;16 July 1990: A 7.8
earthquake strikes the island of Luzon, Philippines. The epicenter was near the town of
Rizal, Nueva Ecija, roughly from Mount Pinatubo. This earthquake caused a landslide, some local tremors, and a brief increase in steam emissions from a preexisting
geothermal area at Mount Pinatubo. ;March–June 1991: Magma rising toward the surface from more than beneath Mount Pinatubo triggered small earthquakes and caused powerful steam explosions that blasted three craters on the north flank of the volcano. Thousands of small earthquakes occurred beneath Pinatubo, and many thousands of tons of
sulfur dioxide gas were emitted by the volcano. ;7 June 1991: First
magmatic eruptions, resulting in the formation of a high
lava dome at the summit of the volcano. ;10 June 1991: after receiving final authorization from the
Secretary of Defense, all non-essential military and Department of Defense civilian personnel and their dependents initiated land evacuation from Clark Air Base at 0600 local time. This land evacuation brought an estimated 15,000 personnel and several thousand vehicles onto U.S. Naval Base Subic Bay. ;12–14 June 1991: Several waves of eruptions generated
eruption columns up to in altitude and
pyroclastic flows (high speed avalanches of superheated gas and
tephra) extending out to from the summit. These eruptions were accompanied by nearly continuous
seismic activity and expulsion of huge quantities of ash, tephra, and
volcanic bombs. ;15 June 1991: Major eruption of Mount Pinatubo, sending ash and tephra over into the air. Clark Air Base and Subic Bay Naval Station, the two largest U.S. military bases in the Philippines, were heavily damaged by ash from this volcanic eruption. Nearly one foot of ash sodden by rain from
Typhoon Yunya (1991) accumulated on both Clark Air Base and U.S. Naval Base Subic Bay. Many buildings collapsed under the weight of the accumulated ash, and all flight operations were suspended at both bases for many days or even weeks afterwards. == Aftermath ==