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Hawaiian landslides have been catastrophic
Volcanic activity and gentle erosion have not been the only forces to shape the Hawaiian islands. Landslide debris has now been mapped off of all the islands. Enormous amounts of material have traveled great distances, indicating that the slides were truly catastrophic. The Nuuanu and Wailau landslides, shown in the map, tore the volcanoes forming eastern Oahu and northern Molokai, respectively, in half, and deposited blocks large enough to have been given names as seamounts. Tsunamis generated during these slides would have been devastating around the entire Pacific Basin.
Our research on Hawaiian landslides
The discussions below are paraphrased from abstracts of papers published by the Submarine Volcanism group.
Geologic history of Wai'anae Volcano
OAHU - Wai'anae Volcano comprises the western half of O'ahu Island, but until recently little was known about the submarine portion of this volcano. Seven submersible dives conducted in 2001 and 2002, and multibeam bathymetry offshore of Wai'anae provide evidence pertaining to the overall growth of the volcano's edifice as well as the timing of collapses that formed the Wai'anae slump complex.
A prominent slope break at ~1400 meters below sea level marks the paleoshoreline of Wai'anae at the end of its shield-building stage and wraps around Ka'ena Ridge, suggesting that this may have been an extension of Wai'anae's northwest rift zone. Subaerially erupted tholeiitic lavas were collected from a small shield along the crest of Ka'ena Ridge, now submerged. To the south, tholeiitic pillow lavas have been recovered 65 km from the volcano's center, indicating the south rift zone extended at least this distance. Sediment cores collected from north of Ka'ena Ridge contain pelagic sediment with volcaniclastic grains and volcanic glass that originated from Wai'anae's postshield stage and eastern Oahu's Ko'olau Volcano's shield stage, respectively.
Multiple collapses and deformation events occurred during and after the shield stage, resulting in compound mass wasting features on the volcano's southwest flank, the Wai'anae slump complex. This slump complex is the largest in Hawai'i, covering an area of ~5500 km2. It is composed of several distinct sections based on morphology and lithologies of collected samples. The outer bench of the slump complex contains tholeiites that correlate with subaerial lavas erupted early during the volcano's shield stage, from 3.9 to 3.5 million years ago (Ma), and probably formed during and shortly after the early shield stage. To the southwest of the outer bench lies a broad debris field of subaerially derived volcaniclastic rocks containing tholeiites with early shield compositions, interpreted to have formed by a catastrophic collapse event that breached the outer bench. The breach may have then been filled by slumping material from the main volcanic edifice. Finally, on top of the northern main body of the slump is a rotated landslide block that detached from the proximal part of the Ka'ena Ridge after the volcano's late shield stage (3.2 to 3.0 Ma), containing higher alkali rocks that correlate with late shield-stage subaerial lavas. None of the slump complex samples correlate with alkalic subaerial postshield lavas.
