Mount Pinatubo Eruption (1991): The Philippines' Great Eruption

The eruption of Mount Pinatubo on 15 June 1991 was one of the largest volcanic eruptions of the 20th century. The volcano, on the island of Luzon about 90 kilometers northwest of Manila, had been quiet for around 500 years. Thanks to scientific monitoring and large-scale evacuations, thousands of lives were saved, but the eruption buried the surrounding lowlands in ash and mudflows and cooled the whole planet for more than a year.

Background

Mount Pinatubo rises in the Zambales Mountains of western Luzon. Before 1991 it was a forested, little-known peak about 1,745 meters high, and many people did not regard it as an active volcano. Its slopes were home to the Indigenous Aeta people, and around a million people lived in the surrounding provinces, including the cities of Angeles and Olongapo.

Two major US military bases lay nearby: Clark Air Base, about 25 kilometers east of the summit, and the Subic Bay Naval Base to the southwest. On 16 July 1990 a magnitude 7.7 earthquake struck northern Luzon about 100 kilometers away. Some scientists have suggested it helped trigger the volcano’s reawakening, though this remains uncertain.

Warnings and evacuations

On 2 April 1991 steam explosions blasted a line of vents on the upper slopes. The Philippine Institute of Volcanology and Seismology (PHIVOLCS) quickly installed seismometers, and in late April a team from the U.S. Geological Survey’s Volcano Disaster Assistance Program joined it. The joint team measured earthquakes and sulfur dioxide emissions and mapped deposits from earlier eruptions, which showed that Pinatubo had produced very large explosive eruptions in the past.

The scientists created a five-level alert system and hazard maps and worked to convince officials and residents of the danger. Evacuations widened as the alert rose. On 7 June the alert reached level 4, and on 9 June level 5, meaning an eruption was in progress. On 10 June about 14,500 American personnel and dependants were evacuated from Clark Air Base, leaving only a small security and scientific team.

The climactic eruption

A large explosive eruption on 12 June, Philippine Independence Day, sent an ash column about 19 kilometers into the sky. Further explosions followed over the next three days. On the afternoon of 15 June the climactic eruption began and continued for about nine hours. Its eruption column reached about 35 kilometers, and the ash cloud spread over some 125,000 square kilometers.

At the same moment, Typhoon Yunya (known locally as Diding) passed over Luzon. Its rain soaked the falling ash, making it heavy and sticky like wet cement. Roofs collapsed across the region, causing many of the deaths. Pyroclastic flows, avalanches of hot gas and rock, filled valleys around the volcano to depths of up to 200 meters.

The eruption expelled roughly 10 cubic kilometers of material, making it about ten times larger than the eruption of Mount St. Helens in 1980. It is rated 6 on the Volcanic Explosivity Index. The summit collapsed to form a caldera about 2.5 kilometers wide, lowering the mountain by roughly 260 meters. The caldera later filled with rainwater to form Lake Pinatubo.

Deaths and destruction

Around 850 people died as a result of the 1991 eruption, according to commonly cited estimates. About 300 were killed during the climactic phase, mostly by collapsing roofs. Many others, especially Aeta children, died from measles and other diseases in crowded evacuation centers. More than a million people were affected, and tens of thousands of homes were destroyed or damaged.

Without the evacuations, the toll would have been far higher. The success depended on scientists persuading local and military officials to act on uncertain evidence, using hazard maps, briefings and a widely shown video about volcanic dangers. The U.S. Geological Survey estimated that forecasts saved at least 5,000 lives and at least $250 million in property.

Lahars: a disaster lasting years

The eruption left several cubic kilometers of loose volcanic debris on the mountain’s slopes. Every rainy season, monsoon rains and typhoons remobilized it as lahars, fast-moving flows of volcanic mud that behave like wet concrete. Lahars buried farmland, roads and towns in the provinces of Pampanga, Tarlac and Zambales for more than a decade.

In October 1995 lahars triggered by Typhoon Sybil killed at least 100 people, many of them in the village of Cabalantian, and flows buried much of the town of Bacolor, Pampanga. The government built dikes and sediment-control structures, but many communities had to be permanently relocated.

Global effects

The eruption injected about 15 to 20 million tonnes of sulfur dioxide into the stratosphere, the largest volcanic sulfur dioxide cloud observed by satellites up to that time. Earlier eruptions such as Tambora in 1815 were larger, but none had been measured with modern instruments. The gas turned into a haze of sulfuric acid droplets that circled the globe within weeks and reflected sunlight back to space.

Average global surface temperatures fell by about 0.5 °C (0.9 °F) in 1992, a cooling that climate scientists had predicted using computer models and later used to test them. The particles also speeded up ozone depletion and produced vivid sunsets worldwide. Like the Year Without a Summer after Tambora, though on a smaller scale, Pinatubo showed how a single eruption can affect the climate of the whole planet.

The Aeta and displaced communities

The Aeta, who had farmed and hunted on Pinatubo’s slopes for generations, suffered especially heavily. Their villages were destroyed, their land was buried under thick deposits, and many spent months or years in resettlement camps far from the mountain. Disease and the loss of traditional livelihoods caused lasting hardship, and some families later returned to the volcano’s slopes to rebuild their communities.

Lowland towns faced a different problem. As lahars raised river beds by many meters, places that had survived the eruption itself were flooded and buried in later years. Displaced residents had to choose between rebuilding behind dikes and moving to new government settlements.

Political consequences

Clark Air Base was buried in ash, and the United States decided that repairing it was not worthwhile. It was handed over to the Philippines in November 1991. In September 1991 the Philippine Senate rejected a new treaty that would have extended the lease on Subic Bay, and the last US forces left in November 1992, ending nearly a century of American bases in the country. Clark and Subic were later redeveloped as special economic zones.

Timeline

• 1990, 16 July: A magnitude 7.7 earthquake strikes northern Luzon.
• 1991, 2 April: Steam explosions mark Pinatubo’s reawakening.
• 1991, 7 June: Alert level 4 is declared as magma rises.
• 1991, 10 June: Clark Air Base is evacuated.
• 1991, 12 June: The first major explosive eruption sends ash 19 km high.
• 1991, 15 June: The climactic eruption coincides with Typhoon Yunya.
• 1991, 26 November: The United States hands Clark Air Base to the Philippines.
• 1992: Global average temperatures are about 0.5 °C lower than before the eruption.
• 1995, October: Lahars bury communities in Pampanga and Tarlac.

Why it matters

Mount Pinatubo showed that volcanic science, combined with effective communication, can save thousands of lives. Its global cooling provided a natural experiment that helped confirm climate models. For the people of central Luzon, however, the eruption was the beginning of years of displacement and loss.