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...