Published online by Cambridge University Press: 05 August 2015
Mount Pinatubo (Philippines) – asleep for ∼ 500 years – began to stir in mid-March 1991, and produced a giant eruption on 15 June 1991, second largest of the twentieth century. Only that of remote Katmai-Novarupta, Alaska in 1912 was larger. About 20,000 indigenous Aeta lived on the volcano, and ∼1,000,000 lowland Filipinos lived around it. Two large American military bases, Clark Air Base and Subic Bay Naval Station, added about 40,000 Americans to those at risk. With centuries' of volcanic gas (supply) accumulated in tens of cubic kilometres of molten rock (magma), and with so many innocent people nearby, a disaster was waiting to happen.
Thick deposits from pumice-rich pyroclastic flows formed the lower slopes of the volcano and told a history of infrequent but very large eruptions - larger than any eruption in the history of modern volcano monitoring. Scientists warned that a giant eruption was possible, perhaps even likely, but none had ever been monitored, much less successfully forecast. For two months after the volcano began to stir, small earthquakes and other signs fluctuated without clear, systematic trends. The volcano was teasing the scientists, and the public was profoundly sceptical.
Against the odds, a team of scientists from the Philippine Institute of Volcanology and Seismology (PHIVOLCS), assisted by the US Geological Survey, correctly forecast a giant eruption. Evacuations that had been recommended earlier were now enforced and expanded. Over the course of a few days, small precursory eruptions escalated to a spectacular climax on 15 June that swept the whole volcano, killing virtually everything in its path. Avalanches of searingly hot ash and pumice (pyroclastic flows) filled valleys and swept over ridge crests. Tens of centimetres of ash, with weight nearly doubled by rain from simultaneous Typhoon Yunya, caused many roofs to collapse. Loss of life was relatively modest considering the population at risk and the enormous size of the events (∼400 died during the eruption, and ∼500 Aeta children died in evacuation camps from measles). Warnings, coupled with strong visible clues from pre-climactic eruptions, had saved nearly all of the Aeta population, plus an unknown number of lowlanders. Some damage was unavoidable, but much was also averted, especially damage to military assets and commercial jets.