THE KOHALA LANDSLIDE: A NEW MEGA-LANDSLIDE INTERPRETATION REGARDING THE NORTHEAST FLANK OF KOHALA VOLCANO, HAWAII

With respect to the Pololu mega-landslide off the northeast coast of the island of Hawaii, two interpretations have been proposed for the anomalously eroded subaerial northeast slope of Kohala Mountain between Waipio and Pololu Valleys. The older interpretation is that the slope forms the upper part of the landslide (Moore et al., 1989), whereas the more recent model is that the slope is not part of the landslide and that the head of the Pololu landslide is located at the high coastal cliffs below the subaerial slope (e.g., Smith et al., 2002).

Contour analysis of the slope supports the following two conclusions: 1) the subaerial slope has experienced landslide movement; and 2) that this landslide is separate from the Pololu landslide. Evidence for the first conclusion is recognition that the trends of contours lines north and south of Pololu Valley, interpreted to delineate the northern lateral boundary of the landslide, are misaligned. Contours south of the valley are shifted seaward from those to the north. This misalignment is readily explained by landslide displacement along a slide plane that dips less steeply seaward than the topographic slope.

The second conclusion derives from analysis of summit area contours. At the crest of Kohala volcano are 3 sizeable faults that bound grabens. Assuming these grabens developed from movement at the head of the landslide, mass balance analysis of a topographic profile suggests that the slide displacement in this area is 225 m and that the slide plane is 1000 m below the summit surface. Using this depth and the constraint that the slide plane is less steep than the topographic slope (noted above), the slide plane is projected to daylight at the base of the coastal cliffs. Daylighting of the low gradient slide plane at this location indicates the slide cannot be part of the Pololu landslide further offshore and must be a separate landslide. The proposed name for this newly recognized landslide is the Kohala slump/debris avalanche.

The volcanic mass missing from the area northeast of the cliffs is proposed to be a part of the Kohala landslide that mobilized into a debris avalanche and travelled far offshore, thereby creating the coastal cliffs. The difference in behavior of the upper and lower parts of the Kohala landslide might be explained by buoyancy affects of ocean water on the lower part.