Table 9 Table summarising basaltic lava flow events at Izu-Oshima Volcano that have threatened inhabited areas since 1950

1986

Three ‘a’ā lava flows were formed. On the first day of the eruption, a clastogenic, ‘a’ā basaltic-andesite lava flow advanced 1.5 km, to within 1 km of Motomachi. The town’s airport was threatened although never inundated. The flow stopped advancing 200 m from town. The second lava flow in this eruption did not threaten any structures or necessitate evacuations. The third and final lava flow, an andesitic, clastogenic lava flow, approached the local volcanic observatory maintained by the University of Tokyo. It stopped flowing less than half a kilometre before reaching the observatory. (1 month)

In less than half a day, 12,000 people were evacuated on 39 vessels. In order to model the potential threat to the town, a new lava flow model was developed. Although lava flow attribute data (such as temperature and viscosity) are necessary to run the model presented in Ishihara et al. (1989), the data used was not measured. Rather, the values were set based on assumptions relative to previous flows (i.e. the silica content was higher, so the viscosity was assumed to be higher).

The evacuations lasted less than one month as the eruption was short.