanu Wailau Debris Field (Fig. F18).

The Nuuanu Landslide, which broke away from the northeast flank of Koolau Volcano on the island of Oahu, is the largest Hawaiian landslide. It is a debris avalanche that contains enormous blocks such as the Tuscaloosa Seamount, which is ~30 km long, 17 km wide, and at least 2 km tall. The landslide is spread over a 23,000-km² area (Normark et al., 1993; Naka et al., 2000), with distal portions extending up the Hawaiian Arch. 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 over 100 km uphill.

Reaching the landslide deposit by gravity or piston coring has proven difficult because the deposit is overlain by a carapace of younger debris such as turbidites and associated deposits. Thus, the thickness and depositional history of the landslide are poorly known. Prior to drilling, the thickness of the distal portion of the landslide was estimated to be from 1 to 100 m (Rees et al., 1993; Naka et al., 2000). Similarly, the age of the landslide is poorly constrained, although it apparently occurred near the end or after the formation of the Koolau Volcano, which has surface flows with ages 1.8–2.6 Ma based on K-Ar dating by Doell and Dalrymple (1973).

The objectives of drilling at the Nuuanu Landslide Site are

1. To resolve whether the Nuuanu landslide occurred as a single distinct event or as multiple collapses;
2. To determine the age of the landslide(s);
3. To determine the thickness of the landslide deposit at the distal site and to obtain ground truth for the available seismic data in order to estimate the volume of the slide;
4. To study the deposition history of the landslide(s); and
5. To gain insight into potential hazards related to giant landslides on the flanks of ocean island volcanoes.

The JOIDES Resolution departed from the Honolulu Harbor at 1404 hr on 20 December for the Nuuanu Landslide Site NU-1 (ODP Site 1223). The 170-nmi voyage to Site 1223 required 17.0 hr at an average speed of 10.0 kt.

Hole 1223A

Hole 1223A was spudded with the APC at 2030 hr on 21 December at a depth of 4235.1 m (4245.8 mbrf). We took two APC and four XCB cores. We cored 41 m and recovered 23.54 m of core (57.4% recovery), with 12.7 m cored and 10.87 m recovered (85.6% recovery) with the APC and 28.3 m cored and 12.67 m recovered (44.8% recovery) with the XCB (Table T4). After two APC cores, we switched to XCB coring. The use of the XCB system at these shallow depths and the long time needed for coring was unexpected as was the presence of lithified volcanic rocks. Core 5X was advanced only 1.0 m when it was recovered because there were indications of jamming. Core 6X was advanced 8.0 m to a depth of 41.0 mbsf when it was recovered because the time on site had expired. The drill bit cleared the rig floor at 0130 hr on 23 December, and we departed for Site 1224 (H2O).