Stage 3
Stage 3 is the giant landslide stage. In the 1980s, the U.S. Geological Survey used sonar to map the sea floor surrounding the Hawaiian Islands. Geologists discovered about 70 major giant landslides that cover half of the flanks of the Hawaiian Ridge (Moore and others, 1989; 1994).
These landslides are among the largest on Earth, attaining lengths of 125 miles (200 km) and volumes of 1,200 cubic miles (5,000 cubic km). Moore and others (1989) identified two types of landslides: slumps and debris avalanches. Slumps moved on an overall slope >3¼, caused little disruption of the structural coherence of the volcano's flanks, and extended back to the volcanic rift zones and down to the base of the volcanic pile. Slumps may move slowly or surge abruptly forward several meters, causing large earthquakes. The 1868 and 1975 earthquakes, the largest of Hawaii's historic earthquakes, resulted from movement of the Hilina slump in south Hawaii.
