15 June

Mount Pinatubo, in the Philippines, explodes in the second largest volcanic eruption of the 20th Century.

Mount Pinatubo, an active stratovolcano in the Cabusilan Mountains on the island of Luzon, the Philippines had, before the volcanic activities of 1991, an unknown eruptive history. It was heavily eroded, inconspicuous and obscured from view. It was covered with dense forest which supported a population of several thousand indigenous people belonging to the Aetas.

The volcano’s Plinian / Ultra-Plinian eruption on 15 June, 1991 produced the second largest terrestrial eruption of the 20th century after the 1912 eruption of Novarupta in the Alaska Peninsula. Complicating the eruption was the arrival of Typhoon Yunya, bringing a lethal mix of ash and rain to areas surrounding the volcano. Successful predictions at the onset of the climactic eruption led to the evacuation of tens of thousands of people from the surrounding areas, saving many lives, but the surrounding areas were severely damaged by pyroclastic flows, ash deposits, and subsequently, by the lahars caused by rainwaters re-mobilizing earlier volcanic deposits causing extensive destruction to infrastructure and changing the river systems months to years after the eruption.

The effects of the eruption were felt worldwide. It ejected roughly 10,000,000,000 tonnes or 10 km3 of magma, and 20,000,000 tonnes SO2 (sulphur dioxide), bringing vast quantities of minerals and metals to the surface environment. It injected more particulate into the stratosphere than any eruption since Krakatoa in 1883. Over the following months, the aerosols formed a global layer of sulfuric acid haze. Global temperatures dropped by about 0.5 °C (0.9 °F) in the years 1991-93, and ozone depletion temporarily increased substantially.