Geologic Background:
Prior to 1991 Pinatubo volcano was a relatively
unknown, heavily forested lava dome complex with no
records of historical eruptions. The 1991 eruption, one
of the world's largest of the 20th century, ejected
massive amounts of tephra and produced massive
pyroclastic flows, forming a small, 2.5-km-wide summit
caldera. The eruption caused hundreds of fatalities and
extensive damage. Widespread lahars that redistributed
products of the 1991 eruption have continued to cause
severe disruption. Six major eruptive periods,
interrupted by lengthy quiescence, have occurred from
modern Pinatubo volcano during the past 40,000 years,
most of which produced major pyroclastic flows and lahars
that were more extensive than in 1991.
Pinatubo is composed of a high-silica hornblende
andesite-dacite dome complex, possibly located within a
small caldera. The last eruption prior to 1991
(635±90 years ago: C14), produced pumiceous
pyroclastic flows that filled the Marella River valley,
leading SW from the summit, and part of the E flank's
Sacabia River valley. Similar deposits of pyroclastic
flows and mudflows from older eruptions fill other
canyons around the volcano.
Historic Activity:
Recent Activity:
- An explosion on April 2, 1991 deposited ash over an
area of about 10,000 sq. m. The explosion devastated
about 1 sq. km. of forested land, stripped leaves and
vegetation over several square kilometers, and ejected
small steam/ash clouds, depositing ash 10 km away. No
injuries or deaths were reported but about 2000 people
were evacuated. After the explosion, a line of new
fumaroles, roughly 1 km long with 6 main vents, had
developed. Emissions, voluminous and at extremely high
pressure, were carried W onto a zone of dead and dying
vegetation.
- From April to mid-May earthquakes were centered about
4-8 km NW of summit at 3-6 km depth. Seismicity, harmonic
tremor, and emissions from vents began to increase in
late May and continued to into early June.
- A major pyroclastic phase began with explosinons on
June 12, 1991 producing a tephra column rising to 20 km.
The explosions were preceded by around 12-16 hours of
continuous tremor and several smaller explosions. Nine
additional explosions occurred from June 12 to June 15
producing columns rising 20 to 25 km high. Major
sustained activity began at 5:55 a.m. on June 15 and
continued until about 3:30 a.m. on June 16. The eruptive
column reached a maximum height of 40 km. The bulk of the
tephra fell to the SE, S, and SW, but airfall
distribution was complicated by a typhoon. Pyroclastic
flows reached radial distances of 12-18 km from the vent.
Smaller explosions continued during the next several
days.
- Infrared video taken from the Clark Air Base control
tower on June 14 suggests that the summit began to fail
at about 11:20 p.m. as a fissure propagated tangentially
across the summit. The 5:55 a.m. explosion and apparent
lateral blast the next day removed the summit and
produced a 25 km ash column. The eruption's most violent
explosive phase began at 1:42 p.m. on June 15 and
continued for more that 15 hours at full strength, with
30-40 km columns. Intense seismicity began at 2:40 p.m.
on June 15 and lasted until about midnight. This
seismicity was probably associated with the collapse that
formed a nearly circular caldera, 2 km in diameter. The
new summit is 145 m lower than before the eruption. The
caldera wall ranges in height from 150- 200 m at the
highest point to 0 m on the E side. No vegetation
remained within 1-2 km of the caldera and trees 6-7 km
away were defoliated.
- Following the paroxysmal explosions on June 14-16
activity decreased dramatically with nearly continuous
3-6 km columns. Occasional stronger explosions produced
columns upto 15 km high into mid-August. Debris-flows
were a problem during most of July and August due the
heavy rains related to typhoons.
- Explosions and ashflows continued thru Sept. 2, 1991.
Secondary explosions in hot ashflow deposits continued
into mid-October 1991. Vapor emission continues into Mar.
1992 along with rainfall induced lahars. Rain induced
lahars continue into June 1992. Rains have resulted in a
caldera lake.
- On July 9, 1992 a small lava dome began to emerge
from the caldera lake. By mid Sept. 1992 the lava dome
was 320 m across and 100 m high in the center of the 600
x 800 m crater lake. Lake depth was estimated at < 5
m. On the flanks secondary explosions were still being
triggered as water from rainfall encounters hot deposits
from the June 1991 eruption.
- The second rainy season since the June 1991 eruption
began in June 1993, resulting in numerous lahars from
June to October. Secondary explosions were still being
triggered as water from rainfall encounters hot deposits
in September.
- Seismic activity significantly increased starting on
Feb. 8, 1994 and peaked at >200 high-frequency
volcanic earthquakes on Feb. 11. Aerial surveys on Jan.
25 and Feb. 8 revealed phreatic cones and explosion pits
on the NW shore of the lake and on the caldera floor near
the north wall. The west half of the caldera lake also
appered muddy, but there was no sign of dome growth.
- Explosion-type earthquakes occurred between Jan.
7-10, 1995 but only minor evidence of surface activity
could be observed from the air within the caldera. In the
first half of July, a secondary explosion and several
lahars occurred on Pinatubo's flanks. The secondary
explosion vented from a still-hot pyroclastic-flow
deposit when recently introduced rainwater penetrated
into the pyroclastic-flow deposit's interior and flashed
into steam. Continuous and intense rainfall during July
28-30 caused by the passage of a tropical storm triggered
moderate to large lahars on the slopes of Mt. Pinatubo
along four river systems.
Data Sources:
- Global Volcanism Network (V. 16, No. 3 to V. 21, No.
3).
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