A huge cloud of volcanic ash and gas rises above Mount Pinatubo, Philippines, on June 12, 1991. Three days later, the volcano exploded in the second-largest volcanic eruption on Earth in this century. Timely forecasts of this eruption by scientists from the Philippine Institute of Volcanology and Seismology and the U.S. Geological Survey enabled people living near the volcano to evacuate to safer distances, saving at least 5,000 lives.
Precursors to the 1991 Eruptions
On July 16, 1990, a magnitude
7.8 earthquake (comparable in size to the great 1906 San Francisco, California,
earthquake) struck about 60 miles (100 kilometers) northeast of Mount Pinatubo
on the island of Luzon in the Philippines, shaking and squeezing the Earth's
crust beneath the volcano. At Mount Pinatubo, this major earthquake caused
a landslide, some local earthquakes, and a short-lived increase in steam
emissions from a preexisting geothermal area, but otherwise the volcano
seemed to be continuing its 500-year-old slumber undisturbed. In March
and April 1991, however, molten rock (magma) rising toward the surface
from more than 20 miles (32 kilometers) beneath Pinatubo triggered small
earthquakes and caused powerful steam explosions that blasted three craters
on the north flank of the volcano. Thousands of small earthquakes occurred
beneath Pinatubo through April, May, and early June, and many thousand
tons of noxious sulfur dioxide gas were also emitted by the volcano.
Impacts of the Eruptions
Fortunately, scientists from
the Philippine Institute of Volcanology and Seismology and the U.S. Geological
Survey had forecast Pinatubo's 1991 climactic eruption, resulting in the
saving of at least 5,000 lives and at least $250 million in property. Commercial
aircraft were warned about the hazard of the ash cloud from the June 15
eruption, and most avoided it, but a number of jets flying far to the west
of the Philippines encountered ash and sustained about $100 million in
damage. Although much equipment was successfully protected, structures
on the two largest U.S. military bases in the Philippines--Clark Air Base
and Subic Bay Naval Station--were heavily damaged by ash from the volcano's
climactic eruption.
Nearly 20 million tons of
sulfur dioxide were injected into the stratosphere in Pinatubo's 1991 eruptions,
and dispersal of this gas cloud around the world caused global temperatures
to drop temporarily (1991 through 1993) by about 1 degree Fahrenheit (0.5
degree Celsius). The eruptions have dramatically changed the face of central
Luzon, home to about 3 million people. About 20,000 indigenous Aeta highlanders,
who had lived on the slopes of the volcano, were completely displaced,
and most still wait in resettlement camps for the day when they can return
home. About 200,000 people who evacuated from the lowlands surrounding
Pinatubo before and during the eruptions have returned home but face continuing
threats from lahars that have already buried numerous towns and villages.
Rice paddies and sugar-cane fields that have not been buried by lahars
have recovered; those buried by lahars will be out of use for years to
come.
The June 15, 1991, explosive eruption of Mount Pinatubo, Philippines, was the second largest volcanic eruption of this century and by far the largest eruption to affect a densely populated area. The eruption produced high-speed avalanches of hot ash and gas (pyroclastic flows), giant mudflows (lahars), and a cloud of volcanic ash hundreds of miles across. (U.S. bases have reverted to Philippine control since 1991.)
Following Mount Pinatubo's cataclysmic June 15, 1991, eruption, thousands of roofs collapsed under the weight of ash made wet by heavy rains (see example in photo above). Ash deposits from the eruption have also been remobilized by monsoon and typhoon rains to form giant mudflows of volcanic materials (lahars), which have caused more destruction than the eruption itself (photo at right shows village buried by lahars). (Photo above courtesy of Peter Baxter, University of Cambridge.
The Eruptions
From June 7 to 12, the first
magma reached the surface of Mount Pinatubo. Because it had lost most of
the gas contained in it on the way to the surface (like a bottle of soda
pop gone flat), the magma oozed out to form a lava dome but did not cause
an explosive eruption. However, on June 12 (Philippine Independence Day),
millions of cubic yards of gas-charged magma reached the surface and exploded
in the reawakening volcano's first spectacular eruption.
When even more highly gas
charged magma reached Pinatubo's surface on June 15, the volcano exploded
in a cataclysmic eruption that ejected more than 1 cubic mile (5 cubic
kilometers) of material. The ash cloud from this climactic eruption rose
22 miles (35 kilometers) into the air. At lower altitudes, the ash was
blown in all directions by the intense cyclonic winds of a coincidentally
occurring typhoon, and winds at higher altitudes blew the ash southwestward.
A blanket of volcanic ash (sand- and silt-size grains of volcanic minerals
and glass) and larger pumice lapilli (frothy pebbles) blanketed the countryside.
Fine ash fell as far away as the Indian Ocean, and satellites tracked the
ash cloud several times around the globe.
|
|