Abstract

A large field of blocky sea-floor hills, up to 10 km long and 500 m high, are gigantic slide blocks derived from the west flank of Mauna Loa volcano on the island of Hawaii. These megablocks are embedded in the toe of the South Kona landslide, which extends ∼80 km seaward from the present coastline to depths of nearly 5 km. A 10–15-km-wide belt of numerous, smaller, 1–3-km-long slide blocks separates the area of giant blocks from two submarine benches at depths of 2600 and 3700 m depth that terminate seaward 20 to 30 km from the shoreline. Similar giant blocks are found on several other major submarine Hawaiian landslides, including those north of Oahu and Molokai, but the South Kona blocks are the first to be examined in detail using high-resolution bathymetry, dredging, and submersible diving. Dredging of two of the giant blocks brought up pillowed tholeiitic lava. Observations from the U.S. Navy submersible Sea Cliff on the asymmetrically steep eastern flank of one block 10 km long and 300 m high revealed a succession of fractured massive basalt, laminar lava flows, hyaloclastite, and pillow lavas. Chemical analyses of dredged lava identified 19 units that overlap compositionally with lavas from the south rift-zone ridge of Mauna Loa. Sulfur content indicates that most of the lavas were erupted in subaerial and shallow submarine (<200 m depth) sites, but some were erupted in deeper submarine sites. These results indicate that the megablocks were carried by a late Pleistocene giant landslide 40–80 km west from the ancestral shoreline of Mauna Loa volcano before growth of the midslope benches by later slump movement.

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