Eruption of Mount Pinatubo in the Philippines in June 1991

Evaluation and Lessons Learned
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Emmanuel M. de Guzman, Consultant (Philippines)

The Pinatubo eruption of June 1991: The nature and impact of the disaster

Nature of the disaster

Reawakened after more than 500 years of slumber, Mount Pinatubo in the island of Luzon in the Philippines showed signs of imminent eruption early April 1991. On 12 June 1991 (Philippine Independence Day), its intermittent eruptions began. Three days after, on 15 June 1991, its most powerful eruption happened. Mount Pinatubo ejected massive volcanic materials of more than one cubic mile and created an enormous cloud of volcanic ash that rose as high as 22 miles into the air and grew to more than 300 miles across, turning day into night over Central Luzon. At lower altitude, the ash was blown in all direction by intense winds of a coincidental typhoon. At higher altitudes, the ash was blown southwestward. Volcanic ash and frothy pebbles blanketed the countryside. Fine ash fell as far as the Indian Ocean and satellites tracked the ash clouds several times around the globe. Nearly 20 million tons of sulfur dioxide were injected into the stratosphere and dispersed around the world causing global temperature to drop temporarily by 1*F from 1991 through 1993. Mount Pinatubo’s eruption was considered the largest volcanic eruption of the century to affect a densely populated area.

After the explosive eruptions, posing a more serious and lingering threat to life, property and environment were the onslaught of lahars. Within hours after the eruption, heavy rains began to wash deposits of volcanic ash and debris from the slopes down into the surrounding lowlands in giant, fast-moving mudflows. Containing 40% (by weight) volcanic ash and rock, lahars flow faster than clearwater streams. These steaming mudflows cascade as fast as 40 miles per hour and can travel more than 50 miles. With 90% volcanic debris, lahars move fastest and are most destructive. When they reach the lowlands, they have speeds of more than 20 miles per hour and are as much as 30 feet thick and 300 feet wide. They can transport more than 35,000 cubic feet of debris and mud per second.

For years, lahars continued to flow down the major river systems around the volcano and out into densely populated, adjoining lowlands. They destroyed and buried everything along their path: people and animals, farm and forest lands, bridges and natural waterways, houses and cars. They also rampage with terrifying rumbling sounds. By 1997, lahars had deposited more than 0.7 cubic miles (about 300 million dump-truck loads) of debris onto the lowlands, burying hundreds of square miles of land and causing greater destruction than the eruption itself. With the volume of volcanic debris deposited on the slopes of Mt. Pinatubo, the threat of lahars is expected to continue until year 2010.

The disaster brought about by the eruption of Mount Pinatubo had assumed a unique nature in view of the following: the widespread devastation that impacts on society and economy, the continuing threat of lahars and flooding, the destruction of endemic species of flora and fauna, the alteration of landscapes and land uses, and its impact on the global environment.

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