2-5. South Kona Slide Complex 　[P Lipman]

Studies on the west (Mauna Loa side) of Hawaii Island, where there are multiple young landslide features (especially South Kona and Alika), can provide counterparts to compare and contrast with the existing detailed Hilina and Nu’uanu results. The large distal blocks of the South Kona landslide have never been systematically sampled. The deeper parts of several of them provide exceptional places to search for lavas that may extend our view of Mauna Loa magmatism back in time. This would provide new views on the evolution of the Hawaiian plume. Evidence may also be obtained on the relative proportions of pillow lava vs hyaloclastite involved in construction of the submarine flanks of large oceanic-island volcanoes, a current topic of controversy.

The South Kona landslide complex (Fig. 2-5-1) involves multiple ages and styles of slope failure. It includes three major morphologic elements: (1) several geometrically intricate proximal mid-slope benches and basins, (2) a mid-distance field of many intermediate-size (mostly <2 km) isolated blocks, and (3) a distal region containing eight large blocks (up to 10 km across and 500 m high). This slide complex was originally recognized during the USGS GLORIA side-scan surveys, and an analogy was made between the mid-slope bench and the Hilina slump (Lipman et al., 1988), but detailed SeaBeam bathymetry was not available until more recently. Dredging of two distal blocks in 1991 brought up pillowed tholeiitic lava, and observations during a dive with the submersible SEA CLIFF in 1995 disclosed massive fractured rocks interpreted as basalt, overlain by hyaloclastite and pillow lava (Moore et al., 1995). No samples were obtained during the single dive, however, because of manipulator malfunction. Several dives by the Monterey Bay Aquarium Research Institute (MBARI) were carried out just north of the mid-slope bench in May 2001, but no results are yet available.
(Fig.2-5-1)
Analyses of glass from dredged samples have compositions similar to modern Mauna Loa, and Moore et al. interpreted the distal blocks as emplaced by a long-traveled landslide from this volcano before growth of the mid-slope benches by later slump movement. No direct visual observations or samples have previously been obtained from the mid-slope benches, however, nor have in-place samples been collected from any of the distal blocks. Various alternative interpretations seem possible. Rocks described (but not sampled) during the SEA CLIFF dive as “massive basalt (possibly intrusive and intensely fractured)” (Moore et al., 1995, p. 126) may alternatively constitute massive turbidite sandstone, such as widely exposed on the Hilina lower scarp. Rather than slump blocks, the mid-slope benches may alternatively represent toes of volcano flank that were compressed and uplifted b y oceanward spreading on the west slope of Mauna Loa. The sequence of landslides and the lithologic succession on the mid-slope benches, if studied in detail, may provide critical information on (1) the early growth of Mauna Loa, (2) any possible involvement of rocks from Hualalai volcano, and (3) frequency of large submarine landslides from the largest volcanic edifice in the world.

The 2001 KAIKO dives were designed to provide pilot information on the feasibility of such studies. One dive (K210) traversed an 1100 m section up the mid-slope bench, and a second dive (K211) a previously unexplored outlying distal block (Fig. 2-5-1). The resulting video images and in-places samples, when adequately analyzed, should provide important information on the primary volcanic sources (compositional comparisons with young Mauna Loa) and eruption environment (subaerial vs. submarine).