USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
The 1991 Eruptions of Mount Pinatubo, Philippines
-- Edward W. Wolfe, 1992,
The 1991 Eruptions of Mount Pinatubo, Philippines:
Earthquakes and Volcanoes, v.23, no.1.
Introduction
Recognition of the volcanic unrest at Mount Pinatubo in
the Philippines began when steam explosions occurred on
April 2, 1991. The unrest culminated ten weeks later in the
world's largest eruption in more than half a century. Volcanologists
of the Philippine Institute of Volcanology and Seismology
(PHIVOLCS), joined in late April by colleagues
from the U.S. Geological Survey (USGS), successfully forecast
the eruptive events and their effects, enabling Philippine
civil leaders to organize massive evacuations that saved
thousands of lives. The forecasts also led to the evacuation
of Clark Air Base (U.S. Air Force), located just east of the
volcano. Nevertheless, coincidence of the climactic eruption
on June 15, 1991, with a typhoon led to more than 300
deaths and extensive property damage, both caused primarily
by the extraordinarily broad distribution of
heavy, water-saturated tephra-fall deposits. Runoff
from monsoon and typhoon rains is eroding and redistributing
the voluminous pyroclastic deposits emplaced
during the eruption. The resulting lahars, floods, and
sedimentation have exacerbated the already severe
social disruption and will be an enormous recurring
problem in future years.
Mount Pinatubo is one of a chain of composite volcanoes
known as the Luzon volcanic arc.
The arc parallels the west coast of the Island of Luzon
and reflects eastward-dipping, subduction along the
Manila Trench to the west. Mount Pinatubo is among
the highest peaks in west-central Luzon. Its former summit
lay at the 1,745-m crest of a 3-km-diameter dacite
dome formed during, an earlier eruptive
period. The volcano's flanks, made up largely of fragmental
(pyroclastic) deposits from prehistoric eruptions,
were intricately dissected and sheathed in tropical vegetation
(fig. 4 and 5).
Before the eruption, about 15,000 people lived in
small villages on the volcano's flanks. A much larger
population, about 500,000, lives in cities and villages on
broad, gently sloping alluvial fans surrounding, the volcano.
Clark Air Base is at the east base of the volcano,
within 25 km of the summit, and Subic Bay Naval Station
is about 40 km to the southwest.
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08/15/00, Lyn Topinka