GEOLOGIC HISTORY
Aso, with a summit elevation of 1,592 m, is located 75 km
east of Unzen and 150 km north of Sakura-jima volcanoes. The
24-km-wide Aso Caldera was formed during four major
explosive eruptions from 300,000 to 50,000 years B.P.;
pyroclastic-flow deposits covered much of Kyushu. A group of
15 central cones was constructed within the caldera, one of
which, Naka-dake, ranks among Japan's most active
volcanoes.
Historic Activity: Nake-dake has erupted more than
165 times since 553, the first documented historical
eruption in Japan.
Recent Activity:
- Areas of red glow on crater floor, the first since
June 1985, have been observed since Oct. 16th. and ash
was ejected from the crater on Dec. 28th. Two ash
emissions in April 1989. Strong ash eruptions began in
late July 1989 and 39 strong eruptions have occured thru
Dec. Continued eruptions thru Feb. 1990. No activity
March thru April. Renewed activity in May 1990 thru
February 1991. Isolated volcanic tremor episodes began to
increase in October 1991, reaching about 100 events/day
by the end of May 1992. Ejections of mud and water were
observed within the active crater lake during April 1992.
Blocks were ejected during the night of June 30th-July
1st from crater no. 1 of Nakadake cone for the first time
since the previous eruption ended in December 1990.
Vigorous steam emission followed for about 10 days.
Ejections of water, mud, and blocks above the surface of
the crater lake were observed through mid September.
Block were again ejected from the crater twice during
September 1992 (8th & 29th). An eruption occurred on
Oct. 26, 1992 and produced a 2500-m plume. No additional
activity during Nov. but more block eruptions during
early Dec. Eruption on Dec. 25, 1992 produced new vent.
Continuing ash plumes during Jan. 1993 with a small
eruption ejecting many scoria blocks on Jan. 22. Scoria
eruption on Feb. 20.
- During June and July 1994 there was phreatic activity
from Crater 1. Additional phreatic activity in Sept.
ejected rocks beyond the crater rim.
- During Dec. 1994 and Jan. 1995 there were
intermittent mud ejections and white plumes from Crater 1
and seismic tremor. Ash cloud to 1-km on March 17. Mud
ejections continued Jan. to Oct. 1995. Tremor continued
into 1996
- Crater glow was reported on April 27, 1996 and
reports continued into July.
Data Sources
- Smithsonian's SEAN Bulletin (V. 9, No. 10 to V. 14,
No. 12)
- Global Volcanism Network (V. 15, No. 1 to V. 22, No.
11).
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