Teachers Guide to Stratovolcanoes of the World
Eruption Feature

Mt. Pinatubo, Philippines:
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Ash Cloud

The eruption of Mt. Pinatubo in 1991 stands as the second largest eruption in the 20th Century (Mt. Novarupta, Alaska 1912 claims largest eruption fame). Ash deposits 5 cm (2 in) thick or more covered a land area of about 4,000 square kilometers (1,544 squares miles) burning crops and other plant life around Pinatubo. A typhoon struck the area after the eruption. The weight of the rain-saturated ash, earthquake shaking and strong winds, caused numerous roofs to collapse in the communities around the volcano, including at the two large U.S. military bases Clark and Subic Bay.

The effects of the eruption were not limited to the area around Pinatubo. The eruption of Mt. Pinatubo affected weather around the globe. Huge quantities of particles from Pinatubo's tall ash cloud injected into the global wind system in the stratosphere. These particles affected the weather in two ways. Tiny aerosol droplets reflected sunlight away from Earth causing cooling at the surface. Scientists observed a maximum global cooling of about 1.5°C. Sunsets and sunrises were more brilliant because of the fine ash and gases high in the air. In addition, the aerosols from the eruption had a chemical effect that reduced the density of the ozone layer in the stratosphere. Until the ozone reforms, it cannot shield that portion of Earth as effectivelyfrom the sun.

More than 350 people died during the eruption, most of them from collapsing roofs. Disease that broke out in evacuation camps and the continuing mud flows in the area caused additional deaths, bringing the total death toll to 722 people. The event left more than 200,000 people homeless. Before the eruption, more than 30,000 people lived in small villages on the volcano.