Geologic Background:
Prior to 1980, Mount St. Helens formed a
conical, youthful volcano known as the Fuji-san of
America. Helens was formed during nine eruptive periods
beginning about 40-50,000 years ago, and has been the
most active volcano in the Cascade Range during the
Holocene. Prior to 2200 years ago, tephra, lava domes,
and pyroclastic flows were erupted, forming the older St.
Helens edifice, but few lava flows extended beyond the
base of the volcano. The modern edifice was constructed
during the last 2200 years, when the volcano produced
basaltic as well as andesitic and dacitic products from
summit and flank vents. During the 1980 eruption the
upper 400 m of the summit was removed by slope failure,
leaving a 2 x 3.5 km horseshoe-shaped crater now
partially filled by a lava dome.
Historic Activity:
- Mount St. Historical eruptions in the 19th century
originated from the Goat Rocks area on the north flank,
and were witnessed by early settlers.
- Contemporary accounts indicate activity several times
during the 1840s and 1850s but are not specific and in
part contradictory.
- The last significant pre-1980 activity was "dense
smoke and fire" in 1857, although minor, unconfirmed
eruptions were reported in 1898, 1903, and 1921.
Recent Activity:
- After over 120 years of quite, earthquakes and
phreatic steam eruptions began in March 1980. On May 18,
1980 a major eruption occurred, in which the top of the
cone was destroyed by a 2.7 cu km landslide which
suddenly removed pressure on a growing cryptodome,
leading to a massive debris avalanche, lateral blast,
plinian column, and dacitic pyroclastic flows and tephra.
The blast devastated about 600 sq km. The debris
avalanche buried the upper 24 km of the North Fork Toutle
valley to a depth of 50 m with material from the
collapsed volcano. The upper 400 m of the cone were
removed, leaving a 600 m deep, 2 km wide crater exposing
rocks as old as about 4,000 yrs. Periodic explosive
eruptions occurred until October 1980.
- Periodic short periods of dome growth occurred from
June 1980 to August 1982.
- Nearly continuous dome growth occurred from February
1983 to September 1984.
- Dome building episodes occurred in May to June 1985,
April to May 1986 and September to October 1986.
- Activity at Mt. St. Helens has generally remained at
background levels since a new lobe was added to the
composite lava dome in October 1986.
- Three small bursts of seismicity occurred in late
August 1989. Increase in seismicity began on Oct. 19 and
continued for about 10 days. Epicenters were generally in
the summit area with focal depths ranging from 1.6 to 11
km involving the entire magma conduit. A small ash
emission episode occurred on Dec. 6, 1989 along with
associated shallow seismicity.
- A brief explosive episode occurred on Jan. 6, 1990
ejecting ballistic tephra and a small amount of ash, and
triggered rock avalanches. Explosion-type seismic signal
on April 25, 1990. Brief explosive episode and ash
emission from north side of dome on Nov. 5, 1990. Another
small explosion from dome occurred on Dec. 20, 1990.
- Two episodes of dome activity occurred in February
1991. The first on Feb. 5 was an explosive event with a
column to about 7 km. altitude. The second on Feb. 14 was
an avalanche from the dome which formed a small lithic
pyroclastic flow.
Data Sources:
- Smithsonian's SEAN Bulletin (V. 8 to 12 &14)
- Global Volcanism Network (V. 15 to 23).
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