Mount Pinatubo Eruption

In June 1991, Mount Pinatubo on the Philippine island of Luzon erupted in one of the largest volcanic events of the twentieth century, second only to the 1912 eruption of Novarupta in Alaska by most measures. The eruption killed several hundred people, displaced hundreds of thousands, and injected so much material into the stratosphere that it measurably cooled the entire planet for about two years. It is also remembered as a major success in volcano forecasting, since early warnings saved many thousands of lives.

Pinatubo had been dormant for roughly 500 years and was heavily forested, so many residents did not even recognize it as a volcano. It lies about 90 kilometers northwest of Manila, near Clark Air Base and Subic Bay Naval Base, both then used by the United States military, and was home to Indigenous Aeta communities. In the spring of 1991, steam explosions, earthquakes, and rising sulfur dioxide emissions alarmed scientists. Philippine volcanologists from PHIVOLCS, working with the United States Geological Survey, installed monitoring equipment and set up a system of alert levels and evacuation zones.

The climactic eruption came in mid-June, with the largest explosive phase occurring on about 15 June. A towering ash column rose tens of kilometers into the sky, and pyroclastic flows swept down the volcano's flanks. The summit collapsed to form a caldera a few kilometers wide. The eruption coincided with the arrival of Typhoon Yunya, whose rain mixed with falling ash, making it heavy enough to collapse roofs. Many of the deaths, commonly put at around 800 when later disease in evacuation camps is included, resulted from collapsing buildings rather than from the eruption's direct blast.

The most far-reaching effects were global. Pinatubo released roughly 20 million tons of sulfur dioxide, which formed a veil of sulfate aerosols that reflected sunlight. Global average temperatures dropped by around half a degree Celsius over the following year or two, and the aerosol layer also contributed to vivid sunsets and temporarily affected stratospheric ozone chemistry. The event gave climate scientists a natural experiment, helping test computer models of climate and strengthening confidence in their ability to predict the effects of changes in the atmosphere.

Locally, the aftermath lasted years. Rain repeatedly remobilized the thick ash deposits into destructive mudflows called lahars, which buried towns and farmland in the surrounding lowlands. Clark Air Base was heavily damaged, and the United States abandoned it; Subic Bay was handed over to the Philippines the following year after negotiations over the bases failed. Today a crater lake fills the caldera, and Pinatubo is studied as a model of both volcanic hazard and successful disaster preparedness.