A forgotten Luzon peak dimmed the planet in 1991
Mount Pinatubo, an active stratovolcano on the Zambales–Tarlac–Pampanga tripoint in Luzon, was so forest-cloaked that few knew its history until unrest in early 1991. On 15 June 1991 it produced a VEI-6 blast—the century’s second-largest after Alaska’s 1912 Novarupta—just as Typhoon Yunya soaked the ash.
Roughly 10 cubic kilometres of magma and about 20 million tonnes of sulphur dioxide reached the stratosphere, more particulate than any eruption since Krakatoa in 1883, seeding a worldwide sulphuric-acid haze that trimmed global temperatures by about 0.5 °C over 1991–1993. Early forecasts evacuated tens of thousands. The volcano sits about 87 kilometres northwest of Manila, 14 kilometres from Clark Air Base and 37 from Subic Bay, amid some six million neighbours.
Before 1991 the summit was only 1,745 metres high and barely 200 metres above surrounding ridges. Subduction along the Manila Trench feeds the Cabusilan chain. F. G. Delfin’s 1983–84 geothermal survey split ancestral Pinatubo (active from about 1.1 million years ago, milder andesite–dacite) from explosive modern Pinatubo inside the Tayawan Caldera.
Aeta communities had lived in the dense forests that hid the somma landscape.
Magma rising from more than 32 kilometres down set off steam blasts in March and April 1991, and on 15 June the climactic explosion drove an ash column 35 kilometres high; the summit caldera that collapsed afterward is 2.5 kilometres wide, and fresh flows filled valleys up to 200 metres deep. Remarkably, 1991 ranks among the smaller eruptions in the volcano's own history: its birth blast around 79,000 BC was about five times bigger. Rain then filled the new caldera as Lake Pinatubo, and after a controlled draining in 2001, the crater's west wall gave way on 10 July 2002, releasing some 160 million cubic metres of water and sediment.
Source: Mount Pinatubo
