Geologic Background:
Api Siau, locally known as Karangetang, sits at the north
end of the island of Siau, north of Sulawesi, and contains
five summit craters strung along a N-S line. This
stratovolcano, the most active in north Sulawesi Provence.
Twentieth-century eruptions have included frequent explosive
activity sometimes accompanied by pyroclastic flows and
lahars. Lava dome growth has occurred in the summit craters;
collapse of lava flow fronts has also produced pyroclastic
flows.
Historic Activity:
- Api Siau is the most active volcano in N Sulawesi
province, with eruptions in 1921-1926, 1947-1949,
1961-1967, 1970-1971, 1975-1978, and almost every year
since 1983. These have been characterized by Strombolian
activity and lava flows; Merapi-style pyroclastic flows
have been common and have resulted in several
casualties.
Recent Activity:
- In May 1983, an eruption began with ejection of
incandescent tephra. Intermittent explosions began in
January 1984, and strong explosions in September fed
pyroclastic flows. In 1985, activity at summit and S
flank vents produced lava flows. Three small explosions
were reported in July 1987.
- Glowing lava was seen in July 1989.
- After an increase in seismicity in April 1991, ash
explosions began in the main central crater, ejecting
incandescent projectiles to 50-75 m height. Strombolian
activity lasted until August, when lava emission began in
the main crater. During September, explosive activity
decreased to ash emissions 25-75 m high, accompanied by
audible explosions and some incandescence.
- Activity increased in February 1992, and incandescent
ash emissions became continuous. An estimated 6,000,000
cu m of lava had accumulated in the crater and lava flows
began to advance down the south flank's Kali Keting
valley. On May 11, a pyroclastic flow caused by the
collapse of the lava flow front travelled 4 km from the
main crater down the Kali Keting, burning 7 farmers (6 of
whom later died in hospitals) and destroying more than 30
houses and about 2 sq km of coconut, cassava, and nutmeg
farms. Glowing rockfalls advanced down the Keting river
during July.
- Activity began on Jan. 21, 1993, when a hot mudflow
was observed flowing south along the Bahembang River to
4.5 km from the summit. Activity increased with ejection
of incandescent lava fragments and gray ash clouds.
Avalanches and rumbling noises were continuing as of Feb.
10.
- Weak, rhythmic explosions from an area located
between the two active lava domes were reported in July
1994.
- On 9 and 16 November, 1995 "thunderclaps" were heard
from the summit and a gray ash plume 500 m high was
observed along with incandescent ejecta. Repeated on Dec.
17, 1995.
- Ash emissions and crater glow were reported from
March to August 1999.
- Activity in 2000 included explosions, ash falls, lava
flows, and debris flows.
- Activity in 2001 included lava flows, lava
avalanches, and ash plumes.
Data Sources:
- Smithsonian's SEAN Bulletin (V. 1, No. 13 to V. 14,
No. 7)
- Global Volcanism Network (V. 16, No.8 to V. 26, No.
10).
|