Mount Pinatubo

Mount Pinatubo in the Philippines erupted on June 15, 1991. When the 5,770-ft (1760 m) mountain shot sulfur dioxide 25 mi (40 km) into the atmosphere , the cloud mixed with water vapor and circled the globe in 21 days, temporarily offsetting the effects of global warming. Satellite images taken of the area after the eruption showed a dustlike smudge in the stratosphere . The sulfur dioxide cloud deflected 2% of the earth's incoming sunlight and lowered temperatures on worldwide average. Although the effects on global temperatures were significant, they are thought to be temporary. These light sulfur dioxides are expected to remain in the stratosphere for years and contribute to damage to the ozone layer.

The Philippine islands originated as volcanoes built up from the ocean floor. Most volcanoes erupt along plate edges where ocean floors plunge under continents and melting rock rises to the surface as magma. The earth's crust pulls apart, creating gaps where the magma can rise. The island of Luzon, where Mount Pinatubo is located, has thirteen active volcanoes. The pattern of volcanoes around the rim of the Pacific Ocean is called the Ring of Fire.

Mount Pinatubo sits in the center of a 3-mi (5-km) wide caldera,a depression from an earlier eruption that made the volcano collapse in on itself. A new cone formed over time, and geothermal vents gave a clue that the volcano was active. Before the 1991 eruption, Mount Pinatubo last erupted 600 years ago. In April 1991 steam eruptions, earthquakes, increasing sulfur dioxide emissions, and rapid growth of a lava dome all indicated a powerful impending blast. Minor explosions began on the mountain on June 12, 1991.

This major eruption occurred in a country with an already shaky economy, and the human effects are likely to be significant for a long period of time. A total of 42,000 homes and 100,000 acres (40,500 ha) of crops were destroyed. Nine hundred people died and 200,000 were relocated, with 20,000 people remaining in tent cities. More than 500 of those have died from disease and exposure. The country suffered over $1 billion in economic losses. Nevertheless, many lives were saved as a consequence of scientists' predictions of the eruption.

The destruction from Mount Pinatubo came mostly from lahars, rushs of cementlike mud, formed when heavy rains loosened the tons of ash dumped on the mountain's sides. These lahars can bury towns and roads and virtually anything else in the way. Also deadly were the pyroclastic flows, killer clouds of hot gases, pumice, and ash that traveled up to 80 mi (130 km) per hour across the countryside, up to 11 mi (18 km) away from the volcano.

See also Geothermal energy; Greenhouse effect; Ozone layer depletion; Plate tectonics

[Linda Rehkopf ]

RESOURCES

PERIODICALS

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Brasseur, G. "Mount Pinatubo Aerosols, Chlorofluorocarbons, and Ozone Depletion." Science 257 (28 August 1992): 1239–42.

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Kerr, R. A. "Pinatubo Global Cooling on Target." Science 259 (January 29, 1993): 594.

Monastersky, R. "Pinatubo Deepens the Antarctic Ozone Hole." Science News 142 (October 24, 1992): 278–79.