Report on Toya (Japan) — July 1978
Scientific Event Alert Network Bulletin, vol. 3, no. 7 (July 1978)
Managing Editor: David Squires.
Toya (Japan) Uplift slows, but steam explosions continue
Please cite this report as:
Global Volcanism Program, 1978. Report on Toya (Japan) (Squires, D., ed.). Scientific Event Alert Network Bulletin, 3:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN197807-285030
42.544°N, 140.839°E; summit elev. 733 m
All times are local (unless otherwise noted)
Steam explosions continued through late July, with ashfalls around the volcano from those observed on 2, 3, 4, 7, 11, 22, 26, 28, and 30 June, and 2 July. The rate of uplift at the new cryptodome decreased to 17.6 cm/day in May and 13.6 cm/day in June. Subsidence of Ko-Usu dome has virtually stopped.
The Japan Times reported a 5-minute eruption on 16 July, with ejecta rising about 1,000 m above the vent. Reuters reported that eruption columns reached 2,000 m on  July. [Ash eruptions occurred frequently 15-31 July.]
Geological Summary. Usuzan, one of Hokkaido's most well-known volcanoes, is a small stratovolcano located astride the southern topographic rim of the 110,000-year-old Toya caldera. The center of the 10-km-wide, lake-filled caldera contains Nakajima, a group of forested Pleistocene andesitic lava domes. The summit of the basaltic-to-andesitic edifice of Usu is cut by a somma formed about 20-30,000 years ago when collapse of the volcano produced a debris avalanche that reached the sea. Dacitic domes erupted along two NW-SE-trending lines fill and flank the summit caldera. Three of these domes, O-Usu, Ko-Usu and Showashinzan, along with seven crypto-domes, were erupted during historical time. The 1663 eruption of Usu was one of the largest in Hokkaido during historical time. The war-time growth of Showashinzan from 1943-45 was painstakingly documented by the local postmaster, who created the first detailed record of growth of a lava dome.
Information Contacts: JMA, Tokyo; Japan Times; Reuters.