Mount Pinatubo is an active stratovolcano placed on the island of Luzon, at the intersection of the borders of the Philippine provinces of Zambales, Tarlac, & Pampanga. The volcano’s ultra-Plinian eruption in June 1991 produced the second largest terrestrial eruption of the 20th century (after the 1912 eruption of Novarupta) and the largest eruption in living memory. The colossal 1991 eruption had a Volcanic Explosivity Index (VEI) of 6, & came around 450–500 years after the volcano’s last known eruptive activity (estimated as VEI 5, the level of the 1980 eruption of Mount St. Helens), and some 1000 yrs after previous VEI 6 eruptive activity. Successful predictions of the onset of the climactic eruption led to the evacuation of tens of thousands of people from the surrounding areas, saving many lives, but surrounding areas were severely damaged by pyroclastic flows, ash deposits, and later by lahars caused by rainwater remobilizing earlier volcanic deposits. Thousands of houses & other buildings were destroyed.
The effects of the eruption were felt worldwide. It ejected roughly 10 billion metric tonnes /10 cubic km of magma, & 20 million tons of SO2, bringing great amounts of minerals & metals to the surface environment. It injected large amounts of aerosols into the stratosphere more than any eruption since that of Krakatoa in 1883. Over the next months, the aerosols formed a global layer of sulfuric acid haze. Global temperatures dropped by about 0.5 °C (0.9 °F), and ozone depletion temporarily increased substantially. The volcano is based in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is 42 km/26 miles west of the dormant and more prominent Mount Arayat, occasionally mistaken for Pinatubo.