Submarine Canyons and the Shelf along the North Coast of Molokai Island, Hawaiian Ridge

Abstract

The north insular shelf of Molokai is a smooth plain, gently dipping seaward, with three slight steps, one occurring between the 30- and 60-foot isobaths, one between the 150- and 180-foot isobaths, and one near the 300-foot isobath. The shelf break occurs near the 500-foot isobath. Off East Molokai Volcano the shelf is cut by eleven submarine canyons; along West Molokai it is unbroken except for one canyon. About half the canyons have bowl-shaped heads; the remainder have V-shaped heads. The canyons originate about 1 mile offshore. Seismic reflection data show that the insular shelf is covered by a thin veneer of sediments, 0.005 to 0.025 seconds of reflection time, thickening seaward. The veneer is underlain by another series of reflectors, the deepest being 0.05 seconds 1 mile from shore and 0.25 seconds 3 miles from shore. The Molokai submarine canyons appear to have originated from subaerial erosion, which was followed by island subsidence with sediment deposition on the shelf and transport in the canyons. The geomorphology of the north slope of Molokai appears to have developed through erosion and deposition operating upon a subsiding volcanic island, rather than through the action of a giant submarine landslide.