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Report on Toya (Japan) — October 1977


Natural Science Event Bulletin, vol. 2, no. 10 (October 1977)
Managing Editor: David Squires.

Toya (Japan) Continued cryptodome uplift and weak emission of steam clouds

Please cite this report as:

Global Volcanism Program, 1977. Report on Toya (Japan) (Squires, D., ed.). Natural Science Event Bulletin, 2:10. Smithsonian Institution.



42.544°N, 140.839°E; summit elev. 733 m

All times are local (unless otherwise noted)

Surface activity during September was confined to continued cryptodome uplift and weak emission of steam clouds, which rose a maximum of 600 m from summit vents [produced by the August eruption]. Daily recorded seismic events, including many felt shocks, declined irregularly through September, but remained high at the end of the month (figure 6).

Figure (see Caption) Figure 6. Graph of monthly volcanic earthquakes at Usu, July 1966-July 1977 [showing a gradual increase in seismicity]. The monthly mean is 9. Courtesy of JMA.

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.