GEOLOGIC HISTORY
Unzen is located on, and comprises much of, the Shimbara
Peninsula east of Nagasaki. It forms a large volcanic
complex that includes 3 large stratovolcanoes. Among the
many lava domes of the Unzen complex is Mayu=yama, the
source of the devastating 1792 avalanche and tsunami.
Historic Activity: The first recorded historical
eruption at Unzen was from Tsukumo-jima Pond at the summit
of Fugen-dake in 1663. Unzen last erupted in 1792. A huge
debris avalanche associated with the eruption slid into the
sea, causing a tsunami that killed 15,000 people.
Recent Activity:
- Increased seismicity began in November 1989.
- Small gas/ash eruption in November 1990.
- On May 20, 1991 a high-silica dacite lava extrusion
began to form a dome. Dome growth continued and
additional pyroclastic flows occurred almost daily. On
June 3 a large pyroclastic flow traveled about 4 km down
the Mizunashi River valley and killed 43 people
(including Harry Glicken and Maurice and Katia
Krafft).
- Dome #4 began forming on Sept. 17, 1991. Growth of
the lava dome in Jigoku-ato Crater continued through
mid-November and partial dome collapses frequently
generated pyroclastic flows.
- Dome #5 began forming on Nov. 21, 1991 and grew
upward about 50 m in mid December to become the highest
of the six 1991 domes.
- Dome #6 began forming on Dec. 3, 1991. Pyroclastic
flows, generated mainly from dome 6, advanced as much as
2.5 km SE down valleys at a rate of 10 to 20 per day
through mid January 1992.
- Dome 7 began forming on March 2, 1992 and was
continuing to grow till September 1992. Almost daily dome
collapse pyroclastic flows.
- Dome 8 began to emerge on August 11, 1992 and domes 7
& 8 continued to grow into September. Exogenous
growth of dome 8 continued through mid October. Partial
collapses of the dome complex often generated pyroclastic
flows. Domes 5 & 8 were continuing to grow as of
mid-Nov., 1992.
- A new dome (#9) began forming on Dec. 3, 1992.
- A new lava dome (dome 10) formed by Feb. 3, 1993.
Continued exogenous growth of Dome 10 until mid-March
when its NE half began collapsing, generating relatively
large pyroclastic flows that traveled 4 km east.
- Dome 11 began forming on the cliff face resulting
from the collapse of Dome 10.
- Beginning in August 1993 the terminology for lava
domes at Unzen changed. The numbered "lava domes" will be
called lobes, and the "lava dome complex" will be
referred to as the lava dome.
- The growth of lobe 12 began on Jan. 15, 1994.
- New lobe (#13) appeared on July 10 and grew
endogenously until late July. Continued slow endogenous
dome growth with accompanying pyroclastic flows through
mid-Dec. then slowed and stopped by end of Jan.
1995.
- No changes in dome, no large rockfalls, and no
pyroclastic flows have occurred since early Feb. 1995
through Jan 1996. Increased seismicity in July.
- Small block-and-ash flow in Feb. 1996. A partial dome
collapse on May 1 triggered a pyroclastic flow.
Data Sources
- Smithsonian's SEAN Bulletin (V. 9, No. 9)
- Global Volcanism Network (V. 15, No. 7 to V. 21, No.
5).
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