Sixty-eight landslides more than 20 km long are present along a 2200 km segment of the Hawaiian Ridge from near Midway to Hawaii. Some of the landslides exceed 200 km in length and 5000 km3 in volume, ranking them among the largest on Earth. Most of these giant landslides were discovered during a mapping program of the U.S. Hawaiian Exclusive Economic zone from 1986 to 1991 utilizing the GLORIA side-looking sonar mapping system. Two general types of landslides are present: slumps and debris avalanches. Many intermediate forms occur and some debris avalanches form from oversteepened slumps. The slumps are deeply rooted in the volcanoes and may extend back to volcanic rift zones and down to the base of the volcanic pile at about 10 km depth. Debris avalanches are thinner, longer, and move on lower gradients than slumps. Their rapid movement is indicated by the fact that some have moved uphill for tens of kilometers, and are believed to have produced major tsunamis. The debris avalanches left large amphitheaters at their heads and produce broad hummocky distals lobes at their toes. Commonly, major canyons have incised the amphitheaters. Giant landslides have recently been discovered on many other marine volcanoes where they also can be related to volcanic structure and eruptive activity.
