GEOLOGIC HISTORY
Usu is a small stratovolcano located astride the southern
topographic rim of the 110,000-year-old Toya caldera. The
summit is cut by a somma formed ~ 7,000-8,000 years ago when
edifice collapse produced a debris avalanche that reached
the sea. The main edifice consists of basaltic to
basaltic-andesites with a small summit caldera. Ten dacitic
lava or cryptodomes lie on the summit and N slope arranged
in two lines trending NW-SE.
Historic Activity:
- The eruptions that occurred at the summit (in 1663,
1769, 1822, 1853 and 1977-82) commenced with a strong
Plinian phase and, apart from 1977-82, were accompanied
by pyroclastic flows. All but perhaps the 1769 eruption
also involved the growth of lava or cryptodomes in the
middle to final stages.
- Both summit (O-Usu and Ko-Usu) and flank
(Showa-Shinzan) lava domes, along with seven cryptodomes,
were erupted in historical time. The war-time growth of
Showa-Shinzan was painstakingly documented by the local
postmaster, who created the first detailed record of
lava-dome growth.
- During the flank eruptions (in 1910 and 1943-45) the
building of lava or cryptodomes was preceded by phreatic
explosions in the initial stage. Each eruption lasted
from one month to two years, with between thirty and one
hundred years of repose between them.
Recent Activity:
- Usu's major explosions of August 1977 were followed
by rapid cryptodome growth and seismicity. Smaller
explosions started in Nov. 1977, became more vigorous and
frequent during the following summer, and ended in
October. The rate of cryptodome growth increased briefly
in early 1978 but slowed gradually, as did accompanying
seismicity, for the next several years, ceasing rather
abruptly in early 1982.
- Seismic activity increased during first half of May
1996.
- An eruption that began on March 31, 2000 was
preceeded by several days of increased seismicity. The
eruption opened multiple vents and continued till at
least July 10.
Data Sources
- Smithsonian's SEAN Bulletin (V. 8, No. 12)
- Global Volcanism Network (V. 21, No. 5 to V. 26, No.
3).
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