Giant Hawaiian underwater landslides

Moore, James; Normark, William; Holcomb, Robin

'The giant Hawaiian landslides have provided new insight into structural features of the volcanoes, but the role of cause and effect remains blurred. Much of the seismic activity shallower than 15 km near the active volcanoes is presumed to be generated by landslide-related processes, and the largest quakes in Hawaiian history (magnitude M>7) are apparently associated with gravity failures. Major normal fault systems, some previously difficult to interpret, can now be related to the upper tensional part of slumps, and anomalous slope changes can be attributed to lava-buried, landslide-related fault systems. The volcanic rift zones, which conduct magma laterally from the summit reservoirs, are regarded as the pull-apart zone at the slump headwall. Large, rapidly cut canyons are common in the newly excavated amphitheaters of debris avalanches, and the presence of such canyons is now employed as evidence of ancient landslides. Broad benches and closed depressions half-way down the submarine slopes are attributed to thrusting in the lower compressional parts of slumps. The giant submarine landslides have stimulated interest in the concept of gravitationally induced volcanic spreading, a topic that was the subject of an entire session at the fall American Geophysical Union meeting. The close parallel in the structure of marine volcanic rift zones and oceanic spreading ridges has led to the proposal that large oceanic volcanoes are small analogs of global plate tectonics (14). Many questions have been raised by the discovery of these large gravity failures. How do the debris avalanches carry blocks 10 km in size, perhaps rapidly, more than 50 km down a slope averaging less than 3°? What effects do such landslides have on the sedimentary record? How might their deposits be recognized in ancient deposits? How are they related to the life history of large marine volcanoes? What is the frequency of such landslides, how are they triggered, and what sites favor them—in short, what hazards do they pose? These and many other issues are matters of debate, and the answers, at present, remain elusive. The mapping of the Hawaiian EEZ helped us to identify some of the questions and maps provide a consistent, reliable basis for future studies.'

Giant Hawaiian underwater landslides

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