The 1986 Iwato slide occurred during November 1986, at Iwato, situated in the southwestern part of Ikutahara Town, northeastern Hokkaido, Japan. This slide took place over an area at the foot of an ancient slide configuration that was approximately 140 m wide and 100 m long. The dimensions of the 1986 Iwato slide were approximately 25 m wide and 45 m long.
The geology in the vicinity of the Iwato slide area consists of the Lower Cretaceous-Eocene Hidaka Supergroup, Upper Miocene Ikutawara Formation, Neogene rhyolite and basalt dikes, as well as Quaternary river terrace deposit, slide deposit, alluvial fan deposit and Alluvium.
Hydrothermal alteration is mainly characterized by smectitization (smectite ± quartz ± opal-CT ± pyrite), zeolitization (mordenite+heulandite-clinoptilolite series mineral ± smectite ± quartz ± opal-CT ± pyrite), propylitization (quartz + albite + chlorite ± smectite ± calcite ± pyrite), illitization (quartz + illite + pyrite ± chlorite) and Kfeldsparization (quartz+adularia+pyrite ± illite ± chlorite).
The bedrock of the slide area is reconsolidated crushed shale of the Hidaka Supergroup in a crush zone . The shale is believed to have been subjected to hydrothermal smectitization. The slide, therefore, can be characterized as both shear zone slide and hydrothermal alteration zone slide. The slide deposit consists chiefly of debris, such as sand or silt mixed with rubble that consists primarily of shale fragments of the Hidaka Supergroup and tuffaceous conglomerate fragments of the Ikutawara Formation, and is approximately 6 to 7 m in thickness. The displaced material moved along a curved surface. The movement of the 1986 Iwato slide was a slump.In addition, the slip surface is estimated to be inside the debris, or the detritus. Thus the slide can also be characterized as a debris slump, or a detritus slump.