Abstract

The Hawaiian Island volcanic edifices have shed at least 15 giant submarine landslides, each classified as either a slump or debris avalanche. Controversy exists regarding the number, size, and type of landslides on the northeast flank of Kohala Volcano. This study provides a new interpretation for the Kohala flank based on contour and balanced cross-section analysis. Specifically, contours indicate that there is a landslide extending from the summit to the coast between Pololu and Waipio Valleys. The contour evidence also shows that the slide plane is planar and dips less steeply than the topographic slope. Balanced cross sections show the slide plane to be approximately 950 m deep immediately downhill from the zone of depletion, and the slide plane presumably reaches the surface at the base of the coastal cliffs on the northeast coast of Kohala mountain. The lower part of the landslide once extended from the coast to approximately 10 km offshore, but this portion now has been completely removed, apparently as a debris avalanche. Removal of this distal landslide mass created a 200 to 450 m headwall that is now topographically represented by sea cliffs. This newly identified slide/debris avalanche is informally named the “Kohala landslide.” Based on cross-cutting relations of landslide faults with Hawi series lava flows, the upper slide part of the landslide moved sometime between 270 and 60 ka. The age of the lower, debris avalanche part is even less certain and depends on whether canyons cut in the seafloor after the avalanche movement were eroded in the subaerial or submarine environment.

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